Predicting the deposition spot radius and the nanoparticle concentration distribution in an electrostatic precipitator

Abstract

Deposition of aerosol nanoparticles using an electrostatic precipitator is widely used in the aerosol community. Despite this, basic knowledge regarding what governs the deposition has been missing. This concerns the prediction of the size of the particle collection zone, but also, perhaps more importantly, prediction of the nanoparticle concentration distribution on the substrate, both of which are necessary to achieve faster and more precise deposition. In this article, we have used COMSOL Multiphysics simulations, experimental depositions, and two analytical models to describe the deposition. Based on that, we propose a simple equation that can be used to predict the size of the deposition spot as well as the particle concentration on the substrate. The equation we derive concludes that the size of the deposition spot only depends on the gas flow rate into the precipitator, and on the constant drift velocity of a particle in an electric field. The equation also displays that the deposited particle concentration is independent of the gas flow rate. Our general mathematical analysis has great applicability, as it can be used to model different geometries and different types of deposition methods than the one described in this article. We can therefore also propose that the drift velocity in this model easily could be replaced by another velocity acting on the particles at other deposition conditions, for instance, the thermophoretic velocity during thermophoretic deposition. This would result in the same dependence as presented in this article. Finally, we demonstrate analytically and through experiment that the particle distribution inside the spot will be homogenous and follows a top hat profile.     

Kinetics of Templated Grain Growth of 0.65Pb(Mg1/3Nb2/3)O3·0.35PbTiO3

Single-crystal layers of 0.65Pb(Mg_1/3Nb_2/3)O₃·0.35PbTiO₃ (PMN-35PT) were grown heteroepitaxially on {001}-BaTiO₃ template crystals. A {001}-BaTiO₃ crystal was embedded in a fine-grained matrix of PMN-35PT containing excess PbO and heated between 950° and 1150°C for 0–5 h. The initial growth of the PMN-35PT on the {001} surface and the growth of the matrix grains both displayed a t ^1/3 dependence which is characteristic of diffusion-controlled growth. Growth was limited to ∼100–150 μm due to the significantly reduced driving force at longer times because of matrix coarsening and porosity evolution.     

Seeding of the Reaction-Bonded Aluminum Oxide Process

The effect of the initial α-Al₂O₃ particle size in the reaction-bonded aluminum oxide (RBAO) process on the phase transformation of aluminum-derived γ-Al₂O₃ to α-Al₂O₃, and subsequently densification, was investigated. It has been demonstrated that if the initial α-Al₂O₃ particles are fine (∼0.2 μm, i.e., 2.9 × 10¹⁴γ-Al₂O₃ particles/cm³), then they seed the phase transformation. The fine α-Al₂O₃ decreases the transformation temperature to ∼962°C and results in a finer microstructure. The smaller particle size of the seeded RBAO decreases the sintering temperature to as low as ∼1135°C. The results confirm that seeding can be utilized to improve phase transformations and densification and subsequently to tailor final microstructures in RBAO-derived ceramics.     

Particle Formation from Solution and Slurry Sprays

ABSTRACT

The initial morphology of particles formed by spray drying or spray pyrolsis of solutions, sols or slurries, is determined by the relative velocity between the atomized droplet and the ambient gas in the furnace. A1 high relative velocities, the droplet can be disintegrated or flattened, whereas at low relative velocities the droplet remains spherical throughout the drying process. The time for respheroidization depends on the liquid properties and the solid content of the drying droplet. When the droplet reaches the percolation limit, the higher viscosity inhibits respheroidization, and the morphology of the deformed droplet is fixed. A new model based on the Weber number, percolation criteria, and the drying kinetics, is presented to predict particle morphology for various drying conditions. In addition, the effects of segregation during drying will be discussed in regard to its effect on particle formation during spray formulation. We will show how physical and chemical segregation processes occur in drying droplets, and how these segregation processes can be controlled to yield particles of designed morphology and internal phase distribution.     

Simultaneous growth mechanisms for Cu-seeded InP nanowires

We report on epitaxial growth of InP nanowires (NWs) from Cu seed particles by metal-organic vapor phase epitaxy (MOVPE). Vertically-aligned straight nanowires can be achieved in a limited temperature range between 340 °C and 370 °C as reported earlier. In this paper we present the effect of the V/III ratio on nanowire morphology, growth rate, and particle configuration at a growth temperature of 350 °C. Two regimes can be observed in the investigated range of molar fractions. At high V/III ratios nanowires grow from a solid Cu₂In particle. At low V/III ratios, nanowire growth from two particle types occurs simultaneously: Growth from solid Cu₂In particles, and significantly faster growth from In-rich particles. We discuss a possible growth mechanism relying on a dynamic interplay between vapor-liquid-solid (VLS) and vapor-solid-solid (VSS) growth. Our results bring us one step closer to the replacement of Au as seed particle material as well as towards a deeper understanding of particle-assisted nanowire growth.      

The Reaction-Bonded Aluminum Oxide (RBAO) Process: II, The Solid-State Oxidation of RBAO Compacts

The oxidation kinetics and the fraction of aluminum that is oxidized via solid–gas reaction in reaction-bonded aluminum oxide (RBAO) compacts are shown to be strongly dependent on the oxidation temperature and the characteristics (size and green density) of the RBAO compact. Based on the Biot number, the oxidation process of RBAO compacts is controlled by convective heat transfer. Low heat transfer from the surface of the compact results in too-rapid oxidation, thermal gradients, and core–shell oxidation of the compacts. Uniform oxidation of RBAO compacts is possible by oxidizing at low temperatures (400°–470°C), where slow surface reaction of the aluminum particles controls the oxidation of the compact. A grain model is presented to cover both linear and nonlinear oxidation regimes during the oxidation of a RBAO compact, and this model predicts the experimental results when surface reaction of the aluminum particles is the rate-controlling mechanism and oxidation of the compact occurs uniformly.     

Molten salt synthesis of anisometric particles in the SrO-Nb2O5-BaO system

Acicular particles of KSr₂Nb₅O₁₅, Sr_0.5Ba_0.5Nb₂O₆, and SrNb₂O₆ were synthesized in the SrO-BaO-Nb₂O₅ system, using KCl or SrCl₂·6H₂O salts. In the SrCl₂·6H₂O + Nb₂O₅ system, acicular SrNb₂O₆ particles were formed by a solid state reaction between the salt and Nb₂O₅. Large, irregularly shaped (Sr-rich) Sr₂Nb₂O₇ particles formed with increasing reaction temperature and time. Small but finite solubility of SrO in the KCl + SrNb₂O₆ system favored the formation of acicular KSr₂Nb₅O₁₅ and blade-like Sr₂Nb₂O₇ particles (at higher temperatures). Uniformly sized, acicular KSr₂Nb₅O₁₅ particles were easier to reproduce compared to the formation of acicular Sr_0.5Ba_0.5Nb₂O₆ and SrNb₂O₆ particles.     

Measurement of Viscosity of Densifying Glass-Based Systems by Isothermal Cyclic Loading Dilatometry

This study describes the isothermal cyclic loading dilatometry (ICLD) technique to measure the viscosity of glass-based materials. We demonstrate its merit relative to constant-load techniques in minimizing the stress history effects (changes in shrinkage anisotropy and sample microstructure) that arise due to the application of an external load. A constant-load test overestimates the viscosity by an order of magnitude compared with a cyclic load test. To obtain accurate viscosity data, maximum loading rates and longer unloading periods are desirable as they reduce effects of shrinkage anisotropy on viscosity values. Representative data for a low-temperature cofired ceramic (LTCC) material are reported. Nonparametric statistical tests revealed insignificant differences between the viscosity data sets at 5% significance level and thus indicate good reproducibility of the testing methodology.     

The 120-S minute: using analysis of work activity to prevent psychological distress among elementary school teachers

To avoid exposure to unpleasant or unwanted emotional material, some people may distract themselves by summoning up pleasant thoughts such as happy memories. Manipulation of negative affect might therefore result in heightened accessibility of pleasant thoughts and memories, contrary to hypotheses of mood-congruent recall. In Experiment 1, repressors were faster to recall happy memories after watching an unpleasant film than after watching a neutral film. Nonrepressors showed the opposite effect (i.e., mood-congruent memory). In Experiment 2, after an unpleasant film, repressors were faster to recall a happy memory than to recall a sad memory. In Experiment 3, repressors spontaneously generated pleasant thoughts after watching an unpleasant film, whereas nonrepressors did not. Thus, repressors apparently cope with exposure to negative affective material by accessing pleasant thoughts. Results are discussed in terms of cognitive defenses against emotional distress and the associative structure of repression.