Size Of Agglomeration Research Articles

Flow behaviour of semi-solid metal alloys

Semi-solid metal alloys, as used in industrial thixoforming, have a special microstructure of globular grains suspended in a liquid metal matrix. The complex rheological properties are strongly influenced by the local solid fraction, particle shape, particle size and state of agglomeration. It was analysed how the microstructure develops in dependence of the shear rate and cooling rate during the solidification and it was observed that the average particle size increased with increasing shear rate and decreasing cooling rate. In order to account for those phenomena, the rate of crystal growth and the relationship between average particle diameter and viscosity was modelled by applying the Sherwood two-film model for the mass transport. The dependence of the viscosity from the particle size were modelled with a modified Krieger–Dougherty model. Based on the rheological and microstructural observations an evaluation method was elaborated that allows for the construction of objective master curves that are independent of the particle growth during the experimentation. The isothermal experiments for the characterisation of the rheological behaviour consisted of step-change of shear-rate and yield-stress experiments. From the experimental data the steady-state flow curves could be determined as well as the time-dependent relaxation of the shear stress after a change of shear rate. The steady-state rheological behaviour was found to be shear thinning. Nevertheless, immediately after a shear-rate change an overshoot was observed that resulted from a short-time shear-thickening behaviour. The yield stress was found to strongly depend on the microstructure and the degree of agglomeration of the solid phase. With increasing rest time the yield stress was increasing strongly, because of the agglomeration of the solid particles. Based on the step-change of shear-rate experiments a single-phase flow has been developed that consists of a modified Herschel–Bulkley approach and accounts for the thixotropic as well as for the yield-stress behaviour of the alloys.

Are South Africa’s cities too small?

Are South African cities to small? Given the history of South Africa’s spatial development, one might expect that South African cities might be under-sized, and not over-sized as in many other developing countries. It is found that the rank-size distribution explains the sizes of South Africa’s cities but that Zipf’s Law does not hold for the country’s cities. The so-called q-coefficient was found to be equal to −0.75 for the 123 places with population in excess of 100 000. It was also found that urbanisation in South Africa over the past decade seems to have taken the form of the parallel (slow, 1.04%) growth of five large cities. Finally, calculating the “ H-measure” for 19 metropolitan areas in South Africa yields an inverse H-measure of 11.3. This suggests a reasonable degree of dispersal, which would only be consistent with optimal city size if transport costs were low and manufacturing not in need of scale economies; two conditions unlikely to apply to South Africa. Finally, the primacy ratio for South Africa’s largest urban agglomeration was found to be 38%. This suggests that the size of the Johannesburg-East Rand urban agglomeration (the primate city) may be relatively too large, whereas more efficient growth may come from larger harbour cities.

화학기상응축 공정으로 제조한 TiO2나노분말의 광촉매 특성

<TEX>$TiO_2$</TEX> nanopowder was synthesized by chemical vapor condensation (CVC) process and its photocatalytic property depending on microstructure was considered in terns of decomposition rate of organic compound. In order to control microstructure of <TEX>$TiO_2$</TEX> nanopowder such as particle size and degree of agglomeration, precursor flow rate representing number concentration was changed as a process variable. In TEM observation, spherical <TEX>$TiO_2$</TEX> nanoparticles with average size of 20 nm showed gradual increases in particle size and degree of agglomeration with increase of precursor flow rate. Also decomposition rate of organic compound increased with decreasing precursor flow rate. Thus, it was concluded that photocatalytic property was enhanced by targe surface area of disperse <TEX>$TiO_2$</TEX> nanoparticles synthesized at lower precursor flow rate condition in CVC process.

Microstructure and properties of alumina-SiC nanocomposites prepared from ultrafine powders

Alumina-Silicon Carbide nanocomposites were produced and studied under different aspects: characteristics of the starting materials, processing, microstructure and mechanical properties. The raw materials were two kinds of fine SiC powders (30 and 45 nm) and two Al2O3 powders (60 and 140 nm). Different compositions (amounts of SiC in the range 0.5–5 vol%) were performed and the characteristics of the resulting materials compared. The oxygen enrichment in SiC nanopowder due to specific powder treatments was controlled, in order to optimize powder processing routes. Densification tests of Al2O3-SiC powder mixtures were performed both by pressureless sintering and hot pressing route. The addition of SiC reduced the densification rate and favoured a refinement of the matrix. Improvement of mechanical properties over monolithic alumina was obtained in composites with the 45 nm SiC. The study pointed out that the critical factor for the success of these materials is the choice of the raw SiC powders in terms of grain size and state of agglomeration. The addition of this ultrafine SiC strongly affected the microstructural evolution, even at low volumetric fractions. The results do not substantiate any remarkable effect by dispersoids in the tested nanosize range.

Agglomeration of 3 mol% Y–TZP powders sythesized by hydrothermal treatment

Mechanical mixtures of zirconia gel and crystalline Y 2O 3 (3 mol%) have been hydrothermally treated at 110°C for 7 days in the presence of different mineralizer solutions. Taking the K 2CO 3/KOH molar ratio constant and equal 3, the total concentration of the mineralizer solutions was changed between 0.25 and 3.0 M. Predominant tetragonal (T) ZrO 2, characterized by different crystal size and different degree of agglomeration resulted by changing the concentration of mineralizer solution. The crystallization rate of Y-TPZ based powders, as well as the texture of the corresponding sintered products, are discussed in terms of concentration of the mineralizer solutions.

The effect of boron nitride on the rheology and processing of polyolefins

The influence of a new processing additive (fine particles of boron nitride) on the rheology and processability of polyolefins is studied. The equipment used includes an Instron capillary rheometer equipped with capillary and special annular dies (Nokia Maillefer wire coating crosshead) and two rheometers, namely a parallel-plate and a sliding-plate rheometer. Several types of boron nitride powders, varying in average particle size and distribution and in morphology are tested at various concentration levels. The additive with the smallest average particle size and free of agglomeration was found to have the greatest influence on the processability (melt fracture performance) of the polyolefins tested. Specifically, it was found that boron nitride not only eliminates surface melt fracture but also postpones the critical shear rate for the onset of gross melt fracture to significantly higher values, depending on the additive concentration, surface energy, and morphology. A flow visualization technique was used to visualize the polymer flow at the entrance of a transparent capillary die in order to determine the mechanism by which boron nitride eliminates gross melt fracture.

Effect of Temperature and Vapor-phase Encapsulation on Particle Growth and Morphology

The effect of in situ vapor phase salt-encapsulation on particle size and morphology was systematically investigated in a sodium co-flow/furnace reactor. The temperature of the furnace was varied, and the primary particle size and degree of agglomeration of the resulting silicon and germanium particles were determined from transmission electron micrograph images of particles sampled in situ. Particle size increased with increasing temperature, a trend expected from our understanding of particle formation in a high-temperature process in the absence of an encapsulant. Germanium, which coalesces faster than silicon, formed larger particles than silicon at the same temperatures, also in agreement with observations of particle growth in more traditional aerosol processes. At the highest temperatures, unagglomerated particles were formed, while at low temperatures, agglomerated particles were formed, with agglomerate shape following the shape of the salt coating.

Time-dependent rheological properties of semi-solid metal alloys

Semi-solid metal alloys, as used in thixoforming, have a special microstructure of globular grains suspended in a liquid metal matrix. The complex rheological properties are strongly influenced by the local solid fraction, particle shape, particle size and state of agglomeration. There is a high demand for models and software tools allowing the simulation of semi-solid casting processes. The material under investigation is a tin-lead alloy (Sn-15%Pb) which exhibits a similar microstructure to aluminium alloys. The experiments were performed with a concentric cylinder rheometer of the Searle type. Initially, the liquid alloy is cooled down to the semi-solid range under constant shearing and then kept under isothermal conditions for further experimentation. Based on the experimental data, a single-phase model has been derived where the semi-solid alloy is regarded as a homogeneous material with thixotropic properties and the microstructure is characterised by a structural parameter. The model consists of two parts: the equation of state, including a finite yield stress, and a rate equation for the structural parameter. The model equations are employed in numerical software and used for the simulation of characteristic filling cases and the comparison with the conventional filling.

Electrostatic effect on the flow behavior of a dilute gas/cohesive particle flow system

AbstractCohesive (Group C) particles have been widely used in various industries. To handle and process such fine particles, a clear understanding of the flow behavior and interparticle force, is needed. To achieve that objective, a Laser Doppler Anemometer system was used to measure particle velocity, fluctuating velocity, and size and extent of agglomeration or cluster formation of particles in a dilute gas/fine oil shale particle flow system with particle density of 2,082 kg/m3, average particle volumetric concentration of 1.5%, and average particle mass flux of about 100 kg/m2·s in a controlled‐moisture environment. The flow behavior of the particles was also studied for a mixture of 99% shale particles and 1% antistatic agent (Larostat powder, a quaternary ammonium compound) to examine the role of electrostatic force in gas/cohesive particle flow behavior. The addition of Larostat powder significantly reduced the electrostatic force and, in turn, made Group C particles behave similar to Group A or in some cases to Group B particles. In addition, our experimental data showed that the Maxwellian distribution function is a reasonable assumption to describe the velocity probability density function of the shale particles with or without antistatic agents.

Strategies for control of ceramic powder synthesis by gas-to-particle conversion

Powder production by gas-to-particle conversion is a commonly used but poorly understood industrial process. The effects of process conditions, mixing, turbulence and chemical additives on coagulation- and condensation-controlled gas-to-powder conversion processes are elucidated. Strategies for control of average particle size, polydispersity and degree of agglomeration are outlined. Coagulation-controlled processes usually result in agglomerate powders of solid primary particles, while condensation-controlled processes result in nonagglomerate powders. In coagulation-controlled processes, the narrowest possible size distribution is set by the self-preserving limit for complete coalescence, σ g≈1.45. A possible way of further narrowing this distribution is by electrical charging of the aerosol. The size distribution broadens by shear-induced (turbulent) coagulation and by broad residence time distributions caused by mixing or gas recirculation. Sintering makes the primary particle size distribution narrower than that of the parent agglomerates. In principle, condensation-controlled processes may result in narrower size distributions than coagulation-controlled ones. Here, high reaction rates and low particle surface energies favor narrow size distributions.

Dip coating of titanium ethoxide solutions

Titania coatings are prepared by dip coating anhydrous titanium ethoxide solutions on glass substrates. Titanium ethoxide reacts with atmospheric water during deposition and drying to form coatings with a variety of microstructures. Transparent gel layers are prepared by coating in dry nitrogen and then exposing the coating to humid atmosphere or using slow withdrawal rates. Hydrated titania particles form in coatings deposited in atmospheres with relative humidity levels greater than ≈ 15%. These coatings are typically composed of agglomerates of small particles (≈0.3 μm); increases in substrate withdrawal rate and Ti ethoxide concentration in the solution bath lead to increases in thickness, particle size and extent of agglomeration. Particle aggregation is prevented by adding hydroxypropyl cellulose (HPC) to the coating solution before deposition.

Microstructure and properties of nanostructured oxides

The current knowledge on the microstructure, sintering and creep properties of the nanostructured oxides TiO 2, Y 2o 3 AND zRo 2 synthesized by gas condensation in summarized. Size dependent phase stability in nanostructured oxide particles with average diameter in the range from 3 to 20 nm and phase transformations during particle growth are discussed. The influence of various microstructural parameter of the powder, i.e. grain size and degree of agglomeration, on consolidation and sintering is discussed. Plastic deformation by creep processes under compressive and tensile stresses at temperatures below half the melting point is presented.

Physical characteristics of ultrastable lipid-coated microbubbles

Surfactant-stabilized microbubbles can be rapidly and easily produced in high concentrations within the narrow size range of 1–5 μm. Such synthetic microbubbles, which find use in medical and other applications, persist in various aqueous media for periods up to one year. Microbubble size distribution analyses at different salt concentrations and after the addition of thiourea clearly indicate that electrostatic interactions are much less important than hydrogen bonding in stabilizing the lipid monolayer surrounding each microbubble. Direct optical confirmation of the typical (1–3 μm) microbubble size and lack of bubble agglomeration was provided by phase-measurement interferometric microscopy data. In addition, freeze-fracture electron microscopy data further indicated a high degree of size uniformity for the lipid-coated microbubbles.

Effect of particulate agglomeration and the residual stress state on the modulus of filled resin. Part II: Moduli of untreated sand and glass bead filled composites

The effect of particle size, particle shape and degree of agglomeration on the modulus of filled resin has been studied. At the same volume fraction, a higher modulus was achieved with non-regularly shaped sand rather than with glass beads. The relatively higher modulus for the sand filled resin can be attributed to larger degrees of heterogeneity associated with high degrees of agglomeration at low volume fractions. The narrowly distributed sand composites showed a better correlation to the modified law of mixtures, I equation of Paul.

Influence of primary QI and particulate matter on pitch carbonizations

The carbonization behaviour of coal-tar and petroleum pitches is influenced by the presence of particulate matter in the pitch. Several types of particulate matter consisting of carbon blacks of different oxygen contents and degrees of agglomeration and silica with hydrophobic and hydrophilic surfaces were added to pitches. Particle sizes were less than 100 nm in diameter. The influence on optical texture of coke structure was most pronounced for the agglomerated carbon blacks which trapped pitch in inter-particle voids creating an isotropic carbon at low wt % additions. The non-agglomerated hydrophobic silica progressively reduced the size of anisotropy with additions. The hydrophilic silica caused intermediate effects. Surface activity of additives influences carbonization chemistry. Reactivities of cokes to carbon dioxide ( k/ s −1) were not affected by structural changes but coke microstrengths were very dependent upon size of agglomeration of carbon black additives.

The Effect of Temperature on the Sooting Behavior of Laminar Diffusion Flames

Abstract Abstract–The effect of temperature on soot formation in a laminar propane diffusion flame was investigated. The temperature of the flame on a Parker Wolfhard burner was varied by adjusting the inert nitrogen content in the oxidizer Row. Local soot aggregate properties, such as aggregate diameters and their number densities as well as soot volume fractions were determined using simultaneous laser light scattering and extinction methods, while spherule diameters were obtained using sample extraction and transmission electron microscopy. Soot volume fractions and aggregate diameters were seen to increase with height above the burner, while the opposite trend was noted for aggregate number densities for the three flame temperatures investigated. Spherule diameters were seen to increase with height particularly in the early part of the flame. An increase in temperature resulted in a considerable increase in local soot volume fractions and aggregate diameters. Spherule size and degree of agglomeration ...

An Ecological Approach to the Theory of Suburbanization

Suburbanization is a result of variation in two theoretically elemental aspects of urban population distribution, congestion and concentration, which can be derived from ecological theory about the structure of the city. Measures of these aspects need not vary together; variation in either may lead to increase in the areal size of the population agglomeration. Both are related to the population size of the agglomeration. Variables suggested by the ecological theory of the structure of the city are associated with variation in the aspects of suburbanization.

Method of Making Population Density Map by Landform Division and Accumulated Result

Intending to make a population density map based on the natural regional unit instead of the administrative one of Shi (city) and Machi (town) and Mura (village), we divide the territory of Japan into landform divisions and calculate the population density by each landform division. As for the elements which will be useful for establishing the natural regional units, we have to consider climate, soil, vegetation etc.. However, in case of Japan which belongs to the Circurn Pacific Orogenic Zone and whose lands are divided into many pieces of block mountains and small planes, landform should be considered having the first priority as the base of population distribution and suitable to represent the detailed regional differences of population distribution. Establishment of landform division The whole land is classified as following landform areas based on its morphology and developmental mechanism. mountain : mountain, volcano, hill land flat land : piedmont gentle slope, volcanic flank, upland, lowland When one kind of landform area is too large, we divide it into minor landform units according to the difference of form and construction material such as fan, delta, peat-bog or sand dune in lowland area, and maturely dissected mountain or elevated peneplain in mountain area. The areas of each landform unit is measured by using planimeter on the map of 1:200, 000 and decided by giving the necessary corrections. Calculation of population of landform unit Population of each landform unit is readjusted from the population of Shi, Machi and Mura illustrated in the population census. When the administrative unit is completely included in one landform unit, its population is simply tabulated, but when it extends over two or more landform units, its population is divided into each landform unit basing upon the number of dots found on the population map 1. Urban population The urban podulation living in the urban agglomerations with area of 0.25km2 or more is excluded in the tabulation of population of landform unit, because it spreads over the boundary of urban agglomeration and makes the population density of the whole unit up too high. The population of urban agglomeration is estimated by multiplying the definite coefficient as following classification getting from sampling to the total population of Shi, Ku (ward) and Machi. 1) Five major Shi 2) Shi with inhabitants of more than 100, 000 or established before 1940 except the five major Shi 3) Other Shi 4) Machi 2) and 3) are subdivided into two according to the area of urban agglomeration and population growth caused by amalgamation of Shi, Machi and Mura from 1950 to 1955. Map and appendix (See Fig. 1 and Table 1) The scale of the map is 1:800, 000 and the map covers the whole of Japan. There are three sheets in one set with 7 colors including boundaries of landform divisions, symbols of the kinds of landf orms and index number of landform units. Population density is presented in the map in grades from yellow, tan to dark brown tone by shading method. The situation and size of urban agglomeration are presented according to its size in its physical shape or definite round shape in brownish red. The base map which includes contour lines, limits of water body, administrative boundaries, settlements and place names are doubly printed in grey color. The appendix includes index number of landform divisions, place names, areas, number of inhabitants and population density, These were published as population census of Japan 1955, population maps of Japan 3. Population density by landform devision in JapanPopulation density by landform division of whole Japan and each district are shown in Table 2 and Table 3. Mountains and Volcanoes which occupy 61% of the whole land are almost low density areas, especially in a volcanic area.