Oxidation of nickel and transport properties of nickel oxide

Abstract

Oxidation kinetics of high purity nickel, as well as the nonstoichiometry and chemical diffusion in nickel oxide have been studied as a function of temperature (1373–1673 K) and oxygen pressure (10–10 5 Pa) using microthermogravimetric techniques. In order to eliminate the possible participation of grain boundary diffusion in scale growth at lower temperatures, the oxidation rate measurements have always been started at the highest temperature (1673 K), when coarse-grained scale was formed, and the temperature and pressure dependence of the oxidation rate was determined by step-wise lowering the temperature of such pre-oxidized sample. Nonstoichiometry and the chemical diffusion coefficient in Ni 1− y O have also been determined on such coarse-grained oxide samples, obtained by complete oxidation of nickel at highest temperature (1673 K). It has been found, that under such conditions oxidation of nickel follows strictly the parabolic rate law, and the parabolic rate constant of this reaction is the following function of temperature and oxygen pressure: k p =0.142p O 2 1/6 exp(−(239 kJ/ mol)/RT). The results of nonstoichiometry measurements, in turn, may be described by the following relationship y=0.153p O 2 1/6 exp(−(80 kJ/ mol)/RT). Finally, chemical diffusion coefficient in Ni 1− y O has been found to be independent on oxygen activity, indicating that the mobility of point defects in this oxide does not depend on their concentration, being the following function of temperature: D ̃ =0.186 exp(−(152 kJ/ mol)/RT). It has been shown, that the parabolic rate constants of nickel oxidation, calculated from nonstoichiometry and chemical diffusion data are in excellent agreement with experimentally determined k p values. All these results clearly indicate that the predominant defects in nonstoichiometric nickel oxide (Ni 1− y O) are double ionized cation vacancies and electron holes and the oxide scale on nickel growths by the outward volume diffusion of cations.