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Tamsin O'Reilly

Tamsin O'Reilly

Tamsin joined the CDT in September 2017, having previously completed an MSci in Chemistry at Queen's University Belfast.

In Semester 1 of 2017-18 Tamsin completed a short exploratory research project at the University of Glasgow in Low temperature testing of superconducting thin films and devices, supervised by Professor Robert Hadfield. In Semester 2 of 2017-18, she conducted a practical research project at Queen's University Belfast supervised by Dr Bob PollardSpatial ellipsometric mapping of thin films.

 

 

CDT PhD Project

EXPLORING DOMAIN CROSS-TALK AND CYCLING EFFECTS IN POLYCRYSTALLINE FERROICS

Supervisors:

Dr Miryam Arredondo, Queen's University Belfast

Dr Damien McGrouther, University of Glasgow

Polycrystalline ferroics are essential materials that form the basis of many of today’s device technologies. They have a wide range of applications, including as sensors and non-volatile memory devices because of their low-cost and fast preparation. The key characteristic of a ferroic material is the development of equivalent ground states, called domains, that form to minimise the free energy of the system when the material undergoes a ferroic phase transition. Ferroic materials exhibit at least one of the primary ferroic properties, where in which domains appear from origins in structural shear (ferroelastics), magnetic moments (ferromagnetics) or electric dipole moments (ferroelectrics). The functionality of ferroics arise from the reversible switching of domains, which are physically separated by domain walls. Many of the electrical and mechanical properties of ferroelectric materials, for example, are governed by the formation and mobility of domains and their domain walls. The domain pattern greatly influences the final properties of the material and it is now widely accepted that the macroscopic properties of ferroic materials are due to the domains and domain walls’ collective phenomena that occurs on the nanoscale.

Recently, there has been increasing interest in the investigation of the domain patterns in polycrystalline ferroics, since these are frequently used in many of today’s devices such as capacitors, transducers and thermistors. In comparison to their single crystal counterparts, the polycrystallinity of these materials adds an additional parameter that will influence the final domain structure within the single crystal grains, due to the elastic and electrostatic compatibility that exists between grains. As a result, polycrystalline ferroics have intricate domain patterns and the characterisation of the domains are a challenge and generally considered to be difficult. Most studies on these materials have been mainly driven by piezoresponse force microscopy (PFM) experiments. Despite its great advantage to probe domain structures, artefacts often appear in PFM imaging due to tip-surface interactions and the technique has spatial and time resolution limitations. In recent research, evidence has been found that there is a profound grain size/domain size relationship in ceramic ferroelectric materials. Therefore, grain orientation and grain size play a crucial role in domain switching and evolution which will ultimately govern the material’s properties. Moreover, the possibility of domain cross-talk across grains has been elucidated and macroscopic observations of domain structures in polycrystalline ferroelectrics show that domain walls will regularly traverse across grain boundaries; suggesting domain continuity across grains. However, the conditions needed for this to occur are not yet fully understood. However, in order to optimise the use of polycrystalline ferroics and gain full control over domains for future technological applications, it is imperative to improve our understanding of domain compatibility across grain boundaries in a static and dynamic manner (during the application of external stimuli).

In this study, polycrystalline BaTiO3 is used as a model system to investigate domain compatibility across grains and their associated domain dynamics under heating and biasing conditions. Studies will be conducted to explore whether domains of individual grains of a polycrystalline ferroic behave in a similar way to that of single crystals, and the conditions needed for domains to couple across grain boundaries in a stress-free manner will be investigated. This will involve characterisation of the grains by electron backscatter diffraction (EBSD), lamellae fabrication fabricated by focused ion beam (FIB) and subsequently in-situ heating transmission electron microscopy (TEM) studies.