We have developed a novel method of producing bulk scale bicrystals of the high temperature superconductor YBa2Cu3O7-d using a dual-seeded melt textured growth process. Such bicrystals that contain a single grain boundary are essential to a variety of studies of superconductivity, particularly the issue of superconducting transport properties across grain boundaries. In contrast to more conventional methods of synthesizing similar bicrystal samples, this new method allows for the reproducible and controlled fabrication of bicrystals of virtually any misorientation. Equally as significant, the grain boundaries contained in these bicrystals contain large, straight facets that are ideal for basic studies of superconducting transport behavior. The unique structure of the grain boundaries contained in these bicrystals lend themselves to a number of significant experimental studies that are not possible with other types of samples, including measurements of the symmetry of the order parameter, the influence of the order parameter on grain boundary transport, the interplay between intragrain pinning and grain boundary transport properties, and the influence of Josephson vortex pinning on grain boundary transport.
The ability to fabricate bicrystals with uniform, well-defined grain boundaries in a systematic manner has been an important development that has helped improve both the fundamental science and the technological applications of high temperature superconductors. In the area of the fundamental mechanisms of superconducting transport across grain boundaries, samples with well-defined grain boundaries provide the only means to test various theories and models of current transport across boundaries. In relationship to the development of practical conductors, single grain boundaries in bulk samples provide an excellent means of studying transport properties in a system that mimics a bulk conductor. For example, the influence of the boundary configuration on transport properties that has been learned through studying these bulk samples has direct relevance to coated conductors. Finally, this new process may be used to develop devices based on grain boundary junctions with properties that may tailored for a particular application by the growth process.
There are 6 published articles in refereed journals
pertaining to the development of this new process and on new results obtained
from these samples. In addition, two patent applications have been filed
and one additional disclosure that is related to seeded melt textured growth
is currently under review by the Laboratory.
Understanding the current transport mechanisms across grain boundaries in high temperature cuprate superconductors has been one of the most outstanding technological and scientific issues facing the field almost since the discovery of these materials. The strong dependence of critical current density carried across a grain boundary as a function of the boundary misorientation angle was first demonstrated in YBa2Cu3O7-x (YBCO) by Chaudhari, et al. [1] and more firmly established through systematic studies in YBCO thin film bicrystals by Dimos, et al. [2] This "weak link" behavior in which the critical current across a grain boundary can be significantly lower than that within the adjacent grains has been a major impediment to the commercialization of high temperature superconductors for high current applications. Conversely, controlling the "weak link" behavior of grain boundaries has been a goal for implementation of grain boundary junctions as an essential element of superconducting electronic devices. In each case, a firm understanding of the mechanism leading to this weak link behavior is an important step in improving the properties and commercialization potential of high temperature superconductors - leading to high current conductors for electric power generation and transmission or to grain boundary junctions with transport characteristics that are sufficiently uniform and reproducible to be useful in microelectronic applications.
One of the most successful approaches to understanding grain boundary transport behavior involves isolating and measuring the transport properties across single grain boundaries. Traditionally, bulk or thin film bicrystals have been used in these studies as a method of preparing a single grain boundary with a well defined structure. However, one of the principal limitations of this approach has been the difficulty in reproducibly fabricating bicrystals with the desired misorientations and grain boundary structure. For example, thin film bicrystals of the type used by Dimos et al. have been most widely utilized in this type of experiment. However, studies of the microstructure of these thin film bicrystals have shown them to be highly non-uniform and subject to contamination by impurities, as shown in Fig. 1.[3] In addition, the cost of synthesizing samples by this method is rather high, largely due to the high cost of the bicrystal substrates required. As an alternative to thin film bicrystals, some researchers have used bulk bicrystals that occur naturally during flux growth of YBCO crystals. The synthesis method for this type of bicrystal is efficient and economical but provides no control over the specific types of bicrystals that are produced. As a result, the researcher faces the tedious and difficult task of hand-selecting bicrystals of the appropriate misorientations, not all of which are available. In addition, the boundaries in these types of samples can contain residual flux and small facets so that the boundary consists of more than one single plane. Furthermore, the critical current density within the grains that compose the bicrystal is very low, significantly limiting the ability to measure the properties of the grain boundary independently. Thus, there is a strong motivation to develop a method to synthesize bicrystals with well defined, clean grain boundaries in an efficient and economical manner.
Figure 1. Bright-field transmission electron micrograph the typical structure of grain boundaries in thin film bicrystals of YBCO. The meandering configuration and impurities are a result of an island-like growth mechanism.
Argonne has developed a new process for the growth of YBCO bicrystals based on the principles of solidification processing and melt textured growth. [4] In this process, seeded directional growth is used to nucleate two single domains of YBCO which then grow together to form a bicrystal, as illustrated schematically in Fig. 2. During growth of the bicrystal, the two grains grow together in the direction of a temperature gradient imposed upon the sample. The magnitude of the gradient is controlled in order to maintain a planar growth front which allows solute and impurity rejection into the liquid ahead of the growth front, yielding grain boundaries that a free of these contaminants, as shown in Fig. 3. Furthermore, the stable growth conditions result in a grain boundary with very long, straight facets that consist of a macroscopically and microscopically single grain boundary plane. [5]
Figure 2. Schematic representation of the dual seeded melt textured growth process developed to produce bulk bicrystals of YBCO.
Figure 3. Schematic illustration of the mechanism of crystal growth that leads to clean, planar boundaries. Once the two crystal domains have grown together, plane front growth and solute rejection to the liquid result in long, straight boundary facets.
The straight facets that comprise the boundary can be as long a several millimeters. As a consequence, the bicrystals can be sectioned so that a sample containing a single grain boundary plane can be easily isolated. Microstructural characterization of a large number of grain boundaries produced by this dual-seeded melt textured growth process has demonstrated that the grain boundary is indeed highly planar over a wide range of length scales. For example, Fig. 4 show a scanning electron micrograph of a small bar isolated from a bicrystal. The boundary separating the two grains can be seen to consist of a macroscopically single plane through the entire sample. Equally as important, the boundaries are also planar and well defined on the microscopic scale. An example of the typical structure observed in these boundaries at the nanometer scale is shown in the transmission electron micrograph of Fig. 5. In this micrograph, the two grains are separated by a smooth, well defined boundary that exhibits only a very small deviation from planarity. This is in sharp contrast to the highly non-uniform boundary structure typically observed in thin films (Fig. 1) and is a critical feature of these boundaries.[5] The presence of a well defined, planar grain boundary is an important aspect in helping to provide the best understanding of the mechanisms of grain boundary transport.
Figure 4. Scanning electron micrograph of a bicrystal sample isolated from a dual-seeded pellet. The grain boundary consists of a macroscopically single plane through the entire sample.
One of the main applications of dual-seeded melt
textured growth for the synthesis of bicrystals is in the area of basic
research since these bicrystals provide samples that can yield unique information
regarding the mechanisms of current transport across grain boundaries.
In addition, there are a variety of practical applications for the process
and variations on the process.
Among the issues that can be studied more effectively using these bicrystals
with well defined boundaries are measurements of the symmetry of the order
parameter, the influence of the order parameter on grain boundary transport,
the interplay between intragrain pinning and grain boundary transport properties,
and the influence of Josephson vortex pinning on grain boundary transport.
Measurements of the symmetry of the superconducting order parameter and
its effect on grain boundary transport have been carried out on thin film
bicrystal and tricrystal samples. As a result of the meandering boundary
configuration in these thin film samples, it has been difficult to deconvolute
the effect of the order parameter from other possible factors that may influence
the transport properties. The use of samples with planar boundaries can
greatly simplify the interpretation of these types of experiments. Likewise,
distinguishing between the effects of intragrain and intergrain dissipation
in measurements of transport properties across grain boundary samples can
be challenging. In these dual-seeded bulk bicrystals, the intragrain critical
currents are sufficiently high that direct measurement of the grain boundary
dissipation free from any effects of dissipation within the grains can be
made over a wide range of misorientation angles.[4,6-8] However, for very
low misorientation angles, dissipation within the grains becomes a factor.
The ability to measure both of these effects by synthesizing samples encompassing
this entire range of misorientations is allowing an improved understanding
of the transition from "grain limited" behavior to "grain
boundary limited" behavior to be gained. Furthermore, comparison with
transport properties of thin film bicrystals with the same misorientation
angles has allowed the influence of Josephson vortex pinning at the grain
boundary to be evaluated.[8]
Practical applications of dual-seeded melt textured growth include devices
based on grain boundary junctions and monolithic components for high current
or levitation applications.
Superconducting quantum interference devices (SQUIDs) are presently the
most sensitive detectors of magnetic flux available and as such are the
basic element of a wide range of sensitive and useful instruments. Dual-seeded
bulk bicrystals are the ideal candidate from which to fabricate the essential
element of a SQUID - a superconducting loop with one or more weak links
- since the boundaries consist of long, straight facets and the critical
current across the grain boundary weak link can be controlled in this processing
method by controlling the misorientation between the two grains. In addition,
bulk SQUIDs offer significant savings in cost over thin film counterparts
due to lower capital equipment costs and are likely to be more stable during
thermal cycling. The technology is of particular use to the military and
civilian transportation industry (aerospace, naval, and land-based) for
the non-destructive evaluation of various craft and components for cracks
and defects. In addition, these devices could be used in combat for the
detection of submarines and mines. (Low temperature SQUIDs have been in
use in the Navy since the 1990's, but there is a strong desire to move to
liquid nitrogen from liquid helium for logistical reasons). Likewise, the
devices could be used in the energy and power industry for the non-destructive
in-field evaluation of parts and components, for prospecting, and
as an aid in maintenance of pipelines.
A variation of dual-seeded melt textured growth using multiple seeds to
produce monolithic components for high current or levitation purposes has
already been developed at Argonne by Zhang, et al.[9] In this process, instead
of joining two grains to make a bicrystal, many grains can be joined together
following the same principles. Sharp curvature in the sample can be accommodated
by a series of small misorientations between grains, yielding lower angle
grain boundaries that have less detrimental effect on properties. It is
anticipated that this adaptation of the process will be particularly useful
in the synthesis of levitating rings.
Figure 5. Bright-field transmission electron microscope image of a grain boundary contained in a dual-seeded bulk bicrystal showing the nearly planar configuration typical of these boundaries.
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2. D. Dimos, P. Chaudhari, J. Mannhart and F. K. LeGoues, Phys. Rev. Lett. 61, 219 (1988).; D. Dimos, P. Chaudhari and J. Mannhart, Phys. Rev. B 41, 4108 (1990).
3. D.J. Miller, T.A. Roberts, J.H. Kang, J. Talvacchio, D.B. Buchholz and R.P.H. Chang, Appl. Phys. Lett. 66, 2561 (1995); J.A. Alarco, E. Olsson, Z.G. Ivanov, D. Winkler, E.A. Stepantsov, O.I. Lebedev, A.L. Vasiliev, A.Ya. Tzalenchuk and N.A. Kiselev, Physica C 247, 263 (1995); C. Traeholt, J.G. Wen, H.W. Zandbergen, Y. Shen, J.W.M. Hilgenkamp, Physica C 230, 425 (1994).
4. V.R. Todt, X.F. Zhang, D.J. Miller, M. St. Louis-Weber, V. P. Dravid, Appl. Phys. Lett. 69 (24) 3746 (1996).
5. X.F. Zhang, V.R. Todt, D.J. Miller, J. Mater. Res. 12 (11) 3029-3035 (1997).
6. M. St. Louis-Weber, V.P. Dravid, V.R. Todt, X.F. Zhang, D.J. Miller, U. Balachandran, Phys. Rev. B 54 (22) 16238 (1996).
7. D.J. Miller, V.R. Todt, M. St. Louis-Weber, D.G. Steel, M.B. Field, K.E. Gray, accepted for J. Mater. Sci. & Eng. B (1997).
8. K.E. Gray, D.J. Miller, M.B. Field, submitted to Phys. Rev. B., March, 1998.
9. H. Zhang, M. Jiang, B.W. Veal, H. Claus, in preparation.