Thermodynamics of the System Cu‐O and Ruby Formation in Borate Glass

Abstract

The CuO‐Cu2O and Cu2O‐Cu equilibria in glass solution were studied in 30Na2O 70B2O, and 25Na2O · 10Al2O3· 65B2O3 glasses at varied partial pressures of O2 and temperatures. The CuO‐Cu2O equilibrium was studied in a Pt crucible atpO2= 1 to 0.05 atm with ∼0.5 wt% Cu; the Cu2O‐Cu equilibrium was studied in Cu crucibles at much lower pO2's (10−14 to 10−17 atm), where no detectable amount of CuO was present. Plots of log [CuO]/[Cu2O]1/2) and log ([Cu2O]1/2/[Cu]) (where the brackets designate concentrations in glass solution) vs log pO2 gave best‐fit lines having slopes of ∼ 1/4. From the variation of the equilibrium constant with temperature, the enthalpies of the reactions °Cu2O(glass soln) + 1/4 O2 (g)→ CuO(glass soln) and Cu(glass soln)1/4 O2(g)→ Cu2O(glass soln) in the glasses were estimated to be − 14.57 and − 54.34 kcal/mol, respectively; these values are near the data for isolated Cu‐O systems. The enthalpy for the solution of Cu in glass was estimated to be + 23.7 kcal/mol. On heat treatment, the best ruby was formed in glasses melted at pO2= 10−14 atm; at higher pO2's no ruby was formed, and at pO2≤10−15 the glass produced progressively poorer rubies. Using the thermodynamic data estimated for the system Cu‐O in glass, possibilities for the precipitation of pure solid Cu and Cu2O are discussed. It was shown that Cu2O is primarily responsible for the bright red ruby color in glass.