JP3041529B2 - Bi-based oxide superconducting thin film - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a Bi-based superconducting thin film having superconducting properties and a high critical current density, and particularly to a physical method such as a sputtering method and a vapor deposition method. Bi-Pb-Sr-Ca produced
The present invention relates to a Bi-based oxide superconducting thin film based on -Cu-O and Bi-Sr-Ca-Cu-O.

[Conventional technology]

It is widely known that Bi-based superconductors are excellent materials having a critical temperature of 100K or more. In order to apply this material to electronic materials and devices, thinning is performed in various places. However, this material has three types of polymorphs, 110K class, 80K class, and semiconductor phase, and it is difficult to form a single phase due to the narrow temperature range in which a 110K class superconductor is formed.

Conventionally, Bi-Pb-
In Sr-Ca-Cu-O-based superconductor, the composition before heat treatment
Bi: Pb: Sr: Ca: Cu = 1: 1: 1: 1: 1.5 or a thin film containing excess Ca was heat-treated at about 850 ° C. (for example, Japanese Journa
l of Applied Physics, 28 (1989), L818-822).

[Problems to be solved by the invention]

When a thin film is synthesized by the above method, a 110K class superconducting thin film having a single phase in X-ray diffraction, good orientation, and a critical temperature exceeding 100K can be obtained.

However, even if the orientation of the resulting plate-like 110K-class superconducting particles is good, the amorphous phase generated during heat treatment remains between the superconducting particles after heat treatment and acts as an insulating phase. However, there is a problem that a superconducting current flowing through the liquid crystal is interrupted and a current density due to superconductivity at a liquid nitrogen temperature which is practically important (hereinafter, this is treated as a critical current density) is reduced.

[Means for solving the problem]

The inventors of the present invention heat-treated the manufactured thin film under various conditions, and examined the relationship between the orientation of the obtained thin film and the critical current density. The inventors have found that the present invention greatly affects the density, and have completed the present invention.

That is, the present invention is a thin film of an oxide superconductor having the following composition manufactured by a physical method such as a sputtering method or a vapor deposition method, and the C axis of the superconducting particles is defined as a C axis in a direction perpendicular to the substrate. A Bi-based oxide superconducting thin film having a degree of orientation in the range of 1 to 10 °.

Bi _a Pb _b Sr _1.00 Ca _c Cu _d O _x where 0.5 ≦ a ≦ 1.20 ≦ b ≦ 1.2 0.4 ≦ c ≦ 1.0 1.3 ≦ d ≦ 2.0 Here, the degree of orientation is the rocking measured by the X-ray diffraction method. Means the half width of the curve.

Rocking curve: This curve is used to evaluate the crystallinity of thin films and epitaxial films with a certain degree of orientation.
The diffraction intensity obtained by fixing the detector at an angle (2θ) corresponding to a certain crystal plane interval and changing the angle (θ) of the sample is shown. When the thin film is gradually made up of small crystal pieces having different orientations, a width corresponding to the spread of the crystal orientation is generated on the diffraction peak. A smaller width (half-width of the rocking curve) means that the crystal axes are aligned, and a smaller width indicates higher orientation.

[Action]

The effect of the degree of C-axis orientation of the superconducting thin film obtained by the heat treatment on the critical current density is not necessarily clear, but the thin film manufactured by a physical method such as a sputtering method or a vapor deposition method has a predetermined thickness. Is heat-treated at the temperature of and crystallized. At this time, the half width of the rocking curve of the superconductor, that is, the degree of orientation of the C axis is smaller than 1 °,
It is presumed that when the particles are stacked with good orientation, the superconducting particles become independent, the amorphous phase generated during the heat treatment is interposed between the particles, and the bonding between the particles is rather inhibited.

Figure 1 is a scanning electron microscope (SEM) photograph showing the particle structure of a cross section of a Bi-based oxide superconducting thin film at a high current density at the temperature of liquid nitrogen. Many joints between them are observed. Figure 2 shows the grain structure of a cross section of a Bi-based oxide superconducting thin film of the same composition at the same liquid nitrogen temperature and low current density. The C-axis orientation is 0.5 °, the grains are independent, and the bonding between grains is achieved. Less is.

When the degree of C-axis orientation is larger than 10 °, the orientation is remarkably reduced, and as a result, the coupling between the superconducting particles is deteriorated, and the critical current density is reduced.

Further, regarding the manufacturing method for keeping the C-axis orientation of the obtained superconducting thin film within the above range, the thin film composition, particularly the Ca / Sr atomic ratio, the sintering temperature, the sintering time, and the like are considered to influence each other.

(Bi-based oxide superconductor) In the present invention, the Bi-based oxide superconductor constituting the Bi-based oxide superconducting thin film is a Bi-Pb-Sr-Ca-Cu-O-based and Bi-Sr-Ca
-Cu-O-based superconductor whose composition is Bi _a Pb _b Sr _1.00 Ca
_c Cu _d O _x , where 0.5 ≦ a ≦ 1.20 ≦ b ≦ 1.2 0.4 ≦ c ≦ 1.0 1.3 ≦ d ≦ 2.0.

If Bi is less than 0.5, it is difficult to synthesize a superconductor,
If it exceeds 1.2, an amorphous phase which adversely affects the superconductivity is easily generated, and the degree of C-axis orientation is smaller than 1 °. Pb too
If it exceeds 1.2, the degree of C-axis orientation will be smaller than 1 °.

Ca is preferably less than Sr, but if it is less than 0.4, a semiconductor is easily formed, the generation of a 110K-class superconductor phase is small, and the critical current density is also reduced. 110K when Ca exceeds 1.0
Class superconductor phase is formed, but Ca compound precipitates between particles and lowers the critical current density.

If Cu is less than 1.3, a 110K phase is hardly generated, and if it exceeds 2.0, the degree of C-axis orientation is smaller than 1 °.

As the Bi-based oxide superconductor of the present invention, a part of the above composition is replaced with another element, for example, a part of Sr is Ba and a part of Ca is Y.
And those in which a part of Cu is replaced by Cd, respectively.

(Production of Thin Film) The production of a thin film in the present invention is performed by a physical method such as a sputtering method and a vapor deposition method.

In the case of the sputtering method, the number of targets does not matter, but when the composition contains Pb, a plurality of targets are used,
It is preferable to use a mixture of Bi and Pb as one of the targets, and the combination of elements other than Bi and Pb in other targets and the number of targets are free.

As a raw material of the sputtering target, an oxide,
Inorganic compounds such as nitrates, sulfates, and carbonates or metals are used.

Substrates used include oxide single crystals such as MgO, SrTiO ₃ , LaGaO ₃ and LaAlO ₃ which do not react with elements in the thin film during heat treatment, polycrystalline metals such as Ag, Au, Pt, Cu, Si, GaAs, etc. Or a combination thereof.

In the production of the thin film, the substrate may be heated, but in that case, the substrate temperature is preferably 700 ° C. or less. When the substrate is heated to a temperature exceeding 700 ° C. to deposit a film, crystallization with a high C-axis orientation occurs at the same time as the deposition, but an amorphous phase remains between the particles, and the resistance curve of this film has a tail. , The critical temperature tends to decrease, and the current density at the temperature of liquid nitrogen becomes almost zero.

To form a thin film on a substrate by sputtering using a plurality of targets, a target using Ar, a gas such as O ₂ by sputtering, the film of the composition comprising Bi on the substrate, the Pb, other constituent elements Are sequentially laminated while controlling the deposition time and the RF power so that the target film composition is obtained.

It is preferable that the thickness of the film when the layers are sequentially laminated in this way and each composition is cycled is 100 mm or less, and preferably about 50 mm. When the thickness is more than 100 mm, the film is easily melted, and it is difficult to produce a 110K-class superconductor.

The thickness of the thin film is manufactured according to the purpose of use, but is preferably about 0.1 to 10 μm. When the thickness of the thin film is 0.1 μm or less, the bond between the superconducting particles becomes weak after heat treatment irrespective of the degree of orientation, and when the thickness is 10 μm or more, the orientation of the film is remarkably reduced, so that the critical current density becomes small.

(Heat Treatment of Thin Film) In the present invention, the heat treatment of the thin film produced as described above is preferably performed at a temperature of 5 to 40 ° C., particularly preferably 10 to 20 ° C. lower than the synthesis temperature of the 110 K class Bi-based superconductor. .

Here, the synthesis temperature is a temperature at which the growth rate of the 110K phase is maximum in a Bi-based oxide superconductor having a desired composition, and Pb
Is about 850 ° C. in the case of containing Pb, and about 870 ° C. in the system not containing Pb.

Although the heat treatment time depends on the temperature, at least 10 hours or more are necessary because the temperature is lower than the synthesis temperature. Also,
Since a heat treatment that is too long results in a decrease in critical temperature and critical current density, a preferable heat treatment time is 50 to 80 hours at a temperature 10 to 20 ° C. lower than the synthesis temperature.

For the above heat treatment, 2-10
It is effective to perform the primary heat treatment for a long time, and the characteristics of the obtained superconducting thin film are stabilized.

 After the heat treatment, it is gradually cooled in the furnace.

Hereinafter, the two-step heat treatment of the Bi-Pb-Sr-Ca-Cu-O-based superconducting thin film will be described.

Primary heat treatment In the primary heat treatment, the temperature is about 50 to 150 ° C lower than the optimum heat treatment temperature for generating the Bi-Pb-Sr-Ca-Cu-O-based superconductor, preferably, about 50 to 100 ° C lower for about 1 hour or more. Hold. As a specific temperature, about 700-800 ° C
It is preferable to heat and hold for 10 hours. Although the atmosphere for the heat treatment is sufficient in the air, preferably, the heat treatment is performed in an atmosphere in which PbO vapor is present, so that volatilization of PbO during the heat treatment can be prevented.

Secondary heat treatment In the second heat treatment, the temperature is maintained at 5 to 40C, preferably 10 to 20C lower than the generation temperature of the Bi-Pb-Sr-Ca-Cu-O-based superconductor, and the heat treatment is performed for 15 hours or more. If the heat treatment is performed for a long time, the film quality is deteriorated and the characteristics are lowered. Therefore, it is preferable that the heat treatment temperature does not exceed 100 hours depending on the heat treatment temperature. The atmosphere during the heat treatment may be the same as in the case of the primary heat treatment, either in air or
Performed in the presence of PbO vapor. The rate of temperature rise from the primary heat treatment to the secondary heat treatment may be fast or slow without any particular problem. After the primary heat treatment, the secondary heat treatment may be performed after cooling to room temperature.

 After the completion of the second heat treatment, the mixture is slowly cooled in the furnace.

〔Example〕

Examples 1-4 Sputtering was performed with the following three types of targets and deposition times.

As a first target, a mixture of Bi ₂ O ₃ and PbO powder mixed at an atomic ratio of Bi: Pb = 1: 1 and mixed in methanol for 24 hours was used. The unit deposition time was 10 seconds.

As a second target, a powder of CaCO ₃ and CuO was blended in an atomic ratio of Ca: Cu = 1: 0.75, and the powder mixed in the same manner as in the above method was baked and crushed in air at 950 ° C. for 10 hours. Was used. The unit deposition time was 50 seconds.

As a third target, a powder of SrCO ₃ and CuO was blended in an atomic ratio of Sr: Cu = 1: 0.75, and the powder mixed in the same manner as above was fired at 950 ° C. for 10 hours in air. And pulverized. The unit deposition time was 40 seconds.

On a MgO single crystal substrate heated to the substrate temperature in Table 1,
Using the above three types of targets, the RF power was set to 100 W, and sputtering was performed with Ar gas. When the deposition of each target completed one cycle, this was repeated 400 times to obtain a thin film of about 2 μm.

The result of analyzing the composition of the obtained thin film by EPMA,
(Bi + Pb) _1.27 Sr _1.00 Ca _0.76 Cu _1.37 O _x .

After these were subjected to primary heat treatment at 780 ° C. for 2 hours, they were each subjected to secondary heat treatment at 835 ° C. for the time shown in Table 1 to obtain samples having various degrees of C-axis orientation.

For the degree of orientation of the thin film, a rocking curve of a (0014) diffraction line (2θ CuKα = 33.4 °) of a 110K-class superconductor was used.

The critical current density was measured in liquid nitrogen by the four probe method.

In addition, the proportion of the 110K superconductor phase in the formed superconductor (110K
The phase ratio was determined from the measurement results of the (002) diffraction line intensity of the 110K-class superconductor phase and the 80K-class superconductor phase in the thin film using an X-ray diffractometer.

 Table 1 shows the results.

Comparative Example 1 The same procedure as in Example 1 was performed except that the secondary firing was performed at 835 ° C. for 2 hours, and the superconducting properties of the obtained fired thin film were measured.

 The results are shown in Table 1.

Comparative Example 2 The substrate temperature was heated to 750 ° C., and sputtering was performed in the same manner as in Example 1 to produce a laminated thin film.

 Table 1 shows the superconducting properties of the obtained thin films.

Examples 5-8, the first target in Example 1 into a powder of only Bi ₂ O _3, the second and third targets using the same,
Sputtering was performed in the same manner as in Example 1 except that the unit deposition time was set to 6 seconds for the first target, 50 seconds for the second target, and 40 seconds for the third target, to obtain a thin film having a thickness of about 2 μm.

The result of EPMA analysis of the composition of the obtained thin film was Bi
_0.80 Sr _1.00 Ca _0.74 Cu _1.55 O _x .

After these were subjected to primary heat treatment at 800 ° C. for 2 hours, each was subjected to secondary heat treatment at 856 ° C. for the time shown in Table 2 to obtain samples having various degrees of C-axis orientation.

With respect to the obtained thin film, its critical temperature, current density in liquid nitrogen, and the ratio of the 110K class superconductor phase in the formed superconductor were measured.

 Table 2 shows the results.

Comparative Example 3 The procedure of Example 1 was repeated except that the secondary firing was performed at 856 ° C. for 1 hour, and the superconducting properties of the obtained fired thin film were measured.

 The results are shown in Table 2.

Comparative Example 4 A substrate was heated to 800 ° C., and sputtering was performed in the same manner as in Example 1 to produce a laminated thin film. Table 2 shows the superconducting properties of the obtained thin film.

[Effect of the Invention] As is clear from the results of the above examples, the heat-treated Bi
When the degree of C-axis orientation (half-width of the rocking curve) determined by the X-ray diffraction method is in the range of 1 to 10 °, the current density at the temperature of liquid nitrogen is several thousand A / It is an excellent superconducting thin film exhibiting superconductivity of not less than cm ² . Therefore, when producing a Bi-based superconducting thin film, by selecting the composition, heat treatment conditions, and the like, and keeping the degree of C-axis orientation of the thin film obtained by firing within the above range, several thousand A / cm ² at liquid nitrogen temperature. A superconducting thin film having the above current density can be obtained.

Before measuring the superconducting properties of the superconducting thin film, a primary evaluation is performed by measuring the degree of C-axis orientation of the thin film to select those having a high superconducting effect.

[Brief description of the drawings]

Figure 1 shows a scanning electron microscope (SE) showing the cross-sectional particle structure of a Bi-based oxide superconducting thin film with a high current density at the temperature of liquid nitrogen.
M) It is a photograph. FIG. 2 is a scanning electron microscope (SEM) photograph showing the particle structure of a cross section of a Bi-based oxide superconducting thin film of the same composition having a low current density.

Continuation of the front page (56) References JP-A-2-55299 (JP, A) JP-A-2-55228 (JP, A) JP-A-3-268406 (JP, A) JP-A-2-153581 (JP) , A) JP-A-2-188426 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C01G 1/00

Claims (1)

(57) [Claims] 1. A thin film of an oxide superconductor having the following composition manufactured by a physical method such as a sputtering method or a vapor deposition method, wherein a C axis of a superconducting particle is defined as a C axis in a direction perpendicular to a substrate. Characterized in that the degree of orientation is in the range of 1 to 10 °
Bi-based oxide superconducting thin film. Bi _a Pb _b Sr _1.00 Ca _c Cu _d O _x where 0.5 ≦ a ≦ 1.20 ≦ b ≦ 1.2 0.4 ≦ c ≦ 1.0 1.3 ≦ d ≦ 2.0