Effects of Li 2O doping on solid-solid interactions in the CuO-Al 2O 3 system

Abstract

The results obtained reveal that pure CuO-Al 2O 3 solids, containing 16.3 wt.% copper oxide and preheated in air at 500 °C, are amorphous and change to crystalline phases on doping with Li 2O (0.3–3.57 wt.% Li 2O). The diffraction lines of the CuO phase are only detected in the XRD patterns of the doped solids precalcined at 500°C. The degree of crystallinity of this phase increases on increasing the amount of Li 2O added. Solid-solid interaction between pure or doped alumina and CuO occurs at 800°C to yield a massive CuAl 2O 4 phase with moderate crystallinity, whereas the excess alumina remains as an amorphous phase. However, in the doped solids increasing amounts of unreacted crystalline CuO phase are found in the patterns, indicating that the doping process opposes the solid-solid interaction between CuO and Al 2O 3. These results can be attributed to the production of lithium aluminate and to the formation of an Li 2O-CuO solid solution. The aluminate compound produced 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 portion of the CuO phase that dissolves Li 2O cannot contribute easily to the interaction with Al 2O 3 to give copper aluminate. Copper oxide stimulates the crystallization of alumina into α-Al 2O 3 (corundum) at temperatures starting from 900°C, whereas lithium oxide doping hinders this crystallization process to an extent proportional to the amount of dopant oxide added. The addition of 3.57 wt.% Li 2O is sufficient to suppress completely the crystallization of α-Al 2O 3 or any other crystalline form of the doped alumina even when firing at 1000° C.