Silicon source structure

After the calcination step, experimental data (XRD, 29 i maSNMR) show that a zeolite with the silicalite structure has been formed. 29 i MASNMR indicates for the zeolite material a Si/Al ratio depending on the sample prepared it has been observed that both the natures of the silicon source and of the alumina supports may originate these fluctuations. 

Tetravalent silicon is the only structural feature in all silicon sources in nature, e.g. the silicates and silica even elemental silicon exhibits tetravalency. Tetravalent silicon is considered to be an ana-logon to its group 14 homologue carbon and in fact there are a lot of similarities in the chemistry of both elements. Furthermore, silicon is tetravalent in all industrially used compounds, e.g. silanes, polymers, ceramics, and fumed silica. Also the reactions of subvalent and / or low coordinated silicon compounds normally lead back to tetravalent silicon species. It is therefore not surprising that more than 90% of the relevant literature deals with tetravalent silicon. The following examples illustrate why "ordinary" tetravalent silicon is still an attractive field for research activities Simple and small tetravalent silicon compounds - sometimes very difficult to synthesize - are used by theoreticians and preparative chemists as model compounds for a deeper insight into structural features and the study of the reactivity influenced by different substituents on the silicon center. As an example for industrial applications, the chemical vapor decomposition (CVD) of appropriate silicon precursors to produce thin ceramic coatings on various substrates may be mentioned. 

Synthesis of MCM-41 with Additives. The hydrothermal crystallization procedure as described earlier [10] was modified by adding additional salts like tetraalkylammonium (TAA+) bromide or alkali bromides to the synthesis gel [11]. Sodium silicate solution ( 14% NaOH, 27% Si02) was used as the silicon source. Cetyltrimethylammonium (CTA) bromide was used as the surfactant (Cl6). Other surfactants like octadecylltrimethylammonium (ODA) bromide (C,8), myristyltrimethylammonium (MTA) bromide (C,4) were also used to get MCM-41 structures with different pore diameter. Different tetralkylammonium or alkali halide salts were dissolved in little water and added to the gel before addition of the silica source. The final gel mixture was stirred for 2 h at room temperature and then transferred into polypropylene bottles and statically heated at 100°C for 4 days under autogeneous pressure. The final solid material obtained was washed with plenty of water, dried and calcined (heating rate l°C/min) at 560°C for 6 h. 

Figure 9 Channel structure of a phase separator generated by ASE deep etching of silicon. (Source IMM.)...

Effect of the Structure of Silicon Sources. Hoebbel et al. used silicic acid sols, silicic acid gels, or Aerosil as a silica source of tetramethylammonium silicate aqueous solutions (9). In the solutions at the conditions that a N/Si ratio is 1.0 and Si02 concentration is ca. 1.4 mol dm-, the distributions of silicate anions are almost the same, and the cubic octamer is a dominant species, although the degradation rates of the silica sources are different. This suggests that the cubic octamer is formed in the tetramethylammonium silicate aqueous solution, regardless of the type of silica source with tetra-functionality. Tetraalkoxysilanes (Si(0R), R denotes an alkyl group) can be used as a silica source as well (4,12,14). 

There are several structurally different types or polymers that are suitable precursors for ternary Si-C-N ceramics. By far the most investigated precursors are polysilazanes of the general type [Si(R )(R°)N(R°)] (R, R°, R° = H, alkyl, aryl, alkenyl, etc.). In contrast to the limited number of starting compounds, H SiCl(4 ) (x = 0-3) as the silicon source and NH3 or H2N-NH2 as the nitrogen source for synthesis of polysilazanes as precursors for binary Si-N ceramics, the chemistry of polycarbosilazanes, that is, carbon-containing or modified polysilazanes, is very multifaceted. The attachment of various organic groups to the silicon atoms allows adjustment of their physicochemical properties, to control their thermolysis chemistry, and also to influence materials properties. The first... 

Figure 1.23. Cross-sections of FET structures employing LB films as gate insulators. The top diagram illustrates a thin-film FET using an amorphous silicon source and drain structures. The bottom diagram illustrates a chemically sensitive FET (CHEMFET) in which the electrical characteristics of the LB gate insulator are sensitive...

The synthesis of titanium silicates by basic and acidic hydrolysis in an organic-water media were carried out at 80-150 C in an autoclave with stirring (150-200 rpm) at autogenic pressure (2.5-4.5 at.). As silicon sources tetraethoxysilane and silicic acid have been used. As metal sources the metal-organic compounds alcoholates, stearates, acetylacetonates, acetates or mineral salts have been used. Hexamethylenetetramine, N,N-dimethylocteIamine, monoethanolamine were used as the structure-directing agents. 

Structures and Preparation Methods for Commonly Used Silicon Sources... 

Table 10.2 summarizes some examples of zeolites based on their classification by chemical composition. Low-silica zeolites (Si/Al < 5) are synthesized in basic conditions (pH >13) using a silicon source, an aluminum source, and alkali hydroxides at moderate temperatures, typically less than 120°C. The identity of the alkaU species used is a determining factor in which phase is obtained from synthesis, as the relative rates of (alumino)silicate hydrolysis and condensation reactions are dependent on the identity of the alkali cation. It is also believed that hydrated alkali cations effectively direct the assembly of (alumino)silicate precursors into fuUy connected three-dimensional structures. Sodium and potassium hydroxide have been used most frequently in low-silica zeolite syntheses due to their low cost and high solubility in... 

Material Structure Metal source Silicon source Template Specific information (temperature, time) (first author, year)... 

Figure 1.6 Macroporous silicon structure generated by means of electrochemical etching of single-ciystalline silicon [Source V. Lehmann, Siemens AG) [108]. Reproduced by kind permission from the publisher.

Traditionally the synthesis of ordered mesoporous silica materials, with pores in the 2-50 nm range, is templated by surfactants, hi this soft-templating approach, quaternary ammonium salts and non-ionic polyether-based surfactants are the most used templates, leading to pore distributions of 2-6 and 4-30 nm respectively [6,7]. Under specific reaction conditions, a mixture of the silica precursor and the surfactant yields a meso-structured solid, which, after removal of the template, becomes an open mesoporous structure. Many synthetic protocols have been developed by varying the surfactant, silicon source or synthesis conditions, but most preparations occur at low or high pH. Extensive reviews on the synthesis of ordered mesoporous materials are available [8-10]. 

Templates or SDAs formed by a single molecule or ion can be used in hydro-thermal methods to shape the final products. An assembly of molecules can also act as a template to direct the formation of new materials. The well-known silica MCM-41, with a unidimensional structure of hexagonal pores, is a good example. A surfectant (e.g., NR4 cations) is used as SDA, TEOS or a similar compound as a silicon source, and water as a solvent a catalyst (acid or basic) is also added. At concentrations above the critical micellar concentration, the surfactant molecules are ordered in micelles (layers, spheres, cylinders, etc.), where the molecules are weakly bonded by van der Waals or hydrogen bonds. Supramicel-lar interactions lead to a liquid crystal (LC) structure, on whose walls the inorganic species are formed, oligomerized, and finally polymerized [44]. 

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