Particle shape, characterization

RFS analysis is not used by itself, but as a second test that expands the particle detection capability of spectroscopy to provide a comprehensive tool to direct the laboratory to identify and solve a maintenance problem. Frequently, the RFS analysis is also used as a screening tool to determine if additional tests, which are too time-consuming or expensive to perform on all samples, should be performed. Supplementary techniques such as analytical ferrography or particle shape characterization and particle counting are then applied to verify a severe wear condition. 

Heffels, C. M. G., Heitzmann, D., Hirleman, E. D., Scarlett, B., The Use of Azimuthal Intensity Variation in Diffraction Patterns for Particle Shape Characterization, Part. Part. Syst. Charact, 1994, 11, 194-199. 

Particle Size. Wet sieve analyses are commonly used in the 20 )J.m (using microsieves) to 150 )J.m size range. Sizes in the 1—10 )J.m range are analyzed by light-transmission Hquid-phase sedimentation, laser beam diffraction, or potentiometric variation methods. Electron microscopy is the only rehable procedure for characterizing submicrometer particles. Scanning electron microscopy is useful for characterizing particle shape, and the relation of particle shape to slurry stabiUty. 

The characteristics of a powder that determine its apparent density are rather complex, but some general statements with respect to powder variables and their effect on the density of the loose powder can be made. (/) The smaller the particles, the greater the specific surface area of the powder. This increases the friction between the particles and lowers the apparent density but enhances the rate of sintering. (2) Powders having very irregular-shaped particles are usually characterized by a lower apparent density than more regular or spherical ones. This is shown in Table 4 for three different types of copper powders having identical particle size distribution but different particle shape. These data illustrate the decisive influence of particle shape on apparent density. (J) In any mixture of coarse and fine powder particles, an optimum mixture results in maximum apparent density. This optimum mixture is reached when the fine particles fill the voids between the coarse particles. 

Microscopy (qv) is appHed when particle identification and, perhaps, shape evaluation ate important in addition to size. Shape characterization is used in the abrasives (qv) industries, pollution or contamination assessment, and forensic studies (see Forensic CHEMISTRY). 

Characterization of the particle shape is generally described by the deviation from sphericity, as in the case of ellipsoids where the ratio of the two radii is the measure of deviation. The surface and volume are important properties aflected by the overall shape of a particle. A more complicated relationship for particle characterization was described by Heywood, who introduced shape coefficients such as surface and volume coefficients and elongation and flatness ratios [42]. 

In the course of further work on characterizing the MCC sols it was found that the CCC of a variety of salts varied both with the solids content and temperature. Investigation of these parameters forms the basis of the study. It will be shown that as a result of particle shape, concentration and surface characteristics, coagulation leads to a gel-like structure. On further addition of salt the coagulated gel-like structure aggregates into floes that are irreversible. In this paper, we outline the experimental parameters which lead to these phenomena and present some possible explanations. 

The studies discussed above deal with highly dispersed and therefore well-defined rhodium particles with which fundamental questions on particle shape, chemisorption and metal-support interactions can be addressed. Practical rhodium catalysts, for example those used in the three-way catalyst for reduction of NO by CO, have significantly larger particle sizes, however. In fact, large rhodium particles with diameters above 10 nm are much more active for the NO+CO reaction than the particles we discussed here, because of the large ensembles of Rh surface atoms needed for this reaction [28]. Such particles have also been extensively characterized with spectroscopic techniques and electron microscopy we mention in particular the work of Wong and McCabe [29] and Burkhardt and Schmidt [30], These studies deal with the materials science of rhodium catalysts that are closer to the ones used in practice, which is of great interest from an industrial point of view. 

In the present paper non-conventional TEM methods to characterize small metallic particles are presented. The topographic information on the particles shape can be combined with micro-diffraction (using STEM) data to obtain a full characterization of the particle. The case of gold particles evaporated on a NaCl substrate is used as example. The particle shapes observed are discussed. It is shown that many particles have a crystal structure which is different from the bulk (Fee). 

In the present section we will present the shape characterization of different types of gold particles which are present on an evaporated film grown on a NaCl substrate. This is an interest model not only for epitaxy studies but also because the same shapes are found in real catalyst. 

Automatic techniques for characterizing particle shape without operator error are also under development, based primarily on fiber optics with automatic signal processing. Kaye (Kl) has given a useful review of recent developments. 

It is no wonder that the particles are spherical but crystalline, if one considers the formation mechanism. The rather smooth surface of the spherical magnetite may be due to the rapid contact recrystallization of the constituent primary particles (5), forming the rigid polycrystalline structure. Flowever, it must be noted that polycrystalline spheres are also prepared by normal deposition of monomeric solute, as shown in the formation of the uniform spherical polycrystalline particles of metal sulfides in Chapters 3.1-3.3. Thus, while we may be able to predict the final particle shape and structure from the formation mechanism, it is risky to conclude the formation mechanism only from characterization of the product. As a rule, scrupulous analyses are needed for concluding the growth mechanism in a particle system. 

Numerous techniques have been applied for the characterization of StOber silica particles. The primary characterization is with respect to particle size, and mostly transmission electron microscopy has been used to determine the size distribution as well as shape and any kind of aggregation behavior. Figure 2.1.7 shows a typical example. As is obvious from the micrograph, the StOber silica particles attract a great deal of attention due to their extreme uniformity. The spread (standard distribution) of the particle size distribution (number) can be as small as 1%. For particle sizes below SO nm the particle size distribution becomes wider and the particle shape is not as perfectly spherical as for all larger particles. Recently, high-resolution transmission electron microscopy (TEM) has also revealed the microporous substructure within the particles (see Fig. 2.1.8) (51), which is further discussed in the section about particle formation mechanisms. 

At low water content from vv = 2 to 5.5, a homogeneous reverse micellar solution (the L2 phase) is formed. In this range, the shape of the water droplets changes from spheres (below ir = 4) to cylinders. At tv — 4, the gyration radius has been determined by SAXS and found equal to 4 nm. Syntheses in isolated water-in-oil droplets show formation of a relatively small amount of copper metallic particles. Most of the particles are spherical (87%) with a low percentage (13%) of cylinders. The average size of spherical particles is characterized by a diameter of 12 nm with a size polydispersity of 14%. 

All these principles are easily found in animals and vegetables, but not so easily in minerals. Fire analysis does not separate gold and silver into their principles at all, yet he believes that these metals contain them. Lemery suggests that in these metals the principles are so tightly bound that they cannot be separated except by breaking their Figures by which the principles are themselves characterized. Lemery does not say what would result if indeed the particle shapes of the principles were broken. ... 

Single-bilayer phosphatidylcholine (14) vesicles Magnetic particles prepared in situ from Fe2+/Fe3+ by OH Particles were characterized by transmission electron microscopy, electron diffraction, and X-ray microanalysis morphologies of intravesicular particles (spherical or disk-shaped) differed from those precipitated in bulk (acircular) 791... 

Data collected for each run included acid analysis using inductively coupled plasma (ICF) to determine cation concentration and titration to determine H concentration. Filtering characteristics were determined using solid and filtrate yield rates, as well as back pressures during the filtration cycle. The filter cake was characterized by moisture content and composition. Solid samples were analyzed with scanning electron microscopy (SEM) to determine changes in particle shape and size under various process conditions, and X-ray diffraction (XRD) was used to determine the solids composition. 

This is liquid-solid chromatography in which the surface of microparticulate silica or other adsorbent constitutes the polar stationary phase. The silica particles are characterized by their shape (irregular or spherical), size and size distribution, and pore structure (mean pore diameter,... 

The performance of a catalyst is well known to be sensitive to its preparation procedure. For this reason, ideally an oxide-supported metal catalyst should be subjected to a number of characterization procedures. These may include measurements of the metal loading within the overall catalyst (usually expressed in wt%), the degree of metal dispersion (the proportion of metal atoms in the particle surfaces), the mean value and the distribution of metal particle diameters, and qualitative assessments of morphology including the particle shapes and evidence for crystallinity. These properties in turn can depend on experimental variables used in the preparation, such as the choice and amounts of originating metal salts, prereduction, calcination or oxygen treatments, and the temperature and duration of hydrogen reduction procedures. 

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