Thickness of mica sheets

Absorbance spectra monolayer on mica, 244/ thickness of mica sheet, 241 Absorptivity ratios, definition, 28 Acyl chains, monolayer films, 198 Adatom(s) definition, 210 surface migration, 211214... 

Many of the various techniques associated with metal film preparation have recently been reviewed by Klemperer (76). Much of the catalytic work with thick continuous films has used a cylindrical reaction vessel (Fig. 7a). This cylindrical geometry permits a cylindrical sleeve of mica sheet to be inserved and used as the film substrate for epitaxial film growth... 

The sample preparation of these mica sheets is a delicate process that requires some experience and often takes 1-2 days prior to an SFA experiment. Tln-ough successive cleaving, one has to prepare 1-5 p.m thick and imifonn sheets of mica. Mica is a natural material that is available m different qualities [44]. 

Each newly cleaved mica surface is very clean. Flowever, it is known that mica has a strong tendency to spontaneously adsorb particles [45] or organic contaminants [46], which may affect subsequent measurements. The mica sheets are cut into 10 nun x 10 nun sized samples using a hot platinum wire, then laid down onto a thick and clean 100 nun x 100 nun mica backing sheet for protection. On the backing sheet, the mica samples can be transferred into a vacuum chamber for themial evaporation of typically 50-55 mn thick silver mirrors. 

Sheet Mica. Pockets of mica crystals ranging in size from a few square centimeters to several square meters are found in pegmatite sills and dikes or granodiorite (alaskite) ore bodies. In order to be used industrially, manufacturers must be able to cut a 6 cm pattern in the mica. "Books" of mica, ranging from 12.9 to 645 cm or more, are cut from the crystals. The books can be punched into various shapes and spHt into thicknesses varying from 0.0031 to 0.010 cm (12). The highest quaUty micas maybe used in aerospace computers, and those of lower quaUty find use as insulators in electrical apphances. 

Mica splittings are processed from lower quaUty blocks and from sheets too thin for blocks and unsatisfactory for producing film. The splittings are packed for sale in three ways book form, which are laminae spHt to the desired thickness from the same book of mica, then dusted with mica dust, and restacked in book form pan packed, in which splitting layers are placed evenly in a pan, and each layer separated by a thin sheet of paper then pressed together and loose packed, in which splittings are si2ed with screens then padded loosely in a wooden box for shipment. 

Before being glued to the glass sample, one face of each mica sheet has to be coated or spread with a thin layer of silver in 50 — 60 nm thick (reflectivity 96 % —98 %) for facilitating the optical interference. As shown in Fig. 16(a), when a beam of white light goes up vertically through the lower sample and reaches the silver film, the beam is partly re-... 

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

The structure, the thickness, and the interactions generated by the presence of the adsorbed polymer layers have been extensively studied [17], In particular, the force between two polymer-covered mica sheets in various solvency conditions has been probed via the SEA technique [27,28], The force is purely repulsive in a good solvent and becomes attractive as the solvent gets poorer. However, these studies concern only a regime of large interaction compared to the thermal energy and are restricted to interactions between solid surfaces. 

To exploit this latter alternative, mica sheets were cleaved from larger crystals of Muscovite mica until a thickness of the order of 1-5 pm was reached as judged by the brightness and color of the fringes seen under room light. Actual film thickness was determined from the infrared fringe spacing. Mica at these thicknesses is delicate but can be handled if care is taken it is possible to cleave mica to thicknesses below 1 pm but at these thicknesses the mica becomes too delicate to use unsupported. The cleaved mica sheets were mounted in standard transmission holders constructed from Teflon. 

The repeating dimensions of the mica crystal along the a-, b-, and c-axes, which are nearly perpendicular to one another as shown in Figures 2.9a and 2.9b, are about j = 5.3 A, Z = 9.2 A, and c = 10.2 A. The c-dimension reflects the fact that the individual mica sheets are 9.6 A thick, and the embedded ions prevent these sheets from stacking together in the most space-efficient manner. Consequently, the 10.2-A spacing (as determined by x-ray diffraction of the mineral) is diagnostic for mica and micalike clay minerals. 

Fig. 3. The thermostatized multicompartment cell used in chromatin experiments. Each compartment has a volumeof 100 pi anda thickness of 1 mm. Thewindows of the cells consist of thin (25 pm) mica sheets. A compartment can be filled from the top using a syringe fitted with a hypodemic needle. Temperature is monitored by means of a sensor placed in the first compartment...

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