Elsevier

Micron

Volume 30, Issue 5, October 1999, Pages 425-436
Micron

Investigating the atomic scale structure and chemistry of grain boundaries in high-Tc superconductors

https://doi.org/10.1016/S0968-4328(99)00044-XGet rights and content

Abstract

The short superconducting coherence length in high-Tc materials makes them extremely susceptible to the deleterious effect of atomic scale defects. Perhaps the most important of these defects for large-scale technological applications, are grain boundaries. Here we describe an atomic resolution investigation of structural and chemical changes that occur at grain boundaries in high-Tc materials using scanning transmission electron microscopy (STEM). STEM is ideally suited to this analysis, as atomic resolution Z-contrast images and electron energy loss spectra (EELS) can be acquired simultaneously. This permits a direct correlation between the structural images and the local electronic structure information in the spectrum. From this detailed experimental characterization of the grain boundaries, simple theoretical models can be derived that allow the structure-property relationships in high-Tc superconductors to be inferred. Results obtained from YBa2Cu3O7−δ and (Bi/Pb)2Sr2Ca2Cu3O10 show that there is a charge depletion zone formed at grain boundaries. This charge depletion zone can act as a tunnel barrier to the flow of superconducting charge carriers and appears to increase in width with increasing misorientation angle. The magnitude of the critical current across grain boundaries in high-Tc materials predicted from these models is in excellent agreement with the widely reported electrical transport results.

Introduction

Grain boundaries have long been known to have a deleterious effect on the transport properties of high-Tc superconductors. For example, in systematic studies of high-angle [001] tilt grain boundaries in YBa2Cu3O7−δ (YBCO) thin-film bicrystals, it has been found that there is an exponential decrease in the critical current as the misorientation angle of the boundary increases (Dimos et al., 1990, Gross and Mayer, 1991, Ivanov et al., 1991). While this overall trend in the data is clear, attempts to ascertain the fundamental origin of the transport properties have been confused by the considerable scatter in the experimental measurements (results can vary by several orders of magnitude for the same misorientation angle). Many explanations for the transport properties have been linked to the chemistry of these ceramic oxide materials. It may be that as YBCO is highly susceptible to the formation of oxygen vacancies, an oxygen deficient boundary layer is formed, thereby reducing the charge carrier concentration (Browning et al., 1992, Zhu et al., 1993, Babcock et al., 1994). Alternatively, there may be second phases, precipitates or the preferential segregation of impurities to the boundary that cause a barrier to transport. However, as all of these possibilities are a function of the processing conditions in ceramic oxides, it is not clear why they should cause the overall trend of an exponential decrease in critical current with misorientation angle. The large scatter in the experimental transport measurements is an obvious consequence of variable processing conditions and the chemistry of the oxide systems, but if it is the dominant effect, high conductivity should be readily achieved by preparing clean, oxygen-rich grain boundaries. This has not been reported in the literature, despite many attempts to oxidize boundaries with excellent cation stoichiometry.

The widely observed transport behavior of grain boundaries in high-Tc superconductors therefore suggests that there is an underlying mechanism responsible for the exponential decrease in critical current with increasing misorientation angle. As the superconducting coherence length in these materials is ∼1 nm, this mechanism is likely to be related to the atomic scale fluctuations in structure and chemistry that can occur at all grain boundaries (even those without second phases). One of the ways to measure these changes on this fundamental atomic scale is through the combination of Z-contrast imaging and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) (Browning et al., 1993a, Batson, 1993). Unlike conventional phase contrast imaging in a transmission electron microscope (TEM), the Z-contrast technique provides a direct image of the structure of grain boundaries that can be intuitively interpreted. Atomic column positions can be determined to be within 0.02 nm accuracy and column compositions inferred from their intensity in the image. Additionally, as the experimental conditions are the same for high-resolution microanalysis, EELS can be acquired from atomic locations defined in the image (see experimental techniques). This spectroscopic technique can detect changes in chemical composition and local electronic structure which can be quantified from variations in the spectral fine-structure (Egerton, 1996, Brydson et al. 1992, Pearson et al., 1988). In particular, core-loss spectra probe transitions from deep core-levels to unoccupied states above the Fermi-level. As each chemical species and type of bond results in a characteristic position for the Fermi-level and density of unoccupied states, EELS is a very accurate probe of the type of bonding and chemical species present in the material. Therefore, by correlating directly with the atomic structure observed in the Z-contrast image, the structure, composition and chemistry can all be quantified on the atomic scale.

In this article we present results from grain boundaries in (Bi/Pb)2Sr2Ca2Cu3O10 (Bi-2223) superconducting wires and YBCO thin-film bicrystals. For both materials there is observed to be a charge carrier depletion layer occurring at the grain boundaries. In the case of the less well controlled bulk Bi-2223 system, the presence of extrinsic effects associated with the processing conditions, i.e. second phases and precipitates, is observed. However, in the YBCO thin-films, a charge carrier depletion layer is observed despite the boundaries being stoichiometric. Using crystal chemistry principles, the reason for the charge carrier depletion is determined to be the presence of a reconstructed boundary plane. The boundary reconstruction leads to copper (Cu) sites in the grain boundary plane that are under-coordinated to oxygen (O). As the Cu–O interaction is the essential ingredient to the formation of charge carrying holes in high-Tc materials, this automatically leads to the presence of a carrier depletion zone at the boundary. The number of under-coordinated sites in the grain boundary plane increases linearly with misorientation angle, naturally explaining the orientation dependence of the transport properties through a tunneling current argument. These models indicate that there is an intrinsic limitation to grain boundary conductivity in high-Tc materials irrespective of processing conditions.

Section snippets

Experimental techniques

The results presented in this article were obtained on a VG HB501 dedicated STEM operating at 100 kV with a 0.22 nm probe size, and a VG HB603 dedicated STEM operating at 300 kV with a 0.13 nm probe size. In both of these instruments the electron optics are primarily designed to form a small probe on the surface of the specimen (Fig. 1). The transmitted electrons are collected in a variety of detectors and correlated with the position of the probe as it scans over the surface of the specimen.

(Bi/Pb)2Sr2Ca2Cu3O10

The Bi-2223 materials examined in the STEM (Prouteau et al., 1998) were 19 filament composites fabricated at American Superconductor Corporation (ASC) using the powder-in-tube technique (Li et al., 1997) with a powder stoichiometry of Bi1.7Pb0.3Sr1.9Ca2Cu3.1Ox. A thermomechanical process consisting of a sequence of roll deformation and heat treatment steps was used to promote Bi-2223 phase formation, texture, and densification. Transport critical current (1 μV/cm) measurements at 77 K under

Discussion

The results from the two systems show that in both cases there can be significant changes in the local electronic structure at high-angle grain boundaries. However, if we consider the processing mechanism involved with each sample, it is reasonably straightforward to see that the origin of the electronic structure changes is different in the two cases. In the case of the Bi-2223 wires, the bulk processing methods mean that the grain boundaries are much more likely to be decorated with

Conclusions

The results from both Bi-2223 and YBCO show that there can be large changes in the local electronic structure associated with grain boundaries. The most prominent changes occur when there is a stoichiometry change at the grain boundary associated with a second phase, precipitate, segregated impurity or oxygen deficiency. Analysis of the intrinsic structure of the clean, stoichiometric grain boundaries indicates that there exists a reconstructed boundary plane. This boundary reconstruction, that

Acknowledgements

Aspects of this work were performed in collaboration with M.F. Chisholm, P.D. Nellist, M. Teplitsky and G.N. Riley Jr. This research is funded by NSF under grant No's. DMR-9503877 and DMR-9803021, by the US DOE under contract no. DE-ACOS-96OR22464 with Lockheed Martin Energy Research Corporation and by a partial appointment to the ORNL postdoctoral program administered by ORISE.

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    Current address: University of Birmingham, Birmingham, UK.

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