Oxygen window

The cavity of the A-type zeolite is approximately spherical with diameter 11.4 A. If the occluded molecule is regarded as a hard sphere, the free diameter of the cavity (the distance through which the center of the occluded molecule can move) will be reduced by a distance a corresponding to the van der Waals diameter of the molecule. The free volume of the cavity, and hence the value of K0 may be estimated on this basis. This method of calculation is equally applicable to other zeolites which consist of discrete cavities interconnected through small (eight-membered) oxygen windows. 

The absence of initial contraction in zeolite 8 Li-4 NaA is probably caused by the fact that Li+ cations may be situated in other positions inside the voids because of their small sizes. This is corroborated by the fact that Li exchange increases the lattice parameter a of zeolite LiA by 2.6% compared with zeolite NaA. Then, the same processes occur as in NaA, leading to crystal expansion. Perhaps at high fillings Li cations extend from six-membered oxygen windows into the large voids this would decrease the contraction of their positive charges and expand the crystal lattice. 

A series of papers by Beran et al. (102-112) was devoted to quantum-chemical studies of the cationic forms of zeolites using cluster models and the CNDO/2 technique. As a rule, cyclic clusters consisting of four or six T04 (T = Si, Al) tetrahedra were considered as models for four-membered (S, sites) and six-membered (S S, and S,r sites) oxygen windows in the zeolite structures. 

In some zeolite-type stmctures, the spatial linkages between these SBUs produce oxygen windows that define the usefiil pore diameters within the zeolite stmctures. These provide another way of classifying zeolite frameworks but it must be realized that these oxygen windows arise from the geometric arrangements of SBUs and are not necessarily the SBUs themselves. 

Table VI. Propylene, Carbon Monoxide, and Nitric Oxide Conversions and Ammonia Formation within the Oxygen Window...

Molecular sieves are natural or synthetic zeolites (alumina silicates of alkaline or alkaline earth metals, commonly, sodium or calcium). Their general formula is 2/nP AI2O3 n Si02 x H2O, where m is the cation valency. They have a well-ordered porous structure [4, 5, 15, 63]. The diameter of zeolite A cavities is 11.4 A and of type X is 11.6 A. Large cavities of A zeolites are connected by means of 8-membered oxygen windows with 4-5 A diameters. Type X cavities are joined by 12-mem-bered oxygen windows of 8-9 A diameter. Cavities form the primary porous structure of dehydrated zeolite crystals. Molecules of substances with diameters smaller than those of the windows are easily adsorbed when molecules are too large in... 

THERMOELECTRICENERGYCONVERSION] (Vol23) -oxygen-generating system for [OXYGEN-GENERATION SYSTEMS] (Vol 17) -vitreous silica windows [SILICA - VITREOUS SILICA] (Vol 21)... 

The best fibers, installed in systems operating at 0.9 )J.m, had losses of 5 dB/km. The lower intrinsic losses in the 1.3 to 1.55-)J.m window were unattainable by this technique. Fundamental cation—oxygen vibrational modes as well as OH contamination were intrinsic to the compositions. 

An example of interface analysis by EDXS line profiling at high lateral resolution is given in Eig. 4.29. It is of particular importance, because the distribution of light elements like carbon, nitrogen, and oxygen is also revealed. This was possible by means of an Si (Li) EDXS detector (Kevex) with an ultrathin window attached to a dedicated STEM HB 501, from Vacuum Generators, with a cold field-emission cathode. 

The rate law for a reaction is a window into the changes that take place at the molecular level in the course of the reaction. Knowing how those changes take place provides answers to many important questions. For example, what controls the rate of formation of the DNA double helix from its individual strands What molecular events convert ozone into oxygen or turn a mixture of fuel and air into carbon dioxide and water when it ignites in an engine ... 

Code Oxygen rings window or channel Access. lun Dim. Si/Al... 

Lou M, Eschenfelder CC, Herdegen T, Brecht S, Deuschl G. Therapeutic window for use of hyperbaric oxygenation in focal transient ischemia in rats. Stroke 2004 35 578-583. 

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