Synthesis And Physicochemical Properties Of ZrC Composite Ceramic Precursors

Polymer-derived method is superior in the fabrication of ultra-high-temperature ceramics with the designable composition and structure,low sintering temperature and easy densifying process.Thus polymer-derived method is one of most important method to fabrication ceramic materials with structural and functional properties combined.In order to solve the disadvantage of oxygen-rich precursors with low ceramic yield,high porosity and high cost,which is caused by the carbothermal reduction when pyrolyzd at high temperature,low-oxygen content ultra-high-temperature ceramic precursors was fabricated in this paper.We synthesized three kinds of low-oxygen content precursors for ZrC/C,ZrC/SiC and ZrC/SiBNC ultra-high-temperature ceramics via radical polymerization.The synthesis of ZrC/C precursors is the premise,and the synthesis of ZrC/SiC or ZrC/SiBNC precursors is based on the former by adding low molecular weight polycarbosilane（LPCS）or polyborosilazane（LPBSZ）for copolymerization.All of the synthesized precursors have a rather high ceramic yield,low ceramization temperature and favourable molding property.The monomer which is synthesized by reacting Cp₂ZrCl₂ with Allyl-MgCl,was used to fabricate ZrC/C ceramic precursors（PZC）.The ceramic yield of PZC is high to72wt%at 1000^oC,with oxygen content lower than 3.5wt%.Precursor PZC has a soft point of 160^oC,and can dissolve in common organic solvent,such as xylene,methylbenzene,tetrahydrofuran and dimethylformamide.PZC can convert into ZrC/C ceramic nanocomposites at a rather low temperature of 1200^oC.In ZrC/C ceramic nanocomposites,ZrC is in nanoscale,which is embedded uniformly in carbon matrix.The ZrC/C ceramic nanocomposites have excellent high temperature resistance,which can restrain the grain growth of ZrC nanoparticles because of the carbon matrix.After treated at 2400°C for 1h,the ZrC/C ceramic exhibit little mass loss of 4.4 wt%and a fine and homogeneous microstructure.However,the molar ratio of Zr to C is 1:3.5,making ZrC/C ceramics contain much free carbon.ZrC/SiC ceramic precursors（PZC-PCS）were synthesized by copolymerization between the monomer of PZC and various proportion of LPCS,thus we can control the Si content of PZC-PCS through adjusting the adding of LPCS.The ceramic yield of PZC-PCS is high to 78wt%at 1000^oC,with oxygen content lower than 3.0wt%.Precursor PZC-PCS has a soft point of 180^oC,and can dissolve in common organic solvent,such as xylene,methylbenzene,tetrahydrofuran and dimethylformamide.During the polymeration process,the C=C double bond in the uncrossing part can reacted with Si-H in LPCS to cause the hydrosilylation reaction,or substituted by Si-H to form the Zr-Si.PZC-PCS can convert into ZrC/SiC ceramic nanocomposites at a rather low temperature of 1400^oC.In ZrC/SiC ceramic nanocomposites,ZrC and SiC is in nanoscale,which is embedded uniformly in carbon matrix.The fabricated ZrC/SiC ceramic nanocomposites have excellent high temperature resistance.After treated at2200°C for 1h,the ZrC/SiC ceramic exhibit little mass loss of 5 wt%and a fine and homogeneous microstructure.The adding of Si element can restrain the grain growth of ZrC nanoparticles.ZrC/SiBNCceramicprecursors（PZC-PBSZ）weresynthesizedby copolymerization with the monomer of PZC and LPBSZ.PZC-PBSZ has a rather low oxygen content of 2.0wt%and a soft point of 140^oC,which can dissolve in common organicsolvent,suchasxylene,methylbenzene,tetrahydrofuranand dimethylformamide.During the polymeration process,the C=C double bond in the uncrossing part can reacted with Si-H in LPBSZ to cause the hydrosilylation reaction,or substituted by Si-H to form the Zr-Si.PZC-PBSZ can convert into ZrC/SiBNC ceramic nanocomposites at a rather low temperature of 1200 ^oC.In ZrC/SiBNC ceramic nanocomposites,ZrC is in nanoscale,which is embedded uniformly in amorphous SiBNC matrix.Because of the introduction of ZrC nanoparticles,the high temperature resistance of SiBNC has improved.Meanwhile,the existence of SiBNC can enhance the oxidation resistance of ZrC.After treated at 1800^oC,ZrC nanoparticles can react with SiBNC matrix to convert into ZrB₂ phase.Ceramics with different composition can with different microstructure,but all remained a fine and homogeneous one.