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

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.

Atomic Scale Analysis of a YSZ Bicrystal Grain Boundary

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.

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

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.

scholarly journals The Atomic-Scale Origins of Grain Boundary Superconducting Properties

1998 ◽  
Vol 4 (S2) ◽  
pp. 688-689
Author(s):  
S. J. Pennycook ◽  
J. Buban ◽  
C. Prouteau ◽  
M. F. Chisholm ◽  
P. D. Nellist ◽  
...  
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Atomic Scale ◽  
Contrast Imaging ◽  
Qualitative Understanding ◽  
Quantitative Understanding ◽  
Bond Valence Sum ◽  
Z Contrast ◽  
Coherence Lengths ◽  
Contrast Image

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.

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

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.

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

2007 ◽  
Vol 22 (4) ◽  
pp. 950-957 ◽  
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.

scholarly journals First-principles simulations and Z-contrast imaging of impurities at tilt grain boundaries in MgO

1997 ◽  
Author(s):  
Y. Yan ◽  
M.F. Chisholm ◽  
S.J. Pennycook ◽  
G. Duscher ◽  
A. Maiti ◽  
...  
Keyword(s):  
Grain Boundaries ◽  
First Principles ◽  
Contrast Imaging ◽  
Z Contrast

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

MRS Bulletin ◽  
1997 ◽  
Vol 22 (8) ◽  
pp. 53-57 ◽  
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.

Non-Stoichiometry at Tilt Grain Boundaries in SrTiO3

2000 ◽  
Vol 6 (S2) ◽  
pp. 114-115
Author(s):  
G. Duscher ◽  
M. Kim ◽  
N. D. Browning ◽  
S. T. Pantelides ◽  
S. J. Pennycook
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Spatial Resolution ◽  
Oxygen Depletion ◽  
High Energy ◽  
High Energy Resolution ◽  
Contrast Imaging ◽  
Atomic Column ◽  
Key Issues ◽  
Z Contrast

The origin of electrically active grain boundaries in perovskite oxides and related materials remains controversial. The stoichiometry of the grain boundary core structure and the role of oxygen vacancies are the key issues involved. Previous results have given no indications of any non-stoichiometry at SrTiO3 grain boundaries, despite the fact that SrTiO3 is the system where atomic column EELS resolution has been demonstrated [1], and where atomic resolution images of the core structures have been obtained by Z-contrast imaging [2,3]. Here we show EELS spectra from individual dislocation cores in an 8° [100] tilt grain boundary in SrTiO3 that show significant oxygen depletion.The low angle grain boundary with well-separated dislocation cores makes it possible to study individual cores with high energy resolution in EELS. Previous work either averaged over all structural units in the grain boundary, sacrificing spatial resolution [2,3], or, used a very low probe current to obtain the highest spatial resolution [1].

The Origin of Electrical Activity at Grain Boundaries in Perovskites and Related Materials

2000 ◽  
Vol 654 ◽  
Author(s):  
S. J. Pennycook ◽  
M. Kim ◽  
G. Duscher ◽  
N. D. Browning ◽  
K. Sohlberg ◽  
...  
Keyword(s):  
High Temperature ◽  
Grain Boundary ◽  
Energy Loss ◽  
Electrical Activity ◽  
Grain Boundaries ◽  
Electron Energy Loss Spectroscopy ◽  
High Temperature Superconductor ◽  
Atomic Resolution ◽  
Z Contrast ◽  
Electron Energy Loss

In the last few years, the combination of atomic-resolution Z-contrast microscopy, electron energy loss spectroscopy and first-principles theory has proved to be a powerful means for structure property correlations in complex materials1. Here we demonstrate the effectiveness of this combined approach by demonstrating the origins of electrical activity at grain boundaries in the prototypical perovskite SrTiO3 and the high-temperature superconductor YBa2Cu3O7-x, materials that are closely related in structure. We show, both experimentally and theoretically, that grain boundaries in SrTiO3 are intrinsically non-stoichiometric. Electron energy-loss spectroscopy (EELS) provides direct evidence of non-stoichiometry, in agreement with total- energy calculations that predict non-stoichiometric grain boundaries to be energetically favorable. The predicted structures are consistent with atomic-resolution Z-contrast micrographs. These results provide a consistent explanation of the grain boundary charge that was previously inferred from electrical measurements, and provides a microscopic explanation of the resulting “double-Schottky barriers”. We also present experimental evidence for non-stoichiometry at grain boundaries in the high-temperature superconductor YBa2Cu3O7-x, where the same phenomenon explains the observed exponential reduction of critical currents with grain boundary misorientation.

Determining Atomic Structure-Property Relationships at Grain Boundaries

1996 ◽  
Vol 54 ◽  
pp. 334-335
Author(s):  
N. D. Browning ◽  
D. J. Wallis ◽  
P. D. Nellist ◽  
S. J. Pennycook
Keyword(s):  
Energy Loss ◽  
Grain Boundaries ◽  
Multiple Scattering ◽  
Scale Effects ◽  
Dominant Role ◽  
Boundary Plane ◽  
Atomic Scale ◽  
Atomic Resolution ◽  
Contrast Imaging ◽  
Z Contrast

Atomic scale effects at grain boundaries are known to play a dominant role in controlling the bulk properties of many materials. However, a detailed understanding of this role is complicated by the tendency for boundaries to behave in a “non-ideal” manner, i.e. the boundary plane can change on the scale of a few nanometers, altering the number of vacancies and impurities and the presence of second phases. The crucial first step to engineering boundary properties is therefore the ability to observe these changes experimentally with both atomic resolution and sensitivity. Such a capability is provided by the combination of Z-contrast imaging and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM).The Z-contrast technique generates an incoherent, atomic resolution structural image of a grain boundary which can be used to position the electron probe with atomic precision for spectroscopy. As the spectrum has nearly the same resolution as the image for core-losses >100eV, this arrangement has the unique advantage of allowing compositional fluctuations to be correlated directly with structural features in the boundary plane. Furthermore, multiple-scattering (MS) analysis can be utilized to extract 3- dimensional structural information from the spectrum. MS techniques consider the fine-structure of the spectrum to arise from interference effects occurring when a photoelectron created during the energy loss process is reflected from neighboring atoms. The real-space clusters used in this methodology allow the flexibility to determine whether the contributions to the spectrum arise from single or multiple scattering paths and from which atomic neighbors they originate. This allows the different structural relaxations that occur at boundaries, i.e. vacancies or structural disorder, to be distinguished from the spectrum.

Sign in / Sign up

Export Citation Format

Share Document