The Atomic-Scale Origins of Grain Boundary Superconducting Properties

Due to the extemely short coherence lengths of the high-Tc superconductors (around 30 Å in the a-b plane), defects such as grain boundaries are obvious barriers to the flow of supercurrent. Within a few months of the discovery of these materials, it was shown how the critical current dropped four orders of magnitude as the grain boundary misorientaion increased from zero to 45°. Even today, there is no quantitative understanding of this behavior. A qualitative understanding is however possible through atomic resolution Z-contrast imaging on YBa2cu3O7-δ and SrTiO3 bicrystal grain boundaries, combined with bond-valence-sum analysis.The Z-contrast image of a YBa2cu3O7-δ low angle grain boundary in Fig. 1 shows the same kind of reconstructed dislocation cores as seen in SrTiO3, containing reconstructions on both the Cu and Y/Ba sublattices.

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Atomic Scale Analysis of a YSZ Bicrystal Grain Boundary

Microscopy and Microanalysis ◽

10.1017/s1431927600028075 ◽

2001 ◽

Vol 7 (S2) ◽

pp. 400-401

Author(s):

Y. Lei ◽

Y. Ito ◽

N. D. Browning

Keyword(s):

Grain Boundary ◽

Grain Boundaries ◽

Bulk Material ◽

Atomic Scale ◽

Structure Property ◽

Contrast Imaging ◽

Structure Property Relationships ◽

Commercial Applications ◽

Z Contrast ◽

Electron Energy Loss

Yttria-stabilized zirconia (YSZ) has been the subject of many experimental and theoretical studies, due to the commercial applications of zirconia-based ceramics in solid state oxide fuel cells. Since the grain boundaries usually dominate the overall macroscopic performance of the bulk material, it is essential to develop a fundamental understanding of their structure-property relationships. Previous research has been performed on the atomic structure of grain boundaries in YSZ, but no precise atomic scale compositional and chemistry characterization has been carried out. Here we report a detailed analytical study of an [001] symmetric 24° bicrystal tilt grain boundary in YSZ prepared with ∼10 mol % Y2O3 by Shinkosha Co., Ltd by the combination of Z-contrast imaging and electron energy loss spectroscopy (EELS).The experimental analysis of the YSZ sample was carried out on a 200kV Schottky field emission JEOL 201 OF STEM/TEM4.

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Z-Contrast Imaging of Grain-Boundary Core Structures in Semiconductors

MRS Bulletin ◽

10.1557/s0883769400033820 ◽

1997 ◽

Vol 22 (8) ◽

pp. 53-57 ◽

Cited By ~ 17

Author(s):

M.F. Chisholm ◽

S.J. Pennycook

Keyword(s):

High Resolution ◽

Grain Boundary ◽

Grain Boundaries ◽

Phase Contrast ◽

Atomic Scale ◽

Resolving Power ◽

Boundary Structure ◽

Contrast Imaging ◽

Contrast Technique ◽

Z Contrast

Interest in semiconductor grain boundaries relates to the development of polycrystalline materials for photovoltaics and integrated-circuit interconnects. Although these structures are responsible for deleterious electrical effects, there are few experimental techniques available to study them at the required atomic scale. Therefore models of the physical processes occurring at grain boundaries have necessarily taken a macroscopic approach. Fortunately recent developments have resulted in tools that provide unprecedented glimpses into these interfaces and that will allow us to address anew the connection between grain-boundary structure and properties.Z-Contrast ImagingWhen exploring the unknown, we rely heavily on our eyes (incoherent imaging) to provide a direct image of a new object. In order to explore the unforeseen atomic configurations present at extended defects in materials, it again would be desirable if one could obtain a directly interpretable image of the unfamiliar structures present in the defect cores. Z-contrast electron microscopy provides such a view with both atomic resolution and compositional sensitivity.This high-resolution imaging technique differs from conventional high-resolution phase-contrast imaging. The phase-contrast technique produces a coherent image, an interference pattern formed by recombining the waves diffracted by the specimen. In the Z-contrast technique, the image is incoherent; it is essentially a map of the scattering power of the specimen. Additionally as was first determined by Lord Rayleigh, the incoherent mode of image formation has double the resolving power of the coherent mode.

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Atomic Scale Analysis of Cubic Zirconia Grain Boundaries

MRS Proceedings ◽

10.1557/proc-589-323 ◽

1999 ◽

Vol 589 ◽

Author(s):

E.C. Dickey ◽

X. Fan ◽

M. Yong ◽

S.B. Sinnott ◽

S.J. Pennycook

Keyword(s):

Grain Boundary ◽

Grain Boundaries ◽

Mirror Symmetry ◽

Fluorite Structure ◽

Atomic Scale ◽

Contrast Imaging ◽

Cubic Zirconia ◽

Atomic Structures ◽

Periodic Arrays ◽

Z Contrast

AbstractThe core structures of two symmetric tilt [001” grain boundaries in yttria- stabilized cubic zirconia are determined by Z-contrast imaging microscopy. In particular, near-σ=13 (510) and σ=5 (310) boundaries are studied. Both grain boundaries are found to be composed of periodic arrays of basic grain-boundary structural units, whose atomic structures are determined from the Z-contrast images. While both grain boundaries maintain mirror symmetry across the boundary plane, the 36° boundary is found to have a more compact structural unit than the 24° boundary. Partially filled cation columns in the 24° boundary are believed to prevent cation crowding in the boundary core. The derived grain boundary structural models are the first developed for ionic crystals having the fluorite structure

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Quantitative structure determination of interfaces through Z-contrast imaging and electron energy loss spectroscopy

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100162983 ◽

1996 ◽

Vol 54 ◽

pp. 104-105

Author(s):

S. J. Pennycook ◽

P. D. Nellist ◽

N. D. Browning ◽

P. A. Langjahr ◽

M. Rühle

Keyword(s):

Grain Boundaries ◽

Cuprate Superconductors ◽

Structure Refinement ◽

3D Structure ◽

Real Space ◽

Contrast Imaging ◽

Coordination Polyhedra ◽

Burgers Vector ◽

Z Contrast ◽

Contrast Image

The simultaneous use of Z-contrast imaging with parallel detection EELS in the STEM provides a powerful means for determining the atomic structure of grain boundaries. The incoherent Z-contrast image of the high atomic number columns can be directly inverted to their real space arrangement, without the use of preconceived structure models. Positions and intensities may be accurately quantified through a maximum entropy analysis. Light elements that are not visible in the Z-contrast image can be studied through EELS; their coordination polyhedra determined from the spectral fine structure. It even appears feasible to contemplate 3D structure refinement through multiple scattering calculations.The power of this approach is illustrated by the recent study of a series of SrTiC>3 bicrystals, which has provided significant insight into some of the basic issues of grain boundaries in ceramics. Figure 1 shows the structural units deduced from a set of 24°, 36° and 65° symmetric boundaries, and 24° and 45° asymmetric boundaries. It can be seen that apart from unit cells and fragments from the perfect crystal, only three units are needed to construct any arbitrary tilt boundary. For symmetric boundaries, only two units are required, each having the same Burgers, vector of a<100>. Both units are pentagons, on either the Sr or Ti sublattice, and both contain two columns of the other sublattice, imaging in positions too close for the atoms in each column to be coplanar. Each column was therefore assumed to be half full, with the pair forming a single zig-zag column. For asymmetric boundaries, crystal geometry requires two types of dislocations; the additional unit was found to have a Burgers’ vector of a<110>. Such a unit is a larger source of strain, and is especially important to the transport characteristics of cuprate superconductors. These zig-zag columns avoid the problem of like-ion repulsion; they have also been seen in TiO2 and YBa2Cu3O7-x and may be a general feature of ionic materials.

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Quantitative Characterization of Grain Boundary Films In Si3n4 by Eels and Z-Contrast Imaging

Microscopy and Microanalysis ◽

10.1017/s1431927600010199 ◽

1997 ◽

Vol 3 (S2) ◽

pp. 661-662

Author(s):

H. Gu

Keyword(s):

Grain Boundary ◽

Grain Boundaries ◽

Surface Oxidation ◽

Force Balance ◽

Contrast Imaging ◽

Full Spectrum ◽

High Temperature Mechanical Properties ◽

Boundary Films ◽

Z Contrast

High temperature mechanical properties of structural ceramics Si3N4 are controlled by ∼1 nm thick silicate amorphous films covering all grain boundaries. The composition of the film dictates the equilibrium film thickness resulted from a force balance at grain boundary. Many efforts arc brought to alter film chemistry and thickness, and this system offers ideal model materials to understand grain boundary and property relationship. Using a dedicated STEM (VG HB601) with high spatial resolution EELS analysis and high resolution Z-contrast imaging, various novel quantification data of the grain boundary in Si3N4 can be obtained. The methods described here can also be applied to other types of grain boundaries.EELS profiling was performed to acquire a full spectrum from each position at a lateral increment of 1Å across a boundary in a pure Si3N4 sample with only SiO2 impurities from surface oxidation. It gives directly elemental distributions near the boundary such as Si, N and O profiles shown in Fig. 1.

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STEM Investigation of Segregation and Local Structure at Grain Boundary in Tetragonal Zirconia

Microscopy and Microanalysis ◽

10.1017/s1431927600009636 ◽

1997 ◽

Vol 3 (S2) ◽

pp. 549-550

Author(s):

H. Gu ◽

F. Wakai

Keyword(s):

Grain Boundary ◽

Grain Boundaries ◽

Solid Electrolytes ◽

Tetragonal Zirconia ◽

Boundary Structure ◽

Contrast Imaging ◽

Amorphous Films ◽

Grain Boundary Structure ◽

Z Contrast ◽

Segregation Of Impurities

Y or Ca stabilized tetragonal ZrO2 (TZP) exhibits superplasticity at high temperature, and can also be used as solid electrolytes. Those properties are dictated by structure and chemistry of grain boundaries, which can be controlled by segregation of impurities or additives. The grain boundaries were found either covered by amorphous films or free of the film. Co-segragation of additives and stabilizers has also been observed. To fully understand the correlation between segregation and grain boundary structure, a dedicated STEM (VG HB601) capable of EDX/EELS analysis and phase/Z-contrast imaging is employed to study 3Y-TZP doped with 0.3 and 0.9 mol% SiO2.Although Y-L lines arc dominated by overlapping Zr-L lines in EDX, Y excess at grain boundaries can still be measured by “spatial difference” which removes Zr signal with a spectrum from the bulk. The co-segregation of Si and Y is also observed (Fig. 1) at many boundaries. Their average excesses arc 5±2 nm−2and 25±10 run−2 respectively, close to 1 monolayer each of SiO2 and Y2O3.

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(110) facets and dislocation structure of low-angle grain boundaries in YBa2Cu3O7−δ and Y0.7Ca0.3Ba2Cu3O7−δ thin film bicrystals

Journal of Materials Research ◽

10.1557/jmr.2007.0110 ◽

2007 ◽

Vol 22 (4) ◽

pp. 950-957 ◽

Cited By ~ 2

Author(s):

Xueyan Song

Keyword(s):

Thin Film ◽

Grain Boundary ◽

Grain Boundaries ◽

Dislocation Structure ◽

Current Flow ◽

Dislocation Core ◽

Current Transport ◽

Contrast Imaging ◽

Grain Boundary Plane ◽

Z Contrast

The facet and dislocation structure of 5° and 7° [001]-tilt grain boundaries of YBa2Cu3O7−δ (YBCO) and Y0.7Ca0.3Ba2Cu3O7−δ (YCaBCO) thin film bicrystals were studied. A 24° [001]-tilt YBCO grain boundary was also examined to contrast with the low angle grain boundary faceting behavior. All the low-angle grain boundaries exhibit strong faceting along (100)/(010) and (110) and possess both straight symmetric segments containing equally spaced [100] unit dislocations and step asymmetric segments composed of (110) and (100)/(010) facets. Grain boundaries with a higher degree of meander acquired up to 40% (110) facets. The atomic structure of (110) facets was revealed by the atomic resolution Z-contrast imaging. The (110) facets are dissociated for both the YBCO and YCaBCO grain boundaries. We also found the Ca-doped (110) facets to be more extended along the grain boundary plane, consistent with our earlier finding of a dissociated dislocation core in Ca-doped (100) facets. These 5° and 7° misorientations that we studied are just in the range at which YBCO grain boundaries start to become obstacles to current flow. The above results will be helpful for understanding the current transport across YBCO low-angle grain boundaries.

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Atomic resolution determination of the structure and chemistry of ceramic grain boundaries

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100137975 ◽

1995 ◽

Vol 53 ◽

pp. 320-321

Author(s):

N. D. Browning ◽

M. M. McGibbon ◽

M. F. Chisholm ◽

S. J. Pennycook

Keyword(s):

Grain Boundaries ◽

Scanning Transmission Electron Microscope ◽

Atomic Scale ◽

Secondary Phases ◽

Contrast Imaging ◽

Scanning Transmission ◽

Recent Developments ◽

Contrast Technique ◽

Z Contrast

Characterization of grain boundaries in ceramics is complicated by the multicomponent nature of the materials, the presence of secondary phases, and the tendency for the grain boundary plane to “wander” on the length scale of a few nanometers. However, recent developments in the scanning transmission electron microscope (STEM) have now made it possible to correlate directly the structure, composition and bonding at grain boundaries on the atomic scale. This direct experimental characterization of grain boundaries is achieved through the combination of Z-contrast imaging (structure) and electron energy loss spectroscopy (EELS) (composition and bonding). For crystalline materials in zone-axis orientations, where the atomic spacing is larger than the probe size, the Z-contrast technique provides a direct image of the metal (high Z) columns. This image, being formed from only the high-angle scattering, can be used to position the electron probe with atomic precision for simultaneous EELS. Under certain collection conditions, the spectrum can have the same atomic spatial resolution as the image, thus permitting the spectra to be correlated with a known atomic location.

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First-Principles Simulations and Z-Contrast Imaging of Impurities at Tilt Grain Boundaries in Mgo

MRS Proceedings ◽

10.1557/proc-492-181 ◽

1997 ◽

Vol 492 ◽

Author(s):

Y. Yan ◽

M. F. Chisholm ◽

G. Duscher ◽

S. J. Pennycook ◽

A. Maiti ◽

...

Keyword(s):

Grain Boundary ◽

Grain Boundaries ◽

First Principles ◽

Density Functional Calculations ◽

Density Functional ◽

Equilibrium Structure ◽

Atomic Resolution ◽

Contrast Imaging ◽

Metastable Structures ◽

Z Contrast

ABSTRACTFirst-principles density-functional calculations were used to study the effects of Ca impurities on the Σ=5 (310) <001> tilt grain boundaries in MgO. An equilibrium structure and two metastable structures of the grain boundaries in pure MgO have been established. The calculations further demonstrated that Ca impurities segregate at particular sites in the metastable grain boundary and induce a structural transformation. This result is consistent with atomic resolution Z-contrast imaging. The calculations also found that the impurities at the grain boundaries do not induce states in the band gap. The mechanism of the transformation is also discussed.

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