Large electrocaloric effect with high thermal and electric field cycling stability in solution-processed Y:HfO2 thin films

Abstract

The electrocaloric effect (ECE) – the zero-emission energy-efficient process in which an applied electric field can reversibly change the entropy in a polar material, is promising for environment-friendly and compact applications like microelectronic cooling and solid-state refrigeration. For such applications, the ECE material must endure numerous thermal and electric field cycles, and long-term thermal and electric field cycling stability of the ECE material should be investigated in detail. We investigated the performance and reliability of the ECE of solution-processed Y:HfO₂ thin films in terms of isothermal entropy change, adiabatic temperature change, isothermal heat and refrigerant capacity under both thermal and electric field cycling. The ECE responses are investigated under thermal (303–423 K) and electric field (10⁶) cycling processes. A large positive ECE response (temperature change, ΔT_max) of up to 24.8 K (with an ECE strength of 0.7 K cm MV⁻¹) was achieved, originating from high polarization and sharp variation in polarization through the phase transition in Y:HfO₂. The isothermal heat (Q) and refrigerant capacity (RC) were 7755 J kg⁻¹ and 822 J kg⁻¹, respectively. The Y:HfO₂ thin films exhibited robust thermal cycling stability with negligible ΔT_max, Q, and RC variations after 40 thermal cycling processes. After 10⁶ electric field cycles, ΔT_max, Q, and RC were 19.4 K, 6060 J kg⁻¹ and 595 J kg⁻¹, respectively. The large and reliable ECE in environment-friendly lead-free Y:HfO₂ thin films deposited directly on a Si-substrate using a facile solution process outperformed the other HfO₂-based and Pb-free ECE materials and will find applications in on-chip microelectronic cooling devices.