Thermal stability of high-temperature materials. Part 2

Abstract

Experimental methods for determining thermal stability of high-temperature materials under dynamic and static thermal loading conditions are considered. Experimentally, thermal stability is characterized by the destructive temperature difference or critical (destructive) rate of change of the temperature field in the bulk or at the surface of the material. It is shown that, with the observance of special conditions, the thermal-shock method, based on sharp cooling of heated specimens in water, provides an acceptable destructive temperature difference for a particular material, in agreement with theory. At present, however, no unique method for experimental determination of the thermal stability of high-temperature materials has been developed. A route towards standardizing static methods for determining the destructive temperature difference is proposed. THERMAL STABILITY OF HIGH-TEMPERATURE MATERIALS. GENERALITIESHigh-temperature materials 2 determine technological, economic, and environmental conditions and the dynamics of development of the leading research and industrial complexes of the world community [1]. While differing in chemical and phase composition, structure and properties, manufacturing technology, and application, these materials share general characteristics of which the thermal stability is of prime importance.The high-temperature materials [2] are divided into the following groups:1. Whisker-type single crystals. 2. Bulky single crystals.3. High-density (with a density close to theoretical) fine-grained polycrystalline materials.4. Porous fine-grained polycrystalline materials. 5. Porous coarse-grained polycrystalline materials. 6. High-porosity polycrystalline materials. 7. Fibrous (filamentary) polycrystalline materials. Except for materials of the 1st and 2nd groups (which are single-phase), the rest of materials may be single-or multiphase, quasi-homogeneous of heterogeneous.Knowledge of the relations between fundamental properties of materials, principles of design of engineering structures intended for service under thermal shock conditions is of importance both for theory and practice. A theory of thermal stability that has been developed within the framework of the maximum stress theory and. quantum field theory of thermal states provides an adequate description of the behavior of materials in a nonstationary temperature field [3,4]. Thermal stability was substantiated as a physical property of materials related to the fundamental properties of matter. However, numerical analysis of the thermally stressed state of high-temperature materials in a high-intensity nonstationary temperature field and associated therewith technological implications is an arduous task; for this reason, experimental determination of thermal stability is frequently a preferable alternative.As an example of an approximate numerical analysis one may refer to the determination of the critical heating rate of a refractory lining by solving the nonstationary heat conduction equation using destructive temperature grad...