Peltier effect principle

The Peltier effect has been shown to be reversible, and is. therefore subject to Le Chatelier s principle, like all reversible processes. Thus, when the boundary between two metals is heated, an electromotive force must be produced. The direction of this E.M.F. must be such as to oppose a current which would produce a positive Peltier effect (that is, a heating effect) at the boundary. Thus a current will flow in a closed circuit made up of two different metals when the two boundaries are at different temperatures. No current will flow when the two boundaries are at the same temperature, even when the temperature at other parts of the circuit is not uniform. In the first case the E.M.F. in the circuit is E = — E2+E — E2, where E and 2... 

Figure 12.8 The two mtegories of detectors used for energy dispersive X-ray fluorescence spectrometry. (a) Proportional counter used in pulse mode (b) Cooled Si/Li diode detector using Peltier effect (XR detector by Amptek Inc.) (c) Functioning principle of a scintillation detector containing a large size reverse polarized semi-conductor crystal. Each incident photon generates a variable number of electron-hole pairs. The very high quantum yield enables the use of low power primary sources of X-rays (a few watts or radio-isotopic sources).

PELTIER EFFECT - When direct current is passed through two adjacent metals one junction will become cooler and the other will become warmer. This principle is the basis of thermoelectric refrigeration. 

Another method of moderate cooling, working at ambient temperature and above, uses the Peltier effect. The principle of operation is shown graphically in Figure 7.1. Two different conductors, A and B, are connected in electric contact with one another (thermocouple see Section 2.3.2). If the circuit is connected with a current source, one junction becomes warm while the other cools down. The effect is reversible that is, when the current is reversed, the previously warm junction becomes cold. Moreover, the power absorbed at the cold junction equals that released at the warm one, namely... 

Here are two examples of coupling. Suppose you solder together two different metals to make a junction. If you apply a voltage to drive a current flow, it can cause heating or cooling of the junction. This is called the Peltier effect. But the existence of a reciprocal relation means that if you heat or cool a bimetallic junction instead, an electrical current will flow. This is the principle of operation for thermocouples, which convert temperature changes to electrical signals. 

Building a heat flow microcalorimeter is not trivial. Fortunately, a variety of modern commercial instruments are available. Some of these differ significantly from those just described, but the basic principles prevail. The main difference concerns the thermopiles, which are now semiconducting thermocouple plates instead of a series of wire thermocouples. This important modification was introduced by Wadso in 1968 [161], The thermocouple plates have a high thermal conductivity and a low electrical resistance and are sensitive to temperature differences of about 10-6 K. Their high thermal conductivity ensures that the heat transfer occurs fast enough to avoid the need for the Peltier or Joule effects. 

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