JPH02116615A - Oxide superconducting material - Google Patents

Abstract

PURPOSE:To increase the critical current density of a Bi-Sr-Ca-Cu oxide superconducting material by incorporating boron into the oxide superconducting material. CONSTITUTION:Boron is incorporated into a Bi-Sr-Ca-Cu oxide superconducting material to obtain an oxide superconducting material represented by a formula BxBiaSrbCacCudOe (where x:a=(0.005-7):1, especially (0.005-3.5):1 and a:b:c:d=1:1:1:2).

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to improvements in B1-3r-Ca-Cu based oxide superconducting materials.

<Prior Art> Bismuth-based layered compounds constitute various related compounds including B14TisO+t.

Conventionally, these bismuth-based layered compounds have been considered mainly as dielectrics. However, in recent years, bismuth-based copper oxides, specifically BL-3r-Ca-Cu-based oxides, have a layered crystal structure similar to that of conventional bismuth-based layered compounds and exhibit electrical conductivity. It has been reported that the material has a superconducting phase exhibiting a critical temperature above (Ibid. et al., KinR5B Vol. 2, p. 2, 1988).

Generally, superconductors become superconducting below a critical temperature,
Exhibits zero electrical resistance and perfect diamagnetism.

Furthermore, when a current higher than the critical current density is passed through a superconductor in a superconducting state, the superconducting state is destroyed and the superconductor becomes normal conductive. High critical temperatures and critical current densities are required to obtain practical superconductors.

<Problems to be Solved by the Invention> Generally, the above B1-3r-Ca-Cu-based oxide superconducting material has a phase having a critical temperature of about 100 to 110K,
Phases with critical temperatures of the order of 70-80 and 10-2
It is composed of phases with a critical temperature of about 0, and is not a single phase.

Moreover, since the volume fraction of the phase exhibiting a critical temperature of about 100 to 110 degrees Celsius is small, even if a critical temperature of about 110K is achieved, the critical current density at that temperature is small. Further, even when used at 70 to 80K, a high critical current density cannot be obtained due to the presence of a phase exhibiting a critical temperature of about 10 to 20K. Furthermore, the B1-3r-Ca-Cu-based oxide superconducting material is difficult to form a dense microstructure, which is also a cause of not being able to obtain a high critical current density.

Based on these facts, even in superconducting materials such as BL-3r-Ca-Cu oxides, which have multiple phases with different critical temperatures, each phase has a dense and continuous structure, and a particularly high critical temperature It is considered that a sufficient critical current density can be obtained if the volume fraction of the phase having temperature is large.

To solve these problems of B1-3r-Ca-Cu-based oxide superconducting materials, after adding PB and sintering, 89
By annealing at about 0℃ for 10 days, the critical temperature 11
It has been reported that the volume fraction of the phase can be increased to about 0 [M, Takano, Japan
se Journal of Applied Phys.
sics, 27 L1041. (1988)).

However, since Pb easily evaporates, it not only tends to contaminate the furnace wall during firing, but also causes loss of uniformity inside the sintered body.

The present invention was made under these circumstances, and an object of the present invention is to provide a B1-3r-Ca-Cu-based oxide superconducting material that exhibits a high critical current density.

Means for Solving the Problems> The above object is achieved by the present invention as described in (1) and (2) below.

(1) An oxide superconducting material containing bismuth, strontium, calcium, and copper, which is characterized by containing boron.

(2) The oxide superconducting material according to (1) above, wherein the boron content is boron:bismuth=0.005:1r-7:1 in atomic ratio.

For use> The oxide superconducting material of the present invention is a B1-3r-Ca containing bismuth, strontium, calcium and copper.
- Constructed by adding boron to a Cu-based oxide superconducting material.

In the oxide superconducting material of the present invention, boron exists as insulating strontium borate, which is considered to form a composite structure with the B1-5r-Ca-Cu-based oxide having superconductivity.

In this composite structure, strontium borate, which forms spherical crystals, is thought to tightly bond the plate-shaped crystals of the B1-5r-Ca-Cu-based oxide.

In addition, the B1-5r-Ca-Cu-based oxide in this composite structure can be made into a superconducting material composed of almost a single phase at a critical temperature of 80°C by selecting the firing temperature. By firing at a lower temperature, a superconducting material composed of two or more phases with different critical temperatures can be obtained. Moreover, when it is composed of two or more phases, the volume fraction of the phases at a critical temperature of about 110 can be increased.

Specific composition> Hereinafter, a specific configuration of the present invention will be explained in detail.

The oxide superconducting material of the present invention contains bismuth, strontium, calcium, and copper, and in addition to these, further contains boron.

The preferred composition (atomic ratio) of the oxide superconducting material of the present invention is within the range expressed by the following formula.

[Formula] %Formula% In the above formula, x:a=o, 005 to 7:1, especially 0
．． The ratio is preferably 0.005 to 3.5:1. If the B content is less than the above range, the effect of the present invention will be low, and if it exceeds the above range, the second phase will have a large proportion and will not clearly exhibit superconducting properties.

Moreover, the preferable ranges of a, b, c and d are as follows.

0.5≦a≦2.5 0.7≦b≦1.3 0.7≦C≦1.3 0.3≦d≦2.5 If a, b, c, and d are within the above ranges , the oxide represented by the above formula has superconductivity. In addition, in order to obtain good superconducting properties, in the above formula,
It is preferable that a:b:c:d=1+l:1:2.

In the above formula, x=O1, that is, an oxide superconducting material that does not contain B is, for example, "metal.

58, No. 2, p. 2, (1988), supra.

The composition of the oxide superconducting material of the present invention as described above is E
It can be measured using PMA or the like.

Note that the oxygen content ratio e in the above formula is usually about 4 to 5, but it may be outside this range as long as the superconducting properties are not impaired.

Such an oxide superconducting material of the present invention has a layered crystal structure, and is observed to be composed of an aggregate of plate-shaped crystals under a scanning electron microscope or the like.

In the oxide superconducting material of the present invention, boron is present as strontium borate.

Strontium borate has a stoichiometric composition of Srs Bz
Although the composition is Oa, in the present invention, a composition deviating from this composition may be used.

Note that the presence of strontium borate can be confirmed by X-ray diffraction.

In the oxide superconducting material of the present invention, strontium borate forms a complex with a B1-5r-Ca-Cu based oxide containing bismuth, strontium, calcium and copper.

Specifically, this complex consists of a bismuth-based oxide in the form of plate-like crystals and strontium borate in the form of nearly spherical crystals.

Such a composite structure can be confirmed by scanning electron microscopy, X-ray diffraction, etc.

Because it has the above-mentioned composite structure, B1-3r-Ca
The -Cu-based oxide has a dense and continuous structure, and as a result, the critical current density is improved.

In this case, the denseness can be seen from the fact that the shrinkage rate during firing is greater than that of a normal bismuth-based oxide superconductor.

The B1-3r-Ca-Cu-based oxide can be composed of almost a single phase by selecting the firing conditions as described below, or it can be composed of multiple phases having different critical temperatures. You can also.

When composed of multiple phases, each phase has a dense and continuous structure, and the volume fraction of the phase at a critical temperature of about 110 ℃ is significantly improved compared to the case without boron. A high critical current density is achieved when using as critical temperature.

The oxide superconducting material of the present invention can be manufactured according to known ceramic manufacturing processes. In that case, the effects of the present invention are realized.

Specifically, the oxide superconducting material of the present invention is preferably manufactured, for example, as follows.

As raw materials for bismuth, strontium, calcium, copper, and boron, oxides or carbonates thereof are preferably used.

Specifically, it is preferable to use oxides for bismuth and copper, and carbonates for calcium and strontium. For example, bismuth dioxide, strontium carbonate, calcium carbonate, and cupric oxide are preferable.

Further, as the raw material for boron, it is preferable to use boron oxide.

These raw material compounds are ground and mixed by a known means such as a ball mill so that bismuth, strontium, calcium, copper and boron have the composition ratio of the present invention.
Incidentally, it is preferable that the grinding and mixing be carried out in a wet manner.

The obtained mixture is pulverized using a ball mill or the like, and further finely pulverized to an average particle size of about 2 to 3 μm to obtain a raw material powder.

The obtained raw material powder is calcined at 700 to 780° C. for 5 to 25 hours in air or in an atmosphere with a controlled oxygen atmosphere. The obtained calcined powder is pulverized to an average particle size of 1 to 2 μm using a grinder, a ball mill, etc., mixed with a binder if necessary, and then molded into a desired shape. There are no particular restrictions on the binder used, and any known organic binder such as polyvinyl alcohol may be used.

The obtained molded body is fired.

The firing atmosphere is preferably air or an atmosphere with a controlled oxygen atmosphere.

Moreover, it is preferable that the holding temperature during firing is 840 to 900°C. Although it varies depending on conditions such as composition, by setting the firing temperature to 860°C or more and 900°C or less, the B1-3r-Ca-Cu-based oxide can be heated to a critical temperature of about 80°C.
It can be almost a single phase. Further, by setting the firing temperature to 840° C. or higher and lower than 860° C., it is possible to configure the structure from a plurality of phases having different critical temperatures. And in this case, the volume fraction of the phase at a critical temperature of about 110 is
This can be significantly improved compared to the case where boron is not included.

The volume fraction of the 110 phase in this case can be 10% or more, particularly 15% or more.

The temperature holding time is preferably 5 to 50 hours.

The superconducting material of the present invention can be applied not only to the sintered body produced in this manner but also to thick or thin films.

The thick film may be formed by applying a paste prepared by mixing a raw material powder having the above composition with a binder, a solvent, etc. onto a substrate by a printing method, and firing the paste. Further, appropriate heat treatment may be performed as necessary.

In the case of forming a thin film, it is preferable to form the film by a vapor phase film forming method such as sputtering, and then heat-treat the film to impart superconductivity.

<Example> Hereinafter, the present invention will be explained in further detail by giving specific examples of the present invention.

Bismuth trioxide, strontium carbonate, calcium carbonate, and cupric oxide in an atomic ratio of Bi:Sr:Ca:Cu=1
:1:1:2. Also, B: Bi
=1.5: It was weighed so that it would be 1.

These were wet mixed using a porcelain ball mill and then coarsely ground. Next, fine pulverization was performed to obtain a raw material powder with an average particle size of 2 μm.

This raw material powder was calcined in air at 780°C for 10 hours. The calcined powder was pulverized to an average particle size of 2 μm using a ball mill, and using polyvinyl alcohol as a binder, a pellet-shaped molded body with a diameter of 10 mm and a thickness of 2 mm was obtained.

The obtained molded body was placed in a zirconia pod, covered with a lid, and fired in an electric furnace for 10 hours to obtain an oxide superconducting material sample of the present invention (sample No. 1). The firing temperature was 870°C.

In addition, the raw material composition was Bi:Sr:Ca:Cu=1:1:
Sample No. 12 was produced in the same manner as Sample No. 091, except that the ratio of B:Bi was 1:1.5, and the firing temperature was 860° C..

Furthermore, for comparison, Sample No. 11 and Sample No. 12 were prepared in the same manner as Sample No. 1 and Sample No. 2, except that B was not added.

A silver electrode was attached to each of the obtained samples, and the superconducting properties were evaluated using the DC four-terminal method.

FIG. 1a shows the temperature dependence of the resistivity of samples Nos. 1, 1 and 11, and FIG. 1b shows the temperature dependence of the resistivity of samples Nos. 2 and 12. In addition, Figure 1a and Figure 1
Figure b shows the results measured at a current density IA/cm''.

In addition, the critical current density of these samples at 40K was measured by a DC four-terminal method.

In addition, the structure of the cut surface of the sample was observed using a scanning electron microscope.

Samples No. 1 and 1 are shown in FIG. 2a and FIG. 2b, respectively.
Figures 3a and 3b show traces of scanning electron micrographs of sample No. 2 and sample No. 12 section, respectively. . Note that these are the results of observation at a magnification of 1000 times.

Furthermore, the X-ray diffraction pattern of the sample was analyzed.

Figures 4a and 4b show X-ray diffraction charts for each sample.

From FIG. 1a, it can be seen that the critical temperature Tc at which the resistivity of sample N091 becomes 0 is 73, and from the appearance of resistance change, it is found that it is composed of almost a single phase. The Tc of sample No. 11 is 40, and the resistivity in the normal conduction state is low, but a phase presumed to be an impurity phase precipitates, and it is not a single phase ratio like sample No. 1. I understand that.

Further, from FIG. 1b, the critical temperature Tc at which the licking resistance of sample No. 2 becomes O is 72, and a rapid decrease in resistance is observed near 110K. In this measurement, the current density was set as IA/cm'', but 0.5A/cm
When measured at a current density of ”, sample No. 2
The resistance value was zero at 10K.

The Tc of sample No. 12 is 45, and the resistance value in the normal conduction state is lower than that of sample No. 2, but l
It can be seen that there is almost no l0K phase.

Further, the volume fraction of the phase at the critical temperature 110 of Sample No. 2 and Sample No. 12 was measured by alternating current magnetic susceptibility. As a result, it is 15% for sample No. 2,
In sample No. 12, it was 2%.

In addition, the critical current density is 57A/ for sample No. 1.
cm", 42A/cm2 for sample No. 62. 2A/cm" for sample No. 11, -sample No. 12
2 A/cm'', and it can be seen that the sample of the present invention containing B has a critical current density that is 20 times or more higher than that of the comparative sample that does not contain B.

From FIGS. 2a and 3a, it can be seen that the sample of the present invention is composed of a composite structure of plate-like crystals and spherical crystals. In addition, as a result of composition analysis, the plate-like crystals are B1-
The spherical crystals were considered to be composed of strontium borate.

In contrast, the comparative samples shown in FIGS. 2b and 3b consist only of plate-shaped crystals.

Furthermore, in the X-ray diffraction charts shown in Figures 4a and 4b, the peak of strontium borate is B1-3.
It is observed clearly separated from the r-Ca-Cu-based oxide peak. In these figures, A is the (113) peak of strontium borate, and B is the (104) peak of strontium borate.

In addition, in the above example, B:Bi is 1.5=1 and 3
Although the effect of the present invention was confirmed for the oxide superconducting material of B:Bi=0.005 to 7:1, the present invention is not limited to these.
It was confirmed that within the range of , the critical current density can be improved without significantly impairing the superconducting properties.

The effects of the present invention are clear from the above examples.

<Effects of the Invention> According to the present invention, B1- having a dense continuous structure
A 3r-Ca-Cu based oxide superconducting material is realized and the critical current density is improved.

The oxide superconducting material of the present invention can be composed of almost a single phase by selecting firing conditions, and this also improves the critical current density.

Furthermore, by selecting different firing conditions, it is also possible to construct the film from a plurality of phases having different critical temperatures. In this case, each phase has a dense and continuous structure, and in particular, the volume fraction of the phase at a critical temperature of about 110 is significantly improved compared to the case without boron, so that temperature is used as the critical temperature. In this case, high critical current densities are achieved.

Further, the oxide superconducting material of the present invention can be manufactured by a normal ceramic firing process, and therefore has excellent productivity.

[Brief explanation of the drawing]

Resistance (xlO-3n) Figures 1a and 1b are graphs showing the temperature dependence of resistivity of oxide superconducting materials. FIGS. 2a, 2b, 3a, and 3b are photographs substituted for drawings showing the crystal structure, and are scanning electron micrographs of the oxide superconducting material. Figures 4a and 4b are X-ray diffraction charts of oxide superconducting materials. F I G, lb degree (to) ■ G. ■G.

Claims (2)

[Claims] (1) An oxide superconducting material containing bismuth, strontium, calcium, and copper, which is characterized by containing boron. (2) The oxide superconducting material according to claim 1, wherein the boron content is in an atomic ratio of boron:bismuth=0.005:1 to 7:1.