Amorphous materials porous glass

Figure 9.1 Examples of texture of the materials formed on a short range from amorphous Si02 (a) silica gel (b) hydrothermally treated silica gel (c) porous glass (d) mesoporous mesophases type of MCM-41 and (e) opal.

The properties of some of the materials suitable for HPLC are summarized in Table I. It should be borne in mind that these materials, including the occasionally used porous glasses, are amorphous xerogels whose properties vary not only from brand to brand but may also vary from batch to batch, and may additionally change during storage and treatment used for purification. ... 

The amorphous (nonordered) mesoporous materials such as ordinary SiC>2 aerogel and porous glass possess mesopores, but the channels or pores are irregular and the pore sizes distribute over a wide range. Most macroporous materials such as ceramics and cement have the same characteristics irregular pores and wide pore-size distribution. 

In nature, there are large deposits of silica-rich volcanic ashes in coarse amorphous colloidal particles, some of which are fairly pure and quite readily soluble to alkali solutions. They are used often as an industrial source of silica. Impure volcanic ash which contains alumina is used as a detergent, filter aid and mild ion-exchange agent after alkali or acid treatment and as a raw material for the synthesis of zeolite and porous glass. 

Another classification can be made by considering the structure-determining step in manufacturing. These include precipitation (amorphous silica), melting (porous glass), and hydrothermal crystallization (layered materials, zeolites). The... 

We measured the temperature-dependance of the spin-lattice relaxation time, for various alumino-silicate aerogels, corresponding porous glasses and crystalline counterparts. The purpose of these experiments is threefold (i) to compare the relaxation response of these very porous amorphous materials to the general one of more classical glasses, (ii) to see whether fractons, whose vibrationnal amplitudes are large, contribute to relaxation mechanisms, (iii) to follow - through variations of the density - the dependance of this dynamical property on the structural parameters, (iv) to test the theoretical predictions about relaxation in disordered systems proposed by R. Orbach and S. Alexander. 

The temperature-dependance of the nuclear-spin relaxation in our very porous amorphous materials is qualitatively the same as in most of the explored classical glasses. . s. In the latters, the relaxation rates behave as T o+ ), with a lying between 0.1 and 0.5. As showed by previous studies, the faster ionic conductor the glass is, the faster the relaxation. When compared to non ionic-conducting amorphous materials, our oxydes present particularly short relaxation times. 

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