| Thermal expansion coefficient, electrical resistivity and thermal diffusivity are the three important parameters in evaluating the thermophysical performance of materials, and are of great significance in the structural design of some key parts of astronautical and aeronautical vehicles. It is well-known that thermal expansion coefficient and electrical resistivity measurement theory is established for the uniform temperature specimen. However, the temperature distribution in big-size sample is not uniform in practical measuring instrument under high temperature, so it is an important issue to measure the thermal expansion coefficient and electrical resistivity with high accuracy. Thermal diffusivity is usually measured by using flash method that depends on the transient temperature of the rear surface of the specimen. However, the accuracy of this method can hardly be improved because of the limited spatial resolution in temperature measurement. The objective of this project is to develop a measuring apparatus that can measure thermophysical properties of big-size samples under high temperature. For thermal expansion coefficient and electrical resistivity, the measurement uncertainties are less than 6%, and for thermal diffusivity it is less than 5%. The thermophysical parameters of various materials under high temperature can be determined by this apparatus so as to present reliable information for designing configuration and optimizing performance in some practical applications.In order to establish the measuring methods of thermophysical properties of big-size samples under high temperature, the principle and technique for measuring the thermal expansion coefficient, electrical resistivity, and thermal diffusivity were systemically investigated in this dissertation. A new apparatus, which can simultaneously measure the three thermophysical properties, was developed, and the data processing methods in this measurement were also discussed. This work can be categorized as follows:In order to eliminate the effects of nonuniform temperature distribution on the steady state measurement, a mathematical model was established for measuring the thermal expansion coefficient and electrical resistivity of nonuniform temperature body according to the temperature dependence of thermophysical properties. The relationship between the whole and parts of the nonuniform temperature body was discussed. The mathematical formula and corresponding data processing methods for calculating thermal expansion coefficient and electrical resistivity were presented.A multi-spectral flash method mathematical model that can measure thermal diffusivity by multiple spectral radiation energy was established based on the experimental process of multi-spectral method. This model decreases the errors induced by the linearization of the relationship between temperature and radiation energy. The least square method was used in the data processing, and the high-precision measurement of thermal diffusion was realized. The finite element simulation of multi-spectral flash method and flash method was carried out. Compared with the measuring results of thermal diffusivity, the correctness and feasibility of the multi-spectral flash theory was justified.An apparatus combing optical, mechanical, electrical measurement for high temperature thermophysical properties parameters, which can measure simultaneously these three parameters, is developed according to the above principles. The work for designing this apparatus is studied as follows: (1) Considering the different heating requirements for different sized samples, we designed a special experiment chamber that can heats rod- or circle-shaped samples to thermal equilibrium state under high temperature; (2) A high-speed multi-spectral pyrometer that can simultaneously measure the thermal diffusivity and temperature of the specimen was developed; (3) A scanning and measuring system that can rapidly measure the rod-like axial temperature distribution was developed; (4) An even pulse heating system that can meet the needs of different measurement requirements for material thermal diffusivity was designed; (5) A high-speed data acquisition and processing system that realizes multi-channel high-speed acquisition and the real-time control of different instruments was improved. (6) The optimum testing area of the instrument was decided through the analysis and comparison of measuring results of thermal diffusivity in different areas.The high temperature thermal properties were measured for different materials by this apparatus. Temperature dependence on the thermal expansion coefficient, electrical resistivity, and thermal diffusivity of a carbon / carbon materials were investigated. The temperature dependence on thermal diffusivity of the SRM 8424 and Pyrocream 9606 standard samples were studied in the temperature range of 800℃~3000℃and 800℃~ 1000℃, respectively. The scope and upper limit of the measuring temperature as well as the precision of the thermal diffusivity measurement compared fairly well with those obtained from the similar equipments in the world. The reasons that impact the measurement results were summarized, and the uncertainty in measuring the thermal expansion coefficient, electrical resistivity, and thermal diffusivity under two typical temperatures were discussed.This work presented the theoretical foundation and established the measurement techniques for building the database of thermal expansion coefficient, electrical resistivity and thermal diffusivity of big sized samples under high temperature.
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