| With the rapid development of military detection methods such as data transmission and radar,new aerospace vehicles and other weapons and equipment have put forward higher requirements on the structure and performance of materials.In addition to the"stealth technology"with electromagnetic absorbing properties,materials are also required with high temperature resistance,corrosion resistance,and oxidation resistance.Polymer-derived ceramics(PDCs)have received extensive attention due to their unique molecular design,excellent high temperature stability,corrosion and oxidation resistance,and adjustable processing properties.At present,the research of PDCs in the field of microwave absorption is mostly about bulk dense ceramics or ceramic fibers,etc.There are few studies on the preparation of PDCs aerogels and the properties of microwave absorption.In this paper,starting from the molecular design of the polymer precursor,based on the research of the group,a method for preparing the polymer ceramic precursor with high ceramic yield is selected,and polymer-derived ceramic aerogels(PDCAs)is prepared by combining freeze drying and high temperature pyrolysis methods.We explored the influence of process parameters on the structure and performance of the material during the synthesis and preparation process,and studied the relationship between composition-structure-performance,and then realized the regulation of the electromagnetic absorbing performance of the material.The specific research content and results are as follows:(1)Synthesis and control of SiBNC ceramic precursors:using trichlorosilane,boron trichloride and hexamethyldisilazane as raw materials,through the"one-step"copolycondensation reaction,polymer ceramic precursor PBSZ was synthesized,with Si,B,C,N target element,containing Si-H,N-H and other active groups,which has good air stability and cross-linking stable structure.Chemical structure of the intermediate product during preparation process at different polymerization temperatures was studied,and determined the final polymerization temperature.(2)Preparation of SiBNC ceramic aerogel:using the synthesized PBSZ precursor as the raw material,divinylbenzene as the crosslinking agent,and cyclohexane as the solvent,the gel is formed by the hydrosilylation reaction in a highly diluted solution containing Karstedt catalyst.After freeze-drying and high-temperature pyrolysis,a three-dimensional network structure porous SiBNC ceramic aerogel with light weight(0.38 g/cm~3),high specific surface area(121.43 m~2/g)and wide pore size distribution(3～140 nm)was prepared.,Explored the influence of different solvent fractions,pyrolysis temperature,pyrolysis rate process parameters on the aerogel structure,and realized the control of the composition and structure of SiBNC ceramic aerogel.The study found that when the solvent fraction is 90%,the maximum pyrolysis temperature is 1000~oC,while the rate is 5~oC/min,the aerogel pore structure is more uniform.(3)Research on the microwave absorbing properties of SiBNC ceramic aerogels:by studying the influence of different process parameters on the dielectric and absorbing properties of SiBNC ceramic aerogels,exploring the relationship between the composition and structure of the aerogel and the ratio of paraffin wax to the absorbing properties.When the fraction is 90%,the pyrolysis temperature is 1000~oC,and the sample thickness is 3.0 mm,the minimum RL value when the ratio of SiBNC to paraffin is 1/1 can reach-36.40 d B@11.97 GHz.The SiBNC ceramic aerogel prepared by PDC method can remain amorphous at 1400~oC,and the structure contains defects such as free carbon and carbon dangling bonds,which mainly contribute to the microwave absorption performance.In addition,the unique three-dimensional network structure of the ceramic aerogel is conducive to electromagnetic wave propagation The extension of the path causes multiple reflections and scattering of electromagnetic waves in the aerogel material and improves the wave-absorbing performance. |
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