Direct measurement of electrocaloric effect based on multi-harmonic lock-in thermography

We report on a direct measurement method for electrocaloric effects, the heating/cooling upon application/removal of an electric field in dielectric materials, based on a lock-in thermography technique. By use of sinusoidal excitation and multi-harmonic detection, the actual temperature change can be measured by a single measurement in the frequency domain even when the electrocaloric effect shows a nonlinear response to the excitation field. We demonstrated the method by measuring the temperature dependence of the electric-field-induced temperature change in two Sr-doped BaTiO₃ systems with different ferroelectric-paraelectric phase transition temperatures, where we introduce the procedure for extracting the pure electrocaloric contribution free from heat losses and Joule heating due to leakage currents. This method can be used irrespective of the type of dielectric material and enables simultaneous estimation of the polarization change and power dissipation during the application of an electric field, making it a convenient imaging measurement method for the electrocaloric effect.