CA1339720C

CA1339720C - High temperature processing of cuprate oxide superconducting

Info

Publication number: CA1339720C
Application number: CA000592182A
Authority: CA
Inventors: Maw-Kuen Wu
Original assignee: University of Alabama in Huntsville
Current assignee: University of Alabama in Huntsville
Priority date: 1988-02-26
Filing date: 1989-02-27
Publication date: 1998-03-17
Anticipated expiration: 2015-03-17
Also published as: WO1989008331A1; AU3430389A

Abstract

A high temperature treatment process whereby superconducting phases and materials can be obtained based on a treatment by high temperature of a non-superconducting phase of a cupric oxide compound. The high temperature processing provides an alternate synthetic route in the search for a new high Tc superconductors and a new high Tc copper oxide material is formed with non-rare earth elements Bi-Sr-Cu-O.
Similarly a nominal composition YSrCuO4-y is high temperature processed to exhibit superconducting transitions at temperatures previously unattainable with low temperature heat treatment methods.

Description

TITLE OF THE INVENTION

HIGH TEMPERATURE PROCESSING OF
CUPRATE OXIDE SUPERCONDUCTORS
BACKGROUND OF THE INVENTION
Field of the Invention:
The present invention is addressed to a process which brings about improved superconductor characteristics .
Discussion of Background:
Since the discovery of the first 90K multiphase Y-Ba-Cu-O compound as disclosed in copending application serial number 014,359 filed February 13, 1987 and in the article by Wu et al., Phys. Rev. Lett. 58, 908 (1987), there has been much activity in the area of cuprate oxide superconductors. The substitution or attempted substitution of copper ions by transition metal ions and the replacement of the Ba ions with smaller alkaline earth metal ions such as Sr, Ca or Mg has produced mixed results. Many of these attempts have resulted in either a lowered transition temperature Tc or the complete disappearance of superconductivity. Even in areas where substitutions have been made which provide adequate Tc, or an improved Tc, the bulk conductivity has been severely depressed or the critical current density has been too low to be as useful or any more useful than the original 90K multiphase Y-Ba-Cu-O compound.

Although some advances have been made in the critical current density of the 123 phase of the superconducting YlBa2Cu307 phase as indicated by the article of Jin et al in the Applied Physics letters 51, 943 (1987), there exists up to this time no reliable system for improving the transition temperature Tc of a given compound by correlating its high temperature treatment during the formation of the superconducting material.
It is to these issues that the present invention is specifically addressed.

SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a process for the production of superconductors which provides improved characteristics and which in some materials provides superconductivity where none had previously existed when said materials were prepared with traditional or low temperature treatments.
It is an object of the present invention to provide fabrication of superconducting films wherein the transition width and the transition temperature Tc are improved.
It is another object of the present invention to provide a processing technique for fabricating materials which provides an alternate synthetic search tool for new high Tc superconductors.

More specifically, it is an object of the present invention to provide a high temperature heat processing to a sintered semiconducting material or low temperature superconducting material in order to convert said material into a superconducting material or raise the transition temperature of the superconducting material.
It is another object of the present invention to provide a high temperature processing whereby the superconducting phase of a superconductor is a more stable phase than the other phases of the compound at these higher temperatures.
It is another object of the invention to provide a heat treatment at a high temperature whereby non-rare earth ions can be used to form a new phase of a copper oxide compound which exhibits high temperature superconductivity.

BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
FIGURE 1 provides a resistance curve of a sintered Y2BaCuO5 disc after being fired at 1300~C both with and without ~2 annealing; and 133~72n -FIGURE 2 indicates a resistance curve of the sample with a nominal composition of BiSrCuO4_y;
~ IGURE 3a is a diagram of the electrical resistance of a sample composition YSrCuO4_y heat treated according to the present invention;
FIGURE 3b is a diagram illustrating the current/voltage characteristics at 4.2~K.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The first 90K multiphase Y-Ba-Cu-O compound was composed of the green Y2BaCuO5 phase known as the "211" phase and the superconducting YlBa2Cu3O7 phase (the "123" phase).
Samples which reacted at a highertemperature than 950~C
but with a comparatively shorter firing time have been observed to give sharper Tc transitions. It has been found that in order to obtain a single phase 123 compound, an extremely careful heat procedure was required. These results led to the observation that the presence of the 211 phase is thermodynamically favorable to the formation of the superconductor 123 phase and the processing at high temperature stabilizes some otherwise unstable phases. The high temperature processing converts the 211 phase into the 123 phase and allows for the synthesizing of new cuprate oxide superconductors involving only non-rare earth elements.

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The processes and the superconductors formed therefrom involve compounds which were prepared in the following manner. These compounds were used for the high temperature processing. These compounds were prepared by using appropriate amounts of metal oxide which were mixed, pressed into pellets, heated at 950~C
for 12 hours and then quenched to room temperature.
The annealing procedures which followed depend on the particular study and are described in the appropriate areas below. Electrical resistivity is measured using a conventional 4-probe technique. An AC inductance method is used to measure the magnetic susceptibility and a standard 4-probe which uses pulse current determines the critical current density at zero field. The structural and phased determinations were made by X-ray diffraction and Raman microprobe (MOLE
U1000) analysis.
High Temperature Processing:
A starting material of a sintered 211 phase of the cupric oxide semiconductor 1/2 BuCu05 (green phase) was fired at 1300~C for 15 minutes. Part of the green phase was converted into the black 123 YlBa2Cu307 phase. The fired sample had a small resistivity at room temperature and behaves like a semiconductor as indicated in FIGURE la. The sample became superconducting after it was oxygen-annealed at 950~
for several hours and furnace-cooled as indicated in FIGURE lb. The results from the Raman test indicate that the black region is the 123 phase and using the same processing method, other rare earth 123 phases were obtained from the corresponding 211 phases. It is to be noted that this method is only restricted by the rare earth elements (e.g., Nd, Pr and Ce) which do not form 211 phases and therefore cannot be used with this process, i.e., they will not be converted to the 123 phase if there is no 211 phase.
The thermodynamics of the Y-Ba-Cu-O system at 950~C
and most particularly the equilibrium phase diagram is well established (Advanced Ceramic Material, 2, 295 (1987) K.G. Frase et al; Advanced Ceramic Material, 2, 303 (1987) R.S. Roth et al; and Advanced Ceramic Material, 2, 313 (1987) G. Wang et al). However, the thermodynamics of this system at temperatures higher than 950 have not been thoroughly investigated previously and the results of the above process shows that the 123 phase is a more stable phase than the 211 phase at higher temperatures. This technique provides for the fabrication of granular thin films because in these types of films there is no stringent requirement for homogeneity. Furthermore the previously discussed article by Jin provides the interesting piece of the puzzle that the critical current density of the 123 compound can be raised to approximately 7000A/cm2 by using high temperature processing techniques. Thus the ' ~3 _7_ 1 3 39 720 resultant high temperature fabrication provides a conversion from a 211 phase to a 123 phase as well as a stable 123 phase.
Superconductivity in Bi-Sr-Cu-O;
The Bi-Sr-Cu-O compound, when fabricated under certain conditions, exhibits an anomaly indicative of a superconductivity with an onset temperature of approximately 60~K. However, an equilibrium phase of this system has a Tc of only 20~K. Thus, the above disclosed high-temperature processing can be used to re-examine this Bi-Sr-Cu-O system. A sample with a nominal composition of Bi-Sr-Cu-O 4_y was prepared with appropriate amounts of Bi2O3, SrO, and CuO being mixed and pressed into a pellet and heated to 800~-850~C for 12 hours. The sample was then quenched, reground, and annealed in an oxygen environment for 2 hours at 1200~C
followed by subsequent furnace cooling. Then the sample was re-annealed for 5 to 6 hours at 850~C. The samples melted during the high temperature heat treatment, however, inside of the melt there were needle-like crystals. The electrical measurement of these crystals yielded the results shown in Figure 2.
This Figure 2 shows an onset of superconductivity at approximately 70~K. The AC magnetic susceptibility showed a small diamagnetic signal at approximately 60~K, which confirmed a superconducting transition.
However, this transition was not bulk in the sense that the superconductive portion was estimated to be less than 5 percent of the bulk material. The X-ray defraction patterns of these materials were different from those of the 214 phase (the La-Ba-Cu-O system) or the 123 phase, indicating the existence of a new phase. Thus, the high-temperature treatment provided a raising of the TC from 20~K to 60~K.
Superconductivity in Y-Sr-Cu-O
Based upon the conversion of the semiconducting 211 phase of Y2BaCuO5 to the superconducting 123 phase YBa2Cu3O7 and the synthesizing of the new cuprate oxide semiconductor BiSrCuOy which involves only non-rare earth metals, a generalized use of a high temperature process to convert a semiconductor to a superconductor is illustrated by the processing of yttrium strontium copper oxide samples.
A sample with a nominal composition YSrCuO4_y was prepared by mixing appropriate amounts of Y2O3, SrO and CuO. The mixture was ground and pressed into pellets, heated to 1300~C for 2 hours and then quenched to room temperature. The material was then reground, pressed reheated to 1200~C for 6 hours in ~2 and then slowly cooled to room temperature. Samples were cut into lxlx3mm3 bars for resistivity and magnetic moment measurements. The magnetic moment measurements were made with a SQUID magnetometer at the national magnetic laboratory at MIT. Structural and phase determinations g were provided by X-ray diffraction and the previously mentioned Raman microprobe analysis.
The electrical resistance R of the sample as a function of temperature is shown in Figure 3a. The current used was lmA. Superconductivity transition is illustrated as being very sharp with an onset at 92~K
and a 0 resistance at 85~K. A linear resistance temperature dependency, before the onset of superconductivity, was observed. This behavior is similar to that of the 123 phase YBa2Cu307 except that the initial resistance was about 2 orders of magnitude larger. The current voltage curve of the sample at 4.2 K exhibits characteristics of a superconductor as illustrated in Figure 3b. The critical current density was estimated to be 145 A/Cm2 which indicates that the sample is not a single phase.
The sample after the heat treatment is quite different from the sample tested by Mei et al as reported in the "Proceedings of International Workshop on Novel Mechanisms of High Temperature Superconductors, ed. v. K Reshin, page 1041 (1987) which indicated superconductivity at 40~K in a sample with a novel composition of Y0 3SrO 7Cu03_y which was processed at a temperature of 900~C. The superconductivity transition reported by Mei et al was broad and had a width of approximately 20 K. X-ray diffraction results show that its structure is similar to that of La2_x(Ba, Sr)xCuO4_y (the "214 phase").

-lO- 1339720 The utilization of high temperature processing as a tool is generically illustrated by the synthesizing of the new cuprate oxide semiconductor involving only non-rare earth elements, i.e., BiSrCuOy and the processing of yttrium strontium copper oxide samples which provided a high Tc semiconductivity at 80~K and a anamolous magnetic moment at 14K in the Y-Sr-Cu-O
system. This provides a clear example of a synthetic alternate route in the search for new high Tc superconductors. The formation of the 123 phase using high temperature processing of the 211 phase in the Y-Ba-Cu-O compound system provides the specific evidence of conversion of a semiconductor (211 phase) to a superconductor 123 phase and serves as the basis for a theoretical consideration of phase conversion occurring upon high temperature processing of either low temperature superconductors or semiconductors to superconductors at a higher temperature, i.e., above 77~K. The mechanism shows that the 123 phase or perhaps any superconducting phase of any superconducting or semiconducting material is a more stable phase than the 211 phase, or whatever is a semiconducting phase or (non-superconducting phase), at higher temperatures. The technique lends itself to fabrication of granular thin films because the requirement of the homogeneity of the film is not as stringent.

-ll- 1339720 Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may practice otherwise than as specifically described herein.

Claims

1. A process for converting the semiconductor phase of a cuprous oxide system into a superconducting phase, comprising the steps of:
preparing amounts of oxides of metals of each metal present in said cuprous oxide system, including cuprous oxide, in order to form at least a semiconducting phase of cuprous oxide material;
annealing said at least semiconductor phase in an oxygen environment at a temperature substantially equal to its melting temperature for a period of time between 15 minutes and 2 hours to form said superconducting phase;
with the proviso that said cuprous oxide system does not comprise rare earth elements.

2. The process according to Claim 1 wherein at least a semiconducting phase of said cuprous oxide material is the 211 phase Y2BaCuO5.

3. The process according to Claim 1 wherein at least a semiconducting phase of said cuprous oxide material is BiSrCuO4-y, wherein y is a number between 0 and 4, and wherein said temperature is 1200°C and said time is 2 hours.

4. The process according to Claim 1 wherein at least a semiconducting phase of said cuprous oxide material is YSrCuO4-y prepared by mixing appropriate amounts of Y2O3, SrO and CuO wherein said temperature is 1300°C and said time is 2 hours and wherein said process further includes the step of grinding and reheating said material subsequent to said step of annealing wherein said reheating is at is 1200°C for 6 hours in oxygen.

5. The process according to Claim 1 further including the step of grinding said material and reheating said material subsequent to the step of annealing.

6. The process according to Claim 5 wherein said step of reheating is from 6 hours at 850°C.

7. The process according to Claim 2 wherein said superconducting phase is the 123 phase YlBa2Cu3O1.