Critical particle concentration in electrophoretic deposition

The role of particle concentration in electrophoretic deposition (EPD) was investigated with two different suspension systems. The first system consisted of positively charged TiO₂ nanoparticles dispersed in isopropanol with 1 vol% water. The second system consisted of negatively charged polystyrene (PS) microbeads dispersed in isopropanol. Constant voltage EPD was performed using suspensions with variable particle concentration (0.013–0.43 vol% TiO₂ and 0.06–11.4 vol% PS). Threshold concentration values were identified for both systems after EPD at 100 V (250 V cm^?1) for 1 min. Below these values the deposited mass deviated from the trend dictated by Hamaker's equation. Higher applied voltages and longer deposition times were tested and the results suggested that the threshold concentration did not depend on those parameters. A phenomenological model of particle deposition was proposed, which accounts for the local electrochemical conditions close to the substrate in relation to particle size.     

Microscopic aspects of the deposition of nanoparticles from the gas phase

The deposition process in a homogeneous electric field, and the subsequent microscopic arrangement of charged, metallic nanoparticles in the size range below $\text{100}\text{nm}$ on flat substrate surfaces is described. The main aspect of the investigation is the transfer of the particles from a three-dimensional distribution in the gas phase into their arrangement on the substrate surface, in dependence on particle–particle interactions and on Brownian motion. For this purpose, a trajectory model has been developed, which takes into account the flow field above the substrate surface, the electric field, the interactions of incoming particles with the substrate surface and with already deposited particles, as well as Brownian motion. The results from the trajectory calculations are compared with experimental results, obtained by scanning electron microscopy investigations of deposition patterns, created by deposition of indium and gold nanoparticles in an electrostatic precipitator. The particle diameter, the particle charge, the substrate material, the electric field strength and the number of particles deposited per unit area have been varied.     

Particle deposition velocity onto a face-up flat surface in a laminar parallel flow considering Brownian diffusion and gravitational settling

The Gaussian Diffusion Sphere Model (GDSM) was developed and improved to reflect the effects of gravitational settling as well as Brownian diffusion of aerosol particles on deposition velocity onto a face-up flat surface in a laminar parallel flow. The model improvement also includes the applicability of the GDSM to a flat surface of any shape with finite dimensions. When deposition velocity for a face-up circular flat plate of 45 cm diameter, representing e.g. a semiconductor wafer in a laminar parallel flow, was calculated by the GDSM and compared with that by the theory of Liu and Ahn (1987). Particle deposition on semiconductor wafers. Aerosol Science and Technology, 6, 215–224, the agreement was good for the tested particle sizes ranging 0.003–1 μm and free stream velocities ranging 5–500 cm/s. Based on this result, deposition velocities onto the face-up square flat plates with different orientations in a laminar parallel flow, simulating e.g. photomasks, were predicted.     

Filtration and inactivation of aerosolized bacteriophage MS2 by a CNT air filter fabricated using electro-aerodynamic deposition

Carbon nanotubes (CNTs) were coated on a sample of glass fiber air filter medium at atmospheric pressure and room temperature using electro-aerodynamic deposition (EAD). In the EAD method, CNTs (diameter: 50 nm, length: 2–3 μm) were aerosolized, electrically charged, and injected through a nozzle. A voltage was applied externally between the ground nozzle and a planar electrode on which the sample was located. The charged CNTs were deposited on the sample in a vertically standing posture even at a low flow velocity. Before the deposition experiment, a calculation was performed to determine the applied voltage by simulating the electric field, flow field, and particle trajectory. Using CNT-coated filter samples, virus aerosol filtration and anti-viral tests were carried out using the aerosol number counting method and the plaque counting method, respectively. For this purpose, bacteriophage MS2 was aerosolized with an atomizer. The particle filtration efficiency was increased to 33.3% in the most penetration particle size zone (100 nm) and the antiviral efficiency of the CNT filter was 92% when the coating areal density was 1.5 × 10⁹ #/cm². The susceptibility constant of virus to CNTs was 0.2 cm²/μg.     

Size dependence of optical and magnetic properties of nickel oxide nanoparticles fabricated by electric arc discharge method

Nickel oxide nanoparticles with an average size of between 28 and 62 nm were fabricated by electric arc discharge method. The electric currents of 10, 100, 200, 300 and 400 A and oxygen pressures of 1, 2 and 3 atm. were tested. High yield production was observed for the samples prepared at low arc current. The samples were characterized using XRD and FESEM measurements. XRD results showed that the samples were pure and single phase of nickel oxide with cubic structure. The produced nanoparticles were cubic shaped and the average particle sizes increased by increasing the arc pressure, but decreased by increasing the arc current and their size distributions were uniform. The magnetic measurements confirmed a soft ferromagnetic behavior for the nickel oxide nanoparticles at low field region but the hysteresis loop tended to be antiferromagnetic like for the higher fields. By decreasing the particle size from 62 nm, the coercivity (H_c) increased but decreased when the particle size was less than about 57 nm. Such magnetic behavior which can be common for antiferromagnetic nanoparticles was interpreted based on a core-shell model.     

Nanoparticle collection efficiency of capillary pore membrane filters

The surface and overall collection efficiencies of capillary pore membrane filters were measured for sub-micrometer particles. Collection efficiencies were derived from the surface loadings of particles on filters measured by scanning electron microscopy and from airborne particle concentrations measured with a scanning mobility particle sizer. Tests used filters with nominal pore diameters of 0.4 and 0.8 μm and face velocities of 3.7 and 18.4 cm/s. Surface collection efficiencies were below 100% for particles smaller than 316 nm and below 55% for particles smaller than 100 nm. Overall collection efficiencies reached as low as 45% for 70 nm particles. For nanoparticles, collection efficiencies overall were substantially higher than those to the filter surface, indicating that deposition occurs to a large extent inside the filter pores. These results underscore the need to account for surface collection efficiency when deriving airborne concentrations from microscopic analysis of nanoparticles on capillary pore membrane filters.     

Computational fluid dynamics simulations of submicrometer and micrometer particle deposition in the nasal passages of a Sprague-Dawley rat

Computational fluid dynamics (CFD) simulations were conducted in a model of the complete nasal passages of an adult male Sprague-Dawley rat to predict regional deposition patterns of inhaled particles in the size range of 1 nm to 10 μm. Steady-state inspiratory airflow rates of 185, 369, and 738 ml/min (equal to 50%, 100%, and 200% of the estimated minute volume during resting breathing) were simulated using Fluent?. The Lagrangian particle tracking method was used to calculate trajectories of individual particles that were passively released from the nostrils. Computational predictions of total nasal deposition compared well with experimental data from the literature when deposition fractions were plotted against the Stokes and Peclet numbers for micro- and nanoparticles, respectively. Regional deposition was assessed by computing deposition efficiency curves for major nasal epithelial cell types. For micrometer particles, maximum olfactory deposition was 27% and occurred at the lowest flow rate with a particle diameter of 7 μm. Maximum deposition on mucus-coated non-olfactory epithelium was 27% for 3.25 μm particles at the highest flow rate. For submicrometer particles, olfactory deposition reached a maximum of 20% with a particle size of 5 nm at the highest flow rate, whereas deposition on mucus-coated non-olfactory epithelium reached a peak of approximately 60% for 1–4 nm particles at all flow rates. These simulations show that regional particle deposition patterns are highly dependent on particle size and flow rate, indicating the importance of accurate quantification of deposition in the rat for extrapolation of results to humans.     

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.     

Fabrication of textured alumina by orienting template particles during electrophoretic deposition

(0 0 1)-Textured α-alumina has been processed by electrophoretic deposition (EPD) and templated grain growth. The mechanism of platelet template orientation during EPD was examined with respect to the impact of the electric field force, gravity and hydrodynamic force in two different deposition cells with vertically or horizontally positioned deposition electrodes. A sharp (0 0 1) ‘fibre texture’ was obtained after templated grain growth during sintering of a deposit formed from a stirred 5 vol% platelet containing suspension in a vertical deposition cell. The texture was characterized by means of the Lotgering factor, texture index and electron backscattering diffraction (EBSD).     

Effect of dispersant on chain formation capability of TiO2 nanoparticles under low frequency electric fields for NO2 gas sensing applications

In this study, the effect of Dolapix CE64 as dispersant on the deposition mechanism and chain formation of TiO₂ nanoparticles in the different frequencies under non-uniform AC electric field was investigated. The optical microscope (OM) images of the deposition patterns obtained at the frequency of 1 Hz and 100 Hz in non-aqueous media one containing dispersant and the other being dispersant free revealed that the addition of charge inducing agents (dispersant) has drastically changed deposition mechanism enabling particles to enter the gap leaving the electrodes surfaces uncoated. At 10 kHz, it was observed that by the introduction of Dolapix CE64 the TiO₂ nanoparticles were able to form chain-like patterns along the electric field lines bridging the interelectrode gap. The obtained pearl chain formation (PCF) was employed to fabricate an NO₂ gas sensor which showed a good response to the target gas at 450, 500 and 550 ˚C.     

Coupled CFD–DEM of particle-laden flows in a turning flow with a moving wall

The nature of the particle–solid interactions and particle–fluid interactions in rectangular duct bend geometry with/without a moving wall is studied, taking into account particle collision, colloidal, and hydrodynamic forces, and four way coupling between the fluid flow and particles. The focus is on systems where particles and fluid phase have similar length scales, fluid Reynolds number (Re_f) ∼ 1, and particle's Stokes number (St) ∼ 1. Particles move toward the walls of the channel near the bend, and have long residence times in these regions. Buoyancy force has negligible effect on particle motion, where adhesion and drag forces lead to particle motion and agglomeration patterns. The effect of a free surface on agglomeration sites in the turning flow is elucidated.     

Application of a modified Eulerian model to study the particle deposition on a vertical surface in turbulent flow

In this study the v2-f model was used with the two-phase Eulerian approach to predict the particle deposition rate on a vertical surface in a turbulent flow. The standard Eulerian particle model was adopted from the literature and modified, considering the majority of particle transport mechanisms in the particle deposition rate. The performance of the modified model was examined by comparing the rate of particle deposition on a vertical surface with the experimental and numerical data in a turbulent channel flow available in the literature. The model took into account the effects of drag force, lift force, turbophoretic force, electrostatic force, inertia force and Brownian/turbulent diffusion on the particle deposition rate. Electrostatic forces due to mirror charging and charged particles under the influence of an electric field were considered. The predictions of the modified particle model were in good agreement with the experimental data. It was observed that when both electrostatic forces are present they are the dominant factor in the deposition rate in a wider range of particle sizes.     

Transformation of silica fume into chemical mechanical polishing (CMP) nano-slurries for advanced semiconductor manufacturing

Silica nanoparticles have been synthesized from silica fume using alkali dissolution–precipitation process. The dissolution efficiency of 99% at a temperature of 80 °C and a time of 20 min was achieved. Sodium silicate solution was obtained by dissolving the fume with NaOH solution. Then, silica nanoparticles were precipitated using sulfuric acid. Silica nanoparticles (175 nm) were achieved using 12% sulfuric acid at pH 7 and 200 ppm sodium dodecyl sulfate (SDS). The silica morphologies appeared as a spherical shape with narrow particle size distribution. The silica samples were used for the formulation and testing of chemical mechanical polishing (CMP) slurries. The morphology of the polished wafer surface and its roughness were examined by atomic force microscope (AFM).The results indicated that the surface roughness was greatly improved after application of CMP. It was found that the surface roughness of the polished wafer is 0.226 nm at an applied pressure of 7 psi. The removal rate was found to be 1200 Å. These values confirm the quality of polished wafers.     

The effect of the surface nanostructure and composition on the antiwear properties of zirconia–titania coatings

This study describes the preparation, surface imaging and tribological properties of titania coatings modified by zirconia nanoparticles agglomerated in the form of island-like structures on the titania surface. Titania coatings and titania coatings with embedded zirconia nanoparticles were prepared by the sol–gel spin coating process on silicon wafers. After deposition the coatings were heat-treated at 500 °C or 1000 °C. The natural tendency of nanoparticles to form agglomerates was used to build separated island-like structures unevenly distributed over the titania surface having the size of 1.0–1.2 μm. Surface characterization of coatings before and after frictional tests was performed by atomic force microscopy (AFM) and optical microscopy. Zirconia nanoparticles were imaged with the use of transmission electron microscopy (TEM). The tribological properties were evaluated with the use of microtribometer operating in ambient air at technical dry friction conditions under normal load of 80 mN. It was found that nanocomposite coatings exhibit lower coefficient of friction (CoF) and considerably lower wear compared to titania coating without nanoparticles. The lowering of CoF is about 40% for coatings heated at 500 °C and 33% for the coatings heated at 1000 °C. For nanocomposites the wear stability was enhanced by a factor of 100 as compared to pure titania coatings. We claim that enhanced tribological properties are closely related to the reduction of the real contact area, lowering of the adhesive forces in frictional contacts and increasing of the composite hardness. The changes in materials composition in frictional contact has secondary effect.     

Particle transport in a small square enclosure in laminar natural convection

The transport of particles with diameters in the range of 50 nm to 1 μm in laminar free convection of air in square enclosures was numerically investigated by an Eulerian–Lagrangian method. Two-dimensional square enclosures with widths from 2.5 mm to 5 cm, with two adiabatic surfaces and 100 and 200 °C temperature difference between the other two surfaces, were considered. The Rayleigh numbers varied from 100 to 8×10⁵. The air flow was simulated in Eulerian frame using a commercial CFD software, whose predictions were compared with published benchmark results. Lagrangian particle transport calculations were carried out by tracking 1000 particles that were initially randomly distributed in the flow field, and assuming one-way coupling between the particles and the carrier gas. Particle motion mechanisms considered included gravity, drag, lift force, thermophoresis and Brownian dispersion.The results showed that at Rayleigh numbers lower than about 10 000 the entire flow field was dominated by a single recirculation pattern. For these low Rayleigh number cases most of the particles disperse towards the walls, while a fraction of particles were trapped in a quasi-steady recirculation zone. Inside this recirculation zone the particles were at quasi-equilibrium with respect to the hydrodynamic and dispersive forces that acted on them, and left the zone due to Brownian dispersion only at a very low rate. This quasi-equilibrium zone was not observed at the higher Rayleigh numbers where a single recirculation pattern no longer governed the entire flow field. The results also confirmed the important role of thermophoresis and Brownian dispersion, in particular for submicron size particles.     

In-situ study of electrophoretic deposition of zinc oxide nanosheets and nanorods

In the present work, the effects of two different morphologies of zinc oxide nanoparticles (nanosheets and nanorods) were investigated by in-situ measurement of deposition weight, and current density. ZnO nanosheets and nanorods were synthesized by microwave-assisted method using co-surfactant route. The average thickness of obtained nanosheets, and the average diameter of nanorods were measured to be about 26 nm and 139 nm, respectively. ZnO films were obtained by electrophoretic deposition from suspension of nanoparticles in ethanol under different voltages. Results indicated that ZnO nanosheets tend to have greater deposition rate than ZnO nanorods under similar conditions. The compactness of the film obtained from nanosheet suspension was higher than the one obtained from nanorod suspension. However, the film obtained from ZnO nanorods displayed more uniformity at different voltages in comparison to the film obtained from ZnO nanosheets, which can be due to different active surface area, and also different way of motion under hydrodynamic forces in the suspension.     

Effect of Ag nanoparticles seeding on the properties of silica spheres

In this study, the effect of seed-recrystallization on the Ag deposition onto silica sphere surfaces has been investigated. It was found that increasing the seed-recrystallization cycles resulted in higher atomic deposition at ca. 84% coverage of silica surfaces with the same mole ratio of Ag precursor characterized by the Ag/Si atomic ratio obtained by XPS analyses. The addition of straight-chain palm oil derived fatty alcohols (PODFA) in the sol-gel prior to seed-recrystallization aided the deposition of Ag. Thus, PODFA play the role of nonsurfactant surface modification agent to produce Ag-silica nanocomposite. Structural analyses showed that the resulting Ag nanocrystallines having a face centre cubic structure and particle size of 5–20 nm were deposited homogeneously on silica spheres. Chemical state analyses from XPS indicated that the increasing number of seed repetition process increased the seeding of Ag nanoparticles on silica surface with the same molar of Ag atoms. XPS spectra at O1s orbital elucidated that the binding energy of three oxides components were determined at ca. 533.8 eV (Si-O-H), 532.8 eV (Si-O-Si) and 530.8 eV (Si-O-Ag), respectively. The high sensitivity of surface plasmon resonance observed in the nanocomposites prepared in this study are useful in optical applications.     

Synthesis of palladium nanoparticles on carbon nanotubes and graphene for the chemoselective hydrogenation of para-chloronitrobenzene

We have studied the synthesis of palladium nanoparticles over carbon nanotubes (Pd/CNT) and graphene (Pd/G) and we have tested their catalytic performance in the liquid phase chemoselective hydrogenation of para-chloronitrobenzene at room temperature. The catalysts were characterized by N₂ adsorption/desorption isotherms, TEM, X-ray diffraction, infrared and X-ray photoelectron spectroscopy and ICP-OES. The palladium particle size on Pd/G (3.4 nm) and Pd/CNT (2.8 nm) was similar though the deposition was higher on Pd/G. Pd/CNT was more active which can be ascribed to the different surface area and electronic properties of the Pd nanoparticles over CNT, while the selectivity was 100% to the corresponding haloaniline over both catalysts and they were quite stable upon recycling.     

Halloysite nanotubes coated with magnetic nanoparticles

Co nanoparticles were assembled on the surface of halloysite nanotubes (HNTs) to prepare one-dimensional magnetic Co-HNTs via electroless deposition. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDXS) and vibrating sample magnetometer (VSM). The cobalt nanoparticles of 3–7 nm in size were uniformly deposited on the surface of the nanotubes. The remanent magnetization (M_r), saturation magnetization (M_s) and coercivity (H_c) values of the Co-HNTs were 13.9 emu/g, 27.05 emu/g and 1659 O_e, respectively, larger than that of the pure Co nanoparticles (580.72 O_e). A mechanism of the deposition of the magnetic nanoparticles on the surface of the halloysite nanotubes is suggested. Co-HNTs showed an interesting potential in the field of magnetic devices.