Significance of grain boundaries for transport phenomena in graphene and proton-conducting barium zirconate

Doctoral thesis, 2015

Grain boundaries can have a significant influence on the properties of polycrystalline materials. When determining the type and extent of this influence it is frequently useful to employ computational methods such as density functional theory and molecular dynamics, which can provide models of the grain boundary structure at the atomistic level. This work investigates the influence of grain boundaries in two different materials, barium zirconate and graphene, using atomistic simulations. Barium zirconate is a proton conducting material with a potential application as a fuel cell electrolyte. However, the presence of grain boundaries has been found to lower the proton conductivity. Here, density functional theory has been used to investigate the segregation of positively charged defects, such as oxygen vacancies and protons, to the grain boundaries. It has been found that both defect types segregate strongly to the grain boundaries, which gives rise to an electrostatic potential that depletes the surrounding region of protons and impedes transport across the grain boundary. A thermodynamical space-charge model has been employed to relate the theoretical results to experimentally measurable quantities. The carbon allotrope graphene has many potential applications in for example electronics, sensors and catalysis. It has also been mentioned as a possible material for phononics and heat management applications due to its unique vibrational properties, which give it a high thermal conductivity. Grain boundaries have been found to decrease the thermal conductivity, but they may also provide a method for manipulating the vibrational properties. The work included in this thesis investigates the scattering of long-wavelength flexural phonons, i.e. phonons with polarization vectors pointing out of the graphene plane, at grain boundaries. Grain boundaries in graphene frequently cause out-of-plane deformation, buckling, of the graphene sheet, and it is found that this buckling is the main cause of scattering of longwavelength flexural phonons. Based on this result a continuum mechanical model of the scattering has been constructed, with a view to facilitating the study of systems too large to be modelled by molecular dynamics.