Solubility of NiO in Al₂O₃

Abstract

Solubility of NiO in A1203 was determined by electron probe microanalysis. A difluusion couple method was used by coupling an NiO-doped A1203 polycrystal to u pure single crystal of A1203. The solubiliq of NiO in A1203 in air was 230 wt ppm (157 at. ppm of cations) and 170 wt ppm (116 at. ppm) at 2073 and 1973 K , respectively. The solubility of NiO in A1203 obtained in this work was compared with our previous work of the solubility of MgO in AlzOj.ANDO and F m o d a have determined the solubility limit of MgO in A1203 in air using a new diffusion couple method which was composed of a high-purity singlecrystal Alz03 and a small amount of MgOdoped A1203 polycrystal.' The values of the solubility limit are 9 5 wt ppm (120 cationic ppm) and 5 5 wt ppm (70 cationic ppm) at 2073 K (1800°C) and 1973 K (1700"C), respectively. The solubility limit is much smaller than that previously determined by Roy and Coble.' Ando and Momoda attributed the cause of the large difference to the difference in physical states of the samples. Powder was used in the study of Roy and Coble, while single-crystal and fully sintered and large grain-sized polycrystal were used in the work of Ando and Momoda. The speculation for the cause is consistent with the result of the recent work done by Baik.3 According to his work, the Mg is highly segregated at surfaces of single-crystal A1203 of the order of from several to several tens of percent depending on the annealing temperature. It is conceivable, therefore, that the surface of the powder and the interfaces which were introduced by the neck growth of the powder during the high-temperature annealing would effectively contribute to the high solubility of MgO in AI20, obtained by Roy and Coble.In this work, the solubility limit of NiO in A1,03 has been determined using the same method as that used for the determination of the solubility limit of MgO to compare the solubility limit of NiO with MgO in A1203. For the NiO-MgO system, as is well-known, both end members have CONTRIBUTING EDITOR-A. H. HEUER