Tl-Ca-Ba-Cu-O Compositions Electrically Superconducting Above 120 Degree K and Processes for Their Preparation

The present invention is concerned with compositions which are stable, bulk electrical superconductors with zero resistance above 120.degree. K., proven by magnetic susceptibility measurements.

The discovery of superconductivity above liquid nitrogen temperature (77.degree. K.) in Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y and related derivatives opens up the possibility for numerous applications in electrical and magnetic devices. The critical temperature for the transition from the normal metal state at high temperature to the superconducting state occurs in the range of 90.degree.-95.degree. K. Some of the properties and historical developments in this field are described by E. M. Engler in a review article (Chemtech, 17, 542 (1987)). The method of making these new superconductors which are a class of compounds known as perovskites is very important to being able to obtain the high temperature superconductivity (that is above 77.degree. K.), and superior properties (for example, sharp transitions to zero resistance, bulk superconducting behavior). An earlier patent application by E. M. Engler et al. describes a method for fabricating improved superconducting materials based on Y and rare earth compounds of the general composition M.sub.1 Ba.sub.2 Cu.sub.3 O.sub.z where M equals Y or an appropriate rare earth element (U.S. patent application Ser. No. 07/024,653 filed Mar. 11, 1987.)

Many reports in the open literature (see for example references 12-18 cited in the first reference above) claim much higher temperature superconducting transitions in Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y and its chemically modified derivatives and in some new compound variations. To our knowledge these results have not been confirmed with unambiguous and reproducible experimental characterization. Most reports are about resistance anomalies (that is, drops in electrical resistance) and not with zero electrical resistance which is needed for use of these materials in superconducting applications. Further, such observations are typically unstable where an initially observed electrical resistance anomaly disappears with time.

Press reports from Japan (H. Maeda from National Research Institute for Metals at Tsukuba, Jan. 21, 1988) and the U.S. (C. W. Chu cited in New York Times, Jan. 28, 1988, p. C2) claimed that the new oxide compounds of Bi-Sr-Ca-Cu of an undisclosed composition and processing procedure, show electrical resistance drops starting near 118.degree. K., but not going to zero until 70.degree.-80.degree. K. Subsequently, we and other research groups confirmed these reports and demonstrated that a superconducting transition for a minor dispersed phase was occurring around 118.degree. K., but that not enough of the phase was present to provide zero resistance (preprints by Parkin et al., Tarascon et al., Maeda et al., Torrance et al., Sunshine et al., Veblen et al., Hazen et al.) The major phase is the 70.degree.-80.degree. K. superconductor.

The Bi-Sr-Ca-Cu-O compound showed reproducible and bulk superconducting properties at about 80.degree. K. which were confirmed by many research groups. However, the 118.degree. K. phase remains a very minor fraction of the overall material and zero resistance had not been demonstrated. As with the 90.degree. K. Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y superconductors, Bi compounds require a specific processing procedure. In particular, the 118.degree. K. resistance anomaly is very sensitive to the specific annealing temperatures and duration of heating used in its preparation.

Recently, we became aware of a preprint article (Hazen, Finger et al., "100K Superconducting Phases in the Tl-Ca-Ba-Cu-O System") which describes the preparation of two Tl-Ba-Ca-Cu oxide compositions which had a superconducting transition of 107.degree. K., which unlike the Bi-compounds, dropped to zero resistance. We have repeated and confirmed these results. The processing conditions needed to make this thallium based superconductor have to be controlled even more carefully than for the earlier perovskite superconductors. Rapid heating for 5 minutes at 890.degree. C. was claimed by Hazen, Finger et al. as necessary to stabilize the 107.degree. K. superconductivity.

Stable, bulk electrical superconductors with zero resistance above 120.degree. K., confirmed with magnetic susceptibility measurements have now been prepared from the elements thallium, calcium, barium, copper and oxygen. Although these are the same elements in the compositions reported by Hazen, Finger et al., mentioned above, both the relative amounts of the elements and the process conditions are different. These differences are necessary to provide superconductivity above 120.degree. K. According to the present invention, the materials in the form of oxides are mixed together and heated in a preheated oven in a closed vessel in the presence of oxygen for from 1-5 hours at a temperature from 850.degree.-900.degree. C. The heating is followed by cooling to room temperature over a period of 1-5 hours. The metals are present at the beginning of the process in the ratio Tl.sub.0.75-1.25 Ca.sub.2-3 Ba.sub.0.75-1.25 Cu.sub.2-3.

In the case of barium, either barium oxide or barium peroxide can be employed as the starting material. Barium peroxide is preferred.

The preparation of the superconductor is carried out in a closed vessel. It is most preferred that the closed vessel be a sealed quartz vessel. The sample of the admixed metal oxides is held in a crucible, made for example from gold, silver, platinum, aluminum oxide or zirconium oxide and sealed inside the quartz vessel. Even when the reaction is carried out in a sealed vessel, approximately 20% of the thallium is lost due to volatilization and reaction with the quartz. There is some indication that perhaps this reaction with the quartz helps the reaction to go in the desired way. In any case, it is preferred that the closed vessel be a sealed quartz one.

It should be emphasized that when the compositions specified by Hazen, Finger et al., mentioned in the reference cited above are employed, zero resistance is not obtained above 107.degree. K. These authors also reported it was necessary to use prereacted BaCu.sub.3 O.sub.4 or Ba.sub.2 Cu.sub.3 O.sub.5 as starting materials in their process. The use of these materials is not necessary in the process of the present invention.

The exact starting admixture of elements employed is important in achieving bulk electrical superconductors above 120.degree. K. For example, when too much or too little thallium is used in the starting mixture, the desired results are not obtained. The other elements, barium, calcium, and copper also have optimum ranges of compositions necessary but not as critical as thallium, for providing the best superconducting results. The preferred compositions cover the range of relative atomic ratios in the starting admixture of Tl from 0.75 to 1.25; Ca from 2 to 3; Ba from 0.75 to 1.25; and Cu from 2 to 3. Some examples of starting admixtures, that after processing as described in this application, do not provide bulk superconductors with zero resistance above 120.degree. K. are: Tl.sub.2 Ca.sub.3 Ba.sub.1 Cu.sub.3, Tl.sub.1 Ca.sub.1 Ba.sub.1 Cu.sub.2, Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3, Tl.sub.2 Ca.sub.1 Ba.sub.2 Cu.sub.2. These examples are not meant to be inclusive, but only to illustrate the importance of the starting composition in stabilizing superconductivity above 120.degree. K.

The heating should be carried out in the presence of oxygen. Oxygen at a pressure of approximately one atmosphere is preferred. It should be emphasized that the final oxygen content of the compositions is very susceptible to the process conditions.

A preferred starting composition has the metal elements present in the ratio Tl.sub.1 Ca.sub.3 Ba.sub.1 Cu.sub.3. During processing, approximately 20% of the thallium is lost due to volatilization. The amount of oxygen present in this final composition is close to 50 atomic percent. This corresponds to a final elemental composition of Tl.sub.0.8 Ca.sub.3 Ba.sub.1 Cu.sub.3 O.sub.8+.delta. where .delta. is less than one. Small variations in .delta. are not important for the occurrence of bulk superconductivity above 120.degree. K.

The compositions of the final material are in the range Tl.sub.0.6-1.1 Ca.sub.2-3 Ba.sub.0.75-1.25 Cu.sub.2-3 O.sub.(5+.delta.)-(9+.delta.) where .delta. is less than one. The most preferred final composition is Tl.sub.0.8 Ca.sub.3 Ba.sub.1 Cu.sub.3 O.sub.8+.delta. where .delta. is less than one. Another preferred example is Tl.sub.0.8 Ca.sub.2 Ba.sub.1 Cu.sub.3 O.sub.7+.delta. where .delta. is less than one.

The compositions of the present invention exhibit bulk, electrical superconductivity. The materials are stable. The measurements indicate reproducibility of results. The materials are perovskite-like but they are not single phase; rather they are composites.

It is an essential aspect of the present application that the process for preparation is carefully followed. The oxides of the metals are admixed by ball milling, grinding or other mixing techniques, and sealed in a vessel, such as a quartz vessel, containing oxygen. They are placed in a preheated oven at a temperature between 850.degree.-900.degree. C. for 1-5 hours. The electrical measurements were carried out by the standard, low frequency, AC lock-in, four-probe technique. The materials showed a sharp drop of electrical resistance below the detection limit of 10.sup.-8 ohms at temperatures above 120.degree. K. Proof that this resistance drop is actually due to bulk superconductivity, rather than filamentary or interfacial superconductivity, is demonstrated by magnetic susceptibility measurements showing substantial diamagnetic shielding and Meissner signal with sharp onsets at temperatures above 120.degree. K.

Some specific starting compositions which are favored include: Tl.sub.0.75 Ca.sub.3 Ba.sub.1 Cu.sub.3 ; Tl.sub.1 Ca.sub.2.5 Ba.sub.1 Cu.sub.3 ; Tl.sub.1 Ca.sub.2.5 Ba.sub.1 Cu.sub.2.5 ; Tl.sub.1.25 Ca.sub.3 Ba.sub.1.25 Cu.sub.3 ; and Tl.sub.1 Ca.sub.2 Ba.sub.1 Cu.sub.2. The most preferred are Tl.sub.1 Ca.sub.3 Ba.sub.1 Cu.sub.3 and Tl.sub.1 Ca.sub.2 Ba.sub.1 Cu.sub.3 where bulk superconductivity was reproducibly obtained at 125.degree. K. which is, as far as we are aware, the highest ever obtained .