Quangui Guo

Relationship between oxidation resistance and structure of B4C-SIC/C composites with self-healing properties

Abstract The oxidation behavior of B 4 C–SiC/C composites of various compositions at temperatures up to 1500°C was analyzed by the thermal gravimetric/differential thermal analysis (TG/DTA) technique and the surface morphology of the composites after isothermal oxidation at 800, 1000 and 1200°C was examined by scanning electron microscopy. The results indicated that the composites exhibited variable oxidation resistance at high temperatures depending on composition and oxidation temperature. The variance of self-healing properties was attributed to the difference of the compositions and the properties of the decarbonized layers including wetting ability, viscosity, volatility and oxygen permeability. The rate-limiting steps of oxidation and the concentration distribution of oxygen across the decarbonized layers of the composites after oxidation are discussed.

Thermophysical properties of high-density graphite foams and their paraffin composites

Abstract High-density graphite foams (GFs) were prepared from mesophase pitch with or without mesocarbon microbeads at different foaming temperatures and pressures, followed by carbonization and graphitization at 1 273 and 2 973 K, respectively. In one case, pitch was repeatedly infiltrated into the graphitized foam at 573 K followed by carbonization to increase its density. Paraffin was infiltrated into the GFs to form GF/paraffin composites. Factors determining the thermophysical properties of the GFs and thermal behavior of the GF/paraffin composites were investigated. The microstructure and thermophysical properties of the foams were found to be greatly influenced by the pitch fraction, foaming temperature and foaming pressure. The thermal conductivity of the foams determines the thermal behavior of the GF/paraffin composites. The thermal diffusivity of the GF/paraffin composites investigated can be increased 768 to 1588-fold compared with that of paraffin. The latent heat of the composites has an almost linear relationship with the mass fraction of paraffin in the composites. The composites are suitable candidates for passive cooling of electronics.

Preparation and electrochemical performance of heteroatom-enriched electrospun carbon nanofibers from melamine formaldehyde resin

Melamine formaldehyde resin was used to prepare heteroatom-enriched carbon nanofibers by electrospinning for the first time. The melamine formaldehyde resin-based carbon fibers without any activation treatment showed a moderate specific surface area ranging from 130 to 479 m2/g and rich surface functionalities (2.56-5.34 wt.% nitrogen and 10.39-11.2 9 wt.% oxygen). Both the specific surface area and surface functionality greatly depended on the carbonization temperature. The capacitive performance was evaluated in 6M KOH aqueous solution. The electrochemically active surface functionalities played an important role in improving the surface capacitance of the electrodes. The sample carbonized at 600°C showed the highest specific surface capacitance of 1.4 F/m2, which was attributed to the most active functionalities (10.69 wt.% of N and O). In addition, the sample carbonized at 750°C exhibited the highest specific capacitance of 206 F/g.

Selection of candidate doped graphite materials as plasma facing components for HT-7U device

Selection of candidate materials for plasma facing material (PFM) in HT-7U device and plasma–wall interactions are critically important to reach high plasma performance. Based on concentrated research on multi-element doped graphite containing B, Si and Ti, two kinds of doped graphites have been chosen as candidates for PFM in HT-7U. Doped graphite GBST1308 with the dopant concentration of 1% B, 2.5% Si, 7.5% Ti was developed as low-Z PFM for reducing the chemical sputtering and suppressing the radiation enhanced sublimation, and successfully used as the new limiter material in last two campaigns of HT-7 tokamak experiments. Doped graphite with the composition of 2.5% Si, 7.5% Ti has improved mechanical properties and thermal conductivity of 314 W/m K at room temperature. TDS and high heat flux experiments results demonstrated that such doped graphites are promising candidate plasma facing components for HT-7U. 2003 Elsevier Science B.V. All rights reserved.

Graphene microsheets from natural microcrystalline graphite minerals: scalable synthesis and unusual energy storage

Mass production of graphene from graphite at a low cost is essential for its practical application since there is huge storage of natural graphite minerals on earth. However, extracting graphite from the minerals usually involves a complex and polluted purification process. Here, natural microcrystalline graphite minerals were directly used to produce high-quality graphene microsheets at a high yield of >70% through a scalable electrochemical & mechanical exfoliation approach. The graphene microsheets present the features of small sheet sizes of 0.2–0.6 μm2 and <5 atomic layers, few defects and high purity. The graphene microsheets can be highly dispersible in various solvents (the absorption coefficient of graphene microsheets dispersed in isopropanol is around 11.00 cm−1) and printable/paintable to make conductive films with a low sheet resistance of ∼10 ohm sq−1. The graphene products were used for energy-storage electrodes for a supercapacitor and a lithium ion battery. The supercapacitor reaches a high-rate areal performance of 77 mF cm−2 area capacity at a high charge/discharge rate of 20 mA cm−2. Notably, graphene anode batteries have a high coulombic efficiency of 99.2% and a high reversible specific capacity of 390 mA h g−1 (after 220 cycles) at 40 mA g−1 and of 200 mA h g−1 at 595 mA g−1 for a fast charge/discharge time of 17 min. This investigation demonstrates that graphene microsheets can be directly prepared from natural graphite minerals at high yield and low cost and potentially used for high-rate energy storage.

An electrochemical DNA sensor based on polyaniline/graphene: high sensitivity to DNA sequences in a wide range

A label-free electrochemical DNA sensor was fabricated by deposition of polyaniline and pristine graphene nanosheet (P/G(ratios)) composites in different mass ratios, DNA probe and bovine serum albumin (BSA) layer by layer on the surface of a glassy carbon electrode (GCE). Electrochemical impedance spectroscopy (EIS) was employed to monitor every step of fabrication of P/G(ratio)-based DNA sensors and to evaluate the detection results in terms of the hybridization of complementary DNA, mutant DNA and non-complementary DNA. The results illustrate that the P/G(ratio)-based DNA sensor could highly efficiently detect complementary DNA from 0.01 pm to 1 μm and discriminate single-nucleotide polymorphisms (SNPs). In the process of detection, double-stranded DNA (dsDNA), resulting from hybridization of a DNA probe, escaping from or remaining on the sensor surface, was monitored by changing the ratio of polyaniline (PANI) to graphene, which was decided by the competition between the electrostatic interaction and Brownian motion.

Graphene quantum dots cut from graphene flakes: high electrocatalytic activity for oxygen reduction and low cytotoxicity

3–8 nm sized high quality graphene quantum dots with zigzag edges and multi-heteroatom doping were synthesized through a green process of electrochemically cutting pristine few-layer graphene flakes. The graphene quantum dots exhibit the structural features of monodisperse and shaped nanocrystals co-doped by O, N and F elements or chemical groups mainly at the zigzag edges. The cutting and in situ doping of the graphene flakes into dots were realized in the potential/current exchangeable electrochemical etching process without any thermal treatment. It was found that the graphene dots showed high electrocatalytic activity for the oxygen reduction reaction including 70 times enhancement in voltammetric current in oxygen saturated KOH compared to that in nitrogen. In addition, low in vitro cytotoxicity and fluorescent labeling to Vero cells and NRK cells of the graphene dots were presented. The Gdots synthesized may potentially be applied in the fields of electrocatalysis and biomedicine.

Properties of mesoporous carbons prepared from different carbon precursors using nanosize silica as a template

Mesoporous carbons (MCs) with high-specific surface area and pore volume were synthesized from different carbon precursors (thermosetting phenol resin (TPR), mesophase pitch (MP), and polyacrylonitrile (PAN)) by using nanosize silica particles as a template. The influence of carbon precursors on the properties of as-prepared MCs was investigated by nitrogen adsorption, elemental analysis, and X-ray photoelectron spectroscopy. Results indicated that the pore structure as well as the surface chemical character of the resultant MCs differed greatly despite the preparation conditions being the same. Because the thermal stability of MP was higher and the dispersion ability of silica particles in MP/pyridine solution was poor, MC prepared from MP contained fewer micropores and mesopores compared with that prepared from TPR. In addition, MC derived from PAN had abundant nitrogen functional groups.

The primary results for the mixed carbon material used for high flux steady-state tokamak operation in China

Abstract Several types of carbon mixed materials have been developed in China to be used for high flux steady-state tokamak operation. Performance evaluation of these materials is necessary to determine their applicability as PFCs for high flux steady state. This paper describes the primary results of carbon mixed materials and the effects of dopants on properties are primarily discussed. Test results reveal that bulk boronized graphite has excellent physical and mechanical properties while their thermal conductivity is no more than 73 W/m K due to the formation of a uniform boron–carbon solid solution. In case of multi-element doped graphite, titanium dopant or a decreased boron content is favorable to enhance thermal conductivity. A kind of doped graphite has been developed with thermal conductivity as high as 278 W/m K by optimizing the compositions. Correlations among compositions, microstructure and properties of such doped graphite are discussed.

Double Core–Shell Si@C@SiO2 for Anode Material of Lithium-Ion Batteries with Excellent Cycling Stability

Lithium-ion batteries (LIBs) composed of silicon (Si) anodes suffer from severe capacity decay because of the volume expansion deriving from the formation of Li15 Si4 alloy. In this study, we prepared a double core-shell Si@C@SiO2 nanostructure by the modified Stöber method. In the process of Si lithiation, the carbon layer alleviates the large pressure slightly then the silica shell restricts the lithiation degree of Si. The combination of carbon interlayer and silica shell guarantees structural integrity and avoids further decay of capacity because of the formation of stable solid-electrolyte interphase (SEI) films. The resultant Si@C@SiO2 presents remarkable cycling stability with capacity decay of averagely 0.03 % per cycle over 305 cycles at 200 mA g-1 , an improvement on Si@C (0.22 %) by more than a factor of 7. This encouraging result demonstrates that the designation involved in this work is effective for mitigating the capacity decay of Si-based anodes for LIBs.