Electrophoretic deposition of hydroxyapatite-chitosan-CNTs nanocomposite coatings

Three component suspensions of hydroxyapatite (HA), chitosan and CNTs were prepared in ethanol base solution (15 vol% water and 0.05 vol% acetic acid). The adsorption of HA nanoparticles on CNTs was investigated by FTIR and SEM analysis. It was found that HA nanoparticles are adsorbed on CNTs via chemical bonding between -NH₂ groups of chitosan (adsorbed on their surface) and -COOH groups of CNTs. Current density as well as kinetics of EPD was studied at 60 V. It was found that current density increases or remains nearly constant during EPD due to the rise in water electrolysis as deposit grows on the substrate. Deposition weight against EPD time showed a linear trend due to the absence of any voltage drop over the deposit during EPD. The incorporation of chitosan and CNTs in the microstructure of coatings was confirmed by TG/DTA and SEM analysis. CNTs exhibited high efficiency in reinforcing the microstructure of coatings and preventing from their cracking. CNTs incorporation in the coatings improved their mechanical properties (adhesion strength, hardness and elastic modulus) and corrosion resistance.     

Electrophoretic deposition of titania–carbon nanotubes nanocomposite coatings in different alcohols

The suspensions of titania nanoparticles in different alcohols (methanol, ethanol and butanol) were prepared using triethanolamine (TEA) as a dispersant. The optimum concentration of TEA was 16.67, 8 and 0.33 mL/L in methanol, ethanol and butanol, respectively. Two component suspensions of titania (20 g/L) and carbon nanotubes (CNTs) (0.1, 0.2, 0.5 and 1 g/L) were prepared in different alcohols without and with optimum concentration of TEA. The finer and positively charged titania nanoparticles were heterocoagulated on the surface of coarser and negatively charged CNTs and generated the titania–CNT composite particles with the net positive charge. In the presence of TEA, titania nanoparticles completely covered CNTs surface due to their higher positive surface charge. At same CNT concentration, the deposition rate was faster for suspensions with TEA additive due to the faster mobility of the composite particles. The photocatalysis efficiency of coatings for methylene blue degradation increased as CNTs were incorporated in their microstructure.     

Electrophoretic deposition of hydroxyapatite nanostructured coatings with controlled porosity

Hydroxyapatite (HA) coatings with controlled porosity were prepared by electrophoretic deposition (EPD) method. Carbon black (CB) particles were used as the sacrificial template (porogen agent). Two component suspensions containing different concentrations of HA and CB particles were prepared in isopropanol. It was found that the finer and positively charged HA nanoparticles are heterocoagulated on the coarser and negatively charged CB particles to form CB–HA composite particles with net positive charge. The deposition rate from the suspensions with WR (C_CB/C_HA ratio) of 0.25 was faster than that of those with WR: 0.5 at initial times of EPD. However the situation was reversed at longer EPD times. It was also found that the amount of porosity in the coatings increases as the CB concentration in the suspension increases (15%, 24%, 31%, 43% for the coatings deposited from the suspensions with 20 g/L HA nanoparticles and 0, 5, 10 and 20 g/L CB particles, respectively).     

Electrophoretic deposition of fiber hydroxyapatite/titania nanocomposite coatings

Hydroxyapatite/titania nanocomposite coatings were electrophoretically deposited from ethanolic suspensions of titania and fiber shaped hydroxyapatite (FHA) nanoparticles. Triethanolamine (TEA) was used to enhance the colloidal stability of particles in suspensions. Electrophoretic deposition (EPD) was performed using the suspensions with different concentrations (wt%) of titania/FHA particles. EPD rate decreased more rapidly with time for suspensions with higher wt% of FHA due to the higher voltage drop over the deposits shaped from them. Stacking of long FHA particles on the substrate during EPD resulted in the formation of coarse pores in the deposits. It was found that titania nanoparticles can more efficiently infiltrate through and fill the pores in TEA containing suspensions due to the stronger electrostatic repulsion force between pore walls (FHA) and titania nanoparticles in them. The coatings deposited from the suspensions with 50 wt% of FHA or more did not crack during drying due to the significant reinforcement action provided by high wt% of FHA in them. Nanocomposite coatings deposited from TEA containing (2 mL/L) suspensions with 50 and 75 wt% of FHA had the best corrosion resistance in simulated body fluid (SBF) solution due to their crack-free microstructure and efficiently filled pores.     

Dynamic behavior of carbon nanotube field emitters observed by in situ transmission electron microscopy

We examined dynamic behavior of field-emitting carbon nanotubes (CNTs) by in situ transmission electron microscopy (TEM). CNTs employed in the present study were multi-wall CNTs prepared by chemical vapor deposition, double-wall and single-wall CNTs produced by arc discharge. Orientation of CNTs, being random when no electric field was applied, were aligned parallel to the electric field and returned reversibly to their original direction when the field was turned off. In addition to this reversible behavior without serious structural damage in CNTs, sublimation and violent oscillation of CNTs were observed. When CNTs were bundled, branching of the bundle by electric static force was also observed.     

Fabrication of CNT-SiC/SiC composites by electrophoretic deposition

Due to the outstanding mechanical and thermal properties of carbon nanotubes (CNTs), they are considered suitable reinforcement for structural materials. In this study, for the first time, electrophoretic deposition (EPD) was used to deposit (multi-walled) CNTs onto SiC fibres (SiC_f) to form an effective CNT interphase layer for SiC_f/SiC composites. This deposition was followed by electrophoretic infiltration of the CNT-coated SiC fibre mats with SiC powder to fabricate a new CNT-SiC-fibre-reinforced SiC-matrix (SiC_f/SiC) composite for fusion applications. In these EPD experiments, a commercial aqueous suspension of negatively charged CNTs and an optimized aqueous suspension of negatively charged SiC particles were used. The CNT-coatings on the SiC fibres were firm and homogenous, and uniformly distributed nanotubes were observed on the fibre surfaces. In a following step of EPD, a thick SiC layer was formed on the fibre mat when the CNT-coated SiC fibres were in contact with the positive electrode of the EPD cell; however, spaces between the fibres were not fully filled with SiC. Conversely, when CNT-coated SiC fibres were isolated from the electrode, the SiC particles were able to gradually fill the fibre mat resulting in relatively high infiltration, which leads to dense composites.     

Enrichment of metallic carbon nanotubes by electric field-assisted chemical vapor deposition

We report herein a rational approach to increase the proportion of metallic carbon nanotubes (CNTs) in horizontally aligned ultralong CNT arrays by electric field-assisted chemical vapor deposition. In a gas flow-directed growth mode, the buoyancy caused by temperature differences near the substrate can lift catalyst particles or CNTs from the substrate into the laminar flow so that ultralong CNT arrays with mixed metallic (m-) and semiconducting (s-) CNTs can be obtained. It was verified that the percentage of m-CNTs was about 47% for pristine CNTs. When an electric field was introduced during CNT growth, the grown CNTs were polarized and the generated electric field force assisted them into the laminar flow. The greater polarizability of m-CNTs compared to s-CNTs resulted in more m-CNTs lifted and an increased m- to s-CNT ratio in the array. Measurements of CNT electrical properties showed that the percentage of m-CNTs could reach 80% when the electric field intensity was set at 200 V/cm.     

Direct measurement of aerosol glass fiber alignment in a DC electric field

We report non-conducting aerosol fiber (i.e., glass fiber) alignment in a DC electric field. Direct observation of fiber orientation state is demonstrated and quantitative analysis of fiber alignment is made using phase contrast microscopy in four different conditions: (i) dry air and naturally charged fibers, (ii) humid and naturally charged, (iii) humid and neutralized (Boltzmann charge distribution), and (iv) humid and neutralized with an electrostatic precipitator upstream electrodes (i.e., non-charged). The glass fiber aerosols generated by a vortex shaking method were conditioned using a Po-210 neutralizer or humidifier and were provided into a test unit where cylindrical or parallel plate electrodes are used and high voltage is applied to them. Fibers were collected on a filter immediately downstream from the electrodes and their images were taken through an optical microscope to visualize the fiber orientation and measure the alignment angles and lengths of the fibers. The results showed that under all four conditions tested, airborne glass fibers could be aligned to the electric field with different alignment quality, indicating that the glass fibers can be polarized in a steady electric field. In humid air, the fiber alignment along the field direction was observed to be much better and the number of uniform background particles (i.e., randomly oriented fibers) in angular distributions is smaller than that in dry air. Also, it was found that charged fibers in humid air could be better aligned with negligible uniform background than neutralized and non-charged fibers. Possible mechanisms about humidity and charge effects on enhanced fiber alignment are discussed to support the observations. The results indicate that the enhancement of alignment in an electric field would be possible in humid air for other non-conducting fibrous particles having surface chemistry similar to glass fibers.     

Beta-sialon phosphor deposits fabricated by electrophoretic deposition (EPD) process in a magnetic field

Phosphor deposits of β-sialon:Eu²⁺ were prepared by electrophoretic deposition (EPD) process within a magnetic field. Under the action of the magnetic force, which was parallel to the direction of the electric field of the EPD, the β-sialon:Eu²⁺ crystals were aligned along the c-axis of the hexagonal cell unit to form an oriented deposit via the EPD fabrication. Higher orientation degree was obtained at longer depositing time (300 s) and stronger applied magnetic field (12 T). The oriented deposit aligned along the c-axis obtained higher relative deposit density than the randomly fabricated deposit. Due to the improved relative density, the oriented deposit prepared within the magnetic field possessed an enhanced external quantum efficiency (η_ex). Also, because of different relative densities of the deposits prepared within and without the magnetic field, they presented different chromaticity coordinates.     

Electrophoretic deposition of carbon nanotubes

Electrophoretic deposition (EPD) has been gaining increasing interest as an economical and versatile processing technique for the production of novel coatings or films of carbon nanotubes (CNTs) on conductive substrates. The purpose of the paper is to present an up-to-date comprehensive overview of current research progress in the field of EPD of CNTs. The paper specifically reviews the preparation and characterisation of stable CNT suspensions, and the mechanism of the EPD process; it includes discussion of pure CNT coatings and CNT/nanoparticle composite films. A complete discussion of the EPD parameters is presented, including electrode materials, deposition time, electrode separation, deposition voltage and resultant electric field. The paper highlights potential applications of the resulting CNT and CNT/composite structures, in areas such as field emission devices, fuel cells, and supercapacitors.     

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.     

Synthesis and field emission properties of vertically aligned carbon nanotube arrays on copper

We report the synthesis of periodic arrays of carbon nanotubes (CNTs) with different densities on copper substrate by employing nanosphere lithography (NSL) and plasma enhanced chemical vapor deposition. At a growth pressure of 8 torr and temperature of 520 °C, vertically aligned bamboo-like CNTs were formed with a catalyst particle on the tip. Electrical properties of CNTs with different densities were investigated for the possible applications in field emission (FE). The investigation of FE properties reveals a strong dependence on the density of CNTs. Experimental results show that NSL patterned low density CNTs exhibit better field emission properties as compared to the high density CNTs. Low-density CNTs exhibit lower turn-on and threshold electric fields, and a higher field enhancement factor. The high density of CNTs results in the deterioration of the FE properties due to the screening of the electric field by the neighboring CNTs.     

Effects of particle size on the molecular orientation and birefringence of magnetic nanoparticles/polyimide composites

Size dependency of nanoparticles for birefringence and molecular orientation of nanocomposite films have been studied using a prism coupler and near‐edge X‐ray absorption fine structure spectroscopy (NEXAFS). We synthesized two different sizes of magnetic nanoparticles, Ni_0.6Zn_0.4Fe₂O₄. The smaller ones were 6.1 ± 1.3 nm‐diameter nanoparticles showing superparamagnetism and the larger ones were 20.7 ± 6.1 nm‐diameter nanoparticles showing ferrimagnetism. To make nanocomposites, we incorporated these particles into poly(N,N′‐bis(phenoxyphenyl)pyromellitimide) (PMDA‐ODA PI). From the prism coupler study, pristine PI without nanoparticles had higher out‐of‐plane birefringence, which indicated high in‐plane orientation of polyimide. However, the birefringence of PI nanocomposites decreased with the increase of particle content. The birefringence of PI nanocomposite with small nanoparticles was smaller than that of PI nanocomposite with large nanoparticles. The birefringence of PI nanocomposite with 1 wt % of small nanoparticles was reduced to almost half of that of pristine PI due to the decreased orientation of PI molecules. NEXAFS spectra of N K‐edge were the same as the birefringence results. Imide and phenyl rings of pristine PI aligned more parallel to the in‐plane direction, but those of PI with nanoparticles aligned less parallel to the in‐plane direction. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3433–3440, 2006     

Effect of Particle Size on the Electrophoretic Deposition of Hydroxyapatite Coatings: A Kinetic Study Based on a Statistical Analysis

This work reports on the effect of particle size on the electrophoretic deposition (EPD) kinetics of submicrometer and nano‐sized hydroxyapatite (HA) employing a design of experiments strategy namely as response surface methodology (RSM). Zeta potential measurements and particle size analysis were used to study the electrostatic interaction between HA particles in an ethanol‐based suspension. The results of deposition kinetics under constant DC voltages showed a lower deposition rate for nano‐sized HA coating. Furthermore, the effects of voltage and time on EPD kinetics of HA coatings were quantified via RSM based on a central composite design (CCD).     

The production of large scale ultrathin aligned CNT films by combining AC electric field with liquid flow

We report a simple and versatile technique combining the use of an AC electric field with a liquid shear force to prepare ultrathin aligned CNT films on solid substrates. Multiwalled carbon nanotubes (MWCNTs), which were synthesized by a template method and acid-treated single walled carbon nanotubes (SWCNTs) were dispersed in water and used for the ultrathin film fabrication. A solid substrate was immersed in the CNT dispersions and withdrawn at constant speed under AC electric field. SEM images of the substrate showed that CNTs were aligned with the AC electric field and the withdrawal direction and formed uniform films with a thickness around 10 nm for SWCNTs and 90 nm for MWCNTs. Repeating the deposition process increases the density and size of the film while also maintaining nanometer-scale thickness. Unidirectional alignment of CNTs was also confirmed by Raman spectra and electric conductivity measurements. It was found that ultrathin films of aligned SWCNTs exhibited very high anisotropic electrical conductivity with conductivity measured parallel to the alignment direction 3.3 × 10⁵ times higher than that measured in the perpendicular direction. We demonstrate that use of the aligned ultrathin SWCNT film for a unidirectional alignment of liquid crystal.     

Evaluating the effect of hydroxyapatite nanoparticles on morphology,thermal stability and dynamic mechanical properties of multicomponent blend systems based on polylactic acid/Starch/Polycaprolactone

A hybrid of hydroxyapatite (HA) nanoparticles and encapsulated triclosan (LATC) was utilized to improve the overall performance of Polylactic acid/Starch/Polycaprolactone ternary blends. The morphological evaluations demonstrated that the starch component becomes gelatinized during the melt mixing process in the absence of LATC particles. However, once encapsulated triclosan particles were also added to the formulation, the granular structure of starch phase was retained indicating no gelatinization during the preparation process. In the case of nanocomposite samples, only 1 wt% of HA nanoparticles was found to exhibit a uniform distribution throughout the whole system whereas the higher concentrations resulted in the aggregation of nanoparticles dividing the system into HA‐poor and HA‐rich areas. Thermogravimetric analysis (TGA) was utilized to prove that those HA‐rich areas were formed around the starch domains due to the strong chemical affinity. Moreover, TGA also showed that only 1 wt% of HA nanoparticles acts efficiently in delaying the initiation of thermal decomposition. Dynamic mechanical analysis results also confirmed that HA nanoparticles had a strong tendency to be absorbed onto the starch domains within the system. J. VINYL ADDIT. TECHNOL., 25:E83–E90, 2019. © 2018 Society of Plastics Engineers     

Enhanced field emission properties from titanium-coated carbon nanotubes

The effect of titanium (Ti) coating over the surface of carbon nanotubes (CNTs) on field emission characteristics was investigated. Vertically aligned CNTs were grown by inductively-coupled plasma-enhanced chemical vapor deposition (ICP-CVD). In order to reduce the screening effect of electric field due to densely packed CNTs, as-grown CNTs were partly etched back by DC plasma of N₂. Ti with various thicknesses from 5 nm to 150 nm was coated on CNTs by a sputtering method. Since thick Ti coating with thickness of 100 nm or more resulted in the shape of a metal post by merging an individual CNT in a bundle, it was inadequate to a field emission application. On the other hand, thin Ti-coated CNTs with thickness of 10 nm or less showed a lower turn-on field, a higher emission current density, and improved emission uniformity compared with pristine CNTs. The improved emission performance was mainly attributed to the low work function of Ti and the reliable and lower resistance contact between CNTs and substrates.     

Generation of negative-charge carriers in the gas phase and their contribution to the growth of carbon nanotubes during hot-filament chemical vapor deposition

Experiments reported here suggest that charged nanoparticles are generated during the synthesis of carbon nanotubes (CNTs) by hot-filament chemical vapor deposition, and it is these which interact with catalytic metal particles to produce CNTs. During the deposition process, at a filament temperature of 1900 °C, the presence of negative-charge carriers in the form of an electric current measuring 2 μA cm⁻² is detected at the substrate position. A bias applied to the stainless steel substrate affects both the growth rate and the morphology of the deposited carbon. The masses of CNTs or carbon nanoparticles (CNPs) grown under an applied bias of +25 V are found to be two and ten times larger than those grown under applied biases of 0 and −200 V, respectively. The CNPs in the gas phase have been observed using transmission electron microscopy (TEM) by capturing them through differential pumping through the orifice onto TEM grids placed in a second chamber, in which the vacuum is higher than in the reactor.     

Characterisation of carbon nanotube films deposited by electrophoretic deposition

Electrophoretic deposition (EPD) was shown to be a convenient method to fabricate uniform coatings of carbon nanotubes (CNTs) with desired thickness and excellent macroscopic homogeneity. The CNT deposition kinetics are controlled by the applied electric field and deposition time which, in turn, prove to be linearly correlated with the deposition yield and thickness. The CNT films were characterised by using a range of techniques including high resolution scanning electron microscopy, nanoindentation and atomic force microscopy. Nanoindentation results revealed differences in the local microstructure of CNT deposits leading to variations of Young’s modulus and hardness, which were ascribed to differences in the packing density of CNTs, as observed also by AFM. A mathematical model for the kinetic of EPD of CNTs based on Hamaker’s law was proposed and the predictions of the model were shown to be in good agreement with experimental results.     

Multi-walled carbon nanotube/polyimide composite film fabricated through electrophoretic deposition

Multi-walled carbon nanotube (MWCNT)/polyimide composite films were fabricated through electrophoretic deposition (EPD) of MWCNT-polyamic acid colloidal suspension which was derived from carboxylated-MWCNTs and poly(pyromellitic dianhydride-co-4,4′-oxydianiline) (PMDA-ODA). Under electric field, both negatively charged MWCNTs and PMDA-ODA colloid particles migrate onto a positively charged anode simultaneously, and are converted to a coherent MWCNT/polyimide composite film in the ensuing imidization reaction. Uniform dispersion of MWCNTs in the composite film was observed using transmission electron microscopy. The thickness of the prepared composite film can be tuned by varying processing conditions such as deposition time and anode conductivity. The electrical conductivity of the composite film increased with increasing the concentration of MWCNTs in EPD suspension. The mechanical reinforcement of polyimide using MWCNTs was evaluated by tensile testing and nanoindentation testing.