The effect of vacuum on the reflectivity of alumina ceramics under conditions of melting by concentrated laser radiation and subsequent free cooling

The results are given of measurements of the normal-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 in vacuum both in the process of fast heating by concentrated radiation of a $\text{CO}_2$ laser 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 switched off. The heating time is approximately $2.25$ s, with the density of heating radiation flux of approximately $1600$ W/cm$^2$. The effect of abrupt variation of the absorption coefficient during melting and solidification on the thermal-radiation characteristics is analyzed. The results demonstrate that the absorption coefficient of the melt in the semitransparency region is higher in vacuum than in the air.