非晶Si3N4氧化晶化法制备Si2N2O透波陶瓷及其晶化-致密化耦合机制

Quartz-based and porous silicon nitride material are two major categories of high-temperature wave-transparent materials for high speed aircraft radomes though the moisture absorption, poor mechanical impact and thermal shock resistance are their disadvantages. Recent studies show that Si2N2O is an promising candidate material for high-temperature wave-transparent application, for its better performance in general. However, it is difficult to produce fully dense Si2N2O ceramics because Si2N2O tends to decompose at the required sintering temperature. In this research, the novel process of preparation of Si2N2O ceramics by amorphous oxidation crystallization is proposed, that is, amorphous silicon nitride powder was used to synthesize Si2N2O phase at a low temperature by controlling oxygen in raw material and micro-oxygen pressure in sintering atmosphere, followed by SPS (spark plasma sintering) by which the decomposition of Si2N2O is inhibited at high temperature. The densification of Si2N2O ceramic can be achieved in low temperature without sintering additives and, therefore, the decrease of high-temperature properties can be prevented. Various and complex changes are involved in this process including Si3N4 to Si2N2O transformation, amorphous to polycrystalline formation, mass coupling diffusion and in situ reaction. In this research, the main factors that influence amorphous silicon nitride crystallization and crystalline transformation, Si2N2O synthetic reaction and phase-change mechanism, coupling mechanism in crystallization and densification will be illustrated, and a new technology to produce fully dense Si2N2O ceramics can be built to obtain the high-temperature wave-transparent materials with more excellent performance.

石英类材料和多孔氮化硅是针对高速飞行器天线罩/窗应用的两类主流高温透波材料，但存在吸潮、抗机械冲击及热冲击性能差等问题。近期研究发现Si2N2O是综合性能更优的高温透波候选材料。但Si2N2O高温易分解，难以制备高纯致密的Si2N2O陶瓷。本项目提出非晶氧化晶化法低温制备Si2N2O陶瓷的新思路，即以非晶Si3N4为原料，控制原料中氧含量、气氛中微氧分压，低温合成Si2N2O相；SPS超快烧结，动力学抑制其高温分解。无需添加烧结助剂即可低温实现致密化，有效避免材料高温性能衰减。该过程涉及Si3N4到Si2N2O、非晶到多晶的转变、元素的耦合扩散与原位反应等复杂变化，目前对其机理不清楚。本研究力图阐明影响非晶Si3N4结晶过程及晶型转变的主要因素、Si2N2O的合成反应及相演变机理、晶化与致密化过程耦合机制等关键科学问题，建立一种制备高纯Si2N2O陶瓷的新方法，获得性能更优的高温透波材料

高温透波材料是保护飞行器雷达系统正常工作的一种多功能介质材料,石英类材料和多孔氮化硅是目前应用与研究的主流。但石英类材料易吸潮且强度低，多孔氮化硅高温性能不稳定。为此，研究、开发一种综合性能更优的高温透波材料，使其兼具二者的优点十分必要与迫切。. Si2N2O是SiO2-Si3N4系统中唯一的化合物，性能介于二者之间。但目前Si2N2O陶瓷的常规制备方法是利用SiO2和Si3N4直接反应，由于反应温度高导致Si2N2O分解，因而难以获得单相Si2N2O。为此本项目以纳米非晶氮化硅为原料，通过控制非晶氮化硅表面的含氧量，通过氧化晶化直接获得Si2N2O相。主要研究涉及以下三个方面：（1）非晶氮化硅的氧化过程、机理及控制因素；（2）Si2N2O的低温形成及其致密化；（3）Si2N2O陶瓷室温/高温力学、介电性能测试与分析。. 研究发现，由于非晶Si3N4中存在大量氮空位，因而其氧化可能存在两种途径，即氧进入氮空位的空位氧化和氧取代氮原子的置换氧化，其氧化活化能分别为9.09 kJ/mol和118.25 kJ/mol。而实验发现，实际非晶氮化硅的氧化活化能为45.96 kJ/mol，介于二者之间，这说明单纯依靠氧化后非晶Si3N4的增重来表征其氧化过程是不完全合理的，还需要结合非晶Si3N4的结构综合分析，这为非晶材料氧化行为的评价提供了思路。. 将氧化后的非晶纳米氮化硅通过热压，在不添加烧结助剂的情况下，获得R.D>98%，σf>400MPa的单相Si2N2O陶瓷，其室温介电常数为5左右，介电损耗介于0.002-0.008之间。与传统反应烧结法相比，该方法可以将Si2N2O陶瓷的制备温度下降50-100℃，有效避免Si2N2O的分解。在此过程中，硅、氮、氧三种元素的迁移对Si2N2O的形成及发育有很大的影响，外加压力和保温时间对其生成和晶粒发育具有重要作用，Si2N2O首先在成分满足SiN3O结构单元的地方形核析出。. 稀土阳离子半径对反应烧结过程中相组成与致密化、微观组织及力学性能也有较大影响。随着稀土阳离子半径的增大，Si2N2O的初始形成温度逐渐降低，而致密化程度逐渐增高。当气孔率相近时，强度随着稀土阳离子半径的增大逐渐增大。利用反应烧结制得的 Si2N2O陶瓷表现出了很好的抗氧化性，但是抗热震性能不理想。