Surface and interfacial morphologies of PAN-CVI carbon–carbon composite

Scanning electron microscopy (SEM), polarized light microscopy (PLM), and transmission electron microscopy (TEM) techniques have been used to characterize the normal surface and flank surface microstructure of a polyacrylonitrile (PAN)-based carbon fiber reinforced chemical vapor infiltrated (CVI) matrix carbon–carbon composite. Optical and SEM results indicate that the CVI deposit consists of two structures: an isotropic phase is present in the fiber bundle-bundle junctions and a second highly oriented lamellar structure is present in the intrabundle matrix. TEM shows that matrix platelets are highly parallel to the fiber axis and the crystallites of the matrix near the fiber surface exhibit better alignment than those farther away from fibers.     

Preparation and performance of sulfur–carbon composite based on hollow carbon nanofiber for lithium-sulfur batteries

A unique structured hollow carbon nanofiber–sulfur composite material (HCF–S) was fabricated and characterized in lithium-sulfur batteries. It is found that a part of spherical sulfur particles are located in the voids formed by the intertwined fibers and the others are confined in hollow channel of the HCF. The high conductive and porous HCF favors the construction of stable three-dimensional conducting network and convenient infiltration of the electrolytes into the cathode. The HCF–S cathode exhibits excellent electrochemical performance in the electrolyte with LiNO₃. By contrast, the ionic liquid electrolyte provides insufficient shuttle suppression and weakens ion transport, which leads to poor cycle and rate capability.     

Novel silicon–oxygen–carbon composite with excellent cycling steady performance as anode for lithium-ion batteries

The novel anode material for lithium-ion batteries, silicon–oxygen–carbon (Si–O–C) composite, is prepared by a liquid solidification combined with following pyrolysis process, in which silicon dioxide (SiO₂) is used as an additive agent to enhance the electrochemical performance of the composite. While the structure of the composite is confirmed by X-ray diffraction (XRD) and Fourier transform infrared spectra (FT-IR), the morphology and microstructure were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. SEM and TEM observations reveal that the Si–O–C powders are about 1 μm in diameter, and there is a homogenous pyrolyzed carbon layer about 5 nm thick on the surface of the particle. The Si–O–C sample as anode material can deliver a high initial charge capacity of 753.4 mAh g^?1, and the capacity keeps above 500.0 mAh g^?1 after 40 cycles at 100.0 mA g^?1. The electrochemical impedance spectroscopy results show that the composite exhibits lower charge transfer resistance and higher lithium-ion diffusion rate compared with the Si–C anode, which indicates that the composite Si–O–C could be used as a promising anode material for lithium-ion batteries.

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Graphical Abstract SEM images of the Si-C (a) and Si-O-C (b) samples and the TEM (c) and HRTEM (d) image of the Si-O-C sample

     

Stoichiometric controlling of pulsed laser deposited boron–carbon thin films

The stoichiometry of B–C thin films was controlled via pulsed laser deposition using a series of ceramic B–C targets (B/C ratio was 3.04–5.92). The effects of B/C ratio in target, laser power and substrate-to-target distance on deposition rate, microstructure, stoichiometry and chemical structure were investigated. The maximum deposition rate was obtained at laser power of 90 mJ and substrate-to-target distance of 50 mm. Boron rich B–C films were obtained and the stoichiometry in B–C thin films was controlled in the range 2.9–4.6. Carbon atoms were bonded with only sp³ hybridization when boron was rich,but with sp² and sp³ hybridizations when carbon was rich.     

The impact of carbon shell on a Sn–C composite anode for lithium-ion batteries

Spherical Sn–carbon core-shell powders (CSCM/Sn) were synthesized through a resorcinol–formaldehyde microemulsion polymerization performed in the presence of SnO₂ powders, followed by carbonization in an inert atmosphere. Scanning electron microscope and X-ray diffractometry analyses showed that the Sn powders were thoroughly encapsulated within the carbon microspheres. The CSCM/Sn presented much better cyclability than the conventional Sn–carbon microsphere composite. In core-shell-structured composite, most of the Sn particles were encased inside carbon microspheres and not easy to aggregate or fall off from the microspheres. The carbon shell suppressed the aggregation of tin particles and alleviated the volume change of tin, and the conductive carbon shell effectively decreased the polarization during cycling, giving rise to better high rate performance and excellent capacity retention ability. It is shown that surface structure plays an important role in alloy/C composite anode materials for lithium-ion battery.     

Influence of defects in SiC （0001） on epitaxial graphene

Defects in silicon carbide(SiC) substrate are crucial to the properties of the epitaxial graphene(EG) grown on it. Here we report the effect of defects in SiC on the crystalline quality of EGs through comparative studies of the characteristics of the EGs grown on SiC(0001) substrates with different defect densities. It is found that EGs on high quality SiC possess regular steps on the surface of the SiC and there is no discernible D peak in its Raman spectrum. Conversely, the EG on the SiC with a high density of defects has a strong D peak, irregular stepped morphology and poor uniformity in graphene layer numbers. It is the defects in the SiC that are responsible for the irregular stepped morphology and lead to the small domain size in the EG.     

Study on the effect of laser parameters on wakefield excitation in femtosecond pulsed laser–plasma interaction using PIC method

Propagation of an intense short laser pulse through under-dense plasma can produce huge amplitude plasma wake field. A 3D particle in cell (PIC) method was used to simulate the wakefield generation for different laser parameters such as intensity, pulse duration, spot size and temporal pulse shape. Our study shows that the amplitude of wakefield is increased with laser intensity, but it is decreased with spot size. The results for pulse shape and pulse duration depend on their optimum values.     

Influence of pH on inactivation of aquatic microorganism with a gas–liquid pulsed electrical discharge

Inactivation of Escherichia coli in water was experimentally studied, with pulsed electrical discharges in a hybrid gas–liquid reactor. The pH was dramatically decreased from 7 to ～3 within 60 min, accompanying with a 6-log reduction. To evaluate the contribution of pH on inactivation, a set of experiments were designed and tested. Results indicate that the contribution of low pH to the inactivation could be neglected compared to that of electrical discharges. On the other hand, the decrease of pH could be eased as carbonate or phosphate buffer was added to the treated water. However, the inactivation efficiency was greatly reduced because the buffers could deplete the active species formed in electrical discharges. Besides, a new finding is addressed in this paper that the water after plasma treatment still owns a certain extent of inactivation ability, functioning like the free chlorine residual. The environmental adaptation ability of E. coli to electrical discharges was also investigated.     

Structure of connections of single-walled carbon nanotubes with the use of the combined 5–7 and 4–8 topological defects

The structures of zigzag-zigzag, armchair-zigzag, zigzag-chiral, armchair-armchair, armchair-chiral, and chiral-chiral pair connections produced by single-walled carbon nanotubes 1.5–5.0 diameter with the use of the combined 5–7 and 4–8 topological defects have been calculated by molecular mechanics methods. It has been established that the use of the combined 5–7 topological defect makes it possible to produce connections between pairs of single-walled carbon nanotubes with any conductivities, chiralities, and diameters, whereas the use of the combined 4–8 topological defect provides a means for forming connections between nanotubes only with the same type of conductivity. The angles between the axes of nanotubes connected by the combined 5–7 and 4–8 topological defects lie in the ranges 145°–180° and 112°–178°, respectively. It has been revealed that there are correlations between structural parameters of the connections and the relative arrangement of the simple topological defects in the combined topological defects.     

Mode-locked L-band bismuth–erbium fiber laser using carbon nanotubes

A passively mode-locked, bismuth–erbium-co-doped fiber (Bi-EDF) with a pulse width of 460 fs is proposed and demonstrated. A highly doped, 180-cm Bi-EDF with an erbium concentration of 3,250 ppm/wt and an absorption rate of 133 dB/m at 1,530 nm serves as the gain medium. The cavity is 11.6 m long with an overall group velocity dispersion of +0.063 ps². The output pulses have a repetition rate, average output power, pulse energy and peak power of 11.18 MHz, 5 mW, 448 pJ and 1 kW, respectively. The system has a high beam quality and a narrow pulse width output in the L-band region.     

Passivation of carbon dimer defects in amorphous SiO_2/4H–SiC(0001) interface:A first-principles study

An amorphous SiO_2/4 H–Si C(0001) interface model with carbon dimer defects is established based on density functional theory of the first-principle plane wave pseudopotential method.The structures of carbon dimer defects after passivation by H_2 and NO molecules are established,and the interface states before and after passivation are calculated by the Heyd–Scuseria–Ernzerhof(HSE06) hybrid functional scheme.Calculation results indicate that H_2 can be adsorbed on the O_2–C = C–O_2 defect and the carbon–carbon double bond is converted into a single bond.However,H_2 cannot be adsorbed on the O_2–(C = C) –O_2 defect.The NO molecules can be bonded by N and C atoms to transform the carbon–carbon double bonds,thereby passivating the two defects.This study shows that the mechanism for the passivation of Si O_2/4 H–SiC(0001) interface carbon dimer defects is to convert the carbon–carbon double bonds into carbon dimers.Moreover,some intermediate structures that can be introduced into the interface state in the band gap should be avoided.     

Periodically modulated interaction effect on transport of Bose–Einstein condensates in lattice with local defects

We theoretically investigate the periodically modulated interaction effect on the propagation properties of a traveling plane wave in a Bose–Einstein condensate(BEC) trapped in a deep annular lattice with local defects both analytically and numerically. By using the two-mode ansatz and the tight-binding approximation, a critical condition for the system preserving the superfluidity is obtained analytically and confirmed numerically. We find that the coupled effects of periodic modulated atomic interactions, the quasi-momentum of the plane wave, and the defect can control the superfluidity of the system. Particularly, when we consider the periodic modulation in the system with single defect, the critical condition for the system entering the superfluid regime depends on both the defect and the momentum of the plane wave. This is different from the case for the system without the periodic modulation, where the critical condition is only determined by the defect. The modulation and quasi-momentum of the plane wave can enhance the system entering the superfluid regime. Interestingly, when the modulated amplitude/frequency, the defect strength, and the quasi-momentum of the plane wave satisfy a certain condition, the system will always be in the superfluid region. This engineering provides a possible means for studying the periodic modulation effect on propagation properties and the corresponding dynamics of BECs in disordered optical lattices.     

Nickel–cobalt oxide/activated carbon composite electrodes for electrochemical capacitors

Nanostructured synthesis of nickel–cobalt oxide/activated carbon composite by adapting a co-precipitation protocol was revealed by transmission electron microscopy. X-ray diffraction analysis confirmed that nickel–cobalt oxide spinel phase was maintained in the pure and composite phases. Cyclic voltammetry, galvanostatic charge–discharge tests and ac impedance spectroscopy were employed to elucidate the electrochemical properties of the composite electrodes in 1.0 M KCl. The specific capacitance which was the sum of double-layer capacitance of the activated carbon and pseudocapacitance of the metal oxide increased with the composition of nickel–cobalt oxide before showing a decrement for heavily-loaded electrodes. Utilisation of nickel–cobalt oxide component in the composite with 50 wt. % loading displayed a capacitance value of ～59 F g^?1. The prepared composite electrodes exhibited good electrochemical stability.     

Investigation of V-shaped extended defects in a 4H–SiC epitaxial film

Abstract

We investigated two types of V-shaped extended defects on the basal plane in epitaxial 4H-SiC by synchrotron X-ray topography, photoluminescence imaging/spectroscopy and transmission electron microscopy (TEM). One is the (2, 5) stacking fault (in Zhdanov notation) bounded by two partial dislocations with the Burgers vector b ± 1/4[0?0?0?1]; the other is the (2, 3, 3, 5) stacking fault bounded by partial dislocations with b = ±1/4[0?0?0?1]. The core of the partial dislocations associated with the (2, 3, 3, 5) fault has an out-of-plane component (Frank component) and three in-plane components (Shockley components); the three Shockley components are cancelled out in total. The electronic structures of the (2, 5) and (2, 3, 3, 5) stacking faults were further examined by photoluminescence spectroscopy and first-principles calculations. It is suggested that the (2, 5) and (2, 3, 3, 5) stacking faults both have an interband state at a similar energy level, although they differ structurally.     

Optical properties of PMN–PT thin films prepared using pulsed laser deposition

(1 ? x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ (PMN–PT) thin films have been deposited on quartz substrates using pulsed laser deposition (PLD). Crystalline microstructure of the deposited PMN–PT thin films has been investigated with X-ray diffraction (XRD). Optical transmission spectroscopy and Raman spectroscopy are used to characterize optical properties of the deposited PMN–PT thin films. The results show that the PMN–PT thin films of perovskite structure have been formed, and the crystalline and optical properties of the PMN–PT thin films can be improved as increasing the annealing temperature to 750 °C, but further increasing the annealing temperature to 950 °C may lead to a degradation of the crystallinity and the optical properties of the PMN–PT thin films. In addition, a weak second harmonic intensity (SHG) has been observed for the PMN–PT thin film formed at the optimum annealing temperature of 750 °C according to Maker fringe method. All these suggest that the annealing temperature has significant effect on the structural and optical properties of the PMN–PT thin films.     

L–H power threshold studies with tungsten/carbon divertor on the EAST tokamak

The power threshold for low (L) to high (H) confinement mode transition achieved by radio-frequency heating and molybdenum first wall with lithium coating has been experimentally investigated on the EAST tokamak for two sets of divertor geometries and materials: tungsten/carbon divertor and full carbon divertor. For both sets of divertors, the power threshold was found to decrease with gradual accumulation of the lithium wall coating, suggesting the important role played by the low Z impurities and/or the edge neutral density on the L–H power threshold. When operating in the upper single null configuration, with the ion grad-B drift direction away from the primary X-point, a lower normalized power threshold is observed in EAST with the tungsten/carbon divertor, compared to the carbon divertor after intensive lithium wall coating. A newly installed cryopump increasing the pumping efficiency also plays an important part in the observed lower threshold. In addition, the H-mode in the Quasi-Snowflake divertor configuration has been obtained on EAST, exhibiting higher L–H power threshold compared to the lower single null configuration with similar I_P/B_T pairs.     

Creep behaviour in a discontinuous SiC–Zn-22% Al composite

Creep experiments were conducted on Zn-22%?Al in which SiC particulates were introduced by variable co-deposition of multi-phase materials (VCM). The objective of the investigation is to determine the effect of SiC particulates on the creep behaviour in region I (the low-stress region) and region II (the intermediate-stress or superplastic region) of the sigmoidal plot between stress and strain rate, which was previously reported for the reinforcement-free Zn-22%?Al. The creep data show that the presence of SiC particulates has no effect on the sigmoidal trend between stress and strain rate; and that in region II, the stress exponent, n, and the activation energy for creep, Q, agree well with those reported for SiC-free grades of Zn-22%?Al; n?=?2.5 and Q?～?Q _gb, where Q _gb is energy for grain boundary diffusion in the alloy. However, the data indicate that the presence of the particulates results in narrowing region II and reducing maximum ductility. An analysis of the creep data reveals the presence of a threshold stress that depends strongly on temperature. The microstructural data inferred from an examination of the crept specimens by the means of transmission electron microscopy (TEM) suggest that the origin of τ ₀ may be related to the interaction between moving dislocations and dispersion particles. These particles are introduced in the material as a result of processing the material by thermal spray and deposition.     

Study on defects associated with interstitial oxygen in PbWO4 crystal

The defects associated with interstitial oxygen in lead tungstate crystals (PbWO4) are investigated by the relativistic self-consistent discrete variational embedded cluster method. The research work is focused on the density of states of interstitial oxygen defects and relational Frankel defects. The transition state method is used to calculate excitation energy of different electron orbits. Simulation results show that the existence of defects related to interstitial oxygen can diminish the bandwidth of the WO4^2- group, and it might produce the green luminescence, Frankel defects associated with interstitial oxygen could result in the absorption at 420nm.     

Microstructural evolution of SiC joints soldered using Zn–Al filler metals with the assistance of ultrasound

SiC ceramics were successfully soldered with the assistance of ultrasound. Two kinds of filler metals, namely non-eutectic Zn–5Al–3Cu and eutectic Zn–5Al alloys, were used. The effects of ultrasonic action on the microstructure and mechanical properties of the soldered joints were investigated. The results showed that ultrasound could promote the wetting and bonding between the SiC ceramic and filler metals within tens of seconds. For the Zn–5Al–3Cu solder, a fully grain-refined structure in the bond layer was obtained as the ultrasonic action time increased. This may lead to a substantial enhancement in the strength of the soldered joints. For the Zn–5Al solder, the shear strength of the soldered joints was only ∼102 MPa when the ultrasonic action time was shorter, and fractures occurred in the brittle lamellar eutectic phases in the center of the bond layer. With increasing ultrasonic action time, the lamellar eutectic phase in the bond layer of SiC joints could be completely transformed to a fine non-lamellar eutectic structure. Meanwhile, the grains in the bond layer were obviously refined. Those results led to the remarkable enhancement of the shear strength of the joints (∼138 MPa) using the Zn–5Al solder, which had approached that enhancement using the Zn–5Al–3Cu solder. The enhanced mechanical properties of the joints were attributed to the significant refinement of the grains and the change in the eutectic structure in the bond layer. Prolonged enhanced heterogeneous nucleation triggered by ultrasonic cavitation is the predominant refinement mechanism of the bond metals of the SiC joints.     

Studies of diamond-like carbon （DLC） films deposited on stainless steel substrate with Si/SiC intermediate layers

In this work, diamond-like carbon （DLC） films were deposited on stainless steel substrates with Si/SiC intermediate layers by combining plasma enhanced sputtering physical vapour deposition （PEUMS-PVD） and microwave electron cyclotron resonance plasma enhanced chemical vapour deposition （MW-ECRPECVD） techniques. The influence of substrate negative self-bias voltage and Si target power on the structure and nano-mechanical behaviour of the DLC films were investigated by Raman spectroscopy, nano-indentation, and the film structural morphology by atomic force microscopy （AFM）. With the increase of deposition bias voltage, the G band shifted to higher wave-number and the integrated intensity ratio ID/IG increased. We considered these as evidences for the development of graphitization in the films. As the substrate negative self-bias voltage increased, particle bombardment function was enhanced and the sp^3-bond carbon density reducing, resulted in the peak values of hardness （H） and elastic modulus （E）. Silicon addition promoted the formation of sp^3 bonding and reduced the hardness. The incorporated Si atoms substituted sp^2- bond carbon atoms in ring structures, which promoted the formation of sp^3-bond. The structural transition from C-C to C-Si bonds resulted in relaxation of the residual stress which led to the decrease of internal stress and hardness. The results of AFM indicated that the films was dense and homogeneous, the roughness of the films was decreased due to the increase of substrate negative self-bias voltage and the Si target power.