Preparation Of Microwave Absorption & Bulletproof Composites And The Study Of Their Performance

With the rapid development of information technology,in order to improve the survivability and combat capability of weapons and meet the needs of the new generation of weapons,there is an urgent need for weapons with stealth function and also good ballistic performance.Among them,carbon materials have various shapes,good electrical conductivity,and adjustable performance,etc.,and the advantages of high strength and high modulus of ballistic fibers represented by aramid fiber and polyethylene fiber,so it is of great significance to study the microwave absorption and bulletproof composites prepared by using different forms of carbon fibers as wave absorbers and ballistic fibers as reinforcing materials.In this thesis,porous carbon fibers（PCF）,porous carbon nanofibers（PCNF）and hollow porous carbon nanofibers（HPCNF）were prepared by polymer blending method and their morphology and structure were characterized.Small samples of coaxial rings of absorbing composites were prepared using carbon materials with different morphological structures as absorbers,the electromagnetic parameters were tested,and the absorbing performance of the corresponding composites was studied to guide the design and preparation of composite plates.The mechanism of microwave absorption of carbon fibers was initially explored,and the microwave absorption performance of multilayer composites were explored.In order to improve the mechanical performance of microwave absorbing composites,an attempt was made to modify the surface of aramid and after treatment with polyethylene reinforcement,microwave absorbers and polyurethane resins to prepare a microwave absorbing and bulletproof composite laminate.The specific studies are as follows.（1）Preparation of carbon fiber absorbers with different morphological structures.The spinning solution was prepared by polymer blending method,in which the carbon precursor was polyacrylonitrile（PAN）,the pyrolysis polymer was polymethyl methacrylate（PMMA）,and the co-solvent was N,N-dimethylformamide（DMF）.The PAN/PMMA blended fibers were obtained by wet spinning and electrostatic spinning techniques,and PCF,PCNF and HPCNF were prepared by pre-oxidation and carbonization.The morphology of the absorbers was characterized by SEM,and the resulting PCF,PCNF and HPCNF have porous and hollow structures.Raman spectroscopy and high-resistance instrumentation were used to characterize the internal structure of the fibers at different carbonization temperatures,and the results showed that the graphitization and electrical conductivity of the resulting carbon fibers increased with increasing carbonization temperature.（2）The microwave absorbing properties of carbon materials with different morphological structures and the influencing factors were investigated.Small samples of coaxial rings were prepared by blending the microwave absorbers（CB,PCF,PCNF,HPCNF）with paraffin wax at certain additions（5%-9%）,and the effects of carbonization temperature,absorber content,as well as pore size and thickness on the absorbing properties of the composite monolayer were investigated by the measured electromagnetic parameters.The results show that the carbonization temperature should not be too high or too low.If the temperature is too low,the graphitization of the fibers will be low and the electromagnetic loss of the material will be poor;if the temperature is too high,the graphitization of the material will be too high and the impedance matching will not be met,the electromagnetic waves will be easily reflected and cannot enter the material smoothly,and the absorption performance will be poor.Therefore,the optimized carbonization temperature is 1000℃,when the absorption band is wide and the comprehensive absorption performance is excellent.The results of microwave absorption performance obtained from electromagnetic parameter calculations show that the effective absorption band moves to lower frequencies as the thickness of the composite monolayer increases,and different absorbers have different optimized contents and thicknesses.When the CB content is 5%and the optimized thickness is 3 mm,the effective absorption band of less than-10 d B is 5.28 GHz;The lowest reflection loss of-31.84 d B and an effective absorption bandwidth of 2.64 GHz when the PCF content is 7%with a thickness of 6.5 mm;PCNF and HPCNF have better performance,where PCNF has the lowest reflection loss of-42.88 d B at thinner conditions;the HPCNF has a hollow structure and the effective absorption band is as wide as 4.56GHz at lower content.PCNF and HPCNF are of nanometer size and have a larger specific surface area than PCF.As the specific surface area increases and hollow structures emerge,the defects and interfaces of the fibers increase,providing more polarization centers and interfaces where interference phase extinction occurs,the degree of polarization and the possibility of interference phase extinction increases,and the loss capacity for electromagnetic waves increases.The absorbing performance of double-layer and multi-layer composites was also investigated.By changing the absorber content and thickness of each layer and adjusting the impedance matching,the best absorbing performance could be obtained,in which the widest absorbing band could reach 14.8GHz.（3）Design and preparation of microwave absorption and bulletproof composites and their performance studies.Composite monolayers with optimized thicknesses were prepared based on the properties of the composites calculated from electromagnetic parameters.The reflectivity measured by the bowing method basically matched with the calculated values,using different reinforcement fibers and matrix resins to obtain consistent results.When CB and PCF composite absorbers are used,the lowest reflectivity of the composite panel reaches-31.32 d B at 5%addition and 3 mm thickness,with an effective absorption band of 4.22 GHz.In order to improve the bonding of the reinforcing fibers to the resin,KH550 silane coupling agent and H₂SO₄ were selected to modify its surface,and SEM,contact angle,monofilament stretching and micro debonding tests were used to characterize the changes in morphology,hydrophilicity and mechanical properties of aramid before and after modification.According to the results,KH550 with a concentration of 15%can effectively improve the mechanical performance of the composite material.The tensile fracture strength reached 366.7 MPa,which was 25%higher than the untreated one.And the interfacial shear strength of 20.6 MPa is 72%higher than that of untreated,making it more suitable as a reinforcing fiber for composite laminates.The modified aramid was molded with absorber to obtain microwave absorbing composite laminate,which was molded with polyethylene weft-free fabric as microwave transparent layer and bulletproof layer to obtain microwave absorbing and bulletproof integrated composite laminate.Bulletproof performance tests were conducted,the results showed that the composite material was not penetrated,the bulletproof damage level reached 2-3,and the specific energy absorption value（SEA）reached 124.7 J.m².kg^-1.The composite laminate has excellent microwave absorption and bulletproof performance.