**6.3 Improvement of crystallographic perfection in Bi12SiO20 crystal by axial vibration**

The authors have experimentally investigated the effect of axial vibration on the free surface flows in a vertical Bridgman model under isothermal conditions (X. H. Pan, 2005a, 2005b). Steady forced flows are obtained on the free surface of the liquid phase driven by pure axial vibration. Based on this hinting, axial vibration is introduced into the Bi12SiO20 crystal growth in our laboratory (Y. Zhang et al., 2008, 2009), in order to suppress the unsteady thermocapillary or buoyancy convections. Some results are described in the following.

Axial vibration has been introduced in the growth of Bi12SiO20 single crystal by vertical Bridgman technique. The frequency of the axial vibration applied is 50 Hz and its amplitude varies. The quality of the Bi12SiO20 single crystal is identified by the high-resolution X-ray rocking curves as shown in Fig. 24. It can be found that the crystal grown with 70 μm

Interfacial Mass Transfer and Morphological Instability of Oxide Crystal Growth 551

for oxide crystal growth. Besides the buoyancy and Marangoni convections induced by temperature gradient, diffusion-induced micro-convection in the boundary layer also exists. The style of mass transfer near the growing interface has great effect on the interfacial kinetics and the interfacial instability. The mass transport governed by pure diffusion can induce inhomogeneous concentration distribution along the growing interface of the single crystal and thus leading to morphological instability, where dendritic or skeletal shape occurs easily. Nevertheless, if the mass transport is governed by both convection and diffusion, the faceted interfacial shape is easy to maintain. The mass transport can be effectively improved by external forces such as magnetic field and vibration, and thus single

The authors would like to thank their colleagues and collaborators. The author would also like to acknowledge the supports from National Natural Science Foundation of China under

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crystal with high crystallographic perfection can be obtained.

**8. Acknowledgements** 

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pp.42-46 (in Chinese)

**9. References** 

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vibration has the strongest diffraction intensity and the smallest full-width at half-maximum (FWHM), indicating the highest crystal quality. This means that crystallographic perfection of oxide crystal can be effectively improved by axial vibration with proper amplitude.

Fig. 24. X-ray rocking curves of Bi12SiO20 crystals grown by vertical Bridgman method with different vibration amplitudes

Fig. 25 shows the etch pit pattern of the as-grown Bi12SiO20 crystals by vertical Bridgman method. The etch pit density of the crystal grown without vibration is about 4.8×104/cm2 (Fig. 25(a)). However, when an axial vibration of 70 μm amplitude is applied, the etch pit density of the crystal grown with vibration is only 2.2×104/cm2 (Fig. 25(b)). This is attributed to the enhanced mass exchange and the diminished radial temperature gradient in front of the solid-liquid interface by vibration-induced forced convection.

Fig. 25. Etch pit patterns of Bi12SiO20 crystals grown without vibration (a) and with vibration of 70 μm amplitude (b)
