Synthesis of nanostructured tungsten and tungsten-base phases

Reductive decomposition of spray dried ammonium metatungstate gives rise to nanocrystalline {dollar}alpha{dollar}-W (bcc structure) or nanocrystalline {dollar}beta{dollar}-W (A15 structure), depending on the specifics of the processing conditions.; By controlling the reaction rate of the high surface area {dollar}alpha{dollar}-W and {dollar}beta{dollar}-W phases with oxygen at low temperatures ({dollar}le{dollar}300{dollar}spcirc{dollar}C) it is possible to transform both phases to an amorphous tungsten oxide. Furthermore, reduction of the amorphous oxide in hydrogen at {dollar}le{dollar}400{dollar}spcirc{dollar}C yields {dollar}gamma{dollar}-W (amorphous structure), in which all or nearly all of the oxygen atoms are removed.; The high surface area {dollar}alpha{dollar}-W and {dollar}beta{dollar}-W phases show striking differences in susceptibility to gas-solid reactions. Reaction of {dollar}beta{dollar}-W with ammonia at low temperatures (100{dollar}spcirc{dollar}-300{dollar}spcirc{dollar}C) results in the formation of an intermediate amorphous {dollar}delta{dollar}-WN{dollar}sb{lcub}rm x{rcub}{dollar} phase, which decomposes at higher temperatures ({dollar}ge{dollar}650{dollar}spcirc{dollar}C) into nanocrystalline {dollar}rm Wsb2Nsb{lcub}x{rcub}{dollar} phase. On the other hand, if nitridation is initiated at room temperature and continues as the temperature gradually increases to 300{dollar}spcirc{dollar}C, another amorphous phase ({dollar}gamma{dollar}-WN{dollar}sb{lcub}rm x{rcub}{dollar}) is formed.; A similar behavior occurs when {dollar}beta{dollar}-W is reacted with carbon monoxide at low temperatures, starting at room temperature and continuing as the temperature gradually increases to 300{dollar}spcirc{dollar}C. The resulting amorphous phase {dollar}delta{dollar}-{dollar}rm WCsb{lcub}x{rcub}Osb{lcub}y{rcub}{dollar} is exceptionally stable. Only upon heating to 800{dollar}spcirc{dollar}C in carbon monoxide does it decompose to {dollar}rm WCsb{lcub}x{rcub}.{dollar}; The unusual chemical activity of high surface area {dollar}beta{dollar}-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 {dollar}beta{dollar}-W to form a metastable solid solution. Some diffusional disordering evidently occurs because of the disappearance of the high order peaks of {dollar}beta{dollar}-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 {dollar}beta{dollar}-W formed by reductive decomposition of ammonium metatungstate.