多孔氮化硅陶瓷是结构功能一体化的先进材料代表，在国防、军事和工业领域都有着不可替代的应用前景。由于氮化硅低自扩散系数和高温相转变的本征特性，导致多孔氮化硅陶瓷在烧结中不仅存在α→β相转变的过程，而且还具有β-Si3N4晶核生长发育过程，同时后期液相烧结还将导致晶粒重排与结构致密化。多孔氮化硅在烧结过程中面临结构收缩、晶粒形貌不均匀和晶粒生长欠佳的问题最为突出，因此，探索能够解决上述问题的新型制备技术就成了提升与发展多孔氮化硅陶瓷性能和潜力的关键所在。本论文主要围绕凝胶注模成型工艺结合高温自蔓延烧结技术制备多孔氮化硅基陶瓷的新技术展开：通过凝胶注模和高温自蔓延工艺融合路线，解决了多孔氮化硅陶瓷晶粒发育不完善、结构收缩大的问题，制备了形貌结构均匀和高长径比晶粒的多孔氮化硅陶瓷。研究了多孔坯体在高温自蔓延中多孔结构转变，并通过固相量实现了对燃烧反应和形貌结构的调控。同时探究了SiC第二相的生成原因是凝胶注模过程中聚丙烯酰胺不完全分解造成的碳残留在高温下反应造成的。基于碳残留的现象，采用反应烧结制备了高性能的多孔Si3N4-SiC复合陶瓷。研究了其微观形貌和结构特征，结果显示溶解在液相中的炭黑高温转变形成石墨化碳层对晶粒均匀包覆，并且整体结构为多级孔。进一步研究了多孔结构对力学性能的影响。此外还评估了多孔Si3N4-SiC复合陶瓷在电磁波吸收方面的表现，并对衰减损耗机理进行了研究，界面极化和传导损耗为主要损耗机制，同时Si3N4和SiC相对含量的调控使吸波材料获得了近100%的阻抗匹配。研究了排胶过程对高温自蔓延的影响，提出排胶气氛转变策略解决了碳残留问题。在此基础通过第二相复合方式制备了高气孔率的多孔BN-Si3N4复合陶瓷，并对第二相调控燃烧反应参数和形貌结构进行了研究。探究了微观结构并对力学性能和介电性能进行了评估，整体表现为高性能的透波材料。研究了抗热震测试后结构和力学性能变化，发现晶间低共熔物结构性破坏是强度下降的主要原因。为进一步实现高效、便捷、环保和经济的合成理念和高强度指标，采用了水基自发凝固体系结合高温自蔓延技术。提出表面水解策略制备了稳定均匀的多相水基自发凝固体系浆料，研究了水解层对浆料稳定性的影响和多相水基浆料的形成机理。实现了超高强度的多孔氮化硅陶瓷制备。结果显示均匀的亚微米长柱状晶粒形成的互嵌网络结构是高强度表现的重要结构基础。
Porous silicon nitride ceramics is a representative of advanced materials with integrated structure and function. It has an irreplaceable application prospect in national defence, military and industry. Due to the intrinsic characteristics of low self-diffusion coefficient and high temperature phase transition of Si3N4, porous Si3N4 ceramics sintering not only has α→β phase transition process, but also accompanied by β-Si3N4 grain growth, and the densification effect of liquid phase sintering. Porous silicon nitride ceramics face the most prominent problems of structural shrinkage, uneven morphology and poor grain growth. Therefore, exploring new preparation technology that can solve these problems has become the key to improve and develop the performance and potential of porous silicon nitride ceramics. This paper mainly focuses on the new technology of preparing porous silicon nitride-based ceramics by gel casting-self-propagating high temperatures synthesis:Through technology combination, the problems of imperfect grain development and large structural shrinkage of porous Si3N4 ceramics were solved, and the porous Si3N4 ceramics with uniform morphology and structure and high aspect ratio were prepared. The transformation of porous structure in self-propagating porous green body at high temperature was studied. The combustion reaction and morphology were controlled by solid content. At the same time, the formation of SiC second phase is caused by carbon residual reaction at high temperature.Based on carbon residue, porous Si3N4-SiC composite ceramics with high performance were prepared by reaction sintering. The microstructure and structure characteristics of the carbon black dissolved in the liquid phase were studied. The results show that the carbon black transformed into graphitized carbon layer uniformly coated the grain at high temperature, and the overall structure is hierarchical pore. The effect of porous structure on mechanical properties was further studied. In addition, the performance of porous Si3N4-SiC composite ceramics in electromagnetic wave absorption was evaluated, and the attenuation loss mechanism was studied. Interfacial polarization and conduction loss are the main loss mechanisms, and the regulate the relative content of Si3N4 and SiC makes the impedance matching nearly 100%.The effect of debinding process on self-propagating high temperatures synthesis was studied, and the change strategy of debinding atmosphere was proposed to solve the carbon residual problem. On this basis, porous BN-Si3N4 composite ceramics with high porosity were prepared by the second phase composite method. The microstructure was investigated and the mechanical and dielectric properties were evaluated. The overall performance of the porous BN-Si3N4 composite ceramics was high performance. The structural and mechanical properties change after thermal shock test were studied. It was found that the structural failure of intergranular low eutectic was the main reason for the strength decline.In order to achieve an efficient, convenient, environmentally friendly and economical synthesis concept, the spontaneous solidification casting system combined with self-propagating high temperatures synthesis is adopted. A surface hydrolysis strategy was proposed to prepare a stable and uniform aqueous multiphase slurry, the effect of hydrolyzed layer on the stability of slurry and the formation mechanism of polyphase aqueous slurry were studied. The high strength porous silicon nitride ceramics were prepared by surface hydrolysis strategy. The results show that the network formed by interlock and intergrown of uniform submicron grains is an important structural basis for high strength.
