| The performance of C/C composites prepared by liquid impregnation method isclosely related to the char yield of resin impregnation resin and the structure of resincarbon. Therefore, the research on the resin preparation with high char yield andresearch on catalytic graphitization of resin carbon become two most important partsin the research field of C/C composites prepared by liquid impregnation method.This study used ammonia as catalyst, used formaldehyde and phenol to prepareC/C composites with high char yield phenolic resin. And the study systematicallyexamined the impact of reaction conditions such as the ratio of formaldehyde/phenolmolar, synthetic temperature, holding time on the yield of phenolic resin, solidcontent, free formaldehyde content, viscosity, curing loss rate and char yield, theoptimum conditions to prepare the phenolic resin with high char yield wereestablished. Three methods had been used to modify the prepared phenolic resin inorder to further improve its char yield, which had laid a good foundation for thepreparation of C/C composites. TG-MS, FTIR and solid C-NMR techniques were usedto conduct comparative study on the pyrolysis process of conventional phenolic resinand boron phenolic resin (BPF) and determine the temperature range of phenolic resinpyrolysis char. The relationship between the structure of phenolic resin and itsthermal stability and char yield was explored. Six catalytic elements of boron,chromium, manganese, iron, cobalt and nickel were added into phenolic resin, AndX-ray diffraction (XRD), scanning electron microscope (SEM) and metallographicmicroscope observation and Raman spectra were used to characterize the catalyticgraphitization effect of its catalytic resin carbon, and the catalytic mechanism of eachelement was analyzed. On the basis of contrasting phenolic resin carbon catalyticgraphitization effect, C/C composites prepared by phenolic resin carbon by threeelements of Boron, chromium, and iron catalysis were optimally selected to be theresearch object. The relationship of bending strength, resistivity, friction coefficientand wear volume of C/C composites and graphitization degree of matrix carbonmaterials was studied. A low cost method to prepare high performance C/Ccomposites was found.The results showed that the formaldehyde/phenol molar ratio, reactiontemperature and holding time were the main factors influencing the structure and performance of phenolic resin, the structure of phenolic resin was closely related toits char performance. It was obtained after optimization that the optimum processunder the condition of ammonium hydroxide as catalyst was as follows: the ratio offormaldehyde/phenol molar was1.2:1, the reaction temperature was90℃, theholding time was25min.The cured phenolic resin prepared by this process after heattreatment at700℃could reach73.8%.Modification is a common method to improve the char yield of C/C compositesresin carbon phenolic resin matrix; this study adopted three methods for synthesis ofammonia phenolic resin for further modification. The first method was to addsalicylaldehyde and benzaldehyde reactive diluents in ammonia phenolic resin, thefunctional groups in molecular chain reactive diluent and phenolic resin formedconjugate system, which improved char yield of phenolic resin; The second methodwas to introduce boron element in phenolic resin to change its molecular structure, thegenerated key could improve boron ester bond in order to improve the char yield ofphenolic resin; The third method was to conduct the compound modification ofphenolic resin, namely adding reactive diluent in boron modified phenolic resin, thusfurther improve char yield of phenolic resin. Preliminary experimental results showedthat the composites modification had higher char yield compared with the singlemodification phenolic resin, its char yield was larger than80%after700℃heattreatment.TG-MS, FTIR and solid C-NMR technology was used to study the pyrolysisprocess of PF1.2resin, the relationship between phenolic resin char yield and itsstructure was discussed. Results showed that the relative content of irreplaceablemethylene in the structure of phenolic resin was a main factor influencing phenolicresin heat resistance and the char yield. The thermal stability of (P-P ’) methylene washigher than that of O-O’ methylene, O-O ’ methylene could be naphthenic intoxanthene ring with two adjacent phenolic hydroxyl before pyrolysis of the resin,thereby increasing the resin char yield.The pyrolysis processes of boron phenolic resin (BPF) and PF1.2werecomparatively studied, the impact of introducing boron element on the phenolic resinthermal stability and char yield was explored, and the major structural factorsinfluencing boron phenolic resin char yield and the temperature range of its pyrolysischar were identified. The results showed that the introduced boronic acid changedmolecular structure of the phenol resin by generating boron ester bond. It was becausethe generated keys in Boronic acid-modified phenolic resin that made it had a higher heat resistance and char yield than the homemade ammonia phenolic resin.The analysis results of XRD, Raman spectroscopy and metallographicobservation showed that after the heat treatment of high temperature above1600℃,six elements had different degrees of catalytic graphitization effect on phenolic resincarbon. The catalytic graphitization process of boron on phenolic resin carbonmaterials was as follows: Boron atoms formed a solid solution with carbon bydiffusion at high temperature; the defect position of boron atoms into thesix-membered ring carbon material plane promoted the improvement of the graphitestructure. Therefore, amorphous carbon material catalyzed by boron could beconverted into a homogeneous Component A. Catalytic mechanism of manganese wassimilar to that of boron, but its catalytic graphitization was worse than boron;Chromium and carbon materials form the carbide at high temperatures, chromiumcarbide was decomposed and transformed into Ts component; Phenolic resin carbonwas dissolved first in elemental iron, elemental cobalt and elemental nickel and thengraphite Component G and Component Ts were precipitated.The resistance, bending strength, friction coefficient and wear of blank sampleadded respectively to boron, iron and chromium, phenolic carbon-based C/Ccomposites were comparatively studied. Experimental results showed that theresistivity of blank sample was up to39.44μ m because the carbon matrix wasdifficult to graphitize, the resin carbon containing catalyst had a higher degree ofgraphitization; among them, the resistivity of boron catalytic was16.82μ m, theresistivity of the iron catalyzed sample was19.82μ m, the resistivity of chromiumcatalyzed sample was30.72μ m, the resistivity matrix carbon after catalyticgraphitization reduced significantly. The interface bonding strength of matrix carbonafter catalytic graphitization and carbon fiber weakened, leading to the decrease ofC/C composites bending strength. The bending strength of C/CBlankwas up to176MPa, the bending strength of C/CB、C/CFeand C/CCrwere115MPa、66MPa and53Mpa, respectively, indicating the bending strength of C/C composites in the matrixcarbon containing catalyst reduced. The catalytic graphitization had a distinct effecton the tribological properties on phenolic carbon matrix C/C composites. The frictioncoefficient of blank sample was0.71, and the friction coefficients of boron catalyticsamples and iron catalytic sample were low, they were0.27and0.28respectively.Wear extent was related to the interface bonding situation of matrix carbon and carbonfiber, weak interfacial bonding C/CFecomposites materials had the biggest wearextent, and strong interfacial bonding C/CBlankand C/CBcomposites had the smallest wear extent. |
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