Studies of the local atomic ordering in nanostructured amorphous materials using advanced scanning transmission electron microscopy techniques

Doye, Alastair Kelway (2022) Studies of the local atomic ordering in nanostructured amorphous materials using advanced scanning transmission electron microscopy techniques. PhD thesis, University of Glasgow.

Abstract

In this thesis, the short and medium range order of a range of glasses are investigated using scanning transmission electron microscopy.

One of the most powerful tools for achieving this is fluctuation microscopy, where a set of diffraction patterns are recorded using a small, but fairly parallel electron probe that is scanned over the sample using a small step size. Calculation of the normalised variance between diffraction patterns allows the determination of the range over which the medium range order persists.

A new direct electron detector was used, and a balance between exposure times that are long enough to not be dominated by shot nose and those short enough to allow rapid data acquisition was struck.

This technique was applied to the study of short range order and medium range order in two different classes of materials - amorphous metal silicides (MoSix, NbSix and WSix), and amorphous oxides (TiO2 doped Ta2O5 and SiO2).

In each case, short range order in the amorphous metal silicides was found to be A15 like, with atom spacing that varied with atomic number, as expected. Medium range order was found to be typically around 1 nm or below, but in a few regions it extended over a few nm. Electron energy loss spectroscopy results found that the chemistry of the materials was consistent with A15 structures.

Time stability of each glass was investigated by using a static electron beam and recording a sequence of diffraction patterns at short exposure times, then analysing the variations in these. Intensity of diffraction spots was tracked as a function of time, electron correlation microscopy was used, where g2(t) was calculated allowing for the calculation of a characteristic decay time τ and stretching parameter β describing how long the diffraction pattern remains stable. In all of the glasses observed the structure seems to vary with time, but the timescale varies in each of them, τ values were estimated to be around 80-150 ms in SiO2, around 800 ms in NbSix, and 2400 ms in TiO2 doped Ta2O5.

A lack of time stability makes fluctuation microscopy of materials more problematic, and means that consideration is necessary to ensure that apparent spatial variances are correctly interpreted. In the case of SiO2 an exposure time of 100 ms per diffraction pattern would mean that any spatial variance measurements would actually contain very little spatial information. As the metal silicides and TiO2 doped Ta2O5 have a much higher τ, time
stability is enough to that spatial variances are meaningful. Time variance in glasses needs further research and understanding it is essential to understanding spatial variance correctly.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: Q Science > QC Physics
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Supervisor's Name: MacLaren, Dr. Ian
• Date of Award: 2022
• Depositing User: Theses Team
• Unique ID: glathesis:2022-82891
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 May 2022 13:57
• Last Modified: 19 May 2022 14:17
• Thesis DOI: 10.5525/gla.thesis.82891
• URI: https://theses.gla.ac.uk/id/eprint/82891

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