Characterisation of the NPL Thermal Conductivity Reference Material Inconel 600

Abstract

Due to the depletion of the thermal conductivity reference materials, the National Physical Laboratory (NPL) has certified a new thermal conductivity reference material, NPL code 2I09 that is based on Inconel 600. This batch of material has been thermally and electrically characterised over the temperature range 100 °C to 500 °C. The primary thermal conductivity measurements used to generate the certified values were carried out in the NPL Axial Heat Flow apparatus, which is a UK national standard measurement facility. It is based on a steady-state absolute technique and is suitable for measuring specimens with thermal conductivities in the range 10 W·m⁻¹·K⁻¹ to 240 W·m⁻¹·K⁻¹. Comparisons were made between the reference thermal conductivity values and the derived thermal conductivity values obtained from two indirect methods based upon electrical resistivity and thermal diffusivity measurements, respectively. The reference thermal conductivity values of NPL 2I09 compare with those calculated from the measurements of electrical resistivity within 2 % and those calculated from the measurements of thermal diffusivity, specific heat capacity, density and thermal expansion better than 4 %. In addition, the certified thermal conductivity values of the new batch of Inconel 600, the NPL code 2I09, were compared with the representative reference thermal conductivity values of Inconel 600 reported by J Clark and R Tye, and the agreement is within 2 %. The new reference material NPL 2I09 is available from NPL and can be used to calibrate or check apparatus that provides measurements of thermal conductivity which includes the range 14 W·m⁻¹·K⁻¹ to 22 W·m⁻¹·K⁻¹ covering the temperature range from 100 °C to 500 °C. The overall uncertainty on the certified values is estimated to be within ± 4.8 %, based on a standard uncertainty multiplied by a coverage factor k = 2, providing a level of confidence of approximately 95 %.