Carbon Wall Research Articles

New doping process mode to synthesize in situ N-MWNTs in novel coaxial nanostructure

Nitrogen-doped MWNT “N-MWNT” have gained an increasing interest because particularly of their electronic properties allowing their use in specific applications such as catalysis, sensors, nano-electronics and energy storage.In this investigation, novel coaxial structures consisting of concentric shells of pure carbon MWNT cores with external nitrogen-doped carbon walls (N-MWNT) were produced using a catalytic CVD process, with ethane and ammonia as carbon and nitrogen sources over Fe–Al2O3 support. In order to optimize the growth conditions of such N-MWNT nanostructures, we investigate the influence of different key parameters, i.e., ammonia injection moment in two doping process modes “Instantaneously doping and Mid-doping”, growth temperature (750 and 850°C) and the presence of H2 as reducer gas in the reaction environment. A strong correlation between ammonia injection duration (60, 90 and 120min) and nitrogen content (from 0.4at.% to 2.2at.%) incorporated in the nanotube products was observed and different coordination between C and/or O atoms have been described by XPS. These functionalized MWNTs were characterized using HR-TEM, FESEM, XPS, BET, TG analyses and Raman spectroscopy in order to determine their structural characteristics (graphitization and crystallinity) in quantitative and qualitative ways.

Flexible P-Doped Carbon Cloth: Vacuum-Sealed Preparation and Enhanced Na-Storage Properties as Binder-Free Anode for Sodium Ion Batteries.

In this work, a flexible and self-supporting P-doped carbon cloth (FPCC), which is composed of interwoven mesh of hollow microtubules with porous carbon walls, is prepared via a vacuum-sealed doping technology by employing the commercially available cotton cloth as sustainable and scalable raw material. When directly used as binder-free anode for sodium-ion batteries, the as-prepared FPCC delivers superior Na-storage properties in terms of specific capacity up to 242.4 mA h g-1, high initial Coulombic efficiency of ∼72%, excellent rate capabilities (e.g., 123.1 mA h g-1 at a high current of 1 A g-1), and long-term cycle life (e.g., ∼88% capacity retention after even 600 cycles). All these electrochemical data are better than the undoped carbon cloth control, demonstrating the significance of P-doping to enhance the Na-storage properties of cotton-derived carbon anode. Furthermore, the technologies of electrochemical impedance spectroscopy and galvanostatic intermittent titration technique are implemented to disclose the decrease of charge transfer resistance and improvement of Na-migration kinetics, respectively.

Erosion at the inner wall of JET during the discharge campaign 2013–2014

The erosion of Be and W marker layers was investigated using long-term samples containing marker layers during the second ITER-like wall discharge campaign 2013–2014 (ILW-2). The samples were mounted in Be coated Inconel tiles between the inner wall guard limiters (IWGL). They were analyzed using elastic backscattering (EBS) before and after exposure. All samples showed noticeable erosion. The results were compared to the data for Be and W erosion rates for the first 2011–2012 JET ITER-like wall (ILW-1) campaign, and to the data for C erosion during the 2005–2009 campaign when JET was operated with a carbon wall. The mean W erosion rates and the toroidal and poloidal distributions of the W erosion were nearly the same for the ILW-1 and ILW-2 campaigns. The mean erosion rate of Be during the ILW-2 campaign was smaller by a factor of about two compared to the ILW-1 campaign.

Effective micro-spray cooling for light-emitting diode with graphene nanoporous layers

A graphene nanoplatelet (GNP) coating is utilized as a functionalized surface in enhancing the evaporation rate of micro-spray cooling for light-emitting diodes (LEDs). In micro-spray cooling, water is atomized into micro-sized droplets to reduce the surface energy and to increase the surface area for evaporation. The GNP coating facilitates the effective filmwise evaporation through the attribute of fast water permeation. The oxygenated functional groups of GNPs provide the driving force that initiates the intercalation of water molecules through the carbon nanostructure. The water molecules slip through the frictionless passages between the hydrophobic carbon walls, resulting an effective filmwise evaporation. The enhancement of evaporation leads to an enormous temperature reduction of 61.3 °C. The performance of the LED is greatly enhanced: a maximum increase in illuminance of 25% and an extension of power rating from 9 W to 12 W can be achieved. With the application of GNP coating, the high-temperature region is eliminated while maintaining the LED surface temperature for optimal operation. This study paves the way for employing the effective hybrid spray–evaporation–nanostructure technique in the development of a compact, low-power-consumption cooling system.

Quartz micro-balance results of pulse-resolved erosion/deposition in the JET-ILW divertor

A set of quartz crystal microbalances (QMB) was used at JET with full carbon wall to monitor mass erosion/deposition rates in the remote areas of the divertor. After introduction of the ITER- like wall (ILW) in JET with beryllium main wall and tungsten divertor, strong reduction of the material deposition and accompanied fuel retention was observed. Therefore the existing QMB electronics have been modified to improve the accuracy of frequency measurements by a factor of ten down to 0.1Hz which corresponds to 1.4ngcm−2. The averaged deposition rates of 1.2–3ngcm−2s−1 and erosion rates of 5.6–8.1ngcm−2s−1 were observed in the inner divertor of JET -ILW with the inner strike point positions close to the bottom edge of vertical tile 3 and at the horizontal tile 4 respectively. The erosion with averaged rates of ≈2.1ngcm−2s−1 and ≈120ngcm−2s−1 were observed in the outer divertor for the outer strike point positions at tile 5 and tile 6 respectively.

Single Molecular Junction Study on H2 O@C60 : H2 O is "Electrostatically Isolated".

A water molecule exhibits characteristic properties on the basis of hydrogen bonding. In the past decade, single water molecules placed in non-hydrogen-bonding environments have attracted growing attention. To reveal the fundamental properties of a single water molecule, endohedral fullerene H2 O@C60 is an ideal and suitable model. We examined the electronic properties of H2 O@C60 by performing single-molecule measurements. The conductance of a single molecular junction based on H2 O@C60 was found to be comparable to that of empty C60 . The observed values were remarkably higher than those obtained for conventional molecular junctions due to the effective hybridization of the π-conjugated system to the metal electrode. Additionally, the results undoubtedly exclude the possibility of electrostatic contact of entrapped H2 O with the carbon wall of C60 . We finally concluded that H2 O entrapped inside a C60 cage can be regarded as an electrostatically isolated molecule.

EDGE2D-EIRENE modelling of near SOL Er: possible impact on the H-mode power threshold

Recent EDGE2D-EIRENE simulations of JET plasmas showed a significant difference between radial electric field (Er) profiles across the separatrix in two divertor configurations, with the outer strike point on the horizontal target (HT) and vertical target (VT) (Chankin et al 2016 Nucl. Mater. Energy, doi: 10.1016/j.nme.2016.10.004). Under conditions (input power, plasma density) where the HT plasma went into the H-mode, a large positive Er spike in the near scrape-off layer (SOL) was seen in the code output, leading to a very large E × B shear across the separatrix over a narrow region of a fraction of a cm width. No such Er feature was obtained in the code solution for the VT configuration, where the H-mode power threshold was found to be twice as high as in the HT configuration. It was hypothesised that the large E × B shear across the separatrix in the HT configuration could be responsible for the turbulence suppression leading to an earlier (at lower input power) L–H transition compared to the VT configuration. In the present work these ideas are extended to cover some other experimental observations on the H-mode power threshold variation with parameters which typically are not included in the multi-machine H-mode power threshold scalings, namely: ion mass dependence (isotope H–D–T exchange), dependence on the ion ∇B drift direction, and dependence on the wall material composition (ITER-like wall versus carbon wall in JET). In all these cases EDGE2D-EIRENE modelling shows larger positive Er spikes in the near SOL under conditions where the H-mode power threshold is lower, at least in the HT configuration.

1H NMR Investigations of Activated Carbon Loaded with Volatile Organic Compounds: Quantification, Mechanisms, and Diffusivity Determination.

Three volatile organic compounds (VOCs), benzene, cyclohexane, and dichloromethane, were adsorbed onto activated carbon fiber cloth. 1H (magic-angle spinning (MAS) and pulsed field gradient (PFG)) NMR techniques were carried out, and the signals were analyzed in terms of peak surface areas and shifts. These techniques were shown to be very useful for determining (i) the intrinsic quantification of adsorbed molecules (VOCs and/or water) in the porosity of the materials (the adsorption capacities ranged from 0.2 to 4 mol·kg-1); (ii) the mechanisms of interactions between adsorbed organic molecules and the carbon walls (illustrations of positions of the molecule inside the pore volume are proposed; the proton-wall distance was less than 0.15 nm); and (iii) the diffusivities (surface diffusion coefficients (DS) were estimated at ≈4.10-12 m2·s-1 for cyclohexane, ≈1.10-11 m2·s-1 for benzene, and ≈4.10-11 m2·s-1 for dichloromethane).

Carbon-oxygen isotopic geochemistry of the Yangla Cu skarn deposit, SW China: Implications for the source and evolution of hydrothermal fluids

The Yangla Cu deposit is the largest Cu skarn deposit in the Jinshajiang tectonic belt. Based on the detailed observation of crosscutting relationships, three mineralization stages (i.e., pre-ore, ore and supergene) have been identified in the Yangla deposit. The pre-ore stage is dominated by prograde skarn. The ore stage is characterized by the precipitation of hydrous silicate minerals, Fe-oxides, Fe-Cu-Mo-sulfides, quartz and calcite, whose mineral assemblages were formed in the early and late sub-ore stages. The early sub-ore stage is marked by retrograde alteration with the deposition of hydrous silicate minerals (e.g., actinolite, epidote and chlorite), Fe-oxides, abundant Fe-Cu-Mo-sulfides, quartz and minor calcite. Whilst, the late sub-ore stage, associated with silicic and carbonate alteration, is represented by widespread thick quartz or calcite veins with disseminated pyrite, chalcopyrite, galena and sphalerite. We present new carbon-oxygen (C-O) isotopic compositions of the ore-hosting marble and hydrothermal calcite of this deposit. The hydrothermal calcite in the Yangla deposit was precipitated from both the early and late sub-ore stages. Calcite I from the early sub-ore stage is anhedral, and occurs as spot in the skarn or locally replaces the skarn minerals. Calcite II from the late sub-ore stage is distinguished by being coarse-grained, subhedral to euhedral and its occurrence in thick veins. Calcite I contains lower δ13CPDB (−7.0‰ to −5.0‰) and δ18OSMOW (7.2‰ to 12.7‰) than Calcite II (δ13CPDB=−4.5‰ to −2.3‰; δ18OSMOW=10.7‰ to 19.4‰). In the δ13CPDB vs. δ18OSMOW diagram, the Calcite I and Calcite II data fall close to the igneous carbonatite field and between the fields of igneous carbonatite and marine carbonates, respectively. This suggests a dominantly magmatic origin for the early sub-ore fluids, and there might have been increasing carbonate wall rock involvement towards the late sub-ore stage. The ore-hosting marble (δ13CPDB=−4.8‰ to −0.3‰; δ18OSMOW=10.2‰ to 23.9‰) also shows a positive δ13CPDB vs. δ18OSMOW correlation, which is interpreted to reflect the decreasing alteration intensity during the interactions between the hydrothermal fluids and ore-hosting carbonates. Simulated calculation suggests that both the Calcite I and Calcite II precipitated at 350°C to 250°C and 250°C to 150°C, respectively. We proposed that CO2 degassing and water/rock interactions were likely the two major processes that precipitated the calcite and led to the observed C-O isotopic features of the Yangla Cu deposit.

A Catalytic Etching-Wetting-Dewetting Mechanism in the Formation of Hollow Graphitic Carbon Fiber

Summary Hollow graphitic carbon nanofibers (HGCNFs) have great promise for many important applications, such as catalysis, sensors, energy conversion, gas storage, and electronic devices. Here, we report a catalytic etching-wetting-dewetting mechanism in synthesizing HGCNFs with both hollow spherical-like and tunnel-like pores. With in situ transmission electron microscope imaging, we show that the spherical pores are formed by the evaporation of encapsulated Ni on the surface of amorphous carbon nanofiber and that hollow tunnels are developed through continuous etching of the amorphous carbon under catalysis of Ni nanoparticles. Theoretical calculations and simulations reveal that continuous tunnel etching is driven by the wetting-to-dewetting transition of the Ni-tunnel wall interaction during the catalytic graphitization of the amorphous carbon wall. In a typical application, we demonstrate that the synthesized HGCNFs, with a capacity about three times higher than that of their non-hollow counterparts, are excellent potential candidates for CO 2 capture.

3D Ordered Macroporous MoS2 @C Nanostructure for Flexible Li-Ion Batteries.

A flexible and robust electrode is demonstrated by assembling the 3D ordered macroporous MoS2 @C nanostructure on carbon cloth with ultrasmall few-layered MoS2 nanosheets homogenously embedded into the interconnected carbon wall. Such unique nanostructures are favorable for enhancing lithium storage capacity, directly applied as a flexible electrode, demonstrating a very high electrochemical performance and superior cycling stability for lithium-ion batteries.

Numerical investigation of plasma-wall interaction during burst of ELMs in a tokamak device

In the tokamak high confinement mode (H-mode), the transient heat fluxes caused by edge localized modes (ELMs) will eventually irradiate the plasma-facing components and may erode, even melt them. To study the performance of the plasma-wall interaction during ELMs, interactions among heat flux from plasma, surface temperature, and electron emission are simulated by considering self-consistency among plasma transport in the sheath, deposited heat at the wall, and material thermal response for carbon (C) and tungsten (W) walls. It is found that the sheath structure determines the surface temperature, which may in turn influence on the sheath. A large amount of electron emission can change the heat load from the plasma to the material surface due to the variation of the ELMs-induced electron temperature and the surface temperature. During the burst of ELMs, the surface temperature rises rapidly at first and then reaches a saturation state with a certain range of fluctuation. The development of these processes depends strongly on the characteristic of ELMs, deposited heat at the wall, and material properties. Simulation results also show that the erosion of the Experimental Advanced Superconducting Tokamak (EAST) divertor target is of no concern in H-mode operation with ELMs for the current and possible future operation parameters.

Capacity Enhancement of Porous Carbon Electrodes during Long-Term Cycling in Lithium-Ion Batteries

Capacity enhancements are sometimes observed when carbon materials, especially porous carbons, are used as lithium-ion batteries anodes. However, the correlative mechanism is unclear now. Here, porous carbon (PC) is used to reveal the mechanism by using a combination of structure analysis and electrochemical calculations. The coexistence of blind pores and closed pores with fractal porous structure is confirmed by N2 sorption and small angle X-ray scattering. When the PC with main pore diameter of 30 nm is used as lithium-ion batteries anode, the specific capacity increases from 700 mAh g−1 (40th cycle) to 1300 mAh g−1 (110th cycle) at a current density of 50 mA g−1. It could be ascribed to the stepwise diffusion of Li atoms inside the pore structure, including the following two effects. (1) The diffusion of solvated Li+ in the blind pores results in the continuous formation of solid electrolyte interface, providing more activated sites on the inside surface; (2) Li atoms diffuse into the inner closed pores via disordered carbon walls to further enhance the reversible capacity. Our finding provides a deep insight on the mechanism of capacity enhancement of porous carbon electrodes, which could benefit the design of advanced electrode materials.

Polydopamine derived porous N-doped carbon nanofibers for lithium ion storage

Porous N-doped carbon nanofibers derived from polydopamine are fabricated by a facile carbonized technique and a subsequent template-removing process. Superior to the other porous carbon nanomaterials that voids are directly exposed in the electrolyte, the nanopores in our designed nanofibers are confined by the inner carbon walls and outer carbon shells. The prepared porous N-doped carbon nanofibers electrode shows high reversible capacities of 1324mAhg−1 after 150 cycles at 0.2Ag−1 and 780mAhg−1 after 500 cycles at 1Ag−1.

Synthesis, Structure, and Electrochemical Properties of a Sulfur-Carbon Replica Composite Electrode for All-Solid-State Li-Sulfur Batteries

Carbon replica constructed of three-dimensional, ordered mesopores was employed as the conductive framework for the composite electrode in Li-sulfur all-solid-state batteries. Using a gas-phase mixing method under various conditions, elemental sulfur was introduced into the mesopores with an average diameter of 12 nm. The all-solid-state cells consisted of the sulfur-carbon replica composite cathode, thio-LISICON solid electrolyte, and Li-Al alloy anode. The cells produced discharge capacities of approximately 1500 mAh g−1 in the 1st cycle, which is comparable to the theoretical capacity of sulfur. Thermogravimetric and X-ray diffraction analyses revealed that sulfur deposited inside the mesopores is the main contributor to the excellent performance of the battery. However, sulfur strongly interacted with the carbon replica, reducing the thickness of the carbon wall from 7 to 4 nm. This structural change in the carbon matrix triggered the deterioration of battery performance, especially the cycling capability. Optimizing the sulfur deposition conditions could therefore enhance the performance of batteries using such composite electrodes.

Benchmarking the GENE and GYRO codes through the relative roles of electromagnetic and E × B stabilization in JET high-performance discharges

Nonlinear gyrokinetic simulations using the GENE code have previously predicted a significant nonlinear enhanced electromagnetic stabilization in certain JET discharges with high neutral-beam power and low core magnetic shear (Citrin et al 2013 Phys. Rev. Lett. 111 155001, 2015 Plasma Phys. Control. Fusion 57 014032). This dominates over the impact of E × B flow shear in these discharges. Furthermore, fast ions were shown to be a major contributor to the electromagnetic stabilization. These conclusions were based on results from the GENE gyrokinetic turbulence code. In this work we verify these results using the GYRO code. Comparing results (linear frequencies, eigenfunctions, and nonlinear fluxes) from different gyrokinetic codes as a means of verification (benchmarking) is only convincing if the codes agree for more than one discharge. Otherwise, agreement may simply be fortuitous. Therefore, we analyze three discharges, all with a carbon wall: a simplified, two-species, circular geometry case based on an actual JET discharge; an L-mode discharge with a significant fast-ion pressure fraction; and a low-triangularity high-β hybrid discharge. All discharges were analyzed at normalized toroidal flux coordinate ρ = 0.33 where significant ion temperature peaking is observed. The GYRO simulations support the conclusion that electromagnetic stabilization is strong, and dominates E × B shear stabilization.

Theoretical Studies of the Stone-Wales Defect in C36 Fullerene Embedded inside Zigzag Carbon Nanotube

We apply density functional theory at PBE/6-311G(d) level as well as nonorthogonal tight-binding model to study the Stone-Wales transformation in C36 fullerene embedded inside the (14,0) zigzag carbon nanotube. We optimize geometries of two different isomers with the D2d and the D6h symmetries and the transition state dividing them. The mechanism of Stone-Wales transformation from D2d to D6h symmetry for the encapsulated C36 is calculated to be the same as for the isolated one. It is found that the outer carbon wall significantly stabilizes the D6h isomer. However, carbon nanotube reduces the activation barrier of Stone-Wales rearrangement by 0.4 eV compared with the corresponding value for the isolated C36.

A comprehensive model for the prediction of coal swelling induced by methane and carbon dioxide adsorption

The phenomenon of coal swelling caused by gas adsorption is well known. To date, three main kinds of models have been used to predict coal swelling caused by gas adsorption, including the linear relationship between volumetric strain and amount of gas adsorbed, the Langmuir-type equation expressing the relationship between volumetric strain and gas pressure, and the energy balance approach. However, the specific interaction between various pores of coal and gas molecules has not been well studied to explain and predict coal swelling or shrinkage caused by gas adsorption. In this paper, the distribution of gas molecules in carbon pores under different pressures is studied using Grand Canonical Monte Carlo (GCMC). The Lennard-Jones potential is then used to analyze the effects of gas molecules on the swelling of coal matrix. Based on the distribution of gas molecules in different pores, the stress exerted by the adsorbed particles on the carbon walls is obtained. Finally, the pore size distribution in coal matrix and the interaction between gas molecules and carbon walls are studied comprehensively to predict coal swelling. Pores with different sizes can exhibit different behaviors during adsorption as pore pressure increases. At low pressures (lower than 1 MPa for CO2), gas molecules mainly form monolayers in coal pores of any size. Micropores, which can hold no more than one layer of gas molecules, contribute most to the swelling of coal matrix. Some larger pores have a negative effect or no effect on the swelling of coal matrix. With an increase in bulk pressure, multilayer adsorption gradually occurs in the larger pores. However, the stress exerted by the adsorbed gas molecules on carbon walls is still weak in these pores. Finally, a two-segment linear model expressing the relationship between adsorption swelling and adsorption amount of coal matrix is obtained. The slopes of the two line segments depend strongly on the pore size distribution of coal matrix and the gas species adsorbed. The experimental data show that the new comprehensive model can accurately explain and predict coal matrix swelling caused by methane and carbon dioxide adsorption.

Dimensionless scalings of confinement, heat transport and pedestal stability in JET-ILW and comparison with JET-C

Three dimensionless scans in the normalized Larmor radius ρ*, normalized collisionality ν* and normalized plasma pressure β have been performed in JET with the ITER-like wall (JET-ILW).The normalized energy confinement and the thermal diffusivity exhibit a scaling with ρ* consistent with the earlier results obtained in the carbon wall JET (JET-C) and with a gyro-Bohm scaling. In the pedestal, experimental results show that the stability is not dependent on ρ*, qualitatively in agreement with the peeling–ballooning (P–B) model.The ν* dimensionless scaling shows that JET-ILW normalized confinement has a stronger dependence on collisionality than JET-C. This leads to a reduction of the difference in the confinement between JET-ILW and JET-C to ≈10% at low ν*. The pedestal stability shows an improvement with decreasing ν*. This is ascribed to the increase of the bootstrap current, to the reduction of the pedestal width and to the reduction of the relative shift between pedestal density and temperature position.The β dimensionless scan shows that, at low collisionality, JET-ILW normalized confinement has no clear dependence with β, in agreement with part of the earlier scalings. At high collisionality, a reduction of the normalized confinement with increasing β is observed. This behaviour is driven mainly by the pedestal where the stability is reduced with increasing β. The P–B analysis shows that the stability reduction with increasing β at high ν* is due to the destabilizing effect of the increased relative shift.

Ion temperature and toroidal rotation in JET's low torque plasmas.

This paper reports on the procedure developed as the best method to provide an accurate and reliable estimation of the ion temperature Ti and the toroidal velocity vϕ from Charge-eXchange Recombination Spectroscopy (CXRS) data from intrinsic rotation experiments at the Joint European Torus with the carbon wall. The low impurity content observed in such plasmas, resulting in low active CXRS signal, alongside low Doppler shifts makes the determination of Ti and vϕ particularly difficult. The beam modulation method will be discussed along with the measures taken to increase photon statistics and minimise errors from the absolute calibration and magneto-hydro-dynamics effects that may impact the CXRS passive emission.