Reduction and sintering of alumina/tungsten nanocomposites - Powder processing, reduction behavior and microstructural characterization

Reduction behavior of tungsten oxide mixed with alumina powder and its sintering processes were investigated to fabricate Al₂O₃/W nanocomposites. Submicron-sized WO₃ powder could be obtained by a conventional ball-milling technique. Hygrometry analysis of Al₂O₃/WO₃ mixture revealed that WO₃ powder was reduced in two steps at 873 and 973 K. X-ray diffraction and transmission electron microscopy (TEM) analysis confirmed that Magnéli phases, such as WO₂ and WO_x (x = 2.89 to 2.92), were obtained by the reduction of WO₃ at 873 K. However, WO₃ was completely reduced to metallic W when the reduction temperature was higher than 1173 K. A thermodynamic analysis qualitatively agreed with the experimental result that WO₂ and WO_x coexisted at intermediate temperatures. Reduction followed by hot-press sintering of Al₂O₃/WO₃ mixtures was carried out to obtain Al₂O₃/W nanocomposites. Agglomeration and/or incomplete reduction was found at high W content; however, dense Al₂O₃/W composites with >98% of their theoretical density were obtained at low W content (i.e., <10 vol%). A microstructural investigation of the dense composite revealed that it consisted of both micro- and nano-sized W dispersions. Furthermore, a large number of W particles of around 40 nm were found to be homogeneously dispersed in the Al₂O₃ matrix. These particles were much finer than the WO₃ particles obtained by ball-milling.