Improved hot-wire procedure for thermophysical measurements under pressure

TLDR

A new, simplified circuitry for the transient hot‑wire method is introduced. The method employs a versatile circuitry allowing variable currents and wire diameters, analyzes temperature rise using the exact solution for a heated wire, corrects for power variations, and is validated through simulations and measurements of thermal conductivity and heat capacity of glycerol, CsCl, and NaCl up to 2 GPa. The measurements of glycerol, CsCl, and NaCl agree with prior studies, achieving 1–2 % accuracy for λ and 3–5 % for ρcp with low variability, and the improved procedure reveals systematic errors from heat‑pulse reflections—particularly affecting ρcp—as confirmed experimentally in NaCl and by theoretical analysis.

Abstract

A new and simplified version of the circuitry for the transient hot-wire method is presented. The circuitry provides a wide range of currents allowing probe wires of various diameters to be used in order to match the thermal properties of the specimen to be investigated. The analysis of the temperature increase during the heat pulse is based on the exact solution for a heated wire immersed in a medium. Data are corrected for varying power. The method was tested by computer simulations and by measurements of the thermal conductivity (λ) and the heat capacity per unit volume (ρcp ) of glycerol at room temperature and atmospheric pressure, and for CsCl and NaCl at room temperature and at pressures up to 2 GPa. The results on glycerol and CsCl are in excellent agreement with previous works. The inaccuracy in λ and ρcp is estimated as 1%–2% and 3%–5%, respectively, but the standard deviation of the measurements is as low as 0.2% for λ and 1% for ρcp. The improved procedure makes it possible to detect systematic errors caused by reflection of the heat pulse from the walls of the high-pressure cell. This error, which reveals itself by a curvature of the residual, defined as the difference between fitted function and data, was demonstrated in the case of NaCl. A theoretical estimate of the influence of perturbations due to reflection was also carried out and it was found that the error mainly affects the value of ρcp.