3.3 Characterisation
3.3.4 X-Ray Diffraction
Solid samples were held in place by plasticine to achieve a level surface. Powder samples (approximately 50 µm average particle diameter, ± 20µm) were placed on a single crystal; zero background substrate of silicon, the single crystal was cut such that the diffraction peaks would be at a large 2θ and so not appear on the trace. The powder was then flattened out using a glass slide.
Diffraction was carried out using a Bruker D2 Phaser, using a step size of 0.0249 2θ between 20-70 2θ. The total time for each analysis varied from 1 to 8 h. Following this, the program X-Pert HighScore Plus was used to identify the phases based on the detected diffraction lines. HighScore Plus uses the International Centre of Diffraction Data (ICDD) to do this.
X-ray diffraction (XRD) is a technique that utilises X-rays to probe the crystalline structure of a material, and provides unique information of its structure, allowing for identification of materials based on the measurements taken, by comparing to known crystalline XRD patterns. This analysis method works in an almost identical way to fingerprint analysis, and has the same drawbacks: it only works well for materials that are known and have been previously documented.
X-rays are electromagnetic radiation produced by electronic transitions in the atom and have wavelengths in the range of 0.02 Å -100 Å. Diffraction of these waves by regularly spaced atoms can then give information on the crystal. To achieve diffraction, the distance between the atomic slits, has to be comparable to that of the wavelength of the propagated radiation. X-rays meet these criteria, and as such are perfect for investigating the distances between atoms in any crystalline media. When the X-rays interact with the crystal, they are diffracted by a diffraction angle θ determined by the Bragg equation:
2𝑑 sin Ɵ = 𝑛𝜆 (10)
Where “d” is the spacing in the crystal, “θ” is the diffraction angle (between the beam and the lattice planes) “n” is an integer, “λ” is the wavelength of the radiation [182].
The interaction of X-rays for the same crystallographic plane produces constructive interference leading to an increase in detected X-rays for the incident angle used to achieve this. By plotting incident angle (measured in 2θ by Equation 10) versus X-ray counts, a set of peaks, representative of that particular crystal, gives the diagnostic “fingerprint”.
85 3.3.5 Differential Thermal Analysis/Thermo Gravimetric Analysis
About one g of powdered samples (~50 µm particle diameter, ±20 µm) were used in an inert alumina crucible, a heating regime of 5°C min-1 was used to a maximum temperature of 1600°C followed by a cooling rate of 20°C min-1. This was performed in an argon atmosphere. The device used was the Netzsch STA 449 F1 Jupiter.
Differential Thermal Analysis (DTA) and Thermo Gravimetric Analysis (TGA) are two techniques, often used simultaneously, to help measure thermal attributes of a material e.g. melting
temperature. The technique involves placing two inert crucibles in a sealed chamber, one containing the sample (powder or monolith) whilst the other one is empty and used as a reference. Both samples are then subject to a heating regime in a predetermined environment (argon or oxygen). A precise thermocouple and weighing scale detects differences in sample temperature, and any weight gain/loss [183].
3.4 Leach tests
3.4.1 Durability tests
The first step to understand how GCM degrade is to use ideal conditions, in this case de-ionised water, in a controlled temperature environment.
Several durability tests have been constructed to assess chemical resistance, mainly those devised by the Materials Characterisation Centre (MCC) at the PNNL in the USA. The MCC-2 (ASTM C1220-10) was selected, and comprises of a monolithic sample placed in a closed container with a volume of deionised water dependant on the surface area of the wasteform. For these tests the Surface Area to Volume ratio (SA/V) was 0.05 m-1, on average this resulted in 10 ml of deionised water for every 0.5 g of wasteform. Once prepared the closed samples were placed in an oven at 90 oC for up to 14 weeks.
3.4.2 Sample preparation
Monolithic samples were first roughly cut into cuboids using a copper saw, and put through a grinding and polishing regime as discussed in section 3.2. The initial grinding regime (coarse) was performed in such a way so as to make a regularly shaped cuboid. The subsequent grinding stages were performed as usual apart from the final grinding stage where all sides were equally ground until the specimen weighed approximately 0.5 g (±0.0001) determined from a Sartorius mass balance. This process was repeated 5 times, resulting in 5 distinct cuboids of the same wasteform which were washed in acetone, allowed to dry, and were then collectively placed in one Teflon digestive vessel. A volume of de-ionised water was then added to the digestion vessel, the amount dependant on the surface area of the sample; the amount of water was calculated such that the SA/V ratio was the same across all wasteforms. The Teflon digestion vessel was then placed in an airtight oven set to maintain a temperature of 90°C ± 1°C.
The desired corrosion times were 2,4,6,10, and 14 weeks. At each time interval, the digestion vessel was removed from the oven (but not opened) and allowed to cool to room temperature in a water bath. A thermocouple was used to determine that the outside of the Teflon digestion vessel was within 1°C of room temperature (measured from the base), and then left to cool for an additional 1 h to ensure the entirety of the vessel was at room temperature. The vessel was then removed from
86 the water bath and opened. One specimen was removed from the vessel using sterilised plastic tweezers. A syringe was then used to remove a portion of the leachate, the amount removed was dependant on the intial SA/V ratio; the volume of water removed was such that it conserved the SA/V ratio throughout the experiment. The vessel was then resealed, and placed back into the oven.
This procedure took no more than 2 h, the oven was noted to have dropped no more than 1 °C during the procedure, and recovered to 90 °C within 5-10 mins.
The whole procedure was repeated at each corrosion time until week 14 when the last specimen was removed along with the remaining water. The amount of water removed from the last specimen was measured to ensure that no excessive evaporation had occurred during the removal process for the entire corrosion time. For example, if 50 ml of water was the initial amount of water added, at each removal stage, 10 ml was removed; if less than 9 ml was present at week 14 then the results from the entire experiment was discarded.
The corroded specimen was immediately removed for electron-microscope analysis as described in section 3.3.3. The leachate was removed for both ICP-OES and pH analysis (on separate aliquots) as described in section 3.5.1 and 3.5.2.
Each wasteform had the above description preformed 7 times, resulting in 6 teflon digestion vessels, each containing 5 specimens for one wasteform. However, 3 of these were dedicated to pH analysis solely, and were removed much more frequently to measure the progression more accurately. The values were checked against those determined from the “main” experiment, as described above.
Also of note, the pH analysis was performed after the main experiment, and did not interfere with the oven temperature.
At least one Teflon digestion vessel from each wasteform (30 samples across 6 wasteforms) failed the “test” on the leachate at week 14 and had all their results discarded, equating to a 84% success rate.
3.4.3 Surface areas
Various surface areas were calculated during the course of the experiments, most prominently for SA/V ratios, however other surface areas are given. All surface areas were measured using the same technique; a combination of high contrast SEM images, and pixel counting. Contrast on an SEM image was altered do make the desired shape appear as one colour (usually white), with everything else appearing as the opposite colour (usually black). The number of white pixels in the image was known (calculated by adobe photoshop) and compared with the scale bar to give a surface area.
Error was limited to the contrast; this was performed manually and susceptible to human error, the contrast was altered until the operator was satisfied that the area only involving white/black pixels was associated with the desired area to be calculated.
When calculating the surface area occupied by a crystalline phase to determine total crystal%, BSE allowed for distinguishing the different phases, and using the above method the various surface areas calculated. The maximum error calculated for this method was ±5%, mostly due to the “grey”
area at the crystal-glass boundaries. However, greater error is cited due to the size of the crystalline phases, and the size of the image; the inhomogeneity of some samples meant that certain SEM images contained more/less crystal % than the average. To lower the error associated with this, multiple sites were analysed.
87 Surface areas for SA/V ratio used this method, each side was imaged and the total surface area calculated. Error using this method was less than that for surfaces areas calculated elsewhere, as no boundary existed (these samples were not mounted in epoxy).
Average pore size utilised this method; one pore at a time was isolated and its pixel count calculated. Additionally, the software also allows for lines of known pixel length to be drawn, this allowed for the dimensions of the pores to be calculated when compared with the pixel length of the scale bar. A large number of pores were then analysed giving an average.
3.5 Solution Analysis
3.5.1 Inductively Coupled Plasma Optical Emission Spectroscopy
Inductively coupled plasma optical emission spectroscopy or ICP-OES was the used for determining leachant composition, this was used to analyse the elements released into solution during MCC-2 tests.
ICP-OES uses an argon based plasma torch, maintained at temperatures upwards of 7000K to completely ionise any element placed into the plasma chamber [184]. Liquids analysed using this technique must first be passed through a nebuliser, via a peristaltic pump; this aerosols the liquid changing it into a mist. This is then injected into the plasma chamber and “swirled” around by an electromagnetic field.
When the sample is passed through the plasma all compounds are broken down into their constituent elements, which are then further broken down to ions, however this de-ionisation process is continuous, with electrons joining the nucleus before being removed from it again. The plasma emits light based on the elements present due to recombination of electrons, the
wavelengths of which are unique to that element. The light is passed through a spectrometer to help separate each wavelength before it reaches the main detector. Most modern detectors are a solid state type, based on charge coupled devices, thus removing the limitations on previous detectors which worked on a single wavelength basis requiring several detectors and were fixed once installed thus limiting the number of elements that can be to analysed.
Once the detector analyses the various wavelengths, a program compares the intensity of these wavelengths with known standards, thus allowing a high degree of accuracy on the order of ppm for analysed solutions. Some issues can arise from this such as an overlap of certain elements
wavelengths; the intensities of certain elements can make it difficult for the computer software to accurately detect other elements with similar wavelengths. As each element produces more than one wavelength of emitted light, several wavelengths were selected to avoid these issues as far as possible.
Leachant samples were removed via a syringe and passed through a filter to remove any solid particulates which could damage the apparatus. These were then immediately moved to the instrument, an iCAP 6000 series ICP-OES and analysed. ICP multi-element standard solution VIII (24 elements in dilute nitric acid, 100ppm) was used to determine the presence of Al, B, Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, K, Li, Mg, Mn, Na, Ni, Pb, Se, Sr, Te, Tl, Zn. A separate standard was used to determine the concentration of silicon (1000ppm in sodium hydroxide solution) due to
88 incompatibility between the two sets of standards. The standards were prepared at concentrations of: 5, 10, 20 and 40ppm. These were then double checked by placing a separately constructed sample of known concentration among the leachant samples analysed. Any deviation from the separately prepared standard would indicate a mistake in the reference standards preparation and it would be performed again with new standards.
3.5.2 pH tests
Long term pH tests were conducted using the MCC-2 test, after a fixed time a 10 ml (±2 ml based on SA/V ratio) portion of liquid was withdrawn and analysed. The vessels were removed from the oven (at 90 oC) and allowed to cool to room temperature without breaking the seal; this was to avoid any evaporation of the leachant. The vessels were then opened and the solution analysed using the Thermo Scientific Orion star A111 pH meter to an accuracy of 0.02. The results were run in triplicate and the vessel resealed and returned to the oven. Error was either the max difference from the average of 3 repeat results on each 10 ml of solution, totalling 9 results for one time, or the inherent error of pH (0.02) whichever was greater.
3.5.3 Normalised Leach Rates
Normalised elemental release rates (NLr) are commonly used to calculate the rate loss of a material subject to a corrosion test as a function of time; normalising these results allows for comparison with other materials subject to the same test. The equation used for calculating NLr of an element i present in the solid [185] is shown below (Equation 11):
𝑁𝐿𝑟 = (𝐶𝑖−𝐶0)∗𝑉
𝑓𝑖∗𝑆𝐴∗𝑡 (11)
Where “Ci” is the concentration of element “i” in solution (g/l), C0 is the initial concentration of element i in solution however as doubly deionised water was used the value was zero (confirmed by ICP-OES). “V” is the volume of leachant used (l), “SA” is the reactive surface area of the sample (cm2), and “t” is the time of the corrosion experiment (days).
In glass corrosion experiments, fi is the mass fraction of element “i” in the material, typically calculated using the following formula:
𝑓𝑖 = 𝑤𝑤𝑖
0 (12)
Where wi is the mass of element i in the wasteform (g), and wo is the total mass of the sample used (g).
However, due to uneven partitioning of elements into different phases in a GCM, Equation 12 could not calculate the total mass fraction of element “i” in the wasteform, instead Equation 13 was used to determine the mass fraction of element “i” in the wasteform:
𝑓𝑖 = ∑ 𝑤𝑤𝑖
𝑜𝑚𝑖
𝑛𝑖=1 (13)
Where “n” is the total number of phases present in the wasteform, wi/wo is the mass fraction of element “i” in phase “n”, and mi is the volume fraction of phase “i” in the wasteform.
Averaged EDX wt% results were used to calculated the mass fraction of element “i” in each phase.
The volume fraction of each phase was calculated from SEM images; a low magnification was used
89 to allow full view of the analysed sample (approximately 1 mm2) and the surface area occupied by each phase was calculated, this was preformed 4-6 times to get as accurate a value as possible.
Error was calculated using the larger value from the maximum and minimum values determined from Equation 11, this involved maximising the values for Ci , whilst minimising the values for fi and SA for calculating the total maximum value, and vice versa for determining the total minimum value.
90
4.0 Results
This chapter aims to present the results collected for each sample whilst chapter 5 will discuss the results presented here in more detail.
4.1 Joule Heated-In Container Vitrification Wasteform Characterisation
To help prediction of how each material will perform under repository conditions, identification of the phases present in them is needed. In addition, microstructural and phase characterisation will help further experiments using this technology by indicating which wastes need to be retreated to avoid the formation of harmful phases (if any). This chapter documents the various results collected from the characterisation techniques used before and after simple corrosion testing in aqueous environments (see section 3.2-3.4).
4.1.1 Joule Heated-Plutonium Contaminated Material
XRD results for this sample were split into two due to the “spots” that were visually observable on the sample. First, a sample was prepared for XRD without any segregation of these spots, the second XRD specimen was prepared by removing and segregating these “spots” so that none were believed to be present for the XRD analysis. Figure 31 displays the results for the non-segregated portion, revealing that the material is crystalline; the main phases suggested using the XRD database described in section 3.3.4 were clinoenstatite, enstatite, pigeonite, and augite. Substitution of various simulated waste elements would be enough to shift the peak locations and their associated intensities from those used to originally determine the XRD pattern for each specific crystal. The use of XRD to determine the exact phase present is well known, however a lesser known use of XRD for more difficult materials is the ability to use the peak locations and intensities, even if changed from a stoichiometric crystal, to assign a family of phases to the crystal in question. In this case, it can be said with a high degree of accuracy that the phase present is from the pyroxene family of phases.
A 2 g (±0.1 g) portion of the sample that was determined to have no crystalline phase was isolated using a combination of a copper saw and subsequent grinding until no more crystalline phases were visually observed. This was then analysed using XRD (figure 32) revealing it to be amorphous.
91 Figure 31: XRD trace of a powdered portion of JH-PCM without separation of the visibly crystalline phase(s).
Peaks shown are pigeonite (P)[PDF 0543] , augite (A)[PDF 0544], enstatite (E) [PDF 01-076-0524], diopside (D)[PDF 01-071-0994] , clinoferrosite (CFS) [PDF 017-0548], and clinoenstatite (C) [PDF 00-019-0769]. Due to the large amount of pyroxene peaks, it is likely the main phase is from the pyroxene family of
phases, with substitution causing shifts in peaks, leading to the convoluted XRD pattern.
Figure 32: XRD trace of the powdered portion of sample JH-PCM which did not contain any of the visually identified crystalline phase. The XRD trace revealed a characteristic amorphous hump and absence of crystal
peaks
92 BSE imaging suggests these spots were crystalline by morphology, due to the dendritic shape, as can be seen in Figure 33a). Not so readily seen is the presence of a second phase observed as the light phase surrounding, and intermixed, with the dendritic crystalline phase Figure 33b). A few areas were large enough (>2 µm2) for EDX to be collected without a significant error associated with particles this small. The EDX maps displayed showed a relatively constant composition compared with the glass portion, with the exception of magnesium and calcium, shown in the EDX maps in Figure 33c) and d). The results obtained for the Ca-depleted phase (herein referred to as phase 1) are displayed in Table 20 but are not truly representative of the elements present; it is probable that small amounts of EDX signals from the surrounding glass and dendritic phase (herein referred to as phase 2) contaminated the results.
Figure 33: BSE images and EDX maps of the crystalline portion of sample JH-PCM. a) shows a smaller (100-200µm) more symmetrical version of this crystal, whilst b) illustrates the shape of the more larger (400µm) crystals.(c) Ca and (d) Mg EDX maps of image a) are shown to highlight the difference in these two phases i.e.
Ca enriched and Mg depleted for phase 1 and vice versa for phase 2. Bright spots are artefacts of the gold coating.
93 Table 20: Composition of the phases present in the sample in oxide wt%. Results were obtained from the average of points taken where the lines indicate in figure 28. Error is displayed as either standard deviation from 10 different point scans, or the inherent error of EDX analysis (0.1 wt%), whichever was greater.
Phase 1 Na2O MgO Al2O3 SiO2 K2O CaO TiO2 Fe2O3
Compound % 2.2 3.5 11.4 62.0 1.3 17.5 1.2 0.4
Error ± 0.1 1.2 0.3 0.6 0.2 1.4 0.1 0.1
Phase 2
Compound % 2.5 11.3 12.3 67.3 2.4 2.4 1.1 ND
Error ± 0.1 0.9 0.3 0.4 0.1 0.8 0.2 -
BSE-SEM showed the glass to be homogeneous (Figure 34). However, EDX evaluation of the glass
BSE-SEM showed the glass to be homogeneous (Figure 34). However, EDX evaluation of the glass