Jin Wei-Qing

Thermalcapillary Convection in NaBi(WO4)2Melt

Surface-tension driven flow in NaBi(WO4)2 melt is visualized by means of the already proved method of high temperature in situ optical observation. The transition phenomena of the melt from steady spatial to oscillatory behaviour in melt are observed. The oscillatory thermocapillary convection is accompanied by synchronous temperature oscillations with the frequency 10 Hz. The onset temperature of thermocapillary convection varies with the change of meniscus curvature of the melt free surface, which probably due to the liquid pressure at the surface associated with the curvature of meniscus. The oscillatory frequency is proportional to the temperature. Finally, another kind of oscillatory thermocapillary convection with the frequency about 5 Hz dominating in the undercooled melt is also observed.

Solute distribution in KNbO3 melt-solution and its effect on dendrite growth during rapid solidification

This paper reports that the rapid solidification of mixed Li2B4O7 and KNbO3 melted in a Pt loop heater has been performed experimentally by the method of quenching, and various morphologies of KNbO3 crystals have been observed in different regions of the quenched melt-solution. Dendrites were formed in the central region where mass transfer is performed by diffusion, whereas polygonal crystals with smooth surface grew in the marginal region where convection dominates mass transport. Based on measurement of KNbO3 concentration along crystal interface by electronic probe analysis, it finds the variety of crystal morphologies, which is the result of different solute distributions: in the central region the inhomogeneity of solute concentration is much sharper and morphological instability is easier to take place; nevertheless in the marginal region the concentration homogeneity has been greatly enhanced by convection which prevents the occurrence of morphological instability. Additional solute distribution in the melt along the primary dendrite trunk axis as well as that in mushy zones has also been determined. Results show that the solute concentration in the liquid increases linearly with distance from the trunk tip and more solutes were found to be concentrated in mushy zones. The closer the mushy zone is to trunk tip, the lower the solute concentration will be there.

Experimental study on the free surface deformation in the oxide melt

Experiments on the thermocapillary convection of high temperature Bi12SiO20 melts have been carried out in the in situ observation system. The steady flow pattern comprises of the main trunk and branches, which is deemed as the surface deformation. The oscillatory thermocapillary convection is characterized by the oscillatory main trunk and traveling branches. The transition of the melt from steady spatial to oscillatory behavior under the temperature differences 120, 60 and 10 K has been considered. The free surface deformation is observed to transform with the change of the applied temperature difference, which manifests that the thermocapillary convection is sensitive to the temperature difference. Moreover, taking the temperature distribution into account, it is noted that the deformation is formed in the colder area of the melt. The oscillatory frequency of the main trunk, which is also sensitive to the applied temperature difference, increases with the rise of temperature.Experiments on the thermocapillary convection of high temperature Bi12SiO20 melts have been carried out in the in situ observation system. The steady flow pattern comprises of the main trunk and branches, which is deemed as the surface deformation. The oscillatory thermocapillary convection is characterized by the oscillatory main trunk and traveling branches. The transition of the melt from steady spatial to oscillatory behavior under the temperature differences 120, 60 and 10 K has been considered. The free surface deformation is observed to transform with the change of the applied temperature difference, which manifests that the thermocapillary convection is sensitive to the temperature difference. Moreover, taking the temperature distribution into account, it is noted that the deformation is formed in the colder area of the melt. The oscillatory frequency of the main trunk, which is also sensitive to the applied temperature difference, increases with the rise of temperature.

Thermal Diffusivity Measurements of Li2B4O7 and KNbO3 Solution by Laser Flash Method

The thermal diffusivities of Li2B4O7+KNbO3(20 wt%) solution have been determined at temperatures between 900°C and 990°C using a three-layered laser flash method on the differential scheme. The thermal diffusivity of this Li2B4O7 and KNbO3 solution ranges from 3.6.×10-3cm2/s to 4.1×10-3cm2/s. A positive temperature dependence of thermal diffusivity of solution is obtained.

Experimental and Theoretical Studies on Velocity Field of Buoyancy Convection in KNbO3 Melt

The Schlieren technique coupling with a differential interference microscope was applied to visualize the KNbO3 melt motion in a loop-shaped Pt wire heater. The natural convection in KNbO3 melt was traced by observing the movement of the tiny KNbO3 crystals (~10 µm) and the stream velocities of these tracer crystals were measured. In theoretical analysis, the Navier-Stokes equation was solved as a stable field. The general solution for this system of the differential equation was expressed by an approximate power series of azimuth and radius vector. The expression was substituted in the differential equation; a non-trivial solution was obtained exactly. The velocity distribution in the vertical section was obtained which is in qualitative agreement with the experimental result.

In Situ Observation of Cell-to-Dendrite Transition

The cell-to-dendrite transition of succinonitrile melt suspended on a loop-shaped Pt heater is observed in real time by a differential interference microscope coupled with Schlieren technique. The transition is divided into two parts: a dendrite coalition process and a subsequent dendrite elimination process. Firstly the dendrites from the same cell are united into a single dendrite. Secondly the competitive growth of dendrites from different cells leads to the elimination of dendrites. The two processes can be understood when involving crystallographic orientation. In addition, the tip velocity and primary spacing of a cell/dendrite are also measured. It turns out that the primary spacing has a significant jump, whereas the growth velocity has no abrupt change during the cell-to-dendrite transition.

Effect of axial vibration on free surface flows in cylindrical liquid

The influence of axial vibration on free surface flows in an open cylindrical container was studied by optical in situ observation method under isothermal conditions. This ground-based experiment was performed on an electromagnetic vibrator with oscillatory frequency of 100Hz. Water–glycerol mixture was chosen as the model liquid. Results showed that small amplitude (<100μm) could generate a new type of steady streaming flows on a free surface, which were mainly driven by the combination of propagating surface wave and Stokes layer effect. The steady flow manifested various patterns according to the vibration amplitude level. Higher amplitude made steady flow periodical or turbulent, which could be characterized by the critical vibrational dimensionless Reynolds number (Nre)c. The calculated value of (Nre)c was of the magnitude of 10−2−10−1. In addition, surface streaming velocities were measured by the particle scattering technique. It was found that the velocity increased parabolically with vibration amplitude and decreased with viscosity for a fixed flow pattern.

NUMERICAL MODELLING OF VELOCITY AND TEMPERATURE DISTRIBUTIONS OF THE BUOYANCY CONVECTION EFFECT IN KNbO3 MELT

Numerical modelling of velocity and temperature fields in high-temperature KNbO3 melt of a loop-shaped Pt wire heater is carried out by using the commercial computational code ANSYS for the mathematical solution of the governing equations. Based on the experimental boundary conditions and the Boussinesq approximation, the numerical modelling of a steady and two-dimensional model is applied to study the process under consideration of the buoyancy-driven convection condition. The result is compared with the previous experimental and theoretical data obtained in our laboratory, and the former is in agreement with the latter. Thus a theoretical guide for reasonable growth conditions is provided by studying in depth the real fluid flow effects in the crystal growth from the melt.

The growth and melting morphology of B12SiO20 crystals

Abstract The differences in morphologies between the {001}, {011} a , {011} b , and {111} faces has been observed. The {001} faces are the only faces on which freely developed growth can take place. Throughout the whole process of growth, these faces maintain a smooth interface. Such growth processes have not taken place on the {011} faces. {011} a faces can appear either stepped or nearly flat, depending upon the growth conditions. In any case, {011} b faces appear as stepped faces. In our experiments, {111} faces do not appear during the crystal growth. However, we observe a combination of {001} and {011} b steps that migrate along the {111} faces. This is a typical morphology of k -faces. The variations in the melting morphology of the {011} and {001} faces are also observed. {011} faces show a melting form with a stepped nature, but the {001} faces are often flattened and finally divided into several faces.

Growth of NaBi(WO4)2 Dendrite and Mechanism

The solid–liquid interface motion of NaBi(WO4)2 (NBWO) melt crystal growth is observed in an in situ system, in which the whole processes of interface transition from flat interface and cellular to dendrite are visualized. The spacing of the dendrite under smaller temperature gradient turns out to be larger than that under larger temperature gradient, which is found to be sensitive to the temperature distribution. The mechanism of dendrite growth of NBWO is studied based on the model of the growth units of anion coordination polyhedra. The { 001} face has two apex links, so it shows higher stability and has high growth rate and forms the arm of dendrite, whereas the { 010} face has only one apex link, and thus shows relative slower growth rate and firstly forms the branches.