Spherical Metal Particles Research Articles

Comment on “Effective thermal conductivity of metal and non-metal particulate composites with interfacial thermal resistance at high volume fraction of nano to macro-sized spheres” [J. Appl. Phys. 117, 055104 (2015)

In a recent article, Faroughi and Huber [J. Appl. Phys. 117, 055104 (2015)] propose two theoretical models to compute the effective thermal conductivity of metal and dielectric spherical particle reinforced composites with interfacial thermal resistance. The models are based on the differential effective medium (DEM) theory. The authors have failed to cite and discuss the paper of Pal [Mater. Sci. Eng., A 498, 135–141 (2008)] where similar models have been derived using the same approach (DEM theory). Furthermore, the models proposed by Faroughi and Huber are seriously flawed in that the “excluded volume effect” is taken into account two times, instead of once, in their derivations. Last but not least, there are typos in their models.

Flower-like carbonyl iron powder modified by nanoflakes: Preparation and microwave absorption properties

In this paper, flower-like carbonyl iron powder (CIP) is prepared under normal temperature and pressure by a simple method of chemical reduction. This flower-like morphology is conducive to forming discontinuous network, enhancing diffuse scattering of the incident microwave and polarizing more interface charges. Those are all in favor of electromagnetic wave penetration and absorption. The test results show that compared with the unmodified CIP, the electromagnetic wave absorbing property of flower-like CIP is significantly improved in X-band. It is concluded that this research paves a way to enhance the microwave absorption properties of spherical metal particles.

Effective thermal conductivity of metal and non-metal particulate composites with interfacial thermal resistance at high volume fraction of nano to macro-sized spheres

In this study, we propose a theoretical model to compute the effective thermal conductivity of metal and dielectric spherical particle reinforced composites with interfacial thermal resistance. We consider a wide range of filler volume fraction with sizes ranging from nano- to macro-scale. The model, based on the differential effective medium theory, accounts for particle interactions through two sets of volume fraction corrections. The first correction accounts for a finite volume of composite and the second correction introduces a self-crowding factor that allows us to develop an accurate model for particle interaction even for high volume fraction of fillers. The model is examined to other published models, experiments, and numerical simulations for different types of composites. We observe an excellent agreement between the model and published datasets over a wide range of particle volume fractions and material properties of the composite constituents.

Experimental and Theoretical Study on Melting Kinetics of Spherical Aluminum Particles in Liquid Aluminum

ABSTRACTIn this study a process analysis of the melting process of solid particles in a bath of same composition is performed using both experimental information and theoretical computations. An experimental setup was used to measure the thermal histories and to follow the evolution with time of the size of solid metallic spherical particles being melted in a metallic bath of same composition. For such a purpose, pure aluminum was used during the experiments for both solid particles and liquid bath. A mathematical model was also developed based on first principles of heat transfer to simulate the melting kinetics of a cold metallic spherical particle immersed in a hot liquid bath of same composition. The mathematical model was reasonably validated when compared against the experimental results obtained in this work. A process analysis of the melting process was performed to determine the effect of the initial temperature and size of the solid particle, the bath temperature and the convective heat transfer coefficient on the melting time and on the energy consumption.The analysis showed that the variable presenting the most significant effect on both the melting time and the energy consumption is the convective heat transfer coefficient between the particle and the bath, since an increment in such a parameter accelerates the melting process and saves energy. Therefore, proper stirring of the bath is highly recommended to enhance the melting of metallic alloying additions in the metallic baths.

Srećko Stopić: Sinteza nanočestica metala ultrazvučnim raspršivanjem u uslovima pirolize

Nanotechnology belongs to the key innovative technologies for powder production. Ultrasonic spray pyrolysis is a versatile method for the formation of nanosized particles of metals, oxides and composites. This work deals with Ag, Cu and Au nanoparticles formed by ultrasonic spray pyrolysis using the horizontal and vertical reactor. Furthermore, a direct synthesis of Ru-TiO2 and RuO2-TiO2 nanoparticles with the core and shell structure was investigated. The molar fractions of precursors, solvent type, and the process temperature play the crucial role in the formation of core and shell structures. Moreover, the influence of the reaction parameters (temperature, residence time, solution concentration and ultrasonic frequency) on the morphological characteristics of the prepared nanoparticles was studied. A decrease in the solution concentration decreases the final nanoparticle size. An increase in temperature from 150°C to 1000°C leads from an irregular form to a more spherical one. Subsequently, a model of metallic nanoparticle formation from an aerosol droplet could be proposed. Using ultrasonic spray pyrolysis, ideal spherical metallic particles were obtained at temperatures above the melting point. A scanning mobility particle sizer (SMPS) was used for the on-line determination of nanoparticle size distribution. The collection of nanosized particles was performed in an electrostatic field. The scale up of the ultrasonic spray pyrolysis method was successfully applied for the synthesis of silver powder from a water solution of silver nitrate.

Niobium powder synthesized by calciothermic reduction of niobium hydroxide for use in capacitors

Metallic niobium powder was produced for applications in electric capacitors via calciothermic reduction of niobium hydroxide in molten CaCl2. Sub-micrometer spherical metallic particles with coral-like morphologies reflected the particle size of the starting oxide powder. A fine powder was obtained from the mixtures of niobium hydroxide and CaO or Ca(OH)2, respectively.Sintered pellets of the metallic powder showed a higher capacitance (CV) than those of the simply reduced powder without pre-treatment, because the shrinkage during sintering was smaller. The CV was as large as that of commercially sintered pellets for tantalum capacitors. Therefore, this niobium powder would act as a higher-voltage capacitor by applying chemical anodic treatment at higher voltages, and lower oxygen content in the reduced power could realize a lower leak current.

Composition and hygroscopicity of aerosol particles at Mt. Lu in South China: Implications for acid precipitation

Physicochemical properties of aerosol particles were studied at Mt. Lu, an elevated site (115°59′E, 29°35′N, 1165 m) within the acid precipitation area. Northeast winds transport copious amounts of air pollutants and water vapor from the Yangtze River Delta into this acid precipitation area. NH4+ and SO42− are the dominant ions in PM2.5 and determine aerosol acidity. Individual particle analysis shows abundant S-rich and metals (i.e. Fe-, Zn-, Mn-, and Pb-rich) particles. Unlike aerosol particles in North China and urban areas, there are little soot and mineral particles at Mt. Lu. Lack of mineral particles contributed to the higher acidity in precipitation in the research area. Nano-sized spherical metal particles were observed to be embedded in 37% of S-rich particles. These metal particles were likely originated from heavy industries and fired-power plants. Hygroscopic experiments show that most particles start to deliquesce at 73–76% but organic coating lowers the particle deliquescence relative humidity (DRH) to 63–73%. The DRHs of these aerosol particles are clearly smaller than that of pure ammonium sulfate particles which is 80%. Since RH in ambient air was relatively high, ranging from 65% to 85% during our study period, most particles at our sampling site were in liquid phase. Our results suggest that liquid phase reactions in aerosol particles may contribute to SO2 to sulfuric acid conversion in the acid precipitation area.

Magnetic dipole radiation tailored by substrates: numerical investigation.

Nanoparticles of high refractive index materials can possess strong magnetic polarizabilities and give rise to artificial magnetism in the optical spectral range. While the response of individual dielectric or metal spherical particles can be described analytically via multipole decomposition in the Mie series, the influence of substrates, in many cases present in experimental observations, requires different approaches. Here, the comprehensive numerical studies of the influence of a substrate on the spectral response of high-index dielectric nanoparticles were performed. In particular, glass, perfect electric conductor, gold, and hyperbolic metamaterial substrates were investigated. Optical properties of nanoparticles were characterized via scattering cross-section spectra, electric field profiles, and induced electric and magnetic moments. The presence of substrates was shown to have significant impact on particle's magnetic resonances and resonant scattering cross-sections. Variation of substrate material provides an additional degree of freedom in tailoring optical properties of magnetic multipoles, important in many applications.

Scattering of electromagnetic radiation by a metal nanoparticle

The scattering cross section of electromagnetic radiation by a small spherical metal particle has been calculated in the framework of the standard kinetic theory in a dipole approximation. The calculation has been performed for relatively small (∼10 nm) particles, which allows the skin effect to be ignored. A mechanism of mixed specular-diffuse reflection of conduction electrons from the particle surface is considered. It is established that, at certain angles of scattering, the mechanism of magnetic-dipole scattering becomes dominating. The influence of kinetic effects on the differential scattering cross section is analyzed.

Resonance in metallic nanoparticles: a rigorous formulation of the dipolar approximation

In classical electromagnetism, the interaction of light with matter is introduced through the polarizability of a dipole subject to the local field of nearby atoms, leading to the Clausius–Mossotti rule. The dipolar model is also used to introduce plasmonics and fluorescence to undergraduates in electromagnetism courses: the scattering, absorption and extinction of light by spherical particles are examples of analytical solutions in spherical coordinates. The possible resonance of the interaction between light and metallic nanoparticles is commonly described in terms of the infinite limit of the dipole when its relative permittivity tends towards −2. However, this divergence is physically disruptive and therefore a mathematically sustainable approach is required to describe the resonance that occurs in the interaction between light and a metallic spherical particle. The proposed method removes infinity, enabling a rigorous discussion of the resonance.

Membrane Emulsification of Liquid Metal for the Generation of Spherical Metal Particles

AbstractPorous membrane structures are used in emulsification processes for the formulation of almost monodispersed emulsion droplets with focus on the melt emulsification of liquid metal melts. The melt is emulsified through nozzle plates and particulate sintered porous structures into a temperature controlled liquid where the liquid metal droplets solidify to form spherical shaped solid particles. Related to melt atomization techniques for powder production, the membrane emulsification processing route obtains an interesting alternative for the production of monodispersed spherical particles with low energy consumption.

液中レーザー溶融法によるサブミクロン球状金属粒子作製

液中レーザー溶融法によるサブミクロン球状金属粒子作製

Solution plasma synthesis of ZnO flowers and their photoluminescence properties

ZnO nanoflowers, amorphous ZnO nanospheres and metallic Zn particles have been controllably synthesized using solution plasma technique at different reaction temperatures and agitation. X-ray diffractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that ZnO nanoflowers were synthesized at a high electrolyte temperature in a static system (no agitation). On the other hand, the use of agitation led to both amorphous ZnO nanospheres and spherical metallic zinc particles. The excitation temperature of plasma was 4000K from the light emission using spectrometer. From these result, melting and vaporization of the Zn electrode produced particles. ZnO nanoflowers grew on the surface of spherical particles by precipitation from Zn(OH)42− ions. Photoluminescence (PL) measurement indicated that as-synthesized ZnO flowers and amorphous ZnO nanospheres showed strong green emissions, which was attributable to their surface defect structure. Furthermore, the annealing at 700°C led to a reduction in green emissions, attributable to a decrease in the number of surface defects. Photocatalytic activity of ZnO nanoflowers was increased by the annealing.

Light emission by point dipole located inside spherical (semiconductor) particle in the vicinity of a spherical metal particle

Light emission by point dipole located inside spherical (semiconductor) particle in the vicinity of a spherical metal particle

Designing photonic structures of nanosphere arrays on reflectors for total absorption

By means of full electrodynamic simulations, we investigate structures that can totally absorb light minimizing all reflections. Such efficient absorbers of visible and infrared light are useful in photovoltaic and sensor applications. Our study provides a simple and transparent analysis of the optical properties of structures comprising a resonant cavity and a reflector, which are the basic ingredients of a resonant absorber, based on general principles of scattering theory. We concentrate on periodic arrays of metallic or dielectric spherical particles in front of metallic or dielectric mirrors and show that tuning the material absorption could turn resonances in the structures into total absorption bands. Perfect absorption is predicted in metallic sphere arrays but also for Si spheres on a metallic substrate, moreover, by replacing the substrate below the Si spheres with a lossless dielectric Bragg mirror an all-dielectric-perfect-absorber is designed.

Casimir force between composite materials containing nonspherical particles

The Casimir force between metal-dielectric composite slabs containing nonspherical particles is investigated. The composite slab may have the symmetric (the nonspherical metal particles and spherical dielectric particles are randomly distributed) or asymmetric microstructure (the nonspherical metal particles are randomly embedded in the dielectric host medium), and the corresponding effective permittivity is described by the generalized Bruggeman effective medium approximation or generalized Maxwell-Garnett approximation. As a consequence, the Casimir force can be controlled by the volume fraction and the particles' shape. It is found that the Casimir force achieves a minimal value for spherical particles, and the magnitude of Casimir force can become strong for nonspherical particles. In addition, the Casimir force for the metal-dielectric composites with the symmetric microstructure shows a fast change at the shape-dependent percolation threshold, above (below) which the composite slab is metallic (dielectric). Our study may be of great interest for making precise comparisons between theoretical and experimental results on the Casimir force.

Material dependence of the wire-particle Casimir interaction

We study the Casimir interaction between a metallic cylindrical wire and a metallic spherical particle by employing the scattering formalism. At large separations, we derive the asymptotic form of the interaction. In addition, we find the interaction between a metallic wire and an isotropic atom, both in the non-retarded and retarded limits. We identify the conditions under which the asymptotic Casimir interaction does not depend on the material properties of the metallic wire and the particle. Moreover, we compute the exact Casimir interaction between the particle and the wire numerically. We show that there is a complete agreement between the numerics and the asymptotic energies at large separations. For short separations, our numerical results show good agreement with the proximity force approximation.

Secondary Electron Emission from a Negatively Charged Spherical Dust Particle by Electron Incidence

Secondary electron emission (SEE) from a spherical metallic dust particle by electron incidence was analyzed according to the OML (Orbit Motion Limited) model. As the observed dust speed in fusion plasmas (< 100m/s) is much slower than electron thermal speed, its effect to the SEE current is negligibly small. It is clarified that there is a window of electron temperature, where the ratio of the SEE current to the absorbed electron current exceeds unity; for graphite 80 eV < Te < 326 eV and the maximum of the ratio ∼ 1.10 at Te ∼ 165 eV, for tungsten 32 eV < Te, the maximum ∼ 2.47 at Te ∼ 358 eV. These excesses come from the inverse proportion of the incident angle to the dust particle.

Fe 系および Pd 系金属ガラス球形粒子作製時の冷却速度と容器壁の影響

Preparation of Fe-based and Pd-based metallic glassy particles with a diameter of around 500 μm were investigated. Firstly, we measured the critical cooling rate of Pd42.5Cu30Ni7.5P20 metallic glass and our results showed that its critical cooling rate was about 0.4 K/s. Pd-based metallic glassy spherical particles with fully amorphous structure were successfully prepared by heating the samples on the Al2O3 substrate up to their melting points and cooling under the cooling rate of around 3 K/s. In contrast, [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 metallic glassy particles cannot be prepared by the same process. This seemed to be caused by a reaction between the Fe-based metallic glass and the Al2O3 substrate around its melting point. This result suggested that containerless solidification method is expected to be used to prepare Fe-based metallic glassy particles. We successfully prepared mono-sized spherical particles of [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 metallic glass by Pulsated Orifice Ejection Method (POEM) as an ideal process for Fe-based metallic glasses.

Photomediated assembly of single crystalline silver spherical particles with enhanced electrochemical performance

A bottom-up based photomediated strategy is presented to create single crystalline silver spherical particles by pulsed laser irradiation of silver nanoparticles in liquid, where the unique selective laser heating is responsible for the formation of spherical particles. The present approach is facile and flexible for the design of silver spherical particles with tunable sizes. Systematical studies reveal that the experimental parameters including laser fluence, laser irradiation time, dispersing liquid, and concentration of starting material are important factors that influence the formation of silver spherical particles. As a demonstration of the as-synthesized silver spheres for electrochemical applications, the performance of silver spherical particles as potential supercapacitors was investigated by cyclic voltammetry, and the results indicated a higher capacity of silver spherical particles compared to that of silver raw nanoparticles, which is considered to originate from the enhanced electronic conductivity due to the single crystalline feature of silver spherical particles. We believe that the photomediated method presented in this work can be set as a new alternative in creating a variety of noble metallic spherical particles.