Mica insulators, thickness

Mica insulators, having a thickness of 0.002-0.003 in., have been used for many years in mounting power devices to heat sinks. Used in conjunction with thermal grease, they provide a low-cost method of reducing the thermal resistance caused by air gaps. Mica has a thermal conductivity of 0.75 W/m K [9]. 

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. 

Most SECM measurements are carried out with the sample under a thick liquid layer, and thus the tip must be sheathed in an insulator to achieve high resolution. SECM measurements can also be carried out within a thin layer of water that forms on the surface of a sample in air. In this case very high resolution can be attained using tips without insulation (e.g., the usual W or Pt-Ir STM-type tips) because the tip area is defined by the small part of the tip that touches the liquid layer (34,35). Studies of mica surfaces, polymer films, and some biological samples as described below are possible by this technique. With this mode it is also possible to fabricate small metal structures in polymer films as demonstrated previously (36). High-resolution electrochemical deposition of silver nanostructures on mica surfaces in humid air was also achieved (35). For detailed discussion on SECM applications for fabrication, see Chapter 13. 

From Table 11-1 we can see that mica creates higher parasitic capacitances despite a lower K, and that is clearly attributable to the smaller thickness of insulator typically required. The same happens when we use some of the modern, expensive, and yet popular polyimide (not polyamide ) insulators which are excellent thermal conductors, but are also very thin. These can be recognized by their typically amber color, and they come in various brand names like Kapton, Kinel, Upilex, Upimol, Vespel, and so on. 

FIGURE 4.10 Typical voltammetric curves in the humidity chamber at 100% RH, 25°C. Radius of tip curvature = 10 pm. In all cases, the voltage was scanned from OV (point S) in either direction and then returned to 0 V the scan rate was 0.2V/s. (A) Pt-Ir tip, Nafion film ( 200nm thick) on mica. (B) Pt-Ir tip, mica substrate treated with solution A. Curves C1-C3 first, second, and third negative scans. Curves A1-A4 first, third, fourth, and fifth positive scans after C3. Curve C4 first negative scan after A4. (C) W tip, mica substrate treated with solution A. (D) W tip, mica substrate treated only with water. (From Fan, F.-R.F. and Bard, A.J., STM on wet insulators Electrochemistry or tunneling Science, 270, 1849-1851, 1995. Reprinted with permission of American Association for the Advancement of Science.)... 

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