High critical temperature superconductors in the system La.sub.3-z . Me.subz Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x where Me=a rare earth or alkaline metal ION and Nc is a Mg, Cd ION

Abstract

Compounds of the of the general formula La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x, wherein Me can be a rare earth metal or an alkaline metal ion selected from the group consisting of yttrium (Y), ytterbium (Yb), sodium (Na) and Nc can be a 2+ion selected from the group consisting of magnesium (Mg) and cadmium (Cd) have been prepared as the HTSC in thin film superconductors. These compounds can be used as thin film high critical superconductors in thin film high critical temperature superconducting structures and antennas and in multilayered structures and devices such as Josephson junctions, broadband impedance transformers and both flux flow and field effect transistors.

Description

FIELD OF INVENTION

The invention relates in general to several new and useful in High Critical Temperature Superconducting (HTSC) compounds of the general formula La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x for use as HTSC's in HTSC devices, and more particularly to compounds of that general formula where Me is a rare earth or alkaline metal ion and Nc is a 2+ion selected from the group consisting of magnesium (Mg) and cadmium (Cd).

BACKGROUND OF THE INVENTION

Heretofore, a most useful HTSC in thin film superconductor technology has been YBa.sub.2 Cu.sub.3 O.sub.6+x, (YBCO). However, there have been problems with the use of YBCO, including the fact that it undergoes a phase transition leading to twinning and stress. A more significant shortcoming with YBCO is the loss of oxygen when heated, which causes a difficult problem in the fabrication of multilayered devices. While it has been well-known that all HTSC cuprate materials lose oxygen upon heating, typically in the range around 450.degree. to 500.degree. C., up until now there has been no effective or practical superconductor material that retains oxygen when heated above about 500.degree. C. This oxygen loss is significant because superconducting properties, critical currently transition temperatures (T.sub.c) depend upon oxygen stoichiometry. The compounds of the general formula La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x, where Me is a rare earth or alkaline metal ion and Nc is a 2+ion selected from the group consisting of magnesium (Mg) and cadmium (Cd) overcome the shortcomings, drawbacks and limitations of other cuprate HTSC materials by being significantly more stable regarding oxygen loss, particularly in the temperature ranges at which thin film multilayered structures are fabricated. Typically for growth of multilyered structures, temperatures in the range about 550.degree. to 750.degree. C. are required for epitaxial growth of the dielectric and a temperature range of about 650.degree. to 800.degree. C. is required for the HTSC (YBCO).

The HTSC system La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x of the present invention is not only attractive for numerous device applications because of its superior oxygen stability as compared with YBCO, further such compounds do not undergo phase transition between the sintering and room temperatures. Additionally, thermal cycling in La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x compounds has demonstrated reversible oxygen losses of less than 1% when heated up to 1000.degree. C. in bulk samples, according to M. Nicolas et. al, which should permit easier preparation of multilayered structures and multilayered devices such as Josephson junctions, broadband impedance transformers and both flux flow and field effect transistors. The inventors herein have prepared thin films with compounds of the general formula La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x, where Me is a rare earth or alkaline metal ion and Nc is a 2+ion selected from the group consisting of magnesium (Mg) and cadmium (Cd), with substitutions for lanthanum (La) and calcium (Ca) and have been able to increase T.sub.c. The compounds of the present invention solve the oxygen loss problem of cuprate HTSC materials without suffering from any of the disadvantages, drawbacks and limitations of prior art compounds.

A superconducting bulk compound in the system La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x where z=0 and v=0, namely La.sub.3 Ba.sub.3 CaCu.sub.7 O.sub.16+x, known as La3317, was first prepared by Engelsbereg and the material was found to be a superconductor with a T.sub.c =80.2 K and to have a triple perovskite structure. The compound was tetragonal with c=3a (c=11.61, a=3.87). An earlier study by H. Fujishita et. al performed on a composition close to La3317 deduced that most of the calcium atoms occupy the central position of the unit cell occupied by the yttrium atoms in YBa.sub.2 Cu.sub.3 O.sub.6+x (YBCO). Recently, Kao et. al reported that about half of the calcium atoms enter the yttrium sites and that the remainder are on barium (Ba) sites. Similar results have been achieved in studies of other La--Ba--Ca--Cu oxides by Fujishita and Wang, where much of the lanthanum is found in the barium sites. Engelsberg already studied the substitution of magnesium (Mg) and scandium (Sc) for calcium and he found a slight increase in lattice parameter when magnesium is substituted, c=11.71 .ANG. and a=3.91 .ANG., and he observed superconductivity below 30 K. Additionally, scandium substitution led to a multiphase material with no observed superconductivity. Wu et. al also investigated rare earth substitution for lanthanum in bulk samples. Based on these superconductor studies, the prior art only indicates a search for higher T.sub.c materials, while still not satisfactorily resolving the long-felt shortcomings and disadvantages of cuprate HTSC materials' oxygen loss and phase transition problems. The compounds of the present invention now resolve those problems by being more stable with respect to oxygen such that these compounds can provide more effective, practical, cheaper and longer lasting superconductors.

Prior art describing similar compounds is found at A. W. Sleight and R. Ward, Inorganic Chemistry, 3 rd edition, p. 292 (1964);

S. Engelsberg, 176 Physica C. 451, 1991; H. Fujishita et. al, 28 Japanese Journal of Applied Physics 754 (1989); H. C. Kao et. al, 9 Superconductor Science Technology 10 (1996); M. Nicolas et. al, 249 Physica C 377 (1995); C. M. Wang et. al, 39 Journal of Chinese Chemical Society 67 (1992); and D. S. Wu et. al, 212 Physica C 32 (1996).

SUMMARY OF THE INVENTION

The general object of this invention is to provide La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x superconductors that are more stable with respect to oxygen and hence overcome the shortcomings of YBCO. A more particular object of the invention is to provide new HTSC materials with a higher level of oxygen loss stability as compared to YBCO and further does not undergo a phase transition that leads to twinning and stress. A further object of this invention is to provide compounds for thin films having lanthanum and calcium substitutes which possess none of the disadvantages of utilizing YBCO compounds.

It has now been found that the aforementioned objects can be attained using a compound of the general formula La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x, as the HTSC in thin film superconductor technology. In the above formula, Me can be a rare earth metal or an alkaline metal ion selected from the group consisting of yttrium (Y), ytterbium (Yb), sodium (Na) and Nc can be a 2+ion selected from the group consisting of magnesium (Mg) and cadmium (Cd). These compounds can be used as thin film high critical superconductors in thin film high critical temperature superconducting structures and multilayered structures and devices such as Josephson junctions, broadband impedance transformers and both flux flow and field effect transistors without any of the shortcomings, disadvantages and limitations attributed to oxygen loss in YBCO.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and details of the present invention will become apparent in light of the Detailed Description of the Preferred Embodiment of the Invention and the accompanying FIG. 1 chart indicating the desorption of oxygen from thin films of the compounds of the present invention and Table I which summarizes useful properties of the thin films of the compounds of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following compounds in the series La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x have been synthesized for use as thin film superconductors in thin film high critical temperature superconductor devices: La.sub.2 YBa.sub.3 CaCu.sub.7 O.sub.16+x, La.sub.2 Ba.sub.3 CaCu.sub.7 O.sub.16+x, La.sub.2 YbBa.sub.3 CaCu.sub.7 O.sub.16+x and La.sub.2.5 Na.sub.0.5 Ba.sub.3 CaCu.sub.7 O.sub.16+x. The properties of these compounds are set forth in Table I.

Bulk samples and targets in the system of the general formula La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x, where Me can be a rare earth metal or an alkaline metal ion selected from the group consisting of yttrium (Y), ytterbium (Yb) and sodium (Na) and Nc can be a 2+ion selected from the group consisting of magnesium (Mg) and cadmium (Cd) were prepared in bulk form for use as targets for pulsed laser deposition (PLD) by mixing precursor powders of La.sub.2 O.sub.3, Nd.sub.2 O.sub.3, Yb.sub.2 O.sub.3, Y.sub.2 O.sub.3, Na.sub.2 CO.sub.3, MgO and CdCO.sub.3 in a BC mortar until they were homogeneous. The powder was pressed into 2.54 cm disks and heated in air at 3.degree. C./minute to 975.degree. C., held for 10 hours, then cooled at 3.degree. C./minute and removed from the furnace. Subsequently, each disk was reground to a powder of 100 .mu.m or less, pressed into 2.54 cm diameter disks and repressed isostatically at 344 MPa. Then the disks were heated at 3.degree. C./minute to 1000.degree. C. and held for 24 hours in flowing oxygen gas. Lattice parameters were obtained from powder diffractometer scans taken between 15-155.degree. 2.theta. using CuK.alpha. radiation, where .lambda.=1.54056 .ANG.. Lattice parameters are shown in Table I.

Thin films prepared by PLD in this manner averaged a thickness of 300 nm. In this connection, the term "thin film" means a body of material that can be of any size with a thickness of between 3.8 .ANG. and 100 .mu.m, which may also be polycrystalline, epitaxial or single crystal and may be grown using any of the numerous known deposition techniques including physical vapor deposition, liquid phase epitaxy, sol gel process, silk screening and so on. The observed reflections from thin films obtained by .theta. to 2.theta. x-ray scans are also reported in Table I. Deposition parameters were as follows: a KrF excimer laser (.lambda.=248 nm) was used at a pulse repetition rate of 10 Hz and a laser fluence of 1-2 J/cm.sup.2 at the target. The chamber was maintained at an oxygen pressure of 13.6 Pa and the heater block holding the substrates was held at 850.degree. C., with deposition times of about 10 minutes. Samples were cooled rapidly to 650.degree. C. for 2 minutes and then to 400.degree. C. in 5.17 kP of oxygen for 30 minutes. The substrates employed were single crystal (211) GGG, (100) LaAlO.sub.3 and (100) LSAT, which is a solid solution of 30 mole % LaAlO.sub.3 and 70 mole % Sr.sub.2 AlTaO.sub.6. The superconducting transition temperature and transition width were measured using an ac Eddy current apparatus. In this connection, the term "by bulk" means dense sintered polycrystalline bodies from 1 to 1.25 inches in diameter and about 0.125 to 0.25 inches in thickness.

Desorption of oxygen from several films was investigated as a function of temperature using thermal deposition mass spectrometry, which involved detecting desorbing O.sub.2 using a double focusing mass spectrometry. The spectrometer has high mass resolution which allows the oxygen signal to be separated from interfering species such as hydrocarbons of the same nominal mass, i.e. 32 AMU. For analysis, a piece of thin film coated substrate was introduced into a tantalum foil cell that had a 0.005 in. 5% Re--W/26% Re--W thermocouple spotwelded to it. The cell was attached to a vacuum feed through that allowed samples to be moved into a roughing chamber and then into a an analysis chamber that contained the mass spectrometer. The base pressure of the analysis chamber at the onset of desorption was in the mid 10.sup.-9 Torr range, which provided a low oxygen background and greater sensitivity for detecting oxygen originating from the content of the cell. The cell temperature could be ramped to 1500.degree. C., but typically a maximum of 800.degree. C. was used. A linear temperature ramp of 1.4 K/sec. was maintained using a feedback temperature controller and a 100 amp power supply. The area of the thin film was determined by computer processing of a scanned image of the sample and sample thickness was determined by fracturing a coated substrate and measuring film cross section with scanning electron microscopy. The oxygen desorption spectra were then normalized to the calculated volume of the film for each of the samples, thus allowing direct comparison of oxygen desorption from the films.

All substitutions of either La or Ca in accordance with the present invention resulted in the preservation of a tetragonal structure. In bulk samples, changes in intensity of x-ray diffraction lines are consistent with the increase or decrease in the scattering power of the substituent. Very small changes in the c lattice parameter were also noted, consistent with the changes in ionic radii of substituent ions, while the a parameter exhibited a slight expansion. For thin films, contractions in the c parameter were observed when the substrate was LaAlO.sub.3, while slight expansion was found when the LSAT or GGG substrates were used. The results of lattice expansion or contraction of the thin film are consistent with known lattice parameters for the substrates. It is noted that the following compounds in the series La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x synthesized in accordance with the present invention constitute new compositions of matter: La.sub.2 YBa.sub.3 CaCu.sub.7 O.sub.16+x, LaY.sub.2 Ba.sub.3 CaCu.sub.7 O.sub.16+x, La.sub.2 YbBa.sub.3 CaCu.sub.7 O.sub.16+x, La.sub.2.5 Na.sub.0.5 Ba.sub.3 CaCu.sub.7 O.sub.16+x, and La.sub.3 Ba.sub.3 Ca.sub.0.5 Cd.sub.0.5 Cu.sub.7 O.sub.16+x.

All thin films prepared by PLD using the bulk targets show the same epitaxial relationship irrespective of the substrate: La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x (001) on (100) LaAlO.sub.3, on (100) LSAT or (211) GGG, as indicated by table I, above. The strongest reflection from each of the thin films was always (006). The best quality films obtained, in terms of the presence of higher order reflections, were those deposited on LaAlO.sub.3. This may be explained by the fact that of all the substrates employed, only LaAlO.sub.3 placed thin films in compression. Another possible explanation is that film deposition conditions were not optimized.

All substituent compounds exhibited high T.sub.c superconductivity, although the transition temperatures were lower than those in the unsubstituted compounds. The highest transition temperatures were obtained when rare earth ions Nd, Yb or Y replaced La. Although Engelsberg found that the substitution of Mg for Ca resulted in a reduced T.sub.c, the inventors herein observed a T.sub.c =35 K upon replacing half of the Ca with Mg, which was close to Engleberg's results. However, the substitution of half of the Ca by Cd by the inventors herein resulted in a much higher transition temperature of 55 K. While the prior art studies sought a higher T.sub.c, the present inventors have discovered that oxygen stability is the key factor for multilayer technology and have proven that surprisingly satisfactory results are now possible with the use of slightly lower T.sub.c materials which are oxygen stable. The fact that divalent Cd is much larger than Mg and has an ionic radius close to Ca, appears to be a major factor influencing superconductivity. Another significant factor could also be the nature of the bonding.

All thin film compounds prepared in accordance with the present invention, when compared with YBCO, desorb significantly less oxygen, as shown in FIG. 1, when heated. Referring now to FIG. 1, the desorption of oxygen from thin films for compounds YBa.sub.2 Cu.sub.3 O.sub.6+x (YBCO), Nd.sub.2 Ba.sub.3 CaCu.sub.7 O.sub.16+x, La.sub.2.5 Na.sub.0.5 Ba.sub.3 CaCu.sub.7 O.sub.16+x, La.sub.3 Ba.sub.3 CaCu.sub.7 O.sub.16+x, and La.sub.3 Ba.sub.3 Ca.sub.0.5 Cd.sub.0.5 O.sub.16+x are compared. Compounds of the present invention, specifically Nd.sub.3 Ba.sub.3 CaCu.sub.7 O.sub.16+x, with losses of about 4%, La.sub.2.5 Na.sub.0.5 Ba.sub.3 CaCu.sub.7 O.sub.16+x, with losses of about 8%, La.sub.3 Ba.sub.3 CaCu.sub.7 O.sub.16+x, with losses of about 1%, and La.sub.3 Ba.sub.3 Ca.sub.0.5 Cd.sub.0.5 O.sub.16+x with losses of about 2%, all demonstrate significantly lower oxygen losses than the YBCO curve, which loses 17% oxygen when heated in the temperature range between 450 to 650.degree. C. When some La is replaced by Na, curve C in FIG. 1, the largest desorption for any substituted compound was observed, while the substitution of Cd and Ca, curve E of FIG. 1, showed a smaller effect. It is theorized that the occupancy by light rare earths which have large ionic radii, in the rare earth site for this triple perovskite structure, strongly influences the retention of oxygen with heating.

In the foregoing disclosure, the term "high critical temperature thin film superconductor device" means a copper oxide superconductor having a critical temperature in excess of 30.degree. K. Examples of HTSC with reduced sensitivity to oxygen loss are La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x, with particular emphasis on those fabricated by the inventors herein and listed on Table I.

The term "single layer device" refers to a device including a single layer of high critical temperature superconductor and any combination of other materials, such as dielectric, pyroeletric, piezoelectric, or ferromagnetic materials. The term "multi-layer device" refers to a device including at least two layers of a high critical temperature superconductor, and any combination of other materials, including but not limited to materials with ferroelectric, pyroeletric, piezoelectric, dielectric or ferromagnetic layers.

High critical temperature superconducting compounds of this invention can be used in devices including flux flow transistors, current limiters, broadband impedance transformers, diodes, delay lines, resonators, antenna, antenna feeds, switches, phase shifters, mixers, amplifiers, balometers and magneto-resistors, but are not limited to those devices.

The compounds of the invention can be made in the form of a bulk single crystal, a dense polycrystalline disc, a crystalline expitaxial thin film or a polycrystalline thin film. In the manufacture of thin films, some form of laser ablation is preferred, but the compounds can also be made by techniques such as sputtering, MOCVD, MBE, evaporation, and so on. The compounds of the present invention can be used in bulk form to serve as targets for the deposition of thin films by such methods as sputtering or laser ablation. The thin films may serve as superconductors in single and multilayer structures. As single crystals the compounds of this invention can be used as bulk substrates. Melt textured materials can be used for flux trapping, magnets, energy storage devices and so on.

The following examples illustrate uses of the La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x compounds of this invention.

An antenna can be made according to the invention by depositing a single layer of high critical temperature superconductor (HTSC) directly onto a suitable single crystal substrate. The HTSC is then patterned to complete the device.

A superconductor insulator superconductor step edge Josephson junction, which is a multilayer superconducting device, is fabricated according to the invention using the superconductor and appropriate metals or dielectrics. More particularly, the device is made by depositing a single layer of HTSC on a single crystal substrate or a substrate of other composition buffered with a layer of appropriate dielectric. Next, the HTSC is patterned by ion milling at a 45.degree. angle. A HTSC layer is then deposited. Finally, another HTSC layer is deposited and patterned to complete the device.

We wish it to be understood that we do not desire to be limited to the exact details of construction shown and described for obvious modifications will occur to a person skilled in the art.

Claims

1. Superconductive compounds of the general formula La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x wherein:

Me is selected from the group consisting of Y, Yb and Na;

Nc is selected from the group consisting of Mg and Cd;

z is 0 to about 2; and

v is 0 to about 0.5, with the proviso that at least one of z and v is greater than 0.

2. Compounds as recited in claim 1, further comprising:

said Me being Y;

said compounds having metallic behavior above the superconducting transition; and

said compounds retaining oxygen when heated in the range of 450.degree. to 650.degree. C.

3. A compound as recited in claim 2 having the formula La.sub.2 YBa.sub.3 CaCu.sub.7 O.sub.16+x.

4. A compound as recited in claim 3 in bulk form.

5. A compound as recited in claim 3 in single crystal form.

6. A compound as recited in claim 3 in thin film form.

7. A compound as recited in claim 2 having the formula LaY.sub.2 Ba.sub.3 CaCu.sub.7 O.sub.16+x.

8. A compound as recited in claim 7 in bulk form.

9. A compound as recited in claim 7 in single crystal form.

10. A compound as recited in claim 7 in thin film form.

11. Compounds as recited in claim 2 in bulk form.

12. Compounds as recited in claim 2 in single crystal form.

13. Compounds as recited in claim 2 in thin film form.

14. Compounds as recited in claim 1, further comprising:

said Me being Yb;

said compounds having metallic behavior above the superconducting transition; and

said compounds retaining oxygen when heated in the range of 450.degree. to 650.degree. C.

15. A compound as recited in claim 14 having the formula La.sub.2 YbBa.sub.3 CaCu.sub.7 O.sub.16+x.

16. A compound as recited in claim 15 in bulk form.

17. A compound as recited in claim 15 in single crystal form.

18. A compound as recited in claim 15 in thin film form.

19. Compounds as recited in claim 14 in bulk form.

20. Compounds as recited in claim 14 in single crystal form.

21. Compounds as recited in claim 14 in thin film form.

22. Compounds as recited in claim 1, further comprising:

said Me being Na;

said compounds having metallic behavior above the superconducting transition; and

said compounds retaining oxygen when heated in the range of 450.degree. to 650.degree. C.

23. A compound as recited in claim 22 having the formula La.sub.2.5 Na.sub.0.5 Ba.sub.3 CaCu.sub.7 O.sub.16+x.

24. A compound as recited in claim 23 in bulk form.

25. A compound as recited in claim 23 in single crystal form.

26. A compound as recited in claim 23 in thin film form.

27. Compounds as recited in claim 22 in bulk form.

28. Compounds as recited in claim 22 in single crystal form.

29. Compounds as recited in claim 22 in thin film form.

30. Compounds as recited in claim 1 in bulk form.

31. Compounds as recited in claim 1 in single crystal form.

32. Compounds as recited in claim 1 in thin film form.

33. Superconductive compounds of the general formula La.sub.3-z Me.sub.z Ba.sub.3 Ca.sub.1-v Nc.sub.v Cu.sub.7 O.sub.16+x wherein:

Me is selected from the group consisting of Y, Yb and Na;

Nc is a 2+ion selected from the group consisting of Mg and Cd;

z is 0 to about 2; and

v is 0 to about 0.5, with the proviso that at least one of z and v is greater than 0.

34. Compounds as recited in claim 33, further comprising:

said Nc being Cd;

said compounds having metallic behavior above the superconducting transition; and

said compounds retaining oxygen when heated in the range of 450.degree. to 650.degree. C.

35. A compound as recited in claim 34 having the formula La.sub.3 Ba.sub.3 Ca.sub.0.5 Cd.sub.0.5 Cu.sub.7 O.sub.16+x.

36. A compound as recited in claim 35 in bulk form.

37. A compound as recited in claim 35 in single crystal form.

38. A compound as recited in claim 35 in thin film form.

39. Compounds as recited in claim 34 in bulk form.

40. Compounds as recited in claim 34 in single crystal form.

41. Compounds as recited in claim 34 in thin film form.

42. Compounds as recited in claim 33 in bulk form.

43. Compounds as recited in claim 33 in single crystal form.

44. Compounds as recited in claim 33 in thin film form.