Ab-initio calculations of fission product diffusion on graphene

A clear understanding of the diffusive behaviour of a wide variety of impurities is essential for the construction and safe operation of the class of nuclear reactors which employ graphite as a shielding material. As a means of gaining insight into this important problem, the bonding, activation energy and structural properties of a variety of the most common nuclear fission products on graphene have been examined using Density Functional Theory (DFT), illustrating the attendant mechanisms of bonding and ionic transport of the different species, as well as the tendency to form nanoscale clusters in bulk graphite. Simulations have been conducted using a variety of approximations to the exchange-correlation functional, and the relative importance of functional choice is discussed in the context of the adsorption and activation energies. Finally, our calculations are compared to the relevant experimental results, allowing us to draw some conclusions about the likely transport mechanisms at larger length and time scales

Borosilicate glass potentials for radiation damage simulations

Three borosilicate glass (SiO2-B2O3) fixed charge potentials from the literature are compared (Delaye and Ghaleb, 1996; Kieu et al., 2011; Rushton, 2006) and their suitability for use in simulations of radiation damage is assessed.For a range of densities, we generate glass structures by quenching at 5×1012 K/s using constant volume Molecular Dynamics. In each case, the bond lengths, mean bond angles, bulk modulus, melting point and displacement energy thresholds are calculated, and where possible compared to experimental data. Whereas the bond lengths and mean bond angles are reasonably well predicted, we find that the potentials predict melting temperatures, bulk moduli and densities that are higher than experimental data.The displacement energy thresholds are generally lower than those for ionic crystalline materials, but show a wider spread of values. However, the barriers for atomic rearrangements, after atoms have been displaced in the equilibrium structures, are very high. This indicates, that the radiation damage produced in the ballistic phase of a collision cascade, is likely to persist for extended time scales. This is in contrast to crystals, where interstitials and vacancies can diffuse rapidly between successive radiation events

Inter-atomic potentials for radiation damage studies in CePO4 monazite

An original empirical potential used for modelling phosphate glasses is adapted to be suitable for use with monazite (CePO4) so as to have a consistent formulation for radiation damage studies of phosphates. This is done by adding a parameterisation for the Ce–O interaction to the existing potential set. The thermal and structural properties of the resulting computer model are compared to experimental results. The parameter set gives a stable monazite structure where the volume of the unit cell is almost identical to that measured experimentally, but with some shrinkage in the a and b lengths and a small expansion in the c direction compared to experiment. The thermal expansion, specific heat capacity and estimates of the melting point are also determined. The estimate of the melting temperature of 2500 K is comparable to the experimental value of 2318 ± 20 K, but the simulated thermal expansion of 49 106 K1 is larger than the usually reported value. The simulated specific heat capacity at constant pressure was found to be approximately constant at 657 J kg1 K1 in the range 300–1000 K, however, this is not observed experimentally or in more detailed ab initio calculations

Iron phosphate glasses: structure determination and displacement energy thresholds, using a fixed charge potential model

Iron phosphate glass is a versatile matrix for the immobilisation of various radioactive elements found in high-level nuclear waste (HLW). Quenched glass structures of iron phosphate glasses with Fe/P ratios of 0.33, 0.67 and 0.75 and with a composition of 40 mol% Fe2O3 and 60 mol% P2O5, with 4% and 17% Fe2 + ion concentrations were generated using molecular dynamics and the threshold displacement energies calculated. In the minimum energy structures, we found that in nearly all cases the P atoms were 4-fold coordinated. The potential energy per atom increased with increasing concentration of Fe2 + ions with similar Fe/P ratio, suggesting that decreasing the Fe2 + content is a stabilising factor. The average bond distances between Fe2 +-O, Fe3 +-O, P-O and O-O were calculated as 2.12, 1.88, 1.5 and 2.5 Å respectively. The threshold displacement energy (Ed) was found to be dependent upon the ion specie, less for Fe2 + ions compared to Fe3 + ions, and was overall slightly lower than that determined for borosilicate glass

Near-surface structure and residual stress in as-machined synthetic graphite

We have used optical and electron microscopy and Raman spectroscopy to study the structural changes and residual stress induced by typical industrial machining and laboratory polishing of a synthetic graphite. An abrasion layer of up to 35 nm in thickness formed on both machined and polished surfaces, giving the same ID/IG ratios evidencing graphite crystal refinement from an La of ~110 nm down to an average of 21 nm, but with different residual compression levels. For the as-polished sample, structural change was limited to the near surface region. Underneath the as-machined surface, large pores were filled with crushed material; graphite crystals were split into multi-layered graphene units that were rearranged through kinking. Graphite crystal refinement in the sub-surface region, measured by La, showed an exponential relationship with depth (z) to a depth of 35–40 μm. The positive shift of the G band in the Raman spectrum indicates a residual compression accompanied by refinement with the highest average of ~2.5 GPa on top, followed by an exponential decay inside the refined region; beyond that depth, the compression decreased linearly down to a depth of ~200 μm. Mechanisms for the refinement and residual compression are discussed with the support of atomistic modelling

Sub-monolayer growth of Ag on flat and nanorippled SiO2 surfaces

In-situ Rutherford Backscattering Spectrometry (RBS) and Molecular Dynamics (MD) simulations have been used to investigate the growth dynamics of silver on a flat and the rippled silica surface. The calculated sticking coeficient of silver over a range of incidence angles shows a similar behaviour to the experimental results for an average surface binding energy of a silver adatom of 0.2 eV. This value was used to parameterise the MD model of the cumulative deposition of silver in order to understand the growth mechanisms. Both the model and the RBS results show marginal difference between the atomic concentration of silver on the at and the rippled silica surface, for the same growth conditions. For oblique incidence, cluster growth occurs mainly on the leading edge of the rippled structure

A new potential for radiation studies of borosilicate glass

Borosilicate glass containing 70 mol% SiO2 and 30 mol% B2O3 is investigated theoretically using fixed charge potentials. An existing potential parameterisation for borosilicate glass is found to give good agreement for the bond angle and bond length distributions compared to experimental values but the optimal density is 30% higher than experiment. Therefore the potential parameters are refitted to give an optimal density of 2.1 g=cm3, in line with experiment. To determine the optimal density, a series of random initial structures are quenched at a rate of 5 1012 K/s using constant volume molecular dynamics. An average of 10 such quenches is carried out for each fixed volume. For each quenched structure, the bond angles, bond lengths, mechanical properties and melting points are determined. The new parameterisation is found to give the density, bond angles, bond lengths and Young’s modulus comparable with experimental data, however, the melting points and Poisson’s ratio are higher than the reported experimental values. The displacement energy thresholds are computed to be similar to those determined with the earlier parameterisation, which is lower than those for ionic crystalline materials

Thermal dynamics of silver clusters grown on rippled silica surfaces

Silver nanoparticles have been deposited on silicon rippled patterned templates at an angle of incidence of 70° to the surface normal. The templates are produced by oblique incidence argon ion bombardment and as the fluence increases, the periods and heights of the structures increase. Structures with periods of 20 nm, 35 nm and 45 nm have been produced. Moderate temperature vacuum annealing shows the phenomenon of cluster coalescence following the contour of the more exposed faces of the ripple for the case of 35 nm and 45 nm but not at 20 nm where the silver aggregates into larger randomly distributed clusters. In order to understand this effect, the morphological changes of silver nanoparticles deposited on an asymmetric rippled silica surface are investigated through the use of molecular dynamics simulations for different deposition angles of incidence between 0° and 70° and annealing temperatures between 500 K and 900 K. Near to normal incidence, clusters are observed to migrate over the entire surface but for deposition at 70°, a similar patterning is observed as in the experiment. The random distribution of clusters for the periodicity of 20 nm is linked to the geometry of the silica surface which has a lower ripple height than the longer wavelength structures. Calculations carried out on a surface with such a lower ripple height also demonstrate a similar effect

Data related to the mesoscopic structure of iso-graphite for nuclear applications

The data in this article are related to the research article “Mesoscopic structure features in synthetic graphite” (Maerz et al., 2018) [1]. Details of the manufacture of isostatically moulded graphite (iso-graphite), thin foil preparation by focused ion beams (FIB) for analysis, and characterisation methods are provided. The detailed structures of coke filler and binding carbon are presented through scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and Raman spectroscopy characterisation. Atomistic modelling results of mesoscopic structural features are included

Mesoscopic structure features in synthetic graphite

The mesocopic structure features in the coke fillers and binding carbon regions of a synthetic graphite grade have been examined by high resolution transmission electron microscopy (TEM) and Raman spectroscopy. Within the fillers, the three-dimensional structure is composed of crystal laminae with the basal plane dimensions (La) of hundreds nanometres, and thicknesses (Lc) of tens of nanometres. These laminae have a nearly perfect graphite structure with almost parallel c-axes, but their a-b planes are orientated randomly to form a “crazy paving” structure. A similar structure exists in the binding carbon regions, with a smaller La. Significantly bent laminae are widely seen in quinoline insoluble inclusions and the graphite regions developed around them. The La values measured by TEM are consistent with estimates from the intensity ratios of the D to G Raman peak in these regions. Atomistic modelling finds that the lowest energy interfaces in the crazy paving structure comprise 5, 6 and 7 member carbon rings. The bent laminae tend to maintain the 6 member rings, but are strained elastically. We suggest that a 7 member carbon ring leaves a cavity representing an arm-chair graphite edge contributing to the Raman spectra D peak