The identification of particles using diagrams and distributions of shape indices

Using five kinds of real powder paricles, we obtained their shape indices from their pictures, attempted to identify the powder particles morphologically and studied the distribution functions of particle shapes and their relationships. The results were indicated as the follwing.It is possible to identify powder particles on the diagrams of the shape indices. The distributions of the shape indices are expressed by some distribution functions. The sample material and the particle size of sample particles affect the distribution of the shape indices in a different way. The distributions of two characteristic diameters and the distribution of shape index defined by ratio of these diameters are accurately connected by an equation.     

Monitoring the particle size and shape in the crystallization of paracetamol from water

In this work, a technique capable of restoring bidimensional particle size distributions from images of the particles in suspension is applied to the seeded cooling crystallization of paracetamol from water. The effects of cooling rate and stirring rate on the final particle size and shape are studied and the average growth rates along different directions of particles are found to be strongly dependend on supersaturation. This observation is in line with previous studies, though in this work it has been established for the first time using populations of particles. The technique was capable of quantifying changes in particle size and shape, indicating particle sizes and shapes that correlated well with observations from electron microscopy images.     

Particle Shape and Size Effects on Anisotropic Shrinkage in Tape-Cast Ceramic Layers

The particle shape of a commercial low-temperature cofired ceramic (LTCC) composite powder was determined quantitatively in the as-received and milled state using a new particle image analyzer. All grades of the milled powder with average particle sizes of 3.0, 2.4, and 1.8 μm, respectively, exhibit a considerable stretched particle shape, because 40% of their particles have circularity values below 0.95. On the basis of the fast particle image analyzer, the influence of the raw materials on particle alignment during tape casting was investigated using "design of experiments" (DOE). In the cast LTCC green tapes, the degree of particle orientation was measured and correlated with the information from the particle shape analyses and with other material and process factors from the DOE. The results showed that the degree of particle alignment correlates significantly with the measured particle shape and size; more than 80% of the particles were oriented in the casting direction if their shape factor was below 0.5. The particle orientation causes shrinkage anisotropy. The use of a coarser LTCC powder with an average particle size d ₅₀ of 3.0 μm instead of 1.8 μm increased the sintering anisotropy factor of LTCC tapes and laminates significantly from 1.0% to 1.85% and from 3.6% to 7.6%, respectively. The use of more binder or less solvent led to higher shrinkage anisotropies too. The casting velocity showed only a minor effect on the degree of particle orientation and sintering anisotropy, which is due to the shorter shearing period in which particle rotation can take place.     

Use of numerical modeling in design for co-firing biomass in wall-fired burners

Co-firing biomass with coal or gas in the existing units has gained increasing interest in the recent past to increase the production of environmentally friendly, renewable green power. This paper presents design considerations for co-firing biomass with natural gas in wall-fired burners by use of numerical modeling. The models currently used to predict solid fuel combustion rely on a spherical particle shape assumption, which may deviate a lot from reality for big biomass particles. A sphere gives a minimum in terms of the surface-area-to-volume ratio, which impacts significantly both motion and reaction of a particle. To better understand the biomass combustion and thus improve the design for co-firing biomass in wall-fired burners, non-sphericity of biomass particles is considered. To ease comparison, two cases are numerically studied in a long gas/biomass co-fired burner model. (1) The biomass particles are assumed as solid or hollow cylinders in shape, depending on the particle group. To model accurately the motion of biomass particles, the forces that could be important are all considered in the particle force balance, which includes a drag for non-spherical particles, an additional lift due to particle non-sphericity, and a “virtual-mass” force due to relatively light biomass particles, as well as gravity and a pressure-gradient force. Since the drag and lift forces are both shape factor- and orientation-dependent, coupled particle rotation equations are resolved to update particle orientation. To better model the reaction of biomass particles, the actual particle surface area available and the average oxygen mass flux at particle surface are considered, both of which are shape factor-dependent. (2) The non-spherical biomass particles are simplified as equal-volume spheres, without any modification to the motion and reaction due to their non-sphericity. The simulation results show a big difference between the two cases and indicate it is very significant to take into account the non-sphericity of biomass particles in order to model biomass combustion more accurately. Methods to improve the design for co-firing biomass in wall-fired burners are finally suggested.     

Precipitation of calcium carbonate in the presence of citrate and EDTA

The influence of process conditions such as feed rate, calcium/carbonate ratio, pH, complexing agents [ethylenediaminetetraacetic acid (EDTA), citrate (CIT)] and their concentration on the average particle size and shape of precipitated calcium carbonate was studied. The precipitation was performed in a semi-batch operated agitated vessel at constant pH by adding sodium hydrogen carbonate to a solution containing calcium chloride. In the absence of a complexing agent, agglomerates of needle-shaped crystals, probably aragonite, are obtained. Increasing feed time and the calcium/carbonate ratio increases the average particle size, whereas the opposite effect is observed for increasing pH. The observations can be related to the level of supersaturation. In the presence of complexing agents and at a concentration ratio of calcium vs. a complexing agent of 6, differently shaped and smaller particles were obtained. Furthermore, the effect of the other parameters on particle size becomes much weaker in the presence of complexing agents. In the presence of EDTA mostly spherical particles were obtained, and in the presence of citrate mainly rhombic particles corresponding to calcite were obtained. The effect on particle shape and size is attributed to interactions of the complexing agents with the faces of the crystalline calcium carbonate.     

Evaluation of Particle Bounce in Various Collection Substrates to be Used as Vaporizer in Aerosol Mass Spectrometer

The determination of the collection efficiency (CE) of particles during transport, vaporization, and ionization in the aerosol mass spectrometer (AMS), which uses vaporizer to evaporate non-refractory particles with subsequent ionization, is important for accurately quantifying the concentrations of chemical constituents. Particle bounce in the vaporizer can be considered as one of the most important parameters influencing the CE of particles. Substrates with various shapes (flat, cylindrical, reverse-conical, cup, trapezoidal, and reverse-T), materials (stainless steel, copper, tungsten, and molybdenum), pores with average sizes of 0.2, 1, 5, 20, and 100 μm, and mesh with a size of 79 μm, which can be a possible candidate for the vaporizer in the AMS, were constructed. Bounce fractions of sub-micrometer particles (polystyrene latex, oleic acid, and dioctyl phthalate) were determined using the differential mobility analyzer (DMA)-impactor technique under a constant impact velocity. For the porous substrate, the particle bounce fraction significantly decreased with increasing pore size and porosity, but there was an upper limit for the pore size above which the particle bounce fraction no longer decreased significantly (i.e., the rebounded particles successfully escaped from the pores). The mesh substrate also had a lower particle bounce fraction than the flat substrate. Among the tested materials, the copper substrate having the lowest hardness and elasticity had the lowest particle bounce fraction. In addition, the reverse-T shape substrate having more available surfaces for particle entrapment led to the reduction of particle bounce fraction. In terms of phase, the liquid particles had lower particle bounce fractions than the solid particles. Our results suggest that the vaporizer in the AMS should provide traps for multiple collisions of the rebounding particles with an appropriate porosity or mesh and should be made of low-hardness materials to minimize particle bounce.

Copyright 2015 American Association for Aerosol Research     

反应时间对CTA粒径的影响

在对各种CTA样品处理方法和分析方法进行比较后,对CTA进行了粒子形态和平均粒度分析。结果表明:CTA晶体呈现无规则的形态,无清晰的晶体边缘;CTA粒子大都是由更小的粒子聚集而成的聚集体。用Malvern激光粒度分析仪分析CTA聚集体粒径时,加超声波后CTA聚集体将破碎而使测得的平均粒径偏低;湿法测定时,CTA聚集体周围吸附一层水膜,致使测得的粒径比干法测得的结果偏高。拟和得到了分批式反应器中CTA聚集体平均粒径与反应时间的关系为:L/μm=95.61-198.33exp(-t/11.384)     

Experimental investigation of pressure drop in packed beds of irregular shaped wood particles

The knowledge of the pressure drop across a packed bed of irregular shaped wood particles is of great importance for achieving optimal control and maximum efficiency in many applications, such as wood drying, pyrolysis, gasification and combustion. In this work the effect of porosity, average particle size and main particle orientation on the pressure drop in a packed bed is investigated. To this end, particle size distributions and porosities are determined experimentally.Based on the experimental results obtained in this study, the form coefficient C and the permeability K of the Forcheimer equation are calculated for different packed beds. The Ergun equation requires an average equivalent particle diameter that is derived from the measured particle size distribution. This equivalent diameter and the corresponding bed porosity are used in the well known Ergun equation in order to derive adapted shape factors A and B.Since a change in bed porosity and particle size, caused by the degradation of the wood particles and gravity, can be expected in a reacting packed bed, a set of shape factors for use with the Ergun equation is determined that are independent of porosity and particle diameter and fit the experimental data very well.     

Entrainment behaviour of high-density Geldart A powders with different shapes

Atomised and milled Ferrosilicon with average particle diameters of 38 and 50 µm respectively were fluidised with air at ambient conditions. The entrainment rate of the more spherical atomised particles was on average six times that of the irregularly shaped milled particles over the range of superficial velocities investigated. In an attempt to decouple the effect of particle size from shape, the bed was divided into theoretically isolated bins based on the distributions of particle sizes. This indicated that the atomised particles had a higher entrainment rate for particles smaller than approximately 25 µm whereas the opposite was true for particles greater than this size. None of the entrainment correlations investigated was able to predict the switch in entrainment behaviour as a function of particle sphericity and diameter. Furthermore, the traditional critical particle diameter associated with cohesive Geldart A particles was not observed for either of the two particle shapes. It is therefore concluded that neither the hydrodynamic nor Van der Waals forces acting on the particles can adequately explain the entrainment rate behaviour of differently shaped high-density Geldart A particles.     

CHARACTERIZATION OF ATMOSPHERIC DRY DEPOSITED PARTICLES AT URBAN AND NON-URBAN LOCATIONS

Microscopic techniques were used to size particles collected on a deposition surface and to generate mass size distributions of deposited particles sampled from urban and non-urban locations. The volume shape factor was defined as a conversion factor between the projected area diameter and equivalent volume diameter of a particle and was used as an indication of the irregularity of the particle shape. The average volume shape factor of deposited particles at the urban location (1.61±0.21) was higher than the average at the non-urban locations (1.16±0.10). This suggests that particles are more irregular in the urban areas. Since non-urban areas have less larger particles in ambient air, depositional mass-size distributions at the urban location had a larger average peak (58 μm) and average mass median diameter (49 μm) of coarse particle mode of the distributions than they did at non-urban locations (averaged 33 and 27 μm). Evaluation of correlation coefficients between parameters (wind speed, deposition flux, peak diamater, mass median diameter) indicates that there are more airborne coarse particles at urban locations than at non-urban locations and this distribution plays an important role in dry deposition. By directly observing the deposited particles, it was found that particles larger than 10 μm diameter contributed to more than 90% (in mass) of the atmospheric dry deposition even when ambient coarse particle concentration is low at non-urban locations.     

Analysis of particle contamination in plasma reactor by 2-sized particle growth model

Rapid particle growth in the silane plasma reactor by coagulation between 2-sized particles was analyzed for various process conditions. The particle coagulation rate was calculated considering the effects of particle charge distribution based on the Gaussian distribution function. The large size particles are charged more negatively than the small size particles. Some fractions of small size particles are in neutral state or charged positively, depending on the plasma conditions. The small size particle concentration increases at first and decreases later and reaches the steady state by the balance of generation rate and coagulation rate. The large size particles grow with discharge time by coagulation with small size particles and their size reaches the steady state, while the large size particle concentration increases with discharge time by faster generation rate and reaches the steady state by the balance of generation and disappearance rates. As the diameter of small size particles decreases, the diameter of large size particles increases more quickly by the faster coagulation with small size particles of higher concentration. As the residence time increases, the concentration and size of large size particles increase more quickly and the average charges per small size and large size particle decrease.     

Aerosol measurement by laser doppler spectroscopy—I. Theory and experimental results for aerosols homogeneous

The basic theory, experimental techniques and results are presented describing a technique for sizing aerosol particles in situ using laser Doppler spectroscopy. Unlike conventional light scattering procedures which use average intensity information, this technique utilizes the Doppler shifted frequency of the scattered light produced by the Brownian motion of the aerosol particles to determine particle diffusion coefficients and size. Experiments were carried out using monodisperse dibutylpthalate aerosols and monodisperse polystyrene latex spheres, in concentrations ranging from 10³ to 10⁶ particles per cubic centimeter. Measured particle sizes were within 10 per cent of the size predicted by conventional light scattering methods for the DBP particles and the reported sizes of the PSL particles. Based on these results it is concluded that laser Doppler spectroscopy can be utilized to accurately measure aerosol particle size in situ.     

Particle size determination from acoustic emissions

A new method for the determination of particle size and size distribution in powders of rigid particles is described. The method is based on the fact that rigid particles when impinging upon each other emit sound signals whose features are related to the size of the particles, to the size distribution in a powder and also to the shape of the particles. The acoustic frequencies of interest lie in the ultrasonic range and audio-noise can be filtered out. So far, the size of metallic spheres and of glass beads has been measured accurately in the range 3 cm down to 50 μm diameter with less than 10 millisec required to take data for one complete size analysis. The method appears to be applicable to continuous on-line monitoring in crushing, powder processing and in mixing and blending.     

Size-dependent luminescent properties of hollow and dense BaMgAl<Subscript>10</Subscript>O<Subscript>17</Subscript>: Eu blue phosphor particles prepared by spray pyrolysis

Hollow and dense BaMgAl10O17: Eu₂₊ (BAM) phosphor particles were synthesized by a spray pyrolysis process and their luminescent properties were investigated under vacuum ultraviolet (VUV) excitation as varying the average particle size. The dependence of the luminescent intensity on the particle size was greatly influenced by the morphology of BAM particles. For the BAM particles with a hollow structure, the luminescent intensity linearly increased with increasing the particle size. However, no significant change in the luminescent intensity was observed for dense particles as the particle size changed. Also, all dense BAM particles had higher photoluminescence intensity than that of the hollow ones regardless of the particle size. The luminescent intensity of BAM phosphor particles prepared by spray pyrolysis was found to have a linear relationship with the crystallite size. Therefore, it was concluded that suppressing the formation of a hollow structure and increasing the crystallite size are needed to obtain high luminous BAM phosphor particles with a spherical shape and fine size of less than 1 Μm. On the basis of penetration depth of VUV, a simple relation equation between the particle size and the luminescent intensity was derived and correlated with experimental results in order to interpret the luminescent behavior of BAM phosphor as the particle size changes.     

Measurement of size-dependent dynamic shape factors of quartz particles in two flow regimes

Understanding and modeling the behavior of quartz dust particles, commonly found in the atmosphere, requires knowledge of many relevant particle properties, including particle shape. This study uses a single particle mass spectrometer, a differential mobility analyzer, and an aerosol particle mass analyzer to measure quartz aerosol particles mobility (d_m), vacuum aerodynamic, and volume equivalent diameters, mass, composition, effective density, and dynamic shape factor as a function of particle size, in both the free molecular and transition flow regimes. The results clearly demonstrate that dynamic shape factors can vary significantly as a function of particle size. For the quartz samples studied here, the dynamic shape factors increase with size, indicating that larger particles are significantly more aspherical than smaller particles. In addition, dynamic shape factors measured in the free-molecular (χ_v) and transition (χ_t) flow regimes can be significantly different, and these differences vary with the size of the quartz particles. For quartz, χ_v of small (d_m < 200 nm) particles is 1.25, while χ_v of larger particles (d_m ～ 440 nm) is 1.6, with a continuously increasing trend with particle size. In contrast, χ_t of small particles starts at 1.1 increasing slowly to 1.34 for 550 nm diameter particles. The multidimensional particle characterization approach used here goes beyond determination of average properties for each size, to provide additional information about how the particle dynamic shape factor may vary even for particles with the same mass and volume equivalent diameter.

© 2016 American Association for Aerosol Research     

Microparticle characterization using digital holography

Digital holography is an effective 3D imaging technique, with the potential to be used for particle size measurements. A digital hologram can provide reconstructions of volume samples focused at different depths, overcoming the focusing problems encountered by other imaging based techniques. Several particle analysis methods discussed in the literature consider spherical particles only. With the object sphericity assumption in place, analysis of the holographic data can be significantly simplified. However, there are applications, such as particle analysis and crystallization monitoring, where non-spherical particles are often encountered. This paper discusses the processing of digital holograms for particle size and shape measurement for both spherical and arbitrarily shaped particles. An automated algorithm for identification of particles from recorded hologram and subsequent size and shape measurement is described. Experimental results using holograms of spherical and non-spherical particles demonstrate the performance of the proposed measuring algorithm.     

Controlled Wind Tunnel Experiments for Particle Bounceoff and Resuspension

The dynamics of particle rebound and resuspension were examined by using uranine particles, polymer microspheres, spores, and pollen in wind tunnel experiments. Particle diameters were 5–42 μm. Results show that both the fraction of rebound and the resuspension rate are strongly dependent on the free stream velocity, particle size, and relative humidity. The effects of relative humidity are more significant at lower windspeeds; a greater relative humidity appears to change the shape of the distribution of adhesion force, mainly affecting the lower range but not greatly affecting the upper end of the distribution. Resuspension rates decrease with time, essentially defining two regimes. The first regime lasts for < 1 min; after this time, the most easily resuspended particles have been removed, leaving only particles with much smaller resuspension rates for the second regime. At a windspeed of 6 m / s, the upper 20% of the distribution of turbulent fluctuations is responsible for ～ 65% of the particle resuspension. Once resuspended, the particles have trajectories which depend on characteristics of the turbulent airflow and not on the initial velocity of release from the surface. Overall, the data show that resuspension is more sensitive to the type of particle than to the type of surface; particle shape and composition may be more important than particle size.     

Modeling of chord length distributions

A procedure for the calculation of chord length distributions (CLD) of populations of rigid and opaque particles of any size and shape distribution is given. It combines the capabilities of a virtual reality renderer to create 2D projections of particles and of an image analysis software which determine their chord lengths. The procedure has been validated on simple shapes (spheres, ellipsoids, parallelepipeds, cuboids, uniform polyhedra) that can be combined to simulate agglomerates or twinned crystals. The procedure has been used to discuss the experimental results obtained on gibbsite particles in different size ranges and to compare the mean chord length to the average particle size.     

Preparation of Nanosized Hematite Particles by Mechanical Activation of Goethite Samples

Nanosized single crystals of hematite with a very narrow particle size distribution were prepared by mechanical activation of two different goethite samples. Both goethite samples transformed completely into hematite after 70 h grinding time. Transmission electron microscopy showed that the final particles were spherical in shape and of ∼17 nm average particle size. This particle size was coincident with that estimated from specific surface measurements, indicating that the hematite samples consisted of nonporous and nonaggregated particles. The crystallite size, calculated from the broadening of the XRD peaks, in the hematite samples indicated that particles consisted of single crystals. No influence of the precursor was observed in the products, so both goethite samples yielded identical rounded single crystals with a narrow particle size distribution.