| Tantalum carbide(TaC)has many unique properties,such as extremely high melting point,elastic modulus and hardness,which is considered as one of the most important UHTC family members.These properties make TaC a suitable candidate for high-temperature applications in the fields of aerospace,metallurgy and machinery.However,similar to other related UHTC systems,TaC suffers from rather poor oxidation resistance at high temperatures.An effective way to solve the above-mentioned problem is to form composites by introducing a second phase to TaC.When the grain size is reduced to the nanoscale in the ceramic,the components within the composites can play a better role,so that the performance of ceramic(such as thermal mechanical and oxidation/ablation resistance properties)can be improved.The main contents are as follows:(1)In this paper,a novel single-source-precursor for SiC-TaC-C nanocomposites was successfully prepared by the chemical reaction between tantalum(Ⅴ)chloride(TaCl5)which served as the tantalum source and the allylhydridopolycarbosilane(AHPCS),subsequently the reaction mechanism was measured by FT-IR.Ultimately,the unique core-shell structured TaC@C nanoparticles were obtained by PDC method.The phase evolution of ceramic nanocomposites was investigated by XRD and TEM.After pyrolysis at 900℃,amorphous ceramic powders were acquired.Annealing at different temperatures in the range of 1200-1600℃ to obtain SiC-TaC-C nanocomposites.The results indicate that the TaC starts to crystallize at lower temperature than the β-SiC.It is particularly worth pointing out that the unique core-shell structured TaC@C nanoparticles were in-situ formed and homogeneously distributed in the ceramic matrix after annealing at 1400℃ and 1600℃.Even at high temperature of 1600℃,the grain sizes of β-SiC and TaC are smaller than 31 nm,indicating the formation of nanocomposite materials.(2)On the basis of the above studies,divinylbenzene(DVB)was added to the system to increase the carbon content.Finally,high-carbon-content SiC-TaC-C ceramic nanocomposites were obtained after heat treatment,and their electromagnetic absorbing properties were investigated.Similarly,the phase evolution and microstructure of ceramic nanocomposites were studied by XRD and TEM.The core-shell structured TaC@C nanoparticles were more clearly observed than that of the low-carbon-content SiC-TaC-C ceramics prepared without adding DVB.Different from the low-carbon-content SiC-TaC-C ceramics,the high-carbon-content ceramics annealed at 1200℃ are amorphous,and the TaC and β-SiC didn’t crystallize until 1400℃,which indicated that the increase of carbon content inhibited the grain growth.According to the results of element analysis,with the increase of DVB addition,the free carbon content of the system significantly increased,so that the conductivity and conduction loss enhanced,and the dielectric properties of the ceramics improved.For the 1600℃ ceramics mixed with paraffin wax,the minimum reflection coefficient(RCmin)reaches-58.89 dB with the thickness of 1.50 mm at 14.32 GHz.By adjusting the sample thickness in the range of 1.00-5.00 mm,the effective bandwidth(the bandwidth of RC values lower than-10 dB)of the obtained sample amounts 13.58 GHz in the range of 2-18 GHz.This result show that the electromagnetic absorption properties of the ceramics significantly improve comparing to the sample without DVB and TaCl5 addition. |
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