Recently, I read an article on Seebeck coefficience(Article Determination of the thermopower of microscale samples with ...
). In this article, the authors used an AC tech to measure the Seebeck of film sample. However, in the '2.Experimental methodology' section, the heater was droven by current at a frequency of ω, while both thermal and electronical data was acquired at a frequancy of 2ω. I heared that there are also 3ω or more if needed. I wonder know how to choose the right frequncy or what difference between them when doing research?
Hello Ziming
An electrical current with the frequency w generates a temperature variation of 2w due to the fact, that negativ current amplitudes generate positive temperature variations. (A filament lamp driven with a ac current of frequency w generates light with the frequency 2w.)
If the temperatur varies with 2w, the resistance of circuit pathes varies with 2w too. The Fourier spectrum of such a signal is not harmonic and has harmonics of higher order, like 3w and so on. But the signal amplitude is smaller than for 2w.
The problem by using ac-currents for seebeck measurements are the capacities (all blind resistancies) of your circuit. This could produce undesirable effects which are difficult to control.
For thermal measurements is important: Wait for the achievment of equilibrium case. The time constants can be high and collide with frequency. If the heater current is zero, the heat flow is not.
With Regard
R. Mitdank
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