Effect of electric field on the hydrodynamics of fluidized nanoparticles

The effect of electric field on the hydrodynamics of nanoparticles was studied in a fluidized rectangular bed, with electrodes attached to two parallel walls. It was shown that the electric field of the order of 3 times the gravity markedly decreased the bed expansion and increased the solids volume fraction of nanoparticles fluidized by air. In these experiments, a light diode assembly was utilized to infer the local solids volume fractions within a rectangular bed of 10 nm silica particles. These experimental measurements yielded a two dimensional solids volume fraction distribution within the rectangular bed. The experimental results provided some new insights into the distribution of solids within the bed. The agglomerate diameters were computed using a momentum balance with the drag given by the Ergun equation and the empirical Richardson-Zaki method. Both methods yielded agglomerate diameters of the order of 100 μm and showed dependence on the strength of the electric field. The electric field decreased the granular temperature of the nanoparticles.     

纳米CeO2的制备方法研究进展

介绍了近年来纳米CeO2的制备方法的研究进展情况，重点分析对比了沉淀法、溶胶—凝胶法、水热法、微乳液法、喷雾热分解法、喷雾反应法等制备纳米CeO2的优缺点，指出了在今后应重点研究和解决的主要问题。     

氧化铝超细粉末中团聚产生的原因及消除机理

应用胶体和表面化学的基本原理对用Ｓｏｌ－Ｇｅｌ法制备氧化铝超细粉末过程团聚现象产生的原因和消除机理进行了研究；采用无水乙醇洗涤和冰冻－干燥两种方法来消除粉末中的团聚体，获得了较好的效果。其中用无水乙醇洗涤剂，可以有效地减少可消除粉末的团聚体；用冰冻－干燥法也起到了减少或消除粉末团聚的作用。     

Combustion of Aluminum Particles near the Burning Surface in AP/AN Composite Propellants

Aluminum (Al) particles are commonly used in ammonium perchlorate (AP) composite propellants of solid rockets for increasing performance. When propellants including Al particles burn, Al particles easily agglomerate on the burning surface of the propellant. The diameters of agglomerated Al particles are greater than those of mixed particles. The combustion efficiency of the propellant decreases with increasing burning time of the agglomerated Al particles. Therefore, it is important to observe how the agglomerated Al particles burn on the burning surface of AP composite propellant. A lot of researchers have studied Al agglomerate characteristics. Previous studies clarified the relation between the agglomerated Al particle diameter and luminous flame diameter around Al particles near the burning surface. The shapes of luminous flames around agglomerated Al particles are spherical or elliptical. This study evaluates the shapes of the luminous flame around agglomerated Al particles at a constant diameter or a different diameter. When the proportion of the luminous flame diameter (D_f) to the diameter of agglomerated Al particles (D₀) is 1.54–1.71 at a constant D₀, the luminous flames are almost perfectly spherical. Otherwise, the luminous flames are elliptical at a constant D₀. Furthermore, when D_f/D₀ is close to the mean value, the luminous flame is more spherical than elliptical at different D₀. The evaporation rate and the burning rate of Al vapor are inversely proportional to D₀. The oxidation gas temperatures were changed and the activation energy of Al vapor was obtained as 39.2 kJ mol⁻¹.     

稳定氧化锆超细粉末的团聚现象

本文对氧化镁部分稳定氧化锆超细粉末的制备过程中的团聚现象进行了研究。采用了氨水和乙醇洗涤氢氧化锆沉淀,以消除粉末团聚颗粒,获得了较好的结果。其中,乙醇作为氢氧化锆沉淀洗涤介质时,可以有效地消除粉末团聚颗粒。文中对团聚颗粒的产生和消除的原因作了讨论。     

7A52合金铸锭漏斗底结物形成原因

7A52合金铸锭时,分液漏斗底部常产生底结物。通过化学及SEM-EDS分析,发现底结物中Mn、Cr、Ti元素浓度分别超正常值约5倍、10倍、15倍。XRD分析表明其主要化合物相组成为Al7Cr、MnAl6、TiAl3。结果表明,底结物产生和形成原因主要与分配漏斗与液穴底的间距、分配漏斗的材质、铝熔体中的杂质等因素有关,同时也与铸造温度、分配漏斗预热等工艺条件有关。根据分析结果,提出了提高铸造速度、改变分配漏斗尺寸、对分配漏斗进行喷涂和预热等消除底结物的具体措施,并成功应用于生产实践。     

粘附性颗粒添加组份流态化进展

综述了近年来国内外粘附性颗粒添加组份流态化的进展，并对粘附性颗粒流化床中颗粒受力分析及模型进行了评价．指出了粘附性颗粒添加组份流态化存在的问题及发展方向，提出应开展添加组份的性质、种类、添加比例及其与粘附性颗粒之间相互作用的机理研究．     

Simulation of entrainment of agglomerates from plate surfaces by shear flows

The entrainment process of agglomerates deposited on plate surfaces by shear flows was simulated using the three-dimensional modified discrete element method (mDEM) and influences of several factors on entrainment process were examined. In the case shear induced force is too weak, deposits are only deformed and particles are barely entrained, however, above some critical value particles are entrained by flows forming agglomerates. It was also clarified that the steric-bulky deposit undergoes the stronger hydrodynamic force and is easy to be entrained. There are two entrainment mechanisms corresponding to the parameter A_s/A which indicates the relative strength of adhesive force between particle and plate surface to that between particles. In case of large A_s/A where the adhesion between particle and plate surface is predominant, the number of entrained particles monotonically decreases as A_s/A increases due to the enhanced binding force. By contrast for small A_s/A, the number of entrained particles is not heavily dependent on A_s/A due to the mechanism in which the upstream side of deposit is lifted and the deposit is deformed extensively then large agglomerates are entrained. The boundary between those two entrainment mechanisms exists at A_s/A=0.5-0.6 which is in good agreement with the theoretical prediction.     

Effect of the impact angle on the breakage of agglomerates: a numerical study using DEM

The study of agglomerate strength is of vital importance in several industrial applications such as pharmaceutical, detergent and food manufacturing. Agglomerates could experience a size reduction during the production and handling processes due to collisions with other agglomerates or with the moving components and walls as well as during bulk flow due to shear deformation. In this analysis, we focus on the agglomerate damage due to oblique impact on walls, as this is a common damage process during, for example, pneumatic conveying and size reduction in pin mills.

Computer simulations have been carried out using Distinct Element Analysis, where the breakage characteristics of oblique impacts and the effect of the interparticle bond strength have been analysed. The procedure adopted here provides an isotropic and spherical agglomerate (uniform mass distribution and coordination number within radial segments of the agglomerate). The results indicate that the damage ratio (i.e. the number fraction of the broken bonds) depends on the normal component of the impact velocity only, i.e. the tangential component has little effect. However, the position of the clusters produced on impact does depend on the impact angle, which influences the pattern of breakage and in turn the size distribution of the large clusters.     

Cathode catalyst layer using supported Pt catalyst on ordered mesoporous carbon for direct methanol fuel cell

The development of a cathode catalyst layer based on a supported Pt catalyst using an ordered mesoporous carbon (OMC) for direct methanol fuel cell is reported. An OMC with a mesopore structure between hexagonally arranged carbon nanorods is prepared using a template method. Platinum nanoparticles are supported on the OMC (Pt/OMC) with high metal loading of 60 wt.%. Compositional and morphological variations are made by varying the ionomer content and by compressing the catalyst layer to detect a parameter that determines the power performance. Increase in power density with decrease in the volume fraction of ionomer in the agglomerate comprising the Pt/OMC and the ionomer indicates that mass transport through the ionomer phase governs the kinetics of oxygen reduction. Impedance spectroscopic analysis suggests that a significant mass-transport limitation occurs at high ionomer content and in the compressed cathode. The power density of the optimum cathode layer, which employs a Pt/OMC catalyst with a Pt loading of 2 mg cm⁻², is greater than that of a catalyst layer with 6 mg cm⁻² Pt-black catalyst at a voltage higher than 0.4 V. This would lead to a significant reduction in the cost of the membrane electrode assembly.