Preparation And Microwave-absorbing Properties Of Non-circular Section Silicon Carbide Fibers

Microwave-absorbing materials have found widespread applications both in the military and civil fields. Structural microwave-absorbing materials perform both as reinforcements to withstand strength and also act as absorbents with excellent absorption ability of electromagnetic wave. Besides having the same excellent mechanical property and resistance to elevated temperature as circular silicon carbide (SiC) fibers, non-circular section SiC fibers also have good microwave-absorbing properties, which make them have potential to be the ideal reinforcement and absorbent in structural microwave-absorbing materials.In this presentation, spinnerets with specific non-circular holes were designed. Starting from polycarbosilane (PCS), after the process of melt-spinning, air-curing, high-temperature treatment, ribbon-like, trilobal, quatrefoil, pentalobal, sexfoil, septfoil, swirl-shaped, T-shaped and C-shaped SiC fibers were elaborated. Influences of spinning procedures on degree of complex profile (Dp) and equivalent diameter (Ed) of non-circular section SiC fibers were studied, and the optimum fabrication processes of them were determined subsequently in this paper. Electromagnetic properties of SiC fibers with different cross-sections were studied, influences of form and content of non-circular SiC fibers as well as electromagnetic matching on microwave-absorbing properties of the composites were also analyzed. Based on these studies, structural microwave-absorbing materials reinforced by non-circular section SiC fibers were prepared, and the microwave-absorbing mechanism of non-circular section SiC fibers was discussed.Ribbon-like, trilobal, quatrefoil, pentalobal, sexfoil, septfoil, swirl-shaped, T-shaped and C-shaped spinneret holes were designed according to polymer moulding theory and rheologic behaviour of PCS. Through the experiments, it was discovered that fibers could have good Dp and suitable Ed when the wide of slit is 0.12-0.14mm and the length-width ratio of slit is 3-4.According to the study of spinning process, Dp of non-circular section SiC fibers reduces while Ed of fibers increases with higher spinning temperature. Extrusion speed and fiber taken-up rate have little influence on Dp of them, while Ed of non-circular fibers reduces if extrusion is slowed down or collection is speeded up. Spinning processes were established like these: the spinning temperature is 75-95℃higher than the softening point of PCS, spinning pressure is between 0.5MPa and 1.3MPa, taken-up rate is between 150rpm and 300rpm.Through suitable air-curing and high-temperature treatment, ribbon-like, trilobal, quatrefoil, pentalobal, sexfoil, septfoil, swirl-shaped, T-shaped SiC fibers and C-shaped fibers with different sectional radian were prepared. Radius of curvature of vanes-shaped SiC fibers changes following secant formula, and the radius of curvature of vane head as well as end of C-shaped fibers is least. With carbon-rich on the surface, non-circular section SiC fibers have the same elements composition and internal structures as circular fibers.Electromagnetic properties of circular, vane-shaped and C-shaped SiC fibers were studied. It was found that real part of normalized dielectric constant of vane-shaped SiC fibers increases 10% while imaginary part increases 15% compared with circular fibers when a pair of vanes is added. The celebration of each vane to the normalized dielectric constant of the fibers with even number vanes is higher than the fibers having odd number vanes. Fiber loss tangent increases with increasing number of the vanes, while absorption peak moves to lower frequency. Real part and imaginary part of normalized dielectric constant as well as loss tangent of C-shaped SiC fibers reduce when the sectional radian increases, the absorption peak moves to higher frequency at the same time.Influences of length, arrangement and content of fibers on electrical parameters and microwave absorbing properties of composites were studied. When length of the fiber increases, electrical parameters of the composites increase. Loss tangent of non-circular SiC fibers increases first along with increase of length, and then decreases with continuative increase. Non-circular SiC fibers distributing randomly have more obvious frequency dispersion effect and larger imaginary part of electrical parameters as well as larger loss tangent compared with order unidirectional fibers. Electrical parameters of composites increase along with increasing of content of fibers, dielectric constant and dielectric loss tangent of the composites also increase.Multilayer structural microwave-absorbing materials reinforced by non-circular section SiC fibers were designed using different SiC fibers through impedance matching. By this method, we can exploit the advantages of different SiC fibers at the same time. The band width of reflectivity can be widened, or the material thickness can be decreased when the microwave-absorbing property is not weakened. And in particular, the microwave-absorbing property at low frequency can be improved. A double layer structural microwave- absorbing composite was fabricated: thickness of the composite was 3.5mm, the minimum reflectivity was -18.62dB while the band width of reflectivity below -10dB was beyond 8GHz.Trilobal SiC fibers/carbonyl iron compound fiber bundles were prepared through adding carbonyl iron in sizing agent. The band width of reflectivity lower than -10dB of composite reinforced by fiber bundles modified by magnetic agent moves to low frequency 2GHz compared with composite reinforced by trilobal SiC fibers. With the additive, the microwave-absorbing properties of composites at low frequency were improved.The microwave-absorbing mechanism of non-circular SiC fibers was discussed preliminarily. Because of the evolution of the surface curvatures of the fibers, non-circular section SiC fibers have different polarization effect under electromagnetic wave. Vane head of vane-shaped SiC fibers and ends of C-shaped fibers have higher surface curvature, more charges gather there, macro-dipoles can be formed afterwards. The formation of macro-dipoles enhances the polarization capacity of the fibers, subsequently increases the electrical parameters of them. When there are more vanes in the vane-shaped SiC fibers, the number of dipoles inside them increases, the normalized dielectric constant and loss tangent increase, while absorption peak moves to lower frequency and microwave-absorbing band width increases. When the sectional radian of C-shaped SiC fibers increases, dipole distance and surface area of effective electric conduction reduces, thus dielectric constant reduces, loss tangent of C-shaped SiC fibers reduce, and absorption peak moves to higher frequency. Dielectric constant of non-circular section SiC fibers increases with surface area.