Development of a Scanning Thermal Microscopy method for high-resolution spatial mapping of the electrocaloric effect

Abstract

Electrocaloric materials have gained attention in recent years for their potential use in environmentally friendly solid-state refrigeration. The electrocaloric effect (ECE) describes a reversible change in temperature upon application and removal of an applied electric field. While the electrocaloric effect has been extensively studied at the macroscopic scale in bulk materials, there are comparatively very few studies addressing the effect at the nanoscale. This thesis has revolved around the development of a novel, Scanning Thermal Microscopy (SThM) based approach for direct, spatially resolved measurements of the electrocaloric effect to explore the effect of naturally-occurring and artificially-induced microstructural features on the spatial dependence of the ECE.

The first part of this thesis discusses the development of the novel SThM-based approach to resolve the ECE On microscopic length scales. Proof-of-concept measurements are presented for a barium titanate (BTO) multi-layer ceramic capacitor (MLCC) to show that the technique is capable of measuring the EC response with sub-micron spatial resolution, with the ultimate maximum resolution possible likely to be limited by the tip radius. Subsequent chapters focus on the implementation of this new electrocaloric mapping technique on Prototype lead scandium tantalate (PST) MLCCs possessing high electrocaloric performance. Local electrocaloric measurements have been compared to existing indirect and direct measurements of the same samples from literature and were found to be in good agreement. Clear spatial variation in the ECE was observed in a polished cross section of the samples. High voltage Kelvin Probe Force Microscopy Measurements performed in tandem with EC measurements to elucidate how the surface potential and local electric field influenced the EC response. Finally, the effect of naturally-occurring and artificially-induced microstructural features on the spatial homogeneity of the ECE are trialled and discussed.

Thesis is embargoed until 31 July 2030.

Date of Award	Jul 2025
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Engineering and Physical Sciences Research Council
Supervisor	Marty Gregg (Supervisor) & Raymond McQuaid (Supervisor)