Chemical characterization of melt-textured YBCO produced by hybrid powder metallurgy

Monolithic YBCO samples were made by traditional top-seeded melt-texturing processes from cold-milled mixtures of Y123 (YBa₂Cu₃O_7−δ) and elemental Y. The bulk composition does not lie on the Y211 (Y₂BaCuO₅)–Y123 join so, formation of Y123 from liquid and Y211 is not an essentially isothermal process on cooling. The reaction liquid + Y211 = Y123 is a ternary reaction and occurs over several tens of degrees. The Y123 thus produced has a range in compositions – particularly in Cu:(Y + Ba) – which may reflect crystallization over the thermal interval. The liquid migrates to an invariant point at which CuO also crystallizes with complete loss of liquid. The presence of trains of CuO grains in the YBCO indicates the locations of the last liquids to be preserved in the sample. These trains are dominantly in an annulus 1–3 mm from the edge of the 20-mm diameter sample. Mapping the compositional variation in Y123 may allow mapping the path of crystallization of these monolithic YBCO samples.     

Effects of powder processing on the characterization of magnesia derived from alkoxide precursors

Magnesia powders have been prepared via alkoxide sol-gel and alkoxide-hydrolysis processes. Gel-derived powders exhibit porous aggregates, and extensive hard agglomerates are regarded as an intrinsic nature of gel powders; the morphology of the powders obtained from the hydrolysis-precipitate route is a function of water content and retains highly agglomerated particles, suggesting that localized microgels are formed during processing. However, a simple modified sol-gel approach proposed to synthesize fine-sized powders with a narrower size-distribution and with less hard agglomerates, has been shown to be feasible. The thermal behaviour and crystallinities of as-prepared powders are similar in all cases, however, there exists a significant difference in particle morphology. The obtained powders have been characterized and compared in the Part I of this work. Their compaction and sintering behaviour will be reported in Part II.     

Effects of powder processing on the characterization of magnesia derived from alkoxide precursors

Compaction and sintering behaviour of three magnesia powders derived from three different processes were compared. Powder preparation and properties have been reported in Part I. It was found in this study that the starting powder morphologies, especially the state of hard agglomeration, strongly affect the uniformity and microstructural evolution during forming and densification. However, a uniform, high-strength, dense (98–99%) and translucent magnesia can be achieved by sintering the modified powder compacts at temperatures much lower than those required for conventional magnesia powders.     

Oxygen evolution during the formation and sintering of nickel-manganese oxide spinels for thermistor applications

Nickel-manganese spinel, prepared from 20 wt % NiO and 80 wt % Mn₂O₃, forms at 950°C by the intermediate formation of Mn₃O₄ with evolution of oxygen, determined by mass-spectrometric evolved gas analysis. On heating to higher temperatures, further oxygen is evolved resulting in pore formation and bloating of pressed sintered samples and anomalies in their densification and electrical properties. Thermodynamic considerations and X-ray diffraction intensity measurements suggest that all the Mn²⁺ is located in the tetrahedral spinel sites, this cation configuration remaining unchanged by higher-temperature treatments such as sintering.     

前驱体反应物浓度对NTC纳米粉体及其热敏电阻特性的影响

重点研究共沉淀法制备CoMnNiO系NTC纳米粉体时前驱体反应物浓度对粉体及材料性能的影响,获得了NTC热敏材料制备的优化条件.采用DTA/TG、TEM等方法,对粉体性能进行了研究,测量了不同反应物浓度对粉料烧结致密度及材料B值和阻值的影响.研究发现随着反应物浓度的增加,粉体颗粒尺寸减小,但胶粒聚合程度增加,网状结构变密实.当浓度控制在0.9～1.2mol/L时,粉体粒度分散、均匀性较好,平均颗粒尺寸为20nm左右,反应物浓度太高或太低,都不利于粉料的烧结.综合分析DTA/TG、TEM得出,将煅烧温度控制在(700±20)℃为宜.浓度对材料的B值和阻值的大小影响很小,但对它们的一致性仍有较大影响.浓度为0.9～1.2mol/L时,粉体的烧结性能较好,材料的B值和阻值的一致性较好.     

Sol-gel processing of tellurite powder

Tellurium ethoxide is highly sensitive to moisture in air and water. The hydrolysis and condensation of tellurium ethoxide exposed under different levers of water and moisture are investigated in this work and the hydrolysis products were characterised by infrared spectrometry and X-ray diffraction. The decomposition process of the product of tellurium ethoxide hydrolysed in air was analysed by differential scanning calorimetry and thermogravimetry. This work also studied the influence of sodium component on the phase transformation of the hydrolysis product in the decomposition process and on the morphology of the heat-treated hydrolysis products.     

Preparation and characterization of lithium nickel cobalt oxide powders via a wet chemistry processing

Nickel-rich phases of the solid solutions, LiNi_1−yCo_yO₂ (y=0.1, 0.2, 0.3), were synthesized by a sol–gel method with citric acid as a chelating agent. Various initial conditions were studied in order to find the optimal conditions for the synthesis of LiNi_0.8Co_0.2O₂. The discharge capacity for the compound synthesized under an optimal synthesis condition of 800 °C for 12 h was found to be 187 mAh g⁻¹ in the 1st cycle and it was 176 mAh g⁻¹ after 10 cycles. The other nickel-rich phases, namely, LiNi_0.9Co_0.1O₂ and LiNi_0.7Co_0.3O₂ showed 1st-cycle discharge capacities of 144 and 163 mAh g⁻¹, respectively. The corresponding capacity values were 140 and 159 mAh g⁻¹ in the 10th cycle. Excess lithium stoichiometric phases, Li_xNi_0.8Co_0.2O₂, where x=1.10, 1.15 and 1.20, resulted in decreased capacity. Structural and electrochemical properties of the synthesized compounds were compared with those of a commercial LiNi_0.8Co_0.2O₂ sample. The effect of calcination temperature and duration, excess lithium stoichiometry and divalent strontium doping in LiNi_0.8Co_0.2O₂ are described. Doping with strontium improved both the capacity and cycling performance of LiNi_0.8Co_0.2O₂.     

PTC热敏电阻在空分设备DCS系统中的应用

为实现对低温液体泵实行变频器和DCS系统的双重保护,将电机及其后轴承保护元件——PTC热敏电阻的阻值信号转换为干接点信号,接入DCS系统的DI通道。简介PTC热敏电阻的工作原理,介绍信号转换方案,分析热敏继电器的工作原理。     

Synthesis and electrical behavior study of Mn3O4 nanoceramic powder for low temperature NTC thermistor

Monodispersed Mn₃O₄ nanoparticles (NPs) were synthesized by reducing KMnO₄ at room temperature in the presence of cetyltetrabutylammonium bromide surfactant and short chain tetra-n-butylammonium bromide co-surfactant. Structural characterization done through XRD, TEM and FT-IR analysis techniques showed mono dispersity (5–8 nm) and capping of the NPs with surfactants. The temperature dependent behavior of dc resistivity of the nanopowder pellets showed reproducible NTC characteristics over a range of 40–200 °C with two thermistor constants (β1 = 10,897 K for 40 °C < T < 107 °C and β₂ = 1,529 K for 107 °C < T < 190 °C) and a negative temperature co-efficient of resistance (α = ?0.111 K^?1 at 40 °C). The thermistor constant (β₁) and NTC values are found to be higher than that of bulk Mn₃O₄ in range of 40 °C < T < 107 °C. This observed behavior implies an enhanced sensitivity in nano-powder based thermistors. Temperature and frequency dependent impedance behavior of the as-synthesized samples evaluated over a temperature range of 40–140 °C and a frequency range of 1 kHz to 1 MHz delineates the role of electron hopping between Mn²⁺ and Mn³⁺ in the conduction process. These studies present monodispersed Mn₃O₄ NPs as promising material for NTC thermistor in the low temperature range 40 °C < T < 107 °C.     

银铜包覆粉末的制备与表征

采用液相还原法制备银包铜超细粉末。以抗坏血酸为还原剂采用葡萄糖预还原法制备铜粉末,以水合肼为还原剂制备银包铜粉末,引入不同的分散剂来制备超细铜粉末。并对样品进行SEM、TEM-EDS和XRD等检测。结果表明,通过条件控制可得粒度均匀、球形度好的包覆粉末。该方法设备简单,且易于操作,适合工业生产。     

Preparation and characterization of nanocrystalline Nd-YAG powder

Nanocrystalline materials have assumed notable importance in a wide variety of fields owing to numerous possible applications offered by them. These include transparent ceramics wherein they facilitate synthesis as well as sintering at significantly lower temperatures. We report preparation of nanocrystalline neodymium doped yttrium aluminum garnet (YAG) with an ultimate intent to make transparent Nd-YAG ceramic. The Liquid Mix method employed involves mixing of metal nitrates with excess amounts of citric acid followed by dissolution and polymerization in ethylene glycol to form complex chelates. Amorphous powder obtained by firing of polymeric chelate compound at 400 °C permits formation of nanoparticles of Nd:YAG at as low a crystallization temperature as 920 °C as shown by the thermal analysis. Progressive evolution of well crystallized phase-pure YAG was studied by XRD of amorphous powders subjected to different calcination temperatures. Scanning electron microscopic (SEM) study of the crystalline material shows that particle size ranges between 50 and 100 nm.     

纳米ATO粉体的制备及性能表征

以SnCl4.5H2O和SbCl3为原料,采用化学共沉淀法制备了纳米级锑掺杂二氧化锡(ATO)粉体。研究了制备过程溶液pH值对ATO粉体粒径的影响。运用差热-热重(DTA-TG)、X射线衍射(XRD)、扫描电镜(SEM)等测试方法对粉体的热分解情况、晶型、粒径及形貌进行了表征。研究结果表明,ATO粉体为四方金红石型结构,平均粒径为60nm左右,团聚较少。     

Optimisation of high energy ball milling parameters to synthesize oxide dispersion strengthened Alloy 617 powder and its characterization

In the present work, ultra-fine powder of oxide dispersion strengthened Alloy 617 was synthesized by high energy ball milling. Milling parameters such as rpm and milling time were varied in the range of 500–2000 and 5–360 min, respectively. Energy applied to the powder in the milling process (Energy per unit mass per hit, E_c) was estimated using the collision model. Effect of milling parameters on the microstructure of powder and refinement of oxides was investigated using X-ray Diffraction (XRD), Scanning electron Microscopy (SEM), conventional Transmission Electron Microscopy (TEM) and High resolution Transmission Electron Microscopy (HRTEM). Desired convoluted lamellar structure with average particle size ∼33 μm was observed during milling at 1000 rpm (E_c ∼ 0.4 kJ/g.hit) for 6 h. TEM analysis of the powder showed the presence of fine oxide dispersoids in the size range 4–16 nm. HRTEM analysis substantiated the presence of fine dispersoids of size ∼4 nm and showed the presence of deformation twins in the matrix. The fine dispersoids in a nanocrystalline matrix is expected to provide superior creep strength to the material at high temperatures.     

Temperature measurement using thermistor with pulsed operation of circuit containing thermistor and linear resistor

A method is considered for determining the basic parameters characterizing a pulsed thermistor-linear resistor temperature-measuring circuit and ensuring increased sensitivity to temperature changes while conserving a given accuracy of measurement.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 30, No. 6, pp. 1081–1088, June, 1976.     

Mo-8wt%Cu纳米复合粉末的烧结与加工

用机械合金化法制取Mo-8%Cu(质量分数)纳米复合粉末,采用液相烧结和致密化后处理工艺制备了Mo-8%Cu(质量分数)合金。通过扫描电镜对Mo-Cu液相烧结和变形加工后合金显微组织进行了分析,研究了各种工艺参数对Mo-Cu合金致密性、拉伸强度、延伸率和晶粒尺寸的影响。结果表明,高能球磨的Mo-8%Cu(质量分数)纳米复合粉末坯体,经液相烧结后,其烧结态为Mo、Cu复合网状组织,可获得相对密度高达98.6%的Mo-Cu合金,再经静液挤压变形加工处理后,可获得全致密的Mo-8%Cu(质量分数)合金,在室温静液挤压40%形变率的条件下,拉伸强度可达到576MPa,延伸率5.8%。     

Surface oxide debonding in field assisted powder sintering

Field activated sintering techniques (FAST) have been applied to two high-temperature powder materials: tungsten and NiAl. High and atomic resolution electron microscopy (HREM/ARM) of tungsten powder sintered via FAST showed essentially clean boundaries. Analytical transmission electron microscopy (TEM) of FAST sintered NiAl also showed boundaries free of surface oxide layer(s). However, small alumina precipitates were found at and near prior particle (grain) boundaries. The boundary cleaning and precipitation phenomena are manifestations of an applied pulsed electric field.     

An overview of the powder processing of soft magnetic composites

The technique of powder metallurgy is becoming increasingly common in the electrical manufacturing industry. It not only eliminates a number of steps in the manufacturing process, but uses less energy and gives near-net shape results which require few, or no finishing steps. The process is suitable for soft magnetic composites, and appropriate heat treatments can further tailor properties to a specific application. This article gives an overview of the technology and makes reference to some advances in the materials used and the processes involved