Effect Of Strain On The Electrocaloric Effect Of Ferroelectrics Materials

The electrocaloric effect (ECE) refers to the temperature change of a material under adiabatic condition in response to an applied electric field. Compared with existing vapor compression technology, electriccaloric technology is a new, efficient and environmentally friendly way. Due to their significantly temperature-dependent spontaneous polarization, ferroelectric materials based on the perovskite-structured oxides possess large ECE, and have been considered as prime candidates for solid state cooling devices. As we all know, the change of domain structure in ferroelectric materials can significantly alter its piezoelectric, dielectric properties, and then the domain transition of the ferroelectric materials will have a significant effect on its ECE. Furthermore, the previous study revealed that there exists multi-domain to mono-domain transition driven by temperature in strained ferroelectrics. Therefore, the domain transition driven by strain can improve the electrocaloric properties of multi-domain ferroelectrics. Based on the thermodynamic theory of the ferroelectric materials, the phase-field model was employed to study the effects of strain on ECE of ferroelectric material, such as multi-domain ferroelectrics, ferroelectric nanotubes and ferroelectric polycrystals.The simulations show that the transition from multi-domain to mono-domain in ferroelectrics can induce the large ECE, and the huge ECE induced by domain transition exhibits only in a range of temperature close to the domain transition temperature, which is similar to that the phase transition induced large ECE in ferroelectrics. The domain transition temperature with large ECE can be tuned by external strains as the phase transition temperature. The domain transition temperature is lower than the Curie temperature, which enlarges the range of temperature where the large ECE exhibits in ferroelectrics. A tensile strain not only increases the domain transition temperature but also induces a much larger average temperature change than a compressive one. However, for the ferroelectric nanotubes, a giant adiabatic temperature change exists near room temperature in the ferroelectric nanotubes subjected to an in-plane compressive strain. The giant adiabatic temperature is induced by the extrinsic contribution from multi-domain to mono-domain transition, which is much larger than the intrinsic one. This finding provides a novel insight into the electrocaloric response of ferroelectric nanostructures. Finally, for ferroelectric polycrystals, the evolution of domain structure indicates that domain switching occurs preferentially both in the grain boundaries with lower orientation difference and the grain’s interior which has a relative higher orientation with respect to the global coordinate in ferroelectric polycrystals. Compared no strain, the multi-domain can be easily formed under tensile strain to some extent. However, the tensile strain weakens the electrocaloric properties of ferroelectric polycrystals than no strain.In summary, the epitaxial strain plays an essential role in the domain transition, which can be used to tune the magnitude of the adiabatic temperature change. Careful selection of external strain allows us to harness the extrinsic contribution of domain transition to obtain large adiabatic temperature change.