High-Temperature Microwave Absorption Property Of Boron-Nitrogen Polymer-Derived Ceramics

Stealth is an important strategy for improving survival and penetration capabilities of weapons.Microwave absorption materials are key to achieve stealth technology.According to the microwave absorption mechanism,absorption materials can be divided into magnetic-loss materials,resistance-loss materials,magnetoelectric-loss complex materials and dielectric-loss materials.However,with the development of space detection technology and the upgradation of defense systems,there is a new challenge to microwave absorption materials for high-speed supersonic aircraft used in the high temperature atmosphere.The resistance-loss type materials are generally not resistant to high temperature and oxidizing environment.The magnetic-loss materials are limited by their low Curie temperature.Therefore,the dielectric-loss ceramic-based materials are the best choice for high temperature microwave absorption,which also possess reliable high temperature and oxidation resistance.Polymer-derived ceramics?PDCs?are effective to prepare ceramic matrix composites.In this dissertation,the polymer precursors were designed and synthesized.First,we prepared cobalt-containing silicon carbide?Si C?pyrolyzed from cobalt coordinated poly?dimethylsilylene?diacetylenes.Subsequebtly,we prepared hyperbranched polyborosilazane with an introduction of boron element.The metal?iron,cobalt?element was used to promote the formation of in-situ graphite carbon,carbon nanotubes,Si C and other nanocrystals.These high dielectric constant nanocrystals were uniformly dispersed in the low dielectric phase,such as Si BCN amorphous matrix phase.Finally,we prepared PDC-ceramic microwave absorption materials in high temperature and oxidation environment.Chapter 2:The cobalt-containing precursor-derived ceramics were obtained via pyrolysis of carbon-rich poly?dimethylsilylene?diacetylenes coordinated with octacarbonyldicobalt.The introduction of cobalt led to in situ formation of cobalt silicide nanocrystals,crystallized carbons?graphitic and tubular carbons?,and cobalt silicide?Co Si and Co₂Si?.The crystallization behavior,relative complex permittivity,microwave absorption property and bandwidth could be tuned by controlling the cobalt content and annealing temperature of ceramics.The minimum reflection coefficient(RC_min)value of-42.43 d B at 10.55 GHz.The effective absorption bandwidth?EAB,RC<-10 d B?almost covers the whole X-band?8.2-12.4GHz?.Chapter 3:Two kinds of carbon-rich hyperbranched polyborosilazanes were designed and synthesized with boron trichloride,the side chain of which are methyl group and phenyl groups,respectively.The two siliconborocarbonitride?Si BCN?ceramic pyrolyzed from hyperbranched polyborosilazanes possess different ceramic microstructure,dielectric property and microwave absorption property.The microwave absorption property of Si BCN ceramics is closely related to the carbon content and hybrid structure.When the methyl groups on the hyperbranched polyborosilazanes are replaced by phenyl groups,the carbon content in the pyrolyzed Si BCN ceramics are greatly increased.At the same time,the main hybrid morphology of carbon in Si BCN ceramics are changed from sp³ hybrid structure to sp² hybrid structure.And the Si BCN ceramics show amorphous state.The microwave absorption property of Si BCN ceramics pyrolyzed from hyperbranched polyborosilazanes with methyl groups are poor.However,the microwave absorption property of Si BCN ceramics pyrolyzed from hyperbranched polyborosilazanes with phenyl groups,with a RC_min value of-71.8 d B and a EAB of 3.65 GHz?8.2-11.85 GHz?.Chapter 4:The hyperbranched polyborosilazane with Si–H bonds termini was designed and synthesized using tris?dichlorosilylethyl?borane as the boron element unit.The synthesized 1,1'-bis?dimethylvinylsilyl?ferrocene was employed for cross-linking of preceramic precursor hyperbranched polyborosilazane.The introduction of iron promoted the yield and crystallization of Si BCN ceramic.And the high temperature microwave absorption property of iron-containing Si BCN ceramic was excellent.The iron-containing Si BCN ceramic was stable before 1400°C under argon and 900°C in air.At 600°C,The iron-containing Si BCN ceramic showed good microwave absorption property,with an RC_min of-35.74 d B and an effective absorption bandwidth almost covering the whole X-band.At 885°C,the RC_min was-12.62d B with an effective absorption bandwidth of 3.2GHz.The graphene@Fe₃O₄/Si BCN composite was prepared by using this precursor.After the material was oxidized for 2 h at 300 ^oC and 600 ^oC in air environment,the microwave absorption property was also well,even improved.Chapter 5:The cobalt-containing meta-organic frameworkmaterials ZIF-67 was used to modify the hyperbranched polyborosilazane.In this system,the ZIF-67 itself was acted as EM dielectric-loss agents.The cobalt element of ZIF-67 promoted the in situ formation of nanocrystals.The Si BCN ceramic matrix composites with good microwave absorption property at room temperature and high temperature were obtained by controlling the amount of ZIF-67 and annealing temperature.The MOFs@Si BCN with special microstructure showed a RC_min of-51.6 d B and an EAB of 3.93GHz?8.47-12.4 GHz?at room temperature.And the EM absorption property become better at elevated temperature from 100 ^oC to 600 ^oC.When the test temperature at 600 ^oC,the RC_min was-30.29 d B and EAB was 3.95 GHz?8.45-12.4 GHz?.