Haisheng Fang

A Revisit to High Thermoelectric Performance of Single-layer MoS2

Both electron and phonon transport properties of single layer MoS2 (SLMoS2) are studied. Based on first-principles calculations, the electrical conductivity of SLMoS2 is calculated by Boltzmann equations. The thermal conductivity of SLMoS2 is calculated to be as high as 116.8 Wm−1K−1 by equilibrium molecular dynamics simulations. The predicted value of ZT is as high as 0.11 at 500 K. As the thermal conductivity could be reduced largely by phonon engineering, there should be a high possibility to enhance ZT in the SLMoS2-based materials.

Characteristics of Thermal Stress Evolution During the Cooling Stage of Multicrystalline Silicon

Directional solidification is one of the most popular techniques for the massive production of multicrystalline silicon (mc-Si) for solar cell application due to its well-balanced high conversion efficiency and low production cost. The grown crystal suffers from several types of defects that significantly degrade the photovoltaic performance of solar cells, among which dislocation is the most critical caused by thermal stress. To examine the characteristics of thermal stress and associated fields, therefore, it is significant for the understanding and optimization of the cooling process. In the article, an integrated simulation tool has been developed and used to investigate heat transfer, fluid flow, and thermal stress during the cooling process of mc-silicon. The simulation results were further proved by experimental observations. According to the distortion energy theory, the total strain energy consists of the volumetric strain energy and the shear strain energy, and yield occurs when the shear component exceeds that at the yield point, which is the major cause of the dislocation. Therefore, by analyzing von Mises stress aligned in the direction that has to support the maximum shear load, the regions in the ingot with dislocation generation and multiplication can be evaluated. The displacement indicates the motion of the crystal ingot, and reveals the regions of deformation due to the existence of thermal stress from uneven cooling. Based on the complete investigation of the characteristics of thermal stress and associated displacement, the cooling process could be well comprehended and further optimized with the minimization of dislocation density.

Adsorption Mechanism of Oxide Gas Pollutants by Single-walled Carbon Nanotubes

Carbon nanotubes (CNTs) have exhibited great potentials in removal of toxic gases due to their large-specific surface area, high porosity, hollow structure, and light density. In this paper, adsorption mechanisms of single-walled carbon nanotubes (SWCNTs) trapping oxide gas pollutants are examined using Density Functional Theory (DFT). Both physisorption and chemisorption are determined through investigation of the preferred adsorption sites of the gases at specific nanotubes. According to the calculated electrical field, the physisorption is found relating to the potential well in the internal channel of the nanotube as well as to the rapid switch of the electrical field at the surface of the nanotube. It is further found that a larger diameter of the nanotube leads to a stronger adsorption capacity of the internal channel, but a weaker adsorption ability of the external surface. The chemisorption process is determined from interactions between the frontier orbitals of the gas molecules and those of the nanotubes. Quantitative analysis tells that molecules with a frontier-orbital energy ranging from −0.23 Ha to −0.15 Ha are more likely to be absorbed by the nanotube. A further examination shows that overlap of electron clouds and transfer of electrons occur during chemisorption, and that change of bond length of the gas molecules depends on the number of transferred electrons per bond. The order of adsorption strength is predicted as SO2, SO3, NO, and NO2 sequentially from the view of the binding energy.

Characteristics of melt convection during Kyropoulos sapphire crystal growth

Abstract Melt flows in the Kyropoulos sapphire growth system include Marangoni convection and buoyancy convection. From the seeding stage to the final growth stage, both the area of the free surface and melt height decrease to zero from initial values, and hence the flow pattern in the melt varies during growth. To study systematically melt convection and its influence on the growth process, a non-dimensional parameter is introduced to denote the relative strength of Marangoni convection to buoyancy convection. Temperature and stream function distributions for the different growth stages are simulated. It is found that Marangoni convection is negligible during the early growth. However, it plays an important role in the late growth. The remelting of the crystal surface starts when Marangoni convection becomes dominant, and thus it may be related to this convection mode. The interface shape is found to be changing during the whole growth process, which reduces the industrial productivity of sapphire. Interface convexity, crystal diameter and the non-dimensional parameter are presented as functions of the melt height. Interface convexity is further described as a function of the non-dimensional parameter. The results are consistent with experimentally observed phenomena.

Numerical Study and Optimization of GaN Thin-Film Growth by MOCVD Method

Light-emitting diode (LED) is considered as the “green light” of the twenty-first century, and the white high-power LED is mainly achieved by the excitation of yellow fluorescent powder with GaN-based blue light. Therefore, the quality of GaN films directly influences the reliability, light efficiency and durability of the Led devices. In the paper, a coupled model has been developed and applied on the simulation of transport phenomena and chemical kinetics during the GaN growth process by a vertical metal-organic chemical vapor deposition (MOCVD) reactor. The effects of the carrier gas type, gas flow rate, and the disk rotation rate on the species distributions, GaN deposition rate and temperature field are investigated. The results indicate that nitrogen is better than hydrogen as carrier gas in consideration of the GaN deposition rate, and that, at the current range of the growth parameters, fast crystal rotation rate and gas inlet velocity are favorable for the large deposition rate and for the improvement of the averaged growth uniformity. However, the edge effect becomes more critical. The results also show that an intermediate gas inlet velocity and a fast disk rotation rate are better conditions from an averaged evaluation of the deposition rate and growth uniformity. The analyses have provided important guide for a practical GaN thin-film growth.Copyright

Role of Internal Radiation With or Without Melt Inclusions During Czochralski Sapphire Crystal Growth

Sapphire is the most popular substrate material for GaN thin-film crystal growth during the Light-emitting diode (LED) fabrication. The performance of GaN films is directly influenced by the quality of the substrates. The control of the growth front, i.e., solid /liquid interface, is critical to improve the quality of the sapphire. As a semi-transparent material, sapphire has an intermediate optical thickness, which requires considering of internal radiation for an accurate prediction of temperature field and interface shape. In the paper, a coupled model has been applied on the modeling of transport phenomena during the Czochralski (Cz) sapphire growth. Especially, the role of the internal radiation with or without melt inclusions has been examined carefully by Discrete Ordinates (DO) method. The interface convexity is influenced by the parameters of the models as well as by the melt inclusions. By setting the different optical properties at the inclusion regions, as observed in the experiments, the entrapment of gas or solid inclusions inside the crystal and its effect on the interface shape are examined.Copyright

Numerical Study of von Mises Stress During Continuous Casting of Multicrystalline Silicon With Large-Size Grains

Continuous casting is a promising technique for massive production of multicrystalline silicon (mc-Si). A theoretically advanced study is performed here to investigate the growth of mc-Si with large grain size, which has much higher photoelectric efficiency than normal mc-Si. However, the casting technique results in high thermal stresses due to its inherent features, and limits the photovoltaic applications of mc-Si because of the stress-induced dislocations. For the analysis and optimization of dislocation formation, a computer-aided method has been applied to investigate thermal stress distribution in the growing ingot of continuous casting. The regions of dislocation multiplication are evaluated by comparing von Mises stress to the critical resolved shear stress. It is found that the stress levels are especially high in the regions close to the solid and liquid (S/L) interface, and that the mold wall has a significant effect on the von Mises stress distribution if the billet were attached on the wall. The triple point is better to keep below the mould bottom to avoid its effect during the growth by certain techniques during the industrial production. Parametric studies were further performed to discuss the effects of growth conditions, such as sheath height, environment temperature, and pulling rate on the distribution of the maximum von Mises stress in the billet. The results imply theoretically that multicrystalline silicon with low stress-induced dislocation could be produced by continuous casting with strictly controlled growth parameters.Copyright

To Control Interface Shape and to Reduce Thermal Stress During Cz-Growth of Sapphire Single Crystal

A global modeling is conducted to predict heat transfer, fluid flow, the shape of solid/liquid (S/L) interface, and electromagnetic field during a Radio Frequency (RF)-heated Cz-growth of sapphire single crystal process. The relationships between the convexity of the S/L interface and furnace design/growth parameters are established. Thermal stress distributions of each case are modeled, and the one with the least level of the maximum von Mises stress is proposed. Specific efforts are further made to achieve the flat S/L shape by modifying the furnace design. It is found that a flat interface is crucial for the reduction of thermal stress. Therefore, stress-related defects in sapphire single crystals grown by Czochralski (Cz) method could be optimized from the discussion.© 2012 ASME