Electrochemical behavior of tungsten in （NaCI-KCI-NaF-W03） molten salt

Abstract The electrochemical reaction mechanism and electrocrystaUization process of tungsten in the NaCl- KCl-NaF-WO3 molten salt were investigated at 973 K （700℃） by means of cyclic voltammetry, chronopotentiometry, and chronoamperometry techniques. The results show that the electrochemical reaction process of tungsten in the NaCl-KCl-NaF-WO3 molten salt system is a quasireversible process mix-controlled by ion diffusion rate and electron transport rate. Tungsten ion in this system is reduced to W（0） in two steps. The electrocrystallization process of tungsten is found to be an instantaneous, hemispheroid three-dimensional nucleation process and the tungsten ion diffusion coefficient of 2.361 × 10^-4 cm2.s^-1 is obtained at experimental conditions.     

Initial stages of nucleation of molybdenum and tungsten carbide phases in tungstate–molybdate–carbonate melts

The electrochemical nucleation of Mo₂C and W₂C crystals for tungstate–molybdate–carbonate melts of specific compositions on Ag, Au, Cu, Pt, and Ni substrates was studied by the electrodeposition method. The influence of electrocrystallization conditions (temperature, deposition time, initial current pulse, and current density) was investigated. Experimental measurements indicate that crystallization overvoltage is associated with three-dimensional nucleation. Carbide deposition onto the substrates prepared from the same solid materials was not associated with crystallization overvoltage. The maximum value of the initial overvoltage, η_max, which includes crystallization and electrolysis overvoltages, is proportional to the electrode surface area. Under these conditions, surface diffusion limits the electrode process rate. An increase in carbide deposition rate leads to an increase in the number of crystallization centers. This reduces the duration of surface diffusion. A rise in the melt temperature complicates the crystallization process by alloy-formation phenomena. Overvoltage maximum height for metals, which do not form alloys, is proportional to the reciprocal of their formation time. Melt temperature increase promotes interdiffusion of refractory metal, carbon, and substrate, and also intensifies their chemical interaction. Structural mismatch is observed during molybdenum and tungsten carbide electrodeposition onto different single-crystal substrates.     

Eliminating Molybdenum from Sodium Tungstate Solution by Selective Precipitation

Eliminating Molybdenum from Sodium Tungstate Solution by Selective Precipitation     

Effect of the fluorine-addition order on the hydrodesulfurization activity of fluorinated NiW/Al2O3 catalysts

Fluorinated NiW/Al₂O₃ catalysts with different orders of fluorine addition have been prepared, tested for hydrodesulfurization (HDS) of thiophene, and characterized using nitric oxide chemisorption and temperature-programmed sulfidation. The catalyst surface area has been affected by fluorine addition but not by the order of fluorination. The fluorine addition-order does not affect the amount of fluorine retained in the catalysts after the calcination and the reaction steps, either. On the other hand, the order of fluorine addition changes the dispersion of the nickel and the tungsten species, incorporation of nickel with the tungsten edge sites, and consequently the HDS activity of the catalysts. The catalyst fluorinated in the last step, i.e., after addition of both tungsten and nickel, shows the highest activity in thiophene HDS, which is supported by other experimental results indicating the most nitric oxide chemisorption and the largest incorporation of nickel with the tungsten species. Accordingly, enhancement of the catalyst activity by fluorination is due to the repartition of the metal species rather than to partial solubilization of alumina in the fluorine-addition step.     

TiCp/W金属基复合材料的制备与性能研究

采用滴定法,在偏钨酸铵（AMT）饱和溶液与碳化钛（TiC）微粉和分散剂组成的悬浮液中,滴入无水乙醇获得AMT包覆碳化钛前驱体;然后对前驱体进行高纯氢还原制备出钨包覆TiC复合粉体,对复合粉体进行了XRD物相和SEM形貌分析。结果表明,当N,N-甲基甲酰胺(DMF)和聚乙二醇(PEG)作为TiC与AMT饱和溶液的分散剂,经600 ℃保温1 h和800 ℃保温30 min H2还原,得到分散良好的类球形复合粉体。采用放电等离子体烧结（SPS）技术制得块体样品,对其进行了SEM形貌分析,并测试了其相对密度和力学性能。结果表明,在1600 ℃,保温2 min,SPS烧结后, TiC在基体中分布均匀,TiCp/W复合材料的相对密度达到94.6%,抗弯强度达到739 MPa,显微硬度达到4.86GPa,断裂韧性达到7.87MPa?m1/2。     

原位生成WC/TaC复合激光熔覆层组织研究

利用激光熔覆技术,将氧化钽和石墨的混合粉末添加到Ni60包WC的镍基合金粉末中,成功制备了TaC/WC复合涂层。使用扫描电镜(SEM)、X射线衍射仪(XRD)等仪器分析了镍基合金复合涂层横断面的显微组织,并对其硬度进行研究分析。结果显示在基体与涂层之间形成了良好的冶金结合,复合涂层不仅含有γ-Ni树枝晶、W₂C、M₇C₃、以及大量的弥散分布的TaC颗粒。复合涂层的硬度可达HV965,是Ni60涂层的1.3倍,主要是因为TaC颗粒的分布促使其内部组织结构改变以及相变引起的硬度上升。     

钨单晶纳米压痕尺寸效应研究

利用纳米压痕仪和扫描探针显微镜对高纯钨单晶的载荷-位移曲线、弹性模量、压痕形貌、纳米硬度-加载深度以及弹性回复率的变化情况进行了研究。结果表明,W(111)晶面在加载和卸载过程中分别经历了弹性变形和塑性变形阶段,荷载-位移曲线未出现不连续现象,表明在加载过程中压痕内部未产生裂纹或脆性断裂;钨单晶的残余压痕表现出堆积形貌,表明钨单晶有较低的加工应变硬化趋势;采用连续刚度法测量了钨单晶的纳米压痕硬度以及弹性模量,结果表明,钨单晶纳米压痕硬度和弹性模量存在尺寸效应,即随着加载深度的增加,单晶的纳米压痕硬度和弹性模量减小;采用 Nix-Gao 模型对钨单晶的纳米压痕力学特征和进行了分析,计算了钨单晶的微观特征长度（h^*）为1490nm,无压痕尺寸效应时的纳米硬度值（H_0）为6.79GPa,尺寸效应因子（m）为0.18,即压入深度小于1490nm时,钨单晶具有明显的尺寸效应,当压入深度超过1490nm时,尺寸效应将减弱。当压入深度超过2450nm时,钨单晶的纳米尺寸效应将消失。     

In situ synchrotron tomography estimation of toughening effect by semi-ductile fibre reinforcement in a tungsten-fibre-reinforced tungsten composite system

Tungsten-fibre-reinforced tungsten composites (W_f/W) are supposed to enable enhanced toughness owing to extrinsic energy dissipation mechanisms such as interface debonding and plastic deformation of fibre. In particular, the latter is an effective source of toughening, since ductile tungsten fibres can absorb a considerable amount of plastic work. For a precise evaluation of the toughening capability, the energy dissipation mechanisms need to be analysed in detail. To this end, single-fibre tungsten composite specimens are fabricated and the stress–strain behaviour of the tungsten fibre bridging a matrix crack is measured by means of in situ high-energy synchrotron microtomography during a uniaxial tensile test. Despite the high X-ray attenuation in tungsten, a sufficiently high resolution is achieved and clear images of crack extension and deformation are obtained. The amount of absorbed energy due to plastic deformation of the tungsten fibre is determined and compared with values obtained conventionally from single-fibre tensile tests.     

Oxide contributions on arc plasma in tungsten inert gas welding of magnesium alloy

Abstract

To study the effects of oxide activating flux on the arc plasma, the investigation is carried out in tungsten inert gas (TIG) welding on magnesium alloy. In this study, five oxides, ZnO, MnO₂, Cr₂O₃, CeO₂ and CdO, are selected to study the roles of activating flux on the plasma characteristics (shape, electron temperature and electron density) and the arc voltage. The mechanism is also discussed by comparisons between single TIG welding processes with and without fluxes. Results reveal that the electron temperature of arc plasma decreases sharply under the influence of oxide; however, the electron density and the arc voltage get enhanced. The reasons for the increase in arc voltage are the greater electricity resistances that result from the improved welding current density and the constriction of conduction channel.     

Petrochemicals from ethanol over a W-Si-based nanocomposite bidisperse solid acid catalyst

Very high ethylene selectivity values approaching 100% and very high ethanol conversion values approaching 85% were obtained in dehydration of ethanol over a new W-silicate-based nanocomposite catalyst having both meso and macropores and containing a W/Si atomic ratio of 0.85. Silicotungsticacid was successfully incorporated into the catalyst structure following a one-pot hydrothermal synthesis procedure. This catalyst is highly stable and does not loose activity in polar solvents and it has a sufficiently high surface area for catalytic applications. Calcination temperature of the catalyst was found to have a very significant effect on the catalyst structure and also on its catalytic performance in ethanol dehydration. Maximum selectivity of the second major reaction product diethylether was obtained as 0.7 at 200 °C, with the catalyst which was calcined at 400 °C. Very high ethylene and diethylether yield values obtained in this study at different reaction conditions are highly promising for the production of petrochemicals from ethanol.