Atomistic models, grain boundaries

Overall, the results from the cathode-only KMC simulations [118-120] were found to be qualitatively consistent with experimental trends, with a great deal of the atomistic-level details preserved. However, in order to improve the results, and approach quantitative agreement with experiments, additional features must be incorporated, such as the anode-side reactions, correlation of the ion-vacancy and vacancy-vacancy interactions, grain boundaries, and explicit structural treatment of the anode and cathode. In order to incorporate some of these necessary features, two KMC-based SOFC simulation studies have recently emerged [126,127] along with some close experimental collaboration [128], In all of these more recent studies, a complete SOFC model (anode+cathode) was assembled. 

Harris DJ, Watson GW, Parker SC (1997) Vacancy migration at the 410 /[001] symmetric tilt grain boundary of MgO An atomistic simulation stndy. Phys Rev B-Cond Mat 56 11477-11484 Harrison NM, Leslie M (1992) The derivation of shell-model potentials for MgCb from ab-initio theory Mol Sim 9 171-174... 

Corrales LR (1999) Dissociative model of water clusters. J Chemical Physics 110 9071-9080 Curtiss LA, Halley JW, Hautman J, Rahman A (1987) Nonadditivity of ab-initio pair potentials for molecular dynamics of multivalent transition metal ions in water. J Chem Phys 86 2319-2327 de Leeuw NH, Parker SC, Catlow CRA, Price GD (2000) Proton-containing defects at forsterite (010) tilt grain boundaries and stepped surfaces. Am Min 85 1143-1154 de Leeuw NH, Parker SC (1998) Surface stracture and morphology of calcium carbonate polymorphs calcite, aragonite, andvaterite An atomistic approach. JPhys ChemB 102 2914-2922... 

The microstructure plays a pivotal role with respect to the chemical properties of a material. Accordingly, if one wishes to use atomistic models to explore the reactivity, then the microstructure must be captured within the atomistic model including the morphology and surfaces exposed, intrinsic and extrinsic point defects, dislocations and grain boundaries. In the following we use selected examples to describe how such features are introduced. 

Figure 5.5 Images of a [001] (210) Z5 grain boundary in a Ce02 thin film. Left Scanning tunnelling electron micrographs. Right Atomistic models. Reproduced with permission from Hojo et Copyright 2010 American Chemical Society.

Figure 5.6 Top Atomistic model of a Ce02 nanotube, which includes microstruc-tural features such as dislocations and grain boundaries. Bottom The method used to generate such models. Reprinted with permission from Martin et al ° Copyright 2007 American Chemical Society.

Figure 5.12 Atomistic models and HRTEM images of ceria nanorods, (a) and (b) Model CeOj nanorods, which extend along [110] and (b), respective, (c) Nanorod with twin-grain boundaries the atomistic structure of the grain-boundary region is shown enlarged in the inset. HRTEM images of nanorods with [110] and [211] growth directions are shown in (d) and (e), respectively, (a)-(c) Reprinted with permission from Sayle et al. Copyright 2007 American Chemical Society, (d), (e) Reprinted with permission from Du et all Copyright 2007 American Chemical Society.

Ramanathan on MgO-supported YSZ thin films, revealed a two orders of magnitude increase in the ionic conductivity in YSZ as the YSZ film size decreased from 9 to 3 nm owing to a decrease in the activation energy barrier from 0.54 to 0.35 eV in the 1200-2000 K temperature range. Simulated amorphisation and crystallisation were used to generate the atomistic model, which enabled the authors to capture the microstructure — particularly a grain-boundary network. 

The operation of ceria under catalytic conditions can place the material under severe mechanical duress and therefore it is important to understand the behaviour of the material under operational conditions, such as vibration, friction, thermal cycling, etc. The mechanical properties of the material may prove pivotal. In particular, it is well known that microstructural features, such as dislocations, defects and grain boundaries, govern the mechanical properties and result in the measured mechanical strength being considerably lower than that predicted based upon the pristine, defect-free material. If one is to simulate the mechanical properties directly then atomistic models, which include all such microstructural features including their synergy of interaction, are needed. And while there are considerable efforts focused in this direction. 

In the case where the membrane is deformed, the deformation profiles can be compared to a variety of theories [16,17,27, 33, 245-247]. Both in coarse-grained [30,234] and atomistic [248] simulations, it was reported that membrane thickness profiles as a function of the distance to the protein are not strictly monotonic, but exhibit a weakly oscillatory behavior. This feature is not compatible with membrane models that predict an exponential decay [16,17,27], but it is nicely captured by the coupled elastic monolayer models discussed earlier [22, 28, 30]. Coarsegrained simulations of the Lenz model showed that the coupled monolayer models describe the profile data at a quantitative level, with almost no fit parameters except the boundary conditions [30, 244]. 

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