Vinokour, Valerii M.
Vinokour, Valerii M.
| 1. | A method for improving the superconducting properties of a type II superconductor comprising: (1) providing a type II superconducting material; and (2) irradiating said type II superconducting material with an ion beam so that splayed columnar defects are created in said type II superconducting material. |
| 2. | The method of claim 1 wherein step (1) of providing a type II superconducting material further includes a step of selecting said type II superconductor so that it is characterized by anisotropy in electronic effective mass, .a short coherence length and a London penetration depth of in the range of from about 10 to about lOOAngstroms. |
| 3. | The method of claim 1 wherein step (1) of providing a type II superconductor further includes a step of selecting a single crystal superconducting material. |
| 4. | The method of claim 3 wherein said single crystal superconducting material is further characterized by a layered crystal structure. |
| 5. | •. |
| 6. | The method of claim 3 wherein said single crystal superconductor material is further characterized by a single crystal material form and said single crystal material form is selected from the group consisting of thin film, thick film, bulk and filament forms. |
| 7. | The method of claim 3 wherein said single crystal superconducting material is a high temperature superconducting material selected from the group consisting of YBa2Cu307, Tl2Ba2Ca2Cu3O10, BiSr2CaCu208 Tl2Ba2CaCu30β and superconducting C60 doped with an alkali metal. |
| 8. | The method of claim 3 wherein said single crystal superconducting material is a low temperature superconducting material selected from the group consisting of Nb, NbTi, Nb3Sn, and amorphous MoGe. |
| 9. | The method of claim 1 wherein step (1) of providing a type II superconductor further includes a step of selecting a polycrystalline superconducting material. |
| 10. | The method of claim 8 wherein said polycrystalline superconducting material is further characterized by a layered crystal structure. |
| 11. | The method of claim 8 wherein said polycrystalline superconductor material is further characterized by a polycrystalline material form and said polycrystalline material form is selected from the group consisting of thin film, thick film, bulk and filament forms. |
| 12. | The method of claim 8 wherein said polycrystalline superconducting material is a high temperature superconducting material selected from the group consisting of YBa2Cu307, Tl2Ba2Ca2Cu3O10, BiSr2CaCu2Oe, Tl2Ba2CaCu308 and superconducting C60 doped with an alkali metal . |
| 13. | The method of claim 8 wherein said polycrystalline superconducting material is a low temperature superconducting material selected from the group consisting of Nb, NbTi, Nb3Sn and amorphous MoGe. |
| 14. | The method of claim 1 wherein step (2) of irradiating said type II superconductor material includes a further step of generating said ion beam so that said ion beam produces wellaligned, continuous columnar tracks which penetrate substantially through said type II superconductor material. |
| 15. | The method of claim 13 wherein said columnar tracks are further characterized by a defect density and said defect density is selected to match the density of magnetic flux vortex lines associated with a particular effective field, Bφ. |
| 16. | The method of claim 14 wherein said columnar tracks are further characterized by a columnar track length. and said columnar track length is in the range of from about lOmicrons to about 70microns and more preferably in the range of from about lOmicrons to about 30microns and most preferably in the range of from about 20microns to about 30microns and by a columnar track width and the columnar track width of each of said columnar tracks is in the range of from about 20Angstroms to about lOOAngstroms, more preferably in the range of from about 40Angstroms to about δOAngstroms and most preferably in the range of from about 50Angstroms to about 70Angstroms. |
| 17. | The method of claim 13 wherein said ion beam further includes heavy ions and said heavy ions are selected from the group consisting of silver, gold, lead and tin ions. |
| 18. | The method of claim 16 wherein said heavy ions are further characterized by an ion beam energy and by an ion beam energy loss rate and said ion beam energy is in the range of from about lOOMev to about lOGev and said ion beam energy loss rate is in the range of from about 2Kev/Angstrom to about 2Mev/Angstrom. |
| 19. | The method of claim 17 further including a step of selecting an ion irradiation time period so that overheating and melting of said type II superconducting material is avoided. |
| 20. | The method of claim 17 further including a step of cooling said type II superconducting material to avoid overheating by said ion beam. |
| 21. | The method of claim 1 wherein said splayed columnar defects are further characterized by a splay angle so that Jc is increased with respect to Jc of said type II superconductor material having parallel columnar defects. |
| 22. | The method of claim 20 wherein said type II superconductor is further characterized by a high temperature flux liquid phase and further including a step of controlling said splay angle so that said splay angle does not exceed root mean square thermal fluctuations in said high temperature flux liquid phase. |
| 23. | The method of claim 21 wherein said step of controlling said splay angle is carried out so that said splay angle is in the range of from about 2degrees to about 20degrees, more preferably in the range of from about 5degrees to about 15degrees and most preferably in the range of from about δdegrees to about 12degrees. |
| 24. | ' The method of claim 1 wherein said type II superconductor material is further characterized by a layered crystal structure having a caxis symmetry axis and said ion beam is. further characterized by an ion beam axis and step (2) of irradiating said type II superconductor material further comprises a step of orienting said ion beam axis so that it is slightly displaced from said caxis symmetry axis of said type II superconductor to avoid generating columnar tracks oriented along said caxis symmetry axis. |
| 25. | The method of claim 23 wherein said columnar tracks are further characterized by a splay angle which is symmetric about said caxis symmetry axis. |
| 26. | The method of claim 1 wherein said type II superconductor material is further characterized by a layered crystal structure having a caxis symmetry axis and step (2) of irradiating said type II superconductor material further comprises steps of providing a first ion beam further characterized by a first ion beam axis and a second ion beam further characterized by a second ion beam axis and orienting said first ion beam axis at a first ion beam axis angle and orienting said second ion beam axis at a second ion beam axis angle equal to said first ion beam axis angle so that said first and second ion beam axis angles are symmetric about said caxis symmetry axis. |
| 27. | The method of claim 1 wherein said type II superconductor material is further characterized by a layered crystal structure and includes a crystal plane associated with a layer of said layered crystal structure and step (2) of irradiating said type II superconductor material further comprises steps of providing a first ion beam further characterized by a first ion beam axis and a second ion beam further characterized by a second ion beam axis and orienting said first ion beam axis at a first ion beam axis angle and orienting said second ion beam axis at a second ion beam axis angle equal to said first ion beam axis angle so that said first and second ion beam axis angles are symmetric with respect to said crystal plane. |
| 28. | The method of claim 1 wherein said ion beam is further characterized by an ion beam axis and step (2) further comprises a step of rotating said type II superconductor material with respect to said ion beam axis so that an isotropic dispersion in orientation of said splayed columnar defects is produced. |
| 29. | The method of claim 1 wherein said ion beam is further characterized by an average beam direction and said splayed columnar defects are further characterized by a splay distribution which is a random distribution confined to a narrow cone about said average beam direction. |
| 30. | The method of claim 28 further comprising a step of combining said splayed columnar defects with a proportion of parallel columnar defects selected to yield a desired Tc. |
| 31. | The method of claim 1 wherein said ion beam is further characterized by a weakly collimated ion beam flux and step (2) further includes a step of translating said type II superconducting material across said weakly collimated ion beam flux to produce said splayed columnar defects. |
| 32. | The method of claim 30 wherein said type II superconducting material is a continuous material and further comprising a step of continuously translating said continuous type II superconducting material across said weakly collimated beam and collecting said material after irradiation by said weakly collimated beam. |
| 33. | The method of claim 30 further comprising a step of applying a magnetic field to said type II superconducting material containing said splayed columnar defects at a first temperature and rapidly cooling said said type II superconducting material to a second temperature below Tc of said type II superconducting material. |
| 34. | A type II superconducting article produced according to the method of claim 1. |
| 35. | A type II superconductor including splayed columnar defects and characterized by Jc which is increased with respect to Jc of said type II superconductor material having parallel columnar defects. |
| 36. | The type II superconductor of claim 34 wherein said Jc is greater than about lxl05amps/cm2 at 77K and greater than about lxl0 amps/cm2 at 5K. |
| 37. | The material of claim 34 wherein said type II superconductor is characterized by anisotropy in electronic effective mass, a short coherence length and a London penetration depth of in the range of from about 10 to about lOOAngstroms. |
| 38. | The material of claim 34 wherein said type 11 superconductor is a single crystal superconducting material. |
| 39. | The material of claim 37 wherein said single crystal superconducting material is further characterized by a layered crystal structure. |
| 40. | The material of claim 37 wherein said single crystal superconductor material is further characterized by a single crystal material form and said single crystal material form is selected from the group consisting of thin film, thick film, bulk and filament forms. |
| 41. | The material of claim 39 wherein said single crystal superconducting material is a high temperature superconducting material selected from the group consisting of YBa2Cu307, Tl2Ba2Ca2Cu3O10, BiSr2CaCu208, Tl2Ba2CaCu308 and superconducting C60 doped with an alkali metal. |
| 42. | The material of claim 39 wherein said single crystal superconducting material is a low temperature superconducting material selected from the group consisting of Nb, NbTi, Nb3Sn and amorphous MoGe. |
| 43. | The material of claim 34 wherein said type II superconductor material is a polycrystalline superconducting material. |
| 44. | The material of claim 42 wherein said polycrystalline superconducting material is further characterized by a layered crystal structure. |
| 45. | The material of claim 42 wherein said polycrystalline superconductor material is further characterized by a polycrystalline material form and said polycrystalline material form is selected from the group consisting of thin film, hick film, bulk and filament forms. |
| 46. | The material of claim 42 wherein said polycrystalline superconducting material is a high temperature superconducting material selected from the group consisting of YBa2Cu307, Tl2Ba2Ca2Cu3010, BiSr2CaCu2Oe, Tl2Ba2CaCu308 and superconducting C60 doped with an alkali metal. |
| 47. | The material • of claim 42 wherein said polycrystalline superconducting material is a low temperature superconducting material selected from the group consisting of Nb, NbTi, Nb3Sn, and amorphous MoGe. |
| 48. | The material of claim 34 wherein said columnar tracks are wellaligned, continuous and penetrate substantially through said type II superconductor material . |
| 49. | The material of claim 47 wherein said columnar tracks are further characterized by a defect density and said defect density is selected to match the density of magnetic flux vortex lines associated with a particular effective field, Bφ. |
| 50. | The material of claim 48 wherein said columnar tracks are further characterized by a columnar track length and said columnar track length is in the range of from about lOmicrons to about 70microns and more preferably in the range of from about lOmicrons to about 30microns and most preferably in the range of from about 20microns to about 30microns and by a columnar track width and the columnar track width of each of said columnar tracks is in the range of from about 20Angstroms to about lOOAngstroms, more preferably in the range of from about 40microns to about δOmicrons and most preferably in the range, of from about 50Angstroms to about 70Angstroms. |
| 51. | The material of claim 49 wherein said type II superconductor is further characterized by a high temperature flux liquid phase and further including a step of controlling said splay angle so that said splay angle does not exceed root mean square thermal fluctuations in said high temperature flux liquid phase. |
| 52. | The material of claim 50 wherein said splay angle is in the range of from about 2degrees to about 20degrees, more preferably in the range of from about 5degrees to about 15degrees and most preferably in the range of from about δdegrees to about 12degrees . |
| 53. | The material of claim 34 wherein said type II superconductor material is further characterized by a layered crystal structure having a caxis symmetry axis and said columnar tracks are displaced from said caxis symmetry axis by a columnar track displacement angle. |
| 54. | The material of claim 52 wherein said columnar tracks are further characterized by a splay angle which is symmetric about said caxis symmetry axis. |
| 55. | The material of claim 34 wherein said type II superconductor material is further characterized by a layered crystal structure and includes a crystal plane associated with a layer of said layered crystal structure and said columnar tracks are splayed with respect to said crystal plane. |
| 56. | The material of claim 34 wherein said splayed columnar defects are further characterized by an isotropic dispersion. |
DMR89-20490 and DMR91-07752 from the National Science
Foundation and grant W-31-109-ENG-38 from the United
States Department of Energy.
Background of the Invention
1. Field of the Invention
The invention relates to a method for enhancing the properties of a superconducting material and to a superconducting material having such enhanced properties. 2_- Description of the Prior Art
A superconductor is a material which undergoes a phase transition from a state of normal electrical resistivity behavior to a superconducting state at some transition temperature, T c . In the superconducting state, the material exhibits a dc (direct current) electrical resistivity .of zero, or so nearly zero that electrical currents can persist in the material for extended periods of time. Other important parameters for characterizing the performance of a superconducting material include the temperature dependent critical field, H C (T), which is defined as the threshold or critical magnetic field which results in the disappearance of superconducting behavior in the sample and the critical current density, J c . The current density J c can be transformed to a critical current I c by. multiplying the cross-section area (A) through which the current flows. Thus, I c =J c xA. The quantity I c is the current above which a linear relation between voltage (V) and current (I) is observed. In the
superconducting region, the relation between voltage (V) and current density (J) becomes non-linear in such a way that very small voltages are generated when J is less than the critical current density. A useful definition of J c is a voltage criterion. J c is then defined as the current density such that the voltage drop becomes less than lxlO "6 volts/cm.
There exist two types of superconductors, type I and type II superconductors, distinguished, in part, by their response to an external applied magnetic field. A type I superconductor is one wherein the superconducting state is destroyed and the normal state restored by application of an external magnetic field in excess of a critical field-, H C (T), described above. By contrast, a type II superconductor is characterized by two critical fields, H cl and H c2 , with H cl less than H c2 . Between H cl and H c2 , the superconductor magnetic flux lines extend through the superconductor in quantized filaments and create what is known as a vortex state. In the new high T c superconductors, the resistance becomes effectively zero only below an irreversibility line which is considerably below H c2 (T) .
It is well known in the art of solenoid production from conventional low temperature type II superconductor materials, i.e., superconducting materials characterized by T c 's less than approximately 30K, that the materials can be "hardened" that is have the amount of magnetic hysteresis behavior which they exhibit increased by pinning the flux lines by some means, including mechanical means. It is the motion of flux lines which produces a voltage and dissipation of energy. Flux lines must be pinned in place for applications.
In the art of high temperature superconductor, i.e., superconducting materials characterized by T c 's in excess
of about 30K, fabrication, it has been observed that the performance and technological utility of these materials have been limited by decreases in their T c 's when exposed to applied external magnetic fields such as those typically encountered in technological applications of these materials. For example, layered cuprate high temperature superconductors have failed to fulfill much of their initial promise because the very high T c 's which they exhibit in zero applied external magnetic field are dramatically decreased, upon application of an applied external magnetic field approximately of the order of O.lTesla, approximately ten to one hundred times smaller than the large magnetic field strength which is typically encountered in most technological applications for such high temperature superconducting materials. One class of high temperature superconducting materials, the thallium- based high temperature superconducting compounds, specifically Tl 2 Ba 2 Ca 2 Cu 3 O 10/ have a zero applied external magnetic field T c of approximately 108K, while in an applied external magnetic field of about 2.5Tesla, they have a T c of only 55K. The critical current density in this material at 77K drops from about 2xl0 4 amp/cm 2 at 0. OTesla to about lxl0 2 amp/cm 2 at 0.5Tesla.
Recent experimental work, such as that of Civale et al., Phys. Rev. Lett. £7, 648 (1991), Budhani et al . , Phys. Rev. Lett. £9, 3816 (1992) and Hardy et al . , Physica C, 191, 85 (1992) , has shown that degradation (i.e., decrease) of high T c 's in the presence of strong applied external magnetic fields can be reduced by creating defects in the form of parallel columnar damage tracks by irradiating a superconductor sample with heavy ion radiation. According to the foregoing experiments, the parallel columnar damage tracks are from approximately 10-lOOmicrons long and 70Angstroms across
and serve as "columnar pins" to trap or pin magnetic flux lines which are also known as vortex lines. The presence of these parallel damage tracks was found to extend the temperature and applied external magnetic field range of usable critical current. For example, efficient pinning of vortex lines by parallel columnar defects in a thallium-based high temperature superconductor compound resulted in an increase in T c at an applied magnetic field of 2.5Tesla from about 55K to about 88K, thereby bringing T c into a temperature range in excess of 77K, which allows use of common and inexpensive liquid nitrogen as a coolant to maintain the material in the superconducting state.
Thus, there exists a need for a method for manipulation df superconductor defect structure to optimally improve superconductor properties, including J c and flux creep, at high applied external magnetic field as required for their fullest exploitation in technological applications as well as for superconductor materials with such controlled defect structure and desirable superconducting properties.
Summary of the Invention
The invention provides a method for improving the superconducting properties of a type II superconductor by irradiating the type II superconductor with an ion beam so that columnar defects splayed about an average direction are created in the type II superconductor material. By "splayed columnar defect", we mean defects in the form of columns, not all columns being parallel to a single direction but, instead, with a prescribed variety of orientations about the average orientation.
In another aspect of the invention, a type II superconductor having splayed columnar defects and
characterized by improved critical current, J c , and flux creep properties at small currents less than about 0.1J C to about 0.01J C by comparison with those of a type II superconductor having parallel columnar defects is provided.
Objects of this invention include providing a method for manipulating the defect structure of a superconductor by irradiating the material to create splayed columnar defects to optimize the superconducting properties of the material including J c and flux creep at very small currents much less than J c and at high applied external magnetic fields such as those which would be encountered in typical technological applications.
A further object of the invention is provision of a type II superconductor material having splayed columnar defects and characterized by the already-described improved superconducting properties which surpass those of type II superconducting materials having parallel columnar defects. Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art in reading the description of the preferred embodiments which follows.
Brief Description of the Drawings FIG. 1 is schematic representation of the entangled ground state of vortices resulting form splayed columnar defects.
FIG. 2 is a graph showing the theoretically predicted current-voltage characteristics of a high temperature superconductor having parallel columnar defects.
FIG. 3 is a graph showing the theoretically predicted current-voltage characteristics of a high
temperature superconductor having splayed columnar defects.
Detailed Description of the Invention The invention provides a method for improving the superconducting properties of a type II superconductor such as J c and flux creep at J<J C by comparison with the values of those properties exhibited by a type II superconductor having a parallel columnar defect structure by irradiating the type II superconductor with an ion beam to produce a splayed columnar defect structure. Such an improved superconductor material can be used, for example, in motors; generators; transmission lines; high field magnets, such as needed for magnetic resonance imaging devices; large scale magnetic energy storage devices, such as solenoids; magnetic levitation applications; microwave cavities as filters and new electronic devices, such as SQUID and Josephson junction- based circuits. In low field applications such as cavities, SQUIDS and circuits, time constants are determined by flux creep at small currents, which makes suppression of flux creep desirable.
While the applicants do not wish to be. limited in any way by theory, it is believed that the resistive response of a superconductor is substantially improved, thus leading to effectively zero electrical resistance, by deliberately splaying the columnar defects and producing a controlled dispersion in orientation. Such a dispersion creates pinned vortices 10 between first surface 12 and second surface 14 of a type II superconducting material 16 wherein the vortices 10 can become entangled in their magnetically preferred configuration as shown schematically in Fig. 1 to form a novel "splayed glass" state which is physically different
from the "Bose glass" of parallel columnar defects in several important ways. The term "entanglement", is used herein in the specification and claims to refer to a single vortex 10, also known as a magnetic flux line, breaking free from a columnar defect 18 and being impeded from further motion by interaction with other magnetic flux lines 10 which remain pinned by other columnar defects 18. These additional barriers reduce the voltage by a factor exp[-U x /k B T] , where U x is the flux line crossing energy, k B/ is Boltzmann's constant and T is the temperature. The improved properties described below will result even if the magnetic flux lines do not jump from pin to pin as shown in Fig. 1, but, instead, each magnetic flux line follows a distinct tilted columnar defect.
The entanglement of flux lines below T c in a magnetic field can be further enhanced by rapidly cooling a flux liquid, which refers to a "liquid" as used in a statistical mechanical sense, i.e., an assembly of statistically fluctuating magnetic flux lines which are moving with respect to a fixed crystalline lattice of atoms, in a magnetic field down from a temperature below the zero field T c , but above T c in that particular applied magnetic field, to a temperature below T c in that particular applied magnetic field where resistivity vanishes. Since flux liquids are already entangled by thermal fluctuations at high temperatures, rapid quenching will preserve existing entanglements and produce an even more entangled state which will then be captured by the entangled template of splayed columnar pins. The type II superconducting material can also be rapidly cooled in an applied magnetic field from the
"normal" state such as at room temperature to its zero applied magnetic field T c and cooling terminated below its
SUBSTITUTE SHEET (RULE 20)
T c in an applied magnetic field. The cooling rate can be varied to produce different superconducting samples which can then characterized according to techniques well known to one skilled in the art to determine which cooling rate yields optimized superconducting properties. The cooling rate can exceed one degree per minute. Entanglements will be prevented from relaxing upon rapid cooling if the line crossing energy U x is large compared to the temperature. First, random misorientation of columnar defects in the splayed glass strongly inhibits large scale low energy excitations which dissipate energy. At a fixed current density, J, excitations of size L o l/J are required for flux motion. Energy barriers for an excitation of size L parallel to the external applied magnetic field grow as L 3 s as L tends to infinity compared to an L 13 dependence for the parallel columnar defect case. This leads to a much reduced dissipation rate for low-current transport at finite temperatures. Hence, "vortex creep" over long time scales in weak magnetic field gradients will be reduced by more than two orders of magnitude.
Secondly, flux lines in the splayed glass state are entangled, as shown in Fig. 1, in sharp contrast to the unentangled parallel columnar defect/parallel pin case. Entanglements strongly enhance vortex line pinning leading to much larger critical currents and further improvement of transport properties. This enhancement results because a few isolated vortices breaking free from their tilted columnar pins will be impeded by the remaining vortices still strongly attached to their columnar pinning sites. This effect arises even for vortices which do not jump from pin to pin, but, instead, follow a single tilted columnar defect .
Finally, splay will also produce diverging barriers which prevent the low lying excitations needed to relax small tilts of less than lOdegrees and thus splay will stabilize vortices in superconductors so-processed with respect to transverse displacement. Such stability is particularly important for magnetic levitation applications.
However, uncontrolled, excessive splay could have the detrimental effect of actually degrading the transition temperature, even though the low temperature transport properties are enhanced. Therefore, splay must be controlled with angular deviations not exceeding the root mean square thermal fluctuations in the high temperature flux liquid phase close to the transition temperature obtained with parallel pins to insure minimal degradation of the transition temperature.
Splayed columnar defects/ pins lead to a new splayed glass phase with improved magnetic flux vortex creep properties as current (I) tends to zero and polymer-like entanglement of vortex lines which leads to high critical currents. For a given sample cross-sectional area, A, the current I is related to the current density J by the relation already discussed I=JxA and, in particular, I c =AxJ c . The relation between voltage (V) and current (I) for a high temperature superconductors having parallel columnar defects in an external applied magnetic field is shown schematically in Fig. 2. For comparison, the relation between voltage and current for a high temperature superconductor in a magnetic field having splayed columnar defects is shown schematically in Fig.
3.
The two most important effects demonstrated by Figs. 2 and 3 are (1) that splay-induced entanglement of vortex lines increases the critical current, referred to 3.. I c , in
Figs. 2 and 3, above which there is a conventional linear relation between V and I and the materials exhibit a normal state resistance and below which the resistance drops to a value much smaller than its normal state value; and (2) that splay also leads to much smaller non¬ linear flux creep voltages for currents well below I c . In practice, I must be less than from about 0.1I C to about 0.01I C for the specific relation V oc exp [- (I 0 /I) 3/s ] to apply. Creep will be reduced relative to parallel columnar pins, however., for all I<I C . The small voltages below I c as well as large values for I c shown in Fig. 3 for the splayed columnar defect structure are essential for many high field devices which require nearly dissipationless supercurrents. The type II superconducting material used in the method of the invention can be characterized by anisotropy in electronic effective mass, a short coherence length <=10-100Angstroms and a London penetration depth, λ«1000-10, OOOAngstroms. The efficacy of the method is greatest for highly anisotropic crystals such as the Bi- and Tl-based high temperature superconductor compounds, because these compounds exhibit the greatest reduction in their critical temperatures and critical currents with the application of a magnetic field. In general, the angular deviation characterizing the extent of splay should be adjusted to maximize J c and reduce flux creep without degrading the T c significantly, as dictated by the requirements of a particular application. The high temperature superconductors are usually layered compounds with weak interlayer couplings which lead to a particularly low irreversibility line, i.e., the locus of transition temperatures as a function of magnetic field strength. Columnar pins are especially
important in raising the critical temperature in high magnetic fields in these compounds. However, the utility of columnar pins, and the usefulness of deliberately splaying them can also be applied to isotropic materials such as isotropic compounds liκe C 60 Fullerenes doped with an alkali metal such as potassium.
The type II superconductor can be a single crystal superconducting material having a layered crystal structure and can have a single crystal material form such as thin film, thick film, bulk or filament forms . A thin film can have a thickness in the range of about lOOAngstroms to about lOOOAngstroms. A thick film can be characterized by a thickness in the range of from about lOOOAngstroms to about lmicron and a bulk sample can have a thickness or other relevant dimension in the range of from about lmicron to about 10cm. In one embodiment, the single crystal superconducting material can be a high temperature superconducting material such as YBa 2 Cu 3 0 7 , Tl 2 Ba 2 Ca 2 Cu 3 O 10/ BiSr 2 CaCu 2 0 8 , Tl 2 Ba 2 CaCu 3 0 8 or superconducting C 60 doped with alkali metals. In another embodiment, the single crystal superconducting material can be a low temperature superconducting material such as Nb, NbTi, Nb 3 Sn, or amorphous MoGe.
Polycrystalline type II superconducting materials are also appropriate for use in the method of the invention and can have a layered crystal structure with texturing in a direction corresponding to the c-axis of the single crystal superconductor. The polycrystalline type II superconductor material can be characterized by a polycrystalline material form including thin film, thick film, bulk, composite, with a metallic and ceramic superconductor phase or filament form. Appropriate polycrystalline superconducting materials for use in the method of the invention include YBa 2 Cu 3 0 7 , Tl 2 Ba 2 Ca 2 Cu 3 O 10 ,
SUBSTITUTE SHEET (RULE 20}
BiSr 2 CaCu 2 0 8 or Tl 2 Ba 2 CaCu 3 0 8 high temperature superconductors as well as low temperature superconducting materials such as Nb, NbTi, Nb 3 Sn, or amorphous MoGe. In a preferred embodime'ht, the ion beam produces well-aligned, continuous columnar tracks which penetrate substantially through the entire type II superconductor material sample undergoing irradiation. In the specification and claims which follow, the term "well- aligned" as used to describe columnar tracks refers to a controlled angular dispersion about an average direction in the range of from about 20degress down to less than about 2degrees. These columnar tracks are further characterized by a defect density which can be determined according to the equation B φ = 0 x overall columnar defect density per unit area wherein B φ is the equivalent field at which the magnetic flux vortex density matches the density of columnar defects and Φ 0 =2xl0" 9 gauss/cm 2 is the flux quantum, and by a columnar track length in the range of from about lOmicrons to about lOOmicrons, more preferably in the range of from about 40microns to about lOOmicrons and most preferably in the range of from about 60microns to about lOOmicrons and by a columnar track width of each of said columnar tracks in the range of from about 20Angstroms to about lOOAngstro s, more preferably in the range of from about 40Angstroms to about δOAngstroms and most preferably in the range of from about 50Angstroms to about 70Angstroms.
The ions which make up the ion beam are heavy ions such as silver, gold, lead and tin ions with the valency of the ions selected to be as positively charged as possible so that the energy imparted to them is very large. Typical valencies can be in the range of from about 20-30 as for example, Sn 30* . The ion beam energy
can be in the range of from about lOOMev to about lOGev per ion and the ion beam energy loss rate can be in the range of from about 2Kev/Angstrom to about 2Mev/Angstrom. The ion irradiation flux is chosen so that overheating and melting of the type II superconducting material is avoided. Another way to avoid beam heating damage to the type II superconducting material is to cool the mate-rial using an active cooling apparatus.
The splay angle of the columnar defects is chosen so that J c is increased with respect to J c of the same type II superconductor material having parallel columnar defects so that flux creep properties are enhanced. The splay can be selected so that the splay angle does not exceed the root mean square thermal fluctuations in the high temperature flux liquid phase. Based on the foregoing considerations, the splay angle can be in the range of from about 2degrees to about 20degrees, more preferably in the range of from about 5degrees to about 15degrees and most preferably in the range of from about δdegrees to about 12degrees. The splay variance in radians can be estimated at all fields according to the relationship v 0 «a 0 /l j . where a 0 is the magnetic flux vortex spacing in the type II superconductor and l z is the entanglement length, which refers to the average spacing between collisions or close encounters between neighboring vortex lines along the field direction. The term "splay variance" as used herein in the specification and claims, refers to the root mean square angle the columnar defects make with respect to their average direction. Since a 0 /l z is of order 0.3 with T = 77K in an applied external magnetic field,. B, approximately equal to 2Tesla in high T c superconductor materials, the splay variance v 0 can be of order lOdegrees without degrading the superconducting transition temperature. This splay
variance is sufficient and greatly enhances low temperature transport properties .
When the type II superconductor material has a layered crystal structure having a c-axis symmetry axis the ion beam axis is oriented so that it is slightly displaced from the c-axis symmetry axis of the type II superconductor to avoid channeling, i.e., generating columnar tracks which inflict minimal damage on the crystal by moving exactly along the c-axis symmetry axis. When the type II superconductor material is a polycrystalline material having a corresponding layered single crystal form and is textured along the c-axis symmetry axis, the ion beam can also be oriented so that it is slightly displaced from the c-axis texturing.The columnar tracks can have a splay angle which is symmetric about the c-axis symmetry axis.
According to an embodiment of the invention, a first ion beam having a- first ion beam axis and a second ion beam having a second ion beam axis can be oriented so that the first ion beam axis is at a first ion beam axis angle and the second ion beam axis is at a second ion beam axis angle equal to the first ion beam axis angle so that the first and second ion beam axis angles are symmetric about the c-axis symmetry axis. According to another embodiment, the columnar defects can be splayed about the plane of a layer of a layered superconductor structure. This configuration is produced by orienting the first ion beam axis at a first ion beam axis angle and orienting the second ion beam axis at a second ion beam axis angle equal to the first ion beam axis angle so that the first and second ion beam axis angles are symmetric with respect to the crystal plane .
Alternatively, an isotropic dispersion in orientation of the splayed columnar defects can be produced by rotating the type II superconductor material with respect to the ion beam axis to produce a very broad dispersion. Splayed columnar defects can also be generated in a type II superconductor material using a weakly collimated ion beam flux and translating the superconductor sample across the weakly collimated ion beam flux. As used herein in the specification and claims, the term "weakly collimated" refers to an ion beam wherein the directions of ions in .the beam vary from the average direction by approximately 2degreees to approximately 20degrees.
According t.o another embodiment, the splay can be reduced to a random distribution confined to a narrow cone about the average direction, which need not be close to the c-axis or the plane of the layers . Such a random distribution can be created by randomly wobbling the type II superconductor sample or by slightly decollimating the ion beam using a lead foil deflector which will result in an ion beam slightly decollimated about a narrow cone surrounding the beam axis.
Yet another embodiment provides for the above- described splayed defects to be combined with an optimized proportion of parallel defects to insure that the critical temperature remains high.
A type II superconductor including splayed columnar defects and characterized by J c which is increased with respect to J c of the same type II superconductor material having parallel columnar defects as well as characterized by enhanced flux creep properties is also provided. Enhanced flux creep at low currents less than J=0.1J C to J=0.01J C will lead to voltages approximately 0.1 or 0.01 times smaller than in the same superconductor with
parallel columnar pins. The type II superconductor can have J c greater than about 1x10 s amps/cm 2 at 77K and greater than about lxlO 7 amps/cm 2 at 5K
The type II superconductor can be characterized by anisotropy in electronic effective mass, a short coherence length, <=10-100Angstroms and a London penetration depth λ«-»1000-10, OOOAngstroms and can be a single crystal or polycrystalline superconducting material. The single crystal superconducting material can have a layered crystal structure and be in the form of a thin film, thick film or a bulk sample such as a filament. The single crystal superconductor can be a high temperature superconducting material like YBa 2 Cu 3 0 7 , Tl 2 Ba 2 Ca 2 Cu 3 O 10 , BiSr 2 CaCu 2 0 8 or Tl 2 Ba 2 CaCu 3 0 8 or a low temperature superconducting material like Nb, NbTi, Nb 3 Sn or amorphous MoGe.
The polycrystalline superconducting material can also be characterized by a layered crystal structure and have a polycrystalline material form such as thin film, thick film, bulk or filament form. The polycrystalline superconducting material can be a high temperature superconducting material such as YBa 2 Cu 3 0 7 , Tl 2 Ba 2 Ca 2 Cu 3 O 10 , BiSr 2 CaCu 2 0 8 or Tl 2 Ba 2 CaCu 3 0 8 or a low temperature superconducting material such as Nb, NbTi, Nb 3 Sn or amorphous MoGe.
The columnar tracks are well-aligned, continuous and penetrate substantially throughout the extent of the type II superconductor material with a defect density which can be determined based on the relationship between the effective field B φ and the magnetic flux vortex density already discussed. The columnar tracks are further characterized by a columnar track length and the columnar track length is in the range of from about lOmicrons to
SUBSmUTE SHEET (RULE 26)
about lOOmicrons and more preferably in the range of from about 40microns to about lOOmicrons and most preferably in the range of from about 60microns to about lOOmicrons and by a columnar track width of each of the columnar tracks in the range of from about 20Angstroms to about lOOAngstroms, more preferably in the range of from about 40Angstroms to about δOAngstroms and most preferably in the range of from about 50Angstroms to about 70Angstroms. The splay angle is controlled so that the splay angle does not exceed the root mean square thermal fluctuations in the high temperature flux liquid phase and can be in the range of from about 2degrees to about 20degrees, more preferably in the range of from about 5degrees to about 15degrees and most preferably in the range of from about δdegrees to about 12degrees.
The type II superconductor material can be further characterized by a layered crystal structure with a c- axis symmetry axis and- have columnar tracks displaced from the c-axis symmetry axis by a columnar track displacement angle of in the range of from about 2degrees to about 20degrees. The columnar tracks can also have a splay angle which is symmetric about the c-axis symmetry axis.
The type II superconductor material can have a layered crystal structure and the columnar tracks can be splayed with respect to the crystal planes.
The columnar defects can also have an isotropic dispersion.
In order to further illustrate the present invention, the following examples are provided. The particular compounds, processes and conditions utilized in the examples are meant to be illustrative of the present invention and not limiting thereto.
EXAMPLE 1 The following example is provided to show how a high temperature superconductor having splayed columnar defects can be prepared according to the method of the invention and tested to produce optimal properties.
A YBa 2 Cu 3 0 7 (YBCO) single crystal can be grown using a flux-melt growth technique. A thin sample of YBCO having dimensions lmmxlmmx20microns can be cut from the flux-melt grown single crystal and mounted in a vacuum chamber on a cold plate in order to prevent overheating of the sample during subsequent irradiation processing.
The cold plate and attached YBCO sample can then be mounted on a goniometer which provides precisely controlled sample orientation during rotation of the YBCO sample during irradiation processing with a heavy ion beam.
The YBCO sample can be irradiated with an ion beam of gold ions having energies of about lGev. Such a beam can be produced by any standard heavy ion accelerator such as the ATLAS at Argonne National Laboratory, Argonne
IL. The gold ion beam can be made more parallel using standard collimators. Gold ions are preferred because they provide more homogeneous columnar defects along the beam direction than other heavy ions such as lead and tin which can produce columnar defects consisting, instead, of amorphous bubbles. Irradiation rates are selected to avoid beam-heating-effects and an irradiation rate of approximately 2xl0 8 ions/cm 2 sec can be used. Also, the total irradiation dose should be selected so that T c at zero external applied magnetic field is not degraded
(i.e., decreased) by more than 10K. Any radiation dose which fulfills this requirement can be used and a possible total radiation dose can be 2.5xlO ions/cm 2 and a B φ «=5Tesla.
The controlled splay of columnar defects can be obtained by tuning the already-described collimator so that the gold ion beam makes an angle theta with the c- axis symmetry axis of the YBCO crystal. After an initial irradiation process is completed, the YBCO sample can be rotated over an interval of 2theta in order to provide a set of columnar defects which enclose an angle theta with the c-axis of the YBCO crystal. The irradiation process is then repeated with the YBCO crystal sample in this second orientation. In this way, a sample with splayed columnar defects consisting of two sets of columnar defects tilted over 2theta with respect to each other and tilted over theta with respect to the YBCO crystal c-axis can be obtained. The above-described procedure can then be repeated by rotating the sample about the c-axis to vary the azimuthal angle phi.
The value of the angle theta can be varied for different samples of YBCO crystal. Possible values for theta include 5degrees, lOdegrees, 15degrees, 20degrees and 30degrees. Samples irradiated at different values for theta can then be evaluated in an external magnetic field applied along the c-axis of the YBCO crystal to determine their superconducting properties as relevant for a particular device or application to determine which irradiation angle theta results in optimized properties.
More uniform irradiation can be obtained by irradiation for a smaller amount of time but at a larger number of different sample orientation angles ranging from about ±3 to about ± 20degrees. The different sample orientation angles can be chosen at random in a solid angle around the YBCO sample c-axis with the exclusion of orientations at which the gold ion beam makes an angle of
less than two degrees with a crystal symmetry axis to avoid channeling.
EXAMPLE 2 The following example is provided to show how a bulk sample having a splayed columnar defect structure such as a spool of filamentary superconductor wire can be continuously processed according to the method of the invention. First, a weakly collimated beam of ions with an angular dispersion about an average direction θ is created. A superconductor wire or tape is then spooled across the weakly collimated bea"m to produce the splayed columnar defect structure. By varying the thickness of the lead foil, the degree of decollimation of the ion beam can be controlled. One technique for beam decollimation is to allow a conventional collimated beam to pass first through lead foil to partially deflect the ions before they strike the superconductor. For superconductor tapes which will be wound to make solenoids, the radiation should strike the first side of the superconductor tape at a grazing angle, preferably of lOdegrees or less, to the plane of the tape and along a direction perpendicular to the motion of the tape as it is spooled so that it passes through the decollimated beam. The above-described procedure can be repeated so that the ion beam strikes the second side of the superconductor tape, as well.
What is claimed is:
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