Patent Application
20240416416
Embodiments relate to methods for producing metallic nanoparticles that are free or nearly free of oxide. Some embodiments include coating the metallic nanoparticles with a passivating layer to prevents re-oxidation.
Metallic nanoparticles can release large amounts of energy during combustion. The amount of energy release can be more than most common fuels. Such metallic nanoparticles can be used as additives to common fuels to improve the energy content of the fuels. These type of fuels are known as nanofuels. One technical problem that plagues existing nanofuels and methods of producing the same is the existence of oxide on the metallic nanoparticles within the additive. For instance, metallic nanoparticles will oxidize and form a native oxide layer on a surface thereof. The native oxide layer can significantly reduce the amount of energy that can be extracted from the nanoparticles when used as additives in a nanofuel. Known methods of reducing the native oxide layer from nanoparticles include hydrogen reduction of oxide and solvent washing. Solvent washing is successful at removing the oxide layer, but it cannot prevent re-oxidation of the surface—i.e., the metallic nanoparticle quickly re-oxidizes when stored. Furthermore, solvent washing has only been successfully demonstrated for born nanoparticles.
Disclosed are methods for producing metallic nanoparticles that are: (1) nearly free or are free of oxide; (2) coated with a passivating layer that prevents re-oxidation; (3) decomposes during combustion; and/or (4) improves combustion of the metal. Embodiments involve reduction or removal of native oxide on the surface of metal nanoparticles by treatment in hydrogen plasma at low pressure (e.g., glow discharge). In the alternative, some embodiments involve generating a mechanical blend of nanoparticle metals to remove or reduce native oxide on the surface of metal nanoparticles. After reduction or removal of native oxide, the resultant nanoparticle metal or nanoparticle metal blend can be coated to prevent re-oxidation. This can involve deposition of a fluorocarbon film on the nanoparticle metal or nanoparticle metal blend.
It should be noted that embodiments of the method can be applied to nearly any metal. As noted herein, the metal nanoparticle metal is highly energetic during oxidation (e.g., when mixed with a fuel as an additive and burned), but a native oxide layer formed thereon limits the available amount of energy per gram of metal nanoparticle. The inventive method, however, eliminates or significantly reduces the oxide. In some embodiments, the inventive method effectively turns the oxide into a fuel.
The inventive method ameliorates energy release characteristics and the storage life of metal nanoparticles. In addition, the inventive method is a dry process that exposes metal nanoparticles to a minimum number of reagents. The inventive process also obviates postprocessing operations. Furthermore, the byproduct of the inventive process is water vapor.
An exemplary method can relate to processing a metal nanoparticle. The method can involve reducing or eliminating metal oxide formed on a surface of a metal nanoparticle via non-thermal hydrogen plasma treatment to generate an oxide-free metal nanoparticle. The method can involve passivating the surface of the oxide-free metal nanoparticle.
In some embodiments, the non-thermal hydrogen plasma treatment can be performed on a plurality of metal nanoparticles. In some embodiments, the passivation can be performed on the plurality of metal nanoparticles.
In some embodiments, the non-thermal hydrogen plasma treatment can involve a glow discharge plasma formation technique.
In some embodiments, the non-thermal hydrogen plasma treatment can be performed: at temperatures ≤50° C.; and at pressures less than 0.2 Torr.
In some embodiments, the non-thermal hydrogen plasma treatment can generate hydrogen plasma that reduces metal oxides on the surface.
In some embodiments, the hydrogen plasma can comprise reactive H atoms, H ions, and vibrationally excited H2 that react with the metal oxide to reduce the metal oxide via a reduction reaction, the reduction reaction yielding elemental metal as the oxide-free metal nanoparticle and H2O.
In some embodiments, the passivating can involve functionalization the surface or encapsulation of the oxide-free metal nanoparticle.
In some embodiments, the passivation can generate a coating that inhibits or prevents re-oxidization of the surface.
In some embodiments, the passivating can involve plasma-enhanced chemical vapor deposition (PECVD).
In some embodiments, the passivating can involve plasma-enhanced chemical vapor deposition (PECVD). The passivation can generate a coating that inhibits or prevents re-oxidization of the surface. Tailoring coating properties can be via selection of a precursor for the PECVD, selection of a carrier for the precursor for PECVD, and/or adjustment of residence time of vapors of the precursor during PECVD.
In some embodiments, the metal nanoparticle can comprise boron, aluminum, copper, iron, or magnesium. The metal nanoparticle size can range from 70 nm to 80 nm.
In some embodiments, the metal nanoparticle can be boron. Passivation can involve functionalization via alkoxy groups, halogens, silanes, organic acid, or polymers. In some embodiments, the metal nanoparticle can be aluminum. Passivation can involve plasma-enhanced chemical vapor deposition (PECVD) using isopropyl alcohol (IPA), toluene, and perfluorodecalin (PFD) precursors.
An exemplary embodiment can relate to a method for reducing or eliminating oxide on a metal nanoparticle. The method can involve mechanically blending a first metal nanoparticle and a second metal nanoparticle to generate a mechanical blend, the first metal nanoparticle being an oxide-free metal nanoparticle, the second metal nanoparticle having metal oxide formed on a surface thereof. The first metal nanoparticle can reduce the metal oxide of the second metal nanoparticle via a redox reaction.
In some embodiments, the mechanical blending can involve mechanically blending a plurality of first metal nanoparticles and a plurality of second metal nanoparticles.
In some embodiments, the redox reaction can produce an oxide-free second nanoparticle.
In some embodiments, the first metal nanoparticle can be different from the second metal nanoparticle.
In some embodiments, the first metal nanoparticle can be aluminum and the second metal nanoparticle is boron.
In some embodiments, mechanical blending can involve magnetic agitation.
In some embodiments, the method involves passivation of the mechanical blend.
An exemplary embodiment can relate to a method for producing an additive for a nanofuel. The method can involve reducing or eliminating oxide formed on surfaces of metal nanoparticles via non-thermal hydrogen plasma treatment to generate oxide-free metal nanoparticles. The method can involve passivating the surfaces of the oxide-free metal nanoparticles. The method can involve forming an additive composition comprising the passivated oxide-free metal nanoparticles.
An exemplary embodiment can relate to a method for producing an additive for a nanofuel. The method can involve mechanically blending first metal nanoparticles and second metal nanoparticles to generate a mechanical blend, the first metal nanoparticles being oxide-free metal nanoparticles, the second metal nanoparticles having metal oxides formed on surfaces thereof. The first metal nanoparticles reduce the metal oxides of the second metal nanoparticles via a redox reaction. The method can involve forming an additive composition comprising the mechanical blend.
An exemplary embodiment can relate to a method for improving energy release of a metal. The method can involve depositing a fluorinated film on metal nanoparticles via plasma deposition.
In some embodiments, the metal nanoparticles can have metal oxides on surfaces thereof and the fluorinated film is deposited on the metal oxides. In some embodiments, metal oxides can have been removed from the metal nanoparticles or metal oxides on the metal nanoparticles have been reduced before deposition of the fluorinated film.
Further features, aspects, objects, advantages, and possible applications of the present invention will become apparent from a study of the exemplary embodiments and examples described below, in combination with the Figures, and the appended claims.
The above and other objects, aspects, features, advantages, and possible applications of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings. It should be understood that like reference numbers used in the drawings may identify like components.
The following description is of an embodiment presently contemplated for carrying out the present invention. This description is not to be taken in a limiting sense but is made merely for the purpose of describing the general principles and features of the present invention. The scope of the present invention should be determined with reference to the claims.
Referring to
As noted above, embodiments of the method reduce or eliminate the metal oxide of the metal nanoparticle. It is contemplated for the method to generate an oxide-free metal nanoparticle, but it is understood that the method can be used to reduce the metal oxide to any level or degree. “Oxide-free” for purposes of this disclosure refers to generating elemental metal of the metal nanoparticle. This can include a metal nanoparticle that has no surface upon which a metal oxide is present, a metal nanoparticle that has a majority of its surface upon which no metal oxide is present, a surface that has a reduced thickness of metal oxide, etc. such that the resultant meal nanoparticle constitutes elemental metal. It is further contemplated for the inventive method to be applied to a sample or bulk comprising a plurality of metal nanoparticles. “Oxide-free” can include a sample or bulk of metal nanoparticles wherein each metal nanoparticle has no surface upon which a metal oxide is present, a sample or bulk of metal nanoparticles wherein a majority of metal nanoparticles has no metal oxide, a sample or bulk of metal nanoparticles wherein a majority (or other predetermined amount) of metal nanoparticles has a reduced amount of metal oxide etc. such that the sample or bulk constitutes elemental metal.
The method can involve reducing or eliminating metal oxide formed on a surface of a metal nanoparticle via non-thermal hydrogen plasma treatment to generate an oxide-free metal nanoparticle. The method can further involve passivating the surface of the oxide-free metal nanoparticle. Again, it is contemplated the inventive method to be applied to a sample or bulk of metal nanoparticles, and thus the method can be applied to a plurality of metal nanoparticles-i.e., the non-thermal hydrogen plasma treatment can be performed on a plurality of metal nanoparticles, and the passivation can be performed on the plurality of metal nanoparticles.
The non-thermal hydrogen plasma treatment can involve a glow discharge plasma formation technique. For instance, hydrogen plasma can be formed by passage of electric current through hydrogen gas. This can include applying a voltage between two electrodes in a vessel containing hydrogen gas at low-pressure so as to induce gas ionization. When the voltage exceeds the striking voltage, gas ionization becomes self-sustaining, leading to hydrogen plasma formation. It is contemplated for the non-thermal hydrogen plasma treatment to be performed at temperatures ≤50° C., and at pressures below 0.2 Torr and preferably between 0.1 Torr and 0.2 Torr. Such temperature and pressure ranges prevent aggregation and sintering of the metal nanoparticles.
The non-thermal hydrogen plasma treatment generates hydrogen plasma that reduces metal oxide(s) on the surface(s) of the metal nanoparticle(s). For instance, the hydrogen plasma comprises reactive H atoms, H ions, and vibrationally excited H2 that react with the metal oxide to reduce the metal oxide via a reduction reaction. The reduction reaction (discussed in detail later) yields elemental metal as the oxide-free metal nanoparticle and H2O. Non-thermal hydrogen plasma treatment, as opposed to thermal hydrogen plasma treatment, offers inherent thermodynamic and kinetic advantages for the reduction of metal oxide due to the formation of highly reactive atomic and ionic species at low temperatures. Thus, non-thermal hydrogen plasma treatment may be preferred in some situations.
Hydrogen plasma can reduce almost every metal oxide at low temperatures because of the highly negative Gibbs energy change involved with the reaction. For instance, with boron as the metal nanoparticle, formation of highly reactive hydrogen atoms, ions, and vibrationally excited H2 molecules that react with the oxide layer at the plasma-boron oxide interface lead to the reduction reaction forming elemental boron and water vapor with a highly negative Gibbs energy at 298 K for atomic hydrogen. The low temperature (e.g., at or below 100° C. and preferably at or below 50° C.) of non-thermal hydrogen plasma enables native oxide removal without sintering of the metal nanoparticles.
The passivating can involve functionalization the oxide-free surface or encapsulation of the oxide-free metal nanoparticle. Passivation can be done to prevent re-oxidization of the oxide-free area(s). This can include preventing re-oxidization via by air and/or humidity, for example. Passivation can include passivation of the entire surface or a portion of the surface that is oxide-free. This can also include passivation of the entire surface or a portion thereof of the metal nanoparticle, regardless of how much of the metal nanoparticle has been processed to be oxide-free. Again, as noted herein, it is contemplated for the inventive method to be applied to a sample of bulk of metal nanoparticles. Passivation can include applying functionalization or encapsulation of all of the metal nanoparticles in the sample or bulk, functionalization or encapsulation of a majority of the metal nanoparticles in the sample or bulk, functionalization or encapsulation of a predetermined amount of the metal nanoparticles in the sample or bulk, etc.
The passivation generates a coating that inhibits or prevents re-oxidization of the oxide-free surface. The coating can be from 1.5 nm to more than 100 nm or even 1μ, but preferably between 2.5 nm to 30 nm thick.
The passivating can involve plasma-enhanced chemical vapor deposition (PECVD). Some embodiments involve argon PECVD. Other passivation methods can include liquid phase processing: (a) by grafting a molecule on the particle surface, (b) via ball milling, where mechanical force induces reactions at the surface of the particle thought to offer passivation, and (c) or in some cases by mixing a material that is thought to cover the particle surface and provide passivation. The advantages of the methods disclosed herein are: (1) avoidance of contamination because of the avoidance of solvents and other chemical that would otherwise have to be removed later, (2) delivering the passivating material in the very small doses (i.e., thin coatings), which is required for optimum performance, and (3) it is performed in the same device where the removal of oxide takes place without risking exposing the reduced metal to air before passivation.
PECVD can be used to deposit a thin film from a gas state (vapor) to a solid state on a surface of the metal nanoparticle, the thin film being the coating. The PECVD techniques generally involve use of precursors. The inventive method can involve tailoring coating properties via selection of a precursor for the PECVD, selection of a carrier for the precursor for PECVD, and/or adjustment of residence time of vapors of the precursor during PECVD. For instance, coating chemistry can be tailored to control interfacial properties by merely changing the precursor. Deposition of the precursor from the gas phase enables nanometer-level control of the coating through adjustment of the residence time of precursor vapors in the plasma reactor. Argon (or any other inert gas) gas can be used as a carrier of precursor vapors to the plasma reactor, and may serve as a plasma generator gas because of its inert nature.
It is contemplated for passivation to be performed at room temperature (20° C. to 100° C., but preferably at or below 50° C.) so as to avoid aggregation and sintering of the metal nanoparticles. The passivating agent should not interfere with combustion (when the coated metal nanoparticle is used as an additive in a nanofuel), and should be effective in amounts sufficiently small such that the volumetric energy density of the metal nanoparticles is not affected.
Referring to
As noted herein, the method can be used on any metal nanoparticle. It is contemplated for the metal nanoparticle to be boron, aluminum, copper, iron, or magnesium, as these are most useful as components for nanofuel additives. It is further contemplated for the metal nanoparticle size to be within a range from 10 nm to 1 micron.
A particular metal nanoparticle of interest is boron. With boron as the metal nanoparticle, passivation can involve functionalization to modify the surface chemistry of the metal nanoparticle. This can be achieved via use of alkoxy groups, halogens, silanes, organic acid, or polymers. Another particular metal nanoparticle of interest is aluminum. With aluminum, passivation can involve PECVD using isopropyl alcohol (IPA), toluene, and/or perfluorodecalin (PFD) precursors. While functionalization of born and PECVD of aluminum is discussed as examples, it is understood that the inventive method is not limited to these types of passivation for these particular elements.
In the alternative to non-thermal hydrogen plasma treatment, a method for reducing or eliminating oxide on a metal nanoparticle can involve mechanically blending a first metal nanoparticle and a second metal nanoparticle to generate a mechanical blend. The mechanical blending can be via any mechanical means, but it is contemplated for the mechanical blending to be via magnetic agitation, shaking, tumbling in rotating cylinders, use of ultrasonic energy, etc. The first metal nanoparticle is an oxide-free metal nanoparticle, and the second metal nanoparticle is a metal nanoparticle having metal oxide formed on a surface thereof. The first metal nanoparticle reduces the metal oxide of the second metal nanoparticle via a redox reaction (discussed in detail later). The redox reaction can produce an oxide-free second nanoparticle. Again, the method is contemplated for use on a sample or bulk of a plurality of metal nanoparticles, and thus mechanical blending can involve mechanically blending a plurality of first metal nanoparticles and a plurality of second metal nanoparticles. Thus, embodiments of the method reduces or removes metal oxide of the second metal nanoparticle in situ during oxidation via an overall exothermic redox reaction with the first metal nanoparticle that enriches the second metal nanoparticle at the expense of first metal nanoparticle.
It is contemplated for the first metal nanoparticle to be different from the second metal nanoparticle. For instance, first metal nanoparticle can be aluminum and the second metal nanoparticle can be boron. Metals can be chosen so that one metal reduces the other. This is done based on the standard reduction potential: The metal with high reduction potential (Al, 1.676 Volts*) can reduce the metal oxide of a metal with relatively less reduction potential (B, 0.890 Volts*). The reduction potential listed above are standard reduction potentials (measured at standard conditions). Similarly, Mg (2.38 V) can reduce oxides of Al (1.676 V) and B also (0.89 V). So, the idea is to use a metal with less energy (say Mg) than the primary metal (say Al) that can reduce the oxide of the primary metal. The reaction liberates Al from its oxide, but this can only work if Mg reduces Al oxide and not the other way around.
The relative percentages of the first and second metal nanoparticles in the mechanical blend can be adjusted to meet desired design criteria. For instance, and as will be explained in more detail later, with Al as the first metal nanoparticle and B as the second metal nanoparticle, an Al-10% to B-90% mechanical blend outperforms 100% B nanoparticle by 40% with respect to energy release under combustion. Chemical bonding between the two elements would lead to significant ignition delays, which is detrimental in applications that require fast energy release. These limitations are overcome if Al and B form a mechanical blend rather than a chemical compound.
Some embodiments involve passivation (via any of the passivation techniques disclosed herein) of the mechanical blend.
As noted herein, embodiments of the methods disclosed herein can be used to produce an additive for a nanofuel. The additive can include a composition comprising the oxide-free metal nanoparticles and/or mechanical blend disclosed herein. For instance, a method for producing an additive for a nanofuel can involve reducing or eliminating oxide formed on surfaces of metal nanoparticles via non-thermal hydrogen plasma treatment to generate oxide-free metal nanoparticles. The method can involve passivating the surfaces of the oxide-free metal nanoparticles. The method can involve forming an additive composition comprising the passivated oxide-free metal nanoparticles.
As another example, a method for producing an additive for a nanofuel can involve mechanically blending first metal nanoparticles and second metal nanoparticles to generate a mechanical blend. The first metal nanoparticles are oxide-free metal nanoparticles, and the second metal nanoparticles have metal oxides formed on surfaces thereof. The first metal nanoparticles reduce the metal oxides of the second metal nanoparticles via a redox reaction. The method can involve forming an additive composition comprising the mechanical blend.
The additive composition can include the oxide-free metal nanoparticles and/or the mechanical blend. The oxide-free metal nanoparticles and/or the mechanical blend can be in an emulsion. Other ingredients of the additive may include a catalyst, a surface active agent, an emulsifying aid, an interphase modifying agent, etc. used to generate a predetermined material property for the nanofuel.
The following example demonstrates development of an in situ non-thermal plasma technology to improve the oxidation and energy release of boron nanoparticles. The example shows reduction of the native oxide layer on the surface of boron nanoparticles (70 nm) by treatment in a non-thermal hydrogen plasma, followed by the formation of a passivation barrier by argon plasma-enhanced chemical vapor deposition (PECVD) using perfluorodecalin (C10F18). Both processes occur near room temperature, thus avoiding aggregation and sintering of the nanoparticles. High-resolution transmission electron microscopy (HRTEM), high-angular annular dark-field imaging (HAADF)-scanning TEM (STEM)-energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) demonstrated a significant reduction in surface oxide concentration due to hydrogen plasma treatment and the formation of a 2.5 nm thick passivation coating on the surface due to PECVD treatment. These results correlated with the thermal analysis results, which demonstrated a 19% increase in energy release and an increase in metallic boron content after 120 min of hydrogen plasma treatment and 15 min of PECVD of perfluorodecalin. The PECVD coating provided excellent passivation against air and humidity for 60 days. Results conclude that in situ non-thermal plasma reduction and passivation lead to the amelioration of energy release characteristics and the storage life of boron nanoparticles, which can benefit conducive for nanoenergetic applications.
Metal nanoparticles have great potential as additives to liquid and solid fuels and even as fuels themselves since they oxidize readily and release large amounts of heat while producing no greenhouse gas emissions. Compared to liquid fuels, the volumetric energy content of metals is much higher. It has been estimated that a vehicle utilizing aluminum as fuel would cover three times the distance without refueling compared to a vehicle running on gasoline. Current research focuses on their use as a secondary fuel in volume-limited propulsion with the goal of improving the energy efficiency of existing engines without adversely impacting other fuel properties such as freezing point, flash point, and viscosity.
Nanometer-sized particles are especially advantageous. They exhibit higher reactivity, lower melting points, enhanced heat and mass transfer properties, lower sintering temperatures, and faster heat release as compared to micron and larger-sized particles as well as better overall combustion characteristics. Thus, it is possible to improve the performance of conventional liquid fuels by the addition of rather small amounts of energetic nanomaterials. In order to utilize the fuel without the need to modify the engine, it is important to maintain a low volume fraction of solids. On the other hand, new challenges arise when the particle size is too small. The main limitation is due to the presence of the native oxide layer. While this layer provides passivation during storage for nanometer sized particles, it represents a significant fraction of the particle mass. Additionally, nanoparticles are prone to aggregation, leading to unstable dispersions that cause precipitation, deposition on pipe walls, and pump erosion during fuel transportation.
Aluminum, boron, copper, iron, and magnesium are among the most extensively studied energetic materials. Boron is of particular interest because of its exceptionally high gravimetric (58 kJ/g) and volumetric (140 KJ/mL) energy densities, which are much greater than hydrocarbon fuels (˜25-40 KJ/g), 1,5 explosives (˜10-15 kJ/g), and other high energy density metals like magnesium (25 kJ/g) and aluminum (31 kJ/g). Boron has applications in various fields including medicine (mineral for building strong bones), the military (aircraft, missiles, and other specialized weapons), protective coatings, and semiconductors. A number of studies demonstrated the liquid-phase and vapor-phase synthesis of boron nanoparticles (BNPs), but they were unable to circumvent the formation of a native oxide layer. The main impediment in the application of boron nanoparticles is the reduced energy output and lagged combustion reaction kinetics due to the formation of a native oxide shell that acts as a barrier to further oxidation of the core. Strong reducing agents such as hydrogen and carbon may be used to reduce the oxide, but the reaction is thermodynamically infeasible even at very high temperatures. Hydrogen plasma, through either non-thermal routes, can be used to reduce the thickness of the native oxide layer. Non-thermal plasma offers inherent thermodynamic and kinetic advantages for the reduction of metal oxide due to the formation of highly reactive atomic and ionic species at low temperatures. Non-thermal hydrogen plasma has been shown to successfully remove the native oxide layer from germanium, copper, silicon, and ruthenium surfaces. Hydrogen plasma can reduce almost every metal oxide at low temperatures because of the highly negative Gibbs energy change. The driving force for reduction is the formation of highly reactive hydrogen atoms, ions, and vibrationally excited hydrogen molecules that react with the oxide layer at the plasma-boron oxide interface leading to the reduction reaction forming elemental boron and water vapor with a highly negative Gibbs energy at 298 K for atomic hydrogen. The low temperature of non-thermal hydrogen plasma enables native oxide removal without the sintering of nanoparticles.
In the absence of the oxide layer, the boron surface reoxidizes rapidly upon contact with air and moisture.
Passivation by surface functionalization or encapsulation is necessary to prevent reoxidation. BNPs have been functionalized using alkoxy groups, halogens, silanes, organic acid, and polymers. A simple synthesis route of functionalized BNPs at room temperature was suggested by Pickering et al. Boron tribromide was reduced to form a sticky, pale yellow compound rich in boron followed by a reaction with excess octanol to produce octyloxy-capped BNPs. Gas-phase pyrolysis of decaborane followed by surface functionalization with halides (Br and F) was studied by Bellott et al. Passivation of BNPs against oxidation is possible at the cost of reduced energy release as suggested by their thermal analysis results. The energy release decreased by 18% and 2% after forming F and Br capping, respectively. This decrease is the result of the chemical bonding of the boron surface with halogens, which reduce the percentage of metallic boron available for the oxidation reaction. Shin et al. synthesized BNPs in the range of ˜30-70 nm by gas-phase nucleation of BCl3 by rapid expansion through a nozzle within a thermal plasma. As-made particles were metallic boron with a hydride layer on the surface, but under storage in ambient air, oxidation occurred and the thickness of the oxide layer stabilized within 2 days. A different approach was used by Van Devener et al. to produce air-stable boron with low oxide content. High energy ball milling was used to reduce the size of commercial micron-sized boron particles down to ˜50 nm in the presence of oleic acid. Size reduction exposes boron on the particle surface, and even though the oxide itself is not removed, fine particles are highly enriched in metallic boron. It was shown that capping with oleic acid was successful in producing air-stable BNPs with no measurable oxygen at the detection limit of X-ray photoelectron spectroscopy (XPS). Chintersingh et al. reported a method to remove the hydrated surface oxide from boron particles by washing them in acetonitrile with and without hydrocarbon fluids. They observed that the resulting boron powder displayed shorter ignition delays as compared to starting commercial boron powder. Combustion characteristics of the boron powder remained unaffected after this modification process. Valluri et al. studied a double displacement, salt metathesis reaction between boron and bismuth fluoride to passivate the surface of boron. The experimental results suggested the ignition temperature for the oxidation of the material was reduced after the employment of the bismuth fluoride coating. Additionally, single-particle combustion experiments showed an accelerated burn rate from the coated sample as compared to boron. The present disclosure demonstrates that the surface functionalization of boron using fluoride-based coatings is an efficient way to optimize its combustion characteristics.
To fully utilize the energy density of BNPs, one must address two problems: eliminate the native oxide at the particle surface while protecting the particle from further oxidation under normal storage conditions. Additionally, the passivating agent should not interfere with combustion and should be effective in amounts sufficiently small, such that the volumetric energy density of the particles is not affected. Ideally, the removal of the oxide layer and passivation of the surface should be done in an integrated process that avoids exposure to air, is capable of utilizing commercially available powders, and minimizes the use of chemical reagents. Disclosed herein is a low-pressure plasma process that accomplishes these goals.
Plasma-enhanced chemical vapor deposition (PECVD) is a versatile technique for the fabrication of surface films since it can utilize chemical precursors in any physical form. It is a dry and readily scalable process that does not require postprocessing separations. The advantage of PECVD lies in its ability to tailor the coating chemistry to control interfacial properties by merely changing the precursor. The deposition of the precursor from the gas phase enables nanometer-level control of the coating through adjustment of the residence time of precursor vapors in the plasma reactor. Argon gas is used as a carrier of precursor vapors to the plasma reactor and serves as a plasma generator gas because of its inert nature. The films obtained by PECVD are pinhole-free, and the thickness of the deposited film can be controlled via deposition conditions. As a result of the cross-linked architectures, PECVD coatings are chemically, mechanically, and thermally stable. PECVD was shown to prevent the oxidation of freshly synthesized aluminum nanoparticles by Matsoukas and co-workers. They observed that coatings from precursors such as toluene, isopropanol, and perfluorodecalin (PFD) provided excellent protection against oxidation by air and humidity by imparting hydrophobic properties to the surface of the NPs. The films made from PFD have a water contact angle of 125°, and the coatings formed were observed to provide a hydrophobic barrier against humidity to increase shelf life under normal storage conditions.
Disclosed herein is an embodiment of a unique in situ method in which a non-thermal hydrogen plasma is used to reduce the native oxide layer from the surface of BNPs, and PECVD is used to form a passivation barrier that protects metallic BNPs against oxidation by air and humidity. This method is a dry process that exposes particles to a minimum number of reagents and does not require postprocessing operations.
The setup of an exemplary embodiment of a non-thermal plasma process is shown in
Before connecting the reactor to the vacuum system, 0.5 g of BNPs (Nanoshel; purity: 99.5%; average particle size: 70 nm) was transferred to the reactor using a disposable spatula for the conducted experimentation. To promote uniform exposure of particles to the plasma and to minimize aggregation, a magnetic micro stir bar was placed inside the reactor, and the magnetic stir plate located outside the reactor was set to 100 rpm, which stirred the particles through spinning the stir bar. The tubular reactor was connected to a vacuum pump. A liquid nitrogen trap was used to condense all organic vapors escaping the reactor before entering the pump and contributed to maintaining a low plasma pressure. Check valves between the vacuum pump and reactor were used to control and regulate a high vacuum in the reactor to avoid attrition of the BNPs. Two external electrodes separated 1 in. from each other were used to generate the non-thermal plasma. The plasma formed when the source impedance matched the load impedance through a matching network.45
BNPs were treated with hydrogen plasma for 60, 85, and 120 min at a pressure of ˜0.1 Torr. After treatment with the hydrogen plasma, the hydrogen gas valve was turned off, and the argon valve was opened after placement of the glass bubbler containing perfluorodecalin (PFD; Acros Organics; purity: 90%) in a water bath at 50° C. After generation of the argon plasma, PFD saturated in argon passed through the reactor for the application of a PECVD coating on the BNPs for 15 min. Hydrogen plasma treatments were performed for different periods, while the PECVD treatment was kept constant with a treatment time of 15 min. This procedure was followed to observe the effect of the hydrogen plasma treatment time on the reduction of the native oxide. At the conclusion of the PECVD process, particles were collected from the reactor and stored under ambient conditions for further characterization. Thermal analyses of the BNPs were done over time to detect any loss in energy release due to oxidation during storage.
For analysis by transmission electron microscopy (TEM) and scanning TEM (STEM), the particles were dispersed in hexane and sonicated for 10 min with a Branson ultrasonicator (Model: CPX3800H). A few drops of the dispersion were deposited on a TEM lacey carbon copper grid (Electron Microscopy Sciences). Both the TEM and STEM-EDS (energy dispersive spectroscopy) analyses were performed on a Talos F200X at 200 kV with a XFEG source and an integrated SuperX EDS. It is also equipped with high-angular annular dark-field imaging (HAADF). STEM-EDS provides both qualitative and quantitative information about the elemental composition in the sample. The thicknesses of the organic coating and the surface oxide were estimated by high-resolution TEM (HRTEM) imaging. Scanning electron microscopy (SEM) images were collected on a Zeiss Sigma Variable Pressure (VP) field emission SEM. The BNPs were placed on copper tape for SEM imaging.
X-ray photoelectron spectroscopy (XPS) was employed to obtain the chemical composition and elemental state of the near-surface region of the particles. XPS experiments were performed using a Physical Electronics VersaProbe II instrument equipped with a monochromatic Al Kα X-ray source (hv=1486.7 eV) and a concentric hemispherical analyzer. Charge neutralization was performed using Ar ions and low energy electrons (<5 cV). A takcoff angle of 45° to the sample surface plane was used for all measurements. Quantification was done using instrumental relative sensitivity factors (RSFs) that account for the X-ray cross-section and inelastic mean free path of the electrons.
Thermogravimetric analysis and differential scanning calorimetry (TGA/DSC) were performed on a TA Instruments Model Q600 SDT, which provides simultaneous measurement of heat flow and weight change on the same sample from ˜20 to 1400° C. TGA measures the weight gain due to the oxidation of boron, and DSC yields the heat released during the oxidation of active boron.
Oxidation tests were conducted in dry air (100 mL/min) for all samples studied. Samples were placed in alumina sample cups (90 μL, TA Instruments). A heating rate of 30° C./min was used up to a maximum temperature of 1400° C.
In non-thermal plasma experiments, hydrogen plasma treatment times were varied (0, 60, 85, and 120 min) and the PECVD time was kept constant at 15 min. Characterization experiments, such as TEM, STEM-EDS, and XPS, were performed on the samples to analyze the surface oxide composition, both qualitatively and quantitatively. Thermal analysis experiments (TGA and DSC) were also performed on these samples to measure and compare the quantity of metal oxidized and the energy release.
The quantitative EDS results are summarized in
Plasma treatment does not cause any visible change to the structure of the particles as seen in ((c)-(d)) of
To probe the presence of elements and chemical environment at the particle surface, XPS was used. It has a probe depth of 10-15 nm, which extends beyond the deposited PECVD film and oxide layer and into the metallic boron phase. (a) of
The weight gain on oxidation in air of untreated and treated BNPs was measured by TGA. Four samples were analyzed three times each in a span of 60 days to check the storage life and to ensure the reproducibility of the results: (a) untreated BNPs, (b) 15 min PECVD treated BNPs, (c) 85 min hydrogen plasma and 15 min PECVD treated BNPs, and (d) 120 min hydrogen plasma and 15 min PECVD treated BNPs. A heating rate of 30° C./min was used with a volumetric flow rate of air of 100 mL/min. Thermogravimetric profiles of the untreated BNPs and plasma-treated samples are shown in
In parallel with TGA, DSC was performed to measure the amount of heat released during oxidation. The DSC results are shown in
Table 1 summarizes the results from the thermal analysis (TGA-DSC). The as-received BNPs show the lowest gravimetric heat release as well as the lowest weight gain. Both the amount of heat released and the weight gain increase with increasing treatment time in hydrogen plasma.
The examples and results demonstrate that treatment under hydrogen plasma is successful in removing the native oxide layer from the surface of BNPs. Hydrogen plasma produces reactive hydrogen species including atomic (H) and ionic hydrogen (H+) to reduce boron oxide as shown in reactions 1 and 2, respectively.
B2O3(s)+6H(g)→2B(s)+3H2O(g) (1)
B2O3(s)+6H+(g)+6e−→2B(s)+3H2O(g) (2)
The feasibility of a reaction is established by a negative Gibbs energy change (AG). The thermodynamic calculations were performed with HSC, an industrial thermodynamic software package (https://www.hsc-chemistry.com/). The data shows the negative Gibbs energy (favorable reaction) when using H and H+, especially at low temperatures, as shown in
In addition to the thermodynamic feasibility of non-thermal hydrogen plasma, stable boron oxide is reduced because of the lower activation energy of reactions 1 and 2 in comparison with the corresponding molecular reactions as studied by Sabat et al. and others. The rate of the reaction is proportional to the exposed oxide surface, which is essentially constant during the reaction. Accordingly, the amount of oxide removed is a linear function of time (HRTEM images in
The performance of nanoenergetic materials is sensitive to the degree of agglomeration. Chintersingh et al. used acetonitrile with other hydrocarbons to remove the oxide layer and showed that boron under TGA oxidizes more completely when less agglomerated. As a dry, low-temperature process, plasma processing does not induce agglomeration. Agglomeration could increase due to the passivation coating, if multiple clusters are encapsulated by the depositing film as a single unit. This is not the case here. As
The removal of the oxide layer and improvement in the oxidation characteristics are further confirmed by TGA. The weight gain in the TGA experiments (see
The weight gain is the lowest for untreated BNPs and increases systematically with treatment time under hydrogen plasma (See Table 1). A corresponding increase is observed in the heat flow measured by DSC (see Table 1 and
BNPs treated for 120 min show a weight gain that is 11.2% higher than the untreated BNPs. This is accompanied by a 19% increase in the energy release (from 24.7 to 29.4 KJ/g). Stoichiometric calculations demonstrate that an 11.2% weight gain corresponds to the oxidation of an additional 5% metallic boron in thermal analysis. This extra 5% metallic boron content improves the energy release by ˜15% (3.6 KJ/g), caused by the additional metal oxidation as well as the better interfacial contact between an oxidizer and boron due to the reduced concentration of B2O3 after hydrogen plasma treatment.
The increased energy release occurs due to the reduction of the oxide layer, which eliminated the diffusion barrier for the oxidizer to access a greater volume fraction of the metallic boron in the BNPs. No significant exothermic peak is observed after ˜750° C. because of the presence of liquid boron oxide that clogs the porosity as the oxidation reaction progresses. This decreases the overall rate of oxidation of the boron sample.
The deposition of the PFD coating following the removal of the oxide layer is critical for maintaining the reduced state of the particle surface. A thickness of 2.5 nm as shown in
A further advantage of the presence of fluorine at the particle surface is an additional contribution to the energy release on the order of 4% (25.6 KJ/g from DSC analysis) as compared to the untreated BNPs (control) (24.7 KJ/g from DSC analysis). The PECVD coating containing CFx triggers their exothermic reaction with boron oxide (formed due to oxidation by air) to form gaseous boron fluoride that increases the apparent enthalpy near the oxidation temperature of boron (˜600° C.). This effect has been reported in several studies of perfluoro-based coatings and fluorocarbon additives (though not by plasma) for the combustion of metal oxides. In other studies, surface films formed from alkoxy groups, halogens, silanes, and oleic acid by solution-based methods adversely affect the heat release from BNPs due to the fact that they do not react chemically with the boron oxide and the elements present on the surface have much lower energy density compared to boron. The presence of fluorocarbon on the surface is therefore important, not only for providing passivation but also for its combustion chemistry. DSC results in this paper also suggest a possibility that fluorocarbon-based plasma films can improve the energy release from the BNPs.
The example demonstrates development of an in situ non-thermal plasma processing method to produce BNPs with improved energetic performance and storage life. Non-thermal hydrogen plasma can be used to produce reactive species of hydrogen, which reduced the oxidized boron surface. PECVD processing after the reduction of the oxide prevents the reoxidation of the surface while improving the storage life of the BNPs under ambient conditions. HRTEM, STEM-EDS, and XPS were used to characterize the oxide reduction both qualitatively and quantitatively. The reduction in native oxide content correlates with the thermal analysis results, which display an increase in active boron content and energy release of the hydrogen plasma-treated samples. The native oxide layer is a diffusion barrier, and upon reduction of this barrier, leading to an increase in the fraction of metallic boron, a 19% higher energy release was observed during oxidation in air. Passivation by PECVD increased the storage time of the BNPs since no change was observed in the measured energy release over a span of 60 days. It can be concluded that non-thermal plasma processing is a highly attractive technique to improve the performance of advanced nanoenergetic materials.
The second example demonstrates passivation of 80-nm aluminum nanoparticles by PECVD. Three organic precursors-isopropyl alcohol, toluene, and perfluorodecalin-were used to fabricate thin films with thicknesses ranging from 5 nm to 30 nm. The coated samples and one untreated sample were exposed to 85% humidity at 25° C. for two months, and the active Al content was determined by thermogravimetric analysis (TGA) in the presence of oxygen. The results were compared with an uncoated sample stored in a glovebox under argon for the same period. It was found that all three coatings provide protection against humidity, compared to the control, and their efficacy ranks in the following order: isopropyl alcohol <toluene <perfluorodecalin. This order also correlates with increasing water contact angle of the three solid coatings. The amount of heat released in the oxidation, measured by differential scanning calorimetry (DSC), was found to increase in the same order. Perfluorodecalin resulted in providing the best protection, and it produced the maximum enthalpy of combustion, ΔH=4.65 KJ/g. This value is higher than that of uncoated aluminum stored in the glovebox, indicating that the coatings promote more complete oxidation of the core. Overall, it can be concluded that the plasma polymer coatings of this study are suitable passivating thin film for aluminum nanoparticles by providing protection against oxidation while facilitating the complete oxidation of the metallic core at elevated temperature.
High-performance energetic materials (e.g., explosives, rocket fuels) are designed to store large amounts of chemical energy with the ability to release it instantly on demand. Metal particles are prime candidates as additives to energetic materials, because they oxidize readily and release large amounts of heat. Aluminum particles are currently being used in solid rocket booster fuels, but a major drawback is their low rate of energy release, compared to other carbon-based energetic compounds, e.g., TNT, HMX, and RDX. Several approaches have been developed to overcome this limitation and improve the rate of oxidation. One strategy is to utilize very fine particles (<100 nm in size). In addition to exhibiting higher rate of oxidation, small particles oxidize more completely, unlike micrometer-sized particles, whose oxidation is eventually arrested by the formation of a thick oxide layer. This layer offers natural protection of the inner metallic core but also adds dead weight. For particles in the nanometer range, this can be a serious problem. For example, for a particle 40 nm in diameter, the oxide layer (typically ˜4 nm) occupies 50% of the total mass of the nanoparticle. This has led to various other methods to passivate the particle surface to protect against oxidation or other contamination of the metal.
The ideal coating should enhance nanoparticle properties. It should protect the metal from oxidation and other contamination and increase the rate of energy release at elevated temperatures. Noble metals and metal oxides have been shown to provide protection for the aluminum core, as well as enhanced energy content, because of intermetallic reactions between the coatings and the core. Boron has been used to stabilize aluminum propellants by providing desirable surface characteristics, such as high corrosion resistance. Carbon offers similar protection at low temperature and is stable at elevated temperatures. More elaborate surface modifications involve in situ surface functionalization of freshly synthesized (oxide-free) aluminum nanoparticles using compounds, such as perfluoroalkyl carboxylic acids (C13F27COOH), formic acid, and aldehydes. Some organic materials have been also used to stabilize aluminum nanoparticles, including waxes, ethanol, and fluoropolymer, but the coatings were found to be permeable to oxygen and thus lacking in their capacity to provide passivation. The present disclosure focuses on a different approach that utilizes plasma-enhanced chemical vapor deposition (PECVD) to produce a surface coating of controllable thickness that provides superior passivation against environmental oxygen and moisture during storage but also enhances the energetic content of the particles.
Plasma-deposited solids have unique properties that are especially advantageous as passivating barriers for nanoenergetic materials. Most notably, plasma polymers produce hydrophobic surfaces. Hydrophobicity adds a chemical interaction to the physical barrier, which alone cannot provide satisfactory protection against moisture. Plasma deposited solids are chemically inert and thermally stable up to 250° C. They contain elements of their precursor molecules, typically carbon, oxygen, and fluorine. These elements oxidize readily under combustion conditions, thereby exposing the aluminum core, and they may also contribute to the overall enthalpy of reaction. As a dry gas-phase process, plasma offers a further advantage of a well-controlled environment. Unlike liquid phase processing, which exposes particles to a complex reaction medium and requires drying and additional separation steps to recover the particles, plasma reaction occurs in an inert environment. The deposition process is flexible and can be applied to any solid substrate, including metallic and nonmetallic materials. A broad choice of organic precursors may be used, including hydrocarbons, alcohols, and fluorocarbons, which offers a degree of flexibility in controlling the interfacial properties of the film. Finally, the process provides very good control of the thickness of the coatings, which is a linear function of the deposition time.
Aluminum nanoparticles (99.9+%, 80 nm) with particle diameters in the narrow range of 80-100 nm were purchased from Nanostructured and Amorphous Materials, Inc. Nanoparticles were incubated in a desiccator under an inert atmosphere. They were transferred from their original container into small vials and stored in a glovebox under argon until time to use. Three organic precursors were used in this study: isopropyl alcohol (IPA 99.5% obtained from VWR), toluene (EMD chemicals), and perfluorodecalin (PFD 99% VWR).
The setup for the deposition process is shown in
The thickness of the coating was measured by transmission electron microscopy (TEM), using a Philips Model (FEI) EM420T system. The morphology of the coatings and the effect of exposure to moisture was also studied by field-emission scanning electron microscopy (FESEM) (Leo, Model 1530). Micrographs of aluminum wafers were collected by scanning electron microscopy (SEM) (Hitachi, Model S-3500N) equipped with a diffraction energy microscopy (EDS). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on a TA Instruments Model SDT 2960 system equipped with a simultaneous differential scanning calorimetry-thermogravimetric analysis (DSC-TGA) system that operates under an air flow of 40 mL/min.
As noted above, three precursors were used in this study: isopropyl alcohol (IPA), perfluorodecalin (PFD), and toluene. TEM micrographs confirm the formation of smooth solid coatings from all three precursors. The coatings appear as a lightly shaded layer surrounding darker particles (see
A unique characteristic of plasma deposition of hydrocarbon-based solid is the water-repellent properties of the surface.
The next phase was to characterize the ability of plasma-deposited coatings to protect aluminum nanoparticles against a humid atmosphere. A sample of uncoated aluminum and three samples of coated nanoparticles with each precursor were kept in a closed container under 85% relative humidity at 25±5° C. for two months. As a control, a sample of uncoated nanoparticles stored in the glovebox during this period was also examined. These particles are seen in (a) of
Energy-dispersive spectroscopy (EDS) provides direct evidence of oxidation of the metal (see
To measure the aluminum content of different samples, TGA was performed by heating in air. The sample was oxidized by slow heating in air and the amount of aluminum was calculated from the weight gain due to the formation of the oxide. Therefore, the method gives a direct measure of the aluminum content of the particles and provides a quantitative measure of the coating to provide passivation. Other possible gain weight due to oxinitride and aluminum nitride formation may take place if self-ignition occurs. This is avoided by a slow rate of heating. For these experiments, heating was done according to the following schedule: 20° C./min, up until 350° C.; 5° C./min, from 350° C. to 600° C.; followed by 20° C./min, from 600° C. to 850° C. The sample was kept at 850° C. for 4 h before cooling to room temperature to ensure that all of the aluminum has reacted. The TGA experiments were done three times and the results are reproducible within ±5% error.
Five samples were analyzed using this method: three coated samples after exposure to air (isopropyl alcohol (IPA), toluene, PFD), uncoated aluminum after exposure to air, and uncoated aluminum stored in the glovebox (control, no exposure to air beyond that during handling). The thermogravimetric profiles of these samples are shown in
The next change in weight is an increase observed between 350° C. to 500° C. (˜50 min) and is due to the oxidation of aluminum. A small weight loss is expected due to the decomposition of the coatings, but this is clearly overshadowed by the large weight gain due to the oxidation of aluminum. Near 500° C., all the samples reach a plateau for almost 20 min, because of the buildup of an oxide layer that prevents further oxidation. Another weight gain is observed at near 650° C. The melting point for 100-nm aluminum is reported to be 656° C.; accordingly, this gain is attributed to the melting of the Al core, which facilitates further oxidation. By the end of the experiment (340 min) the weight gain practically levels off and reaches its maximum amount. The exposed uncoated sample shows a weight gain of only 20% weight. The weight gain of the coated particles are all higher. Coated particles with IPA, toluene, and PFD show weight increases of 52%, 58%, and 60%, respectively, indicating increasing degree of protection by the corresponding coatings.
Notably, all coated samples gained more weight than the uncoated sample stored in the glovebox. This result is surprising, because the uncoated sample kept under inert atmosphere is expected to register at least the same aluminum content as coated samples that were exposed to humidity. The weight gain of the glovebox sample is ˜46% and agrees with similar published studies on bare aluminum nanoparticles. The results suggest that the oxidation of the uncoated aluminum is not complete and that the coating contributes to more-complete oxidation and, thus, higher weight gain. To investigate this possibility further, these samples were studied using DSC. This method measures the heat flow and temperature associated with phase transitions or reactions, as a function of temperature, as shown in
aStored in glovebox.
The next change in weight is an increase observed between 350° C. to 500° C. (˜50 min) and is due to the oxidation of aluminum. A small weight loss is expected due to the decomposition of the coatings, but this is clearly overshadowed by the large weight gain due to the oxidation of aluminum. Near 500° C., all the samples reach a plateau for almost 20 min, because of the buildup of an oxide layer that prevents further oxidation. Another weight gain is observed at near 650° C. The melting point for 100-nm aluminum is reported to be 656° C.; accordingly, this gain is attributed to the melting of the Al core, which facilitates further oxidation. By the end of the experiment (340 min) the weight gain practically levels off and reaches its maximum amount. The exposed uncoated sample shows a weight gain of only 20% weight. The weight gain of the coated particles are all higher. Coated particles with IPA, toluene, and PFD show weight increases of 52%, 58%, and 60%, respectively, indicating increasing degree of protection by the corresponding coatings.
Example 2 demonstrates development of a process to passivate aluminum nanoparticle surfaces via a dry state process. 5-nm coatings were produced on 80-nm aluminum nanoparticles by plasma deposition of isopropyl alcohol (IPA), toluene, and perfluorodecalin (PFD). The coatings provide excellent protection against contact with NaOH and against two month-long exposure to high humidity, and they preserve a higher amount of metallic aluminum, compared to samples stored in inert atmosphere for the same period of time. The materials are ranked in the following order: PFD>toluene>IPA. This order is observed with respect to the contact angle of water; the amount of metallic aluminum, as determined by TGA; and heat of reaction, as determined by DSC. Therefore, the performance of the coatings, with respect to passivation and energy release, correlates with the measured contact angle. This suggests that a hydrophobic interaction is important in building a barrier against humidity. This property can be fine-tuned by proper selection of the chemical precursor.
The third example demonstrates enhanced energy release from boron/aluminum blends at lower temperatures.
Boron has the highest enthalpy of oxidation per unit mass or unit volume among metals and metalloids and is an excellent candidate as a solid fuel or additive to liquid fuels. The native oxide present on the surface limits the available energy and rate of its release during oxidation. Disclosed is a simple and effective method that removes the oxide in situ during oxidation via an overall exothermic redox reaction with aluminum that enriches the particle in B at the expense of Al. Al/B blends with different compositions are studied using thermochemical analysis and stoichiometric calculations with the help of HRTEM and XPS analyses. All blends release more energy than the individual components, and the blend containing 10% Al by weight outperforms pure B by 40%. The high energy release is due to the synergistic effect of B oxidation and redox reaction between Al and B2O3. HAADF-STEM-EDS and XRD of oxidation products of Al/B blends indicate the formation of ternary oxide in the system, which provides porous channels for oxidation of B, thereby maximizing the contact of metal and oxidizer. STEM-EDS of the blends demonstrates qualitatively and quantitatively that Al and B particles stay closer in hydrocarbons, and therefore, they can benefit propellants as secondary fuel additives apart from their use as solid fuels.
Boron has shown high promise as fuel additive for propulsion and energetic applications due to its high gravimetric (58 KJ/g) and volumetric (140 KJ/mL) energy densities. Its ignition performance, however, is hindered by the presence of a native oxide on the surface, which melts at relatively low temperatures (450° C. at atmospheric pressure). The melting of the oxide shell before the solid core causes the clogging of pores, leads to particle agglomeration and acts as a diffusion barrier to boron (B) oxidation. Attempts to overcome these limitations include surface functionalization of B by organic compounds, reduction of the oxide followed by surface passivation using non-thermal plasma processing, or coating with metals to form composites and metal borides by ball milling and high-temperature sintering methods. Functionalization with organic compounds results in reduction of the amount of energy released per unit mass because of the presence of less energetic materials on the B surface. Non-thermal plasma processing has shown to be successful in enhancing the energetic performance over untreated boron, but requires low-pressure equipment that is harder to scale up. Coating with other metals by solution-based methods require multiple chemicals and processing steps prone to introducing contaminants. High temperature sintering leads to agglomeration of B powder due to the low melting temperature of B2O3 (450° C.). In the quest to accelerate the combustion performance of B, metals such as Mg, Al, Zr, Fe, Ti, and Li have been used in conjunction with B. The presence of these metals relieves the accumulation of liquid boron oxide films by forming porous ternary oxides and their preignition raises the local temperature of the reaction interface, thereby facilitating the more complete oxidation of boron. Among these metals, aluminum (Al), whose presence on the earth is ubiquitous, has been studied extensively on its own merit, due to its better reactivity, high gravimetric energy density (31 KJ/g), and relatively low melting point. Al can combine with B to form Al borides, which show better thermal stability during storage but release less energy during combustion (40 KJ/g) compared to boron (58 kJ/g). The chemical bonding between the two elements leads to significant ignition delays, adding a further detriment in applications that require fast energy release. These limitations could be overcome if Al and B form a mechanical blend rather than a chemical compound. Studies on Al/B blends are scarce but the few that are available suggest that the ignition performance of Al/B blends at a weight ratio=1:1 with micron size particles is better than that of aluminum diboride. This improvement is attributed to the less serious accumulation of liquid B2O3 films during the combustion of the blends.
Al is capable of reducing boron oxide to produce elemental boron and Al oxide according to the following redox reaction:
2Al(l)+B2O3(s)→Al2O3(s)+2B(s);ΔH°=−8kJ/g,ΔG°=−859kJ/mol
The reaction is exothermic and frees B from its oxide that may further oxidize to release significantly more energy than the parent material. Al effectively acts as a sacrificial element that extracts the energy of B, which is trapped in the form of B2O3. The reduction of boron oxide by Al can be leveraged to produce Al/B blends with substantially superior performance than B alone. Optimum performance can be realized with nanometer size B, since the oxide makes significant contribution to the total particle volume and mass in this size range. Nanoparticles have their own advantages in energetic applications. They exhibit fast ignition, enhanced reaction kinetics and more complete oxidation relative to micron size particles. In this Communication, we demonstrate superior performance of Al/B blends with respect to energy release at lower temperatures and identify the optimum amount of Al that must be added to maximize the energy of the blend.
Boron particles (99.5%, 500 nm, Nanoshel) and Al particles (99.9%, 70 nm, US Research Nanomaterials) were used. Al and B particles were mixed together in different weight proportions (Al weight (wt) % of 5%, 10%, 20%, 33%, 50%, 67%, and 80% with the rest B) in glass vials by magnetic agitation using stirrer and stir bar to form a homogeneous mixture. The blends were characterized using thermal analysis-thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) and compared to pure Al and B. Additionally, the samples were characterized by X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field (HAADF)-scanning TEM (STEM)-energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) analysis. They are briefly discussed in the supporting information.
Stoichiometric calculations were performed with the support of XPS and HRTEM analyses to determine the amount of Al NPs (70 nm) required to eliminate the native B2O3 present on the surface of B (500 nm). From HRTEM in (a) of
The 10 wt % Al/B blend is characterized in further detail, and is referred to herein as BAL10.
2B(s)+1.502(g)→B2O3(l);ΔH°=−58kJ/g,ΔG°=−832kJ/mol
Based on this information we attribute the first exothermic peak of the blend to the combined effect of the reduction of B2O3 by Al (ΔG°=−859 KJ/mol) plus the oxidation of Al (ΔG°=−1691 KJ/mol), and the second peak to the oxidation of B (ΔG°=−832 KJ/mol). These conclusions are corroborated by the DSC profiles of blends at other compositions, which show the first peak to decrease systematically and the second peak to increase as more Al is added to the blend.
Elemental distribution of the oxidation product of sample BAL10 by HAADF-STEM-EDS is shown in the micrographs of
Dry mixing of metal powders by magnetic agitation is a simple and highly time-effective process as compared to solvent-phase mixing methods. It typically takes 3 min to produce a homogeneous mixture without any organic contamination. The maximum energy release is from BAL10 blend (42 KJ/g), which is due to the reduction of native B2O3 from the surface of B particles while Al is being oxidized to Al2O3 and forms metallic B, which is further oxidized and contributes to the overall energy release. This leads to the conclusion that we are able to extract more energy from B at lower temperatures by removing a kinetic barrier without losing energy. Thus, a larger fraction of energy from B is released at lower temperatures due to the blending of Al NPs with B powder. The weight gain of BAL10 is the highest (see (a) of
In order to test the broader spectrum of the applications of Al/B blends in liquid propellants, these blends were dispersed in dodecane (C12H26) using ultrasonication and were analysed in EDS to see the relative location of Al and B particles. Dodecane was chosen because its chemical composition resembles hydrocarbon fuels.
In summary, example 3 shows development of a simple but highly efficient method to extract higher amounts of chemical energy from B at low temperatures by removing the kinetic barrier using Al. The process can be easily scaled up without causing any chemical contamination and takes only a few minutes, unlike other techniques in the literature that take several hours to days. The significant improvement in the energy release from the blends is primarily due to the synergistic effect of exotherms from B oxidation and redox reaction between Al and B2O3 and secondarily due to the formation of porous channels of ternary oxides of Al and B that increases the contact of B and oxidizer. Characterization studies of blends demonstrate that Al and B particles stay closer in hydrocarbons, and therefore, they can be used as fuel additives in propellants other than as solid fuels. These metal blends can find significant application in solid fuels as they undergo “green” combustion by not emitting greenhouse gases.
The following example demonstrates effectiveness of fluorinated films can be used for passivation.
With this example, aluminum particles are introduced to the energetic systems to equip them with enhanced energy release and faster reaction rates and that makes them attractive for propulsion and fuel-related applications. The native oxide shell on the surface of aluminum nanoparticles (Al NPs) provides passivation during storage but forms a diffusion barrier for oxygen and Al contact that inhibits oxidation during combustion. For nanoparticles, the oxide layer occupies a significant fraction of the particle's mass that adds weight but does not contribute to its energy content. This example demonstrates a perfluoro-based nonthermal plasma process to prepare core/shell nanostructures of Al with significantly improved energy release. The native oxide layer reacts with a fluorocarbon-based plasma film formed on the surface using plasma-enhanced chemical vapor deposition (PECVD) of perfluorodecalin (C10F18). During combustion, this plasma film reacts with native oxide (Al2O3) to form AlF3 with the release of energy at the interface. The nano-explosion at Al/Al2O3 interface due to the exothermic reaction provides new reaction channels for the reaction of Al and the O2, thereby enhancing the energy release and extent of oxidation as illustrated by thermal analysis measurements. Characterization of core/shell nanostructures was performed using various methods. Thermogravimetric analysis/Differential scanning calorimetry combined with high-resolution transmission electron microscopy (TEM), was used to determine the plasma film's optimum thickness (˜10 nm) that maximizes the energy release (˜50%). TEM and STEM-EDS (Energy Dispersive Spectroscopy) in HAADF (high-angle annular dark-field) demonstrated uniformity and distribution of the surface plasma films. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) characterize the respective phases and the chemical bonds present in the plasma films. The combined picture formed from these results is used to explain the mechanism of the interfacial reactions between plasma films and the native oxide, leading to energy release enhancement. The mechanism is supported by characterizing the samples using XRD, SEM, and TEM analyses. This method of eliminating the surface oxide layer and enhancing the energy release by triggering surface fluorination reactions during thermal oxidation can be applied to the cutting-edge design of metal-based nanoenergetic systems.
Metal-based energetic materials store large amounts of chemical energy and they can undergo spontaneous and highly exothermic reactions generating light, heat, and thrust expeditiously. These properties can be harnessed in a number of applications such as propellants, solid rocket fuels, pyrotechnics, and synthesis of nanoenergetic materials. Aluminum (Al)-based nanoenergetic materials have emerged as prime candidates for propellants and fuel additives in civilian and military applications due to their superior gravimetric energy density (31 KJ/g), earth abundance, and green combustion. The size of Al particles is an important factor in enhancing their energy release. Nano-sized Al particles possess high reactivity, lower ignition temperature and ability to undergo more complete oxidation, leading to faster kinetics and higher energy release compared to micron-sized particles. The surface of Al nanoparticle (NP) is covered with a native oxide (Al2O3) layer with an average thickness of 1.7-6 nm. Although this shell acts as a passivating coating, prolonged exposure to air and humidity will further oxidize the Al NPs, thus depleting the metallic Al content. The oxide shell occupies 30-50% of the mass of the particle, depending upon the particle size, which represents mass that does not contribute to the energy release during oxidation reaction. Its high melting point of ˜2100° C. further hinders oxidation leading to slow kinetics and lesser energy release.
Surface functionalization of the freshly synthesized oxide-free Al NPs is one way to address the above-mentioned problems. However, this method is unsuitable for large-scale production unless done in-situ because Al NPs without surface oxide can cause sudden explosions by undergoing rapid oxidation reactions. Alternatively, the native oxide may be removed by reduction followed by surface passivation using secondary coatings. Secondary coatings can provide stability and extended shelf life to Al NPs and can be divided into two categories. One category is coatings with non-energetic components such as silanes, carboxylic acids, and glycols that do not contribute to the energy release of the system. These can stabilize Al NPs but decrease the overall energy density due to the large mass loadings (20-40%) required to provide adequate passivation. Another category of secondary coatings, which is preferred over the previous one, constitutes energetic components such as metals (Fe, Ni) and nitrocellulose, which can contribute to the energy release as a result of intermetallic or thermite reactions between the coatings, the oxide layer, and the core. However, the energy contribution is very small because the energy densities of Fe, Ni, and nitrocellulose are not very high as compared to Al. Therefore, there is no significant improvement in the overall energy release. Hence, there is a need for surface functionalization with a material that is capable of interacting chemically with the oxide shell by releasing greater amounts of energy during oxidation to contribute significantly to the overall energy release. It should be able to form uniform coatings, which can passivate Al NPs against air and humidity to provide storage stability for a longer duration of time.
Known methods focus on developing coating technologies with the material that can enhance the energy release with improved reactivity and lead to the longer shelf life of Al NPs. Fluorine-based polymers are of particular interest as passivating materials due to their excellent thermal and chemical stability compared to hydrocarbon-based coatings. They can improve both the stability and energetic performance of Al by inducing surface reactions that contribute to the overall energy release. Polyvinylidene fluoride (PVDF), perfluoropolyether (PFPE), and polytetrafluoroethylene (PTFE) have been shown to improve the extent of oxidation of Al particles, as F species react exothermically with surface oxide layer to form AlF3.
Fluoropolymers can be used with Al particles as additives or surface coatings. Known methods have been shown to use 5 μm Al particles to reduce the oxide using etching in hydrofluoric acid medium followed by coating PVDF molecules on the surface. These coatings improved the shelf life of the particles and the thermal analysis confirmed that PVDF/Al particles show superior energy release as compared to Al2O3 passivated Al particles. This method provides the ability to extract ˜11 KJ/g of the chemical energy (31 KJ/g) stored in Al particles. Other known methods have showed that the oxide shell on the Al particles can undergo reaction with F species from a decomposing fluorocarbon, causing an exothermic surface reaction that adds to the energy release of the system. Possible reasons for limited energy release in available studies are using Al microparticles with the relatively low specific surface area available for the reaction and the potential contamination from using several reagents in solution-phase methods. Similar studies are not available on nano-sized Al particles, with minimum contamination, lesser time consumption, and nanometer-level control on fluoropolymer deposition.
It is clear from the literature that fluorocarbon coatings have the advantage of providing surface passivation along with effective surface reactions as compared to fluorocarbon additives. Core/shell architectures are preferred over mechanical blends because they maximize the degree of contact relative to the mechanical blends that leads to efficient interfacial reaction between reactive F from fluorocarbon and Al2O3. Thus, there is a need to device a coating technique that takes lesser time, does not cause contamination, is not limited to the particle size, and provides control on the amount of fluorocarbon and its deposition rate on the Al surface. Dry gas-phase nonthermal plasma process, which is known as plasma-enhanced chemical vapor deposition (PECVD), can produce an effective surface coating with switchable interfacial chemistry by using a broad variety of organic precursors. This process provides a nanometer level control on deposition rate of the vapor, takes place in an inert environment, uses minimum chemical species, and thus avoids contamination and undesirable side reactions with the metal surfaces. It is a flexible process and is not limited to particle size and can be applied to the particles of any size. Last but not the least, this process takes much less time as compared to other available methods such as solvent-phase coating process.
The ideal coating should protect the metal from oxidation by acting as a passivation barrier, thereby increasing its shelf life; it should not interfere with combustion and ideally should contribute energy during oxidation. Disclosed is a dry gas-phase nonthermal plasma process to produce core/shell Al NPs with superior energy release characteristics. PECVD is employed with perfluorodecalin (C10F18) as a coating precursor for surface functionalization, which can be utilized during the oxidation process to eliminate surface oxide by an exothermic surface reaction increasing the contact area between the metallic Al and the oxygen. Optimization studies using thermal analysis and high-resolution transmission electron microscopy have been performed to find the optimum plasma film thickness to achieve maximum energy release from Al/Fluorocarbon core-shell NPs. The in-depth mechanism of the interfacial reactions is proposed with the support of material characterization experiments.
Al powder (99.9%, 70 nm) was purchased from US Research Nanomaterials Inc. Perfluorodecalin (PFD, 90%, C10F18, Acros Organics) is used as an organic precursor for PECVD. The setup of PECVD process is shown in
The thickness of the coating was measured by high-resolution transmission electron microscopy (HRTEM) using FEI Talos F200X. Scanning transmission electron microscopy (STEM) and Energy Dispersive Spectroscopy (EDS) analyses were performed on the same instrument at 200 kV with an XFEG source fitted with an integrated SuperX EDS detectors. Imaging was performed in high-angle annular dark-field (HAADF) to obtain information about the morphology and atomic distribution of the elements at the surface. X-ray diffraction (XRD) spectra were obtained using a Malvern Panalytical XPert Pro MPD theta-theta diffractometer equipment was used for this experiment with a Cu kα x-ray source. X-ray photoelectron spectroscopy (XPS) was performed in an ultrahigh vacuum to obtain detailed information about the elemental composition and chemical bonding on the surface of Al NPs. Physical Electronics VersaProbe II instrument equipped with a monochromatic Al kα x-ray source (hv=1,486.7 eV) and a concentric hemispherical analyzer with a take-off angle of 45° was used. Thermogravimetric analysis (TGA) analysis and differential scanning calorimetry (DSC) were performed on a TA Instruments Model Q600 SDT, which provides simultaneous measurement of heat flow (DSC) and weight change (TGA) on the same sample from ˜25° C. to 1000° C. Analyses were conducted in ultra-zero air at a volumetric flow rate of 100 mL/min for all samples studied. Alumina sample cups (90 μL, TA Instruments) were used for the NPs sample. A heating rate of 20° C./min was used till 1000° C. after maintaining isothermal conditions for first 10 min. Scanning electron microscopy (SEM) images were collected on a Verios G4 field emission (FE) SEM. The Al NPs were placed on a copper tape for FE-SEM imaging.
Aluminum nanoparticles were treated with perfluoro-PECVD for various periods (30, 45, 55, 85, 110, and 165 min), and were analyzed with HRTEM, TGA, and DSC to determine their film thicknesses and to compare weight gain and energy release during their thermal oxidation.
HRTEM analysis results showing plasma film thickness as a function of PECVD treatment time are summarized in
In order to discover the chemistry behind the changes in thermal oxidation behaviors, the sample with optimum weight gain and energy release (55 min PECVD; film thickness: 10 nm) was chosen for the further characterization.
High-Resolution Transmission Electron Microscopy (HRTEM) and STEM micrographs are used to study the plasma film thickness and aggregation state.
Chemical maps at the particle surface were obtained by Scanning Transmission Electron Microscopy (STEM) in high-angle annular dark-field (HAADF) imaging and are shown in
The XRD pattern of a sample treated in the plasma for 55 min is shown in
Thermal analysis experiments were performed on the untreated, 30 min, 45 min, 55 min, 85 min, 110 min, and 165 min plasma-treated Al NPs in the temperature range of ˜25° C. to 1000° C.
2Al(l)+3/2O2(g){grave over (a)}Al2O3(s)ΔH°=−31kJ/g
When the metallic content oxidized is calculated, it can be concluded that in case of untreated Al NPs, 52% of metal was oxidized, which increased to 71% after depositing ˜10 nm thick plasma film. The presence of this film leads to the oxidation of additional 19% Al. As was observe in TGA (
Al2O3(s)+6F{grave over (a)}2AlF3(s)+1.5O2ΔH°=−13.2kJ/g
The above reaction illustrates the reaction of Al2O3 with reactive F species from the plasma film. Since, their interfaces are having maximum contact area due to core/shell architecture, it helps in uniform and efficient reaction between them forming AlF3 with the release of energy at the interface. The additional 7 KJ/g energy released in plasma-treated sample is due to two reactions: (a) oxidation of extra 19% Al, as suggested by TGA, and (b) surface fluorination of native oxide of Al. Calculation of energy release explains that additional 19% Al oxidation releases ˜6 KJ/g of energy and the fluorination reaction of Al2O3 releases ˜1 KJ/g of energy.
In
As observed from the thermal analysis results in
Al NPs are covered with native oxide (Al2O3) shell (red) on the surface with a thickness of ˜5 nm as calculated by XPS analysis. They undergo weak oxidation due to the presence of oxide shell, which acts as a diffusion barrier for oxidizer and Al to get into contact with each other for the reaction. The spontaneously formed oxide due to the reaction at high temperatures hinders further oxidation. After PECVD application of fluorocarbon, plasma film (green) is formed on the surface of Al NPs. When thermal analysis is performed in the presence of air as an oxidizer, the Al core starts melting and expanding at ˜600° C. The surface reaction between Al2O3 and fluorine from the plasma film is also triggered at around the same temperature. This results into reaction 2 with a release of energy at the interface. This reaction leads to the elimination of native oxide layer of Al2O3, thereby forming AlF3 on the surface of Al NPs. This is evident by the XRD analysis performed after the oxidation process. The representative peaks of AlF3 are shown in
In summary, the disclosed nonthermal plasma process can be used to perform PECVD on Al NPs, which could significantly improve the oxidation efficiency and heat release from Al NPs and other energetic materials. The process showed a nanometer level control on the deposition of fluorocarbon required to reduce Al2O3, which minimizes the metal contamination by avoiding excess fluorocarbon on the surface of Al NPs. The removal of Al2O3 layer by surface fluorination reaction with fluorocarbon-based plasma film can increase the contact area and diffusion rates of Al and the oxidizer and can therefore improve the oxidation reaction performance. The energy release was increased to ˜1.5 times as compared to the untreated Al NPs with native oxide layer on their surface. The oxidation efficiency was improved by ˜19% without facing the safety issues due to sudden explosion as a result of rapid oxidation. This is possible as there is already a native oxide layer to prevent further oxidation. Additionally, forming a fluorocarbon coating on the surface serves as an extra layer of passivation on highly reactive Al NPs. This can lead to the elongated shelf life of the Al NPs. An in-depth mechanism was proposed to explain the reason for enhanced oxidation and energy release. Thus, it can be concluded that nonthermal plasma processing has a great potential in improving the performance of nanoenergetic materials.
Example 5 demonstrates coated particles can show superior heat release, with a maximum enhancement of 50% at a thickness of 10 nm.
The performance of Al as nanoenergetic material in solid fuel propulsion or additive in liquid fuels is limited by the presence of the native oxide layer at the surface, which represents a significant weight fraction, does not contribute to heat release during oxidation, and acts as a diffusion barrier to Al oxidation. We developed an efficient technique in which the oxide layer is effectively turned into an energetic component via a reaction with fluorine that is coated in the form of a fluorocarbon nanofilm on the Al surface by plasma-enhanced chemical vapor deposition. Perfluorodecalin vapors are introduced in a low-pressure plasma reactor to produce nanofilms on the surface of Al nanoparticles, whose thickness is controlled with nanolevel precision as demonstrated by high-resolution transmission electron microscopy images. Coated particles show superior heat release, with a maximum enhancement of 50% at a thickness of 10 nm. This significant improvement is attributed to the chemical interaction between Al2O3 and F to form AlF3, which removes the oxide barrier via an exothermic reaction and contributes to the amount of heat released during thermal oxidation. The chemistry and mechanism of the enhancement effect of the plasma nanofilms are explained with the help of X-ray photoelectron spectroscopy, X-ray diffraction, high-angle annular dark-field scanning transmission electron microscopy-energy dispersive spectroscopy, thermogravimetric analysis, and differential scanning calorimetry.
As noted herein, metal-based energetic materials store large amounts of chemical energy and undergo highly exothermic reactions generating light, heat, and thrust. These properties are important for a number of applications that include propellants, solid rocket fuels, and pyrotechnics. Aluminum (Al)-based materials have emerged as prime candidates for propellants and fuel additives in civilian and military applications due to superior gravimetric energy density (31 KJ/g), high reactivity, and abundance on earth. The size of the particles is an essential factor in the performance of Al as an energetic material. Nanosized particles exhibit higher reactivity, lower ignition temperature, and the ability to undergo faster and more complete oxidation, leading to enhanced heat release compared to micrometer-sized particles. The surface of the Al nanoparticle (NP) is covered with a native oxide (Al2O3) shell with an average thickness of 2-6 nm. This shell acts as a passivation coating, but under prolonged exposure to air and humidity it will further oxidize, thus depleting the metallic content of the particles. The oxide shell occupies 30-50% of the mass of the particles less than 100 nm. Thus, a substantial fraction of the particle mass does not contribute to heat release under oxidation. The high melting point of Al2O3 (˜2100° C.) further hinders oxidation, leading to the slow kinetics and lesser heat release.
To overcome these problems, one approach is to produce oxide-free Al NPs immediately followed by surface functionalization. This method is unsuitable for large-scale production due to the high reactivity of bare Al NPs, which can cause explosions and therefore has safety issues involved. Alternatively, the native oxide may be removed after particle synthesis by reduction, followed by in situ surface passivation before exposure to air using secondary coatings that provide stability and extended shelf life to Al NPs. Coatings with nonenergetic components, such as silanes, carboxylic acids, and glycols, inhibit the growth of native oxide by providing passivation, but their contribution to the heat released during oxidation is negligible. They generally result in a decrease of the overall energy density due to the significant mass loadings (20-40%) required to provide adequate passivation. Another category constitutes energetic components, such as metals (Fe, Ni) and nitrocellulose, that contribute to the heat release via intermetallic or thermite reactions between the coatings, the oxide, and the metal core. However, the energy contribution is relatively small because the energy density of these additives is low compared to that of Al.
Several studies have focused on coating technologies with materials that enhance heat release, improve reactivity, and prolonged shelf life. Fluorine-based polymers are of particular interest because of their superior thermal and chemical stability than hydrocarbon-based coatings. In addition to providing passivation and stability during storage, they add to the energetic performance of the metal via exothermic reactions with Al and Al2O3. Poly(vinylidene fluoride) (PVDF), perfluoropolyether (PFPE), and polytetrafluoroethylene (PTFE) have been shown to improve the extent of oxidation of Al particles, as F species react exothermically with a surface oxide layer to form AlF3. Fluoropolymers can be used as blends with Al powders or as surface coatings. Coatings have advantage over the blends because they can provide surface passivation and brings F in intimate contact with the Al particle leading to faster reactions. PVDF coating on Al microparticles was stable and released a higher amount of heat during oxidation than Al alone due to an exothermic reaction between surface Al2O3 and F in the coating. The overall amount of heat released is modest, less than 35% of the gravimetric energy density of Al. A likely reason is that the microparticles used in these studies have a low specific surface area that affects their reactivity and causes incomplete metal oxidation, which affects the percentage of heat release. In the available methods, there is limited control on the deposition of fluorocarbon over the surface of Al particles. If the fluorocarbon on the particle is present in excess, it results in lesser heat release per unit mass due to its much lower gravimetric energy density relative to Al.
It is evident from the literature that fluorocarbon coatings have the advantage of providing surface passivation along with effective surface reactions as compared to fluorocarbon blends. Core-shell architectures maximize contact between F and particles and make more efficient use of the added mass without compromising the gravimetric energy density of the particles. The ideal coating should protect the metal from oxidation by acting as a passivation barrier, thereby increasing its shelf life; it should not interfere with combustion, and it should contribute energy during oxidation at a level comparable to that of Al. In previous studies, we have demonstrated a dry gas-phase plasma-enhanced chemical vapor deposition (PECVD) process for depositing thin films from various organic vapor precursors onto the surface of nano- and microparticles. PECVD can produce an effective surface coating with switchable interfacial chemistry by using a broad variety of organic precursors. This process provides a nanometer-level control on the deposition rate of the vapor, takes place in an inert environment, uses minimum chemical species, and thus avoids contamination and undesirable side reactions with the metal surfaces.
Here, we present the application of PECVD to produce core-shell nanostructures in which Al NPs are encapsulated within the thin layers of fluorinated films formed by using perfluorodecalin (C10F18) as a coating precursor. Optimization studies using thermal analysis and high-resolution transmission electron microscopy have been performed to determine the optimum thickness of the plasma coating that achieves superior heat release during oxidation. The composition of the plasma nanofilms was analyzed by using X-ray photoelectron spectroscopy, and the bulk particles were analyzed by using X-ray diffraction. A mechanism is proposed to explain the chemistry of fluorinated Al NPs during oxidation with the help of scanning transmission electron microscopy and X-ray diffraction analysis of the oxidation products, which will also elucidate the observation of optimum heat release as a function of the thickness of the fluorinated coating.
Al powder (99.9%, 70 nm) was purchased from US Research Nanomaterials Inc. Perfluorodecalin (PFD, 90%, C10F18, Acros Organics) is used as an organic precursor for PECVD. The setup of PECVD process is shown in
The setup includes of five main components: (a) a tubular reactor, (b) an argon-assisted precursor delivery system, (c) a 13.56 MHz radio-frequency (RF) generator equipped with a matching box, (d) a vacuum system with a liquid nitrogen (LN) trap, and (e) a magnetic plate placed below the plasma reactor and stir bar located in the reactor to agitate the particles mechanically. Deposition takes place in a tubular reactor connected to a vacuum with an liquid nitrogen (LN) trap and argon-assisted precursor delivery system from the other side. Check valves between the reactor, the vacuum system, and the gas supply control the pressure inside the reactor. Argon gas (flow rate: 5.1 cm3/min) is used as a plasma generator and a carrier for precursor vapors into the reactor. Its supply line passes through a bubbler, whose temperature is maintained at 50° C. by immersion in a hot oil bath. Two external electrodes from the RF generator are used to create an electric field across the reactor to generate plasma inside the reactor. The vacuum pump maintains low pressure in the reactor; the LN trap is used to condense all outgoing organic vapors, and it also contributes to maintaining low pressure inside the reactor. A magnetic plate is used to stir the bar inside the reactor, ensuring uniform exposure of NPs to the plasma. Al NPs were treated with fluorocarbon-based plasma for 30, 45, 55, 85, 110, and 165 min. The RF power was maintained to 20 W and pressure inside the reactor to 0.06-0.1 Torr.
The coating thickness was measured by high-resolution transmission electron microscopy (HRTEM) using an FEI Talos F200X. Scanning transmission electron microscopy (STEM) and energy dispersive spectroscopy (EDS) analyses were performed on the same instrument at 200 kV with an XFEG source fitted with an integrated SuperX EDS detector. Imaging was performed in high-angle annular dark field (HAADF) to obtain information about the morphology and atomic distribution of the elements at the surface. X-ray diffraction (XRD) spectra were obtained by using a Malvern Panalytical XPert Pro MPD theta-theta diffractometer equipment was used for this experiment with a Cu Kα X-ray source. X-ray photoelectron spectroscopy (XPS) was performed in an ultrahigh vacuum to obtain detailed information about the elemental composition and chemical bonding on the surface of Al NPs. A Physical Electronics VersaProbe II instrument equipped with a monochromatic Al Kα X-ray source (hv=1486.7 eV) and a concentric hemispherical analyzer with a takeoff angle of 45° was used. Thermogravimetric analysis (TGA) analysis and differential scanning calorimetry (DSC) were performed on a TA Instruments Model Q600 SDT, which provides simultaneous measurement of heat flow (DSC) and weight change (TGA) on the same sample from ˜20 to 1000° C. Analyses were conducted in ultrazero air at a volumetric flow rate of 100 mL/min for all samples studied. Alumina sample cups (90 μL, TA Instruments) were used in the analyses. A heating rate of 10° C./min was used until 1000° C. after maintaining isothermal conditions for the first 5 min. Scanning electron microscopy (SEM) images were collected on a Verios G4 field emission (FE) SEM. The Al NPs were placed on copper tape for FE-SEM imaging. To determine the passivation effect of plasma nanofilms on Al NPs, accelerated aging tests were conducted. The untreated and plasma-coated (55 min) samples were placed in an oven for 120 min with temperature and relative humidity of 100° C. and 70%, respectively. The samples were then taken for DSC analysis to measure the heat released from the plasma-coated samples and compare them with the untreated sample. The changes in the heat release are noted before and after the aging experiment to quantify the passivation effect of plasma nanofilms on the surface.
Aluminum nanoparticles were treated with perfluoro-PECVD for six different periods (30, 45, 55, 85, 110, and 165 min) and were analyzed by using HRTEM, TGA, and DSC to determine their coating thicknesses and to compare weight gain and heat release with untreated Al NPs (control) during their thermal oxidation.
High-resolution transmission electron microscopy (HRTEM) is used to observe and measure the plasma coating thickness.
The extent of oxidation and heat release were analyzed at low heating rates by using TGA and DSC, which demonstrate the effect of coating Al NPs with plasma nanofilms on their oxidation properties. These properties provide more detailed information about the enhanced effect of perfluoro-based coatings on oxidation. The effect may or may not be related to combustion in which high heating rates are used for the complete oxidation of Al. However, recently, Zheng et al. demonstrated the direct relationship between low heating TGA/DSC and high heating laser combustion. The enhancement effect of the fluorine-based coatings on boron can be observed in similar proportions in low heating as well as high heating oxidation. A more detailed analysis can be extracted from TGA/DSC results, making them a highly significant technique for studying such effects.
The thermogravimetric analysis and differential scanning calorimetry track the progress of the oxidation reaction
2Al(l)+1.5O2(g)→Al2O3(s)ΔH°=−31kJ/g
by registering the weight gain and amount of heat released, respectively. TGA in
Oxidation of untreated Al NPs releases 15 KJ/g of energy, while oxidation of plasma-coated Al NPs releases 22.5 KJ/g of energy. The heat released is measured from both the exothermic peaks 1 and 2 for each sample as shown in FIG. XX (graph b). A significant amount of the additional energy comes from the oxidation of additional metallic Al. Another contribution comes from the fluorination of native oxide, Al2O3, shown by the reaction
Al2O3(s)+3CF2→2AlF3(s)+3COΔH°=−5kJ/g
Both TGA and DSC show similar trends: weight gain and heat release begin to increase with the plasma coating thickness, reach a maximum at a coating thickness of 10 nm, and gradually decrease past that point. At optimum thickness (10 nm), the heat released is 1.5 times that of untreated Al. Even the thickest sample (22 nm) registers higher weight gain and heat release than untreated Al. The maximum weight gain and the heat release correlate with each other, and both identify the same optimum coating thickness, that is, 10 nm. Clearly, more metallic Al is oxidized to release higher heat when F is present at the particle surface. Under optimum thickness, the total weight gain increases from 60.50 to 72%, implying the overall weight gain as a result of weight gain due to reaction 1 (oxidation of Al) and reaction 2 (fluorination of alumina). The metallic Al oxidized in the untreated sample is 68% while the heat release is just 48%, which is due to the inhibiting effects of the Al2O3 present near the surface. On the other hand, 81% metallic Al is oxidized for the plasma-coated (PECVD 55 min) sample, and the heat release is 72.60% of the theoretical heat release of Al because of the promoting effects of the fluorination of surface Al2O3 discussed in the next section. This suggests that the untreated Al releases only 70% (heat released/metal oxidized) of heat relative to the metal oxidized, while the plasma-coated Al releases 90% (heat released/metal oxidized) of heat with respect to the metal oxidized. The slopes of the weight gain in the plasma-coated sample are higher than that of the untreated sample, which indicates that the rate of weight gain due to oxidation is higher in the case of plasma-coated particles when compared to the untreated particles. Hence, the presence of plasma nanofilms enhances the rate of reaction. In untreated Al, weight loss is observed at lower temperatures until 450° C. because of the loss of volatile species such as water molecules. No weight loss is observed in the case of plasma-coated particles, which suggests that the coatings, due to their hydrophobic nature, improve the shelf life by not allowing water molecules to adsorb on the surface of Al NPs as claimed by other studies in the literature. The weight gain due to oxidation becomes negligible after 1100° C., suggesting the significant oxidation is slowed due to a change in the particle morphology and the presence of a lot of Al2O3.
To investigate the chemistry behind the changes in thermal oxidation behaviors, the sample with optimum weight gain and heat release (10 nm thickness at 55 min PECVD) was chosen for further characterization. Chemical maps at the particle surface were obtained by scanning transmission electron microscopy (STEM) in high-angle annular dark-field (HAADF) imaging and are shown in
The high-resolution XPS analysis in (d) of
To demonstrate the effect of plasma nanofilms on the passivation ability of Al NPs, aging tests have been performed on the untreated and the PECVD (55 min) treated Al NPs. The heat release is compared for these samples before and after the aging tests as shown in
A significant decrease of 60% is observed in the heat release of untreated Al NPs. However, the plasma-coated NPs exhibit a negligible decrease in the heat release and therefore validate the surface passivation effects of the 10 nm thick plasma films. These effects are due to the hydrophobic nature of the CFx-based coatings, which create a barrier between the metal and air/humidity. Therefore, the plasma nanofilms can preserve a higher amount of metallic Al in plasma-coated particles as compared to the untreated particles. The passivation effect of these films is corroborated by other studies, demonstrating that a hydrophobic coating is important in building a barrier against air and humidity.
As observed from the thermal analysis results, significantly improved heat release was achieved for plasma-coated Al NPs. The chemistry behind the significant improvement can be explained by using thermal analysis combined with XRD and SEM/TEM. A schematic of the possible interfacial mechanism resulting from PECVD treatment and thermal oxidation of Al NPs is shown in
When coated NPs are subjected to thermal oxidation, the Al core begins to melt and expand at ˜600° C. The surface reaction between Al2O3 and F from the plasma coating is also triggered at around the same temperature. This initiates the fluorination of the oxide according to the reaction disclosed above and is accompanied by heat release. The reaction eliminates the native oxide layer and forms AlF3. The formation of AlF3 is corroborated by comparing the XRD diffractograms of the oxidation products of untreated and plasma-coated samples in
The untreated sample shows representative peaks of Al2O3 (product of oxidation) and unreacted Al, whereas the plasma-coated sample indicates the presence of AlF3 (product of fluorination) in addition to Al2O3 and Al. The formation of the AlF3 shell with the release of energy at the interface leads to the formation of channels due to nanoexplosions that facilitate contact between molten Al and oxidizer and lead to more efficient oxidation. This mechanistic interpretation is supported by comparing the SEM and STEM micrographs of the untreated and plasma-coated particles before and after oxidation (
Plasma-coated particles imaged after TGA show a collapsed but compact structure with a surface morphology that suggests the presence of channels leading into the core of the particle (
In summary, we proposed a PECVD process to form fluorinated nanofilms on the surface of Al NPs, which could by converting native oxide into an energetic component. The process showed a nanometer-level precision on the deposition of fluorinated films required to reduce the native Al2O3, which maximizes the heat release from core-shell nanostructures of Al by avoiding the deposition of excess mass of the films. Removing the Al2O3 layer by surface fluorination reaction with fluorinated plasma coating can increase the contact area and diffusion rates of Al and the oxidizer by forming channels, thereby enhancing the oxidation process. More metallic Al has oxidized due to the presence of plasma nanofilms with a higher rate of oxidation. The heat release of plasma-coated Al NPs was enhanced by 50% as compared to the untreated Al NPs. There is no need to remove native oxide beforehand, and therefore there are no safety issues due to the presence of bare-surfaced Al NPs that can cause explosions. Because of the exothermic reaction between fluorinated films and native oxide during thermal oxidation, the kinetic and thermodynamic barriers due to native oxide are minimized. Using this approach, we can extract 72.5% of the theoretical energy density of Al. The presence of native oxide and fluorinated films together provides a double layer of passivation and can significantly improve the shelf life of the Al NPs. Thus, we can conclude that nonthermal plasma processing in the form of PECVD with appropriate coating materials has great potential in enhancing the performance of nanoenergetic materials.
It should be understood that the disclosure of a range of values is a disclosure of every numerical value within that range, including the end points. It should also be appreciated that some components, features, and/or configurations may be described in connection with only one particular embodiment, but these same components, features, and/or configurations can be applied or used with many other embodiments and should be considered applicable to the other embodiments, unless stated otherwise or unless such a component, feature, and/or configuration is technically impossible to use with the other embodiment. Thus, the components, features, and/or configurations of the various embodiments can be combined together in any manner and such combinations are expressly contemplated and disclosed by this statement.
It will be apparent to those skilled in the art that numerous modifications and variations of the described examples and embodiments are possible considering the above teachings of the disclosure. The disclosed examples and embodiments are presented for purposes of illustration only. Other alternate embodiments may include some or all of the features disclosed herein. Therefore, it is the intent to cover all such modifications and alternate embodiments as may come within the true scope of this invention, which is to be given the full breadth thereof.
It should be understood that modifications to the embodiments disclosed herein can be made to meet a particular set of design criteria. Therefore, while certain exemplary embodiments of the device and methods of using and making the same disclosed herein have been discussed and illustrated, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.
The following references are incorporated herein by reference in their entireties.
This application is related to and claims the benefit of U.S. Provisional Application 63/263,098, filed on Oct. 27, 2021, the entire contents of which is incorporated herein by reference.
This invention was made with government support under Contract No. N689636-19-C-0015 awarded by the United States Navy. The Government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2022/046826 | 10/17/2022 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63263098 | Oct 2021 | US |
