Asymmetric etching

For asymmetric etching of PC or PET, one side of the membrane is put in contact with an alkaline etching solution, usually 9 M NaOH or KOH, and the other side is in contact with the stopping medium, typically a weak acid solution such as 1 M HCOOH in 1 M KCl (30). For asymmetric etching of polyimide (Kapton), the etching solution is NaClO... 

The application of a potential across the membrane is stopped as soon as a monitoring ammeter records an increase in the current that passes through the membrane up to reaching a pre-set value, typically 1 mA (40). The membrane is then immediately immersed (both sides) in the stopping medium, thus blocking the asymmetric etching. At the end of the process, conical nanopores of controlled shape are obtained (37,40). It was also shown that the addition of alcohols (such as methanol, ethanol, or propanol) to the etch solution allows further control of the etching rate (24) and the Vb/ t (40, 41). 

The generalized Prony analysis of END trajectories for this system yield total and state resolved differential cross-sections. In Figure 5, we show the results. The theoretical analysis, which has no problem distinguishing between the symmetric and asymmetric str etch, shows that the asymmetric mode is only excited to a minor extent. The corresponding state resolved cross-section is about two orders of magnitude less than that of the symmetric stretch. 

All waveforms were collected using a custom-built, asymmetric, capacitively coupled (13.56MHz), reactive-ion etching apparatus. A circulating bath maintains the aluminum powered electrode temper cure at 24.0 0.5°C. The apparatus is equipped with a He-Ne laser, A-632.8nm) interferometer as depicted in Figure 4. The laser beam was aimed at the substrate surface at an angle of 10° off the normal. Laser intensity was measured by a photodiode whose... 

Verkleij, A.J., Zwaal, R.F., Roelofsen, B., Comfurius, P., KasteUjn, D. and van Deenen, L.L.M., 1973, The asymmetric distribution ofphosphoUpids in the human red cell membrane. A combined study using phospholipases and freeze-etch electron microscopy. 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

Symmetric membranes and asymmetric membranes are two basic types of membrane based on their structure. Symmetric membranes include non-porous (dense) symmetric membranes and porous symmetric membranes, while asymmetric membranes include integrally skinned asymmetric membranes, coated asymmetric membranes, and composite membranes. A number of different methods are used to prepare these membranes. The most important techniques are sintering, stretching, track-etching, template leaching, phase inversion, and coating (13,33). 

Selection of optically pure crystals can be accomplished with the aid of a polariscope in which the thin plates of quartz are examined between crossed Nicol prisms in polarized light. These plates are sawed perpendicular to the optical axis of well formed natural crystals. Brasilian twins exhibiting bright interference patterns in crossed Nicol prisms must be broken out. Only optically pure areas are selected for further use, for example, for asymmetric adsorption and for asymmetric catalysis. Piezoquartz as plates of an oblique cut, called the "Cut of Curie", are used in radio electronics and can be identified by the sign of their optically rotation according to the method of Lemmlein According to this method, the etched plate of piezoquartz is... 

An injection slit etched by Bosch avoids any formation of asymmetrically shaped plugs, which typically occurs during loading of the sample in wide channels. The slit is cormected to a vacuum system and syringes for the supply of the analyte and the mobile phase. A CFD simulation was first performed to compare different injection well shapes and configurations in order to account for less profitable configurations. 

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