INTERFACES IN COMPOSITES OF ALUMINA IN A MOLYBDENUM MATRIX

Composites of alumina in a molybdenum matrix are candidate electrode materials for thermionic energy converters. The composites are produced by sintering a mixture of alumina and molybdenum powders in vacuum. The polished composite surface was heated for 2000 h at 1400-degrees-C in a caesium atmosphere (1 mbar). After heating, no alumina was present at the composite-atmosphere interface. No aluminium was detectable at the surface of the molybdenum matrix. Alumina particles were present in the bulk of the composite. Aluminium did not dissolve in the molybdenum matrix. Composites of Al2O3 and molybdenum were also sintered in dry hydrogen. In this case as well, no aluminium dissolved in the molybdenum matrix. The possibility of a chemical reaction of Al2O3 and molybdenum is thermodynamically investigated. An isothermal section of the phase diagram of the Mo-Al-O system is presented. The rate of the reaction of Al2O3 with molybdenum is determined by the oxygen permeability of the molybdenum matrix. The aluminium permeability of the molybdenum matrix is much higher. Because of the non-equilibrium conditions both in the testing procedure in the thermionic energy converter and during the sintering process in hydrogen or vacuum, the aluminium diffuses through the molybdenum matrix and evaporates at the composite surface. As a result no aluminium is detectable in the molybdenum matrix.