SiO2 Framework Research Articles

Synthesis of Ceramics/Polymer Nanocomposites Protonic Conducting Membrane for Fuel Cells Application

High temperature protonic conducting polymer membrane provides new technological applications in the electrochemical devices including electrochromic displays, chemical sensors, fuel cells and others. Organic/inorganic nanocomposites membrane consists of SiO2/PEO (Polyethylene Oxides) hybrid are remarkable family of isotropic, amorphous polymer materials, which has been synthesized through sol-gel processes. The hybrid membrane doped with acidic surfactant molecules shows good protonic conductivities at high temperatures above 100C. The membrane was found to be thermally stable at high temperatures because of the inorganic SiO2 framework in the composites matrix.

Clusters stuffed inside frameworks: Electronic structure theory

A zeolite, or other framework material, can be used as a template to form regular 3D arrays of nanosize clusters stuffed inside the framework cages. The aluminosilicate zeolite framework materials have wide electronic bandgaps, so that the stuffed species can produce new active electronic states within that bandgap. We discuss several real and model examples of such systems. We start with a framework of pure silicon, which is the silicon clathrate, and discuss its properties when stuffed with a single alkali atom in each cage. Caged structures of clathrates are similar to those of zeolites and can house alkali metal atoms in a periodic fashion with spacing varying from 5 to 15 A Next we investigate a model system of semiconductor clusters (Si) in the SiO2 framework in the sodalite geometry. Sodalite contains \-cages which are building blocks common to the structure of many important zeolites. We investigate theoretically atomic geometries, energetics and electronic properties of small semiconductor clusters in silica-sodalite. We also consider metal atom Na clusters in naturally occurring sodalite. We find that introducing Si semiconductor clusters within the cages of zeolites gives rise to flat electronic bands in the bandgap of the host, while alkali metal clusters give broad bands. The electronic properties are found in many cases to be dominated by many-body electronic correlation effects.

Characterization of microporous Si by flow calorimetry: Comparison with a hydrophobic SiO2 molecular sieve

Flow microcalorimetry has been used to study microporous silicon produced by electrochemical corrosion of bulk p-type silicon wafers in highly concentrated (50 wt %) aqueous hydrofluoric acid. Calorimetry data on pore size and hydrophobicity of freshly etched crystalline silicon structures are compared with similar measurements on silicalite, a well-studied microporous form of crystalline silicon dioxide. Silicalite has a tetrahedral SiO2 framework with interconnected ‘‘ultramicropores’’ that only readily admit molecules of less than 6 Å diameter. Its measured heat of immersion in n-heptane (kinetic diameter 4.3 Å) consequently far exceeds that in iso-octane (kinetic diameter 6.2 Å) and it preferentially adsorbs the normal alkane from the branched alkane. In direct contrast the microporous Si layers studied exhibited comparable heats of immersion for n-heptane and iso-octane, and did not show any preferential adsorption of the narrower molecule. In addition, the microporous Si layers studied exhibited appreciable heats of immersion in 1, 3, 5 tri-isopropylbenzene (kinetic diameter 8.5 Å). The majority of their pore volume is thus constrained to the ‘‘supermicropore’’ size regime of 10–20 Å width. Both silicalite and freshly etched microporous Si are shown, however, to be highly hydrophobic and organophilic materials. Their exothermic heats of immersion in n-heptane far exceed those in water and both materials preferentially interact with the polar alcohol (n-butanol) more strongly from water than from n-heptane.

Studies on clathrasils. III.*

Abstract The high temperature form of melanophlogite, 46 SiO2 · 6M14 · 2M12(M12 = CH4, N2; M14 = CO2, N2), is cubic with space group Pm3n and a = 13.436(3) Å at 200°C. The mineral is isostructural with the cubic gas hydrates of type I. Structure refinement with 667 independent reflections led to a weighted Rw = 0.040. Corner-sharing [SiO4] tetrahedra form a 3-dimensional framework which contains two types of cages: 2 pentagondodecahedra (V∼97 Å3) and 6 tetrakaidecahedra (V∼136 Å3) per unit cell. From difference Fourier syntheses it is concluded that the guest molecules M12 and M14 within the dodecahedral and tetrakaidecahedral cages are oriented in such a way that the van der Waals contacts between the guest molecules and the SiO2 framework are optimized. The mean value of the Si – O – Si angles (168.8°) is considerably higher and that of the Si – O distances (1.576 Å) is considerably lower than corresponding angles and distances in the known silica polymorphs.

A nuclear magnetic resonance study of the phase transition in anorthite, CaAl2Si2O8

Abstract The nuclear magnetic resonance (NMR) of 27A1 in anorthite (CaAl2Si2O8) has been studied as a function of temperature. At the critical temperature Tc = 241 ± 4°C a complete reversible structural phase transition takes place: two sites each out of the eight inequivalent Al sites of the low-temperature phase become equal at Tc resulting in only four inequivalent sites in the high-temperature phase. The field gradient tensors at the Al nuclear sites have been determined for temperatures below and above the transition. From the results follows that the (Al,Si)O2 framework becomes body-centered at Tc . The NMR results are consistent with x-ray structure investigations if one assumes for the high-temperature phase a “jumping” of the Ca atoms between the two split positions. The existence of two kinds of domains with different Bravais lattices would contradict the NMR results.