微波合成碳化硅晶体可控生长动力学研究

Microwave synthesis of silicon carbide crystals refers to the new theoretical system of materials preparation. In this project, coal mineral is selected as raw materials, which is directly mixed with silica sand or coated with silica by a sol-gel method. The key scientific problems such as the mutual interaction mechanisms between microwave and materials, high temperature physical and chemical reaction, the crystallization process control of silicon carbide crystal etc. will be solved by adjusting the reaction raw materials to control heating effect, optimizing heating process to control high temperature reaction and other critical technologies. The changes in reflected power and temperature during microwave heating will be monitorred to investigate the mechanism, performance characteristics, influenced factors and control methods of various types heating effects such as microwave coupling heating effect, plasma heating effect etc.. The geometrical characteristics of silicon carbide crystals will be analyzed under different heating conditions to analyze the process of high temperature solid phase reaction and gas phase reaction and the effect on crystal nucleation and growth mode. The comprehensive characteristics of silicon carbide crystals obtained under different physical and chemical reaction conditions will be examined to reveal local enhancement of microwave electric field and its effect on the crystal growth. Atomic diffusion process of silicon carbide crystals will be studied under multi effects of hybrid conditions such as complicated microwave thermal effect, high frequency electric field, the high temperature liquid phase etc.. The crystal growth dynamics and controlled growth mechanism of silicon carbide crystals synthesized by microwave sintering will be proposed, so that controllable SiC crystals with different crystalline types, different morphology and different sizes could be prepared.

微波合成碳化硅晶体涉及材料制备新理论体系。本项目以工业煤为原材料，利用石英砂直接混合或硅溶胶包裹制备混合原料；通过调节反应原料控制加热效应、优化加热工艺控制高温反应等关键技术，解决微波与材料相互作用机理、高温物理化学反应、碳化硅晶体的结晶过程控制等相关的关键科学问题；分析微波加热过程中反射功率与温度变化，研究微波耦合热效应、等离子体热等各类热效应的产生机理、表现特征、影响因素及控制方法；分析不同加热条件下碳化硅晶体的几何特征，研究高温固相反应与气相反应的发生过程及其对晶体形核过程与生长方式的影响；分析不同高温物理化学反应条件下获得的碳化硅晶体的综合特征，研究微波电场局部集聚效应及其对晶体生长模式的影响；揭示微波综合热效应、高频电场、高温液相等复杂条件协同作用下碳化硅晶体原子扩散过程，提出微波合成碳化硅晶体的生长动力学过程与可控生长机理，从而获得不同晶体结构、不同形貌、不同尺寸的碳化硅晶体。

本项目以工业煤为原材料，利用石英砂直接混合或硅溶胶包裹制备混合原料；通过调节反应原料控制加热效应、优化加热工艺控制高温反应等关键技术，解决了微波与材料相互作用机理、高温物理化学反应、碳化硅晶体的结晶过程控制等相关的关键科学问题；分析了微波加热过程中反射功率与温度变化，研究了微波耦合热效应、等离子体热等各类热效应和非热效应的产生机理、表现特征、影响因素及控制方法。结果表明微波合成碳化硅加热过程大致出现4个阶段：加热起始阶段的热积累（<600 oC），快速加热阶段的热剧变（600~900 oC），高温生长阶段（900~1100 oC）和保温阶段。不同的微波加热行为以及微波加热效应会引起晶体生长机理的变化，最终引起SiC样品组分、结构、形貌以及介电和吸波性能的改变。通常情况下，微波合成SiC呈现颗粒状、晶须状等，分别对应于不同的形核与生长机理，与微波加热效应有关。加热低温阶段的微波耦合热效应引起合成原料内热积累，到一定温度时激发C@SiO2界面反应，生成SiO和CO气体，大量气体积累激发微波等离子体，引起局部瞬间高温，促使SiO和CO气相反应形成SiC晶核和O2气体。一方面，SiC晶核在添加的晶种表面沉积生长，形成大的颗粒；另一方面，新生成的O2沿C@SiO2界面扩散，带动SiC晶核沿扩散方向定向排列并持续生长，形成SiC晶须。本研究揭示了微波综合热效应、高频电场、高温液相等复杂条件协同作用下碳化硅晶体原子扩散过程，提出微波合成碳化硅晶体的生长动力学过程与可控生长机理，从而制备了不同晶体结构、不同形貌、不同尺寸的碳化硅晶体。