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Über dieses Buch

EPD Congress is an annual collection that addresses extraction and processing metallurgy. The papers in this book are drawn from symposia held at the 2015 Annual Meeting of The Minerals, Metals & Materials Society. The 2015 edition includes papers from the following symposia: •Materials Processing Fundamentals •Solar Cell Silicon •High-Temperature Electrochemistry II



Materials Processing Fundamentals


Extractive Materials Processing

Arsenic and Antimony Removal from Copper Concentrates by Digestion with NaHS-NaOH

Copper concentrates with high arsenic contents must be pretreated before conventional smelting to prevent environmental pollution with arsenic compounds. In this work, experimental results concerning the selective removal of arsenic and antimony from copper concentrates are presented. The process consists of an alkaline digestion using concentrated NaHS-NaOH solutions to transform the arsenic and antimony sulfides into soluble compounds. A water leaching follows the digestion to dissolve the arsenic and antimony, leaving clean copper sulfide solid residues. The laboratory scale tests were carried out using a copper-arsenic concentrate with 15.05% As and 1.42% Sb. The results showed that the most important digestion variables were temperature and concentrations of NaHS and NaOH. Over 97% of arsenic and 92% of antimony could be removed in 10 min of digestion using 8.9 M NaOH and 100% excess of NaHS at 80 °C. The subsequent water leaching was performed at 80°C for 20 min.

Maria C. Ruiz, Felipe Daroch, Rafael Padilla

Rate of Metal Deposition from Aqueous Solutions

The electrodeposition of metal ions from aqueous electrolyte solutions has been a well-known process for the last half century. With numerous industrial applications such as electroplating, electrowinning and electrorefining, the kinetics of electrochemical reactions involved in the process has been extensively researched. Arrhenius-type rate equations, the Butler-Volmer equation, and the empirical Tafel equation are among the most important models to explain the rate of electrochemical reactions. The study of the kinetics of metal deposition is critical to understanding the underlying mechanisms through which a certain morphology of metal deposit is formed on the cathode. The purpose of this paper is to elucidate the influence of experimental parameters such as pH and additives on the rate and the reaction mechanism determining the rate limiting step of the electrodeposition reactions.

Anne-Marie Suriano, Stanley Howard, Sadegh Safarzadeh

Electrochemical Reduction of Cobalt Tetrafluoroborate in 1-Butyl-3-Methylimidazolium Tetrafluoroborate Ionic Liquid

Electrodeposition of cobalt from cobalt tetrafluoroborate (Co(BF4)2) was investigated using 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) ionic liquid. The experiments were conducted at 353 K. Chronoamperometry experiments confirm that the electrodeposition of cobalt on tungsten electrode proceeds via three-dimensional instantaneous nucleation with diffusion-controlled growth process. The average diffusion coefficient of Co(II) was found to be 7.5 × 10-8 cm2s-1 at 353 K, which is in good agreement with the estimated value from cyclic voltammetry. The electrochemical deposit was characterized using SEM-EDS and XRD methods. The SEM image shows formation of a dense and compact deposit at — 0.75 V. The EDS and XRD analysis confirm that the obtained deposit is pure cobalt metal.

Min Li, Zhongning Shi, Zhaowen Wang, Ramana G. Reddy

Extracting Alumina from Coal Fly Ash with Concentrated Sulfuric Acid Sintering and Ultrasound Aided Leaching

Extracting alumina from coal fly ash (CFA) is of great interest for both environmental protection and recycling valuable alumina content. In this study, ultrasonic aided leaching process as an effective method was investigated to enhance extracting Al2O3 from CFA via concentrated sulfuric acid sintering. The influence of ultrasonic power and processing time were tested under the conditions as the same as those without ultrasound. It is found that the alumina extraction rate can reach 90.5 % with ultrasound assistance, while the ultrasonic treatment can reduce almost 15 ~ 30 min in leaching time and 5 ~ 10 °C in leaching temperature than those without ultrasonic operation at the same Al2O3 extraction rate. In addition, the ultrasonic can reduce calcium content by 6 ~ 8 % dissolved into the leaching liquid. SEM analysis also shows that the residue particle size with ultrasound is smaller than that without ultrasonic treatment.

Wenbo Luo, Jilai Xue, Jun Zhu, Kang Liu, Chunlei Yang, Fusheng Mao

Casting, Solidification, and Steel Processing

A Cellular Automaton Model Coupled with Finite Element Method for Solidification Process of Beam Blank Continuous Casting

A two dimensional cellular automaton (CA) model coupled with finite element (FE) method has been developed for simulating the formation of solidified microstructure during beam blank continuous casting, which represents columnar and equiaxed dendrites growth in the case of detailed secondary cooling boundary conditions. In this model, a new adaptive mesh division is proposed to couple CA cubic lattice and irregular FE mesh of beam blank. A double-accuracy liner interpolation method is employed to obtain micro CA temperature field via macro FE calculation. The nucleation process and the velocity of dendrite tip are calculated by continuous nucleation model and KGT model, respectively. Moreover, the preferred orientation of dendrite growth is determined using a probabilistic model. The effects of super heat and casting speed on the microstructure characteristics are analyzed. This model can be applied to optimize processing parameters for beam blank continuous casting basing on microstructure evaluation.

Yan Zhao, Dengfu Chen, Mingtu Ma, Xinming Wan, Yang Li, Rongshan Qin

Numerical Simulation of the Coupled Turbulent Flow, Heat and Solute Transport in the Turbulent Flow Region of Slab Continuous Casting

In the current work, a three-dimensional model coupling turbulent flow, heat and solute transport was developed to investigate the solute transport and redistribution in the mold of slab continuous casting where the Reynolds number is very high and the molten steel flows strongly. The conservation equations of momentum, energy and species for a multicomponent system which includes solute element C, Si, Mn, P, S were solved with the commercial software ANSYS Fluent. The parameters used in the model are based on the actual continuous casting. The fluid flow, temperature distribution and solute elements distribution were analyzed. Results showed that negative segregation occurred near the strand surface, and the species concentration reached a peak value at the solidification front during solidification. The segregation degree of solute element S is higher than the other solute elements because of its lower equilibrium partition coefficient. The species concentration in the liquid pool is homogeneous because of the turbulent flow effect.

Huabiao Chen, Dengfu Chen, Lintao Gui, Mujun Long, Yunwei Huang, Youguang Ma

Interphase Evolution and Defect Formation During Horizontally Directional Solidification Process of Sn-Zn Alloys

In the present research, commercially pure Sn and Zn, and Sn-Zn alloys (Sn-1wt.%Zn, Sn-2wt.%Zn, Sn-4wt.%Zn, Sn-8wt.%Zn and Sn-8.9wt.%Zn, weight percent) were obtained by a horizontal directional solidification process with two opposite senses and heat extraction. The solidification process was realized using a horizontal furnace with two heat extraction systems at both ends. The temperature was measured using eight K-type thermocouples and an electronic recorder of temperature data. The resulting structures were analyzed using optical microscopy. From the solidification process the thermal and metallographic parameters were determined in all samples. The presence of defects in the solidified pieces was observed. Internal defects pretended to be dependent not only on the composition of the alloys under consideration but also on the size of the structures formed, also, on the velocities and accelerations of interphases, and on the variation of thermal gradients. A model of the phenomenon from the first principles is presented.

Alex I. Kociubczyk, Wilky Desrosin, Lucía Mabel Boycho, Carlos E. Schvezov, Alicia E. Ares

Effect of Technological Parameters on Mold Powder Entrainment by Water Model Study

In continuous casting process, mold powder has lots of metallurgical effects, such as covering the molten steel to prevent liquid steel from oxidation by air, preserving heat for top layer liquid steel, lubricating initial shell and so on. However, mold powder also can deteriorate the quality of final products obviously after the entrapment of mold powder in steel. In the present work, based on the similarity theory the effect of technological parameters on mold powder entrainment was investigated by water model. Experiment was carried out in the water mold using oil and water to simulate slag and steel, respectively. The results showed that increasing casting speed increased the water-oil interfacial fluctuation and entrainment frequency; viscosity of oil also had a great impact on the interface fluctuation and entrainment; water-oil interfacial tension effect the entrainment while not changing the interfacial fluctuation; submergence depth had little impact on entrainment and interfacial fluctuation.

Lizhi Zhang, Yugang Li, Qian Wang, Cheng Yan

Equivalency Comparison of Heat Transfer Coefficient in Liquid and Gas Quenches

With the development of quench technology, there is a trend of using gas quench to replace liquid quench for less distortion and residual stress. The fundamental difference between the liquid and gas quench is the heat transfer coefficient, not only the values but also the shape of the curve as a function of temperature. The equivalent heat transfer coefficient for the liquid and gas quench is analyzed by simulations and experiments. Even it may result in the same hardness in the liquid and gas quench, the steel microstructure may be different because of the different cooling processes, and therefore other steel properties, such as toughness, may be different. The cooling process, microstructures and properties such as hardness and toughness should be examined when designing the liquid or gas quench processes.

Yuan Lu, Yiming Rong, Richard D. Sisson

Determination of Cavity Profile Induced by Supersonic Jets Impinging Onto Liquids Surface

The impingement onto the surface of bath by top-blown jets is a significant process characteristic in BOF and EAF steelmaking process. The cavity is one of the most important outcomes of the interaction and plays an important role on reaction kinetics and reactors performance. But to date, the understanding to cavity is still so limited. In present study, a water model for a BOF converter is established and the dimension of cavity profile is investigated. The effects of lance height, gas flow rate, nozzle inclination angle and the amounts of slag are discussed. The results show that penetration depth increases with the increase of gas flow rate and the amount of slag, and the decrease of lance height and nozzle inclination angle. Furthermore, a theoretical model of cavity dimension is proposed for multiply jets impinging liquid bath on the basis of energy balance at the stagnation point of cavity.

Qiang Li, Mingming Li, Mingxia Feng, Zongshu Zou

Materials Processing

In-Situ Gas Monitoring by Emission Spectroscopy

Off-gas monitoring of experimental reactors give little information about intermediate reaction products and reflect chemical equilibria at reduced gas temperatures. A reactor design with optical access was designed and tested in order to be able to qualify and quantify gaseous species from their emission spectra. By use of a sequence of sapphire, UV fused silica and IR transparent sapphire and CaF2 windows and lenses optical access to a hot, maximum 1800 °C, reaction zone was gained. A cooled area behind the reaction zone was used as a background in order to avoid signal saturation from reactor walls. In addition to molecular emission spectra, atomic emission lines are used to characterize stable gas constituents and radicals. The setup has been designed for the study of methane dehydrogenation experiments in order to investigate the carbon activity of reduction of oxide with methane, but has also been used to monitor gaseous silicon suboxide.

Thor Anders Aarhaug, Alain Marc Ferber, Pål Tetlie, Halvor Dalaker

Poster Session

Thermodynamic Study on Vanadium Extraction with CO2 and O2 Mixed Blowing

Carbon dioxide (CO2), a major component of the greenhouse gases, could be comprehensive utilized as a valuable resource to oxidize vanadium during the converter vanadium extraction process. The thermodynamic software, FactSage, was utilized to study the oxidation ability and theoretical cooling ability of using weak oxidant CO2 as coolant in vanadium-containing hot metal. Besides, experimental research on vanadium extraction in CO2 and O2 mixed blowing extracting process had been undertaken. The results indicated that the elements C, Si, V, Mn etc. could be oxidized by weak oxidant CO2, and the content of V2O3 in slag was approached to pure O2 blowing. Furthermore, CO2 had a remarkable cooling ability and the molten bath temperature kept on decreasing as CO2 blowing rate increases. This research has proved that CO2 and O2 mixed blowing in vanadium extraction process are effective.

Wei-Tong Du, Yu Wang, Gang Wen

Effects of Power Ultrasound on Precipitation Process of Sodium Silicate Solutions

Sodium silicate solutions for producing silica have been ultrasound treated to enhance the precipitation process in laboratory scale. Higher precipitation rate was found with ultrasound treated solutions than those without ultrasound. The obtained SiO2 powder products were characterized using XRD, SEM, BET and laser particle size analyzer. The particle size distribution and the specific surface area of the powder products varied with the power input level and the processing time of the ultrasonic treatment. The results indicate that the efficiency of the precipitation process can be improved optimally when the ultrasound is conducted into the solutions before the nucleation stage.

Tiepeng Li, Jilai Xue, Wenbo Luo, Jun Zhu

CO2 Capture and Conversion Using a Cobalt(III) Schiff Base Complex as a Catalyst at Ambient Conditions

CO2 emissions must be reduced by at least 50 % by 2050, and hence the technical solutions to capture and convert CO2 into value-added products should be considered. A Cobalt(III) Schiff base complex (Salen-Co(III)) has been investigated as a catalyst for synthesis of cyclic carbonate from CO2 and epichlorohydrin(ECH) with tetrabutylammonium bromide (TBAB) as co-catalyst. To recover Salen-Co(III) for the next cycling operation, it was immobilized onto zeolite 13X through excessive impregnation method. The immobilized catalyst was characterized using XRD, SEM, BET and ICP-AES techniques. Catalytic tests showed that yield of cyclic carbonate reached 90 % using 0.5 mol % Salen-Co(III) at ambient conditions. The immobilized Salen-Co(III) exhibited better catalytic activity than the homogenous one when used at the first cycle, but the yield decreased by some 20 % after five cycles.

Jun Miao, Jilai Xue, Jun Zhu, Kang Liu

Effect of Laminar Cooling Parameters on Martensite Volume Fraction and Mechanical Properties of Hot Rolled Dual Phase Steel

In this study, effect of rolling and coiling temperature in thermo mechanical controlled process (TMCP), on the microstructure and mechanical properties of dual phase (DP) steel according to martensite volume fraction (MVF) rate was investigated. By using various finishing mill exit temperature and cooling conditions that is controlled laminar cooling system, the microstructural evolution of coil improves the mechanical properties with lower costs and higher productivities, in comparison to the heat treatment after rolling. The main part of the paper contains the result of tensile, yield and microstructure properties of DP600 steel produced on hot strip rolling plant and run- out table line. The results show that a higher MVF and an increasing cooling rate after the last deformation raised strength of the DP600 steel. The yield strength and ultimate tensile strength increase by decreasing coiling temperature.

Sibel Daglılar, Isıl Kerti, Sinem Yıldırım

The Effect of Graphene on the Microstructure and Mechanical Properties of Aluminum/Graphene Produced by HPT

The aluminum matrix composite reinforced by graphene was successfully fabricated using high pressure torsion (HPT). With only 0.5wt% graphene, the grain size was reduced to 100nm while that of pure Al was about 500nm. The hardness of the composite increased from the center (64HV) to the edge (120HV) of the sample. Moreover, the tensile strength of this composite reached 197MPa, while that of pure Al was 157MPa. The enhancement in mechanical properties was induced by grain refinement, load transfer from the matrix to the reinforcement and dislocations piled up at the grain boundaries.

Liyuan Zhao, Huimin Lu, Zhijiang Gao

The Extent of Dopant Activation after Microwave and Rapid Thermal Anneals Using Similar Heating Profiles

Many sustainability issues arise during the manufacturing processes that are currently used for solar cells. Solar energy is a renewable energy source that is independent of the earth’s resources, and it is therefore important for the development of more sustainable technologies. Microwave annealing (MW) has been proposed as a technically feasible fabrication scheme for large area silicon solar cells. Apart from that, microwave annealing has been demonstrated to be a promising alternative for repairing damage and electrically activating dopants in ion-implanted semiconductors for integrated circuit manufacturing. A microwave oven is cheaper than conventional furnace systems. In addition, microwave heating is much more efficient than conventional furnace heating, as heating is directly produced inside the material. This minimizes the loss of energy due to heating of the ambient. There is a need for more efficient processing techniques. In this study, microwave annealing is used as an alternative to the current post-implantation processing. Arsenic-doped silicon was microwave annealed (with an alumina-coated silicon carbide susceptor) to activate dopant atoms and to repair damage that was caused by ion implantation. Sheet resistance and Hall effect measurements were used to assess the extent of dopant activation. Rutherford backscattering spectrometry (RBS) with ion channeling was conducted to determine the extent of recrystallization. The dopant activation and recrystallization resulting from microwave annealing is compared with that resulting from conventional rapid thermal annealing (RTA) with the same heating profile. The results show that when compared to RTA, susceptor-assisted microwave annealing results in better dopant activation for shorter anneal times under the same heating conditions.

T. Gunawansa, Zhao Zhao, N. David Theodore, A. R. Lanz, T. L. Alford

Application of Computational Thermodynamics to Steel Processing: The Case of Steel Cleanness

Steel cleanness is of great importance to the performance of almost all steel products. From very clean steel for bearings to long products used in civil construction, processing variables — in special those in steelmaking operations- must be properly balanced to achieve adequate cleanness for the desired performance. With the present complexity of steels as alloy systems, it is not efficient nor appropriate to develop these processes on empirical basis alone. Computational thermodynamics can greatly improve process development to achieve the desired cleanness level at reasonable costs. In this work, examples of these calculations and their applications to real steel processing are presented and discussed. The advantages, limitations and areas in which improvement in these calculations is desired are highlighted and discussed.

A. Costa e Silva, L. Goulart, E. Araujo, R. Batista , A. Martins

Reaction Between MnO-SiO2 Oxides with Low FeO Content and Solid Steel Deoxidized by Si and Mn During Heat Treatment

In order to clarify the reaction between MnO-SiO2 oxides with low FeO content and Fe-Mn-Si alloy, two diffusion couples were produced by the new method using confocal scanning laser microscopy (CSLM). The interface of the alloy and oxide, content of Mn and Si in the alloy near the interface, size distribution and composition of the particles which precipitated in the alloy were observed and analyzed using the electron probe microanalysis (EPMA). Results show that though the FeO content in the oxides decrease to 1% which is lower than the equilibrium with the molten steel at 1873K, the diffusion of oxygen from oxide to alloy still exists during the heat treatment at 1473K and causes shorter Particle Precipitated Zone (PPZ) and Manganese Depleted Zone (MDZ) normally for the reduction of diffusion flux of oxygen.

Chengsong Liu, Jingshe Li, Haiyan Tang, Xiaojie Gao

Nucleation and Growth in the Equiaxed Zone of Metal Matrix Composites

In the present work it was observed the nucleation and growth of grains in the equiaxed zone of metal matrix composites (MMCs) samples directionally solidified wit the presence of columnar-to-equiaxed transition (CET) in the samples. In order to do so, first, it was defined the difference between the real temperature at a given instant, which it is used in the analysis, its variation and furthermore, the variation in the solid fraction with time between the beginning and the end of solidification. It was determined a growth law assuming a lineal variation of equiaxed grain radius as a function of time, and also, it was expressed a law of grain density as a function of time, considering the presence of SiC and Al2O3 particles in the matrix. Finally, the results obtained were validated with measured values of grain density at each position of the thermocouples in the equiaxed zone of the solidified samples.

Alicia E. Ares, Carlos E. Schvezov

Combined Effects of Silicon (Si) and Low Temperature Annealing on the Tensile Properties of Cartridge (70/30) Brass with Nickel (Ni) and Iron (Fe) Contaminants

The combined effects of the addition of Silicon (Si) as alloying element and low temperature annealing on the tensile properties of 70/30 brass with Nickel (Ni) and Iron (Fe) contaminants is investigated in this paper. Melts of cartridge brass were made to which 0, 0.5, 1, 2, 3 and 4 wt % of Si was added. These were sand cast into rods of 600 mm by 10 mm diameter and were thereafter machined into standard tensile test samples. Some of these samples were annealed in a muffle furnace at temperatures of 250, 300, 350, 400 and 450 °C respectively and the others were used as control. Subsequently, all the heat treated and non heat treated samples were subjected to tensile tests on a Hounsfield Extensometer and the load — extension plots retrieved were analyzed. Microstructural characterizations of the samples were carried out using Accu-Scope Optical Microscope. The results showed that cartridge brasses subjected to alloying and annealing treatments had improved tensile and yield strengths. However, the tensile properties were increased and maintained within acceptable limits at stress relieve annealing temperatures.

Adegbenjo Adewale Oladapo, Adegbola Adekunle Amos, Raji Fatai Olasunkanmi, Ladiipo Olufemi Akinbiyi, Adeboje Taiwo Bode, Ibitoye Simeon Ademola

Effect of Lime on Alumina Extracting Property of Calcium Aluminate Slag

The effect of lime ratio on phase composition and alumina leaching property of MgO-contained calcium aluminate clinker were investigated by analytical reagent, the XRD analysis of its mechanism was discussed. The results showed that: when the sintering temperature was 1350 °C, C/A was 1.4, A/S was 1.3 and the content of MgO was 4%, the increase of the C/A promoted the formation of C12A7 and restrained the formation of Q-phase. The alumina-leaching ratio of the clinker (A/S was 1.3) was increased from 74.33% to 91.34% when the content of CaO was 5.21%.

Wang Bo, Zhang Yubing, Ma Lijuan, Sun Huilan

Prepare for U3O8 from Ammonium Uranyl Carbonate Using Microwave Calcination

The novel technology of calcination to prepare triuranium octaoxide (U3O8) from ammonium uranyl carbonate in microwave fields was investigated on the basis of the temperature rising characteristics of ammonium uranyl carbonate, triuranium octaoxide (U3O8), and their mixture. The result of experiments show that ammonium uranyl carbonate had weak capability to absorb microwave energy, while triuranium octaoxide has the very strong capability to absorb microwave energy and the sample temperature increased rapidly with an increase mixture ratio of triuranium octaoxide. The optimal calcination conditions were as follows: microwave power 700 W, calcination time 10 min and 60 g in this experiments range, respectively. Under these conditions the value of total uranium andU4+ of triuranium octaoxide was 84.28% and 31.02%, respectively. It is feasible to prepare triuranium octaoxide by calcination from ammonium uranyl carbonate, which mixed with small amounts of triuranium octaoxide under microwave fields.

Liu bing-guo, Peng jin-hui, Zhang li-bo, Hu jin-ming, Guo sheng-hui, Kong dong-cheng

Solar Cell Silicon


Silicon Production and Refining

Directional Growth of Bulk Silicon from Silicon-Aluminum-Tin Melts

Molten Si-Al alloy, with the addition of Sn, was used for metallurgical grade Si purification by a low-temperature solidification method, with the aim of improving the recovery rate of Si. In this ternary melt, bulk Si was grown with a planar front by a directional solidification process under a well-controlled thermal gradient and growth rate conditions, thereby reducing the contamination of solvent metals. Effects of cooling rate, amount of Sn addition, and temperature gradient on the quality of bulk Si were determined. The criterion for constitutional supercooling for single-phase growth from multicomponent melts was employed to evaluate the growth process.

Yaqiong Li, Yi Tan, Kazuki Morita

Preparation of Solar Grade Silicon Precursor by Silicon Dioxide Electrolysis in Molten Salts

Al-Si alloy, a precursor of solar grade silicon, was prepared by direct electrolysis in cryolite molten salt at 950 °C using high purity silica as material, liquid aluminum as the cathode and high purity graphite as the anode. The electrochemical behavior of Si(IV) ion was investigated using cyclic voltammetry method. The electrolysis products were characterized by XRD, SEM/EDS and ICP. The results indicate that the reduction process of Si(IV) on tungsten electrode is a two-step process and there is about 0.6 V gap between the two steps. The contents of boron and phosphorus in the aluminum-silicon alloy are 3 ppmw and 8 ppmw, which will make the directional solidification purification effectively and reduce the cost of preparation solar grade silicon from metallurgical grade silicon.

Liangxing Li, Jinzhao Guan, Aimin Liu, Zhongning Shi, Michal Korenko, Junli Xu, Bingliang Gao, Zhaowen Wang

Effect of Temperature in Extraction of High Purity Amorphous Silica from Rice Husk for Silicon Production

This work shows how pure silica was derived directly from rice husks ash (RHA) in different temperatures for reduction to Silicon (Si). The processes to form Si materials are usually complex, costly and energy-intensive. The silica in the ash undergoes structural transformations depending on the conditions of combustion. A high purity silica was produced at 650°C after burning the rice husk (RH) for up to 7 hours. The purity of the silica was determined by dissolving the RHA with alkali solution to form sodium silicate solution of pH 11.0. This was added to hydrochloric acid of pH 1.5 which lowered the pH of the solution to 4.0 to form silica gels. The silica gels produced were washed and dried to powder and then characterized using XRD and EDX techniques. This study was done to reduce energy waste and environmental pollution in Nigeria as silica is used to develop many materials.

C. E. Ilochonwu, C. O. Nwajagu, I. U. Onyenanu, E. I. Nwonye

Crystallization and Mechanical Properties

Numerical Modeling of Stress Distribution in a Bi-Grain Small Scale Silicon Ingot Including Crucible Deformation

In a previous work, a small scale Bridgman furnace has been used to study silicon bi-grain crystallization at different cooling rate. This work expands the analysis by studying the mechanical interaction between the crucible and ingot during the solidification and cooling. The thermal model is based on a 2D-axisymetric heat-transfer model. The flux histories are then transferred to the ingot-crucible 3D-model. Anisotropic Elastic and Crystal Plasticity model are used to model the silicon deformation. Four different assumptions are applied to model the mechanical contact crucible-ingot and three grain misorientations are considered. The results show the strong impact of the alumina crucible contraction on the stresses and deformations in the silicon ingot.

Sylvain Gouttebroze, Mohammed M’Hamdi

Mechanical Strength of Silicon Solar Wafers Characterized by Ring-On-Ring Test in Combination with Digital Image Correlation

Avoiding wafer breakage is a big challenge in the photovoltaic silicon industry, limiting production yield and further price reduction. Special fracture strength tests suitable for thin silicon solar wafers and solar cells, to be used in combination with Weibull statistics, finite-element (FE) modelling and digital image correlation have been developed in order to study the mechanical stability of solar wafers. The results show that removal of the saw damage significantly increases the strength of crystalline silicon wafers. Furthermore, it was found that silicon crystallinity and the location where the wafer is extracted from the cast Si ingot have a significant effect on the strength, namely samples taken from the bottom of the ingot are 30% stronger than those taken from the top. The study also showed that there is a decrease in fracture strength when an anti-reflective SiNx coating is applied, which is caused by high thermal stresses.

V. A. Popovich, W. Geerstma, M. Janssen, I. J. Bennett, I. M. Richardson

Behavior for Nitrogen and Iron in the Bottom of Casting Multicrystalline Silicon Ingot

The paper studied the diffusion process of nitrogen and characteristic distribution of iron impurities in the bottom of cast crystalline silicon ingot along the direction of crystal growth. According to the dynamics analysis of the decomposition of Si3N4 coating, the diffusion process of nitrogen into multicrystalline silicon ingot during casting process should be divided into three steps. The diffusion length of nitrogen in multicrystalline silicon ingot is 700 µm. The results of EDS analysis indicate that the distribution characteristic of iron in multicrystalline silicon ingot is concerned with the diffusion process of nitrogen in this length. Based on the above research results, the distribution model of iron in casting multicrystalline silicon ingot was established and verified by the experimental results.

Cong Zhang, Kuixian Wei, Wenhui Ma, Jiao Li, Yongnian Dai

Thermal Field Design and Optimization of Directional Solidification for Multicrystalline Silicon Growth

Cast multicrystalline silicon ingots are widely used in photovoltaic manufacturing. A key issue to achieve high solar cell efficiencies is to attain an optimized temperature field during directional solidification (DS) process. This paper reports numerical investigation of multicrystalline silicon (mc-Si) ingot production using two major types of DS furnace. Specific examination is made on thermal distribution, interface shape and stress field. Evaluation is performed for the applicability of thermal system design to reduce thermal stress, improve crystal quality and enhance energy efficiency. The effects of procedure parameters and geometric configuration on temperature distribution are discussed as well to provide the viable solutions for systems optimization.

Wenhui Ma, Xi Yang, Guoqiang Lv

Microstructure and Mechanical Properties of a Screen-Printed Silver Front Side Solar Cell Contact

The most critical processing step during the manufacture of screen-printed crystalline solar cells is firing aluminium and silver contacts, which generates residual stresses and solar cell bowing. In this paper, an alternative Ag contact formation mechanism is proposed and aspects related to electrical contact properties, residual stresses and layer delamination are investigated. It is found that there are two main processing parameters affecting the uniformity and delamination of the Ag/Si interface, namely the peak firing temperature and the silicon surface roughness. Silicon surface polishing gives a better wetting of the silicon surface by the glass layer, resulting in a good contact and lower incidence of large voids, compared to the case of highly textured surfaces. The non-uniformity in the glass layer and large voids at the Ag/Si interface (in the case of a textured surface) are expected to have a negative effect on the mechanical strength of the solar cell.

V. A. Popovich, M. Janssen, I. J. Bennett, I. M. Richardson

High-Temperature Electrochemistry II


Molten Salt Technology

Studies on the Purification of Na3AlF6−K2SiF6−AlF3 Melt

The purity of the electrolyte has an important influence on the content of the impurities in electrodeposited silicon. In this paper, effects of electrolysis temperature, current density and the time on the concentrations of two important impurities: phosphorus and iron in the electrolyte were investigated during the purification of Na3AlF6−10%K2SiF6−AlF3 melt. It was observed that the concentrations of P and Fe in the electrolyte decreased with the electrolysis temperature, current density and the electrolysis time increasing. X-ray fluorescence (XRF) of the electrolyte shows that the concentrations of P and Fe decrease from 4.2ppmw to 1.3ppmw and 204.0ppmw to 42.6ppmw, respectively. The removal rates of P and Fe are 69.08% and 79.12% when the electrolysis was performed for the duration of 3 hours and at a current density of 20mA•cm−2 while the operating temperature was 1225K.

Zhongliang Tian, Shu Yang, Yanqing Lai, Xun Hu, Jie Li

Nuclear and Rare Earth Technology

Electrochemical Separation of Dy and Nd From Nd Magnet Scrap in a Molten LiCl-KCl

The Dy-Nd-Pr-Ni alloy sample was prepared by cathodic potentiostatic electrolysis at 0.65 V (vs. Li+/Li) for 1 h using a Ni plate in a molten LiCl-KCl-DyCl3−NdCl3−PrCl3 system at 723 K. The highest mass ratio (Dy/Nd+Pr) in the alloy sample was observed to be 50 at 0.65 V. Anodic potentiostatic electrolysis at 2.20 V for 12 h was conducted using the Nd-Fe-B magnet electrode in a molten LiCl-KCl system. All elements were almost dissolved from the magnet, and the original form of the magnet disintegrated. After anodic potentiostatic electrolysis at 2.20 V, cathodic potentiostatic electrolysis was conducted at 1.00 V for 5 h using a Mo plate in order to remove the dissolved Fe from the bath. Finally, cathodic potentiostatic electrolysis was conducted at 0.65 V for 12 h using a Ni plate. The mass ratio of Dy/Nd in the alloy sample was determined to be about 18.

Hirokazu Konishi, Hideki Ono, Eiichi Takeuchi, Toshiyuki Nohira, Tetsuo Oishi

Sensors and Advanced Materials

Production of Titanium Oxycarbide from Titania-Rich Mineral Sands

The production of titanium oxycarbide — a consumable anode material used in molten-salt processes for electrowinning titanium — from heavy mineral concentrate, such as a high-titania leucoxene and natural rutile is described. The oxycarbide is prepared by carbothermic reduction of the oxide at elevated temperature. Particle size distribution (of the titanium oxide raw material) and temperature have large effects on the kinetics of the reaction, which seem to follow a “shrinking-core” model. It is anticipated that low porosity (in the anode pellets) would be advantageous during electrolysis. Particle size distribution, extent of reaction during carbothermic reduction, and sintering temperature are expected to affect pellet porosity after sintering, and are being tested experimentally.

Farzin Fatollahi-Fard, Petrus Christiaan Pistorius

Experimental Thermodynamic Study on the Ag-Sb System at Elevated Temperatures

In this paper, the standard thermodynamic properties of Ag3Sb and Ag6Sb intermetallic compounds have been studied by the solid state EMF-method, using Ag+ ion conducting ß-alumina and AgI. The intermetallic compounds were synthesized from pure substances in evacuated silica ampoules and their formation were confirmed by the SEM-EDS analyses. The EMF measurements were made on electrochemical cells of the type [Ag | ß-alumina | Ag-Sb] and [Ag | AgI | Ag-Sb]. The usage of ß-alumina electrolyte, in this study, enabled thermodynamic measurements of the Ag-Sb-intermetallic compounds above 710 K, hitherto unreported. Based on the new experimental data, thermodynamic properties of the Ag-Sb-intermetallic compounds have been determined. The obtained experimental values have been compared with the available literature data.

Markus Aspiala, Fiseha Tesfaye, Pekka Taskinen

Energy Storage Devices, Corrosion and Molten Salt Science

Synthesis of High Performance LiMn0.8Fe0.2PO4/C Cathode Material for Lithium ion Batteries: Effect of Calcination Temperature

LiMn0.8Fe0.2PO4/C composite was prepared by an improved solid-state method and the effect of calcination temperature on properties of the obtained materials was investigated. The results showed that increasing calcination temperature from 600 to 700 °C improved the performance of the LiMn0.8Fe0.2PO4/C due to enhanced crystallinity and increased conductivity, but further increase in calcination temperature to 800 °C led to degraded performance due to particle growth and decrease in porosity. Therefore, the LiMn0.8Fe0.2PO4/C composite prepared at 700 °C exhibited the best electrochemical performance, and could deliver a high capacity of 152 mAh g−1 at 0.1 C, 147 mAh g−1 at 1 C and 114 mAh g−1 at 10 C. In addition, the performance of the LiMn0.8Fe0.2PO4/C and LiMn0.8Fe0.19Mg0.01PO4/C was compared when they were obtained at the optimum calcination temperature.

Enrui Dai, Weibing Chen, Haisheng Fang, Hui Wang, Bin Yang, Wenhui Ma


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