WO1999003159A1

WO1999003159A1 - High temperature superconducting device having secondary microtwin structures that provide strong pinning

Info

Publication number: WO1999003159A1
Application number: PCT/US1997/011899
Authority: WO
Inventors: Manoj Chopra; Siu-Wai Chan
Original assignee: The Trustees Of Columbia University In The City Of New York
Priority date: 1997-07-10
Filing date: 1997-07-10
Publication date: 1999-01-21
Also published as: CA2295848A1

Abstract

A high-temperature superconductor capable of maintaining a large critical current density in the presence of an applied magnetic field is described. The superconductor comprises crystalline YBa2Cu3O7-d and a plurality of small-sized non-reactive non-deformable particles, such as Y2BaCuO5, which make up at least 10 % of the initial volume of the superconductor material. The non-reactive non-deformable particles have a sufficiently small radius of curvature so as to increase localized stress in the superconductor and induce the formation of intersecting twins and secondary microtwin structures, which act as pinning centers for applied magnetic flux, in the crystalline YBa2Cu3O7-d.

Description

HIGH TEMPERATURE SUPERCONDUCTING DEVICE HAVING SECONDARY MICROTWIN STRUCTURES THAT PROVIDE STRONG PINNING

Notice of Government Rights

The U.S. Government has certain rights in this invention pursuant to the terms of the National Science Foundation award DMR-93-50464.

Background of the Invention

I. Field of the invention.

The present invention relates to high temperature cuprate-compound superconductor devices, and more specifically, to superconductor devices which maintain a large critical current even in the presence of an applied magnetic field.

II. Description of the related art. In recent years, extensive work has been conducted in the area of high temperature superconductors. Researchers have strived to develop materials which not only exhibit superconductivity at high temperature T_c., but which also are able to carry a large critical current density J_c. In particular, those skilled in the art have conducted numerous experiments with cuprate compounds in an attempt to develop superconductor devices which meet the foregoing criteria.

It is generally known that mechanical structures called "twins" form during certain crystal lattice transformations in cuprate compounds in order to reduce transformation strains. For example, in the article by A. Rosova et al., "Role of Microtwins in Twin Lamella Intersections and Interconnections in YBa₂Cu₃O₇._v," Physica, vol. C 214, p. 274 (1993), it is disclosed that mechanical twins form spontaneously in order to relieve strains during a ferroelastic phase transition when YBa₂Cu₃O₇._d material is slowly cooled in the presence of Oxygen. Rosova et al. disclose a technique for making a high temperature superconductor (T_c = 92-93K) by preparing doped YBa₂Cu₃O₇._d material, i.e., crystalline YBa₂(Cu₀₉₇Auo ₀₃)₃O₇__d, in a gold crucible and annealing the material in oxygen to induce twinning. Microscope images of the superconductor reveal that when a twin layer intersects and goes through a twin layer of differing oriention, "secondary" microtwins are created in the vicinity of the area of intersection of the principal twins.

Further discussions of the mechanical effects of twinning and the formation of secondary microtwins are presented in articles by Y. Zhu et al., "The Interface of Orthogonally Oriented Twins in YBa₂Cu₃O₇._d," Philosophical Magazine, vol. 67, A 1057 (1993), K.N.R. Taylor et al., "Intersections of Twins and of Optical Domains in Crystalline YBa₂Cu₃O₇._d," J. Crystal Growth, vol. 117, p. 221 (1992), and T.M. Shaw et al., "The Effect of Grain Size on Microstructure and Stress Relaxation in Polycrystalline YBa₂Cu₃O₇._d," J. Matter. Res., vol. 4, p. 248 (1989).

Although the phenomenon of twinning in YBa₂Cu₃O₇._d superconductors is well documented, the mechanism by which twins impact superconducting behavior has been far less understood. It is known that when a current of density J flows in a mixed-state superconductor along a direction normal to an applied magnetic field B, the individual lines of magnetic flux are subjected to the Lorentz force, also called the Magnus force, F = J x B. Under most circumstances, as the applied magnetic field B increases, the superconducting current density J is degraded by the resulting Lorentz force. When there are few pinning centers to provide enough pinning, flux-lines will move with the Magnus force and there will be finite resistance and J,. will drop. However, in cuprate superconductors where twinning has occurred, lines of magnetic flux are "pinned" by the twin boundaries in the superconductor, and the magnetic flux is, at least to some extent, prevented from moving with the Lorentz force.

One article which examines the impact of twinning in YBa₂Cu₃O_7.d superconductors is V.K. Vlasko-Vlasov et al., "Study of Influence of Individual Twin Boundaries on the Magnetic Flux Penetration in YBa₂Cu₃O_7.d," 72 Physical Review Letters 3246 (1994). That article discloses a YBa₂Cu₃O_7.d high temperature superconductor formed by growing a YBa₂Cu₃O₇._d crystal in a gold crucible. According to V.K. Vlasko-Vlasov et al., twin boundaries in the crystal act as pinning barriers which disrupt magnetic flux penetration of the superconductor, i.e., vortices of flux cannot easily pass through the boundaries and pile up on one side of the superconductor. Thus, with an applied magnetic field of 73 Oe, a critical current J_c = 1.7 X 10⁴ A/cm² was measured by the authors in a twinned region of the superconductor as compared to an expected critical current of J_c = 1.7 X 10³ A/cm² for an untwinned region.

Although V.K. Vlasko-Vlasov et al. explain that twin regions in a YBa₂Cu₃O₇__d superconductor serve to disrupt magnetic flux penetration of the superconductor, this reference does not suggest any way of inducing or effectively utilizing such twin regions in a superconducting device. Thus, there remains a need in the art for a superconductor device which is able to maintain a large critical current density in the presence of an applied magnetic field.

Summary of the Invention

An object of the present invention is to provide a superconductor device which is able to maintain a large critical current density in the presence of an applied magnetic field.

Another object of the present invention is to provide a technique for inducing secondary microtwin structures in a cuprate-compound superconductor. A further object of the present invention is to provide a YBa₂Cu₃O₇._d superconductor which has an optimum pattern of secondary microtwin structures that serve to effectively pin magnetic flux penetration of the superconductor.

In order to meet these and other objects which will become apparent with reference to further disclosure set forth below, the present invention is a high- temperature superconductor device capable of maintaining a large critical current density in the presence of an applied magnetic field. To achieve this capability, the device advantageously comprises crystalline YBa₂Cu₃O₇__d and a plurality of non- reactive particles which make up at least 10% of the initial volume of the device, where the particles are less deformable than the YBa₂Cu₃O₇._d. With this arrangement, twinning and secondary microtwinning of at least a portion of the crystalline YBa₂Cu₃O₇._d is induced by the inclusion of the particles. In accordance with one aspect of the present invention, the non-reactive particles advantageously are Y₂BaCuO₅ particles that comprise at least 20% of the initial volume of the device, and preferably comprise from 30% to 50% of the initial volume of the device. In a preferred embodiment, the particles have an average diameter of approximately 0.5 μm.

A different aspect of the present invention beneficially provides for the inclusion of a doping material which acts to refine the distribution of the non- reactive particles, so that the average size of the non-reactive particles is kept from coarsening during processing. In another preferred embodiment, the doping material is PtO₂ in amount equal to approximately 0.5% of the total weight of the superconductor. When the non-reactive particles are profitably chosen to be Y₂BaCuO₅ particles which comprise at least 20% of the initial volume of the device, the PtO₂ doping reduces the average size of the Y₂BaCuO₅ particles to thus increase the inducement of twinning and secondary microtwinning in the crystalline YBa₂Cu₃O₇._d .

In yet another preferred embodiment, the doping material is CeO₂ in amount equal to approximately 1% of the total weight of the superconductor device. When the non-reactive particles are profitably chosen to be Y₂BaCuO₅ particles which comprise at least 20% of the initial volume of the superconductor material of the device, the CeO₂ doping reduces the average size of the Y₂BaCuO₅ particles to thereby increase the inducement of twinning and secondary microtwinning in the crystalline YBa₂Cu₃O₇._d .

By including small, non-reactive and relatively non-deformable particles in the YBa₂Cu₃O₇._d, localized stress zones are created in the superconductor as it undergoes a phase transformation. The increased stress within the superconductor serves to induce twinning and to effect the morphology of twinning by inducing twin intersections and, accordingly, secondary twin structures which serve to pin magnetic flux-lines.

The accompanying drawings, which are incorporated and constitute part of this disclosure, illustrate preferred embodiments of the invention and serve to explain the principles of the invention. Brief Description of the Drawings

Fig. 1 is a diagram representing a 2-dimensional flux-line structure showing the three close-packed slip directions of flux line movement in a type-II superconductor;

Fig. 2 is a diagram representing two twin variants that are 90° misoriented with respect to each other;

Fig. 3 is a diagram representing a different twin variant boundary separating two twin varients where twinning has occurred;

Fig. 4 is a transmission electron micrograph showing a twin boundary where two twin variants penetrate one another and form multiple secondary microtwin structures;

Fig. 5 is a diagram representing a portion of a YBa₂Cu₃O₇._d superconductor where perpendicular twin regions have intersected and penetrated one another to form multiple secondary microtwin structures;

Fig. 6 is a graph plotting the area fraction of twin intersections against the maximum magnetic field that may be applied to a superconductor before its superconducting current decreases as the Lorentz force has increased beyond the volume pinning force;

Fig. 7 is a three-dimensional graphical representation of a typical stress distribution around a rigid particle having a diameter of 0.5 μm that is subjected to a sheer stress;

Fig. 8 is a transmission electron micrograph illustrating two neighboring Y₂BaCuO₅ particles and the resulting twin structure which formed between the particles;

Fig. 9 is a graph plotting twin spacing against the square root of local interparticle spacing showing finer twins as interparticle spacing decreases;

Fig. 10 is a schematic representation of twin spacing in the vicinity of a Y₂BaCuO₅/YBa₂Cu₃O₇._d interface;

Fig 11 is a graph plotting the ratio of the product of twin spacing (Tw) and local particle curvature (a) over the square root of Y₂BaCuO₅ interparticle spacing (S_2U) against effective distance (R); Fig. 12 is a graph plotting the pinning force of the superconductor twin structures against the applied magnetic field withstood by several superconductor devices fabricated in accordance with the present invention;

Fig. 13 is a graph plotting the number of Y₂BaCuO₅ particles against particle size for undoped and PtO₂ doped samples of a superconductor devices fabricated in accordance with one aspect of the present invention;

Fig. 14 is a graph plotting the number of Y₂BaCuO₅ particles against particle size for undoped and CeO₂ doped samples of a superconductor devices fabricated in accordance with another aspect of the present invention; and

Fig. 15 is a graph plotting local twin spacing against the square root of interparticle spacing for undoped, PtO₂ doped and CeO₂ doped samples of a superconductor devices fabricated in accordance with still another aspect of the present invention.

Description of the Preferred Embodiments

In general, the present invention provides a technique for effectively utilizing twinning and microtwinning in certain cuprate compounds, e.g., YBa₂Cu₃O₇._d, in order to provide a superconducting device which is able to maintain a large critical current even in the presence of an applied magnetic field. In other words, the present invention provides a technique for obstructing or "pinning" lines of magnetic flux in a crystalline superconductor material in order to prevent the above-mentioned Lorentz force from limiting the superconducting critical current density of the device.

Referring to Fig. 1, the principals which underlie the present invention will now be described. Fig. 1 illustrates a so-called Abrikosov lattice 100 with the three closed-packed directions of a flux line movement 110, 120, 130 inherent in any type II superconductor where no twinning has occurred. In such a superconductor, lines of magnetic flux are free to slip in any of the three illustrated directions, thereby causing energy dissipation and degrading the critical current density. In cuprate-compound superconductor, one of the three close-pack directions of the magnetic flux-line structure will align itself with the parallel twin boundaries. Thus particular close-packed direction will remain as an easy slip direction, while the other two will not.

However, where two twin variants exist, dislocations of magnetic flux-line structures are not so free to penetrate the superconductor. Referring to Fig. 2, a boundary 220 separates two twin variant 200, 210 of a cuprate superconductor where twinning has occurred. Variant 210 is perpendicular to varient 220, so none of the three close-packed directions of variant 210 are parallel with any one ofthe three close-packed directions ofthe flux-line lattice in variant 200.

With the structure shown in Fig. 2, magnetic flux lines which move from left to right in varient 200 along lines 250 reach an obstruction at the varient boundary 220 and cannot continue to travel along the same path; instead, the lines of magnetic flux will be forced to pile up at the variant boundary. The varient boundary 220 is therefore said to have pinned the motion of magnetic flux 250.

The structure described with reference to Fig. 2 will not completely pin lines of magnetic flux which traverse from right to left along 250; those lines are pinned from penetrating the twin boundary plane 220, but not along the parallel twins 250. Another arrangement is illustrated in Fig. 3, where two touching variants 300 and 310 have been twinned in perpendicular planes so as to form a 45° angle along twin variant boundary 320. With the structure of Fig. 3, any left to right flux flow that is pinned in twin variant 300 will have to jump to a different pin position in twin variant 310, and hence the flow of magnetic flux through a superconductor having the structure shown in Fig. 3 will also be limited.

In general a twin variant boundary is a good barrier for flux line motion as flux-lines are moving towards the variant boundary instead of away from it. The importance is to have a high density of twin variant boundaries for effective obstruction of flux-line motions.

Of course, if the motion ofthe magnetic flux-lines is parallel to the twin boundary illustrated in Figs. 2 or 3, there will be no pinning by the twin boundary. Only where the Lorentz force includes a component which tends to traverse the twin boundary will the pinning phenomenon be exhibited. Moreover, there will be a certain amount of "leakage" of magnetic flux along the actual twin variant itself. Therefore, it is evident that a superconducting device which has a compact arrangement of multiple twin boundaries of differing twin plane orientations would more effectively pin flux lines and thus prevent the magnetic flux-lines from moving within the superconductor.

Referring next to Fig. 4, a transmission electron micrograph showing an structure where two twin variants not only border but penetrate one another will now be described. As explained in the above-discussed article by Rosova et al., when one twin variant intersects and penetrates through a second twin variant, "secondary" twins are created in the vicinity ofthe area of intersection ofthe principal twins. The secondary twins 400 are microscopic or nanoscopic in size, and are formed spontaneously in order to relieve stress when YBa₂Cu₃O₇._d material undergoes a tetragonal to orthombic phase transition.

Heretofore, the existence of such secondary microtwin structures has not been related to the superconducting behavior of YBa₂Cu₃O₇._d material. However, as more fully describe below in accordance with the present invention, such secondary microtwin structures can advantageously be utilized to improve superconductive behavior of YBa₂Cu₃O₇._d material in the presence of an external magnetic field.

Fig. 5 illustrates a portion of a YBa₂Cu₃O₇._d superconductor where perpendicular twin variants have intersected and penetrated one another, thereby forming multiple secondary microtwin structures 530 of lenticular shape. Clearly defined twin planes 510, 520 separate the two orthogonal twin variants. Dotted line 540 represents the boundary between the twinned region 550 and the region which includes the microtwin structure 530, where a two degree misorientation occurs across 540.

In the superconductor illustrated in Fig. 5, points 560 are areas of potential high flux pinning, as they represent areas where a closed-packed direction of possible flux line flow runs into effective flux traps. As those skilled in the art will appreciate, the higher concentration of potential flux pinning centers enabled by the microtwin structure permits the superconductor to withstand a greater applied magnetic field before the superconducting current sustainable by the superconductor is subjected to degradation. This behavior is illustrated in Fig. 6, where the total area fraction of twin intersections, i.e., the area where microtwinning occurs, is plotted against the forth power ofthe maximum magnetic field that may be applied to the superconductor before the superconducting current is degraded. Accordingly, by inducing microtwinning in YBa₂Cu₃O₇._d superconductors, one may fabricate a superconducting device which is able to maintain a large critical current in the presence of high magnetic fields.

In accordance with one aspect ofthe present invention, a technique for inducing microtwin structures in a YBa₂Cu₃O₇._d superconductor will now be described. As discussed above, twins form in YBa₂Cu₃O₇._d in order to reduce transformation stress in the crystal during cooling. Thus, if the cooling crystal can be subjected to additional localized stress, twinning may be induced in the vicinity of such localized stress points.

One technique to create localized stress points is to add relatively non- deformable particles to the YBa₂Cu₃O₇__d material prior to cooling and crystal formation. As referred herein, a non-deformable particle shall mean a particle which is not necessary rigid, but is less deformable that the superconductor material itself. To this end, Fig. 7 illustrates a typical stress distribution around a non-deformable particle having a diameter of 0.5 μm that is subjected to a sheer stress. If particles are added to YBa₂Cu₃O₇._d material prior to cooling and crystal formation, the transforming crystal will be subjected to numerous points of localized stress which may potentially act as centers for inducing the formation of finer twins, finer twin variants, and secondary twins.

Any insulating non-deformable particle which is non-reactive with YBa₂Cu₃O_7.d is a suitable potential candidate for such non-deformable particle inclusion. We have found that Y₂BaCuO₅ particles are especially adapted to inclusion into YBa₂Cu₃O₇._d material for the purpose of altering the stress patterns in that material. One suitable method of preparing a sample of YBa₂Cu₃O₇._d material with included Y₂BaCuO₅ particles is to place a pellet of a correct mixture of compacted powder YBa₂Cu₃O₇._d and Y₂BaCuO₅, and add a SmBa₂Cu₃O₇._d or a NbBa₂Cu₃O₇._d seed on top ofthe sample. The seed implanted pellet may then be melt-textured by, e.g., a top seeded melt textured process, as those skilled in the art will be familiar with. Both SmBa₂Cu₃O₇._d and NbBa₂Cu₃O₇._d have a higher peritectic temperature than YBa₂Cu₃O₇._d .

The amount of Y₂BaCuO₅ added to the sample will greatly impact any induced twinning which results from such non-deformable particle inclusion. For optimum results, Y₂BaCuO₅ particles should be added to the YBa₂Cu₃O₇._d sample such that a fairly uniform dispersion of a homogenous array of closely spaced Y₂BaCuO₅ particles is achieved. Neighboring Y₂BaCuO₅ particles should be close enough to induce twins and rotations in twinning, to be more fully described below, while not so close so as to choke off the superconducting current J flowing in the device. Fig. 8 illustrates an exemplary structure of two neighboring Y₂BaCuO₅ particles and the resulting twin structure which formed between the particles.

In order to determine the preferable concentration of Y₂BaCuO₅ particles, large grained YBa₂Cu₃O₇._d samples containing an initial Y₂BaCuO₅ volume of 10, 20, 30, 40 and 50 percent were analyzed. Transmission electron microscope samples were prepared using conventional polishing and dimpling to approximately 50μm. These dimpled samples were then ion-milled at 5kV to perforation and later polished at 3kV to minimize artifacts due to ion-milling. The samples were observed under a JEOL (JEM 100C) transmission electron microscope. The variation ofthe twin spacing and its dependence on the local interparticle spacing (S_{2] ]}) were both studied.

The local twin spacing and the corresponding local Y₂BaCuO₅ interparticle spacing were measured for the 40% Y₂BaCuO₅ sample and for a sample containing 30% Y₂BaCuO₅ with 0.5wt% PtO₂ doping. As further discussed below, PtO₂ was added to the mixture in order to refine the Y₂BaCuO₅ particles and produce a more homogeneous range of sized of Y₂BaCuO₅ particles. The results, illustrated in Fig. 9, show that local mean twin spacing varies linearly with the square root ofthe local interparticle spacing in both 40% Y₂BaCuO₅ and the 30% Y₂BaCuO₅ samples. Theoretically, it can be shown that the average local twin spacing is dependant on the Y₂BaCuO₅ interparticle spacing in accordance with equation (1):

4πγ S tw 211

Tw= (1) μe

where the twin spacing (Tw) is the distance between two consecutive twin boundaries in the same plane, twin boundary energy is denoted by γ_tw the shortest local distance between two YBa₂Cu₃O₇._d/Y₂BaCuO₅ interfaces is S_{21 !}, μ is the shear modulus (59GPa for YBa₂Cu₃O₇._d) and e is the shear strain due to twinning and is given by (b/a -1), where 'b' and 'a' refer to the basal plane lattice parameters of YBa₂Cu₃O₇._d.

Using equation 1, the best fit lines for the data shown in Fig. 9 is shown as lines 900, 910. The line 900 for the 40% Y₂BaCuO₅ sample yields a slope of 4.664 nm"². Using this magnitude ofthe slope, the mean twin boundary energy is computed to be approximately 28.9 ± 0.6 mJ/m².

Referring now to Fig. 10, twin spacing in the vicinity of a Y₂BaCuO₅/ YBa₂Cu₃O₇._d interface is illustrated. As can be seen, the change of twin spacing is a highly localized phenomenon about the Y₂BaCuO₅ particles. We have determined that the twin spacing around a YBa₂Cu₃O₇._d/Y₂BaCuO₅ interface increases linearly as R² ,where R is the effective distance between the twin and the Y₂BaCuO₅ particle 1000, i.e., the sum ofthe radius of curvature ofthe Y₂BaCuO₅ particle (a) and the distance from the Y₂BaCuO₅/YBa₂Cu₃O₇._d interface (r). Twins near a YBa₂Cu₃O₇._d/Y₂BaCuO₅ interface 1010 were observed to be finer than those 1020 in the matrix.

This is due to increased stress interactions between particles with local microstructures being dominated by the effect of such localized stresses which is increasingly evident with the initial Y₂BaCuO₅ content increasing beyond 30%. Here, the stress state ofthe YBa₂Cu₃O₇._d matrix around a Y₂BaCuO₅ particle can be thought of as a perturbation of a mean stress at the YBa₂Cu₃O₇._d/Y₂BaCuO₅ interface. This perturbation in the mean stress leads to the nucleation of additional twin boundaries at the interface and translates to a local variation in twin spacing at a YBa₂Cu₃O₇._d/Y₂BaCuO₅ interface. When S₂₁₁ is small, i.e. less than about three times the particle-diameter, and for Yba₂Cu₃O₇._d regions around the YBa₂Cu₃O₇. _d/Y₂BaCuO₅ interface various types of twin structures evolve, e.g., twin intersections and variations in the twin density at the YBa₂Cu₃O₇._d/Y₂BaCuO₅ interface.

The stress at the YBa₂Cu₃O₇._d/Y₂BaCuO₅ interface originates from a combination of coherency strain between the YBa₂Cu₃O₇._d/Y₂BaCuO₅ interface, difference in thermal expansion coefficients and the result ofthe tetragonal to orthorhombic (t→o) transformation. If we assume that the stress at the YBa₂Cu₃O₇. _d/Y₂BaCuO₅ interface is entirely due to the t-o transformation, a higher strain energy density at the YBa₂Cu₃O₇._d/Y₂BaCuO₅ interface is expected around particles with a smaller radius of curvature. Furthermore, since the Y₂BaCuO₅ particles are comparatively stiffer (E=213GPa) than the bulk YBa₂Cu₃O₇._d (E=182GPa), they do not plastically yield during the phase transformation. Hence, the t→o transformation leads to a non uniform stress field, with a higher stress gradient at the YBa₂Cu₃O₇._d/Y₂BaCuO₅ interface.

As discussed above, the magnitude ofthe stress at a distance R from a Y₂BaCuO₅ particle varies as R² where R is the effective distance ofthe YBa₂Cu₃O₇. _d/Y₂BaCuO₅ interface. As illustrated in Fig. 7, the stress continues to decrease with increasing R until the interfacial stress approaches the average bulk stress. A consequence ofthe stress variation at the interface is the creation of a gradient in the strain energy density along the interface. One manifestation of this strain energy gradient is the variation in the twin spacing at the interface. The twin density would be higher at the interface to compensate for a locally higher strain energy density. This implies that the twin density at a smaller or alternatively sharper Y₂BaCuO₅/YBa₂Cu₃O₇._d interface would lead to a higher twin density at the interface or to the nucleation of an additional twin variant.

A qualitative understanding on such behavior can be obtained by considering the various stresses which arise from the t→o transformation and the presence of the Y₂BaCuO₅ particles in the matrix. For an ellipsoidal inclusion in an infinite matrix, the stress variation at the interface is a complex function ofthe local radius ofthe inclusion and can be expressed as follows:

Ba ^' Ca σ R (A+ (2)

R ^' R

where a is the local radius of curvature ofthe inclusion, R is the effective distance, σ(R) is the local stress at a distance R from the Y₂BaCuO₅/YBa₂Cu₃O₇._d interface and A, B and C are constants. A minimization ofthe total strain energy with the stress given by equation 1 yields:

Y.

(3) f R

211

From the foregoing, the stress at the interface can be approximated to σ(R)<*A+Ba² or σ(R)=σ(0)l+a² where σ(0) is the average bulk value ofthe stress.

R² R²

This assumption is valid for the case where the contribution to the total stress from the higher order terms can be neglected. Substituting this value of stress in the above equation results as follows:

T a Y.

'-R (4) f e

211

Since R = r + a, a substitution can be made:

gives the ratio ofthe

particle sizes over the experimental data. Experimental results are plotted against this theoretical calculation in Fig. 11.

Referring next to Fig. 12, the ability of YBa₂Cu₃O₇._d samples which include an initial amount of 10%, 20%,30%, 40% and 50% of Y₂BaCuO₅ by volume to pin an applied magnetic field is now described. The percentages given represent the initial volume ofthe sample that is Y₂BaCuO₅. Due to liquid loss, the actual final superconductor can have a much higher concentration of Y₂BaCuO₅.

From Fig. 12, it can be seen that the flux pinning force decreases rapidly with the magnetic field beyond 0.2T for the sample containing an initial Y₂BaCuO₅ addition of 10%. Samples containing initial Y₂BaCuO₅ additions of 20% and 30% exhibit maximum pinning forces at magnetic fields of 2T and IT, respectively. However, for the samples containing an initial Y₂BaCuO₅ volume of 40% the true pinning force is found to be the highest in magnitude for the entire range of magnetic fields. The true pinning force for the 40% Y₂BaCuO₅ sample shows a point of inflection at a magnetic field of 2.2T while the 50% Y₂BaCuO₅ sample shows a inflection point at 3T indicating that the flux pinning at higher field for the latter sample has actually improved. This can be substantiated by the fact that for the 50% Y₂BaCuO₅ sample, the true pinning force is practically unaltered for the magnetic field range from 0.8T to 3.5T for the 50% Y₂BaCuO₅ sample.

Hence, the magnetic field at maximum true pinning force increases as the amount of Y₂BaCuO₅ included in the sample increases. The effective flux pinning efficiency ofthe YBa₂Cu₃O₇._d matrix tends to improve with an increasing Y₂BaCuO₅ addition, although the electrical connectivity is substantially reduced when the actual volume of YBa₂Cu₃O₇._d falls below 20%. Of course, as the actual percentage of Y₂BaCuO₅ increases beyond 80%, the electrical characteristics ofthe superconductor are impacted by the increasing quantity of insulating material in the superconductor. Accordingly, in fabricating a superconducting device in accordance with the present invention, the amount of Y₂BaCuO₅ included in the superconducting device should be selected depending on the both the strength of the applied magnetic field and the desired electrical characteristics ofthe device. For potentially large magnetic fields, 40, 50 or even 80% Y₂BaCuO₅ inclusion may be appropriate. For smaller magnetic fields, adding Y₂BaCuO₅ so that 20% ofthe initial sample comprises that compound may be sufficient while yielding maximum electrical benefits.

In accordance with a further aspect ofthe present invention, YBa₂Cu₃O₇._d material with Y₂BaCuO₅ inclusions may be doped or loaded with certain materials which can refine the Y₂BaCuO₅ particles and thereby enhance the impact of the Y₂BaCuO₅ particles on inducing twinning and secondary microtwinning. As discussed above, Y₂BaCuO₅ particles having a smaller radius of curvature lead to a higher strain energy density at the YBa₂Cu₃O₇._d/Y₂BaCuO₅ interface than do Y₂BaCuO₅ particles having a larger radius of curvature. When Y₂BaCuO₅ material is simply added to bulk YBa₂Cu₃O₇._d and a superconducting device is fabricated from that mixture, the Y₂BaCuO₅ material will congeal into particles of a wide range of sizes. It is therefore desirable to employ a technique which produces a more homogeneous distribution of Y₂BaCuO₅ particles which are small in size, e.g., within the range of 0.05 - 2 μ, and accordingly have a smaller radius of curvature.

Referring now to Fig. 13, one technique to fabricate a YBa₂Cu₃O₇._d superconductor with a more homogeneous distribution of small sized Y₂BaCuO₅ particles is by doping the material with a small quantity of PtO₂. The data points in Fig. 13 represented by squares indicate the number of particles that were observed at different sizes when Y₂BaCuO₅ is added to YBa₂Cu₃O₇._d to initially form 30%) ofthe material. Data points represented by circles indicate the same 30%) Y₂BaCuO₅ sample but with the addition of PtO₂, in an amount totaling 0.5% ofthe sample by weight. As shown in the graph, the inclusion of PtO₂ in the sample substantially reduces the mean Y₂BaCuO₅ particle size, and appears to prevent large Y₂BaCuO₅ particles having a radius of lOμm or greater from forming in the sample.

Referring next to Fig. 14, an alternate technique to fabricate a YBa₂Cu₃O₇._d superconductor with an improved distribution of small sized Y₂BaCuO₅ particles requires doping the material with a small quantity of CeO₂. In Fig. 14, the data represented by circles indicate the number of particles that were observed at different sizes when Y₂BaCuO₅ is added to YBa₂Cu₃O₇._d to initially form 40% of the material, while the data represented by squares indicate the same sample but with the addition of CeO₂ , in an amount totaling 1% ofthe sample by weight. With the addition of CeO₂ , the average Y₂BaCuO₅ particle size is also reduced.

The data shown in Figs. 13 and 14 indicate a considerable refinement ofthe Y₂BaCuO₅ particles and twin spacing in YBa₂Cu₃0₇._d with PtO₂ or CeO₂ addition. Referring to Fig. 15, the localized spacing of twins observed in the samples is plotted against the square root ofthe local interparticle spacing S_{2I 1}. As shown in Fig. 15, the sample containing 30% Y₂BaCuO₅ with 0.5wt% PtO₂ shows a much shallower slope, approximately 2.92 nm"² , when compared to the sample containing 40% Y₂BaCuO₅ with no doping. Likewise, the sample which included CeO₂ doping showed improvement over the undoped sample.

The addition of PtO₂ or CeO₂ to Y₂BaCuO₅ refines the Y₂BaCuO₅ particle size and reduces the average interparticle spacing. Hence, for a fixed initial volume of Y₂BaCuO₅ addition, a smaller Y₂BaCuO₅ particle size would allow a larger Y₂BaCuO₅ particle density. For such a case, the corresponding increase in the YBa₂Cu₃O₇._d/Y₂BaCuO₅ interface area is about ten times larger than in the undoped sample.

We have measured the Y₂BaCuO₅ particle density in a Ice sample without PtO₂ doping to be about 3 x 10⁸ particles/cc, while for the sample with PtO₂ doping the Y₂BaCuO₅ particle density rises substantially to 2 x 10" particle/cc. This difference in particle density leads to approximately a 10 fold enhancement in Y₂BaCuO₅ surface area. Hence, for the same amount of initial Y₂BaCuO₅ addition, the final Y₂BaCuO₅ densities for the PtO₂ doped YBa₂Cu₃O₇._d sample are substantially higher.

The morphology ofthe twin spacing in structures changes with a smaller and relatively homogeneous distribution of Y₂BaCuO₅ particles. Intersections of twin spacings appear particularly in the 30% Y₂BaCuO₅ with PtO₂ sample and in 40% and 50% Y₂BaCuO₅ samples without any PtO₂. It was observed that twin intersections begin to form when the Y₂BaCuO₅ interparticle spacing is about 740nm for the samples without PtO₂ , and 300nm for the case when PtO₂ is added. Twin morphologies such as intersecting twins begin to appear because ofthe stress fields associated with adjacent YBa₂Cu₃O₇._d/Y₂BaCuO₅ interfaces. They can also form because ofthe presence of submicron Y₂BaCuO₅ particles which have a locally high strain energy density as compared to larger particles (>lμm) leading to the nucleation of both twin variants.

In summary, the major factors affecting the inducement of twin intersections and, accordingly, the formation of secondary twin spacings are the proximity of a Y₂BaCuO₅ particle to its neighbors and the radius of curvature of the Y₂BaCuO₅/YBa₂Cu₃O₇._d interface. By controlling the size and distribution of the Y₂BaCuO₅ particles, it is possible to control both twin colony size and the degree of extended defects in the twin structures.

The foregoing merely illustrates the principles ofthe invention. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view ofthe inventors teachings herein. For example, while the above description was directed to PtO₂ or CeO₂ doping, other noble metals or noble metal oxides, such as Ag, Au and their oxides may be used to refine the distribution of Y₂BaCuO₅ particles in the YBa₂Cu₃O₇._d superconductor. Moreover, insulating materials other than Y₂BaCuO₅ which are non-reactive with YBa₂Cu₃O₇._d may be used to induce twin intersections and microtwinning. For example, Ba₂ZrO₃ or SrZTiO₃ are suitable non-deformable particles for inclusion in YNa₂Cu₃O₇._d. Likewise, although the foregoing embodiment was described with reference to YBa₂Cu₃O₇._d, other cuprate compounds which generally follow the chemical formula XBa₂Cu₃O₇._d where "X" is a rare earth metal, e.g., Yb, Sm, Nd or La, may be used to form the high temperature superconductor device. It will thus be appreciated that those skilled in the art will be able to devise numerous systems and methods which, although not explicitly shown or described herein, embody the principles ofthe invention and are thus within the spirit and scope ofthe invention.

Claims

Claims 1. A high-temperature superconductor capable of maintaining a large critical current density in the presence of an applied magnetic field, comprising: (a) a crystalline cuprate compound; and (b) non-reactive particles comprising at least 10% ofthe initial volume of said superconductor device dispersed in the crystalline cuprate compound, said particles being less deformable than said cuprate compound, and having a sufficiently small radius of curvature so that intersecting twins and secondary microtwins in at least a portion of said cuprate compound is generated by the stress caused by the inclusion of said particles.

2. The high-temperature superconductor of claim 1 , wherein said cuprate compound comprises YBa₂Cu₃O₇._d.

3. The high-temperature superconductor of claim 2, wherein said non- reactive particles comprise Y₂BaCuO₅.

4. The high-temperature superconductor of claim 3, wherein said Y₂BaCuO₅ particles comprise at least 20% ofthe initial volume of said superconductor.

5. The high-temperature superconductor of claim 3, wherein said Y₂BaCuO₅ particles comprise from 30% to 50% ofthe initial volume of said superconductor .

6. The high-temperature superconductor of claim 1, wherein said non- reactive particles are particles having an average diameter in the range of 0.05 μm - 2 μm.

7. The high-temperature superconductor of claim 6, wherein said non- reactive particles are particles having an average diameter of approximately 0.5 μm.

8. The high-temperature superconductor of claim 1, further comprising a doping material selected from the group consisting of PtO₂, CeO₂, Au, Ag, Au₂O or Ag₂O, wherein said doping material acts to refine the distribution of said non-reactive particles such that the average size of said non-reactive particles is decreased.

9. The high-temperature superconductor of claim 8, wherein said doping material is PtO₂ in amount equal to approximately 0.5%) of the total weight of said superconductor .

10. The high-temperature superconductor of claim 9, wherein said cuprate compound comprises YBa₂Cu₃O₇._d and said non-reactive particles are Y₂BaCuO₅ particles which comprise at least 20% ofthe initial volume of said superconductor, and wherein said PtO₂ doping reduces the average size of said Y₂BaCuO₅ particles to thereby increase the inducement of intersecting twins and secondary microtwin structures in said crystalline YBa₂Cu₃O_7.d .

11. The high-temperature superconductor of claim 10, wherein said Y₂BaCuO₅ particles comprise from 30% to 50% ofthe initial volume of said superconductot.

12. The high-temperature superconductor of claim 8, wherein said doping material is CeO₂ in amount equal to approximately 1% of the total weight of said superconductor.

13. The high-temperature superconductor of claim 12, wherein said cuprate compound comprises YBa₂Cu₃O₇._d and said non-reactive particles are Y₂BaCuO₅ particles which comprise at least 20% ofthe initial volume of said superconductor, and wherein said CeO₂ doping reduces the average size of said Y₂BaCuO₅ particles to thereby increase the inducement of intersecting twins and secondary microtwin structures in said crystalline YBa₂Cu₃O₇._d .

14. The high-temperature superconductor of claim 13, wherein said Y₂BaCuO₅ particles comprise from 30% to 50% ofthe initial volume of said superconductor .