Microstructure of refractories with participation of lanthanum chromite

Abstract

Questions are considered concerning the microstructure of electrode refractories based on lanthanum chromite. On the example of a refractory composition of lanthanum chromite -yttrium oxide the formation of stochastic phase-mosaic structure and its evolution during high-temperature exposure is demonstrated. The main features of crystal phase and porosity transformation of refractory composite constituents are revealed.Multiphase refractory composites based on lanthanum chromite are used as high-temperature electrical conducting materials in oxidizing gas atmospheres [1,2]. In [1] the possibility is demonstrated of multiphase refractory materials based on lanthanum chromite in heating modules for operation in an air atmosphere. These heating modules have a series of advantages over analogs within which as the electrical conducting resistive material there is use, for example, of silicon carbide, molybdenum disilicide, and single-phase materials based on lanthanum chromite. Heating modules fitted with resistive elements of multiphase materials based on lanthanum chromite, have an operating capacity up to 1750°C. In publication [2] preparation is demonstrated for resistance materials based on lanthanum chromite for resistance heating elements with high heat liberation from the surface. for this extrusion was used to prepare a U-shaped structure for resistive objects made of lanthanum chromite. This molding method for electric heaters is promising fro creating heaters with a single current lead.The resistance properties of multiphase materials based on lanthanum chromite govern not only the electrical conductivity of lanthanum chromite, but also distribution within the volume of the refractory of dielectric and electrically conducting components [3,4]. A mechanism was presented in [3] for lanthanum chromite electrical conductivity. The leading role was demonstrated in electrical conductivity of polarons that primarily diffuse over grain boundaries. In this connection material electrical conductivity is affected considerably by grain surface development, and also the stability of the surface chemical composition. In [4] surface phenomena were considered in electrically conducting multiphase material based on lanthanum chromite. In particular, the x-ray electron spectrum was analyzed for the surface of grains in a two-phase resistive composite of lanthanum chromite -yttrium oxide. Analysis of the spectrum made it possible to identify a different valent state of chromium in the lanthanum chromite structure, responsible for the appearance of electrical conducting properties of the material.The aim of this work is to study the microstructure of refractory composites based on lanthanum chromite and its evolution during high-temperature exposure in an air atmosphere. The microstructure of refractory composites was studied in polished sections in reflected light.Analysis of the microstructure of refractory composites (Table 1), showed that the linear size of lanthanum chromite crystallites is independent of the presence of C...