过渡金属掺杂与堆垛缺陷协同增强SiC晶须介电性能研究

Silicon carbide (SiC) has a lower density and better high-temperature stability than conventional absorbing materials, which is very promising as high-temperature microwave absorbing materials. However, the conduction loss and polarization loss of SiC is too poor to meet the requirement of the strong absorption and broadband absorption. The current researches reveal that those properties could be improved by doping or introducing of stacking faults, however, single enhancement mechanism is still not sufficient. So this project intends to synergistically improve the dielectric properties of SiC whiskers by doping and stacking faults. Firstly, the effect of transition-metal doping (such as Fe, Mn, Ni and etc.) on the dielectric properties of SiC will be mastered according to the investigation of SiC band structure via first-principle theory calculation. Secondary, the doped SiC whiskers will be synthesized, and their polarization loss will be enhanced by forming stacking faults through microwave rapid heating method. Based on this, the relationship among doping, stacking faults and dielectric properties will be investigated, and the synergistic effect mechanism of conduction loss and polarization loss on dielectric properties of SiC whiskers will be mastered. After that, the microwave absorbing properties of the as-synthesized SiC whiskers will be investigated and optimized, and then the absolute value of minimum reflection loss in X-band should higher than 20 dB, and the 10 dB bandwidth should larger than 3 GHz. This work is very important for the research and application of SiC microwave absorbing materials.

相对于传统吸波材料，SiC具有密度低、高温稳定性优异的特点，在高温电磁波吸收领域极具潜力。但SiC由于电导损耗和极化损耗都较低，难以满足吸波材料的要求。研究表明，掺杂或引入堆垛缺陷均是有效提升SiC介电和吸波性能的有效手段，但依靠单一增强机制性能提升有限。因此本项目拟采用掺杂提高电导损耗和引入堆垛缺陷提高极化损耗，通过二者协同增强机制提高SiC晶须介电性能。在理论计算过渡金属如Fe、Mn、Ni等掺杂对SiC能带结构影响的基础上，采用微波快速加热，通过对SiC晶须形核长大过程动力学的调控，解决掺杂引起的电导升高的同时难以避免的缺陷浓度下降引起的极化损耗降低的矛盾，合成掺杂SiC晶须；研究掺杂-堆垛缺陷-介电性能之间的关系，揭示电导损耗和极化损耗协同增强SiC晶须介电性能的作用机制，使其在X波段最小理论反射损耗>20dB，10dB以上吸收频宽≥3GHz，为SiC吸波材料的研究和应用奠定基础。

与受限于低居里温度的磁损耗材料和易氧化的碳材料相比，SiC具有密度低、高温稳定性优异的特点，在高温电磁波吸收领域极具潜力。但SiC由于电导损耗和极化损耗较低，难以满足高性能吸波的要求。因此，本课题采用金属掺杂的方式提高SiC晶须的介电和吸波性能。第一性原理计算表明，过渡金属Fe、Ni和Mn掺杂SiC的态密度中，上自旋穿过费米能级，而下自旋则形成带隙，即导带和价带部分重叠，这将使过渡金属掺杂的SiC具有半金属特性，从而显著提高SiC的电导率，增强其电导损耗。基于计算结果，采用硅、碳和金属粉作为原料，通过微波加热快速合成了不同名义掺杂浓度的SiC晶须。研究表明，过高的名义掺杂浓度将降低SiC晶须的产率，因而不同金属掺杂存在不同的较优名义掺杂浓度。所制备的掺杂SiC晶须中仍存在堆垛缺陷，但随着掺杂浓度的增大而减少。介电参数表征和反射损耗计算结果表明，具有堆垛缺陷的掺杂SiC晶须相对于仅具有堆垛缺陷的SiC晶须，展现了更高的介电损耗和吸波性能，且其性能随着掺杂浓度增大而逐步增强。这是由于堆垛缺陷引起的界面极化损耗和掺杂引起的电导损耗协同增强了SiC晶须的介电和吸波性能。当SiC晶须填充量为20wt%时，在X波段范围内Fe掺杂SiC晶须的最小反射损耗可达–21.2dB，Mn掺杂SiC晶须的有效吸收频宽（≤–10dB）可达3.1GHz。在此基础上，研究制备了Al掺杂SiC晶须，其最小反射损耗可达–25.4dB。此外，为了进一步研究结构组成、微观形貌对界面极化弛豫行为和介电参数的影响规律，本课题采用碳纤维和硅粉制备了SiC/CF同轴结构材料。结果表明，界面偶极子的谐振频率显著依赖于体系的电阻值；随着电阻值的增高，偶极子谐振频率向高频移动。另一方面，SiC壳层不仅引起了多重界面极化损耗，提高了材料的介电损耗，并且具有半导体性质的壳层显著改善了材料的阻抗匹配性能，进而增强了材料的吸波性能。当样品填充量为35wt%时，SiC/CF同轴结构材料的最小反射损耗为–31.6 dB，有效带宽为4.5 GHz (13GHz-17.5GHz)。SiC壳层也有效地提高了碳纤维的抗氧化性能和亲水性。