Particle Breakup in Shock Waves Studies by single Particle Light Scattering

The breakup of suspended, agglomerated submicron particles was studied by exposing the aerosol to weak shock waves of varying strength under conditions 400 ms^?1?v?880 ms^?1. A newly developed laser light-scattering diagnostic employing a top hat laser profile was used to size the particles passing through a very small scattering volume. By Comparing the optically measured particle size in front of and behind shock waves, the breakup of agglomerated particles could be clearly identified. The experiments indicate that the aerodynamic forces behind an incident shock overcome the particle binding force resulting in disintegration of the submicron agglomerates. The results are presented in form of a modified Weber number.     

Low-Concentration Photon Correlation Spectroscopy

Photon correlation spectroscopy (PCS) is a technique to measure rapidly particle size in the sub-micrometre region. The use of PCS is, however, limited by concentration. The upper limit is due to multiple scattering of the incident light and the lower limit is determined by the fact that fluctuations of the number of particles in the measuring zone have a significant influence on the apparent diffusion coefficient. In this paper a signal processing method is described which differentiates this influence. With this system the lower limit is no longer limited to about 100 particles in the measuring volume corresponding to a concentration of 10⁹ particles/cm³. The limitation is now the intensity of the scattered light, which becomes too weak at a concentration of about 50 particles/cm³. As a consequence of this work, a revision to the basic theory of PCS may be necessary. Moreover, the new processing method also permits the measurement of the particle concentration in the sample.     

Nanoparticle production by UV irradiation of combustion generated soot particles

Laser ablation of surfaces normally produce high temperature plasmas that are difficult to control. By irradiating small particles in the gas phase, we can better control the size and concentration of the resulting particles when different materials are photofragmented. Here, we irradiate soot with 193 nm light from an ArF excimer laser. Irradiating the original agglomerated particles at fluences ranging from 0.07 to 0.26 J/cm² with repetition rates of 20 and 100 Hz produces a large number of small, unagglomerated particles, and a smaller number of spherical agglomerated particles. Mean particle diameters from 20 to 50 nm are produced from soot originally having a mean electric mobility diameter of 265 nm. We use a non-dimensional parameter, called the photon–atom ratio (PAR), to aid in understanding the photofragmentation process. This parameter is the ratio of the number of photons striking the soot particles to the number of the carbon atoms contained in the soot particles, and is a better metric than the laser fluence for analyzing laser–particle interactions. These results suggest that UV photofragmentation can be effective in controlling particle size and morphology, and can be a useful diagnostic for studying elements of the laser ablation process.     

A statistical model for ESR line shape of lithium colloids created by γ irradiation in HLi

Electron paramagnetic resonance results of small lithium particles created in lithium hydride by γ irradiations are presented. The EPR line was found to be inhomogeneous and the spin lattice relaxation time for conduction electrons to be unusually large (10^?4?10^?6s). We point out that the EPR line width is induced by the dispersion of the total nuclear magnetic moment inside the small particle. We assume that the size distribution of lithium particles created by γ irradiation is a logarithmiconormal distribution. Different experimental results confirm this model.     

Fabrication of Al nanoparticles and their electrical properties studied by capacitance-voltage measurements

Nanometer-scale Al particles are fabricated and are embedded in a GaAs matrix using molecular beam epitaxial technique. The Al particle is self-assembled on GaAs by supplying an Al molecular beam. The average particle size is found to be 25 nm. The density is 7 × 10¹⁰ cm⁻² when Al of 6.2 × 10¹⁵ atoms/cm² is supplied on (1 0 0)GaAs at a substrate temperature of 300 °C. Clear hysteresis and plateaus in capacitance-voltage (C-V) curves are found in an Al-embedded sample, whereas monotonic increase of capacitance is obtained in a reference sample having an AlAs layer instead of Al. This difference results from trapping of electrons by the Al particles, suggesting that the particles have metallic character.     

One-step solid-state synthesis of nanosized LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathode material with power properties

A simple one-step solid state reaction way of preparing nanosized LiMn₂O₄ powders with high-rate properties is investigated. Oxalic acid is used as a functional material to lose volatile gases during the process of calcining in order to control the morphology and change the particle size of materials. The results of X-ray diffraction and scanning electron microscopy show that particle size of materials decreases with the increase of the oxalic acid content. The electrochemical test results indicate that optimal LiMn₂O₄ particles (S_0.5) is synthesized when the molar ratios of oxalic acid and total Mn source are 0.5:1. It also manifests that LiMn₂O₄ sample with middle size has the optimal electrochemical performance among five samples instead of the smallest LiMn₂O₄ sample. The obtained sample S_0.5 with middle size exhibits a high initial discharge capacity of 125.8 mAh g^?1 at 0.2C and 91.4% capacity retention over 100 cycles at 0.5C, superior to any one of other samples. In addition, when cycling at the high rate of 10C, the optimal S_0.5 in this work could still reach a discharge capacity of 80.8 mAh g^?1. This observation can be addressed to the fact that the middle size particles balance the contradictory of diffusion length in solid phase and particle agglomeration, which leads to perfect contacts with the conductive additive, considerable apparent Li-ion diffusion rate, and the optimal performance of S_0.5.     

Die Verteilung der natürlichen Radioaktivität auf das Größenspektrum des natürlichen Aerosols

The apparatus described allows to measure the distribution of natural radioactivity on aerosol particles. Careful measures were taken that the aerosol concentration and its size spectrum were not altered before being examined. To increase the accuracy of the measurements the natural aerosol was charged with thoron decay products. The resulting electric charge-distribution by ion diffusion on dust particles is known, therefore the radii of these particles could be calculated by mobility measurements. By means of the law, governing attachment of emanation decay products on dust particles — as derived byLassen orWieser andStierstadt (afterBricard), it is possible to calculate the size spectrum of natural aerosol from the measured activity spectrum. Until now we have onlyion spectra for the range of size 6×10^?7 to 10^?5 cm. With the quoted apparatus however it is possible to measure thesize spectrum of natural aerosol. So we have a simple size-spectrometer, that covers a large range of particles. Among other things can be deduced from these measurements, that the natural radioactivity of air is attached to particles of radii from 10^?6 up to 10^?5cm, — the main part of the activity being carried by aerosols with radii between 4 and 6×10^?6cm. From the calculated aerosol spectrum (r from 6×10^?7 to 10^?5 cm) follows a mean particle radius ofr=2,5×10^?6cm at a particle concentration of 3×10⁴cm^?3.     

Particle Size of Pneumatically Conveyed Powders Measured Using Impact Duration

CSIRO Minerals has developed a technique for measuring particle size in pneumatically conveyed powders [1] by measurement of the acoustic waves produced by particle impacts upon a specially designed transducer. Previous work has focused on using the peak acoustic wave amplitude to determine particle size. This produces a spectrum that is hard to determine the particle size from, as the peak amplitude is a non‐linear function of particle diameter, and is strongly affected by angle of incidence and velocity of the impacting particle. In this paper impact duration measurements are used to overcome these difficulties while retaining the advantages of being able to measure in high solids loadings of up to at least 0.5 kg/m³ of powder. In laboratory tests the impact size monitor's (ISM) results have been correlated with optical diffraction measurements of the mean (by number) powder size with a correlation coefficient of 0.985 and a relative error of 5.5 %. The ISM operated successfully in the laboratory at a loading of 0.5 kg/m³ of powder and measured particles down to 50 microns in size.     

A mössbauer spectroscopy study of superparamagnetic amorphous alloy particles in a ferrofluid

A ferrofluid prepared by thermal decomposition of Fe(CO)₅ has been studied by use of Mössbauer spectroscopy. From the magnetic field dependence of the spectra obtained above the superparamagnetic blocking temperature, we estimate a particle size of about 4.2 nm. At low temperatures the magnetic properties of the sample seem to be influenced by magnetic coupling among the particles. An upper limit for the magnetic anisotropy energy constant of about 0.6x10⁵ J m^-3 is estimated. This is considerably smaller than that of crystalline α-iron particles prepared on a support.     

Diagnostics of High-Temperature,High-Density Plasma by Radiation Analysis

This article will be restricted to the diagnostics of laboratory plasmas having an average particle energy of higher than 30 eV per particle and a density of greater than 10¹⁹ particles per cm³. Common features of such plasmas related to applied spectroscopy are their complete ionization, the high excitation and ionization levels the particles, the large emission coefficients for continuum radiation over a wide spectral range from the X-ray region up to the infrared, their small size, high temperature and density gradients to the environment, and their transient nature.     

Error Estimates in the Sampling From Particle Size Distributions

It is often necessary to estimate the properties of particle size distributions from limited samples taken from large populations. When the distributions are broad, and higher order moments required, as in the case of volume based particle size distributions, the inferred parameters d_3,50 (volume median diameter) and GSD (geometric standard deviation) can have high intrinsic errors not immediately obvious to the measuring scientist. We show that there is a critical number of particles, N_crit, which must be counted or else the error may blow up catastrophically. N_crit is very sensitive to the width of the distribution, and is approximately proportional to GSD¹¹ We develop formulae to estimate the random sampling error inherent in measured values of the d_3,50 and GSD for the log-normal distribution; compare the predictions to a typical experimental particle size measurement; and then generalize to the median of any arbitrary moment, d_{r, 50}.     

Quantum diffusion and scaling of localized interacting electrons

A new numerical method is introduced that enables a reliable study of disorder‐induced localization of interacting particles. It is based on a quantum mechanical time evolution calculation combined with a finite size scaling analysis. The time evolution of up to four particles in one dimension is studied and localization lengths are defined via the long‐time saturation values of the mean radius, the inverse participation ratio and the center of mass extension. A systematic study of finite size effects using the finite size scaling method is performed in order to extract the localization lengths in the limit of an infinite system size. For a single particle, the well‐known scaling of the localization length λ₁ with disorder strength W is observed, λ₁ ∝ W^—2. For two particles, an interaction‐induced delocalization is found, confirming previous results obtained by numerically calculating matrix elements of the two‐particle Green's function: in the limit of small disorder, the localization length increases with decreasing disorder as λ₂ ∝ W^—4 and can be much larger than <$>\mitlambda λ₁. For three and four particles, delocalization is even stronger. Based on analytical arguments, an upper bound for the n‐particle localization length λ_n is derived and shown to be in agreement with the numerical data, λ_n ∝ λ₁. Although the localization length increases superexponentially with particle number and can become arbitrarily large for small disorder, it does not diverge for finite λ₁ and n. Hence, no extendedstates exist in one dimension, at least for spinless fermions.     

400 AMeV 12C诱发乳胶核反应重电离粒子前后关联研究

对400 AMeV 12C诱发乳胶核反应重电离粒子的前后关联进行了研究。重电离粒子来源于靶核碎片,分为灰径迹粒子和黑径迹粒子两种。实验结果很好地验证了核-核碰撞几何模型。灰径迹粒子的发射在前、后半球是各向异性的,而黑径迹粒子的发射基本是一致的。向前、向后发射的灰径迹粒子、黑径迹粒子平均多重数对重电离粒子数n_h呈线性依赖,黑径迹粒子、灰径迹粒子的向前发射随n_h的增加而增加的幅度分别大于其向后发射随n_h的增加而增加的幅度,但核内级联效应受靶核大小的影响,向后半球内发射的灰径迹粒子平均多重数随n_h的增加有饱和现象,这些多重数关联特性基本上可以利用基于碰撞几何的旁观体-反应体模型及级联蒸发模型来解释。The forward-backward multiplicity correlation of heavily ionized particles produced in 12C-emulsion interactions at 400 AMeV is investigated. The heavy ionized particles, come from the target fragments, are divided into grey track particle and black track particle. The experimental results can be well explained by the geometry model of the nucleus-nucleus collisions. The emission of grey track particles in forward and backward hemisphere is not isotropic, but the emission of black track particle is almost isotropic. The averaged multiplicity of grey track particles and black track particles in forward and backward hemisphere linearly depend on the number of heavily ionized particle n_h, the correlation strength in forward hemisphere is greater than that in backward hemisphere, but the dependence of grey track particle in backward hemisphere on the number of heavily ionized particle n_h shows the saturation because of the intranuclar cascade effect is influenced by target size. The characteristics of multiplicity correlations can be well explained by the participant-spectator model based on the colliding geometrical picture and the cascade evaporation model of high energy nucleus-nucleus collisions.     

Properties of nanocrystalline TiO2 synthesized in premixed flames stabilized on a rotating surface

The properties of TiO₂ nanoparticles synthesized using the flame stabilized on a rotating surface method (FSRS) are investigated. The method uses a laminar, premixed, stagnation flame, combining particle synthesis and film deposition in a single step. The current study examines the effects of flame properties on particle characteristics. Synthesized particles were characterized using X-ray diffractometry, Transmission Electron Microscopy and UV–vis spectrometry in order to quantify the effects of equivalence ratio and precursor loading on particle size, crystallinity and optical band-gap. Results show that flame stoichiometry significantly affects crystal phase, but it has little to no effect on particle size and light absorption band edge. In addition, precursor loading impacts both the particle size and the crystal phase. The study demonstrates the potential of the FSRS method for producing tailored nanoscale TiO₂ particles for a variety of applications.     

Uniform nanoparticles building Ce1?x Pr x O2?δ mesoarchitectures: structure, morphology, surface chemistry, and catalytic performance

The design of cell-based bioreactors for inorganic particles formation requires both a better understanding of the underlying processes and the identification of most suitable organisms. With this purpose, the process of Au³⁺ incorporation, intracellular reduction, and Au⁰ nanoparticle release in the culture medium was compared for four photosynthetic microorganisms, Klebsormidium flaccidum and Cosmarium impressulum green algae, Euglena gracilis euglenoid and Anabaena flos-aquae cyanobacteria. At low gold content, the two green algae show maintained photosynthetic activity and recovered particles (ca. 10?nm in size) are similar to internal colloids, indicating a full biological control over the whole process. In similar conditions, the euglenoid exhibits a rapid loss of biological activity, due to the absence of protective extracellular polysaccharide, but could grow again after an adaptation period. This results in a larger particle size dispersity but larger reduction yield. The cyanobacteria undergo rapid cell death, due to their prokaryotic nature, leading to high gold incorporation rate but poor control over released particle size. Similar observations can be made after addition of a larger gold salt concentration when all organisms rapidly die, suggesting that part of the process is not under biological control anymore but also involves extracellular chemical reactions. Overall, fruitful information on the whole biocrystallogenesis process is gained and most suitable species for further bioreactor design can be identified, i.e., green algae with external coating.     

Formation of small particles of gold on alumina support films and their behaviour in oxygen and hydrogen atmospheres

Attempt has been successfully made to disperse ultrafine gold particles on alumina support films by keeping the substrate at elevated temperatures during metal deposition under vacuum of ≈ 10^-5 Torr. It is found that reasonable dispersion of particles occurs for a mean thickness of 0.5 nm at a substrate temperature of 125 °C. The resulting samples turn out to be quite suitable for model studies of the behaviour of gold/alumina catalyst. Electron microscope observations coupled with diffraction have provided evidence for emergence of gold particles without any compound formation with the residual gases present during deposition at 10^-5 Torr. The nature of dispersion and average particle size are shown to depend on substrate temperature and the amount of metal evaporated. Heat treatments in hydrogen and oxygen at 200–500 °C for various lengths of time led to an increase in particle size with a simultaneous decrease in number density (i.e. coarsening of particles) following, in all probability, the Ostwald ripening mechanism. The oxidizing atmosphere has been shown to be more favourable to coarsening or deactivation of the catalyst. Finally, it is indicated that gold particles prefer to facet giving well-defined shapes (e.g. hexagonal, pentagonal or rhombohedral) with faces corresponding to planes of relatively smaller interfacial energies.     

Stress state of silver nanoparticles embedded in a silicate glass matrix investigated by HREM and EXAFS spectroscopy

Ag particles of 3.9 and 5.1 nm mean size in silicate glasses were produced by ion exchange and subsequent annealing at 480 and 600 ^°C. These thermal treatments may induce stresses in matrix and particles in addition to the well known effect of surface atoms because of the thermal expansion mismatch of both materials. Structural characterisation of the particles by high-resolution electron microscopy revealed a size-dependent lattice dilatation quite opposite to the so far observed lattice contraction of similar metal/glass composites. This result, confirmed by X-ray absorption spectroscopy at the Ag K-edge, is discussed in terms of an Ag-Ag bond length increase near the particle surface. The temperature-dependent EXAFS spectra (10-300 K) indicate an increased thermal expansion coefficient of the particles with an increased mean particle size calculated on the basis of an anharmonic Einstein model. With that the bond length increase can be explained. The results can be interpreted by a combination of both the particle size effects and the influence of the surrounding matrix. Received 30 November 2000     

Analysis of Single Particle Attrition during Impact Experiments†

Particle breakage can be characterised as attrition, chipping, fracture, abrasion and wear. All these types of breakage mechanisms are the effect of the damage caused to these particles. These mechanisms can be differentiated not just on the basis of magnitude and direction of the force but also by the damage caused to the particles. The damage is measured by change in the size distribution and the change in shape of the particles. In the current research, experiments were performed on the newly developed Repeated Impact Test. The unique feature of this test is that about hundred particles can be subjected simultaneously to a monitored number of impacts, without particle‐particle interactions at regulated velocities. The preliminary experiments were performed with single crystalline particles of different shapes and sizes. After fixed number of impacts, the images of the particles were taken. The volume and shape of the particles were determined by image analysis. It was observed that the rate of attrition was very high when the particles are irregular. The rate decreased as the particles became more spherical.     

Drift velocity in the necklace model for reptation in a two-dimensional square lattice

We report the chain dynamics in the necklace model that mimics the reptation of a chain of N particles in a two-dimensional square lattice. We focus on the drift velocity under an applied static field. The characteristics of the model allow us to determine the effects of the forces on the chains and the resulting mechanisms that affect the drift velocity. Results obtained through Monte Carlo simulations were analyzed and discussed and distinct regimes as a function of the force strength and N were identified. We found that for small total applied forces, the drift velocity scales as 1/N. When the applied force to every particle is small but the total applied force is not, the tube deforms in such a way that the drift velocity does not depend on N. Large forces, applied to every particle, can straight chains such that the distance between the chain ends increases faster than the number of particles. Also, large forces can deform the chain within the tube what is directly related to a decrease of the drift velocity.     

Enhanced magneto-optic activity of magnetite-based ferrofluids subjected to gamma irradiation

We report here the effect of γ-irradiation on the particle size and size distribution dependent spectroscopic and magneto-optic properties of ferrofluids, synthesized by a co-precipitation method. The X-ray diffraction (XRD) study exhibits magnetite (Fe₃O₄) phase of the particles while electron microscopic and dynamic light scattering (DLS) studies have predicted particle growth upon γ-irradiation. Further, Fourier transform infrared (FT-IR) spectroscopy studies ensured that no dissociation has occurred due to irradiation effect. As a consequence of magneto-optic behavior reflected in the Faraday rotation (FR) measurement, the Verdet constant increased from a value of 0.64×10⁻² for the pristine sample to 5.6×10⁻² deg/Gauss-cm for the sample irradiated with the highest dose (2.635 kGy). The substantial enhancement in the FR is assigned to the improvement in associated chaining effect owing to adequate particle growth where an increased stoichiometry variation of Fe²⁺/Fe³⁺ is assured.