Z-meter: Easy-to-use Application and Theory

Abstract

s The paper is divided into two parts. The first part is an applying one. We present a handy, easy-to-operate useraddressing Z-meter. The device provides measurement of thermoelectric (TE) modules parameters: AC resistance (R), thermoelectric figure-of-merit (Z) and maximum temperature difference (∆Tmax). The second part is of a theoretical value. As a rule, the device exposure does not happen to be ideally insulated and vacuumlike. Consequently need to take into consideration heat losses of various sorts. In this part we are to discuss specific formulae for estimating heat-exchanging uncertainties involved by Z-metering. All the expressions are explicit On the basis of theoretical estimations it is advised and realized in Z-meter new availabilities to measure performance parameters of single stage TE modules with corrections factors, as well as modules mounted into packages and twostage modules. Introduction Applications of thermoelectrical (TE) modules are becoming wider from year to year. Having started from commercial use mainly in consumer fields (TE refrigerators, cool boxes and so on) now applications of TE modules increase dramatically in optoelectronics, electronics laser technique and come from specialized fields, such as military and airspace, to commercial mass production. In many emerging applications TE modules are critical components because they affect the temperature of the whole device, can have an effect on its correct operation, and impact on the heat dissipation. That is why reliability requirements to TE modules are very high and are still growing even if comparing with consumer applications. That is why severe test procedures (precise and express) are required when a specific TEC type needs to be used in experimental and serial production. Any TEC must provide high performance and a long operation lifetime without failure. The suggested failure criteria for reliability tests are the following: A drop in TEC maximum cooling capacity (∆Tmax) below its specified rating. Measurements of Figure-of-Merit Z is used here as ∆Tmax =f(Z) An increase (5% is usual value) or higher in TEC resistance. The reason of setting forward the two parameters is due to the AC Resistance (R) and Figure-of-Merit Z (and, therefore, ∆Tmax calculated based on the Z value) being very sensitive to latent defects or damage of TE modules. The slightest change of these parameters during operation or storage could be the result of destruction of the TE module. That is why AC R and Z are very useful for control of TE modules quality and reliability. Although there are a lot of methods for measurement of TE modules parameters only express methods are useful for certifications and mass production quality control. On the basis of Harman method our company RMT developed its own series of testing facilities (Z-meters) and has developed corresponding methods to examine a range of TE modules, as well as modules mounted into complete devices. 1. Classical Review of Harman Approach Papers describe and develop an approach for measuring thermoelectric (TE) properties and Figure-of-Merit of Peltier modules. This method was first suggested by T.C. Harman in 1958 and bears his name. A small current I passes through the system generating a slight temperature differential along the module pellets. By measuring the Joule and Seebeck voltage drops one can find some thermoelectric parameters. Let us show it. If the Peltier effect results in the temperature gradient T0<T1, in the simplest case the thermal rate equations for a 1stage TEC can be written this way: