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.     

Mechanical properties of sol–gel prepared nanocomposite coatings in the presence of titania and alumina-derived nanoparticles

Hybrid nanocomposite coatings were prepared by sol–gel method using silica, titania and alumina nanoparticles derived from their alkoxides precursors; in the presence of 3-glycidoxypropyl-trimethoxysilane (GPTMS) and bisphenol A (BPA) on 1050 aluminium alloy substrate. The effect of type and ratio of nanoparticles on mechanical behaviour of the coatings were investigated by dynamic mechanical thermal analysis (DMA) and nanoindentation experiments. DMA results demonstrated that the values of the glass transition temperature (T_g) and the temperature at maximum tan (δ), (T_t) as well as the storage modulus of the hybrid samples depend mainly on the silane content and titania to alumina molar ratio of nanoparticles in the coating composition. In addition, nanoindentaion experiments were performed to study the mechanical properties such as hardness, elastic modulus and E/H ratio for the nanocomposite hybrid coatings. Nanoindentation results indicate that the homogenous reinforced structure was formed in the surface of nanocomposite coating with incorporation of titania and alumina-derived nanoparticles. The incorporation of TiO₂ in comparison with AlOOH nanoparticles in the GPTMS-based coatings showed an improving effect on E/H ratio.     

Study of the electrophoretic deposition copper–carbon nanotubes composite coatings in deep eutectic solvent using a Taguchi experimental design approach

ABSTRACT

The present work discusses the electrophoretic deposition (EPD) of copper–carbon nanotubes (Cu–CNTs) composite coatings in a Deep Eutectic Solvent (DES) media, using a non-symmetric deposition process. A Taguchi experimental design is implemented in order to assess the effect of the different parameters on the microstructural characteristics of the coatings. The analysis of the design of experiments (DOE) is performed with the signal to noise (S/N) ratio and the analysis of variance. The results clearly reveal that the time of deposition is the most influential parameter on crystallite size, whereas the asymmetric factor has the highest effect on the preferential deposition of Cu or C and thus on the chemical composition. It is therefore concluded that by changing some of the parameters, EPD can be implemented to develop nanostructured composite coating having a desired crystallite size and morphology.     

Corrosion resistance of Pb–Sn composite coatings reinforced by carbon nanotubes

Carbon nanotubes/Pb–Sn composite coatings were prepared by electrodeposition technology. The polarization curves and electrochemical impedance of the Pb–Sn coatings and carbon nanotube/Pb–Sn composite coatings were studied in 3.0 wt% HCl, 10 wt% NaOH, and 3.5 wt% NaCl electrolyte solutions, respectively. The results show that the corrosion potential of carbon nanotubes/Pb–Sn composite coatings were improved in the three kinds of corrosive medium, especially in 3.5 wt% NaCl electrolyte solution, where it increased significantly from −0.592 V (vs SCE) to −0.535 V (vs SCE). In addition, composite coatings have higher electrochemical impedance. Carbon nanotubes can improve the corrosion resistance of lead–tin electroplated coatings.     

Deposition,structure and tribological behavior of silver–carbon nanocomposite coatings

Silver–carbon nanocomposite coatings were deposited by plasma-enhanced chemical vapor deposition and d.c. magnetron sputtering of a silver target. Coatings with various metal concentrations were prepared by changing of acetylene and argon gas mixture ratio (C₂H₂/Ar), and concentrations of more than 40 at.%Ag was achieved in this study. Transmission electron microscope revealed that silver metallic grains with typically 15 nm were dispersed in amorphous carbon host matrix. Size of the grains increased with decrease of the gas mixture ratio due to secondary or triangularly formed metal grains. Tribological behavior of the coatings was investigated using reciprocating tribometer with in-situ electrical contact resistance measurement. Low and stable friction coefficient was achieved in the specimen with relatively low Ag concentration. Elemental mapping results on the ball after the friction tests reveal that tribofilm was formed on the ball when low and stable friction was achieved, and the tribofilm was mainly composed of C and Ag. It can be concluded that formation of the tribofilm is necessary for achieving low and stable friction.     

Electrophoretic deposition of bioactive wollastonite and porcelain–wollastonite coatings on 316L stainless steel

Wollastonite and porcelain–wollastonite coatings on stainless steel were obtained by electrophoretic deposition using acetone as dispersive medium. A direct electric current of 800 V for 3 min was used for obtaining the single wollastonite coating. A well-sintered layer was observed after heat treatment at 1050 °C for 1 h in air. The two-layer coating was obtained by depositing dental porcelain at 400 V for 30 s followed by the deposition of wollastonite at 400 V for 3 min. After forming the two layers, this complex coating was heat treated at 800 °C for 5 min. Under these conditions, strong bonds of both the interface wollastonite–porcelain and that of porcelain–metallic substrate were observed. The in vitro bioactivity assessment of the coatings was performed by immersing the deposited substrates in simulated body fluid (SBF) for 21 days. All the materials showed to be highly bioactive through the formation of a homogeneous apatite layer.     

Electrodeposition of Zn–SiC nanocomposite coatings

Zn–SiC composite coatings were obtained on mild steel substrate by electrodeposition technique with high-current efficiency. A slightly acidic chloride bath, containing SiC nanoparticles and gelatine as additive, was used. The electrodeposition was carried out under galvanostatic control with pulsed direct current; the effect of experimental parameters (temperature, average current density and particles concentration) on composition, morphology and structure of the deposit was studied. Coatings were characterized by means of scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffractometry and Vickers microhardness measurements. Zn–SiC electrodeposits with the best characteristics were obtained by performing electrodepositions at 45 °C, with 20 g L^?1 SiC in the bath and with average current density in the range 100–150 mA cm^?2. Under these experimental conditions, homogeneous and compact coatings, with low-grain size and SiC content ranging from 1.7 to 2.1 wt%, were found to be electrodeposited. Microhardness measurements showed for these deposits an increase of about 50 % with respect to those without nanoparticles obtained in the same experimental conditions.     

Electrophoretic co-deposition of Bi2O3–multiwalled carbon nanotubes coating as supercapacitor electrode

Nafion is suggested as an efficient assistant in preparing supercapacitor by employing nanoparticles. In this work, using a bi-additive of 0.10-mM NaOH + 0.10 g L⁻¹ Nafion, Nafion-assisted electrophoretic co-deposition of Bi₂O₃–multiwalled carbon nanotubes (MWCNTs) coating is successfully realized in ethanol solvent. The capacitance performances of the electrophoretic coatings in 6.0-M KOH electrolyte are investigated by cyclic voltammetry and galvanostatic charge–discharge techniques. Comparing with Bi₂O₃ coating prepared with electrophoretic deposition (EPD) by employing other additive (such as polyethyleneimine), the Bi₂O₃ coating prepared by Nafion-assisted EPD shows a better capacitance performance. Benefiting from the improvement in coating conductivity caused by MWCNTs, with a small additional amount of 4.0 wt.%, the Bi₂O₃–MWCNTs coating exhibits an amazing 164% increase of mass-specific capacitance (473 F g⁻¹ at the current density of 1.0 A g⁻¹) in comparison with pure Bi₂O₃ coating (179 F g⁻¹ at the current density of 1.0 A g⁻¹). The cyclic stability test exhibits excellent capacitance retention of 88.7% over 3000 cycles at a constant current density of 10.0 A g⁻¹. This work combines the advantages of MWCNTs, Nafion, and EPD to provide a facile route for preparing Bi₂O₃-based coating as a high-performance supercapacitor electrode.     

In situ growth of carbon nanotubes in alumina–zirconia nanocomposite matrix prepared by solution combustion method

In this work successful synthesis of multiwall carbon nanotube (CNT) using solution combustion and chemical vapor deposition (CVD) methods are reported. Ceramic nanocomposite samples of (Al_2?xFe_xO₃)–(y)ZrO₂ formula with x = 0.017, 0.034 and 0.17 and y = 0.15 were initially prepared. These were then subjected to CVD process during which the in situ reduction of iron oxide to metallic iron (Fe/Fe₃C) phase/s provided the necessary catalyst for the CNT formation. The formation of long flexible filaments with a smooth and regular surface bridging between alumina–zirconia (AZ) grains could be detected. The diameters of the formed filaments were in the range of ～70 to ～320 nm and length of the order of some tens of micrometers. However, transmission electron microscope (TEM) examinations also revealed the existence of small amounts of Bamboo-like carbon along with more or less straight CNTs. This could be related to the lack of strong interactions between the metallic iron phase/s and the nanocomposite support.     

Electrophoretic deposition of functionally-graded NiO–YSZ composite films

Functionally-graded NiO–8 mol % YSZ composite films were prepared by a controlled voltage-decay electophoretic deposition (EPD) process. The films consisted of three layers with varying NiO concentrations and porosities. Effects of different parameters including the type of the organic media, solid concentration, NiO:YSZ ratio, and iodine on the stability of EPD suspensions and deposition kinetics were studied. A stable NiO–YSZ suspension was attained in isopropanol with NiO–YSZ ratio of 60:40 and iodine concentration of 0.5 mM. The composite film contained varying NiO concentration from 46 wt.% near the substrate to 32 wt.% close to the electrolyte with 42 wt% NiO in the intermediate region. The thickness of each layer is about 10, 44 and 68 μm, respectively. The prepared anode could be promising for solid oxide full cells as it compromises good contact to the electrode with higher corrosion resistance and active reaction zone with the electrolyte.     

The application of Langmuir–Blodgett technique in preparation of the macroporous titania coatings

The aim of this work was preparation of the macroporous titania coatings with the use of the sol–gel process and poly(methylmetacrylate) beads as a template. The effectiveness of the Langmuir–Blodgett (LB) and dip-coating (DC) methods in deposition of polymer beads on the silicon wafers was compared. Resulted polymer layers and final porous titania coatings were analyzed with the use of the atomic force microscope. It was found, that application of the LB is possible only when arachidic acid is present in the subphase. It should be highlighted, that the application of the LB method is the novelty between the methods of the polymer beads arrangement having the diameter of 200–300 nm. Main factors which influence the structure and the arrangement of polymer templates were the concentration of the polymer suspension and the rate of the substrate immersion/withdrawal from the suspension. We established, that the optimal concentrations for preparation of polymer templates, exhibiting good arrangement of individual beads, were 0.5 and 6 % for LB and DC methods, respectively. The size of pores of the obtained macroporous titania (200–330 nm) corresponds well with the size of the polymer beads used as the template (200–235 nm).     

A kinetic study on the electrophoretic deposition of hydroxyapatite–titania nanocomposite based on a statistical approach

In the present study, the electrophoretic deposition (EPD) process of hydroxyapatite–titania nanocomposite was kinetically described by the use of response surface methodology (RSM). The electrostatic interaction between particles in ethanol based suspensions was determined by Zeta potential and particle size analyses. After successful electrophoretic deposition from hydroxyapatite–titania suspensions with 0, 10 and 20 wt% of titania nanoparticles, it was shown that Baldisserri model can well reproduce the experimental data among the other semi-empirical kinetic equations. The as-deposited hydroxyapatite–titania nanocomposites were characterized employing SEM, AFM, XRD, and FT-IR analyses. Then, the effects of deposition voltage, deposition time and wt% TiO₂ on the kinetic of EPD at two time intervals (10–60 s and 60–300 s) were identified and quantified via RSM based on a central composite design (CCD). According to the results obtained from the statistical analysis, it was found that the deposition rate decreases by an increase in wt% TiO₂ and time. Also, a transition in deposition mechanism from linear to parabolic mode was observed and two second order polynomial equations were fitted to the response (deposit weight) at each time intervals.     

Selective synthesis of single-walled carbon nanotubes on Fe–MgO catalyst by chemical vapor deposition of methane

The selective synthesis of single-walled carbon nanotubes (SWCNTs) with narrow chirality and diameter distribution by methane decomposition over Fe–MgO catalyst is reported. The catalyst was examined by nitrogen physisorption, X-ray diffraction, temperature programmed reduction, X-ray photoelectron spectroscopy, and UV–Vis diffuse reflectance spectroscopy to elucidate the structure and chemical state of the species responsible for SWCNT growth. High resolution electron microscopy, Raman and optical absorption spectroscopy, temperature programmed oxidation, energy dispersive X-ray spectroscopy and nitrogen physisorption were used to probe reaction selectivity, SWCNT chirality and diameter distribution, carbon yield and effectiveness of purification protocols. The yield of carbon increased with an increase in temperature, although SWCNTs selectivity decreased above the optimum synthesis temperature. Results established a clear link between the degree of dispersion of iron oxide species inside the MgO lattice and the catalyst selectivity for SWCNT growth.     

Influence of different additives on anatase–rutile transformation in titania system

Abstract

Hydrated titanium oxide gel samples containing 5% Fe, La, Ce, and Y oxides were prepared. Discs of size 10 mm dia. and ～1·5 mm thickness pressed from dry precursor gels were heated to temperatures in the range of 400–1000°C at intervals of 200 K for 2 h. The thermal behaviour of the doped and undoped gels was studied by DTA. The phase changes occurring in the gel precursors were identified using X-ray diffraction (XRD). The changes in electrical response of doped and undoped samples were monitored at room temperature by impedance spectral measurements. La₂O₃ was found to be the most effective in increasing the anatase to rutile transformation temperature while Fe₂O₃ has the reverse effect. Impedance spectroscopy studies reflected the conducting nature of the anatase matrix and the insulating rutile phase and appear to be simple tool for characterisation of such systems by bulk measurements.     

Selective growth of single-walled carbon nanotubes over Co–MgO catalyst by chemical vapor deposition of methane

The selective synthesis of SWCNTs with narrow chirality and diameter distribution by methane decomposition over a Co–MgO catalyst is reported. Raman spectroscopy, temperature programmed oxidation (TPO), UV–Vis–NIR absorption spectroscopy, and nitrogen physisorption were used to probe SWCNTs morphology, reaction selectivity, SWCNTs chirality and diameter distribution, and carbon yield. The catalyst was examined by nitrogen physisorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), and UV–Vis-diffuse reflectance spectroscopy to elucidate the structure and chemical state of the species responsible for SWCNT growth. The results established a clear link between the degree of dispersion of Co species inside the MgO lattice and the catalyst activity and selectivity for SWCNT growth. High dispersion and stabilization of Co species influenced catalytic activity for methane decomposition and the high SWCNT selectivity. The yield of carbon and SWCNT selectivity increased with an increase in temperature, however, SWCNTs diameter distribution shifts to larger diameter tubes as synthesis temperature was increased.     

Structure–property relationships in thermally-annealed multi-walled carbon nanotubes

The mechanical properties of individual multi-walled carbon nanotubes (MWCNTs) synthesized by a catalytic chemical vapor deposition (CVD) method followed by a series of high temperature annealing steps at 1200, 1800, 2200 and 2600 °C are investigated by a manipulator tool operated inside a scanning electron microscope. To investigate the relationship between the MWCNT structure and mechanical properties, such MWCNTs with a significantly different nanostructure are separately tested in tension, and subsequently observed their nanostructure and fracture morphology by a transmission electron microscope. The results show that the thermal annealing is effective for improving both the strength and modulus of the catalytic CVD-grown MWCNTs. The MWCNTs annealed at 1800, 2200 and 2600 °C display enhancements to their strengths by factors of ∼5.4, ∼5.1 and ∼15.6, and moduli by factors of ∼5.9, ∼13.2 and ∼18.9, respectively, compared to the MWCNTs annealed at 1200 °C. This effect is associated with the degree of waviness of the graphitic planes along the nanotube axis as well as the degree of crystallinity of the MWCNTs: the strength and modulus of the MWCNTs increases with a higher degree of orientation of the 0 0 2 graphitic planes and with a lower degree of defect concentration in the MWCNT structure.     

Electrophoretic deposition of carbon nanotube-reinforced hydroxyapatite bioactive layers on Ti–6Al–4V alloys for biomedical applications

Ultra-fine (20 nm) hydroxyapatite powders-reinforced with multi-walled carbon nanotubes were coated on Ti–6Al–4V medical alloy using electrophoretic deposition in an attempt to increase poor mechanical properties of hydroxyapatite, in particular inter-laminar shear strength between coating layer and implant surface. It is shown that the addition of carbon nanotubes increases both hardness, elastic modulus and inter-laminar shear strength of monolithic hydroxyapatite layers. A deposit thickness of 25 μm is also found to be critical for preventing crack formation during sintering.     

Adhesion and residual stress of plasma sprayed Alumina–titania coatings

Plasma-sprayed coatings are formed by the impacting of particles onto a fixed substrate layer-by-layer. Residual stresses inside the coatings are essential for their influencing on the coatings’ performance and durability during service life. In the present work, heat transfer and elastic–plastic residual stresses generation during plasma spraying in Al₂O₃–13wt.%TiO₂/NiCrAl (AT13) coating system were analyzed by finite element analysis (FEA). The sophisticated spraying process was simulated and the laminated structure of the coating was modeled under three-dimension. In this simulation, radial and axial compressive stresses were concentrated at the interfaces and inside the bond layer. Besides, at the specimen corner of the free edge, there were high tensile radial and axial stress concentrations. Such remarkable stresses, no matter tensile or compressive, may lead to the delamination and failure of coatings. Comparing with the numerical results, X-ray diffraction measurement was conducted on the AT13 coatings. As a result, the tested values matched well with the FEA simulated results.     

Visible light photocatalytic degradation of thiophene using Ag–TiO2/multi-walled carbon nanotubes nanocomposite

Ag–TiO₂ nanocatalyst, supported on multi-walled carbon nanotubes, was synthesized successfully via a modified sol–gel method, and the prepared photocatalyst was used to remediate aqueous thiophene environmentally by photocatalytic oxidation under visible light. The prepared Ag–TiO₂/multi-walled carbon nanotubes nanocomposite photocatalyst was characterized through X-ray diffraction, Brunauer–Emmett–Teller (BET), transmission electron microscopy, and UV–vis spectra (UV–vis). The results showed that both Ag and TiO₂ nanoparticles were well-dispersed over the MWCNTs and formed a uniform nanocomposite. Ag doping can eliminate the recombination of electron–hole pairs in the catalyst, and the presence of MWCNTs in the TiO₂ composite can change surface properties to achieve sensitivity to visible light. The optimum mass ratio of MWCNT:TiO₂:Ag was 0.02:1.0:0.05, which resulted in the photocatalyst's experimental performance in oxidizing about 100% of the thiophene in a 600 mg/L solution within 30 min and with 1.4 g L⁻¹ amount of catalyst used.     

Evaluation of corrosion resistance of polypyrrole/functionalized multi-walled carbon nanotubes composite coatings on 60Cu–40Zn brass alloy

Polypyrrole/multi-walled carbon nanotubes (PPy/MWCNT) and its carboxylic functionalized (PPy/MWCNT-COO⁻) composite films were successfully electropolymerized by cyclic voltammetry as protective coating against corrosion on 60Cu–40Zn brass alloy surface. It yielded to strongly adherent and smooth nanocomposite films. Kinetics of the corrosion protection was investigated in 3.5 wt% NaCl solutions by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests. The results showed that the presence of MWCNT in PPy coat considerably reduces the corrosion rate of 60Cu–40Zn brass alloy. The enhanced inhibition is most likely due to interaction between MWCNT and PPy. This in turn, improves the alloy passivation improvement and alters the permselectivity of the coating from anionic selectivity to the cationic selectivity. Moreover, PPy/MWCNT-COO⁻ functionalized nanocomposite provided higher corrosion resistance coating than PPy/MWCNT alone.