Hard-particle methods preparation

Before applying the ideas summarised in the first section to polymer latices it is appropriate to consider the nature of polymer latex particles. We know, for example, that each particle is composed of a large number of polymer chains, with the chains having molecular weights in the range of about 105 to 107. Moreover, the particles themselves can be amorphous, crystalline, rubbery, glassy or monomer swollen, either extensively or minutely. It follows, therefore, that the properties of the system on drying depends directly on the physical state of the particles, for example, if the particles are soft, coalescence can occur to form a continuous film, whereas with hard particles their individuality is retained. The nature of the particle obtained is directly related to the preparative method employed and the surface properties are often determined by s-... 

Due to the variety in porous structure, particle size and surface area, pure silica gels and powders find a very wide range of applications. Variation in preparation methods and parameters allows the tailoring of the substrate properties for specific application needs. The main features in the silica applications are its porosity, active surface, hardness, particle size and the viscous and thixotropic properties. Although most applications are based on a combination of those, a classification according to the main properties of interest may be set up. For references, the reader is referred to the works of Iler6 and Unger7 and to the references cited in chapter 8. 

Figure 11.10 A schematic representation of the hard-template method used to prepare CPCs (a) porous membrane, (b) colloidal particles, (c) nanowires...

Powder Preparation. The goal in powder preparation is to achieve a ceramic powder which yields a product satisfying specified performance standards. Examples of the most important powder preparation methods for electronic ceramics include mixing/calcination, coprecipitation from solvents, hydrothermal processing, and metal organic decomposition. The trend in powder synthesis is toward powders having particle sizes less than 1 p.m and Httie or no hard agglomerates for enhanced reactivity and uniformity. Examples of the four basic methods are presented in Table 2 for the preparation of BaTiO powder. Reviews of these synthesis techniques can be found in the Hterature (2,5). 

Roark D, Yphantis DA (1969) Ann New York Acad Sci 164 245 Rinde H (1928) The Distribuhon of the Sizes of Particles in Gold Sols Prepared According to the Nuclear Method. PhD Thesis, University of Uppsala, Uppsala, Sweden Harding SE (1988) Gums and Stabilisers for the Food Industry 4 15 http //www.wyatt.com/... 

The now classical methods used for the preparation of supported gold catalysts are hardly capable of giving particles that are both small and bimetallic, when the precursors in solution do not interact strongly with each other. During the subsequent thermal treatment performed to get metal particles, the metals must have enough mobility to migrate on the support, interact with each other, and form bimetallic particles. However, phase separation can be a common problem, especially when the metal ratio falls in the miscibility gap (Section 2.6), or if the intended composition is not thermodynamically stable. 

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