Hemispherical Particles Research Articles

Nucleation and Growth Kinetics of Electrodeposited Sulfate-Doped Polypyrrole: Determination of the Diffusion Coefficient of SO42− in the Polymeric Membrane

A kinetic study for the electrosynthesis of polypyrrole (Ppy) doped with SO(4)(2-) ions is presented. Ppy films were electrochemically polymerized onto a graphite-epoxy resin electrode. Experimental current density transients (j-t) were obtained for three different potentiometric behaviors: anionic, cationic, and a combination. Theoretical models were used to fit the experimental j-t data to determine the nucleation and growth processes controlling the polymer synthesis. It was encountered that, in all cases, pyrrole electropolimerization involves two concomitant processes, namely, a Ppy diffusion limited multiple 3D nucleation and growth and pyrrole electro-oxidation on the growing surface of the Ppy nuclei. SEM analysis of the electrodes surfaces reveals that Ppy deposition occurred over most of the electrode surface by multiple nucleation of hemispheres, as the theoretical model used for the analysis of the current transients required. Hemispherical particles formed the polymeric film displaying different sizes. The order for the particle size was as follows: anionic > anionic-cationic > cationic. These results are congruent with those obtained by theoretical analysis of the corresponding current transients. Analysis of the impedance measurements recorded on the anionic Ppy film, immersed in an aqueous solution with different sulfate ion concentrations evidenced that SO(4)(2-) ions diffuse through the Ppy film provoking a decrease of its electrical resistance and an increase of its dielectric constant. From the Warburg impedance coefficient, the sulfate coefficient of diffusion in the Ppy film was 1.38 x 10(-9) cm(2) s(-1).

A two-step roll coating phosphate/molybdate passivation treatment for hot-dip galvanized steel sheet

A two-step roll coating passivation treatment employing phosphate followed by molybdate solutions has been performed on hot-dip galvanized (GI) steel sheet. The phosphate coating was primarily porous, amorphous Zn phosphate. A second step coating treatment resulted in more hemispherical Zn phosphate particles and the incorporation of molybdate ions and molybdenum oxide into the existing phosphate coating, giving rise to an improved corrosion resistance. The coating reaction during the second step roll coating treatment and the corrosion protection afforded by the second step molybdate treatment are discussed, with emphases on the comparison with the coating formed via immersion.

Diamond plates on dome-like particles: preparation, characterization and field emission properties

Thin diamond microplates have been grown on dome-like/hemispherical carbon particles on titanium carbide by a microwave plasma chemical vapour deposition (MPCVD) method using a gas mixture of methane and hydrogen. The diamond microplates have a thickness of about 200 nm. A thin (300 nm) film of titanium carbide was formed during carburization of sputtered titanium on an Si(100) substrate in MPCVD. The hemispherical carbon particles were covered with diamond microplates. The diamond microplates are isolated electron-emitting spherules and exhibit a low threshold (50 V µm−1) and high current density (0.92 mA cm−2) in their field emission properties. A possible mechanism for the formation of the diamond microplates and hemispherical carbon particles is presented.

Influence of Die-Chill Skin on the Formation of Stannate Conversion Coating on AZ91D Magnesium Alloy

The microstructure of die-chill skin on a die-cast AZ91D plate has been characterized, and its effect on the formation and corrosion resistance of stannate conversion coating has been investigated. The die-chill skin contained interconnected pores, around which carbon–silicon-containing magnesium/aluminum oxides were detected. The stannate conversion coating on the die-cast AZ91D consisted of hemispherical magnesium tin oxide particles. When compared with the substrate whose die-chill skin had been removed by mechanical polishing, the presence of die-chill skin enhanced the nucleation of magnesium tin oxide particles and reduced the thickness of the conversion coating layer. The intact sites on the substrate with die-chill skin, however, led to the formation of uncoated spots after 10 min of stannate conversion coating treatment. The thinner conversion coating containing uncoated spots was less effective in retarding the reduction of water during cathodic polarization in a mixture of 0.1 M sodium sulfate and 0.05 M sodium chloride solution. These uncoated spots, which were resistant in the alkaline stannate solution, seemed to prohibit the oxidation and dissolution of the substrate during anodic polarization.

Scattering of light into silicon by spherical and hemispherical silver nanoparticles

The interaction of light with noble metal nanoparticles deposited onto the top surface of a semiconductor has been investigated using the finite-difference time-domain method. The scattering is calculated for spherical and hemispherical silver nanoparticles placed in a periodic two-dimensional array on the upper surface of a semi-infinite silicon substrate. The results show that the contact area between hemispherical particles and the silicon significantly reduces the forward scattering. The use of an oxide buffer layer to separate the particle from the semiconductor is investigated and is seen to be important if the forward scattering of light is to be enhanced.

In-situ study of the formation process of stannate conversion coatings on AZ91D magnesium alloy using electrochemical noise

The formation process of stannate conversion coating (CC) on AZ91D alloy was in-situ investigated by electrochemical noise (EN). The wavelet transform, as well as noise resistance ( R n ) and spectral noise resistance ( R sn ), had been employed to analyze the EN data. It was revealed that there exist two distinguishing stages of stannate CC formation process on AZ91D alloy, including an incubation stage companying with the nucleation and nuclei dissolution process, a periodical growth stage involving hemispherical particles growth and coating dissolution process. Furthermore, the results demonstrated that EN was a powerful tool to investigate rapid electrochemical process, such as CC formation process.

A Novel Approach for Preparation of Micrometer-sized, Monodisperse Dimple and Hemispherical Polystyrene Particles†Part CCCXXX of the series “Studies on Suspension and Emulsion”

Micrometer-sized, monodisperse dimple and hemispherical polystyrene (PS) particles were successfully prepared by heating (55-70 degrees C) of spherical PS particles dispersed in methanol/water media (40/60 to 80/20, w/w) in the presence of decane droplets, and subsequent cooling down to room temperature. Decane was absorbed by the PS particles during the heating process. Decane-absorbed PS particles phase-separated into PS and decane phases in the inside during the cooling process, and eventually dimple and/or hemispherical particles were formed by removal of the decane phase from phase-separated PS/decane particles by evaporation. The size of the dimple, which is determined by the volume of decane phase-separated from decane-absorbed PS particles during the cooling process, increased with increases in the heating temperature and the methanol content.

Ab Initio Study of Stability and Site-Specific Oxygen Adsorption Energies of Pt Nanoparticles

We have employed ab initio calculations based on density functional theory in order to study stability and oxygen adsorption energies of Pt nanoparticles. For particles with sizes up to 200 atoms and various geometric shapes, we have explored the dependence of cohesive energies on atomic coordination number and on lattice strain effects. A simple empirical relation, which is consistent with the well-known Gibbs−Thomson relation, represents the cohesive energy over the range of considered sizes and shapes. For hemispherical cuboctahedral particles with 37 and 92 atoms, we have generated contour plots of the adsorption energy of atomic oxygen on all nanofacets. These plots furnish the known trend of strongly enhanced oxygen adsorption energies in comparison to extended surfaces. We found that the interplay of geometric effects, involving the periodic arrangement of surface atoms and edge effects on nanofacets, causes the high site-selectivity of Pt−oxygen interaction energies, with the largest adsorption energies found at the edges. Particle relaxation upon oxygen adsorption exhibits a significant influence on adsorption energies. The presented results provide a map of the peculiar site-selectivity of adsorption at Pt nanoparticles, which should be accounted for in building detailed models of reaction mechanisms and reactivity.

Molecular Dynamics Simulations of Nanoimprinting Lithography

We have performed coarse-grained molecular dynamics simulations of molding and replication of nanometer-size objects. Nanoimprinting of hemispherical particles was modeled as a three-step process: (1) a mold was created by pressing a hard hemispherical particle (master template) into a polymeric film; (2) a polymeric film was cross-linked fixing a negative image of a master mold into a polymeric film; (3) a polymeric mold was pressed into a monomeric liquid replicating an original master. The quality of the replication process was analyzed by comparing the shape of the replica with the shape of the master. It is shown that deformation of a polymeric stamp during the replication process (Step 3) is a result of optimization of the surface energy of the mold-liquid interface and the elastic energy of the polymeric mold. The relative deformation, epsilon, of the replica is a function of the universal parameter gamma/(GR(0)), where gamma is the surface energy of the polymer-liquid interface, G is the shear modulus of the polymer network, and R(0) is the radius of the master. In the case of small deformations, this function reduces to epsilon proportional to gamma/(GR(0)).

Experimental study of sedimentation characteristics of spheroidal particles

The drag of non-spherical particles is a basic, important parameter for multi-phase flow. As the first step in research in this area, the terminal velocities, U t, of hemispherical and spherical segment particles with maximal diameters of 6–21 mm were measured in static fluids by using a high-speed video camera. The drag coefficient, C D, measured for Reynolds number, Re of 10 1–10 5, has been obtained and compared with those for a sphere. The Re based on the terminal velocity has a logarithmic linear relationship with Ar number for both the facet facing upwards or downwards for the two experimental spheroidal particles, and their C D values are greater than those of spheres. A shape function that depends on the initial orientation of the particle facet is presented to correct for the shape effects.

Design principles for particle plasmon enhanced solar cells

We develop fundamental design principles for increasing the efficiency of solar cells using light trapping by scattering from metal nanoparticles. We show that cylindrical and hemispherical particles lead to much higher path length enhancements than spherical particles, due to enhanced near-field coupling, and that the path length enhancement for an electric point dipole is even higher than the Lambertian value. Silver particles give much higher path length enhancements than gold particles. The scattering cross section of the particles is very sensitive to the thickness of a spacer layer at the substrate, which provides additional tunability in the design of particle arrays.

Stannate conversion coatings on Mg–8Li alloy

Abstract The stannate conversion coatings (SnCC) on Mg–8Li alloy were investigated by simple immersion method. The surface morphology and composition were studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction spectroscopy (XRD) techniques. The corrosion resistance was assessed by means of potentiodynamic polarization measurements and electrochemical impedance spectra (EIS). The effects of time of a stannate bath on the quality of stannate conversion coatings were investigated by SEM and EIS. It was found that the coating particles were mainly composed of hemispherical particles MgSnO 3 ·3H 2 O. A comparison of results revealed the coating treated for 60 min exhibited the most uniform, dense and corrosion-resistant.

Unusual size-dependent magnetization in near hemispherical Co nanomagnets on SiO2 from fast pulsed laser processing

Nanosecond pulsed laser melting of ultrathin metal films can lead to self-organized arrays of spherical nanoparticles. We have applied this technique to assemble arrays of nanoparticles of the soft elemental ferromagnet Co on SiO2. Surface morphology studies by using scanning electron microscopy and atomic force microscopy established that the nanoparticles were nearly hemispherical with an average contact angle of ∼104±22°. Magnetic properties of these nanoparticles in the size range of 30–250nm diameter were investigated by magnetic force microscopy under zero applied field in conjunction with simulations of the magnetic tip-particle interaction. Particles up to 180nm diameter were found to be single domain with the magnetization direction oriented predominantly in-plane for the smaller particles (⩽75nm) and out-of-plane for the larger particles (⩽180nm). Multidomain behavior was observed for particles larger than 180nm. Magnetic hysteresis measurements at room temperature confirmed that the arrays consisted of a mixture of in-plane and out-of-plane orientations. Microstructural analysis by transmission electron microscopy revealed that the nanoparticles had a granular microstructure with the average grain size increasing with particle size. This size-dependent magnetic orientation is inconsistent with the expected in-plane orientation due to shape anisotropy. We suggest that a size-dependent residual strain and the microstructure formed by rapid laser processing determine the orientation of nanomagnets. This idea was supported by the significant increase in in-plane orientation of larger particles following thermal annealing. These studies suggest that anisotropic nanomagnets of near hemispherical polycrystalline particles with desired magnetic orientation can be prepared by fast laser thermal processing.

Synthesis by a single‐step swelling process and characterization of micrometer‐sized polychloromethylstyrene/poly(butyl methacrylate) hemispherical composite particles of narrow size distribution

AbstractPolychloromethylstyrene (PCMS) micrometer‐sized particles of narrow size distribution were prepared by the dispersion polymerization of chloromethylstyrene in a mixture of ethanol and dimethyl sulfoxide. Micrometer‐sized PCMS/poly(butyl methacrylate) hemispherical composite particles of narrow size distribution were prepared by a single‐step swelling process of the uniform PCMS template particles with emulsion droplets of butyl methacrylate (BMA) containing benzoyl peroxide, followed by the polymerization of BMA at 73°C within the swollen template particles. The effects of various polymerization parameters, for example, BMA volume, initiator type and concentration, and toluene as the swelling solvent, on the properties (size and size distribution, morphology, polymerization yield, and composition) of the hemispherical composite particles were elucidated. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Formation processes of iron silicide nanoparticles studied by ex situ and in situ transmission electron microscopy

The formation processes of iron silicide nanoparticles dependent on thermal annealing were examined by ex situ and in situ transmission electron microscopy (TEM). An Fe thin film with a thickness of ∼2 nm was deposited on a Si(100) substrate at room temperature using an electron-beam evaporation method, followed by thermal annealing at temperatures ranging from 573 to 1173 K. Ex situ TEM observations showed that pyramidal β-FeSi2 particles (1173 K) were formed via the ε-FeSi layer (573 K) and the γ-FeSi2 particle (873 K). Detailed observations by in situ TEM indicated that an amorphous Fe-Si layer was formed on the Si substrate in the as-deposited specimen. This amorphous layer was crystallized into ε-FeSi after thermal annealing and then hemispherical ε-FeSi particles together with metastable γ-FeSi2 were formed just beneath the surface of the Si substrate. With increasing annealing temperature, the ε phase changed to hemispherical γ-FeSi2 nanoparticles and finally the γ-FeSi2 particles transformed into pyramidal β-FeSi2 particles. We discuss the formation mechanism of the iron silicide nanoparticles at the atomic scale.

Hemispherical polymer nano-particles of polyisoprene–poly(methyl methacrylate) blend with core–shell structure

We show a simple preparation procedure of particles with unique shape and inner structures of polymer blend. Submicron sized particles consisting of polyisoprene (PI) and poly(methyl methacrylate) (PMMA) were prepared by adding a poor solvent (water) into a tetrahydrofuran (THF) solution of these polymers and following evaporation of THF. The diameter and shape of particles were controlled by changing the concentration of the solution. After selective staining of PI by osmium tetraoxide, particles were observed by scanning transmission electron microscopy (STEM). The observation showed the hemispherical particles with PI core and PMMA shell were formed. After dispersion of the stained particles into THF, the hemispherical particles of the cross-linked PI core were obtained by the selective dissolution of PMMA shell.

Synthesis of polystyrene encapsulated nanosaponite composite latex via miniemulsion polymerization

The synthesis and characterization of polystyrene encapsulated nanosaponite composite suspension via miniemulsion polymerization are reported in this study. The particle size of nanoclay and its pre-modification are critical to successfully producing a stable complex suspension. The final products were characterized by X-ray diffraction spectra, transmission electron microscopy (TEM), scanning electron microscopy (SEM), thin window energy dispersive spectroscopy (EDS), and light scattering. The results show that ar-vinylbenzyltrimethylammonium chloride (VBTAC) modified nanosaponite could be fully exfoliated and encapsulated inside the polystyrene latex via in situ miniemulsion polymerization. When the concentration of hexadecane (a co-stabilizer used in the miniemulsion polymerization) was high, the final composite particles are composed mainly of spherical particles with size less than 100nm, and a small number of hemispherical or bowl-structured particles of size ∼100nm to 1000nm. The phase separation due to the existence of large amounts of hexadecane accounted for the formation of a variety of morphologies.

Physical and mathematical models of an inert macroelectrode modified with active hemispherical microelectrodes

The physical model of an inert electrode partially activated with hemispherical microelectrodes was formed by the deposition of silver grains on a graphite substrate. It is shown that the process on the microelectrodes can be under activation control despite the fact that the overall rate is determined by the diffusion layer of the macroelectrode. On the basis of this conclusion a mathematical model of mass transfer on an inert electrode partially covered with active hemispherical particles (microelectrodes) is given and verified qualitatively by appropriate experiments. It was found that the degree of activation does not depend on the size of the particles, but on the ratio of the radius of the particles to the interparticle distance. This means that the quantity of catalyst required for the transformation of an inert electrode into an active one decreases strongly with decreasing active particle size. It was also shown that the maximum current density to the activated inert electrode was equal to the limiting diffusion one to the massive active electrode, as well as that the activity of the modified inert electrode at the same coverage of catalyst strongly depends on the exchange current density of the electrochemical process taking place on it. The larger is the exchange current density, the lower is the quantity of catalyst required for the same effect on the activity of the modified electrode.

自己組織化による高分子ナノ微粒子の作製

We found a simple method to prepare polymer nanoparticles by evaporation of solvent from a polymer solution. This method is applicable to prepare of nanoparticles of many kinds of functional materials (eg. engineering plastics, fluorescence polymers, conductive polymers, biodegradable polymers and so on.) with controlling their size ranging from several tens nm to several µm. By using this method, we fabricated particles with phase-separation structures from polymer blends or block-copolymers. Furthermore, it is shown that novel nanostructures including hemispherical particles and nano-disks by selective cross-linking and selective elution of polymer moieties that forms phase-separation structures are prepared.

Surface Selective Deposition of Mo(IV) on Ni/TiO2 Particles in Aqueous Solutions

The selective depositions of MoS2 and MoO2 over Ni surfaces are demonstrated on Ni/TiO2 particles in a mild electroless deposition process. High-resolution transmission electron microscopy (HRTEM) images show the uniform distribution of 10-30 nm spherical and hemispherical Ni particles on TiO2 surface, and three to six layers of MoS2 on the surface of Ni particles. The as-prepared MoS2-Ni/TiO2 is used as a catalyst for the hydrodesulfurization (HDS) reaction of dibenzothiophene (DBT), and shows a significant increase over commercial catalysts in turnover frequencies as the result of unique distribution of active components in the binary catalyst. The selective material deposition is explained in the context of Ni catalyzed KBH4 decomposition, which produces strong reducing species responsible for the site selective deposition of Mo. The synthetic method can be potentially used to prepare bimetallic materials with similar nanostructures such as those of Mo-Co, Mo-Pd, and Mo-Rh.