强吸波SiC(N)-Si3N4陶瓷泡沫的构筑、介电特性与构效关系研究

High temperature wave-absorbing materials are pivotal radar stealth materials for high-speed weapons, such as fighter aircraft, cruise missile and so on. SiC-based ceramic foams are considered as a novel wave-absorbing material of high temperature, which wave-absorbing property depends on their composition of pore struts and pore structures (pore size and porosity). Till now, there are still some challenges left, including a limited wave-absorbing capability of pore struts, oversized pore diameter and uncontrollability of pore structures. SiC(N)-Si3N4 ceramics are constituted of a wave-loss N-doped-SiC phase and a wave-transparent Si3N4 phase, and possess a synergistic dielectric loss mechanism of interfacial polarization and relaxation polarization, which thus suit as pore struts of strong wave-absorbing capacity. Moreover, foaming-gelcasting process can be adopted to realize the thinning of pore size and tailoring of pore structures. In this project, new-style SiC(N)-Si3N4 ceramic foam with excellent wave-absorbing property will be constructed through this process. We will investigate the effects of N-doping, phase composition and grain size of SiC(N)-Si3N4 multiphase ceramics on their dielectric characters and wave-absorbing property, illuminate the regulatory mechanisms of dielectric loss, and select the optimized composition of pore struts with excellent wave-absorbing property. The foaming-gelcasting process for SiC(N)-Si3N4 ceramic foams will be built, and the wave-absorbing property will be optimized through tailoring their pore structures. And then we will study the effects of pore structures on the dielectric characters and wave-absorbing properties, and reveal their structure-function relationship. The research achievement will provide theoretical supports for the design and preparation of high-performance wave-absorbing ceramic foam materials.

高温吸波材料是战斗机、巡航导弹等高速武器的关键隐身材料。SiC基陶瓷泡沫作为一种理想高温吸波材料，其吸波性能取决于孔壁组成与孔结构（孔径与孔隙率），目前存在孔壁组成吸波能力有限、孔径偏大、孔结构调控困难等问题。由N掺杂SiC损耗相与Si3N4透波相组成的SiC(N)-Si3N4陶瓷具有界面极化与松弛极化协同的介电损耗机制，有望作为吸波能力强的孔壁材料。发泡-凝胶注模工艺可实现陶瓷泡沫的孔径细化与孔结构调控，本项目拟采用该工艺构筑新型强吸波SiC(N)-Si3N4陶瓷泡沫。探究N掺杂、相组成、晶粒尺寸对SiC(N)-Si3N4陶瓷介电特性的影响规律，阐明介电损耗调控机制，据此优选吸波能力强的孔壁组成。建立SiC(N)-Si3N4陶瓷泡沫的发泡-凝胶注模制备工艺，通过孔结构调控优化吸波性能，揭示孔结构与介电特性及吸波性能之间的构效关系，研究成果将为高性能陶瓷泡沫吸波材料的设计制备提供理论支撑。

高温吸波材料是高速武器的关键隐身材料，高温下吸波机制主要为介电损耗。SiC基多孔陶瓷是优异的高温介电损耗吸波材料，其吸波性能取决于孔壁组成与孔结构，目前主要通过模板法、冷冻干燥法等方法制备，存在孔壁组成吸波能力有限、孔结构调控困难等问题。本项目在高介电常数SiC孔壁中引入介电常数低、透波性能好、耐高温Si3N4第二相，形成吸波-透波复相孔壁组成，增强孔壁电磁波吸收；同时，采用颗粒堆积、固态造孔剂、发泡等多种成孔机制，调控优化孔结构，实现强吸波SiC-Si3N4多孔陶瓷材料制备，同时深入研究SiC-Si3N4多孔陶瓷的吸波性能、构效关系与吸波机制。 通过材料组分设计调控了SiC与Si3N4的两相含量，使其等效电磁参数满足优化设计要求，当SiC含量为40wt%时阻抗匹配与吸收损耗达到较佳平衡，多孔陶瓷吸波性能最佳。通过发泡-凝胶注模工艺制备球形孔SiC-Si3N4多孔陶瓷吸波材料，通过调整固含量与发泡剂添加量可以实现孔结构的调控，发现双级孔结构SiC-Si3N4多孔陶瓷吸波性能最佳，最小反射系数可达-22.78dB，即电磁波吸收率达99.47%。进一步引入铁基催化剂，最小反射系数可达-74.09 dB。吸波机理为：宏观尺度上大孔的存在可以改善界面阻抗匹配，使电磁波能更多地进入材料内部，小孔对电磁波的多重反射、散射，延长了电磁波的传输路径，为孔壁实现对电磁波的多次吸收提供了机会；微观尺度上，纳米线的大量生长提供了众多的界面，大大增加了界面极化损耗；纳米尺度上，材料中存在的晶界、缺陷以及堆叠层错等在电磁场作用下会产生极化弛豫，极化损耗会导致电磁波的吸收衰减。通过本项目研究，获得了性能优异的SiC基陶瓷泡沫材料，并明晰了其孔结构与吸波性能调控优化规律，为高性能的高温吸波材料的开发奠定了良好的基础。