Aluminothermic reduction of zirconia

Results are presented for the reaction of aluminium metal with zirconia ceramics under (reducing) sintering conditions. During hot-pressing the principal phase formed is zirconium monoxide which is shown to have a rock-salt cubic structure and a unit-cell dimension of 0.46258 nm. When pressureless sintered or hot-pressed using a yttria partially-stabilised zirconia, composite structures of ZrO, alumina and cubic zirconia are produced which are the result of reduction reactions which proceed through the liquid metal phase. The results are interpreted through X-ray diffraction, microstructural and phase equilibria considerations which show that the sintering gas also plays a role in reaction through the oxygen partial pressure. The results allow calculation of an approximate value of the free energy of formation of zirconium monoxide (-294,500 J mol(-1)) which is consistent with the reported values for the monatomic carbide and nitride of zirconium and with other similar transition metal carbide, nitride and monoxide series. (c) 2004 Elsevier Ltd. All rights reserved.