Production of nanocrystalline metal powders via combustion reaction synthesis
Nanocrystalline metal powders comprising tungsten, molybdenum, rhenium and/or niobium can be synthesized using a combustion reaction. Methods for synthesizing the nanocrystalline metal powders are characterized by forming a combustion synthesis solution by dissolving in water an oxidizer, a fuel, and a base-soluble, ammonium precursor of tungsten, molybdenum, rhenium, or niobium in amounts that yield a stoichiometric burn when combusted. The combustion synthesis solution is then heated to a temperature sufficient to substantially remove water and to initiate a self-sustaining combustion reaction. The resulting powder can be subsequently reduced to metal form by heating in a reducing gas environment.
1. A method for synthesizing powders by a combustion reaction, the method comprising:
forming a combustion synthesis solution by dissolving in water an oxidizer, a fuel, and at least one base-soluble ammonium metatungstate (AMT) in amounts that yield a stoichiometric burn when combusted;
heating the combustion synthesis solution to a temperature sufficient to substantially remove the water and to initiate a self-sustaining combustion reaction to form a combustion product of WO 2 crystallites of a size less than 60 nm; and
heating the combustion product for less than 6 hours in a reducing atmosphere at a temperature lower than 850° C. to form the W powder.
2. The method of claim 1 , further comprising dissolving a nitrate reagent of an alloying metal in the combustion synthesis solution.
3. The method of claim 2 , wherein the oxidizer comprises the nitrate reagent.
4. The method of claim 1 , wherein the oxidizer comprises nitric acid.
5. The method of claim 1 , wherein the oxidizer comprises ammonium nitrate.
6. The method of claim 1 , wherein the fuel comprises glycine.
7. The method of claim 1 , further comprising cooling the combustion product to a temperature below 100° C. and then introducing an oxidizing gas to passivate the surface of W powder.
8. A method for synthesizing W nanocrystalline metal powders by a combustion reaction, the method characterized by the steps of:
forming a combustion synthesis solution by dissolving in water an oxidizer, a fuel, and at least one base-soluble ammonium metatungstate (AMT) in amounts that yield a stoichiometric burn when combusted;
heating the combustion synthesis solution to a temperature sufficient to substantially remove the water and to initiate a self-sustaining combustion reaction to form a combustion product of WO 2 crystallites; and
heating the combustion product to a temperature below 850° C. in a reducing atmosphere to reduce the WO 2 crystallites to W nanocrystalline metal powder.
9. The method of claim 8 wherein the reducing comprises exposing the WO 2 crystallites to hydrogen.
10. The method of claim 9 wherein the reducing further comprises heating the WO 2 crystallites to a temperature between 600° C. and 800° C.
11. The method of claim 10 wherein the reducing further comprises rapidly heating the WO 2 crystallites to the temperature and rapidly cooling the W nanocrystalline metal powder to room temperature.
12. The method of claim 11 wherein the rapid heating and/or cooling is performed at a rate up to 100° C./min.
13. The method of claim 8 wherein the W nanocrystalline metal powder has an average particle size of less than 60 nm.
14. The method of claim 8 wherein the W nanocrystalline metal powder has an average particle size of less than 30 nm.