Direct Local Measurements of the Electrocaloric Effect by Scanning Thermal Microscopy

Interest in using the electrocaloric effect for environmentally friendly solid-state refrigeration has been rejuvenated by studies in multilayer ceramic capacitors [1,2] and discovery of outsized effects in thin films [3]. While the electrocaloric effect can be well described macroscopically through a thermodynamic approach, the effect is not as well characterized or understood at microscopic length scales. In this regard, infrared thermal imaging has led the way by enabling in-situ imaging of electrocaloric temperature distributions [4], although spatial resolution is diffraction limited to a few microns.

Scanning Thermal Microscopy is emerging as a useful Atomic Force Microscope based platform for mapping temperature distributions on the nanoscale. In this technique, temperature-induced variations of the resistance of a sharp metallic probe are recorded as it is rastered over the sample surface, allowing maps of temperature to be obtained. To date, however, its use for studying the electrocaloric effect has been limited [5,6], likely in part due to the challenge of performing contact thermometry while electric fields are being applied. We build on previous studies to show that Scanning Thermal Microscopy can be used to spatially map electrocaloric temperature changes across microscopic length scales on the active material, here demonstrated in cross-sectioned BaTiO₃-based and PST multilayer ceramic capacitors. In our approach [7], the electrocaloric response is measured at discrete points on the active ceramic surface with separation on the order of 100 nm, allowing for reconstruction of spatial maps of heating and cooling as well as their temporal evolution. In ongoing work, we are also carrying out high voltage Kelvin Probe Force Microscopy to determine the surface electric field distributions, so that any temperature and electric field heterogeneity can be correlated. From this point of view, the Scanning Probe Microscope can provide a complementary platform for studying electrocaloric effects on microstructural length scales, that might otherwise be challenging for infrared camera investigations.

[1] S. Kar-Narayan & N. D. Mathur J. Phys. D: Appl. Phys. 43, 032002 (2010). [2] B. Nair et al. Nature 575, 468 (2019). [3] A. S. Mischenko et al. Science 311, 1270–1 (2006). [4] P. Vales-Castro, et al. Adv. Electron. Mater. 7, 2100380 (2021). [5] S. Kar-Narayan, et al. Appl. Phys. Lett. 102, 032903 (2013). [6] D. Shan et al. Nano Energy 67, 104203 (2020). [7] O. E. Baxter, et al., J. Phys. Energy 5, 045009 (2023).