MedeA® Application Notes for Energy & Power Generation

Interfaces are present in most materials and have a large impact on mechanical properties such as stiffness and yield strength. Given that the properties of an interface can radically change by the presence of even minute amounts of impurities, it is of great interest to predict the effect of segregated atoms at interfaces.

As systematic experimental information on the impact of specific defect types on the grain boundary strength is hard to obtain, computational modelling is of great help.

Strength of Ni Grain Boundary and the Effect of Boron

Accurate measurements of diffusion coefficients of atoms in solids are difficult and deviations between different experiments can be several orders of magnitude. For the benchmark case of hydrogen diffusion in nickel first-principles calculations give a remarkable agreement with available experimental data especially near room temperature. Thus, computations of diffusion coefficients can be comparable in reliability with measured data. Simulations are possible for situations such as high strain, or slow processes where measurements are difficult or impractical.

Diffusion of Hydrogen in Nickel

First-principles calculations reveal the atomistic structures of the active phases of CoMoS and NiMoS hydrodesulfurization catalysts. The reliable determination of the catalyst surface is critical, as it represents the starting point for subsequent adsorption and reaction path simulations. The predicted dominant structures are consistent with experimental STM, EXAFS and magnetic susceptibility measurements.

Atomic Structure of Hydrodesulfurization (HDS) Catalysts

First-principles calculations reveal a three-fold increase in the Young’s modulus of graphite as it is lithiated (C→LiC₆). A linear expression is determined that describes the approximate stiffness of Li intercalated graphite as a function of loading which may lead to greatly improved continuum models of electrode deformation and failure.

Graphite Electrode Elastic Properties upon Li Intercalation

First-principles calculations using VASP reveal the lowest energy reaction pathway for the catalyzed skeletal isomerization of 2-pentene by the acidic zeolite H-ZSM. Three potential mechanisms were evaluated: an ethyl-shift pathway, a dimethylcyclopropane (DMCP) intermediate pathway and a pathway involving an edge-protonated DMCP species. The results indicate that the DMCP intermediate pathway is the kinetically preferred pathway with a classical barrier height of 98 kJ/mol. Evident in the calculations is the influence of the transient intermediate stability along the reaction path; with secondary carbenium ions leading to energetically favored mechanisms.

Catalytic Isomerization of 2-pentene in H-ZSM-22

The physics and chemistry of materials containing defects is of great interest in areas such as semiconductors, metal alloys and compounds, magnetic systems and optical materials.
Destabilizing or stabilizing crystalline bulk systems, surfaces and interfaces through additives is a common technique, for example recent research into complex hydrides aims to tune the kinetics of the adsorption/desorption changing the stoichiometry and composition of the base materials.

Building and Analyzing Indium defects in GaAs

The energy of adsorption and dissociation of molecules on surfaces plays a critical role in technological processes such as chemical vapor deposition, catalysis, and corrosion. The present case shows the calculation of the energy of the dissociative chemisorption of a silane molecule on a Si (001) surface.

Energy of Dissociative Chemisorption of SiH₄ on Si (001) Surface

This application shows the interaction of carbon monoxide with rutile.
An answer is given to the question whether CO binds with the carbon or the oxygen molecule to the surface.

CO Adsorption on a TiO₂ Surface

Hydrides containing alkaline-earth metals are prototypes for hydrogen storage materials. For this purpose the heat of formation and the mechanical properties are of fundamental interest. First- principles electronic structure methods provide systematic values for these materials properties. The agreement with experimental data for the heat of formation is good. Presently, no experimental data for the elastic coefficients of these metal hydrides are available thus leaving the computed data as the sole source.

Alkaline-earth hydrides

Iodine is a fission product of uranium. It can attack the inner side of zircaloy cladding in nuclear power reactors leading to cracking and fracture. Computations show that iodine molecules adsorb and dissociate on a zirconium surface without an energy barrier. The binding energy of iodine on this surface is large (nearly 300 kJ/mol per iodine atom), but the barriers for surface diffusion is only 6.8 kJ/mol. This gives rise to rapid surface diffusion allowing iodine atoms to reach the crack tips faster than the propagation of cracks.

Adsorption and Dissociation of Iodine Molecules on a Zr Surface