A study of thermal conductivity of structural ceramic materials. Part I. State of research of thermal conductivity of structural materials

At the present time, experiment is a reliable method for studying the thermal conductivity of new ceramic materials and especially refractories. However, the range and possibilities of existing devices do not meet the requirements for measuring thermal conductivity, especially at a high temperature. At very high thermal loads under the conditions of formation of surface columnar crystal structures, thermoelastic stresses, disturbances in vibration of the elementary lattice, and other phenomena, the thermal conductivity can be a function of the temperature drop. The present paper concerns the physical fundamentals of heat conduction in current ceramic materials and refractories based on MgO, A1₂O₃, and Si₃N₄. The classical stationary and nonstationary methods for determining thermal conductivity are considered. Special attention is devoted high-temperature processes and the difficulties arising in this case. It is recommended to solve high-temperature problems by using methods based on solving inverse problems of heat conduction     

A study of heat conduction in structural ceramic materials. Part III. Mathematical model of measuring cell

The process of heat propagation in a specimen is considered in an approximation of a one-dimensional heat flow with side leakages of heat. They are modelled as a function of the heat sources (sinks). The chosen stationary heating and the temperature field in the specimen are described by a nonlinear one-dimensional differential equation. The boundary conditions and the source function are determined from experimental data. The nonlinear one-dimensional differential equation is used in an implicit identification method and solved numerically; a minimum of the quality criterion is determined at each iteration step in the search procedure. The identification procedure is performed by explicit and implicit methods of solution of inverse problems of heat conduction. A numerical simulation has shown that the method of component-wise minimization is the most efficient. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 4, pp. 39–42, April, 2000. Part I appeared in No. 8, 1999, and Part II in No. 2, 2000.     

Studies of heat conduction in structural ceramic materials. Part II. A study based of the solution of inverse problems of heat conduction

The process of energy transfer in refractory materials is studied under conditions of intense high-temperature heating. The coefficients of inverse problems of heat conduction (IPHC) are analyzed. The extremum coefficients of IPHC are determined and classified as explicit and implicit in accordance with the methods used for their estimation. Coefficient-type IPHC make it possible to bring the processing of results as close as possible to the measuring cell and to diminish the effect of errors of measurement on the solution of the problems and to increase simultaneously the efficiency of the experiments. A method for performing physical experiments, high-temperature heating sources, and measuring cells are described. The boundaries of the confidence interval of the instrumental error are shown. Results of single measurements of the thermal conductivity of MgO-base ceramics in an optical furnace are presented. Computation of the errors shows that the total relative instrumental error of measurement of the thermal conductivity is quite acceptable for high-temperature studies. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 2, pp. 33–43, February, 2000. For the beginning of the series see No. 8 of 1999, p. 22.     

Heat-Conductivity Studies of Structural Ceramic Materials. Part VII. Heat Conductivity of an Si3N4-Based Ceramic

The heat conductivity of an Si₃N₄-based ceramic is studied. A sintered silicon carbide, prepared from ultrafine (0.5 μm) powder Si₃N₄ with aluminum and yttrium oxides added to form a liquid phase and thus to activate the sintering process, is used for the study. The Si₃N₄-based ceramic has a heat conductivity higher than that of an oxide-based ceramic. The approximation function for heat conductivity at temperatures below 1400 K has a negative slope, that is, the effective heat conductivity tends to decrease with temperature. Above 1400 K, the effective heat conductivity tends to increase.