Thermodynamic activity of nickel oxide in alkali nickel silicate glasses

Abstract

The thermodynamic activities of NiO were studied experimentally by equilibrating Ni0-Ni2+ redox in alkali nickel silicate glasses at different temperatures, melt compositions as well as different NiO concentrations, respectively, in air as furnace atmosphere. The relation between log γNiO and NiO concentrations, logγNiO and temperature as well as logγNiO and glass composition were investigated. The activity coefficient of nickel oxide was found to decrease with increasing basicity of alkali silicate glasses containing 1·0-5·0 mol% NiO whereas it decreased with decreasing basicity of an alkali silicate glass containing more than 5·0 mol% NiO. These observations were explained on the basis of octahedral and tetrahedral ligand field considerations of Ni2+ ion in R2O.NiO.SiO2 (where R+=Li+, Na+ and K+ ions) glasses. In the lower concentration range of NiO Henry's law was found to obey at a constant temperature whereas in the higher concentration range of NiO the relation between logγNiO versus NiO concentration deviates from Henry's law. The activity coefficient of nickel oxide was found to decrease with increasing temperature for a glass of constant composition and constant NiO concentration. The enthalpy and entropy change for partial molar mixing of NiO during Ni-Ni2+ redox equilibrium reaction in nonideal molten glass solutions were calculated and the process of nonideal mixing was found to be endothermic on the basis of Δ values. Further, based on entropy data (ΔHM) it was inferred that the dissolution of NiO in an alkali nickel silicate glass solution was more homogeneous with increasing temperature. The results were discussed thermodynamically.