SPREADING BEHAVIOR OF MOLYBDENUM TRIOXIDE ON ALUMINA AND SILICA - A RAMAN MICROSCOPY STUDY

Tje wetting of alumina by molybdenum trioxide has been demonstrated by Raman microscopy. Special specimens, which consisted of an alumina wafer in contact with a molybdenum trioxide wafer, were prepared for this purpose. Concentration profiles across the sharp dividing line between the two oxides were measured with a Raman microscope after thermal treatment at 800 K in a stream of dry O2 or O2 saturated with water vapor. The molybdenum oxide was transported over the alumina surface for several hundred micrometers. In contrast, in a similar wafer with silica no transport of molybdenum oxide could be observed. The results suggest that neither gas-phase transport nor surface diffusion in a concentration gradient provide major contributions to the spreading process. A solid-solid wetting process is considered to best describe the observed phenomena, the decrease in surface free energy being the dominant driving force for the spreading of molybdenum oxide over alumina. © 1990.