WO1991003059A1

WO1991003059A1 - PRODUCTION OF ReQ2Cu4Zx(1-2-4) SUPERCONDUCTOR IN BULK FORM

Info

Publication number: WO1991003059A1
Application number: PCT/US1990/003227
Authority: WO
Inventors: John B. Vander Sande; Toshihiro Kogure
Original assignee: Massachusetts Institute of Technology
Current assignee: Massachusetts Institute of Technology
Priority date: 1989-08-18
Filing date: 1990-06-07
Publication date: 1991-03-07
Anticipated expiration: 1992-02-18

Abstract

A method for fabrication of ReQ2Cu4Zx(1-2-4) superconductor in bulk form, where 'Re' is a rare earth metal, 'Q' is an alkaline earth element and 'Z' is a chalcogen element, is provided. According to this method, a metallic precursor having a chemical composition near ReQ2Cu4 is oxidized by exposing it to an oxidizing agent for 1 - 10 hours.

Description

PRODUCTION OF ReQ₂Cu₄Z_x (1-2-4) SUPERCONDUCTOR IN BULK FORM

Background of the Invention

This invention relates to the fabrication of ReQ₂Cu₄Z_x (1-2-4) superconductor in bulk form.

Superconductors are materials having zero resistance to the flow of current below a certain critical temperature, T₃. It is known that certain metal oxides including metallic constituents such as Y, Yb, La, Ba, Ca, Ce, Cu, Sr, Eu, and Lu exhibit superconductivity. In addition, a class of compounds containing thallium, Tl, or bismuth, Bi, as constituents also display superconductive behavior. Examples of superconductive compounds include YBa₂Cu₃0₇_--, La₂__xSr_xCu₄0₄__x, and YbBa₂Cu₄O_x. It is well established in the art that the properties of these new oxide superconductors depend strongly on their processing histories. As such, many efforts have been directed toward the development of processing techniques to optimize these desirable properties. Current production methods for the deposition of thin film oxide superconductors include sputtering and electron beam evaporation techniques .

The production of thin film YbBa₂Cu₄O_κ (1-2-4) phase has been reported (Marshall et al . Abstr. Mater. Res. Soc. Meeting (1988) 197) and a T_c of 81 K has been observed for this material. Summary of the Invention

The invention provides a method for synthesis of ReQ₂Cu₄Z_x (1-2-4) superconductor in bulk form where "Re" is a rare earth metal, "Z" is a chalcogen element and "Q" is an alkaline earth element including Ca, Sr, and Ba. Additionally, Ba may be replaced partially or completely by other alkaline earth elements including Sr and Ca.

According to the method of the invention, a metallic precursor containing the elemental constituents of the superconductor, having a composition near ReBa₂Cu₄, is oxidized at an elevated temperature, 500-950°C, by exposing it to an oxidizing agent for 1 - 10 hours. An "oxidizing agent" is a substance which reacts with the elements of the metallic precursor, causing them to acquire a more positive valence.

The method of the invention may be applied to metallic precursors including any of the rare earth metals, Ce, Y, La, Pr, Nd, Pm, S , Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu. The oxidizing agent may be any chalcogen element including 0, S, Se or Te. The oxidizing agent may also be air or dessicated air.

The metallic precursor may include a noble metal elemental constituent and may be prepared according to any rapid solidification technique. The metallic precursor may be prepared using melt spinning and may be in ribbon form.

Bulk ReQ₂Cu₄O_x (1-2-4) , synthesized according to the method of the invention, has a morphology characterized by plate-like grains and hence can support a higher critical current density, J_c, than ReQ₂Cu,0_;. (1-2-3) .

Brief Description of the Drawing

In the drawing: Fig. 1 are electron micrographs showing two areas exhibiting selected area diffraction (SAD) patterns characteristic of the YbBa₂Cu₄O_x (1-2-4) phase;

Fig. 2 is a high resolution electron microscope (HREM) image obtained using the same diffraction conditions shown in the "B" pattern of Fig. 1.

Fig. 3a is a schematic structural model of the (1-2-3) structure viewed from the [100] direction.

Fig. 3b is a schematic structural model of the (1-2-4) structure viewed from the [100] direction.

Example 1

An alloy having the composition YbBa₂Cu₃ and also containing 30 wt % silver was prepared for use as a metallic precursor. This YbBa₂Cu₃ metallic precursor was prepared in ribbon form using a copper wheel melt spinner provided with an inert argon atmosphere. The resulting, approximately 70% amorphous, ribbons were oxidized in a furnace using dessicated air as the oxidizing agent. Oxidized ribbons were typically 3-10mm long, l-2mm wide, and 20-50μm thick. X-ray powder pattern diffraction techniques were used to characterize the crystal phase assembly present in the oxidized specimens. These studies indicated the presence of YbBa₂Cu₃O_x (1-2-3), BaCu0₂, Yb₂BaCu0₅ (2-1-1) and Ag. The volume fraction of the 1-2-3 phase present depended upon the sample oxidation temperature. Its volume fraction was 90% in a sample oxidized at 900°C for three hours in air and about 70% for one oxidized for three hours in air at 800°C. Oxidation in air at temperatures in the range 850- 875°C is preferred to promote formation of the 1-2-4 phase, which appears primarily at high angle grain boundaries. Critical temperature measurements indicate that materials prepared according to this method have T_c in a range 88-93 K.

Polished cross sections of the ribbons were prepared for further electron microscopy investigations using ion milling techniques. Electron microscopy studies were performed using JEOL-200CX electron microschop (Cs = 2.9mm) operated at 200kV with a LaB₆ filament and a side entry double-tilting stage. Images obtained in these electron microscopy studies are shown as diffraction patterns in Figs. 1 and 2. Fig. 1 shows two areas "A" and "B" exhibiting selected area diffraction (SAD) patterns from 13.5A lattice fringe spaces. Arrows indicate reflections characterisitic of the (1-2-4) phase diffraction-pattern. The designation of a^* and b^* axes in Fig. 1 is not confirmed. Patterns expected for the (1-2-3) phase crystal structure are superimposed to aid in identification of the (1-2-4) phase. The reflections indicated by arrows are not allowed by the (1-2-3) structure; however, they are consistent with a (1-2-4) structure which results when a Cu-0 layer is inserted in the basal plane of every unit cell of the (1-2-3) structure (Figs. 3a and 3b) . Figs. 3a and 3b schematically illustrate the (1-2-3) and (1-2-4) phase crystalline structures. The bold lines represent unit cells of each structure and broken lines represent the unit cells of the (1-2-3) structure.

The crystalline strcuture illustrations of Figs. 3a and 3b are idealized and slight distortions of these structures with respect to bond angles, atomic positions, and atomic substitutions may appear in the actual crystalline lattice. The present invention of the (1-2-4) phase is therefore not restricted solely to embodiments with the exact, non-varying periodic structure illustrated in Fig. 3b and claimed herein.

The high resolution microscopy image (Fig. 2), obtained using the same diffraction conditions as used for the "B" area of Fig. 1 shows the crystal structure of a unit cell for the (1-2-4) phase near the center of the image. A square in the bottom left corner of the image corresponds to a unit cell for the (1-2-3) phase and is shown for comparison. The unit cell of the YbBa₂Cu₄O_x (1-2-4) phase, shown in Fig. 3b, has dimensions a = 3.81A, b = 3.87A and c = 27.1A, corresponding to the Ammm or Bmmm space groups .

Example 2 An alloy having the composition YbBa₂Cu₃ with 30 wt.% silver can be prepared for use as a metallic precursor by a rapid solidification processing technique such as melt spinning. This metallic precursor may be oxidized in sulfur gas at temperatures in the range 850-875°C for between 1 - 10 hours.

Critical temperature measurements may be used to characterize the superconducting properties of the resulting material. High resolution electron microscopy may be utilized to determine the crystal structure of this material.

What is claimed is:

Claims

1. A method for fabrication of ReQ₂Cu₄Z_x (1-2-4) superconductor comprising: providing a metallic precursor containing the elemental constituents of the superconductor with a composition near ReQ₂Cu₄; and oxidizing the metallic precursor at an elevated temperature in a range 500-950°C by exposing it to an oxidizing agent for 1 - 10 hours.

2. The method of claim 1 wherein the rare earth metal (Re) includes one or more rare earth metals selected from the grup consisting of: Ce, Y, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu.

3. The method of claim 1 wherein the oxidizing agent is a chalcogen element.

4. The method of claim 3 wherein the chalcogen element (Z) includes one or more chalcogen elements selected from the group consisting of: 0, S, Se or Te.

5. The method of claim 1 wherein the metallic precursor is prepared using rapid solidification techniques.

6. The method of claim 5 wherein the rapid solidification technique is melt spinning.

7. The method of claim 1 wherein the metallic percursor further includes noble metal elemental constituents.

8. The method of claim 7 wherein the noble metal includes one or more noble metals selected from the group consisting of Ag, Au or Pt.

9. The method of claim 1 wherein the metallic precursor has ribbon form.

10. The method of claim 1 wherein the oxidizing agent is air.

11. The method of claim 1 wherein the oxidizing agent is dessicated air.

12. The method of claim 1 wherein the ReQ₂Cu₄Z_x (1-2-4) superconductor is YbBa₂Cu₄O_x (1-2-4) .

13. A composition according to the following crystalline structure:

.* o^B' • Cu o ^"&

wherein Re is one or more of the rare earth ions Ce, La, Y, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu, Q is one or more alkaline earth ions Be, Mg, Ca, Sr or Ba, and Z is one or more of the chalcogen ions 0, S, Se, or Te.

14. The method of claim 1 wherein the alkaline earth metal includes one or more alkaline earth metals selected from the group consisting of Be, Mg, Ca, Sr, or Ba.