Temperature Dependence of Thermal Diffusivity and Conductivity of FeO Scale Produced on Iron by Thermal Oxidation

Abstract

The thermal diffusivity/conductivity of FeO scales produced on iron substrates by thermal oxidation have been determined as functions of temperature. Iron plates (99.99%) were oxidised at 973 K in air to obtain oxide scales of FeO, Fe3O4, and Fe2O3, and then were reduced at 1273 K in nitrogen to obtain FeO-only. The densities of these scales were 5.85–6.05 g cm−3. The laser flash method was used to measure the apparent thermal diffusivity of the whole sample from room temperature to 1164 K during the heating/cooling cycles. This was converted to the thermal diffusivity of the scale only, which in turn was converted to the thermal conductivity. However, these values depended on the scale thickness, which suggests an interfacial heat resistance occurs between the scale layer and iron substrate. In addition, scanning electron microscopy (SEM) observations revealed that the scales contained Fe and Fe3O4 phases after heating. The scale thermal diffusivity/conductivity were corrected considering the interfacial heat resistance and dispersed phases to derive the corresponding values for FeO only. The interfacial heat resistance derived from the thickness dependence of the scale thermal conductivity was 8.3 × 10−6 m2 K W−1. Using this value, the thermal diffusivity of FeO was derived as 3.7 × 10−7–5.8 × 10−7 m2 s−1 and the thermal conductivity as 1.8–2.5 W m−1 K−1 between room temperature and 1164 K. The temperature coefficients of the thermal conductivity were mostly negative, which would be dominated by the phonon mean free path.