Amorphous SiBCN Microwave Absorption Ceramics Derived From Hyperbranched Polyborosilazane Containing Benzene Rings

Microwave absorption materials can effectively reduce the harm of electromagnetic radiation to living organisms and electronic devices,and are also key materials for stealth technology in defence security.It is an important research issue in the field of materials science.Ceramic materials have become best candidate in extreme environments,due to their excellent mechanical properties,high temperature resistance,oxidation resistance,corrosion resistance and chemical stability.Polymer-derived ceramics（PDCs）have the advantages of designable molecular composition,controlled elemental composition and phase size,various material curing and forming methods,good process applicability.In this paper,a new hyperbranched polyborosilazanes containing benzene rings were prepared by molecular precision design.The phase structure of the ceramics was regulated by molecular topology.The amorphous PDCs-SiBCN complex phase ceramics with good oxidation resistance,high temperature stability and excellent microwave absorption properties were prepared.The main research contents and conclusions of this thesis are as follows:（1）Hyperbranched polyborosilazane precursors were designed and synthesized using boron trichloride（BCl₃）as the boron source.The benzene ring was introduced into the precursor by a hydrosilylation reaction between the Si-H bond in hyperbranched polyborosilazane and the vinyl bond in divinylbenzene（DVB）.Amorphous SiBCN ceramics with high temperature stability and good microwave absorption properties were obtained by high temperature pyrolysis.The adjustment of the DVB content led directly to a change in carbon content and hybridization.The ceramics obtained by pyrolysis of the benzene ring containing precursors have remained amorphous state at up to 1400℃.By regulating the DVB content and pyrolysis temperature,the amorphous SiBCN ceramics exhibited optimized microwave absorption property with a minimum reflection coefficient(RC_min)of-54.24 d B and an effective absorption bandwidth(EAB_max)of 3.15 GHz.In addition,the prepared amorphous SiBCN ceramics shown excellent high-temperature resistance in both nitrogen（N₂）and air atmosphere up to 1400℃.（2）Using the A₂+B₆ synthesis strategy,hyperbranched polyborosilazanes containing benzene rings were prepared from borane dimethyl sulfide complexes,dichloromethylvinylsilane,methyl dichlorosilane,dichlorodiphenylsilane and hexamethyldisilazane by the one-pot method.By adjusting the monomer ratio,reaction conditions and pyrolysis temperature,the PDCs-SiBCN ceramics with excellent microwave absorption performance were obtained.The carbon element and sp² hybridised carbon content of the SiBCN ceramics was enhanced with an increase in the proportion of dichlorodiphenylsilane.At temperature of up to 1400℃,SiBCN ceramics have remained in an amorphous state.Compared to SiBCN ceramics obtained by pyrolysis of precursors without benzene rings,SiBCN ceramics obtained by pyrolysis of precursors containing benzene rings,with RC_min of-30.79 d B and an EAB_max of 4.07 GHz.SiBCN ceramics shown outstanding high temperature stability performance with essentially no mass loss in argon（Ar）and air atmospheres at 1400℃.