Date of Completion

5-11-2017

Embargo Period

11-6-2017

Keywords

Ferroelectric, Electrocaloric, Mechanocaloric, Elastocaloric, Barocaloric, Flexoelectric, Flexocaloric

Major Advisor

S. Pamir Alpay

Associate Advisor

Mark Aindow

Associate Advisor

Serge M. Nakhmanson

Associate Advisor

Puxian Gao

Associate Advisor

Seok–Woo Lee

Field of Study

Materials Science and Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

There exist multiple driving forces in solid–state materials that can be utilized for entropy changes and hence stronger caloric response. In multiferroic materials, adiabatic temperature changes (ΔTad) can be obtained via combined application of electric, stress, and magnetic fields. These external stimuli provide additional channels of entropy variations resulting in a multi–caloric response. In ferroelectric (FE) materials, caloric responses can be obtained with the application of electric and mechanical fields. Here, we compute the intrinsic electrocaloric and elastocaloric of prototypical FE materials using Landau–Devonshire theory of phase transformations with appropriate electrical and electro–mechanical boundary conditions. Also, the flexocaloric response of FE material systems are computed due to generation of strain gradient induced misfit dislocations. Our electrocaloric calculations indicate that the intrinsic ΔTad in relaxor FEs are substantial and do not vary much over a large temperature interval. Also, we show that an elastocaloric ΔTad of 12.7 ◦C can be obtained in PbTiO3 with the application of uniaxial tensile stress of 500 MPa near its Curie point. Moreover, flexocaloric ΔTad exceeding 1.81 °C can be realized in 20 nm thick barium titanate films. We show a strong link between strain relaxation and strain gradients in epitaxial films and their caloric response. These findings indicate that caloric responses in ferroic materials can be deterministically controlled and enhanced by utilizing a variety of external stimuli. Our results suggest a promising perspective to find solid–state systems with giant caloric responses to be used as alternatives for conventional refrigeration technologies.

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