In Situ Mesophase Transformation In Densification Of C/C Composites

Carbon/carbon （C/C） composites, consisting of carbon matrix reinforcedwith carbon fibers, are advanced materials that have widespread applicationsattributed to their superior mechanical properties, high thermal conductivity,low thermal expansion, excellent friction properties and good performanceunder extreme conditions.Mesophase pitch, with high carbon yeild, high density and highgraphitizability, can be used as good precursor of carbon matrix in makingC/C composites. But due to its high viscosity, the mesophase pitch blocks theaccess channels to the inner part of the preform. The extent of mesophasepenetration decreases from the outer edge to the center of the composites,resulting in non-uniform filling. The in situ mesophase transformation indensification of C/C composites is a new, efficient, and low-cost process toaddress this issue. The mechanism of the process is that the mixture of moltenmonomer （pure aromatic hydrocarbons such as naphthalene） and catalyst easily penetrate fiber bundles and void space of the preform. When thepreform is completely filled, the in situ poly-condensation leads the monomerto mesophase pitches. After poly-condensation and carbonization, mostmesophase pitches retained in the preform are transformed into carbon matrix.In this work, mesophase pitches were prepared by the aid of AlCl₃andZrCl₄as catalyst. We investigated the effect of catalysts, temperature andholding time on the mesophase formation and transformation behavior. Thechemical state and transformation behavior of catalyst during carbonizationand graphitization were also investigated. On the basis of the above researchwe found the optimum conditions for in situ mesophase transformation indensification of C/C composites. The C/C composites doped with catalystswere prepared and characterized. The effect of retained catalyst andtransformation of catalyst on the properties of C/C composites were alsostudied in detail.Under the same conditions, the catalytic activity of AlCl₃was superior tothat of ZrCl₄, and with increasing the amount of ZrCl₄, the catalytic effect alsoincreased. AlCl₃could make the mesophase pitches have better opticalstructure and higher carbonization yield. In certain conditions, the mesophasepitches with good optical texture could also be prepared by useing ZrCl₄ascatalyst. Although the poly-condensation reaction rate of samples using ZrCl₄was much slower than that using AlCl₃, the large amount of small moleculesand alkyl groups in the pitches greatly decreased the viscosity of the pitches. After comparing different reaction temperatures, we found that mesophasepitches with best properties were obtained at350℃. The volatilization of lightcomponent in mesophase pitches easily destroyed the microcrystallinestructure of the samples during carbonization.After poly-condensation, AlCl₃was transformed into amorphous Al（OH）₃,this may be due to the reaction between AlCl₃and a small amount of waterwithin the raw materials. After carbonizing, a part of Al（OH）₃was transformedinto Al₂O₃, but both substances were amorphous, which can not be detectedfrom XRD patterns. In the above process, AlCl₃was transformed into othersubstance, so it was saved after heat treatment at1000℃. After graphitizationall the Al containing substance was completely removed. ZrCl₄wastransformed into t-ZrO₂druing poly-condensation, and the crystal structure oft-ZrO₂was improved and some of t-ZrO₂was transformed into m-ZrO₂aftercarbonization. During graphitization, a majority of ZrO₂was transformed intoZrC, others transformed to zirconium oxycarbide （ZrCxOy）. Since ZrCl₄wastransformed into ZrO₂, ZrC and zirconium oxycarbide after carbonization andgraphitization, the Zr containing catalyst perfectly retained in the pitches. TheZrC generated during graphitization promoted the growth of graphite structure.While the AlCl₃and ZrCl₄were converted into the oxides and hydroxides, theCl element was transformed into unstable Cl-ions, which adsorbed at thesurface of catalyst. With increasing the temperature, the Cl elementseventually disappear. After three in situ desification at350℃using different catalysts （AlCl₃13wt%, ZrCl₄13wt%, ZrCl₄20wt.%）, the bulk density of low and high densityC/C composites increased from0.625g/cm3and1.715g/cm3to1.289g/cm3,1.383g/cm3,1.373g/cm3and1.790g/cm3,1.810g/cm3,1.798g/cm3, respectively.Due to the higher catalytic activity of AlCl₃, the high viscosity greatly reducedthe efficiency of the impregnation process, which made the densificationefficiency using AlCl₃was much lower than that using ZrCl₄. After three insitu densification, the flexural strength of low density composites rose fromlower than5MPa to74.1Ma,68.3MPa and55.0MPa, while the flexuralstrength of high density composites increased from101.5MPa to200.8MPa,234.5MPa and220.6MPa, respectively. Although the densification efficiencyof high density C/C composites was only about5%, the flexural strengthincreased more than100%. The mechanical properties of low densitycomposites decreased when increasing ZrCl₄content. This is because thegrowth of ZrO₂grain and phase transformation of ZrO₂during carbonizationdestroyed the carbon matrix structure, which decreased the mechanicalproperties of the composites. Attributed to the very low content of catalyst incomposites, the mechanical properties of high density composites did notchange when the content of ZrCl₄increased.Due to the retained Zr-containing catalysts brought damage to themicrocrystalline structure of the carbon matrix, the electrical conductivity ofthe composites decreased. The oxidation properties of the C/C composites at low temperatures were mainly determined by their density, the higher densityand fewer defects increased the oxidation resistance. Although the ZrO₂particles partly reduced the mechanical properties of the composite, it couldreact with carbon into ZrC, which would greatly reduce the ablation rates ofC/C composites. The ZrO₂and ZrC doped C/C composites fabricated throughin situ mesophase transformation will have broad applications.