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2015, 30(3): .

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Three dimensional graphene networks for supercapacitor electrode materials

LI Chen, ZHANG Xiong, WANG Kai, ZHANG Hai-tao, SUN Xian-zhong, MA Yan-wei

2015, 30(3): 193-206. doi: 10.1016/S1872-5805(15)60185-8

Abstract(1047)

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Three dimensional graphene networks (3DGNs) are promising electrode materials in electrochemical applications because they provide short diffusion pathways for electrolyte ions and fast transport channels for electrons, and act as an ideal scaffold for forming composites with pseudocapacitive materials to obtain a synergistic effect. This review offers an overview of recent advances in the synthesis of 3DGNs and their composites as electrodes in supercapacitors. In particular, methods to enhance the capacitive performance of 3DGN-based supercapacitors are summarized. Prospects for the future development of 3DGNs are presented at the end.

Time-dependent electrical properties of carbon nanotube yarns

H. E. Misak, S. Mall

2015, 30(3): 207-213. doi: 10.1016/S1872-5805(15)60186-X

Abstract(632)

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Carbon nanotubes (CNTs) are projected as a desirable option to transmit electrical power where specific conductivity is an important consideration. A CNT-yarn was therefore characterized for its ability to transmit power as a function of time. A current-voltage (I-V) relationship was established that showed three distinct regions: linear, non-linear and finally its degradation. The linear region shows a low and constant resistance. The output current did not change with time when the applied constant voltage was in the linear region of the I-V relationship. However, when the applied constant voltage was in the non-linear region the current decreased exponentially and leveled off with time. Under constant current tests, voltage increased only when current level was in the non-linear region. These time-dependent electrical conducting properties can be accounted for by short circuits occurring from the development of a localized red-hot spots in the CNT-yarn in the non-linear region, leading to thermal degradation, as revealed by thermal imaging of the yarn, thermal gravimetric analysis of the CNTs, and SEM images and EDAX of the thermally degraded CNT-yarn.

Synthesis of SWCNT-GO/MnO₂ nanocomposites for use as electrodes of electrochemical capacitors by microwave and hydrothermal methods

WANG Ying-hui, QIU Han-xun, WANG Zhao, LI Jing, SHEN Xiao, YANG Jun-he

2015, 30(3): 214-221.

Abstract(854)

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HNO₃-oxidized single-wall carbon nanotubes (SWCNTs) and graphene oxide (GO) were ultrasonically dispersed in water to which potassium permanganate was added, and the resulting mixtures were treated under microwave radiation at 100℃ for 1h or hydrothermal conditions at 100℃ for 1, 5 and 10h to prepare SWCNT-GO/MnO₂ composites as electrode materials of electrochemical capacitors. Results indicate that the composite prepared using the microwave method has a higher specific capacitance (173F/g at 0.2A/g), lower pseudo-charge transfer resistance (1.425 Ω) and longer cyclic stability (3.74% capacitance loss after 1000 cycles at the scan rate of 20mV/s) than the ones prepared using the hydrothermal method. Moreover, the microwave method has the advantages of simplicity of operation and shorter heating time.

Synthesis and characterization of silicon carbide nanowires from lignin-phenolic resin and silicon powder with an in-situ formed molten salt as catalyst

WANG Fu-cheng, ZHAO Lei, FANG Wei, HE Xuan, LIANG Feng, CHEN Hui, CHEN Huan, DU Xing

2015, 30(3): 222-229. doi: 10.1016/S1872-5805(15)60187-1

Abstract(809)

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Silicon carbide (SiC) nanowires were synthesized using lignin-phenolic resin (LPR) and silicon powder as raw materials. The morphology and structures of the nanowires were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The influence of reaction conditions on the growth of the nanowires was investigated by thermodynamic analysis. It was revealed that the growth of the nanowires starts at around 1 100 ℃ by a vapor-liquid-solid (VLS) mechanism and the growth temperature is lower than that for the formation of SiC nanowires from commercial phenolic resin and silicon powder. The nanowires have diameters of 30-100 nm and lengths of several microns. It was revealed that the inorganic salts are formed in-situ from LPR in the lignin segment as liquid catalyst droplets during pyrolysis. These dissolve SiC and become supersaturated, from which SiC nanowires grow along the [1 1 1] direction by the stacking of (1 1 1) planes.

Structural characterization and photoluminescence properties of SiC nanowires prepared by microwave method

HUANG Shan, WANG Ji-gang, LIU Song, ZHANG Yue-chen, QIAN Liu, LIANG Jie

2015, 30(3): 230-235.

Abstract(614)

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SiC nanowires were synthesized by a microwave-heating method at 1480 ℃ for 80 min under vacuum, using silicon powder, silica dioxide powder and artificial graphite as raw materials. SEM, TEM and XRD were used to investigate the microstructure of the samples and excitation light with wavelength of 240 nm was used to test the photoluminescence properties of the products. Results indicated that β-SiC can be synthesized directly without using a catalyst by the vapor-solid growth mechanism. The samples exhibited different morphologies and sizes at different zones due to the temperature differences. The products in an upper crucible were bright-green, relatively pure SiC, consisting of mainly nano-rods with a diameter of about 150 nm and small amount of SiC micro-crystals, and surface oxidation was not obvious. The products in other zones were grey-green with lots of SiC/SiO₂ coaxial nanowires with a diameter around 20-50 nm and a SiO₂ surface layer of thickness about 2 nm, and there was also some un-reacted graphite and silica dioxide. Both the SiC nano-rods and SiC/SiO₂ coaxial nanowires exhibited a strong broad photoluminescence peak at a wavelength of about 390 nm and a high degree of blue-shift compared with the reported luminescence of β-SiC nano-materials.

Improving the oxidation resistance of carbon fibers using silicon oxycarbide coatings

XIA Ke-dong, LU Chun-xiang, YANG Yu

2015, 30(3): 236-243. doi: 10.1016/S1872-5805(15)60188-3

Abstract(837)

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Silicon oxycarbide (SiOC) ceramic was coated on carbon fibers using a vinyl group-modified silicon alkoxide as the sol precursor. The coatings were characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and thermogravimetry. The mechanical properties of the carbon fibers were investigated by single fiber tensile tests. Results showed that the amorphous SiOC coating was composed of SiC_xO_4-x and a free carbon phase. The oxidation resistance of the carbon fibers was improved by the SiOC coatings. However, debonding of the coatings and surface cracks led to a reduction of tensile strength and an increase of Weibull modulus. The coating with a thickness of 200 nm increased the onset oxidation temperature by about 150℃, but decreased the tensile strength from 3.18 to 2.32 GPa.

Potato starch-derived almond-shaped carbon nanoparticles for non enzymatic detection of sucrose

Soma Das, Mitali Saha

2015, 30(3): 244-251. doi: 10.1016/S1872-5805(15)60189-5

Abstract(568)

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We report the formation of almond-shaped carbon nanoparticles (ASCNs) from peeled potatoes and the fabrication of a highly sensitive and a selective non enzymatic sucrose sensor based on this carbon nanoparticle electrode. The potato was pyrolyzed initially at 400-500 ℃ in vacuum, followed by slow heating at around 800 ℃, which produces the ASCNs. The ASCNs were examined by SEM, XRD, EDX and AFM and were further characterized by fluorescence microscopy, which clearly suggested their fluorescent nature. Electrochemical detection of sucrose was examined by cyclic voltammetry, differential pulse voltammetry and linear sweep voltammetry in an acidic solution. The new sensor shows a good response towards the sucrose oxidation, with a wide linear range (R² = 0.996 79), a high sensitivity of ~41.737 25±0.01 μA·M^-1·cm^-2and a low detection limit of 1 μmol/L. Moreover, it is also stable and has a short response time (9 s).

Preparation of activated carbon from carbonized rice husk by ozone activation for Cr(VI) removal

Sivaraju Sugashini, Kadhar Mohamed Meera Sheriffa Begum

2015, 30(3): 252-261. doi: 10.1016/S1872-5805(15)60190-1

Abstract(807)

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Activated carbon (AC) was prepared from carbonized rice husks using ozone as an activating agent. The AC was characterized by nitrogen adsorption, SEM-EDAX and FT-IR. The Brunauer-Emmett-Teller surface area of the carbons was increased from 20 to 380 m²/g by the activation. It was observed that the silica attached to the carbonaceous material is loosened, leading to a release of carbon during the ozone activation. Ozone exists as both molecular and atomic oxygen on the surface of carbon. Atomic oxygen, as a powerful oxidizing agent, oxidizes the carbon surface into acidic functional groups such as carboxylic, ketonic and phenolic. The rice husk AC was used for the adsorption of Cr (VI) ions. A maximum removal percentage (94%) of Cr (VI) ions was obtained for a 100 mg/L aqueous solution at the optimized conditions of pH value of 2.0, adsorbent dosage of 0.2 g, time of 2.5 h and stirring speed of 300 r/min. Adsorption equilibrium and kinetic models were used to investigate the adsorption mechanism. It was found that the adsorption isotherms were well fitted by the Freundlich equation. The adsorption rate follows pseudo second order kinetics and the adsorption is spontaneous and exothermic. It is possible to recover Cr and carbon by NaOH stripping.

Preparation and thermal properties of graphite foam/eutectic salt composite as a phase change energy storage material

GUO Cha-xiu, HU Gao-lin, LUO Zhi-jun

2015, 30(3): 262-268.

Abstract(592)

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A graphite foam/eutectic salt composite was prepared by the infiltration of the foam by a molten binary eutectic nitrate (KNO₃/NaNO₃) to improve its thermal conductivity as a phase change energy storage material. The thermal properties and stability of the composite and the eutectic salt were investigated by differential scanning calorimetry and Raman spectroscopy. Results indicate that infiltration using a molten salt is an effective method to prepare the composite. The phase change temperature of the composite (221.3℃) is similar to that of the salt (222.4℃), and its latent heat is 3.74% lower. The composite has a thermal conductivity 102 times higher than the pure eutectic salt, because of the high thermal conductivity of the graphite. The microstructure of the composite remains unchanged after 100 phase change cycles.

Adsorption dynamics of phenol in a fixed bed packed with activated carbon and stainless steel fiber-reinforced activated carbon paper

SHAO Yan, YAN Ying, ZHANG Hui-ping

2015, 30(3): 269-274.

Abstract(789)

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Stainless steel fiber-reinforced activated carbon paper was prepared by a wet papermaking method, followed by a high temperature treatment, using stainless steel fibers as the reinforcement and coniferous wood pulp cellulose as the binder. A fixed bed for phenol adsorption was made by first packing granular activated carbon to a depth of 13 cm followed by 2 cm of the as-made paper near the outlet. The adsorption dynamics of phenol in the bed were investigated under different experimental conditions. Results show that the breakthrough curve of phenol in the bed is steeper than that in a bed packed only with activated carbon with the same bed height. The breakthrough time of phenol in the bed decreases and the breakthrough curves become sharper with increasing flow rate and inlet concentration. The length of the unused bed decreases by 14% compared with the bed packed with activated carbon only, indicating that mass transfer in the bed and its utilization ratio are improved by the paper packing.

Microstructure of a pyrolytic carbon coating on a nuclear graphite substrate IG-110

FENG Shang-lei, YANG Ying-guo, BAI Shuo, XU Li, YANG Xin-mei, XIA Hui-hao, ZHOU Xing-tai

2015, 30(3): 275-281.

Abstract(776)

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The molten salt reactor (MSR) is one of the six Generation IV reactors that is being reexamined today, owing to its unique fuel cycle capabilities and safety characteristics. IG-110 nuclear graphite a candidate material for constructing a MSR. However, the existence of large pores at its surface is a big problem due to the impregnation of molten salts and the diffusion of fission product gases into the graphite through the pores. A pyrolytic carbon (PyC) coating can act as a barrier coating on the nuclear graphite. Investigation of the microstructure and growth characteristics of PyC is very important for an understanding of the relationship between microstructure and performance. In this study, polarized light microscopy, scanning electron microscopy, transmission electron microscopy and synchrotron based grazing incidence X-ray diffraction were used to study the microstructure and growth characteristics of the PyC coating. Results show that the PyC coating shows three growth cones (a large cone, a smaller one and a regenerative cone) and exhibits a wave-like layered structure. The resulting structure is fairly dense. There are two kinds of textures in the PyC coating: smooth laminar and regenerative laminar, each of which contains two crystal structures with different interlayer spacings. The smooth laminar carbon has mostly a low degree of graphitization, while the regenerative laminar structure mainly has a high degree of graphitization. The PyC coating is a perfect barrier to gas infiltration due to its compact structure and it containing only nanopores rather than large pores.

Electrodeposition of an aluminum coating on a graphite surface from a molten AlCl₃-NaCl-KCl mixture

CAO Rui-xiong, TAO Ze-chao, WANG Hong-bao, GUO Quan-gui

2015, 30(3): 282-288.

Abstract(846)

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An electrochemical method was used to deposit aluminum on a graphite plate in a molten NaCl-KCl-AlCl₃ mixture with a weight ratio of 1:1:8 to form a coatings with different thicknesses. The thickness and morphology of the coating were controlled by the current density and electrochemical deposition time. Results indicated that the thickness of the coating increased with deposition time up to 240 min at a current density of 1.06 A/dm² and a dendritic structure coating was formed by increasing the deposition time beyond 300 min. The greater the current density, the faster the deposition rate. When the current density was increased to 3.28 A/dm², the thickness of the coating reached a maximum of 148 μm for an electrochemical deposition time of 120 min. Excellent adhesion strength between the aluminum coating and graphite substrate was proved by thermal shock resistance and scratch hardness tests. The thermal conductivity of the samples was increased significantly from an initial value of 115.7 to 199.0 W/(m·K).

2015 Vol. 30, No. 3

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