Two-phase zone at Al2O3 meltin by laser radiation and at melt solidification under the conditions of free cooling

Temperature distribution across the growth zone of sapphire (Al 2 O 3 ) and yttrium–aluminum garnet (YAG) single crystal fibers

“…Similar sudden changes in temperature (specifically the singularity of heat radiation power being measured by a pyrometer) were observed during sapphire crystallization in several experiments [7,8,16]. The existence of so abrupt change in heat radiation power was explained in these studies by steep drop of the sapphire emissivity during crystallization.…”

“…It was found that at the melting temperature the absorption coefficient in the visual range is 70 times higher for the melt then for the sapphire crystal [17]. It should be noted, that in the experiments being cited [7,8,16] the growing crystal and surrounding (or covering) melt were in the same line of sight.…”

“…Experimental data on spectral and temperature dependence of the crystal sapphire absorption coefficient are known up to the melting temperature [13,16]. Unfortunately the values of spectral emissivity for the sapphire melt are unknown.…”

“…In this case the single crystal and two-phase medium (which includes the melt and microcrystals) makes the contribution to the measured heat radiation signal both [7,16]. Besides the similar effect of local temperature minimum in the melt was obtained after computer modeling of melting/crystallization processes with the plane interfaces for the sapphire crystal [18].…”