Equilibrium density of irradiated silica
Silica or silicon dioxide (SiO2) is one of the main constituents (60%) of the earth's crust in the form of sand or rock. A transparent material in the visible range, it is widely used in optical components (lenses, prisms, optical fibers, etc.).
Silica has many polymorphs: its structure can vary from the most compact crystallised form, i.e. a single quartz crystal, to more open or even amorphous structures, involving variations in the material’s properties (density, mechanical properties, optical index, etc.).
Silica glass, which has a large free volume, can be densified to a significant degree under high pressure (up to 20%), but the mechanisms and associated structural changes are still the subject of controversy, particularly between experiments and theoretical models.
By irradiating heat-treated or compression-densified silica with 2.5 MeV electrons, it has been shown that an equilibrium density of the order of 2.26 g/cm3 can be achieved, for an irradiation dose of the order of 10 GGy.
These results will lead to a better understanding of the structure of silica thin films, which are fundamental to the glass industry, and more specifically to the development and behaviour under extreme conditions (high temperature or irradiation) of optical fibres of the ‘Bragg grating sensor’ type.
Various methods are used to modulate the density of silica glass:
- by heat treatment, it is possible to modify the temperature at which the glass sets (fictitious temperature), which has a slight effect on its density.
- by UV, X or gamma irradiation, the densification remains low, of the order of 3% at the most…
- Densification of up to 20% can be achieved under high pressure (greater than 9 GPa at room temperature).
A study of relaxation under irradiation of silicas weakly densified by heat treatment (fictitious temperature: 1050, 1250 and 1400°C) and others compressed under high pressure at high temperature (initial density: 2.2, applied pressure: 5 GPa at 350 and 1000°C) was carried out by a team of researchers, using the SIRIUS electron accelerator (2.5 MeV) at the Laboratoire des Solides Irradiés (LSI).
The figure opposite shows how the density of these silicas changes with the irradiation dose. For silicas slightly densified by heat treatment, irradiation has the effect of increasing their density, whereas it decreases for hot-pressed glasses. The figure also shows an equilibrium density towards which all the glasses seem to converge: 2.26 g/cm3. This value is remarkable because it is identical to that of quartz or amorphous silica irradiated by fast neutrons.
The silica unit cell is tetrahedral, with a silicon atom in the centre and an oxygen atom on each vertex. In the solid, these tetrahedra connect to each other at the vertices to form local cycles made up of a variable number of tetrahedra, ranging from 3 to 9. Raman spectroscopy provides information about the short- and medium-range order of silica glass, in particular the number of tetrahedra per ring.
In particular, it shows an increase in the number of short cycles (3 to 5 tetrahedra) accompanying an increase in density, as long as the latter remains below 2.26 (in agreement with the literature on glasses densified by irradiation). More surprisingly, it also shows an increase in the number of short cycles as the density of hot-pressed glasses decreases with irradiation.
This last result is interesting because it proves that the reduction in the number of 6- or 7-tetrahedron cycles in favour of 3- or 4-tetrahedron cycles is not the main factor in the densification mechanism of silica glass. Finally, it should be noted that there is a high degree of similarity in terms of structure and ring size statistics between glasses densified and relaxed by electron irradiation and thin layers of silica (which are generally denser) deposited by ion-beam sputtering (IBS) or magnetron sputtering.
These results complement previous studies aimed at gaining a better understanding of the link between density and structure in silica glasses, data needed to identify the right processes for modulating silica density. Irradiation thus appears to be a good way of adjusting the material to the required properties, particularly for its applications in optics (fibre optics in particular). These studies are continuing on the effect of temperature (irradiation at 600 K) and the irradiation regime (105-108 Gy), in search of more precise data on the Si-O-Si angles, the ring size statistics and the free volume of the silica, in particular by NMR of 17O and 29Si.
Ref:
- Relaxation study of pre-densified silica glasses under 2.5 MeV electron irradiation, N. Ollier, M. Lancry, C. Martinet, V. Martinez, S. Le Floch, D. R. Neuville, Scientific reports 9, 1227 (2019).
- Fast-neutron-induced changes in quartz and vitreous silica, W. Primak, Phys. Rev. 110, 1240 (1958)
- Original news on the website of the CEA’s Institut Rayonnement-Matière de Saclay (Iramis)
