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

Alumina-spinel refractory bricks, composed of 86 wt.% alumina phase and 13 wt.% MgAl2O4 spinel phase, are used in steel making 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. Measurements of thermal conductivity were made with the laser flash technique from 20 °C to 1100 °C. Analysis of the influence of the microstructure on the thermal conductivity of the alumina-spinel refractory bricks was based on simplified analytical relations and validated by comparing the behaviour with four different alumina model materials. A model of thermal resistors in series was used to describe the combined effect of grains and grain boundaries on the solid phase conductivity whereas the effect of porosity was calculated with Landauer’s relation. Though the overall conductivity of the refractory brick was evaluated as 6.5 W m-1 K-1 at room temperature, the thermal conductivity of the alumina grains was deduced to be 33 W m-1 K-1, close to that of single crystal sapphire at 36 W m-1 K-1. The strongly attenuated conductivity of alumina-spinel refractory is explained by the roles of porosity, grain boundary thermal resistance and the spinel phase thermal conductivity.