Amorphous particle size

In contrast to the material seen in the previous figures. Fig. 5 shows untransformed preceramic particles which were collected on a cold substrate. In this TEM, the preceramic particles are supported on a carbon film. This micrograph shows the transparency of the individual particles and the dark contrast that arises firom particle overlapping. Electron diffiaction reveals the particles to be entirely amorphous. Particle size distribution analysis using TEM, indicates that the preceramic particles have a mean diameter of 65 nm. The ceramic powder, obtained by subsequent thermal treatment has a similar particle size distribution. 

The precursor glass powders may be produced by various methods, the simplest being the milling of quenched glass to an average particle size of 3—15 p.m. Sol gel processes, in which highly uniform, ultrafine amorphous particles are grown in a chemical solution, may be preferable for certain apphcations. 

Texture. All limestones are crystalline, but there is tremendous variance in the size, uniformity, and arrangement of their crystal lattices. The crystals of the minerals calcite, magnesite, and dolomite are rhombohedral those of aragonite are orthorhombic. The crystals of chalk and of most quick and hydrated limes are so minute that these products appear amorphous, but high powered microscopy proves them to be cryptocrystalline. Hydrated lime is invariably a white, fluffy powder of micrometer and submicrometer particle size. Commercial quicklime is used in lump, pebble, ground, and pulverized forms. 

Properties. CoUoidal siUca is a stable aqueous dispersion or sol of discrete amorphous siUca particles having diameters of 1 to 100 nm. SiUca sols do not gel or setde out of solution for at least several years of storage. Aqueous sols that contain up to 50% siUca have been developed (30,31). Particle sizes of approximately 130 nm in diameter are possible (32), but slowly settle out of solution. 

In the absence of a suitable soHd phase for deposition and in supersaturated solutions of pH values from 7 to 10, monosilicic acid polymerizes to form discrete particles. Electrostatic repulsion of the particles prevents aggregation if the concentration of electrolyte is below ca 0.2 N. The particle size that can be attained is dependent on the temperature. Particle size increases significantly with increasing temperature. For example, particles of 4—8 nm in diameter are obtained at 50—100°C, whereas particles of up to 150 nm in diameter are formed at 350°C in an autoclave. However, the size of the particles obtained in an autoclave is limited by the conversion of amorphous siUca to quartz at high temperatures. Particle size influences the stabiUty of the sol because particles <7 nm in diameter tend to grow spontaneously in storage, which may affect the sol properties. However, sols can be stabilized by the addition of sufficient alkaU (1,33). 

X-ray diffraction consists of the measurement of the coherent scattering of x-rays (phenomenon 4 above). X-ray diffraction is used to determine the identity of crystalline phases in a multiphase powder sample and the atomic and molecular stmctures of single crystals. It can also be used to determine stmctural details of polymers, fibers, thin films, and amorphous soflds and to study stress, texture, and particle size. 

To produce amorphous anhydrous borax, the molten borax is mn between two large water-cooled roUs, forming sheets about 1.6 mm thick, which ate then cmshed and screened to the desired particle size. Because the borax is cooled rapidly by the roUs, it remains largely amorphous, though it may contain some crystalline anhydrous borax. 

Following wet processing, fine particle size kaolins may be calcined, ie, heat treated at about 1000°C. This treatment converts the kaolin to an amorphous pigment of significantly higher brightness and opacity (8). Properties of the various types of kaolins used in paper are shown in Table 2. 

An abrasive is usually chemically inert, neither interacting with other dentifrice ingredients nor dissolving in the paste or the mouth. Substances used as dentifrice abrasives include amorphous hydrated silica, dicalcium phosphate dihydrate [7789-77-7] anhydrous dicalcium phosphate [7757-93-9] insoluble sodium metaphosphate [10361-03-2], calcium pyrophosphate [35405-51-7], a-alumina trihydrate, and calcium carbonate [471-34-1]. These materials are usually synthesized to specifications for purity, particle size, and other characteristics naturally occurring minerals are used infrequently. Sodium bicarbonate [144-55-8] and sodium chloride [7647-14-5] have also been employed as dentifrice abrasives. 

Measurements [113,368] of interfacial (contact) potentials or calculated values of the relative work functions of reactant and of solid decomposition product under conditions expected to apply during pyrolysis have been correlated with rates of reaction by Zakharov et al. [369]. There are reservations about this approach, however, since the magnitudes of work functions of substances have been shown to vary with structure and particle size especially high values have been reported for amorphous compounds [370,371]. Kabanov [351] estimates that the electrical field in the interfacial zone of contact between reactant and decomposition product may be of the order of 104 106 V cm 1. This is sufficient to bring about decomposition. 

Amorphous Si3N4 powder from silicon halides and ammonia at high temperature.P lP l The powder can also be produced by using the same reaction at 1000°C in an RF plasma with a mean particle size of 0.05-0.1... 

There is actually no sharp distinction between the crystalline and amorphous states. Each sample of a pharmaceutical solid or other organic material exhibits an X-ray diffraction pattern of a certain sharpness or diffuseness corresponding to a certain mosaic spread, a certain content of crystal defects, and a certain degree of crystallinity. When comparing the X-ray diffuseness or mosaic spread of finely divided (powdered) solids, the particle size should exceed 1 um or should be held constant. The reason is that the X-ray diffuseness increases with decreasing particle size below about 0.1 J,m until the limit of molecular dimension is reached at 1-0.1 nm (10-1 A), when the concept of the crystal with regular repetition of the unit cell ceases to be appropriate. 

The release rate and therefore the duration of action of injectable insulin, in the form of insulin zinc, is controlled by its crystallinity coupled with its particle size. The crystallinity and particle size of insulin zinc, which is precipitated as an insoluble complex when insulin is reacted with zinc chloride, is controlled by the pH. The amorphous complex of small particle size, Prompt Insulin Zinc Suspension USP, is rapidly absorbed and has a relatively short duration of action. In contrast, the crystalline complex of large particle size, Extended Insulin Zinc Suspension USP, is slowly absorbed and has a relatively long duration of action. The intermediate form, Insulin Zinc Suspension USP, consists of seven parts of the crystalline form to three parts of the amorphous form and has an intermediate rate of absorption and duration of action. 

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