Size-characterization studies

Elutriators for Size-Characterization Studies and Fine-Powder Fractionation... 

Size characterization studies have often evolved in parallel in many of these industries and sometimes there is vocabulary confusion because of the different perspectives of scientists from the various industries. We will attempt to develop and use a consistent terminology as we study the muldmdes of powders used in various industries. 

Dispersing Powder Samples for Size Characterization Studies... 

In horizontal elutriators a stream of suspension is passed over a reservoir of clear fluid. The larger fineparticles leave the suspension stream and are collected at the bottom of the settling chamber. The fractionation efficiency of this device is not good since some fineparticles of all sizes will leave the suspension stream as it moves over the clear fluid in the collection chamber. The solids concentration of the feed stream must be kept low, otherwise, bulk penetration by the feed layer into the fluid in the container will occur (see discussion of the stability of the line start suspension system in Chapter 4). It is possible to use a more dense fluid in the reservoir to improve the stability of the system but this system has not been used in size characterization studies. Fineparticles that drop through the full height of the fluid stream in the time taken for the fluid stream to move across the elutriating chamber are completely removed from the feed stream. 

The NiO phase could be reduced to metallic Ni by hydrogen treatment (723 K, 1 h). The crystallite size of Ni metal (estimated from XRD pattern of the reduced sample) is similar to that of the NiO in the unreduced sample (e.g., for 7B, the Ni metal crystallite size is 8.2 nm). The textural characterization studies reveal that for the samples prepared by coprecipitation + digestion and hydrothermal synthesis (methods B and C) the pore sizes are in the mesopore range (2.9 to 6.8 nm) (Table 11.2). 

Transmission electron microscopy (TEM) has been an underutilized yet valuable too in particle size characterization of MC particles in LB films. Monolayer films of trioctylphosphine oxide-capped CdSe (18), spread as a monolayer on an aqueous subphase, were transferred to a TEM grid. A close-packed hexagonal arrangement of 5.3-nm (cr —4%) crystallites was found. TEM images were also obtained for HMP-stabilized CdS incorporated in BeH/octadecylamine films (79) and for CdS formed under an amine-based surfactant monolayer and transferred to a TEM grid (14). In one study, direct viewing of CdS and CdSe particles made from Cd2+-FA films on TEM grids was not possible due to poor phase contrast between the particles and the film (30). Diffraction patterns were observed, however, that were consistent with crystalline (3-CdS or CdSe. Approximately spherical particles of CdSe could... 

Each of these considerations must be resolved before a personal monitor can be applied to air pollution research and characterization studies. Some advances have been made for carbon monoxide, volatile organic compounds, acid aerosols, and particulate matter (PM-10 and RSP these represent the masses of all particles collected in samplers with 50% cut sizes of 10 and 25 xm, respectively) and its components (12, 22-31). Each advance is still undergoing development, and further advances can be anticipated for these as well as for other pollutants. The next generation of monitors will probably include devices for some pollutants that incorporate the use of microsensors. Currently, microsensors are being examined for detection of nitrogen dioxide and ozone, but the range of sensors available suggests that they can be used for a number of compounds (10, 32) ... 

While the simple process of comparing fractograms with appended particle diameter scales will provide all the information needed in many particle characterization studies, the fractogram does not constitute a finished particle size distribution (PSD) curve. To obtain a quantitative PSD curve, corrections must be applied. The first correction, which we call a scale correction, is necessary because elution volume and particle diameter are not proportional to one another. Thus the particle content of a fixed volume of eluted sample will correspond to a different increment in d depending upon the point of collection. A simple correction factor, detailed elsewhere (3), can be applied to this scale problem. 

It is concluded from the characterization studies, that the extraction technique which we have employed for the preparation of zeolite-supported iron catalysts results in the formation of highly dispersed, small particle-sized Y-Fe2°3 on the support surface and, in addition, a small amount ( 1%) of iron present as a spe-... 

Recently, petroleum residua have been studied extensively (I, 2) because of the increasing importance of heavier fuels. Both the asphaltene (pentane-insoluble) and maltene (pentane-soluble) components of residua are of interest, and since their properties overlap, a complete study of petroleum residua must consider both asphaltenes and maltenes. One area that has received considerable attention has been the size characterization of asphaltenes and maltenes (3, 4, 5). Size distribution data are useful both in understanding the fundamental chemistry of asphaltenes and maltenes and in observing the effects of various processes on residua sizes. 

Size characterization measurements have provided useful information on the importance of the hydroprocessing catalyst pore size distribution and on the effects of visbreaking and hydroprocessing on the residua molecular size distributions. It is apparent that asphaltenes and maltenes are not unique entities, but instead have considerable overlap in their size distributions. A complete study of the effects of processing conditions would require consideration of all components of a residuum. 

In addition to structural analysis and purity evaluation, Raman spectroscopy can also be used to estimate the crystal size of nanostructured solids. In most cases size characterization using Raman spectroscopy is based on the phonon confinement model (PCM), which uses changes in Raman frequency and Raman peak shape to estimate the crystal size. Although several attempts have been made to relate confinement-induced changes in the Raman spectrum of ND to the crystal size, the agreement between calculated and experimental data and the accuracy of the fitting procedure are still unsatisfactory. A detailed discussion of the limitations of the PCM and the accuracy of previous studies on ND powders is given in Ref [86]. 

The results of fractionation and characterization studies of naturally occurring organo-clay complexes in a B t-horizon of a black solonetz silty clay loam were reported by Arshad and Lowe (1966). This soil was predominantly montmorillonitic with relatively large amounts of kaolinite in the coarse fraction and only traces in the fine clay. The data indicated that more organic matter was associated with the coarse clay fraction than with the fine clay. The percentage of extractable organic matter increased with decrease in particle size. There was no evidence of adsorption of organic matter in the clay lattice. As... 

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