Verneuil Method for Growing Sapphire Crystals

In the Verneuil sapphire crystal growth system, the alumina powder is molten during its fall into an oxygen-hydrogen flame. The liquid droplets fall on the top of the slowly-pulled crystal. From a physical point of view, the Verneuil process is complex because of the coupling of the turbulent combustion between hydrogen and oxygen, the hydrodynamics of the gas phase and the heat transport by convection, conduction and radiation.

Numerical simulations of the Verneuil crystal growth process have been carried out using the FIDAP finite element software, and the results have been presented in a previous paper. In order to determine the effect of the transparency of the sapphire, some assumptions were used: the process is considered in quasi-steady state, the model is axi-symmetric and the latent heat of fusion is negligible, i.e. the powder is not included in the model. The results obtained in the case where the crystal is considered as opaque have been compared with the simulation of a fully transparent material. Numerical simulation of heat transfer in transparent and semitransparent crystal growth processes.

The transparent crystal appears to be cooler than the opaque one, because in a transparent body the absorption of heat is reduced. The axial thermal gradient in the opaque crystal is smaller than that calculated in the case of a transparent crystal. By comparing the numerical results with experimental thermocouple data, it has been concluded that the simulation of a fully transparent sapphire crystal is more realistic than that of an opaque one.

A further step in this study will be to analyse the effect of the participating media radiation through the sapphire.

Vertical Bridgman method for growing fluoride crystals.


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