Fine Particles: Science and Technology
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sily seen that the dominance of thèse contributions dépends on the géométrie factor. The above équation, expressed per unit volume, reads (£.o.)„ = e, + y{AIV) Since ex and y are intrinsic properties of a material, the only variable quantity is A/V. If this ratio is large, the second term becomes significant (e.g., A/V —104 — 107 cm'1). The properties of matter are greatly affected by the surface energy contribution to the total energy of the System. It is obvious that the large value of AIV can be achieved by having one, two, or three dimensions rather small, resulting in films, fibers, and fine particles. This séries of articles concerns itself only with the last case, where the above cited AIV range corresponds to particle sizes of a few nanometers to a few micrometers. Monodispersed Systems In reading the articles to follow, note the consistent emphasis on the uniformity of particles. Indeed, a major effort is currently being placed on developing techniques to prépare dispersions of powders of différent shapes and chemical composition, consisting of particles of narrow size distribution. This trend has been promoted for both académie and application reasons. Scientists engaged in colloid research hâve been fascinated with "monodispersed" Systems ever since this state of matter was recognized last century. Faraday prepared the now famous gold sols of différent color as early as 1857 and understood their particulate nature. The synthesis of many différent uniform dispersions of éléments (sulfur, sélénium, silver, etc.) or compounds (silica, tungstic acid, barium sulfate,
etc.) has been reported in the literature over the years. Ail thèse préparations were based on the "trial and error," with no common underlying scientific principles. The only exceptions were polymer colloids of différent chemical compositions, commonly known as latexes, which hâve been produced in large quantifies as exceedingly uniform spherical particles by emulsion or dispersion polymerizations. In view of the interest in such Systems, much effort has been invested in developing processes and techniques that would yield well-defined colloidal dispersions. The emphasis in thèse, more récent, studies has been on certain scientific principles which include physical, chemical, or combined physicalchemical approaches. As a resuit, a large library of inorganic or organic materials is now available, which consists of amorphous or crystalline, simple or composite particles of différent shapes, of narrow size distribution with modal sizes in the colloidal range. Some récent review articles describe and refer to many such Systems.1'5 Dispersions of uniform particles hâve been made of metals, of métal (hydrous) oxides, sulfides, selenides, phosphates, carbonates, etc., and of nonmetallic compounds. Some of thèse fine particles may hâve more than one anion or cation in their structure. It is also possible to generate uniform coated particles, in which the core and the surface layers differ in chemical composition. T. Sugimoto's article reviews some of the
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