Fraction Of Primary Particles Research Articles

Formation of primary intermetallic phases in the interaction of Fe-containing melts of recycled Al-Si alloys with Al2O3 and Al2O3-C filter materials: A µ-CT study

Fe is the most detrimental impurity element in recycled Al-Si alloys due to the formation of brittle, primary, Fe-containing, intermetallic particles during solidification. Their removal from the Al-Si melts e.g. by filtration can reduce the Fe content. New active filter materials can facilitate the formation of these particles for their removal and contribute to the production of high-quality, recycled Al-Si alloys. The interaction of the alloy with the filter material can lead to modification of the thermodynamics of the alloy or of the kinetics of the particle formation. Time-resolved, three-dimensional microstructural investigations have been carried out to study the formation of primary intermetallic particles in Al7.1Si1.5Fe and Al7.1Si0.75Fe0.75Mn alloy melts in contact with Al2O3 and Al2O3-C filter substrate material during a melt conditioning treatment at 620 °C. The microstructures, in particular the primary intermetallic particles αc and αh, have been characterized by computed tomography (CT) and supplementary scanning electron microscopy (SEM). As expected by thermodynamics, the total volume fraction of primary particles remains unchanged by the interaction with the substrate materials. However, kinetic advantages for Fe-removal efficiency can be achieved by an accelerated and preferred selective particle formation in contact with the Al2O3-C material. Furthermore, particle formation is discussed in view of its different stages: nucleation, growth, and ripening.

An investigation on quantitative detector characteristics of novel flexible skin dosimeter using Monte Carlo simulation method

Novel lead oxide-based flexible dosimeters with superior performance were experimentally tested for electron therapy. However, absorbed/transmitted primary particle fraction and secondary radiation distribution from the dosimeter surface have not been reported. These features should be specified to improve the dosimeter’s reliability for medical applications. Hence, the absorbed primary particle fraction, transmitted particle and secondary radiations distributions of lead oxide-based flexible skin dosimeter under the incident 6 MeV electron radiation have been investigated by pyPenelope Monte Carlo Simulation. The results have demonstrated that the generated secondary irradiation probabilities are not significantly high to enhance the therapeutic dose abnormally. In addition, the angular distribution of the scattered secondary irradiations is low. No abnormal changes were observed in the fraction and energy distribution of the transmitted primary electrons. Hence, it can be concluded that the designed structure has promising potential to be used as dosimeters in electron beam therapy.

Estimating effective density of cohesive sediment using shape factors from holographic images

To reveal the influence of shape factors on the effective density (ρe) of cohesive sediment, a series of laboratory experiments has been conducted under different turbulence intensities using a submersible digital holographic camera (LISST-Holo). Shape factors representing axis ratio, boundary roughness, and sturdiness of flocs exhibited characteristic variations with turbulence intensity. For turbulence intensities from 0.54 to 1.42 Pa, micro-flocs were broken into single grains, resulting in a constant axis ratio, a 13% decrease in boundary roughness, and a 16% increase in sturdiness. Out of these shape factors, ρe was mainly derived from the concept of sturdiness, which represents the fraction of primary particles within flocs. In fact, the 16% increase in sturdiness added 9% to ρe. When the flocs became sturdier, ρe had lower dependence on floc size due to the increase in the fraction of primary particles. Furthermore, ρe as a function of floc size might be affected by the response of the optical instrument. In the case of fine particles (<25 μm), not every suspended particle was reconstructed from the holographic image, inducing an underestimation of ρe. This is because insufficient scattering from floc edges impacts the particle detection ability. Despite the uncertainties, LISST-Holo is well suited to obtain a series of data for shape factors of flocs. These outcomes give insight into means for estimating ρe.

Liquid segregation behaviour of a semi-solid squeeze cast A356 aluminium cup-shaped part

ABSTRACTIn this study, the effects of pouring temperature (solid fraction of primary particles) on the structure and liquid segregation behaviour of a cup-shaped semi-solid rheocast processed casting made from the A356 aluminium alloy were investigated. Results showed that the solid fraction of primary particles at the bottom of the castings was greater than in other parts of the castings, and the highest liquid segregation occurred in the mid-height (wall) of the castings. Liquid segregation was increased with an increase in the initial solid fraction of primary particles. The critical solid fraction at pouring for minimum liquid segregation was found to be approximately 25%.This paper is part of a thematic issue on Light Alloys.

Modeling coupled nanoparticle aggregation and transport in porous media: A Lagrangian approach

Changes in nanoparticle size and shape due to particle–particle interactions (i.e., aggregation or agglomeration) may significantly alter particle mobility and retention in porous media. To date, however, few modeling studies have considered the coupling of transport and particle aggregation processes. The majority of particle transport models employ an Eulerian modeling framework and are, consequently, limited in the types of collisions and aggregate sizes that can be considered. In this work, a more general Lagrangian modeling framework is developed and implemented to explore coupled nanoparticle aggregation and transport processes. The model was verified through comparison of model simulations to published results of an experimental and Eulerian modeling study (Raychoudhury et al., 2012) of carboxymethyl cellulose (CMC)-modified nano-sized zero-valent iron particle (nZVI) transport and retention in water-saturated sand columns. A model sensitivity analysis reveals the influence of influent particle concentration (ca. 70 to 700mg/L), primary particle size (10–100nm) and pore water velocity (ca. 1–6m/day) on particle–particle, and, consequently, particle–collector interactions. Model simulations demonstrate that, when environmental conditions promote particle–particle interactions, neglecting aggregation effects can lead to under- or over-estimation of nanoparticle mobility. Results also suggest that the extent to which higher order particle–particle collisions influence aggregation kinetics will increase with the fraction of primary particles. This work demonstrates the potential importance of time-dependent aggregation processes on nanoparticle mobility and provides a numerical model capable of capturing/describing these interactions in water-saturated porous media.

Semisolid melt squeezing procedure for production of open-cell Al–Si foams

Melt squeezing process in semisolid state was used for the first time for production of open-cell Al–Si foams with improved microstructural and mechanical characteristics. First a given amount of preheated NaCl particles was stirred into molten A356 alloy. Stirring continued during solidification of the slurry until reaching a given solid fraction of primary particles. The resulting mixture was pressed by a perforated piston to squeeze a controlled amount of the residual meltout. Open-cell foams were achieved by leaching the final Al–Si–NaCl composites in water. The suitable values of the NaCl particle size, pressurizing temperature, melt to salt ratio and piston pressure for production of uniform and high integrity foams were obtained to be 3400μm, 605°C, 1.5:1 and 10MPa, respectively. The results showed that the semisolid processing employed could refine and modify the microstructure of the cell ligaments in the final foams. Mechanical properties of the foams such as energy absorption and fracture toughness were also improved by the semisolid processing.

Coagulation and stabilization of sterically functionalized magnetite nanoparticles in an organic solvent with different technical polymers

This experimental study deals with the colloidal stability of sterically functionalized magnetite nanoparticles in a low dielectric constant organic solvent with different concentrations of technical grade polymers. Those dispersions are the starting point of a solution and spray drying process chain to synthesize highly filled nanocomposite materials with nanoparticle volume concentrations exceeding 10%. We introduce a thermo gravimetric method together with light extinction and dynamic light scattering measurements to gain quantitative information on the concentration of primary particles and the mechanism of destabilization or stabilization by polymer addition. Poly(vinyl butyral) is found to stabilize the dispersion considerably caused by stronger interactions with the fatty acid coated magnetite particles quantified by means of adsorption measurements. Both poly(methyl methacrylate) as well as two grades of poly(bisphenol A carbonate) are found to destabilize the dispersion due to depletion flocculation over the entire concentration range investigated However there is a significant quantity of a stable fraction of primary nanoparticles in the supernatant after depletion flocculation occurred. This fraction of primary particles is increasing with decreasing polymer concentration. We furthermore point out important concerns and limitations for the composition of and concentrations in such complex colloidal systems for use in industrially relevant processes.

Semi solid metal processing: The fraction solid dilemma

One of the most challenging aspects in semi solid metal (SSM) processing is to determine the actual volume fraction of the solid at the processing temperature. The fraction has great impact on the SSM slurry viscosity and the subsequent filling of the mold in the casting stage. Three methods, namely quantitative metallography, thermodynamic calculation, and thermal analysis are employed to investigate and clarify the contradictory open literature reports about the real value of the volume fraction of primary particles. It is reported that the discrepancies between the results obtained by different methods are caused mainly by variations in cooling rates and by coarsening of the primaries during the quenching process.

Effects of processing parameters on microstructure of thixoformed ZA27 alloy

In this paper, the effects of processing parameters on microstructures of thixoformed ZA27 alloy cylindrical rods were investigated. The results indicated that the microstructure of the formed rod was basically uniform except that of the section near the inner gate. But the detailed microstructure observation or quantitative examination showed that the primary particle fraction slightly decreased along the axial direction but it relatively more obviously decreased along the radial direction. The primary particle size increased and the primary particle fraction decreased with the increase of reheating duration. Both the size and fraction of primary particles decreased with elevating the reheating temperature and increased with increasing the die temperature. The secondary solidification began with the direct growth of the secondary primary phase on the surface of the primary α′ particles and completed by forming lamellar eutectics. The secondarily solidified structure was significantly affected by die temperature while the other two parameters, reheating temperature and reheating time had no obvious effect.

Phenomenological observations on thixoformability of a zinc alloy ZA27 and the resulting microstructures

The thixoformability to cylindrical rods of a Zr-modified zinc alloy ZA27 and the resulting microstructures were observed with different reheating temperatures, reheating durations and mould temperatures. The results were compared with those of an unmodified ZA27 alloy formed by semi-solid die-casting. It was found that the formability of the Zr-modified alloy was significantly superior to that of the unmodified one. The microstructures of the thixoformed rods were basically uniform along their axial direction and the primary particle fraction obviously decreased along radial direction. The primary particle size slightly increased and the primary particle fraction decreased with increasing reheating duration. Both the size and fraction decreased with the increase of reheating temperature, while both of them increased as the die temperature increased. The mould-filling process and subsequent secondary solidification were also discussed.

Effects of processing parameters on tensile properties and hardness of thixoformed ZA27 alloy

Abstract The influences of processing parameters, reheating temperature, reheating period of time and die temperature on tensile properties of thixoformed Zr modified ZA27 alloy were investigated. Simultaneously, the hardness of the alloy was also examined. The results indicated that the thixoforming process significantly improved both ultimate tensile strength and elongation of the alloy because of obvious decreasing of porosities, compared with the semi-solid die-cast no modified ZA27 alloy and permanent mould cast Zr modified ZA27 alloy. The tensile cracks initiated from the defects, such as voids, porosities and inclusions, and eutectic cell boundaries between primary particles, and then preferentially grew along eutectic structure, regardless of the processing parameters. The hardness was mainly affected by the solubility of Zn in the primary particles, the primary particle fraction and the amount of the decomposed primary particles. The hardness HB could be calculated from the micro-hardness HB p of the primary particles, HB e of the matrix and the primary particle fraction f p according to an equation of HB = HB p × f p + HB e (1− f p ).

Evolution of Sodium Silicate Sols through the Sol-to-Gel Transition Assessed by the Fluorescence-Based Nanoparticle Metrology Approach

The fluorescence-based nanosize metrology approach, proposed recently by Geddes and Birch (Geddes, C. D.; Birch, D. J. S. J. Non-Cryst. Solids 2000, 270, 191), was used to characterize the evolution of primary silica particles through the sol-to-gel transition and during aging of sodium silicate (SS) derived silica. In this study, the evolution of silica particles within SS derived silica was examined as a function of pH and glycerol doping through the sol-to-gel transition and up to 10 months after gelation. Time-resolved anisotropy decays were measured for the cationic dye rhodamine 6G, which was strongly adsorbed to the silica nanoparticles, and for the anionic probe pyranine, which provided accurate data on the microviscosity of the internal aqueous solution within sols and gels. The data provide evidence for the presence of nonaggregated primary particles far beyond the gelation point and even after prolonged aging of the resulting silica when aging is done either at low pH or in the presence of glycerol. Both the fraction of stable primary particles and the final size of the primary particles within the aged silica were dependent on the pH and the presence of glycerol. In general, lower pH values (pH 3.5) or the presence of glycerol increased the fraction of primary particles relative to samples at prepared at pH 6.5. Gelation pH did not have an affect on the final particle size, which was 1.5 ± 0.4 nm at both pH 3.5 and 6.5. On the other hand, a smaller particle size (0.9 ± 0.2 nm) was observed for SS sols and hydrogels containing 50% glycerol.

Characterisation of through-thickness microstructure in centrifugally cast hypereutectic Al-Si and Al-Al3Ni alloys

In this study, a series of hypereutectic Al-Si and Al-Al3Ni alloys were manufactured using the centrifugal casting technique. Experimental results showed that there are significant differences in the micro-structural morphology and hardness distribution of centrifugally-cast hypereutectic Al-Si and Al-Al3Ni alloys resulting mainly from the difference in densities of the non-metallic particles (ρ Al3Ni = 4000 kg/m3; ρSi = 2330kg/m3).The solidification structure of the hypereutectic Al-Si pipes can be divided into four specific zones along the through-thickness direction. The primary silicon area fraction and hardness at the outer area are higher than those at the inner area, which are in turn higher than those at the interior region. On the other hand, the solidification structure of the hypereutectic Al-Al3Ni pipes can be roughly divided into two or three solidification zones in order of decreasing primary Al3Ni area fraction from the outer peripheral zone to the inner peripheral zone.In addition, the area fraction of primary particles and the hardness along the through-thickness direction of the pipes could be influenced by the solute content.

Aggregation studied by laser diffraction in relation to plowing and liming in the Cerrado region in Brazil

In the Cerrado savanna region in Brazil, the effects of different land use on Oxisol aggregation were studied using laser diffraction grain size analyses. The topsoil of plowed systems had significantly lower amount of macroaggregates (2000–194 μm) and a significantly higher amount of soil in the microaggregate size and primary particle fraction (<76 μm) compared to pastures and the native Cerrado. In plowed systems low in soil organic carbon (SOC), lime had a negative effect on aggregate stability. Lime addition had no effect on topsoil aggregation in land use systems that were not plowed regularly and generally had a higher SOC content. For all the studied topsoils, pH KCl was positively correlated with the amount of clay dispersed after 3 h of shaking in water. SOC did not influence clay dispersion in the range of soils studied. In continuous cropping systems in the Cerrado region, with a combination of mechanical stress and low SOC, liming will cause an increase in small aggregates and primary particles and hence contribute to a destabilization of the soil structure.

Rheological model of semi-solid A356-SiC composite alloys Part II: Reconstitution of agglomerate structures at rest

This study complementary to the previous paper on the dissociation of the particulate structure during shear of semi-solid A356 alloy reinforced with SiC particles and focuses on the reconstitution of the structure at rest. Semi-solid composite alloys with 26% primary solid fraction are initially sheared under specific conditions in order to establish a dynamic equilibrium state and viscosities were then measured after varying periods of rest. The kinetics of reconstitution are presented using an analogous model to the dissociation of the particulate structure by defining a reconstitution parameter. The model was applied successfully to the results of this study as well as to the data of similar studies in the literature. The kinetic reconstitution constant was observed to be about 4–9 times smaller than the dissociation constant and was found to increase with a decrease in the solid fraction of primary particles, an increase in the shear rate prior to rest and a decrease in content of SiC particles. The presence of the SiC particles was also found to oppose the tendency for the primary particles to agglomerate thus slowing down the reconstitution kinetics.

Elastic properties of networks of fractal clusters

Elastic properties of casein gels were followed by dynamic measurements for various volume fractions of casein micelles far above the percolation threshold. Elastic and loss storage moduli versus volume fractions obey a power law with an exponent close to 3. The previous theoretical models for networks made of a homogeneous packing of fractal flocs do not predict the observed behavior. A model is proposed to estimate the vectorial contribution to the network elasticity. This model is in agreement with our experimental results. The value of the fractal dimension of the flocs derived from this analysis is discussed as a function of the volume fraction of primary particles.

非水分散系の押し出しにおけるレオロジー的研究

Extrusion tests have been carried out for non-aqueous dispersions of clay with adsorbed water under a variety of solid concentrations as well as the amounts of water adsorbed. The extrusion pressure can be divided into two components; one is the pressure drop in nozzle cone, which is independent on the extrusion speed, and the other is that in nozzle. From the rheological analysis of the dependence of the pressure drop in nozzle on the extrusion speed, flow properties of these dispersions were found to be represented approximately by the Bingham plastic model.With increasing a solid concentration, the yield stress increased exponentially, whereas the plastic viscosity remained unchanged above a critical solid concentration. This can be interpreted by the saturation of the effective volume fraction of particles including a part of dispersion medium immobilized within flocs, which may be resulted from a decrease in a void fraction between the particles. A further increase in the yield stress above the critical solid concentration can be attributed to an increase in a packing fraction of primary particles in flocs.The yield stress and the plastic viscosity were also dependent on the amount of water adsorbed, and showed maximum values at different amounts of water, respectively. These can be interpreted by considering the changes not only in a volume fraction of flocs but also in a packing fraction of primary particles in flocs, which may be resulted from the changes in the particle interactions associated with the cohesive force of water bridge as well as the repulsive force of the diffuse double layers between adjacent particles with water film. In addition, it is necessary to take into account the London-van der Waals' attractive force in order to interpret the rheological properties for the dispersions of dry powders.