The Kinetics of Boron Transfer in Slag Refining of Silicon

Slag refining is one of the few metallurgical methods for efficient removal of boron from silicon. In industrial slag refining for production of silicon for photovoltaic applications, the relationship between the slag composition and the mass transfer rate of boron from metal to slag is of great importance. The kinetics of boron removal from liquid silicon during slag refining has been investigated in the present work by means of several small-scale experimental series at temperatures between 1,600°C and 1,650°C. Slag and metal, in batch weights of 30?g, were heated together in a graphite crucible placed in a resistance-heated tube furnace. The slags were produced from powdered SiO₂, CaO, and MgO. The oxides applied were of high purity. Experiments were carried out at slag-to-metal ratios of 1 or 2, where the silicon initially contained approximately 250?ppm boron. Metal?Cslag mass transfer coefficients were calculated based on chemical analyses of the silicon and ranged from 1.7???m/s for slag consisting of equal amounts of SiO₂ and CaO by weight, to 4.3???m/s for a 40%?C40%?C20% SiO₂-CaO-MgO slag.     

Mechanism of boron removal from Si–Al melt by Ar–H2 gas mixtures

A new method about purification of metallurgical grade silicon (MG-Si) by a combination of Si–Al solvent refining and gas blowing treatment was proposed. The morphologies and transformation of impurity phases, especially for boron and iron in Si–Al melt were investigated during Ar–H₂ gas blowing treatment. The mechanism of boron removal was discussed. The results indicate that gas blowing can refine grain size and increase nucleation of the primary Si. Boron can be effectively removed from MG-Si using the Ar–H₂ gas blowing technique during the Si–Al solvent refining. Compared with the sample without gas blowing, the removal efficiency of boron increases from 45.83% to 74.73% after 2.5 h gas blowing. The main impurity phases containing boron are in the form of TiB₂, AlB₂ and VB compounds and iron-containing one is in the form of β-Al₅FeSi intermetallic compound. Part of boron combines [H] to transform into gas B_xH_y (BH, BH₂) and diffuses towards the surface of the melt and is volatilized by Ar–H₂ gas blowing treatment under electromagnetic stirring.     

Temperature Effects on the Oxidation of Low Carbon Steel in N2–H2–H2O at 800–1200?°C

A low carbon, low silicon steel was reacted with flowing N₂–H₂–H₂O gases at temperatures of 800–1,200?°C, to produce scales of fast-growing wüstite, the only stable iron oxide under these conditions. Scaling kinetics were parabolic, after an initial period of linear reaction. The parabolic rate constants measured in the range 800–1,100?°C were two orders of magnitude lower than values predicted from Wagner’s diffusion theory, and activation energies were higher than expected. At 1,200?°C, however, the measured parabolic rate constant was in agreement with prediction. The initial period of linear kinetics was extensive at 1,100?°C, and rates are in agreement with those predicted for surface reaction rate control. Because the surface reaction is much less sensitive to temperature than the solid-state diffusion process, the extent of the linear kinetic regime is smaller at lower temperatures.     

Active Oxidation of Liquid Silicon: Experimental Investigation of Kinetics

Small scale laboratory experiments on the oxidation of liquid silicon have reproduced important features of the industrial refining of liquid silicon: active oxidation led to the formation of amorphous silica spheres as a reaction product. The boundary condition for active oxidation in terms of maximum oxygen partial pressure in the bulk gas was found to lie between 2·10^?3 and 5·10^?3?atm at T?=?1,500?°C. The active oxidation of liquid silicon had linear kinetics, and the rate was proportional to bulk oxygen partial pressure and the square root of the linear gas flow rate, consistent with viscous flow mass transfer theory. Classical theory for unconstrained flow over a flat plate led to mass transfer rates for SiO_(g) which were 2–3 times slower than observed. However, computational fluid dynamic modeling to take into account the effects of reactor tube walls on flow patterns yielded satisfactory agreement with measured volatilization rates.     

Evaluation of Factors on Removing Boron from Silicon by Slag Refining Under Atmospheric Conditions

Upgraded metallurgical grade silicon (Si) is available for high-efficiency solar cells. In the present article, Si refining with CaO-SiO₂-CaF₂ slags is carried out under open atmosphere, and orthogonal analysis is applied to evaluate the influencing factors. The results show that the influence order of the objective factors is: (1) slag/Si ratio?>?(2) refining time?>?(3) original slag basicity?>?(4) CaF₂ content and that the slag/Si ratio acts as the key factor in boron removal. It is observed that boron decreases effectively within 30 min refining period. Moreover, the slags are pulverized when the basicity is?>?1.39 because of the β-γ phase transition of Ca₂SiO₄, and the pulverization is beneficial to the separation of Si from slag.

     

CaO-SiO_2熔渣精炼去除冶金级硅中杂质硼(英文)

利用CaO-SiO2熔渣去除冶金级硅(MG-Si)中的杂质硼。热力学分析和实验结果表明:纯SiO2基本上不能去除冶金级硅中的杂质硼。通过建立硼的分配系数与熔渣中SiO2和CaO活度之间的关系,从热力学上对CaO-SiO2熔渣的除硼能力进行表征。结果表明:随着渣中CaO配比的升高,硼的分配系数和去除效率大大提高。当熔渣组成为60%CaO-40%SiO2(质量分数)时,硼的分配系数达到最大值1.57。当渣硅比为2.5,精炼温度为1600°C以及精炼时间为3 h时,利用60%CaO-40%SiO2熔渣可以将冶金级硅中的硼含量从18×10-6降低至1.8×10-6,去除效率达到90%。     

Heat treatment of superbainitic steels

Abstract

Two experimental high silicon high carbon steels (with 5 and 24 ppm of boron separately) have been investigated for the development of superbainite structure. After austenitisation, the specimens were held respectively at three different isothermal transformation temperatures (150, 200, and 300°C) for a variety of time intervals. The microstructures were examined via optical metallography (with microhardness measurement) and transmission electron microscopy. It was found that after isothermal transformation at 200°C for 10 days, both steels produced a high volume fraction of sheaf structures with nanometre scaled bainitic ferrite subunits, which contributed to an ultrahigh microhardness, up to 675 HV. It was also found that adding 24 ppm of boron accelerated the bainitic transformation in the early stage of isothermal transformation at 200°C, but did not have a significant effect on reducing the finish transformation time. Both isothermal temperature of 150 and 300°C could not lead to the development of high amount of bainite.     

Structural Evolution of Silicon Oxynitride Fiber Reinforced Boron Nitride Matrix Composite at High Temperatures

The structural evolution of a silicon oxynitride fiber reinforced boron nitride matrix (Si-N-O_f/BN) wave-transparent composite at high temperatures was investigated. When heat treated at 1600 °C, the composite retained a favorable bending strength of 55.3 MPa while partially crystallizing to Si₂N₂O and h-BN from the as-received amorphous structure. The Si-N-O fibers still performed as effective reinforcements despite the presence of small pores due to fiber decomposition. Upon heat treatment at 1800 °C, the Si-N-O fibers already lost their reinforcing function and rough hollow microstructure formed within the fibers because of the accelerated decomposition. Further heating to 2000 °C led to the complete decomposition of the reinforcing fibers and only h-BN particles survived. The crystallization and decomposition behaviors of the composite at high temperatures are discussed.     

Effect of Thermal Pretreatment and Acid Leaching on the Removal of Boron from Metallurgical Grade Silicon

The present work aims at investigating the impact of thermal pretreatment and acid leaching (HCl–HF) on the boron removal efficiency of metallurgical grade silicon (MG-Si). The impact of various parameters, involving oxidation temperature (700–1200 °C), oxidation time (1–5 h) and acid leaching (4 mol L^?1 HCl–3 mol L^?1 HF), on the removal of boron from MG-Si was thoroughly explored. It was found that thermal oxidation resulted in an enhanced removal efficiency of boron from MG-Si. By employing MG-Si particles in the range of 75–106 μm in conjunction with acid leaching at 65 °C for 6 h, the boron content was decreased from 19.60 to 14.10 ppmw, offering a removal efficiency of ca. 28%. When the MG-Si powder was subjected to thermal oxidation at 1100 °C for 5 h before leaching, the boron concentration in the purified Si was reduced from 19.60 to 8.90 ppmw, giving an extraction efficiency of 54.59%. An extended characterization study, regarding the microstructure, morphology and chemical composition of both un-treated and treated samples, was conducted to gain insight into the underlying mechanism of boron removal under different conditions.

     

Grain refining of Al-Si alloys using Al-10% Ti master alloy: role of Zr addition

The present study was carried out on ultra-pure aluminum, 1050, and 356 alloys. The results revealed that when Al-4%B is added to ultra-pure aluminum, it forms AlB₁₂ and AlB₂ which have no grain refining effect in pure aluminum. When no grain refiner is added to pure aluminum, the microstructure is a mixture of columnar and equiaxed grains. The addition of 30 ppm Ti to pure aluminum is sufficient to promote equiaxed grains when the metal is solidified at high rate ~ 10°C/s but requires an addition of 1000 ppm B at 0.8°C. Increasing the Si content to 7% reduces the initial grain size of pure aluminum from 2800 μm to 1200 μm. In commercial aluminum, B reacts with traces of Ti to form TiB₂ phases which are active grain refining agents. However, in Al-7%Si, Ti reacts with Si forming (Al,Si)₂Ti phase which is a poor refining agent.     

Matrix Transformation in Boron Containing High-Temperature Co–Re–Cr Alloys

An addition of boron largely increases the ductility in polycrystalline high-temperature Co–Re alloys. Therefore, the effect of boron on the alloy structural characteristics is of high importance for the stability of the matrix at operational temperatures. Volume fractions of ε (hexagonal close-packed—hcp), γ (face-centered cubic—fcc) and σ (Cr₂Re₃ type) phases were measured at ambient and high temperatures (up to 1500 °C) for a boron-containing Co–17Re–23Cr alloy using neutron diffraction. The matrix phase undergoes an allotropic transformation from ε to γ structure at high temperatures, similar to pure cobalt and to the previously investigated, more complex Co–17Re–23Cr–1.2Ta–2.6C alloy. It was determined in this study that the transformation temperature depends on the boron content (0–1000 wt. ppm). Nevertheless, the transformation temperature did not change monotonically with the increase in the boron content but reached a minimum at approximately 200 ppm of boron. A probable reason is the interplay between the amount of boron in the matrix and the amount of σ phase, which binds hcp-stabilizing elements (Cr and Re). Moreover, borides were identified in alloys with high boron content.     

Laboratory investigation on the condensation and corrosion rates of top of line corrosion in carbon steel: a case study from pipeline transporting wet gas in elevated temperature

A glass cell was designed to simulate the condition for top of line corrosion encountered in wet gas transportation pipelines. Aqueous solution of 3 wt-% NaCl saturated with CO₂ at atmospheric pressure was employed. Effect of temperature gradient in the formation of condensation and its rate was investigated. API 5L Grade X65 carbon steel material was used as the working electrode for the experiment. The condensation rate was measured for the temperature gradient of 20°C, 30°C and 50°C under atmospheric condition for 24?h duration of the experiment. The corrosion rate of the specimens was measured using weight loss and Linear Polarisation Resistance (LPR) techniques. The LPR probe was immersed in the collected condensed water from the experiment to calculate the corrosion rate. The measured corrosion rate from the weight loss technique was in agreement with the corrosion rate measured from LPR. The corrosion rate measurement was repeated with the addition of 1000?ppm of pH modifying agent. This study indicates that the corrosion rate of pipeline and piping when subject to temperature gradient of 50°C and above is very high and alarming.     

Notch toughness in hot-rolled low carbon steel wire rod

Charpy V-notch toughness has been investigated in four hot-rolled, low carbon steels with different grain sizes and carbon contents between 0.019 and 0.057%. The raw material was wire rod designed for drawing and possible subsequent cold heading operations and manufactured from continuous cast billets. In this study, the influence of microstructure, mechanical properties, and alloying elements on the ductile-brittle transition behavior has been assessed. A particular emphasis has been given to the influence of boron with contents up to 0.0097%. As a result, transition temperatures between −29 and +50°C explicated by the material properties have been obtained. The examination also shows that the transition temperature raises with circa 0.5°C for each added ppm boron most likely as a consequence of an enlargement of the ferrite grain size and the reduction of yield and tensile strength. The highest upper shelf energy and lowest transition temperature can be observed in a steel without boron additions and with maximum contents of carbon, silicon, and manganese.     

CO gas-sensing properties of CuO-TiN and CuO-TiO2 prepared via an oxidizing process of a Cu-TiN composite synthesized by a mechanically induced gas-solid reaction

A Cu-TiN composite powder was synthesized by ball milling a Ti₂Cu alloy for 25 h in an N₂ pressurized atmosphere for use in CuO-TiN and CuO-TiO₂ p-type sensing materials. The sensing materials were prepared via a two-step oxidizing process following an investigation of their CO gas-sensing properties. The resistances of both sensing materials increased as CO was introduced, which is typical for a p-type semiconductor at temperature higher than 200 °C. It was found that the CuO-TiN powder shows a better response above 200 °C when compared to the CuO-TiO₂ powder. The highest response shown by CuO-TiN was 3.18, while CuO-TiO₂ shows a response of 2.20 for 1000 ppm of CO gas at 250 °C. It is believed that the lower responses of CuO-TiO₂ at temperature higher than 200 °C was due to the falloff of p-type resistances by simultaneous operation of p- and n-type semiconductors in the presence of CO gas.     

Characteristics of TiN thin films grown by ALD using TiCl4 and NH3

Titanium nitride thin film was deposited on a silicon wafer by the Atomic Layer Deposition (ALD) method using TiCl₄ and NH₃ as source chemicals. Nitrogen gas was used for carrying the TiCl₄ and purging the reactants. The gases were introduced into the reaction chamber in the sequence of TiCl₄?N₂?NH₃?N₂ for the saturated surface reaction on the wafer. TiN film was grown with [100] preferred orientation at 350°C, while with [111] preferred orientation at 450°C and higher temperatures. The deposition rate was constant as 0.17 Å/cycle irrespective of deposition temperature, which demonstrates TiN film was grown by the ALD growth mechanism. TiN thin films grown at a temperature higher than 450°C with thickness of 320 Å showed electrical resistivity as low as 72×10^?6 Ωcm.     

Gas sensing properties of nano-crystalline SnO<Subscript>2</Subscript>-CuO compounds

We fabricated a micro gas sensor for hydrogen sulfide (H₂S) gas using MEMS technology and the sol-gel process, and synthesized SnO₂-CuO as a sensing material by the sol-gel method. Synthesized particles of SnO₂-CuO were characterized with an average particle size of about 40 nm as measured by FE-SEM imagery and XRD peaks. The sensing material was coated on the micro platform and annealed at 400 °C. The maximum gas sensitivity (R_s= R_g/R_a) was 0.005 at 300 °C for 1.0 ppm — H₂S. The gas sensitivity showed linear behavior with increasing H₂S concentration.     

Structural and morphological characteristics of nanocrystalline copper electrodeposits from acid sulphate electrolytes

Copper is the presently favoured and future interconnect material in high-end microprocessors and memory devices because of its low electrical resistivity and higher electromigration than aluminium. The present investigation deals with the electrodeposition of nanocrystalline copper onto brass metallic foil from electrolytes containing copper sulphate (CuSO₄·5H₂O) as the source of metal ion and sulphuric acid (H₂SO₄). Benzotriazole (0.5?g?L^??1) and sodium lauryl sulphate (0.1?g?L^??1) were used as additives. The electrolyte was mechanically agitated and the temperature was maintained at 3°C?±?2°C. These additives have been found to be effective in reducing the grain size, grain boundaries and improving surface morphology of the copper films. They also improve the throwing power of the deposition electrolytes and hardness of deposits. X-ray diffraction (XRD) patterns obtained for the electrodeposited copper films showed polycrystalline cubic structure. The crystal size of the copper films was calculated by both XRD and atomic force microscopy (AFM) analysis. A uniform and pore free surface morphology was observed under SEM, and AFM investigation revealed the grain refining brought about by the additives.     

Etching mechanisms during plasma jet machining of silicon carbide

The material removal of the C- and Si-face of 4H-SiC using a 13.56 MHz RF excited plasma jet source at atmospheric pressure using helium as feed gas and CF₄ as reactive gas has been investigated. Additionally O₂ is provided together with the peripherally injected N₂ shielding gas and it is shown that a decrease of the etching rate with an increase of the O₂ gas flow occurs.Furthermore, etching experiments under sample heating have been carried out for different [CF₄]/[O₂] mixtures to obtain the activation energy of fluorine and oxygen with the surface. A minimum in the etching rate at a temperature of approximately 150 °C has been found. Therefore XPS and SEM analyses have been carried out for surfaces etched at sample temperatures of 25 °C, 150 °C and 400 °C showing an elevated fraction of silicon oxides and film thickness at 150 °C.     

Influence of thermal treatment on structure and microhardness of Fe75Ni2Si8B13C2 amorphous alloy

Correlation between hardness of amorphous Fe₇₅Ni₂Si₈B₁₃C₂ alloy and thermally induced structural transformations has been investigated by measuring microhardness in a series of samples heated at different temperatures from 25 to 1000 °C. The alloy has a relatively high hardness in the amorphous state, due to its chemical composition involving silicon, boron and carbon. As the alloy begins to crystallize, microhardness increased and reached a plateau in 500–650 °C temperature region, due to formation composite structure involving the small nanocrystals of α-Fe(Si) and Fe₂B phases dispersed in the amorphous matrix. After treatment at higher temperatures, the nanocomposite structure is replaced by a more granulated structure, leading to decline in microhardness.     

Chemical and mechanical alterations of SiC Nicalon fiber properties during the CVD/CVI process for boron nitride

To improve the interfacial properties in SiC/SiC composites, BN is an appropriate interphase material to control the fiber/matrix bond. Unfortunately, the gaseous phase (NH₃,BF₃,HF,Ar) used to deposit BN acts aggressively upon Si–C–O (ex-PCS) Nicalon fiber surfaces, and weakens that bond through the formation of a complex interfacial sequence (SiO₂/C), which actually controls the localization of debonds. The reactions between each gas involved and the fiber surface have been studied. Further, if the fiber surface consists of SiC or any silicon-containing compound, the BF₃ gas reacts through a substitution of the silicon by boron in the initial fiber composition. Then, the surface evolves from a (C,O,Si) composition to a (B,C,O,Si) glassy layer. Such a reaction occurs mainly before the BN nucleation, and it alters the reinforcing potential of fibers in composites. This boron-containing glass is shown to be very unstable in the presence of HF gas (the main reaction product).