Surfaces With Dense Defects

The diameter of hollow fibers varies over a wide range, from 50 to 3000 xm. Fibers can be made with a uniformly dense structure, but preferably are formed as a microporous structure having a dense selective layer on either the outside or the inside surface. The dense surface layer can be either integral with the fiber or a separate layer coated onto the porous support fiber. Many fibers must be packed into bundles and potted into tubes to form a membrane module modules with a surface area of even a few square meters require many kilometers of fibers. Because a module must contain no broken or defective fibers, hollow fiber production requires high reproducibility and stringent quality control. 

Mixed oxides have the ability to form paracrystalline phases that represent an intermediate phase between the crystalline and the amorphous phase. Because of the disordered structure paracrystallinity causes defects that occur due to the inhibition of recrystallization and are initiated by the introduction of additional anions or molecules into the lattice structure. The formation of these deformed crystalline structures depends on the used synthesis method as well as the nature of the cation precursor. Thermal decomposition of the LDHs leads to the formation of the metastable phase that contains divalent and trivalent cations with densely packed configuration [2,32]. After the reduction, tri-valent ions are trapped inside the crystallites of divalent ions causing the formation of active centers. The centers that exhibit higher reactivity are believed to be formed on the surface of the paracrystalline crystals. After deactivation, the grain size of these mixed oxides increases due to the gradual removal of the defects in the crystal lattice [2]. 

On the contrary, once the Pd and Pd-alloy membrane thickness is thin, the porous substrate may become the main factor contributing to the cost of the membranes. It is generally acknowledged that the minimum thickness of a dense Pd-based membrane is approximately three times the size of the largest pore present in the substrate surface. This implies that ultra-thin Pd-based membranes with high H2 permeance and low cost can only be deposited on defect-free surfaces with a small and narrow pore size distribution. However, the fabrication of asymmetric ceramic composite tubes also presents high costs because multiple fabrication steps are required to obtain gradually reduced porosity. 

Densely packed oxide surfaces, such as MgO(lOO), are largely inactive, but defects, particularly those associated with oxygen vacancies, provide sites where adsorbates may bind strongly. Figure 5.10 shows the adsorption of different molecules on defects in a Ti02 surface. 

The Raman spectra (0-1400 cm l) shown in Fig re 6 illustrate the structural changes which accompany the consolidation of silica gels. The 1100°C sample is fully dense, whereas the 50 and 600°C samples have high surface areas (1050 and 890 m2/g), respectively. The important features of the Raman spectra attributable to siloxane bond formation are the broad band at about 430 cm 1 and the sharp bands at 490 and 608 cm 1(which in the literature have been ascribed to defects denoted as D1 and D2, respectively). The D2 band is absent in the dried gel. It appears at about 200°C and becomes very intense at intermediate temperatures, 600-800°C. Its relative intensity in the fully consolidated gel is low and comparable to that in conventional vitreous silica. By comparison the intensities of the 430 and 490 cm 1 bands are much more constant. Both bands are present at each temperature, and the relative intensity of the 430 cm 1 band increases only slightly with respect to D1 as the temperature is increased. Figure 7 shows that in addition to elevated temperatures the relative intensity of D2 also decreases upon exposure to water vapor. 

Myelin basic protein. In PNS myelin, MBP varies from approximately 5% to 18% of total protein, in contrast to the CNS, where it is close to 30% [ 1 ]. In rodents, the same four 21,18.5,17 and 14kDa MBPs found in the CNS are present in the PNS. In adult rodents, the 14kDa MBP is the most prominent component and is termed Pr in the PNS nomenclature. The 18.5 kDa component is present and is often referred to as the P, protein in the nomenclature of peripheral myelin proteins. Another species-specific variation in human PNS is that the major basic protein is not the 18.5 kDa isoform that is most prominent in the CNS but rather a form of about 17 kDa. It appears that MBP does not play as critical a role in myelin structure in the PNS as it does in the CNS. For example, the shiverer mutant mouse, which expresses no MBP (Table 4-2), has a greatly reduced amount of CNS myelin, with no compaction of the major dense line. By contrast, shiverer PNS has essentially normal myelin,both in amount and structure, despite the absence of MBP. This CNS/PNS difference in the role of MBP is probably because the cytoplasmic domain of P0 has an important role in stabilizing the major dense line of PNS myelin. Animals doubly deficient for P0 and MBP have a more severe defect in compaction of the PNS major dense line than P0-null mice, which indicates that both proteins contribute to compaction of the cytoplasmic surfaces in PNS myelin [23],... 

Ion-beam thinning is usually used for dense bulk specimens where particular regions must be analyzed. It can be useful in AEM for thinning the same single crystals used in surface analysis to make direct comparisons with results from AES, XPS, etc. Ion-beam thinning can also be useful in analysis of interfaces and defects within bulk metallic catalysts such as Pt and Pd and their alloys. 

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