真空碳热还原制备Mg-Li合金热力学分析

提出了真空碳热还原制备Mg-Li合金的新思路,并对还原反应进行了热力学分析,研究了还原反应的反应式、吉布斯自由能及临界还原温度。结果表明:真空碳热还原制备Mg-Li合金具备热力学可行性,且其吉布斯自由能随真空度和反应温度的升高而降低;相同真空度下,该反应的临界反应温度低于真空碳热还原制备金属Mg、金属Li的临界温度,反应更容易进行;当真空度为10 Pa,Li_2O的相对比例为0.1时,真空碳热还原制备Mg-Li合金的临界反应温度为1345 K;在常规皮江法(真空硅热还原法)制镁的反应条件下,不论反应物料中Mg O、Li_2O相对比例为多少,真空碳热还原制备Mg-Li合金均具有热力学可行性。     

Kinetic study on carbothermic reduction of ilmenite with activated carbon

The carbothermic reduction of Panzhihua ilmenite with various additions of activated carbon was investigated by isothermal experiments over the temperature range of 1373 to 1773 K in the argon atmosphere. According to the reaction kinetics recorded by the infrared gas analyzer, it was found that the amount of carbon addition had little influence on the reaction rates at various temperatures except 1473 K. When the reaction temperature was above the eutectic temperature of 1427 K of Fe–C binary system, part of carbon would dissolve into Fe to form a liquid phase, which made the liquid Fe as a diffusion channel of carbon to diffuse to the reaction interface. The carbothermic reduction above 1573 K obeyed the shrinking-core model. The mass fraction of TiC could be determined by the standard addition technique.     

Reduction of Nickel Oxide with Ethanol

This work aims to investigate the reduction behavior of NiO powder by ethanol vapor at 600–1100 K for the reaction times up to 60 min. The products were characterized by mass measurement, x-ray diffraction, and scanning electron microscopy techniques. The reaction of NiO with ethanol essentially consisted of oxide reduction followed by C deposition. At 600 K, significant oxide reduction was attained. Full oxide reduction was observed at 650–1100 K within 10–15 min. At this temperature range, the reduction reaction was controlled by external mass transfer of gaseous species. At the lower temperature range 600–650 K, the reduction rate was sensitive to the temperature change and influenced by the total gas flow rate to a lesser degree. The temperature dependence of C uptake was explained by Boudouard reaction. The results of this study demonstrate that NiO can be completely reduced to Ni by ethanol as predicted by the thermodynamic analysis.     

CaF_2催化真空碳热还原异极矿制备金属锌(英文)

通过异极矿真空碳热还原试验,研究添加CaF2和对碳热还原硅酸锌的的影响。结果表明:CaF2能催化硅酸锌的碳热还原,降低还原温度,缩短反应时间;温度越低,催化效果越好;CaF2的添加量越多,催化效果越明显。CaF2催化真空碳热还原异极矿的较佳工艺条件是:CaF2的添加量约10%,还原蒸馏温度1373K,C/Zn总的物质的量比2.5,系统压强低于20kPa,反应时间约40min。在较佳工艺条件下,异极矿中约93%的锌被还原蒸馏出来。     

Application of vacuum distillation in refining crude indium

Vacuum distillation is a technique suitable for low boiling and melting point materials,to remove the heavy and low vapor pressure impurities at low level.As indium has low melting point and high boiling point,it is suitable for refining by vacuum distillation.First,saturation vapor pressure for major elements in crude indium was calculated by the Clausius–Clay Prang equation,which could approximately predict the temperature and pressure during vacuum distillation process.Second,the activity coefficients for In–Cd,In–Zn,In–Pb,In–Tl at 1373 K,and In–Sn at 1573 K were acquired by means of molecular interaction on volume model.Vapor–liquid equilibrium composition diagrams of those above systems in crude indium were drawn based on activity coefficients.These diagrams could estimate the compositions of products in each process during the refinement of crude indium.Finally,1.2–1.6 ton crude indium was used per day when vacuum distillation experiments were carried out,and experimental results are in good agreement with the predicted values of the vapor–liquid equilibrium composition diagrams.     

Microstructures and hydrogen embrittlement of Ti–49Al alloy

Microstructures and hydrogen embrittlement of Ti–49 at% Al were investigated. Results showed that there were three kinds of microstructures formed by heat treatment at 1423, 1573 and 1703 K. The specimens which were heat-treated at 1573 K, showed better elongation than the others on tensile tests at room temperature, and this kind of specimens were used to investigate the effect of hydrogen on tensile properties of this alloy. After heat-treatment in hydrogen gas at 1073 K for 3 h, the specimens were divided into three groups. In the first group, they were tensile-tested at room temperature in vacuum; in the second group, they were heat-treated at 823 K for 1.5 h in argon gas followed by tensile-testing at room temperature in vaccum; and in the third group, they were tensile-tested at 473 or 573 K in vacuum. Results showed that for the specimens precharged with hydrogen, the elongation was decreased significantly at room temperature, and that the decreased elongation was recovered by removing hydrogen at 823 K in Ar gas, although it was not recovered to that of the specimens without hydrogen. This means that hydrogen decreases the room temperature elongation of this alloy. For the precharged specimens the elongation was also decreased at 473 and 573 K in vacuum. This may indicate that hydrides in the precharged specimens affect the tensile properties in vacuum.     

Study of Reaction Mechanisms for Copper-Cobalt-Iron Sulfide Concentrates in the Presence of Lime and Carbon

The reaction mechanisms for the carbothermic reduction of complex mineral sulfide concentrates in the presence of lime were studied between 1073 K and 1323 K. The reaction mechanisms were studied by stopping the reduction experiments at different times and analyzing the reaction products by x-ray diffraction and scanning electron microscopy techniques. Magnetite (Fe₃O₄) and digenite (Cu_1.8S) were the initial phases formed during reduction of CuFeS₂ and Cu₅FeS₄ mineral particles, such that metallization of iron occurred before copper above 1173 K and at an equal stoichiometric ratio of CaO and C. The metallization of iron was found to take place via reduction of intermediate oxide phase (Fe₃O₄/FeO), whereas metallization of copper occurred via diffusion of S^2? ions away from the mineral particles or via formation of Cu-O-S liquid phase. Metallic iron and cobalt were embedded in the copper matrix due to a preferential reduction of iron and cobalt from the Cu-Fe-S and Cu-Co-S type of mineral particles. The effects of CaO/C ratio were analyzed and the rate of reactions was increasing with an increase in the CaO/C ratio. The formation of liquid phase has been discussed. The experimental results were found to be in good agreement with the thermodynamic predictions.     

Process of producing magnesium by thermal vacuum reduction using silicocalcium as reductant

A new process of producing magnesium by thermal vacuum reduction using dolomite and magnesite as materials and silicocalcium as reductant was studied in this study. The reduction process of MgO by silicocalcium was analyzed by phases analysis of reduction slag through X-ray diffraction(XRD) and the factors influencing the reduction ratio of MgO were investigated. The experimental results show that when using silicocalcium as reductant, the reduction ratio of MgO can be over 93%. In the reduction process, calcium in silicocalcium takes part in the reduction reaction of MgO firstly below 1,000 ℃ and it makes CaSi_2 decompose. It also releases elemental silicon which has more reactive activity and improves the reduction reaction of MgO. That is the main cause that the reduction ratio of MgO using silicocalcium as reductant is 8%–10% higher than that by Pidgeon process using ferrosilicon as reductant under the same conditions.     

High-Temperature Oxidation Kinetics of TiB2 Powders in Air

The oxidation kinetics of TiB₂ powders in air at temperatures up to 1,673 K was investigated using thermal analysis [thermogravimetry and differential scanning calorimetry (DSC)]. DSC-curves had three well distinguishable exo-effects in the temperature ranges of about 760–780, 960–980 and 1,630–1,650 K and a slightly lower intensity in the temperature range 1,000–1,600 K. With the temperature (T) and conversion fraction (α) increasing, the thickness of a scale, its composition and microstructure continuously changed. This led to a change in the limiting-rate step mechanism of the oxygen diffusion transport to the diboride-scale boundary. The powders oxidation macro-mechanism is slightly dependent on their dispersion and morphology, but these factors substantially influence the kinetics. It has been determined that the reaction is of the first formal order. The activation energy is 300–390 kJ/mol at low temperatures and α ≤5–7 % in the beginning of the reaction. In the end of the reaction the activation energy is ~20 kJ/mol, when T and α are high.     

Synthesis of Tin Powder Using Tin Oxide and Ethanol

The reduction behavior of SnO₂ powder has been investigated in the temperature range of 900–1200 K under ethanol flow. Scanning electron microscopy, x-ray diffraction, and mass measurement techniques were used to characterize the products. Full oxide reduction was attained at 1000 K, 1100 K, and 1200 K within about 20 min, 15 min, and 7.5 min, respectively. At 900 K, the extent of reduction increased with the reaction time up to 20 min, but further increases in the time (30 min and 60 min) resulted in a slight mass gain. This was attributable to the C uptake. Spherical Sn particles (diameter ~ 1 μm) were observed at 1000 K and 1100 K. At 1200 K, large beads of Sn (diameter 400–800 μm) were obtained. The spherical particle morphology was attributed to the liquid metallic phase formed during the reaction. The reduction mechanism of SnO₂ in ethanol has been discussed in the light of thermodynamic and experimental results.     

Development of Re‐based diffusion barrier coatings on nickel based superalloys

A diffusion barrier type coating with a duplex layer structure, an inner σ‐(Re, W, Cr, Ni) as a diffusion barrier and outer Ni‐aluminide as an Al reservoir, was formed on a Nickel based, single crystal, superalloy (TMS‐82 +) and on Hastelloy X. Oxidation properties of both the alloys with or without the diffusion barrier coating were investigated in air under thermal cycling between room temperature and 1423 K for up to 360 ks. The inner σ layer with a composition (at%) of (35–40) Re, (15–20) W, (15–25) Cr and (15–25) Ni was produced by electrodeposition of Ni‐70Re and Ni‐20W films from aqueous solutions followed by Cr‐pack cementation at temperatures between 1473 and 1573 K, and the outer Ni‐aluminides of β‐(Ni,Cr)Al + γ′‐(Ni,Cr)₃Al was formed by electrodeposition of a Ni film, followed by Al pack cementation. After the 360 ks oxidation it was found that the structure and composition of both σ layer and alloy substrate were retained with little change. Furthermore, there was little Al in the σ layer. It could be concluded that the Re‐based alloys such as σ (Re(W),Cr,Ni) are very promising candidates as a diffusion barrier between the outer Al‐reservoir layer and alloy substrate at temperature of 1423 K. It was found that the Re(W)‐Cr‐Ni acts as a diffusion barrier for both inward diffusion of Al and outward diffusion of alloying elements in the alloy substrate.     

Efficient Utilization of Nickel Laterite to Produce Master Alloy

To lower the smelting temperature associated with the carbothermic reduction processing of laterite, the optimization of slag and alloy systems was investigated to enable the reduction of laterite ore in the molten state at 1723 K. The master Fe-Ni-Mo alloy was successfully produced at a lower temperature (1723 K). The liquidus of the slag decreased with the addition of oxide flux (Fe₂O₃ and CaO) and that of the ferronickel alloy decreased with the addition of Mo/MoO₃. More effective metal–slag separation was achieved at 1723 K, which reduces the smelting temperature by 100 K compared with the current electric furnace process. A small addition of Mo/MoO₃ not only decreased the melting point of ferronickel alloys but also served as a collector to aggregate the ferronickel sponges allowing them to grow larger. The FeO concentration in the slag and the nickel grade of the alloy decreased with increasing graphite reductant addition.     

The carbothermic route to magnesium

The carbothermic reduction of magnesia to produce magnesium offers the potential of a lower energy and higher productivity route for metal production compared to existing industrial routes. The reaction of magnesia and carbon produces a magnesium and carbon monoxide vapor. Slow cooling of that vapor will allow the reaction to quickly revert and the prevention of this reversion reaction is a major technical challenge. Two main approaches can be taken to prevent reversion and allow recovery of the metal product: rapid quenching of the vapor and dissolving the magnesium directly in a suitable metal solvent before reversion can occur. The commercial viability of either carbothermic route to magnesium is closely connected to the physical chemistry of each system.     

Improvement in oxidation resistance of a Ni3Al-based superalloy IC6 by rhenium-based diffusion barrier coatings

The oxidation behavior of a Ni₃Al-based superalloy IC6 coated with a duplex Re–Cr–Ni–Mo diffusion barrier layer and an Al reservoir layer was investigated in air at 1423 K for up to 1080 ks. The diffusion barrier layer was formed by electroplating Re(Ni) and Ni films on the alloy, followed by Cr pack cementation at 1573 K, and as a result, forms a continuous inner Re–Cr–Ni–Mo diffusion barrier layer and an outer Ni(Cr,Mo,Al) layer. Then a Ni film was electroplated on the Ni(Cr,Mo,Al) layer, followed by Al-pack cementation at 1273 K for 18 ks, to form an Al reservoir layer with a duplex Ni₂Al₃ and γ-Ni(Cr,Mo,Al) layers. After oxidation at 1423 K in air for 1080 ks, the Al reservoir layer changed to a γ-Ni–4Cr–5Mo–12Al (all in at%) layer, on which a protective α-Al₂O₃ scale formed. The Re–Cr(Mo)–Ni layer was stable and effectively retarded the interdiffusion between the Al reservoir layer and the alloy, as a result, the depth of the microstructural change zone of the alloy was less than 15 μm. In contrast, the bare and the coated IC6 superalloy only with an Al reservoir layer were significantly oxidized, accompanied by serious spallation of oxide scales. After oxidation at 1423 K for 1080 ks, the depth of the microstructural change zone of the alloy was about 200 μm for the bare and coated alloy only with an Al reservoir layer. These results indicate that the oxidation resistance of IC6 superalloy can be effectively improved by coating with a Re–Cr–Ni–Mo diffusion barrier layer and an Al reservoir layer.     

Stirring brazing of dissimilar Al/Mg alloy without flux in air

AZ31B magnesium alloy and 2024 aluminum alloy were successfully jointed at aid of mechanical stirring with Sn-Zn-Al filler metal. The microstructure, fracture morphologies, and mechanical properties of joint were investigated. The results show that Mg-Al intermetallic compounds can be avoided by the process. But, a small quantity of porosity is found in the joint. The sheafing strength of joint interface adjacent to magnesium alloy is 35.4 MPa for formation of Mg-Sn intermetallic compounds, which is about 46 % of that of filler metal. While, the shearing strength of joint interfaces adjacent to aluminum alloy is 70.4 MPa for formation of Zn-Sn-Al solid solution, which is about 92 % of that of filler metal.     

Oxidation of Stainless Steel Powder

To understand the corrosion behavior of a model 304L(p)–ZrO₂(s) composite, a 304L stainless steel powder was studied under oxygen at high temperature. Oxidation tests were performed with thermogravimetry. The so-called jumps method, which involves a sudden change of the temperature, was also applied to propose a kinetic model. Two periods with different rate-determining steps could be distinguished for short (<12 h) and long time experiments (12–20 h). SEM observations of oxidized particles revealed an oxide layer structure similar to that of alloy plates of same composition: during the first ten hours period, the external scale surrounding stainless steel particles was found to be chromium oxide; for the second oxidation period, the outer oxide layer was enriched in iron. Considering the relatively short-term oxidation period, a kinetic model based on an outward growth of chromia from oxidation of Cr in solution in the spherical alloy particles was successfully compared to the experimental mass gain curve. The k_p value deduced from this modeling was found to be in agreement with the literature data. The diffusion of interstitial chromium ions is the rate-determining step in agreement with the absence of influence of the oxygen partial pressure.     

Mechanism of Iron Oxide Scale Reduction in 5%H<Subscript>2</Subscript>–N<Subscript>2</Subscript> Gas at 650–900 °C

The reduction behaviour of the oxide scale on hot-rolled, low-carbon steel strip in 5%H₂–N₂ gas at 650–900 °C was studied. In general, the reduction rate of the oxide scale at the centre location was more rapid than that at the near-edge location. In both cases, the reduction rates at 650 °C were extremely low and the rates increased with increased temperature, reaching their maxima at 850 °C. Arrhenius plot of the rate constant derived from the early parabolic stage revealed that the reduction mechanism at 650–750 °C differed from that at 750–850 °C, with the former being oxygen diffusion in α-Fe and the latter most likely iron diffusion in wustite. In all cases, a thin iron layer formed on the scale surface within a very short time and then the thickness of this layer remained essentially unchanged, while the scale layer was gradually reduced via outward migration of the inner wustite–steel interface, as a result of inward iron diffusion through the wustite layer to that interface. More rapid oxygen diffusion through the thin surface iron layer than the oxygen supply rate through interface reaction was believed to result in a lower oxygen potential at the outer iron–wustite interface, thus providing a driving force for iron to diffuse through the wustite layer. The inner wustite–iron interface became undulating initially; then with the rapid advance of some protruding sections, some parts of the wustite layer were reduced through first, and finally the remaining wustite islands were reduced to complete the reduction process. Porosities were generated when wustite islands were reduced due to localized volume shrinkage. Higher oxygen concentrations in the scales of the near-edge samples were believed to be responsible for their slower reduction rates than those of the centre location samples.     

Interdiffusion and Atomic Mobilities in fcc Co-Ga and Co-V Alloys

On the basis of Co/Co-10Ga, Co/Co-12Ga, and Co/Co-10V (at.%) diffusion couples, interdiffusion coefficients in the face-centered cubic (fcc) phase of the Co-Ga and Co-V binary systems were investigated in the temperature range between 1273 and 1573 K by means of the den Broeder method. Based on available thermodynamic information, the interdiffusion data were assessed to develop the atomic mobilities for the fcc Co-Ga and Co-V alloys using the DICTRA software package, and their validity was tested by simulating concentration–distance profiles.     

Size-controlled synthesis of high-purity tungsten carbide powders via a carbothermic reduction–carburization process

In this paper, a novel method is proposed to synthesize high-purity tungsten carbide (WC) powders with different sizes via carbothermic reduction of yellow tungsten trioxide (WO₃) followed by the further carbonization process. The effects of the reaction temperature, reaction time and C/WO₃ molar ratio on the phase transition and morphology evolution of the products are investigated in detail. The results reveal that the morphology of the final products is mainly determined at the carbothermic reduction stage, and the particle size of WC is significantly affected by the C/WO₃ molar ratio and reaction temperature. It can be concluded from experimental results that particle size of WC increased with the increased of temperature, but decreased with increased of C/WO₃ molar ratio. When the C/WO₃ molar ratio is 2.7–3.5, the single phase WC with a size of 178–825 nm can be obtained after further carbonization at 1200 °C.     

Characterization of diffusion-bonded joint between Al and Mg using a Ni interlayer

Aluminum and magnesium were joined through diffusion bonding using Ni interlayer. The microstructure and mechanical performance of the Al/Ni/Mg joints at different temperatures was investigated by means of scanning electron microscope(SEM), electro-probe microanalyzer(EPMA), X-ray diffraction(XRD), Vickers hardness testing, and shear testing. The results show that the addition of Ni interlayer eliminates the formation of Mg–Al intermetallic compounds and improves the bonding strength of the Al/Mg joints. The Al/Ni/Mg joints are formed by the diffusion of Al, Ni and Mg, Ni. The microstructure at the joint interface from Al side to Mg side is Al substrate/Al–Ni reaction layer/Ni interlayer/Mg–Ni reaction layer/Mg substrate multilayer structure. The microhardness of the Mg–Ni reaction layer has the largest value of HV 255.0 owing to the existence of Mg_2Ni phase.With the increase of bonding temperature, the shear strength of the joints increases firstly and then decreases.The Al/Ni/Mg joint bonds at 713 K for 90 min, exhibiting the maximum shear strength of 20.5 MPa, which is greater than that of bonding joint bonded directly or with Ag interlayer. The fracture of the joints takes place at the Mg–Ni interface rather than the Al–Ni interface, and the fracture way of the joints is brittle fracture.