射频等离子体球化中钼粉颗粒加热过程的数值模拟

对射频等离子体球化中钼粉颗粒的加热过程进行研究可以为优化等离子体制备球形钼粉的工艺过程提供参考。采用了数值模拟的方法研究了线圈电流频率、粉枪位置及送粉速率等参数对钼粉颗粒在射频等离子体中的运动轨迹及加热过程的影响效应。结果表明:线圈电流频率较低时,等离子体炬轴线附近的温度更高,钼粉颗粒在等离子体中运动时能够达到的温度也更高;改变粉枪位置仅对粒径较小颗粒的运动和加热有较大的影响;降低送粉速率可以提高颗粒从等离子体中获得的能量,从而在一定程度上提升钼粉的球化率。     

Densification behaviour of pure molybdenum powder by spark plasma sintering

Pure molybdenum was sintered with SPS under various temperatures, external pressures and heating rates. The microstructure of the specimens representing the different sintering conditions was investigated by classical metallographic methods. The relative density, the microhardness and the chord length distribution were measured. Linear shrinkage, depending on time or temperature, was calculated from piston travel, which was recorded during sintering process. These results show that the main part of consolidation takes place during fast heating up. The densification behaviour is controlled mainly by sintering temperatures and applied pressure. The molybdenum powder was successfully consolidated by SPS in very short times. A relative density of 95% was reached by sintering temperatures of 1600 °C and external pressure of 67 MPa.     

Spark plasma sintering of a commercially available granulated zirconia powder: Comparison with hot-pressing

A commercially available granulated TZ3Y powder has been sintered by hot-pressing (HP). The “grain size/relative density” relationship, referred to here as the “sintering path”, has been established for a constant value of the heating rate (25 °C min^?1) and a constant value of the macroscopic applied pressure (100 MPa). It has then been compared to that obtained previously on the same powder but sintered by spark plasma sintering (SPS, heating rate of 50 °C min^?1, same applied macroscopic pressure). By coupling the analysis of a sintering law (derived from creep rate equations) and comparative observations of sintered samples using transmission electron microscopy, a hypothesis about the densification mechanism(s) involved in SPS and HP has been proposed. Slight differences in the densification mechanisms lead to scars in the microstructure that explain the higher total ionic conductivity measured, in the temperature range 300–550 °C, when SPS is used for sintering.     

选区激光熔化成形射频等离子体球化钽粉的微观组织与力学性能研究

本文采用射频等离子体球化法制备的球形钽粉进行选区激光熔化(SLM)成形致密化研究。通过工艺参数优化,在扫描速度为650 mm/s和激光能量为240 W条件下,获得了完全致密的钽样件。由于原始粉末具有较高的流动性,激光成形钽样件表现出较好的成形性能和表面质量。微观组织表征结果显示,样件顶面和侧面分别呈现等轴晶和柱状晶。EBSD结果表明,在制造方向呈现<111>择优取向。最高显微硬度和抗拉强度分别达到296.2 Hv和697 MPa,致密样件的延伸率也显著提高到28.5%。     

电感耦合等离子体发射光谱法测定金饰品中铼元素

选取346.046 nm作为铼元素的测试谱线,建立了电感耦合等离子体发射光谱法(ICP-OES)测定金饰品中铼元素含量的方法。在0～5.0μg/mL线性范围内,标准曲线相关系数0.9984,检出限为0.0224μg/mL。加标量为1.0, 3.0, 5.0μg/mL时,相对标准偏差(RSD,n=6)分别为0.64%,0.55%,0.78%,平均加标回收率分别为102.7%,97.7%,93.7%。结果表明,该方法适用于金饰品中铼元素的测定。     

Ultrafast synthesis of nanocrystalline molybdenum powder by thermal plasma and its sintering behavior

Nanocrystals sintering is a critical way to achieve full-densified compact with fine grain size, which is highly desired to improve its mechanical properties by grain boundaries strengthening. In this work, a one-step plasma synthesis strategy was developed to synthesize Mo nanopowder using ammonium molybdate as precursor, and the reaction could be completed by an ultrafast way in several seconds. As obtained products exhibit single-crystalline structure and quasi-spherical shape with size of about 30 nm. The sintering behavior of Mo nanoparticles was further investigated by isothermal and nonisothermal processes, and the results reveal that Mo compact experiences different grain growth and microstructure evolution at different sintering stages. Fully densified Mo compact (99.13%) with well suppressed grain growth of about 7 μm was obtained at a rather low temperature of 1500 °C, and kinetics analysis illustrates that the surface diffusion with low activation energy is responsible for the high sintering activity of nanosized powder at low temperature.     

Columnar grain growth of yttria-stabilized-zirconia in inductively coupled plasma spraying

Yttria-stabilized-zirconia (YSZ) was deposited 7 mm thick by inductively coupled plasma spraying (ICP) and a theory of nucleation and solidification of YSZ was introduced. The concentration was homogeneous within a particle, but different from particle to particle. The solute rejection by diffusion occurred in a layer during solidification, but the interface condition of (dT _q/dz)_z=0 > m(dC/dz)_z=0 led columnar grains to facilitate. The microstructure of the bottom part showed small equiaxed grains. In the middle part, large columnar grains, about 100 μm thick and 300 μm long, were developed through the layers with strong adhesion. Heat of droplets, latent heat of solidification, small pore array in the splat boundaries, and low thermal conductivity retard the heat transfer, and thus thick and long columnar grains could be facilitated. The long columnar grain growth through the layers was supposed to be possible when the previously solidified surface of the deposit acts as seed.     

A study of the microstructure of yttria-stabilized zirconia deposited by inductively coupled plasma spraying

Zirconia stabilized with 20 wt.% yttria was deposited to a thick free-standing type, ∼7 mm, by (inductively coupled plasma spraying) (ICPS). The spheroidization of particles and the microstructure of deposits were analyzed. Spheroidization fraction dependence on spray parameters such as particle size, H₂ gas mixing quantity, and probe position was studied. Effects of parameters on the spheroidization of particles were analyzed by ANOVA (analysis of variance) (ANOVA). ANOVA results showed that the spheroidization fraction largely depend on H₂ gas mixing quantity and particle size, and there are also some dependence on probe position and H₂ gas mixing quantity. After melting, particles kept their chemical composition homogeneously from the center to their surface without segregation or evaporation. The degree of deformation of the diameter of the splat over the diameter of the spheroidized particle was approximately 320%, and splat thickness in the deposit varies between 2 μm and 3 μm depending on the deposition condition. The yttrium concentration gradient of the interlayer boundary appeared linear in the range of 0.5 to 1 μm. X-ray diffraction analysis and a transmission electron microscope (TEM) micrograph showed that low yttrium content particles resulted in tetragonal phase in deposit. The major characteristics of the microstructure of the thick free-standing deposit and solidification mode were studied. Microstructure of the bottom part of the deposit represented equiaxed or cellular structure. Equiaxed small grains prevailed when the droplets were quenched rapidly on substrate. The middle part of the deposit showed large columnar grains, of about 100 μm thick and 300 μm long. This may be due to high substrate or deposit temperatures and results in recrystallization and grain growth. The effects of the parameters, such as H₂ gas mixing quantity, particle size, spraying distance, and probe position, on the microstructure of the deposits were evaluated. The H₂ gas mixing quantity of Ar/H₂=120/20 L/min compared to Ar/H₂=120/10 L/min resulted in larger grain size and thicker cellular in chill. Grain shapes were affected by the heat removal rate from the deposit to its surrounding. Deposition with larger particle size showed heterogeneous grain size, insufficient particle melting, and incomplete recrystallization. The effect of probe position was less than the others.     

Syntheses of carbon nanomaterials by radio frequency plasma enhanced chemical vapor deposition

We report a simple and effective method for syntheses of carbon nanomaterials, including a hybrid CNTs/cobalt particles shelled with graphitic layers, pure CNTs, a CNTs/carbon nanosheets (CNSs) composite and pure CNSs, on O₂-thermal etched Co substrate films, in discharging different mixed gases (CH₄/H₂, CH₄/H₂/O₂, CH₄/Ar and CH₄/Ar/O_2, respectively), using radio frequency plasma enhanced chemical vapor deposition (PECVD). Being etched by O₂ during the pretreatment stage, Co thin film transformed into small CoO particles and consequently the deoxidization of CoO particles may occur in PECVD process when different reactive gases are used to obtain various carbon nanomaterials. We find that the reactive gases govern the degree of how the CoO particles can be deoxidized and determine the final carbon nanomaterial products. In this work, the possible growth mechanisms for obtaining various carbon nanomaterials have been explored.     

Preparation of perovskite powders and coatings by radio frequency suspension plasma spraying

Perovskite-type LaMnO₃ powders and coatings have been prepared by a novel technique: reactive suspension plasma spraying (SPS) using an inductively coupled plasma of approximately 40 kW plate power and an oxygen plasma sheath gas. Suitable precursor mixtures were found on the basis of solid state reactions, solubility, and the phases obtained during the spray process. Best results were achieved by spraying a suspension of fine MnO₂ powder in a saturated ethanol solution of LaCl₃ with a 1 to 1 molar ratio of lanthanum and manganese. A low reactor pressure was helpful in diminishing the amount of corrosive chlorine compounds in the reactor. As-sprayed coatings and collected powders showed perovskite contents of 70 to 90%. After a posttreatment with an 80% oxygen plasma, an almost pure LaMnO₃ deposit was achieved in the center of the incident plasma jet. This paper originally appeared in Thermal Spray: Meeting the Challenges of the 21st Century; Proceedings of the 15th International Thermal Spray Conference, C. Coddet, Ed., ASM International, Materials Park, OH, 1998. This proceedings paper has been extensively reviewed according to the editorial policy of the Journal of Thermal Spray Technology.     

感应等离子体工艺对制备致密球形钼粉的影响

研究了原料钼粉携带气体流量和钼粉加料速率对感应等离子球化钼粉形貌和粒度的影响。球化前后钼粉的微观形貌用扫描电镜(SEM)观测,粉体物相用X射线衍射仪(XRD)分析,等离子体处理前后钼粉的松装密度和振实密度分别用斯科特容量计和振实密度仪检测。结果表明,等离子体处理后产品仍为纯金属钼粉,形状由不规则变为球形,颗粒平均粒径由原料的40～70 μm减小至35~60 μm,振实密度由2.5 g/cm³提高到5.8 g/cm³。制备球形钼粉最佳工艺参数为：加料速率45 g/min,携带气体流量0.6 m³/h。感应等离子体是制备高纯致密球形钼粉的有效技术。     

Properties evolution of additive manufacture used tungsten powders prepared by radio frequency induction plasma

In this study, radio frequency induction plasma was adopted to improve tungsten powder properties to meet the requirements of additive manufacturing process. After plasma spheroidization process operated, tungsten particles changed from irregular shape to uniform spherical shape. The average size and oxygen content of raw tungsten powders were both decreased, meanwhile, the laser absorption coefficient was increased by plasma spheroidization process. Furthermore, a series of tests were carried out for raw and spherical tungsten powders to investigate the influence of plasma spheroidization process on powder flowability evolution. The results displayed that, plasma spheroidized powders had a better comprehensive flowability, such as basic flow property, aerated flow property, compressibility, permeability and shear property, comparing to raw tungsten powders. In conclusion, plasma spheroidized tungsten powders met demands of additive manufacturing process very well.     

Spark plasma sintering of a commercially available granulated zirconia powder—II. Microstructure after sintering and ionic conductivity

The microstructure of TZ3Y zirconia samples sintered by spark plasma sintering was investigated using transmission electron microscopy. The results of these observations were used to confirm the mechanisms involved in the control of densification. For the second time, the ionic conductivity of some samples obtained by SPS was investigated as a function of temperature. The results were compared with the best results found in the literature and discussed.     

Hydrogen plasma induced crystallization of Si thin films by remote inductively coupled plasma source assistant pulsed dc twin magnetron sputtering

Hydrogenated microcrystalline silicon thin films (μc-Si:H) were deposited by remote inductively coupled plasma assistant pulsed dc twin magnetron sputtering at temperatures below 300 °C. The formation of μc-Si:H was only found in the environment of hydrogen plasma, where Ar and H₂ mixed gas was used. In pure argon plasma or without the assistance of ICP in the Ar/H₂ gas mixtures, all the samples were amorphous structure. It suggested that ICP hydrogen plasma which enhanced the density and energy of H radicals played the key role in the formation of μc-Si:H films.     

Analytical characterization of the corrosion mechanisms of WC-Co by electrochemical methods and inductively coupled plasma mass spectroscopy

WC-Co hardmetals are used for their combined high hardness and toughness. However, their poor corrosion resistance in aqueous solutions reduces the spectrum of their application. The goal of this work was a systematic investigation of the corrosion mechanisms of the WC-Co composite with electrochemical methods and analytical chemistry solution analysis characterization. WC-Co, Co and WC samples were investigated by electrochemical impedance spectroscopy, potentiodynamic polarization and inductively coupled plasma mass spectroscopy (ICP-MS). Concerning the corrosion susceptibility, the solution pH dominates the effect of specific ions. In neutral and acidic solution, the corrosion process of WC-Co consists mainly of Co dissolution. WC dissolution becomes more significant at alkaline pH. Degradation is mainly the result of selective uniform dissolution of the phases (Co or WC) not of localized corrosion because of the poor passivating ability of Co (except in alkaline pH). Synergistic effects due to galvanic coupling between the Co binder and WC are accelerating Co dissolution and hindering WC dissolution in the hardmetals compared to the pure compounds. This although the Co binder phase contains W and C, making it more corrosion resistant than pure Co. A further influence of the locally separated anodic and cathodic reactions is that the cathodic reduction on WC induces local pH increase which causes chemical dissolution of WC detected only by ICP-MS.     

射频感应等离子体反应沉积碳化钨

用射频感应等离子体将钨粉炭化成碳化钨，对炭化过后的粉末在氩气保护下，于l900℃进行烧结处理，并对在感应等离子体中钨粉被甲烷炭化的反应过程和在高温基体上碳与沉积物进一步反应的机理进行了研究。采用扫描电镜和X射线衍射仪分析了原始钨粉、炭化粉末和反应沉积物的显微组织和成分。结果表明，快速炭化反应开始于金属钨粉熔化后，颗粒尺寸为5μm的钨粉在等离子体中被部分炭化，完全炭化要靠反应沉积物在高温基体上与碳的进一步反应来实现，经炭化的钨粉在热处理时存在脱碳现象。     

Study on behaviors of tungsten powders in radio frequency thermal plasma

This paper describes an investigational study of the spheroidization of refractory metal tungsten powders by radio frequency thermal plasma, with emphasis on the melting, solidification and growth behavior of the tungsten powder particles during the spheroidization process. The flight time and melting time of tungsten powder particles in the plasma were estimated, and the growth behavior of the tungsten powder particles was analyzed in detail by investigating the change in the average particle size before and after plasma spheroidization. The morphology and spheroidization rate were analyzed using field emission scanning electron microscopy. The flight time and melting time for tungsten powder particles with radius of 7.8 μm were calculated to be 9.3 ms and 2.9 ms, respectively. The change in powder particle size during the process showed that the growth of tungsten powder particles was mainly caused by the coalescence of droplets in the thermal plasma system. The experimental results demonstrated that the spheroidization rate can reach up to 95% under the operating conditions used in this work.     

Quantification of trace amounts of impurities in high purity cobalt by high resolution inductively coupled plasma mass spectrometry

An analytical method using high resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) for rapid simultaneous determination of 24 elements (Be, Mg, Al, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Ga, Ge, As, Se, Mo, Ag, Cd, Sn, Sb, Ba, Pt, Au, and Pb) in high purity cobalt was described. Sample digestions were performed in closed microwave vessels using HNO3 and HCl. The matrix effects because of the presence of excess HCl and Co were evaluated. The usefulness of high mass resolution for overcoming some spectral interference was demonstrated. The optimum conditions for the deter- mination were tested and discussed. The standard addition method was employed for quantitative analysis. The detection limits were 0.016-1.50 μg?g?1, the recovery ratios were 92.2%-111.2%, and the RSD was less than 3.6%. The method was accurate, quick, and convenient. It was applied to the determination of trace impurities in high purity cobalt with satisfactory results.     

Hydrophobic over-recovery during aging of polyethylene modified by oxygen capacitively coupled radio frequency plasma: A new approach for stable superhydrophobic surface with high water adhesion

In this work, a new approach for the stable superhydrophobic surface with a high water adhesion has been found from an aging process at a high aging temperature of 90 °C during an aging time up to 24 h for the low density polyethylene (LDPE) modified by the oxygen capacitively coupled radio frequency plasma (CCP) under a radio frequency (RF) power of 200 W for an exposure time of 5 min. The plasma nanotexturing produced the nanofibrils array on the LDPE surface showing the contact angle of approximately 0°. During the aging process, the as-modified superhydrophilic LDPE surface underwent a hydrophobic over-recovery into the superhydrophobicity with a pinned water droplet showing the contact angle higher than 150°. A time-dependent contact angle model on the nanotextured surface was developed from the Cassie's equation for the heterogeneous surface containing different wetting patches, to describe the hydrophobic over-recovery due to the surface restructuring on the nanofibrils array. The stable superhydrophobicity with the high water adhesion was attributed to the CCP modified LDPE surface with the nanofibrils composed of the heterogeneous chemical compositions of polar and nonpolar chains/groups after the aging process.