**2. High-temperature in situ observation system**

Crystal growth is a dynamic process which is composed of the mass and heat transport and interface kinetics. In this part, a high temperature in situ observation method coupling differential interference microscope and the Schlieren techniques will be introduced. The

Interfacial Mass Transfer and Morphological Instability of Oxide Crystal Growth 533

of mass flow. Fig. 3 shows the schematic of the Schlieren optical system includes two lens. The fore lens L1 is used to form parallel rays, and a knife edge is placed at the rear focal point of lens L2. Similarly, these parallel lights are also used to pass through the objective of the optical system of differential interference microscope. If a knife edge is installed at the rear focal point of the objective of the microscope, part of the light which has passed through the ununiform region of the object will be refracted and shielded, and the Schlieren effect can be obtained, i.e., the mass flow can be observed. So with this method, the growth pattern and the mass transportation phenomenon can be visualized simultaneously. The video from the microscope (through CCD) is recorded and visualized by the monitor. The video signal can also be transferred digitally into a computer directly for further analysis.

Fig. 2. The photograph of the crystal growth cell. a: loop-shaped Pt wire heater; b: Pt-10%Rh

Fig. 3. Schematic figure of the Schlieren optical system. S: light source; L0, L1, L2: lens;

D0,D1: diaphragms; P: specimen; K: knife edge; P': image of P

thermocouple; c: V-typed electrode

kinetic and morphological behaviour of the growing crystals can be observed by the differential interference microscope, and Schlieren technique is applied here to visualize simultaneously convective flow in the liquid phase. Melting and growing of oxide crystal taking place in high temperature up to 1400 °C can be observed and recorded by this system.

Fig. 1 shows the photograph of the in situ observation system. The system consists of a crystal growth part, a differential interference microscope coupling with Schlieren system, and the controlling part.

Fig. 1. The photograph of the optical in situ observation system. a: differential interference microscope coupling with Schlieren system; b: crystal growth part; c: optical light source; d: mechanical controlling part

The crystal growth part consists of a growth cell, a digital volt meter and a stabilized DC power supply. Fig. 2 shows the photo of the micro-floating-zone growth cell. A loop-like Pt wire (φ0.2-0.5 mm) is employed to heat and suspend the oxide melt/solution. The temperature of the loop is measured by a Pt-10%Rh thermocouple (φ0.08 mm). Temperature fluctuations of less than ±1 °C are obtained for high temperature melts. Two V-typed electrodes are used to prevent the loop from deformation at high temperature. For crystal growth experiment, oxide material is firstly heated and suspended horizontally on the loop heater to form a melt/solution film. The amount of the melt should be precisely adjusted so that the upper and lower planes of the thin film of the liquid are parallel to each other. Moreover, to avoid the contamination of the volatile materials, the top window in the growth cell should be eccentrically placed and rotatable. Then nucleation and crystal growth in two-dimensions are performed by decreasing the temperature of the loop heater.

A differential interference microscope is applied to visualize the crystal growth processes from the melt/solution. Schlieren technique is coupled into the microscrope for visualization

kinetic and morphological behaviour of the growing crystals can be observed by the differential interference microscope, and Schlieren technique is applied here to visualize simultaneously convective flow in the liquid phase. Melting and growing of oxide crystal taking place in high temperature up to 1400 °C can be observed and recorded by this

Fig. 1 shows the photograph of the in situ observation system. The system consists of a crystal growth part, a differential interference microscope coupling with Schlieren system,

Fig. 1. The photograph of the optical in situ observation system. a: differential interference microscope coupling with Schlieren system; b: crystal growth part; c: optical light source;

The crystal growth part consists of a growth cell, a digital volt meter and a stabilized DC power supply. Fig. 2 shows the photo of the micro-floating-zone growth cell. A loop-like Pt wire (φ0.2-0.5 mm) is employed to heat and suspend the oxide melt/solution. The temperature of the loop is measured by a Pt-10%Rh thermocouple (φ0.08 mm). Temperature fluctuations of less than ±1 °C are obtained for high temperature melts. Two V-typed electrodes are used to prevent the loop from deformation at high temperature. For crystal growth experiment, oxide material is firstly heated and suspended horizontally on the loop heater to form a melt/solution film. The amount of the melt should be precisely adjusted so that the upper and lower planes of the thin film of the liquid are parallel to each other. Moreover, to avoid the contamination of the volatile materials, the top window in the growth cell should be eccentrically placed and rotatable. Then nucleation and crystal growth

in two-dimensions are performed by decreasing the temperature of the loop heater.

A differential interference microscope is applied to visualize the crystal growth processes from the melt/solution. Schlieren technique is coupled into the microscrope for visualization

system.

and the controlling part.

d: mechanical controlling part

of mass flow. Fig. 3 shows the schematic of the Schlieren optical system includes two lens. The fore lens L1 is used to form parallel rays, and a knife edge is placed at the rear focal point of lens L2. Similarly, these parallel lights are also used to pass through the objective of the optical system of differential interference microscope. If a knife edge is installed at the rear focal point of the objective of the microscope, part of the light which has passed through the ununiform region of the object will be refracted and shielded, and the Schlieren effect can be obtained, i.e., the mass flow can be observed. So with this method, the growth pattern and the mass transportation phenomenon can be visualized simultaneously. The video from the microscope (through CCD) is recorded and visualized by the monitor. The video signal can also be transferred digitally into a computer directly for further analysis.

Fig. 2. The photograph of the crystal growth cell. a: loop-shaped Pt wire heater; b: Pt-10%Rh thermocouple; c: V-typed electrode

Fig. 3. Schematic figure of the Schlieren optical system. S: light source; L0, L1, L2: lens; D0,D1: diaphragms; P: specimen; K: knife edge; P': image of P

Interfacial Mass Transfer and Morphological Instability of Oxide Crystal Growth 535

driven flow caused by the melt rising along the hot wall and descending in the center of vessel which is heated from the side and cooled from the top surface. Fluid flow velocity measurement shows that the momentum profile in the melt is similar to the thermal one. In the central portion of the loop, the flow is steady because of low value of the applied

Fig. 5. The buoyancy driven convection indicated by the movement of tiny KNbO3 crystals

Marangoni convection is driven by the variation of surface tension along the free surface. The temperature distribution along the azimuthal coordinate of the loop-like heater is measured by our developed non-contact method (X. A. Liang et al., 2000), which is based on the fact that dissolvability is one-valued function of the temperature. The result is shown in Fig. 6. Here ΔTx=TB-TA. Clearly, the temperature along the free surface is not uniform, which is caused by existence of thermocouple and other attachment. The side with thermocouple

Fig. 6. Temperature difference along the azimuthal coordinate of the heater

temperature gradient.

**3.2 Marangoni convection** 

on has a lower temperature.
