Average Agglomerate Size Research Articles

Sol−Gel Synthesis and Characterization of SrFeCo0.5O3.25-δ Powder

SrFeCo0.5O3.25-δ powder was synthesized by a sol−gel processing from metal alkoxides. The powder synthesized was characterized by X-ray diffraction, scanning electron microscopy (SEM), BET surface area measurements, particle size analysis, and thermogravimetric analysis (TGA). The synthesized oxide consisted of three phases: a main perovskite phase with an amount of a perovskite-related layered phase and a trace spinel phase. The powder calcined at 850 °C in air for 4 h exhibited a uniform rod-shaped morphology with a particle size of about 300 nm and a surface area of 2.1 m2/g. The average agglomerate size was about 3 μm. TGA data showed that the powder was of larger oxygen nonstoichiometry at high temperature in air. Experimental results demonstrate that sol−gel processing is a very effective technique for synthesizing submicron SrFeCo0.5O3.25-δ powders.

Modelling of rheological behaviour of semisolid metal slurries Part 2 – Steady state behaviour

In this part of the current series, the rheological model presented in Part 1 is applied to study the steady state behaviour of semisolid metal (SSM) slurries. Experimental results on the steady state viscosity of Sn–15 wt-%Pb slurries in the literature are used to determine the model parameters. Application of the developed model to Sn–15 wt-%Pb slurries has revealed a number of microstructural and rheological characteristics of SSM slurries. Under shear flow, the steady state represents the dynamic equilibrium between two opposite processes, agglomeration and deagglomeration. Under steady state, the microstructure of a SSM slurry is characterised by spherical particle morphology, uniform particle size, constant average agglomerate size and constant effective solid fraction. The theoretical analysis shows that there is an strong coupling between the microstructure and the steady state viscosity. Shear rate affects viscosity of SSM slurries by changing the amount of entrapped liquid between solid particles in the agglomerates and altering therefore the effective solid fraction. It is shown that steady state viscosity of a SSM slurry decreases with increasing shear rate and approaches an asymptotic viscosity. The asymptotic viscosity is only a function of solid fraction and corresponds to a microstructure in which there is no particle agglomeration. Finally, the current model predicts that particle size has no significant effect on apparent viscosity when particle size is either less than a few micrometres or greater than a few hundred micrometres. However, in the intermediate particle size range, apparent viscosity decreases with increasing particle size. This theoretical prediction is in good agreement with experimental results in the literature.

Temporal distributions: The basis for the development of mixing indexes for scale‐up of polymer processing equipment

AbstractNew and better mixing criteria are needed to assess dispersive and distributive mixing efficiency in polymer processing equipment. Such criteria can serve the purpose of process optimization and machine scale‐up. In this work, the history of flow strength and shear stresses experienced by a number of particles in a twin‐flight, single‐screw extruder serve as the basis to produce temporal distributions of these parameters. In turn, the temporal distributions can be used for developing new mixing indexes for process optimization and scale‐up. Using models for dispersion kinetics and experimental data, calculations of agglomerate size distribution and average agglomerate size can be used as a dispersive mixing criterion.

Force balance modelling for agglomerating fluidization of cohesive particles

To estimate the size of agglomerates formed in fluidized beds of cohesive particles, a model of force balance is developed. The average agglomerate size estimated by this model is in reasonable agreement with experimental data. An agglomeration criterion is proposed by analyzing the model. Theoretical analysis shows that higher superficial gas velocity and fluid density, lower particle cohesion, and the collision between agglomerates are advantageous to the agglomerate fluidization of cohesive particles.

Dispersion of Alumina and Silicon Carbide Powders in Alumina Sol

Dispersion of Al2O3 and SiC particles in an alumina sol has been investigated through determination of particle‐size distribution, zeta potential, and agglomerate morphology. The particle size of Al2O3 and SiC (as determined by the particle‐size analyzer) is strongly affected by the presence of alumina sol in the solution. The average agglomerate size is decreased by at least 50%. The zeta potential of Al2O3 in 1Malumina sol increases slightly, whereas that of SiC reverses its sign over a wide range of pH values. It is proposed that these effects are caused by AlO4Al12(OH)24(H2O)7+12 sol clusters (1‐2 nm in size) that are absorbed on the surface of the large (1‐5 µm) ceramic particles. The electrostatic and steric effects of the cluster absorption help to control the dispersion and stabilize the suspension of ceramic particles in the alumina sol during wet processing. It is expected that the alumina‐sol clusters can be used as an efficient, clean dispersant for single‐phase and composite ceramic powders.

Structure of Aerosil in Liquid Crystal Polymer

The structure of aerosil in a glassy state of liquid crystalline polymer was investigated using the method of atomic force microscopy. Rod-like agglomerates of aerosil were found. Average agglomerate size, anisotropy rate and inter-agglomerate distance were estimated.

Demixing effects in laminar shearing flows: A practically useful approach

AbstractThe phenomena of shear‐induced agglomeration and shear‐induced migration in a non‐ colloidal particulate suspension were examined in different flow fields. The shear flow of a model suspension consisting of spherical glass beads in an aqueous corn syrup with added surfactant was studied in homogeneous and non‐homogeneous flow fields (cone‐and‐plate, parallel plate and laminar flow in a tube). It was found that there was a rapid increase in the average agglomerate size followed by leveling off at long times resulting in an equilibrium particle size. This equilibrium particle size was found to be a strong function of the applied shear rate. It was further found that agglomerate data from the different flow fields superposed well, and, at least in the range of solid volume fractions considered, particle migration was negligible. This work provides a simple technique for quantifying demixing in laboratory experiments and applying the results to industrially relevant situations.

Oxalate coprecipitation of doped ceria powder for tape casting

Twenty cation per cent yttria doped ceria Ce 0.8Y 0.2O 1.9 was prepared by an optimised oxalate coprecipitation route to give fine sinterable powder. This was formulated into slurries for tape casting of the doped ceria solid electrolyte used in solid oxide fuel cells. Prior dry milling of the powder was found to be most effective to further reduce the average agglomerate size, which allowed to sinter dense membranes at 1400°C for 2 h, among the best results obtained so far for tape cast doped ceria. Ball milling of the powder, composition and viscosity of the slurry as well as sintering conditions of different tapes are discussed.

Preparation and characterisation of forsterite (Mg 2SiO 4) aerogels

2MgO·SiO 2 gels were prepared from alkoxide precursors using different prehydrolysis conditions for TEOS, and dried supercritically. The resultant powders were subjected to heat treatments to 1200°C, and the formation of the forsterite crystal phase was studied. The powders were characterised using differential thermal analysis, thermogravimetric analysis, BET surface area analysis, X-ray diffraction and transmission electron microscopy. Formation of forsterite between 795 and 805°C indicated a high degree of homogeneity in the gels, although the initial formation of MgO at 490°C suggested that there were some regions which were MgO or SiO 2 rich. Surface areas between 207 and 340 m 2 g −1 were observed. TEM revealed loosely packed agglomerates with primary particle sizes between 10 and 15 nm. On heating to 1200°C the primary particle size increased to 200 nm, although the average agglomerate size did not increase significantly.

Synthesis and dielectric characterisation of lead magnesium niobate from precipitation and freeze-drying methods

Lead magnesium niobate, Pb(Mg1/3Nb2/3)O3 (PMN), has been prepared from aqueous nitrate solutions by freeze-drying and stepwise precipitation using an ammonia solution as the precipitant. Perovskite PMN powders with less than 5% pyrochlore phase were obtained when the resulting precursors were calcined at temperatures in the range of 800 to 1000 °C. They exhibited a rounded powder morphology and an average agglomerate size of <1.0 µm. Freeze-dried PMN offered a lower formation temperature for the perovskite phase (700 °C) and a higher relative permittivity than those of precipitated PMN. On sintering at 1150 °C, the maximum relative permittivity observed for the precipitated PMN was 8019 at 1 kHz with a Curie temperature of –11 °C, while that of freeze-dried PMN was 14542 with a Curie temperature of –3 °C at the same frequency.

Combustion synthesis and properties of the NASICON family of materials

The fine-particle NASICON family of materials, MZr2P3O12(where M = Na, K, ½Ca and ¼Zr) and NbZrP3O12, have been prepared by the combustion of aqueous heterogeneous mixtures of stoichiometric amounts of metal nitrate, zirconyl nitrate, niobium phosphate, diammonium hydrogen phosphate, ammonium perchlorate and carbohydrazide (CH) at 400 °C. The formation of NASICON materials was confirmed by powder X-ray diffraction (XRD), IR, solid-state (31P) NMR spectroscopy and thermal expansion coefficient measurements. The combustion-synthesized NASICON powders have an average agglomerate size of 9–13 µm with a specific surface area varying from 8 to 28 m2 g–1. The powders pelletized and sintered in the range 1100–1200 °C for 5 h achieved 95–97% theoretical density and showed fine-grain microstructure. The coefficient of thermal expansion of a sintered compact was measured up to 500 °C and ranged from –1.5 × 10–6°C–1 to 1.0 × 10–6°C–1 depending on the composition.

Effects of Temperature and Pressure on Asphaltene Particle Size Distributions in Crude Oils Diluted with n-Pentane

The effects of temperature (0--150 C) and pressure (0--5.6 MPa) on the size distribution of asphaltene particles (or agglomerates), formed as a result of diluting the crude oils with n-pentane, were studied using a modified laser particle analyzer. Four crude oils, ranging from an asphaltic condensate to a heavy oil-sand bitumen, were tested in this investigation. The average size of asphaltene agglomerates ranged from 266 to 495 Am. The results suggest that the mean asphaltene particle size increases with pressure and decreases slightly with temperature; however, no trends were evident with the molar mass of crude oils. Although the particle size distributions in most cases were unimodal and described adequately by the log-normal distribution function, bimodal distributions were also identified in certain cases.

A Novel Autoignited Combustion Process for the Synthesis of Bi-Pb-Sr-Ca-Cu-O Superconductors with a Tc (0) of 125 K

Zero resistivity at 125 K has been observed for the first time in bismuth system with the nominal composition Bi 1.75Pb 0.25Sr 2Ca 2 Cu 3O 10+δ by applying a novel autoignited combustion process. The thermal decomposition characteristics of the gel and the sequence of formation of the superconducting phases are described. This combustion process is very simple, convenient, and nonexplosive, as well as economical for large scale production, of fine powder samples with higher degree of homogeneity, excellent sinterability, and reproducibility. The low T c and the high T c phases started forming at temperatures as low as 750 and 800°C, respectively. Powders formed have an average agglomerate size of around 0.8 μm.

Controlled combustion synthesis and properties of fine-particle NASICON materials

Fine-particle NASICON materials, Na1 +xZr2P3 –xSixO12(where x= 0.0, 0.5, 1.0, 1.5, 2.0 and 2.5), have been prepared by controlled combustion of an aqueous solution containing stoichiometric amounts of sodium nitrate, zirconyl nitrate, ammonium perchlorate, diammonium hydrogen phosphate, fumed silica and carbonohydrazide. Formation of NASICON has been confirmed by powder XRD, 29Si NMR and IR spectroscopy. These NASICON powders are fine (average agglomerate size 5–12 µm) with a surface area varying from 8 to 30 m2 g–1. NASICON powders pelletized and sintered at 1100–1200 °C for 5 h achieved 90–95% theoretical density and show fine-grain microstructure. The coefficient of thermal expansion of sintered NASICON compact was measured up to 500 °C and changes from –3.4 ×10–6 to 4.1 × 10–6 K–1. The conductivity of sintered Na3Zr2PSi2O12 compact at 300 °C is 0.236 Ω–1 cm–1.

High purity zirconia powders via wet chemical processing: A comparative study

Comparison has been made of the achievable purity levels and agglomeration characteristics of zirconia powders obtained via six different wet chemical routes where the starting material was an impure Zr-oxychloride solution obtained from Indian zircon sand. For the highest purity (2–15 ppm of other oxides except Hf), two routes were found to be effective: crystallization of the oxychloride via the basic sulphate, or double crystallization of the oxychloride, followed by dissolution and hydroxide precipitation. The average agglomerate size was the largest (10–15 μm) for the powders obtained by direct decomposition of the basic sulphate at 1000°C, though the size distribution was narrow and distinctly monomodal. For smaller agglomerates (1·5–2·5 μm), on the other hand, the size distribution was very wide. Calcination at 800°C of powders obtained via hydroxide precipitation from a solution always yielded only tetragonal zirconia as the crystalline phase.

Particle Design of Enoxacin by Spherical Crystallization Technique. II. Characteristics of Agglomerated Crystals.

The characteristics of agglomerated crystals of enoxacin prepared by a spherical crystallization technique with the acetone-ammonia water-dichloromethane solvent system were investigated. Enoxacin forms a sesquihydrate, but has two different pseudopolymorphs, i.e. anhydrate and trihydrate. The crystalline form of the resultant agglomerates could be controlled by selecting the composition ratio of the three solvents and their mixing procedure. In order to obtain the sesquihydrous agglomerates, the mixing of an ammonia water solution of enoxacin with acetone in the first stage was required. When ammonia concentration in ammonia water used as a bridging liquid was higher, agglomerated crystals became smaller in size and less spherical in shape. The average size of agglomerates decreased with an increase in agitation speed and with a decrease in the ammonia water fraction in the solvent system. Primary crystals composing the agglomerates grew larger in size in the solvent system where the crystallization rate was reduced, resulting in less spherical agglomerates. Spherically agglomerated crystals prepared with a low fraction of ammonia water improved their flowability and packability without much delay in their dissolution rate, as compared with the primary crystals.

Combustion synthesis of fine particle rare earth orthoaluminates and yttrium aluminum garnet

Fine particle rare earth orthoaluminates, LnAlO 3, where Ln = La, Pr, Nd, Sm, Gd, Tb, and Dy, and yttrium aluminum garnet, Y 3Al 5O 12 (YAG), have been prepared by a novel combustion process using corresponding rare earth nitrate, aluminum nitrate, and urea/carbohydrazide mixtures. A saturated aqueous solution of the mixture when rapidly heated at 500°C foams and ignites to yield voluminous, crystalline single phase rare earth orthoaluminates in less than 5 min. The solid combustion products were characterized by XRD, SEM, TEM, particle size analysis, and surface area measurements. Rare earth orthoaluminates obtained have 3–4 m 2/g surface area, with particle size in the range of 0.1–0.6 μm and an average agglomerate size of 4–5 μm.

ゾル−ゲル法によるＹ系超電導粉末の低温合成

We have succeeded in producing of yttrium-barium-cuprate powders using metal alkoxides at below 650°C by sol-gel process. In this work, the basic experiments were focussed on the selection of metal alkoxides as starting materials and suitable solvents to prepare chemical homogeneous solution. A highly concentrated mixed alkoxides solution was prepared using Y(n-OC4H9)3, Ba(OC2H4)2, and Cu(N03)2 as starting materials and that the formation of superconducting 123 powders fired only at 650°C.The powders were characterized by X-ray diffraction, TEM, and magnetic susceptibility measurements. X-ray diffraction patterns of the powders showed a presence of 123 single phase. TEM observation of this powder revealed small spherical partices with an average agglomerate size to be less than 0.05μm.

Agglomeration of High Calcium Fly Ash for Utilization II. Binding Mechanisms

AbstractFly ashes from the Lansing and Ottumwa power plants in Iowa were agglomerated by means of a continuous pan agglomerator, a continuous auger and a batch turbine agglomerator. In order to compare agglomeration mechanisms the following parameters were determined: (a) particle size distributions of the untreated fly ashes; (b) particle size distributions of the agglomerated fly ashes; (c) pore size distribution of agglomerates; (d) crystalline hydration products by X-ray diffraction; and (e) morphological characterization by scanning electron microscopy.In the batch system coalescence mechanisms were favoured. The agglomerates were fairly irregular in shape and had a rough surface texture. As residence time in the system increased breakage of agglomerates occurred, reducing the average agglomerate size. In the continuous systems layering of the fine feed particles onto established agglomerates was the predominant growth mechanism. The agglomerates were smooth and spherical. The layer structure was observed by scanning electron microscopy. Agglomerates of widely varying size, strength, and pore matrix can be produced in both systems. It is envisaged that while agglomerates could be produced with characteristics essential for their proposed end use by either method, continuous pan agglomeration would be the most versatile system to utilize.

CHARACTERIZATION OF CARBON BLACK FILLED POLYMERS USING SMALL ANGLE X-RAY SCATTERING AND TRANSMISSION ELECTRON MICROSCOPY: A COMPARATIVE STUDY

Abstract Carbon black (CB) is often incorporated in polymeric materials in order to modify their properties. The final properties of composite materials containing CB depend on, among other things, the nature of the carbon black, its concentration and its degree of dispersion. Therefore, there is a need to accurately characterize the degree of dispersion of carbon black in polymers. Although techniques based on optical microscopy can be used for detecting large agglomerates, they do not have the resolution to accurately quantify dispersions of particles having diameters less than 0.25 microns. Small angle X-ray scattering (SAXS) has the potential of being a useful and efficient tool for characterizing dispersions on length scales ranging from the order of 0.01 to 0.1 micron. The method is quick and provides an average value of the average agglomerate size through indirect measurements of the correlation length and the inner specific surface area. On the other hand. Transmission Electron Microscopy (TEM) a...