Zur Kinetik der Spinellbildung von β-Ga2O3 mit zweiwertigen Oxiden. IV. Reaktionen 2. Art im System CoO-β-Ga2O3

The solid-state reaction of the second kind in a sandwich type diffusion couple of $\text{Co}{}_{\mathrm{1?z}}\text{Ga}{}_{\mathrm{<mtext>2z</mtext><mtext>3</mtext>}}\text{O}$ and β-Ga₂O₃ has been investigated between 1249 and 1550°C in air. The quantity z, which corresponds to the saturation concentration of β-Ga₂O₃ in CoO, was determined as a function of temperature by X-ray methods and the optical microscope; the homogeneity range of the spinel phase $\text{Co}{}_{\mathrm{1?y}}\text{Ga}{}_{\mathrm{<mtext>2+</mtext><mtext>2y</mtext><mtext>3</mtext>}}\text{O}{}_4$ was investigated also. The growth of the thickness of the reaction layer follows a parabolic rate law; the activation energy is 71.6 kcal/mole. A comparison of reaction rate constants of the first and second kind in connection with experimental results, achieved with a modified marker technique, leads to confirmation of the Wagner mechanism for the formation of CoGa₂O₄ spinel as supposed before by Laqua. Reaction rate constants of the second kind, calculated from interdiffusion profiles in CoO-β-Ga₂O₃ diffusion couples, are in good agreement with experimental values. Presented data are used for estimating interdiffusion coefficients for the CoO-β-Ga₂O₃ system according to theoretical aspects developed by Pelton, Schmalzried, and Greskovich.