The Radiation of Alumina Melt in the Middle IR Spectral Region at Solidification in the Course of Free Cooling

The results are presented of radiation intensity measurements in the middle IR spectral region from $3$ to $11$ $\mu$m at cooling and solidification in ambient air for both “thick” layers several millimeters thick and “thin” layers several tenths of a millimeter thick of a pure aluminum oxide melt. In the former case, the melt was held in a powder bed of the same material; in the latter case, a pool with the melt was created on a ceramic surface. The results on different wavelengths are presented as time dependences of the infrared spectrometer signals, proportional to the intensity of radiation emitted by the melt. In addition, the data on the time dependence of the emissivity for thick melt layers at a solidification plateau are presented. On the basis of the obtained results and the results of previous investigations and radiative-conductive heat transfer calculations at cooling and solidification, it is shown that the intensity of emitted radiation at crystallization is defined not only by optical properties of a melt and a crystal, but depends also on the thickness and the temperature field of the melt before the start of cooling. An almost horizontal section on the solidification plateau is caused by the presence of an isothermal two-phase zone; however, with a melt thickness of several millimeters, such a section of different length is observed only in the range of wavelengths from $5$ to $11$ $\mu$m.