The reflectivity of alumina ceramics under intense surface heating and subsequent free cooling

The results are given of measurements of the normally hemispherical reflectivity of alumina ceramics in the semitransparency region for the wavelengths of $0.488$, $0.6328$, $1.15$, and $3.39$ $\mu$m. The measurements are performed both in the process of fast heating of ceramics in the air by concentrated radiation of a $\text{CO}_2$ laser with fluxes of different densities from room temperature to $\sim2900$ K with the formation of a thin layer of melt and in the process of subsequent cooling after the heating radiation is discontinued. The heating time is approximately $2.25$ s, with the density of heating radiation flux of approximately $1200$ W/cm$^2$ and $1600$ W/cm$^2$. It is demonstrated that the reflectivity in the process of heating depends significantly on the flux density. By the end of the heating period and under the effect of a flux of higher density, the layer of melt formed on the ceramic surface is optically infinite for reflection at all wavelengths, while no optical infinity is attained at a lower flux density. The effect of abrupt variation of the absorption coefficient during melting and solidification on the thermal-radiation characteristics is analyzed.