Apparent oxygen solubility in refractory carbides

The occurrence of an apparent solubility of oxygen in polycrystalline NbC, TiC, VC and ZrC is discussed. This is shown by experiments in which the oxy gen consumed by the sample is directly measured as a function of temperatur e in an argon stream at 1.6 bar (rel) where the oxygen partial pressure was as low as 0.8 Pa. The parameter Gamma, as the ratio between the weight cha nge of the sample and the number of moles of oxygen consumed, has been prop osed to discriminate the nature of the processes related to the gas/solid i nteraction. The limit values of Gamma, theoretically expected to be 0 and 3 2 g mol(-1), mean that the gas/solid interaction changes from a surface rea ction without any weight change to a complete retention of oxygen in the so lid. Intermediate values indicate the formation of the oxide at a given sto ichiometry as found experimentally as a function of thr oxygen partial pres sure. (C) 1999 Elsevier Science B.V. All rights reserved.