Surface Reaction between (Ba, Sr, Ca)CO3 Powder and a Ni-28W-0.4Zr Ternary Alloy Plate

Products formed on the interface between a thermionic emissive material (Ba0.56Sr0.40Ca0.04)CO3 powder and a Ni-28 mass%W-0.4 mass%Zr metal plate were examined by X-ray diffraction method, the scanning electron microscopy and the EPMA analysis. The reaction was performed under the condition of 1073 K∼1373 K and 1.8 ks∼36 ks. The results are as follows. (1) At the early stage of the reaction, Ba3WO6 was formed remarkably on the metal surface. The formation of Ba3WO6 was greatly enhanced by the oxidation of the metal due to CO2 gas produced at the time of BaCO3 decomposition. The Ba3WO6 has the same needle like shape as a starting material of the carbonate. The Ba3WO6 seems to be formed by the following solid-vapor reaction. (This article is not displayable. Please see full text pdf.) (2) As the reaction proceeded, Ba3WO6 began to decrease and BaZrO3 gradually increased. The BaZrO3 was formed along the grain boundary inside the metal. The rate of the formation was found to be controlled by a diffusion process. The BaZrO3 seems to be formed by the following reaction. (This article is not displayable. Please see full text pdf.) \oindentwhere the W component is alloyed with the metal and remains as small particles on the metal surface.