Phase Equilibria in the System NiO–Al2O3–SiO2

Abstract

Equilibrium diagrams for the systems NiO‐SiO2, NiO‐Al2O3, NiAl2O4‐SiO2, Ni2SiO4‐NiAl2O4, and NiAl2O4‐Al6Si2O13 were drawn from data obtained by quenching and direct observational techniques. The only intermediate compound in the binary system NiO‐SiO2 is Ni2SiO4, which has the olivine structure. Unlike other olivines which melt congruently, nickel olivine has an upper temperature of stability (1545°C) and at temperatures between 1545° and 1650°C, NiO and SiO2 coexist in equilibrium. The only compound in the binary system NiO‐Al2O3 is NiAl2O4, which has a spinel structure. The nickel aluminate spinel varies in composition from 50 to 35 mole % Al2O3 at 1800°C, and the stoichiometric NiAl2O4 composition has a melting point near 2110°C. Of the joins within the ternary system NiO‐Al2O3‐SiO2 which were studied, only Ni2SiO4‐NiAl2O4 is not binary. In this join, crystals of NiO exist in equilibrium with liquid and a ternary assemblage of NiO + NiAl2O4+ liquid is stable to 1775°C. The decomposition temperature of Ni2SiO4 is decreased from 1545°C in the binary system to approximately 1490°C, presumably the result of solubility of NiAl2O4 in Ni2SiO4. The join NiAl2O4‐SiO2 is binary in that the compositions of crystalline phases can be expressed in terms of the chosen components. The eutectic temperature in the system is 1495°C. The join NiAl2O4‐Al6Si2O13 is binary for the same reasons and has a eutectic temperature at 1720°C. Using the data obtained in this study and those published for the well‐known system Al2O3‐SiO2, a liquidus surface diagram for the system NiO‐Al2O3‐SiO2 is proposed. Nickel olivine, even though it has an upper limit of stability in the binary system, has a primary field in the ternary system NiO‐Al2O3‐SiO2. This is the only refractory oxide system known to illustrate this so‐called “typical case,” the governing principles of which have been clearly presented in discussions of phase equilibria.