R_2Fe_(17)的气相碳化作用

R_2Fe_(17)系与丁烷在780K 发生反应生成中间碳化物 R_2Fe_(17)C_(3-δ)后,其居里温度提高了约330K。只有Sm 碳化物在室温表现出易轴各向异性,各向异性场为10T。Er 和 Tm 碳化物在较低的温度下呈自旋重新取向。

碳化钨粉制备新技术

介绍了乌克兰国家科学院超硬材料研究所VPBondarenko教授研究小组开发的一种气相法制备WC粉新技术。该技术不采用固体炭粉为原料,而用CH4为原材料,将W粉经气相碳化后,制得含C量可控制的WC粉末,再经常规压制、烧结后制成硬质合金,产品性能符合前苏联国家标准(GOST)3882—74。并对该技术的生产基本原理、粉末特性、及所制取合金的性能作了重点介绍。

Improved Epitaxy of 3C-SiC Layers on Si(100) by New CVD/LPCVD System

Single crystalline 3C-SiC epitaxial layers are grown on φ 50mm Si wafers by a new resistively heated CVD/LPCVD system,using SiH_4,C_2H_4 and H_2 as gas precursors.X-ray diffraction and Raman scattering measurements are used to investigate the crystallinity of the grown films.Electrical properties of the epitaxial 3C-SiC layers with thickness of 1～3μm are measured by Van der Pauw method.The improved Hall mobility reaches the highest value of 470cm 2/(V·s) at the carrier concentration of 7.7×10 17 cm -3 .

In-situ homogeneous synthesis of carbon nanotubes on aluminum matrix and properties of their composites

Using nickel catalyst supported on aluminum powders, carbon nanotubes （CNTs） were successfully synthesized in aluminum powders by in-situ chemical vapor deposition at 650 ℃. Structural characterization revealed that the as-grown CNTs possessed higher graphitization degree and straight graphite shell. By this approach, more homogeneous dispersion of CNTs in aluminum powders was achieved compared with the traditional mechanical mixture methods. Using the in-situ synthesized CNTs/Al composite powders and powder metallurgy process, CNTs/Al bulk composites were prepared. Performance testing showed that the mechanical properties and dimensional stability of the composites were improved obviously, which was attributed to the superior dispersion of CNTs in aluminum matrix and the strong interfacial bonding between CNTs and matrix.

Modification of ACFs by chemical vapor deposition and its application for removal of methyl orange from aqueous solution

Viscose activated carbon fibers （ACFs） were characterized using specific surface area, scanning electron modified with chemical vapor deposition （CVD）. The samples were microscopy （SEM）, pore size distribution and Fourier transform infrared spectroscopy （FTIR）. Batch adsorption experiments were carried out to investigate the adsorption behavior of modified ACFs for methyl orange（MO） from its aqueous solutions. The results show that the adsorption isotherms of MO onto modified ACFs well follows the Langmuir isotherm equation. The adsorption kinetics of MO can be well described by the pseudo second-order kinetic model. The adsorption process involves the intra-particle diffusion, but is not the only rate-controlling step. Thermodynamic parameters including AG, AH and AS were calculated, suggesting that the adsorption of MO onto modified ACFs is a spontaneous, exothermic and physisorption process. FTIR result indicates that the major adsorption mechanism of modified ACFs for MO is hydrogen bond.

Improved oxidation resistance of chemical vapor reaction SiC coating modified with silica for carbon/carbon composites

To protect carbon/carbon (C/C) composites from oxidation, a SiC coating modified with SiO2 was prepared by a complex technology. The inner SiC coating with thickness varying from 150 to 300 μm was initially coated by chemical vapor reaction (CVR): a simple and cheap technique to prepare the SiC coating via siliconizing the substrate that was exposed to the mixed vapor (Si and SiO2) at high temperatures (1 923?2 273 K). Then the as-prepared coating was processed by a dipping and drying procedure with tetraethoxysilane as source materials to form SiO2 to fill the cracks and holes. Oxidation tests show that, after oxidation in air at 1 623 K for 10 h and thermal cycling between 1 623 K and room temperature 5 times, the mass loss of the CVR coated sample is up to 18.21%, while the sample coated with modified coating is only 5.96%, exhibiting an obvious improvement of oxidation and thermal shock resistance of the coating. The mass loss of the modified sample is mainly contributed to the reaction of C/C substrate with oxygen diffusing through the penetrating cracks formed in thermal shock tests.

Chemical vapor deposition of SiC at different molar ratios of hydrogen to methyltrichlorosilane

Chemical vapor deposition(CVD) of SiC from methyltrichlorosilane(MTS) was studied at two different molar ratios of H2 to MTS(n(H2) /n(MTS) ) . The total pressure was kept as 100 kPa and the temperature was varied from 850 to 1 100 ℃ at a total residence time of 1 s. Steady-state deposition rates as functions of reactor length and of temperature,investigated at different n(H2) /n(MTS) values,show that hydrogen exhibits strongly influences on the deposition rate. Especially,the deposition of Si co-deposit can be obtained in broader substrate length and at higher temperatures with increasing hydrogen partial pressure. Influence of hydrogen on the deposition process was also studied using gas phase composition and deposit composition analysis at various n(H2) /n(MTS) . SEM micrographs directly show the variation of surface morphologies at various n(H2) /n(MTS) . It can be found that the crystal grain of the deposit at 1 100 ℃ is better developed and the crystallization is also improved with increasing n(H2) /n(MTS) .

Promotion of activation ability of N vacancies to N2 molecules on sulfur-doped graphitic carbon nitride with outstanding photocatalytic nitrogen fixation ability

Nitrogen vacancies and sulfur co-doped g-C3N4 with outstanding N2 photofixation ability was synthesized via dielectric barrier discharge plasma treatment. X-ray diffraction, ultraviolet–visible spectroscopy, N2 adsorption, scanning electron microscopy, X-ray photoelectron spectroscopy, photoluminescence spectroscopy, and temperature-programmed desorption were used to characterize the as-prepared catalyst. The results showed that plasma treatment cannot change the morphology of the as-prepared catalyst but introduces nitrogen vacancies and sulfur into g-C3N4 lattice simultaneously. The as-prepared co-doped g-C3N4 displays an ammonium ion production rate as high as 6.2 mg·L^-1·h^-1·gcat^-1, which is 2.3 and 25.8 times higher than that of individual N-vacancy-doped g-C3N4 and neat g-C3N4, respectively, as well as showing good catalytic stability. Experimental and density functional theory calculation results indicate that, compared with individual N vacancy doping, the introduction of sulfur can promote the activation ability of N vacancies to N2 molecules, leading to promoted N2 photofixation performance.

Characters of the P_(CO)_2 and CO_2 Flux in the East China Sea in Autumn

P CO 2 of air and seawater samples from the East China Sea (ECS) were measured in situ in autumn, 1994. Ocean currents, terrestrial fluviation, biological activities, etc.,P CO 2 characters in air and seawater were investigated. CO 2 flux and its character in the East China Sea are discussed on the basis of theP CO 2 profiles of air and seawater. It was clear that the nearshore was the source of CO 2; and that the outer sea area was the sink of CO 2; and that the shelf area of the ECS is a net sink for atmospheric CO 2 in autumn.

Kinetics of Vapor-Phase Carbonylation of Ethanol on Ni-Zn/C Catalyst

A novel heterogeneous Ni-Zn/C catalyst was used for vapor-phasecarbonylation of ethanol under at- mospheric pressure. Experimentswere designed with the elimination of mass-transfer resistances. Thedata of primary reactions in the carbonylation were collected with adifferential tubular reactor. Power law rate models were employed toexpress the conversion of ethanol and the yields of ethyl propionateand diethyl ether. The results obtained with the models were inagreement with the experimental data.

Field emission from Si tips coated with nanocrystalline diamond films

The electron field emission from Si tips coated with nanocrystalline diamond films was investigated. The Si tips were formed by plasma etching, and nano-diamond films were deposited on the Si tips by hot filament chemical vapor deposition. The radius of curvature for the Si tips was averagely about 50 nm. The microstructure of the diamond films was examined by scanning electron microscopy and Raman spectroscopy. The field emission properties of the samples were measured in an ion-pumped vacuum chamber at a pressure of 106 Pa. The experimental results showed that the nanostructured films on Si tips exhibited a lower value of the turn-on electric field than those on flat Si substrates. It was found that the tip shape and non-diamond phase in the films had a significant effect on the field emission properties of the films.

In-situ preparation, modulation and mechanism of refractory metal carbide gradient coating

TiC, ZrC and TaC modified layers were in-situ prepared on graphite matrix by chemical vapor infiltration method with metal salts as the activator. Taking the TiC modified layer as an example, through thermodynamic calculation and experiment, the thermal decomposition process of raw materials(Ti/K_(2)TiF_(6)) was analyzed, the formation mechanism of TiC was determined, and the distribution of TiC modified layer was modulated. The results show that activator K_(2)TiF_(6)has higher decomposition temperature than NH4Cl, which is conducive to improving the utilization rate of raw materials in the gas infiltration process. Increasing the content of Ti powder can increase the concentration of reaction gas and contribute to the formation of TiC modified layer. When the molar ratio of Ti to K_(2)TiF_(6)is 3:1, the surface thickness and infiltration depth of Ti C are 5.42 and 136.24 μm, respectively. Increasing the reaction temperature can improve the rate of in-situ reaction and the thickness of TiC surface layer. When the experimental temperature rises to 1600 °C, the TiC surface layer thickness increases to 20.27 μm.

Characteristic evaluation of Al_2O_3/CNTs hybrid materials for micro-electrical discharge machining

The characteristic evaluation of aluminum oxide （A1203）/carbon nanotubes （CNTs） hybrid composites for micro-electrical discharge machining （EDM） was described. Alumina matrix composites reinforced with CNTs were fabricated by a catalytic chemical vapor deposition method. A1203 composites with different CNT concentrations were synthesized. The electrical characteristic of A1203/CNTs composites was examined. These composites were machined by the EDM process according to the various EDM parameters, and the characteristics of machining were analyzed using field emission scanning electron microscope （FESEM）. The electrical conductivity has a increasing tendency as the CNTs content is increased and has a critical point at 5% A1203 （volume fraction）. In the machining accuracy, many tangles of CNT in A1203/CNTs composites cause violent spark. Thus, it causes the poor dimensional accuracy and circularity. The results show that conductivity of the materials and homogeneous distribution of CNTs in the matrix are important factors for micro-EDM of A1203/CNTs hybrid composites.

Effect of nanofibers at surface of carbon fibers on microstructure of carbon/carbon composites during chemical vapor infiltration

Before densification by chemical vapor infiltration,carbon or SiC nanofibers were grown on the surface of carbon fibers by catalytic chemical vapor deposition using electroplated Ni as catalyst.The modification and mechanism of nanofibers on the pyrocarbon deposition during chemical vapor infiltration were investigated.The results show that the nanofibers improve the surface activity of the carbon fibers and become active nucleation centers during chemical vapor infiltration.They can induce the ordered deposition of pyrocarbon and adjust the interface bonding between pyrocarbon and carbon fibers during the infiltration.

Future biomass carbon sequestration capacity of Chinese forests

Chinese forests, characterized by relatively young stand age, represent a significant biomass carbon （C） sink over the past several decades. Nevertheless, it is unclear how forest biomass C sequestration capacity in China will evolve as forest age, climate and atmospheric CO2 concentration change continuously. Here, we present a semi-empirical model that incorporates forest age and climatic factors for each lbrest type to estimate the effects of forest age and climate change on total forest biomass, under three different sce-narios based on the fifth phase of the Coupled Model Intercomparison Project （CMIPS）. We estimate that age-related forest biomass C sequestration to be 6.69 Pg C （~0.17 Pg C a^-1） from the 2000s to the 2040s. Climate change induces a rather weak increase in total forest biomass C sequestration （0.52-0.60 Pg C by tile 2040s）. We show that rising CO2 concentrations could further increase tile total forest biomass C sequestration by 1.68-3.12 Pg C in the 2040s across all three scenarios. Overall, the total forest biomass in China would increase by 8.89-10.37 Pg C by the end of 2040s. Our findings highlight the benefits of Chinese afforestation programs, continued climate change and increasing CO2. concentration in sustaining the forest biomass C sink in the near future, and could therefore be useful for designing more realistic climate change mitigation policies such as continuous forestation programs and careful choice of tree species.

Tailoring the Diameter of Single-Walled Carbon Nanotubes for Optical Applications

Single-walled carbon nanotubes （SWCNTs） with specific diameters are required for various applications particularly in electronics and photonics, since the diameter is an essential characteristic determining their electronic and optical properties. In this work, the selective growth of SWCNTs with a certain mean diameter is achieved by the addition of appropriate amounts of CO2 mixed with the carbon source （CO） into the aerosol （floating catalyst） chemical vapor deposition reactor. The noticeable shift of the peaks in the absorption spectra reveals that the mean diameters of the as-deposited SWCNTs are efficiently altered from 1.2 to 1.9 nm with increasing CO2 concentration. It is believed that CO2 acts as an etching agent and can selectively etch small diameter tubes due to their highly curved carbon surfaces. Polymer-free as-deposited SWCNT films with the desired diameters are used as saturable absorbers after stamping onto a highly reflecting Ag-mirror using a simple dry-transfer technique. Sub-picosecond mode-locked fiber laser operations at -1.56μm and -2 μm are demonstrated, showing improvements in the performance after the optimization of the SWCNT properties.

SnSb@carbon nanocable anchored on graphene sheets for sodium ion batteries

The development of materials with unique nanostructures is an effective strategy for the improvement of sodium storage in sodium ion batteries to achieve stable cycling performance and good rate capability. In this work, SnSb- core/carbon-shell nanocables directly anchored on graphene sheets （GS） were synthesized by the hydrothermal technique and chemical vapor deposition. The simultaneous carbon coating and the encapsulation of SnSb alloy is effective for alleviating the volume-change problem in sodium ion batteries. After optimizing the electrolyte for SnSb in the sodium ion batteries, the optimized coaxial SnSb/carbon nanocable/GS （SnSb/CNT@GS） nanostructure demonstrated stable cycling capability and rate performance in 1 M NaClO4 with propylene carbonate （PC） ＋ 5% fluoroethylene carbonate （FEC）. The SnSb/CNT@GS electrode can retain a capacity of 360 mAh/g for up to 100 cycles, which is 71% of the theoretical capacity. This is higher than in the other three electrolytes tested （1 M NaClO4 in PC, 1 M NaC104 in PC/FEC （1：1 v/v） and 1 M NaPF6 ＋ PC）, and higher than that of the sample without the addition of graphene. The good electrochemical performance can be attributed to the efficient buffering provided by the outer carbon nanocable layer and the graphene inhibiting the agglomeration of SnSb particles, as well as its high conductivity.

Superconductivity in two-dimensionalη-Mo3C2 films

Two-dimensional(2D)superconductors have intriguing physical properties and abundant potential applications.Recently,2D superconductingα-Mo2C and facecentered cubic Mo2C have been controllably prepared and they bring new viewpoints to carbon-based superconductivity.Although molybdenum carbides(Mo-Cs)have multiple crystalline stacking orders,there are still few structures reported for the lack of higher energy supply during growth.In this study,we report a two-step vapor deposition method to grow superconducting η-Mo3C2 films with different thicknesses,with the assistance of controllable plasma power.The grownη-Mo3C2 films show polycrystalline characteristics,but they still present superior superconductivity.The 3.0-nm-thick film has the superconducting transition temperature of 5.38 K,and its electrical performances follow truly 2D superconducting transitions.This study will not only exhibit a robust superconductingη-Mo3C2 ultrathin film,but also provides a convenient growth way to realize more carbide-based heterostructures for future device applications.

Heating graphene to incandescence and the measurement of its work function by the thermionic emission method

The work function （WF） of graphene is an essential parameter in graphene electronics. We have derived the WF of graphene by the thermionic emission method. Chemical vapor deposition （CVD）-grown single-layered polycrystalline graphene on copper foil is transferred to a cross-stacked carbon nanotube （CNT） film drawn from a super-aligned multiwalled CNT array. By decreasing the pore size of the CNT film, the as-prepared CNT-graphene film （CGF） can be Joule heated to a temperature as high as 1,800 K in vacuum without obvious destruction in the graphene structure. By studying the thermionic emission, we derive the WF of graphene, ranging from 4.7 to 4.8 eV with the average value being 4.74 eV. Because the substrate influence can be minimized by virtue of the porous nature of the CNT film and the influence of adsorbents can be excluded due to the high temperature during the thermionic emission, the measured WF of graphene can be regarded as intrinsic.

Chemical vapor infiltration of pyrocarbon from methane pyrolysis: kinetic modeling with texture formation

A complete mechanism of methane pyrolysis is proposed for chemical vapor infiltration of pyrocarbon with different textures, which contains a detailed homogeneous mechanism for gas reactions and a lumped heterogeneous mechanism for pyrocarbon deposition. This model is easily applied to simulate gas compositions and pyrocarbon deposition in a vertical hot-wall flow reactor in the temperature range of 1,323–1,398 K without any adjusting parameters and presents better results than previous mechanisms. Results have shown that the consumption of methane and the production of hydrogen are well enhanced due to pyrocarbon deposition. Pyrocarbon deposition prevents the continuously increasing of acetylene composition and leads to the reduction in the mole fraction of benzene at long residence times in the gas phase. The carbon growth with active sites on the surface is the controlling mechanism of pyrocarbon deposition. C1 species is the precursor of pyrocarbon deposition at 1,323 K,and the primary source over the whole temperature range. As temperature increases, gas phase becomes more mature and depositions from acetylene, benzene and polyaromatic hydrocarbons become more prevalent. A general pyrocarbon formation mechanism is derived with the specific precursors and illustrates that the maturation of gas compositions is beneficial to forming planar structures with hexagonal rings or pentagon-heptagon pairs, namely, high textured pyrocarbon. The results are in well agreement with experiments.