Mutual solid-solid interaction between CuO and sodium oxide-doped alumina

Abstract

The solid-solid interactions between CuO and Al 2O 3 doped with Na 2O were investigated using DTA and XRD techniques. The content of copper oxide was fixed at 16.3 wt.% and that of sodium oxide was varied between 1.87 and 7.49 wt.%. The different solids were preheated in air at 500–1000 °C for 8 h. The results obtained reveal that the pure mixed solids were amorphous at 500°C. Doping with Na 2O at this temperature resulted in the appearance of CuO diffraction lines having intensities that increase with the amount of the oxide dope added. Thermal treatment of the pure mixed solid samples at 800°C brought about a solid-solid interaction between CuO and Al 2O 3 producing CuAl 2O 4. The excess alumina remained as an amorphous phase. The presence of sodium oxide was found to inhibit copper aluminate formation to an extent proportional to the amount of Na 2O present. This effect was attributed to the formation of a layer of sodium aluminate that covers the surfaces of the Al 2O 3 grains and hinders the thermal diffusion of Cu 2+ ions into the Al 2O 3 matrix. The inhibition effect of Na 2O toward CuAl 2O 4 formation was also observed for the doped solids heated at 900 and 1000 °C. At 1100°C Al 2O 3 was found to exist as a mixture of θ-, κ- and α-aluminas and the transformation of these phases into α-Al 2O 3 required heating at > 1100°C. However, the presence of CuO was found to stimulate the crystallization of α-alumina at temperatures > 900 ° C, while the presence of Na 2O prevented completely the formation of α-Al 2O 3 at 900 ° C and also decreased the amount produced at 1000 ° C. The addition of 7.49% Na 2O prevented completely the appearance of α-alumina even after heating at 1000 ° C, and led to the formation of β-alumina, NaAl 2O 8, at between 900 and 1000 ° C.