A ZrB2–SiC/SiC oxidation protective dual-layer coating for carbon/carbon composites

In order to improve the oxidation resistance of C/C composites, a ZrB₂–SiC/SiC oxidation protective dual-layer coating was prepared by a pack cementation combined with the slurry paste method. The phase and microstructure of the coating were characterised by X-ray diffraction, scanning electron microscope and energy-dispersive spectrometer analyses. The anti-oxidation and thermal shock resistance of the coating were also investigated. It was found that the ZrB₂–SiC/SiC coating could effectively improve the oxidation resistance of the C/C composites. The weight loss of the coated samples was only 1.8% after oxidation at 1773?K for 18?h in air. The coating endured 20 thermal shock cycles between 1773?K and room temperature with only 4.6% weight loss.     

Ablation behavior of ZrB2–SiC protective coating for carbon/carbon composites

Ablation behavior of ZrB₂–SiC protective coating for carbon/carbon composites during oxyacetylene flame test at 2500 °C was investigated by analyzing the microstructure differentiation caused by the increasing intensity of ablation from the border to the center of the surface. After ablation, a continuous SiO₂ scale, a porous SiO₂ layer inlaid with fine ZrO₂ nuclei, and a continuous ZrO₂ scale respectively emerged in the border region, the transitional region, and the center region. In order to investigate the ablation microstructure in the initial stage, the sub-layer microstructure was characterized and found to be mainly formed by coral-like structures of ZrO₂, which showed huge difference with the continuous structure of ZrO₂ on the surface layer. A kinetic model concerning the thickness change induced by volatilization and oxidation during ablation was built to explain the different growth mechanisms of the continuous ZrO₂ scale and the coral-like ZrO₂ structure.     

Functional polymer–polymer/carbon nanotube bi-component fibers

Bi-component fibers typically combine multiple functions that arise from at least two distinct components. As a result, these fibers can incorporate carbon nanotubes, which impart specific and controllable mechanical, electrical, and thermal transport properties to the fibers. Using gel spinning, sheath-core polyacrylonitrile–polyacrylonitrile/carbon nanotube bi-component fibers with a diameter of less than 20 μm and carbon nanotube concentrations of up to 10 wt% were produced. In these fibers, the carbon nanotubes were well dispersed and aligned along the fiber axis. The fibers exhibited a tensile strength as high as 700 MPa, and a tensile modulus as high as 20 GPa, as well as enhanced electrical and thermal conductivities when compared to the fibers without carbon nanotubes.     

A SiC–Si–ZrB2 multiphase oxidation protective ceramic coating for SiC-coated carbon/carbon composites

In order to improve the oxidation protective ability of SiC-coated carbon/carbon (C/C) composites, a SiC–Si–ZrB₂ multiphase ceramic coating was prepared on the surface of SiC-coated C/C composite by the process of pack cementation. The microstructures of the coating were characterized using X-ray diffraction and scanning electron microscopy. The coating was found to be composed of SiC, Si and ZrB₂. The oxidation resistance of the coated specimens was investigated at 1773 K. The results show that the SiC–Si–ZrB₂ can protect C/C against oxidation at 1773 K for more than 386 h. The excellent oxidation protective performance is attributed to the integrity and stability of SiO₂ glass improved by the formation of ZrSiO₄ phase during oxidation. The coated specimens were given thermal shocks between 1773 K and room temperature for 20 times. After thermal shocks, the residual flexural strength of the coated C/C composites was decreased by 16.3%.     

A Si–SiC oxidation protective coating for carbon/carbon composites prepared by a two-step pack cementation

To prevent carbon/carbon (C/C) composites from oxidation, a Si–SiC coating has been prepared by a two-step pack cementation technique. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis show that the coating obtained by the first step pack cementation is a porous β-SiC structure, and a dense structure consisting α-SiC, β-SiC and Si is obtained after heat-treatment by the second step pack cementation. By energy dispersive spectroscopy (EDS) analysis, a gradient C–SiC transition layer can be formed at the C/C-coating interface. The as-received coating has excellent oxidation protection ability and can protect C/C composites from oxidation for 166 h at 1773 K in air. The weigh loss of the coated C/C is due to the formation of bubble holes on the coating surface and through-coating cracks in the coating.     

Ultra-high-temperature ablation behavior of SiC–ZrC–TiC modified carbon/carbon composites fabricated via reactive melt infiltration

To improve the ablation resistance under the ultra-high temperature, the matrix of the carbon/carbon (C/C) composite was modified with a ternary ceramic of SiC–ZrC–TiC via reactive melt infiltration. The obtained ceramic matrix was composed of Zr-rich and Ti-rich solid solution phases of Zr_1−xTi_xC and SiC. This composite exhibited an excellent ablation property at 2500 °C with low mass and linear ablation rates of 0.008 mg s⁻¹ cm⁻² and 0.000 μm s⁻¹, respectively. The ablation mechanism was revealed with various microstructure characterizations and compared with those of C/C–SiC and C/C–TiC composites. Results showed that the degradations of these composites were primarily caused by the loss of the protective oxide scale via volatilization under the ultra-high temperature and flushing by high-speed airflow. The high ablation resistance of the C/C–SiC–ZrC–TiC composite was attributed to the protection of a multiphase oxide scale with high viscosity and low volatility.     

Microstructures,mechanical properties and corrosion resistance of sprayed Ca–P coating for micropatterning carbon/carbon substrate surface

Carbon/carbon (C/C) surface micropatterning is a method of modifying the surface into the complete and regular geometry. In this work, we introduce a positive effect on bonding strength between sprayed Ca–P coating and surface micropatterning C/C substrate. Interestingly, C/C substrate coated by Ca–P coating provides textured surface for a new bone ingrowth. The sprayed Ca–P coating is then subjected to microwave-hydrothermal (MH) treatment with the aim of eliminating surface defects and obtaining a uniform purity phase. These objectives were achieved in our previous study by the MH method. The molar ratio of Ca/P in the coatings is nearly close to 1, which is far below that of Ca/P for hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HA, 1.67). The purpose of this article is to transform the phases in the sprayed Ca–P coating, which owns the better bioactivity and high corrosion resistance. In order to raise the molar ratio of Ca/P, the coatings are treated under high-temperature (around 700 °C). They are analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and a fourier transform infrared spectra (FTIR). The bonding strength (coating/substrate), biological activity and corrosion resistance of the coatings are investigated. The resulting coatings own the different microstructures and phase compositions from the original sprayed Ca–P coating. Especially, results show that the shear strength of the sprayed Ca–P coating deposited on surface micropatterning C/C substrate increases by 61% which is more than that of the coating on non-surface micropatterning C/C substrate. Additionally, high-temperature treated coating presents a good biological activity and an excellent corrosion resistance of current density (1.3078 × 10^-6 A/cm²) and potential (−0.17 V_SCE).     

Microstructure of friction surface developed on carbon fibre reinforced carbon–silicon carbide (Cf/C–SiC)

We have used TEM to study the microstructure of friction surface of carbon fibre/carbon–silicon carbide composites brake discs after multi braking stop by using organic pads. A friction surface layer was developed consistently on the top of Si regions of the composites, but inconsistently on that of SiC and C. Inside the layer, amorphous silicon/silicon oxides appeared extensively with various non-metallic and metallic crystallites dispersed inside with sizes ranging from a few nanometers to several microns. A coherent interface between the friction layer and the composite surface was established under the braking conditions, whilst its sustainability varied notably in SiC and C regions. Microcracking near the friction surface appeared in SiC and C_f/C regions largely due to the extensive ductile deformation of SiC and weak interfaces between C and C_f. Material joining mechanisms were discussed to enlighten the friction transfer layer development on the surface of the composite discs.     

Effect of heat treatment at 1300°C on W coating prepared by double-glow plasma on carbon/carbon composite

W-coated carbon/carbon composite has been considered as an attractive ITER plasma facing material in fusion devices. In this paper, W coating was prepared on the carbon/carbon composite substrate by double-glow plasma method using pure W as target. Argon was input into the chamber as the plasma and the reactive gas. W-coated carbon/carbon composite was heat-treated in vacuum furnace at 1300°C for 1 h. Phase and microstructure of W coating were examined by X-ray diffraction and scanning electron microscopy, respectively. The chemical composition of W coating was analyzed by energy dispersive spectroscopy. The micro-hardness of the W coating was estimated by nano-indentation instrument. The results indicated that a continuous and dense W-modified layer could be successfully coated on the carbon/carbon composite surface by double-glow plasma method. W coating completely reacted with C to form WC at 1300°C. WC almost completely diffused into the carbon/carbon composite after heat treatment. The great decrease in elemental W on the carbon/carbon composite surface after heat treatment led to a significant reduction in micro-hardness.     

Preparation of carbon nanotubes/hydrogenated nitrile rubber composite by ultrasonic technique

将低相对分子质量的液体氢化丁腈橡胶(HNBR)溶于丙酮溶剂中,加入碳纳米管,用超声波技术制备了液体HNBR与碳纳米管的复合母料,然后再与HNBR混炼、硫化,获得碳纳米管/HNBR复合材料.结果表明,碳纳米管在HNBR中分散性好,对HNBR有较好的增强性,但在后期机械加工中产生了断裂.     

Maturation of a ketoprofen/β-cyclodextrin mixture with supercritical carbon dioxide

This work studies the maturation process of an association complex between an active compound, ketoprofen (KP) and a β-cyclodextrin (CD) with supercritical carbon dioxide (SC-CO₂). The process involves putting these two compounds into contact with a certain quantity of water, over a certain duration, to obtain the complex. We have studied the phenomena involved and the influence of several operating parameters on the complexation rate. The results enable us to suggest explanations for the phenomena involved in inclusion formation. An increase in the parameters related to the process: pressure, temperature, maturation period, agitation and density of SC-CO₂ resulted in an increase in the association rate of KP with CD in all cases. The water added at the end of the mixing procedure allowed a surface solubilization of CD where a ripening phenomenon can occur, but it is also a destabilizing agent for the water already adsorbed on the CD. The added water then favours an evolution towards a more stable energy state by enhancing the emptying of the CD cavities, and being replaced by KP. The mass ratio of the SC-CO₂/mixture had an effect on association, as the use of great volumes of CO₂ caused dilution of KP and was deleterious to the complexation rate. It was also found that the method of mixture preparation influenced the formation of the complex: adding water before KP onto CD inhibited complexation. The stoichiometry of complexation was found to be one molecule of KP with two molecules of CD. With control of both the operating conditions (pressure, temperature, maturation period, agitation and density of SC-CO₂) and the preparation of the mixing, this process leads to high percentages of complexation without the use of organic solvent.     

A microstructural study of carbon–carbon composites impregnated with SiC filaments

Porous multidirectional carbon/carbon composite obtained by pulse chemical vapour infiltration (PCVI) was impregnated with silicon carbide (SiC) derived from pyrolysis of polymethylsiloxane resin (PMS). The impregnation process was made to improve oxidation resistance and mechanical properties of MD C/C composite. The resin was used as a source of silicon carbide component of the composite forming after heat treatment above 1000 °C. During this process SiC thin filaments were formed inside the porous carbon phase. The aim of this work was to investigate the structure and microstructure of the constituents of carbon composite obtained after pyrolysis of SiC PMS precursor. Microscopic observations revealed that during careful heat treatment of crosslinked polymethylsiloxane resin up to 1700 °C, the filaments (diameter 200–400 nm) crystallized within porous carbon phase. The filaments were randomly oriented on the composite surface and inside the pores. FTIR spectra and XRD analysis of the modified C/C composite showed that filaments had silicon carbide structure with the crystallite size of silicon carbide phase of about 45 nm. The Raman spectra revealed that the composite contains two carbon components distinctly differing in their structural order, and SiC filaments present nanocrystalline structure.     

Production of carbon molecular sieves by plasma treated activated carbon fibers☆

Carbon molecular sieves (CMS) are valuable materials for the separation and purification of gas mixtures. In this work, plasma deposition was used aiming to the formation of pore constrictions, by narrowing the surface pore system of commercial activated carbon fibers (ACF). For this reason propylene/nitrogen or ethylene/nitrogen discharges of 80 and 120 W were used. The molecular sieving properties of the plasma treated ACF were evaluated by measuring the adsorption of CO₂ and CH₄. The CO₂/CH₄ selectivity was significantly improved and depended on plasma treatment conditions (discharge gas and power). The optimum CO₂/CH₄ selectivity (26) was observed for C₂H₄/N₂ plasma treated ACF at 80 W. Sample scanning electron microscopy (SEM) analysis after plasma treatment revealed an external film formation and X-ray photoelectron spectroscopy (XPS) analysis showed the incorporation of nitrogen functional groups in the film, which probably interact with CO₂, thereby altering CO₂/CH₄ selectivity.     

Thermal conductivity of a graphene oxide–carbon nanotube hybrid/epoxy composite

Thermally conductive graphene oxide (GO)–multi-wall carbon nanotube (MWCNT)/epoxy composite materials were fabricated by epoxy wetting. The polar functionality on the GO surface allowed the permeation of the epoxy resin due to a secondary interaction between them, which allowed the fabrication of a composite containing a high concentration of this hybrid filler. The thermal transport properties of the composites were maximized at 50 wt.% of filler due to fixed pore volume fraction in filtrated GO cake. When the total amount of filler was fixed 50 wt.% while changing the amount of MWCNTs, a maximum thermal conductivity was obtained with the addition of about 0.36 wt.% of MWCNTs in the filler. Measured thermal conductivity was higher than the predicted value based on the by Maxwell–Garnett (M–G) approximation and decreased for MWCNT concentrations above 0.4%. The increased thermal conductivity was due to the formation of 3-D heat conduction paths by the addition of MWCNTs. Too high a MWCNT concentration led to increased phonon scattering, which in turn led to decreased thermal conductivity. The measured storage modulus was higher than that of the solvent mixed composite because of the insufficient interface between the large amount of filler and the epoxy.     

Diamond-like carbon — present status

Hydrogen free diamond-like carbon (DLC) films have been the subject of investigation all over the world during the past 25 years due to the unique combination of their properties that can be found between those of diamond and those of graphite. Intensive work throughout the world in the past 10 years has led to a much better understanding of the complicated mechanisms involved in the deposition of these films. This led to a significant improvement in the deposition processes in the variety of systems employed, enabling fabrication of films with better properties. The present work gives the author's assessment of the current status of DLC film deposition. Topics addressed include: deposition systems; characterization methods; film properties; and possible applications.     

Effects of γ-radiation versus H2O2 on carbon steel corrosion

The effect of ionizing radiation on steel corrosion is an important materials issue in nuclear reactors. In the presence of ionizing radiation water decomposes into both oxidizing and reducing species (e.g., OH, H₂O₂, O₂⁻) whose net interactions with steels are not fully understood. The effect of radiation on the corrosion kinetics of carbon steel has been studied at pH 10.6 and room temperature, using electrochemical and chemical speciation analyses. The present study investigates the effect of γ-radiation on carbon steel corrosion and compares it with that of chemically added H₂O₂, which is considered to be the key radiolytically produced oxidant at room temperature. Various oxide films were pre-grown potentiostatically on carbon steel electrodes, and then exposed to either γ-radiation at a dose rate of ∼6.8 kGy h⁻¹ or to H₂O₂ in a concentration range of 10⁻⁶ to 10⁻² M. The corrosion kinetics were studied by monitoring the corrosion potential (E_CORR), and periodically performing linear polarization (LP) and electrochemical impedance spectroscopy (EIS) measurements.     

Effect of specific surface area on capacitance in asymmetric carbon/α-MnO2 supercapacitors

α-MnO₂ has been made using a solid state synthesis and the specific surface area then modified through milling. The formation of α-MnO₂ (specific surface area 96 m² g⁻¹) has been studied by SEM and powder XRD prior to milling. Electrode films (cast using MnO₂, graphite and PVDF) have been investigated using N₂ sorption at 77 K and show a more complex relationship than their parent oxides. Specific capacitances of 235 F g⁻¹ were observed in cyclic voltammetry studies in (NH₄)₂SO₄ (aq.) electrolyte. Good cyclability was observed in hybrid C/MnO₂ cells investigated through both galvanostatic and electrochemical impedance techniques. The specific capacitances of the cells were found to correlate with S_BET of the electrode films and not that of the parent MnO₂ powders.     

A high-performance asymmetric supercapacitor based on carbon and carbon–MnO2 nanofiber electrodes

An asymmetric supercapacitor with high energy and power densities has been fabricated using MnO₂/carbon nanofiber composites as positive electrode and activated carbon nanofibers as negative electrode in Na₂SO₄ aqueous electrolyte. Both electrode materials are freestanding in nature without any conductive additives or binders and exhibit outstanding electrochemical performances. The as-assembled asymmetric supercapacitor with optimal mass ratio can be operated reversibly over a wide voltage range of 0–2.0 V, and presents a maximum energy density of 30.6 Wh kg⁻¹, which is much higher than those of symmetric supercapacitors. Moreover, the supercapacitor exhibits excellent rate capability (high power density of 20.8 kW kg⁻¹ at 8.7 Wh kg⁻¹) and long-term cycling stability with only 6% loss of its initial capacitance after 5000 cycles. These attractive results make these freestanding materials promising for applications in aqueous electrolyte-based asymmetric supercapacitors with high energy and power densities delivery.