Synthesis of nanophase tungsten carbide by electrical discharge machining

Tungsten carbide nanopowders were synthesized successfully by electric discharge machining followed by annealing under a nitrogen atmosphere. The tungsten workpieces were initially melted and evaporated on the working surface during the electric discharge machining process, and then the tungsten powders were reacted with the carbon electrode and the working medium of kerosene to form the nanocrystalline WC_1−x powders. The powders produced were characterized by XRD, SEM, and TEM. When annealing the powders under an N₂ atmosphere, the cubic phases of WC_1−x gradually changed to hexagonal W₂C and then were transformed fully to nanocrystalline hexagonal WC at 1200 °C, with the nanocrystalline tungsten carbide encapsulated in a carbon shell. On the other hand, under an H₂ atmosphere, the WC_1−x phase changed via a W₂C phase to reduced powders of pure tungsten at 1000 °C or were reduced directly from WC_1−x to elemental W.     

THE EFFECT OF PRECIPITATION PARAMETERS ON PREPARATION OF LITHIUM FLUORIDE (LIF) NANO-POWDER

Lithium fluoride powder (LiF) is a white powder with a density of 2.64 gr/cm³ and a melting point of 848°C. This powder has several applications such as flux, glaze, soldering, and aluminum melting process, but one of the most important uses of this powder is its application in dosimetry. The commercial powders currently used for this purpose have average sizes of 5 to 10 micrometers; the objective of this research is to produce LiF powder with nano-metric particle size. In this study, the reaction of LiOH + HF → LiF + H₂O has been selected from among several reactions that were able to produce LiF powder, and some precipitation parameters such as temperature, time, agitation type, and supersaturation degree have been controlled. The morphology, phase analysis, and particle size distribution of the resulting powders were analyzed by SEM, XRD, and LPSA. Finally, lithium fluoride nano-powder was synthesized at a temperature of 25°C, pH about 2-3, reaction time less than 1 s, and agitation by ultrasonic bath.     

超声电沉积制备纳米铜粉末的机理研究

利用超声电沉积法制备金属纳米铜粉,平均粒径30nm,分散性较好;利用XRD、TEM等进行了成分、粒度、形貌及结构分析,对影响纳米粉末制备的主要工艺因素进行分析和优化。试验表明,电流密度对纳米粉末形成起控制作用,表面活性剂和超声场对粉末分散更为重要。     

Role of a low level of La2O3 dopant on the tetragonal-to-monoclinic phase transformation of ceria-yttria co-stabilized zirconia

Yttria, yttria-ceria and yttria-ceria-lanthana stabilized zirconia powders were prepared by coprecipitation. Their tetragonal-to-monoclinic phase (t→m) transformation was investigated by calcining the powders in a temperature range of 400–1400 °C for 2 h. The results show that after doping with 0.1 to 0.3 mol.% La₂O₃ and calcining at 1400 °C in air, unusual redox behaviours of cerium were detected in the 1.5 mol.% Y₂O₃+5.5 mol.% CeO₂ co-stabilized zirconia. Grain refinement and a sharp reduction in oxygen vacancy concentration were observed simultaneously. The t→m transition was not found in the 0.1 mol.% La₂O₃ doped zirconia but appeared in the cases with a higher dopant content. The changes are discussed with regards to the grain size, valence change of cerium, presence of oxygen vacancies, and segregation of the dopants at grain boundaries.     

添加A12O3纳米粉对中空钢55SiMnMo力学性能与夹杂物的影响

研究了中空钢55SiMnMo还原前期加0、018％、铸锭时加0．002％的120nmAl2O3纳米粉后钢的力学性能和夹杂物。结果表明，添加0．02％Al2O3纳米粉使锻后空冷55SiMnMo钢的屈服强度提高了13．9％，常温冲击韧性提高了70．8％；场发射SEM观察得出Al2O3纳米颗粒大多数成为非金属夹杂物的核心，钢中大部分夹杂物得到了细化，尺寸均在300nm～3μm之间。     

纳米VO2粉体的制备及性能和应用

VO2是一种相变型金属氧化物,在相变点(68℃)附近发生金属-半导体状态的可逆转变,同时伴随着光学、电学及磁学等性能的变化.因此作为表面功能制膜材料以及陶瓷材料具有广泛的应用前景.综述了纳米VO2粉体的制备方法、性能特点及其应用的研究进展.     

Bed collapse dynamics of fluidized beds of nano-powder

Bed collapse experiments have been carried out in fluidized beds of agglomerated nano-powders possessing high void fractions. Transient responses for the local pressure-drop as well as the gas velocity during the collapse were recorded. From the local pressure transient data, occurrences of key events of the collapse dynamics were identified. The first event is the zero pressure-drop condition soon after the flow interruption while the second is the global minimum on the pressure-drop profile that marks the end of the first stage of the bed collapse characterized by fast bed transients. The second stage is a relatively slow process, which ends with the onset of the zero pressure-drop condition again (third event) that reflects particles becoming stationary, thus signaling the completion of the collapse dynamics. The time of the first stage of the collapse is found to vary linearly with the initial fluidization velocity at which the collapse experiment was initiated whereas the total collapse time shows a rather weak dependence on the fluidization velocity. On the other hand, the velocity transients during the collapse follow first-order time lag that can be accurately modeled using a single time constant irrespective of the initial fluidization velocity.     

Synthesis of titanium carbide nano-powders by thermal plasma

The paper presents a thermodynamic analysis for predicting the conditions for the plasma synthesis of TiC powders. The paper also investigates the effects of feeding rate and molar ratio. The experimental results show that TiC powders are synthesized by thermal plasma and the average size of the TiC powders is less than 100 nm.     

纳米SnO2粉体制备及影响因素

采用液相沉淀法，通过工艺参数的优化设置，成功制备出纯净的SnO2纳米粉体；经过X光衍射、扫描电镜分析，认为在合适工艺控制条件下，弱碱液相沉淀法可成功制备出粒度在100nm以下、金红石结构、稳定锡石相的纳米SnO2粉体。同时，本文对制备工艺中的影响参数进行了对比讨论。     

超细粉制备方法及其团聚问题

超细粉泛指尺度在纳米级的微小固体微粒．本文扼要介绍了超细粉的几种典型的制备方法及其特点，以及超细粉在精细陶瓷，光，电，磁，催化，生物等领域的应用状况．并着重讨论了超细粉制备及加工中的团聚现象，团聚的表征，团聚体强度及团聚控制