An Improved Apparatus for Measuring Thermal Conductivity of Hard, High-Conductivity Ceramics

Abstract

An apparatus for determining thermal conductivity by an absolute (calorimetric) method of steady-state hea tflow through small cylindrical (20 or 30mmφ) or prismatic (22mm square) samples is described (Fig. 1). Some estimate of the accuracy and precision obtained is evidenced by comparing the values with that calculated from a comparative method experiment which the apparatus can also afford to perform with the aid of long silver rod beneath the sample. The results of the study are as follows;(1) It has been shown that if proper precautions are taken, linear heat flow and low heat loss can be so easily attained by the absolute method using a water flow calorimeter as to obtain reliable values on the high-conductivity materials.(2) An experimental results (Fig. 2) has shown that, unless the specimen is not soldered, interfacial temperature drop will grow so high that it can not be neglected in determining the thermal conductivity of these high-conductivity materials.(3) Measurements on electrolytic lead (>99.997%) have given data in good agreement with previous values reported for melting-point lead in literatures. Reproducibility obtained by using lead samples of various size and shape has also proved to be satisfactory (Table 1 and Fig. 4).(4) Two series of experiments were conducted in which the temperature of guard ring was made too high or too low relative to that of the sample, and the effect of these temperature gap on conductivity values obtained were examined. Examples obtained on lead are shown (Fig. 4).(5) It is very difficult to prepare accurate cylindrical samples of hard ceramic materials and to drill thermocouple holes. The difficulties were removed by preparing prismatic samples with narrow but deep groove on a vertical side face (Photo. 2 & 3). The holes for thermocouples were drilled into insulating material filled in the groove. The new method of sample preparation has been proved correct through the experiments made with lead (as well as with SiC brick) (Fig. 4, P).(5) The thermal conductivity of Carbofrax brick decreases from 0.052 at 100°C to 0.033 (cal. sec-1. cm-2. °C-1. cm.) at 750°C. The result is in good agreement with that of F. Holler for clay-bonded SiC bricks, but it is somewhat higher and has negative gradient to the contrary of the data given in the manufacturer's catalogue. The same is true on a domestic SiC brick (Fig. 5).