Analysis of Microstructural Effects on the Thermal Conductivity of Alumina-Spinel Refractories Compared to Alumina Ceramics

Alumina-spinel refractory bricks, composed of 82 wt.% alumina and 18 wt.% MgAl2O4 spinel phases, are used in steel ladles due to their ability to resist chemical attack and thermal shock. Thermal shock resistance is determined, in part, by the thermal conductivity of the material. Thermal conductivity measurements for alumina-spinel refractory, three model alumina ceramics, and single crystal sapphire were made with the laser-flash technique from 20 °C to 1000 °C. At room temperature, these gave 6.5 W m−1 K−1 for the refractory, 5.8 to 22 W m−1 K−1 for the alumina ceramics, and 36 W m−1 K−1 for sapphire, despite all materials containing >81 vol.% of alumina. The differences are explained by the roles of porosity, grain boundary thermal resistance, and the spinel phase (refractory). In order to estimate the thermal conductivity of alumina grains in each material, these microstructural effects are modelled with Landauer’s relation for porosity and thermal resistors in series for grains combined with grain boundaries. For two alumina ceramics, the grains yielded similar behaviour to the single crystal. By taking the spinel phase into account with a two-phase mixture relation, the alumina grains in the refractory were estimated with a value of 31 ± 2 W m−1 K−1, close to sapphire.