JPH013052A - superconductor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] This invention relates to a novel superconductor, and particularly to an oxide ceramic superconductor.

[Prior Art] Conventionally, there are metal-based, ceramic-based, and organic-based materials as materials exhibiting superconductivity, and among these, ceramic-based materials have been attracting attention in recent years.

As such ceramic-based superconducting materials, those having a layered perovskite type (K2NiF4 type) structure are known. For example, [LaS r] 2 Cu
Oxide ceramic-based superconducting materials such as Cub4 or Cub4 exhibit a critical temperature of 30°C or higher.

[Problems to be Solved by the Invention] An object of the present invention is to provide an oxide ceramic superconductor that can exhibit a higher critical temperature than that obtained with conventional layered perovskite superconducting materials. It is.

[Means for Solving the Problems] The present invention provides an oxide ceramic comprising a sintered body containing at least one element selected from the Ua group elements of the periodic table, at least one element selected from the Ma group elements, copper, and oxygen. In the system superconductor, the surfaces and interfaces of crystal grains are composed of components exhibiting a superconducting critical temperature higher than the interior of the crystal grains, and the first two high critical temperature components form a three-dimensional network structure. It is characterized by:

Note that the above-mentioned "surface of crystal grains" refers not only to the surface of the sintered body itself, but also to the large number of pores that exist inside the sintered body (regardless of whether or not they communicate with the outside). This is a concept that also includes surfaces that are in contact with. Furthermore, the term "crystal grain interface" refers to the boundary between a crystal grain having one crystal orientation and a crystal grain having another crystal orientation.

[Operations and Effects of the Invention] In the oxide ceramic superconductor made of the sintered body according to the present invention, the high critical temperature component exists in a three-dimensional network structure. It is possible to incorporate magnetic flux lines (fluxoids), and vortex threads [Vor
tex], it can have a very high critical magnetic field (HC2).

In addition, since the three-dimensional network structure can be regarded as an aggregate structure of thin or thin filament-like or sheet-like materials, it has a large number of superconducting-normal conducting interfaces, and the critical temperature (Tc ) is also called “
higher than the "bulk" state.

For example, P., W. Anders theoretically proposed the mechanism of ceramic oxide superconductors.

According to the Anderson model by Dr. N., it is thought that the lower the dimensionality of a material, the easier the attraction between electrons due to the magnetic interaction between electron spins will be, and the easier it will be to generate electron pairs. Based on this theory, the three-dimensional network structure that is the structure of the high critical temperature component according to the present invention is
Since it is filament-like or sheet-like and has low dimensionality, it is presumed that the critical temperature is high from this point of view as well.

FIG. 1 is a model diagram showing crystal grains existing in a superconductor according to the present invention.

Referring to FIG. 1, a superconductor 1 includes a large number of crystal grains 2,
3. ...exist in contact with each other. These crystal grains 2, 3. ... includes a surface 4 forming the surface of the sintered body itself and a surface 6 in contact with the pores 5 existing inside the sintered body. Further, if crystal grains 2 and 3 have different crystal orientations, an interface 7 is formed at the boundary between them.

According to this invention, as explained based on FIG. 1, the surfaces 4, 6 and interface 7 of the crystal grains 2.3 are composed of a component exhibiting a higher superconducting critical temperature than the interiors 8, 9 of the crystal grains 2, 3. This high critical temperature component forms a three-dimensional network structure.

Incidentally, the high critical temperature component forming the three-dimensional network structure mentioned above actually flows from the surfaces 4 and 6 of the crystal grains 2.3 and the interface 7 to a predetermined point as shown by the dotted line in FIG. Distributed in areas up to depth.

The above-mentioned group na element of the periodic table is Be.

Examples include Mg, Ca, Sr, Ba, and Ra. In addition, as group ma elements of the periodic table, Sc, Y, La, Ac
, Ce, Pr, Nd, Pm, Sm, Eu.

Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu
can be mentioned. Preferably, the Ila group element is barium or strontium, and the Ila group element is lanthanum,
Yttrium or scandium.

Note that some of the group IIa elements are replaced by elements that can have a valence of +2, and some of the group I[[a elements are replaced by elements that can have a valence of +3.

Furthermore, in order to obtain a sintered body constituting the superconductor according to the present invention, for example, when a mixed powder containing barium, yttrium, and copper is fired in air, the surface of the sintered body has a metallic luster. It is black, and it is 0.0 mm from the surface. 1~0. It appeared to a depth of about 5mm. It is presumed that this surface area with a black metallic luster was produced during sintering under a specific oxygen partial pressure, and the material that makes up the surface area of the sintered body has a temperature that exceeds the critical temperature of superconductivity. It was also found that it is more important to increase As can be seen from the above, it is presumed that oxygen present during firing plays an important role.

Furthermore, the firing temperature for obtaining the sintered body constituting the superconductor according to the present invention is selected to be lower than the melting temperature or decomposition temperature of the material to be fired, whichever is lower, and higher than the temperature at which sintering is possible. I also know that this is preferable. Note that the "melting temperature" refers to the temperature at which even a slight liquid phase occurs during firing. Furthermore, the term "decomposition temperature" refers to the temperature at which at least a portion of the material is separated into elemental or component units and a crystalline structure cannot be formed. It should be noted that the vertical relationship between "melting temperature" and "decomposition temperature" varies depending on material components, composition ratios, etc., and it is not possible to uniformly determine which one is higher. Regarding the calcination temperature, it is particularly preferable to select a temperature just below the melting temperature or the decomposition temperature, whichever is lower.

For example, it has been found that by selecting the calcination temperature between the lower of the melting temperature or the decomposition temperature and a temperature 30° C. lower than the melting temperature or the decomposition temperature, a larger portion of the material having a higher critical temperature can be obtained.

However, in addition, the superconducting properties and other electromagnetic properties of the sintered body according to the present invention, thermal properties such as specific heat, specific gravity, physicochemical properties such as X-ray structural analysis, and mechanical properties such as Vickers surface hardness, etc. As a result of deep consideration, the following findings were obtained.

For example, as mentioned above, materials with excellent superconducting properties exist in large numbers at a depth of about 0.5 mm from the surface, but this is more essentially due to the fact that crystal grains are exposed to oxygen in the atmosphere. It was found that this is a substance that is generated on the surface of crystal grains when sintered in an atmosphere that contains it. In addition, since the superconductor according to the present invention is a sintered body, there are minute pores inside, so substances with excellent superconducting properties have been confirmed even in depths deeper than 0.15 mm, although in small amounts. Ta. For this reason, it is thought that since fresh oxygen is always supplied to the surface of the sintered body at a predetermined partial pressure, more substances with excellent superconducting properties are generated on the surface.

In this way, the surfaces and interfaces of crystal grains are important factors in determining electrical resistance, and in the end, superconducting current is generated by the crystal grains, which are composed of components that exhibit a higher superconducting critical temperature than the interior of the crystal grains. It has become clear that the water flows through a three-dimensional network structure consisting of surfaces and interfaces.

[Explanation of Examples] As an example of the present invention, fine powder containing Ba5Y and Cu oxides was solidified at 1000 atmospheres, and then heated to 90% in the atmosphere.
Baked at 40°C for 24 hours, finally B a Cuo, 7
A sample was obtained which was adjusted so that Y(,,30,).

When we investigated the change in electrical resistance of this sample due to temperature, we found that three inflection points appeared in the electrical resistance change, the first inflection point was near 100, and the second inflection point was around 20.
The third inflection point was near 300. Then, it reached a completely superconducting state around 82.

Of the three inflection points in the electrical resistance characteristics mentioned above, the third inflection point that appears at the highest temperature is the high critical temperature that forms the surfaces and interfaces of crystal grains, which are mostly distributed on the surface of the sintered body. It is estimated that the components contribute.

As mentioned above, it was confirmed as follows that the materials constituting the surfaces and interfaces of crystal grains have an extremely high critical temperature of around 300℃, which becomes superconducting at room temperature. .

That is, when the bulk sample obtained as described above was ground to a depth of 0.5 mm from the surface,
Although the results of the X-ray structural analysis remained almost unchanged, the amount of the substance that contributed to the extremely high critical temperature mentioned above had decreased to such an extent that it was no longer discernible.

This means that the substance that gives the highest critical temperature is mainly the substance that is formed up to a depth of 0.5 mm from the surface of the sintered body, and that new oxygen is constantly supplied to the surfaces and interfaces of the crystal grains formed. This suggests that it was a constituent substance.

Furthermore, measurement of magnetic susceptibility also confirms that the substance that provides the highest critical temperature is formed in a larger amount on the surface of the sintered body. In materials that exhibit uniform superconductivity throughout the bulk, such as ordinary superconductors, there is a point where the electrical resistance begins to decrease, the so-called "Tc on set."
Therefore, the Meissner effect should be recognized, but in the sample obtained as described above, the Meissner effect began to appear after the electrical resistance became completely zero.

This suggests that the substance that gives the highest critical temperature is more distributed on the surface of the sintered body.

Furthermore, when the critical magnetic field (HC2) of this sample was measured, it was confirmed that it had an extremely high value exceeding 100T.

[Brief explanation of the drawing]

FIG. 1 is a model diagram showing crystal grains existing in a superconductor according to the present invention. In the figure, 1 is a superconductor, 2.3 is a crystal grain, 4.6 is a surface, 5 is a hole, 7 is an interface, and 8 and 9 are the interior.

Claims (1)

[Claims] (1) Oxide ceramics consisting of a sintered body containing at least one element selected from group IIa elements of the periodic table, at least one element selected from group IIIa elements, copper and oxygen, and containing a large number of crystal grains. system superconductor, the surfaces and interfaces of the crystal grains are composed of a component exhibiting a superconducting critical temperature higher than the interior of the crystal grains, and the high critical temperature component forms a three-dimensional network structure. A superconductor characterized by: