Use this URL to cite or link to this record in EThOS: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.815068
Title: Precise nanoscale characterisation of novel Heusler thermoelectrics via analytical electron microscopy
Author: Webster, Robert William Henry
ISNI:       0000 0004 9356 4429
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2020
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Abstract:
Thermoelectric power generation presents an opportunity to `scavenge' energy that would otherwise be wasted as heat. Heusler alloys, a class of materials often comprising inexpensive, non-toxic elements, are promising for practical use in a new generation of thermoelectric devices. Recently, efficient thermoelectric Heusler alloys have overcome a performance-limiting thermal conductivity through the introduction of nanostructures that scatter phonons and impede thermal transport. However, the nature and stability of nanostructures can be difficult to discern, especially the minor compositional variations that derive from inhomogeneous phase segregation. Throughout this thesis TiNiSn, which forms the basis for some of the most promising n-type half-Heusler thermoelectrics, is studied through a unique combination of elemental and diffractive analysis in the scanning transmission electron microscope (STEM). Epitaxial thin films of TiNiSn are grown by pulsed laser deposition and FIB-prepared cross-sections of these are characterised in STEM with a focus on aberration-corrected STEM-EELS spectrum imaging and scanning precession electron diffraction (SPED), yielding precise chemical and structural quantification with nanoscale spatial resolution. The results throughout this thesis demonstrate the importance of STEM for quantitative studies of thermoelectric materials, as it can provide the analytical precision required for accurate identification of minority phases in TiNiSn specimens that would otherwise be overlooked in bulk analytical techniques. Sensitivity to very small elemental concentrations is a cornerstone of the use of STEM-EELS for chemical characterisation. Precisions of 0.3 % were achieved through adoption and development of refined, reference-based, absolute elemental quantification protocols which were essential in overcoming difficulties with large uncertainties posed by conventional methods. The success of this approach, in part, is due to advances made in characterisation of experimental conditions including, for the first time, an automated, standard-less approach to the measurement and correction of energy dispersion non-uniformities. Dispersion correction enables reliable, absolute calibration of energy-loss in spectra to yield a precision better than 0.1 eV. These developments in STEM-EELS were then used in three investigations of TiNiSn thin films exploring aspects of nanostructuring, phase segregation and crystrallographic strain and coherency. We discovered the spontaneous formation of nanostructures during thin film growth, gaining some insight into the phase segregation mechanisms that lead to their nucleation. Novel in situ STEM studies of phase segregation facilitated direct observations of the thermal evolution of nanoscale phases and results enabled characterisation of diffusion rates of Ni migration between full- and half-Heusler phases, for which the activation energy was calculated as 0.3~eV. Combining SPED with advances in detector technology, STEM structural investigations highlighted an interesting strain texture associated with nanostructuring of the half-Heusler thin films. Finally, combining SPED results with STEM-EELS measurements is proposed as a route to `correlative-STEM' analysis, which unifies nanoscale chemical and structural information for greater insights into the impact of nanostructures in thermoelectrics.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.815068  DOI: Not available
Keywords: QC Physics
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