Preparation And Properties Of High Temperature And Ultra-thin Microwave Absorption Materials

With the modernization of military equipment and the complicated application environment of military equipment,the new generation of stealth materials need meet more requirements,not only to meet the requirements of"thin,wide,strong,and light",but also to oxidation,high temperature,and corrosion resistance.This work is mainly divided into two parts:（1）preparation of high temperature resistant and light absorbing materials;（2）preparation of high dielectric and ultra-thin absorbing materials.（1）Short carbon fibers（SCFs）/glass,MoSi₂/glass,Ti₃SiC₂/glass coatings were prepared by the plasma spraying system.As the absorbent content increases,the porosity of coatings decreases,and the coating becomes denser,and the complex dielectric constant increases.The absorbents are uniformly dispersed in the glass matrix.Only MoSi₂ has a lamellar structure in the coating.The XRD patterns of SCFs/glass coatings are mainly presented characteristic amorphous.SCFs are oxidized very severely during spraying.The XRD patterns MoSi₂/glass consisted of crystalline phase MoSi2 and amorphous background.The XRD patterns of Ti₃SiC₂/glass are composed of glass matrix,Ti₃SiC₂,TiC,and a small amount of Ti₅Si₃.The SCFs/glass with 30wt%SCFs has the best absorbing performance.When the thickness is 2.2mm,the effective absorption bandwidth（RL≤-5dB）can cover the entire X-band,and the strongest absorption can reach-28.47 dB.The MoSi₂/glass with 22.5 wt%MoSi₂ has the best absorbing performance.When the thickness is 1.4 mm,the strongest absorption can reach-16.42 dB.And the effective absorption bandwidth is 3.88 GHz,but cannot cover the entire X-band.The Ti₃SiC₂/glass coating with 25 wt%Ti₃SiC₂ has the best wave absorbing performance.In the thickness range of 1.37～1.49mm,the effective bandwidth can cover the entire X-band.The coatings with 20wt%SCFs and 25 wt%Ti₃SiC₂ were heat-treated at 400℃.After heat treatment for 2h,the real and imaginary parts of permittivity SCFs/glass are greatly reduced,and the reflection loss is also very poor.After heat treatment for 10h,the real and imaginary parts of Ti₃SiC₂/glass decrease and the reflection loss also is deteriorated.But,with heat treatment continuing,the dielectric and mass of the coating do not change.（2）Highly dense Ca²⁺-doped and Sr²⁺-doped Y₂Ti₂O₇ were prepared by vacuum hot-pressing sintering technology.With the increase of Ca²⁺concentration,the lattice parameter of Y₂Ti₂O₇decreases first and then increases,which will introduce oxygen vacancies and interstitial atoms.However,with the increase of Sr²⁺concentration,the lattice parameter of Y₂Ti₂O₇ only decreases.The Ca²⁺and Sr²⁺doping introduce oxygen vacancies and interstitial atoms.According to the TGA data,Sr²⁺-doped Y₂Ti₂O₇ has a higher concentration of oxygen vacancies introduced when Ti⁴⁺is reduced to Ti³⁺.Ca²⁺and Sr²⁺doping improve the dielectric properties of Y₂Ti₂O₇.The effect of Sr²⁺doping on the dielectric properties is greater than that of Ca²⁺doping.Theε＇of Y_1.8Sr_0.2Ti₂O₇ is four times that of pure Y₂Ti₂O₇ and twice than that of Y_1.8Ca_0.2Ti₂O₇ at 8.2 GHz.Compared with Ca²⁺-doped Y₂Ti₂O₇,Sr²⁺-doped Y₂Ti₂O₇ has better microwave absorbing properties.Y_1.85Sr_0.15Ti₂O₇ has the best microwave absorption performance among them.When the thickness is 0.85 mm,the strongest absorption is-29.68dB and the effective absorbing bandwidth is 2.9 GHz.（3）Highly dense Ca²⁺-doped and Sr²⁺-doped Gd₂Ti₂O₇ were prepared by vacuum hot-pressing sintering technology.With the increase of Ca²⁺concentration,the lattice parameter of Gd₂Ti₂O₇decreases first and then increases.However,with the increase of Sr²⁺concentration,the lattice parameter of Gd₂Ti₂O₇ only decreases.According to the TGA data,Ca²⁺-doped Y₂Ti₂O₇ has a higher concentration of oxygen vacancies introduced when Ti⁴⁺is reduced to Ti³⁺.Ca²⁺and Sr²⁺doping improved the dielectric properties of Gd₂Ti₂O₇.The effect of Sr²⁺doping on the dielectric properties is greater than that of Ca²⁺doping.Among Ca²⁺-doped Gd₂Ti₂O₇,Gd_1.91Ca_0.09Ti₂O₇ has better absorption performance.When the thickness is 0.90mm,the effective absorption bandwidth can cover 3.07 GHz.Among Sr²⁺-doped Gd₂Ti₂O₇,the effective absorption bandwidth of Gd_1.91Ca_0.09Ti₂O₇ can cover 2.78 GHz at the thickness of 0.90mm.（4）Highly dense SrTiO₃/Y₂Ti₂O₇ composite ceramics were prepared by vacuum hot-pressing sintering technology.According to the XRD results,no new phase is generated in the composite ceramics,but mutual doping occurs.Sr²⁺substitutes for Y³⁺in the Y₂Ti₂O₇ phase and Y³⁺substitutes for Sr²⁺in the SrTiO₃ phase.Mutual doping results in oxygen vacancies,electrons,and lattice distortions in the SrTiO₃/Y₂Ti₂O₇ composite ceramics,which lead to the lattice parameters of both phases increased.According to the TGA data,the concentration of oxygen vacancies introduced when Ti⁴⁺is reduced to Ti³⁺in the sample increases with the increase of SrTiO₃.With the increase of SrTiO₃ content,the permittivity of SrTiO₃/Y₂Ti₂O₇ composite ceramics greatly improves.When the SrTiO₃ content is 25 wt%,ε＇can reach 1562.76 at8.2GHz and has a good dispersion effect,butε＇＇increases first and then almost remaine constant with the frequency increases,the maximum value can reach 135.96.When the SrTiO₃content is 25 wt%,it has the best absorbing performance.When the thickness is only 0.226 mm,the strongest absorption is-24.48 dB and the effective absorption bandwidth is 2.84 GHz which cannot cover the entire X-band.（5）Highly dense SrTiO₃/SrAl₁₂O₁₉/SrAl₈Ti₃O₁₉ composite ceramics were prepared by vacuum hot-pressing sintering technology.XRD results shows that the composite ceramics includes SrTiO₃,SrAl₁₂O₁₉ and SrAl₈Ti₃O₁₉.During the preparation process,Al³⁺substitutes for Ti⁴⁺,which results in oxygen vacancies,electrons and lattice distortions in the SrTiO₃ phase,and its lattice parameters increases.With the decrease of SrTiO₃ content,the concentration of oxygen vacancies introduced when Ti⁴⁺is reduced to Ti³⁺in the sample also decreases.The SrTiO₃/SrAl₁₂O₁₉/SrAl₈Ti₃O₁₉ composite ceramics with higher SrTiO₃ content have higher complex permittivity.When the SrTiO₃ content is 59.83 wt%,theε′of the composite ceramics can reach 1662.2 at 8.2 GHz with a good frequency response characteristic,ε＇＇,besides having a peak in the low frequency band,also has good frequency response characteristics,up to205.01.As the content of SrTiO₃ decreases,the complex complex permittivity of SrTiO₃/SrAl₁₂O₁₉/SrAl₈Ti₃O₁₉ composite ceramics also drop sharply.Among the SrTiO₃/SrAl₁₂O₁₉/SrAl₈Ti₃O₁₉ composite ceramics,the composite ceramic with SrTiO₃ content of 59.83 wt%has the best absorbing performance.When the thickness of the composite ceramic is only 0.232 mm,the strongest absorption can reach-10.70 dB,and the effective absorption bandwidth is 3.8 GHz,the areal density is only 1.08 kg/m².SrTiO₃/SrAl₁₂O₁₉/SrAl₈Ti₃O₁₉composite ceramic is very promising to become an ultra-thin microwave absorption material.