Electromagnetic Absorbing Properties And Its Optimization Of PDCs-SiC(N) Ceramics And Composites

Polymer derived SiCN and SiC ceramics (PDCs-SiCN and PDCs-SiC) as well asPDCs-SiC/PDCs-SiCN ceramic matrix composites possess low density, high temperatureresistance, high oxidation resistance, high creep resistance, high strength and excellentelectrical property. Therefore, they are one kinds of the most important materials used as hightemperature absorption materials. A lot of researches have been carried out on the microwaveabsorption properties of PDCs-SiC(N)(PDCs-SiC and PDCs-SiCN) ceramics and ceramicmatrix composites abroad, but few reports can be found. In domestic, the researches are in thefledging period and the issues of poor microwave property and narrow absorption frequencyband (the reflectivity cannot be smaller than-10dB in the8~18GHz) are existing. Thecomposites with excellent microwave absorption properties can be obtained though thedesigns of microstructures of PDCs and the macrostructures of composites.In the paper, the fulfillments of complex relative permittivity for the wide bandabsorption materials are firstly got by the theoretical calculation. On the basis, the dielectricand electromagnetic absorption properties of the PDCs-SiCN and PDCs-SiC ceramics areinvestigated. Then, dielectric properties of PDCs are improved through the modification ofprecursors by ferrocene. Finally, composites with wide frequency absorption band aredesigned and fabricated with laminated structures. The major contents and results aresummarized as follows:(1) The relative complex permittivity when the reflectivity is smaller than-10dB in the8~18GHz are theoretically calculated for the materials with single-layer, two-layer andthree-layer structure.The reflectivity with single-layer structure is rigorous with the samples thickness andcomplex permittivity, which leads to the difficulty for the fabrication of wide frequency bandabsorption materials. The relative complex permittivity of material is reasonable whentwo-layer or three-layer structure materials are used for fabrication of wide frequency bandabsorption composite.(2)The microstructure evolutions and electromagnetic absorption properties ofPDCs-SiCN ceramics are investigated.The real permittivity, imaginary permittivity and loss tangent of amorphous PDCs-SiCNceramics are4.5,0.25and0.06, respectively. Therefore, amorphpous PDCs-SiCN ceramics are excellent high temperature impedance match materials with air. The crystallized SiCNceramics are composed of SiC nanocrystals, Si3N4, free carbon and amorphous SiCN. Aftercrystallization, the real permittivity, imaginary permittivity and loss tangent of SiCN are14.3,7and0.19, respectively. The polarization loss is the primary dielectric loss. The minimumreflectivity and the frequency band when reflectivity is smaller than-10dB of PDCs-SiCNannealed at1500oC are-53dB and3.02GHz, respectively.(3)The microstructure evolutions and electromagnetic absorption properties of PDCs-SiCceramics are investigated.The formation of SiC nanocrystal and free carbon network is the main reason for theincrease of electrical conductivity and dielectric property of PDCs-SiC. The electricalconductivity, real permittivity, imaginary permittivity and loss tangent of PDCs-SiC reach1.4S/m,8.5,10.2,1.24, respectively. The polarization loss is larger than conductivity loss. Thereflectivity of PDCs-SiC is smaller than-8dB in the X band.(4)The SiC/C composite ceramics are fabricated by pyrolysis of ferrocene modified PCS.The microstructure evolutions and electromagnetic absorption properties of SiC/C compositeceramics are investigated. The electromagnetic absorbing properties of Al2O3f/SiC,Al2O3f/SiCN and Al2O3f/SiC-C composites are investigated.The crystallization temperatures of SiC are remarkably decreased and the dielectricproperties increased after modification by ferrocene. The electrical conductivity, realpermittivity, imaginary permittivity and loss tangent of samples can reach14.2S/m,41.9,38.7and0.92, respectively. The free carbon is the main reason for the increase of dielectricproperties of samples. The composites possess good microwave absorption properties in the Xband.(5)The microstructures of SiC fibers and its effects on the electromagnetic absorptionproperties of SiCf/SiCN composites are investigated. The composites with wide frequencyband absorption properties are designed and fabricated through two-layer and three-layerstructure composite.The ZMI-SiC fiber and Amosic-SiC fiber are both composed of SiC nanocrystals andfree carbons. Compared with ZMI-SiC fiber, Amosic-SiC fiber possesses larger crystallizationdegree, which makes Amosic-SiC fiber higher electrical conductivity and reflectivity. The realpermittivity, imaginary permittivity and loss tangent of Amosic-SiCf/SiCN andZMI-SiCf/SiCN composites are17.5,29.4and1.68,10.8,6.3and0.58, respectively. Thefracture strengths and toughnesses of ZMI-SiCf/SiCN and Amosic-SiCf/SiCN composites are287MPa and10.2MPa·m1/2，155MPa and7.9MPa·m1/2，respectively. The reflectivity of ZMI-SiCf/SiCN composite is smaller than-4dB in the whole8~18GHz.The microwave absorption property can be effectively improved when the composites usetwo-layer and three-layer structure. The reflectivity of composite can be smaller than-10dB inthe8~18GHz with sample thickness of3mm, which fulfills the requirements of thinthickness, low density, high strength and wide absorption frequency band.