Crushed Powders

The more rapidly dissolving layer on the outside of finely crushed quartz, tridymite. cristobalite, and vitreous silica was examined by Bergman and Paterson (220) by 

The dissolution rate of amorphous silica in 0.01 /V NaOH solution has been measured at 23-88 C by a rotating disk method by Anatskii and Ratinov (221, 222). 

De Keyser and Wollast (223a) have been able to use the constant of the rate of dissolution of a solid, obtained by the rotating disk method, to calculate the rate of solution of powders of which the particle size has been measured by the cumulative sedimentation method. 

The nature of the disturbed layer on the surface of crushed quartz was described at the beginning of the section on solubility, and its abnormal solubility is discussed in Chapter 7 in relation to a theory of silicosis. When crushed powders are to be used for solubility measurements it is essential that the highly soluble outer layers and fine fractions be eliminated. The problems involved in obtaining quartz with a reproducible dissolution behavior have been described by Baumann (223b), who found that subsequent adsorption of silicic acid from solution or heating the cleaned powder above 800 C can reestablish a layer which is abnormally soluble. 

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Zerstdrung,/, destruction overthrow, ruin. Zerstorungsbombe, /. demoUtion bomb, zerstdrungsfrei, a. nondestructive, zerstossen, v.t. poimd (to pieces), bray, bruise, crush, powder, pulverize. 

Rock samples were collected from measured sections. Weathered samples were avoided as far as possible, and weathered crusts were later removed in the laboratory. The samples were thoroughly cleaned, treated with chromic acid, crushed, powdered in porcelain ball mills, and stored in glass jars. 

Special attention should be given to the photoacoustic signal generation in powders. Due to interstitial gas flow in and out of powders, an important increase of the signal is seen when going from a homogeneous bulk form to that of a crushed powder. 

Monometallic catalysts on alumina supports are ideal cases for X-ray fluorescence analysis the matrix (alumina or alumina containing approximately 2% weight chlorine) remains " constant in terms of the matrix effects, and analysis can be conducted directly on finely crushed powder. Texture or grain effects must, however, be carefully avoided by using, in the preparation of standards, the same support as that of the catalysts. Any change in the nature of the support (granulometry, structural modifications, etc.) should be systematically indicated by the person requesting the analysis. 

Reactant particle size distributions Kinetic characteristics of some reactions of solids depend sensitively on reactant particle sizes (29). Ideally, reactants to be used in kinetic studies should be composed of crystallites of identical (known) sizes and shapes, to which the geometry of interface advance can be related quantitatively. This is not, however, always (or easily) achieved in experimental studies, and most powder samples contain particles of disparate sizes for example, sometimes crystals are mixed with fine powder. The kinetic model giving the best apparent fit to the data then may not accurately represent the reaction. Dependencies of rate on particle size are only rarely investigated. The state of subdivision of a solid reactant is most frequently described in literature reports only by qualitative terms, such as single crystals or crushed powder. 

In addition small-scale exposures at 870"C were used to study further the role of water vapour with 20 vol.-% and 30 vol.-% water vapour in air in an alumina tube furnace. Phase morphology was studied with scanning electron microscope (SEM). To distinguish between the glassy and crystalline phases some samples were etched in 2-% HF-solution. Phase composition was determined by a quantitative X-ray diffraction (XRD) analysis based on the internal standard, Cap2, method. The chemical composition of the amorphous phase in the binder was determined locally with EDS analysis in SEM. A bulk chemical analysis of the amorphous phases in binder was performed by Induction Coupled Plasma Mass-Spectrometry (ICP-MS). The samples were crushed powders of as-received and 500 h at water vapour bulk exposed (w) materials. The glassy phase was dissolved in HF-HNO3 solution. Open porosity, apparent and bulk densities of each specimen were determined by Archimedes method before and after the tests. 

Table I. Extraction of Europium from the Crushed Powder of Fluorescence...

External bacterial or fungal infections Use herb as a finely crushed powder, liberally sprinkled on the site of infection as frequently as needed. 

Formulating Considerations Can tolerate temperatures as high as KKi C for 5 hr Form Supplied Milky white, crushed powder Microbial Considerations Standard for proteins Supplier Croda... 

In transmission EELS the specimens are confined to very thin slices of the material or thin edges of crushed powders. In the case of rare earths this restricts samples to evaporated thin films. Due to the high reactivity of the rare earths, oxide, hydroxide or hydride phases pose serious problems in the conventional vacua of many transmission spectrometers. More flexibility in the nature of samples is possible in reflection EELS. Evaporated thin films, foils or massive bulk samples can all be readily investigated, but the sample surfaces have to be cleaned thoroughly by often tedious procedures. Some surface cleaning and preparation techniques of rare earth materials have been compiled by Netzer and Bertel (1982), and more recently by Netzer and Matthew (1986). 

Fitter, wash, dry, then crush powder and pellet. 

XRD was performed on the compaction surface, cross section and crushed powder of the specimen. The XRD profiles shown in Fig. 14.15(b) reveal the (200) reflection that does not appear in the pristine Cgo phase. The relatively strong intensity of the (220) reflection may be due to the <110> preferred orientation along the compact direction, probably caused by the (111) <110> slip system that allows considerable deformation of the Cgo crystals. The XRD chart in Fig. 14.15(b) indicates that the matrix phase can be indexed as the fee structure with a lattice constant a = 1.31 nm. The lattice constant decreases by approximately 8% relative to that of the pristine Cgo (a = 1.42 nm) and is approximately equal to the minimum value (a 1.32nm) reported by Takahashi et al. In this specimen, an endothermic peak at approximately 563 K has been confirmed by differential thermal analysis. These results suggest that the h-Cgo phase, a Cgo phase that is subjected to high pressure, can be regarded as a polymerised Cgo phase. 

X-ray diffraction chart obtained using monochromated Cu ka for (a) pristine Cso, (b) compaction surface, (c) cross section and (d) crushed powder. 

In thermal power stations used for electricity generation, the coal fuel is carried by an air stream into the furnace combustion zone in the form of a finely crushed powder. The heat released by carbon oxidation converts the water into steam, which then drives turbines that are used to generate the electricity. Various processes take place to remove pollutants from the flue gases before they enter the atmosphere. 

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