Electrophoretic deposition of carbon nanotube–ceramic nanocomposites

The purpose of this paper is to present an up-to-date comprehensive overview of current research progress in the development of carbon nanotube (CNT)–ceramic nanocomposites by electrophoretic deposition (EPD). Micron-sized and nanoscale ceramic particles have been combined with CNTs, both multiwalled and single-walled, using EPD for a variety of functional, structural and biomedical applications. Systems reviewed include SiO₂/CNT, TiO₂/CNT, MnO₂/CNT, Fe₃O₄/CNT, hydroxyapatite (HA)/CNT and bioactive glass/CNT. EPD has been shown to be a very convenient method to manipulate and arrange CNTs from well dispersed suspensions onto conductive substrates. CNT–ceramic composite layers of thickness in the range <1–50 μm have been produced. Sequential EPD of layered nanocomposites as well as electrophoretic co-deposition from diphasic suspensions have been investigated. A critical step for the success of EPD is the prior functionalization of CNTs, usually by their treatment in acid solutions, in order to create functional groups on CNT surfaces so that they can be dispersed uniformly in solvents, for example water or organic media. The preparation and characterisation of stable CNT and CNT/ceramic particle suspensions as well as relevant EPD mechanisms are discussed. Key processing stages, including functionalization of CNTs, tailoring zeta potential of CNTs and ceramic particles in suspension as well as specific EPD parameters, such as deposition voltage and time, are discussed in terms of their influence on the quality of the developed CNT/ceramic nanocomposites. The analysis of the literature confirms that EPD is the technique of choice for the development of complex CNT–ceramic nanocomposite layers and coatings of high structural homogeneity and reproducible properties. Potential and realised applications of the resulting CNT–ceramic composite coatings are highlighted, including fuel cell and supercapacitor electrodes, field emission devices, bioelectrodes, photocatalytic films, sensors as well as a wide range of functional, structural and bioactive coatings.     

Effect of pH and carbon nanotube content on the corrosion behavior of electrophoretically deposited chitosan–hydroxyapatite–carbon nanotube composite coatings

In the first stage, chitosan (CH)–hydroxyapatite (HA)-multiwalled carbon nanotube (MWCNT) composite coatings were synthesized by electrophoretic deposition technique (EPD) on 316L stainless steel substrates at different levels of pH and characterized by X-ray diffraction (XRD), Raman spectroscopy, FTIR and field emission scanning electron microscopy (FESEM). A smooth distribution of HA and MWCNT particles in a chitosan matrix with strong interfacial bonding was obtained. In the next stage, effects of pH and MWCNT content of the suspension on the corrosion behavior and deposition mechanism were studied. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) curves revealed that increasing pH level of the suspension increases the corrosion protection properties of the deposited composite coating in simulated body fluid (SBF). Furthermore, Nyquist plots showed that increasing MWCNT content of the suspension resulted in higher amounts of R_p, but because of the capillary properties of MWCNTs and degradability of the chitosan matrix, corrosion protection level of the coatings containing HA–CH–MWCNT was lower than those of coatings containing solely HA–CH. Amperometric curves in different pH levels of the suspension revealed that the system is diffusion controlled at elevated pH values.     

A high-performance three-dimensional micro supercapacitor based on ripple-like ruthenium oxide–carbon nanotube composite films

A micro-supercapacitor with a three-dimensional configuration has been fabricated using an inductively coupled plasma etching technique. A ruthenium oxide–carbon nanotube (CNT) composite with a ripple-like morphology is successfully synthesized using a cathodic deposition technique while using silica-based three-dimensional microstructures as a template. The desired network of carbon nanotubes in the composite facilitates electrolyte penetration and proton exchange/diffusion. A single three dimensional microelectrode is studied using cyclic voltammetry, and a specific capacitance of 272 mF·cm⁻² is observed at 5 mV s⁻¹ in a neutral Na₂SO₄ solution. The accelerated cycle life is tested at 80 mV s⁻¹, and a satisfactory cyclability is observed. When placed on a chip, the symmetric cell exhibits good supercapacitor properties, the specific capacitance up to 37.23 mF cm⁻² and specific power density up to 19.04 mW cm⁻² were obtained at 50 mA cm⁻².     

Structure–property–processing relationships of single-wall carbon nanotube thin film piezoresistive sensors

In an investigation of structure–property–processing relationships for SWCNT thin film piezoresistive sensors, the gauge factor of the sensors for a small tensile deformation (less than 2% strain) was found to be close to unity and showed negligible dependence on the film thickness and SWCNT bundle length (L) and diameter (d). However, for a large tensile deformation (20–30% strain), the film thickness and the microstructure of SWCNTs had a compounding effect on the piezoresistive behavior. A gauge factor of ∼5 was obtained for the sensors fabricated with SWCNT bundles of short length and thin diameter (L = 549 nm and d = 3.7 nm) with thicker films. Furthermore, the gauge factor of the sensors was found inversely proportional to the excluded volume V_ex of SWCNT bundles (V_ex ∝ 1/L² d).     

Field emission of ribonucleic acid–carbon nanotube films prepared by electrophoretic deposition

Carbon nanotube (CNT) films are fabricated on indium tin oxide (ITO) glass substrates by combining electrophoresis with photolithography using ribonucleic acid (RNA)–CNT hybrids as functionalized CNTs and their emission properties are investigated. The CNTs are well-dispersed by wrapping them with RNA and well-defined RNA–CNT patterns are obtained on the ITO glass substrate. The RNA–CNT films show good field emission properties, such as high current densities, low turn-on fields, and uniform emission images. The RNA–CNT hybrids compare favorably to other functionalized CNTs for use in the electrophoretic deposition.     

Performance evaluation under supercritical conditions of Fe–Mn–Cu–K composite catalysts synthesized by supercritical methods

A series of nanosized Fe–Mn–Cu–K composite catalysts was prepared by a supercritical combined technology. The nanosized catalysts were characterized by means of XRD, TEM and BET techniques, and tested for catalytic performance under Fischer–Tropsch synthesis (FTS) reaction conditions. The catalyst synthesized by the supercritical combined technology has some excellent properties. Additionally, the drying and crystallization of nanosize catalyst could be achieved simultaneously by this supercritical combined technology. The addition of Mn, Cu and K promoters can improve the catalytic performance properties of the catalyst, including lower CH₄ and CO₂ formation rates, and higher production rates of desired light-olefin. The optimal performance with a 95.7% CO conversion and a 46.5% light-olefin yield was obtained by using a catalyst component of Fe/Mn/Cu/K = 60:25:10:8.5. In summary, optimal catalytic performance was obtained by using the nanosized catalyst under supercritical reaction conditions, resulting in higher CO conversion, less byproduct CO₂ formation, and a higher yield of light-olefin.     

Prospective growth region for chemical vapor deposition synthesis of carbon nanotube on C–H–O ternary diagram

Chemical vapor deposition has become a standard process for synthesizing carbon nanotubes. Since the successful use of chemical vapor deposition for the first time, much effort has been expended into exploring various carbon sources that can be used to synthesize carbon nanotubes, such as methane, ethane, and ethanol. However, whole perspectives for suitable carbon sources have not been clear. In this study, we performed experiments in order to determine that the appropriate C–H–O components ratio in raw materials can be used to synthesize carbon nanotubes. We also examined a variety of raw materials in our newly developed round-trip-type vacuum furnace in order to determine whether they could be used to synthesize a carbon nanotube. We used Raman spectroscopy to identify the developed carbon nanotube, and we plotted the component ratios of effective and ineffective materials on a C–H–O ternary diagram; in this diagram, the growth region became highly apparent. It should be noted that for the growth of the carbon nanotube, this region should satisfy the equation O < C < (H + O) in molar ratio. Furthermore, it was observed that adjusting the component ratios by mixing raw materials did not cause an inconsistency in the growth region.     

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.     

Continuous electrodeposition of polypyrrole on carbon nanotube–carbon fiber hybrids as a protective treatment against nanotube dispersion

This paper describes the electrodeposition of nanometer size polypyrrole layers on carbon fibers coated with multi-wall carbon nanotubes. The obtained carbon nanotubes–carbon fiber hybrids are characterized by electrochemistry, electron microscopy, vibrational spectroscopy and thermogravimetry. The electrical properties are measured for various polymerization times and electrolytes. The conductivity is found decrease with increasing polypyrrole thickness, but all the carbon nanotube–carbon fiber hybrids remain more conductive than pristine carbon fibers having a sizing coating. Finally it is shown that polypyrrole acts as a protecting layer against carbon nanotube dispersion when sonicated in ethanol.     

Synthesis of a graphene–carbon nanotube composite and its electrochemical sensing of hydrogen peroxide

Graphene, whose structure consists of a single layer of sp²-hybridized carbon atoms, provides an excellent platform for designing composite nanomaterials. In this study, we have demonstrated a facile process to synthesize graphene–multiwalled carbon nanotube (MWCNT) composite. The graphene–MWCNT composite material is endowed with a large electrochemical surface area and fast electron transfer properties in Fe(CN)₆^3?/4? redox species. A graphene–MWCNT composite modified electrode exhibits good performance in terms of the electrocatalytic reduction of H₂O₂; a sensor constructed from such an electrode shows a good linear dependence on H₂O₂ concentration in the range of 2 × 10^?5 to 2.1 × 10^?3 mol L^?1. The detection limit is estimated to be 9.4 × 10^?6 mol L^?1. This study provides a new kind of composite modified electrode for electrochemical sensors.     

Dry reforming of methane over palladium–platinum on carbon nanotube catalyst

A dry reforming (DR) catalyst based on bimetallic Pd–Pt supported on carbon nanotubes is presented. The catalyst was prepared using a microwave-induced synthesis. It showed enhanced DR activity in the 773–923?K temperature range at 3 atm. Observed carbon balances between the reactant and product gases imply minimal carbon deposition. A global three-reaction (reversible) kinetic model—consisting of DR, reverse water gas shift, and CH₄ decomposition (MD)—adequately simulates the observed concentrations, product H₂/CO ratios, and reactant conversions. Analysis shows that, under the conditions of this study, the DR and MD reactions are net forward and far from equilibrium, while the RWGS is near equilibrium.     

Warpage of carbon–epoxy composite channels

Abstract

Finite element models have been developed of the warpage occurring during the cure of unidirectional carbon fibre-epoxy resin channels. These were based on equivalent experimental channels that were formed on a male mould, with the distortions determined separately after cure and post-cure. To quantify the warpage, the decrease in enclosed angle, or spring forward, of the two corners of the U-shaped cross-sections were calculated; values were determined using displacements from both the finite element predictions and measurements of the experimental channels. The experimental channels were fabricated so that several different factors affecting the distortions could be investigated. These included: fibre orientation; cured or post-cured state; conditions of post-cure; fillet radius of the channel corners; and channel thickness, width and depth. Results across the different channels showed predictions of 1° spring forward where the fibres followed the cross-section profile (0° channels), which were fairly accurate, at 75-85% of the experimental values. However, for the channels that had the fibres aligned parallel to the channel length (90° channels), negligible values were predicted, which were considerably lower in magnitude than the experimental values of 0° to-5° (spring back). Subsequent inhomogeneous models and optical microscopy work indicated that the unpredicted spring back in the more flexible 90° channels was caused by a thin (<0·1 mm) resin layer on the outer surface of all the channels. The small underprediction of spring forward in the stiffer 0° channels was attributed to unmodelled cure shrinkage, which was moderated by some reduced spring back due to the presence of a resin layer.     

Fischer-Tropsch synthesis in the presence∣ of cobalt-containing composite materials based on carbon

The Fischer-Tropsch synthesis in the presence of composite materials prepared by the IR pyrolysis of polyacrylonitrile (PAN) with cobalt salts immobilized on it was studied. The catalysts were small granules containing PAN carbonization products and to 80% cobalt metal particles of size 10–17 nm. The synthesis was performed in flow reactors with a fixed bed and a catalyst bed suspended in a liquid at 2–3 MPa and 200–310°C. It was established that the activity of the catalyst depends on the nature of the cobalt salt used, the temperature of IR pyrolysis, and the synthesis conditions. The catalyst prepared with the use of cobalt carbonate exhibited the greatest activity. The yield of liquid hydrocarbons on it reached ～70 g/m³ at ～60% selectivity. It was found that the test composite materials were characterized by an extremely high productivity of 2–5 kg (kg Co)^?1 h^?1.     

Engineering nanostructured thermal spray coatings: process–property–performance relationships of ceramic based materials

Abstract

Nanostructured powders were deposited using thermal spraying to produce coatings having internal features of nanosized dimensions. Several ceramic based materials were studied, including WC–12 wt-%Co, TiO₂, hydroxyapatite, Al₂O₃–13 wt-%TiO₂ and yttria stabilised zirconia. The effect of the thermal spray conditions on the microstructure, phase composition, properties and performance was investigated. Key nanostructural features of the coatings were identified and their potential benefit in contributing to enhanced behaviour explored. Issues relating to design strategies and process control for engineering these types of coatings with performance characteristics tailored for targeted applications are discussed.     

Improved electrical conductivity of a non-covalently dispersed graphene–carbon nanotube film by chemical p-type doping

Using a high-pressure air spray we developed a method to deposit electrically-conducting thin films consisting of non-covalently dispersed graphene and carbon nanotubes. The graphene–carbon nanotube film was immersed in a nitric acid and followed by exposure to fuming nitric acid. The acid treatment induced an increased concentration of atomic nitrogen on the graphene basal plane and carbon nanotube sidewall. This result indicates chemical p-type doping of the graphene oxide–carbon nanotube film. After the two acid treatments, the spray coated graphene oxide–carbon nanotube films on a glass substrate exhibit a low sheet resistance of 171 Ω/sq, and a high transmittance of 84% at a wavelength of 550 nm.     

Electrodics of methanol oxidation on Pt-f-multiwalled carbon nanotube composite, prepared by γ-radiolysis

We report the electrodics of methanol oxidation on Pt-multiwalled carbon nanotube composites (Pt-f-MWCNTs), prepared by γ-radiolysis of K₂PtCl₆ in the presence of HOOC-functionalized multiwalled carbon nanotubes. The electrocatalytic activity for the methanol oxidation was studied using cyclic and linear sweep voltammetric techniques on the stationary indium tin oxide and rotating gold disc electrodes, respectively. Higher values of oxidative (anodic) current were obtained using Pt-f-MWCNTs compared to the polycrystalline Pt electrode. This phenomenon is attributed to the synergistic effect of oxy groups on MWCNTs, which alleviate CO poisoning. The electrodics of the reaction at various temperatures was studied using linear sweep voltammetry (LSV) on a rotating disc gold electrode, modified with the composite. From the Koutecky-Levich plots, the standard rate constant (k⁰) was determined to be 7.9 ± 1.9 × 10⁻⁸ cm s⁻¹.     

Tension–tension fatigue behavior of carbon nanotube wires

The tension–tension fatigue behavior of three types of as-received carbon nanotube (CNT) wires, comprising of 30-, 60-, and 100-yarn, was investigated. Fatigue tests were conducted at 35%, 50%, 60%, 75% and 80% of their ultimate tensile strengths which provided the fatigue life data (S–N curves). Their electrical conductivities were measured as a function of the number of cycles. Fatigue strength of the CNT wires at a given number of cycles decreased with an increase in the number of yarns. Their electrical conductivity increased with increase of applied fatigue load and number of fatigue cycles. Damage and failure mechanisms involved relative sliding of yarns in CNT wires leading to the formation of kink bands, followed by plastic deformation and then breakage of yarns. Microtomography density measurements provided the evidence that the increase in conductivity was due to the reduction of micro/nano voids between and inside the yarns, which decreased with increasing fatigue load and number of fatigue cycles.     

Electro-deposition and characterizations of nickel coatings on the carbon–polythene composite

Nickel coating on the carbon–polythene composite plate was prepared by electrodeposition in a nickel sulfate solution in this work. The morphology and cross-sectional microstructure of the nickel coating were examined by scanning electron microscope (SEM) and optical microscope (OM), respectively. The influence of bath temperature on the nickel deposition rate was investigated experimentally. The adhesion between the coating and the substrate was evaluated by the pull-off test. The corrosion behavior of the coating in an aqueous solution of NaCl was studied by electrochemical methods. The results showed that the nickel electrodeposition rate could reach up to 0.68 μm min⁻¹ on average under conditions of cathodic current density of 20 mA cm⁻² and bath temperature of 60 °C. It was confirmed that increasing the bath temperature up to 50 °C had a positive effect on the nickel deposit rate, while an adverse effect was observed beyond 60 °C. The adhesion strength between the nickel coating and the substrate can be more than 2.3 MPa. The corrosion potential of the bright coating in the NaCl solution was more positive than that of the dull coating, and the anodic dissolution rate of the bright coating was also far lower at the same polarization potential compared with the dull coating.