Directional Solidification Furnace Research Articles

Schmidt number's impact during the growth of mc-silicon ingot through directional solidification furnace for photovoltaic applications: A computational investigation

The use of multi-crystalline silicon for photovoltaic solar cell applications is highly advantageous due to its low manufacturing costs. In this study, numerical simulations were conducted to investigate mass transport phenomena during the directional solidification of molten silicon for different Schmidt values at the critical Prandtl number of 0.01. The simulations were performed using the diffusion model and convection-conduction equations as the framework for the incompressible Navier-Stokes equation. The finite-volume method was used to carry out computations in a two-dimensional (2D) axisymmetric model. The diffusion flux of impurities (oxygen, carbon, and nitrogen) and dopants (phosphorus and boron) during the mc-silicon growth process was examined for Schmidt numbers Sc=0.5, Sc=5, and Sc=10. It was found that the Schmidt number Sc=0.5 is suitable for producing higher-grade mc-silicon ingots by regulating the production of impurities and dopants. The simulation results obtained in this study may provide a better understanding of the mc-silicon growth process and can be used to improve the production of high-quality mc-silicon ingots for photovoltaic applications.

Experimental determination of solid-liquid interface energy for para- and diamagnetic solid solution in equilibrium with eutectic liquid under a high magnetic field

The solid-liquid interface energy (SLIE) of the Al-Cu and Bi-Cd systems under a high magnetic field (HMF) has been experimentally determined by a Bridgeman-type directional solidification furnace. The equilibrated grain boundary groove shapes for the paramagnetic solid α-Al solution and the diamagnetic solid Bi solution in equilibrium with the eutectic liquid under various HMFs were observed in Al-15.3at%Cu alloy and Bi-50at%Cd alloy. It can be found that the profiles of grain boundary grooves changed significantly under the action of the HMF. The Gibbs-Thomson coefficient and the SLIE for the solid α-Al and solid Bi solution in equilibrium with the eutectic liquid under various HMFs were obtained from obtained grain boundary groove shapes. The results show that the SLIE of the Al-Cu and Bi-Cd alloys increase and decrease under an HMF, respectively. The interaction theory of magnetic dipoles induced by an HMF can be responsible for the above experimental phenomena. Additionally, the results of the differential thermal analysis (DTA) and theoretical analysis indicate that the undercooling of the Al-15.3at%Cu and Bi-50at%Cd alloys increase and decrease under the influence of an HMF, respectively, which is ascribed to the existence of HMF-induced interface energy.

Influence of helium gas flow under the retort bottom to control the impurities in grown mc-Si ingot by DS process for photovoltaic application: Numerical simulation

The quality improvement of the mc-Si ingot is achieved by numerical simulation using the finite volume method. We have done the numerical simulation on a 2D symmetric Directional solidification furnace with helium gas flow at the bottom of the retort. In this process, the first nucleation starts at the bottom centre of the crucible due to spot cooling. The spot cooling process achieves the convex melt crystal interface during the growth process. It will push the impurity from the centre to the peripheral region of the ingot. This spot cooling method results show the acceptable range of the dislocation density and stress of the grown ingot. The melt-crystal interface, thermal stress, dislocation density and impurity formation of spot cooling system grown mc-Si have been analysed and compared with the conventional DS system of the grown mc-Si ingot. It is seen from the results that the LID effect, SiC formation and dislocation density are reduced in the spot cooling system grown mc-Si ingot.

Numerical Analysis of the Influence of Size Upgrading on Oxygen and Carbon Impurities in Casting Silicon

Size upgrading is the main method to increase production capacity and reduce production costs during the directional solidification of silicon ingots. The distributions of oxygen and carbon impurities in G6 and G7 directional solidification furnaces are studied. The simulation results show that the distributions of oxygen and carbon impurities change significantly in the silicon ingot at different growth stages, especially the position of the highest concentration of carbon impurities has shifted. Compared with the G6 furnace, the average concentrations of oxygen and carbon in silicon crystal in the G7 furnace are reduced by 6.7%, and 7.3% respectively. With the growth of silicon crystal, the average concentration of oxygen gradually decreases, while the average concentration of carbon gradually increases.

The impact on mc-Si ingot grown in a directional solidification furnace by partially replacing the susceptor bottom with an insulation material: A numerical investigation

Directional solidification (DS) is the major process for growing the multi-crystalline silicon (mc-Si) ingots with low cost and low power consumption. Here, the investigation of mc-Si growth has been carried out by two-dimensional numerical simulations on the simplified axisymmetric DS furnace. The mc-Si ingots were grown in the DS furnace with conventional susceptor and a furnace with partially replaced susceptor bottom. The susceptor bottom of size 278 mm × 42 mm × 20 mm is partially replaced with an insulation material. The temperature distribution and melt-crystal interface shape of the grown ingots were analysed at 25%, 50% and 75% of the solidification fraction. The von Mises stress and max shear stress were investigated on both the ingots at the end of the DS-Si process. The better quality mc-Si ingot with low power consumption has been attained by the partial replacement of susceptor bottom with insulation block.

Numerical simulation approach to investigate the effect of gas tube design on the impurities distribution and thermal properties of multi-crystalline silicon ingot grown by directional solidification process

Multi-crystalline Silicon (mc-Si) grown by directional solidification (DS) system is considered as a crucial photovoltaic material due to its relatively low cost and high yield. The numerical modelling is very useful for understanding the impurities segregation during mc-Si growth process. In the present paper, numerical simulation is carried out to investigate the impact of Argon gas tube design as shower head. In this Argon gas tube modification act on the non-metallic impurities, melt-crystal interface, temperature distribution and stress levels in the silicon ingot during the different stages of solidification process. The proposed design resulted in remarkable reduction in the back transfer of CO from heater component of the DS furnace. The generated von misses stress is also discussed as the function of different stage of solidification. The results concluded that the stress level at the initial stage of solidification is lower but it is large at above 75 % solidification rate. Also, we found that the modified gas tube design gives better temperature distribution with melt-crystal interface (m-c) shape. The modified argon tube design will be helpful for enhancing the quality of silicon ingot by controlling the chemical reaction between silicon melt and impurity transport.

Influence of Argon Gas Flow Rate on Oxygen and Carbon Impurities Concentration in Multicrystalline Silicon Grown by Directional Solidification Furnace: Numerical and Experimental Investigation

Influence of Argon Gas Flow Rate on Oxygen and Carbon Impurities Concentration in Multicrystalline Silicon Grown by Directional Solidification Furnace: Numerical and Experimental Investigation

Impurity analysis of the effect of partial replacement of retort with an insulation material on mc-silicon grown in directional solidification furnace: Computational modeling

A transient global model has been created for modified directional solidification system. Since the efficiency of the solar cell mainly depends on the quality of the multi-crystalline silicon, it is important to modify the directional solidification system and to optimize the parameters. In multi-crystalline silicon solar cell, the concentration and segregation of impurities and precipitates plays a vital role in the conversion efficiency. Scalar parameters have been used to analyze the concentration and segregation of impurities in the grown mc-Si. The computation was developed to clarify the distributions of oxygen, carbon, nitrogen and silicon carbide. Five cases were discussed whereas except conventional case, all other cases show better results to grow good quality mc-Si ingot for solar cell applications.

Quality improvement of multi-crystalline silicon ingot by the Hot-Zone modification

Directional solidification process is the dominant method for the multi-crystalline silicon wafer production due to the lower production cost, simple operating process and mass production. Numerical simulation is the best tool to understand and enhance the multi-crystalline silicon ingot quality by using the directional solidification process for photovoltaic application. We improved the multi-crystalline silicon ingot quality by adding an additional graphite layer on the hot-zone of directional solidification furnace. We have simulated both modified and conventional directional solidification process and compared the temperature distribution, growth rate and stress distribution of both systems. The modified directional solidification system significantly increases the multi-crystalline silicon ingot quality compared to the conventional directional solidification process grown multi-crystalline silicon ingot. With the modified directional solidification system we have prevented the wall growth during the casting process.

Numerical and experimental investigation on mc-Silicon growth process by varying the Si3N4 coating thickness of crucible

In the present work, the bottom-opening directional solidification technique was adopted to grow multi-crystalline silicon ingot. By varying the thickness of Si3N4 coating on the quartz crucible, the variation in carbon and oxygen concentration in the ingot has been analyzed. The axis-symmetric time-dependent 2D modeling has been done to analyze heat and mass transfer within the directional solidification furnace. We have analyzed the concentration of carbon monoxide, silicon oxide, silicon carbide, carbon and oxygen in the grown mc-Si ingots by varying the Si3N4 thickness. The oxygen and carbon impurities concentration are higher for the Si3N4 coating thickness of 50 μm to 150 μm and lower for 200 μm. The minority carrier lifetimes of the top, middle and bottom wafers have been analysied. The results show that the middle wafer has higher minority carrier lifetime compared to top and bottom wafers.

Effect of Partial Replacement of Retort with an Insulation Material on Mc-Silicon Grown in Directional Solidification Furnace: Numerical Modeling

Effect of Partial Replacement of Retort with an Insulation Material on Mc-Silicon Grown in Directional Solidification Furnace: Numerical Modeling

3D numerical simulation with experimental validation of a traveling magnetic field stirring generated by a Bitter coil for silicon directional solidification process

Silicon is the most widely used raw material in photovoltaic industry; however, the quality of the silicon photovoltaic solar cells depends on the quality of the raw material (i.e. metallurgical silicon or poly-silicon) and the solidification methods used for the production of the silicon ingot from which the solar cells are produced. This study is related to how improve the quality of the final ingot in the directional solidification process; it is necessary to control the impurity segregation of silicon raw material during the processing. This control can be accomplished by adding an electromagnetic Bitter coil which can generate an external traveling magnetic field (TMF) stirring to control the hydrodynamic flow of silicon melt during the solidification process without contaminating it. To carry out this study, we used a Bridgman vertical directional solidification furnace, equipped with a cylindrical Bitter coil stirrer used in order to have the control of the silicon melt convection on the principal parameters of the solidification process, such as the growth rate, the thermal gradient and the natural convection of silicon melt. For the electromagnetic, heat exchange and silicon melt flow modelling, we used 3D numerical Multiphysics coupled models. Parallel to the numerical results we carried out experimental investigations relating to the characterization of the electromagnetic parameters. This study shows a promising effect of the applied traveling magnetic field on the final ingot quality; indeed, we have the ability to control the silicon melt flow which can affect the thermal configuration, the solidification interface shape and the segregation of impurities by changing the electric current input configuration of the Bitter coil stirrer.

EFFECT OF CU CONTENT AND GROWTH VELOCITY ON THE MICROSTRUCTURE PROPERTIES OF THE DIRECTIONALLY SOLIDIFIED AL-MN-CU TERNARY ALLOYS

In this work, influences of composition (Cu content) and growth velocity (V) on the microstructure (dendritic spacing) of Al–Mn–Cu ternary alloys have been investigated. Al–1.9Mn–xCu (x=0.5, 1.5 and 5 wt. %) alloys were prepared using metals of 99.90% high purity in the vacuum atmosphere. These alloys were directionally solidified upwards under various growth velocities (8.3–978 m/s) using a Bridgman-type directional solidification furnace at a constant temperature gradient (7.1 K/mm). Measurements of primary dendrite arm spacing () of the samples were carried out and then expressed as functions of growth velocity and Cu content. Especially, cell-dendritic transition was detected for low growth velocity (41.6 m/s) for alloys containing 0.5 and 1.5Cu. It has been found that the values of  decrease with increasing V and decreasing Cu content. Keywords: Aluminum alloys, Solidification, Cell-dendritic transition, Dendrite arm spacing

Numerical Investigation of Cone Shape Grooved DS Block to Improve the mc–Si Ingot Quality

AbstractMulticrystalline silicon solar cells occupy 62% in crystalline silicon solar cell production. It is grown by the directional solidification process. Solidification control has a vital role in directional solidification process. Cone shape groove is made in the directional solidification block to enhance the outgoing heat flux in the center region than the peripheral region. Five directional solidification furnaces are simulated for making a multicrystalline silicon ingot. First furnace is the conventional furnace, the second furnace has 30 mm × 85 mm groove block, the third furnace has 40 mm × 85 mm groove block, the fourth furnace has 50 mm × 85 mm groove block and the fifth furnace has 60 mm × 85 mm groove block. The von Mises stress in the maximum volume of the conventional and modified grown ingots are below the range of critical value. In conventional case 7% of the ingot volume is above critical stress value and in the modified cases 2.5% of the ingot volume is above critical stress value. If axial and radial temperature gradient is combined in the 50 mm × 85 mm groove block leads to better results.

Transient Simulation on the Growth of Mono-like Silicon Ingot in DS Process Using Crucible with Plano-Concave Bottom for PV Applications

Two-dimensional numerical simulation on axisymmetric directional solidification furnace has been carried out to investigate the growth of mono-like silicon ingot. The silicon ingots were grown in conventional crucible and modified crucibles with Plano-Concave bottom. Four different crucibles with the thickness variation of 5 mm, 10 mm, 15 mm and 20 mm at the bottom center of the crucible were used for the simulation. The thermal field of the grown ingots were analysed during the growth process. The effect of the modification at the crucible bottom on the outward heat flux has been investigated and the melt-crystal interface was analysed during the growth process. The distribution of thermal stress and shear stress in the grown ingots was investigated. The results show that the modified crucible with Plano-Concave bottom gives better melt-crystal interface by enhancing the spot cooling and lead to lower shear stress and lower thermal stress in the grown ingot which can be used for photovoltaic applications.

Effect of Argon Flow on Oxygen and Carbon Coupled Transport in an Industrial Directional Solidification Furnace for Crystalline Silicon Ingots

Transient global simulations were carried out to investigate the effect of argon flow on oxygen and carbon coupled transport in an industrial directional solidification furnace for quasi-single crystalline silicon ingots. Global calculation of impurity transport in the argon gas and silicon melt was based on a fully coupled calculation of the thermal and flow fields. Numerical results show that the argon flow rate affects the flow intensity along the melt–gas surface, but has no significant effect on the flow patterns of silicon melt and argon gas above the melt–gas surface. It was found that the evaporation flux of SiO along the melt–gas surface decreases with the increasing argon flow rate during the solidification process. However, the net flux of oxygen atoms (SiO evaporation flux minus CO dissolution flux) away from the melt–gas surface increases with the increasing argon flow rate, leading to a decrease in the oxygen concentration in the grown ingot. The carbon concentration in the grown ingot shows an exponential decrease with the increase of the argon flow rate, owing to the fact that the dissolution flux of CO significantly decreases with the increasing argon flow rate. The numerical results agree well with the experimental measurements.

Numerical Modeling of Thermal and Flow Field in Directional Solidification Silicon under Vertical Magnetic Field

One of the key issues in the directional solidification (DS) process of multi-crystalline silicon (mc-Si) is to control the melt flow in order to achieve a higher quality of silicon ingot. A steady global model is developed to investigate the effects of vertical magnetic field (VMF) variation in industrial G5 DS furnace. The Solid/Liquid (S/L) interface, thermal and flow field during directional solidification of mc-Si have been simulated. The results show that, with B0 increases, the deflection of S/L interface decreases and melt flow rate is greatly inhibited. Through the analysis of the flow structure, it is found that two small vortexes in melt without B0 become a large vortex as B0 increases. The melt flow distribution is more uniform which is beneficial to obtain a homogeneous distribution of impurities. These results show the VMF can be adjusted to successfully obtain a more uniform flow distribution and a flatter S/L interface shape.

Reduction of Carbon and Oxygen Impurities in mc-Silicon Ingot Using Molybdenum Gas Shield in Directional Solidification Process

Transient global heat transfer simulations are done to analyse the impact of molybdenum shield on carbon and oxygen impurity formation in mc-silicon grown by Directional Solidification (DS) process. Carbon and oxygen are the two main impurities that have a direct impact on affecting the conversion efficiency of the solar cell. The molybdenum shield effectively guides the argon gas flow within the furnace and carries SiO away from the melt free surface and the incorporation of carbon into the melt from the graphite furnace elements is also reduced. The molybdenum shield within the furnace reduces the incorporation of carbon and oxygen concentration in the ingot thereby enhancing the quality of the ingot. For the ingot grown in the conventional furnace, the higher concentration region of carbon ranges between 2.45 × 1017 atom/cm3 to 2.25 × 1017 atom/cm3 and the lower concentration region of carbon ranges between 6.77 × 1016 atom/cm3 to 8.56 × 1015 atom/cm3. For the ingot grown with molybdenum shield, the higher concentration region of carbon ranges between 1.06 × 1017 atom/cm3 to 9.76 × 1016 atom/cm3 and the lower concentration region of carbon ranges between 4.32 × 1016 atom/cm3 to 1.61 × 1016 atom/cm3. Similarly, for the ingot grown in the conventional furnace, the higher concentration region of oxygen ranges between 2.51 × 1017 atom/cm3 to 2.31 × 1017 atom/cm3 and the lower concentration region of oxygen ranges between 6.85 × 1016 atom/cm3 to 7.67 × 1015 atom/cm3. For the ingot grown with molybdenum shield, the higher concentration region of oxygen ranges between 2.49 × 1017 atom/cm3 to 2.25 × 1017 atom/cm3 and the lower concentration region of oxygen ranges between 8.75 × 1016 atom/cm3 to 9.10 × 1015 atom/cm3. Simulations are done for the whole DS growth process for the conventional furnace having no molybdenum shield and for the furnaces having molybdenum shield fixed at three different positions. The molybdenum shield incorporation in DS furnace shows a significant impact on uniformity and reduction of carbon, oxygen impurity distribution in the as-grown mc-silicon ingot.

Comparative analysis of thermal stress induced dislocations in 7 kg, 40 kg and 330 kg multi-crystalline silicon ingots grown by directional solidification

In this paper, transient heat and mass transfer numerical simulations have been done to analyse the variation in thermal stress induced dislocations in 7 kg, 40 kg and 330 kg mc-silicon ingots grown using Directional Solidification (DS) furnace. The thermal parameters such as temperature distribution, principal stress, shear stress and von Mises stress distribution are examined. The effect of ingot size on temperature parameters, solid/melt interface, stress distribution and dislocation generation are investigated to obtain essential information needed for system design. It is observed that with respect to increase in size of the ingot the convexity of solid/melt interface decreases and the uniformity in radial and axial distribution of temperature and thermal stress also increase thereby decreasing the dislocation generation in the grown mc-silicon ingot. It shows that furnace design and ingot size play a crucial role in generation of thermal stress induced dislocation and defects in the grown ingot.

SICAKLIK GRADYENTİNİN MİKROYAPI VE MİKROSERTLİĞE ETKİSİ

Al–6.5Ni–1.5Fe (ağ.) ötektik bileşiminde hazırlanan alaşım sabit katılaştırma hızında (V = 16.57 µm/s) ve beş farklı sıcaklık gradyentinde (G = 3.23–8.52 K/mm) Bridgman Tipi fırın yardımıyla kontrollü doğrusal katılaştırılarak sıcaklık gradyentinin (G) alaşımın mikroyapı (λ) ve mikrosertlik (HV) özelliklerine etkisi araştırılmıştır. Yapılan deneyler sonucunda oluşan mikroyapılar arası mesafe ve mikrosertlik değerleri ölçülerek sıcaklık gradyentinin mikroyapı ve mikrosertlik üzerine etkisi ortaya çıkarılmıştır. Bu fiziksel özellikler arasındaki ilişki ise lineer regresyon analizi ile belirlenerek benzer deneysel sonuçlarla elde edilen veriler kıyaslanmıştır.