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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.

A kinetics model for silicon evaporation is established in order to investigate the dependence of temperature and chamber pressure on silicon loss. The results show that the rate-limiting step is mass transfer of silicon in gas phase at low temperature; while it transits to free evaporation on melt surface as the temperature rises. As for electron beam melting process, silicon free evaporation is considered as the rate-limiting step due to the extremely high temperature of the melt. The relationship between impurity removal efficiency and silicon loss is obtained according to this calculation model. The silicon loss ratio is less than 10% when the removal efficiency of phosphorus is close to 100%, but that of aluminum is still low, indicating this technology is more effective for the removal of phosphorus. Aluminum can be further removed by increasing temperature or prolonging refining time, but leading to more silicon loss.

Electron beam melting was utilized to investigate the behavior of carbon flow by melting 100 g of multi-crystalline silicon in an electron beam furnace for five minutes. Carbon and nitrogen are the constituent impurities in contaminated Si samples with an average weight of 13 % and 9, respectively. The electron beam melting experiment caused redistribution of the impurities along the periphery and bottom of the Si sample with a pie-shaped structure. Investigations through scanning electron microscopy and energy dispersive X-ray spectroscopy confirmed that the impurities were silicon nitride and silicon carbide. It was determined that Si3N4 has a rod-shaped microstructure, whereas SiC has a granular morphology. By segregating the impurities redistributed through this technique, pure Si was obtained in the remaining sample.

Removal of boron from silicon is a tough task by traditional directional solidification and vacuum refining techniques, due to its large and inappropriate segregation coefficient and low saturated vapor pressure. At high temperature boron react with oxygen to form volatile boron oxides which can be evaporated. So, the removal procedure of boron from silicon melt is investigated by incorporating a small amount of water vapor above the melted surface. The results show that boron is oxidized to mainly form BO and evaporated with 28% removal efficiency by average. It is considered that oxygen atoms experience a series of physical and chemical processes, such as a chemical reaction in the bulk of the melt, evaporation from the melt surface, transportation across the gas phase and ionization due the electron beam, which is conducive to the continuous removal of boron.

There are currently ten COVID-19 vaccines being announced their preliminary efficacies from phase 3 clinical trial, and nine of them have been authorized for emergency use or conditional licensed, which brings some issues to present placebocontrolled efficacy trial of other COVID-19 vaccines. The approval of “first wave” COVID-19 vaccines raises concerns about the administration of a placebo in ongoing and future phase 3 trials of COVID-19 vaccine candidates. Comprehensive efficacy assessment strategy for the next steps is now required. This perspective covers challenges for ongoing COVID-19 vaccine clinical studies and alternative clinical study designs in the future, under the placebo use being acceptable or unacceptable circumstances, respectively, in order to ensure the safety, efficacy and effectiveness evaluation of COVID-19 candidate vaccines pre- and post-licensure.

Summary Background This is the first randomised controlled trial for assessment of the immunogenicity and safety of a candidate non-replicating adenovirus type-5 (Ad5)-vectored COVID-19 vaccine, aiming to determine an appropriate dose of the candidate vaccine for an efficacy study. Methods This randomised, double-blind, placebo-controlled, phase 2 trial of the Ad5-vectored COVID-19 vaccine was done in a single centre in Wuhan, China. Healthy adults aged 18 years or older, who were HIV-negative and previous severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection-free, were eligible to participate and were randomly assigned to receive the vaccine at a dose of 1 × 1011 viral particles per mL or 5 × 1010 viral particles per mL, or placebo. Investigators allocated participants at a ratio of 2:1:1 to receive a single injection intramuscularly in the arm. The randomisation list (block size 4) was generated by an independent statistician. Participants, investigators, and staff undertaking laboratory analyses were masked to group allocation. The primary endpoints for immunogenicity were the geometric mean titres (GMTs) of specific ELISA antibody responses to the receptor binding domain (RBD) and neutralising antibody responses at day 28. The primary endpoint for safety evaluation was the incidence of adverse reactions within 14 days. All recruited participants who received at least one dose were included in the primary and safety analyses. This study is registered with ClinicalTrials.gov, NCT04341389. Findings 603 volunteers were recruited and screened for eligibility between April 11 and 16, 2020. 508 eligible participants (50% male; mean age 39·7 years, SD 12·5) consented to participate in the trial and were randomly assigned to receive the vaccine (1 × 1011 viral particles n=253; 5 × 1010 viral particles n=129) or placebo (n=126). In the 1 × 1011 and 5 × 1010 viral particles dose groups, the RBD-specific ELISA antibodies peaked at 656·5 (95% CI 575·2–749·2) and 571·0 (467·6–697·3), with seroconversion rates at 96% (95% CI 93–98) and 97% (92–99), respectively, at day 28. Both doses of the vaccine induced significant neutralising antibody responses to live SARS-CoV-2, with GMTs of 19·5 (95% CI 16·8–22·7) and 18·3 (14·4–23·3) in participants receiving 1 × 1011 and 5 × 1010 viral particles, respectively. Specific interferon γ enzyme-linked immunospot assay responses post vaccination were observed in 227 (90%, 95% CI 85–93) of 253 and 113 (88%, 81–92) of 129 participants in the 1 × 1011 and 5 × 1010 viral particles dose groups, respectively. Solicited adverse reactions were reported by 183 (72%) of 253 and 96 (74%) of 129 participants in the 1 × 1011 and 5 × 1010 viral particles dose groups, respectively. Severe adverse reactions were reported by 24 (9%) participants in the 1 × 1011 viral particles dose group and one (1%) participant in the 5 × 1010 viral particles dose group. No serious adverse reactions were documented. Interpretation The Ad5-vectored COVID-19 vaccine at 5 × 1010 viral particles is safe, and induced significant immune responses in the majority of recipients after a single immunisation. Funding National Key R&D Programme of China, National Science and Technology Major Project, and CanSino Biologics.

Summary Background A vaccine to protect against COVID-19 is urgently needed. We aimed to assess the safety, tolerability, and immunogenicity of a recombinant adenovirus type-5 (Ad5) vectored COVID-19 vaccine expressing the spike glycoprotein of a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) strain. Methods We did a dose-escalation, single-centre, open-label, non-randomised, phase 1 trial of an Ad5 vectored COVID-19 vaccine in Wuhan, China. Healthy adults aged between 18 and 60 years were sequentially enrolled and allocated to one of three dose groups (5 × 1010, 1 × 1011, and 1·5 × 1011 viral particles) to receive an intramuscular injection of vaccine. The primary outcome was adverse events in the 7 days post-vaccination. Safety was assessed over 28 days post-vaccination. Specific antibodies were measured with ELISA, and the neutralising antibody responses induced by vaccination were detected with SARS-CoV-2 virus neutralisation and pseudovirus neutralisation tests. T-cell responses were assessed by enzyme-linked immunospot and flow-cytometry assays. This study is registered with ClinicalTrials.gov, NCT04313127. Findings Between March 16 and March 27, 2020, we screened 195 individuals for eligibility. Of them, 108 participants (51% male, 49% female; mean age 36·3 years) were recruited and received the low dose (n=36), middle dose (n=36), or high dose (n=36) of the vaccine. All enrolled participants were included in the analysis. At least one adverse reaction within the first 7 days after the vaccination was reported in 30 (83%) participants in the low dose group, 30 (83%) participants in the middle dose group, and 27 (75%) participants in the high dose group. The most common injection site adverse reaction was pain, which was reported in 58 (54%) vaccine recipients, and the most commonly reported systematic adverse reactions were fever (50 [46%]), fatigue (47 [44%]), headache (42 [39%]), and muscle pain (18 [17%]. Most adverse reactions that were reported in all dose groups were mild or moderate in severity. No serious adverse event was noted within 28 days post-vaccination. ELISA antibodies and neutralising antibodies increased significantly at day 14, and peaked 28 days post-vaccination. Specific T-cell response peaked at day 14 post-vaccination. Interpretation The Ad5 vectored COVID-19 vaccine is tolerable and immunogenic at 28 days post-vaccination. Humoral responses against SARS-CoV-2 peaked at day 28 post-vaccination in healthy adults, and rapid specific T-cell responses were noted from day 14 post-vaccination. Our findings suggest that the Ad5 vectored COVID-19 vaccine warrants further investigation. Funding National Key R&D Program of China, National Science and Technology Major Project, and CanSino Biologics.

Background: We aimed to assess the safety, tolerability and immunogenicity of a recombinant adenovirus type 5 (Ad5) vectored COVID-19 vaccine expressing the spike glycoprotein of a SARS-Cov-2 strain. This is the first-in-human study of a candidate vaccine against COVID-19. Methods: We conducted a single-center, open-label, dose-escalating clinical trial of Ad5 vectored COVID-19 vaccine. Healthy adults aged between 18-60 years were sequentially enrolled and allocated to receive a single intramuscular injection in one of three dose groups: 5 × 10^10, 1×10^11, and 1·5×10^11 viral particles. Safety was assessed over the next 28 days. Specific antibodies were measured on enzyme-linked immunosorbent assay (ELISA), and the neutralizing antibody responses induced by vaccination were detected by using SARS-CoV-2 virus neutralization and pseudovirus neutralization tests. T-cell responses were accessed by enzyme-linked immunospot (ELISpot) and flow-cytometry assays. Results: A total of 108 participants were recruited and received low dose, middle dose, or high dose vaccine, with 36 in each dose group. 30(83·3%), 30(83·3%), and 27(75·0%) recipients in the low dose, middle dose, and high dose groups reported at least one adverse reaction within the first 7 days after the vaccination. The most common injection-site adverse reaction was pain, the most commonly reported systematic adverse reactions were fever, fatigue, headache, and muscle pain. A majority of the adverse reactions that were reported in all dose groups were mild or moderate in severity. No serious adverse event was noted within 28 days post-vaccination. Both ELISA antibodies and neutralizing antibodies increased significantly at day 14, and peaked 28 days post-vaccination. Specific T cell response peaked at day 14 post-vaccination. Conclusions: The Ad5 vectored COVID-19 vaccine is tolerable and immunogenic. Humoral responses against SARS-CoV-2 peaked at day 28 post-vaccination in health adults, and rapid specific T cell responses were noted since day 14. Trial Registration: The study is registered with ClinicalTrials.gov, number NCT04313127. Funding Statement: National Key R&D Program of China (2020YFC10841400), National Science and Technology Major Project (2016ZX10004001, 2018ZX09201005), and CanSino Biotechnology Inc. Declaration of Interests: Mr. Gou report being employees of Tianjin CanSino Biotechnology Inc, No other potential conflict of interest relevant to this article was reported. Ethics Approval Statement: The protocol and informed consent were approved by the Institutional Review Board of the Jiangsu Provincial Center of Disease Control and Prevention. Written informed consents from all participants were obtained before screening. This study was undertaken by Jiangsu Provincial Center of Disease Control and Prevention, Hubei Provincial Center for Disease Control and Prevention and Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology in accordance with the Declaration of Helsinki and Good Clinical Practice.

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.

The diffusion layer thickness of impurity during directional solidification process was evaluated under different temperature gradient conditions. The thickness of Fe, Cu, Ni and Ti in silicon ingot under 9.74 K/m were 6.19 mm, 4.92 mm, 4.61 mm and 8.70 mm, respectively; The thickness of Fe, Cu, Ni and Ti in silicon ingot under 28.49 K/m were reduced to 3.35 mm, 1.21 mm, 1.85 mm and 5.81, respectively. The diffusion layer thickness was reduced by the large temperature gradient to lead a low effective segregation coefficient. As a result, the impurity concentration of silicon ingot under 28.49 K/m was reduced to 0.94 ppmw. The strong vortex in the solid-liquid boundary interface was enhanced by the large temperature gradient, as the main transport mechanism to accelerate the diffuse of impurity atom in the diffusion layer，which effectively reduced the thickness of diffusion layer.

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.

Uniform doping of phosphorus (P) in silicon is always the key and difficult point in the study of n-type solar cells. This paper aims to study the influencing factors for horizontal distribution for P. It is found that the distribution of P in silicon ingot is the results of the combined effect of melt flow and solid-liquid (S/L) interface. With numerical simulation, the melt flow rate in the edge is lower than that of the middle area, which leads to a higher effective segregation coefficient (Keff). For the slightly concave S/L interface, the concentration of P in the molten silicon is lower when the solidification process began at the edge position. The higher Keff and lower concentration make the slightly concave interface has better performance in uniform doping at the early stage of growth. In addition, with the melt flow taken into account, a mathematical model was built to predict P concentration in the molten silicon. This work is helpful to explain the movement of P and achieve uniform distribution in the silicon ingot during production.

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.

// Xiaoming Li 1, * , Boyu Li 1, * , Jing Gao 1, * , Yunfei Wang 1 , Song Xue 1 , Dachuan Jiang 1 , Qi Hua 1 and Jing Li 1 1 Division of Cardiology, Xuanwu Hospital Capital Medical University, Beijing, China * These authors contributed equally to this work and share co-first authors Correspondence to: Jing Li, email: shpxbb@sina.com Qi Hua, email: huaqi5371@medmail.com.cn Keywords: ST-segment elevation myocardial infarction, spontaneous reperfusion, coronary angiography, prognosis Received: April 05, 2017 Accepted: June 30, 2017 Published: July 18, 2017 ABSTRACT Objective: To explore the influence of angiographic spontaneous coronary reperfusion (SR) on the short- and long-term outcomes in patients with ST-segment elevation myocardial infarction (STEMI). Materials and Methods: Consecutive patients diagnosed as STEMI and undergoing emergent coronary angiography from January 2009 to August 2010 in a single center were enrolled. Patients whose initial coronary blood flow met Thrombolysis in Myocardial Infarction (TIMI) grade 3 were regarded as angiographic SR. Others (TIMI grade < 3) were included into the NSR group and subsequently underwent primary percutaneous coronary intervention (PCI). Patients’ characteristics and outcomes were compared. Results: A total of 207 patients were eligible for analysis. The coincidence rate of patients both with ≥ 70% ST-segment resolution and ≥ 70% relief of chest pain and SR was 100%. Patients in the SR group ( n = 38) were younger, had more smokers, with higher level of platelet count, lower blood glucose and uric acid, and more distal culprit lesions, as compared to the NSR group ( n = 169). Importantly, patients in the SR group had lower rates of in-hospital events (0 vs. 22.5%, p < 0.001) with less stents placed (1.03 ± 1.05 vs. 1.59 ± 1.17, p = 0.007). Moreover, there were comparable long-term outcomes (2.6% vs. 6.9%, p = 0.302) between the two groups during 41-month follow ups. Conclusions: Angiographic SR is associated with significantly favorable short-term outcomes.

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 behavior of boron redistribution in silicon with and without oxide layer after electron beam injection (EBI) was investigated. Special defect shapes were generated on the surface of bare and oxidized silicon wafers. Secondary ion mass spectrometer was used to measure the boron profile. The results showed that after long EBI time, boron tended to be induced from both sides of the transition region between the oxide layer and silicon. For the sample without oxide layer after EBI, boron tended to diffuse towards the surface and its concentration obviously reduced inside the silicon. The results of the study show the potential use of the process in removing boron impurity in silicon.

A full domain control model is established for impurity transportation in the liquid phase, gas-liquid interface and gas phase of silicon to analyze the dynamic mechanics of impurity removal. The results show that the overall mass transfer coefficient mainly depends on the temperature and the chamber pressure. Its value increases with the increase of temperature or the decrease of chamber pressure. Under the same melting condition, the order of the overall mass transfer coefficients for P, Al and Ca is kP > kAl > kCa, indicating that P is easier to remove by evaporation. Mass transfer in the gas phase is the rate-controlling step for volatile impurity removal at the temperature above the melting point of silicon. The rate-controlling step transits to evaporation on the gas-liquid interface then to mass transfer in the liquid boundary layer as the temperature increases. During electron beam melting, the removal of P is controlled by both evaporation on the gas-liquid interface and mass transfer in the liquid boundary layer, and the removal of Al and Ca is controlled by evaporation on the gas-liquid interface.

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.

Silicon ingots with columnar grains and irregular grains as raw material for solar cells were obtained by directional solidification. The relationship between crystal morphology and concentration of metal impurities was studied. The result shows that columnar grains can improve the ratio of the high-purity area, where the concentration of metal impurities is less than 10 ppmw and more consistent with the calculated value by the Scheil’s equation. Meanwhile, the metal silicide precipitates at the irregular grain boundaries. Large temperature gradient at the solid-liquid interface dedicated to the stability of solid-liquid interface, which is conductive to the growth of the columnar grains. High concentration of metal impurities introduces a large constitutional supercooling to reduce the stability of solid-liquid interface, then further form a mushy zone in front of the solid-liquid interface, which leads to the growth of irregular grains.

A theoretical model for investigating the crystal growth rate and the solidified height during silicon purification by directional solidification is proposed. The growth rate is not constant usually and it has profound effects on the distribution of metal impurity in production process. The crystal growth rate and the solidified height, based on thermal equilibrium on the melt–crystal interface, were discussed. The relationship between the surface temperature of silicon melt (T 1 ) and temperature of graphite heater (TC′ 1 ) was found. The result shows that the value of T 1 has an approximate linear relationship with the TC′ 1 . The theoretical model can be used to design or predict the crystal growth rate by controlling the TC 1 according to the different request. Then, the distribution of metal impurity during silicon purification by directional solidification can be calculated according to the crystal growth rate. Thus, the theoretical model can be used to design the growth rate and predict the distribution of impurity to the silicon purification process by directional solidification. The experiments proved that the calculation agreed well with the existing experimental results.

A numerical model is proposed to investigate influences of process parameters, including crucible pulling down rates and heater temperature, on crystal growth rates for silicon purification by vacuum directional solidification. The crystal growth rates of a silicon ingot are analyzed based on the interface energy balance equation combining with the temperature field calculated by software of ProCAST, and the segregation behavior of impurities is investigated with the Scheil's equation. The results show that the crystal growth rates decrease linearly with the increase of heater temperature at a fixed value of the crucible pulling down rate, and increase exponentially with the increase of the crucible pulling down rates at a fixed value of the heater temperature. The numerical model is verified by removal of iron impurity from 300 ppmw to 1 ppmw and by the temperature of melt silicon which is recorded by a thermocouple. The results show that numerical results agree well with experimental results. This research is used for adjusting process parameters to control crystal growth during silicon purification by directional solidification.

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.

In this paper, alternating electromagnetic field and directional solidification are used to separate SiC and Si3N4 in polycrystalline silicon tailings. It is found the inclusion particles move downward at the center of the ingot and moving upward at the edge of the ingot by a variety of forces during the directional solidification process. The electromagnetic force accelerates the melt flow and enhances the lift force, so that larger particles can be pushed to the top of the ingot. Rod-shaped Si3N4 and block-shaped SiC particles show symbiotic relationship between each other. The aggregation of inclusion particles adsorbs metallic impurities, especially volatile metals, due to the effects of mushy region and short-circuit diffusion. The average conversion efficiency of the solar cells (Al-BSF method) prepared using the recycled silicon reached 18.56%, which meets the demand of the solar cells.

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 hollow silicon ingot was obtained by a solid–liquid separation method inducted by the fountain effect and the formation mechanism of the ingot was also discussed. A layer of solidified shell was formed on the melt surface and the gas dissolved in the melt was separated out. Because of this, the thickness of the shell was gradually increased and expanded due to the sudden change of the chamber pressure leading to the silicon melt being squeezed out from the preset hole of the shell. During this process, the melt left behind contains a high concentration of impurities and can be separated or detached completely from the decontaminated solid. This novel approach has of great potential to inhibit the back diffusion of impurities and to produce a silicon ingot at a high yield rate.

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.

Small amounts of multicrystalline silicon were melted in an electron beam furnace in different experimental conditions in order to investigate the oxygen evaporation behavior during the electron beam melting (EBM) process. The oxygen content level before and after EBM was determined by secondary ion mass spectroscopy. The oxygen content was reduced from 6.177 to 1.629 ppmw when silicon was melted completely at 15 kW with removal efficiency up to 73.6 %. After that, it decreased continually to

A new method for boron removal from silicon using electron beam injection (EBI) is proposed. After thermal oxidation on monocrystalline silicon (100) wafer at 1000 °C for 1 h, EBI was used to induce thermal and negative charging effects to enhance boron diffusion in the oxide film and the silicon substrate. This facilitates boron removal from the silicon substrate. The boron concentration in samples was measured by secondary ion mass spectrometry. The results show that EBI reduced the boron concentration in the silicon substrate by 4.83%.

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.

This paper proposed a theoretical model to determine the thermal contact resistance (TCR) on the surfaces of silicon and copper during electron beam melting. The effect of temperature and pressure on TCR, based on specific melting process conditions, was discussed. Hertz's theory was used to analyze the characteristics of material surfaces and to calculate the relationship between the pressure and distance of contact surfaces, the real contact area and the number of contact asperities combined with the physical characteristics of the material. The geometric parameter of the theoretical model was obtained based on theoretical calculations. The TCR of the entire surface was obtained by analyzing the temperature field of silicon and copper using Ansys and TCR equations. The relationship among TCR, pressure, and temperature were found. The computational results agreed with existing experimental results.

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.