As an intriguing alternative to the Peltier effect, the electrocaloric (EC) effect refers to changes in the temperature of a material when an external electric field is applied to and removed from the material. Since the EC effect is a reversible phenomenon, the efficiency of EC cooling can closely approach the Carnot limit. Despite its great potential, technical challenges remain to realize practical coolers based on this giant EC effect. The ∆T achievable in bulk materials is typically small and high ∆T can only be achieved so far in thin films of both ceramics and polymers. Due to their small thermal masses, thin film EC materials must undergo thermodynamic cycles at relatively high frequencies to achieve high cooling power densities per unit area. However, high frequency operations may also lead to significant losses, which in turn pose more stringent limits on the acceptable on/off ratio as well as the off-state thermal resistance of thermal switches. In addition, innovative structures need to be designed to compensate for the small thermal masses by stacking multiple EC thin-films together while avoiding temperature non-uniformity due to the low thermal conductivity of typical EC materials.
In the present work, we first design a switchable thermal interface based on an array of discrete liquid droplets initially confined on hydrophilic islands on a substrate. The rupture distance, loading pressure, thermal resistance in the on-state and the reliability of the interface are evaluated both theoretically and experimentally to find the optimal design for practical applications.
We also present a solid-liquid hybrid thermal interface for reliable low-contact pressure (< 1 kPa) switching with on-state thermal contact resistance < 15 x10-6 m2K/W. Reduction in the thermal resistance of hybrid interfaces created through electroplating is evaluated using transient pulsed heating measurements and thermal time constant characterization.
We then demonstrate our new device concept of a compact solid-state cooling module based on the giant electrocaloric effect. Our novel design realizes controllable thermodynamic cycles for the ferroelectric material thin films with laterally interdigitated electrodes. It also exploits the liquid-based thermal interfaces to enable reliable and high-contrast thermal switching and at the same time offer high off-state resistance to minimize deleterious parasitic heat transfer. We numerically simulate the cooling performance, as well as experimentally confirm ~1 K temperature drop at the operation frequency of ~0.3 Hz.
Since the performance of the current EC cooling device is limited by the material intrinsic properties, we design two methods to characterize the EC effect so that we can find the optimal material for high performance solid-state EC refrigerator. One is a direct thermal characterization of the electrocaloric (EC) heating and cooling in thin films supported on substrates using microfabricated thin-film resistance thermometers. We analyze the temporal temperature profiles through full three-dimensional heat diffusion modeling to obtain effective adiabatic temperature changes.
The other is a direct characterization of the frequency dependent temperature responses in EC thin films under AC bias electric fields using a high-precision lock-in technique. The temperature responses detected by embedded thin-film resistance thermometers are analyzed using the steady periodic solutions of the 3D heat conduction model to extract the equivalent volumetric heat sources, corresponding respectively to the electrocaloric effects and hysteresis losses.