Patent Application
20100321147
1. Field of the Invention
The present invention relates to conductive nanocomposites, and particularly to a vanadium sesquioxide nanocomposite useful for applications in thermistors, current switching devices, static charge dissipation devices, and electromagnetic shielding. The present invention also relates to a novel sol-gel process for preparing nanoparticles of vanadium sesquioxide.
2. Description of the Related Art
Ceramic metal oxides have been shown to possess interesting electrical properties that have potential utility for electrical applications. These properties, e.g., conductivity and its reciprocal, resistivity, often depend upon the particular metal and its oxidation state, the method of forming the ceramic oxide (from gaseous materials, by microwave plasma, by sol-gel, including the temperature range that the metal is heated, the duration of heating, whether the process is performed under vacuum or pressure, the presence or absence of an oxidizing or reducing atmosphere, etc.), the nature of the product (gel, thin film, powder, etc.), and other factors. Since ceramic metal oxides, particularly transition metals, are also brittle and sometimes difficult to draw into a wire, the must also be fixed or embedded into a support, such a plating onto an electrode for use in batteries, applying in a thin layer to the surface of a support for infrared sensors or the like, mixing with electrolytes to form a dielectric material for a capacitor, or dispersing nanosized particles into a polymer matrix to form a conductive nanocomposite. The electrical properties, particularly conductivity and resistivity, of the resulting nanocomposite may often be affected by interactions between the metal oxide and the polymer(s) in which the metal oxide is dispersed.
Vanadium oxide is a transition metal oxide that is sometimes prepared with a valence of two (VO2, or vanadium dioxide), three (V2O3, or vanadium sesquioxide), five (V2O5, or vanadium pentoxide), or mixtures thereof. The forming processes have included powders formed by microwave plasma, thin films formed by sol-gel, etc., but the inventors are not aware of vanadium sesquioxide powder being formed by a sol-gel process, and certainly not by the process conditions described below and having the electrical properties described herein.
Thus, a vanadium sesquioxide nanocomposite solving the aforementioned problems is desired.
The vanadium sesquioxide nanocomposite is useful for applications in thermistors, current switching devices, static charge dissipation devices, and electromagnetic shielding. Vanadium sesquioxide nanoparticles are produced using a sol-gel process that results in a V2O5 gel. The gel is heated in a reducing atmosphere of about 5% H2-95% argon at 850° C. for about four hours. The resulting product is dried at about 50° C. for twenty-four hours to produce V2O3 powder having particles about 23 nm in size. The nanocomposite is prepared by mixing the sesquioxide nanoparticles with epoxy resin and hardener in a centrifuge, casting the mixture in a Teflon mold, heating the mixture at 60° C. for 30 minutes, and curing the product at 150 KN/m2 at 100° C. for two hours. The nanocomposite contains about 80-90 wt % epoxy resin-hardener mixture and about 10-20 wt % vanadium sesquioxide nanoparticles.
Various samples of the nanocomposite were prepared with the vanadium sesquioxide concentration ranging between 0 wt % and 20 wt %. Above about 8 wt %, the nanocomposite sample showed a decrease in resistivity that was 16 orders of magnitude less than pure epoxy resin. Above 20 wt %, the dielectric constant increase to the point that no dielectric effect is observed, and the resistivity and conductivity become constant. Between about 8% and 20%, the dielectric constant is increasing while resistivity decreases.
Testing of nanocomposite samples between about 8 wt % and 20 wt % vanadium sesquioxide showed a sharp, positive increase in the temperature coefficient of resistivity, which suggests applications as thermistors, temperature probes, temperature sensors, and other electrothermal applications. Measurement of current vs. applied voltage at constant temperature for vanadium sesquioxide concentrations in the above range showed a linear increase of current with voltage up to a peak switching value, beyond which the relationship is no longer linear, suggesting the nanocomposite samples are useful for switching applications. The nanocomposite samples between 10 wt % and 20 wt % of vanadium sesquioxide also showed high attenuation values between 1 GHz and 12 GHz, which suggests their use for antistatic charge dissipation and electromagnetic shielding in the microwave region.
These and other features of the present invention will become readily apparent upon further review of the following specification and drawings.
Similar reference characters denote corresponding features consistently throughout the attached drawings.
The vanadium sesquioxide nanocomposite is useful for applications in thermistors, current switching devices, static charge dissipation devices, and electromagnetic shielding. Vanadium sesquioxide nanoparticles are produced using a sol-gel process that results in a V2O5 gel. The gel is heated in a reducing atmosphere of about 5% H2-95% argon at 850° C. for about four hours. The resulting product is dried at about 50° C. for twenty-four hours to produce V2O3 powder having particles about 23 nm in size. The nanocomposite is prepared by mixing the sesquioxide nanoparticles with epoxy resin and hardener in a centrifuge, casting the mixture in a Teflon mold, heating the mixture at 60° C. for 30 minutes, and curing the product at 150 KN/m2 at 100° C. for two hours. The nanocomposite contains about 80-90% epoxy resin-hardener and about 10-20% vanadium sesquioxide nanoparticles.
Various samples of the nanocomposite were prepared with the vanadium sesquioxide concentration ranging between 0 wt % and 20 wt %. Above about 8 wt %, the nanocomposite sample showed a decrease in resistivity that was 16 orders of magnitude less than pure epoxy resin. Above 20 wt %, the dielectric constant increase to the point that no dielectric effect is observed, and the resistivity and conductivity become constant. Between about 8% and 20% the dielectric constant is increasing while resistivity decreases.
Testing of nanocomposite samples between about 8 wt % and 20 wt % vanadium sesquioxide showed a sharp, positive increase in the temperature coefficient of resistivity, which suggests applications as thermistors, temperature probes, temperature sensors, and other electrothermal applications. Measurement of current vs. applied voltage at constant temperature for vanadium sesquioxide concentrations in the above range showed a linear increase of current with voltage up to a peak switching value, beyond which the relationship is no longer linear, suggesting the nanocomposite samples are useful for switching applications. The nanocomposite samples between 10 wt % and 20 wt % of vanadium sesquioxide also showed high attenuation values between 1 GHz and 12 GHz, which suggests their use for antistatic charge dissipation and electromagnetic shielding in the microwave region.
The principles of the vanadium sesquioxide nanocomposite will now be illustrate by a description of the synthesis of a nanoparticle powder of vanadium sesquioxide, and by a description of tests of the properties of the nanocomposite at various concentrations of vanadium sesquioxide in the epoxy resin matrix.
V2O3 nanoparticles were synthesized by heat treating sol-gel derived vanadium oxide nanopowder in reducing atmospheres. In a typical procedure, 1.0 g of V2O5 powder was dissolved in 30 ml of 30% H2O2 solution under vigorous magnetic stirring at room temperature. Red brown viscous gels formed within three days by heating the solution at 85° C., with the pH being controlled at around 2.0 by periodic addition of H2O2 as needed. V2O3 powders were formed by heating the V2O5 gel in a 5% hydrogen-95% argon gas mixture at 850° C. for about 4 hours. After being dried in vacuum at 50° C. for 24 hours, the final black powder products were obtained. The powder had a particle size of about 23 nm.
The polymer used in this investigation was a commercial bisphenol A-type epoxy resin (Epikote 828) and a casting hardener (type B002W), which were supplied by Yuka Shell Epoxy Co., Ltd. of Tokyo, Japan. A stoichiometric resin/hardener ratio of 100:20 by weight was used according to the manufacturer's data sheets. V2O3 powder with particle size of about 23 nm was received from the method of Example 1. Several batches of epoxy resin:V2O3 weight ratios were considered, including 90:10, 95:15, and 80:20, respectively, and abbreviated as V10, V15, and V20, respectively. The green epoxy-hardener with different contents of vanadium sesquioxide filler were prepared by mixing in a centrifuging mixer at 4000 rpm for 2 minutes at room temperature. The bulk samples of nanocomposite were obtained by casting the green composites into Teflon molds, which were placed in an electrical oven that was preheated to 60° C. for 30 minutes. Then, the epoxy resin/vanadium sesquioxide filler nanocomposites were cured under hot uniaxial pressure 150 KN/m2 at 100° C. for 2 hours.
The room temperature XRD patterns of green epoxy (0 wt % V2O3) and epoxy/V2O3 samples with different compositions are shown in
As expected, the V2O3 is more dispersed, sinking into the epoxy matrix and forming a more continuous phase in the nanocomposites as a result of good interfacial adhesion. This was verified by scanning electron microscopy images of the nanocomposites. The crystallinity (C) of the filler in the nanocomposites increases with an increase in the V2O3 loading levels, as shown in the chart in
It is realized that the extent of filler reinforcement increases with increase of V2O3 in epoxy matrix, as shown in
The glass transition temperature (Tg) as a function of V2O3 content is also depicted in
For further confirmation the above facts, the sound velocity (SV) as a function of V2O3 content is shown in
The volume resistivity of the epoxy/V2O3 composites as a function of the weight percentage of the V2O3 is shown in
Further, the reduction of resistivity with the increase of conductive filler content is attributed to the enhanced mobility of charge carriers. The increase of conductive phase content results in smaller intermolecular distance between conductive sites. This leads to the decreasing electrical resistivity of the nanocomposites. It is interesting to note that a conductivity plateau was detected in our results and was attributed to the presence of a superstructure of flocculated filler particles. However, the phenomenon demonstrates that the percolation threshold in the conductivity of the nanocomposite is less than 8 wt %.
The dielectric constant as a function of V2O3 weight percentage of the composites is also plotted in
The positive increase in electrical resistivity of the epoxy/V2O3 nanocomposites at elevated temperatures can be used to design “electrical self-regulating heating” materials. The resistivity vs. temperature characteristics of the nanocomposites is presented in
Clearly, a closer examination of the hopping and activation energies as a function of filler content is necessary to gain insight into the conduction process. The values of activation energies have been calculated using least square fitting of the slopes of the log of resistivity versus the inverse of temperature data for various concentrations in the temperature range 25°-70° C. and are shown in
This fact is confirmed by computation of the concentration of charge carriers as a function of filler content in the nanocomposites, as shown in
To explain the influence of V2O3 on the network structure, an isothermal stability test of the nanocomposites by monitoring the variation of resistivity versus time at a fixed temperature was conducted to obtain more information concerning interfacial bonding between filler and matrix. First, the variation of electrical resistivity of the composites as a function of time at 50° C. was monitored, and the results are shown in
Again, the epoxy matrix charged with V2O3 nanoparticles has a higher thermal stability than the uncharged epoxy polymer. This assertion is based on analysis of the thermal gravimetry (TG) and differential scanning calorimetry (DTA) diagrams of the nanocomposites, as shown in
With an increase of the electric field, the behavior of the I-V curve changes from Ohmic to non-Ohmic. This is attributed to the change in the percolation conductive network across the epoxy matrix, and to thermal fluctuations due to significant Joule heating that took place so that nonlinearity that set in. Increasing the electric field above a certain voltage, termed peak, and which depends on filler content, leads to an increase in the Joule heating effect, and consequently increases the sample's temperature and decreases the current, i.e., showing negative resistance.
According to the results of the V/T experiment, shown in
The shapes of both experimental and theoretically calculated SE, shown in
Hence, the reflectivity properties of the conductive epoxy nanocomposites depend on nanocomposite composition and the microstructure attained after processing. In this case, the V2O3/epoxy nanocomposite agglomerates between the resin phases affect the wave-matter interaction. This shows that these conductive V2O3/epoxy nanocomposites are an effective absorber in the microwave range.
In addition, the V2O3/epoxy nanocomposite with 10% V2O3 content (V10) presents lower microwave absorbing properties between 1 and 12 GHz. However, the conducting nanocomposite presents a shift of the attenuation values to higher frequencies, specifically the maximum of the attenuation values occur at frequencies higher than 12 GHz. These results suggest that the nanocomposite with 20 wt % V2O3 content may be useful for antistatic discharge and electromagnetic shielding applications in the microwave region, achieving attenuation up to 42 dB in the 1-12 GHz region. For electromagnetic shielding, it is preferred that the shielding be applied as a surface coating having a thickness of about 0.1 mm.
In summary, the conductive nanocomposites containing epoxy resin reinforced V2O3 nanoparticles are useful for various technological applications, such as positive temperature coefficient (PTCR) thermistors, current switching devices, and electromagnetic shielding. A new chemical route of synthesizing V2O3 nanoparticles has been presented that is a very simple and economic mode for mass production. Furthermore, the chemical method, i.e., a sol-gel method heating the product in a reducing atmosphere, i.e., in a 5% hydrogen-95% argon gas mixture at 850° C. for 4 hours, provides a powder having a particle size of about 23 nm.
In the nanocomposites, it is observed that the V2O3 nanoparticles are more dispersed, sinking into the epoxy resin matrix and forming a more continuous phase in the nanocomposites as a result of good interfacial adhesion. The crystallinity, packing factor, and extent of filler reinforcement increase with increasing loading levels of V2O3, which indicates that the inclusion of V2O3 nanoparticles improves the molecular structure of the epoxy resin and acts as a bonding and or reinforcing agent in the epoxy resin matrix. Also, the glass transition temperature and hardness of the nanocomposites increase with increasing V2O3 content.
According to the above structure and properties, it becomes possible to provide a good, conductive, nanocomposite structural material having excellent structural and mechanical properties and excellent thermal stability. The percolation threshold for electrical conductivity of the nanocomposite is less than 8 wt %. The dielectric constants of the nanocomposite samples reach 102 when the weight percentage of the V2O3 is 20 wt %, which is 37 times that of plain green epoxy without V2O3 nanoparticles. This high value makes it possible to provide super-capacitor conducting nanocomposites having especially excellent electrical properties.
The positive increase in electrical resistivity of the epoxy/V2O3 nanocomposites at elevated temperatures may be used to design electrical self-regulating heating or thermal materials and devices. A sharp electrical resistivity increase is generally seen at relatively high temperature and has been termed the “positive temperature coefficient” (PTC) effect of electrical resistivity, useful for temperature probes, thermistors, sensors, and the like.
The current versus dc voltage and ultimate temperature characteristic (I/V/T) behaviors of the nanocomposites at a room temperature of 25° C. exhibit a peak point (i.e., a switching point), which is referred to as the negative resistance. This behavior makes it possible to use the vanadium sesquioxide nanocomposites as switching current and/or voltage devices at low applied voltage potentials for microelectronic applications with good reliability.
High SE attenuation values (about 42 dB) in the frequency range of 1-12 GHz, were obtained when the nanocomposites contained 20 wt % of the V2O3 in the epoxy-based nanocomposite (V20). It is preferred that the thickness of the surface of the EMI Shielding be about 0.1 mm. This makes it possible to attenuate electromagnetic waves up to about 42 dB, so that it is possible to provide electromagnetic shielding conducting nanocomposites having especially excellent electro-magnetic properties.
It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.
