WO2006089222A2

WO2006089222A2 - Metal nano-powder compositions and methods for preparing same

Info

Publication number: WO2006089222A2
Application number: PCT/US2006/005835
Authority: WO
Inventors: Qingmin Cheng; Anit Giri; Srikanth Raghunathan
Original assignee: Nanomat, Inc.
Priority date: 2005-02-18
Filing date: 2006-02-17
Publication date: 2006-08-24
Also published as: WO2006089222A3

Abstract

The present invention provides a process for producing metal, metal alloy, and metal composite nano-powders in large volume with a minimum of by-products and process steps. The invention specifically embodies a method comprising providing one or more metal oxide nano-powder(s), mixing the metal oxide nano-powder(s) with an amount of reducing metal or reducing metal precursor to provide an oxide-reducing metal mix; heating the oxide-reducing metal mix in an inert atmosphere to a reducing temperature; followed by washing and drying. The method provides metal nano-powders with average primary particle size of 5 to 500 nm, and a high specific surface area. The invention further comprises unique metal nano-powder compositions of tungsten, zirconium, titanium, and tungsten- hafnium composite that are useful in propellant and explosive formulations. A TEM of a zirconium nano-powder of the invention is illustrated.

Description

Metal Nano-Powder Compositions and Methods for Preparing Same

This invention claims priority back to a United States Provisional application filed on February 18, 2005, and designated U.S. Provisional Application Ser. No. 60/654,747. It was originally made with United States Government support under Agreement No.N00014-04-C-0236 awarded by the Office of Naval Research. The United States Government has certain rights in the invention.

BACKGROUND OF INVENTION Field of Invention. The present invention relates to the field of metal nano-powder manufacture and in particular to manufacture of metal nano-powder compositions useful in the fields of capacitor manufacture, hydrogen storage media, and high energy propellants and explosives. Description of Related Art. Reduction of heavy metal oxides by alkaline earth metals or hydrides is known; see U.S. Patents 4,687,632, 4,149,876, 3,801,307, 2,950,185, 2,516,863, 1,728,941, for example. These patents discuss reducing methods for preparing metal powders with particle size ranging from about 5 to about 300 μm. The fraction of nano-sized particles, from 5 to 200 nm, available from these methods, is too small to be a practical source of submicron-sized nano-powders.

There are many known physical methods and chemical methods for preparing metal nano-powders including high energy mechanical milling of metals, mechanical milling that initiates chemical reactions, high energy irradiation of metals or metal precursors with electron beam or lasers; heat induced evaporative methods, heat induced decomposition of metal precursors, and chemical reduction methods. For instance, Porarnsky, in U.S. 6,688,494, describes a process for the manufacture of metal nanoparticles that calls for forming a vapor of a metal that is solid at room temperature, the vapor being provided in an inert gaseous stream that is contacted with an inert liquid collecting medium. Other known methods for dispersing ultra- fine metal particles in a dispersing medium also are reviewed in the reference. Magnusson (J. Mater. Res., 15, 1564-1569, 2000) describes the thermal decomposition of tungsten (W) hexacarbonyl to produce size-selected, W particles with a size range of 15 to 60 nm Matteazzi, et al, (J. Alloys and Compounds, 358, 71-5, 2003) describes the solid state reduction of tungsten oxide with magnesium metal driven by high energy ball milling. The tungsten powders formed are welded aggregates of small particles with a primary particle size of 70-100 ran All the methods suffer from either a large number of processing steps; particle sizes which are much larger than the primary particle sizes because of agglomeration and/or welding; and a limited throughput due to bottlenecks in the processing and high energy requirements. As a result, nano-metal powders available commercially are expensive and restricted in use to high value applications.

Metal nano-powders are of interest in diverse technology areas including electrical applications, e.g., formation of capacitors; fuel cell applications, e.g., hydrogen gas storage; and explosive and propellant applications. In these applications the characteristics of small primary particle size and high surface area of metal nano- powders offer significant advantages. Other performance requirements such as metal purity, oxygen content, etc. may vary widely, for each specific application. Capacitor Applications. U.S. patent 4,149,876, Rerat, describes a method of producing tantalum powder with a selected particle size and demonstrates that, in the 5 -300 urn range, smaller particle size of metal powders leads to higher capacitance. U.S. patent 6,051,044, Fife, describes a nitrogen containing niobium powder having a nitrogen content of about 300 ppm, and a BET surface area of at least 1.0 m²/g. Surpin, in U.S. patent 6,409,796 describes a method of treatment of a metal powder to increase its surface area for capacitance applications. The method produces tantalum powders with surface areas ranging from 2.5 to 40 m²/g.

US patent 6,136,062, Loffelholz, et al, describes a process for the reduction of niobium and/or tantalum oxides by means of alkaline earth metals in a two stage process, wherein the first reduction stage is carried out to provide an average composition corresponding to (Nb₃Ta)O_x where X = 0.5 to 1.5, and before the second stage the reduction product from the first stage is freed from alkaline earth oxides which are formed by washing with mineral acids. The resulting NbO_x powder which is obtained occurs in the form of agglomerated primary particles. Column 3, line 11 and thereafter, states that the primary particle size is essentially determined by the particle size of the oxide starting material, taking into account the reduction in volume due to the removal of oxygen and the change in crystal structure. The particle size of the oxide starting material is selected according to the desired primary particle size. The aim is to achieve a primary particle size of 0.1 to 1 μm for the acidic earth metal powder obtained after the second reduction stage, by corresponding grinding and sieving of the oxide starting material. In the second stage reduction the elements phorporous and/or nitrogen are added in low levels to improve the capacitance characteristics of the final metal powder. U.S. patent 6,786,951, He, et al, describes a process of the production of high surface area tantalum and/or niobium powders via the reduction of the corresponding tantalum and/or niobium oxides. The powders are suitable for manufacturing electrolytic capacitors. The reduction is carried out in the presence of at least one metal halide selected from the group consisting of halides of Mg, Ca, Sr, Ba and Ce, and an alkali metal at elevated temperature so as to form the tantalum and/or niobium powders. The tantalum and/or niobium oxides used for the process may be any tantalum and/or niobium oxides or their mixture. The primary particle size of powders dervied from the process is in the range of 10-250 nm and the BET surface area of 1 to about 40 m²/g. The process of the '951 patent does not specify a particle size for the starting metal oxide, that is, any source of metal oxide may be used; and the process requires a metal halide diluent, about 6 parts of a salt are present during typical reductions. Figure 1 of the '951 patent shows an SEM that claims the primary particle size of the powder is about 40-60 nm. Example 1 of the '951 patent shows the Ta powder, the product of the reduction, has a BET SSA of 7.2 m²/g and a medium particle size of 2.1 μm. Hydrogen Storage Applications

There are many applications, which can use materials that can effectively absorb hydrogen and then release hydrogen. Upon release of hydrogen, the materials can be used again to absorb hydrogen. U.S. patent 6,797,182, Vergani, et al, describes a process for the purification of organometallic compounds by contacting said organometallic compounds with a hydrogenated getter alloy including those derived from Ti and/or Zr with one or more elements selected among transition metals. U.S. patent 5,543,687, Woyke, et al, describe a hydrogen discharge lamp having a glass lamp enclosure formed with a radiation-emitting window and receiving a body of a Zr-Co alloy forming in part a hydride which constitutes a reservoir for hydrogen or deuterium and enabling controlled liberation of the hydrogen or deuterium from the reservoir.

Hydrogen is one of the most promising fuels being considered to replace petroleum fuels for the transportation sector. Hydrogen fuel-cells that generate electricity from a flow of hydrogen are being used to power electric automobile engines, and combustion engines that burn hydrogen are being used in other applications. Although hydrogen has more energy than gasoline on a per-weight basis, it has a much lower energy/unit volume than gasoline. As a result, conventional hydrogen storage systems require a much larger storage vessel than gasoline tanks to provide the same vehicle operating range. Gas-on-solid adsorption offers the possibility of a more dense storage medium than would be achieved utilizing hydrogen alone. A number of metal alloys are known to reversibly absorb and release hydrogen. For instance, FeTi, LaMs, Mg₂Ni, pure Mg, and ZrCo have all been investigated as hydrogen storage media. U.S. patent 5,525,435, Pourarian, provides a recitation of the prior art on hydrogen storage materials, and describes hydrogen storage materials for use in various hydrogen absorber devices such as electrochemical cells, and hydrogen separator devices. U. S. patent 6,726,892, Au, describes advanced aluminum alloy nano crystalline powders suitable for storing hydrogen.

Propellants and Explosives

Metal powders are the primary fuel for almost all solid rocket motor propellants widely used in a variety of aerospace applications, such as launch vehicles for space craft, tactical missile and short range propulsion systems. The solid propellant formulations most widely used today contain as key ingredients aluminum (Al) particles as the metal fuel and ammonium perchlorate (AP) particles as the oxidizer. The Al and AP particles are held together in a polymer matrix, most often a polyurethane matrix based on hydroxyl-terminated polybutadiene. A number of efforts have been made to improve the efficiency of rocket motor propulsion systems, propellants and explosives. A brief summary of these efforts is outlined in U. S. patent 6,843,868 entitled "Propellants and Explosives with Fluoro-organic Additives to Improve Energy Release Efficiency." U. S. patent 6,454,886, entitled "Compositions and Method for Preparing Oxidizer Matrix Containing Dispersed Metal Particles," hereby incorporated by reference, and 5,912,069 to Yializis et al., entitled "Metal Nanoaluminate Composite" discuss the utility of extremely small- sized particulate matter in the context of propellants. For instance, within the '886 patent, (Table 5, and column 19) is discussed the effect of particle size, on the burn properties of the propellant. Within a given system the propulsion potential (as defined in the patent, and measured at low, near ambient pressures) increases as the particle size of the Al particles and AP particles decreases, indicating that the lower particle size formulations would provide more powerful fuels. Other studies support this finding. For instance, Armstrong, et al., (Nano Letters, 3 253-255, 2003) describe the burn-rate of aluminum nano -powders including "ALEX", the nano-aluminum powder available by the plasma explosion process. The burn rate at the smallest nanometric Al particles size, in the range of 50 to 300 nm, appears to be asymptotically approaching an inverse particle-squared dependence. That is, the smaller the particle size, the higher the burn rate. Kuo, et al, in 'Tropellant Burning Rate Enhancement and Thermal Behavior of Ultrafine Aluminum Powders (ALEX)" (29^th International Annual Conference of ICT, June 30 - July 3, 1998, Karlsruche, DE, July 1998, Report 30) concluded that the beneficial aspects of ALEX addition in terms of enhanced burning rate in propellant formulations are caused by the large specific surface area and small size of ALEX particles compared to regular aluminum powders. Aluminum powder is preferred for most propulsion and explosive applications because it is inexpensive; it has high energy content, 7.4 Kj/g; and is stable in storage. For certain projectile applications, high density and high energy content are desired characteristics and thus, metal powders such as W, Ta, Hf, Nb, and Ti powders are being considered as alternatives to aluminum They have comparable energy content per unit volume to aluminum (about 20 Kj/cc) and densities 2 - 6 times higher than aluminum As with aluminum propellants, there is also a desire to have high burn rates in these materials. Thus, in the field of energetic materials there is a need to explore efficient methods to manufacture metal nano-powders, having high surface area, small particle size and high densities. Thus, there is a need in several diverse technology areas for processes that provide metal nano-powders having a primary particle size on the nano-scale of about 5 to about 200 nm, with a minimum of agglomeration, and high surface area. Preferable would be processes that provide an inexpensive source on nano-powders, in high volume, with a minimum of by-products, and process steps. SUMMARY OF INVENTION

One objective of the invention is to provide efficient processes for producing metal nano-powders in large volume with a minimum of by-products and process steps. Another objective is to provide metal nano-powder compositions with high specific surface area and high reactivity toward oxygen. Another objective is to provide metal nano-powders with high metal content and low oxygen content.

The invention embodies a method for preparing metal nano-powder, characterized by an average primary particle size of about 5 to about 500 ran, comprising: providing one or more metal oxide nano-powder(s), characterized by an average primary particle size of about 5 to about 500 ran, of the general formula:

[M_x(O)_y]_1-z[M_x(O)_y]_zi[M_x(O)_y]_z2[M_x(O)_y]_z3 wherein M is a metal selected, independently, from the group of: Ti, Zr₅ Hf, V, Nb, Ta, Cr, Mo, W, Mn, and rare earth metals, x is an integer equal to the valence of oxygen, y is an integer equal to the valence of M, and z is equal to the summation of zl, z2, and z3, and the summation of 1-z, zl, z2, z3, is equal to 1; mixing the metal oxide nano-powder(s) with an amount of reducing metal or reducing metal precursor to provide an oxide-reducing metal mix; heating said oxide-reducing metal mix in an inert atmosphere to a reducing temperature for a reaction time sufficient to reduce said metal oxide nano-powder(s) to provide a metal nano-powder and a reducing metal oxide; washing said metal nano-powder with reagents to remove said excess of reducing metal and reducing metal oxide to provide a purified metal nano-powder; and drying said purified metal nano-powder at a drying temperature for sufficient time to remove residual reagents. Another embodiment of the invention is a tungsten nano-powder characterized by an average primary particle size of 5 to about 80 nm, a BET specific surface area of greater than 5 niVg; and a DSC/TGA oxidation exotherm peak less than about 450 ⁰C when scanned at 5 ⁰C /min in air at a flow rate of 1 L/min.

Another embodiment of the invention is a zirconium nano-powder characterized by an average primary particle size of 5 to about 200 nm, a BET specific surface area of greater than 5 m²/g, and burning in air at ambient temperature and pressure.

Another embodiment of the invention is a tungsten-hafnium composite nano- powder characterized by an average primary particle size of 5 to about 200 nm, a BET specific surface area of greater than about 5 mVg; and a DSC/TGA oxidation exotherm peak less than about 440⁰C when scanned at 5 ⁰C /min in air at a flow rate of 1 L/min.

Another embodiment of the invention is a titanium nano-powder characterized by an average primary particle size of 5 to about 200 nm, a BET specific surface area of greater than 2 m^z/g; and a DSC/TGA oxidation exotherm peak less than about 600 ⁰C when scanned at 5 ⁰C /min in air at a flow rate of 1 L/min,

Another embodiment of the invention is a zirconium-tungsten alloy nano- powder characterized by an average primary particle size of 5 to about 200 nm, a BET specific surface area of greater than 5 ntVg; and a DSC/TGA oxidation exotherm peak in the range of about 300 to about 500⁰C when scanned at 5 ⁰C /min in air at a flow rate of 1 L/min.

BRIEF DESCRIPTION OF THE FIGURES

Fig. IA illustrates the DSC/TGA scan for Ta nano-powder provided by the process of the invention.

Fig. IB illustrates the DSC/TGA scan for a commercial micron-sized Ta powder.

Fig. 2A illustrates the DSC/TGA scan for tungsten nano-powder of the invention. Fig. 2B illustrates the DSC/TGA scan for a commercial tungsten nano-powder with a nominal particle size of 100 nm

Fig. 3A illustrates the DSC/TGA scan for Ti nano-powder of the invention. Fig. 3B illustrates the DSC/TGA scan for commercial micron-sized Ti powder. Fig. 4 illustrates the DSC/TGA scan for W/Hf nano-powder of the invention.

Fig. 5 illustrates the DSC/TGA scan for W/Zr nano-powder of the invention. Fig. 6 illustrates the TEM of tungsten nano-powder of the invention at 120,00OX.

Fig. 7 illustrates the TEM of Ti nano-powder of the invention at 8O₃OOOX. Fig. 8 illustrates the TEM of Zr nano-powder of the invention at 25,00OX.

DETAILED DESCRIPTION OF INVENTION

Throughout the specification and in the Claims the term "metal nano-powder" is meant to encompass pure metals, metal alloys comprising two or more metals, and metal composite nano-powders. The term "metal oxide nano-powders" is meant to encompass a single metal oxide, mixtures of two or more metal oxides, metal oxide composites, and mixtures thereof.

Metal oxide nano-powders useful in practicing the process of the invention are characterized by an average primary particle size of about 5 to about 500 nm, and preferably 5 to 200 nra The metal oxide nano-powders are available from a variety of well known chemical, mechanical and physical methods including plasma synthesis, sol-gel synthesis, hydrothermal synthesis, and high temperature calcining in reactive atmospheres. For instance, Pedrosa, et al, (Mat. Res. Bull., 39, 683-693, 2004), and references cited therein, reviews several synthetic pathways to WO3 powders, and specifically describes WO₃ powders provided by the thermal decomposition of tungstic acids. U.S. patent application publication 2002/0155059, Maher, et al, describes a process and apparatus for the synthesis of TiO₂ nano-powder by plasma oxidation OfTiCl₄. Li, et al, (J. Solid State Chemistry, 177, 1372-1381, 2004) describe the sol-gel method for preparing TiO₂ nano-powder from tetrabutyl titanate. Ristic, et al, (Materials Letters, 58, 2658-2663, 2004) describe the sol-gel method for preparing Nb₂O₅ fromniobium(V) ethoxide. Kominami, et al, (Phys. Chem. Chem Phys., 2001, 3, 2697-2703) describes the synthesis OfTa₂O₅ by solvothermal reaction of tantalum pentabutoxide in toluene in the presence of water. U.S. patent application now U. S. Patent No. 2004/0022722, to Martin, describes the synthesis OfZrO₂ aggregates with an average size in the range 0.1 μmto 0.6 μmby aqueous ammonia hydrolysis OfZrOCl₂. U.S. patent application now U.S. Patent No. 2002/0182141, to Uchida, describes a synthesis OfZrO₂ powder comprising pre- calcining ZOCl₂ followed by calcining the product in an atmosphere containing air and hydrogen chloride. The product formed has a primary particle diameter of 0.1 μm and a BET SSA of 15.4 m²/g. Larbot, et al, (J. Sol-Gel Sci. & Tech., 17, 99-110, 2000) describe the synthesis of hafnia (HfO₂), with a particle size of about 30 run and a SSA of 27.0 m²/g, by the sol-gel process from hafnium l-methoxy-2-propoxide. A preferred source of metal oxide nano-powders is the hydrolysis of a metal salt or mixture of metal salts with an organic base to form a metal oxide gel. The hydrolysis process comprises: mixing an aqueous solution of a first metal salt, optionally, with an aqueous solution of a second metal salt, optionally, adding a third metal salt and, optionally, adding a forth metal salt, to provide an aqueous metal salt mixture; adding to the aqueous metal salt mixture, with mixing, an organic base in sufficient quantity to convert the metal salt mixture to provide a metal oxide gel; separating the metal oxide gel from the aqueous solution; washing the separated metal oxide gel with water and then a water miscible solvent to provide a purified metal gel; drying the purified metal gel to provide a gel powder; annealing the gel powder at an annealing temperature sufficient to convert the gel powder to metal oxide nano- powders. The process allows the formation of metal oxide nano-powders of average primary particle size of about 5 to about 200 nm.

Metal salts that are useful in forming of metal oxide nano-powders include those selected from the group: TiCl₄, ZrCl₄, ZrOCl₂, Zr(NO₃)₄, ZrOSO₄, HfCl₄, TaCl₅, TaBr₄, NbCl₅, NbBr₄, WCl₆, Na₂WO₄, (NH₄)ioW₁₂0₄i, CoCl₂, CoCl₂-OH₂O, CoBr₂, CoBr₂-XH₂O, Co(NO₃)₄, CO(OAC)₂, CO(OAC)₂- 6H₂O, and the metal alkoxides Ti(OR)₄, Hf(OR)₄, Ta(OR)₅, Nb(OR)₅, and Zr(OR)₄, wherein R is a lower alkyl group having from 1 to about 6 carbon atoms. An aqueous solution of one or more of the metal salts is prepared, preferably at a molar concentration of about 0.02 to about 0.2 molar and, more preferably, about 0.05 to 0.1 molar. A water miscible co-solvent, such as methanol, ethanol, acetone and the like, can be used as a co-solvent, if so desired. The salt solution can be mixed at room temperature or a higher or lower temperature can be used, if so desired. Preferably, the mixture is stirred at room temperature during the addition of the organic base. Organic bases useful in the hydrolysis reaction include: ammonium hydroxide, methylamine, ethylamine, propylamine, dimethylamine, diethylamine, triethylamine. Preferably about a 5 to 20 % stoichiometric excess of base, based on complete hydrolysis of the metal salts is used, and more preferably about 5 to 10 % excess is used. Mixing in the organic base can be accomplished with any type of high speed stirrer capable of maintaining a homogeneous suspension during the addition of the organic base.

Separating the metal oxide gel from the aqueous solution can be most readily accomplished with centrifugation. Washing the separated metal oxide gel with water and then a water miscible solvent to provide a purified metal gel also can be accomplished with centrifugation and decantation of the supernatant. Other separation methods also can be used including membrane filtration, dialysis, and vacuum filtration, if so desired.

Drying of the gel can be accomplished in a hot air oven at a temperature in the range of 80 to 150 ⁰C. However, a lower or higher temperature can be used, if so desired. Annealing the gel powder to an annealing temperature is usually accomplished by heating the gel powder in a furnace at about 250 to about 500 ⁰C, for about 1 to 4 hours. Preferably annealing is accomplished in air, but an inert atmosphere can be used, if so desired. The temperature range and time range may be extended, if so desired, so long as the temperature and time are sufficient to fully convert me gel powder to a metal oxide or metal oxide composite powder. The annealing temperature for respective metal oxides is selected by measuring the weight loss, due to dehydration to the metal oxides, in the DSC/TGA scans. The metal oxide may be amorphous, crystalline, or a mixture thereof. All forms appear to work equally well in the reduction process of the invention.

Metal oxide nano-powders characterized by an average primary particle size of about 5 to about 200 nm, that are available by the above described hydrolysis process include: TiO₂, ZrO₂, HfO₂, Ta₂O₅, Nb₂O₅, WO₃, ZrO₂/Co₃O₄, WO₃/ZrO₂, WO₃/TiO₂, W0₃/Ta₂0₅, WO₃ZNb₂O₅, and WO₃ZHfO₂. One advantage of the above process for preparing metal oxide nano-powders is that the powders can be used in the process of the invention with no further size reduction or separation processes.

Mixing of the metal oxide nano-powders with a reducing metal or reducing metal precursor can be accomplished by any low energy method that provides a relatively homogeneous mixture of the metal oxide nano-powder and the reducing metal or reducing metal precursor, without inducing chemical reaction. Mixing does not require the formation of a completely homogeneous oxide-reducing metal mix. Examples of mixing apparatuses include a mortar and pestle, a vibratory ball mill, such as a Spex 8000 mixer-mill; a tumbler ball mill; and low shear blender. Under vigorous grinding conditions, outside the scope of the mixing contemplated in this process step, the metal oxide may react with the reducing metal. For instance,

Matteazzi, et al., (referenced above) reported that "nanometric particles" of tungsten can be produced by solid state room temperature reduction OfWO₃ with magnesium metal in a vibratory milling over a period of eight minutes. The initial WO₃ particle size was of about 20 μm, and the magnesium particle size was between about 350 and 125 μm The tungsten powder provided are formed of impact welded aggregates of small particles of homogeneous dimension of about 70-100 nm Premature reaction of the metal oxide with a reducing metal or reducing metal precursor in the mixing step should be avoided.

Reducing metals that can be used in the process are Na, Li, K, Mg, Ca, and the rare earth metals, and preferred reducing metals are Na, Mg, and Ca. Reducing metal precursors that can be used in the process are those that, upon heating, generate an active reducing metal. For instance, calcium hydride, sodium hydride, potassium hydride, lithium hydride, all form reducing metals upon heating. The metal hydrides are preferred because they readily form fine powders that can be rapidly dispersed with the metal oxides and metal oxide composite nano-powders. Calcium hydride, having an average particle size of about 100 μm, is a most preferred reducing metal precursor for the process of the invention. Sodium hydride, and potassium hydride can be used, if so desired, if precautions are taken to remove the oil from the dispersion usually used to stabilize the powders.

The reducing metal or reducing metal precursor used in preparing the oxide- reducing metal mix should be at least the stoichiometric quantity required to obtain complete reduction of the metal oxide or metal oxide composite. Preferably, a small excess of reducing metal or reducing metal precursor is used; in the range of 1.01 to 1.5 times the stiochiometric amount required; and more preferably, about 1.1 to 1.3 times the stoichiometric amount required is used.

The oxide-reducing metal mix is heated in an inert atmosphere to a reducing temperature for a reaction time sufficient to reduce the metal oxide to provide a metal nano-powder. The inert atmosphere can be argon, helium or the like. Nitrogen is avoided, and argon is preferred. The reducing temperature is in the range of about 600 to about 1200 ⁰C and a preferred range is 650 to about 1000⁰C. When running a batch type process, the oxide-reducing metal mix is usually ramped to the reducing temperature over a period of time, for instance, 10 °C/min. A higher or lower ramp rate may be used, if so desired. Typically, the oxide-reducing metal mix is placed in a ceramic container, placed in an argon purged furnace, and ramped to the reducing temperature. The temperature is held at the reducing temperature for a period of time to complete the reduction. Usually 0.5 to 6 hours, and preferably 1 to 3 hours, is sufficient to provide complete reduction of the oxide-reducing metal mix to a metal nano-powder. The time required to obtain complete reduction is usually empirically determined for each specific composition. During the heating process, the oxide- reducing metal mix can be further mixed with a stirrer, tumbled in a rotary type furnace or allowed to sit in a ceramic crucible, for instance.

Heating the oxide-reducing metal mix to the reducing temperature is preferably accomplished in the absence of other inorganic diluents such as the metal salts described in the He '951 patent. Applicants have found that diluents are not necessary for preparing metal nano-powders using the process of the invention. The use of diluents to control the exotherm of the reaction or decrease the agglomeration of primary particles in the product adds to the cost of the process, increases the amount of reagents needed in the washing step, and leads to undue contamination of the metal nano-powders with halide.

Washing the metal nano-powders and reducing metal oxide mixture with wash reagents can be used to remove any excess reducing metal and the reducing metal oxide. The wash reagents can be selected from the group of aqueous solutions of mineral acids, preferably dilute hydrochloric acid; aqueous solutions of organic acids, preferably dilute acetic acid; water, preferably deionized water; aqueous solutions of organic alcohols, preferably methanol or ethanol; aqueous miscible organic solvents, preferably, acetone, methyl ethyl ketone, tetrahydrofuran, and the like. Typically, washing is accomplished by first treating the nano-powder and reducing metal oxide mixture with dilute aqueous acid, followed by multiple washing with deionized water to bring the pH of the wash water to about 6 to about 8, and preferably about 7. The water washing can be followed by one or more washings with aqueous miscible organic solvents to substantially remove water from the nano-powder in preparation for the drying process.

The drying process removes residual water and solvent from the metal nano- powder. Suitable drying apparatuses are ovens, vacuum ovens, rotary ovens, and radiant heaters. Typical ambient temperature of the dryer is about 30 to about 100 °C, but lower or higher temperatures may be used, if so desired. In the drying process, a pressure below ambient room pressure can be used; an inert gas stream, such as argon, can be used; and a mixing or other form of agitation can be used, if so desired. Drying of reactive metal nano-powders is preferably done in an inert atmosphere. The metal nano-powders can be further characterized by their behavior in differential scanning calorimetry and thermal gravimetric analysis (DSC/TGA) that are run simultaneously. The DSC/TGA characterization is performed under a standardized set of conditions of 5 ⁰C /min scan rate and an air flow through the test cell of 1 L/min. For propellant and explosives, important attributes of the metal nano- powders are the active metal content, the sensitivity to burning in air, the burn rate, the overall energy release and the oxygen content. In the propellant industry these attributes are usually evaluated in complex formulations and burn tests, for instance, as described in the '886 patent (referenced above). However, some of these attributes can be measured by TGA, wherein the weight gain from oxidation of the metal nano- powder to metal oxide in air is measured; or by DSC by direct measure of the oxidation exotherm. In general, large particle size metal powders (> 1 μm) show slow uptake of oxygen but exhibit high active metal contents (> 95 %). The metal nano-powders of the invention generally exhibit very rapid burning in air, so the true energy release often can not be reliably measured in the DSC. Some metal nano- powders, e.g. zirconium, burn immediately when exposed to air. Those metal nano- powders that do not burn immediately upon exposure to air can be characterized by their oxidation exotherms upon heating. The oxidation exotherm is accompanied by a large increase in weight as determined by the TGA. The oxidation exotherm is the exotherm corresponding to the largest gain in weight in the TGA and is characterized by a weight gain greater than 10 % and usually greater than 15 %. Secondary exotherms of significantly lower energy may be exhibited at lower and higher temperatures.

The metal nano-powders provided by the process of the invention generally exhibit oxidation exotherms at a much lower temperature than conventional micron sized powders. For instance, The Ta nano-powder of Example 1 exhibits an oxidation exotherm at 450 ⁰C (Fig. IA) versus an exotherm of micron-sized Ta powder of about 665 ⁰C (Fig IB). The tungsten nano-powder of Example 2 exhibits an oxidation exotherm of 423 ⁰C (Fig. 2A) versus an exotherm of a commercial tungsten nano-powder that exhibits an exotherm at 514 °C (Fig. 2B). The titanium nano- powder of Example 3 exhibits an oxidation exotherm of 542 ⁰C (Fig. 3A) versus an exotherm of a commercial micron-sized Ti powder of about 1070 ⁰C (Fig. 3B). The zirconium nano-powder of Example 6 exhibits such reactivity toward oxygen that it burns immediately upon exposure to air.

The high reactivity of the metal nano-powders may be a function of their small particle size and relatively high surface area The metal nano-powders are characterized by average primary particle size using transmission electron microscopy (TEM). The primary particles are agglomerated, some metals more than others. Fig. 6 shows a substantially agglomerated tungsten nano-powder of the invention with an average particle size of 50 nm. Fig. 7 shows a lightly agglomerated titanium nano- powder of the invention with an average particle size of about 100 nm. Fig. 8 shows a partially agglomerated zirconium nano-powder of the invention with an average particle size of about 50 nm The nano-powders of the invention are further characterized by BET specific surface area (SSA). The nano-powders have high surface areas relative to commercial micron-sized metal powders. For instance, the tungsten nano-powder of Example 2 has an SSA of 9.74 m²/g versus a commercial tungsten nano-powder exhibiting an SSA of 2.3 m²/g (example 2A). The zirconium nano-powder of the invention exhibits an SSA of 23.8 m²/g versus a commercial micron-sized powder exhibiting an SSA of 0.08 mVg (example 6A).

A further embodiment of the invention is a tungsten nano-powder characterized by an average primary particle size of 5 to about 80 run, a BET specific surface area of greater than 5 m²/g; and a DSC/TGA oxidation exotherm peak less than about 450 ⁰C when scanned at 5 ⁰C /min in air at a flow rate of 1 L/min. Preferably the W nano-powder has a BET specific surface area of about 9 to 15 m²/g, and a DSC/TGA oxidation exotherm peak between about 350 and 450 ⁰C. The tungsten nano-powder is further characterized by a tungsten content of about 98 wt %, or higher, and preferably higher than 99 wt %.

A further embodiment of the invention is a zirconium nano-powder characterized by an average primary particle size of 5 to about 200 nm, a BET specific surface area of greater than 5 m²/g, and burning in air at ambient temperature and pressure. Preferably the Zr nano-powder has a BET specific surface area greater than 10 m²/g and preferably between about 18 to 30 m²/g. The Zr nano-powder is further characterized by a Zr content of greater than 94 wt %.

A further embodiment of the invention is a tungsten-hafnium composite nano- powder characterized by an average primary particle size of 5 to about 200 nm, a BET specific surface area of greater than about 5 m²/g; and a DSC/TGA oxidation exotherm peak less than about 500 ⁰C when scanned at 5 ⁰C /min in air at a flow rate of 1 L/min. Preferably the W/Hf composite nano-powder has a BET specific surface area greater than 10 m²/g, and most preferably, between 10 and about 30 m²/g. The tungsten-hafnium composite nano-powder is further characterized by a mol ratio of tungsten to hafnium of about 0.8 to about 1.2. Preferably the oxygen content of the nano-powder is less than 3 wt % as measured by inert gas fusion analysis. A further embodiment of the invention is a titanium nano-powder characterized by an average primary particle size of 5 to about 200 nm, a BET specific surface area of greater than 2 m²/g; and a DSC/TGA oxidation exotherm peak less than about 600 ⁰C when scanned at 5 ⁰C /min in air at a flow rate of 1 L/min. The Ti nano-powder is further characterized by an oxygen content of less than 2 wt % and a titanium content of greater than about 98 wt %.

The small primary particle size and high surface area exhibited by the metal nano-powders of the invention are characteristics that are of significant value in the development of new propellants and explosives. The same characteristics are sought in other applications such as for catalysis, hydrogen storage materials, and electrical applications such as capacitors.

The following examples are meant to illustrate the invention and are not meant to limit the scope of the invention. Example 1

This example illustrates the formation of Ta metal nano-powder by reduction of tantalum oxide nano-powder.

Anhydrous TaCIs (0.14 mol, Strem Chemicals, Inc.) was dissolved in anhydrous methanol (100 mL) in an argon filled glove box to provide a clear solution with the release of heat. The solution was removed from the glove box and diluted with deionized (DI) water (2000 mL) with stirring in air to provide a clear solution. The solution was stirred vigorously (600 rpms) and ammonium hydroxide (100 mL, 2.57 mol) was added slowly over 10 min. Stirring was continued for an additional 1 h. The resulting gel was centrifuged, washed three times with DI water (500 mL) and twice with acetone (100 mL) The material was dried in air in an oven at 120 ⁰C to give a white powder (34.0 g) that was annealed at 450 ⁰C for 60 min in air to give Ta₂O₅ (29.0 g, 0.065 mol, 94 %) as a nano-powder. The XRD diffraction pattern was consistent with crystalline Ta₂O₅. The transmission electron microscopy (TEM) of the annealed powder showed a spherical average primary particle size of about 5 nm. The Ta₂O₅ nano-powder (5.4 g, 0.012 mol) and CaH₂ (3.5 g, 0.083 mol, 38 % excess) were mixed and ground in a glove box for 10 min. The mixture was covered and transferred to a ceramic alumina boat in an argon purged ceramic tube furnace. The furnace was ramped to 850 ⁰C at 10 °C/min while maintaining an argon purge of 1 L/min, and held there for 3 h. The furnace was cooled to room temperature to obtain a black-gray powder. The powder was stirred with dilute HCl (1.0 M₅ 800 mL) for 4 h at room temperature and centrifuged. The resulting powder was washed with DI water (400 mL) three times to give a final wash water of pH 7; followed by washing two times with acetone (200 mL) and drying under vacuum overnight; to give a black Ta nano-powder (3.5 g, 0.019 mol, 79 %). The XRD was consistent with a phase-pure Ta metal and a small unknown peak at 2Θ = 35 degrees. The TEM showed an average primary particle size of about 20 - 50 nm with agglomeration. The oxygen content, as determined by inert gas fusion analysis using a LECO TCH-600 instrument, was 1.76 %. The BET specific surface area (SSA), as measured by nitrogen absorption with a Beckman Coulter SA-3100 instrument was 5.74 m²/g. DSC/TGA (Fig. IA) in air, at 5 °C/min showed a sharp oxidation exotherm peak at 451 ⁰C with an energy of 3.3 KjVg. The SSA of the Ta powder, the product of the process of the invention described herein, is comparable to that of Example 1 of the '951 patent that exhibits a BET SSA of 7.2 m²/g.

Example IA (Comparative)

This Example illustrates the properties of a tantalum metal powder available commercially from Strem Chemicals, Inc, Newburyport MA.. A Ta powder with a nominal particle size of 325 mesh exhibited a BET SSA of 0.16 m²/g. DSC/TGA (Fig IB) in air exhibited an oxidation exotherm at 665 ⁰C with an energy 6.0 Kj/g.

Example 2

This example illustrates the formation of tungsten metal nano-powder by reduction of WO₃ nano-powder.

Ammonia paratungsten (100 g, 0.033 mol, (NH₄)_IoWnO_4I) was dissolved in water (3 L) and heated to 60-65 ⁰C. Hydrochloric acid (200 mL, 12 M, 2.4 mol) diluted with water (500 mL) was added to the paratungsten solution slowly (30 min) with vigorous stirring (600 rpm). Stirring at 60-65 ⁰C was continued for an additional

1 h. Centrifugation provided a yellow precipitate that was washed 4 times with water

(500 mL) until PH was 7, followed by washing twice with acetone (200 mL). The resulting precipitate was dried overnight and annealed in air at 500 ⁰C for 2 h to provide a yellow nano-WO₃ powder (80 g, 88 %) The powder exhibited an average particle size of about 50 nm with agglomeration. XRD was consistent with crystalline

WO₃.

Nano-WO₃ (85 g, 0.367 mol) was treated with calcium hydride (60 g, 1.43 mol) as described in Example 1 to give a black nano-W powder (54 g, 0.293 mol) The powder exhibited a BET SSA of 9.74 m²/g and oxygen content of 0.941% (LECO). TGA/DSC (Fig 2A) in air exhibited an oxidation exotherm at 423 ⁰C with an energy 2.56 Kj/g. TEM (Fig. 6) showed an average primary particle size of about 60 nm with some agglomeration. Example 2A (Comparative)

This Example illustrates the properties of a tungsten metal nano-powder available commercially from Argonide Corp., Sanford Fl. A nano-W powder with a nominal particle size of 100 nm exhibited a BET SSA of 2.3 m²/g. TGA/DSC (Fig 2B) in air exhibited an oxidation exotherm at 514 ⁰C with an energy 1.0 Kj/g.

Example 3

This example illustrates the formation of nano-Ti.

Nano-TiO₂ (9.0 g, 0.113 mol) was treated with calcium hydride (12.0 g, 0.286 mol) as described in Example 1 to give a black nano-Ti powder (4.5 g, 0.094 mol, 84.5 %). TEM (Fig. 7) of the powder exhibited an average primary particle size of about 100 nm, an SSA of 2.9 m²/g and an oxygen content of 0.97 %. TGA/DSC (Fig. 3A) in air exhibited an oxidation exotherm at 533 ⁰C with an energy of 3.7 Kj/g.

Example 3A (Comparative)

This Example illustrates the properties of a titanium metal powder available commercially from Aldrich Chemical. A Ti powder with a nominal particle size of 100 mesh exhibited a BET SSA of 0.17 m²/g. TGA/DSC (Fig 3B) in air exhibited an oxidation exotherm at 1070 ⁰C with an energy 37 Kj/g.

Example 4

This example illustrates the synthesis of nano-W/Hf composite. Nano -WOsZHfO₂ was prepared in a manner similar to that OfWO₃ZZrO₂ in

Example 5.

Nano-WO3ZHfO₂ (10.0 g) was treated with calcium hydride (7.0 g) as described in Example 1 to give a black nano-WZHf composite powder (4.8 g) The powder exhibited a BET SSA of 12.8 mVg; an oxygen content of 2.68 %, and a TEM average particle size of about 20 nm. TGA/DSC (Fig. 4) in air exhibited an oxidation exotherm at 320 ⁰C with an energy of 5.0 Kj/g.

Example 5

This example illustrates the formation of nano-ZrW₂ composite.

In argon filled glove box, 3O g ZrCl₄ (0.129 mol) was dissolved into 100 ml anhydrous methanol to give a clear solution with release of heat. The clear solution was removed from the glove box and diluted by DI water (2 L) to give a clear colorless solution. Ammonia hydroxide (40 mL, 1.03 mol) was slowly added to this solution with vigorous stirring and stirring continued for 1 h to give a white gel that was centrifuged and washed with DI water (500 ml) three times, until pH was about 7.0. Water (1 L) was added and stirred for 30 min to give solution A.

Ammonia paratungsten (20 g, 0.0066 mol) was dissolved in DI water (1 L) and the solution heated to 60-65 ⁰C. Solution A was added to the APT solution and stirred for another 10 min to provide a gel solution C. Hydrochloric acid (140 mL, 4

M₅ 0,48 mole) was slowly added to solution C with vigorous stirring with heating to about 60-65 ⁰C. Stirring was continued for 1 h, followed by centrifugation, washing with DI water three times (1500 ml) and acetone (200 ml) twice. The white powder was dried in air overnight, followed by annealing at 500 ⁰C for 2 h in air to provide a nano- WCVZrC^ metal oxide composite (33 g). XRD pattern showed the white powder was amorphous. Heating to 700 ⁰C, in air, provided a yellow powder with an

XRD consistent with WO3 and ZrO₂. The white powder is not a simple WO₃ and

ZrO₂ composite. TEM images indicated the white powder was of gel-like structure with particle size less than 5nm Nano-WO₃/ZrO₂ (15 g) was treated with calcium hydride powder (13.0 g,

0.31 mol) as described in Example 1 to give a black nano-ZrW₂ powder (12.5 g, 0.027 mol) The powder exhibited a BET SSA of 9.79 m²/g and a TEM average particle size of about 20-50 run. DSC/TGA (Fig. 5) in air exhibited an oxidation exotherm at 359 ⁰C with energy of about 5.4 Kj/g. Inductively coupled plasma mass spectrometry analysis showed 55.2 wt % W and 34.9 wt % Zr.

Example 6

This example illustrates the formation of zirconium metal nano-powder. ZrCl₄ (30 g, 0.129 mol) was treated with methanol (100 mL)and ammonium hydroxide (40 mL) as described in Example 1, to give a white nano- ZrO₂ . Nano-ZrO₂ (20 g, 0.162 mol) was treated with magnesium powder (10.0 g,

0.648 mol, 325 mesh) as described in Example 1 to give a black nano-Zr powder (12.0 g, 0.132 mol) The powder exhibited a BET SSA of 23.8 m²/g and oxygen content of 5.86 %. DSC/TGA in air of a hexane damp sample exhibited an exotherm at 150⁰C with an energy of about 5 Kj/g. The dry powder burned in air at room temperature. TEM (Fig. 8) showed an average primary particle size of 50 nm with some agglomeration.

Example 6A (Comparative)

This Example illustrates the properties of a zirconium metal powder available commercially from Strem Chemicals, Inc, Newburyport MA. A Zr powder with a nominal particle size of less than 50 mesh and exhibited a BET SSA of 0.08 m²/g. DSC/TGA (shown in Fig 3B) in air exhibited an oxidation exotherm at 741 ⁰C with an energy 2.5 Kj/g.

Example 7 This example illustrates the formation of nano-Hf.

Nano-HfO₂ (8.0 g, 0.113 mol) was treated with calcium hydride (4.0 g, 0.095 mol) as described in Example 1 to give a black nano-Hf powder (3.8 g, 0.021 mol, 56 %). TEM of the powder exhibited an average primary particle size of about 100 nm The powder exhibited an SSA of 1.2 m²/g, and an oxygen content of 0.48 %. DSC/TGA in air exhibited an oxidation exotherm at 461 ⁰C with an energy of 3.5 Kj/g.

Example 8

Nano-Nb₂θ₅ (5.0 g, 0.019 mol) was treated with magnesium powder (2.8 g, 0.115 mol, 325 mesh) as described in Example 1 at a reaction temperature of 650 ⁰C to give a black nano-Nb powder (2.8 g, 0.030 mol, 80 %). TEM of the powder exhibited an average primary particle size of about 20 nm The powder exhibited an SSA of 3.1 m²/g, and an oxygen content of 7.0 %. DSC/TGA in air exhibited an oxidation exotherm at 604 ⁰C with an energy of 1.1 Kj/g.

In the claims the term "metal oxide nano-powder(s)" is meant to encompass both singular and plural forms of the term

Claims

We claim:

1. A method for preparing metal nano-powder, characterized by an average primary particle size of about 5 to about 500 run, comprising: providing one or more metal oxide nano-powder(s), characterized by an average primary particle size of about 5 to about 500 nm, of the general formula:

[M_x(O)_y]_1-z[M_x(O)_y]_zl[M_x(O)_y]_z2[M_x(O)_y]₂₃ wherein M is a metal selected, independently, from the group of: Ti, Zr, Hf, V₃ Nb, Ta, Cr, Mo, W, Mn, and rare earth metals, x is an integer equal to the valence of oxygen, y is an integer equal to the valence of M, and z is equal to the summation of zl, z2, and z3, and the summation of 1-z, zl, z2, z3, is equal to 1; mixing the metal oxide nano-powder(s) with an amount of reducing metal or reducing metal precursor to provide an oxide-reducing metal mix; heating said oxide-reducing metal mix in an inert atmosphere to a reducing temperature for a reaction time sufficient to reduce said metal oxide nano-powder(s) to provide a metal nano-powder and a reducing metal oxide; washing said metal nano-powder with reagents to remove said excess of reducing metal and reducing metal oxide to provide a purified metal nano-powder; and drying said purified metal nano-powder at a drying temperature for sufficient time to remove residual reagents.

2. A method of Claim 1 wherein said metal oxide nano-powder(s) is characterized by an average primary particle size of about 5 to about 200 run.

3. A method of Claim 2 wherein said metal nano-powder is characterized by an average primary particle size of about 5 to about 200 nm

4. A method of Claim 1 wherein M is a metal selected, independently, from the group of: Ti, Zr, Hf, W₅ Co, Nb and Ta.

5. A method of Claim 4 wherein M is a metal selected, independently, from the group of: Ti, Zr, and W.

6. A method of Claim 5 wherein M is a metal selected, independently, from the group of: Ti and W.

7. A method of Claim 6 wherein M is titanium.

8. A method of Claim 1 wherein M is a metal selected, independently, from the group Ta and Nb; and further comprising adding a dopant containing N, P, S, B or Si to said metal oxide nano-powder(s).

9. A method of Claim 1 wherein said reducing metal or reducing metal precursor is selected from the group of: Na, Li, Mg, K, Ca, and alloys thereof; and calcium hydride.

10. A method of Claim 9 wherein the reducing metal precursor is calcium hydride.

11. A method of Claim 9 wherein the reducing metal is Mg.

12. A method of Claim 9, wherein the amount of reducing metal or reducing metal hydride used is 1.1 to about 1.3 times the stoichiometric amount for complete reduction.

13. A method of Claim 9 wherein said reducing temperature is in the range of

650 to about 1000 ⁰C and said inert atmosphere is argon.

14. A method of Claim 1 wherein said mixing comprises a low energy method that provides a homogeneous mixture of the metal oxide nano-powder and the reducing metal or reducing metal precursor, without inducing chemical reaction.

15. A method of Claim 1 wherein providing one or more metal oxide nano- powder(s) comprises: mixing an aqueous solution of a first metal salt, optionally, with an aqueous solution of a second metal salt, optionally, adding a third metal salt and, optionally, adding a forth metal salt, to provide an aqueous metal salt mixture; adding to the aqueous metal salt mixture, with mixing, an organic base in sufficient quantity to convert the metal salt mixture to provide a metal oxide gel; separating the metal oxide gel from the aqueous solution; washing the separated metal oxide gel with water and then a water miscible solvent to provide a purified metal oxide gel; drying the purified metal oxide gel to provide a gel powder; annealing the gel powder at an annealing temperature sufficient to convert the gel powder to a metal oxide powder.

16. A tungsten nano-powder characterized by an average primary particle size of 5 to about 80 ran, a BET specific surface area of greater than 5 m²/g; and a DSC/TGA oxidation exotherm peak less than about 450 ⁰C when scanned at 5 ⁰C /min in air at a flow rate of 1 L/min.

17. A zirconium nano-powder characterized by an average primary particle size of 5 to about 200 nm, a BET specific surface area of greater than 5 m²/g, and burning in air at ambient temperature and pressure.

18, A zirconium nano-powder of Claim 17 wherein the BET specific surface area is greater than 10 m²/g.

19. A zirconium nano-powder of Claim 18 wherein the BET specific surface area is about 18 to 30 m²/g.

20. A zirconium nano-powder of Claim 17 further characterized by an oxygen content, as measured by inert gas fusion analysis, of less than 6 % and a zirconium content of greater than 94 %.

21. A tungsten-hafnium composite nano-powder characterized by an average primary particle size of 5 to about 200 nm, a BET specific surface area of greater than about 5 m²/g; and a DSC/TGA oxidation exotherm peak less than about 440 ⁰C when scanned at 5 ⁰C /min in air at a flow rate of 1 L/min.

22. A tungsten-hafnium composite nano-powder of Claim 21 further characterized by a mole ratio of tungsten to hafnium of about 0.8 to about 1.2.

23. A tungsten-hafnium composite nano-powder of Claim 21 wherein the BET specific surface area is greater than 10 m²/g.

24. A tungsten-hafnium composite nano-powder of Claim 21 further characterized by a oxygen content, as measured by inert gas fusion analysis, of less than 3 wt % and a combined zirconium-hafnium content of greater than 90 wt %.

25. A titanium nano-powder characterized by an average primary particle size of 5 to about 200 nm, a BET specific surface area of greater than 2 m²/g; and a

DSC/TGA oxidation exotherm peak less than about 600 ⁰C when scanned at 5 ⁰C /min in air at a flow rate of 1 L/min.

26. A titanium nano-powder of Claim 25 further characterized by a oxygen content, as measured by inert gas fusion analysis, of less than 2 % and a titanium content of greater than 98 %.