In order to perform a heat transfer analysis for an apparatus that generates heat or for an apparatus used at a high temperature, four thermophysical properties (e.g., specific heat capacity, hemispherical total emissivity, thermal conductivity and thermal diffusivity) of the materials of which the apparatus is made are required. In case that the apparatus is composed of newly developed materials, or those for which reliable data of thermophysical properties are not available, the above described thermophysical properties of the materials need to be experimentally obtained. Since these four thermophysical properties are generally measured using separate apparatuses, multiple expenditures of time and money are required to obtain all the thermophysical properties. Further, if a specimen is heated to temperatures beyond 1000° C. many times to measure all the thermophysical properties, either the physical properties of the specimen will change or deterioration of the measurement apparatus will occur.
In the 1970s, Cezairliyan et al. in the United States developed a method whereby the specific heat capacity and the hemispherical total emissivity of a conductive material at a temperature beyond 1000° C. were measured rapidly using a single measurement apparatus (see Reference 1). The feature of this measuring method is the way to heat a specimen. According to this method, when electric charges stored in a battery or in a capacitor having a large capacity were applied to a conductive specimen, the temperature of the specimen could be raised to 3000° C. or higher in 0.2 seconds by Joule heat which is induced by a large pulsed current that flows through the specimen.
In this measuring method, the temperature of the sample and the Joule heat generated in the specimen are measured during the rapid resistive self-heating and during the subsequent cooling of the specimen. Then, the specific heat capacity and the hemispherical total emissivity are calculated using a heat balance relationship between the Joule heat, the heat capacitance and the heat radiated by the specimen. This measuring method is employed only for electrically conductive materials; neverthless, this was an remarkable method that can be used to rapidly measure thermophysical properties at temperatures beyond 2000° C., which it previously had been very difficult to measure, and that can minimize measurement errors due to changes in the qualities of a specimen and deterioration of the measurement apparatus and can considerably reduce measurement costs.
However, with this measuring method, the thermal conductivity and thermal diffusivities required for a heat transfer analysis can not be measured, and these property values must be measured separately. To resolve this problem, Righini et al. in Italy has developed a method whereby, for a specimen, a change in the temperature distribution with reference to time during rapid resistive self-heating and the subsequent cooling of the specimen is measured to determine the thermal conductivity together. However, since the thermal conductivity is calculated based on an assumption that is not always self-evident, this method has not yet been commonly employed.
In principle, a thermal conductivity is defined as the product of the specific heat capacity, the density and the thermal diffusivity.
In most cases, since the temperature dependency of the density of a solid is very small compared with the temperature dependency of the thermal conductivity or the specific heat capacity, the thermal conductivity at a high temperature is generally calculated based on the density at room temperature and the thermal diffusivity, and the specific heat capacity each measured at the temperature. At present, the thermal diffusivity of a solid is generally measured by using a flash method.
[Reference 1] A. Cezairliyan, J. L. McClure, C. W. Beckett: J. Res. National Bureau of Standards, Vol. 75C-1 (1971), pp. 7-18.
However, the upper limit temperature where the flash method is available is approximately 2700° C., because the temperature of the specimen is generally controlled by using a resistance furnace. Further, changes in the qualities of a specimen or deterioration of a measurement apparatus, as described above, will adversely affect the measurement.
In view of the above, an object of the present invention is to provide a method to measure the specific heat capacity, the hemispherical total emissivity, the thermal conductivity and the thermal diffusivity at once during a rapid resistive self-heating of the specimen, and whereby these physical properties can be measured even at a high temperature at which measurements can not be conducted by a conventional method, and an apparatus for employing this measuring method.