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High-performance p-type silicon target materials of Co-doped B and Al elements were produced using Ga and P contaminated upgraded metallurgical-grade silicon (UMG-Si) at the industrial scale. The purity of silicon ingots is above 5.5 N after the directional solidification process, which meets market demand. The segregation behavior of elements and compensation effect on the resistivity are discussed. The effective segregation coefficients of B, Al, Ga, and P for ingot No. 1 were approximately 0.66, 0.14, 0.38, and 0.49, respectively. The segregation coefficients of P, Ga, and Al become larger, the segregation effect tends to become smaller, which is attributed to the doped and contaminated elements that have the recombination effect on the holes and electrons. The distribution of resistivity can be regulated precisely by the compensation difference [NA–ND] along the solidified fraction. The mean resistivity of the ingots is approximately 0.013 Ω cm. Prolonging melting time is conducive to the uniform distribution of doping elements.

Electron beam melting technology can remove volatile impurities from silicon very effectively. However, the strong cooling capacity of water-cooled copper crucible brings high energy consumption, which confines the wide application of this technology. As far as energy utilization is concerned one can see in this paper, the refining process of silicon is improved based on numerical simulation and experimental results. The energy efficiency is increased through using a graphite lining between silicon and the bottom of water-cooled copper crucible. The results show the addition of graphite lining is resulting in increased thermal resistance and the heat loss is reduced. Compared with traditional electron beam melting technology, the temperature of the melt pool is increased at the same power accelerating purification process. The experiment results show that refining cycle time for same amount of material can be reduced while keeping same purification efficiency. As a consequence, the energy consumption is reduced by 33%.

Arsenic is one of the main impurity elements in silicon. Electron beam melting (EBM) is considered an effective method for the purification of silicon. The removal of arsenic from silicon is performed by electron beam solidification furnace in this paper. The results demonstrated that arsenic could be removed effectively by EBM process with the average removal efficiency up to 95.4% and show the morphology of silicon ingot is less essential to distribution of arsenic impurities in the silicon ingot. The evaporation behavior of arsenic atoms is more important in the process of arsenic removal by EBM. The relationship between the evaporation coefficient of arsenic impurities and temperature was deduced. It provides a reference for further research on removal arsenic in silicon by EBM.

n-type multicrystalline silicon ingot with a doping element of phosphorus was produced by directional solidification under industrial conditions. The evaporation and distribution of phosphorus were studied by content analysis in different positions. The results show that it is mainly controlled by melting and solidification processes. During the melting process, the content of phosphorus reduced from 16.6 and 6.7 ppmw to 11.1 and 6.0 ppmw under different melting times, respectively. The evaporation of phosphorus is mainly affected by the temperature. During the solidification process, the segregation of phosphorus was promoted by evaporation. A formula similar to the Scheil equation was established to calculate the longitudinal distribution of phosphorus in silicon. The results show that its longitudinal distribution is influenced by the evaporation coefficient, the solidification rate, and the temperature gradient. The effective segregation coefficient of phosphorus in ingots a and b was 0.31 and 0.33, respectively, which could be obtained from the fitting result. During the whole process, the phosphorus removal efficiency in ingots a and b was 60.8% and 50.5%, respectively. Its horizontal distribution is related to the morphology of the solid–liquid interface.

High-performance multicrystalline silicon (HPMC-Si) wafers were produced using upgraded metallurgical-grade silicon (UMG-Si) materials in the seed-assisted growth system at the industrial scale. The HPMC-Si wafers yielded low dislocation density and fine and uniform grain size. We observed that fine grain size suppressed the segregation effect of metal impurities. The effective segregation coefficients of Fe, Al, and total metal impurities approximated 0.265, 0.492, and 0.386, respectively. The concentration of impurities within 10–90% of the solidified fraction in the ingot was relatively uniform based on the improved crystal structure control. The heterogeneous nucleation mechanism of concave and planocera nucleation was discussed intensively. HPMC-Si wafers were obtained under the crystal structure control coupled with behavior regulation of impurity segregation in the seed-assisted growth system. The average conversion efficiency of Al-BSF processed solar cells reached 18.65%.

Two 450 kg multicrystalline silicon ingots were obtained by mixing and melting high purity silicon and silicon from the edges and the bottom of the casted ingots together in a casting furnace. For one of the ingot, the silicon from casted ingots was refined in an electron beam melting furnace to remove oxygen. The oxygen content was reduced from 10 to less than 0.0517 ppmw when silicon was refined at 500 kW with removal efficiency up to 99.429% in the most areas. The life time of the ingot after oxygen removal was measured to be far better than another one, whereas in the central parts the value was almost 6.7 µs. The efficiencies of both solar cells were initially 17.55% but after 4 h decreased to 17.05% and 15.55%, respectively. The solar cell after oxygen removal shows a better performance in degradation.

Carbon and their compounds were removed successfully through electron beam melting (EBM), so that those areas (contaminated with carbon) of ingot were recycled and reused. During EBM process, the numerical simulation results show that there is great temperature gradient existing in the melt. During EBM, the melt near copper crucible shows low temperature and bad fluidity. Carbon in silicon melt flows, precipitated and gathered in this area so that it is separated. The flow mechanism of SiC in silicon melt was investigated. After EBM, carbon enriches in the form of SiC at the bottom of the ingot but not in the center. This technology is applied on industrial scale EBM equipment. The results show that a majority of SiC was deposited in the bottom of the refining crucible and the carbon contaminations are not found in the most of the area of the solidified ingot in crucible.

A new method for SiC elimination from multicrystalline silicon with Al addition by using the chemical reaction between Al and SiC is conducted in this study. Inclusion-free region is obtained when Al addition content increases to 5.4% and 7.9%. SiC particles tend to pile up around Al during the process and precipitate at ingot bottom. Furthermore, Al-Si alloying occurs at the inclusion-free region. The electron probe micro analysis mapping illustrates the existence of carbon in the precipitated Al which indicates the occurrence of chemical reaction between SiC particles and Al during the process. The results can be an effective way to eliminate SiC for silicon scraps recycling.

Boron (B) removal by slag refining using Na2O-SiO2 was investigated in industrial applications. The experimental results showed that the reasonable ratio range of slag to silicon is about 0.7–0.8; the suitable holding time is about 30 min; the concentration of B is reduced from 1.90 ppmw to 0.17 ppmw by three times slag refining; and the removal efficiency of B reaches 91.1%. Moreover, it is discussed that B in silicon is more inclined to be oxidized by Na2O than SiO2 according to thermodynamic analysis and then volatilized to the atmosphere in the form of Na2B2O4 according to kinetic analysis.

A multicrystalline silicon ingot with columnar and irregular grains was obtained from metallurgical-grade silicon (MG-Si) by directional solidification. The segregation behaviors of substitutional and interstitial impurities in different grain morphologies have been studied. The concentration distribution of substitutional impurities (B and Al) in the silicon ingot was accord with the Scheil's equation, which depended on the grain morphology. However, the concentration distribution of interstitial impurities (Fe, Ti, Cu, and Ni) was only accord with the Scheil's equation under the columnar grains growth condition. The difference lattice sites of the impurities will result in the disparate segregation behavior of impurities for columnar and irregular grains growth, which leads to the diverse concentration distribution of substitutional and interstitial impurities in the silicon ingot. Furthermore, the transport mechanism of interstitial and substitutional impurities in front of the solid-liquid interface boundary has been revealed.

A large amount of silicon in the form of silicon scraps is wasted annually in photovoltaic industry because of the presence of silicon carbide inclusions. Silicon carbide inclusions must be eliminated so that silicon scraps can be recycled. This study investigated the electromagnetic separation of silicon carbide inclusions in multicrystalline silicon by imposition of supersonic-frequency magnetic field. Silicon carbide particles tend to migrate to the ingot side and to the top and bottom of the central region of the ingot under increasing holding time. During separation process, the penetrating aluminum can react with silicon carbide and then dissolve some of the inclusions. In this work, the separation efficiency reaches up to 89.3% at a holding time of 45 min. This method is a potentially effective means of recycling silicon scraps.

A silicon target material with the purity of 99.999 wt% (99.999%) was prepared by adding Al–B master alloy in directional solidification. The segregation behavior of the dopant and the effect on the resistivity were studied in this work. It was revealed that the AlB2 particles in the Al–B master alloy will generate the clusters of [B] and [Al] in molten silicon at 1723 K spontaneously. The concentrations of B and Al were increasing gradually along the solidified fraction in the silicon ingot. The measured values of B were in good agreement with the curve of the Scheil’s equation below 85% of the solidified fraction. The measured values of Al were fitting well with the curve of the Scheil’s equation when the effective segregation coefficient is 0.00378. It was found that the resistivity of the silicon target material was regulated by B co-doped Al simultaneously in directional solidification.

The development of photovoltaic industry demands great amount of multicrystalline silicon. Carbon and SiC in silicon need to be contained in a limited amount since they can cause great adverse affect to solar cells. The behavior of carbon and its precipitation SiC in silicon by electron beam melting (EBM) with a slow cooling pattern was investigated in this study. SiC is found to sedimentate to ingot bottom after EBM. The presence of Si 3 N 4 can be heterogeneous nucleation agent for SiC to nucleate continually and both of them precipitate to the ingot bottom. The comprehensive effect of slow solidification condition, temperature gradient and melt convection causes the sedimentation of SiC. It is also found that oxygen plays an important role on the migration of the dissolved carbon. The formation of carbon-oxygen complexes tend to migrate to ingot top since oxygen can transfer from silicon melt to vacuum environment during EBM.

Multicrystalline silicon ingots without and with alternating magnetic field during directional solidification process under industrial system were obtained from metallurgical grade silicon (MG-Si). The concentrations and normalized concentrations of metal impurities in the two silicon ingots were studied. The result shows that the concentrations and normalized concentrations in high-purity area of the silicon with alternating magnetic field are lower than those of the ingot without alternating magnetic field. The transport mechanism for metal atoms in the diffusion layer area has been changed due to the alternating magnetic field. Alternating magnetic field introduces a convection to reduce the thickness of diffusion layer in the molten silicon, which results in a decreased effective segregation coefficients. Enhancing transport driving force of metal atoms in molten silicon is the effective way to improve the removal rate of metal impurities.

Redistribution of iron during directional solidification of metallurgical-grade silicon (MG-Si) was conducted at low growth rate. Concentrations of iron were examined by ICP-MS and figured in solid and liquid phases, at grain boundary and in growth direction. Concentrations are significantly different between solid and liquid phases. The thickness of the solute boundary layer is about 4 mm verified by mass balance law, and the effective distribution coefficient is 2.98×10−4. Iron element easily segregates at grain boundary at low growth rate. In growth direction, concentrations are almost constant until 86% ingot height, and they do not meet the Scheil equation completely, which is caused by the low growth rate. The effect of convection on the redistribution of iron was discussed in detail. Especially, the “dead zone” of convection plays an important role in the iron redistribution.

The recycling of Si scraps has become a serious concern in the industry because of the increasing amount of Si waste generated during ingot production. In this work, an operative method is proposed to process Si scraps and separate SiC from Si by coupling electromagnetic field melting with Al/SiC wetting and interaction. This method yields an inclusion-free SiC particle-enriched region that tends to migrate to the top of ingots with increasing melting time. A micron-grade SiC-rich layer is formed at the ingot edge. A petal-shaped SiC particle group is formed by Al addition and precipitates at the bottom of the ingot. A separation efficiency of 88.1% is obtained after melting for 45 min. The results show that the proposed process is an effective way to eliminate SiC from recycled Si scrap.

The behavior of carbon in multicrystalline silicon scraps by electron beam melting was investigated in this study. It was found that the process favors nucleation of SiC on Si 3 N 4. Furthermore, carbon tends to gather to top surface of the ingots with increasing melting time, and the reaction between oxygen and carbon favors carbon migration. The melt convection and temperature gradient caused by electron beam are employed to be the dominate reason the phenomenon occurs. The results can provide guidance in silicon recycling.

A theoretical model for investigating the back diffusion of iron impurity during silicon purification by vacuum directional solidification is proposed in this paper. The back diffusion of iron impurity in directional solidification process can markedly reduce the purifying effect; thus, clarifying the back-diffusion behavior of iron impurity has great significance in silicon purification by directional solidification. Results show that the influence of back diffusion becomes more obvious with increased solidification proportion. At the end of solidification, the degree of back diffusion relative to Scheil equation calculation reaches nearly 200%. Experimental verification reveals that the calculation that considers back diffusion is more consistent with experimental results than when back diffusion is ignored. Thus, the distribution of iron impurity during silicon purification by directional solidification can be more accurately calculated or predicted when back-diffusion is considered in solidification.

This research deals with the study of electron beam melting (EBM) methodology utilized in melting silicon material and subsequently discusses on the effect of oxygen level on electrical resistivity change after EBM process. The oxygen content was reduced from 6.177 to less than 0.0517 ppmw when refining time exceeded 10 min with removal efficiency of more than 99.08%. The average value of electrical resistivity of silicon before EBM processing was recorded to be 2.25 Ω cm but with the increase in melting time that was applied through EBM, the electrical resistivity was recorded to go high in the range of 4–13 Ω cm for different regions. The electrical resistivity values were greater in the top and the bottom regions, whereas lowest in the central region at all conditions of melting time. It is the result of the evaporation of oxygen during melting process and the segregation of metal impurities during solidification.

Aluminum is one of the main impurities in silicon, which can be separated and eliminated by electron beam melting. However, high removal efficiency can be obtained only by increasing melt temperature or extending refining time, resulting in high energy consumption. In this work, the directional solidification of silicon was achieved by electron beam with exponential decreasing power, considering that aluminum has both characteristics of segregation and evaporation. The distributions of aluminum show increasing trend from the bottom to the top of the electron beam melted silicon ingot, which is the same as that after traditional directional solidification. The removal efficiency is improved by the coupling of segregation and evaporation. Compared with traditional electron beam melting, the loss of silicon reduced by more than 52% and the energy consumption reduced by more than 54%. This method is more effective to remove aluminum from silicon with low energy consumption.

A theoretical model to determine distribution of iron impurity during silicon purification by directional solidification with fluctuant crystal growth rate is proposed in this paper. The crystal growth rate is fluctuant usually and it has profound effect on the distribution of iron impurity in practical production. The model validation by the distribution of iron impurity during silicon purification by directional solidification in industrial production and the results show that the calculation agrees with existing experimental results. The results also indicate that distribution of iron impurity is directly correlated with the instantaneous value of crystal growth rate. The high fluctuant distribution of iron impurity during silicon purification by directional solidification can be well explained. Many potential applications of the model in practical production are found, such as predicting the distribution of iron impurity with fluctuant crystal growth rate, evaluating the effect degree of production accident, design and optimization of the process parameters and evaluating of maximum yield for raw silicon with different impurity concentration. Silicon purification with low energy consumption is possible based on the research in this paper.

Purification process of MG-Si was limited by the difficulties involved in reducing boron content. The possibility of removing impurity boron (B) in MG-Si using CaO–Al 2 O 3 –SiO 2 –CaF 2 slags was investigated in the present study. Different from traditional research methods, the whole melting process was carried out under atmosphere condition, which was closer to the actual production conditions. The thermodynamic and kinetic mechanisms of B removal were analyzed. Based on the ionic structure theory, the relation between ionic structure of slags at high temperature and optical basicity can be explained firstly. The parameters, including the slag optical basicity, melting time and CaF 2 content were discussed. The boron content was reduced from original 25 ppmw to 4.4 ppmw through 120 min melting with the optical basicity of 0.551 and 5 wt% CaF 2 additions.

A multicrystalline silicon ingot was obtained from metallurgical-grade silicon by vacuum induction melting and directional solidification. Based on the concentration distributions of aluminum and calcium along the growth direction, the removal mechanism of such impurities with both high saturated vapor pressures and low segregation coefficients is investigated. The results show that the removal of this type of impurities only depends on evaporation during vacuum induction melting process, thus their contents decrease significantly due to the strongly evaporation under the high temperature and high vacuum conditions. During subsequent directional solidification process, a model including both segregation and evaporation is used to simulate the concentration distribution. The results show that the impurity distribution is controlled by both two mechanisms in the initial stage of solidification and is mainly determined by segregation in the end stage due to the decrease of the diffusibility and evaporability of the impurity atoms.

Silicon is widely used as a raw material for production of solar cells. As a major impurity in silicon, phosphorus must be removed to 1 × 10−5 wt.%. In the present study, based on the distribution of phosphorus in a silicon ingot obtained by vacuum refining and directional solidification, the mechanism for removal of phosphorus from silicon is investigated. The results show that the distribution is controlled not only by segregation at the solid–liquid interface but also by evaporation at the gas–liquid interface, showing some deviation from Scheil’s equation. A modified model which considers both segregation and evaporation is used to simulate the distribution, matching quite well with the experimental results. The temperature and solidification rate are two important parameters that affect the overall mass transfer coefficient and the effective segregation coefficient and thus the distribution of phosphorus. A high removal efficiency and a homogeneous distribution can be obtained by adjusting these two parameters.

The purification of metallurgical grade silicon, especially the removal of aluminum, was investigated by electron beam melting and solidification. Small amounts of silicon raw materials were melted in an electron beam furnace with same melting time and different solidification time to obtain the distribution of Al in silicon ingot. The removal mechanisms in different stages were also discussed. The results show that the removal of Al during melting process only depends on evaporation and that during solidification process depends on both segregation and evaporation. The distribution of Al shows an obvious increasing trend from the bottom to the top of the silicon ingot when solidification time is 600 s. The removal efficiency in most area is close to that in the ingot solidified instantaneously, but the energy consumption is less, which is considered to be an effective way for the purification of silicon.

According to the traditional metallurgical theory, the evaporation process of phosphorus and silicon during silicon refining by electron beam melting (EBM) is discussed and a theoretical model is established to obtain the loss rate of silicon, the removal efficiency of phosphorus and the corresponding energy consumption. The results show that phosphorus can be removed from silicon melt efficiently and quickly by EBM. There is a one-to-one correspondence between the loss of silicon and the removal efficiency of phosphorus, indicating that they have obvious effect on each other, whereas the EB power has little influence on the loss rate of silicon. If the EB power is increased from 9 kW to 21 kW, the melting time can be shortened by 68%, the loss of silicon increased by only 0.1% and the energy consumption decreased by 25%. Based on the theoretical and experimental results, a high-power EBM method is considered to be a better way for the removal of phosphorus with high efficiency and low energy consumption under such experiment conditions.

Solidification rate has a significant influence on purification of silicon due to segregation of impurities at a liquid–solid interface of a solidifying silicon ingot. A mathematical model is developed to evaluate time-dependent position of the liquid–solid interface and solidification rate of electron beam melted ingots. A series of solidification experiments with different cooling rates are conducted to measure position of a line which separates directionally grown columnar crystals visible in cross-sections of the solidified ingots. Results show that not the whole ingot solidifies directionally when the reduction rate of the beam current is larger than 1.67 mA/s. The position of the dividing line depends on cooling rate and the experimental trend is consistent with that resulted from theoretical simulations. Modeling shows that the solidification rate changes fast when the beam current reduces linearly that is detrimental for segregation of impurities. It also predicts that an exponential reduction of the beam current leads to a uniform solidification rate which is beneficial to segregation of impurities, though not all exponential current reductions lead to this kind of solidification behavior.

During the refinement of metallurgical-grade silicon (MG-Si), Al and Ca cannot be reduced to a target level by conducting directional solidification only once. Electron beam candle melting (EBCM) is a new method that can be used to further reduce Al and Ca by directly melting Si ingots after directional solidification. A certain stable molten pool with a maximum surface area and a minimum depth exists and is proven to effectively remove Al and Ca. The energy utilization ratio during EBCM is also compared with that of electron beam melting (EBM).

Small amounts of metallurgical grade silicon were melted in an electron beam furnace in different experimental conditions in order to investigate the aluminum (Al) evaporation behavior during the electron beam melting (EBM) process. Impurity was significantly decreased in the early periods of melting at 9, 15, and 21 kW. These changes slowed down with the extension of the melting time. Moreover, the removal reaction of Al by evaporation from molten silicon during the EBM process occurred in accordance with the first order kinetics. The calculated mass transfer coefficients of Al at 1941, 1964, and 2051 K increased with the increase of melting temperature. The removal rate of Al was controlled by the transportation of Al from the bulk of silicon metal to the molten/vacuum interface within the range of the experimental temperature.

The purification of metallurgical-grade silicon (MG-Si) has been investigated during electron beam melting (EBM) process. The results show that the phosphorus, calcium and aluminum contents decrease significantly after melting, and magnesium is partially removed. However, no significant change in content for boron and iron has been found. Langmuir’s equation and Henry law were used to derive the removal efficiency for each impurity element. The free surface temperature was estimated by the Hertz-Knudsen-Langmuir equation and silicon’s vapor pressure equation. Good agreement was found between measured and calculated impurities’ removal efficiency for phosphorus, calcium and aluminum, magnesium, boron and iron. The deviation between the two results was also analyzed in depth.

In order to investigate Ca evaporation behavior in the electron beam melting process, metallurgical-grade silicon was melted in an electron beam furnace with different experimental conditions. The results showed that the content of impurity Ca was significantly decreased in the early time, while these changed slowly with the extension of the melting. The removal rate of Ca was controlled by the transfer of Ca atoms from the bulk liquid silicon to liquid/gas phase interface within the range of experiment temperature.

The effects of melting condition on the dephosphorization behavior in metallurgical grade silicon (MG-Si) by electron beam melting (EBM) were investigated in this study. Experiment results showed that phosphorus content decreased by evaporation with the increase of melting time, and evaporation rate increased with the increase of electron beam power. Phosphorus content decreased to below 0.1 ppmw by EBM treatment at 21 kW for 1800 seconds. The dephosphorization reaction was found to follow the first order kinetics. Dephosphorization rate was controlled by free evaporation.

The electron beam injection (EBI) process involves offering electrons around silicon powder, whose surface was oxidized, and subsequently the powder is washed by HF acid so as to remove the SiO2 film. The new electron beam injection process, in which micro electric filed formed between Si and SiO2 film will accelerate impurities diffusion from Si to SiO2 film, was developed and applied to eliminate the transition-metal impurities of MG-Si. It is proved to be effective to remove transition-metal impurities from metallurgical grade silicon (MG-Si). By applying the electron beam injection method, the removal rate of 10% to 59% was achieved during the refining process. The efficiency of impurity removal originates from two aspects: the impurity concentration gradient on both sides of Si/SiO2 interface; the micro electric field formed from Si to SiO2 film. A further increase in the removal rate can be realized by controlling the processing parameters.

The growth in the solar energy technology caused inshortage solar grade Si. As a lowcost, environmental friendly technology, metallurgical method purity silicon is developed significantly. However, as a typical impurity in Si, B is difficult to be removed by directional refining or vacuum melting due to its large segregation coefficient and less evaporation coefficient. In this paper, the big difference of evaporation pressure between Si and B can be applied to separate B from Si, in which, B is remained in molten Si, while most of Si becomes evaporant. Electron beam is applied to scan molten Si and the Si existed in the form of the evaporant is gather on the watercooled crystallizer. The content B in the evaporant is undetectable by ICP-MS.

An experimental investigation into the removal of aluminum (Al) and calcium (Ca) from molten silicon by using electron beam melting was carried out. Based on the distributions of Al and Ca along the growth direction of the ingot under different solidification conditions, the influence of segregation and evaporation behaviors on the removal of such impurities with both high saturated vapor pressure and low segregation coefficients was investigated. The results showed that the distributions of impurities depend upon the interaction between segregation and evaporation, so that the removal efficiency can be further improved by adjusting the melting parameters. Compared with the traditional electron beam melting process, the energy consumption decreases by 20% during the whole melting and solidification process. It is considered to be a more effective way for the purification of silicon and the reduction of energy consumption by electron beam melting.