A new numerical method and modified apparatus for the simultaneous evaluation of thermo-physical properties above 1500 K: A case study on isostatically pressed graphite

Abstract

This paper presents a new numerical inverse method coupled with an improved apparatus based on the laser flash (LF) technique for the measurement of thermo-physical properties of materials at high temperatures. Using this thermal conductivity, specific heat capacity, thermal diffusivity and spectral emissivity have been measured at temperatures above 1500 K. The method improves the characterization of input parameters such as laser power profile, which was shown to impact thermal conductivity and specific heat capacity by 15–20%. Convective heat losses are characterized semi-empirically and are not fitted. The apparatus has been enhanced via the adoption of a spectropyrometer for the simultaneous measurement of spectral emissivity within an uncertainty of 5% (equivalent to 0.3–0.7% error in temperature in the range 1500–3000 K) The results obtained on isotropic, isostatically pressed, graphite are in good agreement with literature values (around 1500 K) and extend the available data up to 2800 K. Additionally, a model has been developed based on the theory of Debye and Klemens for predicting the temperature dependence of thermal conductivity, specific heat capacity and thermal diffusivity of isotropic graphite. The model is in good agreement with the new experimental data and previous lower temperature data and therefore provides confidence in the new experimental approach.