Vacuum Directional Solidification Research Articles

Removal of metal impurities from diamond wire saw silicon powder by vacuum electromagnetic directional solidification

The utilization of high-purity silicon (∼99.9999 %) is prevalent in the solar photovoltaic sector, and achieving a reduction in its cost has emerged as a pivotal engineering objective within the silicon solar industry. Recycling silicon from diamond wire saw silicon powder (DWSSP) is a promising solution to fulfill the continuously expanding of high purity silicon demand. Vacuum directional solidification technology using high-frequency electromagnetic induction heating was proposed to purify DWSSP for the first time efficiently. The silicon melt and gas interface reaction, as well as metal impurities’ microsegregation in silicon ingot were observed. The effects of vacuum level and electromagnetic force on the separation of silicon and metal impurities were investigated using thermodynamics and electromagnetic mechanics. Results indicate that the content of oxygen in the silicon reduced from 30.91 wt% to 5.08 wt%, and the actual sample composition including for uptake of residual gas contamination steadily declined as the vacuum level increased from 600 Pa to 10 Pa. Vacuum electromagnetic directional solidification reduced the content of metal impurities from 641 ppmw to 29.80 ppmw, increases the proportion of crystalline silicon area, and makes the silicon lattice arrangement more regular.

Effect of orientation relationship on the morphology and strengthening effect of chromium precipitation in Cu-0.67Cr-0.3Zr alloy

High strength and high conductivity Cu-0.67Cr-0.3Zr alloy was prepared by vacuum directional solidification equipment. The face centered cubic (FCC) and body centered cubic (BCC) Cr rich precipitations in Cu matrix were characterized by high resolution transmission electron microscopy. In the as cast and solution treated state, a large amount of coherent FCC Cr precipitates is uniformly distributed in the matrix. These FCC Cr precipitates are covered by close packed {111} crystal planes and presents regular morphology. Due to their small size and cube-cube orientation with the matrix, FCC Cr precipitates follows dislocation shearing mechanism, resulting in limited strengthening effect. After aging treatment, both incoherent ordered nearly spherical and incoherent rod-shaped BCC Cr precipitates were formed in the matrix. These incoherent precipitates always follow dislocation bypassing mechanism. Due to its small size and high-volume content, the spherical precipitation plays a major role in improving the strength of the alloy. On the contrary, the influence of rod-shaped precipitates on strength is minimal. According to the minimum free energy theory and local chemical potential equilibrium requirement, the precipitation with Kurdjumov-Sachs (K-S) orientation relationship tends to be rod-shaped extending along the direction of Cu [110] and Cr [111], while the precipitation with Nishiyama-Wassemann (N-W) orientation relationship tends to be nearly spherical.

Purification of Metallurgical Grade Silicon Via the Mg-Si Alloy Refining and Acid Leaching Process

A novel process by combining Mg-Si alloy refining and acid leaching was suggested to purify metallurgical grade Si (MG-Si) in this paper. The proposed process was mainly composed of three steps, such as electromagnetic solidification, evaporation, and acid leaching. The results show that the precipitated Mg2Si crystals in the Mg-Si melt were successfully separated and enriched at the bottom of the sample because of the electromagnetic stirring and temperature gradient. Then, a porous structure Si with pore sizes ranging from nanometer- to micrometer-scale was obtained by evaporating Mg from the Mg2Si crystals. Finally, the high-purity Si powders with a purity of 99.995% were prepared by employing a successive acid leaching process. Most of the impurities (B, P, Fe, Ca, Ti, Ni, V, Cu) can be removed to less than 1 ppmw after the leaching process, indicating that an efficient elimination of impurities can be achieved with the help of the porous structure. Although bits of Mg was contained in the purified Si, it is expected to be eliminated by using a vacuum directional solidification. During the process, most of the magnesium can be recycled for cyclic utilization, which should be beneficial for the cost reduction. Therefore, it could be proposed that an efficient method of Si purification was obtained, which may be a valuable idea for the fabrication of solar grade silicon (SoG-Si).

Preparation of high-purity Ti–Si alloys by vacuum directional solidification

Abstract To prepare high-purity Ti–Si alloys, we newly used vacuum directional solidification technology to purify and separate Ti–Si alloys, which have different compositions (Ti-8.45 wt%Si, Ti-63.7 wt%Si, Ti-71.6 wt%Si, and Ti-84.1%wt%Si). The pulling-down rates in the directional solidification were to be 3 μm/s, 5 μm/s, and 7 μm/s, respectively, to investigate their effects on the purification. The results showed that vacuum directional solidification can purify Ti–Si alloys efficiently without discharging waste acid, slag and gas, and the slower pulling-down rate could result in better purification results. The impurities, Fe, Al, V, and Ni, could be removed due to their segregation behavior on the boundary of the solid and liquid phases. However, the impurities Ca and Mg could be removed using the vacuum technology because their vapor pressures were significantly higher than those of Si and Ti. TiSi2 could be precipitated and separated from the Ti-63.7 wt%Si and Ti-71.6 wt%Si alloys. The precipitated TiSi2 could be agglomerated at the bottom of the Ti–Si alloys, under the effect of gravity. The phases in the Ti–Si alloys, after vacuum directional solidification, were observed and determined using an electron probe micro-analyzer (EPMA). The results showed that the eutectic Ti-8.45 wt%Si alloy consisted of Ti5Si3 and Ti, the Ti-63.7 wt%Si and Ti-71.6 wt%Si alloys consisted of TiSi2 and eutectic Si–Ti alloy at the Si-rich side, and the Ti-84.1 wt%Si alloy consisted of Si particles and eutectic Si–Ti alloy at the Si-rich side. The study shows that vacuum directional solidification is an efficient and clean technology for the purification of Ti–Si alloys, and for the preparation of TiSi2 through the separation of Ti–Si alloys.

Effect of High Temperature Annealing on Crystal Structure and Electrical Properties of Multicrystalline Silicon by the Metallurgical Method

The defects of dislocations and grain boundaries (GBs) could significantly reduce the electrical properties of multicrystalline silicon (mc-Si) solar cells. In this manuscript, the influence of crystal defects on the electrical properties of mc-Si was investigated, aims to carry out the research about the influence of crystal defects on the electrical properties of mc-Si. Purified metallurgical grade N type mc-Si ingot was obtained with the method of vacuum directional solidification technique by employing industrial production ingot furnace. The variations in grain boundaries and dislocations were characterized by electron backscatter diffraction (EBSD), and their effects on the electrical properties of mc-Si before and after annealing at different heat insulating time were investigated. Experimental results showed that high temperature annealing process could significantly improve the electrical properties of mc-Si, the minority carrier lifetime was improved from 0.599 μs to 0.770 μs after 10 h insulation, with a significant increase of 28.61%. Experimental results indicated that high temperature annealing process could significantly eliminate the crystal defects of multicrystalline silicon. The crystal defects was eliminated more obviously with prolonged heat preservation time. When treated by high temperature annealing, the proportion of small angle grain boundaries in the same region can be increased by as high as 13.45%. Moreover, the whole grain sizes of partly region of silicon wafer could be increased by 20000μm2, increased by 15.89% compared with the original grain sizes. Based on the experiment phenomena, we attributed the improvement in the electrical properties of mc-Si to the elimination of dislocations, increased grain sizes and migration of GBs.

Research on Microstructure Evolution of Al–Al2Cu Eutectic by Regional Melting under Directional Solidification

AbstractAl–Al2Cu eutectic is prepared by regional melting through vacuum directional solidification furnace in an experiment. The effects of different pulling rates, heating power, and transition acceleration on the microstructure evolution of Al–Al2Cu eutectic under directional solidification are analyzed. Four constant growth rates of 3, 6, 9, and 15 mm min−1 are selected for the pulling rate. The primary Al2Cu phase will precede the coupled eutectic phase. The morphology of the irregular faceted primary Al2Cu phase is coarse and the feature of growth is transformed from faceted to non‐faceted as the pulling rate increases. The microstructure at three different heating powers is studied. With the increase of heating power, the primary Al2Cu phase in the alloy decreases, the eutectic structure increases, and the eutectic spacing of the alloy is obviously refined. The pulling rate is changed from 3 to 6 mm min−1, 6 to 9 mm min−1, and 9 to 15 mm min−1, respectively, in directional solidification for transition acceleration. The increase of the pulling rate will change the morphology of the primary Al2Cu phase and produce a partial rod eutectic structure. The lamellar eutectic spacing obtained by transition acceleration is smaller than that obtained by the same constant growth rate.

Boron removal from silicon by hydrogen assistant during the electromagnetic directional solidification of Al Si alloys

Abstract Boron (B) removal from silicon (Si) by using an Ar-20% H2 gas mixture during the Al Si solvent refining was investigated. Electromagnetic directional solidification was employed to enrich the primary Si crystals in the melt and simultaneously drive the produced BxHy gas species to migrate out of the Si enrichment zone. Thermodynamic analysis shows that the formation of BxHy gas species by the reaction of dissolved boron [B] and dissolved hydrogen [H] in Al Si melt is feasible. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) analysis confirmed that the impurity carbon (C), oxygen (O), and B adhered to the wall of the gas cavity, indicating that B removal can be attributed to the migration of gas species. As the flow time of Ar-20% H2 gas mixture increased, the B removal fraction significantly increased, and a maximum value of 96.3% for B removal fraction was achieved when the flow time was 150 min. Finally, an overall process for producing solar grade silicon (SoG-Si) from metallurgical grade silicon (MG-Si) with the combination of Ar H2 refining in Al Si solvent and vacuum directional solidification was proposed.

The influence of Marangoni effect on the growth quality of multi-crystalline silicon during the vacuum directional solidification process

A multi-field coupling model of heat-flow-thermal stress was established to investigate the influence of Marangoni effect on the growth quality of multi-crystalline silicon (mc-Si) during the vacuum directional solidification (VDS) process. The simulation results showed that the Marangoni effect has a significant effect on the distribution of temperature and thermal stress as well during the VDS process due to the increased velocity of melts. The enhanced flow incurred more homogeneous temperature distribution of silicon melts and reduced radial temperature gradient, which then leads to the flatter solid/liquid (s/l) interface. However, as temperature gradient of the crystal increased, thermal stress of the silicon ingot was strengthened as a result. The growth quality of crystal was more desirable when adopting a pulling-down rate of 10 µm/s, in which case the (111) surface was advantageous throughout the entire crystal growth process by XRD detection. Finally, the reliability of the numerical simulation result was verified by the experiment.

Separation and purification of silicon from cutting kerf-loss slurry waste by electromagnetic and slag treatment technology

The recovery of silicon from cutting kerf-loss slurry waste for the polycrystalline silicon production has been becoming a meaningful and urgent issue. In this paper, a united method of combining electromagnetic separation and slag treatment technology was used to separate and purify silicon from silicon kerf. The low melting compound of Na3AlF6 is added to the synthetic binary CaO-SiO2 slag, which can improve the separation effect of silicon from slag phase. The mechanism of physical separation and chemical purification of silicon from cutting kerf-loss slurry waste was investigated. The experimental results show that the electromagnetic stirring has an obvious effect for silicon separation in spite of no slag addition. The addition of CaO-SiO2 or CaO-SiO2-Na3AlF6 slag can further improve the separation rate of silicon. The purity of separated silicon is about 99.47% after the synthetic CaO-SiO2-Na3AlF6 slag treatment and reaches 99.98% followed by a vacuum directional solidification treatment. The recovered silicon from cutting kerf-loss slurry waste by this method can meet the purity requirement to the raw materials for silicon ingot casting.

The interaction and migration of deformation twin in an eutectic high-entropy alloy AlCoCrFeNi2.1

An eutectic high-entropy alloy consisting Al, Co, Cr, Fe and Ni elements was prepared by vacuum directional solidification technology. The alloy exhibits excellent comprehensive mechanical performance during tension at temperature range of 600–700 °C. The microstructure reveals the intersection of twin-twin is the prevailing deformation mechanism and the twins play a dual role in strengthening and toughening the alloy in the thermomechanical process. The deformation twin variants I and Π were formed by the edge dislocation 112¯ and the mixed dislocation 211¯ on the {111} crystal planes, respectively. Besides, the dislocation jogs and kinks caused by twin intersection on the slip planes can strengthen the alloy, which may contribute to the high strength (the tensile strengths at the 600° and 700° tensile tests are respectively780 MPa and 630 MPa.). Moreover, the coherent twin boundary migration has the function of coordinating deformation and contributes to the high ductility of the alloy.

Behavior of Rare-Earth Metals in Vacuum Melting and Directional Solidification of Nickel Superalloys

The interaction of the nickel melt containing REMs (Y, Ce, and La) with the ceramic material of a melting crucible (Al2O3, MgO ∙ Al2O3, Y2O3) during vacuum melting and with a mold (Al2O3) during directional solidification of has been detected experimentally. The REM concentration in a metal decreases as a result of holding of an REM-containing melt in a ceramic crucible or a mold. This should be taken into account to achieve the optimal required REM content in alloys.

Phosphorus removal from upgraded metallurgical-grade silicon by vacuum directional solidification

The removal of phosphorus impurities is among the most important problems in the purification of upgraded metallurgical-grade silicon (UMG-Si) used in the preparation of solar-grade silicon (SOG-Si). Vacuum refining is considered relatively effective for removing phosphorus from molten silicon. Based on the saturated vapor pressure of phosphorus calculated, the removal of phosphorus from UMG-Si at 1823K was performed via an industrial-scale vacuum directional solidification (VDS) furnace in this paper. Amathematical model for phosphorus removal during VDS was proposed to predict the distribution of phosphorus in silicon ingots. An industrial-scale experiment was carried out to confirm the mathematical model. The results demonstrated that phosphorus could be removed effectively main by vacuum evaporation during the VDS process, and thephosphorus concentration was about 4.63–13.41 ppm.The relationship between the phosphorus concentration and the resistivity of the silicon is discussed by experimental and calculated findings. The experimental results show that the resistivity decreases from 0.354 Ω cm to 0.259 Ω cm with increased phosphorus concentration.

Si Purification by Removal of Entrapped Al during Electromagnetic Solidification Refining of Si-Al Alloy

To investigate the effect of the removal of entrapped Al on Si purity during Al-Si electromagnetic solidification refining, the formation mechanism and removal principle of entrapped Al in primary Si were studied. The results showed that the agglomeration and irregular shape of primary Si crystals during electromagnetic solidification were the main reasons for the formation of entrapped Al; a higher initial Si content in an Al-Si alloy may result in higher amounts of entrapped Al. The entrapped Al can be effectively removed by first grinding the primary Si to powder particles of ~20 μm and then leaching the particles with acid, which can simultaneously remove other impurities concentrated in the entrapped Al such as Fe, Ca, Ti, and B. Hence, it is proposed that the combination of electromagnetic solidification, acid leaching, and vacuum directional solidification could be used to obtain solar-grade silicon.

Microstructure evolution and room temperature fracture toughness of as-cast and directionally solidified novel NiAl-Cr(Fe) alloy

The microstructure evolution and room temperature fracture toughness of as-cast and directionally solidified NiAl-Cr(Fe) alloy were investigated using OM, SEM, EDS, DSC and three-point bending tests. From the as-cast microstructure and DSC result, NiAl-34Cr-4Fe (at.%) is a eutectic alloy which consists of eutectic cells in different sizes. The half-baked mesh-like structure is observed at the cell center, and the radial emanating thicker or longer Cr(Fe) phases embedded within NiAl matrix are observed near or at the cell boundary. In the directional solidification process, the solid-liquid interface morphology has an evolutionary process of planar to cellular, even dendritic interface with increasing the withdrawal rates, and the eutectic cell and the microstructure at the cell center refines gradually. From the transverse microstructure, the characteristic of eutectic cell is similar to that of eutectic cell in as-cast alloy. It can be seen from the longitudinal colony/cell center that the broken (short) Cr(Fe) rods are observed at 6 μm/s, and they evolve to granular Cr(Fe) phases when the withdrawal rate increases further. Moreover, regardless of vacuum induction melting (as-cast) and directional solidification, NiAl-34Cr-4Fe (at.%) eutectic alloy possesses a poor fracture toughness due to the inferior brittleness of both NiAl and Cr(Fe) phases. Meanwhile, the crack propagation and fracture surface are observed to better understand the fracture behavior.

A mathematical model for distribution of calcium in silicon by vacuum directional solidification

Calcium is one of the main impurity elements in silicon. The removal of calcium strongly affects the quality of the polycrystalline silicon ingot produced by a vacuum directional solidification method. Based on the considerations of the theory of segregation, mass transfer and evaporation during vacuum directional solidification process, a mathematical model for calcium distribution in silicon was proposed and it can be used to explain the removal mechanism. In order to confirm the mathematical model, an industrial scale experiment on UMG-Si with an initial purity of 99.98 wt. % was performed. Since the reaction temperature strongly influences both the evaporation and segregation of calcium, the dependences of effective segregation coefficient (keff) and the evaporation coefficient (kE) on temperature were carefully investigated. The results showed that the proposed mathematical model was highly consistent with the experimental data and the calcium removal efficiency mainly relied on the evaporation step.

Microstructure of a complex Nb–Si-based alloy and its behavior during high-temperature oxidation

A in-situ composite Nb–Si–Ti–Hf–Cr–Mo–Al composite material alloyed with yttrium and zirconium is studied. The evolution of the structure–phase state of the alloy during oxidation under dynamic and isothermal conditions is considered on samples prepared by vacuum remelting and directional solidification. The phase composition and the microstructure of the alloy are examined by the methods of physico-chemical analysis, and the distribution of alloying elements in initial samples and the products of oxidation is estimated. Thermogravimetric experiments are performed on powders and compacted samples during continuous (in the range 25–1400°C) and isothermal (at 900 and 1100°C) heating in air. The directional solidification of an Nb–Si–Ti–Al–Hf–Cr–Mo–Zr–Y is found to cause the formation of an ultradispersed eutectic consisting of α-Nbss and γ-Nb5Si3ss cells. The as-cast sample prepared by vacuum remelting has a dendritic structure and contains Nb3Si apart from these phases. Oxidation leads to the formation of a double oxide layer and an inner oxidation zone, which retain the two-phase microstructure and the ratio of alloying elements that are characteristic of the initial alloy. Diffusion redistribution is only detected for molybdenum. The cyclicity of heating at the initial stage of oxidation weakly influences the oxidation resistance of the alloy.

Process parameters influence on the growth rate during silicon purification by vacuum directional solidification

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.

The effect of radiative heat transfer characteristics on vacuum directional solidification process of multicrystalline silicon in the vertical Bridgman system

In the vertical Bridgman system for multicrystalline silicon vacuum directional solidification process, the radiation heat transfer characteristics have a significant impact on the quality of crystal growth. In this paper, the effect of heat transfer characteristics under different pulling-down rates on the vacuum directional solidification process was investigated. Simplified radiation view factor dependent on pulling-down rate in pilot-scale vertical Bridgman system was derived. Combined with the transient simulation results, we obtained the heat transfer characteristics inside the furnace, and proposed the optimization scheme by using a rate of slowly first and then faster to pull down the crucible, which can not only satisfy thermal conditions of crystal growth, but also shorten the solidification time and effectively reduce the energy consumption. In addition, according to the characteristics of the radiative heat transfer under vacuum condition and the change of view factor, it was found that an improved design by widening the cold zone of the furnace can narrow the temperature gradient in the silicon ingot, and greatly decrease the thermal stresses, which in turn can aid in the design of future industrial equipment and process improvements.

Back diffusion of iron impurity during silicon purification by vacuum directional solidification

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.

Numerical simulation and experimental verification of vacuum directional solidification process for multicrystalline silicon

In this paper, a transient numerical model was applied to simulate the vacuum directional solidification (VDS) process of multicrystalline silicon (mc-Si) under different pulling-down rates, and the evolution of temperature distribution, thermal stresses and shape of solid/liquid (s/l) interface were simulated and analyzed. The experiments, such as the content and distribution of metal impurity, the crystal growth orientation and quality of mc-Si ingot were investigated to evaluate and validate the relationship between the thermal stresses and shape of s/l interface with simulation results. The results show that thermal stress, shape of interface s/l and temperature distribution in the silicon is determined by the thermal conditions in the furnace during the VDS process, and the crystal growth quality of mc-Si ingot relates closely to these factors. An appropriate pulling-down rate can satisfy thermal conditions to provide ideal temperature gradient in the silicon with lower thermal stresses and suitable s/l interface. We found that the mc-Si ingot produced by VDS process with pulling-down rate of 10 μm s−1 had a larger grain size, a vertical columnar structure and an ideal efficiency of the impurity removal.