The effect of the molecular structure of naphthalene-based mesophase pitch on the properties of carbon fibers derived from it
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摘要: 中间相沥青基炭纤维因具有高模量、低电阻率、高导热等特性,在许多领域有广阔的应用前景。本文分别以采用HF/BF3催化萘一步法制备的中间相沥青(AR-MP)和采用AlCl3催化萘两步法制备的中间相沥青(N-MP)为原料,制备了高性能炭纤维。通过元素分析、TG-MS、FT-IR、13C-NMR、MALDI-TOF-MS、XRD和SEM等手段对上述沥青和纤维进行了分析表征,对比了不同催化聚合工艺制备的中间相沥青的分子结构和性能,并进一步探究了中间相沥青分子结构差异对其炭纤维结构和性能的影响。结果表明:AR-MP分子构型偏向于半刚性的棒状,含有更多的环烷结构和甲基侧链,其预氧化后的纤维显示出更好的碳平面取向,使其石墨化纤维具有更好的热导率(716 W/m·K);而N-MP分子构型偏向于刚性的圆盘状、芳香度高,其纤维在后续热处理过程中产生的缺陷更少,石墨化后具有更大的拉伸强度(3.47 GPa)。Abstract: Mesophase pitch-based carbon fibers (MPCFs) have a high modulus of elasticity, low electrical resistivity and high thermal conductivity, so can be used in many fields. Carbon fibers were prepared from two naphthalene-based mesophase pitches, one synthesized by a HF/BF3 catalytic one-step method (AR-MP) and the other by an AlCl3 catalytic two-step method (N-MP). The mesophase pitches, spun pitch fibers, pre-oxidized fibers, carbonized fibers and graphitized fibers produced from them were characterized by TG-MS, FT-IR, 13C-NMR, MALDI-TOF-MS, XRD, SEM and elemental analysis. The molecular structures and properties of mesophase pitches were compared, and the effects of molecular structures on the structures and properties of the carbon fibers produced from them were measured. In comparison to N-MP, AR-MP has a rod-like semi-rigid molecular configuration containing more naphthenic structures and methyl side chains. The pre-oxidized fibers derived from AR-MP have a better carbon layer orientation, so that their graphitized fibers have a higher thermal conductivity of 716 W/(m·K). N-MP has a higher aromaticity with a disc-like rigid molecular configuration, so that the graphitized fibers prepared from it have a higher tensile strength of 3.47 GPa due to fewer defects being formed during preparation. The molecular structures of AR-MP and N-MP have an obvious influence on the structures and properties of their graphitized fibers.
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Table 1. General properties of AR-MP and N-MP
Sample SP (°C) TSa QI Elemental analysis (wt%) n(H)/n(C) Ash content Spinning viscosity (Pa·s) C H N S AR-MP 234 42.9% 26.7% 95.1 4.80 0 0.05 0.60 2×10−5 22 N-MP 240 37.2% 32.6% 95.8 4.12 0 0.04 0.52 5×10−5 37 Note: SP—softening point, TSa—toluene soluble, QI—quinoline insoluble Table 2. Aromatic carbon and Aliphatic carbon content of AR-MP and N-MP
Sample Aliphatic (%) Aromatic (%) Car3/Car2 fa Cchain CH2 CH3 CHar Car3 Car2 Cars AR-MP 10.06 11.28 5.79 4.88 40.38 16.77 10.84 2.41 0.73 N-MP 4.51 4.10 3.69 6.15 54.51 15.16 11.89 3.59 0.88 Note: Cchain—bridge/hydroaromatic structures (methylene carbons to two aromatic rings); CH2—all the rest of methylene carbons; CH3—methyl carbons; CHar—protonated aromatic carbons; Car3—peri-condensed aromatic carbons; Car2—cata-condensed aromatic carbons; Cars—aromatic carbons joined to aliphatic chains; fa—aromaticity Table 3. The X-ray parameters of mesophase pitches and spun pitch fibers
Sample d002 (nm) FWHM (°) Lc (nm) N AR-MP 0.3520 3.080 2.93 9 N-MP 0.3410 4.606 1.96 6 AR-PFs 0.3517 2.848 3.17 10 N-PFs 0.3488 4.637 1.95 6 Table 4. The properties and X-ray parameters of AR-GFs and N-GFs
Sample Properties Crystal parameters TSb (GPa) TM (GPa) ρ (μΩ·m) TC (W/(m·K)) d002 (nm) La (nm) Lc (nm) AR-GFs 3.02±0.78 634±75 1.77 716±43 0.3369 60.12 30.36 N-GFs 3.47±0.48 652±70 1.82 704±45 0.3370 62.09 29.03 Note: TSb—tensile strength; ρ—resistivity; TM—tensile modulus; TC—thermal conductivity; DC—dispersion coefficient -
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