Interfaces atomistic structures

Now having specified the bulk electrode, the bulk electrolyte, and the interface between them, our aim in this section is to quantify the atomistic structure of the interface and derive an expression that allows us to evaluate its stabUity. Based on (5.5), we wUl extend the ab initio atomistic thermodynamics approach to electrochemical systems. 

Experiments have shown that Aoxide spinel formation is on the order of 10 4cm at ca. 1000°C [C.A. Duckwitz, H. Schmalzried (1971)]. Using Eqns. (10.45) and (10.46) with the accepted cation diffusivities (on the order of 10 10 cm2/s), one can estimate from j% that each A particle crosses the boundary about ten times per second each way. In other words, quenching cannot preserve the atomistic structure of a moving interface which developed during the motion by kinetic processes. This also means that heat conduction is slower than a structural change on the atomic scale, unless one quenches extremely small systems. 

The AIMD models of the surface region represent a necessary starting point for any large-scale MD study of the interface. The limited information available on the atomistic structure of the glass substrate complicates the assessment of the accuracy of surface models obtained by classical force fields. The AIMD models, albeit of limited size, are an essential reference to define the stability of typical surface sites, that should be at least qualitatively reflected in any larger model produced by classical MD simulations. Whereas the latter are generally not suitable to investigate surface... 

I n order to investigate the atomistic structure of the interface and the relaxation-related dislocations, H RTEM analysis was applied. These studies aim to provide information on the relative atomic column positions of the two materials. Since the contrast in HRTEM images depends on defocus, lens aberrations, and foil thickness, [61] it is not possible to directly derive atomic column positions from the images. However, the atomic positions with respect to the... 

D. Wolf, in Materials Interfaces. Atomistic-level Structure and Properties, D. Wolf and S. Yip (eds.). Chapman Hall, London, 1992 A. P. Sutton, R. W. Balluffi, Interfaces in Crystalline Materials, Clarendon Press, Oxford, 1995. 

In this situation computer simulation is useful, since the conditions of the simulation can be chosen such that full equihbrium is established, and one can test the theoretical concepts more stringently than by experiment. Also, it is possible to deal with ideal and perfectly flat surfaces, very suitable for testing the general mechanisms alluded to above, and to disregard in a first step all the complications that real substrate surfaces have (corrugation on the atomistic scale, roughness on the mesoscopic scale, surface steps, adsorbed impurities, etc.). Of course, it may be desirable to add such complications at a later stage, but this will not be considered here. In fact, computer simulations, i.e., molecular dynamics (MD) and Monte Carlo (MC) calculations, have been extensively used to study both static and dynamic properties [11] in particular, structural properties at interfaces have been considered in detail [12]. 

In this chapter, we will give a general description of electrochemical interfaces representing thermodynamically closed systems constrained by the presence of a hnite voltage between electrode and electrolyte, which will then be taken as the basis for extending the ab initio atomistic thermodynamics approach [Kaxiras et ah, 1987 Scheffler and Dabrowski, 1988 Qian et al., 1988 Reuter and Scheffler, 2002] to electrochemical systems. This will enable us to qualitatively and quantitatively investigate and predict the structures and stabilities of full electrochemical systems or single electrode/electrolyte interfaces as a function of temperature, activi-ties/pressures, and external electrode potential. 

Atomistic simulation of an atactic polypropylene/graphite interface has shown that the local structure of the polymer in the vicinity of the surface is different in many ways from that of the corresponding bulk. Near the solid surface the density profile of the polymer displays a local maximum, the backbone bonds of the polymer chains develop considerable parallel orientation to the surface [52]. This parallel orientation due to adsorption can be one of the reasons for the transcrystallinity observed in the case of many anisotropic filler particles. 

There is little question that one of the most active research areas in materials science is studying interfaces. In the past, emphasis in materials science has been placed on relating the bulk properties to the structure and composition of the solid. Today, efforts are in progress that relate surface reactivity and stability to the crystallographic orientation and composition, primarily at the S/G interface. Since a fundamental understanding of interfacial behavior and degradation mechanisms at an atomistic level is necessary if short-time test data are to be extrapolated to 30-year lifetimes, careful studies at the S/S, S/G, and S/L interfaces are required (4). 

B.E. Conway, The Solid/Electrolyte Interface, NATO Conf. Ser. 6, Vol. 5 on Atomistics of Fracture, R.M. Latanision, Ed., Plenum (1983) 497. (Review, emphasis on metals double layers and water structure near charged surfaces.)... 

Theoretical methods offer the opportunity to explore structure-property relationships in ideal metal-ceramic interfaces. Ultimately, improved understanding of the causal sequence leading to a particular interface structure and set of properties would enable further optimization of manufacturing parameters. Atomistic modeling constitutes the perfect laboratory in this respect. Within the limits of the specific approximations used for interatomic interactions, physical properties may be resolved to arbitrary accuracy and competing effects may be separated. 

Often the most important properties of materials are directly or indirectly connected to the presence of defects and in particular of point defects [126,127]. These centers determine the optical, electronic, and transport properties of the material and usually dominate the chemistry of its surface. A detailed understanding and a control at the atomistic level of the nature (and concentration) of point defects in oxides are therefore of fundamental importance also to understand the nature of the metal-oxide interface. The accurate theoretical description of the electronic structure of point defects in oxides is essential for understanding their structure-properties relationship but also for a correct description of the metal-oxide interface and of the early stages of metal deposition on oxide substrates. 

For the number of shells in both structures, each lattice is related to the radius (R) of the nanoparticle [27-29]. Therefore, the value of R contains a number of shells and the size of a nanoparticle increases as the number of shells increases. The shells (R) and their numbers are only bounded to the nearest-neighbour pair exchange interactions (J) between spins. To provide the magnetization of the whole particle, each of the spin sites, which stand for the atomistic moments in the nanoparticle, are described by Ising spin variables that take on the values S1-= l, 0. For a core/surface (C/S) morphology, all spins in the nanoparticle are organized in three components that are core (C, filled circles), interface (or core-surface) (CS) and surface (S, empty circles) parts. The number of spins in these parts within the C/S-type nanoparticle are denoted byNc, Ncs and Ns, respectively. But, the total number of spins (N) in a C/S nanoparticle covers only C and S spin numbers, i.e. N =NC + Ns. On the other hand, the numbers of spin pairs for C, CS and S regions in 2D are defined by N [,=(N (.y(. /2)-Ncs,... 

The structure of the [CjinimHCl] gas-liquid surface was studied using atomistic simulation [128], A region of enhanced density immediately below the interface in which the cations were oriented with their planes perpendicular to the surface and their dipoles in the surface plane was observed [128], A negligible segregation of cations and anions was also found. The temperature dependence of the surface... 

Finally, it is highly desirable to improve the ability to calculate the properties of surfaces and interfaces involving polymers by means of fully atomistic simulations. Such simulations can, potentially, account for much finer details of the chemical structure of a surface than can be expected from simulations on a coarser scale. It is, currently, difficult to obtain quantitatively accurate surface tensions and interfacial tensions for polymers (perhaps with the exception of flexible, saturated hydrocarbon polymers) from atomistic simulations, because of the limitations on the accessible time and length scales [49-51]. It is already possible, however, to obtain very useful qualitative insights as well as predictions of relative trends for problems as complex as the strength and the molecular mechanisms of adhesion of crosslinked epoxy resins [52], Gradual improvements towards quantitative accuracy can also be anticipated in the future. 

Polyamino acids can be considered as models for conformational studies, providing an atomistic description of the secondary structural motifs typically found in proteins [30-39]. Two-dimensional hydrogen-bonded layers and columns in the structures of crystalline amino acids can mimic S-sheets and helices in proteins and amyloids [40 5], and can be compared with two-dimensional crystalline layers at interfaces [46-58]. Nano-porous structures of small peptides can mimic cavities in proteins [24, 59-63]. One can also prepare crystals in which selected functional groups and side chains are located with respect to each other in the same way, as at recognition sites of substrate-receptor complexes, and use the systems to simulate the mutual adaptation of components of the complex responsible for recognition. 

Through comparison of the data to representative atomistic models of the stearate-calcite interface structure, we can learn about the detailed structure of this film, the stearate coverage, and its structural relationship to the substrate lattice. In these calculations, we include the positions, scattering factors fh and rms widths (usually interpreted as vibrational amplitudes) of each atom in the structure. We use the results of... 

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