Single-source-precursor Synthesis Of SiC-based Ultrahigh-temperture Ceramic Nanocomposites

Titanium boride(TiB2)and Titanium carbide(TiC)ceramics are members of a family of ideal materials for use in extreme environments such as high temperature,reaction atmosphere,mechanical thermal load and wear due to excellent properties such as high strength,high melting point,high conductivity and low density.These properties and their light weight make them attractive for the field of aircraft propulsion systems,spacecraft thermal protection and high temperature high performance engines.However,TiB2 and TiC single material have the same toughness and sinterability as other ceramics,and are poorly stable in aerobic environments.Therefore,ultra-high temperature composite ceramics such as MB2/SiC and MC/SiC(M = Ti,Zr,Hf)are often prepared by adding SiC,which can achieve better overall properties such as higher strength and excellent oxidation resistance.For ceramic microstructure and high temperature performance research,still need further exploration.An allyl-containing hyperbranched polycarbosilane(AHPCS)was used as the raw material,and the compounds containing heterogeneous elements(titanium and boron)were introduced to AHPCS to obtain single-source-precursors(SSPs).After crosslinking and high temperature annealing of the resultant SSPs,titanium-containing SiC composites were successfully synthesized.The effects of crosslinking,introduction of metals,annealing temperatures on the high temperature performance and microstructure evolution of SiC composite ceramics were investigated in this thesis.The results provide a basis for the preparation of MB2/SiC and MC/SiC(M = Ti,Zr,Hf)nanocomposites by single-source-precursor method.The main research contents and results are as follows:Firstly,two kinds of crosslinking agents(titanium tetrachloride and butyl titanate)were introduced into AHPCS.The effects of crosslinking agents on crosslinking reaction and ceramic microstructure were studied.The results show that the introduction of TiCl4 or Ti(OBu)4 promotes the hydrosilylation,dehydrogenation and dehydrochlorination of AHPCS,which improves the yield of ceramics.From the crystallization behavior of ceramics,the introduction of TiCl4 promoted the crystallization of ?-SiC in the ceramic product,while the introduction of Ti(OBu)4 inhibited the crystallization of ?-SiC in the ceramic product.And the content of Ti in the ceramic can be controlled by the introduction amount of titanium tetrachloride or butyl titanate in the precursor.Secondly,based on the introduction of butyl titanate into AHPCS,the SSPs were prepared by adding different borane complexes.The results show that the boron hydride reaction of B-H in borane and C=C in AHPCS improves the crosslinking degree of the precursor.Crystallization of ?-SiC,TiB2,TiC in the ceramic pyrolyzed at 1400?.The introduction of heterogeneous elements(titanium,boron)inhibits the crystallization of SiC.Pyridine borane further fixes the butyl titanate in the precursor,which increases the TiB2 content in the ceramics.Compared with the massl loss of ceramics under high temperature annealing,it is found that the ceramic products with triethylamine borane as boron source show better stability at temperatures above 1500?.Finally,TiC-TiB2-SiC ceramic nanocomposite were successfully synthesized by a novel single-source-precursors approach with AHPCS,bis(cyclopentadienyl)titanium dichloride(Cp2TiCl2)and triethylamine borane(TEAB)as starting materials.The obtained single-source-precursor was characterized by Fourier transform infrared spectra(FT-IR),which confirms that hydroboration(C=C/B-H)and dehydrochlorication(Si-H/Cp2TiCl2)reactions were involved to introduce B and Ti elements into the AHPCS chains.The structural evolution of single-source-precursors,phase composition and chemical composition of the obtained ceramics were investigated by FT-IR,X-ray diffraction(XRD)and elemental analysis.High temperature behavior of the resultant TiC-TiB2-SiC ceramic nanocomposites with respect to decomposition as well as crystallization was carefully checked by XRD and mass loss after annealing at high temperatures of 1600 and 1800 ?.Transmission electron microscopy(TEM)was used to further observe the microstructure of TiC-TiB2-SiC nanocomposites,which again confirms the crystalline phases consist of nanoscaled ?-SiC,TiC and TiB2.A single-source-precursor method was developed to synthesize SiC-MC-MB2(M = Ti,Zr,Hf)nanocomposite ceramics.