火焰CVD法合成二氧化钛纳米颗粒的数值模拟

【作者】 王利希；

【导师】 陈石；

【作者基本信息】 大连理工大学 ， 工程热物理， 2004， 硕士

【摘要】 纳米材料具有非常广泛的用途——比如日常用到的碳黑、钛白粉颜料（TiO₂）或者通讯中的光学纤维等，其制备方法的研究越来越受到重视。其中化学沉淀法（CVD）是制备纳米粉体的一种很有效的方法。由于颗粒尺寸、尺寸分布状况以及形态等特性对颗粒产品的性能都产生极大的影响，必须对生产装置的结构和操作参数要有很好的了解和控制，这也是本文研究的重点。 本文应用CFD商业软件FLUENT，对火焰CVD法合成二氧化钛纳米颗粒的过程进行了详细的数值模拟。假设燃烧反应在一步内完成，文章模拟了CVD法中的湍流扩散火焰，计算出的火焰形状与实验中所观察到的基本相符。为了模拟火焰中颗粒的聚结生长过程，采用了如下假设：当气体温度超过一定温度后，所有的先驱物分子将转化为二氧化钛单分子；不考虑先驱物TiCl4氧化反应时放热对温度场的影响；忽略颗粒相对流体的影响。在这些假设的基础上分别用Kruis等人（1993）提出的颗粒动力学模型和谢洪勇（2002）提出的颗粒动力学模型结合FLUENT软件对颗粒的特性进行了预测。在对两者的预测结果的比较中发现，前一个模型在用碰撞半径作为衡量颗粒尺寸的标准时其预测的粒径和实验数据吻合得相当好，而后一模型对颗粒尺寸的积累分数的预测却要相对理想一些。以Kruis等人的颗粒动力学模型的计算结果为例，对火焰温度，先驱物载体气体、燃料、氧化剂流量等对生成颗粒或者颗粒聚集块的尺寸的影响进行了分析，结果发现温度越高就越容易形成球形颗粒，颗粒在火焰中时间越长生成的颗粒或聚集块尺寸就越大。更多还原

【Abstract】 Considerable interest lies in the synthesis and the use of nanosized particles for a variety of applications. Commodities such as carbon blacks, pigmentary titania or optical fibers for telecommunications are typical products of CVD. Particle characteristics like size and size distribution or the morphology mainly influence the final product quality. This emphasizes the need for a tool for optimization of the reactor geometry and the operating parameters.Using the commercial CFD-code FLUENT, the simulation of the growth process of titania nanoparticle synthesized in a flame CVD process for nanoparticles is detailedly performed. With the suppose that the combustion reaction occurs in a single step, the turbulent diffusion flame in the flame CVD process is calculated and the configuration of the turbulent diffusion flame agrees reasonably well with that observed in experiments. To simulate the coagulation and sintering process of nanoparticle in this flame, assumptions are put up as follows: the process of all precursor molecules to free TiO2 "monomer" molecules occurs instantaneously when the gas temperature exceeds a certain given value; the effects of both the oxidation of TiCI4 on the profile of the temperature and the effects of the particle volume fraction on the fluid are negligible. Based on these assumptions, two different particle dynamic models, Kruis’(1993) and Xie’s(2002) are respectively implemented into FLUENT to investigate the growth of particles; the results from these two models are compared with each other. It is discovered that the particles/aggregates collision radius calculated from the Kruis’ model is perfectly consistent with the sizes of particles from the experiment, but the particle size distribution from the other model is better coincident with the experimental results. With the model by Kruis, the effects of flame temperature and the flow rates of fuel, oxygen and precursor on the sizes of primary particles and aggregates also are analyzed. The results indicate a flame of higher temperature more easily leads to spherical particles; the sizes of particle aggregates become bigger with the longer residence time.更多还原