In Cz-process, the crystallization front shape is a strong indicator of the crystal thermal history. The radial non-uniformity of thermal history, associated with the curved (κ≠0) front shape, affects the crystal structure and properties. The ideal flat (κ=0) interface is not generally stable during the growth of high-melting point oxides typically exhibit convex (κ>0) to melt front shapes. The crystal rotation rate, ω generates a forced upward flow in the melt central column and affects the boundary layer thickness in BPS model. Experimentally, the abrupt interface inversion in Cz-growth Nd:YAG and Yb:YAG crystals with A= (r/R)~0.4, could be observed only at the final stages in which both the melt depth, h_m and the fluid flow Ra-number has significantly decreased. From simulations of the melt parabolic temperature, T_w - dependence on h_m along the crucible wall, some critical positions, h_m* ≤ A h_m0 are found for which the buoyancy forces should change the sign. However, at the same positions in the melt central column, the forced upward flow (Re~r²ω) is relatively strong. Around such positions the S/L interface inversion would suddenly occur. In Cz-growth of Nd,Cr,Ca:YAG crystal (V_p~1 mm/hr; ω₁=24 rpm) and after a well-tried shoulder (Φ~38.5 mm) stage, some strong tendency to decrease (~24%) in diameter was observed without any measurable external perturbation. So a critical cross section (CCS) of ~8.5 mm in thickness was formed. This was almost the same as experienced in the case of Cr,Ca:YAG growth leading to abrupt separation of the flat-interface shoulder from the melt. A sharp decrease in rotation rate was needed to regain the Nd,Cr,Ca:YAG growth stability. The process continued safely under the new growth conditions (NGC: Φ₂=29.5 mm, ω₂=12 rpm). However, the detailed spectral study of the crystal (L= 176 mm) revealed that Cr⁴⁺/Al³⁺ tetrahedral site occupation is limited to the initial part (L₁= 48.5 mm) including the up-cone and CCS-region. The rest of the body (L₂=128 mm), grown under the new growth conditions (NGC) and clear green in color, is evidently dominated by Cr³⁺ ions. The shoulder-stage instability might be described as a perturbation developed near by the tri-junction point affecting the fluid flow and heat transfer in a region just close to the S/L interface. However, the observed sharp decrease in diameter, associated with κ → 0 , is mainly attributed to the internal radiation heat transfer which fails for the body of highly coated surface. In fact, the brown/red color of the initial part appeared only after polishing the surface being completely coated with impurities deposited by vapor-phase transport. As well, the applied NGC obviously prevented the charge compensation mechanism (CCM) so that Ca²⁺/Y³⁺ octahedral site occupation couldn’t be realized. In BPS model δ_D/Φ represents the aspect ratio of the boundary layer and the higher δ_D/Φ, the lower is k_eff as the segregation coefficient of the solute. This was the outcome of the applied NGC leading to a sharp decrease in Re-number of the flow field. The layer thickness δ_D should be increased about 42% while the diameter Φ has effectively (24%) decreased. Thus, NGC has resulted in a higher (~ 87%) δ_D/Φ affecting k_eff of Ca²⁺ ions of k<1 and stopped CCM as the prime condition for Cr⁴⁺/Al³⁺ site substitution.

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1. Thermocapillary Effect on the Pattern Transition of Czochralski Natural Convection of Oxide Melt
2. Internal Radiation Effect on the Buoyancy-driven Flow pattern of Czochralski Oxide Melt