C - DTU Nanotech - Danmarks Tekniske Universitet

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24 4 Basic Principles of a Peltier Element α = Seebeck coefficient T = Temperature Due to the heat transfer when a temperature gradient is set up across both sides of the junction, thermal conductivity acts to reduce the temperature gradient. This means that the ideal materials A and B need to have a high electrical and a low thermal conductivity. The high Peltier coefficient ΠAB of semiconductors enables the construction of convenient Peltier elements, because they have a good electrical-to-thermal conductivity ratio. The cooling element consists of alternating p–endowed semiconductor (Π > 0) and a n–endowed semiconductor (Π < 0) which are connected electrically in series by thin junctions and thermally in parallel between two ceramic substrates [25] (illustrated in Fig. 4.1). In the endowing process atoms from another element are inserted to a semiconductor crystal to change the conductivity of the semiconductor. These inserted charge carriers can have a negative charge or a positive charge, dependent on the material which is used for the endowing process. Semiconductors which are endowed with negative charge carriers are n–endowed and have consequently free movable electrons. These, which have been endowed with positive charge carriers are called p–endowed and thus have holes. For further readings on the subject of semiconductors, I refer to [10] and also for the subject of Peltier cooling to [12]. The advantages of Peltier elements are their small size, the absence of wear and their silent running. They are often used in the following systems or because of following reasons: • Micro–constructions of cooling or heating systems • Heat transfer to hermetical closed cabinets • Spot cooling or heating • Temperature stabilization • Compensation of heat flow • Mobile application because of their light weight • Long–life • For fast and dynamic response • No moving parts, therefore they require little or no maintenance, ideal for cooling parts that may be sensitive to mechanical vibration • No refrigerants, such as potentially harmful CFCs, therefore environmental and safety benefits
p-semiconductor p-semiconductor 4 Basic Principles of a Peltier Element 25 ceramic substrate Current flow Hole flow Heat flow cooler side warmer side + _ Current flow metal interconnection Electron flow Heat flow n-semiconductor n-semiconductor Figure 4.1: Illustration of the physical structure of a conventional Peltier effect refrigerator constructed from discrete p– and n–type semiconductors. In addition the polarity of energy and charge flows are shown [22]
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Page 69: Experiments with 20 µl PBS-buffer
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Bibliography [1] Arisawa, H.; Brill
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Bibliography 77 [26] Mark, H. F.: E
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Bibliography 79 [50] Zimmermann, H.
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C - DTU Nanotech - Danmarks Tekniske Universitet

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