Small Metal Spheres Research Articles

Extinction by plasmonic nanoparticles in dispersive and dissipative media.

Extinction of small metallic spheres has been well understood through the classical Mie theory when the host medium is dispersive and transparent. However, the role of host dissipation on the particulate extinction remains a competition between the enhancing and reducing effects on the localized surface plasmonic resonance (LSPR). Here, using a generalized Mie theory, we elaborate on the specific influence mechanisms of host dissipation on the extinction efficiency factors of a plasmonic nanosphere. To this end, we isolate the dissipative effects by comparing the dispersive and dissipative host with its dissipationless counterpart. As a result, we identify the damping effects of host dissipation on the LSPR including the resonance widening and amplitude reducing. The resonance positions are shifted by host dissipation, which cannot be predicted by the classical Fröhlich condition. Finally, we demonstrate that a wideband extinction enhancement due to host dissipation can be realized away from the positions of the LSPR.

Study on Negative Corona Discharge of the Small Spherical Metal Electrode

The negative corona discharge between a small spherical metal electrode (SSME) and a grounded plane electrode (GPE) was often a concern. The ring mode discharge and the dot mode discharge were found in the experimental study. Up until now, these two phenomena are not verified in simulation study. Simulation on the negative corona discharge between an SSME and a GPE is staged in this article. The pulse currents in negative corona discharge are analyzed under different voltages, radius of the SSME, and gap distance coupling fluid dynamic equations and Poisson equation based on the axisymmetric model. The interesting phenomenon found is that the corona discharges sequentially happen at different central angles on the surface of the SSME starting with the point nearest to the GPE. The negative corona discharges generated by the SSME and the hemispherical metal electrode are compared. The mechanism of discharge is analyzed by comparing their current pulses, electric field intensities, and changes in the concentration of ions. The ring mode discharge and the dot mode discharge are shown in our simulation. The reason for the formation of the ring mode discharge is also given.

Calibration to Mitigate Near-Field Antennas Effects for a MIMO Radar Imaging System.

A calibration method for a high-resolution hybrid MIMO turntable radar imaging system is presented. A line of small metal spheres is employed as a test pattern in the calibration process to measure the position shift caused by undesired antenna effects. The unwanted effects in the antenna near-field responses are analysed, modelled and significantly mitigated based on the symmetry and differences in the responses of the MIMO configuration.

Microwave absorption by small dielectric and semi-conductor coated metal particles

The presence of the oxide coatings on conducting metal particles is often ignored in the calculations of microwave absorption. However, we find that the optical properties of the coatings play a significant role in enhancing or suppressing the absorption of electromagnetic energy. Here, we solve the Mie scattering equations numerically to separate and quantify the role of electric and magnetic field absorption from microwaves at 2.45 GHz by small metal spheres coated with dielectric or semi-conducting materials. The range of size and conductivities of the metal particles and the optical properties of the coatings are chosen for their practical importance. We also provide simple approximate expressions for the absorption per unit volume by coated spheres in the small particle limit which agrees very well with the exact Mie solution. We have demonstrated that for highly conducting particles coated with a material of low conductivity, the electric field absorption depends only on the optical properties, and the volume fraction of the coating. In contrast, the magnetic field absorption depends only on the properties of the core which is the same as the bare metal. We find that the power absorbed by coated particles via the electric field is maximized when loss tangent, tanδ~1. A key result of this study is that a thin layer of lossy coating (~15% of the particle size) on highly conductive particles will significantly enhance (up to a factor of 105) both the power absorbed from the E-field and the total power absorbed.

Effects of Interfacial Layer on Refractive Index and Propagation of Waves in Small Spherical Metal/ Dielectric Composite

In this work we have studied the effect of interfacial layer on the refractive index, and propagation of waves in small spherical metal/dielectric composite separated by interfacial layers which is randomly embedded in a linear dielectric host matrix. The theoretical, and numerical descriptions are in terms of the interfacial factor (I) by incorporating Taylor expansion and Drude model. The result shows that both the interfacial layer property, and the percentage of the volume fraction of the metallic particles in the composite has an effect on the refractive index, and propagation of waves of the composite when the dielectric functions of the interfacial layer is more metal-like property than dielectric-like property.

Modeling the growth of a vapor film formed in contact between a hot metal sphere and water in pressure vessels

The contact between a hot object such as the particles of a molten metal and a cool and relatively volatile fluid such as water likely takes place in metal casting, paper manufacturing industries, and nuclear reactors. The highly intensive and quick heat transfer caused by the contact may increase the pressure in the vapor film formed on a hot particle within a short period leading to the vapor explosion followed by a strong pressure wave in a vessel containing a volatile matter. The present study aims to provide a mathematical model for contacting between a small hot copper metal sphere and water and explain the effect of pressure change in water vessel on the vapor film radius, pressure inside vapor film, as well as the saturation temperature at the interface. In addition, it examines the results of vapor film growth and the internal pressure of the film for such a phenomenon in nuclear power plants with fuel rod coolant pressures ranged between 60 and 150 bar. The results of the extremely high pressures of the vessels indicated an increase in the pressure pulse inside the vapor film within a short period which raised the likelihood of vapor explosion.

Temperature dependence of the surface plasmon resonance in small electron gas fragments, self consistent field approximation

The temperature dependence of the surface plasmon resonance in small metal spheres is calculated using an electron gas model within the Random Phase Approximation. The calculation is mainly devoted to the study of spheres with diameters up to at least 10nm, where quantum effects can still be relevant and a simple plasmon pole approximation for the dielectric function is no more appropriate. We find a possible blue shift of the plasmon resonance position when the temperature is increased while keeping the size of the sphere fixed. The blue shift is appreciable only when the temperature is a large fraction of the Fermi energy. These results provide a guide for pump and probe experiments with a high time resolution, tailored to study the excited electron system before thermalisation with the lattice takes place.

Effect of kinetic boundary conditions on the cross section of the scattering of an electromagnetic wave by a small metal particle

The scattering cross section of a small spherical metal particle in the presence of a plane electromagnetic wave is calculated in the dipole approximation. On the assumption that the radius of the particle is less than the skin-layer depth, the skin effect is disregarded. The calculations are performed with the aid of the kinetic approach at an arbitrary ratio of the free-path length of electrons to the radius of the particle. The model of the diffuse—specular Fuchs boundary conditions is used to obtain the nonequilibrium distribution function. Asymptotic expressions for large particles are in agreement with the known results of the macroscopic theory.

An Adjustable Radar Cross Section Doppler Calibration Target

This paper describes the development of a simple low-cost adjustable radar cross section (RCS) Doppler calibration target. It consists of a small metal sphere attached to the diaphragm of an ultrasound transducer. It is shown that the effective RCS of the target is a function of the physical RCS determined by the size of the sphere, and the magnitude of the displacement introduced by the transducer. A simple Michelson's interferometer was constructed to determine the magnitude of these displacements, and a linear relationship between the applied sinusoidal voltage (at resonance) and the displacement is determined. Measurements made using a sensitive Doppler radar confirm the calculated RCS to an accuracy of better than 2 dB, and show that it can be adjusted over 20 dB by adjusting the transducer excitation voltage through one decade. These low RCS targets are ideal for testing radio acoustic sounding systems and entomological radars.

The Effect of Bio-Coal on the Carbothermic Reduction of Laterite Ores

In order to investigate the effect of bio-coal on the carbothermic reduction of laterite ores, carbothermic reduction experiments of two type laterite ores mixed with bio-coal were conducted in this research. The reduction experiments were carried out at 1173­1673K for 1800s in Ar. Compared reduction results of coal and bio-coal show increasing the nickel content with increasing temperature. Bio-coal had become the reductant, the nickel content of limonite specimens and saprolite specimens had been raised from 1.18mass% (before reduction) to 1.64mass% (after reduction at 1373K for 1800s) and from 3.43mass% (before reduction) to 4.93mass% (after reduction at 1473K for 1800s), respectively. The major phases of limonite specimens were Fe1.833(OH)0.5O2.5 and Fe2SiO2. After the reduction at 1473K for 1800s, the major phase was metal Fe. The major phases of the saprolite specimens were Fe(CO3), Fe3O4 ,F e2SiO4 and Al3S4. After the reduction at 1473K for 1800s, the major phases were Fe, Fe0.64Ni0.36 and i-Fe10.8Ni. The morphological observation of limonite and saprolite specimens shows that the pelletshaped specimen became a dense area and the core of the pellet had loose structure at 1173­1374K for 1800s. The core of pellet was produced small spherical metal phase at 1473K for 1800s, this temperature was the soft melting temperature. Finally, the pellet produced nickel-iron alloy phase and iron was separated from the oxide phase at 1673°C for 1800s. [doi:10.2320/matertrans.M2014315]

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.

Localization and Poincaré catastrophe in the problem of a photon scattering on a pair of Rayleigh particles

It is shown that complexities in a problem of elastic scattering of a photon on a pair of Rayleigh particles (two small metallic spheres) are similar to the complexities of the classic problem of three bodies in celestial mechanics. In the latter problem, as is well known, the phase trajectory of a system becomes a nonanalytical function of its variables. In our problem, the trajectory of a virtual photon at some frequency could be considered such as the well-known Antoine set (Antoine's necklace) or a chain with interlaced sections having zero topological dimension and fractal structure. Such a virtual ``zero-dimensional'' photon could be localized between the particles of the pair. The topology suppresses the photon's exit to the real world with dimensional equal-to-or-greater-than units. The physical reason for this type of photon localization is related to the ``mechanical rigidity'' of interlaced sections of the photon trajectory due to a singularity of energy density along these sections. Within the approximations used in this paper, the effect is possible if the frequency of the incident radiation is equal to double the frequency of the dipole surface plasmon in an isolated particle, which is the only character frequency in the problem. This condition and transformation of the photon trajectory to the zero-dimensional Antoine set reminds of some of the simplest variants of Poincar\'e's catastrophe in the dynamics of some nonintegrable systems. The influence of the localization on elastic light scattering by the pair is investigated.

Blueshift of the surface plasmon resonance in silver nanoparticles: substrate effects

We study the blueshift of the surface plasmon (SP) resonance energy of isolated Ag nanoparticles with decreasing particle diameter, which we recently measured using electron energy loss spectroscopy (EELS) [1]. As the particle diameter decreases from 26 down to 3.5 nm, a large blueshift of 0.5 eV of the SP resonance energy is observed. In this paper, we base our theoretical interpretation of our experimental findings on the nonlocal hydrodynamic model, and compare the effect of the substrate on the SP resonance energy to the approach of an effective homogeneous background permittivity. We derive the nonlocal polarizability of a small metal sphere embedded in a homogeneous dielectric environment, leading to the nonlocal generalization of the classical Clausius-Mossotti factor. We also present an exact formalism based on multipole expansions and scattering matrices to determine the optical response of a metal sphere on a dielectric substrate of finite thickness, taking into account retardation and nonlocal effects. We find that the substrate-based calculations show a similar-sized blueshift as calculations based on a sphere in a homogeneous environment, and that they both agree qualitatively with the EELS measurements.

Benchmark measurements and simulations of dose perturbations due to metallic spheres in proton beams

Monte Carlo simulations are increasingly used for dose calculations in proton therapy due to its inherent accuracy. However, dosimetric deviations have been found using Monte Carlo code when high density materials are present in the proton beamline. The purpose of this work was to quantify the magnitude of dose perturbation caused by metal objects. We did this by comparing measurements and Monte Carlo predictions of dose perturbations caused by the presence of small metal spheres in several clinical proton therapy beams as functions of proton beam range and drift space. Monte Carlo codes MCNPX, GEANT4 and Fast Dose Calculator (FDC) were used. Generally good agreement was found between measurements and Monte Carlo predictions, with the average difference within 5% and maximum difference within 17%. The modification of multiple Coulomb scattering model in MCNPX code yielded improvement in accuracy and provided the best overall agreement with measurements. Our results confirmed that Monte Carlo codes are well suited for predicting multiple Coulomb scattering in proton therapy beams when short drift spaces are involved.

Soft hollow particle damping identification in honeycomb structures

The aim of this study is to provide a structural damping solution for space applications to enhance mission performance of honeycomb structures. Classical particle dampers are enclosures partially filled with small metallic or glass spheres, attached to a vibrating structure. The induced damping mechanism is mainly due to frictional losses and collision effects. This paper deals with replacing classical hard particles with soft hollow ones. This study is oriented toward experimental investigations and theoretical validation in order to distinguish dissipation phenomena. The experimental approach first relies on identifying the damping in small honeycomb samples filled with particles. Instead of dissipation by friction and impact, the elliptical shape of the measured hysteresis loops highlights that visco-elastic behavior is dominant with these specific soft particle dampers. Then, experimental and numerical validations are performed on aluminum honeycomb cantilever beams filled with particles. To take into account the effect of the particles, equivalent oscillators, based on the previous experimental damping identification, are added to a finite element model. These kinds of particle dampers are highly nonlinear as a function of excitation frequency and amplitude. It is shown that good damping efficiency is achieved across a large frequency range with low impact on structure stiffness. This paper suggests a convenient method to model the structural damping induced by soft hollow particles.

MAGNETIC RESPONSE FROM A COMPOSITE OF METAL-DIELECTRIC PARTICLES IN THE VISIBLE RANGE: T-MATRIX SIMULATION

Normal 0 false false false MicrosoftInternetExplorer4 The optical response of a particle composed of a dielectric core surrounded by a densely packed shell of small metal spheres is simulated with the superposition T-matrix method for realistic material parameters. In order to compute the electric and magnetic particle polarizabilities a single expansion T-matrix is derived from a particle centered T-matrix. Finally the permeability of a medium comprising such particles is found to deviate considerable from unity resulting in a noticeable optical response.

Experimental and Theoretical Study of Metallic Hollow Sphere with Medium Thick Wall

. Metallic hollow sphere (MHS) material is a new type of cellular material characterized by super-light, high specific stiffness and strength, the ability to absorb noise and energy, thermal insulation. It had been widely used in engineering, i.e. automobile, aircraft, civil and military. The basic unit of MHS cellular material was small metallic hollow sphere in macro, and the instability of sphere shell is always the most difficult problem in mechanical field. In this paper, we explored the internal factors and the principle of this phenomenon though the methods of theoretical and experimental. The equation of deformation of MHS was derived. It had shown that the nominal strain was related to the radius of the shell and loads, and the nominal stress was connected with the radius, load and the thickness of shell wall. A series of quasi-static uniaxial compression tests were progressed for a group of single MHSs, which radii were 4mm, 5mm and 7mm. It was found that the deformation planes are circle and symmetry both top and bottom, and the curves of nominal stress-strain were similar to a half parabola. There was a perfectly uniformity of the theoretical model and the experimental results.

Determination of trajectories of metallic spheres settling in non‐newtonian fluids

AbstractA “videogrammetric” system to measure the trajectory of small metallic spheres falling through a slurry‐filled tank has been developed to support fluid dynamics research. The system used two low‐cost video cameras and a combination of commercially available and custom‐built software for the image acquisition, the measurement and tracking of the spheres in the imagery and the photogrammetric operations. After a review of relevant literature and a description of the system, this paper reports on an extensive testing regime conducted to gauge the performance of the system. The results show that it achieved object space coordinate precision and accuracy better than the required figure of 1 mm. Simulation‐based analyses, to quantify the sensitivity of refraction corrections in object space to the accuracy of the three refractive indices and distance parameters involved in the system, are also reported. The results of these tests demonstrated that some parameters need only be specified to about 56% of their true value whereas others must be accurate to better than 2%. Finally, results from two examples in sphere trajectory determination are presented and analysed.

On the quenching of steel and zircaloy spheres in water-based nanofluids with alumina, silica and diamond nanoparticles

The quenching curves (temperature vs time) for small (∼1 cm) metallic spheres exposed to pure water and water-based nanofluids with alumina, silica and diamond nanoparticles at low concentrations (⩽0.1 vol%) were acquired experimentally. Both saturated (Δ T sub = 0 °C) and highly subcooled (Δ T sub = 70 °C) conditions were explored. The spheres were made of stainless steel and zircaloy, and were quenched from an initial temperature of ∼1000 °C. The results show that the quenching behavior in nanofluids is nearly identical to that in pure water. However, it was found that some nanoparticles accumulate on the sphere surface, which results in destabilization of the vapor film in subsequent tests with the same sphere, thus greatly accelerating the quenching process. The entire boiling curves were obtained from the quenching curves using the inverse heat transfer method, and revealed that alumina and silica nanoparticle deposition on the surface increases the critical heat flux and minimum heat flux temperature, while diamond nanoparticle deposition has a minimal effect on the boiling curve. The possible mechanisms by which the nanoparticles affect the quenching process were analyzed. It appears that surface roughness increase and wettability enhancement due to nanoparticle deposition may be responsible for the premature disruption of film boiling and the acceleration of quenching. The basic results were also confirmed by quench tests with rodlets.

Mechanical Properties and Impact Resistance of a New Transparent Glass‐Ceramic

The focus in ceramic armour development today is towards improved protection capability combined with an overall reduction in production cost. Because of their brittle nature, ceramics and glasses are susceptible to localized surface damage in the form of cracking when subjected to impact by foreign objects. Much work has been carried out on the dynamic impact behaviour of ceramics and glasses. Tests have been carried out using small ceramic or metallic spheres, impacting at high speed in so-called ballistic tests. In these tests, projectiles are accelerated toward their target by utilizing specialised devices called gas guns. The development of transparent glass-ceramics with a reinforced surface layer is a promising approach to novel armour materials since glass-ceramics combine high strength with improved fracture toughness compared with glass and yet they can be reliably produced using cost-effective processing methods. A number of technical reports have been published in the last forty years regarding different types of transparent glass-ceramics, but very few focused on their use as armour materials. The most attractive advantage of glass-ceramics is the possibility of using standard glass processing methods followed by controlled heat treatment, which is a fast and reliable production method, resulting in almost no residual porosity in the materials. Moreover the melting plus heat-treatment route offers very good reproducibility compared with the alternative method based on powder processing and sintering commonly used in polycrystalline ceramics, which is prone to microstructure inhomogeneity. In the majority of previous investigations, transparent glass-ceramics have been obtained, which exhibit the presence of nanocrystals within an amorphous matrix. This nanostructure (with less than 70 % crystalline phase) guarantees optical transparency since the crystal size is smaller than the wavelength of visible light. The two principal conditions for achieving high transparency are low optical scattering and low atomic absorption in the visible range. Transparent glass-ceramics appear to be an ideal material for advanced armour applications due to their relatively good transparency, low density, relatively high fracture strength as well as their thermal and chemical stability. However, compared with the most popular polycrystalline ceramics (e.g. Al2O3), glass-ceramics exhibit lower fracture toughness. [26–30]