Simulations of vibrated fine powders

During the past few decades, several studies have been conducted to understand the behaviour of powders in vibrated beds. This paper introduces a technique of incorporating the agglomeration and deagglomeration phenomena in the simulation of vibrated fine powders. Two-dimensional direct simulations are performed using 300 spheres 2.99 mm in diameter in a trapezoidal container vibrated vertically at an amplitude of 2.5 mm and 20 Hz frequency as preliminary conditions. Under non-cohesive conditions, the results are in agreement with those found in the literature. As a preliminary effort to predict the behaviour of cohesive fine powders under vibrated conditions, agglomeration and deagglomeration processes are modelled as the formation and destruction, respectively, of interparticle bonds during particle collisions. Two parameters used to model agglomeration and deagglomeration are the ease of cohesion and cohesivity of the powder. Dependencies of these parameters on certain physical properties of cohesive powders have been suggested. Simulation results reveal two aggregate populations, one with uniform size aggregates and another population with multi-sized aggregates. The former aggregates were more prevalent in weakly cohesive powders while the latter in highly cohesive powders. Interesting macroscopic bed behaviour such as alternating cycles of agglomeration and deagglomeration were also observed. Further work is needed in which the aerodynamic forces are taken into account and cohesion mechanisms at the particle surface are modelled.     

Synthesis of fine and high purity Cr2AlC powders by novel method

Cr₂AlC powders using Cr/Al/C and Cr/Al/Cr₃C₂ systems as raw materials were successfully synthesised by a microwave hybrid heating method for the first time. The mixtures of Cr, Al and graphite or Cr₃C₂ with different molar ratios were used to investigate the formation of Cr₂AlC phase. For Cr/Al/C with the molar ratios of 2:(1.0–1.2):1 system, Cr₂AlC with a small amount of Cr₇C₃ and Cr₂Al was synthesised at 1000°C for 3 min, and the average particle size was ?1?μm. For Cr/Al/Cr₃C₂ with the molar ratio of 1:2:1 system, high purity Cr₂AlC powders was synthesised at 1000°C for 3 min, and the average particle size was ?1?μm. The synthesis of high purity Cr₂AlC powders for short time was attributed to the combination of the hybrid heating effect and the introduction of Cr₃C₂ as carbon source. Microwave hybrid heating is a promising method for the preparation of various other MAX phases.     

Preparation of barium hafnate fine powders by oxalate method

Barium hafnate (BaHfO₃) precursor was prepared by mixed barium nitrate and hafnium (IV) chloride with ammonium oxalate aqueous solution. Barium hafnate powders were obtained by decomposition of the precipitate precursor at 800 °C for 2 h. As a dispersant, polyethylene glycol (PEG6000) was used in the precipitation reaction and the influence of dispersant dosage on particle size was studied. X-ray diffraction analysis (XRD) showed the powders was pure cubic BaHfO₃ without impurities. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) demonstrated that the size distribution was uniform, and the average grain size was about 50 nm.     

Solid circulation rate in a circulating fluidized bed in the presence of fine powders

Fine powders (Geldart's group C) are added to a circulating fluidized bed (CFB) of coarse particles (Geldart's group A) and the solid circulation rate (SCR) is investigated with addition of fine powders of different sizes and different fractions (different hold-ups) to the bed. Experiments were carried out in a CFB of 2 m in height and 0.052 m in diameter, using FCC catalyst particles of as the coarse particles and cohesive aluminum hydroxide powders of 0.5- as the fine powders. The effects of hold-up of fine powders in the bed, fine powders size, and superficial gas velocity on the SCR were investigated.The SCR strongly depended on the hold-up of fine powders of 0.5- in size and noticeably decreased with increasing the hold-up of fine powders under constant gas velocity. This dependency disappeared when the size of fine powders was larger than . Thus, depending on the size of fine powders added to the CFB, two distinct regions for the changes of SCR could be clearly identified.     

Synthesis of spherical shape borate-based bioactive glass powders prepared by ultrasonic spray pyrolysis

Spherical shape borate-based bioactive glass powders with fine size were directly prepared by high temperature spray pyrolysis. The powders prepared at temperatures between 1200 and 1400 °C had mixed phase with small amounts of fine crystal and an amorphous rich phase. Glass powders with amorphous phase were prepared at a temperature of 1500 °C. The mean size of the glass powders prepared by spray pyrolysis was 0.76 μm. The glass powders prepared at a temperature of 1200 °C had two distinct exothermic peaks (T_c1 and T_c2) at temperatures of 588 and 695 °C indicating crystallization. The glass transition temperature (T_g) of the powders prepared at a temperature of 1200 °C was 480 °C. Phase-separated crystalline phases with spherical shape were observed from the surface of the pellet sintered at a temperature of 550 °C. Crystallization of the pellet was completely occurred at temperatures of 750 and 800 °C. The pellets sintered at temperatures below 700 °C had single crystalline phase of CaNa₃B₅O₁₀. The pellet sintered at a temperature of 800 °C had two crystalline phases of CaNa₃B₅O₁₀ and CaB₂O₄.     

Synthesis of ultra‐fine TaB2 nano powders by liquid phase method

Ultra‐fine TaB₂ powders were synthesized by a liquid phase method using tantalum ethoxide, boric acid and sucrose as the sources of tantalum, boron, and carbon. The TaB₂ precursor powders is a Ta–B–C–O network system, which were heat‐treated at lower temperature (1500°C) in normal argon atmosphere to obtain the TaB₂ powders. XRD confirms the presence of only hexagonal TaB₂, while EDS and XPS spectrums confirm the composition and element chemical states of TaB₂. The TEM images show a platelet shape of the TaB₂ powder and the monocrystal SAED pattern confirms the presence of hexagonal TaB₂. Particle size distribution curves show that particle size of the TaB₂ powders distributes in the range of 30‐160 nm, whose mean particle size is 106 nm.     

Electrical properties of lanthanum doped Bi4Ti3O12 thin films annealed in different atmospheres

Pure and lanthanum doped Bi₄Ti₃O₁₂ thin films were deposited on Pt/Ti/SiO₂/Si substrate using a polymeric precursor solution. Annealing in static air and oxygen atmosphere was performed at 700 °C for 2 h. The obtained films were characterized by X-ray diffraction and atomic force microscopy. The dielectric constant and dissipation factor were measured in the frequency region from 1 kHz to 1 MHz. Electrical characterization of the films pointed to ferroelectricity via hysteresis loop. Films annealed in static air possess a dielectric constant higher than films annealed in oxygen atmosphere due to differences in the grain size, crystallinity and structural defects. A regularly shaped hystereses loop is observed after annealing in static air. The obtained results suggest that the annealing in oxygen atmosphere can increase the trapped charge and the relaxation phenomenon.     

Spectral analysis of force fluctuations during probe penetration into cohesive powders

We investigate force fluctuations during probe penetration into cohesive powders consisting of ∼ 5 μm lactose particles with varying surface properties prepared by spray drying. The results obtained for the more cohesive powders were remarkably similar to those previously reported for orders of magnitude larger noncohesive particles. For the less cohesive powders, the spectral densities were instead found to exhibit two distinct power-law regions. Furthermore, the spectra were found to be independent of the geometry of the penetrating probe and dimensions of the die cavity. These findings suggest that the response is dominated by particle aggregate or agglomerate movement for the more cohesive powders, whereas the behaviour of the less cohesive ones is consistent with a response dominated by relatively weak force chains, with the fluctuations resulting from the recurring creation and collapse of jammed states being damped for length scales > 0.1 mm.     

Synthesis of fine dispersed titanium diboride from nanofibrous carbon

This paper presents the experimental data on the synthesis of titanium diboride (TiB₂) fine dispersed powder carried out in laboratory scale. TiB₂ powder was prepared by the reduction of titanium dioxide with boron carbide and nanofibrous carbon in an argon atmosphere. The powders of TiB₂ were characterized by X-ray diffraction (XRD), elemental analyses, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), low-temperature nitrogen adsorption, particle size analysis, simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC). The resulting material contains a single phase – titanium diboride. The particles of the powder were predominantly aggregated. The average size of the particles and the aggregates were 7.4–8.0 µm with a wide size of distribution. The specific surface values of samples obtained were 2.4–5.8 m²/g. The oxidation of titanium diboride began from the temperature of 450 °C. In this work, the optimal synthesis conditions were estimated: the molar ratio was TiO₂:B₄C:C=2:1:3 (according to stoichiometry), the temperature was 1600 °C, the process duration was 20–30 min.     

Synthesis and characterization of hafnium carbide fine powders

Hafnium carbide powder was synthesized by sol–gel polycondensation of hafnium chloride with citric acid. The starting materials were dissolved in water and mixed homogeneously on a hot plate until precursor gel was formed. Pyrolysis of this gel resulted in monoclinic hafnia which after subsequent heat treatment transformed into hafnium carbide. The obtained materials were analyzed by means of X-ray diffraction and scanning electron microscopy. The results showed that the obtained carbide powders were composed of nearly equiaxed particles with narrow size distribution. Characterization by nitrogen adsorption showed that the obtained powders were micro- and mesoporous materials with high specific surface area.     

Generation of bipolar electric fields during industrial handling of powders

This paper presents the results of some field tests that were carried out to determine the electric fields generated due to bipolar charging that occurs during the filling of a container with conveyed powders in an industrial setting. Approximately 1000 kg of pneumatically conveyed powder was allowed to fall into a cubically shaped container made out of woven fibreglass cloth that was placed on a metallic base electrically connected to ground. During the filling operation, the electric fields at three points close to the outside surface of the container were measured with a calibrated electrostatic probe. The results show that as the container is being filled, the mass of powder that accumulates in the lower part generates outward directed positive electric fields. In contrast, the shrinking, unfilled, upper space in the container generates considerably stronger outward directed negative electric field at the surface. At the level of the powder filling the container, the outward directed electric field was approximately zero. As the container fills, the zero electric field level moves upwards and the negative electric field disappears leaving the entire surface with a positive electric field. The net charge to mass ratio of the conveyed powder was measured in the order of 1.5 E-10 C/g, while falling into the container. Some of the generated negative electric fields at the surface were as high as 30 kV/cm. These results confirm previous measurements with other powders that showed that in a poly-disperse mixture, fine particles tend to charge negatively whereas coarser particles charge positively.     

Synthesis and structural characterization of ultrafine terbium oxide powders

Glycine-nitrate self-propagating high-temperature synthesis was used to synthesize terbium oxide nanopowders whose phase transformations were investigated by methods of high-temperature differential scanning calorimetry, dilatometry, and X-ray diffraction analysis. The as-prepared powders consisting of Tb₇O₁₂ and Tb₁₁O₂₀ phases were converted to Tb₂O₃ after calcination at 600–800 °C in reducing atmosphere. The sintering behavior of Tb₂O₃ was studied under microwave heating up to 1780 °C. The microstructure of the powders and ceramics was investigated by scanning electron microscopy. Near full-density material was obtained at about 1620 °C. Further temperature increases causes a deterioration of the ceramics microstructure due to monoclinic Tb₂O₃ phase formation.     

SiAlON粉体合成的研究进展

概述了SiAlON材料的组成、结构特点、理化性能和应用领域 ,全面阐述了SiAlON粉体的合成方法以及由天然原料合成SiAlON粉体的研究进展     

铁电存储器及其疲劳机制的研究

简述了铁电材料的结构及其性能,比较了铁电存储器的各种制备工艺的优缺点及发展前景;同时介绍了目前较为先进的铁电储存器的优良的读,写等特性及国际市场发展动态对当今高度发展的信息社会的重要作用。同时探讨了铁电材料的疲劳机制研究现状及其深入研究的所应遵循的技术路线。     

Synthesis of TaB2 powders by borothermal reduction

Borothermal reduction processes of Ta₂O₅ with boron under vacuum were investigated. Ta₂O₅ reacted with boron to form various borides (TaB₂, Ta₃B₄, and TaB), depending on the boron/Ta₂O₅ molar ratio and temperature. In order to prepare pure TaB₂ powders, two routes were developed. The first route was one‐step heat treatment at 1550°C. With boron/Ta₂O₅ molar ratio of 9.0, pure TaB₂ powders with strong agglomeration were synthesized by the first route, and the particle size and oxygen content were 0.7 μm and 0.9 wt%, respectively. The second route consisted of two‐step heat treatment at 800°C and 1550°C plus intermediate water washing. With lower boron/Ta₂O₅ molar ratio of 8.2, pure TaB₂ powders with less agglomeration and more uniform distribution were synthesized by the second route, and the particle size and oxygen content were 0.8 μm and 0.8 wt%, respectively. Moreover, the particle size similarity of TaB₂ powders by the two routes suggested that byproduct boron oxides, which were previously reported as the most important factor in promoting the coarsening of ZrB₂ powders by borothermal reduction, did not lead to the significant coarsening of TaB₂ powders.     

Al-doped Ni-rich cathode powders prepared from the precursor powders with fine size and spherical shape

LiNi_0.8Co_0.15Al_0.05O₂ cathode powders with fine size and spherical shape were prepared by solid-state reaction method using the Ni-Co-Al-O precursor powders with fine size and spherical shape. The Ni-Co-Al-O precursor powders with fine size and filled inner structure were prepared by spray pyrolysis from the spray solution with drying control chemical additive (DCCA), citric acid and ethylene glycol. The one LiNi_0.8Co_0.15Al_0.05O₂ cathode powder with fine size and spherical shape was formed from the one precursor powder with spherical shape and filled morphology. The mean size of the spherical shape LiNi_0.8Co_0.15Al_0.05O₂ cathode powders was 1.1 μm. The initial discharge capacity of the LiNi_0.8Co_0.15Al_0.05O₂ cathode powders prepared from the spray solution with citric acid, ethylene glycol and DCCA was 200 mAh g⁻¹. The cycle properties of the cathode powders prepared from the spray solution with and without additives were compared.     

Synthesis of pure phase BiFeO3 powders in molten alkali metal nitrates

Pure phase BiFeO₃ powders were successfully synthesized in molten alkali metal nitrates (KNO₃–NaNO₃) at 500 °C. The as-prepared BiFeO₃ had a rhombohedral structure which was studied using X-ray diffraction. The plate-like morphologies were investigated through scanning electron microscopy and transmission electron microscopy. The average length and width of BiFeO₃ plates were 400 and 200 nm, respectively. Furthermore, the mechanism of formation of BiFeO₃ was also discussed through X-ray diffraction, thermogravimetry, differential thermal analysis and mass spectrometry.     

Gas-solid adsorption of selected volatile organic compounds on titanium dioxide Degussa P25

This paper focuses on the adsorption of gaseous trichloroethylene, toluene and chlorobenzene on the photocatalyst TiO₂ Degussa P25. An optimized EPICS (Equilibrium Partitioning In Closed Systems) methodology was used to study equilibrium partitioning. For the three compounds investigated, equilibrium adsorption was reached within of incubation. Adsorption isotherms, determined at a temperature (T) of and relative humidities (RH) of 0.0% and 57.8% were found to be linear (R²>0.993,n=5), indicating that no monolayer surface coverage was reached in the concentration interval studied ). Within the linear part of the isotherm, the influence of both relative humidity and temperature was investigated in a systematic way and discussed from a thermodynamic point of view. Data analysis resulted in a double linear regression for 22% ?RH?90% and . The equilibrium adsorption coefficient represents the equilibrium concentration ratio and ΔU_ads is the internal energy of adsorption . At RH=0.0%, experimental K values were a factor 5-10 higher than those expected from the regression equation, indicating that another adsorption mechanism becomes important below monolayer surface coverage of TiO₂ by water vapour molecules. Since surface interactions are of primary importance in photocatalytic reactions, this paper contributes to a better understanding of the basic mechanisms of TiO₂ mediated heterogeneous photocatalysis and is an interesting tool for developing optimized mathematical models.     

小分子荧光化合物的合成

有机发光二极管(OLED)作为平板显示新技术,成为近年来平板显示领域的主要研究方向之一。综述了OLED的发光原理、以及影响其性能的主要因素,并合成了一种小分子荧光化合物。     

Nanocrystalline titanium powders by high energy attrition milling

The present investigation deals with the synthesis of nanocrystalline titanium powders by the high energy attrition milling of micron sized titanium powder in an inert atmosphere. Titanium powders of about 12 μm particle size were subjected to high energy attrition milling in an argon atmosphere. Selecting suitable milling parameters, nanosize (< 100 nm) titanium powders were obtained after 15 h of milling. An average particle size of 35 nm was obtained at 30 h of milling after which the particle size stabilized with continuation of milling to 75 h. The powders after milling for various durations were characterized by XRD, SEM, TEM, ICP-AES and DTA and these results are reported and discussed.