Spatially Resolved Imaging of Electrocaloric Effect and the Resultant Heat Flux in Multilayer Capacitors

International audienceMultilayer capacitors (MLCs) are one of the most 11 studied electrocaloric (EC) systems because they are regarded as the 12 most promising configuration for developing electrocaloric prototype 13 devices. Indeed, (1) they combine thin films prone to exhibit the so-14 called giant EC effect; (2) they are made of several hundred active 15 layers; and (3) their internal interdigitated structure alternating highly 16 thermally conductive electrodes with EC layers should dramatically 17 increase their propensity to extract heat. Although the latter has been 18 foreseen by modeling, it has never been observed experimentally. Here 19 we use an infrared camera to observe for the first time the heat flux 20 crossing standard EC MLCs during operation. Interestingly, we 21 observed that heat flux perpendicular to the terminal−terminal 22 direction accounts for only 20% of the total heat flux. This observation 23 strongly suggests that all future electrocaloric cooling devices should rely on a design favoring heat exchange through the 24 MLC terminals. 25 T he electrocaloric effect (ECE) is defined as the 26 adiabatic temperature change, ΔT, or isothermal 27 entropy change, ΔS, of a material upon application 28 or withdrawal of an external electric field, ΔE. 1,2 Electrocaloric 29 refrigeration based on ECE is regarded as a solid-state 30 alternative to conventional vapor-compression cooling technol-31 ogy. 3−7 Interestingly, electrocaloric refrigerator prototypes 32 based on multilayer capacitors (MLCs) (see the typical f1 33 structure as shown in Figure 1a) were recently designed 34 attesting to the potential of MLCs for cooling applications. 8−11 35 The promise of using MLCs is that heat flow can be conducted 36 by the inner interdigitated electrodes. 12,13 Previous works 37 assumed a homogeneous distribution of temperature change in 38 the electrocaloric active ceramic regions, and the behavior of 39 electrocaloric heat flux between the center of ceramic regions 40 and the metallic terminals was only foreseen by modeling. 12,14 41 Direct experiments are still lacking; however, understanding the 42 heat flow behavior in specific caloric prototype devices is crucial 43 to optimize refrigeration efficiency. 6 For instance, in active 44 magnetic regeneration, the control of heat flux direction has 45 been shown to be very useful to enhance the operating 46 frequency and thus the power density of the device. 15 47 Interestingly, infrared (IR) imaging can now enable a direct 48 determination of caloric effects both temporally and 49 spatially. 16−24 For instance, previous studies have demonstrated 50 the ability of this tool to measure real-time caloric responses 51 with good precision, which was confirmed by simultaneously 52 monitoring using a thermocouple 17,19,21 or by comparison with 53 differential scanning calorimetry (DSC). 20 The noncontact IR 54 camera is compact and fast compared to other techniques, i.e., 55 DSC, and its use is rather simple. More importantly, the IR 56 camera can directly image spatial caloric response, which is the 57 main advantage over other techniques such as DSC, 58 thermocouple, and other specifically designed calorimeters. As 59 a result, spatially resolved measurements have been conducted 60 to study the local inhomogeneity of elastocaloric and 61 magnetocaloric responses relevant to the microstructure 62 nucleation dynamics of phase transition 17,23 and correlated to 63 local magnetic field strength. 16 However, spatial imaging of the 64 ECE is rarely reported. The ECE is usually characterized in the 65 literature by isotropic ΔT with homogeneous distribution along 66 samples, except in very few studies. 21,25,26 For instance, 67 anisotropy of electrocaloric cooling (∼0.20−0.25 K) due t