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Abstract:For the need of wave-transparent and the heat resistant materials of the flight vehicle in high march number severe thermal environment, aerogels composites were prepared via sol-gel process and supercritical CO2 drying process and the post-processing that were obtained using wave-transparent quartz fibers as reinforcement and silica sol as precursor. Micro-structure and morphology, thermal insulation properties, thermostability,mechanical property were studied by SEM,plane wave thermal conducticity, the quartz radiation heating system,univeral testing muchire. The result indicated flexible SiO2 wave-transparent aerogels composites has greater dielectric properties,the dielectric constant could be adjusted in 1. 28 to 1. 39, the tanδ is less than 0. 005,the thermostability is greater than 1 100℃,the thermal conductivity is less than 0. 02 W/ ( m·K). It also has better mechanical property, which could make the flight vehicle work normally of communication,telemetry and control in severely flying environment.

Abstract:We have investigated three significant factors affecting emissivity and microwave compatibility of infrared stealth coatings, including type of infrared stealth filler, content and thickness of the coating. It is found that the coating with 30wt% HW-3 aluminum powder as infrared stealth filler and thickness of 20 μm has the lowest infrared emissivity of 0.25. In addition, it can effectively reduce the surface temperature of infrared radiation and remain superior compatibility of microwave absorbance.

Abstract:In this paper, an low-frequency microwave absorbing (LFMA) array with patch & cross multiple-component is designed by electromagnetic simulation, and the microwave absorbing composites (MAC) based on LFMA array is prepared by moulded pressing. The influence of component structure on changing of the intrinsic microwave absorbing properties is studied. The microwave absorbing performance measurement reveals that the intrinsic absorbing bandwidth can be broaden by utilizing multi-component array. Dual absorbing peak is found on multi-component array with patch & loop cell, the frequency of which is located at 3.1 GHz and 4.5 GHz, respectively. The low-frequency absorbing performance of MAC can be obviously improved by intoduction of LFMA array. The reflection coefficient of MAC in 5 mm thickness is lower than -4.7 dB in the frequency range of 2 to 6 GHz as well as lower than -7 dB in the frequency range of 6 to 16 GHz, which is remarkably improved compared with the traditional impedance gradation MAC.

Abstract:Repetitious chemic switchs were used to investigated the transmission performance, electromagnetic shielding performance, switch time,surface morphology of controllable wave-transparent materials before switch and after switch. The results show that at 6 GHz to 18 GHz frequency, the increase of switch degree has little effect on transmission performance(more than 90%)of materials at wave-transparent state, while electromagnetic shielding performance (more than 44%)of materials at shielding state is enhanced, switch time of wave-transparence/ shield is prolonged,the time of switch from stealth to wave-transparence is less than 10 s, the time of switch from wave-transparence to stealth is less than 30 s and the ammonium chloride appears on surface of materials,when the switch times are below ten.

Abstract:A novel polyarylacetylene matrix composites reinforced by hollow silica fiber (HSF/ PAA) were fabricated by resin transfer molding (RTM) using hollow silica fiber (HSF) and polyarylacetylene (PAA). The mechanical properties and dielectric properties between the HSF/ PAA composites and SF/ PAA composites were compared in this paper, and so the effects of fiber reinforcement architecture on the mechanical properties. The results show that the HSF/ PAA composites have good high temperature mechanical properties and excellent dielectric property, and that maximum operating temperature of the HSF/ PAA composites is up to 450℃. Besides,it is hopeful to further improve the over-all properties of HSF/ PAA composites by optimizing the fiber reinforcement architecture. Since the low dielectric constant (3. 1) and dielectric loss (0. 004) seem to have excellent stability to a wide temperature and frequency range, the HSF/ PAA composites could be used as high-temperature wave-transparent materials in the aerospace and many other fields, though, the high temperature mechanical properties of the HSF/ PAA composites are only 55 ~75% of the SF/ PAA composites.

Abstract:FSS is a crucial approach to realize the wave-transparent and stealth performance of randome. Based on the research and practice results of the electric performance of domestic and foreign FSS randome, reseach and analyze of the electric performance problems involved in the wave-transparent and stealth technical indicators of the FSS randome, some ways are put forwarded to deal with the design of the wave-transparent and stealth performance. It points out that the choice of small unit,close cycle and the choice of reasonable randome struc

Abstract:Cyanate ester resins modified by monofunctional cyanate ester had been synthesized. Characterization of the resins had been carried out including polymer reaction activity, rheology and dielectric performance. Results indicated that monofunctional cyanate ester partly inserted the polymer network during the final curing stage and improved the reaction degree of cyanate groups. The cured cyanate ester exhibited a lower dielectric constant ranged from 2. 8 to 2. 9(7 ~18 GHz) and a lower dissipation factor than those of the cured BADCy resin.

Abstract:The rheological behavior, curing reaction characteristic, heat resistance and dielectric properties of modified cyanate ester have been studied by rheometer, differential scanning calorimetry(DSC), dynamic mechanical analyzer(DMA), thermogravimetic analysis and dielectric properties testing. The results show that the viscosity of modified cyanate ester is lower than 500 mPa·s when the temperature is between 90℃ and 170℃, and the viscosity is 280 mPa·s and pot life is longer than 10 h at the temperature of 90℃. The activation energy and the reaction order of the curing reactions are 39. 5 kJ/ mol and 0. 89, respectively. The glass transition temperature and the thermal decomposition is 294℃ and 420℃. The dielectric constant is 2. 85 and loss tangent is less than 5×10-3.

Abstract:In this paper, influences of the dielectric materials on the transmission properties of frequency selective surface are studied with theoretical calculation and experimental verification. The frequency properties, including resonance frequency, transmission bandwidth, and transmission loss, of the FSS with the skin and core of different dielectric constants, different thickness are discussed. The result shows that raising the thickness of the skin and core layer will make the bandpass position shifts to low frequency, reducing the thickness will make the the bandpass shifts to higher frequency. Raising the dielectric constant of the skin and core layer will make the bandpass position shifts to low frequency, reducing the dielectric constant will make the the bandpass shifts to higher frequency. And the transmission properties of the sample flat, which has the same structure the discussed FSS, are tested. Theoretical analysis and experimental results are in good consistency.

Abstract:Several 2. 5D derived structures SiO2f / SiO2 composites were fabricated by sol-gel using warp-laid-in 2. 5D woven structure,z-laid-in 2. 5D woven structure and warp & z-laid-in 2. 5D woven structure. The mechanical properties in the warp direction between these composites and prior 2. 5D SiO2f / SiO2 composite were compared in this paper, and so the effects of the straight warp yarn proportion and fiber volume fraction on the mechanical properties. The fiber reinforcement architecture of warp & Z-laid-in 2. 5D fabric was optimized on this basis. The results show that the mechanical properties of composites in the warp direction have been improved obviously by using warp & Zlaid- in 2. 5D fabric as the reinforcement. While the longitudinal tensile strength remained basically unchanged, the longitudinal compressive strength of the low density composites (1. 6g/ cm3 ) increased by 4. 3 times compared with prior high density 2. 5D composite (1. 65g/ cm3).

Abstract:Two 3D-sandwiched structure reinforced composites were separately prepared by cyanate ester/ phenolic resins with fabric hollow matrix. The mechanical and dielectric properties of composites are performed. The results indicate that compressive strength of the composites is respectively 5. 0 and 3. 5 MPa on cyanate ester or phenolic resins. Compared with traditional corn materials such as honeycomb and foam, the mechanical strength is increased significantly. The dielectric constants are 1. 5 and 1. 6 by different resins, besides the constants are stabilized on varied frequency.

Abstract:To meet the need of thermal protection and high strength for high Ma vehicles, lightweight rigid nanoporous thermal insulation material has been prepared by fiber dispersion, molding pressing and nano-particle composite technology. The relationship between material composition and properties of insulating materials was investigated. The results show that the compressive properties of composites strength with rigid tile is twice than with fiber mat. The compressive properties became large with increasing temperature.

Abstract:This paper investigated the welding process of one-side welding with backing formation of 6 mm 2A14 -T6 aluminum alloy, and analyzed the discipline of the alternating current frequency and assembly accuracy to the welding formation and the joint performance. The results showed that with the AC frequency gradually increased from 60 Hz to 80 Hz, the average tensile strength decreased from 356 MPa to 310 MPa, but the joint elongation did not change significantly. The analysis of assembly accuracy showed that the body material would be burnt through or single cutting by one-side when the assembly clearance exceeded 20% of thickness which was unable to form welding leakage. On the other way, when the larger assembly stagger was come out, the mechanical properties of joint decreased obviously. In addition, the influence of stagger on the performance was greater than the clearance. When the stagger was increased, the backing formation would be worse. The result of joint performance showed that if the stagger was greater, the joint tensile strength and elongation declined gradually. So in practical engineering applications, the local stagger of joints should be controlled within 25% of thickness.

Abstract:In this paper, C/ SiC composites were machined by traditional grinding and ultrasonic vibration-assisted grinding. Surface characterizations and mechanical properties of the C/ SiC composites grinded under different processing parameters are measured. Results of performance test indicated that the composite material machined by ultrasonic assisted grinding process obtained the better surface roughness quality and the greater bending strength,the best processing parameters were: grinding depth 0. 05 mm, feed velocity 600 mm/ h, speed 1 600 r/ min, ultrasonic frequency 14 kHz. It shows that ultrasonic vibration-assisted grinding has the lower damage to material mechanical properties.

Abstract:A method of ultra-precision cutting was used to ensure the concentricity,the spacing between the conductive ring and make sure the deformation of the insulated micro-groove less than 0. 10 mm. A grooving model of precision conductive slip ring was established to simulate and analyze the impact of spindle speed, feed rate, rake angle and depth of cut on the micro groove deformation, and the model was verified by experiments. The results showed that: the deformation of the slip ring increased with the rise of spindle speed, when the speed reached 700 r/ min, the deformation reached 0. 10 mm, which was out of the accuracy requirements; the deformation of the slip ring increased with the rise of feed rate, when the feed rate reached 1. 5 mm/ min, the deformation was out of the accuracy requirements; the deformation of the slip ring decreased with the rise of feed rate; the deformation of the slip ring increased with the rise of the depth of cut. , when the cutting depth was less than 0. 2 mm, the deformation became very little.

Abstract:Considering the weakness of interlayer in traditional composite π-joints, a novel composite π-joint structure is proposed, and joints with different panel thickness are manufactured by prepreg-RTM method. Flexural experiments are tested on composite and aluminum joints, the results show that the flexural strength of composite joints with 6 mm bottom panel thickness are better than that of aluminum joint, and the flexural stiffness is in the same scale. For the joints with 8 mm bottom panel thickness, the flexural strength of composite joints are less than that of aluminum joint, and the flexural stiffness is also in the same scale.

Abstract:In this article, micro-scale structures of a solid part of large-thickness composites embedded with a metal adapter were studied through ultrasonic non-destructive testing and metallographic microscopy. Decay of ultrasonic intensity was observed when the metal adapter was wrapped around by 0 degree unidirectional carbon fibers reinforced prepregs. Mechanisms for the ultrasonic decay were analyzed. First, it was micro-scale air bubbles included in prepregs that caused high porosity and formed internal looseness in composites. Second, micro-cracks caused by inner stress during the curing cycle were found responsible for ultrasonic decay. Third, fiber buckling deformation and localized resin also had effects on ultrasonic waves.

Abstract:A novel method for measuring the volume porosity of the C/ C-SiC composites based on micro-CT technology was proposed. We first disscussed the principle of this new method, and then investigated the influence of the measurement volume on its accuracy. The results showed that, using micro-CT technology, the 3D microstructures of the C/ C-SiC samples in gray histogram mode were obtained. According to which, the volume porosity of the C/ CSiC samples can be determined through threshold segmentation analysis. And it was of importance to find that a critical measurement volume is needed to obtain the volume porosity with a high accuracy.