Comprehensive Performances And Transmittance Structure Of Cyanate-ester Copolymer/fiber Reinforced Composites

Radome transmittance material are used to ensure the antenna system to maintain operating in remote communications under severe environments,which are required high transmissivity of electromagnetic-waves especially in radar bands.It is urgent to develop radome materials with high transmittance performance,heat-resistance,mechanical strength applying for aerospace electronics technology.In the present study,based on the excellent wave-transmittance of cyanate ester（CE）as matrix resin,the epoxy resin（EP）,bismaleimide（BMI）and glass/ceramic fibers are employed as reinforcements to comprehensively promote the heat-resistance,mechanical and dielectric properties of CE transmittance materials,and the sandwich composite structures are designed to further improve wave-transmittance.Conforming to the self-curing mechanism and copolymerization characteristics,EP and BMI are exploited as reinforcements to be mixed into CE matrix resin for preparing CE/BMI/EP radome composites with high heat-resistant and mechanical performances by the heating viscous-flow blend methods under the catalysis of aluminum acetylpyruvate.The crystallization temperature,transformation heat and reaction rate of curing copolymer composites are tested to analyze heat-resistant characteristics and evaluate the curing processes.Scanning electron microscopy is implemented to characterize the micro-morphology of tensile fracture,and combined with the tensile test and dynamic thermomechanical analysis to investigate the composite modifications on tenacity and rigidity.Weibull statistics is applied for electrical breakdown fields,and the dielectric-polarization and wave-transmission performances are analyzed with complex dielectric spectra.In comparison to the pure CE and the EP or BMI reinforced CE composites,the CE/BMI/EP composite achieves the most evident improvements in mechanical and electrical-insulation performances,as manifested by the broken elongation and dielectric breakdown strength being simultaneously increased by 40%,which are consistent to the obvious transverse lines uniformly distributed on the fractured cross-section of CE/BMI/EP composite.Moreover,CE/BMI/EP composite acquires a glass-transition temperature approaching300°C,with the relative dielectric permittivity and dielectric loss being significantly abated to<3.2 and<0.01 respectively in X-band.CE/BMI/EP composite is preferential on the mechanical,heat-resistant and electrical-insulation performances,which are highly-qualified for the low-frequency transmittance in prospective radome applications.By introducing fiber reinforcements into CE/BMI/EP composites（CBE）with polyimide films（PI_f）as the surface-protective layers,the PI_f/fiber-reinforced CBE/PI_fsandwich composites are prepared by heat-curing lamination schemes and analyzed in terms of micro-structure characterization,heat-resistance,mechanical and dielectric performances.It is evident that the glass-fiber reinforced CBE as the core layer is preferable in processing compatibility with PI_fsurface layers than that of ceramic-fiber.Glass-fiber reinforcements and PI_f surface layers can effectively improve tensile strength,storage modulus and dielectric breakdown strength,and no additional dielectric loss will arises from the dielectric polarization at the glass-fiber/CBE interfaces in core-layer and the core-layer/surface-layer interfaces of sandwich structure in the wide range of 10⁴～10¹⁰Hz,as manifested by the dielectric loss factor being less than 0.01 for a high wave-transmittance.Compared with glass-fiber,the composite tightness between ceramic-fiber and CBE matrix is not adequate in the laminated heat-curing process,accounting for the observable alleviation in ameliorating the mechanical and electrical-insulation strengths of CEB composite.Since the ceramic-fiber material present substantial dielectric polarization in low frequency range,the dielectric permittivities of ceramic-fiber reinforced CBE composite and its sandwich composite structure are appreciably higher than that of glass-fiber,with a remarkably higher dielectric loss in low frequency range which implies lacking of qualification in wave-transmittance for low-frequency radomes.According to the electrical properties required for radome materials,an effective method of calculating wave-transmissions through sandwiched composites is established by applying electromagnetic theory of wave-multiple transmission and-reflection to determine the optimal transmittance wave-bands of the reinforced CBE composites in a sandwich structure.In the sandwich structure with PI_f as the surface material,the CE composite with low dielectric permittivity should be specified as core layer for X-band transmittance.PI_f/glass-fiber reinforced CBE/PI_f sandwich composite will represent a higher transmittance performance than its single layer counterpart（glass-fiber reinforced CBE composite）in the frequency range of 1～12GHz with an sufficient transmittivity higher than 70%.