流化床化学气相沉积过程纳米粒子凝聚/合并/分散的介尺度机制及调控

The control of nanoparticle coagulation/coalescence/restructuring is the key of fluidized bed chemical vapor deposition synthesis of nanoparticles. The properties of synthetic nanoparticles depends on the properties of primary nanoparticles from the micro scale, and the multiphase flow dynamics from the macro scale. Thus it is a typical problem of multiscale science. We study the synthesis of silicon carbide nanoparticles, by developing an agglomerate stability mesoscale model, in order to elucidate and control the relationship between the nanoparticle properties and process parameters. In the experiment, we use an in situ sampling tool for nanoparticles and characterization, providing basic data. From the meso scale model, by using temporal and spatial scale decomposition, a collision/adhesion/sintering model of nanoparticle agglomerate will be developed. By using variational extremum principle, we will analyze agglomeration structure, energy balance, and stability mechanisms during dynamic evolution of agglomerates, to obtain a phase diagram which relates the agglomerate size/morphology/structure and operating parameters. Then, according to the phase diagram, the effects of typical operations of temperature field, fluidizing particles and particle residence time on the properties of the synthesized particles will be investigated and optimized. Towards this end, this project will establish the optimized recipes of synthesis nanoparticles by a mesoscale stability methodology. This will be a significant improvement that the nanoparticle synthesis method can be based on regulation mechanism and scientific design, from the current trial and error methods. It should be another demonstration case of application of mesoscience, and will promote the development of mesoscience theory.

控制纳米粒子凝聚/合并/分散是流化床化学气相沉积合成纳米颗粒的核心关键，合成纳米颗粒的性质同时与微观的原生纳米粒子、宏观的多相流场密切相关，是典型的多尺度科学问题。本项目以合成碳化硅纳米颗粒为实例，建立聚团稳定性介尺度模型，研究合成纳米颗粒性质与过程参数联系机理和相应的调控方法。实验方面，采用原位取样纳米颗粒及表征，提供基础数据。模型方面，借鉴介尺度思想按时空尺度将影响纳米颗粒团聚稳定性的参量分解，建立纳米粒子碰撞/粘附/烧结模型，引入变分极值原理，分析聚团的凝聚-合并-分散的介尺度约束条件，探索聚团动态演化结构和机制，获得聚团尺寸/形态/结构和操作参数的相图。依据相图，考察温度场、流化颗粒床存量、粒子停留时间等因素对合成颗粒性质的影响，形成以聚团介尺度稳定性为导向的纳米颗粒合成优化调控方法，使纳米颗粒合成从经验调控发展到机理调控、科学设计，同时促进介尺度科学的实例研究和模型发展。

控制纳米粒子凝聚/合并/分散是流化床化学气相沉积合成纳米颗粒的核心关键，合成纳米颗粒的性质同时与微观的原生纳米粒子、宏观的多相流场密切相关，是典型的多尺度科学问题。本项目以流化床-化学气相沉积合成碳化硅纳米颗粒为实例，进行实验测量、数学建模和数值模拟研究。实验方面，采用原位取样纳米颗粒及表征，提供基础数据。模型方面，借鉴介尺度思想建立纳米粒子碰撞/粘附/烧结模型，获得纳米颗粒凝聚/合并/分散动态演化过程。基于此模型模拟考察了外场（剪切作用）对聚团稳定性的影响，形成纳米颗粒制备及调控方法。最终基于此聚团和粒径调控方法制备了特定尺寸的碳化硅纳米颗粒，进行表面改性，获得新型高热导率核燃料基体低温烧结技术，为纳米颗粒可控制备技术提供了实际应用方向。