Microstructure Design and Electromagnetic Property Optimization of CVI SiCN Ceramic and Composites
Author:Xue Ji Mei
Supervisor:yin xiao zuo
SiCN ceramics with the properties of low density,high temperature resistance,oxidation resistance and adjustable dielectric properties,are promising for the integration of structure and function of ceramic matrix composites,which are used as matrix materials reinforced by fibers.Chemical vapor infiltration method(CVI)is a preferred fabrication method for high-performance ceramic matrix composites due to low preparation temperature,adjustable preparation process and the uniform and compact structure of materials.SiCN ceramics prepared by CVI are amorphous and have a low dielectric constant.Therefore,it is difficult to achieve whole-band absorption(RC<-10dB,90%electromagnetic wave was attenuation)in X band(8.2-12.4GHz).To achieve the whole-band absorption,the dielectric ceramic should have a suitable target values of dielectric constant,which ask the material having a certain wave transparent and absorption nano-heterogeneous interface.How to obtain CVI SiCN ceramic with wave transparent and absorption nano-heterogeneous interface is the key to improve its electromagnetic wave absorbing performance.The nano-phase(CNTs and BN with nanometer thickness)can induce crystallization of SiCN ceramics,the formation of wave transparent and absorption nano-heterogeneous interface could improve the electromagnetic wave absorption performance.Therefore,it is urgent to study the influence of microstructure evolution on the electromagnetic properties of CVI SiCN ceramics modified by nano-phase.In this paper,the deposition conditions of amorphous SiCN ceramics at lower temperatures were obtained by thermodynamic calculations.At the same time,the target electromagnetic parameters of dielectric materials which had broad bandwidth and strong absorption were optimized by theoretical calculation.The influence of microstructure evolution on the electromagnetic properties of CVI SiCN ceramics annealed at high temperatures were investigated.And then,the nano-heterogeneous interphases CNTs and BN phases were introduced into SiCN ceramics to improve the conductive loss and polarization loss of the composite ceramics.The influence of microstructure evolution on the electromagnetic properties of the ceramics was studied.Finally,the optimized SiCN matrix were combined with electromagnetic wave absorbing fiber to prepare SiCN ceramic matrix composites with integration structural and function.The main contents and results are summarized as follows:(1)The influence of deposition conditions on the phase composition and phase content of CVI SiCN ceramics was revealed.The preparation parameters range of co-deposition SiCN ceramic at low temperatures was obtained.The deposition of SiC and Si3N4 phase is facilitated by the increase of H2 dilution ratio.The lower deposition temperature is favorable for the deposition of C and Si3N4 phases,and the deposition efficiency is higher.At the sufficient amount of SiCl4,the bigger C3H6/(C3H6+NH3)ratio is,the deposition efficiencies of SiC and C phases are.To achieve SiCN ceramic at low temperatures,the deposition conditions should be controlled at a lower H2 dilution ratio(10～50),sufficient SiCl4 concentration([SiCl4]/([NH3]+[C3H6])>0.75)and C3H6/(C3H6+NH3)<0.3.(2)Based on the metal back panel model and transmission line theory,the target electromagnetic parameters for electromagnetic absorbing and shielding materials were predicted.The great electromagnetic absorbing and shielding properties were obtained by adjusting the electromagnetic parameters of dielectric materials.It had excellent electromagnetic wave absorbing properties when the real permittivity was less than 20 and the dielectric loss was from 0.25 to 1;while it had excellent electromagnetic shielding properties when the real permittivity was more than 20 and the dielectric loss was higher than 1.(3)The influence of the microstructure evolution of SiCN ceramics on its electromagnetic properties were studied,the model of transparent and absorption nano-heterogeneous interphase were built.Si3N4-SiCN composite ceramics were prepared by CVI SiCN into porous Si3N4 ceramics.The lower permittivity and dielectric loss of both Si3N4 and amorphous SiCN in Si3N4-SiCN composite ceramics result in poor electromagnetic wave absorbing properties.The real permittivity,imaginary permittivity and dielectric loss of the composite ceramics were 4.5,0.2 and 0.04,respectively.Si3N4-SiCN composite ceramics annealed at 1600℃ had been crystallized completely into β-SiC nano-grains,free carbon,α-Si3N4 and the Si3N4-C and Si3N4-SiC nano-heterogeneous interphase.With the increase of the crystallization degree of free carbon,the conductive loss of the composite ceramics increases,and the formation of nano-hetero interphase leads to the increase of the polarization loss.Therefore,fully crystallized composite ceramics have excellent electromagnetic wave absorption properties in the X-band,the minimum RC was-41.7dB and the bandwidth with RC<-10dB was 3.95GHz.(4)The induce crystallization effects of in-situ CNTs on Si3N4-SiCN composite ceramics had been studied.The influence of microstructure evolution on the electromagnetic properties of the composite ceramics during the heat treatment is clarified.CNTs were introduced into the Si3N4-SiCN composite ceramics by catalytic CVI method.The complete crystallization temperature decreased from 1600℃ to 1400℃ due to the induce crystallization effects of in-situ CNTs.The dielectric properties of the composite ceramics were transformed from lower permittivity and dielectric loss to the intermediate permittivity and dielectric loss,because the high conductivity of CNTs and the conductive network lead to the increase of dielectric properties.The real permittivity,imaginary permittivity and dielectric loss of the composite ceramics were 7.4,7.5 and 1.01,respectively.The content of CNTs in the composite ceramic reached the percolation threshold,resulting in the microwave absorption properties was almost unchanged with the increase of crystallization degree.The minimum RC was-11.OdB and the bandwidth with RC<-10dB was 1.9GHz.(5)The influence of amorphous BN phase on the microstructure evolution and electromagnetic wave absorption properties of Si3N4-SiCN composite ceramics was investigated.The nanophase BN were introduced into the Si3N4-SiCN composite ceramics by CVI method.The defects and impurity elements in the composite ceramics was increased by inducing BN phase,leading to the improvement of polarization loss and obtaining the low permittivity and medium dielectric loss materials.Before heat treatment,the real permittivity,imaginary permittivity and dielectric loss of the composite ceramics were 5.3,4.0 and 0.76,respectively.The minimum RC was-20.2dB,and the bandwidth with RC<-10dB covered the whole X-band.During the heat treatment,the composite ceramics were crystallized into P-SiC,C and α-Si3N4 nano-grains,leading to the increase of polarization loss.Moreover,the reaction between oxygen which was released during the crystallization of BN phase and the free carbon in SiCN lead to the decrease of conductive loss.Therefore,the composite ceramics annealed at 1300℃ had great electromagnetic wave absorbing properties,the minimum RC was-47.1 dB,and the bandwidth with RC<-10dB was 3.7GHz.(6)The influence of the content and properties of CVI SiCN matrix on the electromagnetic and mechanical properties of Al2O3f/SiCN and SiCf/SiCN composites was researched.Al2O3f/SiCN and SiCf/SiCN composites were prepared by PIP combined with CVI method.CVI SiCN matrix was infiltrated into the crack formed by PIP SiCN matrix,which not only enhanced the electromagnetic shielding effectiveness but also improved the mechanical properties.Both Al2O3f/SiCN and SiCf/SiCN composites had great electromagnetic shielding properties and mechanical properties.The SET and SEA of Al2O3f/SiCN composites were 27dB and 22dB in X-band,respectively,and the flexural strength and fracture toughness were 279MPa and 9.4MPa-m1/2,respectively.The EMI shielding effectiveness of SiCf/SiCN composites was lower than that of Al2O3f/SiCN composites,while SiCf/SiCN composites had excellent mechanical properties(flexural strength of 384MPa,fracture toughness of 12.8 MPa·m1/2).It can be seen that the structural and functional integrated composites were obtained by combining CVI SiCN matrix with high strength fiber.