Porous materials silicon

Increasing the surface-to-bulk ratio of the sample to be studied. This is easily done in the case of highly porous materials, and has been exploited for the characterization of supported catalysts, zeolites, sol-gels and porous silicon, to mention a few. 

For application of protein-immobilized porous materials to sensor fields, use of an electroactive substance as the framework material is important. DeLouise and Miller demonstrated the immobilization of glutathione-S-transferase in electrochemically etched porous silicon films [134], which are attractive materials for the construction of biosensors and may also have utility for the production of immobilized enzyme bioreactors. Not limited to this case, practical applications of nanohybrids from biomolecules and mesoporous materials have been paid much attention. Examples of the application of such hybrids are summarized in a later section of this chapter. 

Oxidation is a widely used procedure in carbohydrate chemistry, mainly to access sugars that contain a carbonyl function to serve as valuable intermediates for a variety of derivatizations. Many procedures have been developed, employing either chemical or biochemical methodologies.14 148 While most of these methodologies rely on homogeneous catalysis, the use of heterogeneous catalysts has proved to be a feasible alternative.123c However, the utilization of catalysts based on silicon porous materials for the oxidation of carbohydrates is still a field to be further explored. 

The formation of etch pits and tunnels on n-Si during anodization in HF solutions was reported in the early 1970 s. It was found that the solid surface layer is the remaining substrate silicon left after anodic dissolution. The large current observed on n-Si at an anodic potential was postulated to be due to barrier breakdown.5,6 By early 80 s7"11 it was established that the brown films formed by anodization on silicon substrate of all types are a porous material with the same single crystalline structure as the substrate. 

As observed by D. Johnson and J. Stiegler, "Polymer-precursor routes lor fabricating ceramics offer one potential means or producing reliable, cost-effective ceramics. Pyrolysis of polymeric metalloorganic compounds can be used to produce a wide variety of ceramic materials." Silicon carbide and silicon oxycarbide fibers have been produced and sol gel methods have been used In prepare line oxide ceramic powders, such as spherical alumina, as well as porous and fully dense monolithic forms. 

The formation of a sol-gel porous material is through a hydrolysis-polycondensation reaction. An example is given in equation 1 with the methoxide of silicon (tetramethyl-orthosilicate, TMOS), but many other alkoxides, aryl oxides and acyl oxides can be used, as well as Si—N and Si—Cl compounds. 

The following review is concerned with the synthetic and structural chemistry of molecular alumo-siloxanes, which combine in a molecular entity the elements aluminum and silicon connected by oxygen. They may be regarded as molecular counterparts of alumo-silicates, which have attracted considerable attention owing to their solid-state cage structures (see for example zeolites).1 3 Numerous applications have been found for these solid-state materials for instance the holes and pores can be used in different separation techniques.4,5 Recently the channel and pore structures of zeolites and other porous materials have been used as templates for nano-structured materials and for catalytical purposes.6 9... 

Porous silicon was discovered over 35 years ago by Uhlir.28 The porous material is created by electrochemical dissolution in HF-based electrolytes. Hydrofluoric acid, on its own, etches single-crystal Si extremely slowly, at a rate of only nanometers per hour. However, passing an electric current between the acid electrolyte and the Si sample speeds up the process considerably, leaving an array of deep narrow pores that generally run perpendicular to the Si surface. Pores measuring only nanometers across, but micrometers deep, have been achieved under specific etching conditions. 

E.g., inorganic materials (ceramics, glass, porcelain) can be waterproofed with easily hydrolysed alkylchlorosilanes (methyltrichlorosilane, dimethyldichlorosilane, ethyltrichlorosilane, diethyldichlorosilane). Metals and porous materials (paper, leather, textile, plaster, cement, gypsum, etc.) should not be waterproofed with alkylchlorosilanes, because they release hydrogen chloride, which destroys these materials. Alkylchlorosilanes can be successfully replaced with silicone oligomers with aminogroups or hydrogen atoms in the molecule. 

Neckers and coworkers reported a similar polymerization using (chloromethyl)dichloro-methylsilane as a starting material (equation 15)162. Oligomers were obtained in addition to cyclic trimers and tetramers. In the presence of a platinum catalyst and tetravinylsilane, the oligomer crosslinked when irradiated to form a hard, porous material that could be pyrolyzed to a silicon carbide char containing excess carbon. A later report described other platinum photo- and thermal catalysts that could be used in this crosslinking reaction163. 

It is expected that the sensitivity of positrons and positronium to changes in the matrix material will be used extensively in the near future. For example, tantalum-silicon-nitride was found to be an effective diffusion barrier between copper and silicon dioxide [34]. The applicability to porous materials needs to be checked. Work has been carried out, for example, by Gidley et al. on TiN diffusion barriers [35],... 

K. B. Babb, D.A. Lindquist, S.S. Rooke, W.E. Young, and M.G. Kleeve, Porous Solids of Boron Phosphate, Aluminum Phosphate, and Silicon Phosphate, eds. S. Komarneni, D.M. Smith, and J.S. Beck, Advances in Porous Materials, vol. 371. (Materials Research Society, 1995), pp. 279-290. 

In spite of the mentioned disadvantages, useful information has been obtained from SPM imaging of a number of porous materials. To illustrate such point, the present review examines the research that has addressed the visualization of the porous structure of solids by SPM. A wide variety of materials is covered, such as porous silicon, activated carbon materials, aluminas, synthetic membranes or biological materials. 

Non-oxide ceramic materials such as silicon carbide has been used commercially as a membrane support material and studied as a potential membrane material. Silicon nitride has also the potential of being a ceramic membrane material. In fact, both materials have been used in other high-temperature structural ceramic applications. Oxidation resistance of these non-oxide ceramics as membrane materials for membrane reactor applications is obviously very important. The oxidation rate is related to the reactive surface area thus oxidation of porous non-oxide ceramics depends on their open porosity. The generally accepted oxidation mechanism of porous silicon nitride materials consists of two... 

Solid-state NMR spectroscopy has been demonstrated as a well established technique for characterization of zeolites and other porous materials with respect to structure elucidation, pore architecture, catalytic behaviour and mobility properties. The latest progress in the development of NMR techniques, both with respect to software and hardware improvements, has contributed to the present state of the art for NMR within the field of characterization of zeolitic materials. Furthermore, the introduction of NMR imaging (110), two-dimensional quintuple-quantum NMR spectroscopy (111) and transfer of populations in double resonance (TRAPDOR) NMR (112,113) will extent the horizons of zeolite characterization science. As a final example, the Al => Si TEDOR experiment directly proves, for the first time, that silicon substitutes for phosphorous atoms in the framework of SAPO-37 (114). The Al... 

---