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2018, 33(3): .

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Research progress on the characterization of mesophase pitch

DUAN Chun-ting, ZHENG Dong-fang, LIU Jun-qing, WANG Qiu-shi, LIANG Peng, GONG Xiao-yi, SONG Huai-he

2018, 33(3): 193-202.

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Mesophase pitch consists of polycyclic aromatic hydrocarbons with optical anisotropy, which is an important precursor for high-performance industrial carbons. Characterizing the structures and properties of mesophase pitch plays a significant role in its quality control, process optimization and applications. This review summarizes research advances in mesophase pitch characterization, including chemical structure, molecular weight, domain structure and rheological property. The future techniques in mesophase pitch characterization are briefly discussed.

Preparation of high-carbon-yield pitches from 1-methylnaphthalene through I₂O₅-assisted electrophilic iodination

YANG Hai-xiao, HAN He-xiang, WANG Ji-tong, QIAO Wen-ming, LING Li-cheng

2018, 33(3): 203-212. doi: 10.1016/S1872-5805(18)60335-X

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Novel high-carbon-yield pitches were prepared through the iodination of 1-methylnaphthalene (1-MNP) followed by dehydroiodination/polycondensation. 1-MNP was first iodinated by I₂ with the aid of I₂O₅, yielding mainly 1-iodo-4-MNP. Then, a thermal-dehydroiodination reaction at a comparatively low temperature of 130-180℃ was carried out to condense the iodinated products to obtain the pitches. The structures of the iodinated products and pitches were investigated by TG, GC-MS, 1H-NMR and FT-IR. Results indicated that the iodination selectivity (Smb) reached 96.4% when 0.75 mol I₂ and 0.18 mol I₂O₅ reacted with 1 mol 1-MNP at 60℃ for 24 h in acetic acid. Ring-hydrogenation products (tetralin, 5-methyltetralin) and methyl migration products (2-MNP, dimethylnaphthalene and trimethylnaphthalene) were found in the pitches by analyzing their methanol-soluble fractions. Dehydroiodination substantially increased the degree of polymerization and carbon yield of the pitches. The pitches exhibited high polymerization degrees (toluene soluble fraction less than 12 wt%), high carbon yields (54-66 wt%) and complicated 1-MNP molecular assembly patterns including α-α', α-β', β-β', depending on dehydroiodination temperature and time. The iodination/dehydroiodination approach is simple, mild and clean for preparation of high quality pitches for high performance carbon materials.

Molecular dynamics simulation of the thermal conductivity of graphitized graphene/polyimide films

XU Jing-cheng, DAI Si-chang, LI Hao-liang, YANG Jun-he

2018, 33(3): 213-220. doi: 10.1016/S1872-5805(18)60336-1

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Graphitized polyimide strips act as bridges between graphene nanoribbons to form macro-scale graphitized graphene/polyimide films. The effect of the sizes of the two components was investigated by non-equilibrium molecular dynamics simulation using the optimized Tersoff molecular force field. Results indicate that the thermal conductivity of the graphene nanoribbon increases almost linearly with length for lengths below 800 nm, but increases as a power function and converges to about 4 300 W/(m·K) above this. The heat conduction between graphene ribbons in the graphitized graphene/polyimide films is improved by increasing the number or width of the graphitized polyimide strips, but is lowered by increasing their length.

Hydrothermal synthesis of magnetic graphene-BiFeO₃ hybrids and their photocatalytic properties

GAN Lu, XU Li-jie, QIAN Kun, WANG Ya-dong, JIANG Fu-yuan

2018, 33(3): 221-228.

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The graphene-bismuth ferrite (Graphene-BiFeO₃) hybrids with different graphene contents were synthesized by a hydrothermal method. Their structure was characterized by XRD, SEM and FTIR. Their light reflection and absorption were investigated by UV-visible diffuse reflectance spectroscopy and photoluminescence spectroscopy, respectively. Their photocatalytic properties were evaluated by degrading methylene blue (MB) and rhodamine B (RhB) dyes under irradiation with tungsten light. Results showed that the hybrids had a sphere-like morphology, within which the graphene was uniformly dispersed between the BiFeO₃ spheres. The hybrids had lower band gaps and higher absorption intensities in the visible light region (400-800 nm) than pure BiFeO₃, and achieved much higher degradation rates for both MB and RhB. The hybrid with a graphene content of 3.0 wt% exhibited the best photocatalytic performance, and its degradation rate constants for MB and RhB reached to 0.083 and 0.10, respectively, which are about 10 times higher than pure BiFeO₃. The introduction of the graphene effectively inhibited the recombination rate of the excited electrons and the holes in BiFeO₃. Their magnetism made them easy to recover from the dye solutions and their photocatalytic activities remained unchanged after recycling 5 times.

Preparation of porous carbons using a chrysotile template and their electrochemical performance as supercapacitor electrodes

CAO Xi, CHUAN Xiu-yun, LI Ai-jun, HUANG Du-bin

2018, 33(3): 229-236.

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Porous carbons were prepared by the template method using chrysotile as the template and sucrose as the carbon precursor. The effect of the weight ratio of the template to the carbon precursor on the porosity and electrochemical performance of the porous carbons was investigated. Results indicate that the pore size distributions and specific surface areas of the porous carbons are changed by the ratio. The porous carbon prepared at a ratio of 1:3 exhibits the highest specific capacitance at different current densities, and best rate capability and cycling stability, which are ascribed to its highest surface area, highest percentage of meso-and macropore volume and its hierarchical porous structure.

The effect of the oxidation method of an activated carbon on the selective catalytic reduction of NO_x with NH₃ over CeO₂/activated carbon catalysts

HUANG Li-hua, LI Xue, CHEN Yao-qiang

2018, 33(3): 237-244.

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An activated carbon (AC) was oxidized with air at 300℃ for 4 h or with 30 wt% HNO₃ at 60℃ for 4 h under reflux before it was loaded with CeO₂ by an impregnation method to prepare CeO₂/AC catalysts for the selective catalytic reduction of NO_x in flue gas with NH₃. The influence of the oxidation method on the performance of the catalysts was investigated. Both catalysts were characterized by FTIR, N₂ adsorption, XRD, temperature-programmed desorption of NH₃ and temperature-programmed reduction with H₂. Results showed that the HNO₃ oxidation increased both the number of oxygen functional groups and the surface acidity of the AC and the redox activity of the CeO₂/AC-HNO₃, which favored the adsorption and activation of NH₃ with the CeO₂/AC-HNO₃ and hence its catalytic performance. The onset and full conversion temperatures of NO_x over the CeO₂/AC-HNO₃ catalyst were 75 and 190℃, respectively, which were significantly lower than those over the CeO₂/AC-air catalyst. However, the activity of the CeO₂/AC-HNO₃ catalyst was easily inhibited by H₂O.

Hydrolysis and carbonization mechanism of cotton fibers in subcritical water

CUI Li-ping, SHI Sheng, HOU Wen-sheng, YAN Zhi-feng, DAN Jin-ming

2018, 33(3): 245-251. doi: 10.1016/S1872-5805(18)60337-3

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Waste cotton fibers were used as the raw material to prepare carbon microspheres by treatment with subcritical water. Results show that the cotton fibers are hydrolyzed, dehydrated and carbonized to form carbon spheres with a graphitic structure in subcritical water with different pH values. The carbon spheres prepared at a pH value of 3.5 have particle sizes of 0.8-3 μm and a carbon content of 74.99%. High temperature and pressure lead to the destruction of the crystal structure of the cotton fibers. The β-1,4 glycosidic linkage in a cellulose molecule is attacked by H⁺ during hydrolysis. The smaller the pH value, the higher the glucose yield in hydrolysis, and the more regular the morphology of the carbon spheres, the greater the carbon content and calorific value of the carbonized products. It is difficult to prepare spherical carbons under high pH values due to the low hydrolysis yields of glucose.

Large-surface-area carbons derived from lotus stem waste for efficient CO₂ capture

WU Xing-xing, ZHANG Cheng-yun, TIAN Zhong-wei, CAI Jin-jun

2018, 33(3): 252-261. doi: 10.1016/S1872-5805(18)60338-5

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Porous carbons with a large number of micropores were obtained by the hydrothermal carbonization of lotus stems followed by KOH activation, and their CO₂ capture performance was evaluated. The influence of KOH/char ratio on the pore texture and surface chemistry of the carbons was investigated by N₂ adsorption, SEM, TEM and elemental analysis. Results indicate that as-prepared carbons had surface areas up to 2 893 m²/g and pore volumes up to 1.59 cm³/g, and that KOH activation increased their surface areas by forming large micropores and small mesopores. The CO₂ uptake in the carbons at ambient pressure was up to 3.85 and 6.17 mmol/g at 25 and 0℃, respectively, which are among the highest values for biomass-derived carbons reported in the literature. The porous carbon with the highest surface area did not show the highest CO₂ uptake. The decisive factor for their CO₂ uptake at ambient pressure is not the surface area but the microporosity and micropore size distribution. This result provides guidance for further searches for porous adsorbents for CO₂ capture. The porous carbons from lotus stem waste have the advantages of low cost and high capture ability for CO₂.

Preparation and thermal properties of a phase change material modified by a functionalized reduced graphene oxide

CHEN Su-qing, HUANG Guo-bo, BAO Jian-she, ZHUGE Chun, YU Jin-na, ZHU Bing-yu

2018, 33(3): 262-267.

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Graphene oxide (GO) prepared by the Hummers method was first functionalized by N,N,N-trimethyl-1-hexadecanaminium bromide (CTAB), then reduced by hydrazine hydrate to obtain CTAB-RGO. The CTAB-RGO was dispersed in a phase change material, decanoic acid-lauryl alcohol (55/45 wt/wt) at 40℃ under sonication. The latent heat and thermal conductivity of the CTAB-RGO modified phase change material were investigated. Results show that the addition of CTAB-RGO increases its latent heat and thermal conductivity. The latent heat and thermal conductivity are 164.7 J/g and 0.94 W/(m·K), respectively when 1% CTAB-RGO is added, which correspond to increases of 22% and 184% compared with those of the unmodified material.

The effect of Ar ion beam irradiation on mesocarbon microbead-densified graphite as the matrix of fuel elements in molten-salt nuclear reactors

XU Liu-jun, WANG Hao-ran, LIN Jun, XU Hong-xia, ZHANG Feng, ZHONG Ya-juan, ZHU Zhi-yong, GUO Quan-gui

2018, 33(3): 268-275.

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Mesocarbon microbeads (MCMBs) were added to the precursors of a traditional matrix A3-3 graphite to increase its density for use as the matrix of fuel elements to inhibit the impregnation of liquid fluoride salt in molten salt reactors. The physical properties of the densified graphite (MDG) obtained using MCMBs with different sizes were compared with those of the A3-3 graphite. Ar ion-beam irradiation by a 4 MV accelerator was used to obtain surface damage to ~1 μm depth in the specimens. Results show that the median pore sizes of the MDGs increase from 530 to 644 nm with the size of MCMBs increasing from 2 to 16 μm, but all are smaller than that of the A3-3 graphite (924 nm). The density is increased by adding MCMBs. The MDG prepared from MCMBs with a size of 2 μm has the highest resistance to irradiation hardening. Both the A3-3 graphite and the MDGs become amorphous at an Ar ion beam dose of 1.47 dpa.

Rapid and green synthesis of fluorescent carbon dots from starch for white light-emitting diodes

ZHENG Jing-xia, LIU Xing-hua, YANG Yong-zhen, LIU Xu-guang, XU Bing-she

2018, 33(3): 276-288. doi: 10.1016/S1872-5805(18)60339-7

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A simple and green synthesis of fluorescent carbon dots (CDs) was achieved by a microwave-assisted hydrothermal method using potato starch as the carbon source. CDs with a maximum quantum yield of 2.46% were prepared by hydrothermal treatment of a 1 mg/mL starch solution at 220℃ for 30 min. Nitrogen doped CDs (N-CDs) were obtained under the same conditions using starch and ethylenediamine as the carbon and nitrogen sources, respectively, and their quantum yield was twice that of the undoped CDs. Both the CDs and N-CDs exhibit excellent water solubility and good thermal stability, emit blue fluorescence under UV light, but the fluorescent intensity of the latter is obviously higher than that of the former. Two white light-emitting diodes using the CD/starch composite and N-CDs as the phosphors emit yellowish white light and white light, respectively with xy coordinates of (0.38, 0.45) and (0.33, 0.35) in the Chromaticity Diagram of the Commission Internationale de L'Eclairage, suggesting that they have great potential application in optoelectronic devices.

2018 Vol. 33, No. 3

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