Boundary Core Region Research Articles

Localized Phonon Densities of States at Grain Boundaries in Silicon.

Since it is now possible to record vibrational spectra at nanometer scales in the electron microscope, it is of interest to explore whether extended defects in crystals such as dislocations or grain boundaries will result in measurable changes of the phonon densities of states (dos) that are reflected in the spectra. Phonon densities of states were calculated for a set of high angle grain boundaries in silicon. The boundaries are modeled by supercells with up to 160 atoms, and the vibrational densities of states were calculated by taking the Fourier transform of the velocity–velocity autocorrelation function from molecular dynamics simulations with larger supercells doubled in all three directions. In selected cases, the results were checked on the original supercells by comparison with the densities of states obtained by diagonalizing the dynamical matrix calculated using density functional theory. Near the core of the grain boundary, the height of the optic phonon peak in the dos at 60 meV was suppressed relative to features due to acoustic phonons that are largely unchanged relative to their bulk values. This can be attributed to the variation in the strength of bonds in grain boundary core regions where there is a range of bond lengths.

Blocking in Ionic Conduction By Defect Segregation Near Grain-Boundaries in Nanocrystalline Yttria-Doped Ceria Thin Films

Nanocrystalline materials have unusual and exceptional electrical and chemical properties, some of which are due to their nanoscale grain sizes and highly dense grain boundaries. Compared to bulk materials, nanocrystalline films used in solid oxide fuel cells have an extremely high density of grain boundaries, and therefore, understanding the role of grain boundaries in ion transport is crucial. There is evidence that grain boundaries affect the electrochemical properties and behavior of thin film solid oxide fuel cell (SOFC) components in two main ways. First, in terms of cathode interfacial kinetics, grain boundaries at the external surface of the solid oxide electrolyte, e.g.,yttria-stabilized zirconia (YSZ), exhibit a higher surface exchange coefficient for oxygen than the bulk. Due to dopant segregation, grain boundaries contain a higher population of oxygen vacancies than the bulk, and thus facilitate low activation energy for oxygen incorporation. Second, previous work studying the effects of dopant distribution, space charge, as well as grain size on ionic transport across grain boundaries revealed that grain boundary resistivity is usually several orders higher than that of the bulk. Thus, understanding the blocking effect of grain boundaries on ionic conductivity in nanocrystalline materials requires a study of how grain size and thermal history affect ion transport in nanocrystalline films. Yttrium-doped ceria (YDC) is a promising electrolyte material for low temperature solid oxide fuel cells (LT-SOFCs) because of its superior ionic conductivity and lower activation energy for ionic transport than YSZ, which is the electrolyte material most commonly used in SOFCs. Previous studies on nanocrystalline YDC examined the effects of grain size on ionic conductivity by varying the grain size using different thermal conditions to generate different grain boundary densities. However, even though the effects of heat treatment on grain boundary density were considered in these studies, the effects of heat treatment on dopant segregation were not. In this paper, nanocrystalline YDC thin films with grain-sizes ranging from 38 to 93-nm were prepared using pulsed laser deposition followed by thermal annealing. The blocking effect of the grain boundary on ionic conduction in nanocrystalline YDC films was investigated with different grain sizes and thermal histories. Through high resolution TEM analysis accompanied with EDS capability, we found out that dopant segregation at the grain boundary core region became significant after annealing while it was not observed in as-deposited YDC film. Concentration of oxygen vacancies due to dopant segregation may hinder ionic transport by a space charge layer effect, decreasing the ionic conductivity of annealed YDC films. In electrochemical impedance spectroscopy analysis, as-deposited YDC film showed comparable ionic conductivity and activation energy to epitaxial YDC film, but ionic conductivity decreased as the annealing temperature increased. Spectroscopic evidence presented in this study suggests that dopant segregation has a more pronounced impact on ionic conductivity than grain size, i.e., grain boundary density [1]. Reference [1] J. An, et al., "Grain Boundary Blocking of Ionic Conductivity in Nanocrystalline Yttria-doped Ceria Thin Films", Scripta Materialia, Vol. 104, pp. 45-48 (2015)

Chemical diffusion and defect chemistry of grain boundaries in n-type barium titanate ceramics

Conductivity relaxation experiments have been performed on n-type barium titanate ceramics, co-doped with Y (donor) and Mn (acceptor), as a function of temperature (700–900°C) and oxygen partial pressure (0.001–1.0bar) by employing the van der Pauw technique. The chemical diffusion process owing to a change of the oxygen partial pressure in the surrounding atmosphere can be described by fast transport of oxygen vacancies along the grain boundaries (mainly core regions) and slow diffusion of cation vacancies into adjacent bulk regions (including space charge layers). A careful analysis of the experimental conductivity relaxation experiments allows the determination of the chemical diffusion coefficient for oxygen along grain boundary core regions with typical values in the range of 10−8 to 10−9cm2s−1 and an activation energy of 0.9eV (900>T/°C>700). Furthermore, the partial pressure dependence of the electronic conductivity can be interpreted by means of a defect chemical model involving grain boundary core regions where Mn is segregated and oxygen vacancies are the decisive mobile ionic charge carriers.

Modelling of diffusion and conductivity relaxation of oxide ceramics

A two-dimensional square grain model has been applied to simulate simultaneously the diffusion process and relaxation of the dc conduction of polycrystalline oxide materials due to a sudden change of the oxygen partial pressure of the surrounding gas phase. The numerical calculations are performed by employing the finite element approach. The grains are squares of equal side length (average grain size) and the grain boundaries may consist of thin slabs of uniform thickness. An additional (space charge) layer adjacent to the grain boundary cores (thin slabs) either blocking (depletion layer) or highly conductive for electronic charge carriers may surround the grains. The electronic transport number of the mixed ionic-electronic conducting oxide ceramics may be close to unity (predominant electronic conduction). If the chemical diffusion coefficient of the neutral mobile component (oxygen) of the grain boundary core regions is assumed to be higher by many orders of magnitude than that in the bulk, the simulated relaxation curves for mass transport (diffusion) and dc conduction can deviate remarkably from each other. Deviations between the relaxation of mass transport and dc conduction are found in the case of considerably different electronic conductivities of grain boundary core regions, space charge layers, and bulk. On the contrary, the relaxation curves of mass transport and electronic conductivity are in perfect coincidence, when either effective medium diffusion occurs or the effective conductivity is unaffected by the individual conductivities of core regions and possible space charge layers, i.e. the grain boundary resistivity is negligible.

Cover Image, Volume 99, Issue 3

Computed atomic structure of a metastable Σ 7m grain boundary of an Al2O3 bi‐crystal with yttrium atoms segregated to the grain boundary. Red, green and grey spheres denote oxygen, aluminum and yttrium atoms, respectively. The differently colouredpolyhedra highlight the various aluminum‐oxygen coordination in the grain boundary core region: the yellow tetrahedra mark fourfold‐coordinated Al atoms and the pyramidally shaped blue polyhedra indicate five‐fold coordinated Al atoms, while the usual six‐fold coordination of Al atom in bulk Al2O3 is identified with the green octahedra. The yttrium atoms in the boundary have eight oxygen neighbours and are also shown in greater detail in the pink polyhedra. The top view perspective shows clearly the inclination of the two single crystals against each other, and also highlights the high degree of atomic alignment when viewed along the [0001] direction. See feature article by Heuer et al.image

C–Cr segregation at grain boundary before the carbide nucleation in Alloy 690

The grain boundary segregation in Alloy 690 was investigated by atom probe tomography. B, C and Si segregated at the grain boundary. The high concentration regions for each segregation element form a set of straight arrays that are parallel to each other in the grain boundary plane. The concentration fluctuation has a periodicity of about 7 nm in the grain boundary plane. Before the Cr{sub 23}C{sub 6} nucleation at grain boundaries, the C-Cr co-segregate on one side of the grain boundaries while not the exact grain boundary core regions have been detected. The reasons why grain boundary carbides have coherent orientation relationship only with one side of nearby grain which grain boundary is located at high index crystal plane were discussed. - Highlights: Black-Right-Pointing-Pointer Grain boundary segregation in Alloy 690 was investigated by atom probe tomography. Black-Right-Pointing-Pointer B, C and Si segregate at the grain boundary. Black-Right-Pointing-Pointer Concentration of segregated atoms periodicity fluctuated in the grain boundary plane. Black-Right-Pointing-Pointer C and Cr co-segregate on one side of the grain boundary before carbide nucleation.

HAADF-STEM observations of a Σ13 grain boundary in α-Al2O3 from two orthogonal directions

Atomic structure of a Σ = 13, grain boundary in α-Al2O3 was directly observed by aberration-corrected Z-contrast scanning transmission electron microscopy. Combining observations from two orthogonal directions parallel to the grain boundary plane , the three-dimensional atomic configuration of the grain boundary core region can be determined, and proves to be in a good agreement with the stable boundary structure predicted by first-principles calculations.

Modelling of grain boundary resistivities of n-conducting BaTiO 3 ceramics

The grain boundary resistance of donor-doped barium titanate ceramics has been modelled as a function of temperature (90–400 °C) by application of a modified double-Schottky-barrier model. Diffusion profiles of cation vacancies formed during the cooling process after sintering have been calculated numerically for various cooling rates. Deep acceptor states at the grain boundary core (segregated acceptor co-dopants) trap electrons from adjacent n-conducting bulk regions. The cation vacancies created by re-oxidation processes at high temperatures act as immobile shallow acceptor states, which modify the space charge density in the depletion zones. Expressions for the potential barrier height of the depletion zone and the grain boundary resistance have been derived. Optimized values for the temperature dependent energy level and the density of the deep acceptor states at the grain boundary core regions are provided. The present model allows a reliable description of the PTCR (positive temperature coefficient of resistance) effect and theoretical results coincide well with experimental data.

Vacancy Formation and Vacancy-Induced Structural Transformation in Si Grain Boundaries

Atomic resolution Z-contrast scanning transmission electron microscopy reveals preferential nucleation of electron-beam-induced damage in select atomic columns of a Si tilt grain boundary. Atomic scale simulations find that the region of initial damage nucleation corresponds to columns where the formation energies of vacancies and vacancy complexes are very low. The calculations further predict that vacancy accumulation in certain pairs of columns can induce structural transformations of the grain boundary core region.

Lattice imaging study of grain boundaries at normal incidence to the boundary plane

The electron microscope lattice imaging technique has been widely used to study the atomistic structure of tilt grain boundaries. In these studies, the incident electron beam is aligned essentially parallel to the boundary tilt axis (i.e., the beam is parallel to the boundary). A projected image of the atomic columns which run parallel to the tilt axis in the grain boundary core region and the two adjoining crystals (Crystals 1 and 2) is then obtained on a plane perpendicular to the incident beam. Important information about the projected atomic structure of the core region and of the translational state assumed by Crystals 1 and 2 relative to one another in the plane of projection can be obtained from such an image. The paper describes work in which information about the translational state of a boundary is obtained by the lattice imaging technique under conditions where the incident beam is aligned perpendicular to the boundary plane. In this case, the image consists of a superposition of the projections on the boundary plane of the atomic columns in Crystals 1 and 2 which run perpendicular to the boundary. The boundary is the (001) ..sigma..5 twist boundary in gold.

Diffraction study of the influences of bonding type and misorientation angle on the structure of [001] twist boundaries along the direction normal to the interface

Abstract The influences of bonding type and misorientation angle on the structure of [001] twist grain boundaries along the direction normal to the interface have been studied using an electron diffraction approach in which ′streaks′ of diffuse scattering are recorded from the grain boundary core region. The boundaries studied were in NiO (with largely ionic bonding), Au (metallic bonding) and Ge (covalent bonding), in some cases with varying misorientation angles. The electron diffraction patterns have been analysed in terms of changes of plane spacing normal to the interface using the theoretical analysis in the preceding paper (Vaudin, Burkel and Sass 1986). Qualitative conclusions are drawn on the influence of bonding type and misorientation angle on the structure of [001] twist grain boundaries. The results are discussed with particular reference to the limitations of the experimental technique, dynamical diffraction effects, grain boundary segregation and reconstruction of the grain boundary core.