Study On Synthesis Of The Zirconium Containing Polymer Precursor

In recent years,high temperature ceramics prepared by preceramic precursor method have been applied to the high temperature structural components such as wing leading edges,nose tips and rocket nozzles due to their high performance.However,the key difficulty of this method lies in the preparation of the precursor.Since Zr C ceramics possess excellent anti-ablation performance and are widely used,researchers have done a lot of research in the synthesis of Zr C precursor,and also the organic synthesis has become the preferred method.While there are still some drawbacks in this method,such as high cost,complex preparation process,high toxicity and unpossible long-term storage of the products.In addition,multiphase ceramic composites have drawn much attention since they exhibit enhanced mechanical strength and oxidation resistance than any of the individual component.Unfortunately,there are few reports about the precursor synthesis of multiphase ceramics.Therefore,it is necessary to prepare better Zr C preceramic precursor and also precursors with more ceramic components.In this work,Zr C,Si C/Zr C,Zr B₂/Zr C and Si C/Zr B₂/Zr C preceramic precursors were successfully synthesized by in–situ chemical reaction of phenol,paraformaldehyde,tetraethoxysilane,boric acid,acetylacetone and Zr OCl2·8H2O.X-ray photoelectron spectroscopy（XPS）,Fourier transform infrared spectra（FTIR）,nuclear magnetic resonance（NMR）,thermogravimetric analyzer-differential scanning calorimetry（TG-DSC）,the cone and plate rheometer,X-ray diffractometer（XRD）,Laser-Raman spectrometer（Raman）and scanning electron microscopy（SEM）with energy dispersive spectroscopy（EDS）were applied to characterize the obtained precursor and its ceramic products.The most probable reaction mechanisms of the precursors were proposed.And the chemical structures of the precursors were provided;Properties,pyrolysis process and pyrolysis products of the obtained precursors were investigated.Ceramic mechanisms of the precursors were discussed.In addition,Zr C preceramic precursors with different zirconium contents were prepared.The effect of zirconium incorporation on the properties of the precursors was investigated.What is more,effect of the zirconium incorporation on the catalytic graphitization of the carbon phase was discussed and concluded.（1）A zirconium containing precursor for Zr C ceramic（PZC）with different zirconium contents was successfully synthesized by chemical reaction of phenol,paraformaldehyde,Zr OCl2·8H2O and acetylacetone.The obtained precursor might be Zr–O and Zr–O–C chain polymer with hydroxymethyl phenol and acetylacetone as ligands.It showed long temperature range of 40-80 °C and low viscosity of 50-800 m Pa·s for the precursor with a zirconium content of 16 wt%,which was suitable for impregnation;The highest pyrolysis temperature of this precursor was 700 °C;Residue of the precursor was 55 % at 1200 °C.It was completely transformed into ceramics at 800 °C with C and Zr O2 as products.After heat-treated at 1800 °C,highly crystallized Zr C–C was obtained.When the temperature was increased to 2000 °C,the structure order of the pyrolysized carbon was the highest.Compared to the ceramic products without zirconium,its structure order,crystallite size and crystallite height was increased by 57%、133% and 22.3%,respectively.Due to the enchancement of Zr O2 particles and bridging of Zr–C bonds,the final pyrolytic residue（Zr C and C）was a hard and dense massive solid without obvious defects.The Zr C particles were fine and uniformly distributed in the carbon matrix.（2）Microstructures of the carbon phase in Zr C/C ceramics with zirconium content ranging from 0 to 27 wt% were investigated.It shows that zirconium incorporation helped improve structure order and crystalline growth of the carbon phase.With the increasing of zirconium content,the catalytic graphitization of zirconium was enhanced.However,structure order and crystalline growth of the carbon phase were decreased when the zirconium content went up to 27 wt%.In addition,the content of graphite carbon was dramatically increased six times due to zirconium incorporation.However,the content of graphite carbon didn’t change much with the increasing of zirconium content in Zr C–C ceramics.（3）Based on the synthesis of PZC,a silicon and zirconium containing polymer precursor（PSZC）for Si C/Zr C ceramic was successfully synthesized by chemical reaction of acetylacetone and Zr OCl2·8H2O.Decomposition of PSZC was completed at 800 oC,and it gave amorphous carbon,Si O2 and Zr O2 with a yield of 70%.During heat-treatment,Si C was formed at about 1500 oC,followed by the appearance of Zr C.Further heating at 1900 oC induced highly crystallized Zr C,Si C ceramic and structure-ordered carbon with a structure of Zr C and Si C particles in the size of 100 nm uniformly distributed in the carbon matrix,which was dense solid having no obvious defects.（4）A boron and zirconium containing polymer precursor（PBZC）for Zr B₂/Zr C ceramic was successfully synthesized by chemical reaction of phenol,paraformaldehyde,boric acid,acetylacetone and Zr OCl2·8H2O.Decomposition of PBZC was completed at 800 oC with Zr O2,B₂O3 and carbon as pyrolysis products.Since the thermal stability of Zr B₂ is better than Zr C and the reaction Cibbs energy of Zr B₂ is lower,Zr B₂ was formed preferentially in the matrix at about 1500 oC,followed by the appearance of Zr C when there was not enough B₂O3.The Zr B₂/Zr C/C phase with high crystallinity was obtained at 1800 oC.The obtained Zr B₂/Zr C/C composites were hard and dense solids with Zr B₂ and Zr C grains in the size of 200 nm uniformly embedding in the carbon matrix.（5）Based on the reaction of PSZC and PBZC,a kind of zirconium,silicon and boron containing polymer precursor（PBSZ）for Si C/Zr B₂/Zr C ceramics was synthesized.Residue of the precursor was 67% at 800 oC and it gave amorphous carbon,Si O2,B₂O3 and Zr O2 as pyrolysis products.Highly crystallized Si C–Zr B₂–Zr C–C was obtained when the temperature was higher than 1600 oC.Zr B₂/Zr C/C ceramics together with randomly distributed Si C whiskers with a diameter of 2 μm were in-situ synthesized after heat-treated at 1800 oC.The size of Zr B₂ and Zr C grains was 200 nm and they were uniformly distributed in the carbon phase.Further heated at 1900 oC,ceramic grains grew larger,while Si C whiskers showed a much samller diameter of 500 nm and tended to grow on the surface of the bulk matrix.As for the Zr B₂/Zr C/C ceramic matrix,the size of Zr B₂ and Zr C grains was 500 nm and they were uniformly distributed in the carbon phase..