Formation Of Submicron Particles Research Articles

Formation of Calcium Carbonate Sub-Micron Particles in a High Shear Stress Three-Phase Reactor

The production of precipitated calcium carbonate (PCC) was investigated experimentally under industrially relevant conditions, i.e. at high solid concentrations and increasing amount of solid product in the slurry. Temperature is an important parameter since it determines the crystal structure, the particle shape and, as a consequence, the viscosity of the slurry. Of course, the mass concentration of the raw material also has an important influence on the viscosity. From the particle size distributions of primary particles and agglomerates, it can be concluded that the nucleation process is governed by primary nucleation. Also, heterogeneous nucleation occurs on solid calcium hydroxide particles that are present in the slurry. Especially if the raw material contains impurities heterogeneous nucleation occurs and large and unwanted particles are formed. If the slurry is not stabilized, strong agglomeration occurs that can be influenced by the shear stress introduced to the slurry: a high shear stress which is linked to the viscosity of the slurry limits the upper particle diameter and leads to a steep particle size distribution of the product.

Overview of methods for analysing single ultrafine particles

Increasing awareness that structures and attributes on a nanometre scale within aerosol particles may play a significant role in determining their behaviour has highlighted the need for suitable single ultrafine particle analysis methods. By adopting technologies developed within complementary disciplines, together with the development of aerosol–specific methods, a basis for characterizing single sub–100 nm (ultrafine) particles and features in terms of size, morphology, topology, composition, structure and physicochemical properties is established. Size, morphology and surface properties are readily characterized in the scanning transmission electron microscope (STEM), while high–resolution transmission electron microscopy (HRTEM) allows structural information on particles and atomic clusters to sub–0.2 nm resolution. Electron energy loss spectroscopy (EELS) and X–ray emission in the STEM allow the chemical analysis of particles and particle regions down to nanometre diameters. Scanning probe microscopy offers the possibility of analysing nanometre–diameter particles under ambient conditions, thus getting away from some of the constraints imposed by electron microscopy. Imaging methods such as atomic force microscopy and near–field scanning optical microscopy (NSOM) offer novel and exciting possibilities for the characterization of specific aerosols. Developments in aerosol mass spectrometry are providing the means for chemically characterizing size–segregated ultrafine particles down to 10 nm in diameter on–line. By taking a multi–disciplinary approach, the compilation and development of complementary tools allowing both routine and in–depth analysis of individual ultrafine particles is possible.

Formation of ferromagnetic submicron particles of MnAs and MnGa on GaAs substrates by ion implantation

Submicron room-temperature ferromagnets of MnAs and MnGa have been fabricated on GaAs semiconductor substrates by Mn/sup +/ ion implantation and subsequent heat treatment at 900/spl deg/C under a controlled atmosphere. MnAs and MnGa particles are formed under nitrogen gas and forming (90%N/sub 2/,10%H/sub 2/) gas atmospheres, respectively. Their structural, magneto-optical and magnetic properties are characterized by superconducting quantum interference device magnetometry, magnetic circular dichroism spectroscopy, transmission electron microscopy, and atomic and magnetic force microscopies. The MnAs precipitates grow epitaxially on GaAs substrates and show an in-plane uniaxial magnetic anisotropy.

Formation of submicron particles from the combustion of straw

Formation of submicron particles from the combustion of straw

Hydrogel Microspheres I. Preparation of Monodisperse Hydrogel Microspheres of Submicron or Micron Size

Precipitation polymerization of acrylamide derivatives in water resulted in the formation of monodisperse submicron particles. Repetitive seeded polymerizations using the product of precipitation polymerization or precipitation polymerization in ethanol–water media gave larger monodisperse particles. Especially, the latter was very effective for the preparation of micro-size particles.

Characterization and formation of submicron particles in coal-fired plants

Submicron particles resulting from pulverized coal combustion in coal-fired steam plants have been chemically characterized and the results interpreted in terms of various mechanisms for their formation. Elemental analyses using XRF, AA, and INAA techniques are reported for approximately 60 elements for flyash from two large coal-fired plants. The concentrations of elements volatilized during combustion are independent of particle size in the submicron size range, in contrast to the larger particle sizes where an inverse relationship with particle size applies. Scanning electron micrographs show the submicron particles are also much more homogeneous than larger flyash particles, while still showing evidence of some surface enrichment in ESCA studies. Various mechanisms for formation of submicron particles have been considered. The present results are most consistent with formation of submicron particles by the bursting of larger particles due to gas release during rapid heating, followed by coagulation and condensation of volatilized elements to form particles in the 0.1–1.0 μm size range. The importance of this mechanism in other coal-fired plants is probably dependent on both temperature and ash composition.

Characterization and formation of submicron particles in coal-fired plants

Characterization and formation of submicron particles in coal-fired plants