碳化硅陶瓷具有高强度、高硬度、耐高温、低热膨胀系数、良好的化学稳定性以及优异的高温力学性能。但碳化硅陶瓷也存在陶瓷材料共同的脆性问题，使其难以兼具强韧性，直接限制了碳化硅陶瓷在航空发动机等关键领域的应用。通过细化晶粒，设计纳米晶结构，可以同时提高材料的硬度和韧性。因此，制备碳化硅纳米晶陶瓷是同时提高其硬度和韧性的可行之路。而碳化硅纳米晶陶瓷性能很大程度上取决于碳化硅粉体的质量。稳定合成高性能碳化硅纳米陶瓷粉体是制备碳化硅纳米晶陶瓷最为首要和核心的工作。针对现有技术难以低成本、批量化、稳定合成高质量的β-SiC纳米粉体，本论文首先采用氮气催化辅助燃烧合成法制备了一次粒径为20nm、粒度分布较窄、比表面积为20m2/g左右、等轴状形貌的β-SiC纳米粉体。研究Si和C原材料性能对于β-SiC纳米粉体性能的影响。从热动力学角度证明了氮气催化燃烧合成法制备β-SiC纳米粉体的气固反应模式，并结合形核和生长过程，提出β-SiC晶体的限域生长机制。基于此机制，以Si3N4为前驱体制备SiC单晶纤维。进一步强化气相反应的主导作用，以Na2CO3和Si固态反应物为原料，采用硅热还原燃烧合成法成功制备了粒径3-10nm、粒径分布很窄、比表面积超过45m2/g、高烧结活性的β-SiC纳米粉体。研究燃烧合成工艺参数对粉体性能的影响，阐明气相中间产物SiO和CO的形成过程，以及气相反应主导的SiC形核生长过程。接着以促进SiC纳米晶陶瓷的烧结为目的，对合成的β-SiC纳米粉体进行镁热还原燃烧合成法包覆石墨烯层、高能球磨机械活化和无氧酸循环酸洗处理。为验证本论文中合成的SiC纳米粉体的烧结性能，以常规的Al2O3-Y2O3和Al-B-C作为烧结助剂，对氮气催化燃烧合成的SiC纳米粉体进行放电等离子体活化烧结。另外，在常规烧结条件且无助剂情况下，以硅热还原燃烧合成的SiC纳米粉体为原料，成功制备近全致密、晶粒尺寸小于30nm的SiC纳米晶陶瓷。本论文基于燃烧合成法，引入创新工艺，合成高质量的β-SiC纳米粉体，形成精确调控、按需制备β-SiC纳米粉体的理论，证明了制备工艺可规模化放大，SiC纳米粉体批量化生产的可行性。以活化烧结方法进一步验证自主制备的SiC纳米粉体的高烧结活性，为SiC纳米晶陶瓷的制备提供重要保障。
SiC ceramic have the remarkable properties including high strength, high hardness, high temperature resistance, low thermal expansion coefficient, good chemical stability and excellent high temperature mechanical properties. However, the common brittleness of SiC ceramic makes it difficult to have both strength, hardness and toughness, which directly limits the application in key fields such as aeroengine. By refining the grain and designing the nanocrystalline structure, the strength, hardness and toughness of the material can be improved simultaneously. Therefore, preparation of SiC nanocrystalline ceramic is a feasible way to improve its hardness and toughness, which largely depends on the quality of SiC powders. It is the most important and core work to synthesize high performance SiC nanopowders stably.However, it is difficult to synthesize high-quality β-SiC nanopowders in large quantities with the low cost by existing technology. In this paper, β-SiC nanopowders with primary particle size of 20nm, specific surface area of 20m2/g, equiaxed morphology and narrow particle size distribution were prepared by the nitrogen-catalyzed and assisted combustion synthesis method. In addition, combined with the nucleation and growth process, the gas-solid reaction mode of nitrogen-catalyzed combustion synthesis was proved from the thermodynamics. And the confined growth mechanism of β-SiC crystals was proposed. Based on this mechanism, SiC single-crystal fibers were prepared from Si3N4 precursor. To further strengthen the leading role of gas phase reaction, Na2CO3 and Si solid reactants were used as raw materials. β-SiC nanopowders with the particle size of 3-10nm, narrow particle size distribution, specific surface area of more than 45m2/g and high sintering activity were successfully prepared by silicon-thermal reduction combustion synthesis. The effects of combustion synthesis parameters on the properties of nanopowders were studied. The formation process of gas-phase intermediates SiO and CO, and the nucleation and growth process of SiC controlled by gas phase reaction were elucidated. In order to promote the sintering of SiC nanocrystalline ceramic, the synthesized β-SiC nanopowders were coated with graphene by magnesium-thermal reduction combustion synthesis, activated by the high-energy ball mill and pickling with anaerobic acid.In order to verify the sintering properties of SiC nanopowders synthesized in this paper, the spark plasma sintering method was used to sinter SiC nanopowders prepared by nitrogen-catalyzed combustion with the conventional sintering additives of Al2O3-Y2O3 and Al-B-C. Moreover, with the conventional sintering conditions and without the sintering additives, SiC nanocrystalline ceramics with the grain size less than 30nm and high density were successfully prepared from SiC nanopowders synthesized by silicon-thermal reduction combustion synthesis.Based on the combustion synthesis method, in this paper, the innovative technology was introduced to synthesize high-quality β-SiC nanopowders, forming the theory of precise control and on-demand preparation of β-SiC nanopowders. The feasibility of large-scale production of SiC nanopowders was proved. The active sintering method was used to further verify the high sintering activity of the self-prepared SiC nanopowders, which provides an important guarantee for the preparation of SiC nanocrystalline ceramics.
