Ultrathin coating of fluidized particles by means of aerosol

各种用途的包衣颗粒通常是通过在机械搅拌或流态化的芯子上喷洒含固体液体来生产的。沉积在核心颗粒表面的每个喷雾液滴在溶剂或悬浮液(最好是水)蒸发后会留下固体残留物。每个这样的沉积都是涂覆层的积木(BB)。然而，在目前的技术中，喷雾液滴相当大(通常为40微米，采用双流体喷嘴)，因此BBS也很大，导致涂层粗糙而厚实。使用气雾剂(液滴直径约为1微米或更小)而不是普通喷雾，可以在流态化颗粒上生成从根本上更薄、分辨率更好的涂层(达到纳米级)。最近，通过原理验证实验，证明了各自的气溶胶流态化(AFB)涂层工艺的可行性。在此基础上，本项目旨在对新的AFB过程进行彻底的科学调查。这包括随操作参数、材料以及气溶胶产生和进入条件的变化而进行的批量涂层实验。用扫描电子显微镜和各种图像分析技术对包覆颗粒的质量进行了全面的表征，包括颗粒内涂层厚度分布、颗粒间涂层厚度分布、平均孔隙率、孔隙率分布和孔径分布。在这些独特数据的支持下，建立了一个随机(蒙特卡罗)模型，并将其参数化，以准确地模拟单个粒子上涂层的积累和粒子种群中的涂层；此外，在表面覆盖期(可能有岛屿生长)和后来的(涂层成长期)。最后进行了测量，并建立了预测该过程固体产率的模型，该模型相当于流态化床过滤气流中气溶胶液滴的效率。

Coated particles for various applications are usually produced by spraying solid-containing liquid on mechanically agitated or fluidized cores. Every spray droplet which is deposited on the surface of a core particle leaves behind a solid remnant after evaporation of the solvent or suspension liquid (preferably water). Each such deposit is a building block (BB) of the coating layer. However, spray droplets are quite large (typically 40 µm with two-fluid nozzles) in present technology, so that BBs are also large, resulting in coarse and thick coating. Radically thinner and finer resolved coating layers (down to the nanoscale) could be produced on fluidized particles by using aerosol (with droplet diameters around 1 µm or less) instead of common spray. Feasibility of the respective aerosol fluidized bed (AFB) coating process has recently been shown by a proof-of-principle experiment. On this basis, the present project aims at a thorough scientific investigation of the novel AFB process. This includes batch coating experiments with variation of operating parameters, materials, as well as aerosol generation and entrance conditions. The quality of coated particles is characterized thoroughly by scanning electron microscopy and various image analysis techniques in regard of intra-particle coating thickness distribution, inter-particle coating thickness distribution, average porosity, porosity distribution, and pore size distribution. Supported by such unique data, a stochastic (Monte Carlo) model is developed and parameterized to accurately simulate the buildup of coating layers on single particles and in the population of particles; Moreover, in the surface coverage period (possibly with island growth) and later on (in the coating layer growth period). Finally, measurements are conducted and a model is developed to predict solids yield of the process, which is equivalent to the efficiency of the fluidized bed in filtering aerosol droplets out of the gas flow.