Research On The Synthesis Of ZrC/ZrB₂ Organic Precursors And Its Ceramic Modification For High Thermal Conductive C/C Composites

Carbon materials, especially C/C composites, are extensively used in the fields of space flight and aviation materials, friction materials, heat-stable materials, high mechanical property materials, biomedical materials and so on, which are based on their good thermomechanical and thermophysical properties. PAN based carbon fibers are usually used as reinforcement of C/C composites, which greatly limits the thermal comductive property of the C/C composites. On the other hand, the cost of the C/C composites modified by ceramic is high, so that the synthesis of ceramic organic precursors is worth to research thoroughly. Focusing on the above-mentioned problems, a series of research work were performed in this paper with the aim of reducing costs of ceramic organic precursors and improving the thermal conductive and anti-oxidation properties of C/C composites. The main research works and conclusions are exhibited as follows:1. A series of soluble precursors for ultra-fine zirconium carbide(Zr C) were successfully synthesized by using zirconium tetrachloride after alcoholysis reaction with ethanol as zirconium source and 1,4-butanediol, catechol, hydroquinone, bisphenol A, phenol as carbon source with the participation of acetylacetone. The synthesis process and pyrolysis behavior of the precursors were investigated.(1) The Zr O2 are residual in pyrolysis product due to the lack of carbon source derived from 1,4-butanediol. However, the pyrolysis products of the precursor synthesized by above phenols can provide enough carbon for promoting Zr O2 to convert into Zr C phase completely.(2) The product pyrolyzed at 1600 oC of the Zr C precursor synthesized by using catechol as carbon source is near-spherical Zr C ceramic particles with size of 100~200 nm. When using hydroquinone and bisphenol A as carbon sources, the size of Zr C particles is about 100 nm and 200 nm, respectively, and the initial temperature of Zr O2 begaining to convert to Zr C are relatively higher.(3) The product pyrolyzed at 1600 oC of the Zr C precursor synthesized by using phenol as carbon source reveals that Zr C grains also are near-spherical particles in the range of 50~100 nm. The polymer precursor decomposes completely at about 600 oC and its ceramic yieldis about 24 % at 1200 oC. The initial temperature of carbothermal reduction reaction of the precursor(about 1300 °C) is much lower than the temperature of 1660 °C determined by thermodynamics calculation, owing to the good disperstiveness of the compositions in the pyrolysis product which can reduce the reaction energy barrier.2. The pyrolysis behavior of the as-prepared Zr B2 ceramic organic precursor were investigated by using liquid polyborazine(PBN), synthesized by using BCl3, NH4 Cl and isopropylamine as basic raw materials, as boron source and zirconium-containing organic precursor, synthesized by using 1,4-butanediol and phenol as carbon source.(1) The zirconium-containing organic precursor synthesized by using 1,4-butanediol and liquid PBN were dissolved in toluene. Zr B2 can be found in the product of the acquired mixture solution co-pyrolyzed at 1600 oC. Additional carbon source can urge Zr O2 to convert into Zr B2 and Zr C, but Zr O2 still existed in the reaction product, which suggests that the formation of Zr B2 need the participation of carbon and additional carbon source can not disperse uniformly with zirconium and boron source resulting in the surplus of Zr O2.(2) When using Zr C precursor synthesized by using 1,4-butanediol and phenol as carbon source as zirconium source, Zr B2 is the main phase of the product pyrolyzed at 1600 oC and Zr O2 can not be found. The products are uniform Zr C-Zr B2 or Zr B2-BN multiphase ceramic when the zirconium or boron source excessed, respectively. The hexagonal Zr B2 with grain size of 1 μm can be acquired on the condition of proper proportion of zirconium or boron source. The mixture of Zr C precursor and liquid PBN with proper proportion can be regarded as Zr B2 ceramic organic precursor.(3) The pyrolysis behavior of Zr B2 precursor with excess boron source at different temperature illustrates that Zr O2 starts to react with C and BN to form Zr B2 at 1300 °C and the reaction almost completes at 1400 oC. Zr C does not appear in the products pyrolyzed at each temperature, which implies that the reaction between Zr O2 and C, BN is simultaneous. The ceramic yield of Zr B2 precursor is about 50 % and fall to 32 % with the temperature increase to 1500 °C.(4) The lowest temperature of the reaction of Zr O2 react with BN without carbon to form Zr B2 is 3817 oC acquired by thermodynamics calculation, which implies that the reaction hardly happen on this experiment. However, the initial temperature of the reaction has dropped to 1792 oC with the participation of carbon. The results further prove that the formation mechanism of Zr B2 is borothermal reduction reaction with the participation of carbon. The initial temperature of the reaction is lower than the calculated temperature about 500 oC, which reveals that the zirconium source, carbon source and boron source are well distributed in the precursor and its pyrolysis product.3. Round cross-section graphite fibers with diameter about 50 μm have been prepared from mesophase pitch by melting-spinning, oxidative stabilization, carbonization and further graphitization. The graphite fiber preforms prepared by hot-pressing(HP), bundling(BD) or needle punching(NP) process were used to fabricate 1D cylindrical or 1D, 2D and 3D bulk-like high thermal conductive C/C composites undergoing phenolic resin impregnation and pyrolysis(PRIP), chemical vapor infiltration(CVI), mesophase pitch impregnation and pyrolysis(MPIP) process. The microstructure, phase composition, electrical and thermal conductivties as well as mechanical properties were investigated.(1) The 1D cylindrical or bulk-like C/C composites produced by HP+CVI, BD+PRIP+CVI or BD+CVI process all possess highly anisotropy. The thermal conductivities of the C/C composites along the axial direction of the fiber are as high as 600~700 W/m K. The thermal conductive property shows high anisotropy ratio of about 30:1 along the axial and radial directions of the fiber. The mechanical properties of the C/C composites prepared by BD process are closed to those prepared by HP process. With the further densifing by MPIP process, the density of the C/C composite can reach over 1.8 g/cm3 and its thermal conductivity along the axial direction of the fiber exceed 700 W/m K.(2) 1D, 2D and 3D bulk-like high thermal conductive C/C composites with relative low density of 1.0~1.4 g/cm3 were prepared by needle punching combining PRIP and CVI process. With the increase of dimensions of the fiber, the electrical and thermal properties deteriorate along the axial direction of the fiber and slightly improve along normal direction of fiber layer. The thermal conductivities of the C/C composite along above-mentioned directions are in the range of 100~300 W/m K and 5~50 W/m K, respectively.(3) The kind of carbon matrix has obvious influence on the thermal and mechanical properties of the as-prepared C/C composites. The poor mechanical property of the C/C composites resulting from the weak bonding between phensolic resin derived carbon(PRC) or pitch derived carbon(Pi C) and graphite fiber can be noticeably improved by subsequent CVI process. In addition, the pyrolytic carbon(Py C) with shell structure oriented along the axial direction can improve the thermal conductivity of the C/C composites and maintain the high anisotropy. The PRC have a little influence on the the thermal property of the C/C composites, while the influence of Pi C are opposite.4. Effect of Si C/Zr C precursor ratio on the ablation property of C/C-Si C-Zr C composites was investigated. The impregnant with optimum proportion were used to prepare 1D, 2D and 3D high thermal conductive C/C composites and theirs ablation properties were analyzed.(1) The C/C-Si C-Zr C composites with the densities of 1.75~2.10 g/cm3 were prepared by precursor impregnation and pyrolysis(PIP) with different mass ratio of PCS/PZC as impregnant. The results show that the anti-ablation properties of the C/C-Si C-Zr C composites were enhanced firstly and receded subsequently with the increasing of the content of Zr C. The samples(SZ13) possessing the relatively best ablation performance were fabricated with the impregnant with the mass ratio of PCS/PZC=1:3. Appropriate Si C-Zr C multiphase ceramics are favorable to improve the ablation stability of the materials.(2) With the increase of fiber dimensions, the densities of the caron fiber preforms and the subsequent C/C composites increase as well as the anti-ablation property of the C/C composites after ceramic modification have a enhanced trend. Among them, the ablation properties of 1D and 2D samples are similar and that of 3D sample are superior to them. The mass ablation rate and linear ablation rate of the 3D sample are very approach to the sample SZ13. The results show that the ablation properties of pitch-based carbon fiber reinforced C/C composites can be comparable with that of the PAN based carbon fiber reinforced C/C composites after the same ceramic modification process.