Silicates structural properties

N.G. Vasyliev and V.V. Gonacharuk, The Natural Silicates Structure, Properties and the Reaction Ability, Naukova dumka, Kiev, 1992 (in Russian). 

J 8 Distinguish the principal silicate structures and describe their properties. 

As Ti is incorporated in the silicate lattice, the volume of the unit cell expands (consistent with the flexible geometry of the ZSM-5 lattice) (75), but beyond a certain limit, it cannot expand further, and Ti is ejected from the framework, forming extraframework Ti species. Although no theoretical value exists for such a maximum limit in such small crystals, it depends on the type of silicate structure (MFI, beta, MCM, mordenite, Y, etc.) and the extent of defects therein, the latter depending to a limited extent on the preparation procedure. Because of the metastable positions of Ti ions in such locations, they can expand their geometry and coordination number when required (for example, in the presence of adsorbates such as H20, NH3, H2O2, etc.). Such an expansion in coordination number has, indeed, been observed recently (see Section II.B.2). The strain imposed on such 5- and 6-fold coordinated Ti ions by the demand of the framework for four bonds with tetrahedral orientation may possibly account for their remarkable catalytic properties. In fact, the protein moiety in certain metalloproteins imposes such a strain on the active metal center leading to their extraordinary catalytic properties (76). 

As a function of their structural properties, clays interact differently with organic and inorganic contaminants. Two major groups of clay minerals are selected for discussion here (a) kaolinite, with a 1 1 layered structured aluminosilicate and a surface area ranging from 6 to 39 m g" (Schofield and Samson 1954) and (b) smectites with a 2 1 silicate layer and a total surface area of about 800m g" (Borchardt 1989). 

The dynamic mechanical thermal analyzer (DMTA) is an important tool for studying the structure-property relationships in polymer nanocomposites. DMTA essentially probes the relaxations in polymers, thereby providing a method to understand the mechanical behavior and the molecular structure of these materials under various conditions of stress and temperature. The dynamics of polymer chain relaxation or molecular mobility of polymer main chains and side chains is one of the factors that determine the viscoelastic properties of polymeric macromolecules. The temperature dependence of molecular mobility is characterized by different transitions in which a certain mode of chain motion occurs. A reduction of the tan 8 peak height, a shift of the peak position to higher temperatures, an extra hump or peak in the tan 8 curve above the glass transition temperature (Tg), and a relatively high value of the storage modulus often are reported in support of the dispersion process of the layered silicate. 

Previous characterisation (SEM, HREM) of the purely siliceous composite materials [4] revealed the formation of fairly complex aggregates of MFI and MCM-41 type material. The data were not sufficient to assess a more intimate integration of the two structures, although the enhanced hydrothermal stability obtained [5] would indicate the presence of a closer interaction between the two phases than just a physical mixture. The actual structure of the present samples has not been studied in further detail so far. Taking into account the considerable potential within catalytic applications for structures which may possess unique integrations of acidity and structural properties, the synthesis, modification, catalytic activity and structure of these materials should be studied closer. 

Provided in this chapter is an overview on the fundamentals of polymer nanocomposites, including structure, properties, and surface treatment of the nanoadditives, design of the modifiers, modification of the nanoadditives and structure of modified nanoadditives, synthesis and struc-ture/morphology of the polymer nanocomposites, and the effect of nanoadditives on thermal and fire performance of the matrix polymers and mechanism. Trends for the study of polymer nanocomposites are also provided. This covers all kinds of inorganic nanoadditives, but the primary focus is on clays (particularly on the silicate clays and the layered double hydroxides) and carbon nanotubes. The reader who needs to have more detailed information and/or a better picture about nanoadditives and their influence on the matrix polymers, particularly on the thermal and fire performance, may peruse some key reviews, books, and papers in this area, which are listed at the end of the chapter. 

In this chapter, an overview of the fundamentals of PNs is described, according to the author s understanding and experience as well as support from numerous references and review articles. The content of this chapter covers all kinds of inorganic nanoadditives, but, because the most widely investigated and thus understood nanoadditives used to enhance the thermal and fire resistance of the polymers are clays (natural or synthetic) followed by the CNTs and colloidal particles, the focus of the chapter is primarily on clays, particularly on the silicate clays and LDHs, as well as the CNTs. This includes structure, properties, and surface treatment of the nanoadditives, design of the modifiers, synthesis, characterization of the structure/morphology, and thermal and fire... 

Talc is the magnesium silicate structural analog to pyrophyllite. Its properties are nearly identical to pyrophyllite, except that Al3+ cations have been replaced by Mg2+ cations [25], Talc occurs in secondary deposits and is formed by the weathering of magnesium silicate minerals such as olivine and pyroxene [2], In bulk form, talc is also called soapstone and steatite. A typical composition for talc is given in Table 7 [22], Historically, talc has been used extensively in electrical insulator applications, in paints, and as talcum powder [2],... 

Layered aluminosilicates are the most important secondary minerals in the clay fraction of soils. When layer silicate minerals are clay or colloidal size (<2 gm effective diameter), their large surface area greatly influences soil properties. Most of the important clay minerals have similar silicate structures. Inasmuch as clay minerals are such important clay components, and as different clay minerals can change sail properties greatly, an understanding of soil properties begins with an understanding of silicate structures. 

The increasingly indispensable role of atomistic and quantum mechanical simulations in inorganic crystallography is perhaps no more strikingly illustrated than in the field of silicates and zeolitic materials. The two classes of material with which we shall be concerned in this chapter, namely microporous zeolites (including both aluminosilicate-based materials and their sister compounds, the aluminophosphates (ALPOs)), and the dense silicate materials and related oxides which constitute the bulk of the Earth s mantle, have in common the fact that their structural properties may be difficult to determine by conventional experimental means. Yet highly detailed and accurate structural information is critical in understanding the properties of both these important types of material. 

Silicates in Solutions. The distribution of silicate species in aqueous sodium silicate solutions has long been of interest because of the wide variations in properties that these solutions exhibit with different moduli (23—25). Early work led to a dual-nature description of silicates as solutions composed of hydroxide ions, sodium ions, colloidal silicic acid, and so-called crystalloidal silica (26). Crystalloidal silica was assumed to be analogous to the simple species then thought to be the components of crystalline silicate compounds. These include charged aggregates of unit silicate structures and silica... 

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