Flow abilities of powders and granular materials evidenced from dynamical tap density measurement

in Powder Technology (2013), 235

This paper offers an overview of the flow properties of granular systems, including voids, granular porosity and random packing characteristics. For the purposes of the study, the notion of additional ... [more ▼]

This paper offers an overview of the flow properties of granular systems, including voids, granular porosity and random packing characteristics. For the purposes of the study, the notion of additional porous volume is intro-duced. This volume is defined as the additional air volume added to the optimal granular packing. It represents the difference between the volume of the bulk powder bed and that of the same powder but when ideally packed. It appears as the volume of additional air (or voids) trapped/stored between the grains when the powder passes from a dynamical state to a static state (during the filling of a container or the formation of a powder heap, for example). Therefore, if the powder bed traps air, it is then able to restore air partially or completely or not at all, depending on the intergranular cohesion level. This mechanism of the storing and releasing of air can be analysed from the measurement of compressibility curves. If the powder is non-cohesive or free flowing, it traps a small amount of air in its static state. Conversely, if the powder is cohesive, it traps more air. These data can be related to the flow properties of granular materials. Indeed, the compressibility curves obtained for gran-ular materials provide information such as additional porosity, a kinetic parameter which characterizes the com-pressibility dynamics, a granular relaxation index which predicts how far a powder is from its optimal packing state and an index which gauges the de-areation speed of the powder. Measurement of such properties provides a better understanding of the nature of granular materials. Measurements of dynamical compressibility were car-ried out on five granular materials (two different lactose powders, hydrated lime Ca(OH)2, yttrium stabilized zir-conia balls and polystyrene balls). The overall results are presented using a radar graph. The use of this tool and its advantages are discussed in relation to the measurement and characterization of powder flow properties. [less ▲]

Measuring the flowing properties of powders and grains

in Powder Technology (2012), 224

A granular material is a complex system which exhibits non-trivial transitions between the static, the quasi-static and the dynamical states. Indeed, an assembly of grains can behave like a solid or a ... [more ▼]

A granular material is a complex system which exhibits non-trivial transitions between the static, the quasi-static and the dynamical states. Indeed, an assembly of grains can behave like a solid or a fluid according to the applied stress. In between solid and fluid granular states, very slow dynamics are observed. When a complete macroscopic characterization of a powder is needed, all these granular states have to be precisely analyzed. In this paper, we show how three measurement techniques can be used to measure the physical properties of a powder. The measurements are based on classical tests modified to meet the recent fundamental researches on granular materials. The static properties of the powder are analyzed through the shape of a heap. The quasi-static behavior is studied with the analysis of the compaction dynamics. Finally, the dynamical regime is monitored through the flow in a rotating drum. In order to illustrate how these measurements can be used in practical cases, analyses are performed with three types of granular materials: silicon carbide abrasives, flours and rice. These selected materials allow to show the influence of the different parameters (grain size, grain size distribution, grain shape) on the macroscopic properties of the assembly. Moreover, these studies show the pertinence of the parameters obtained with the proposed techniques for the rheological characterization of powders and grains. [less ▲]

Synthesis of LiFePO4/Carbon composite cathode materials for lithium-ion batteries

Conference (2011, June 23)

van der Waals cohesion in Non Smooth Contact Dynamics : application to powder mixtures

in Canadian Journal of Physics (2011)

Preparation of fire-resistant poly(styrene-co-acrylonitrile) foams using supercritical CO2 technology

in Journal of Materials Chemistry (2010), 20

This work deals with the preparation and characterization of fire-resistant poly(styrene coacrylonitrile) (SAN) foams containing (organo)clays and/or melamine polyphosphate (MPP) as fire retardants using ... [more ▼]

This work deals with the preparation and characterization of fire-resistant poly(styrene coacrylonitrile) (SAN) foams containing (organo)clays and/or melamine polyphosphate (MPP) as fire retardants using supercritical CO2 as the foaming agent. The additives dispersion was first characterized with X-ray and transmission electron microscopy (TEM) analyses. Their presence clearly affected the cellular morphology, as observed by scanning electron microscopy (SEM). Then, the peak of heat release rate (PHRR) and total heat evolved (THE) were determined with a cone calorimetry test, performed on each foamed sample as a function of the foam density. Incorporation of clay (3 and 5 wt%) in the exfoliated state into the SAN foam clearly led to a significant decrease of PHRR, while intercalated and aggregated clay had a lower effect. Similar results were obtained with 10 and 20 wt% of MPP. The best results were obtained when exfoliated clay and MPP were combined, with a PHRR drop as large as 75%, thanks to the synergistic action of both additives. The magnitude of PHRR drop, related to the fire resistance, was found to be in direct relationship with the cohesiveness of the protective carbonaceous layer formed at the sample surface during combustion. Clay and MPP, when added together, are thus believed to favour the formation of a highly cohesive protective layer able to act as an efficient shield against the flame, despite the fact that the sample is originally composed of ~90% of voids. [less ▲]

Effect of an electric field on an intermittent granular flow

in Physical Review. E : Statistical, Nonlinear, and Soft Matter Physics (2010), 81

Granular gravity driven flows of glass beads have been observed in a silo with a flat bottom. A dc high electric field has been applied perpendicularly to the silo to tune the cohesion. The outlet mass flow ... [more ▼]

Granular gravity driven flows of glass beads have been observed in a silo with a flat bottom. A dc high electric field has been applied perpendicularly to the silo to tune the cohesion. The outlet mass flow has been measured. An image subtraction technique has been applied to visualize the flow geometry and a spatiotemporal analysis of the flow dynamics has been performed. The outlet mass flow is independent of voltage, but a transition from funnel flow to rathole flow is observed. This transition is of probabilistic nature and an intermediate situation exists between the funnel and the rathole situations. At a given voltage, two kinds of flow dynamics can occur: a continuous flow or an intermittent flow. The electric field increases the probability to observe an intermittent flow. [less ▲]

Mullite coatings on ceramic substrates: Stabilisation of Al2O3-SiO2 suspensions for spray drying of composite granules suitable for reactive plasma spraying

in Journal of the European Ceramic Society (2009), 29(11), 2169-2175

The present work deals with the preparation of stable alumina + silica suspensions with high solid loading for the production of spray-dried composite powders. These composite powders are to be used for ... [more ▼]

The present work deals with the preparation of stable alumina + silica suspensions with high solid loading for the production of spray-dried composite powders. These composite powders are to be used for reactive plasma spraying whereby the formation of mullite and the coating on a ceramic substrate are achieved in a single step process. Electrostatic stabilisation of alumina and silica suspensions has been studied as a function of pH. Silica suspensions are most stable at basic pH whereas alumina suspensions are stable at acidic pH. The addition of ammonium polymethacrylate (APMA) makes it possible to stabilise alumina and prepare a stable 50 wt% alumina + silica suspension at pH 10. The optimum amounts of dispersant and binder have been determined by zeta potential, viscosity and sedimentation measurements. Spray drying of the suspension yields composite powders whose morphology, size distribution and flowability have been characterized before realizing reactive plasma spraying tests. (C) 2009 Elsevier Ltd. All fights reserved. [less ▲]

Influence of the shaping effect on hardness homogeneity by Vickers indentation analysis

in Journal of the European Ceramic Society (2006), 26(15), 3191-3196

In this study, indentation technique (Vickers indentation) has been unconventionally used to evaluate the homogeneity of barium zirconate ceramic samples which have been shaped through different routes ... [more ▼]

In this study, indentation technique (Vickers indentation) has been unconventionally used to evaluate the homogeneity of barium zirconate ceramic samples which have been shaped through different routes. Statistical tools have been used to estimate the con-elation which can be established between heterogeneities within the samples and their shaping ways. (c) 2005 Elsevier Ltd. All rights reserved. [less ▲]

Colloidal stability of aqueous suspensions of barium zirconate

in Journal of the European Ceramic Society (2005), 25(13), 3195-3201

In this article, the colloidal behaviour of aqueous suspensions of barium zirconate is investigated. The variation of zeta-potential as a consequence of changing the pH and the concentration of an anionic ... [more ▼]

In this article, the colloidal behaviour of aqueous suspensions of barium zirconate is investigated. The variation of zeta-potential as a consequence of changing the pH and the concentration of an anionic polyelectrolyte is studied, the isoelectric point occurring at pH 5.3. The IEP shifts down on calcining the powder and also when anionic polyelectrolytes are added. Rheological studies have been made on suspensions prepared to a solids loading of 27 vol.% (72 wt.%). Optimum dispersing conditions are reached for suspensions prepared at basic pH with 1.5 wt.% tetramethylammonium hydroxide (TMAH) and a concentration of ammonium polymethacrylate (PMAA) of 1.6 wt.%. Comparison with similar suspensions prepared with NaOH instead PMAA demonstrated that TMAH gives an extra contribution to stability, probably related to the adsorption of N+(Me)(4) groups. The suspensions were slip cast, and green densities higher than 60% of theoretical were obtained. (c) 2004 Elsevier Ltd. All rights reserved. [less ▲]

Preparation of BaZrO3 powders by a spray-drying process

in Journal of Materials Research (2003), 18(6), 1325-1332

The potential use of barium zirconate for the manufacture of corrosion-resistant substrates emphasizes the need for a simple, inexpensive, and easily scalable process to produce high-quality powders with ... [more ▼]

The potential use of barium zirconate for the manufacture of corrosion-resistant substrates emphasizes the need for a simple, inexpensive, and easily scalable process to produce high-quality powders with well-controlled composition and properties. However, the classical solid-state preparation of barium zirconate leads to an inhomogeneous powder unsuitable for applications in highly corrosive environment. For this paper, the possibility to use the spray-drying technique for the preparation of BaZrO3 powders with a controlled size distribution and morphology was investigated. The influence of the nature and concentration of the precursor solution and the influence of the spray-drying step are discussed on the basis of x-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and dilatometric measurements. [less ▲]