Simultaneous Thermal Imaging of Peltier and Joule Effects

We present a novel thermoreflectance technique to visualise quasi-static Peltier and Joule thermal distributions simultaneously, enabling side-by-side comparison in situ. The main principle is an extension of so called ’-bucket’ imaging techniques well documented in the literature. The method is particularly advantageous in improving both the reliability and turnaround time for the characterisation of thermoelectric materials and related devices such as microcoolers. In essence, separation of Peltier and Joule effects relies on their respective linear and quadratic current dependence. Established techniques employ two distinct runs of data averaging: one with forward and one with reverse supply polarity. Post processing of the two images provides the Peltier and Joule terms, based on the sign change induced to the former. In practice however, the polarity switch often gives rise to issues with probing and modified contact resistance. Sample movement and drifting lab conditions also introduce asymmetry into the runs, further distorting the measurement. Finally, the experimenter must wait for full completion of both runs to obtain actual results. The key to the proposed technique is that a bipolar excitation is used. Namely, we supply a slow sine wave with zero offset to the DUT. This causes the Peltier term to manifest itself in the first temperature harmonic, while the Joule term has a second harmonic component. A phase-locked CCD oversamples the signal  times (instead of ) per period. Appropriate image processing yields magnitude and phase distributions for both harmonics, as well as the DC component. The method thus provides immediate and simultaneous access to the Peltier and Joule terms, and enables real-time fine tuning of the experiment if necessary. Validation against the unipolar approach shows a nearly perfect match. Characterisation of a heater and p-type thermoelectric sample further illustrates the technique.