Construction Of Low-dimension Carbon Nanostructures By In-situ Catalytic Pyrolysis Of Phenolic Resin And Properties Of The Derived Carbon

To improve the performances of phenolic resins used as binders in refractories,and their derived carbons, transition metal Ni and Co were used to modify phenolicresins. The high-temperature structural evolution of the Ni doped phenolic resins andthe microstructures of the derived carbons were investigated. The catalytic effect ofelemental Ni on the pyrolysis of phenolic resin and the microstructure of the derivedcarbon are explored. The results show that the direction of the pyrolysis of phenolicresin can be attuned and the graphitization of the derived carbon can be accomplishedat a lower temperature by the catalysis of Ni catalysts. As a result, the carbon yield, theconductivity and the antioxidation of the derived carbons have been improved. On thebasis, varieties of low-dimensional carbon nanostructures were constructed by the in-situ catalytic pyrolysis of the phenolic resins with different morphologies of NiC2O42H2O and CoC2O42H2O as the catalyst precursors. The formation mechanisms of thelow-dimensional carbon nanostructures were discussed in the dissertation. Therefore, anovel and morphology-controlled method to prepare low-dimension carbonnanostructures by phenolic resin was established. Further, the relationships betweenthe structure and the performance of the phenolic resin-derived carbon werepreliminary investigated. The above research results provide a theoretical basis for theimprovement of performances of phenolic resin-based carbonaceous refractorymaterials and the carbon/carbon composites. The main contents are as follows:With ammonia as catalyst, P1F1.25resin was synthesized by controlling the molarratio of phenol to formaldehyde for1:1.25. Similarly, P1F1.25Nixresins weresynthesized with the same conditions besides the addition of some Ni (NO3)26H2O,where x mains the value of the molar ratio between Ni (NO3)26H2O and phenol. Theprocess of the catalytic pyrolysis and high-temperature structural evolution of Nidoped phenolic resins from473to1473K can be divided into four stages:1)Cross-linking of phenolic resin(473-673K), in which Ni(Ⅱ) promotes the crosslinkingand the formation of crosslinked structure favors the improvement of the char yield.2)Catalytic cracking of methylene (673-968K), during this stage, the weight loss isserious and Ni (Ⅱ) is reduced to zero-valence Ni.3）Catalytic dehydrogenation and carbonization stage (968-1073K), in which the catalytic cracking of C-H by Nireduces the dehydrogenation temperature and favors the formation of a stable structureof polycyclic aromatic hydrocarbons at a lower temperature, which improves the charyield.4) Catalytic graphitization of phenolic resin-derived carbon (1073-1473K), inwhich Ni catalyst shows a distinct catalytic activity on the graphitization of phenolicresin carbon above1173K.Different low-dimension carbon nanostructures, such as carbon nanotube,onion-like carbon, Ni@C core-shell, bamboo-like carbon and graphene were preparedvia in-situ catalytic pyrolysis of the Ni doped phenolic resin. The effects of catalyticpyrolysis temperature and the dosage of the catalyst precursor on the formation oflow-dimension carbon nanostructures were investigated. The results show that thepreferred temperature range to produce carbon nanotubes by the catalytic pyrolysis ofphenolic resin is from1073to1273K and the preferred dosage of the catalystprecursor is under1%accounted by the molars of the phenol. The V-L-S mechanismwas proposed for the formation of the carbon nanotube and S-L-S mechanism wasproposed for the formation of the onion-like carbon and Ni@C core-shell formed inthe body of the derived carbon.The resistivity of the P1F1.25and P1F1.25Ni0.01treated at different temperatures(873-1473K) was measured by four-probe method. The results show that theresistivity drops with the increase of heat treatment temperature. Thecatalytic graphitization of phenolic resin by Ni increased the conductivity of thederived carbon.Based on the formation mechanism of Ni@C core-shell, NiC2O42H2O nanofibreswere prepared by solvothermal for the first time to be used for preparing graphiticcarbon nanofibres via the in situ catalytic graphitization of phenolic resins, in whichthe NiC2O42H2O nanofibres act both as the templates and catalyst precursors. Thehighly crystalline nanostructures trapped in the matrix of the carbonized phenolicresins keep the fibrous morphology of the NiC2O42H2O nanofibres. The homogeneousdispersion of the NiC2O4·2H2O nanofibres in the phenolic resin determines thehomogeneous formation of the graphitic carbon nanofibres in the matrix during in situcatalytic graphitization. The fabrication of the graphitic carbon nanofibres is proposedto occur via S-L-S mechanism. Compared with conductivities of the carbonized NiC2O42H2O/phenolic resin composite and P1F1.25Nixunder the same amount of Ni,the former because of the formation of graphitic carbon nanofibres has a moreexcellent conductivity than the latter.CoC2O4·2H2O nanocrystals with various morphologies, such as nanoparticle andnanorod, were respectively prepared for the precursors to produce carbon onion andcarbon nanofibre by the in-site catalytic pyrolysis of phenolic resin. The in-situformation of the carbon onion and carbon nanofibre in the matrix of the carbonizedphenolic resin shows that the precursors has the template function during the formationof the low-dimensional carbons.The non-isothermal pyrolytic kinetics of P1F1.25shows that the pyrolysis can bedivided into three stages: hydroxyl dehydration, cracking of the crosslinkers, anddehydrogenation and aromatization with the corresponding apparent activationenergies are163.4,241.1,340.9kJ mol-1, respectively. The apparent activationenergies of the reactions corresponding to the four stages P1F1.25Ni0.01are respectively150.5,162.7,232.3and279.1kJ mol-1. The results indicate that the dehydration of thehydroxyl group catalyzed by Ni (II) reduces the apparent activation energy from163.4kJ mol-1to150.5kJ mol-1, which promotes the crosslinking of phenolic resin; Thegenerated Ni during the pyrolysis of P1F1.25Ni0.01shows catalytic activity on thearomatization reaction, which makes the apparent energy decrease from340.9kJ mol-1to279.1kJ mol-1. The Ni catalytic cracking of C-H promotes aromatization andreduces cleavage of the crosslinkers, which results a higher char yield, and is aneffective method for controlling the pyrolytic direction of phenolic resin.The result of the oxidation of the P1F1.25and P1F1.25Ni0.01treated at differenttemperatures indicates that the initial oxidation temperature of the derived carbonsincreases with the heat treatment temperature of their precursors. The catalyticgraphitization of the derived carbon by Ni promotes the increase of the initialoxidation temperature. The oxidation reaction process of the derived carbon obtainedby pyrolysis of P1F1.25at1473K for3h was divided into two stages corresponding totwo different carbon structures: amorphous and crystalline structures. The apparentactivation energies of the two stages are respectively88.2and91.8kJ mol-1calculatedby non-isothermal kinetic equation. However, due to the catalytic graphitization by Niat1473K, the P1F1.25Ni0.01derived carbon only appears one reaction stage with an apparent activation energy for96.0kJ mol-1.In conclusion, the catalyticgraphitization by Ni promotes the antioxidation performance of the phenolicresin-derived carbon.