The various microstructures that can develop during processing for glass ceramics only, were discussed in depth by Holand and Beall [Error! Bookmark not defined.] and are summarised here.
Figure 7a shows an ultrafine grained microstructure in what would now be termed nanoscale. Figure 7b shows a cellular membrane structure resembling an organic cell. The microstructure itself
comprises the crystalline portion as the “cell” and the membrane as residual glass. Using silicate glass as an example, the formation of this structure occurs when the silica content of the crystal forming entity contains less than that of the surrounding glass. Normally, crystallisation would continue until all the glass has been consumed, or until the rejected elements formed their own separate crystal phase in a similar way to eutectic formation in metals. However, for glasses with certain compositions, the rejected elements may themselves form a stable glass (relative to the crystal) as reported by Chyung et al. for a LiO2-Al2O3-SiO2-TiO2 glass crystallising to beta spodumene.
They reported that the remnant glass was roughly 95% SiO2 and 5% Al2O3 which is an extremely stable glass [152].
Figure 7:a) an ultrafine grained microstructure. The black bar represents 1 µm. B) Cellular membrane structure [153].
Dendrites are often defined as “branching tree like structures” (Figure 8, Figure 9). However, the standard definition is insufficient to fully describe the dendritic structures that can form. Whilst it is true that if you isolate one strand it will indeed appear to look similar to that of a Christmas tree.
However, at the point at which it crystallised, whether it was due to large undercooling, nucleating agent, or through surface nucleation, the average angle, length, and spacing between branches will be different. For instance, Wang et al. [154] observed a small (200-500nm) surface-like dendritic crystallisation in between two different areas of composition. This is in contrast to Trivedi and Laorchan [155] who found a variety of bulk-dendrites (Figure 9).
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Figure 8: dendritic structure of a glass ceramic (a) and a computer simulated dendritic structure in a glass (b) [156].
Figure 9: Various dendritic structures observed by Wang et al. (left) and Trivedi (middle,right).
In the case of a nuclear waste matrix, the formation of dendrites could be beneficial, for instance should closely packed surface dendrites that are superior to the waste underneath form, the lifetime of the wasteform would be increased. Dendritic microstructure formation during cooling for glass is much the same as it is in metallurgy, a reasonable degree of undercooling is preferred for the formation of a dendritic microstructure, however a heat treatment may be required to grow them [157].
The house of cards microstructure is not as ordered as the name might suggest, rather, consisting of randomly orientated crystalline flakes (Figure 10). This type of microstructure is sought after for certain applications as the flakes act to stop or redirect cracks such that they cause only local damage [153] and not catastrophic failure, although this is dependent on the nature of the crystal (flake versus spherulitic) [158]. The chemical durability of these types of microstructure have not been studied from a nuclear wasteform perspective but from a dental one, glass ceramics have been looked at for dental applications since 1968 and was finally made a reality with a tetra-silicate mica crystal in a glassy matrix and was found to hold up well to the biological environment [159].
48 Figure 10:House of Card microstructure. a) Illustrating the orientation of this microstructure (no scale bar was
found), b) House of card microstructure at the scale shown (20 µm scale bar) [160].
Glass encapsulated crystal GCMs display a microstructure of a glassy matrix, along with either an altered form of the crystal waste from thermal processing, or the original crystal dispersed in the matrix. The main difference between glass ceramics and glass encapsulated crystal waste forms is from the origin of the crystal, in glass ceramics the pre-treated materials naturally form a crystal component during processing leading to the microstructures previously shown. Glass encapsulated crystals will however result in crystals with differing microstructures dependant on waste loading, pre-treatment of the waste, and thermal treatment [161]. Figure 11 shows how the waste loading of a sintered borosilicate glass with Cs containing clinoptilolite resulted in different microstructures.
When the clinoptilolite was sintered (Figure 11b) there was insufficient glass to fully encapsulate the waste allowing for Cs migration during leaching studies [162]. Further, with different waste loadings additional phases formed, such as sodalite (Na4Al3Si3O12Cl) which occurred due to a reaction
between the clino and the glass. Overall, the effect this will have on the durability of the wasteform will be dependent on both the type of crystal that has formed, and its microstructure. Juoi et al.
[162] found that increased waste loadings of a Cs-clinoptilolite GCM, the formation of wollastonite with different microstructures occurred; higher waste loadings resulted in a more rounded
morphology with better durability than the angular ones that formed at lower waste loadings.
49 Figure 11: BSE image of 3 GCMs (a) GCM wasteform with 1:1 glass to Cs-clino volume ratio (b) sintered Cs-clino only and (c) sintered borosilicate only, accompanied are the EDX graphs for the glass and crystal phases present in each image; energy (keV) versus count rate (A.U) [162].
Another issues that arises from glass encapsulation is incomplete bonding between the crystalline and glassy phase, when encapsulating a crystalline phase it must be chosen such that the thermal expansion co-efficient are similar to avoid this [163].An example where such choices were made by Pace et al., resulted in good bonding between the encapsulated crystal and host glass can be observed (Figure 12); cracks propagated through the material with little deflection when encountering the crystal phase.
50 Figure 12: A glass encapsulated lanthanum zirconate material, where cracks propagated through the crystals, with little deflection, signifying strong bonding between the two [163].