In addition to the x-ray diffraction methodologies described so far, there were also some other analysis methodologies used to determine other parameters of the material or to confirm those found in the x-ray diffraction experiments.
2.3.1 Scanning Electron Microscopy
A scanning electron microscope (SEM) allows analysis of a material at the
micrometre to nanometre scale. In this thesis, a magnification of the order of 13,000x was used. The area to be analysed is irradiated with a finely focussed beam of electrons, generated by the heating of an element such as tungsten. In this thesis, a tungsten hairpin filament is used. This beam can either be swept across the surface in a raster, forming an image, or focussed on one single point to analyse that point specifically, depending on the mode of analysis undertaken. The interaction of the beam with the material produces a number of physical phenomena, most
interestingly for the analysis undertaken in this thesis, backscattered electrons, secondary electrons and characteristic x-rays [31]. Backscattered electrons and secondary electrons allow an image of the surface topography to be generated, and due to the large depth of field, this image has a three-dimensional effect to it, with different heights and shadows visible, making it easier to understand the nature
of the topography [32]. The characteristic x-rays allow analysis of the chemical composition of the material to be conducted.
2.3.1.1 Energy-Dispersive X-Ray Spectrometry (EDX)
Energy-Dispersive X-Ray Spectrometry (known as EDS or EDX; the latter will be used in this thesis) is a method for determining the chemical composition of a material analysed through SEM methods. It can be used both qualitatively and quantitatively. Electrons from the electron beam can eject inner shell electrons from atoms in the material under analysis, which leaves the atom in an excited state. Outer orbital electrons release discrete energies in the form of photons in order to fill the inner shell vacancy, resulting in a discrete set of x-ray frequencies being emitted. By measuring the x-ray frequencies emitted by this process, and their relative intensities, it is possible to build a model for the atoms making up the sample [32].
Since these x-rays will often have to pass through the material in order to be detected, some corrections are necessary to ensure the correct interpretation of the data is achieved. In this thesis, ZAF corrections (corrections due to the atomic number (Z), the absorption (A) and the fluorescence (F)) have been applied automatically by the software used to analyse the EDX data through an EDAX system; a thorough description of ZAF corrections is given by Dekker [33].
By observing the characteristic x-rays as a function of position, as in a raster SEM image, elemental maps can be constructed, which mean that areas with higher or lower concentrations of a given element can be observed, which can allow the characterisation of structures within the sample.
2.3.2 Thermal Analysis
Thermal analysis is a blanket term for various measurements taken as a function of temperature. In this thesis, combined Thermogravimetric Analysis (TGA) and Mass Spectrometry (MS) were used, using aMettler Toledo DSC1-Star system.
TGA is a technique where the mass is measured as a function of time. The temperature is either changed at a constant rate, or held at a given temperature, while the measurements are taken [34]. This allows a measure of the change in mass with temperature, or if the sample is heated and then cooled, a measurement of the change in temperature as a result of the whole heating process.
MS is a varied technique, but there are some broad steps followed in all cases: A material is ionized. The ionized material is then analysed by mass, often separated
by mass-to-charge ratio by utilising a magnetic or electric field. This distribution is then detected, usually both by this ratio and their relative abundance. All of this is operated under high vacuum conditions [35]. From this, the materials can be identified. In the case of MS in the context of thermal analysis, this can be combined with TGA to observe the temperature at which a material is given off from the material, at which point the vapour is ionised, to determine its composition through MS.
2.3.3 Birefringence
Transparent crystals are rarely optically isotropic; in other words, the refractive index varies depending on the direction through which the light is propagating through the crystal. These differences in refractive index with respect to propagation direction are represented by a surface, which generally forms an ellipsoid. This is generally known as an optical indicatrix [36] (though is also known as an index ellipsoid [37]). The major and minor radii of this ellipsoid give the refractive indices experienced by waves propagating in those directions; for any general direction of travel in such an anisotropic material, an elliptical cross section of the optical indicatrix is observed. The difference between the axes gives the linear birefringence. The symmetry of a system defines the form of optical anisotropy possible (uniaxial or biaxial) and also the orientation of the optical indicatrix with respect to the crystal axes; as such, from observing the optical indicatrix, it is possible to derive the crystal system and symmetry to which a material belongs [38].
2.4
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