Tunable and anomalous electrocaloric behaviors in Bi0.5Na0.5TiO3-based relaxor enabled by dynamics of polar nanoregions

Abstract

Eco-friendly electrocaloric (EC) refrigeration technology , by virtue of its high efficiency and a cutting-edge decarbonization strategy, has become a promising alternative for vapor compression refrigeration. The EC effect in Bi 0.5 Na 0.5 TiO 3 -based ferroelectric is prominent since their high ferroelectricity and flexible phase structure regulation. However, the underlying mechanisms responsible for high and anomalous EC performances in Bi 0.5 Na 0.5 TiO 3 -based ceramics are not fully addressed. In this work, a high Δ T with 0.66–0.94 K and a large temperature span (Δ T span ) of 50–60 K are achieved in Bi 0.5 Na 0.5 TiO 3 -Ba(Ti, Hf)O 3 -NaNbO 3 system. The EC effect exhibits a tunable behavior, i.e., the temperature corresponds to the maximal Δ T value (Δ T max ) gradually decreases via downshifting critical freezing temperature ( T f ). This phenomenon is accompanied by the collapse of domains into polar nanoregions (PNRs) and a reduction in correlation length, as supported by inverse fast Fourier transformation analysis and piezoelectric force microscopy images. Furthermore, the asymmetrical EC profiles with a large gap between exothermic and endothermic EC peaks near T f are largely ascribed to the time effect for PNRs evolution, as demonstrated by time-resolved pulsed dielectric spectra and customized polarization test. Interestingly, high Δ T is maintained as T ≥ T f but sharply declines as T < T f , which originates from a crossover from activated to frozen ferroelectric state. These findings reveal that PNRs dynamics strongly impact on EC performances, with high Δ T and broad Δ T span primarily arising from highly polar and mobile PNRs. This work not only gives a deep insight into EC behaviors in Bi 0.5 Na 0.5 TiO 3 -based ceramics but also proposes a refreshing strategy for optimizing EC performance with regulating PNRs dynamics.

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