Al-Si Melt Research Articles

Mechanism of phosphorus removal from Si–Al melt by hydrogen

Phosphorus is a critical impurity in solar cell that deteriorates the photovoltaic convection efficiency of solar cell. Phosphorus removal from metallurgical grade silicon by injecting H2 and Ar-50%H2 mixture during the Al–Si solvent refining was investigated. With gas injecting into the Al–Si melt, the phosphorus concentration in the refined Si decreased with an increase of the injecting time. When the gas injecting time was 2.5 h, the removal fraction of phosphorus increased to 93%, an improvement of 63% compared with 57% without Ar–H2 injection. The effects of H2 proportion, gas refining temperature, and Si proportion in the alloy were investigated. The kinetics of phosphorus removal with Ar–H2 injection were clarified. The mass transfer coefficient of phosphorus was 9.07 × 10−9 m/s during the Ar–H2-assisted solvent refining process. Three mechanisms of phosphorus removal were discussed for this gas-solvent composite refining process. These results demonstrate that H2 or Ar–H2 gas injecting treatment is an effective strategy to further improve the phosphorus removal and reduce the cost during Al–Si solvent refining.

Combined Fe and Sr refining of Al-Si melt strategy for fabrication of hierarchical structure Si anodes

Both industrial and academic research societies have considered silicon (Si) as the most promising anode for next-generation lithium-ion batteries (LIBs). However, the huge volume changes of the Si matrix during the cycling processes damage the lifetime of Si anodes. Numerous studies have demonstrated that the structure of the Si matrix is a vital factor affecting the cycling performance of the Si anodes. Herein, we propose a combined modification strategy by adding Sr and Fe into the Al-Si melt for the fabrication of refined hierarchical structure Si (HSSi-SrFe). Under the poisoning effect of Sr and refining effect of Fe, the morphology of the Si matrix was changed from a coarse flaky to a micron-sized hierarchical structure with 1D nano-secondary branches. As a result, the HSSi-SrFe delivers higher tap density, better dispersion and over 10 times conductivity compared with nano-Si. Meanwhile, the HSSi-SrFe@C electrode maintains a reversible capacity of ∼ 1000 mAh g-1 after 1000 cycles, and the LCO || HSSi-SrFe@C full cell shows a 91 % capacity retention after 100 cycles with a high areal capacity. This work exhibits the practicality of the melt modification strategy to scalable production low-cost and high-quality hierarchical structure Si for the application of high energy density LIBs.

Energy analysis of solar-grade silicon purified by solidification from Al-Si melt

Energy analysis of solar-grade silicon purified by solidification from Al-Si melt

On the mechanism of Si-promoted destabilization of TiCx particles in Al alloys

TiCx is an excellent composite strengthening particle and grain refiner for Al alloys. However, the stability of TiCx is poor when solute Si exists in Al alloy melts, which significantly depresses its strengthening and grain refining effects. In this work, the destabilization mechanisms of the TiCx particles in Al-Si alloy melt with a composition of Al-7Si-7.5TiC were explored via experiments, first-principles calculations and thermodynamic calculations. The experimental results show that Si atoms diffuse into TiCx and Ti atoms are released into the Al melt to form a Ti-rich transition zone during the insulation of TiCx in Al-Si melt, and the TiAlySiz and Al4C3 phases are solidified in the Ti-rich zone and at Ti-rich zone/TiCx interface, respectively. The first principles calculations show that the low formation energy of C vacancies facilitates the rapid diffusion of Si atoms in TiCx, while the doping of Si atoms reduces the energy barrier of diffusion of Ti atoms in TiCx and promotes the formation of Ti-rich zones. The thermodynamic calculations show that the wide crystallization temperature range of the destabilized product TiAlySiz phase is the key to continuous decomposition of TiCx particles. In addition, the driving force of the main destabilization reaction of TiCx in the Al-Si alloys is about 44 times higher than that in the Al alloys without Si addition. This indicates that the presence of solute Si remarkably promotes the subsequent decomposition process of TiCx in the Al-Si alloy melts.

Non-isothermal Wetting of an Al Alloy Pin by Al-Si Melt under Terrestrial and Microgravity Conditions

Non-isothermal Wetting of an Al Alloy Pin by Al-Si Melt under Terrestrial and Microgravity Conditions

Enhancing P removal from primary Si by P-containing Al2Si2Sr phase during Al–Si solvent refining

In this paper, adding small amounts of Sr in Al–Si solvent to enhance the P removal from primary Si by the formation of P-containing Al2Si2Sr phase has been developed. By microstructure and EDS analysis, it is determined that the Al2Si2Sr phase precipitated in the melt is enriched with 0.8–1.2 at% P. The precipitation of the P-containing Al2Si2Sr phase consumes a large amount of P in the melt, which makes the P content in the primary Si decrease. With the addition of Sr-16000 ppmw, the P removal fraction in the primary Si is all over 93% and the highest removal fraction reaches 94.66%. The B content in primary Si does not change significantly with the increase of Sr addition. Some of the Al2Si2Sr phases are trapped and engulfed by the primary Si, which leads to the accumulation of Sr impurity in the primary Si. Moreover, the possibility of P behaving as the solute atom in the Al2Si2Sr solid solution is analyzed through first-principle calculations. The result shows that P replaces Si at its the atomic position in the Al2Si2Sr phase to achieve a more stable crystal structure. Finally, an evolution model of the interaction between the P-containing Al2Si2Sr phase and the primary Si in the Al–Si melt is established.

Study on Oxidization and Mass Transfer of Al from Si-Al Melt Using Top Oxygen Blowing Refining

Study on Oxidization and Mass Transfer of Al from Si-Al Melt Using Top Oxygen Blowing Refining

Microstructure and morphology of Si crystals grown in pure Si and Al–Si melts

Microstructure of Al-40 wt%Si samples solidified in electromagnetic levitation furnace is studied at high melt undercooling. Primary Si with feathery and dendritic structures is observed. As this takes place, single Si crystals either contain secondary dendrite arms or represent faceted structures. Our experiments show that at a certain undercooling, there exists the microstructural transition zone of faceted to non-faceted growth. Also, we analyze the shape of dendritic crystals solidifying from liquid Si as well as from hypereutectic Al–Si melts at high growth undercoolings. The shapes of dendrite tips grown at undercoolings >100 K along the surface of levitated Al-40 wt%Si droplets are compared with pure Si dendrite tips from the literature. The dendrite tips are digitized and superimposed with theoretical shape function recently derived by stitching the Ivantsov and Brener solutions. We show that experimental and theoretical dendrite tips are in good agreement for Si and Al–Si samples.

Electromagnetic self-encapsulation of carbon fiber reinforced Al matrix composite phase change material for high-temperature thermal energy storage

Realizing the effective encapsulation of eutectic Al-Si melt is of paramount significance for developing the Al matrix phase change materials in the high-temperature heat storage field. In this paper, a carbon fiber reinforced Al matrix composite phase change material with a Si-rich◎Si-poor cladding structure ([Cf×Si-rich]◎EAl-Si cPCM) was prepared by controlling the precipitation behavior of primary Si under electromagnetic stirring, which can realize the self-encapsulation of eutectic Al-Si alloy. The thermal and structural properties in steady state and non-steady state of the [Cf×Si-rich]◎EAl-Si cPCMs were systematically investigated. The results show that the [Cf×Si-rich] shell has a high thermal conductivity of 53.3–100.7 W/m·K at 200 °C~500 °C. With the increase of Si content in initial Al-Si melt, the high-temperature damage tolerance of the [Cf×Si-rich]◎EAl-Si40 cPCM prepared by Al-40Si is 62.8% higher than that of the [Cf×Si-rich]◎EAl-Si25 cPCM prepared by Al-25Si, but the heat storage capacity is decreased by 22.7%. The [Cf×Si-rich]◎EAl-Si30 cPCM prepared by Al-30Si melt is considered to have the best comprehensive property. Under the non-steady working condition, the [Cf×Si-rich]◎EAl-Si30 cPCM exhibited excellent structural stability and great phase change characteristics (0.3% fluctuation of the melting latent heat and 1.4% fluctuation of the phase change temperature). This work offers new self-encapsulation strategy for extending the application process of Al matrix PCM to the field of high-temperature thermal energy storage.

Determination of cooling of Al—Si melt flow front in thin vertical cavity of ceramic shell for upper supply

The thermophysical properties of ceramic mould are experimentally determined, which made it possible to refi ne the method for calculating of the temperature of the Al—Si melt flow front when filling thin vertical cavity.

Formation of metastable aluminum silicide as intermediate stage of Al-Si alloy crystallization

The mechanism of Al-Si alloy crystallization from an amorphous state is still unclear. Despite the absence of equilibrium compounds for this binary system, there are several experimental evidences confirming the formation of metastable silicide at annealing of amorphous Si mixed with Al. Classification of this metastable structure is a missing puzzle piece in the description of Al-Si alloy crystallization kinetics. Here, we considered the properties of aluminum silicide Al2Si structure, which is a probable candidate for the role of the observed metastable compound. Our investigation was based on the atomistic simulations with the interatomic potential developed recently and DFT approach. All used methods revealed that there are several crystal structures of Al2Si with close geometry and relatively low formation energies. The chemical ordering in such structures is similar to the one in Si-Al melt. We also showed that the combination of these structures allows to form a crystal with different degree of ordering.

Interaction of Fe‐Containing, Secondary Al–Si Alloy with Oxide and Carbon‐Containing Ceramics for Fe Removal

Fe is a detrimental impurity element in secondary, i.e., recycled, Al–Si cast alloys for castability and mechanical properties. Fe removal can be achieved by binding Fe into Fe‐containing, primary particles and removing these from the, thus, Fe‐depleted Al–Si alloy melt. Expanding the application of filters beyond the removal of nonmetallic inclusions to increase the Fe‐removal efficiency can contribute to production of high‐quality, secondary Al–Si alloys. The usability of Al2O3, 3Al2O3 · 2SiO2, MgAl2O4, Al2O3–C, and SiC filter materials in contact with Al7.1Si, Al7.1Si1.5Fe, and Al7.1Si0.75Fe0.75Mn alloys is evaluated in sessile‐drop and crucible experiments regarding wettability, chemical interaction, nucleation, particle formation, and the subsequent effect on the alloy composition. The Al–Si melts in contact with Al2O3 act as nonreactive, low‐wetting reference systems. MgAl2O4 and SiO2 in 3Al2O3 · 2SiO2 are reduced, forming low‐wetting Al2O3, whereas Mg and Si enrich in the droplet. In contact with SiC and Al2O3–C, a thin layer of high‐wetting Al4C3 is formed. In case of Al2O3–C, primary Fe‐containing particles are specifically attached to the Al4C3 layer, which is associated with oriented growth of that layer. The remaining Fe content in the melt is 0.9 at% and beneficial kinetic effects for the Fe removal are suggested.

Microstructure and mechanical properties of Zr3Al3C5-based ceramics synthesized by Al-Si melt infiltration

In this work, bulk Zr3Al3C5-based ceramics were synthesized by the infiltration of Al-Si melt into zirconium carbide (ZrC) perform. The phase composition, microstructure, and mechanical properties of as-fabricated ceramics were studied. The results demonstrate that Si is more effective to reduce the twin boundary energy of ZrC than Al, and thus promotes the decrease of formation temperature of Zr3Al3C5. With the infiltration temperatures increasing from 1200 to 1500 °C, the Zr3Al3C5 content increases from 10 to 49 vol%, which is contributed to the increase of flexural strength from 62±9 to 222±10 MPa, and fracture toughness (KIC) from 2.8±0.2 to 4.1±0.3 MPa·m1/2. The decrease of mechanical properties for the samples fabricated at 1600 °C is ascribed to the abnormal growth of Zr3Al3C5 grains.

Low Temperature Reactive Flux Growth of SiC and SiC-AlNusing Al-Si Flux

Al - 12% Si alloy system was used as nutrient flux to grow silicon carbide at low temperature by reactive flux growth. Thick films were grown below 900°C on a SiC substrate having Al-Si melt rotated with a speed of 30rpm in a graphite cruciblein nitrogen atmosphere. A constant 2 atmospheric pressure of nitrogen was used on the melt. Using longer soak time of the melt in the presence of carbon impurity, the destruction of dendritic morphology was observed that subsequently resulted into the formation of cellular, colony and facetted crystal morphology. Photoluminescence data from film showed SiC with higher band gap in the presence of aluminum nitride. Keywords: A1.Crystallites; A1. Nucleation; A1. Optical morphology; A1.Solidification; A2. Growth from melt; B2.Semiconducting silicon compound;

Mechanism of boron removal of primary Si phases and morphology evolution of impurity phases during slow cooling solidification refining of Al-30wt.%Si alloy with Zr additions

This work combines the control of kinetic factors and the addition of a small amount of Zr element in Al-Si solvent refining process to obtain a super high B removal rate. Impurity removal of primary Si phases in Al-30wt.%Si alloy is evaluated by solvent refining with slow cooling rates of 0.4, 2, 10, and 50 mK ⋅s−1, and Zr addition contents of 0, 330, 1000, and 3000 ppmw, respectively. With a 3000 ppmw Zr addition, the removal fraction of B in the primary Si is all over 93% and the best value reaches 99.36%. The P content is more sensitive to the cooling rate but does not change significantly with the increase of Zr. By morphology observation and ICP-OES analysis of the primary Si phases, the interaction between Zr, B, and Si, and the morphology evolution of ZrB2 and ZrSi2 phases are revealed. Based on theoretical calculations, the formation temperature of ZrB2 in the Al-Si melt is evaluated and a competitive reaction mechanism is discovered. Due to the excessive Zr addition, Zr first reacts with B and forms ZrB2 before the primary Si phase growth, which causes a decrease of the B content in the melt and contributes to the high B removal fraction. The remaining Zr in the melt combines with Si to form ZrSi2 phases. A model is established to describe the different trapping/ engulfing/ pushing situations of ZrB2 and ZrSi2 phases, which determine the final impurity content in the refined primary Si phases.

Effect of mechanical stirring on silicon purification during Al-Si solvent refining

Al-Si solvent refining is a promising method to purify silicon, and boron and phosphorus are key impurity elements to be eliminated. When primary silicon grows in hypereutectic Al-Si melt, impurity atoms are enriched at the solid-liquid interface and diffuse into the remaining melt. Mechanical stirring was introduced in the Al-Si solvent refining in this work to dilute the concentrated impurity atoms at the solid-liquid interface. A rotating tube with an end closed was used to stir the Al-Si melt. It is found that mechanical stirring is beneficial to impurity atoms removal. Boron and phosphorus contents in purified silicon with mechanical stirring are smaller than that without mechanical stirring. When stirring speed increases, boron and phosphorus contents in purified silicon decrease.

Microstructure evolution during AlSi10Mg molten alloy/BN microflake interactions in metal matrix composites obtained through 3D printing

Utilization of metal/ceramic powders opens new possibilities for 3D printing of metal matrix composites of complex shape with high strength, but it is still a great challenge. In this work, an AlSi10Mg matrix composite embedded with 1 wt% of hexagonal BN phase microflakes (h-BN) was obtained by means of 3D printing. Then the present study elucidated microstructure evolutions occurring at the h-BN/melt interface during selective laser melting (SLM) of an h-BN-AlSi10Mg powder mixture. During short-term (0.15 ms) high-temperature (∼2900 K) processing the BN inclusions partly dissolved in the Al–Si melt. This process was accompanied by the formation of an AlN phase at the BN surfaces. The AlN crystallites, 100–200 nm in size, had spherical/semispherical shape and formed a continuous layer along the BN/metal grain boundaries. The peculiar growth of AlN grains along the metal/BN interfaces was governed by the specific features of localized N diffusion in the vicinity of interfaces. By contrast, B atoms, released from the dissolved BN phase, were randomly distributed over the melt. AlB2 nanocrystallites (∼10 nm in size) precipitated from the supersaturated Al–Si melt during cooling stage. With the addition of h-BN microflakes, the composite hardness and tensile strength increased by 32% and 28%, respectivelly. The observed experimental results were supported by ab initio molecular dynamics simulations. Our study demonstrates the possibility and wide prospects of obtaining a dense BN/AlSi10Mg material reinforced with h-BN, AlN, and AlB2 phases via SLM 3D printing and sheds a new light on fine morphological and microstructural features of thus obtained new composites.

Mechanism for Si Poisoning of Al-Ti-B Grain Refiners in Al Alloys

Al-5Ti-1B is the most widely used grain refiner for Al alloys. However, it is not effective for grain refining Al alloys containing more than 3 wt pct Si. This adverse effect of Si is referred to as Si poisoning. In spite of extensive experimental and theoretical investigations in the past decades, the exact mechanism for Si poisoning is still not clear. In this work, the state-of-the-art electron microscopy was performed to investigate the mechanism for Si poisoning. Our experimental results suggest that Si segregates preferably to the TiB2/Al-Si melt interface and the pre-existing Al3Ti 2-dimensional compound (2DC) layer on TiB2 surface dissolves into the Al-Si melt. Based on the experimental results, we have postulated a new mechanism for Si poisoning: interfacial segregation of Si leads to enrichment of Si at the TiB2/Al-Si melt interface, and this in turn makes the pre-existing Al3Ti 2DC on the TiB2 surface unstable and dissolve gradually in the melt, resulting in a loss of TiB2 nucleation potency and hence a decreased total number of potent TiB2 particles available for heterogeneous nucleation and grain initiation and consequently an increased grain size. This mechanism for Si poisoning can explain consistently the experimentally observed phenomenon reported in the literature.

Silicon separation and purification process from hypereutectic aluminum-silicon for organosilicon use

Industrial silicon is always used for the manufacture of organosilicon. This study evaluated the segregation properties of hypereutectic aluminum–silicon melts and revealed the silicon separation process from hypereutectic aluminum-silicon melts dealt with alternating electromagnetic directional solidification (AEM-DS). During the electromagnetic refining, the electromagnetic force can not penetrate Al–Si melts deeply and melts flow was generated by the outer thin surface, when the frequencies are 3, 30 and 50 kHz, the corresponding skin depths are 5.930, 1.876 and 1.027 mm, which show sharp attenuations. Moreover, the numerical simulation was used to simulate the electromagnetic field distribution. The experimental phenomenon can accord well with the simulation and calculation results. Moreover, this study reveal the middle process of hypereutectic Al–Si melt dealt with AEM-DS. The better separation effect will be helpful to eliminate Si, Al and acid waste in the following acid leaching.

Controlling Segregation Behavior of Primary Si in Hypereutectic Al-Si Alloy by Electromagnetic Stirring

Controlling the segregation behavior of primary Si in the solidification process of hypereutectic Al-Si alloy is crucial for enhancing the design ability of the solidification structure. To explore the separation condition and morphological evolution of primary Si in detail, a series of experiments concerning the coupling effect of a temperature field and electromagnetic stirring on the segregation behavior of primary Si were carried out. Experimental results show that the temperature field and fluid flow in the melt are two key points for controlling the segregation behavior of primary Si. The establishment of a temperature gradient in the Al-Si melt is a precondition for realizing the separation of primary Si. On the basis of the temperature gradient, the electromagnetic stirring can further strengthen the separation effect for primary Si, forming a Si-rich layer with 65~70 wt.% Si content. The formation of the Si-rich layer is a continuous growth process of primary Si by absorbing Si atoms from Al-Si melt with the help of electromagnetic stirring. The separation technology for primary Si is proposed to realize the segregation control of primary Si, which not only broadens the application of Al-Si alloys in the functionally gradient composites but also provides a low-cost supply strategy of Si raw materials for the solar photovoltaic industry.