The present disclosure relates to methods and devices for in situ synthesis of metal oxides in carbon nanotube arrays. The present disclosure further relates to carbon nanotubes foams with controllable mechanical properties.
Nominally-aligned arrays of carbon nanotubes (CNTs) are known to behave as low-density energy dissipative foams under compression. The material can be readily synthesized using standard thermal chemical vapor deposition techniques, resulting in a foam-like bulk material consisting of trillions of CNTs per square centimeter. However, these systems have remained largely unused in practical applications due to large variations in properties that result from the synthesis process.
According to a first aspect of the present disclosure, a method for controlling microstructural arrangement of nominally-aligned arrays of carbon nanotubes (CNTs) is provided. The method comprises modifying or controlling mechanical response of CNT arrays after synthesis of CNTs by synthesizing particles in situ in the nominally-aligned arrays of carbon nanotubes (CNTs).
According to a second aspect of the present disclosure, a method for controlling microstructural arrangement of nominally-aligned arrays of carbon nanotubes (CNTs) is provided where the CNTs have an ordered structure as grown. The method comprises modifying mechanical response of arrays of CNTs after synthesis of CNTs by associating a plurality of particles to the arrays of CNTs, where the arrangement of CNTs with the particles is an arrangement ordered like or equally to the ordered structure of the CNTs as grown.
According to a third aspect of the disclosure, a foam structure comprising nominally-aligned arrays of carbon nanotubes (CNTs) is provided. The foam structure further comprises a plurality of particles associated to the nominally-aligned arrays of CNTs; where the CNTs have an ordered structure as grown, the arrangement of CNTs with particles is an arrangement ordered like or equally to the ordered structure of the CTNs as grown, where a modification of the distribution or number of particles determines a modification of mechanical response of the foam structure.
Further aspects of the disclosure are shown in the specification, drawings and claims of the present application.
Throughout the present disclosure, embodiments and variations are described for the purpose of illustrating uses and implementations of the inventive concept. The illustrative description should be understood as presenting examples of the inventive concept, rather than as limiting the scope of the concept as disclosed herein. The words and phrases used in the present disclosure should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art.
In the present disclosure, the expression “nominally-aligned arrays of carbon nanotubes” can be used to refer ordered structures or arrangements of nanotubes which can naturally align themselves and can be held together by Van der Waals forces and lateral entanglement of the CNTs, which are not perfectly parallel (hence “nominally-aligned”). In this context, the term “alignment” can refer to “bundles” or “groups” of CNTs, and not specifically on the alignment of the individual tubes in the arrangement.
In the present disclosure, the expression “synthesis”, which is, for example, included in the expression “synthesis process”, “synthesis parameters” or “method for synthesizing”, can refer to a process in which volatile or gas-phase precursors including a carbon source, can react on a substrate, leading to nanotube growth. In some embodiments of the present disclosure, the synthesis can be a process based on chemical vapor deposition (CVD). In such cases, CVD synthesis can be achieved by taking carbon species in the gas phase and using an energy source, such as plasma, a resistively heated coil or heat in general, such as, the heat of a heated furnace to impart energy to a gaseous carbon molecule. Gaseous carbon sources can comprise, for example, toluene, methane, carbon monoxide, and acetylene. In such cases, the energy source can be used to “crack” the carbon molecule into a reactive radical species. These radical reactive species can then be diffused down to the substrate, which can be heated and coated in a catalyst (for example, a first row transition metal such as Ni, Fe, or, Co), where it can bond. According to some example embodiments, the synthesis of nominally-aligned CNTs can comprise a floating catalyst thermal chemical vapor deposition (TCVD) system with a reaction zone (furnace), a precursor solution comprising a catalyst and a carbon source, and a carrier gas to move the solution into the reaction zone. The synthesis of the CNTs can take place on a thermally oxidized surface, for example, Si surface, placed inside the furnace prior to the reaction.
In accordance with the present disclosure, mechanical properties of carbon nanotubes (CNTs) can be useful in many applications, [see, for example, reference 1, incorporated herein by reference in its entirety], which can serve as a motivation to design materials that can realize macroscale advantages through integrating these nanoscale structures [see, for example, reference 2, incorporated herein by reference in its entirety]. As known by a person skilled in the art that, among such design approaches, nominally aligned arrays (or “forests”) of millimeter-scale CNTs can be readily synthesized via standard chemical vapor deposition (CVD) techniques. Nominally aligned arrays (or “forests”) of millimeter-scale CNTs can exhibit behavior similar to fatigue-resistant, open-cellular foams under compression [see, for example, references 3 and 4, incorporated herein by reference in their entirety], with significant recovery from deformation and orders of magnitude superior energy dissipation relative to commercial foams of comparable density (0.1-0.3 g cm−3) [see, for example, reference 5, incorporated herein by reference in its entirety].
In some example embodiments, understanding the structure-property relationships for nominally aligned arrays of millimeter-scale CNTs materials, such as, how the bulk mechanical response can be affected by various structural features [see, for example, references 6-8, incorporated herein by reference in their entirety] can be beneficial. In order to study such properties, synthesis parameters can be altered to obtain CNT arrays with different features, allowing the study of how CNT surface roughness (see, for example, reference 6, incorporated herein as reference in its entirety), CNT diameter distribution (see, for example, reference 7, incorporated herein as reference in its entirety), or partially-graphitic layering around individual CNTs (see, for example, reference 8, incorporated herein as reference in their entirety.) can affect the bulk mechanical response. The control of these synthesis parameters combined with the modification of CNT arrays after synthesis (e.g., by the infiltration of polymer into array interstices [see, for example, reference 9, incorporated herein by reference in its entirety], or by the incorporation of surfactants and nanoparticles via solvent wetting [see for example, reference 10, incorporated herein by reference in its entirety]), can allow for tuning of the mechanical response, such as array stiffness and energy dissipation, under compression.
In some embodiments, nanoparticle modification of CNTs can be performed on disordered arrangements of CNTs that have first been dispersed in solution (often an acid) and then filtered. Such procedures can be performed to synthesize particles on disordered arrangements of CNTs such as ZnO [see, for example, reference 12, incorporated herein by reference in its entirety], Au [see, for example, reference 13, incorporated herein by reference in its entirety], Ni [see, for example, reference 14, incorporated herein by reference in its entirety], CaCO3 [see, for example, reference 15, incorporated herein by reference in its entirety], Cu [see, for example, reference 16, incorporated herein by reference in its entirety], and others [see, for example, reference 17, incorporated herein by reference in its entirety]. SnO2 nanoparticles can be integrated with disordered arrangements of CNTs using CVD [see, for example, reference 18, incorporated herein by reference in its entirety] and solution-based techniques [see, for example, reference 19, incorporated herein by reference in its entirety]. Moreover, in some embodiments, MnO2 particles can be integrated with disordered CNTs [see for example, references 20, 21, incorporated herein by reference in their entirety] as well.
As known in the art, such materials can be used for various electrochemical applications, such as aqueous super-capacitors [see, for example, reference 22, incorporated herein by reference in its entirety]. The materials based on disordered agglomerates of surface-modified CNTs can be useful for some applications, without infiltration of particles deep inside CNT arrays. However, this can necessitate the loss of the ordered structure of CNT arrays. In some applications, such a CNT powder can be integrated into another, usually polymeric, matrix. This process can have difficulties of its own, such as the difficulty in obtaining uniform dispersion of the CNTs in the matrix [see, for example, reference 23, incorporated herein by reference in its entirety].
In some embodiments of the present disclosure, inorganic materials can be infiltrated into ordered CNT arrays. In such cases, a sol-gel process can be used to create a CNT-glass composite, however such cases can focus on enhancing thermal and electrical conductivities of the arrays [see, for example, reference 24, incorporated herein by reference in its entirety]. In some embodiments, low pressure CVD can be used as well, for short CNT arrays (for example, approximately 50 μm) due to difficulties in getting reactions to take place more than a few tens of microns deep in the array [see, for example, reference 25, incorporated herein by reference in its entirety]. Moreover, a vapor-assisted technique can be used to synthesize TiO2 uniformly in short CNT arrays [see, for example, reference 26, incorporated herein by reference in its entirety]. However, in to some embodiments, the presence of nanoparticles (for example, metal particles or metal oxide particles) can improve the mechanical performance of CNT arrays without disrupting their ordered structure and can be useful to investigate the mechanical stability of the hybrid CNT-nanoparticle structures, which could be useful in multifunctional applications. Some example of such applications can be found in reference 31, incorporated herein by reference in its entirety. For example, SnO2 and MnO2, or other particles or substances, can be synthesized in CNT arrays without disrupting the ordered structure of the individual CNTs or the overall structure of the arrays themselves. Moreover, under compression the structures can exhibit a hysteretic response, similar to CNT arrays. Such structures modified with nanoparticles can dissipate up to twice the amount of energy as unmodified samples. Modifying CNT arrays with SnO2 can result in brittle deposits of the oxide in the array interstices separated by elastic bundles of CNTs.
In accordance with the present disclosure, in some embodiments, compressing CNT arrays that have been modified with SnO2 can result in lateral fracturing through the oxide deposits, followed by elastic recovery of the CNT bundles. In such cases, after a few compressive cycles, the material with SnO2 responds similarly to unmodified CNT arrays in compression (as compared by quasistatic stress-strain data and energy dissipation). In contrast, when MnO2 particles are synthesized in CNT arrays by emersion of the CNTs in aqueous KMnO4, the particles can form on the individual CNTs themselves. The modifications can result in higher energy dissipation during compression and minimal lateral fracturing after repeated cycling, but can yield more entanglement of the individual CNTs, resulting in less strain recovery after compression.
As known by a person skilled in the art, electrochemical applications have been developed for similar materials and continued study of the mechanical properties of these systems can lead to useful multifunctional materials with simultaneous mechanical and electrochemical uses. Moreover, dispersion of particles deep within millimeter-scale arrays can be obtained without altering the crystalline structure of the individual CNTs or the ordered arrangement of them. In addition to modifying the CNT arrays, the ordered arrangement of CNTs can be tested under quasistatic compression to examine how energy dissipation, strain recovery, loading/unloading modulus, and permanent damage are affected by the modifications. Understanding of these mechanical properties can be a first step toward the use of materials based on nanoparticle-CNT array structures in relevant applications, such as electrochemical applications [see, for example, reference 27, incorporated herein by reference in its entirety].
In accordance with the present disclosure, the previously indicated methods can be a novel approach for modifying the mechanical response of CNT arrays post-synthesis. For example, the CNT arrays can be reinforced by coating the individual CNT surfaces or filling the interstices of the arrays with metal oxide particles, or other particles or substances that can be synthesized in situ in the CNT arrays. In other words, the CNT arrays can have particles or other substances added, which can be extraneous with respect to the CNT material, and can be synthesized in situ in the CNT arrays. These particles or other substance can be synthesized in the CNT material after synthesis of the CNT material.
According to some embodiments of the present disclosure, two different procedures can be used to synthesize MnO2 and SnO2 particles in the CNT arrays. For the synthesis of MnO2 particles, a solution-based approach can be used. This approach is described in more detail in subsequent paragraphs of the present disclosure. For the synthesis of SnO2 particles, a kinetically-controlled catalytic synthesis approach can be used, similar to that used for growing Sn particles in situ in graphitic anodes for electrochemical applications [see, for example, reference 11, incorporated herein by reference in its entirety]. In both cases, the particles can be synthesized in situ in the CNT arrays.
According to several example embodiments of the present disclosure, in relation to CNTs and synthesis of CNTs, arrays of multiwall carbon nanotubes (CNTs) can be synthesized using a thermal chemical vapor deposition (CVD) system and a floating catalyst approach described in references 7 and 28, each of which is incorporated herein by reference in its entirety]. In such cases, the growth substrate can be thermally oxidized Si placed in a CVD furnace set to 827° C. A 0.02 g ml-solution of ferrocene (i.e., a precursor of Fe, a catalyst for CNT synthesis) and toluene (i.e., a carbon source for CNT synthesis) can be injected at a rate of 1 ml min−1 using a syringe pump into the heating zone, with Ar as a carrier gas. This approach can result in continued deposition of new catalyst, and thereby continued initiation of new CNT growth, throughout the synthesis process. CNT array samples (for example, with heights of 1-1.5 mm, a cross sections of 10-20 mm2, volume occupied by CNTs ˜10%, and individual CNT diameters of 40-50 nm, as characterized by transmission electron microscopy in reference 7, incorporated herein as reference in its entirety) can be removed from their growth substrates using a razor blade. In such cases, the mass for each of these samples can be measured using a microbalance, which can be used to calculate the bulk density, both before and after synthesis of the oxide particles.
In some embodiments, loading of SnO2 particles can follow steps similar to those discussed in reference 11, incorporated herein as reference in its entirety. In such cases, CNT samples can be first added to aqueous SnCl2 (for example, 0.2 M, 5 ml) with, for example, 0.6 ml of acetone added to aid absorption into the array. After soaking for, for example, 46 h at room temperature, the CNT samples can be fully wetted with the SnCl2 solution and placed in a sealed container with an open solution of ammonia (for example, 2% wt.). Consequently, the ammonia vapor can gradually diffuse to the sample, initiating hydrolysis of the SnCl2 solution contained inside the CNT array. The samples can then be removed and washed with deionized water, followed by further heat treatment in, for example, N2 at 450° C. for 1 h at the heating rate of 5° C. min−1, yielding the final CNT/SnO2. To load MnO2 particles into the CNT arrays, the CNT samples can be soaked in, for example, aqueous KMnO4 (0.2 M, 5 ml) for 46-120 h (with the variation in time controlling the loading amount) at room temperature. During the soaking, MnO2− can be spontaneously reduced to MnO2 on the surface of the CNTs, which can act as a reducing agent [see, for example, reference 21, incorporated herein as reference in its entirety]. After soaking, the sample can be subjected to further heat treatment in N2 at 450° C. for 1 h at a heating rate of 5° C. min−1, yielding the final CNT/MnO2. Scanning electron microscopy (SEM) can be used to obtain images of sample structure at different magnifications and locations for each sample. By counting the number of CNTs crossing an arbitrary horizontal line at different locations, it can be determined that there are no statistically significant changes in the spacing of individual CNTs.
According to some example embodiments of the present disclosure, two samples with SnO2 and five samples with MnO2 can be synthesized following the procedures as described above, and can be compared to the performance of three unmodified control samples. Samples can be repeatedly compressed quasistatically, using a commercial materials test system (for example, Instron E3000), to 0.8 strain (with strain being defined as the displacement normalized by sample height; i.e., 0.8 strain is equivalent to compressing the sample until it is only 20% of its original height). These compressions can occur at a strain rate of 0.03 s−1 (i.e., 3% of the original sample height every second). In such cases, for each modified CNT array an unmodified “control” sample can be tested that has been removed from the growth substrate directly adjacent to it, and therefore can have almost the exact same height, density, mean CNT diameter, etc., prior to modification.
In such cases, as mentioned in the previous paragraph, energy dissipation per unit volume can be obtained by integrating the area of the stress-strain hysteresis for each loading cycle. The loading modulus can then be calculated by examining the initial slope of the stress-strain curve. Similarly, the unloading modulus can be calculated by taking the slope of the stress-strain curve after unloading from maximum strain has just begun (corresponding to a drop in stress to ⅔ of the maximum stress). Thermo-gravimetric analysis (for example, TGA, Mettler-Toledo 851e instrument) can be conducted at 550° C. in air to quantify the amount (wt. %) of particle loading for each modified sample. Consequently, the type of oxide can be determined after synthesis of the particles using x-ray diffraction (XRD), Philips microscopy using a FEI Quanta 200F, and transmission electron microscopy using a FEI TF30UT at 300 kV.
In some embodiments, after synthesizing oxide nanoparticles in the CNT arrays with various loadings (i.e., different quantities of SnO2 or MnO2 as quantified by wt. %) following relevant experimental procedures, samples can be characterized with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For example, in accordance with the present disclosure, the example embodiments of
As shown in the example embodiment of
As described in the previous paragraphs, the synthesis of SnO2 can be performed from aqueous SnCl2 precursor contained in the CNT array, using a hydrolyzing agent (ammonia) to cause the precipitation of Sn(OH)Cl that is converted to SnO2 in the subsequent heat treatment. In such cases, the CNT array can provide a substrate/space to accommodate the SnO2 but may not play an active role in the reaction. For the synthesis of MnO2, with aqueous KMnO4 as precursor, the CNTs can take a more active role in the reaction, acting as both a reducing agent and a substrate for MnO2 precipitation [see, for example, references 20 and 21, incorporated herein as reference in their entirety]. This can result in particles being formed mainly on CNTs, not everywhere in the interstices. In the example embodiments of
In accordance with the present disclosure, example embodiments of
In accordance with the present disclosure, representative compressive stress-strain responses for samples modified with MnO2 and SnO2 are shown in exemplary embodiments of
In some embodiments, in addition to these differences resulting from the different particle types, an effect from particle morphology can exist within a given category of particle type. As mentioned earlier, the morphological differences between the samples displayed in the exemplary images of
In some embodiments, further examination can be performed on the response of the samples under repeated compressive loading. As known by a person skilled in the art, one of the properties of as-grown CNT arrays synthesized by floating catalyst CVD is their ability to dissipate energy and to recover much of their original height even after many compressive cycles to high strain (0.8 or higher) [see, for example, references 3 and 4, incorporated herein as reference in their entirety]. In such cases, the first cycle can reach the highest peak stress and can dissipate the largest quantity of energy, with a significant drop in these for the second cycle. After only a few compressive cycles, however, the material can begin to reach a steady state response that does not vary significantly from cycle to cycle [see, for example, reference 7, incorporated herein as reference in its entirety]. In some cases, it can be observed that the response to repeated loading can depend on whether the sample was reinforced with MnO2 or instead with SnO2. As shown in the exemplary graph of
In table 1 as shown below, loading and unloading modulus and energy dissipation per unit volume of modified and unmodified samples are provided.
According to example embodiments of the present disclosure,
In the case of samples modified with SnO2, however, by the fourth compressive cycle, the material can behave almost identically to the control, dissipating approximately the same amount of energy and attaining approximately the same peak stress, as shown in the exemplary graph of
In some embodiments, examining the loading and unloading moduli can be useful to understand the compressive response under repeated loading cycles. As summarized in the exemplary table 1, the initial loading moduli for exemplary samples modified with SnO2 have an average value (41±8 MPa) approximately an order of magnitude higher than those of either the unmodified samples or those modified with MnO2. However, by the fourth cycle (see table 1) the average loading modulus for exemplary samples with SnO2 has dropped by an order of magnitude to closely match the average value for unmodified samples. In contrast, the exemplary samples with MnO2 show a substantial increase in loading modulus after a few cycles. In the exemplary table 1, all samples show a decrease in unloading modulus after the first cycle, though the samples with MnO2 show a decrease of a relatively smaller value.
In accordance with the present disclosure, the results discussed above and displayed in
Moreover, in some embodiments, such behavior can be in accordance with the morphology displayed in
In contrast, as shown in the example embodiment of
In accordance with the present disclosure,
In addition to Sn and Mn oxides, Fe oxide and Co oxide particles can be synthesized as well, using corresponding metal salts as precursors and following similar procedures as described earlier. Subjecting the oxide particles to carbothermal reduction can form metallic particles. Moreover, as known by a person skilled in the art that, the versatility of the processes described in the present disclosure as a proof of concept, further work is necessary, including the synthesis of a larger number of such samples, to understand the systematic effects of these different types of particle loadings on the mechanical properties. The integrity of these structures under mechanical stresses can be understood by understanding how the affinities of the various types of particles for CNTs can differ from one another, as discussed in the present disclosure for SnO2 and MnO2.
The examples set forth above are provided to give those of ordinary skill in the art a complete disclosure and description of how to make and use the embodiments of the disclosure, and are not intended to limit the scope of what the inventors regard as their disclosure. Modifications of the above-described modes for carrying out the disclosure may be used by persons of skill in the art, and are intended to be within the scope of the following claims. All patents and publications mentioned in the specification may be indicative of the levels of skill of those skilled in the art to which the disclosure pertains. All references cited in this disclosure are incorporated by reference to the same extent as if each reference had been incorporated by reference in its entirety individually.
It is to be understood that the disclosure is not limited to particular methods or systems, which can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. The term “plurality” includes two or more referents unless the content clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosure pertains.
A number of embodiments of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the present disclosure. Accordingly, other embodiments are within the scope of the following claims.
The present application claims priority to U.S. Provisional Application No. 61/639,747, filed on Apr. 27, 2012, which is incorporated herein by reference in its entirety. The present application may be related to U.S. patent application Ser. No. 13/491,014, filed on Jun. 7, 2012, and U.S. patent application Ser. No. 13/254,402 filed on Mar. 2, 2010, each of which is incorporated herein by reference in its entirety. The present application can also be related to U.S. application Ser. No. 13/866,596, entitled “Multilayer Foam Structures Of Nominally-Aligned Carbon Nanotubes (CNTs)”, filed on Apr. 19, 2013, and U.S. application Ser. No. 13/868,952, entitled “Method For Controlling Microstructural Arrangement Of Nominally-Aligned Arrays Of Carbon Nanotubes” filed on Apr. 23, 2013, by Chiara Daraio, Abha Misra and Jordan R Raney, each of which is incorporated herein by reference in its entirety.
This invention was made with government support under W911NF-09-D-0001 awarded by the Army Research Office. The government has certain rights in the invention.
Number | Date | Country | |
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61639747 | Apr 2012 | US |