Amorphous materials membranes

Basement membrane Layer of dense amorphous material on which cells as-... 

The membrane-coating granules in keratinized epithelia contain electron-dense lipid lamellae [68, 77], and therefore, the intercellular spaces of the stratum corneum are filled with short stacks of lipid lamellae [67, 132], Most of the membrane-coating granules in nonkeratinized epithelia consist of amorphous material [120] however, some studies have shown that a small number of these granules in nonkeratinized epithelia contain lamellae [151]. Therefore,... 

Carbon molecular sieve membranes. Molecular sieve carbons can be produced by controlled pyrolysis of selected polymers as mentioned in 3.2.7 Pyrolysis. Carbon molecular sieves with a mean pore diameter from 025 to 1 nm are known to have high separation selectivities for molecules differing by as little as 0.02 nm in critical dimensions. Besides the separation properties, these amorphous materials with more or less regular pore structures may also provide catalytic properties. Carbon molecular sieve membranes in sheet and hollow fiber (with a fiber outer diameter of 5 pm to 1 mm) forms can be derived from cellulose and its derivatives, certain acrylics, peach-tar mesophase or certain thermosetting polymers such as phenolic resins and oxidized polyacrylonitrile by pyrolysis in an inert atmosphere [Koresh and Soffer, 1983 Soffer et al., 1987 Murphy, 1988]. 

Current electron-microscopic evidence shows that intercellular regions in SC are filled with lipid-rich amorphous material (43, 45, 46). In the dry membrane the intercellular diffusion volume may be as large as 5% and at least 1% in the fully hydrated tissue. . . this intercellular volume is at least an order of magnitude larger than that estimated for the intra-appendageal pathway and allows the possibility of significant intercellular diffusion. Diffusion between cells cannot be ruled out, but various data show that diffusion cannot be primarily intercellular. 

To assess about the quality and purity of the synthesized membranes, several experimental techniques and procedures are available many of those are commonly employed in catalyst characterization. Thus, XRD (x-ray diffraction) analysis of the supported samples is conventionally used to identify the type of zeolite, the proportion of amorphous material and impurities, and the preferential orientation of the crystals (XRD-pole figure). However, for the vast majority of the synthesis procedures described, the XRD spectra of the scrapped membrane or the resulting powder from the liquid phase is supplied to avoid the support contribution. 

Posttreatment processes have been used to improve the quahty of the resulting membranes, such as ion exchange (to provide catalytic properties or change them between hydrophobic and hydrophilic surfaces), liquid or vapor sililation, coke deposition, CVD (chemical vapor deposition), and ALCVD (atomic layer chemical vapor deposition). These techniques are used to reduce the intercrystalline gaps and the pore-mouth size, modify the acid properties of the modified membranes, and remove amorphous material. Some of these modifications have demonstrated very high separation selectivities for the resulting membranes however, in many cases, they are of limited practical application due to the relatively low fluxes obtained. 

Positronium lifetime spectroscopy is particularly well suited for stud)hng defects in crystals and structural fluctuations in amorphous materials and can give an estimate of free volumes in condensed matter [116]. It is a useful technique to estimate the free volume of polymeric membranes [117]. In a study on silica gels, the decay lifetime has been found (Fig. 4.16) to be proportional to the pore diameters (measured by N2 adsorption) between 30 and 100 A [118]. Information on pore size distribution and surface area may also be obtained by means of calibration curves. 

Amorphous silica has also been mentioned as a starting metal oxide material for the preparation of particulate mesoporous membranes. These membranes were prepared from commercial sols, Ludox (DuPont) or Cecasol (Sobret), and coated on a macroporous a-alumina support [35]. In contrast to crystalline membrane materials such as alumina, titania or zirconia, the evolution of pore size with temperature of amorphous silica membranes was revealed to be more sensitive to drying conditions than to firing temperature (Table 7.1). When heat-treated for several hours at 800°C the silica top layer transformed from an amorphous state to cristobalite. 

General criteria for selection of materials for the processing of hydrogen separation membranes are discussed. Performance and stability standards required for applications in high temperature membrane reactors have been focused. The correlations between pore structure and stability issues of membranes made of amorphous materials, specifically silica membranes are discussed in detail. 

The lack of methods for a fast and reliable assessment of membrane quality is still one of the outstanding issues in zeolite-membrane development. The usual meaning of the term quality relates to the ability of the membrane to carry out a given separation with a reasonable flux therefore, a system-specific property and a universal membrane quality test do not exist. In general, specihc permeation measurements at different temperatures, either of single gases (or vapors) or of multicomponent mixtures in the gas or liquid (pervaporation) phase, provide extremely useful information on the effective pore structure of the membrane, on the existence of intercrystalline defects, and amorphous material and permeation fluxes, as well as information about the main transport controlling effect (adsorption or diffusion). 

The packing density of the constituent atoms in the interior regions of protein molecules is, on average, equivalent to that of most organic compounds in the crystalline state. As such, it is possible to consider that membrane proteins may exhibit some of the solid-state electronic properties which have been extensively studied in elemental amorphous materials and organic polymers. Such properties include electronic conduction via localized and delocalized electronic states. When localized states are... 

Another approach is to use amorphous inorganic membranes based on silica, which have an intrinsic lower scaling-up costs. Materials based on pure silica have been proven to be unsuitable for this application because of a too low hydrothermal stability. Recent inventions on membranes that... 

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