Regulation of thermal transport via ferroic interfaces

Abstract

The thesis reports the work done in measuring the influence of domain walls on heat flow in the ferroelastic LaAlO3 material system. Two main approaches were developed to investigate the thermal properties of domain walls. Firstly, macroscopic heat flow experiments were carried out on bulk single crystals and, secondly, local thermal properties were carried out using a Scanning Probe Microscope technique known as Scanning Thermal Microscopy (SThM). Development of a micropatterned heater approach to measuring thermal properties of nanostructured samples is also discussed.For the bulk measurements, the thermal conductivity of LaAlO3 single crystals was measured as a function of domain wall number density and orientation using the steady state heat flow method at cryogenic temperatures. The results indicated that the measurement precision of our home-built system was not sufficient to resolve the effect of domain walls on heat flow. Supporting second-principle simulations of the thermal transport properties of individual domain walls suggested that a higher volume proportion of domain wall material would be required to resolve their cumulative effect in our measurements.For local imaging of thermal properties using SThM, a very sharp, temperature-sensitive probe, is used to collect spatially resolved temperature maps of the LaAlO3 samples under an imposed thermal gradient. Steps in temperature across domain walls were expected due to their thermal boundary resistance, however, these features could not be resolved in the experimentally obtained temperature maps due to challenges in stabilising large enough thermal gradients. In response to this, a micropatterned heater and sample geometry is explored with a view to maximising the achievable thermal gradient. Finite element modelling suggests that three orders of magnitude increase in the thermal gradient can be achieved, compared to the macroscopic geometry

Thesis embargoed until 31st December 2027

Date of Award	Dec 2022
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Northern Ireland Department for the Economy
Supervisor	Raymond McQuaid (Supervisor) & Marty Gregg (Supervisor)