Growth kinetics in particle coating by top-spray fluidized bed systems

The kinetics of growth for the coating of particles in top- spraying fluidized bed systems is reported. The results indicated that only a small amount of particles that visited the spraying region are coated at a time. It was also revealed that different particle sizes are not equally coated during the process. In a polydispersed particle distribution, smaller particles were found to receive more coating than their larger counterparts. This preferential coating, which was associated with a rapid decrease in the distribution variance, is more pronounced in the earlier parts of the process. When a narrower seed distribution was used, the preferential coating was reduced. A segregation factor, f_s, was introduced in the development of a growth kinetics model to represent the chance of each particle size visiting the coating region. The result for the distribution from the model clearly resembled the results obtained experimentally. For the top-spraying process, the segregation factor was found to be an exponentially decaying function of particle weight. For lactose particles coated with hydroxypropyl methyl cellulose, two different rates of growth were observed during the coating process.     

矿渣粉比表面积及粒度分布对水泥强度的影响

采用勃氏透气仪和激光粒度分析仪,对矿渣粉试样进行比表面积和颗粒群粒度分布的测试,研究矿渣粉比表面积及粒度分布对水泥强度的影响。结果表明:矿渣粉总体颗粒越细,则比表面积越大,特征粒径越小,颗粒群分布越宽;细颗粒含量(小于5μm)越多的矿渣粉,其比表面积越大,水泥砂浆的早期强度就越高。     

降低超细高氯酸铵感度的方法研究

为了降低超细高氯酸铵(AP)的机械感度,以十八烷胺为包覆剂,经气流粉碎工艺,制备了超细AP包覆粒子。通过对超细AP包覆粒子进行粒度分析、SEM分析、DSC分析及感度检测,研究了超细AP包覆粒子的包覆效果、热分解特性及感度特性。试验结果表明:超细AP包覆粒子,粒度为D50=5.8μm,表面有包覆层存在;超细AP包覆粒子的感度,与超细AP样品相比,当包覆剂质量分数为1%时,撞击感度降低31.2%,摩擦感度降低12.0%;当包覆剂质量分数为3%时,撞击感度降低34.5%,摩擦感度降低22.0%,且包覆剂用量越大,撞击感度和摩擦感度越低;但包覆剂对AP的热分解会产生一定的负面影响,质量分数应≤1%。     

Microstructure and electrical conductivity in shape and size controlled molybdenum particle thick film

We study the effects of particle morphology on the microstructure and electrical conductivity of Mo particle thick films. In our study, the shape and size of molybdenum (Mo) particles are modified by mechanical ball-milling and atomic layer deposition (ALD). As the total number of collisions between Mo particles and ball-milling media increases, Mo particles are deformed, and the shape of Mo particles changed from irregular polyhedrons to thin flakes. In the ball-milling process, stress frequency is an important processing parameter governing the deformation and breakage of Mo particles. In addition, ALD-grown TiO₂ layer is found to significantly suppress the growth of Mo particles at high temperature. After 1000 °C annealing, the particle size of the TiO₂ layer-coated film is only half of that of bare Mo particle films. The shape of the particles changes electrical conductivity of the Mo thick films. Large contact area between flake shape particles can increase the carrier mobility of the film and the 5-nm thick TiO₂ layer can provide the inter-particle carrier transport path via a tunneling mechanism. Our results show that the combined use of the ball-milling and the ALD coating leads to Mo thick films with high electric conductivity and large surface area.     

A novel approach to a fine particle coating using porous spherical silica as core particles

The applicability of porous spherical silica (PSS) was evaluated as core particles for pharmaceutical products by comparing it with commercial core particles such as mannitol (NP-108), sucrose and microcrystalline cellulose spheres. We investigated the physical properties of core particles, such as particle size distribution, flow properties, crushing strength, plastic limit, drying rate, hygroscopic property and aggregation degree. It was found that PSS was a core particle of small particle size, low friability, high water adsorption capacity, rapid drying rate and lower occurrence of particle aggregation, although wettability is a factor to be carefully considered. The aggregation and taste-masking ability using PSS and NP-108 as core particles were evaluated at a fluidized-bed coating process. The functional coating under the excess spray rate shows different aggregation trends and dissolution profiles between PSS and NP-108; thereby, exhibiting the formation of uniform coating under the excess spray rate in the case of PSS. This expands the range of the acceptable spray feed rates to coat fine particles, and indicates the possibility of decreasing the coating time. The results obtained in this study suggested that the core particle, which has a property like that of PSS, was useful in overcoming such disadvantages as large particle size, which feels gritty in oral cavity; particle aggregation; and the long coating time of the particle coating process. These results will enable the practical fine particle coating method by increasing the range of optimum coating conditions and decreasing the coating time in fluidized bed technology.     

不同形貌碳酸钙颜料粒子的制备及在喷墨印刷中的应用

目的研究碳酸钙颜料粒子的形貌对喷墨打印涂层性能的影响。方法通过改变分散剂的种类制备不同的碳酸钙颜料粒子,并通过扫描电子显微镜(SEM)、傅里叶变换红外光谱仪(FT-IR)、粒径分析仪对制备的颜料粒子进行表征,研究分散剂种类对颜料形貌和粒径的影响。将4种不同形貌的碳酸钙分别作为颜料,将其应用于喷墨打印纸的表面涂层,并测试涂布纸张的动态渗透性、物理性能、喷墨打印性能。结果 CMC作为分散剂有助于形成粒径分布均匀的球形碳酸钙颜料粒子;纺锤形碳酸钙颜料粒子有助于提高涂布纸的平滑度和光泽度;球状体碳酸钙颜料粒子有助于改善涂布纸的喷墨印刷性能和渗透性;颜料粒子的粒径分布均匀性对涂布纸的动态渗透性影响较大,其影响超过颜料粒子形貌对涂布纸渗透性的影响。结论粒径分布均匀的球状体碳酸钙颜料粒子最适合用于喷墨印刷涂层,可以提高纸张的物理性能、印刷性能和动态渗透性能。     

Advanced deposition characteristics of kinetic sprayed bronze/diamond composite by tailoring feedstock properties

In this study, the effect of protective nickel film on diamond particles was studied with the goal of preventing fracture and obtaining a uniform diamond distribution in a bronze/diamond composite coating during kinetic spraying. Two types of bronze/diamond composite were deposited on aluminum substrate. For comparison with experimental results, the impact behavior between diamonds in the gas flow field was simulated by finite element analysis (using ABAQUS/Explicit 6.7-2). Size distribution and deposition efficiency of the diamond particles in the composite coatings were analyzed through scanning electron microscopy and image analysis methods. Diamond fracturing was avoided because the impact energy was mostly absorbed by the outer protective nickel film on the diamond particle during impact. The uniform distribution and deposition efficiency of diamond particles in the coating layer could also be achieved by tailoring the physical properties (density, size, etc.) of the feedstock.     

Calculation of emittance of a coating layer with the Kubelka-Munk theory and the Mie-scattering model

The radiative transfer equation in a coating layer with a not too high concentration of pigment particles, which is attached to a substrate, is resolved in terms of Kubelka-Munk theory and a Mie-scattering model. In the case of a coating layer constituted with aluminum spherical particles, the dependence of emittance of the coating layer on particle radius and thickness of the layer are studied. The optimum radius of a pigment particle is suggested as well.     

Self-healing efficiency of unhydrated cement nuclei for dome-like crack mode in cementitious materials

Nuclei of cement particles left unhydrated in cementitious materials after maturation can provide self-healing capability to micro-cracked matrix under favorable conditions. In this present study under the assumption that self-healing is attributed to the rehydration of the unhydrated cement nuclei in cementitious materials, a model study is conducted to characterize the self-healing efficiency of the hydration reaction of unhydrated cement nuclei on crack. Based on a tortuous crack path around unhydrated cement nuclei in practical cementitious materials, a dome-like crack mode is presented to investigate the self-healing efficiency on cracks. Recurring to a generalized hydration reaction model of cement particles, the analytical self-healing efficiency model quantitatively considers the influence of the volume fraction and particle size distribution of unhydrated cement nuclei randomly distributed in cementitious materials based on the proposed dome-like crack mode. Meanwhile, the healing process of unhydrated cement nuclei in model cementitious materials is simulated and the reliability of these analytical models is verified via computer simulation. Furthermore, model applications suggest that volume fraction of unhydrated cement nuclei in matrix is a key factor and the particle size distribution is also very important for self-healing efficiency in the long time.     

Influence of colloidal silica sol on fresh properties of cement paste as compared to nano-silica powder with agglomerates in micron-scale

Influence of colloidal silica sol (SS) with mono-dispersed nano-particles on fresh properties of cement paste was investigated as compared to nano-silica powder (NS) with agglomerates in micron-scale. The SS addition showed a much greater influence on sedimentation and rheological behavior of the paste than the NS incorporation, because the nano-particles in SS coagulate immediately once cement is mixed into water containing SS, forming loose floc and coating layer around cement particles. The loose floc cannot function as fillers to release free water, but possesses a more open microstructure, leading to a higher free water retention capacity than the agglomerates in NS. However, addition of SS presented an obviously better accelerating effect on cement hydration than that of NS, though the nano-particles in SS are nearly the same as those in NS in primary particle size and the flocs in the paste with SS addition are typically larger than the agglomerates in NS, implying that the acceleration may have nothing to do with the seeding effect. Through detecting calcium-absorbing properties of NS and SS, it is found that the accelerating effect is highly dependent on the rapid depletion of calcium ions in the paste. Finally, it was interestingly found that the CH crystals are even more prone to grow along (0001) plane with larger size in the paste with SS addition, because the coagulated gel network in the paste slows down the diffusion rate of the released ions and eliminate the convection in the system, thus the 3D nucleation and growth of the CH crystals were suppressed.     

Effects of mixing ratio and order of admixed particles with two diameters on improvement of compacted packing fraction

To improve particle flowability, a technique is used in which fine particles are admixed with the main particles. However, the effects of coating structure on the improvement in flowability are not yet fully understood. Thus, predicting the improvement resulting from this technique is difficult. In this study, we focused on the effects of the particle diameter distribution of the admixed particles on coating structures and improvement of flowability in terms of the compacted packing fraction in a particle bed. Main particles of size 397 nm with admixed particles of sizes 8 and 104 nm were used. Bimodal particle diameter distributions were adjusted by changing the mixing ratios of the two admixed particles. Furthermore, the main and admixed particles were mixed in various orders. We examined the compacted packing fractions for these different mixing ratios and orders. Scanning electron microscopy images were obtained in order to analyze the coating structures on the main particle surfaces. The results show that the main particle packing fraction was most greatly improved by pre-mixing the two admixed particles. This can be explained by a linked rigid-3-bodies model with leverage based on increasing the apparent diameter of the main particles.     

三效催化剂涂层制备与性能研究

薄水锅石作为γ-Al₂O₃的前驱体,加入CeO₂-ZrO₂-La₂O₃和扩孔剂等助剂制备蜂窝陶瓷载体涂层,研究了涂层料液的表观粘度、因含量与载体负载量的关系,粘度对料液中粒径分布和涂层性能的影响,利用SEM、BET表面积、超声波震荡和热冲击等方法研究了涂层的性能.结果表明,涂层料液中适宜的固含量为30％～40％;一次浸量>二次浸量>三次浸量,适宜的负载量为8％～15％;低粘度料液以小颗粒为主,颗粒分布均匀,制备的涂层有较好的抗震动和抗热冲击性能;高粘度涂层料液的颗粒较大和粒径分布较宽,有利于提高载体的负载量,但涂层的性能明显下降.堇青石蜂窝陶瓷载体经一步法负载涂层后,可获得约 50 m²/g的表面积.     

水泥粉体粒度分布分维与其流动性的关系研究

为研究水泥粉体粒度分布分维与流动性的相关性,本文中将粒度分布分维作为表征水泥粉体的粒度分布特征,用图像分析法测算了5种水泥颗粒样品的粒度分布分维,5种水泥样品粒度分布分维值处于2.1006～2.2931之间;并用卡尔指数法测量了水泥样品的流动系数,考察了水泥粉体粒度分布分维及其与粉体流动性的关系,研究表明:水泥粉体粒度分布分维与其流动性呈线性负相关性。     

热处理对二氧化钛表面二氧化硅包膜的影响

通过单流匀速滴定法在二氧化钛颗粒表面均匀包覆一层二氧化硅,系统地研究热处理温度对这层硅包覆层的形貌和相组成的影响;采用X射线衍射仪(XRD)、X射线光电子能谱仪(XPS)、透射电子显微镜(TEM)和傅立叶红外光谱仪(FTIR)等手段对包硅二氧化钛颗粒进行表征。结果表明:150℃热处理形成连续致密的硅二氧化硅覆层;500℃热处理3 h后形成岛状二氧化硅膜;700℃热处理后形成不连续的硅二氧化硅覆层,并有脱落的倾向;当温度升到1 000℃时,硅二氧化硅覆层会脱落,二氧化钛的粒径增大,大约在1 000℃,二氧化硅包覆层结晶化。     

Microencapsulation by a Solvent-Free Supercritical Fluid Process: Use of Density, Calorimetric, and Size Analysis to Quantify and Qualify the Coating

Coating of microparticles or microencapsulation is a widely used operation in which a thin layer of a coating agent is deposited onto a solid particle. Currently, the technique faces two challenges: being solvent-free and being applicable for coating particles smaller than 80 microns. In this work, several techniques are used to test the feasibility of a new solvent-free coating process using supercritical fluid technology. Some model microparticles, glass beads, and an active compound (AC) are coated with Precirol®, which is a mixture of fats. The process involves two steps: first supercritical CO₂ is dissolved in molten Precirol®, then the melt solution is sprayed onto the host particles by a rapid expansion. The particles coated in this way are examined by particle size analysis, environmental scanning electron microscopy (ESEM), and IR spectrophotometry. Also mass ratios of host particle to coating are evaluated using both helium pycnometry to determine sample density and calorimetry to measure the heat of fusion of the coating agent. The results show that these analytical techniques can be used for qualitative analysis as well as quantification of the coating. Thus they give relevant information on reproducibility and feasibility of the new process.     

水泥粒度分布对水泥性能影响的研究进展

从两个方面总结了水泥粒度分布对水泥性能影响的研究进展,一方面总结了反映水泥颗粒群整体情况的特征参数比表面积S、特征粒径x′和均匀性指数n对水泥性能的影响;另一方面总结了不同粒径区间水泥颗粒的性能。介绍了描述水泥粒度分布的RRB方程和Fuller曲线,综述了理论上粒度分布对水泥性能的影响情况,认为水泥的粒度分布是与其性能有明确定量关系的细度参数,是水泥粉磨细度控制的最终目标。     

Preparation of electrically conductive fibres by new surface coating method

Abstract

In the present research, a new surface coating method (dissolving-coating method) that is on the basis of dissolution mechanism was designed. Substantially transparent antimony doped tin oxide (ATO) particles were used instead of carbon black as conductive particles. ATO coated polyester (PET) fibres and ATO coated polycaprolactam (PA6) fibres were prepared by the traditional coating method and the dissolving–coating method respectively. The electrical conductivity, mechanical property and launderability of the coated fibres were discussed. For the characterisation, a laser particle size distribution analyser was used to analyse the size distribution of ATO. Scanning electron microscopy (SEM) and transmittance electron microscopy (TEM) were used for the observation of ATO particles and fibre surface. Electron probe was carried out to make an elemental analysis of the fibre.     

Infrared emittance of medium containing nonsphere-shaped particles

The scattering and absorption sections of cylinder- and sheet-shaped particles in a binder have been investigated. The IR emittance of a coating layer composed of randomly arrayed particles embedded in the binder has been calculated in terms of the size of the particles, the volume fraction occupied by the particles, and the complex refractive indices of the particle and the binder as well as the emittance of the substrate. The calculation shows that the inhomogeneous medium including sheet-shaped particles can have lower emittance than that including cylinder- or sphere-shaped particles.     

Sedimentation kinetics of polydisperse second-phase inclusions in molten selenium

The behavior of polydisperse second-phase inclusions in molten selenium during settling is modeled using solid carbon particles as an example. A theoretical limit of the particle concentration in the surface layer of the melt is evaluated as a function of particle size, and the particle concentration is determined as a function of time, melt viscosity, and surface layer thickness. The calculated histograms of the particle size distribution in the surface and bottom layers of molten selenium are in satisfactory agreement with experimental data.     

铜粉粒径对机械镀Cu-Zn镀层性能的影响

为获得更好的防护+装饰双重效果,采用不同粒径(200,400,800,1 000,1 200目)的铜金粉,利用机械镀技术在钢铁基体表面制备了铜-锌复合镀层。采用称重法分析了镀层的致密度,采用贴滤纸法检验了镀层孔隙率,采用划线划格法分析了镀层的结合强度,采用全浸腐蚀法及电化学极化法分析了镀层的耐腐蚀性能。结果表明:不同粒径铜金粉制备的镀层均覆盖完整,随铜粉粒径减小,复合镀层孔隙分布减少;随着铜粉粒径的减小,镀层的致密度逐渐增加,当铜粉粒径为1 000目时,Cu-Zn镀层的致密度已大于金属锌的密度;随着铜粉粒径的减小,镀层的结合强度增加,当铜粉粒径为200目时镀层的结合强度较差,而铜粉粒径小于400目时镀层的结合强度明显提高;随着铜粉粒径的减小,镀层的全浸腐蚀速率逐渐减小,耐腐蚀性增强。