Amorphous particles

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. 

The thermal imidization of a polyamic acid film (PMDA-ODA or BPDA-ODA) obtained by casting an NMP solution leads to an amorphous polyimide. Two different teams have shown that a polyamic acid solutions in NMP heated at 200°C for a short time (20 min) gives polyimide particles fully cyclized and highly crystalline, as shown by X-ray diffraction and solid 13C NMR spectroscopy.151152 The chemical imidization of the same solution gives only amorphous particles. The difference between the cyclization of a solution and a casted film in the same solvent is intriguing. In the case of the solution, the temperature and the heating time are lower than in the case of the casted film as a consequence, a less organized structure would be expected for the particle. 

Light microscopy can be used to detect particles with regular shapes (e.g., crystals) and microbes like yeast and bacterial cells (Glenister, 1971). Microscopy can also detect some irregular particles such as diatomaceous earth or adsorbents (Glenister, 1974). It is much less informative with amorphous particle hazes. The use of specific stains can, however, provide useful information. An excellent book by Glenister (unfortunately no... 

Spherical amorphous particles of zinc oxide were prepared by hydrolyzing zinc acetate solutions in the presence of diethylene glycol (111), or zinc nitrate solutions containing triethanolamine (28). 

Analogously, colloidal tin(lV) oxide with particles of 1-2 (xm in diameter was obtained by the interaction of SnCl4 droplets with water vapor, which required considerably lower temperatures (—15 to -30°C) (62). To promote the rate of hydrolysis at these temperatures, a small amount of ammonia was added into the gas stream. The original amorphous particles started to crystallize into cassiterite on calcination at 400°C, and they retained their sphericity at treatments up to 1000°C. 

The nebulization was also employed to generate composite powders for specific applications, such as in ceramics, by hydrolyzing with water vapor droplets containing Al(5ec-OBu) and silicon methoxide in the atomic ratio Al/Si = 3. This ratio of alkoxides was chosen in order to produce mullite, which was achieved by calcination of the resulting amorphous particles at rather high temperatures (up to I400 C) (52). In another approach a mixed Al-Mg-Si ethoxide was first synthesized, and then nebulized and hydrolyzed as usual (77). Depending on the experimental conditions, the powders calcined at 500 C exhibited structures of pure cordierite, or mixed with forsterite. In all of these described cases the nebulization yielded spherical but polydisperse particles. 

In vivo formation of inorganic crystals and/or amorphous particles in biological systems. 

Unlike crystalline subsystems, one cannot understand the structure of an amorphous particle from a knowledge of the spatial organization of a small group of its constituent atoms. The reason is that the atoms are not arranged in a regular, periodic array which would enable one to define the whole space occupied by the particle by simple translational repetitions of a basic structural motif of atoms. The spatial organization of the ions comprising the amorphous material in bone mineral is, at present, completely unknown. 

In natural anoxic environments, the major alternative oxidants are iron(III) and manga-nese(IV) oxides and hydroxides. Both are common in natural systems, as crystalline or amorphous particles or coatings on other particles. In the absence of photocatalysis, however, iron and manganese oxides are weak oxidants. As a result, they appear to react at significant rates only with phenols and anilines (45, 59-64). 

Besides specific surface area, silicas are also characterised by their porosity. Most of the silica s are made out of dense spherical amorphous particles linked together in a three dimensional network, this crosslinked network building up the porosity of the silica. Where the reactivity of diborane towards the silica surface has been profoundly investigated, little attention has been paid to the effect of those reactions on the pore structure. However different methods are developed to define the porosity and physisorption measurements to characterise the porosity parameters are well established. Adsorption isotherms give the specific surface area using the BET model, while the analysis desorption hysteresis yields the pore size distribution. 

Dubovoy A.G., Perekos, A.E., Chuistov K.V. Structure and magnetic properties of small amorphous particles of metallic Fe-151.% alloy. Physics of Metals 1985 6(5) 1085-8. 

Dubovoy A.G., Zalutskiy V.P., Ignat ev I.Yu. Structure, magnetic parameters and thermal stability for small amorphous particles and amorphous strips of Fe-15 at.% B. Physics of Metals 1990 8(4) 804-7. 

Thus, nanotubes of Mo and W diselenides have been prepared by the decomposition of the triselenide or the ammonium selenometallate at elevated temperatures under a flow of H2.22 Apart from the nanotubes, nanorods of WSe2 were also obtained. Some of the nanorods were attached to an amorphous particle at the tip (Fig. 11a). Several single-walled WSe2... 

The second group includes crystalline compounds with a goethite structure. Among them there are distinguished (a-FeOOH) —hydrogoethite, formed as a result of natural aging of amorphous Fe " hydroxides due both to processes of enlargement and polymerization of amorphous particles, and... 

The SEM micrographs of the kaolin and metakaolins show plate shape particles for both types of solids (Figure 3a). Calcination of the kaolin produces its dehydroxylation but does not destroy its platy shape. For samples treated under reflux conditions the plates are dissolved and the micrographs (Fig 3b,c) show amorphous particles that coexist with the metakaolin plates. There are not many differences between the samples activated at different times, although the chemical composition indicated that the amount of Al removed by activation for 24 h was higher than during activation for 6 h. 

Because dissolution of the whole particle is not expected and because one amorphous particle is expected to transform into a crystalline particle, formation of homogeneous precursor gel is essential. On the other hand, if the real dissolution-recrystallization mechanism takes place, synthesis of homogenous gel does not have any meaning, since preferential dissolution of one component may be expected to occur in the solvothermal reaction. 

Amorphous particles appear dark under polarized light microscope... 

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