PROCESS FOR THE PRODUCTION OF HIGH TEMPERATURE SUPERCONDUCTOR WIRES

Abstract

The present invention is in the field of processes for the production of high temperature super-conductor wires. In particular, the present invention relates to a process for the production of high temperature superconductor wires comprising heating a film comprising yttrium or a rare earth metal, an alkaline earth metal, and a transition metal to a temperature of at least 700 C. and cooling the film to a temperature below 300 C., wherein the heating and cooling is per-formed at least twice.

Claims

1. A process for producing of high temperature superconductor wires, the process comprising heating a film comprising yttrium or a rare earth metal, an alkaline earth metal, and a transition metal to a temperature of at least 700 C. and cooling the film to a temperature below 300 C., wherein the heating and cooling is performed at least twice and a partial pressure of water during a first heating to a temperature of at least 700 C., denoted P.sub.1,w, a total pressure during the first heating to the temperature of at least 700 C., denoted P.sub.1, a partial pressure of water during a second heating to a temperature of at least 700 C., denoted P.sub.2,w, and a total pressure during the second heating to the temperature of at least 700 C., denoted P.sub.2, fulfill the relationship: $\frac{\sqrt{P_{1,W}}}{P_1}<\frac{\sqrt{P_{2,W}}}{P_2}$ when P.sub.1,w, P.sub.1, P.sub.2,w and P.sub.2 are expressed in mbar.

2. The process according to claim 1, wherein the film is heated to a temperature of at least 700 C. and cooled to a temperature below 300 C. such that seed crystals of the high temperature superconductor develop in the film during the first heating to the temperature of at least 700 C. while most of the film constitutes different phases.

3. The process according to claim 1, wherein: the first heating to the temperature of at least 700 C. includes at least one temperature plateau and the second heating to the temperature of at least 700 C. includes at least one temperature plateau; and a time-average temperature of a longest temperature plateau during the first heating to the temperature of at least 700 C., denoted T.sub.I, is lower than a time-average temperature of a longest temperature plateau during the second heating to the temperature of at least 700 C., denoted T.sub.II.

4. The process according to claim 1, wherein: the first heating to the temperature of at least 700 C. includes at least two temperature plateaus with temperatures of at least 700 C.; and a temperature of a first temperature plateau, denoted T.sub.I,1, is higher than a temperature of a second temperature plateau, denoted T.sub.I,2.

5. The process according to claim 1, wherein the film is passed through a furnace at a speed of 1 to 300 m/h.

6. The process according to claim 1, wherein the heating is performed in a furnace in which only one particular pressure and one particular composition of the atmosphere are chosen.

7. The process according to claim 1, wherein the film is consecutively passed through different furnaces.

8. The process according to claim 1, wherein the film comprises yttrium, barium and copper.

9. The process according to claim 1, wherein a molar ratio of the transition metal and the yttrium or rare earth metal in the film is 3:1.0 to 3:1.5.

10. The process according to claim 1, wherein a molar ratio of the transition metal and the earth alkaline metal in the film is 3:1.5 to 3:2.0.

11. The process according to claim 1, wherein the film is on a substrate comprising Ni and 1-10 at-% tungsten.

12. The process according to claim 11, wherein between the substrate and the film there is a buffer layer comprising lanthanum zirconate, cerium oxide, or both.

Description

DESCRIPTION OF THE FIGURES

[0050] FIG. 1: Schematic temperature profile in which each heating to at least 700 C. contains one temperature plateau.

[0051] FIG. 2: Schematic temperature profile in which the first heating to at least 700 C. contains two temperature plateaus.

[0052] FIG. 3: X-ray diffraction pattern of the film of example 1 after passing through the first furnace. Diffraction peaks corresponding to different phases are indicated by different symbols, whereby YBCO stands for YBa.sub.2Cu.sub.3O.sub.7-x. The large peak at 233-33.5 corresponds to a superposition of the (400)La.sub.2Zr.sub.2O.sub.7 (LZO) and (200)CeO.sub.2 buffer layer diffraction peaks.

[0053] FIG. 4: X-ray diffraction peaks of the film of example 2 after passing through the first furnace (line with dots, denoted by 1) and second furnace (line without dots, denoted by 2), respectively. The film passed through the second furnace was subsequently oxidized in pristine oxygen at temperatures below 500 C. Diffraction peaks corresponding to YBa.sub.2Cu.sub.3O.sub.7-x are indicated by their (00L) values.

EXAMPLES

[0054] The examples show two-pass processes, applied to CSD precursors with cation stoichiometry Y.sub.1.3Ba.sub.1.8Cu.sub.3. Layers with a final thickness of about 1 m were produced by the deposition of two successive 500 nm coatings. The solutions contained trifluoroacetate salts of at least one of the cationic constituents. Each coating was pyrolyzed to a maximum temperature of 400 C. according to standardized procedures. The total thickness after pyrolysis of the second layer was about 2.5-3 m. Substrates consisted of a tape of textured Ni containing 5 at % W with a width of 1 cm coated with La.sub.2Zr.sub.2O.sub.7 and CeO.sub.2 buffer layers made by chemical solution deposition. This sample was passed through a first furnace by a reel-to-reel system and subsequently passed through a second furnace again by a reel-to-reel system. The tape motion and the gas flow were in opposite directions in both furnaces. The sample cooled to room temperature between the two heat treatments when being collected on the take-up reel. The tape speeds described in the examples below are based on a heating length of 1.5-2 m for both furnaces. Higher tape speeds can be achieved for example by using longer furnaces.

Example 1

[0055] The first furnace had a temperature of 750 C. In this furnace a mixture of water vapor, nitrogen and oxygen was flown over the sample at a total pressure of 10 mbar. The water flow rate was 40 g/h (which corresponds to a water partial pressure of 3.3 mbar) and the nitrogen flow rate was 100 I/h. The oxygen partial pressure was 0.2 mbar. The sample was moved through the furnace at a speed of 20 m/h corresponding to a high-temperature residence time of 5.5 min in the first furnace. The residence time includes a slower heating ramp from 500 to 700 C. during the initial heating, but excludes the fast part of the heating ramp to 500 C. and the cooling ramp. FIG. 3 shows an XRD spectrum of the sample after exiting from the first furnace.

[0056] The second furnace had a temperature of 780 C. A mixture of water vapor and oxygen was flown over the sample at a total pressure in the furnace of 1.5 mbar. The water flow rate was 100 g/h (corresponding to a water partial pressure of 1.3 mbar) and the oxygen flow rate was 20 l/h (corresponding to an oxygen partial pressure of 0.2 mbar). The sample was moved at 20 m/h resulting in a high-temperature residence time of 4.5 min in the second furnace (heating and cooling ramps not included).

[0057] Not taking into account YBCO seed crystals or nuclei formed during the first pass, a maximum growth rate of 3.7 nm/s may be estimated from the 4.5 min residence time of the second heat treatment and the target YBCO film thickness of 1 m. A critical current I.sub.c value of 176 A was measured at 77 K over the full 1-cm tape width for a 1 m long sample with a continuous Hall sensor technique (Tapestar).

Example 2

[0058] The first furnace had two zones with different temperatures. The zone through which the sample passed first had a temperature T.sub.1 of 775 C., the second zone had a temperature T.sub.2 of 740 C. The residence time of the sample in the first zone was about 0.3 min and in the second zone 1.5 min. In the first furnace a mixture of water vapor, nitrogen and oxygen was flown over the sample at a total pressure of 10 mbar. The water flow rate was 160 g/h and the nitrogen flow rate was 100 I/h. The water partial pressure was 6.5 mbar, the oxygen partial pressure was 0.25 mbar. The sample was moved through the furnace at a speed of 20 m/h corresponding to a residence time of 5.5 min in the first furnace.

[0059] Passing through the second furnace was the same as described in example 1 for the second furnace.

[0060] A critical current of 161 A was measured inductively for this sample over the full 1-cm tape width. Similar I.sub.c values were obtained from transport measurements on comparable samples. In FIG. 4 XRD spectra of the sample after exiting from the first furnace (curve 1dotted) and after exiting from the second furnace (curve 2) are depicted. The XRD data of FIG. 4 indicate that the large majority of the YBCO is formed during the second pass.