Synthesis And Application Of ZrC Ceramic Precursor

Carbon fiber-reinforced ZrC-SiC ceramic matrix composites(CMCs)are one of the candidate materials for use in structural components of hypersonic vehicles due to the advantages of lower density,thermal resistance against ultra-high temperatures,anti-oxidation and anti-ablation performance,etc.Polymeric precursor infiltration pyrolysis(PIP)is an effective process to produce ceramic matrix composites with these excellent properties.Hence,synthesis of ceramic precursors of ZrC which is fully capable of PIP has great significance for preparing CMCs.In this study,aim to the urgent demand of structural components used in hypersonic vehicle and thus,a precursor of zirconium carbide(PZC)for PIP was synthesized via the reaction of zircomiun tetrachloride,ethanol,acetylacetone and 1,4-butylene glycol.At the same time,the synthesis mechanism and pyrolysis behavior of PZC were studied in detail.The structure and microstructure of the composites that prepared by using solution of PZC and PCS(mass ratio 1:1)were also characterized.Major results are summarized as follows:(1)Via an organic polymerization,linear precursor of PZC without chlorine was synthesized via the chelate reaction of alkoxy-substituted ZrC14 and acetylacetone,transesterification with 1,4-butylene glycol.And PZC shows well chemically stablity,excellent solubility in organic solvents,particularly,it has superior intersolubility with PCS in any mixing proportions.The rheological properties of the PZC xylene solution(50 wt%)were tested,and the results showed that the viscosity of the solution was very low in a wide temperature range(40-80 ?),which was beneficial to improve efficiency of impregnation.(2)The products of each stage in the process of synthesis were characterized,furthermore,the migration and residual of impurity elements in the intermediates and PZC were discussed through tracking of these elements.There was a little impurity such as Al,Ca,Mg but more Fe remained in zirconium ethoxide which was prepared by the ethanol and ZrC14,therefore,the content of Fe in ZrC14 should be as low as possible.In addition,the variation of relative molecular weight in S-type indicates that it is a typical step polymerization,however,the low effective collision rate of active groups leads to the slow growth of molecular weight in the later stage of the reaction.(3)Pyrolysis process and conversion mechanism of the precursor to ceramics was studied as well,the results showed that c-ZrC crystalline in nano sizes was formed from PZC as it was heat treated at 1500 ? in an inert atmosphere,in which the reactions that decomposition of the organic molecules and carbothermal reduction between carbon and ZrO2 were completed successively,with a yield of ceramic of 33.45 wt%ultimately.The carbothermal reduction reaction was completed at a temperature lower than that from thermodynamic calculations by possible reasons of the bridging structure of PZC,nanometer effect of pyrolysis products,low free energy of surface,and the co-effect of chemical equilibrium transfer.(4)The mixed precursors were analyzed and characterized,which was found that there was no obvious crosslinking between them.Moreover,the optimum temperature range used for impregnation is 40-80 ?.The investigation of mixed precursor to ceramics indicated that the ceramic yield of mixed precursors was higher than the theoretical value because of the synergistic effect of them in the pyrolysis process,which inhibited the formation of pyrolysis carbon and the volatilization of SiO.(5)Dense C/C-ZrC-SiC composites with a bulk density of 2.254 g/cm3 were prepared by CVI pretreatment and 12 times PIP process,using this mixed precursor solution as impregnating.Through the analysis of the structure and micro-morphology of the composites,it was found that the carbon fibers were coated with approximately 1 ?m thick layers of pyrolytic carbon,which was deposited as an interfacial layer between carbon fibers and continuous dense carbide(ZrC,Sic)around it;the elements of Zr and Si were evenly distributed in the matrix composed of pyrolytic carbon and carbide,and especially the ZrC particles of nano sized were evenly dispersed in the continuous SiC substrate.