Thermoelectric Power in Thick Film Resistors

Advances in thick film technology have progressed up to the present time along empirical lines motivated largely by economic factors, so at present we have a poor and inadequate understanding of the transport mechanism(s) in thick film resistors and conductors. Contributions to a better knowledge of these materials could be derived from the measurement of quantities other than those usually considered, which include temperature-dependence of resistivity and noise. This note presents the main results of thermoelectric power measurements in thick film resistors and discusses the observations on the basis of the current models for electrical conduction in these particle-aggregate components. Thick film resistors of the systems PdO/Ag-Pd and Ru2 Bi2 07 with sheet resistivity ranging from 102 to 10 ohm/square have been investigated. Patterns were prepared 8 mm to 10 mm long and mm to 3 mm wide on alumina substrates, terminated with conductive islands of Pd-Ag and Au-Pt compositions. Two small gauge iron-constantan thermocouples were fixed near the extremes of the resistors in order to measure the mean temperature and the temperature gradient along the resistors. Thermoelectric voltages were measured with a Keithley electrometer Mod. 601 (> 1011 ohm input impedance). The temperature dependence of the resistivity was also measured in order to identify the temperature ranges where the temperature coefficient of the resistance (TCR) has positive or negative sign (Figure 1.). Typical results of thermoelectric power a are shown in Figure 2 for PdO/Ag-Pd based resistors, for which a is positive. Results in Ru2 Bi2 07 based resistors have quite similar behaviour except for the sign of a. Interesting features of the thermoelectric power data are: (i) a has very low values, (ii) a is a monotonic function of temperature with no apparent changes of behaviour (no change in sign nor change in slope) near the temperature Tm at which the sign of TCR changes from negative to positive values,