Surface Modification And Microwave Absorption Properties Of Silicon Carbide Fibers

With the rapid development of radar detection technology on modern aircraft,ships,and other weapons and equipment stealth performance requirements are increasing.In addition,the use of a large number of electronic devices in daily life brought about by electromagnetic interference is also increasing.Therefore,developing"thin thickness,low density,wide bandwidth,strong absorption"of high-performance electromagnetic wave absorbing materials is an important way to achieve radar stealth and solve electromagnetic pollution.Compared with traditional absorbers,Si C fibers are ideal for preparing new absorbers due to their unique characteristics of adjustable resistance,flexibility,oxidation resistance,high-temperature resistance,excellent mechanical properties,and anisotropy.However,the low dielectric loss and the lack of magnetic properties of Si C fibers make them not capable of absorbing waves in the frequency range of 2-18 GHz.Based on the different loss mechanisms of electromagnetic waves,choosing suitable surface modification techniques,depositing modified materials on the surface of Si C fibers,increasing the interaction between the micro and nano-scale phase interfaces inside the materials,improving the impedance matching performance,and increasing the electric loss or introducing magnetic loss of the materials are the keys to enhance the electromagnetic wave absorbing performance of Si C fibers.This paper conducts a systematic study on the surface modification of silicon carbide fibers and their wave absorption properties for the subsequent preparation of Si C fiber wave absorbing materials with structural-functional integration as a preliminary exploration.The main research contents of this thesis are as follows.（1）Si CNWs@Py C@Si C_f composites were prepared by depositing Si C nanowires on Si C fibers coated with pyrolytic carbon by electrophoretic deposition,followed by depositing Si C matrix by chemical vapor permeation method.The content of Si C nanowires increased with the increase of electrophoretic deposition time,and they lapped each other to form a bird’s nest-like structure and anchored on the surface of the Si C fibers with a pyrolytic carbon coating,forming a stable structure.In addition,the introduction of Si C nanowires makes the Si CNWs@Py C@Si C_fcomposites exhibit an obvious porous structure.The minimum value of its reflection loss decreases with the increase of the content of Si C nanowires.For the sample with an electrophoretic deposition time of 9 min,the peak value of the reflection loss of-58.6 d B was reached at a maximum effective absorption bandwidth of 6.13 GHz when the thickness was 2.2 mm.（2）Uniformly distributed and vertically oriented arrays of CNTs were prepared on Si C fibers by chemical vapor deposition.It is shown that the height of CNTs can be obtained by controlling the CVD growth time,and the CNT content has a significant effect on the electromagnetic parameters of CNT@Si C_f.The CNT@Si C_f with a chemical vapor deposition time of10 min has a peak reflection loss of-56.2 d B and a maximum effective absorption bandwidth of 4.3 GHz with a thickness of 3 mm.（3）The CNT arrays and Fe Ni coatings were prepared on the Si C fibers by chemical vapor deposition and magnetron sputtering in sequence.The results show that the deposited Fe Ni films can uniformly wrap around the CNT surface grown on the Si C fibers,and the thickness of Fe Ni on the CNT of the Fe Ni@CNT@Si C_f-15 samples prepared by RF sputtering for15 min is about 5 nm.The peak absorption thickness of Fe Ni@CNT@Si C_f-15 achieves a reflection loss of-63.4 d B and the maximum effective absorption bandwidth reaches 6.26 GHz（11.74～18 GHz）with a thickness of 2.5 mm.（4）Fe Ni-C@Si C_f double-layer coating and Fe Ni-Si O₂@Si Cf multilayer gradient coating materials were prepared on Si C fibers by magnetron sputtering method,respectively.Compared with the Fe Ni@Si C_f,the absorption performance of Fe Ni-C@Si C_f is significantly improved at low frequencies,especially at 3.44 GHz with an absorption thickness of 5 mm,where the peak reflection loss reaches-26.18 d B,while there is almost no difference in its absorption performance at high-frequency bands.For Fe Ni-Si O₂@Si C_f,the Fe Ni content increases sequentially in the five coatings from the outermost layer to the fiber surface with corresponding thicknesses of about 20,110,110,150,and 130nm.At 10.37 GHz and 2 mm absorption thickness,the reflection loss can reach a peak of-55.2 d B with a maximum effective absorption bandwidth of 6.84 GHz（11.08 to 17.92 GHz）.（5）The mechanism of the improved wave absorption performance of modified Si C fibers is analyzed.In terms of loss capability,the synergy of multiple loss mechanisms can significantly enhance the loss capability of Si C fibers.In addition to the loss capability from the modified materials themselves,the interfacial interaction between them is also an important factor to enhance the wave absorption performance of Si C fibers.In addition,the enhancement of dielectric constant and magnetic permeability improves the impedance matching performance of Si C fibers,which is also an important reason for the good wave absorption performance of modified Si C fibers.82 Figures,5 Tables,and 194 References are listed in the dissertation.