Influence of mechanical boundary conditions on the electrocaloric properties of ferroelectric thin films

Abstract

A thermodynamic analysis of the electrocaloric ͑EC͒ effect in BaTiO 3 ferroelectric thin films has been carried out under differing mechanical boundary conditions. It is shown that both the magnitude of the electrocaloric effect and temperature at which it is maximized depend not only on the extent of the applied field change but also on the value of the initial field. For initial fields smaller than a critical value the EC effect is largest at the phase transition temperature but the effect is a strong function of temperature. For external electrical fields larger than this value, conversely, the EC effect is the largest at a higher temperature and is a weak function of temperature. Perfect lateral clamping transforms the first-order phase transition into a second-order transition, lowering the magnitude of the electrocaloric effect and dependence on temperature. Compressive and tensile misfit strains also alter the nature of the phase transition and affect the electrocaloric properties in an analogous way. A compressive misfit strain shifts the maximum in the EC effect to higher temperatures, reduces its magnitude, and reduces its dependence on temperature, while tensile misfit strain results in the opposite effects. Control of the misfit strain by appropriate choice of substrate provides potential means to vary both the magnitude and the temperature sensitivity of the EC effect for use in cooling or thermodielectric power conversion devices.

Influence of epitaxial strain on elastocaloric effect in ferroelectric thin films

“…12 Moreover, changing the mechanical boundary conditions by varying the epitaxial misfit strain can remarkably affect the phase transition, which may be helpful to tune the elastocaloric properties of ferroelectric thin films. [16][17][18][19] This is of great interest and importance to design the elastocaloric cooling devices in the future and therefore motivates us to study the elastocaloric effect in ferroelectric thin films under the influence of epitaxial strain.…”

“…[36][37][38][39] In particular, the electrocaloric properties of BTO thin films have been intensively investigated. 12,18,19,26,[40][41][42][43][44][45][46][47] Interestingly, it was reported that applying a uniaxial compressive stress could enhance and widen the electrocaloric response considerably in ultrathin BTO films due to tuning of the depolarization field. 26 It is also reported that the electrocaloric peak under tensile stresses moves towards higher temperatures with its magnitude slightly enhanced in BTO single crystals.…”

Amplitudon and phason modes of electrocaloric energy interconversion

“…If the direction of E remains fixed during the poling, polarisation azimuthal angle θ stays constant, while only the amplitude P 0 changes value. Such amplitudon mode of polarisation switching is usually seen in conventional EC materials, 5,6,34,35 where the largest variations in P 0 and thus the largest changes in p γ and ΔT occur near T C .…”

“…For the poling field ΔE γ $ 200 kV=cm, we obtain the values of ΔT in the range of 1-2 K (Supplementary Figure 2), which is similar to the performance of conventional EC materials undergoing amplitudon-polarisation switching. 5,6,34,35 In the case of low-field poling, i.e., for E γ;a $ 1 -5 kV=cm and ΔE γ 10-30 kV=cm, the sombrero-hat energy landscape is only slightly perturbed. If the P99 [100] state of Figures 1b and c is taken as a starting point, applying noncollinear E induces a rotation of the polar vector along the minimum-energy groove until P99E.…”

“…The equilibrium state of the system at a given temperature T and strain ε is determined by minimising f with respect to the polarisation vector components P x , P y . System excess heat capacity 34 ΔC and pyroelectric coefficients p γ , γ ¼ x; y, can be obtained from the equilibrium values of polarisation (P 0 ) and energy density f 0 ≡f(P 0 ) as…”