The influence of non-stoichiometry of oxides on their wettability by liquid metals

Abstract

Many oxides of technological importance, such as TiO2, ZrO2, UO2 and Y203, present significant ranges of non-stoichiometry. Despite this fact, there is little or no information available about the influence of non-stoichiometry on the wettabflity of this type of oxide by liquid metals. Hodkin and Nicholas [1] observed that by increasing the value of x in UO2+x from x = 0.001 to 'x = 0.084 the contact angle of copper at 1446 K was reduced from 116 ° to 84 ° . However, this decrease may be explained simply by the effects of dissolved oxygen. Indeed, from reference data [2], it is found that at 1446 K the value of partial oxygen pressure Po2 in equilibrium with UO2+x for x = 0 , is Po2 = 10-16 Pa, and for x = 0.084, Po2 = 10-2 Pa. For a Po2 value as high as 10 -2 Pa, molten copper dissolves oxygen in concentrations large enough to reduce quite considerably the contact angle of copper on oxides (by several tens of degrees [3]). More recently, Chabert [4] studied the effect of non-stoichiometry of TiO2_x on its wettability by molten gold. By decreasing Po2 in the furnace, an increase in x from x = 0 to x = 10 -2 was produced. Contact angles between 130 ° and 115 ° were measured but no systemati c trend was found. In this letter, experimental evidence will be given showing significant effects of changes in the value x on the wettability of Y203-x substrates by molten uranium. These results are part of a study of yttria resistance to corrosion by molten uranium, a study in which long duration experiments (up to 200 h) were performed [5, 6]. The experiments were carried out in a metallic furnace heated by molybdenum resistance under a dynamic vacuum greater than 10 -3 Pa obtained using an oil diffusion pump. In order to decrease the oxygen partial pressure Po2 in the furnace, the specimens were placed in a tantalum isothermal chamber in which the partial oxygen pressure value, Po2, was evaluated to be Po2 = 10 7 Pa [5]. Uranium contains 35 ppm of oxygen and 100 ppm of carbon. Aluminium, chromium, iron and nickel are the major metallic impurities, with a total content of 30ppm. 5 g parallelepipedic uranium samples were mechanically prepared before testing. The Y203 samples were 2 mm thick and 20 mm in diameter. The substrate was polished to a roughness of about 1 nm. Yttria substrates with a grain size of