Electrocaloric (EC) materials show promise in eco-friendly solid-staterefrigeration and integrable on-chip thermal management. While directmeasurement of EC thin-films still remains challenging, a generic theoreticalframework for quantifying the cooling properties of rich EC materials includingnormal-, relaxor-, organic- and anti-ferroelectrics is imperative forexploiting new flexible and room-temperature cooling alternatives. Here, wepresent a versatile theory that combines Master equation with Maxwell relationsand analytically relates the macroscopic cooling responses in EC materials withthe intrinsic diffuseness of phase transitions and correlation characteristics.Under increased electric fields, both EC entropy and adiabatic temperaturechanges increase quadratically initially, followed by further linear growth andeventual gradual saturation. The upper bound of entropy change (dS_max) islimited by distinct correlation volumes (V_cr) and transition diffuseness. Thelinearity between V_cr and the transition diffuseness is emphasized, whiledS_max=300 kJ/(K.m3) is obtained for Pb0.8Ba0.2ZrO3. The dS_max inantiferroelectric Pb0.95Zr0.05TiO3, Pb0.8Ba0.2ZrO3 and polymeric ferroelectricsscales proportionally with V_cr^(-2.2), owing to the one-dimensional structuralconstraint on lattice-scale depolarization dynamics; whereas dS_max in relaxorand normal ferroelectrics scales as dS_max ~ V_cr^(-0.37), which tallies with adipolar interaction exponent of 2/3 in EC materials and the well-provenfractional dimensionality of 2.5 for ferroelectric domain walls.
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