A discrete nonlinear mass transfer equation with applications in solid-state sintering of ceramic materials

The evolution of grain structures in materials is a complex and multiscale process that determines the material's final properties [1]. Understanding the dynamics of grain growth is a key factor for controlling this process. We propose a phenomenological approach, based on a nonlinear, discrete mass transfer equation for the evolution of an arbitrary initial grain size distribution. Transition rates for mass transfer across grains are assumed to follow the Arrhenius law, but the activation energy depends on the degree of amorphization of each grain. We argue that the magnitude of the activation energy controls the final (sintered) grain size distribution, and we verify this prediction by numerical simulation of mass transfer in a one-dimensional grain aggregate.     

Characterization of porous silicon prepared by powder technology

In this study, we have proposed the powder technology as new method for preparation of bulk porous silicon. Formation of porous silicon by high-energy ball milling followed by pressing and sintering was studied by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy (XPS). A crystalline wafer with (1 1 1) orientation was extensively ball milled up to 72 h leading to a decrease in average crystallite size up to 15 nm. The most significant reduction of crystallite size was observed after milling process for about 24 h. The nanopowders were then pressed into pellets at a pressure up to 400 MPa and sintered at 1173 K for 60 min in a high purity argon atmosphere. Results showed that after sintering the material became porous with uniform porosity in whole volume, independently of the sinter size. It is not possible to prepare such porous materials using the conventional electrochemical etching, where the porous structure depth usually does not exceed tens of micrometers. Core-level XPS studies showed very good agreement between peak positions of the sintered porous silicon and in-situ prepared polycrystalline 20 nm-Si thin film or single-crystalline Si (1 1 1) wafer. Furthermore, the valence band spectra measured for sintered samples are broader compared to those measured for the Si (1 1 1) wafer or polycrystalline Si thin film. On the other hand, the shape and broadening of the valence bands measured for the sintered samples are in very good agreement with those reported for electrochemically prepared porous silicon.     

A radial analogue of Poisson’s summation formula with applications to powder diffraction and pinwheel patterns

Diffraction images with continuous rotation symmetry arise from amorphous systems, but also from regular crystals when investigated by powder diffraction. On the theoretical side, pinwheel patterns and their higher dimensional generalisations display such symmetries as well, in spite of being perfectly ordered. We present first steps and results towards a general frame to investigate such systems, with emphasis on statistical properties that are helpful to understand and compare the diffraction images. An alternative substitution rule for the pinwheel tiling, with two different prototiles, permits the derivation of several combinatorial and spectral properties of this still somewhat enigmatic example. These results are compared with properties of the square lattice and its powder diffraction.     

Structural and thermal study of calcium undecanoate

In the present work, an X-ray diffraction (XRD) and thermal study of calcium undecanoate is presented. The measured high-resolution XRD powder pattern of the synthesized salt at room temperature, using synchrotron radiation, showed that the salt is a mixture of monohydrated and anhydrous calcium undecanoate. Calcium undecanoate monohydrate proved to have a monoclinic cell with a symmetry described by the P2₁/a space group. The structure dehydrates at about 100°C. After dehydration, the salt undergoes a phase transformation which results in a thermotropic mesophase. Further heating of the salt leads to decomposition and melting. Ketones are the probable products of decomposition at 400°C.     

SPECTROSCAN-U型便携式波长色散X射线荧光光谱仪现场测定铜矿区的15种元素

采用SPECTROSCAN－U型便携式波长色散X射线荧光光谱仪，使用粉末样品压片制样，现场分析了某铜矿区样品中的Cu、Pb、Zn、Co、Ni、Cr、V、Ti、Mn、Rb、Sr、Zr、Y、Ca、Fe等15种元素，获得了较好的精密度与准确度。用现场分析数据圈出的异常图与室内化学分析数据圈出的异常图符合较好，为野外现场快速分析作了有益的尝试。     

DEM simulations of die filling during pharmaceutical tabletting

The flow behaviour of powders from a stationary shoe into a moving die, which mimics the die filling process in a rotary tablet press, was analysed using a discrete element method （DEM）, in which 2D irregular shaped particles were considered. The influence of the particle shape, size and size distribution, the number of particles used in the simulation, the initial height of powder bed in the shoe, and the filling speed on the average mass flow rate and the critical filling speed （the highest speed at which the die can be completely filled） were explored. It has been found that a maximum flow rate is obtained at the critical filling speed for all systems investigated and poly-disperse systems have higher mass flow rates and higher critical filling speeds than mono-disperse systems. In addition, the powder with particles which can tessellate generally has a lower filling rate and a lower critical titling speed.     

DTA和XRD用于(14-x)SrCO3-xCaCO3-24CuO系统合成spin-ladder化合物Sr14-xCaxCu24O41的研究

The (14-x)SrCO3xCaCO324CuO system was investigated by means of differential thermal analysis (DTA) technique combined with the powder Xray diffraction (XRD) technique, and the relationship of its thermal behavior with the synthesis of spin-ladder compound Sr14-xCaxCu24O41 was first analyzed in detail. It is found that the DTA curve (x≤5.6) exhibits two endothermic peaks and could be divided into three reaction stages, corresponding to the solid state reaction to synthesize single-phase Sr14-xCaxCu24O41. The DTA curve (x≥8.4) shows three big endothermic peaks below 1000 ℃. The intensity and the position of the first endothermic peak around 800 ℃ is depending on the content of CaCO3 in the system, while the third endothermic peak is the crucial factor of the synthesis of single-phase compound.     

Synthesis of lithium-manganese oxide spinel in hydrothermal conditions

Abstract

Lithium-manganese oxide spinel LiMn₂O₄ was synthesized in hydrothermal conditions (400°C, 20 MPa) in the course of thermovaporous treatment mixtures of MnO₂ and LiOH/or Li₂CO₃. The conditions of synthesis of the spinel as monophase product were determined. The obtained product has been characterized by means of various physical and chemical methods. The spinel has been used for manufacturing cathodes of rechargeable lithium cells. The cells discharged in the potential range 2.8-3.5 V. The specific capacity was 100-140mAh/g.     

UV-laser-light-controlled photoluminescence of metal oxide nanoparticles in different gas atmospheres: BaTiO3, SrTiO3 and HfO2

The photoluminescence (PL) enhancement has been studied at room temperature using various specimen atmospheres (O₂ gas, CO₂ gas, CO₂–H₂ mixture gas, Ar–H₂ mixture gas and vacuum) under 325 nm laser light irradiation on various metal oxides. Of them, the results obtained for BaTiO₃ nanocrystals, SrTiO₃ ones and HfO₂ powder crystal are given in the present paper. Their PL were considerably increased in intensity by irradiation of 325 nm laser light in CO₂ gas and CO₂–H₂ mixture gas. The cause of the PL intensity enhancements is discussed in the light of the exciton theory, the defect chemistry and the photocatalytic theory. The results may be applied for the utilization of greenhouse gas (CO₂) and the optical sensor for CO₂ gas.     

The influence of particle size on separation and dustiness in powder mixtures during nonelectrostatic and electrostatic coating

Common food powders and their mixtures, consisting of two powders with the same composition but different in particle size: fine (51–95 μm) and coarse (244–401 μm) NaCl, KCl, sucrose, rice starch, maltodextrin, whey protein, casein and soy protein, were coated on a target at 0 and −25 kV. Over half of the mixtures showed separation due to a difference in particle size. Separation was caused by the difference in individual transfer efficiency of the powders and interactions during coating. Both composition and differences in size were found to be important. Being in a mixture did not change the amount of dust formed.