Ge Pei-Wen

Influence of the interface kinetics of crystallization and dissolution on the scaling laws in solution system for crystal growth

The influence of interface kinetics on scaling laws in a solution system for cry stal growth is studied. Because the variation of the solution density caused by the solute concentration change can be omitted and only that caused by the temperature change is taken into account, the interface kinetics does not affect the scaling laws of the fluid velocity and the temperature distribution index S-theta. By taking the interface kinetics into account, the curves of the concentration distribution index S-phi versus the Rayleigh number Ra, the Prandtl number Pr or the Schmidt number Sc are changed. When Ra and Sc are small. S-phi approaches a constant S-phi(0) independent of Ra, Sc and lambda. When Ra and Sc are large, the influence of the interface kinetics on the curves of S-phi is negligible. The interface kinetics affects the curves of the average dimensionless crystal growth rate (V) over bar(cg) versus Ra, Sc or Pr only when Ra and Sc are large. In this case, (V) over bar(cg) is still a power function of Ra, Pr or Sc in certain regions of the parameter (Ra, Pr and Sc) space, but the exponents and coefficients of power functions are varied.

Effects of interface kinetics and anisotropy on the stability of the growing crystal face and dissolution face during crystallization from solution under microgravity

The stability of the shapes of the growing crystal face and dissolution face in a two-dimensional mathematical model of crystal growth from solution under microgravity is studied. Effects of the interface kinetics and anisotropy of crystallization and dissolution on the stability are also studied. It is proved that the stable shapes of crystal growth face and dissolution face do exist, which are of suitably shaped curves with their upper parts inclined backward properly no matter whether the interface kinetics and the anisotropy are taken into account or not. The stable shapes of the growing crystal faces and dissolution faces are calculated for various cases. The interface kinetics will make the inclination degree of stable crystal growth face reduce and that of stable dissolution face reduce slightly. The anisotropy of crystallization and dissolution may make the inclination degree of stable-growing crystal face smaller or larger, and that of the stable dissolution face varies very slightly.

Computational Analysis of the Vertical Bridgman Growth of Te Doped GaSb Under Microgravity

A transient analysis of Te doped GaSb melt growth process is performed using finite element method. The solute concentration at the growth interface increases with time because of k < 1. The growth interface shape becomes a little flat at the beginning of the growth compared with the initial shape. Radial segregation occurs even under the μg condition. This segregation increases with the increase of gravity when gravity is small, and reaches a maximum at g = 10-3 for our system.

INFLUENCE OF SEED CRYSTAL DIMENSION ON CRYSTAL GROWTH FROM SOLUTION UNDER MICROGRAVITY

The influence of the dimension of seed crystal on the characteristics of crystal growth from solution under microgravity is studied. The dimensionless average velocity of fluid Vav, the dimensionless maximum velocity of fluid Vmax, the temperature distribution index Sθ, the concentration distribution index S and the dimensionless average growth rate of crystal cg are calculated by only taking into account the variation of the solution density caused by the temperature change, that caused by the concentration change being neglected. In certain regions of the parameter (Ra, Pr, Sc and μ) space, some scaling laws are generated: the scales of S and cg are given by power functions of μ with negative exponents. It is shown that the characteristics of crystal growth for small seed crystal are different from those for large seed crystal.

Interface shape and concentration distribution in crystallization from solution under microgravity

The Kerr rotation of MnBixAl0.5(0.4 less than or equal to x 0.9) thin films with and Al protective layer as a function of the Bi concentration, x, has been investigated. Compared with MnBix thin films, it is found that a large enhancement of Kerr rotation appears in the MnBixAl0.15 thin films with 0.4 less than or equal to x less than or equal to 0.7, but no enhancement of Kerr rotation appears when x is greater than 0.7. When x = 0.5, a maximum Kerr rotation of 2.75 degrees is observed at 633 nm for the MnBi0.5Al0.15 thin film, which is much larger than that of 1.56 degrees for the MnBi0.5 thin film. For the MnBixAl0.15 thin films with 0.4 less than or equal to x less than or equal to 0.7, the c lattice shrinking may result in a stronger hybridization between Bi 6p and Mn 3d states, which also should be responsible for the large enhancement of Kerr rotation. In addition, the saturation magnetization M-s is reduced from 4 x 10(5) A/m for the MnBi0.5Al0.15 thin film to 3 x 10(5) A/m for the MnBi0.5Al0.15 thin film, suggesting that some of Al may also substitute for Mn at the octahedrtal sites.

Shape dependence of low-temperature magnetic relaxation of Mn(12)Ac

We report the discovery that the low-temperature magnetic relaxation in Mn(12)Ac single crystals strongly depends on the shape of the samples. The relaxation time exhibits a minimum at the phase transition point between ferromagnetic and antiferromagnetic phases. The shape dependence is attributed to the dipolar interaction between molecular magnets.We report the discovery that the low-temperature magnetic relaxation in Mn(12)Ac single crystals strongly depends on the shape of the samples. The relaxation time exhibits a minimum at the phase transition point between ferromagnetic and antiferromagnetic phases. The shape dependence is attributed to the dipolar interaction between molecular magnets.

Magnetic Quantum Tunnelling in Faster Relaxation Process in Mn12Ac Molecular Magnets

The field-tuned ac susceptibility of Mn12Ac single crystal has been measured as functions of temperature and frequency. Two relaxation processes appear in our measurement. One is related to the magnetic quantum tunnelling of a collective spin S = 12 by a 61 K magnetic barrier and it has been well studied, while the other is faster than the former and its mechanism is unclear. We find that the relaxation time for the faster process shows the minima at H = 0 and H = 3.4 kOe, indicating that the quantum tunnelling also takes place in this process while the resonant fields are different from what we have known before.

Stability of Crystal Growth Face and Dissolution Face in Crystallization from Solution under Microgravity

The stability of the shapes of crystal growth face and dissolution face in a two-dimensional mathematical model of crystal growth from solution under microgravity is studied. It is proved that the stable shapes of crystal growth face and dissolution face do exist, which are suitably shaped curves with their upper parts inclined backward properly. The stable shapes of crystal growth faces and dissolution faces axe calculated for various values of parameters, Ralpha, Ptau and Sc. It is shown that the stronger the convection relative to the diffusion in solution is, the more backward the upper parts of the stable crystal growth face and dissolution face are inclined. The orientation and the shape of dissolution face hardly affect the stable shape of crystal growth face and vice versa.