Diffraction and near-zero transmission of flexural phonons at graphene grain boundaries

Graphene grain boundaries are known to affect phonon transport and thermal conductivity, suggesting that they may be used to engineer the phononic properties of graphene. Here, the effect of two buckled grain boundaries on long-wavelength flexural acoustic phonons has been investigated as a function of angle of incidence using molecular dynamics. The flexural acoustic mode has been chosen due to its importance to thermal transport. It is found that the transmission through the boundaries is strongly suppressed for incidence angles close to 35$^\circ$. Also, the grain boundaries are found to act as diffraction gratings for the phonons

Scattering of flexural acoustic phonons at grain boundaries in graphene

We investigate the scattering of long-wavelength flexural phonons against grain boundaries in graphene using molecular dynamics simulations. Three symmetric tilt grain boundaires are considered: one with a misorientation angle of $17.9^\circ$ displaying an out-of-plane buckling 1.5 nm high and 5 nm wide, one with a misorientation angle of $9.4^\circ$ and an out-of-plane buckling 0.6 nm high and 1.7 nm wide, and one with a misorientation angle of $32.2^\circ$ and no out-of-plane buckling. At the flat grain boundary, the phonon transmission exceeds 95 % for wavelengths above 1 nm. The buckled boundaries have a substantially lower transmission in this wavelength range, with a minimum transmission of 20 % for the $17.9^\circ$ boundary and 40 % for the $9.4^\circ$ boundary. At the buckled boundaries, coupling between flexural and longitudinal phonon modes is also observed. The results indicate that scattering of long-wavelength flexural phonons at grain boundaries in graphene is mainly due to out-of-plane buckling. A continuum mechanical model of the scattering process has been developed, providing a deeper understanding of the scattering process as well as a way to calculate the effect of a grain boundary on long-wavelength flexural phonons based on the buckling size.Comment: 11 pages, 14 figure

Oxygen vacancy segregation and space-charge effects in grain boundaries of dry and hydrated BaZrO3

A space-charge model is applied to describe the equilibrium effects of segregation of double-donor oxygen vacancies to grain boundaries in dry and wet acceptor-doped samples of the perovskite oxide BaZrO3. The grain boundary core vacancy concentrations and electrostatic potential barriers resulting from different vacancy segregation energies are evaluated. Density-functional calculations on vacancy segregation to the mirror-symmetric \Sigma 3 (112) [-110] tilt grain boundary are also presented. Our results indicate that oxygen vacancy segregation can be responsible for the low grain boundary proton conductivity in BaZrO3 reported in the literature

Theoretical modeling of defect segregation and space-charge formation in the BaZrO3 (210) 001 tilt grain boundary

Density-functional theory (DFT) has been used to determine the structure and interface energy of different rigid body translations (RBTs) of the (210)10011 grain boundary (GB) in BaZrO3. There exist several different stable structures with almost equally low interfacial energy. Segregation energies of protons and oxygen vacancies have been determined for the most stable (210)10011 grain boundary structure. The results suggest that both defect species favor segregation to the same site at the boundary interface with minimum segregation energies of - 1.45 eV and - 1.32 eV for vacancies and protons respectively. The segregation energies have been used in a thermodynamic space-charge model to obtain equilibrium defect concentrations and space-charge potentials at a 10% dopant concentration. Space-charge,potential barriers around 0.65 V were obtained at intermediate temperatures under hydrated conditions, where protons are the main contributor to the excess core charge. The potential is slightly lower under dry conditions. (C) 2013 Elsevier B.V. All rights reserved

Atomistic Simulation of Interfaces: Proton transport across BaZrO₃ grain boundaries

<p>Due to the negative environmental effects of fossil fuels it is necessary to develop technology that may reduce or eliminate the need for oil and coal. Fuel cells are highly important in this context as they provide an efficient way of converting chemical energy into electrical energy. However, the development is hampered by a lack of electrolyte materials able to function at temperatures high enough to enable use of hydrocarbon fuels, yet low enough to avoid the wear on component materials caused by high operating temperatures. Solid oxide proton conductors are found to have several of the characteristics of a good electrolyte material in this temperature range, but increasing the conductivity to the level needed in practical applications remains a challenge. </p> <p>The aim of this thesis is to elucidate microscale phenomena that affect the performance of proton-conducting oxides. The material under investigation is BaZrO₃, which is regarded as a promising electrolyte material due to its chemical stability and high grain interior conductivity. However, the grain boundaries in the material are highly resistive and lower the total conductivity. The cause of this high grain boundary resistivity has been investigated using atomistic simulations and thermodynamic modelling. Particular attention is devoted to the role of defect segregregation to the grain boundaries.</p> <p>From atomistic simulations it has been found that positively charged defects such as oxygen vacancies and protons segregate to the grain boundaires of BaZrO₃. The accumulation of positive charge in the grain boundaries creates a potential barrier and leads to depletion of positive mobile defects from the surrounding region, impeding transport across the boundary. Thermodynamic models have been used to determine the height of the potential barrier resulting from segregation of positive defects, and the results compare well with experimental findings.</p

Adsorption of metal atoms at a buckled graphene grain boundary using model potentials

Two model potentials have been evaluated with regard to their ability to model adsorption of single metal atoms on a buckled graphene grain boundary. One of the potentials is a Lennard-Jones potential parametrized for gold and carbon, while the other is a bond-order potential parametrized for the interaction between carbon and platinum. Metals are expected to adsorb more strongly to grain boundaries than to pristine graphene due to their enhanced adsorption at point defects resembling those that constitute the grain boundary. Of the two potentials considered here, only the bond-order potential reproduces this behavior and predicts the energy of the adsorbate to be about 0.8 eV lower at the grain boundary than on pristine graphene. The Lennard-Jones potential predicts no significant difference in energy between adsorbates at the boundary and on pristine graphene. These results indicate that the Lennard-Jones potential is not suitable for studies of metal adsorption on defects in graphene, and that bond-order potentials are preferable

Theoretical modeling of defect segregation and space-charge formation in the BaZrO 3 (210)[001] tilt grain boundary

Abstract Density-functional theory (DFT) has been used to determine the structure and interface energy of different rigid body translations (RBTs) of th

Comparison of Space-Charge Formation at Grain Boundaries in Proton-Conducting BaZrO₃ and BaCeO₃

Acceptor-doped BaZrO₃ (BZO) and BaCeO₃ (BCO) both exhibit considerable bulk proton conductivity, which makes them suitable as electrolytes in electrochemical devices. However, these materials display high grain-boundary (GB) resistance that severely limits the overall proton transport in polycrystalline samples. This effect has been attributed to the presence of space charges at the GBs, which form because of segregation of protons and charged oxygen vacancies. This blocking behavior is less prominent in BCO, but in contrast to BZO, BCO suffers from poor chemical stability. The aim with the present work is to elucidate why GBs in BZO are more resistive than GBs in BCO. We use density-functional theory (DFT) calculations to study proton and oxygen vacancy segregation to several GBs and find that the oxygen vacancy segregation energy is quite similar in both materials while the tendency for proton segregation is larger in BZO compared with that in BCO. This is not related to the GBs, which display similar proton formation energies in both materials, but because of different formation energies for protons in the bulk regions. This can be understood from a stronger hydrogen bond formation in bulk BCO compared with that in bulk BZO. Furthermore, segregation energies are evaluated in a space-charge model, and the resulting space-charge potentials provide a consistent explanation of the experimentally observed difference in GB conductivity