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, Fe 3 O 4 , and Fe 2 O 3 , and then were reduced at 1 273 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 1 164 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 Fe 3 O 4 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 m 2 K W − 1 . Using this value, the thermal diffusivity of FeO was derived as 3.7 × 10 − 7 -5.8 × 10 − 7 m 2 s − 1 and the thermal conductivity as 1.8-2.5 W m − 1 K − 1 between room temperature and 1 164 K. The temperature coefficients of the thermal conductivity were mostly negative, which would be dominated by the phonon mean free path.

High Temperature Thermal Diffusivity Measurement for FeO Scale by Electrical-Optical Hybrid Pulse-Heating Method

“…The value of α FeO,app would be influenced by the thermal boundary resistance between FeO scale and Fe substrate. Li et al 5) have suggested that the boundary resistance is not negligibly small for the FeO scale/Fe at room temperature and they have reported that the boundary resistance for the FeO scale/Fe was 8.3 × 10 − 6 m 2 KW − 1 at room temperature.…”

“…There have been several reports for thermal diffusivities of iron oxides. [2][3][4][5][6] However, the values reported by different laboratories show large discrepancies among them. The discrepancies would be due to the difference in microstructures of samples originating from their preparation methods, i.e., sintering of iron oxide powders 2,3) or thermal oxidation of iron or steel sheets.…”

“…The discrepancies would be due to the difference in microstructures of samples originating from their preparation methods, i.e., sintering of iron oxide powders 2,3) or thermal oxidation of iron or steel sheets. [4][5][6] From the practical viewpoint, the High Temperature Thermal Diffusivity Measurement for FeO Scale by Electrical-Optical Hybrid Pulse-Heating Method measured data on iron oxide samples prepared by the thermal oxidation of iron would be more suitable as the input parameters used in the water-cooling-process simulation. There are two reports on the thermal diffusivity for FeO scales formed on iron sheets as functions of temperature, 4,5) where both reports applied the laser flash method.…”

“…[4][5][6] From the practical viewpoint, the High Temperature Thermal Diffusivity Measurement for FeO Scale by Electrical-Optical Hybrid Pulse-Heating Method measured data on iron oxide samples prepared by the thermal oxidation of iron would be more suitable as the input parameters used in the water-cooling-process simulation. There are two reports on the thermal diffusivity for FeO scales formed on iron sheets as functions of temperature, 4,5) where both reports applied the laser flash method. However, a large discrepancy exists between the values reported by Taylor et al 4) and Li et al, 5) which would be due to the compositional change of the sample during the experiment, as suggested by Taylor et al According to the phase diagram of Fe-O system, 7) FeO decomposes into Fe 3 O 4 and Fe below 838 K. The decomposition of FeO increases the measured thermal diffusivity because the thermal diffusivity of iron is much greater than that of FeO.…”

“…There are two reports on the thermal diffusivity for FeO scales formed on iron sheets as functions of temperature, 4,5) where both reports applied the laser flash method. However, a large discrepancy exists between the values reported by Taylor et al 4) and Li et al, 5) which would be due to the compositional change of the sample during the experiment, as suggested by Taylor et al According to the phase diagram of Fe-O system, 7) FeO decomposes into Fe 3 O 4 and Fe below 838 K. The decomposition of FeO increases the measured thermal diffusivity because the thermal diffusivity of iron is much greater than that of FeO. Li et al reported that their measured value was greater than that reported by Taylor et al at 673 K, and also noticed that their results had been influenced by the decomposition of FeO from the phase identification for the post-experiment samples using a Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) with the target of Co.…”