KEY WORDS:weak links, bicrystals, EELS, YBa2CU307_,S, grain boundary structure A BSTRACT Grain-boundary weak links limit the critical current density of poly crystalline high-Tc superconductors. Themes that have evolved from research on these grain boundaries include (a) an apparent general and dramatic distinction between the behavior of low-angle and high-angle grain boundaries, (b) electrical heterogeneity along as well as among boundaries, (c) indications that the macroscopic current path may or may not cross certain high-angle grain boundaries, (d) a key role for TEM EELS in studies of the electronic structure near grain boundaries, and (e) evidence that current models for grain boundary structure pertain to the high-Tc superconductors and may eventually help us to understand their electromagnetic character. Results regarding each of these topics are reviewed in this paper. 0084-6600/95/0801-0193$05.00 193 Annu. Rev. Mater. Sci. 1995.25:193-222. Downloaded from www.annualreviews.org by University of Massachusetts -Amherst on 09/27/12. For personal use only. Quick links to online content Further ANNUAL REVIEWS Annu. Rev. Mater. Sci. 1995.25:193-222. Downloaded from www.annualreviews.org by University of Massachusetts -Amherst on 09/27/12. For personal use only. (HIGH-Tel SUPERCONDUCTOR GRAIN BOUNDARIES [> EPITAXY OF DISSIMILAR MATERIALS, C. J. Palmstrnm 389 V11 Annu. Rev. Mater. Sci. 1995.25:193-222. Downloaded from www.annualreviews.org by University of Massachusetts -Amherst on 09/27/12. For personal use only. Quick links to online content Further ANNUAL REVIEWS
We propose that the critical currents of nearly stoichiometric YBa,Cu~O~-s single crystals (S < 0.1) are determined by fluxon pinning at ordered oxygen-vacancy domains. Since these domains are also superconducting, albeit with lower Tc and H, than the stoichiometric matrix, their pinning strength at a given temperature will increase as they are driven normal by an increasing applied field. Experimentally, the magnetization hysteresis curves display a secondary maximum, characteristic of pinning by a superconducting second phase. The ordered-vacancy domains are assumed to result from a spinodal decomposition. The density and size of the domains were calculated on this assumption. The bulk pinning force was computed by direct summation. In the limit of low temperatures, the calculated values of critical current density J, are in good agreement with measurements on high-quality single crystals.It has been recently shown that the flux pinning of almost stoichiometric high-quality YBa2CuR0,-s single crystals is directly controlled by the oxygen deficiency.' Pinning by oxygen defects was found to be significant even when 6<0.05. The magnetization hysteresis at fields of several tesla decreased by factors of 3-5 as the equilibrium value of 6 approached zero.' The magnetization curves also displayed an anomalous secondary peak at high-field (referred to sometimes as a "fish-tail" or "bow-tie"). Thus any pinning model for single crystals must take into account that (i) J, is of order lo6 A cm-z at low T, (ii) J, increases as S increases from zero to approximately 0.1, and (iii) at a given temperature, the hysteresis goes through a minimum and then increases as B increases, peaking at high field values.Pinning by atomic scale defects, particularly oxygen vacancies, was previously proposed."*3 Recently, the elementary pinning force of single oxygen vacancies randomly distributed in the CuOa planes was estimated in the limit of zero field and temperature.4 However, neutron diffraction data showed that the oxygen vacancies are not located in the CuOz but in the CuO ("chain") layers5 and that significant vacancy ordering in oxygen-deficient material can occur even at room temperature.6,7 Ordered vacancy domains were detected in oxygen-deficient material by transmission electron microscopy (TEM).8V9 This result was recently confirmed by neutron diffraction."The most commonly observed structure consists of ordered oxygen vacancies located at (0, l/2 b,, 0) every other unit cell. This arrangement generates a superlattice with a doubled unit cell parameter in the a direction and has the YBazCusOb., composition. This orthorhombic II (011) structure has been associated with the 50-60 K transition temperature (T,) plateau that has been observed for S around 0.5.8V9 Electron diffraction experiments demonstrated the three-dimensional coherence of the 011 structure.8'1' A spinodal transformation from a single nonstoichiometric phase into various ordered superlattices was also predicted in theoretical work on oxvaen-deficient We propos...
Field ion specimens of BizSr2CaCu20x (BSCCO) high temperature superconductor (HTS) materials have been prepared using a combination of three different preparation techniques: the method of sharp shards, electropolishing and ion milling. Field ion microscopy (FIM) has demonstrated that samples which exhibit the "striped"-image contrast characteristic of HTS materials can be successfully fabricated using this combination. FIM images have been obtained which show the striped-image contrast much clearer than any previously published images of Pb-free BSCCO. Prelimina~y atom probe (AP) chemical analysis of the material was also performed. Analytical electron microscopy was used to confirm the existence of both the correct crystallographic structure and nominal composition in the near-apex region of the specimen after preparation and FIM.
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