Engineering and mapping entropy changes at the nanoscale in oxide materials

"The transition from order to disorder is a defining characteristic for many structural phase transformations in solids. Changes in entropy associated with ordering processes can elicit changes in temperature, when driven under adiabatic conditions [1]. In the family of electrocaloric materials, cooling is achieved through electric field induced changes in dipole ordering, typically in the vicinity of ferroelectric and antiferroelectric transitions. In recent years, noteworthy electrocaloric performance reported in ferroelectric thin-films [2] has revived interest in these materials for solid state cooling applications and proof of concept heat pumps have been demonstrated. Even though electrocaloric effects have been explored across numerous material systems, the microstructural origins of the effect are not well understood due to challenges in carrying out direct measurements with nanoscale resolution. As a result, the thermal changes that may be expected to accompany the microstructural nucleation and growth processes have not been directly mapped and correlated at the nanoscale. Furthermore, unusual dynamics at the dipole level have been suggested to rationalise large electrocaloric effects in antiferroelectric materials, but these remain to be verified experimentally [3].

Despite the resurgence of interest in electrocaloric materials, little is known about this effect at the microscopic level. This project aims to map and understand dipolar disorder through nanoscale spatially resolved temperature mapping and has two key objectives: firstly, to correlate entropic temperature changes with progressive microstructural development, and secondly, to explore gradient fields as a means to generate local disorder and associated temperature change. Planned experimental investigations will include:

(i) Carrying out nanoscale resolved temperature mapping of caloric materials undergoing transition using Scanning Thermal Microscopy (SThM). The SThM facility is a cutting edge atomic force microscopy (AFM) imaging mode available at the Centre for Nanostructured Media (CNM) and the candidate will build on the existing knowledge base to map spatial nanoscale temperature distributions associated with microstructural development.
(ii) Investigating the effect of gradient fields (e.g. strain gradients) on local disorder and their implications for caloric effects. It is well known that AFM probes can be used to apply up to GPa of stress at the nanoscale; the resulting strain fields are highly inhomogeneous and can affect polar ordering through piezo- and flexo-electric effects [4]. The in-house AFM system at CNM will be used to apply local pressure and to monitor caloric effects that develop because of induced strain gradients.

This project falls primarily within the remit of the 'Functional Ceramics and Inorganics' research area in the Physical Sciences Theme.

References:
[1] Moya et al. Nature Materials 13, 439-450 (2014). [2] Mischenko et al. Science 311, 5765 (2006).[3] W. Geng et al. Advanced Materials 20, 3165-3169 (2015). [4] H. Lu et al. Science 336, 6077 (2012).
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