JPH0474716A - Bi-system superconductive material - Google Patents

Abstract

PURPOSE:To improve critical electric current density by adding Ir to a Bi-Sr-Ca- Cu-O system or a Bi-Pb-Sr-Ca-Cu-O system superconductive substance so as a ratio of Ir/Cu to be a specific weight ratio. CONSTITUTION:Fixed amounts of Bi2O3, SrCO3, CaCO3 and CuO are mixed and further Ir2O3 in an amount making a weight ratio of Ir/Cu to be 0.005-0.08, then crushed, thus blended. Resultant mixed powder is calcined in an atmosphere of Ar.O2 mixed gas and press-molded, then burned in an atmosphere of Ar.O2 mixed gas to obtain a target material. Next, said target is sputtered on a heated base plate of MgO single crystal by Ar.O2 mixed gas in 1.8-2.3X10<-7> Torr initial degree of vacuum to form a film. Thereafter, resultant film is heated in an atmosphere of Ar.O2 mixed gas and subjected to annealing treatment at a fixed temperature in an O2 atmosphere to afford a thin film of Bi-Sr-Ca-Cu- O system superconductive substance having about 3000-3500 Angstrom film thickness. A thin film of Bi-Pb-Sr-Ca-Cu-O system superconductive substance is obtained in the same manner.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is applicable to wiring, electromagnetic wave sensors, superconducting transistors,
It relates to superconducting materials used in current control elements, superconducting magnets, etc.

[Prior Art] In order to expand the application fields of superconductivity, it is essential that superconducting materials have a high critical temperature. Following the discovery of Y-based superconducting material whose critical temperature exceeded the liquid nitrogen temperature for the first time by C, W, and Chu et al. of Houston University, Bi, 4, with a critical temperature exceeding 100 K was discovered by Mae et al. It is believed that the superconductor market will expand rapidly as superconducting materials are discovered. This Bi-based superconducting material is composed of Bi-3 r-Ca-Cu-○, and there are several types of compounds thereof. Its main composition is Bi25r2Ca2C
u30X is raised. Furthermore, some of the constituent elements may be replaced with pb in order to achieve a single phase with a high critical temperature.

These materials are attracting a lot of attention, so please see the recent Physical Review Letter for details.
sya Japanese Journal of
It is described in most of Applied Physics.

[Problems to be Solved by the Invention] However, the critical current density of the superconducting material is as follows: (1) It is a material that tends to become polycrystalline, and a second phase that inhibits current flow is likely to precipitate at grain boundaries.

■Because the coherence length is short, it is easily affected by grain boundaries.

■Strong anisotropy. (It is necessary to orient the crystals.) It was very low due to the following reasons. The value is consistently obtained and is generally in the 10'A/cm2 range for thin films and in the lower 10'A/cm2 range for wire rods. Normally, the critical current density is 10'A/cm2 for thin film-based electronics applications. It is said that 105A/cm2 or more is required for applications in heavy electrical equipment mainly made of wire rods, but as shown above, there is still a two-digit difference in values.

A countermeasure to these problems is to make superconducting materials into single crystals, but single crystals are not only difficult to increase in diameter but also very expensive, so for practical use it is necessary to reduce the critical current density in polycrystals. It is necessary to raise it.

In addition, in superconducting transistors and Josephson devices, several n
Since it is necessary to form an extremely thin insulating layer with a thickness of 500 nm, the surface must be smooth, but at present it has irregularities of several nanometers.

The present invention is intended to solve these problems, and its purpose is to create a Bi film that has a high critical current density in a polycrystalline material that is low in production cost, and that has the surface morphology necessary for device fabrication in a thin film. The aim is to obtain superconducting materials.

[Means for solving the problem] Bi-Sr-Ca-Cu-0 system or Bi-Pb
The present invention is characterized in that Ir is added to the -Sr-Ca-Cu-0 based superconducting material, and the amount of Ir added is preferably within the range of 0°005 to 0.08 in terms of Ir/Cu ratio.

[Example code] The present invention will be described in detail below with reference to Examples.

First, create a target. Predetermined amounts of cupric oxide, bismuth trioxide, strontium carbonate, calcium carbonate, and iridium oxide are mixed and dispersed using a ball mill. Next, this powder was calcined for 15 hours at 800°C in an argon-oxygen mixed gas atmosphere, pressure-molded at 300 kg/cm2, and finally fired for 30 hours at 865°C in an argon-oxygen mixed gas atmosphere. Obtain a 3 mm inch thick target.

Next, a thin film is formed on the Mg0 (100) single crystal substrate by RF magnetron sputtering using the above target. The film forming conditions at this time are: initial degree of vacuum = 1-8 to 2.3*IC
17 Torr, gas used: argon (70%) oxygen (
30%) mixed gas, output 100-155W, gas pressure during sputtering: 1.5-3.2*10-” Torr, substrate temperature = 720°C, film formation rate = 4-8 nm/
min, film thickness is 3000 to 3500 people. In order to suppress the influence of variations in surface roughness on the critical current density, the substrate was not polished but had a cracked surface.

Next, the superconducting thin film is obtained by annealing at 845° C. in an argon-oxygen mixed gas atmosphere for 15 hours and at 500° C. in an oxygen atmosphere for 30 hours.

After the obtained superconducting thin film was patterned by ion beam etching, the critical current density was measured by a four-terminal method. The measurement atmosphere was He gas (He has good thermal conductivity and can uniformize the sample temperature, and is inert and can prevent sample deterioration.) The measurement temperature was 55℃, and a cryogenic refrigerator (manufactured by Daikin Industries) was used for cooling. ) was used. Comparative example (engineering) of the results
Table 1 shows cases in which no addition was made and cases in which the amount added was outside the appropriate range).

As can be seen from Table 1, the critical current density is significantly improved by adding Ir to the Bi-5r-Ca-Cu superconducting material, although it is still slightly insufficient for actual applications.

This is thought to be because the addition of Ir suppresses the precipitation of a second phase that inhibits current flow and because the thin film is made smooth.

The amount added is 0.005 to 0.0 in terms of Ir/Cu ratio.
It is preferably within the range of 8. If the amount added is small, it will not be effective.
If it is too large, the structure of the superconducting material will be destroyed, and the critical current density will decrease.

Furthermore, the improvement in smoothness, which is also useful in increasing the critical current density, also brings about good results in terms of device fabrication, as mentioned earlier.

In the embodiments, a thin film was described, but the same effect can be achieved with a wire rod or a bulk film, and there is no problem. However, since it is expensive, thin films that require less material are more suitable. Furthermore, a system in which pb is substituted to stabilize the high critical temperature phase has the same effect and there is no problem.

[Effects of the Invention] As described above, according to the present invention, it is possible to suppress the second phase that precipitates at the grain boundaries of crystals and inhibit current flow, and to smooth the material, so even polycrystalline materials with low production costs can have high criticality. Current density can be obtained. In addition, the surface morphology required for device fabrication also improves.

This material is used for wiring, electromagnetic wave sensors, magnetic flux memory, Josephson elements, superconducting transistors, magnetic shielding materials,
It can be applied to power transmission cables, communication cables, superconducting motors, superconducting magnets, etc.

that's all Applicant: Seiko Epson Corporation Representative Patent Attorney Kizobe Suzuki and 1 other person

Claims (1)

[Claims] 1) Bi-Sr-Ca-Cu-O system or Bi-Pb-
A Bi-based superconducting material characterized by adding Ir to a Sr--Ca--Cu--O-based superconducting material. 2) The Bi-based superconducting material according to claim 1, wherein Ir/Cu is within the range of 0.005 to 0.08.