Phase relations of solid ZnO-SiO2, ZnO-SiO2-CaO, ZnO-SiO2-Al2O3 and ZnO-SiO2-MgO systems at 1073 K have been determined by X-ray diffraction and X-ray microanalysis studies of quenched samples. e.m.f. measurements using stabilized zirconia solid electrolytes have been carried out to determine the activities of zinc oxide in solid ZnO-SiO2 and ZnO-SiO2-CaO systems, the activities of zinc in liquid Au-Zn alloys and the standard free energy of formation of Cu2O and ZnO. In the ZnO-SiO2 system, the activities of zinc oxide are represented by a single value of aZnO = 1.0 over the range NZnO = 0.67 - 1.0, while at NSiO(2) = 0.33 - 1.0 they showed a single value of around 0.2 at 973 and 1073 K. In the ZnO-SiO2-CaO system, the activities of zinc oxide showed a single value of aZnO = 1.0 in the compositions near pure ZnO at 973 - 1073 K. In the ZnO-SiO2-Al2O3 and ZnO-SiO2-MgO systems, solid solubilities of SiO2, Al2O3, MgO in ZnO phase were small. The activities of zinc oxide showed a single value of aZnO = 1.0 in the compositions near pure ZnO. In liquid Au-Zn alloys, the activities of zinc showed considerable negative deviation from Raoult's law at 1023, 1073 K and agreed with published data. The standard free energies of formation of Cu2O and ZnO were determined.
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