Polydisperse Powder Research Articles

Ultradispersed hardening of metal-ceramic microcomposites and coatings containing borides of transition metals in a low-temperature plasma flow

The results of extended researches of processes of modification of morphology, composition and structure of polydispersed refractory powders and microcomposites based on CrB2, TiB2, TiCrB2, Al2O3 in the flow of low-temperature plasma and its use for the formation of plasma coatings – layered composites are presented.

Formalization of the process and development of an algorithm for solid-phase mixing of components of a heterogeneous composite material

The article considers one of the approaches to modeling the process of mixing of polydisperse powders, which includes three main fractions of different sizes, the shape of which is close to spherical. The work is aimed at reducing material costs at the stage of development of mixing processes by reducing the number of experiments. Aiming to obtain the most uniform mixing with the minimum time of the technological process, the model is based on the target function for a certain number of iterations of mixing of the composition to obtain the required (maximum possible) density of the package of mixed solid particles.To develop a model of the mixing process, one of the heuristic algorithms – the “metal annealing method” – was used. As a representative element of the model, an elementary cell in the form of several hexagonal densely. Packed particles around one introduced into the composition of the composite material (in a small amount, from 5 to 15 %) as a modifier was adopted. The mo del is formalized with the condition of averaging the particle sizes within each fraction, as well as the morphology of their surface. The number of particle repackaging iterations is calculated by the probability of obtaining the minimum amount of voids in the representative element and the uniformity of distribution of the modifying element.Comparison of the values obtained during the simulation with the measured values of the mixing results on a specific mixer will form a scale of compliance of the simulation results with the operating modes of the process equipment. This will make it possible to predict the appropriate mixing modes at the stage of development of the technological process with the possible system of fluctuations in the characteristics of the supplied raw materials and, thereby, to create a methodological basis for the formation of quality management of manufacturing heterogeneous composite material. The model can be adapted for polydisperse powders with the content of the main fractions of more than three.

Comparison between a low-voltage benchtop electron microscope and conventional TEM for number size distribution of nearly spherical shape constituent particles of nanomaterial powders and colloids

Nanomaterial powders and colloids are already a large industry and are expected to continue to grow rapidly. In the context of risk assessment associated with nanomaterials, characterization of nanoparticle size and morphology is required. Until now, the best method giving direct access to these parameters has been electron microscopy (EM), in particular, transmission electron microscopy (TEM). Although this method is widely used, several issues are highlighted such as cost, maintenance, sample representativity and damage for sensitive materials. Low-voltage transmission electron microscopes (LVTEMs) could be an alternative approach to solve some of these issues. This paper presents a first comparison between a benchtop LVTEM and a conventional device to determine the number size distribution of the constitutent particles of two polydispersed industrial powders (TiO2 and SiO2) with particle sizes close to 100 nm and two colloids referenced for their particle size (ERM FD 304 and NM 300 K). The samples were prepared with an optimized deposition protocol involving glow discharging and Alcian blue solution pre-treatment on the EM grids. The benchtop LVTEM produced a rather good resolution and the relative differences obtained for the median diameters D50 are generally within ± 15 %. On the basis of these results, benchtop LVTEM could be promoted for identifying nanomaterials within the framework of risk assessment strategy.

Production of high-quality polydisperse construction mixes for additive 3D technologies.

The paper describes a new design of a mixer allowing production of high quality polydisperse powders, used in additive 3D technologies. A new principle of dry powder particle mixing is considered, implementing a possibility of a close-to-ideal distribution of such particles in common space. A mathematical model of the mixer is presented, allowing evaluating quality indicators of the produced mixture. Experimental results are shown and rational values of process parameters of the mixer are obtained.

High-quality poly-dispersed mixtures applied in additive 3D technologies.

The paper describes the new mixer design to obtain high-quality poly-dispersed powders applied in additive 3D technologies. It also considers a new mixing principle of dry powder particles ensuring the distribution of such particles in the total volume, which is close to ideal. The paper presents the mathematical model of mixer operation providing for the quality assessment of the ready mixtures. Besides, it demonstrates experimental results and obtained rational values of mixer process parameters.

Assessment of different coarse graining strategies to simulate polydisperse gas-solids flow

Continuum methods require the additional development of solids stress closures for polydisperse powders based on complex kinetic theories that are non-trivial to develop, code, and numerically converge for the wide range of fluidization regimes from very dilute to dense/frictional flow limit. On the other hand, it is straightforward to model the flow of polydisperse granular materials by treating particles as discrete rigid bodies that are tracked following simple physical laws of motion. The coarsening of these discrete methods by lumping several particles in a parcel alleviates the significant computational cost associated with these discrete methods while introducing some inaccuracies in the numerical results. In this research, we explore two different coarse graining methods that can be applied to polydisperse powders, namely the same statistic weight method (SSW) and the same size parcel method (SSP), and assess their accuracy by comparison with the finest simulation results obtained with a discrete element method (DEM). For Geldart group B powders fluidized at a relative low superficial velocity, the numerical results indicate that the SSW is more accurate than the SSP method. For type A powders fluidized at relatively high velocity, these two methods predict similar results. Interestingly, up to four times increase in the speed of simulation of the SSP method was obtained because the original polydisperse powder is scaled to a mono-disperse system in terms of particle-particle collision. These results suggest that the SSP method is more favorable for the simulation of fluidized beds due to its accuracy and efficiency while the SSW method may be used for granular flow and dense fluidized bed systems where capturing the size segregation of particles due to collision is important.

Thermogravimetric analysis for rapid assessment of moisture diffusivity in polydisperse powder and thin film matrices

Accurate description of moisture diffusivity is key to precisely understand and predict moisture transfer behaviour in a matrix. Unfortunately, measuring moisture diffusivity is not trivial, especially at low moisture values and/or elevated temperatures. This paper presents a novel experimental procedure to accurately measure moisture diffusivity based on thermogravimetric approach. The procedure is capable to measure diffusivity even at elevated temperatures (>70°C) and low moisture values (>1%). Diffusivity was extracted from experimental data based on “regular regime approach”. The approach was tailored to determine diffusivity from thin film and from poly-dispersed powdered samples. Subsequently, measured diffusivity was validated by comparing to available literature data, showing good agreement. Ability of this approach to accurately measure diffusivity at a wider range of temperatures provides better insight on temperature dependency of diffusivity. Thus, this approach can be crucial to ensure good accuracy of moisture transfer description/prediction especially when involving elevated temperatures.

Computational modeling of electrically-driven deposition of ionized polydisperse particulate powder mixtures in advanced manufacturing processes

A key part of emerging advanced additive manufacturing methods is the deposition of specialized particulate mixtures of materials on substrates. For example, in many cases these materials are polydisperse powder mixtures whereby one set of particles is chosen with the objective to electrically, thermally or mechanically functionalize the overall mixture material and another set of finer-scale particles serves as an interstitial filler/binder. Often, achieving controllable, precise, deposition is difficult or impossible using mechanical means alone. It is for this reason that electromagnetically-driven methods are being pursued in industry, whereby the particles are ionized and an electromagnetic field is used to guide them into place. The goal of this work is to develop a model and simulation framework to investigate the behavior of a deposition as a function of an applied electric field. The approach develops a modular discrete-element type method for the simulation of the particle dynamics, which provides researchers with a framework to construct computational tools for this growing industry.

Influence of dry cohesion on the micro- and macro-mechanical properties of dense polydisperse powders & grains

Understanding how cohesive granular materials behave is of interest for many industrial applications, such as pharmaceutical or food and civil engineering. Models of the behaviour of granular materials on the microscopic scale can be used to obtain macroscopic continuum relations by a micro-macro transition approach. The Discrete Element Method (DEM) is used to inspect the influence of cohesion on the micro and macro behaviour of granular assemblies by using an elasto-plastic cohesive contact model. Interestingly, we observe that frictional samples prepared with different cohesion values show a significant difference in pressure and coordination number in the jammed regime; the differences become more pronounced when packings are closer to the jamming density, i.e. the lowest density where the system is mechanically stable. Furthermore, we observe that cohesion has an influence on the jamming density for frictional samples, but there is no influence on the jamming density for frictionless samples.

Particle-based simulations of powder coating in additive manufacturing suggest increase in powder bed roughness with coating speed

We have developed the first particle-based numerical tool to simulate the coating of powder particles in additive manufacturing devices. Our Discrete Element Method considers realistic particle shapes and incorporates attractive interaction (van-der-Waals) forces between the particles. From simulations of powder coating using a roller as coating device, we find that the surface roughness of the powder bed scales with the square of coating speed. Moreover, we find that using fine, highly polydisperse powders may lead to larger powder bed roughness, compared to process simulations using coarser powders, due to the formation of agglomerates resulting from cohesive forces.

Particle separation by alternating electric fields of quadrupole type

In this article, the possibility of particle separation by alternating electric fields of quadrupole type was demonstrated. It was shown that by varying the parameters of the trap (the distance between electrodes, the magnitude and frequency of alternating voltage) it is possible to separate both specific types of particles from the polydisperse powder as well as spatially separate them inside the trap.

Development and Laboratory Performance Evaluation of a Variable Configuration PM1/PM2.5 Impaction-Based Sampler

This study presents the design and lab evaluation of a compact medium flow multiple round nozzle-based inertial impactor with variable head for collecting particulate matter (PM). The impactor operates at a flow rate of 175 LPM and consists of two different sets of circular acceleration nozzles designed for PM2.5 (particle aerodynamic diameter < 2.5 µm) and PM1 (particle aerodynamic diameter < 1 µm), either of them could be used at a time as per the objective of sampling. High vacuum grade silicone grease is used as an impaction substrate. Lab experiments were performed using a poly-disperse dolomite powder as test aerosol. The nozzle configurations selected were such that the first one provides a 50% cutpoint at 2.50 µm (aerodynamic diameter) with a pressure drop of 0.35 kPa while the second one produces a 50% cutpoint at 1.06 µm (aerodynamic diameter) with a pressure drop of 1.07 kPa. Particle losses through the nozzles were low for particle diameter upto 5.0 µm. Particle bounce-off and re-entrainment losses were found to be insignificant. Owing to its higher flow rate, it has an advantage of collecting appreciable quantity of particles (and higher number of samples) in a relatively smaller time frame as compared to other low flow rate samplers. The samples can be analyzed to obtain finely time-resolved aerosol composition which can help in a better understanding of the role of various physico-chemical processes active during different meteorological conditions (extreme dust and foggy events) that affect fate and transport of PM, especially in the Indo-Gangetic plain (IGP).

Effect of Original Powder Mixture Fractional Composition and Dispersion on Reaction-Bonded Silicon Carbide Physicomechanical Properties

Results are given for an experimental study of the effect of fraction composition and dispersion of an original charge on the main domestic silicon carbide polydispersed powders developed by OAO UNIKhIM with a Test Plant on ceramic physicomechanical properties prepared by reaction sintering.

Measurements of Electrostatic Charging of Powder Mixtures in a Free-fall Test Device

Electrostatic charges generated during powder handling often adversely influence process performance. In cases where significant charges are present, accidental discharge of the accumulated charges may cause sparks, fires, even explosions, affecting process performance and causing significant safety concerns. Electrostatic charging is a complicated phenomenon, especially when handling powder mixtures of poly-disperse particles because of bi-polar charging, with small particles being charged opposite to their larger counterparts. Bipolar charging can lead to agglomeration, segregation and severe adhesion to walls or contact surfaces. Therefore, in order to characterize electrostatic properties of powders, it is desirable to measure not only the net charges on a powder, but also its polarity. In this work, a simple method is developed to investigate charge generation due to particle-particle collisions and particle-wall contact during powder handling. The experimental set-up consisted of two parallel copper plates connected to a high voltage power supply and a vibrating charging device with adjustable contacting surfaces. When subjected to a static electrical field, negatively charged particles moved to the positive side of the parallel plates, whereas positively charged particles migrated to the negative side. The separation of charged particles under investigation indicated bi-polar charging in a polydisperse powder system, with smaller particles carrying charges of polarity opposite to their larger counterparts. However, smaller particles were also found to carry two different charges from a set of fine particle-only experiments in binary powder mixtures. This was due to two different charging mechanisms: charge transfer and charge separation. In a further study with the addition of fine particles to mono-sized powders, the results indicated that the addition of fine particles helped reduce the net charges of the mixtures, with fine particles tending to carry positive charges after powder separation. When a typical antistatic agent (Larostat 519) was added to the mono-sized powders, the net charges of the mixtures decreased. Particle separation experiments revealed that this antistatic agent considerably altered the charging behavior of the host powders. This finding implies that the role of Larostat 519 in neutralizing charges differs from that of simple addition of other fine particles.

Attractive particle interaction forces and packing density of fine glass powders.

We study the packing of fine glass powders of mean particle diameter in the range (4–52) μm both experimentally and by numerical DEM simulations. We obtain quantitative agreement between the experimental and numerical results, if both types of attractive forces of particle interaction, adhesion and non-bonded van der Waals forces are taken into account. Our results suggest that considering only viscoelastic and adhesive forces in DEM simulations may lead to incorrect numerical predictions of the behavior of fine powders. Based on the results from simulations and experiments, we propose a mathematical expression to estimate the packing fraction of fine polydisperse powders as a function of the average particle size.

Self-organisation of regular dusty structures in plasma trap

The formation of ordered dusty structures in the special RF-gas discharge (plasma trap) capable of capturing and confining nano- and micro-particles in electromagnetic field was studied experimentally. Two ways of the dust-plasma structure formation were investigated: (1) The condensation of preliminary evaporated organic or inorganic substances. The plasma trap could confine the vapour clusters during their growth up to the several micrometres in size. (2) The mechanical injection of mono- or poly-dispersed powders into the RF-discharge. To realise the first way a small-size target of acetylsalicylic acid, polyethylene, polypropylene or amorphous carbon was evaporated by a short pulse of the powerful electron beam (EB). Powders of proteins, polysaccharides, amorphous carbon and oxides of various metals were used for mechanical injection. Air, oxygen, noble gases and some hydrocarbons were used for gas-discharge plasma generation. The topology of the ordered dusty structures was proved to be controlled by the EB injection.

Analysis of photocatalytic performance of nanostructured pyrogenic titanium dioxide powders in view of their polydispersity and phase transition: Critical anatase particle size as a factor for suppression of charge recombination

Series of TiO2 nanopowders from flame aerosol synthesis with average particle size from 7.1 to 21nm with increased rutile content were studied for particle size distribution (PSD) and anatase-to-rutile transition to be correlated with their photocatalytic performance. The certain particles’ growth regime favours the formation of defect-rich anatase nanoparticles which were found to condition the overall reactivity of polydispersed nanostructured powders. Two crucial roles of oxygen vacancies as key characteristics of nanoparticles are discussed: (a) they are responsible for anatase-to-rutile transition and (b) are vital for the adsorption of oxygen as electron acceptor, i.e. for the photoelectrochemical efficiency. The linear dependence of the photocatalytic oxidation reaction rate constant on the content of the specific anatase particles fraction amenable to phase transition was revealed by the regression analysis.

Implementation of procedure for centrifugal hydrodynamic fractionation of finely disperse particles

In chemical engineering, methods of micro-element fractionation are based on reduction of the solubility of polymers similar in terms of chemical structure in the same solvents as their molecular mass increases. However, methods more adaptable to production are possible. Methods of analytical fractionation include ultra-centrifuging of a solution, which is based on measurements of the sedimentation rate of a suspension, which is caused by centrifugal and Stoke’s forces, and also turbidimetric titration, which is based on measurement of the optical density of a polymer solution as it is enriched with a precipitant [1]. Investigations [2, 3] are devoted to the basis of an alternative methodology with respect to the circle of problems in question, proceeding from a model of direct-flow (dynamic) fractionation of forms of elements in a rotating chemical column. In the electrotechnical, thermal-power generation, mining, and other branches of industry, various physico-mechanical methods of action are used for fractionation of powders when working with coarse- and medium-disperse particles (milled materials, industrial dusts, etc.). Treatment of a powder suspended in circulating air or liquid flows is used to intensify particle fractionation [4‐7]. In the literature, therefore, fractionation is examined primarily either as a laboratory process, on low-output equipment, or from the standpoint of its variation in fractionation processes of coarse- and medium-disperse particles. Despite the series of advantages offered by mechanical centrifugal fractionation of finely disperse systems (rate of process, operational convenience), however, this method has been subject to insufficient investigation [8‐11]. There are essentially no procedures for predictive analytical analyses and planning of procedures for particle classification based on specific synthetic geometric and physico-mechanical indicators of the process, development of procedures for the formation of fractions of a given size of particles from a polydisperse powder has been meager, etc. This study analyzes questions concerning the basis of the theory and practice of the process in question in an example of quantitative analysis of fractionation based on batch-action centrifuging of particles in a finely disperse suspension without restrictions placed on productivity in the analytical solution, i.e., as applies to a production process. It is suggested that the fraction of particles of the required size (fraction of targeted particles) in a polydisperse powder be formed by particle sedimentation organized in a given manner in a liquid‐powder-particle system. As the liquid

Model of heating and ignition of conductive polydisperse powder in electrostatic discharge

Heating of a conductive polydisperse powder by electrostatic discharge (ESD) is modelled numerically. Powder packing is described using a discrete element model; powder resistance is defined by geometry of particle contacts and properties of plasma produced by electrical breakdown between neighbour particles. A set of parametric calculations in combination with experimental data is used to determine necessary adjustable model parameters. The model predicts the temperature for each powder particle resulting from its heating by the ESD current. Location and packing of individual particles within the powder affects greatly their achieved temperatures and thus the likelihood of ignition. Consistently with experiments, a trend showing that smaller particles are generally heated to higher temperatures at a given ESD energy is detected for coarser powders; this trend becomes less clear for finer powders with particle sizes less than the breakdown distance given by the Paschen curve in air. Comparison of the experimental data and calculations suggests that the transition from single particle to cloud combustion occurs when the distance between the particles ignited by ESD becomes close to the flame size for the individual burning particle. This distance, inversely proportional to the number of ignited particles, is primarily determined by the ESD energy.

Investigation of the processes of refining of ceramic articles in a plasma flow

Results of investigations of the process of dissipation of polydisperse particles in low-temperature plasma flow with the formation of ultrafine, including nanosized, particles are presented. A special method is proposed that can be used for producing polydisperse powders, including ultrafine and nanosized powders, with spherical particles and the amorphous, crystalline, or amorphous, crystalline structure.