Differential ac method of thermopower measurement

Abstract

A new differential ac technique is described for rapid and continuous measurement of thermoelectric power of thin metallic ribbons over a wide temperature range. Sinusoidal heating of one thermocouple junction by means of a small resistive heater results in a measuring principle which eliminates spurious thermoelectric voltages in the lead wires and favorably benefits from ac signal processing. The resultant equilibrium temperature of the heated junction contains a constant term depending on thermal parameters of the system, and an oscillating term with a frequency-dependent amplitude. By choosing a suitable sample configuration no corrections are required for the thermal response of the system. By careful shielding of the measuring circuits and using modern lock-in amplifiers noise voltages can be reduced to less than 1 nV for measuring frequencies below 10 Hz. Measurements accurate to 1% are possible, using temperature differences smaller than 10 mK. The effectiveness of this technique is demonstrated by comparing our measurement of the thermoelectric power of an amorphous Fe80B20 alloy with measurements using conventional techniques and by studying the thermopower of an amorphous Fe90Zr10 alloy in the vicinity of its second-order magnetic phase transition.