Xuejiang Chen

Numerical Analysis of mc-Si Crystal Growth

The content and uniformity of impurities and precipitates have an important role in the efficiency of solar cells made of multicrystalline silicon. We developed a transient global model of heat and mass transfer for directional solidification for multicrystalline silicon and a dynamic model of SiC particles and silicon nitride precipitation in molten silicon based phase diagrams. Computations were carried out to clarify the distributions of carbon, nitrogen and oxygen based on segregation and the particle formation in molten silicon during a directional solidification process. It was shown that the content of SiC precipitated in solidified ingots increases as a function of the fraction solidified. It was also clarified from the results that Si2N2O was first formed near the melt-crystal interface, since oxygen concentration in the melt decreases and nitrogen concentration in the melt increases with solidification of the molten silicon. Si3N4 was formed after Si2N2O had been formed.

Application of a niching genetic algorithm to the optimization of a SiC crystal growth system

It was demonstrated that a niching genetic algorithm (NGA) could be efficient for the optimization of a SiC crystal growth system. And several design parameters of SiC crystal growth system could be optimized at the same time, and high diversity of population was maintained to obtain global optimization solution by NGA. Firstly, the NGA and thermal models were described and applied for SiC crystal growth by physical vapor transport (PVT) method, and the combination method of NGA and thermal models were presented. Then two cases were carried out to demonstrate the automatic optimization of SiC crystal growth system by NGA. One case was a single-objective optimization problem, in which the axial position of coils was optimized to improve the growth rate of crystal. The another was a multi-objective optimization problem, in which the thickness of substrate holder and input current were optimized for uniform temperature distribution along the growth surface for reducing the thermal stresses in growing crystal. Finally, all the optimization results were analyzed.

Experimental and numerical analysis on noise reduction in a multi-blade centrifugal fan

In this work, analysis on noise source and reduction in a multi-blade centrifugal fan used for air-conditioners was carried out by experimental and numerical methods. Firstly, an experimental system using microphone mounted on volute surface for measuring surface pressure fluctuations of volute was designed and introduced, then surface pressure fluctuations of the whole volute for a multi-blade centrifugal fan were measured by this system, and the inlet noise for this fan was also obtained. And then, based on the experimental results, the aerodynamic noise source of the studied fan was analysed. The surface pressure fluctuations of the volute showed that there were largest surface pressure fluctuations near the volute tongue, and peaks appeared at the Blade Passing Frequency (BPF). The spectra of fan inlet noise showed that the peaks also appeared at BPF, and noise levels in a wide range of frequency were also larger. Secondly, the internal flow of the fan was simulated by commercial software under the same conditions with the experiment, and then the fluid flow and acoustic power field were obtained and discussed. The contours of acoustic power level showed that the larger noise was generated at the impeller area close to the outlet of scroll and at the volute tongue, which is same as that from experiment. Based on all of the results, we can find that the vortex noise is an important part of fan noise for the studied fan, and the rotation noise also cannot be neglected. Finally, several reduction methods that are thought to be effective based on experimental and numerical results were suggested.

Heat and impurity transfer mechanisms of Czochralski and directional solidification processes

In this paper, results of global computation in a three-dimensional configuration for transverse magnetic field-applied Czochralski method to study distributions of temperature and impurity concentration are presented. The analysis includes a re-melting and segregation model at an interface between a crystal and the melt. Deflection of an interface between a crystal and the melt is also taken into account. Time-dependent global analysis of the solidification process of a silicon ingot for photovoltaic is described in this paper. This model includes distributions of dislocation, impurity and point defects for a quasi-single silicon ingot grown by the solidification method. Heat and mass transfers in a square crucible are also discussed.

Simulation Analysis of Point Defects in Silicon Ingot during Unidirectional Solidification Process for Solar Cells

A transient global model was used to obtain the solution of thermal field within the entire furnace of a unidirectional solidification process. The melt-solid interface shape was obtained with a dynamic interface tracking method. And then, based on the global solution of heat transfer, influence of growth rate Vg, temperature gradient G and ratio Vg/G on point defects were analyzed. Finally, several different melt-solid interface shapes were obtained by using different solidification times. Then, the effects of solidification times on ratio Vg/G and point defects were also studied.