Control Particle Growth Research Articles

Transition from Five-Fold Symmetric to Twinned FCC Gold Particles by Thermally Induced Growth

Stable gold clusters around 1 nm in size have been prepared by evaporation of the metal into an organosilicon polymer solution. A controlled particle growth of the entrapped gold particles was achieved by annealing in helium at ∼410 °C. Above 460 °C the growth rate is dramatically increased, concurrent with the onset of pyrolysis of polysilazane to form a porous solid. X-ray diffraction and simulation calculations using Debye functions of model clusters of different sizes and both crystallographic and noncrystallographic symmetry were used to characterize these particles. The exclusive presence of noncrystallographic (decahedral and icosahedral) multiply twinned particles (MTPs) in the as-prepared material is confirmed by EXAFS. The high precision of the XRD experiments and theoretical data fits gives detailed insight into the thermally induced growth process, during which the MTPs progressively disappear and are replaced by singly twinned fcc particles of larger size. This transition occurs in the range ...

Ultrafine aerosol particles in aircraft plumes: Analysis of growth mechanisms

An analysis of in situ measurements of ultrafine volatile aerosols in the plume near field of the DLR aircraft ATTAS using low (0.02 g/kg) and high (2.7g/kg) fuel sulfur contents (FSCs) is presented. The observed growth of nanoparticles (diameter 5–10 nm) is reproduced in detail by a microphysical simulation with chemi‐ion emissions of 2.6×1017/kg fuel. Volatile aerosol dynamics is controlled by sulfuric acid (H2SO4) for high FSC, consistent with a S to H2SO4 conversion of 1.8%. The very high conversion for low FSC (55%) prescribed in the model to match the observations contradicts direct in situ measurements of H2SO4 and suggests that species other than H2SO4, likely exhaust hydrocarbons, control particle growth in such cases.

Non-linear diffusion controlled particle growth—A variational and asymptotic approach

Non-linear diffusion controlled particle growth problems, in which the diffusion coefficient in the matrix phase varies with composition, are examined. The growth of an expanding ellipsoid, initially of zero size is considered, limiting cases of which include planar, cylindrical and spherical growth. The theory for dendritic growth is also given.Variational principles are applied to generate numerical solutions to the full non-linear problems. Explicit formulae, relating to fast and slow growth are derived using perturbation techniques, and obtained for a general class of diffusion coefficient, dependent upon composition. The latter serve as a check on the numerical results. In addition, for situations in which a constant diffusion analysis is used as a substitute to the non-linear problem (by assuming the diffusion coefficient to be some weighted average of the variable one), the asymptotic expressions indicate which is the best average to adopt. It is found that for fast ellipsoidal growth (including planar, cylindrical and spherical growth) and for dendritic growth, to a first approximation, the most appropriate average to use for the constant diffusion analysis is when D(C) is replaced by FAV=∫CxCMD(C)dC(CM−Cx) where CM, C∞ denote the concentrations at the particle-matrix interface and at infinity respectively. A similar conclusion obtains for slow spherical growth, although no such simple formulae exist for planar and cylindrical growth in this limit.The theory is also applied to a planar growth problem in which the diffusion profile is discontinuous, and the exact solution is used to judge the accuracy associated with the variational theory.

Sol-gel processing of inorganic membranes

Basic principles involved in sol-gel processing of ceramic membranes are described. This process has been applied to ceramic ultrafiltration membranes and is now investigated to prepare ceramic nanofilters. In this paper special emphasis is put on new developments concerning microporous zirconia membranes obtained by the polymeric route. A zirconium alkoxide precursor modified with an acetylacetone ligand has been used in order to control particle growth in the sols and pore size distribution in the membranes. N2 adsorption and X-ray diffraction analysis have been performed on membrane materials showing the influence of process parameters (molar ratio r = acacH/Zr and sintering temperature T) on membrane structural evolution.

EFFECT OF PVA AND GELATIN ADDITIVES ON BARIUM SULFATE PRECIPITATION IN AN MSMPR REACTOR

Abstract The effect of high molecular weight additives (PVA and gelatin) on barium sulfate precipitation in an MSMPR reactor is investigated. The impeller speed is varied from 0 to 1200 rpm and the additive concentration in the bulk solution is increased up to 5.0 g/1. As the additive concentration is increased, the particle growth rate decreases and the nucleation rate increases. However, the particle morphology is not changed by the additives. The experimental results are explained qualitatively by supposing that the additive is adsorbed on the particle surface and inhibits the mass transfer to the surface. To explain the additive effect quantitatively a diffusion limitation model is developed. The diffusion limitation model predicts the effects of additives over the entire range of additive concentrations and impeller speeds set in the experiments. The model predicts very high supersaturation levels in the reactor which is consistent with a mass transfer controlled particle growth. Furthermore, the mod...

Preparing Monodisperse Metal Powders in Micrometer and Submicrometer Sizes by the Polyol Process

One of the newer tendencies in materials science has been to tailor-make classical products (long associated with old applications) with controlled properties for special uses, especially in high technology. Preparing dispersed systems in which all particles have nearly uniform size (monodisperse solids) is a typical example. This goal can be achieved in some cases through cleverly controlled particle growth from a liquid medium. Examples of such preparations include gold colloids prepared by Zsigmondy and later by Turkevich et al., sulfur sols obtained by LaMer, metal oxides and hydrous oxides prepared by Matijević et al., silica, etc. These dispersions have been used either to check theories of colloid science, or to a lesser extent, for industrial purposes. In the case of fine metal particles, a uniform size distribution associated with a low degree of agglomeration, and sometimes the spherical shape, appear as particularly convenient characteristics for certain applications. The production of conductive inks or pastes for electronic materials and for the preparation of conductive paints are particularly good examples.In so-called thick film technology, conductive inks and pastes are screen printed on a ceramic substrate in order to form, after firing, a conductive film with a thickness less than 10 μm. This technique is, for instance, used to form the network in hybrid integrated circuits or the internal electrodes of multilayer ceramic capacitors.Metallic powders in thick film compositions are usually precious metals (Au, Ag, Pt, Pd), their mixtures, or alloys. Cheaper metals such as copper or nickel are tested and may be potential substitutes for precious metals in different specific applications. Powders for thick film composition are mainly obtained through chemical precipitation from aqueous or organic solutions, which yield high purity powders. Modification of precipitation parameters (such as the nature and the concentration of the starting metallic compound and of the reducing agent, reaction temperature, viscosity of the medium) and the addition of additives and surfactants, can often be used to control particle size and agglomeration.Over the past few years, we have developed a new process for preparing finely divided metal powders of easily reducible metals (such as precious metals and copper) or less reducible metals (such as cobalt, nickel, cadmium, or lead) by precipitation in liquid polyols. This reaction will be used as an example in order to discuss the mechanism of formation of uniform micrometer and submicrometer size metal particles by precipitation reactions.

Ti(CN) precipitation in microalloyed austenite during stress relaxation

Stress relaxation tests were carried out on a 0.05 pct C-0.007 pct N-0.25 pct Ti steel preheated to 1260 °C and then held in the temperature range 900 to 1050 °C. The precipitation of Ti(CN) that took place was followed by extraction replication. Dense precipitates were observed when stress plateaus appeared on the relaxation curves. The cube-shaped Ti(CN) precipitates were heterogeneously distributed in either a chain-like or a cell-like manner, indicating that the precipitates were nucleated on dislocations or on dislocation substructures. The changes in the size distribution of the precipitates during relaxation were also followed and analyzed in terms of the Zener diffusion controlled particle growth theory. The results suggest that Ti(CN) precipitation under the present experimental conditions proceeds in three sequential stages: (i) nucleation and growth according to a parabolic law; (ii) nucleation site saturation accompanied by growth alone; and (iii) Ostwald ripening. The time dependence of the mean Ti(CN) particle size is nonlinear during the first, and becomes linear during the second stage.

Monodisperse colloidal systems, fascinating and useful

Monodisperse colloidal systems may form spontaneously (protein solutions, micellar solutions, micro-emulsions) or be obtained by fractionation (Perrin's suspensions of gamboge) or by cleverly controlled particle growth (Au, S, latex). The controlled particle growth consists of a nucleation or seeding phase and a growth phase in which the size distribution is narrowed. The size and size distribution can be checked by a variety of techniques, such as centrifugation, light scattering, electronmicroscopy. The size distribution of micelles and micro-emulsion droplets is discussed on the basis of the thermodynamics of these systems. Monodisperse systems have many applications, most of them in checking theories of fundamental aspects of colloid science, but also several in industrial applications.