Plasma defect engineered and characterisation of yttria-stabilised zirconia

Shi, Fangzhou ORCID: https://orcid.org/0009-0008-2352-2043 (2024). Plasma defect engineered and characterisation of yttria-stabilised zirconia. University of Birmingham. Ph.D.

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Abstract

Oxygen-deficient zirconia (ZrO\(_{2-α}\)) holds great promise for applications demanding light absorbing materials and has recently become the focus of many high-quality studies. Despite various approaches taken to manufacture this material, ranging from electrical, chemical, and physical methods, achieving commercial success in the production of bulk ZrO\(_{2-α}\) has proven challenging to date. This project aimed to investigate the feasibility and transformation mechanism for plasma-induced blackening of zirconia (ZrO\(_{2-α}\)) and to illustrate the resulting change in properties and potential for applications.
Different types of plasma technologies (DC plasma and active-screen plasma), variations in DC plasma treatment configurations (contact conditions and cathode material), and a range of treatment parameters (potential, temperature, and duration) were explored to study the conditions for plasma-induced blackening of zirconia and to understand the underlying mechanisms involved in the generation of the black oxygen-deficient zirconia. Comprehensively characterisation of black zirconia generated through low-pressure plasma treatment, against untreated equivalent samples, was characterised in terms of structure, microstructure and properties. In addition, a plasma-induced crack-healing phenomenon was also discovered and fully characterised.
The findings reveal that the bulk-transformed black zirconia can be successfully fabricated by low-pressure plasma treatment from industrially available dense zirconia and that the oxygen-deficient zirconia formed is structurally unmodified from the pristine material. The oxygen-deficient nature of the plasma treated zirconia (ZrO\(_{2-α}\)) is revealed using a combination of electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS) and thermal analysis (TGA and DSC) characterisation techniques. Light absorption behaviour of plasma-treated zirconia, measured via diffuse reflectance spectroscopy (DRS), demonstrates >65% absolute light absorption across the tested spectrum (200-3000 nm). Comparisons of the average absorption across the spectrum indicate a substantial enhancement of 66.2% after plasma treatment of the zirconia.
Furthermore, plasma treatment is also found to lead to a significant reduction in both the direct and indirect bandgap values of zirconia. The direct bandgap decreases from 4.84 eV to 2.61 eV, while the indirect bandgap decreases from 3.19 eV to 1.45 eV.
The results also elucidate the conditions and the mechanism involved in the plasma blackening (reduction) process. The initial location and reduction rate of plasma blackening are determined by the contact conditions with the cathode. The initiation of blackening in oxygen-deficient zirconia is found to consistently arise at locations in direct contact with the cathode, before gradually spreading to non-contacting regions. The black areas are always found to expand from the cathode-facing surface towards the anode-facing surfaces, and the opposing migration of oxygen vacancies and lattice oxygen leads to the hemispherical growth of black regions, which differs from traditional current reduction methods. Throughout the plasma treatment, the degree of blackening (reduction) of zirconia samples varies based on plasma treatment parameters (such as temperature, time, voltage, pressure, etc.), with a positive correlation with the blackening process being observed for most of them.
It has also been found that during the plasma reduction of zirconia, indentation-formed microcracks on zirconia can be healed and microcracks formed by a load of 20 kgf are partially repaired under the plasma conditions of N\(_2\) gas, 500 °C, 10 hours, and 3 mbar pressure. The repair of the cracks is attributed to the plasma induced transformation of the tetragonal phase around the cracks to the monoclinic phase with a lower specific volume as compared with the tetragonal phase, which is accompanied by a volume expansion that enables the closing or repair of the cracks. Raman mapping and XRD measurements around an indent produced by a load of 5 kgf provided strong supporting evidence. Moreover, a unique simultaneous increase in both surface hardness and fracture toughness is also measured following the plasma-treatment of zirconia and is also attributed to the formation of the dispersed monoclinic crystal that pre-stress the material.

Type of Work:	Thesis (Doctorates > Ph.D.)
Award Type:	Doctorates > Ph.D.
Supervisor(s):	Supervisor(s) Email ORCID Dong, Hanshan UNSPECIFIED orcid.org/0000-0002-9244-8894 Li, Xiaoying UNSPECIFIED orcid.org/0000-0001-6449-1505 Dashtbozorg, Behnam UNSPECIFIED orcid.org/0000-0002-3809-7325
Licence:	All rights reserved
College/Faculty:	Colleges > College of Engineering & Physical Sciences
School or Department:	School of Metallurgy and Materials
Funders:	Engineering and Physical Sciences Research Council
Subjects:	T Technology > TJ Mechanical engineering and machinery T Technology > TN Mining engineering. Metallurgy T Technology > TS Manufactures
URI:	http://etheses.bham.ac.uk/id/eprint/15066

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