Synthesis of Nanostructured Tungsten and Tungsten - Phases

Reductive decomposition of spray dried ammonium metatungstate gives rise to nanocrystalline alpha -W (bcc structure) or nanocrystalline beta -W (A15 structure), depending on the specifics of the processing conditions. By controlling the reaction rate of the high surface area alpha-W and beta -W phases with oxygen at low temperatures ( <=300^circC) it is possible to transform both phases to an amorphous tungsten oxide. Furthermore, reduction of the amorphous oxide in hydrogen at <=400 ^circC yields gamma -W (amorphous structure), in which all or nearly all of the oxygen atoms are removed. The high surface area alpha -W and beta-W phases show striking differences in susceptibility to gas-solid reactions. Reaction of beta-W with ammonia at low temperatures (100^circ-300^ circC) results in the formation of an intermediate amorphous delta-WN_ {rm x} phase, which decomposes at higher temperatures (>=650 ^circC) into nanocrystalline rm W_2N_{x} phase. On the other hand, if nitridation is initiated at room temperature and continues as the temperature gradually increases to 300^circC, another amorphous phase (gamma-WN_{ rm x}) is formed. A similar behavior occurs when beta -W is reacted with carbon monoxide at low temperatures, starting at room temperature and continuing as the temperature gradually increases to 300^circC. The resulting amorphous phase delta- rm WC_{x}O_{y } is exceptionally stable. Only upon heating to 800^circC in carbon monoxide does it decompose to rm WC_{x }.. The unusual chemical activity of high surface area beta-W led to speculation concerning its susceptibility to solid-solid reactions, in addition to the gas-solid reactions noted above. Tests on the W -Cu system, in which both elements are mutually insoluble in the solid state, clearly showed that Cu can be diffused into beta-W to form a metastable solid solution. Some diffusional disordering evidently occurs because of the disappearance of the high order peaks of beta-W. However, the disordering process evidently never goes to completion to form an amorphous state. To summarize, this research demonstrates the feasibility of synthesizing a wide variety of metastable W and W-base phases by controlled gas-solid or solid-solid reaction at low temperatures, and particularly when the starting material is high surface area beta -W formed by reductive decomposition of ammonium metatungstate.