Preparation of highly dispersed superfine W-20 wt% Cu composite powder with excellent sintering property by highly concentrated wet ball-milled process

The ball milling process and the CuWO_4-WO_3 precursors were investigated, and a new highly concentrated wet ball-milled process(HWM) was designed. W-20 wt% Cu composite powders with excellent sintering property were synthesized by highly concentrated wet ballmilled process and co-reduction. The powders were characterized by scanning electron microscopy(SEM), X-ray diffraction(XRD), field electron transmission electron microscopy(FESEM) and laser-diffraction diameter tester.The results indicate that particle size of W03-CuO powder mixtures decreases to 390 nm rapidly with the milling time increasing to 5 h. The CuWO_4 precursors promote the microstructural homogeneity of W and Cu. W-Cu composite powders have a highly dispersed and well sintering property. The particle size of W-Cu powders milled by HWM for 5 h is about 680 nm. High-resolution transmission electron microscopy(HRTEM) result suggests that W phase and Cu phase are mixed at nanometer scale. The above W-Cu composite powders reach the relative density of about 99.3%.     

Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-CuO powder mixtures

Understanding the microstructure of W–Cu nanocomposite powder is essential for elucidating its sintering mechanism. In this study, the effect of milling time on the structural characteristics and densification behavior of W-Cu composite powders synthesized from WO₃-CuO powder mixtures was investigated. The mixture of WO₃ and CuO powders was ball-milled in a bead mill for 1 h and 10 h followed by reduction by heat-treating the mixture at 800 °C in H₂ atmosphere with a heating rate of 2 °C/min to produce W-Cu composite powder. The microstructure analysis of the reduced powder obtained by milling for 1 h revealed the formation of W–Cu powder consisting of W nanoparticle-attached Cu microparticles. However, Cu-coated W nanocomposite powder consisting of W nanoparticles coated with a Cu layer was formed when the mixture was milled for 10 h. Cu-coated W nanopowder exhibited an excellent sinterability not only in the solid-phase sintering stage (SPS) but also in the liquid-phase sintering stage (LPS). A high relative sintered density of 96.0% was obtained at 1050 °C with a full densification occurring on sintering the sample at 1100 °C. The 1 h-milled W-Cu powder exhibited a high sinterability only in the LPS stage to achieve a nearly full densification at 1200 °C.     

Metal injection molding of W-Cu powders prepared by low energy ball milling

Low energy ball milled W/Cu powders were used for metal injection molding (MIM) in order to overcome the low powder volume fraction of MIM parts after debinding as well as the inherently poor sinterability of the W-Cu powder compacts. Ball milling was carried out using commercial fine W and Cu powders to form a powder mixture suitable for injection molding. W powders showed no change in either size or shape during the milling process, but the ductile Cu powders were easily deformed to a three-dimensional equiaxed shape, having a particle size comparable to that of W powders. This modification of powder characteristics by ball milling resulted in an improvement of the solid loading of roughly 58%, maintaining a high and uniform powder packing density in the feedstock. The densification behavior of W-Cu MIM parts is also discussed on the basis of the relationship between Cu composition and W particle size.     

Mechanochemical synthesis of Mo-Cu nanocomposite powders

Mo-Cu nanocomposite powders were successfully synthesized by the mechanochemical (high energy ball-milling) and hydrogen-reduction process at low temperature (650 °C). MoO₃ and CuO powders were used as precursors, which were calcined in air atmosphere to get CuMoO₄-MoO₃ mixtures. The mechanochemical treatment of the CuMoO₄-MoO₃ powder mixtures caused a substantial increase of both the reduction activity of powder mixtures in hydrogen and the refinement of powders. It was accompanied by a transformation from CuMoO₄ to Cu₃Mo₂O₉, playing a critical role in hydrogen reduction process. By optimizing the experimental parameters, Mo-30 wt.% Cu nanocomposite powders with superfine particles with size ranging from 100 to 200 nm could be successfully obtained by mechanochemical-reduction method.     

Characterization of Ni-W solid solution alloy powders and sintered compacts synthesized via mechanically activated hydrogen reduction of NiO-WO3 mixtures

Blended nickel oxide — tungsten oxide powders corresponding to the compositions of 70 wt% nickel — 30 wt% tungsten were mechanically alloyed (MA’d) for different durations such as 0 h, 6 h, 12 h and 24 h and reduced/alloyed at 550 °C for 1 h followed by 600 °C for 0.5 h under hydrogen (H₂) atmosphere. H₂ reduction of the MA’d fine oxide powders resulted in the fabrication of nanocrystalline Ni(W) solid solution alloy powders, whereas a mixture of Ni and WO₂ powders were obtained via hydrogen reduction of asblended oxide mixtures, which revealed the activation of the reduction process by MA. Obtained powders were sintered at 1300 °C for 1 h under H₂ and Ar gas flowing conditions. X-ray diffraction patterns taken from the sintered samples revealed the presence of the Ni(W) solid solution phase for all samples, whereas the presence of elemental W phase was observed in the sintered as-blended and reduced powders. The lowest relative density value of 92.04% and microhardness value of 1.27 GPa were measured for the sintered as-blended and reduced powders, which increased to between 97.62% and 98.72% and 2.19 GPa and 2.23 GPa, respectively, with the applied MA.     

Synthesis of nanocrystalline yttria powder prepared by a polymer solution route

Nanometer-scaled yttria (Y₂O₃) powders were synthesized by a polymer solution route using polyvinyl alcohol (PVA) as an organic carrier. The PVA polymer contributed to a soft and porous powder microstructure, and homogeneous precursor gels containing PVA polymer were effective in making nanometer-sized yttria powders. In this process, the content of PVA and the calcination temperature strongly affected the microstructure and crystallization behavior of the yttria powders. The homogeneous precursors were crystallized to a stable yttria phase at 600 °C for 1 h. In this paper, a simple solution technique for the fabrication of nanometer-sized yttria powders is introduced. The effects of PVA on the powder morphology and on powder specific surface area were studied. The characterization of the synthesized powders was examined by using XRD, DTA/TG, SEM, TEM, a particle size analyzer and nitrogen gas adsorption. The yttria powder synthesized from PVA with a content ratio of 4:1 revealed a crystallite size of about 15 nm with a high surface area of 34.71 m²/g.     

溶胶-凝胶法合成导电SrVO3 粉末

通过溶胶-凝胶法结合热处理合成了导电SrVO₃粉末。在溶胶配制过程,对Sr:V摩尔比进行精确调控,再通过对凝胶热分解行为的表征,确定其煅烧温度和除去残余碳,从而获得前驱体粉末,再将其在H₂中还原以获得最终产物。研究了煅烧温度、Sr:V摩尔比对产物形貌、结构和组成的影响,并采用标准直流四探针技术对样品的电导率进行测试。结果表明,当Sr:V摩尔比为1:1.06,煅烧温度500 ℃,再在850 ℃氢气还原,可以制备没有残余碳或钒的氧化物杂质的单相SrVO₃粉末。SrVO₃粉末的电导率达到714.3 S/cm,比石墨粉末的电导率（500 S/cm）高。     

Optimizing the synthesis of ultrafine tungsten carbide powders by effective combinations of carbon sources and atmospheres

Nanostructured WC powders can provide technologically attractive properties due to the fine microstructures obtained after sintering. Either W or WO₃ powders are used for the industrial production of WC. In both cases, the contact area between carbon and tungsten precursors has a critical influence on the reaction temperatures, which in turn affects grain growth and agglomeration of particles. Different methods have been studied to increase the reaction rates by enhancing the contact between reactants: carbon coating of tungsten powder, solid-gas reactions of tungsten powders with atmospheres containing CH₄, or mechanical activation followed by thermal activation of tungsten and carbon precursors.In this work WC-powders were obtained by mechanical activation of tungsten and carbon precursors followed by thermal activation of these mixes at temperatures up to 1100 °C. A systematic study has been carried out combining two dissimilar carbon sources (graphite and carbon black), with different atmosphere compositions (Ar, Ar-50H₂, Ar-10CO) and studying the evolution of phases at different stages of the synthesis. The results show how the efficiency of the interaction between carbon sources and atmospheres affects the completion of the synthesis. The synthesis of WC from WO₃ in H₂ containing atmospheres is enhanced when using carbon black sources, however in CO containing atmospheres the most effective interaction is with graphite.     

Synthesis of high-purity ultrafine tungsten and tungsten carbide powders

High-purity ultrafine W or WC powder was prepared via a two-step process composed of the carbothermic pre-reduction of WO_2.9 and the following deep reduction with H₂ or carbonization with CH₄+H₂ mixed gases. The effects of C/WO_2.9 molar ratio and temperature on phase composition, morphology, particle size, and impurity content of products were investigated. The results revealed that when the C/WO_2.9 ratio was in the range from 2.1:1 to 2.5:1, the carbothermic pre-reduction products consisted of W and a small amount of WO₂. With changing C/WO_2.9 ratio from 2.1:1 to 2.5:1, the particle sizes were gradually decreased. In order to prepare ultrafine W or WC powder, a relatively high C/WO_2.9 ratio and a lower reaction temperature at this stage were preferred. After the second reaction, the final products of ultrafine W and WC powders with a high purity could be obtained, respectively.     

A systematic study on the synthesis of Ca, Gd codoped cerium oxide by combustion method

Co-doped of CeO₂ nanopowders are ideal electrolyte materials for intermediate temperature solid oxide fuel cells. In this work, Ce_1−(x+y)Gd_xCa_yO_2−(0.5x+y) nanopowders are successfully synthesized by a glycine-nitrate combustion process. Then calcination was carried out at 450, 700, 850, 950 and we found that calcined powders were single phase by room temperature X-ray diffraction (XRD) and have an average crystallite size of 45 nm (based on Scherrer formula). Scanning electron microscopy (SEM) was employed to characterize the morphology of powder. Finally we studied the effect of fuel to nitrate ratio on the properties of resulting powders.     

Hydrogen reduction behavior and microstructural characteristics of WO3 and WO3-NiO powders

The characterization and understanding of the hydrogen reduction and sintering behavior of powder mixtures prepared from WO₃ and WO₃-NiO have been investigated. The nano-sized W and W-Ni powders were prepared by ball milling and hydrogen reduction of oxide powders. The reduction behavior is analyzed by temperature-programmed reduction method with different heating rates in Ar-10% H₂ atmosphere. X-ray diffractometry analysis revealed that the oxide powders are changed to W and W-Ni powders with an average particle size of about 100 nm by hydrogen reduction at 800 °C for 1 h. The hydrogen reduction kinetics was evaluated by the amount of peak shift with heating rates. The activation energies for the reduction of pure WO₃ and WO₃-NiO, estimated by the slope of the Kissinger plot, were measured as 87.4–117.4 kJ/mol depending on reduction steps. The consolidated W-Ni by spark plasma sintering has relatively dense and large grains with neck growth by enhanced mass transport due to the addition of Ni. These results are help to optimize the powder synthesis process and to understand the hydrogen reduction behavior and Ni addition effect related to microstructure of powders and sintered bodies.     

Synthesis of nanostructured powders by new chemical processes

New manufacturing processes, such as thermochemical, mechanochemical and chemical vapor condensation processes, have been developed to obtain nanostructured materials. Nanoscale size tungsten (W) base composite powders of less than 60 nm particle size such as WC/Co and W/Cu systems can be synthesized by thermochemical and mechanochemical processes using metallic salt precursors as starting materials. Nanostructured W base composite materials showed better sinterability, microstructural uniformity with ultrafine microstructures and mechanical properties than did commercial W base composite materials. Non-agglomerated nanoscale size TiO₂ powder can be synthesized by the Combustion Flame Chemical Vapor Condensation (CF-CVC) process using metallorganic precursors as a starting material. In this paper, scientific and technical issues on the synthesis of nanostructured materials by the new chemical processes are considered.     

Preparation and characterization of yttrium iron garnet (YIG) nanocrystalline powders by auto-combustion of nitrate-citrate gel

The nitrate–citrate gel exhibits auto-catalytic behavior, which can be used to synthesize nanocrystalline YIG powders. In this study, yttrium iron garnet (Y₃Fe₅O₁₂) nanocrystalline powders were prepared by a sol–gel auto-combustion process. The influence of metal nitrates to citric acid molar ratio (MN/CA) of the precursor solution on the combustion behavior and crystallite size of synthesized powders was investigated by scanning electron microscopy (SEM), thermal analyses (DTA/TGA) and X-ray diffraction (XRD). The results show that with increasing MN/CA value, the combustion rate increases and the single-phase YIG forms at a higher temperature. The crystallite size of the single phase YIG prepared with different MN/CA values and calcined at 800 °C for 3 h are in the range of 38–70 nm. In addition, the crystallite size of the powders increased with increasing the calcination temperature.     

Nanocrystalline ZrO<Subscript>2</Subscript> preparation and kinetics research of phase transition

The precursor of nanocrystalline ZrO2 was synthesized by solid-state reaction at low heat using ZrOCl2·8H2O,and Na2CO3·10H2O as raw materials.The nanocrystalline ZrO2 was obtained by calcining the precursor.The precursor and its calcined products were characterized using TG/DTA,FT-IR,XRD,and SEM.The results showed that the precursor dried at 353 K was a zirconyl carbonate compound.When the precursor was calcined at 673 K for 150 min,highly crystallization ZrO2 with tetragonal structure (space group P42/nmc (137)) was obtained with a crystallite size of 24 nm.However,when the precursor was calcined at 1023 K for 150 min,highly crystallization ZrO2 with monoclinic structure (space group P21/c (14)) was obtained with a crystallite size of 20 nm.The mechanism and kinetics of the thermal process of the precursor were studied using DTA and XRD techniques.Based on the Kissinger and Arrhenius equation,the values of the activation energies associated with the thermal process of the precursor were determined to be 26.80 and 566.73 kJ·mol-1 for the first and third steps,respectively.The mechanism of ZrO2 phase transition from tetragonal to monoclinic structure is the random nucleation and growth of nuclei reaction.     

A new route for preparing Mo-10wt.%Cu composite compacts

In this work, high pure Mo-10 wt% Cu composite powders with a ultrafine particle size are prepared by a two-step reduction process (first carbothermal reduction and then hydrogen reduction). Ammonium heptamolybdate and copper nitrate trihydrate powders are used as molybdenum and copper source, respectively. The mixtures of raw materials are calcined at 400 °C firstly to form the composite oxides which are then reacted with insufficient carbon black at 1050 °C for 2 h. The as-prepared powders are further reduced by hydrogen at 750 °C for 2 h to obtain the ultrafine Mo-10 wt% Cu composite powders. The experimental results show that the residual carbon of the Mo-10 wt% Cu powders can be decreased to 0.015 wt%, and the composite powders have an average particle size of 200 nm. The sintering behavior of ultrafine MoCu powders and the properties (vickers hardness and thermal conductivity) of samples after sintering are investigated. High sintering temperature is beneficial to increase the density of the compact. At 1200 °C, the relative density of the MoCu compacts is 98.8%. The vickers hardness and thermal conductivity of the Mo-10 wt% Cu composites sintered at 1200 °C for 3 h are 233 HV and 130 (W/m·k), respectively.     

Precursor solution prepared by evaporation deamination complex method for solvent separation of Mo and W by H2O2-complexation

An novel method on preparation of precursor solution for solvent separation of molybdenum (Mo) and tungsten (W) by hydrogen peroxide (H₂O₂)-complexation from the ammonium tungstate solution containing high Mo was studied. The precursor solution was obtained via evaporation deamination and H₂O₂-complex transformation processes. Then it was extracted with a mixture extractant of tri-alkyl phosphine oxide (TRPO) and tributyl phosphate (TBP) to separate Mo and W. The results indicated that the evaporation deamination complex method reduced the acid consumption by more than 90% in comparison with the traditional directly acid regulation complex method. The transformation rates of W and Mo were higher than 95% and the decomposition rate of H₂O₂ was less than 15% at a 1.8-1.9 times H₂O₂ dosage, 45-50 °C, initial pH of 1.80-1.90, and transformation volume ratio of 100% for 60 min in the H₂O₂-complexation transformation process. The minimum extraction rate of W was 2%, the maximum extraction rate of Mo was 82.6% and the highest separation coefficient was 76.7 in a single-stage extraction.     

Characteristics of Fe powders prepared by spray pyrolysis from various types of Fe precursors as a heat pellet material

Aggregated Fe powders comprising elongated and aggregated particles used in the production of heat pellets for application in thermal batteries were prepared by spray pyrolysis. Iron oxide powders comprising dense and hollow particles were prepared by spray pyrolysis from spray solutions containing various types of Fe precursors. Iron oxide powders prepared from iron chloride and iron nitrate precursors were comprised of spherical and micron-sized particles. On the other hand, iron oxide powders prepared from iron oxalate were comprised of large, hollow, and thin-walled particles. The Brunauer-Emmett-Teller (BET) surface areas of iron oxide powders prepared from iron chloride, iron nitrate, and iron oxalate precursors were 17.5, 71.9, and 78.5 m² g⁻¹, respectively. At a low reduction temperature of 550 °C, iron oxide powders prepared from iron oxalate afforded loosely aggregated Fe powders comprised of elongated and loosely aggregated particles, with a BET surface area of 5.9 m² g⁻¹. The heat pellets prepared from Fe powders reduced at 550 °C and composed of fine primary powders had an ignition sensitivity of 0.9 W and a burn rate of 10 cm s⁻¹.     

80W-Cu合金力学行为的拉压不对称性研究

在不同加载应变率下,对熔渗法制备的80W-Cu合金进行压缩和拉伸力学性能研究,对比分析应力状态对80W-Cu合金力学行为的影响规律。结果表明,80W-Cu合金在准静态和动态加载条件下的力学行为均具有明显的拉压不对称性,在压缩加载条件下80W-Cu合金具有良好的塑性,而在拉伸加载条件下材料的塑性极差。微观分析表明,80W-Cu合金具有高的钨-钨连接度,在拉伸加载时钨-钨界面极易发生开裂,导致材料破坏,因此塑性变形能力极差;而在压缩加载时钨骨架发生坍塌变形,塑性良好的铜在压应力的作用下发生延性流动,填充到钨-钨界面之间,从而使材料具有良好的塑性变形能力     

柠檬酸络合盐燃烧法合成DyFeO3纳米粉体

利用溶胶凝胶自蔓延燃烧合成工艺制备得到了DyFeO3纳米粉体,并通过热综合分析(DTA/TG),X射线衍射(XRD)和扫描电子显微镜(SEM)分析探讨了溶液pH值对单相DyFeO3的形成和粉末晶粒尺寸的影响。结果表明,干凝胶能够在400℃以下燃烧基本完全,当pH值为3时,几乎没有杂相生成,将制得的DyFeO3纳米粉体置于750℃煅烧温度下并保温3h,微晶的尺寸在55～90nm范围内。     

Characterization and sintering of nanocrystalline CeO2 powders synthesized by a mimic alkoxide method

Nanocrystalline CeO₂ powders of high sinterability have been successfully synthesized by a mimic alkoxide method, which employs alcohols as solvent, cerium nitrate hexahydrate as cerium source and diethylamine (DEA) as precipitant. Precipitation participating anions (OH⁻) are generated via the hydrolysis of DEA with the molecular water of the cerium salt. Irrespective of solvent type, the precursors produced by this method are CeO₂·2H₂O, which completely dehydrates to CeO₂ at temperatures >500°C. Calcining the precursors at 600°C for 2 h yields highly reactive CeO₂ powders with average crystallite sizes of ∼15 nm. These powders can be densified to >99% of the theoretical up to 1160°C in air at a constant heating rate of 10°C/min or to the same density by isothermal sintering at 1000°C for 2 h. The effects of calcination temperature and solvent type on powder characteristics and sinterability are also investigated.