Refining of MG-Si by hybrid melting using steam plasma and EMC

In this study, it was aimed to remove boron, one of the major impurities of MG-Si (metallic grade silicon), in molten silicon by a steam plasma method utilizing EMC (electro magnetic casting) process. In order to effectively remove boron, less volatile than silicon, plasma injection gas was systematically varied. As a result, the most effective way to remove boron was additionally injecting hydrogen gas and steam with argon gas, which is a reactive gas of a general plasma injection routine. Although mass loss of silicon increased with increase in plasma injection time, the refining effect of boron and several metallic impurities like aluminium and titanium was improved significantly.     

Kinetics of boron hydrolysis for hydrogen production

This study examines the kinetics of boron hydrolysis. The boron hydrolysis reaction is classified as a noncatalytic heterogeneous reaction. It was found the steam rapidly hydrolyzes the boron, forming an oxide ash layer that then also reacts with the steam, forms gaseous boric acid, and re-exposes the boron substrate to steam. Additionally, the thickness of the ash layer, once formed, was found to be relatively constant through the reaction. In order to facilitate kinetic analysis the reaction was divided into two stages: a fast oxidation stage and a second, slower stage. The corresponding kinetic parameter for the first stage activation energy is 25 kJ/mol. The shrinking core model was used to model the second stage of the hydrolysis reaction where it was found that diffusion of steam through the ash layer was the rate limiting step.     

Thin silicon string ribbon

Evergreen Solar is a new solar cell company that is commercializing a silicon sheet growth method known as String Ribbon, a method for forming continuous polycrystalline silicon ribbon directly from the melt. Recently, modifications to the growth environment by means of a tunable afterheater have enabled Evergreen to grow flat, 5.6 cm wide and 100 μ thick ribbon with low enough stress to permit solar cell fabrication. The development work was aided by using finite element analysis with a thermoelastic model. Initial work on forming cells on this material was quite promising. The best solar cell made on the 100 μm thick, p-type silicon ribbon ( = 0.1 Ω cm) was 14.7% efficient. It had a V_oc of 639 mV and a fill factor of 0.798 with a J_sc of 28.8 mA/cm². Significant increases in lifetime during processing were not unusual. With better starting lifetimes from higher bulk resistivity values, PC1D modeling indicated that 16% efficient cells should be readily achievable on this material.     

Demineralization of Turkish Tosya lignite coal by boric acid leaching

Boric acid is a weak inorganic acid type which is generally being used in glass, ceramics, detergents, agriculture, nuclear energy, and medicine fields, which has white crystal color and which can easily dissolve in water. Boric acid is being produced from colemanite which is a boron mineral having wide reserves in Turkey. In recent years, intense studies have been performed on the alternative usage areas of boric acid in Turkey. In this study, boric acid was used as a leaching reagent for the demineralization of coal with high ash content. For the accurate determination of the success of boric acid in dissolving mineral matter, first a series of leaching tests were performed with boric acid, and then studies for removing mineral matter with strong acids such as hydrofluoric acid (HF), HCl, and H₂SO₄ were performed.     

Experimental characterization of the accuracy of multidetector boron meter for operational safety of reactors

The accuracy of a boron meter, which is used to measure the concentration of boric acid, is 1 of the factors affecting operational safety in nuclear reactors. The concept of using a multisensitivity detector to improve the accuracy of the boron meter is proposed, and the accuracy improvement is confirmed by Monte Carlo N‐Particle simulation. A Monte Carlo simulation using a multidetector showed a 30 times lower accuracy error (~2 ppm) than a single‐detector boron meter system (~100 ppm). In this study, a new concept of boron meter with improved accuracy is experimentally verified. Experimental verification confirmed that the accuracy was 50% higher than that of the single detector system. To improve the accuracy of the system, the detection efficiency of BF₃ detector was measured in boron concentration, source strength, and neutron irradiation time.     

Induction melting process using segmented graphite crucible for silicon melting

Induction melting process using segmented graphite crucible was investigated to melt silicon feedstock for solar cells. Induction melting is the key technology used in silicon melting process, such as ingot-growing, metallurgical refining and ribbon production. However, contamination of silicon from the crucible in induction melting is the main source of degrading the silicon. In this paper, new structure of crucible was suggested to minimize the contamination from the crucible wall. It was the segmentation of graphite crucible wall that introduced the non-contact between melt and inner wall of crucible. Numerical and experimental studies of induction melting process of silicon using the suggested crucible were conducted. For numerical analysis, 3D models of crucible, induction coil and silicon were constructed and electromagnetic force and temperature distribution in silicon and crucible were calculated. To evaluate a process with the suggested crucible, induction melting system was built up based on the simulated results and experiments of silicon melting were carried out.     

An overview of silicon ribbon growth technology

A silicon ribbon growth method, String Ribbon, is discussed and compared with the two other vertical ribbon technologies. Manufacturing advances in production of 300 μm String Ribbon are described along with characterization of this ribbon, particularly dislocation distribution. Progress on the growth of 100 μm ribbon and in the making of higher efficiency cells are detailed.     

氧化铝陶瓷衬底多晶硅薄膜区熔再结晶的研究

采用简化的区熔再结晶工艺,即区熔过程中不采用隔离层和盖帽层结构,直接在陶瓷衬底上制备出高 质量的多晶硅薄膜。实验中给出了用X射线衍射方法得到的晶向图谱和光学显微镜、扫描电子显微镜方法得到 的表面形貌,并采用扩散法和正面引电极的方法初步探索了电池的制备工艺,电池的开路电压达到196mV,短 路电流为200μA。     

A metallurgical route to produce upgraded silicon and monosilane

We studied alumothermic reduction of silica from silicate slag to obtain silicon-containing Alloy-I and Alloy-II. Phosphorous industry waste and synthetic slag are used as a silicate slag that consists of more than 90% silicon and calcium oxides and less than l0% other elemental oxides. Silicon-containing Alloy-I was upgraded by acid leaching to silicon of a fine powder structure. Using this powder, we grew poly- and mono-crystalline p-type silicon, with resistivity of 0.24 Ω cm, by the Czochralski method. Silicon-containing Alloy-II was used for obtaining monosilane by aqueous treatment with hydrochloric acid under atmospheric conditions and without any catalyst. There was no trace of diborane, which is a common source for boron contamination in crude silane.     

Crystalline and thin-film silicon solar cells: state of the art and future potential

Bulk crystalline silicon solar cells have been the workhorse of the photovoltaic industry over the past decades. Recent major investments in new manufacturing facilities for monocrystalline and multicrystalline wafer-based cells, as well as for closely related silicon ribbon and sheet approaches, ensure this role will continue well into the future. Such investments suggest that the silicon wafer-based approach has successfully withstood the challenge mounted by thin-film chalcogenide-based cells, in the form of polycrystalline films of CdTe and CuInSe₂, as well as that mounted by thin-film cells based on amorphous silicon and its alloys with germanium. The encumbent now faces a fresh challenge by a new wave of thin-film technologies developed in the 1990s, more closely related to the bulk approach and with some advantages over the earlier contenders. One new approach is based on a stack of two silicon thin-film cells, one cell using amorphous silicon and the other mixed-phase microcrystalline silicon. The second uses silicon thin-films in polycrystalline form deposited onto glass, even more directly capturing the strengths of the wafer-based approach.     

The diffusion of hydrogen and inert gas in sputtered a-SiC:H alloys: Microstructure study

The microstructure of DC sputtered amorphous silicon carbon (a-SiC:H) is studied by effusion measurements of hydrogen and of implanted inert gases helium, neon, argon and secondary ion mass spectrometry. The results suggest that the motion of inert gas atoms is controlled by the diffusion, greatly depending on a broadening of network openings. Already at carbon concentrations of 25 at%, isolated voids disappeared presumably because interconnected voids are formed. A void formation is mainly attributed to an increase in hydrogen incorporation in the samples.     

Enhancement of the electrocatalytic activity for the oxygen reduction reaction of boron-doped reduced graphene oxide via ultrasonic treatment

Commercial polymer electrolyte membrane fuel cells have relied on scares Platinum to catalyse the kinetically sluggish oxygen reduction reaction occurring at their anodes. Over the last decade organic materials, frequently based on graphitic structures have been demonstrated as promising alternative electrocatalysts to the noble metals. Researchers typically utilize ultrasonic treatment as part of the synthesis procedure to achieve homogeneous dispersion of graphitic carbon prior to. Herein we investigate the implications of the structural and compositional changes induced by the ultrasonication treatment on boron-doped reduced graphene oxide for oxygen reduction reaction. It is shown that ultrasonication pre-treatment prior to the boron doping and reduction of graphene oxide via hydrothermal process step leads to the increase of both substitutional B and electrocatalytic surface area, with associated reduction of average pore size diameter, leading to a significant improvement in the oxygen reduction reaction performance, with respect to the non-ultrasonicated material. It is proposed that the higher degree of substitutional doping of boron is a result of formation of the additional epoxy functionalities on graphitic planes, which act as a doping site for boric acid.     

Extended combustion model for single boron particles - Part I: Theory

Ramjet engines have significant advantages when compared to conventional rocket motors concerning specific impulse, manoeuvrability, and range. Boron particle addition to the propellant of ducted rockets further increases this potential due to a very high heating value. However, the combustion of boron particles is a very complex process because of an inhibiting oxide layer covering the particles. This layer has to be removed before vigorous combustion can start. The boron particle combustion process runs in two distinct stages. In the literature review presented in this article two combustion models for single boron particles are outstanding. A very detailed model by the Princeton/Aerodyne group features hundreds of elementary reactions and considers all physical processes in the particle environment. It is very elaborate and, thus, not suitable for incorporation into three-dimensional CFD-calculations at present. The second model developed at Penn State University takes on a global approach with only a few reactions which makes it promising for CFD applications. A careful analysis of this model revealed some inconsistencies, errors and drawbacks which gave rise to the new model presented in this paper. The new model comprises a consistent formulation of the heat and mass transfer processes in the particle environment based on a quasi-steady approach, accounts for boron evaporation which is a relevant process despite the high boiling point of boron, and it considers the influence of forced convection on the particle conversion. The chemical reaction rates adopted from the original model were revised and are slightly changed, the differential equations to be solved are corrected and an iterative solution algorithm is introduced. A careful validation of the model is presented in Part II of this paper showing that the new model is suitable for boron particle sizes relevant for ramjet combustion chambers.     

Hydrogen storage in boron-doped carbon nanotubes: Effect of dopant concentration

Boron-doped carbon nanotubes (BCNTs) with varying B content (0–8 at%) were prepared by thermo-catalytic decomposition of ethanol in presence of boric acid at 1073 K. It was observed that hydrogen adsorption capacity improved to a critical B content of 3.86 at% and then decreased. Maximum hydrogen adsorption was found to be 0.497 wt% at 273 K and 16 bar with 3.86 at% of boron doping in CNTs. With the help of transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy, it was found that in addition to dopant concentration, dopant bonding with carbon structures, crystallinity and defects play pivotal roles in determining the extent of hydrogen adsorption by BCNTs. The thermogravimetric studies revealed the oxidation stability of the BCNTs. The hydrogen adsorption kinetics was found to follow the pseudo-second-order model. The rate constant value was minimum for the BCNT with the highest hydrogen storage capacity.     

Characterization and the hydrogen storage capacity of titania-coated electrospun boron nitride nanofibers

In the present work, titania-coated (TiO₂) boron nitride nanofibers were produced by the electrospinning method, and the effect of heat treatment on the nanofibers was studied. Electrospinning method is often adopted for the synthesis of one-dimensional nanofibers due to high productivity, simplicity, and cost-effectiveness. In this study, boric oxide was deposited on co-electrospun polyacrylonitrile and TiO₂. TiO₂-coated boron nitride nanofibers, with a diameter of 100 nm, were obtained after heat treatment and nitridation. The effects of heat treatment on the morphology, surface area and hydrogen storage capacity were studied extensively. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) showed long, bead-free nanofibers and the presence of TiO₂ nanoparticles on the nanofibers. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy depicted hexagonal structures of boron nitride. The hydrogen uptake capacities of the nanofibers were investigated by pressure composition isotherm (PCI) in the pressure range of 1–70 bar at room temperature.     

高纯金属硅吹气造渣除硼工艺研究

高纯多晶硅是太阳能光伏产业的主要原材料,现阶段生产高纯多晶硅的主流技术为西门子法,但是该方法生产成本居高不下,因此探索低成本高纯多晶硅生产技术成为国内外的研究热点之一.目前低成本多晶硅生产技术主要是物理法,包括冶金法和重掺硅废料提纯法两种.冶金法生产太阳能级多晶硅的技术关键在于除硼.选择吹气造渣除硼工艺进行探索性试验研究,利用反应气体和熔渣、硅液中的硼发生氧化反应,从而达到除硼的目的,探索冶金法制备高纯多晶硅的新途径.     

Refining of metallurgical-grade silicon by inductive plasma

A new process combining inductive plasma torch and electromagnetic stirring of molten silicon in a cold crucible has been developed to refine upgraded metallurgical silicon. The addition of reactive gases to the plasma leads to volatilisation of impurities at the liquid silicon surface. The concentration of boron impurities decreased from 15 ppmw in the raw material to less than 2 ppmw after the plasma treatment. The most volatile form of boron was BOH, which was obtained by simultaneous treatment with oxygen and hydrogen. The limitation in boron volatilisation is due to the formation, at high oxygen flow rate, of a silica layer at the surface of the molten silicon, which results in a dramatic drop of the volatilisation rate. In contrast to boron, the concentration of phosphorus was reduced by only a factor of two, although the remainder seems to be electrically neutral. Thermodynamic studies suggest that phosphorus could be trapped as phosphate in combination with metallic impurities. Cells made of material produced using this technique exhibited a conversion efficiency of 12.4%.     

Methanogenesis of water hyacinth in the presence of boron compounds

Cowdung, human excreta, distillery effluents etc. are used for biogas generation in India. The addition of various boron compounds at different concentrations was made to study its effect on the cumulative gas production rate. A retention time of 15 days in a small batch digester was used. Additions of borax, boron hydride, boric and borozol were made at appropriate levels. The addition of such compounds affected the methanogenesis and increased gas generation.     

添加物对石蜡相变螺旋盘管蓄热器蓄热和放热性能的影响

对以石蜡为相变材料的螺旋盘管蓄热器的蓄热和放热性能进行了实验研究,探讨了在石蜡中添加铜粉、硅粉和不锈钢丝带对石蜡螺旋盘管蓄热器蓄热和放热性能的影响。结果表明：在蓄热过程中,随着加热时间的增加,蓄热器内的温度分布不均匀性逐渐增大;纯石蜡蓄热器内温度分布不均匀性最为严重;插入不锈钢丝带的蓄热器内温度分布最均匀。在放热过程中,纯石蜡蓄热器的出口水温下降最快;而石蜡加不锈钢丝带的蓄热器出口水温最高。     

Crystalline silicon thin-film solar cells—recent results at Fraunhofer ISE

Crystalline silicon thin-film solar cells combine the advantages of the stability and high-efficiency potential of crystalline silicon solar cell technology with the low material utilization of the thin-film solar cell technology. At Fraunhofer ISE the wafer equivalent concept is currently pursued. Within this concept, the active silicon layers are deposited on high-temperature stable substrates. The resulting substrate/layer sandwich can be processed into a solar cell using the same techniques that are used in conventional crystalline silicon wafer solar cell processing, hence the name wafer equivalent. In the present paper we report on how we realized wafer equivalents and explain in detail our development work on processors for both large-area silicon deposition and for zone melting recrystallization. An overview is given on the solar cell results achieved in this area.