Silicon and the Silicates

The uses in the glass and ceramics industries reflect the diagonal relation between boron and silicon and the similarity of vitreous borate and silicate networks (pp. 203, 206 and 347). In the UK and continental Europe (but not in the USA or Japan) sodium perborate (p. 206) is a major constituent of washing powders since it hydrolyses to H2O2 and acts as a bleaching agent in very hot water ( 90°C) in the USA domestic washing machines rarely operate above 70°, at which temperature perborates are ineffective as bleaches. 

Silicon-containing ceramics include the oxide materials, silica and the silicates the binary compounds of silicon with non-metals, principally silicon carbide and silicon nitride silicon oxynitride and the sialons main group and transition metal silicides, and, finally, elemental silicon itself. There is a vigorous research activity throughout the world on the preparation of all of these classes of solid silicon compounds by the newer preparative techniques. In this report, we will focus on silicon carbide and silicon nitride. 

This process contrasts with the elemental-silicon processes sometimes used for alkyl silicates (8) and the elemental-silicon processes generally used for oligomeric and polymeric organosi-loxanes ( ,7) Since the silicon in these processes is obtained from quartz, these processes entail, in terms of bond cleavage, the destruction of four silicon-oxygen bonds per silicon and the subsequent reformation of the required number of such bonds. In terms of oxidation number, they entail the reduction of the silicon from four to zero and then its reoxidation back to four, Figures 2 and 3. 

Based on dissolved ions only, the titanate waste showed an overall leach rate of x 10 5 g/cm day and a rate of 5 3 x 10 7 g/cm day for the fission waste oxides only. The results indicate that the leaching which is occurring is associated with the silicate phases in the ceramic, i.e., the Si02 formed from the silicon and the zeolite. The glass samples showed overall leach rates of 6-15 x 10 5 g/cm day and fission waste oxide leach rates of 1.8-2.7 x 10 g/cm day, where the higher rates in both cases were observed in the phosphate-containing glass. 

The possibility of ligand substitution reactions in pentacoordinate silicates Sil I31 2 and Sil FfF via hexacoordinate intermediates was studied by Fujimoto, Arita and Tamao73. Attack on each of these silicates by F or hydride produced qualitatively similar reaction pathways, leading to stable hexacoordinate intermediates, without significant breaking of the bond between silicon and the leaving group. It was concluded that a nonconcerted displacement mechanism via a hexacoordinate intermediate is likely. 

Paasche, E. Silicon and the ecology of marine plankton diatoms. I. Thalassiosira pseudo-nana (Cyclotella narta) grown in a chemostat with silicate as limiting nutrient. Mar. Biol. 

Silicon is the most plentiful electropositive element on the earth s crust, being three times as abundant as aluminum and six times as abundant as iron. Yet the only compounds of silicon which have been important to human history are those natural forms of silica and the silicate minerals which are used in the building arts and in ceramic technology. Only within the past 90 years have hydrides and organic derivatives of silicon been synthesized, and the chlorides 30 years before up to a few years ago it could be said that all these substances were still relatively unknown products of the laboratory, unimportant save for their scientific interest. The chemistry and technology of silicon continued to be dominated entirely by consideration of the inorganic silicates. 

In silica and the silicates, each atom of silicon is surrounded tetrahedrally by four atoms of oxygen. This structure is in agreement with the tetra-valency of silicon and with the directional character of the four sp hybrid bonds. In such an SiO group, only one of the two valencies of oxygen... 

In silica and the silicates oxygen atoms are arranged tetrahedrally round silicon atoms. These tetrahedra may be... 

Silicon Solid-state Chemistry.—Contrary to the pattern adopted in previous volumes, the chemistry of aluminosilicates and zeolites will not be discussed here the data published during the period of this Report associated with these materials are considered in detail in Chapter 3. In this section, silicon dioxide and the silicates will be described separately emphasis will be laid on the inorganic chemistry of these compounds, and papers describing solely their catalytic, adsorption, diffusion, and other similar properties will not be considered. 

The biggest chemical differences between silicon and carbon are that (1) silicon does not form stable double bonds, (2) it does not form very stable Si—Si bonds unless the silicon atoms are bonded to very electronegative elements, and (3) it has vacant hd orbitals in its valence shell into which it can accept electrons from donor atoms. The Si—O single bond is the strongest of all silicon bonds and accounts for the stability and prominence of silica and the silicates. 

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