Thermal conductivity of the ZrB 2 – CuCrZr composite related to infiltration time

Abstract

The paper analyses thermal properties of the ZrB₂-CuCrZr composite that was prepared by the gas pressure infiltration technology (GPI) using a ZrB₂ porous preform and CuCrZr alloy. The influence of 2-min and 20-min infiltration time on the composite thermal conductivity was characterised and analysed. The results showed that the infiltration time did not have a significant effect on the thermal conductivity (k) of the ZrB₂-CuCrZr composite in the temperature range from 200°C to 800°C (k = 130 – 115 W/m.K). Only from RT to 200°C an observable effect of longer infiltration time appeared (k _2min. = 145 - 130 W/m.K and k _20min. = 152 – 130 W/m.K). Additionally, the prepared composite was sequentially thermally loaded at temperatures of 800°C; 600°C or 400°C for 250 min in a vacuum in order to simulate working conditions in potential applications as plasma waste burners and fusion reactors. During this treatment thermal conductivity was recorded and was at the level of 125 and 115 W/m.K. The authors compared the experimental thermal conductivity of the ZrB₂-CuCrZr composite with well-known analytical models considering perfect thermal contact between the components. They were e.g. Maxwell’s model and the empirical model by Lewis and Nielsen. In the model by Hasselman and Johnson also the interfacial thermal resistance (ITR) was taken into calculations. The results showed that at room temperature (RT) both, the Lewis-Nielsen and Hasselman-Johnson model using high thermal conductivity at the interface (h_c), come closest to the measurement (k_c ∼ 150 W/m.K). At higher temperatures calculated values were overestimated, compared to the experimental ones. The conductivity obtained using the Rule of Mixtures (ROM) – its upper bound, was at the level of the experiment at 300°C (k_c,300°C ∼ 128 W/m.K). The thermal conductivity of the ZrB₂, was derived from experimental values obtained for the composite. Authors used equations describing upper and lower bounds of the ROM. The calculated lower bound showed high conductivity (k_ZrB2 ∼ 100 W/m.K) and the upper bound gave 100 - 60 W/m.K between RT and 300°C.