Miniaturization problems

Tia, as well as some other metals, can undergo a phenomenon where tiny metal filaments, called whiskers, form randomly on parts used ia electrical apphcafions. In low voltage, miniature circuitry, whiskers can cause short circuits. Alloys having 2% lead minimum or 0.5% bismuth or heat treatments are said to overcome the problem. A specification for electroplated fin coafings is available (133). 

Turboexpanders eurrently in operation range in size from about 1 hp to above 10,000 hp. In the small sizes, the problems are miniaturization, Reynolds Number effeets, heat transfer, seal, and meehanieal problems, and often inelude bearing and eritieal speed eoneerns. In intermediate sizes, these problems beeome less signifieant, but bearing rubbing speeds and vibration beeome inereasingly important. 

Miniaturized catheter-type ISE sensors, such as the implantable probe shown in Figure 5-20 represent the preferred approach for routine clinical in-vivo monitoring of blood electrolytes. For these intravascular measurements the reference electrode is placed outside die artery (in die external arm of die catheter), tints obviating biocompatability and drift problems associated with its direct contact with the blood. 

From the above-given condensed review of the EEP detection methods one can infer that none of these methods can independently satisfy all the requirements specified for the study of heterogeneous processes involving the EEP participation. To our opinion, the application of semiconductor sensors for detection of EEPs can be provided by a combination of required qualities. The sensors are highly sensitive, miniature, can be operated within wide ranges of gas temperatures and pressures, and are made of simple devices. At the same time, a series of problems arise connected with the preliminary preparation of sensors and improving their selectivity. These and other questions of general nature will be considered in the section that follows. 

The electrolyte volume of the STM cells is usually very small (ofthe order of a 100 pi in the above-described case) and evaporation of the solution can create problems in long-term experiments. Miniature reference electrodes, mostly saturated calomel electrodes (SCE), have been described in the literature [25], although they are hardly used anymore in our laboratory for practical reasons Cleaning the glassware in caroic acid becomes cumbersome. For most studies, a simple Pt wire, immersed directly into solution, is a convenient, low-noise quasireference electrode. The Pt wire is readily cleaned by holding it into a Bunsen flame, and it provides a fairly constant reference potential of fcj>i — + 0.55 0.05 V versus SCE for 0.1 M sulfuric or perchloric acid solutions (+ 0.67 0.05 V for 0.1 M nitric acid), which has to be checked from time to time and for different solutions. 

The current trends toward miniaturization and the need of massively parallel measurements led to the development of biochips. In this area, biocatalyzed and electrogenerated chemiluminescence reactions appear attractive and represent an alternative to fluorescence detection which is still widespread used despite the numerous problems of quantitative measurements and interference fluorescence emission. 

Although glass pH electrodes are, in general, simple to use and available at a reasonable cost, they are limited by the potential problems of glass breakage [65] and miniaturization difficulties [60, 66], One of the alternative approaches to preparation of non-glass pH sensors is to use polymer-based pH sensitive membranes to replace solid glass membranes. 

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