Preparation,Dielectric And Microwave Absorbing Properties Of High Temperature Ti₃SiC₂ Absorbent

Developing radar absorbing materials?RAMs?used at high temperature has become a hot spot with the rapid development of stealth technology.Traditional RAMs used at elevated temperature are consisted of high temperature absorbents and glass or ceramic matrix,so the absorbents are keys to the properties of the RAMs.Ti₃SiC₂ is a ternary layer compound with high flexural strength,high fracture toughness,excellent conductivity and oxidation resistance,these merits make it a promising absorbent for high temperature application.In this work,Ti,TiC and Si powders were used as the raw materials to synthesize Ti₃SiC₂ bulks by solid reaction route,then the loose bulks were milled to obtain the high purity Ti₃SiC₂ powders.The dielectric and microwave absorption properties of Ti₃SiC₂ powders were studied.A single layer microwave absorbing material was prepared by adding the Ti₃SiC₂ powders into cordierite?MAS?matrix.The influences of Ti₃SiC₂ content,particle size on the mechanical,dielectric and microwave absorption properties of the Ti₃SiC₂/MAS composite ceramic were studied.The dielectric and microwave absorption properties of the Ti₃SiC₂/MAS composite ceramics at elevated temperature and oxidized in air for a long time were also investigated.The main contents and results are presented as follow:The Ti₃SiC₂ powders with a purity of 96.3% were prepared by controlling the milling method,the weight ratio of raw materials and the reaction parameters.Both the real and imaginary parts of complex permittivity for Ti₃SiC₂ powders were enhanced with the increasing content as well as the milling time,resulted in the variation of the microwave absorbing performance.The layered Ti₃SiC₂ particles were delaminated after oxidation at elevated temperature.The complex permittivity and microwave absorbing properties were decreased with the increasing temperature due to the reduction of Ti₃SiC₂ content and the formation of insulated?TiO2 and SiO2?phases on the outer layer of the particles.A smooth,flat silica layer was coated on the surface of Ti₃SiC₂ powders by TEOS hydrolysis.The SiO2 coating was densified by adding Na+ and heat treated at high temperature.The dense silica layer contributed to improving the oxidation temperature about 100?.But the dielectric and microwave absorption properties were decreased due to the insulated layer hinders the hopping of free electrons.Compared with the raw Ti₃SiC₂ powders,the complex permittivity and microwave absorption properties of the silica coated Ti₃SiC₂ powders were enhanced after oxidation at 600? for 1h.The dense Ti₃SiC₂/MAS microwave absorption ceramics were prepared by hot pressing at 1350? using the Ti₃SiC₂ as the absorbents and cordierite as the ceramic matrix.The flexural strength,fracture toughness,and complex permittivity of the ceramics were enhanced with increasing Ti₃SiC₂ content.When the content of Ti₃SiC₂ increased from 0 to 20wt%,the flexural strength increased from 116 MPa to 179 MPa and the fracture toughness raised from 1.8MPa·m1/2 to 2.5MPa·m1/2.While the microwave absorption properties of the composite ceramics increased firstly then decreased with the enhancement of Ti₃SiC₂ content.The Ti₃SiC₂/MAS composite ceramic containing 15wt% Ti₃SiC₂ exhibited the most favorable absorption performance.The reflection loss below-10 dB was obtained in the frequency range of 9.7-12.4GHz with the lowest value of-16.4d B.In addition,the mechanical properties and complex permittivity of the composite ceramics were both increased with the decreasing of particle size.When the particle size of Ti₃SiC₂ decreased from 10?m to 2?m,the thickness for the 15wt%Ti₃SiC₂/MAS composite ceramic with the most favorable absorption performance was decreased from 1.8mm to 1.5mm.As the temperature rises from 25? to 600?,both the real and imaginary part of complex permittivity for the 15wt%Ti₃SiC₂/MAS composite ceramic increased due to the shortened of relaxation time.Moreover,the absorbing performance was deteriorated and the absorption peak of the reflection loss shifted to the lower frequency range with the increasing temperature.The oxidation behavior of the Ti₃SiC₂/MAS composite ceramic at 800-1000? were also investigated.Although the flexural strength and complex permittivity of the Ti₃SiC₂/MAS composite ceramics were decreased after oxidized at elevated temperature for 60 h,the microwave absorption performance changed little.The results indicated that the microwave absorbing properties of the Ti₃SiC₂/MAS composite ceramics were still stable after oxidation at high temperature for a long time,so it could be used as a RAM for high temperature application.