Controllable preparation of tungsten/tungsten carbide nanowires or nanodots in nanostructured carbon with hollow macroporous core/mesoporous shell

Large scale tungsten nanowires and tungsten nanodots are prepared in a controllable way. The preparation is based on mechanisms of chemical vapor transportation and phase transformation during the reduction of ammonium metatungstate (AMT) in H₂. The AMT is first encapsulated into the hollow core of nanostructured carbon with hollow macroporous core/mesoporous shell (NC-HMC/MS) and forms nanorods, which are the precursors of both tungsten nanowires and tungsten nanodots. Just by controlling H₂ flow rate and heating rate in the reduction process, the AMT nanorods could turn into nanowires (under low rate condition) or nanodots (under high rate condition). Besides, via heat treatment at 1200℃, the as-obtained nano-sized tungsten could convert into W₂C nanorods or WC nanodots respectively. Furthermore, the diameter of the as-obtained tungsten or tungsten carbide is confined within 50 nm by the NC-HMC/MS, and no agglomeration appears in the obtained nanomaterials.     

过渡金属锆铁对钨中氢氦行为的影响

钨被广泛地认为是最具潜力的面向等离子体材料。钨在聚变堆中的服役过程中,会受到强束低能的氢氦粒子流的影响,其结果是钨的性能下降。本工作通过第一性原理计算的方法研究了过渡金属锆铁对钨中氢氦行为的影响。研究结果表明,锆或铁的掺入会使钨的机械强度降低,延展性增加;锆铁的掺入不会改变氢氦在钨中的最佳占位,但是它们对氢氦在钨中的稳定性有一定影响;锆和铁对钨中氢氦的捕获作用存在一个最佳作用半径;铁原子在短距离（< 3.626 Å）时对氦有捕获作用,在长距离（> 3.626 Å）时存在排斥作用,而锆对钨中氢氦均具有捕获作用;钨中氢表现为亲电子的性质,而氦表现出疏电子的特性。总体上讲,锆对钨中氢氦的捕获作用要强于铁对钨中氢氦的捕获作用。本研究工作能够为钨基面向等离子体材料制备提供理论指导。Tungsten was widely considered as a highly promising candidate of plasma facing material, while the presence of hydrogen and helium has an adverse effect on the performance of the tungsten. The effects of transition metals (zirconium, iron) on the behavior of hydrogen and helium in tungsten were investigated by using the first-principles calculation method. The results show that doping of zirconium and iron decreases the mechanical strength of tungsten a little, but they increase the ductility of tungsten; zirconium and iron can't change the best occupied site of hydrogen and helium in tungsten, but they have some effect on the stability of the point defects formed by hydrogen and helium in tungsten; there is the best attraction radius between the transition metals (zirconium, iron) and hydrogen or helium in tungsten; there is an attractive interaction between iron and helium in a short distance (<3.626 Å), but a repulsion interaction in a long distance (>3.626 Å). An attractive interaction exists between zirconium and helium or hydrogen in tungsten whatever the distance is; the hydrogen that in tungsten has an electrophilic nature, while the helium has opposite features. The attraction interaction between zirconium and hydrogen or helium in tungsten is stronger than that of iron. Our works in this paper might provide a theory guide for the selection and preparation of the tungsten based alloy that is used as the plasma facing materials.     

Numerical Study of Shock-Wave Loading of the W- and WC-Based Metal Composites

Technical Physics - Shock-wave loading of metal composites (Elkonites) that represent sintered refractory materials (tungsten, tungsten carbide, or molybdenum) in combinations with low-melting...     

Preparation and characterization of scandia and Re doped tungsten matrix impregnated cathode

The paper describes the preparation and emission property of scandia and Re doped tungsten matrix impregnated cathode. By an easy and reproducible way, solid-liquid doping combined with two-step reduction, powders of tungsten particles covered with scandium oxide were obtained. Compared with scandia mixed tungsten powders prepared by mechanically mixing, scandia and rhenium doped tungsten powders had smaller particle size, for example, scandia (3 wt%) and Re (5 wt%) doped tungsten powders had the average size of about 50 nm in diameter. Based on this kind of powder, scandia and Re doped tungsten matrix with the sub-micrometer sized tungsten grains and a more uniform distribution of Sc₂O₃ were obtained in this paper. Scandia and Re doped tungsten matrix impregnated cathode had shown excellent emission property and good emission uniformity. The space charge limited current densities of more than 58A/cm² at 900 °C_b could be obtained and the work function of this cathode was as low as 1.18 eV.     

Synthesis and electrical characterization of tungsten oxide nanowires

Tungsten oxide nanowires of diameters ranging from 7 to 200~nm are prepared on a tungsten rod substrate by using the chemical vapour deposition (CVD) method with vapour--solid (VS) mechanism. Tin powders are used to control oxygen concentration in the furnace, thereby assisting the growth of the tungsten oxide nanowires. The grown tungsten oxide nanowires are determined to be of crystalline W₁₈O₄₉. I--V curves are measured by an \textit{in situ} transmission electron microscope (TEM) to investigate the electrical properties of the nanowires. All of the I--V curves observed are symmetric, which reveals that the tungsten oxide nanowires are semiconducting. Quantitative analyses of the experimental I--V curves by using a metal--semiconductor--metal (MSM) model give some intrinsic parameters of the tungsten oxide nanowires, such as the carrier concentration, the carrier mobility and the conductivity.     

Ion scattering spectroscopy studies of barium and oxygen on tungsten and tungsten-based dispenser cathodes

In order to use Ion Scattering Spectroscopy (ISS) for studies of tungsten dispenser cathodes, the relevant ISS sensitivities must be measured. Calibrations have been made using a polycrystalline tungsten ribbon with controlled coverages of oxygen, barium and combinations thereof. Auger Electron Spectroscopy (AES) was used to monitor these controlled surfaces and the escape depths of the tungsten Auger electrons in barium and oxygen have been measured. The absolute ISS sensitivities of all three elements were found to be strongly dependent on the barium coverage of the tungsten surface. This effect has been attributed to the lowering of the work function of the tungsten surface caused by the barium adsorption. However, the relative ISS sensitivities of the three elements are not affected in this way when both barium and oxygen (or oxygen alone) are present on the tungsten surface. ISS spectra of such surfaces have been analyzed quantitatively and found to be in reasonable agreement with AES measurements. The analysis has also been applied to ISS spectra of uncoated tungsten matrix dispenser cathodes in an active state and following exposure to oxygen. Compared to AES, these spectra indicate less oxygen on the active cathode surfaces as a result of the oxygen (associated with barium) not contributing to the oxygen ISS signal. Comparisons of the spectra from the active and oxygen poisoned cathodes suggest that oxygen adsorbed during the oxygen exposure sits on the topmost barium layer whereas the oxygen on the active cathode surface does not.     

First principles study of the structural,electronic, mechanical and superconducting properties of WX (X=C,N)

The structural, electronic, mechanical and superconducting properties of tungsten carbide (WC) and tungsten nitride (WN) are investigated using first principles calculations based on density functional theory (DFT). The computed ground state properties, such as equilibrium lattice constant and cell volume, are in good agreement with the available experimental data. A pressure induced structural phase transition is observed in both tungsten carbide and nitride, from a tungsten carbide phase (WC) to a zinc blende phase (ZB), and from a zinc blende phase (ZB) to a wurtzite phase (WZ). The electronic structure reveals that these materials are metallic at ambient conditions. The calculated elastic constants obey the Born-Huang criteria, suggesting that they are mechanically stable at normal and high pressure. Also, the superconducting transition temperature is estimated for the WC and WN in stable structures at atmospheric pressure.     

中能高浓度氦离子注入对钨微观结构的影响

采用15 keV, 剂量1×10¹⁷/cm², 温度为600 ℃氦离子注入钨, 分别以块体钨研究氦离子对钨的表面损伤; 以超薄的钨透射电镜样品直接注入氦离子, 研究该条件下钨的微观结构变化, 以了解氦离子与钨的相互作用过程; 采用扫描电子显微镜、聚焦离子束扫描显微镜、透射电子显微镜、高分辨透射电子显微镜等分析手段研究氦离子注入对钨表面显微结构的影响及氦泡在钨微观结构演化中的作用.     

冷动力喷涂法制备钨和钨合金涂层 及其涂层的计算模拟

采用冷动力喷涂法以纯钨和钨-镍-铁合金为原料在铜合金基体上制备了钨涂层和钨-镍-铁涂层. 研究了冷喷涂过程中钨粉粒径、喷涂距离等因素对涂层性能的影响. 用扫描电子显微镜分析了涂层的表面、断面微观结构, 并用原子力显微镜测量了涂层的粗糙度. 此外, 计算了冷喷涂过程中粉末颗粒的实际速度, 并采用有限元分析软件ANSYS/LS-DYNA模拟了冷喷涂过程中颗粒撞击基体时的变形情况.     

Surface erosion of tungsten and the morphology of erosion products in experiments simulating plasma disruption

The surface erosion of different sorts of tungsten subjected to high-power pulsed plasma streams simulating plasma disruption is studied. With W-13I polycrystalline and (111) single-crystal tungsten samples used as examples, the size distributions for the erosion products collected at different angles to the target are compared. The typical drop erosion of the surface is observed. Fine drops either return to the surface or fly away in a direction parallel to the surface. Coarse drops leave the surface nearly at right angles to the surface. The single-crystal surface displays the absence of fine (<0.125 μm) drops typical of a polycrystalline tungsten surface. The erosion of the single-crystal samples is least among the tungsten sorts considered.     

电解抛光法制备金属钨箔膜

研究了在硫酸甲醇体系中进行电解抛光制备状态方程(EOS)靶用钨薄膜。分析了钨的阳极极化曲线,对薄膜的表面形貌、晶粒取向、密度和厚度一致性进行了测试和分析,并制备出均方根粗糙度小于50nm、厚度一致性好于99%、能够保持原材料密度的钨箔膜,满足激光驱动材料高温高压状态方程研究的标准靶材料的需求,证明电解抛光是制备低表面粗糙度、块材密度的EOS所用金属箔材的有效手段。     

钨-3,5二溴水杨基荧光酮-CTMAB-Tween80多元配合物的分光光度法研究

在混合表面活性剂溴化十六烷基三甲铵 (CTMAB)和吐温 80 (Tween80 )存在下 ,研究了 3,5 二溴水杨基荧光酮 (DBSAF)与钨 (Ⅵ )显色反应的光度特性。在 0 6 0mol·L-1HCl介质中 ,钨 (Ⅵ )与DBSAF及表面活性剂形成胶束络合物 ,采用混合表面活性剂使增溶增敏作用更为显著 ,络合物表观摩尔吸光系数ε =2 6 4× 10 5L·mol-1·cm-1,λmax =5 2 7nm。用摩尔比法和连续变换法测得钨 (Ⅵ )与DBSAF所形成的络合物化学计量比为 1∶2。钨 (Ⅵ )含量在 0～ 4 0 0 μg·L-1范围服从比尔定律 ,拟定分析方法可直接用于合金钢样品中微量钨的测定     

Dependence of work function on coverage for aluminum/tungsten in the field electron microscope

Aluminum adsorption on tungsten has been investigated by field electron microscope techniques. Changes in average work function and work functions of the (001) plane and (111) region were determined as a function of aluminum deposition. The average work function exhibits a minimum of about (4.11 × 0.04)eV (relative to an assumed value of 4.50 eV for the tungsten substrate) and reaches a value of (4.21 ± 0.04) eV for thick deposits. The (001) plane and (111) region exhibit complicated work function dependences on aluminum coverage. Some qualitative observations of aluminum on tungsten surface diffusion are also reported.     

Experiments and analysis of thin tungsten slice and W/Cu brazing for primary collimator scraper in CSNS/RCS

According to the requirements for the beam collimation system of the rapid cycling synchrotron(RCS)of China Spallation Neutron Source(CSNS),the main structure of a scraper of primary collimator is made by W/Cu brazing,in which the thickness of tungsten slice is 0.17 mm.In order to get the best mechanical properties,the brazing temperature is suggested to be controlled under the recrystallization temperature of tungsten,while the recrystallization temperature is affected directly by the thickness of tungsten.Because of little research and application on the brazing of thin tungsten slice of 0.17 mm and copper,tensile tests are done to get the mechanical properties of tungsten slices which experience different brazing temperatures.In keeping the inner relationships between the mechanical properties and temperature,another experiment is done by using SEM to scan the microstructures including the size and distribution of crystals.Finally we determine the recrystallization temperature of tungsten slice of 0.17 mm,and get the best parameters of W/Cu brazing for scrapers of primary collimator in CSNS/RCS.     

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采用射频(RF)等离子体对颗粒形状不规则的钨粉球化,研究了加料速率和钨粉分散方式对球化率的影响。通过用电子扫描显微镜(SEM)观测得到的被球化粉末的百分比评估了球化效率。通过对球化处理的钨粉的X射线衍射谱(XRD)的检测,验证了在球化过程中无氧化发生和其它杂质介入。当钨粉以极短暂时间(约几毫秒)快速穿越等离子体炬时,钨粉颗粒因受热而熔化成液滴,快速冷却后,形成致密的球形固态颗粒。     

射频等离子体制备球形钨粉研究

采用射频(RF)等离子体对颗粒形状不规则的钨粉球化,研究了加料速率和钨粉分散方式对球化率的影响。通过用电子扫描显微镜(SEM)观测得到的被球化粉末的百分比评估了球化效率。通过对球化处理的钨粉的X射线衍射谱(XRD)的检测,验证了在球化过程中无氧化发生和其它杂质介入。当钨粉以极短暂时间(约几毫秒)快速穿越等离子体炬时,钨粉颗粒因受热而熔化成液滴,快速冷却后,形成致密的球形固态颗粒。     

Silicon diffusion through thin tungsten films on silicon,studied with Auger spectroscopy

Silicon out-diffusion through ? 3000 Å tungsten films deposited on silicon by r.f. sputtering was studied using Auger spectroscopy. Silicon first diffuses to the tungsten film surface by grain boundary diffusion and surface migration. The out-diffusion kinetics were most strongly dependent on the thickness of the silicon dioxide layer between tungsten and silicon, and this (native) oxide thickness varied with substrate doping. The out-diffusion rate was independent of tungsten film thickness at 540 Å and 2400 Å. For substrates from which the native oxide was removed by backsputtering just prior to tungsten deposition, no Si out-diffusion to the W film surface was observed until almost the entire film had converted to WSi².     

The study of radiation damage in metals with the field-ion and atom-probe microscopes

The application of the field-ion microscope (FIM) and the atom-probe FIM techniques to the study of radiation damage in metals is reviewed. The FIM research in the following three areas is discussed: (1) the defect structure of the primary state of irradiated metals; (2) the recovery behavior in Stages I, II and III of irradiated body-centered and face-centered cubic metals (tungsten and platinum); and (3) the recovery behavior in Stages I, II and III of irradiated body-centered and face-centered cubic alloys [tungsten (carbon), tungsten (rhenium) and platinum (gold)]. The data obtained in each of these three categories provide fundamental structural, kinetic and thermodynamic information about the properties of point defects and point-defect clusters. The atom-probe FIM research on the direct measurement of the range of 300 eV He⁺ ions and the mobility of interstitial helium atoms in tungsten is also presented and compared with the theoretical predictions of Wilson, Haggmark and Bisson.     

Reconstruction at a metallic interface studied by field ion and field emission microscopy

Reconstruction of a tungsten surface by adsorbed layers of gold, silver and copper has been studied by field emission and field ion microscopy. Gold reconstructs the surface in three ways, termed the α, β and γ rearrangements. The α rearrangement, which results in a smoothing of the tungsten surface, takes place at around 400° K with gold coverages of 5 monolayers (5θ), and is thought to be an increase in structural perfection of the tungsten surface by gold-assisted surface diffusion of tungsten atoms, β-reconstruction takes place in the temperature range 480–950°K at coverages ? 1.7θ, producing a faceted surface which comprises {211} and {110} facets, and is thought to result from the need to minimise the free energy at the gold/tungsten interface. The γ structure, which appears above 1400°K, is believed to represent a change in the shape of the tip by transport of tungsten to the (110) locality. Adsorbed silver produces neither β nor γ structures, and the degree of α rearrangement is very small, being confined to the {230} regions of the substrate. Copper lies between silver and gold in its ability to rearrange the tungsten surface, some degree of α rearrangement is detectable, and the β structure is very poorly developed unlike the γ structure which is clearly formed. The binding strength of copper to tungsten is greater than that of silver, but less than that of gold; the capacity of an adsorbate, to reconstruct the tungsten substrate is therefore thought to be related to the strength of the adsorbate-substrate bond.     

Injection of holes and their localization in defect states at the interface between an electrochromically sensitive single crystal and an amorphous thin film of a complex tungsten oxide

The dynamic current-voltage and estance-voltage characteristics of amorphous thin films of complex tungsten oxides (tungsten oxide bronzes) and, for comparison, tungsten oxide bronze single crystals have been measured with a change in the character of their interaction with electromagnetic radiation. The optoelectronic properties of the samples have been analyzed taking into account the important role of the processes of hole injection, carrier localization in traps, and nanostructurization. A model of the physical processes occurring in the surface and near-surface regions at the interface between a tungsten oxide bronze and a liquid proton electrolyte, different from the generally accepted approach, is considered.