The production of niobium powder and electric properties of niobium capacitors

In order to evaluate the electrical and frequency properties of niobium powder manufactured by the metallothermic reduction method for use as a capacitor, the present study measured capacitor performance evaluation factors such as leakage current, permittivity loss (tanδ) and capacitance, etc. The niobium powder used in this experiment was processed using the external continuous supply method and had large coarse globular particles of 0.5 μm to 1.0 μm, but the granularity distribution was very irregular. Capacitance decreased significantly from 150 μF in electrolyte (wet cap) to 130 μF after carbon (C)/silver (Ag) solution coating, and to around 115 μF after aging, falling within the capacity tolerance of tantalum capacitors. Converted to CV/g, capacitance was around 81,000 CV/g. Permittivity loss (tanδ) decreased significantly from 13.0 % after C/Ag coating to 7.5 % after aging, satisfying the general standard level of 10 % or less. Leakage current was 2.5 μA after C/Ag coating and 3.0 μA after aging, both less than the standard level of 6.3 μA. On the whole, the niobium capacitor showed somewhat more unstable characteristics than commercial tantalum capacitors but is nonetheless considered applicable as a future substitute for tantalum capacitors.     

Kinetics of thermal decomposition of niobium hydride

High-purity niobium powders can be obtained from the well-known hydride-dehydride (HDH) process. The aim of this work was the investigation of the structural phase transition of the niobium hydride to niobium metal as function of temperature, heating rate and time. The niobium powder used in this work was obtained by high-temperature hydriding of niobium machining chips followed by conventional ball milling and sieving. X-ray diffraction measurements were carried out in vacuum using a high-temperature chamber coupled to an X-ray diffractometer. During the dehydriding process, it is possible to follow the phase transition from niobium hydride to niobium metal starting at about 380 °C for a heating rate of 20 °C/min. The heating rate was found to be an important parameter, since complete dehydriding was obtained at 490 °C for a heating rate of 20 °C/min. The higher dehydriding rate was found at 500 °C. Results contribute to a better understanding of the kinetics of thermal decomposition of niobium hydride to niobium metal.     

钽铌电子材料新进展

通过降低杂质含量、改善物理性能等新技术以及Ta2O5钠还原制取高比电容钽粉、TaCl5低温钠还原制取纳米级钽粉的新工艺方法,生产了性能优良的高比容钽粉,并研究了其微观结构;同时开发了制造高比电容铌粉、一氧化铌粉的新技术,制得了高性能的电容器级铌粉和一氧化铌粉,为铌电容器作为一种新类型电容器产业参与竞争提供了优质的基础材料.     

The fabrication of niobium powder by sodiothermic reduction process

Niobium powder was fabricated by sodiothermic reduction process using K₂NbF₇ as a raw materials, KCl and KF as the diluents and sodium as a reducing agent. The apparatus for the experiment was designed and built specifically for the present study. Varying properties of niobium powder depending on reaction temperature and excess of reducing agent were analyzed. The niobium particle size increased significantly as reduction temperature increased from 993 to 1093 K. The particle size was fairly uniform at given reaction temperature, varying from 0.2 μm to 50 nm depending on the reaction temperature. The yield of niobium powder increased from 58% to 83% with a increase in reaction temperature. The average particle size of niobium powder was improved from 70 nm to 0.2 μm with increase in the amount of sodium excess. In addition, the yield of niobium powder was 82% in the 5% sodium excess.     

Dry sliding wear in case-hardened niobium microalloyed steels

Low-carbon steel samples containing a small addition of niobium singly or in combination with nitrogen have been carburized in a natural Titas gas atmosphere at 950 °C and 15 psi gas pressure for different time periods. At the end of the predetermined time period, the specimens were pre-cooled to 860 °C in the furnace and quenched in 10% brine. One set of the quenched specimens was tempered at a low temperature of 160 °C and the other set was sub-zero treated at −195 °C in liquid nitrogen, followed by tempering at the same tempering temperature. Surface hardness was measured by Rockwell hardness testing machine and optical microscopy was performed on etched samples. Using a pin-on-disc type apparatus, wear test was carried out under dry sliding condition to assess the beneficial effect of niobium and niobium with nitrogen on the wear properties of the carburized and hardened low-carbon steels in relation to the resulted surface hardness and microstructures.

It has been found that niobium with or without nitrogen improves the wear resistance under both the heat treatment conditions. Niobium with nitrogen is more effective than niobium in improving the wear resistance. Whatever was the heat treatment condition, the wear rate of the specimens increases for all the steels as the carburizing time increases. It has also been found that samples with sub-zero treatment always have higher wear resistance than that of samples without sub-zero treatment. Niobium singly or in combination with nitrogen has been found as a modifier of the wear mechanism.     

On the sulphidation mechanism of niobium and some of Nb-alloys at high temperatures

The kinetics and mechanism of niobium sulphidation have been studied as a function of temperature (700-1000 °C) and sulphur pressure (10⁻⁴-10⁴ Pa) in pure sulphur vapour and H₂-H₂S gas mixtures, using microthermogravimetric technique. It has been found that in both different sulphidizing atmospheres the sulphidation process follows parabolic kinetics, being thus diffusion controlled. Marker experiments have shown that the slowest step of the overall reaction rate is the outward diffusion of cations. No influence of small amounts of impurities on the sulphidation rate has been observed in this study. Excellent agreement between calculated and experimentally determined parabolic rate constants has been obtained under the assumption, that the correct formula of the sulphide scale on niobium is Nb_2±yS₃ and not Nb_1+xS₂, as suggested by Gesmundo.It has been found that the rate of niobium sulphidation in H₂-H₂S gas mixtures is much higher than in pure sulphur vapour, strongly suggesting that the dissolution of hydrogen in the growing scale influences the defect structure in this sulphide.     

Niobium Powders of Mesoporous Structure

The mesoporous structure of niobium powders of specific surface areas from 32 to 150 m²/g obtained through reduction of niobium-oxide compounds by magnesium vapors has been investigated. A doubling of the specific surface area of magnesium-thermic niobium powders in comparison to tantalum powders has been shown to be caused by the larger volume and smaller size of pores. For a powder with a specific surface area of 150 m²/g, 90% of the surface is governed by pores of sizes smaller than 5 nm. Although the X-ray pattern of the powder corresponds to the metal niobium, 96.5% of this powder weight consists of a natural surface-oxide film, according to the TGA data. The thickness of this oxide decreases in comparison with the surface oxide on the compact metal depending on the powder mesoporous structure.     

Producing titanium powder by continuous vapor-phase reduction

One of the goals of the U.S. Department of Energy’s Albany Research Center is to reduce the cost of titanium parts by developing a continuous titanium process. In this work, titanium powder was produced by feeding liquid TiCl₄, with argon as a carrier gas, and magnesium wire into a shaft reactor at 1,000°C. The magnesium and TiCl₄ vaporized and reacted to produce a mixture of titanium, MgCl₂, and magnesium powder. Ti/Mg/MgCl₂ powder was removed from the argon gas stream by an electrostatic precipitator, and the titanium powder was separated from the magnesium and MgCl₂ by either vacuum distillation or leaching. Vacuum distillation produced sintered titanium powder with lower oxygen levels, but unacceptably high levels of magnesium and chlorine. Leached powder was spherical and free-flowing with low levels of magnesium and chlorine, but the oxygen content was no lower than 0.82%. The high oxygen content of the leached powder is caused by surface oxidation of the submicrometer titanium powder. For more information, contact S.J. Gerdemann, Albany Research Center-Department of Energy, 1450 Queen Avenue S.W., Albany, Oregon 97321-2198; (541) 967-5964; fax (541) 967-5868; e-mail gerdeman@alrc.doe.gov.     

Development and evaluation of a dynamic powder splitting system for the directed energy deposition (DED) process

A dynamic powder splitting system (DPSS) was developed to overcome a slow powder flow rate (PFR) response in conventional powder delivery systems used in the Directed Energy Deposition (DED) additive manufacturing process. Complementary to the main powder delivery system, the DPSS uses a custom valve to split powder flow and cyclone separator to redirect powder flow that is not used in the cladding process. Its potential applications are compensation for clad height inconsistencies caused by machine feed rate errors and increase of powder usage efficiency via a powder recycling system. Various tests were performed to verify performance and cladding.     

Deformation behaviour of nanocrystalline magnesium

The deformation behaviour of nanocrystalline magnesium was studied. Nanocrystalline magnesium powder was prepared by high energy ball milling in a dry inert gas atmosphere and vacuum cold pressed to form fully dense cylindrical specimens. Compression tests were carded out at room temperature. The samples exhibited remarkably high values of ductility, and a significant improvement in yield strength in sintered specimens. Large yield drops were observed on the stress-strain curve of sintered specimens. The strain rate sensitivity of the flow stress was also measured and compared to the values for commercially available pure magnesium bars. The values of activation volume as well as the stress exponent were found to be significantly lower in the nanocrystalline samples. It is speculated that grain boundary sliding controls the deformation mechanism in the nanocrystalline magnesium samples.     

基于STAR-CCM+的旋风分离器下行流量的数值模拟分析

旋风分离器内部气体的下行流量与旋风分离器的性能密切相关。通过采用STAR-CCM+对旋风分离器内部流动情况进行数值分析,发现传统下行流量计算方法存在一定缺陷。在模拟结果基础上提出了一种改进的计算旋风分离器计算下行流量的方法,并采用自定义的量纲一化平均行程的概念分析各参数对下行流量的影响。研究表明:随着进气流速减小、升气管插入深度增加、进气口宽度减小,量纲一化平均行程增加;随着升气管直径减小,量纲一化平均行程先减小,后逐渐增加。     

Experimental Analysis of Spray Dryer Used in Hydroxyapatite Thermal Spray Powder

The spray drying process of hydroxyapatite (HA) powder used as a plasma spray powder on human hip implants was examined. The Niro-Minor mixed spray dryer was studied because it incorporates both co-current and counter-current air mixing systems. The process parameters of the spray drying were investigated: temperature, flow rate of the inlet hot air in the spray dryer, viscosity of feed/HA slurry, and responses (chamber and cyclone powder size, deposition of powder on the wall of spray dryer, and overall thermal efficiency). The statistical analysis (ANOVA test) showed that for the chamber particle size, viscosity was the most significant parameter, while for the cyclone particle size, the main effects were temperature, viscosity, and flow rate, but also their interaction effects were significant. The spray dried HA powder showed the two main shapes were a doughnut and solid sphere shape as a result of the different input.     

Microstructure and properties that change during hard cyclic visco-plastic deformation of bulk high purity niobium

The effect of strain amplitude and strain rate on the microstructure and the properties that change during hard cyclic visco-plastic deformation of bulk niobium of high purity at room temperature are systematically measured. These changes in the refractory metal niobium were studied at different tension-compression strain amplitudes (up to Ɛ = ±2%) and strain rates (up to έ (t) = 0.4 s⁻¹) during hard cyclic visco-plastic deformation in dependence on von Mises strain (up to Ɛ_vM = 13.8) during the equal channel angular pressing, respectively. The pure Nb was specified with respect to microstructure, micromechanical properties, density, gas content, tensile strength, Young's modulus, viability and fracture mechanics at fatigue failure for use in industry. The micro hardness and the indentation modulus of the nanostructured shear bands were significantly higher, but the plasticity was lower than that of the body metal between the shear bands. The decreasing Young's modulus (when increasing the strain rate) is related to the fatigue failure of the niobium during the tension-compression cycling and shows nucleation and thickening of the shear bands, as well as the changes in grain boundaries in the pre-fracture state.     

燃烧合成法制备超细 TiC 粉体(英文)

利用镁热、碳热还原钛铁矿,原位燃烧合成制备了超细 TiC 粉体。通过理论分析、实验研究分析了燃烧合成的规律及镁含量对产物特性的影响。结果表明,钛铁矿-镁-碳是一个强放热体系,体系开始反应的温度为 577.7 ℃,当钛铁矿与镁的摩尔比为 1:4 时,燃烧合成酸洗出的 TiC 微粉纯度最高,并显示了较窄的粒度分布,其平均粒度为 229.6 nm。     

Preparation of niobium borides NbB and NbB2 by self-propagating combustion synthesis

Preparation of niobium borides NbB and NbB₂ was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts in this study. Effects of the sample green density, preheating temperature and starting stoichiometry on combustion characteristics, as well as on the composition of final products were studied. Experimental evidence indicates the self-sustained reaction zone propagating along a spiral trajectory for the reactant compacts without prior heating or preheated at 100 °C. The increase of initial sample temperature to 200 and 300 °C by prior heating brings about a planar flame-front propagating in a steady mode. As the preheating temperature or sample green density increased, the combustion temperature was found to increase and the propagation rate of combustion wave was correspondingly enhanced. According to the temperature dependence of combustion wave velocity, the activation energies associated with the Nb + B and Nb + 2B reactions were determined to be 151.8 and 132.4 kJ/mol, respectively. As indicated by the XRD analysis, the final composition of burned products was essentially governed by the starting stoichiometry of reactant compacts. Synthesized products composed of a single boride phase NbB and a small amount of unreacted Nb were obtained from the reactant compacts of Nb:B = 1:1. In addition, the SHS reaction of powder compacts with an initial composition Nb:B = 1:2 yielded niobium diboride NbB₂ as the dominant phase, along with another boride phase Nb₃B₄ in a minor quantity.     

Effects of niobium additions on the structure,depth, and austenite grain size of the case of carburized 0.07% C steels

Carbon (0.07%) steel samples containing about 0.04% Nb singly and in combination with nitrogen were carburized in a natural Titas gas atmosphere at a temperature of 1223 K (950 °C) and a pressure of about 0.10 MPa for 1/2 to 4 h, followed by slow cooling in the furnace. Their microstructures were studied by optical microscopy. The austenite grain size of the case and the case depths were determined on baseline samples of low-carbon steels and also on niobium and (Nb + N) microalloyed steel samples. It was found that, when compared to the baseline steel, niobium alone or in combination with nitrogen decreased the thickness of cementite network near the surface of the carburized case of the steels. However, niobium in combination with nitrogen was more effective than niobium in reducing the thickness of cementite network. Niobium with or without nitrogen inhibited the formation of Widmanstätten cementite plates at grain boundaries and within the grains near the surface in the hypereutectoid zone of the case. It was also revealed that, when compared to the baseline steel, niobium decreased the case depth of the carburized steels, but that niobium with nitrogen is more effective than niobium alone in reducing the case depth. Niobium as niobium carbide (NbC) and niobium in the presence of nitrogen as niobium carbonitride, [Nb(C,N)] particles refined the austenite grain size of the carburized case, but Nb(C,N) was more effective than NbC in inhibiting austenite grain growth.     

铌在一种高合金化高温合金中的偏析与扩散行为

研究铌在一种高合金化镍基高温合金凝固过程中的偏析和均匀化过程中的扩散行为。铌严重偏析于枝晶间区域,其偏析系数高达4.30。许多富铌相,包括Laves相、δ相、（γ＋γ′）共晶、MC和M6C碳化物等在枝晶间析出。利用差热分析和在不同温度、时间的均匀化处理确定各种富铌相的溶解温度和枝晶偏析的消除程度。元素扩散计算表明：铌的扩散速率随着均匀化温度的提高而显著增加,从而有效地缩短均匀化时间。提出一种无初熔的三步均匀化制度,可以完全消除铌的枝晶偏析,获得均一的组织。     

海绵钛副产品氯化镁高温气相氧化反应器内流场的模拟研究

本文研究基于利用“原位热解—热法还原炼镁”海绵钛清洁生产新工艺,即还原蒸馏产生的气态氯化镁直接氧化热解制备高纯氧化镁及氯气,氧化镁经热法炼镁返回TiCl₄还原环节、氯气返回沸腾氯化环节作为原料,实现海绵钛生产中新的镁、氯循环。,并针对“海绵钛生产中新的镁、氯循环”中的关键步骤—气态氯化镁与氧气的均相热解反应的反应器进行研究。采用数值模拟和物理模拟方法研究了反应器模型内的浓度场和速度场,在氮气与二氧化碳流量比为6：1时,二氧化碳采用环向四口进气方式,气体混合不均匀度为0.02,气体混合程度最佳。     

The corrosion of pure niobium in oxidizing,sulfidizing, and oxidizing-sulfidizing gas mixtures at 600–800°C

The corrosion of pure niobium has been studied at 600–800°C in various environments as part of a study of the corrosion resistance of its alloys with iron, cobalt, and nickel to atmospheres of low-oxygen and/or high-sulfur activities. The results have shown that not only the sulfidation but also the corrosion in mixed atmospheres and particularly the oxidation under low oxygen pressures of pure niobium are quite slow, with kinetics rather similar in the three types of gas mixtures used. The good corrosion resistance of niobium to attack by oxygen under low pressures is quite interesting because this element is corroded very rapidly by oxygen under high oxygen pressures, due to the formation of the nonprotective highest oxide Nb₂O₅ as a main corrosion product.     

Effect of niobium in medium alloyed ductile cast irons

The present work studies the effect of niobium addition on the mechanical properties and microstructure of an as-cast 4·3%Ni alloyed ductile iron. The experimental iron was made from high purity raw materials in a laboratory induction furnace. Eight castings were produced with niobium content in the range 0–0·8%. Silicon and carbon content were fixed at 2·4 and 3·1% respectively for all the castings. A complete micro-structural characterisation was undertaken for all the as-cast alloys. Niobium was observed to be directly related to the formation of polygonal niobium carbides of the type NbC. Such carbides increased in size and amount as niobium content increased in the alloys. The amounts of pearlite and ferrite phases were not affected by the niobium content added in the present study. Neither nodule count nor nodularity was affected by such niobium addition. Mechanical properties such as hardness, tensile strength, yield strength and ductility were measured for the alloy; a small strengthening of the alloys was observed as the niobium amount increased. Such strengthening is explained in terms of NbC formation during solidification. IJCMR/372