Scanning Electron Microscope

Samples were ground and polished, and because of the non-conducting nature were coated with either gold or chromium using:

- Gold: EMITECH K550 at 25 milliamps for 1 min.

- Chromium: Quorum Q150TS ,pre-set program for 20 nm layer thick.

Gold or chromium were chosen to not interfere with elements known to be present in the samples for EDX analysis, based on the chemical composition as was determined from section 3.1. For instance, samples known to contain lanthanum, cerium, praseodymium, and neodymium were coated using gold rather than chromium, due to the Kα peak of Au overlapping with the minor actinide elements Mα. Carbon was not chosen to allow determination of the presence of carbon in the material, if any.

Following this they were then observed using Backscattered Electron Imaging (BEI), Secondary Electron Imaging (SEI), and Energy Dispersive X-Ray (EDX) results were collected. The Jeol JSM 6400 Scanning Electron Microscope (SEM) was used at an accelerating voltage of 20 keV and a working distance of 15 mm, unless stated otherwise. High resolution images were obtained using the LEO Gemini 1520 Field Emission Gun (FEG) SEM, both the working distance and acceleration voltage were varied.

Pre-corrosion tests were performed to allow comparison with the post corrosion SEM. Secondary electron and backscattered electron images were taken of areas of interest (such as a crystal or easily found feature) along with EDX results to allow a before and after comparison.

Post corrosion, samples were gold-coated and analysed using BEI, SEI and EDX. Following these results, the samples were mounted in epoxy, cut in half and gently polished to reveal a cross

sectional view of the sample. This exposed cross section was then coated with gold and re-examined using SEM-EDX to gather chemical data as a function of depth from the original surface. The main focus of this test was EDX line scans. Whilst point scans give qualitative/quantitative information on the chemical content at the specific point selected, line scans give only a profile of how the elements are distributed along a line, showing any increases or decreases as a function of distance. This makes line scans a very useful tool for analysing how the chemical composition changes as a function of depth.

It is important to note that the interaction volume of X-rays under standard conditions is close to 1 µm³, this is obviously undesirable for gel layers with thicknesses of <1 µm. To reduce the interaction volume as much as is possible, the acceleration voltage was lowered to 5 kV. The reduction was estimated to be approximately half, to 0.5 µm using the formula and basic assumptions shown in Equation 8.

Penetration depth equation:

𝑥(µ𝑚) = ^{0.1 𝐸𝑂1.5}

𝜌 (8)

75 Where Eo is the accelerating voltage in kV and ρ the density of the sample in g/cm³ [175].

Typically, borosilicate glass has a ρ around 2.23g/cm³ giving a penetration depth of 0.501 µm using a 5 keV accelerating voltage.

The equation given for the width of the interaction volume is as follows:

𝑦(𝜇𝑚) = ^{0.077 𝐸01.5}

𝜌 (9)

Where Eo is the accelerating voltage in kV and ρ the density of the sample in g/cm³ [176].

The SEM principle is much the same as that of an optical microscope, however instead of light waves and an optical lens, electron and magnetic lenses are used, typically in a high vacuum to limit

interference from dust particles however SEM’s do exist that can operate without a vacuum [177].

The use of electrons is due to their potentially small wavelength. The spatial resolution of an image is directly proportional to the wavelength of the photon used to convey the image; electrons used in this manner can have very high theoretical resolution as their wavelengths are in the region of 10pm in comparison to a limit of 500 nm for visible light. One important difference is the mechanism of image formation; an SEM forms the image with each pixel being formed sequentially, by scanning each spot before moving onto the next.

Electrons are generated from an electron source; three sources currently exist for SEM applications however only two such sources were used in this study, a tungsten filament which produces electrons via thermionic emission, and a FEG, which as the name suggests, emits electrons through field emission; this provides a much higher image resolution than that of a tungsten gun. More information regarding the sources can be found in “Review of electron guns” by Christian Travier [178].

Electrons are then passed through a series of magnetic lenses and focused into a narrow beam and on to the sample. Several signals are emitted as a result of electronic interaction, the ones utilised in this thesis are secondary electrons, back scattered electrons and characteristic X-rays, and the zone from which they are generated is termed the interaction volume. This is summarised in Figure 24.

76 Figure 24: Cross sectional view of the generic set up of an SEM, showing how the electron beam is focused as it passes through several electromagnetic lens’ before being directed onto the sample by a set of deflection coils.

The relative position of the various detectors (BSE/X-ray, SE detector) and coils are shown. [prepared by author].

Although these signals are generated within the interaction volume, they originate from different portions of the interaction volume, the size of which is dependent on the density of the sample and the accelerating voltage applied to the electron source (and thus the energy of the electrons interacting with the sample) [179].

Each of the different signals is discussed in the following sections.

3.3.1.1 Secondary Electrons

Secondary electrons arise from the inelastic interaction of the primary electron with electrons in the sample, removing an electron from the atom it is bound to. The electrons produced are energetically weak, typically less than 50 eV and thus only those from the first new nanometres (~5 nm) escape the sample and are detected. The low energy nature and low penetration depth leads to a signal that primarily yields topographical features as crevices lead to reabsorption of electrons appearing dark, whilst raised surfaces and slopes allow more electrons to reach the detector appearing brighter. In addition, edges appear brighter due to enhanced emission at corners [180].

77 3.3.1.2 Backscattered electrons

Backscattered electrons (BSE) are electrons that have elastically interacted with the sample and have small sample escape angles, relatively to the electron beam. For this reason, the backscattered electron detector is usually a toroidal silicon solid state detector [181] placed directly above the sample, and such that the electron beam passes through the open centre. Production of BSE is dependent on the electron beam spot size, accelerating voltage, density of the material and chemical content of the investigated area. Heavier atoms and atomically denser areas will create more BSE as the nucleus will have a larger field of influence. It is for this reason that BSE imaging shows differences in composition with bright areas being atomically denser than darker areas. BSE originate from the lower half of the interaction volume, typically a depth of 1 µm, giving a similar spatial resolution. This can be influenced by the accelerating voltage and density of the sample.

3.3.1.3 Energy dispersive X-ray spectroscopy

The ejection of electrons in the sample atoms via inelastic primary electron collisions produces an unstable atom. This is instantaneously corrected by a high energy electron falling to the low energy position. By the law of conservation of energy, an X-ray is produced which is unique to that element.

This allows for qualitative information regarding the chemical content of the specimen to be collected however quantitative data can also be obtained using standards calibrated by comparing their relative intensities. The resolution of EDX is similar to that of BSE and affected by the same factors.