Thermal emissivity and high-temperature stability of thermal insulating materials produced by mixing powdered porous MgAl₂O₄ ceramics with commercial insulating castable

Abstract

Conventional industrial thermal insulating materials have a porous structure and provide resistance to thermal conduction. However, this structure permits heat transfer through radiation. Hence, they exhibit high thermal conductivity at temperatures higher than 1000 °C. To achieve high total-thermal-insulation efficiency, we recently developed new insulating materials, called THERMOSCATT^TM, which greatly suppress radiation heat transfer. Their thermal conductivity was measured to be less than 0.3 W/(m·K) at 1500 °C. These insulating materials consist of porous MgAl₂O₄ ceramics having 1–5 µm pores which restrain heat transfer through radiation, which was consistent with the Mie scattering theory. From the thermal emissivity estimated from reflectance measurements, the porous MgAl₂O₄ ceramics had near-zero hemispherical spectral emissivity values in the wavelength range of 0.35–5 µm. Mixing these powdered porous MgAl₂O₄ ceramics with a conventional commercial insulating castable ceramics is shown to successfully reduce heat transfer through radiative Mie scattering. This report describes the experimental result of the powdered porous MgAl₂O₄ ceramics mixed into the commercial insulating castable.