2017 Vol. 32, No. 3

Graphical Contents
2017, 32(3): .
Abstract(169) PDF(374)
Abstract:
Nuclear graphite for high temperature gas-cooled reactors
ZHOU Xiang-wen, TANG Ya-ping, LU Zhen-ming, ZHANG Jie, LIU Bing
2017, 32(3): 193-204. doi: 10.1016/S1872-5805(17)60116-1
Abstract(912) PDF(533)
Abstract:
Since its first successful use in the CP-1 nuclear reactor in 1942,nuclear graphite has played an important role in nuclear reactors especially the high temperature gas-cooled type (HTGRs) owing to its outstanding comprehensive nuclear properties.As the most promising candidate for generation IV reactors,HTGRs have two main designs,the pebble bed reactor and the prismatic reactor.In both designs,the graphite acts as the moderator,fuel matrix,and a major core structural component.However,the mechanical and thermal properties of graphite are generally reduced by the high fluences of neutron irradiation of during reactor operation,making graphite more susceptible to failure after a significant neutron dose.Since the starting raw materials such as the cokes and the subsequent forming method play a critical role in determining the structure and corresponding properties and performance of graphite under irradiation,the judicious selection of high-purity raw materials,forming method,graphitization temperature and any halogen purification are required to obtain the desired properties such as the purity and isotropy.The microstructural and corresponding dimensional changes under irradiation are the underlying mechanism for the changes of most thermal and mechanical properties of graphite,and irradiation temperature and neutron fluence play key roles in determining the microstructural and property changes of the graphite.In this paper,the basic requirements of nuclear graphite as a moderator for HTGRs and its manufacturing process are presented.In addition,changes in the mechanical and thermal properties of graphite at different temperatures and under different neutron fluences are elaborated.Furthermore,the current status of nuclear graphite development in China and abroad is discussed,and long-term problems regarding nuclear graphite such as the sustainable and stable supply of cokes as well as the recycling of used material are discussed.This paper is intended to act as a reference for graphite providers who are interested in developing nuclear graphite for potential applications in future commercial Chinese HTGRs.
Synthesis of tantalum carbide from multiwall carbon nanotubes in a molten salt medium
CUI Zheng-wei, LI Xuan-ke, CONG Ye, DONG Zhi-jun, YUAN Guan-ming, ZHANG Jiang
2017, 32(3): 205-212. doi: 10.1016/S1872-5805(17)60117-3
Abstract(504) PDF(526)
Abstract:
Tantalum carbide (TaC) nanofibers and coatings were synthesized using multiwall carbon nanotubes (MWCNTs) with different structures as templates and the carbon source in a KCl-LiCl molten salt mixture (41.2/58.8 mol/mol).The TaC and MWCNTs were characterized by scanning electron microscopy,transmission electron microscopy,X-ray diffraction and selected area electron diffraction.Results indicate that the microstructure of the MWCNTs has a distinct influence on the formation of a TaC coating on the MWCNTs.MWCNTs heat-treated at 2 900℃ have a higher crystallinity and are harder to react with Ta to form TaC than those without the heat-treatment.The formation of TaC nanofibers or TaC coatings on MWCNTs is dependent on the molar ratio of tantalum to carbon nanotubes.The morphology of the polycrystalline cubic TaC nanofibers and the TaC coating is similar to that of MWCNTs.The reaction time and temperature have a great influence on the conversion of carbon to TaC and its crystallite size.
Fabrication of interconnected mesoporous carbon sheets for use in high-performance supercapacitors
WANG Xiao-ting, MA Hao, HE Xiao-jun, WANG Jing-xian, HAN Jiu-feng, WANG Yong
2017, 32(3): 213-220. doi: 10.1016/S1872-5805(17)60118-5
Abstract(384) PDF(515)
Abstract:
Interconnected mesoporous carbon sheets (IMCSs) for use in supercapacitors were fabricated from coal tar pitch using a metal-organic framework (MOF-5) template method combined with KOH activation.The samples were characterized by transmission electron microscopy,N2 adsorption,X-ray diffraction and X-ray photoelectron spectroscopy.Results show that the specific surface area of IMCSs is in the range of 860 to 1 046 m2·g-1.The IMCS prepared under the optimal conditions presents a high specific capacitance of 242 F·g-1 at 0.05 A·g-1 in a 6 M KOH aqueous electrolyte.It also exhibits an excellent rate capability with a 80.2% capacitance retention rate as the current density increases from 0.05 to 20 A·g-1,and a cycle stability with a 94.2% capacitance retention rate after 10 000 charge-discharge cycles.The good electrochemical performance is ascribed to small hierarchical pores for ion fast transport,abundant accessible micropores for ion storage and interconnected structure for high electron conductivity.This work provides a simple method for the synthesis of IMCSs from cheap polycyclic aromatic hydrocarbons,including coal tar pitch and petroleum pitch,for use in high-performance supercapacitors.
Effect of annealing temperature on the mechanical properties of flexible graphene films
SONG Ning-jing, LU Chun-xiang, CHEN Cheng-meng, MA Can-liang, KONG Qing-qiang
2017, 32(3): 221-226. doi: 10.1016/S1872-5805(17)60119-7
Abstract(526) PDF(512)
Abstract:
Free-standing graphene films (GFs) as functional materials require high mechanical performance for a convenient industrial processing.GFs were prepared from graphene oxide films by annealing temperatures from 1 300 to 1 700℃,and were characterized by AFM,XRD,SEM,Raman spectroscopy and mechanical testing.Results indicate that the samples have a maximum tensile strength and fracture strain of 22.41 MPa and 2.44%,respectively and a minimum thermal conductivity of 744 Wm-1·K-1 for an annealing temperature at 1 500℃.This is related to physical interlocking which results,from surface wrinkles of the graphene layers.
Performance of pitch and glucose pyrocarbons for reversible sodium storage
DONG Wei, YANG Shao-bin, SHEN Ding, WANG Xiao-liang, LI Si-nan, SUN Wen
2017, 32(3): 227-233.
Abstract(545) PDF(645)
Abstract:
Pitch and glucose pyrocarbons were prepared by the carbonization of petroleum pitch and glucose at 800℃ under a nitrogen flow and were used as the electrodes of sodium ion batteries.Results show that the carbon interlayer spacing,and the oxygen content of the glucose carbon are much larger than those of the pitch carbon,while the degree of crystallinity is smaller.The first discharge capacities of the glucose and pitch carbons are 171.9 and 79.2 mAh/g,and their capacity retention rates after 20 cycles are 94.6 and 68.2%,respectively.Cyclic voltammetry analysis shows that the potential of the irreversible reduction peak in the low potential range of the pitch pyrocarbon is lower than that of the glucose pyrocarbon.AC impedance analysis shows that the impedance of a solid-electrolyte interface film,the interface impedance and the diffusion resistance of the glucose pyrocarbon are significantly lower than that of the pitch pyrocarbon.
Preparation of diameter-controlled multi-wall carbon nanotubes by an improved floating-catalyst chemical vapor deposition method
LI Ya-juan, MA Chang, KANG Jian-li, SHI Jing-li, SHI Qiang, WU Da-heng
2017, 32(3): 234-241. doi: 10.1016/S1872-5805(17)60120-3
Abstract(573) PDF(589)
Abstract:
Multi-wall carbon nanotubes (MWCNTs) with controlled diameters were synthesized by an improved-floating catalyst chemical vapor deposition method,using toluene and ferrocene as a carbon source and catalyst precursor,respectively.Ferrocene was sublimed in a heater and carried as a gas mixed with toluene vapor into a reactor where MWCNTs were formed.The effects of the sublimation temperature,hydrogen content in the gas mixture and gas flow rate on the diameter and diameter distribution of the CNTs formed were investigated.Results indicated that the CNT diameter distributions could be controlled by changing the sublimation temperature.The higher the sublimation temperature,the narrower the distribution of CNT diameters.The average CNT diameter decreased and levelled off with increasing hydrogen content in the gas mixture from 0 to 40 vol%.The CNT diameter decreased with increasing gas flow rate.
An insight into the superior performance of a gold nanocatalyst on single wall carbon nanotubes to that on titanium dioxide and amorphous carbon for the green aerobic oxidation of aromatic alcohols
Anne E Shanahan, Mary McNamara, James A Sullivan, Hugh J Byrne
2017, 32(3): 242-251. doi: 10.1016/S1872-5805(17)60121-5
Abstract(417) PDF(329)
Abstract:
Gold nanocomposites based on three supports,single wall carbon nanotubes,carbon black and TiO2,were prepared using an in-situ reduction technique and characterized.They were tested for their suitability as heterogeneous catalysts in the green aerobic oxidation of 1-phenylethanol,2-phenylethanol and benzylalcohol of industrial importance.For all reactions,the use of single wall carbon nanotubes as supports resulted in superior reaction efficiency and specificity for aldehyde to that of TiO2 and carbon black.The gold nanocatalysts can be reused over several reaction cycles with a minimal degeneration in catalytic activity.The activity of the gold nanoparticle catalyst was related to the shape and size of the gold particles and the properties of the support.The selectivity was ascribed to the functional groups on the substrate,the properties of the supports and the particle size distributions of the gold nanoparticles.
Suspension of carbon nanotubes in natural humic acid water
WEI Chao-xian, ZHANG Huang, ZHANG Di, YANG Xiao-lei
2017, 32(3): 252-257.
Abstract(453) PDF(513)
Abstract:
The widespread use of carbon nanotubes (CNTs) may cause them to be released into the environment.Their interaction with natural organic matter enhances their dispersion in water,leading to a severe environmental threat.In this study,the effect of dissolved humic acid (HA) on the dispersion of graphitized or hydroxylated CNTs in water was investigated by repeated dispersion of both CNTs in HA water with a concentration of 100 mg/L for 19 times.Results showed that the cumulative adsorption amounts of HA on the graphitized CNTs increased and leveled off,but those on the hydroxylated ones increased continually with the number of dispersions.The amounts of graphitized CNTs suspended in HA water were always lower than those of the hydroxylated ones,which reached a maximum after 4 of 5 dispersions.TEM images showed that hydroxylated CNTs were shorter than the graphitized ones and the suspended CNTs were shorter than the un-suspended ones for both types of CNTs,indicating that CNTs with more defects are easily suspended.
Synthesis and characterization of Al2O3-C hybrid aerogels by a one-pot sol-gel method
ZHANG Rui, JIANG Ning, DUAN Xiao-jia, JIN Shuang-ling, JIN Ming-lin
2017, 32(3): 258-264. doi: 10.1016/S1872-5805(17)60122-7
Abstract(388) PDF(495)
Abstract:
Al2O3-C hybrid aerogels were prepared by a one-pot sol-gel method,supercritical n-hexane drying and carbonization using Al (NO3)3·9H2O as the Al2O3 source,resorcinol (R) and furfural (F) as carbon precursors,and propylene oxide (PO) as a gelation initiator.The effects of the Al2O3 contents,the R+F concentrations and the PO/Al molar ratios on the porosity of the hybrid aerogels were investigated using a constant R/F molar ratio of 0.5.It was found that the hybrid aerogels are all mesoporous with an average pore size below 20 nm.The crystalline structure of Al2O3 is γ-type,but its diffraction peaks are quite broad.The hybrid aerogels are monolithic when the Al2O3 content is below 5.31 wt% with an R+F concentration of 10 g/100 mL.The mesopore volume and size,BET surface area and external surface area all increase with increasing PO/Al molar ratio under otherwise identical conditions.The volume shrinkage decreases,carbonization yield increases and the density of the hybrid aerogel exhibits a maximum with an Al2O3 content of 4.93 wt%.The size and volume of the mesopores,and the external surface area decrease with the increasing R+F concentration at PO/Al ratios of 5 and 6,but increase with increasing R+F concentration at a PO/Al ratio of 4.
Ablation behavior of a three-dimensional C/C-HfC composite prepared by a precursor infiltration and pyrolysis method
XUE Liang, SU Zhe-an, YANG Xin, HUANG Qi-zhong
2017, 32(3): 265-270.
Abstract(496) PDF(501)
Abstract:
A three-dimensional C/C-HfC composite with a density of 1.95 g/cm3 was fabricated by chemical vapor infiltration of carbon into a 3D woven carbon fiber felt to a density of 1.45 g/cm3 followed by vacuum impregnation and pyrolysis of a solution containing the HfC precursor.Results indicate that HfC particles are uniformly dispersed around the pyrocarbon.The 3D C/C-HfC composite exhibits a good ablation resistance at 2573 K.The mass and linear ablation rates after ablation for 120 s are 0.001 5 g/s and 0.002 4 mm/s,respectively.The resistance to ablation is attributed to the introduction of HfC into the C/C composite.The tree-coral-like HfO2 particles formed during the ablation act as thermal and oxygen diffusion barriers,protecting the composite from further ablation.Also,the oxidation of HfC and the volatilization of the ablation product (CO) absorb a large amount of heat from the composite.
A carbon fiber network/polypropylene composite with a low thermal expansion coefficient and high stiffness
YUAN Jian-min, FENG Yan-rong, WU Zhen-jun, WANG Yan-jun, LI Si-yu, SUN Ping
2017, 32(3): 271-276.
Abstract(365) PDF(580)
Abstract:
A carbon fiber network reinforcement (CFNR) was prepared by bonding short carbon fibers with phenol formaldehyde resin followed by carbonization.The CFNR was vacuum-impregnated with molten polypropylene (PP) to produce a CFNR/PP composite with a low thermal expansion coefficient and high stiffness.The microstructure and thermal mechanical properties of the composite were characterized by scanning electron microscopy and thermal mechanical tests.Results show that CFNR is more effective in improving the stiffness and thermal dimensional stability of polymer matrix composites than short carbon fibers.The deformation rate of a conventional short carbon fiber (SCF)/PP composite is 2.3 times higher than that of the CFNR/PP composite under the same load.The decreasing order of the storage and bend moduli is CFNR/PP > SCF/PP > PP.The bend moduli of the CFNR/PP composite at 30 and 110℃ are about 1.8 and 2.5 times that of the SCF/PP composite,respectively.The average thermal expansion coefficient of the CFNR/PP composite between 30 to 120℃ is 25% of that of the SCF/PP composite.
Simultaneous detection of trace Cd (II),Pb (II),Cu (II),Hg (II) ions by a boron-doped diamond electrode
GAO Cheng-yao, TONG Jian-hua, BIAN Chao, XIA Shan-hong
2017, 32(3): 277-283.
Abstract(531) PDF(692)
Abstract:
A boron-doped diamond (BDD) thin-film electrode was prepared and used for the simultaneous detection and quantification of Cd (II),Pb (II),Cu (II) and Hg (II) ions using an anodic stripping voltammetry method.The BDD thin film was prepared by a hot filament chemical vapor deposition technique and used as the working electrode in the detection.The influence of experimental parameters,such as scanning mode,working electrode,supporting electrolyte,pH,boron concentration in the BDD electrode,accumulation potential and accumulation time were investigated.Under the optimized conditions,the stripping peak currents showed a good linear relationship with the concentrations of the four heavy metal ions from 1 to 7 ppb,and the results were very reproducible.
Fourier transform infrared and dynamic thermomechanical analyses of mesophase pitch fibers during oxidative stabilization
SUN Zhu-lin, XI Li-hua, LI Hong, ZENG Fan-long, LU Yong-gen
2017, 32(3): 284-288.
Abstract(612) PDF(533)
Abstract:
The stress in and elongation of mesophase pitch fibers during oxidative stabilization were investigated by a dynamic thermo-mechanical analyzer under a fixed elongation and strainload,respectively.Their functional groups at different temperatures were characterized by Fourier transformed infrared spectroscopy.Results indicated that when the temperature was below 175℃,a distinct decrease in the stress in the fibers and the number of hydroxyl groups was found,accompanied by dehydrogenation reactions.When the temperature was increased above 250℃,the number of ether bonds and the stress increased,and elongation decreased due to cross-linking reactions.When the temperature was increased further,above 270℃,the fibers began to shrink because of the increase in the extent of stabilization.It was found,based on dynamic thermo-mechanical data,that the effect of gravity on the fiber cannot be neglected in designing a stabilization furnace and a vertical furnace is superior to a horizontal one.