JPH04144919A - Superconducting ceramics - Google Patents

Abstract

PURPOSE:To obtain a superconducting ceramic having further improved critical temperature and stable superconducting characteristics by decreasing the content of impurity alkali metal and/or halogen element existing in a V-based superconducting ceramic having a specific composition below a specific level. CONSTITUTION:The subject superconducting ceramic is expressed by the general formula (A1-xBx)yVzOw (x is 0-10; V is 1.0-3.0; z is 1.0-3.0; w is 2.0-10.0) and contains <=0.2wt.% of alkali metal such as Li, Na and K, <=0.2wt.% of halogen element and <=0.2wt.% of alkali metal and halogen atom in total. In the above formula, A is one or more elements selected from Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ag, Cd, In, Sn, Hg, Tl, Pb and Bi and B is one or more elements selected from Be, Mg, Ca, Sr and Ba.

Description

[Detailed description of the invention] (a) Field of application of the invention The present invention relates to oxide ceramic superconducting materials.

(b) Conventional technology Conventional superconducting materials include elements such as mercury and lead, NbN,
Alloys such as Nbs Ge, Nb2Ga or Nbs (
A16. Metal materials consisting of ternary compounds such as l Geo,t ) have been used. However, the critical temperature Tc at which these materials exhibited superconductivity was up to 25K.

On the other hand, ceramic-based superconducting materials have attracted attention in recent years. This material was first reported by IBM's Zurich Research Institute as a Ba-La-Cu-0 ()-based oxide high-temperature superconductor. After that, La-M-Cu-0 system (
M is Sr, Ba, Ca, etc.), Ba-Ln-Cu-0
system (Ln is Y, etc.).

B1-3r-Ca-Cu-0 system, Tl-Ba-Ca-C
U-0 series 1 etc. have been confirmed so far.

What these ceramic-based superconducting materials have in common is that they all have a perovskite-like structure and contain copper.

However, in these ceramic superconducting materials containing copper, the critical temperature at which they exhibit superconductivity or Tl-Ba-Ca-
The highest degree was 125 in the Cu-0 system. Conventionally, it has been known that ceramic superconducting materials have a layered structure, and as the number of layers increases, the critical temperature at which superconductivity occurs increases. There have been many reports that even if the amount is increased beyond this level, the critical temperature that exhibits superconductivity will not rise. This means that
This is likely to suggest that the currently available critical temperature at which superconductivity is exhibited is the limit for ceramic superconducting materials containing copper. In addition, for cholera's oxide superconducting ceramic materials, only new combinations of materials have been pursued, and almost no materials have achieved stable superconducting properties. The starting raw material for conventional superconducting materials has a purity of 99%.
% was considered sufficient, and no consideration was given to impurities unintentionally introduced during synthesis.

(c) Purpose of the Invention The present invention aims to further improve the critical temperature exhibiting superconductivity, and also aims to exhibit stable superconducting characteristics.

To this end, we attempted to synthesize a ceramic superconducting material containing vanadium as an alternative to the conventional ceramic superconducting material containing copper. As a result, it has become clear that the critical temperature exhibiting superconductivity can be raised to 150 or higher.

(d) Structure of the Invention The superconducting ceramic of the present invention can be generally expressed by the following formula.

(A,,B,),V,O.

However, x=0 to 1. O,y=1. O~3.0. z=1.
0-3.0. w=2.0~10. OA is Y (yttrium), La (lanthanum).

Ce (cerium), Pr (praseodymium), Nd (neodymium), Pm (promethium), Sm (samarium),
Eu (europium), Gd (gadolinium), Tb (
Terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Yb
(ytterbium), Lu (lutetium), Ag (silver)
, Cd (cadmium), In (indium), Sn (tin), Hg (mercury), TI (thallium), Pb (
Consists of one or a combination of two or more elements selected from lead), Bi (bismuth), and Po (boronium), where B is Be (beryllium), Mg (magnesium), Ca (calcium), and Sr (strontium).
, Ba (barium), Ra (radium), and a combination of one or more elements selected from among them, and ■ is vanadium. These superconducting materials contain only 0.2% by weight or less of alkali metals (Na, Li, etc.) as impurities, contain 0.2% by weight or less of halogen elements as impurities, or contain halogen elements and alkali. It is characterized by containing less than 0.2% by weight of both metals.

The superconducting material of the present invention is based on a model in which the role of copper in conventional copper-containing superconducting ceramics is replaced with vanadium, thereby exhibiting superconductivity through a similar superconductivity development mechanism.

A feature of conventional superconducting ceramics containing copper is that, as mentioned above, they all have a perovskite-like structure and contain copper. Reasons for this include the following.

■Reason for containing copper Copper is a 3d transition metal and has several stable valences. Therefore, it is easy to create a mixed valence state, that is, a state doped with holes and electrons, which is considered to be essential for the development of superconductivity.

■Reasons for having a perovskite-like structure The perovskite structure is shown in Figure 1. Perovskite-like type is also the first
The structure is similar to that shown in the figure. The metal located at the center of the oxygen octahedron is usually called the A site, and the metal located at the outer corner of the cubic crystal is called the B site. In the perovskite type structure and the perovskite-like structure, many metals are easily substituted and dissolved in the A site, so it is easy to control the valence of the metal at the B site. Furthermore, since oxygen can easily enter and exit, the valence of the metal at the B site can be controlled by controlling the atmosphere during firing.

There are also examples of oxides made from materials that do not use copper that exhibit superconductivity. For example, it is Ba- to -Bi-〇 system.

However, it is said that the mechanism by which superconductivity develops in this system is different from that of other superconducting ceramics. In order to obtain a critical temperature that exhibits higher superconductivity, a mechanism similar to that of copper systems would be suitable. Therefore, the present inventor focused on vanadium.

Vanadium has an electronic configuration of (Ar)3d"4s" and is known to take the form of many oxides with nonstoichiometry between pentavalent and divalent, and some perovskite-type compounds are also known. Therefore, it is considered suitable for searching for superconducting materials.

In addition, in order to obtain stable superconducting properties, the inventors of the present invention discovered that among the various impurities mixed in during the process of producing superconducting materials, alkali metals (Na, Li, etc.) and halogen elements are particularly important in superconducting materials. It was found that the stable properties of

In particular, if the amount is 0.2% by weight or less, stable superconducting properties can be obtained. These impurities gather at the grain boundaries of the oxide ceramic grains, creating a barrier between the grains and adjacent grains, and inhibiting electrical conduction.

For this reason, it was not possible to improve the maximum current density, and in addition, the critical temperature at which superconductivity was exhibited was also low.

Furthermore, due to the presence of these impurities, the main skeletons of vanadium and oxygen were separated, making it impossible to obtain stable superconducting properties.

The following method was used to synthesize superconducting ceramics in the present invention.

First, a conventionally known perovskite compound containing vanadium at the B site was selected as the parent material, and it was fired. The valence of vanadium in a compound varies depending on each compound, and in order to accurately control the valence and obtain the desired substance, it is essential to control the atmosphere during firing. When fired in the air like conventional superconducting ceramics containing copper, the valence of vanadium is pentavalent, and the valence cannot be changed any further. Therefore, we installed a vacuum pump to evacuate the inside of the electric furnace, and A r and H inside the electric furnace.
2. An apparatus having a structure that allows CO2COt gases to be mixed and flowed at an arbitrary ratio was prepared, and firing was performed using the apparatus. XRD was used to determine whether parent material was formed. The parent material synthesized in this way is doped with a metal that has a different valence than the metal that occupies the A site of the parent material,
The aim was to achieve superconductivity by injecting holes and electrons. In addition, when performing this injection of holes and electrons, by controlling the firing atmosphere, the amount of oxygen was also controlled at the same time, and superconducting ceramics were synthesized.

In the above production, the purity of the starting materials was sufficiently increased, and then they were mixed, washed with ultrapure water (specific resistance of 18 MΩ or more), and then thoroughly dried using ultrasonic waves to remove impurities. .

In addition, the equipment and instruments used were similarly cleaned with ultrapure water.

Furthermore, it was also possible to dissolve and remove these impurities with a solvent that can dissolve other impurities instead of ultrapure water.

EXAMPLES The present invention will be explained in more detail with reference to Examples below.

(e) Example 1 As Example 1 of the present invention, A is Bi and B is Sr.
An example using .

As starting materials, Bit's was used as a Bi compound, 5rCO was used as an sr compound, and vO2 was used as a V compound. These were prepared by preparing a product with a purity of 4N for v02 and a product with a purity of 3N or higher for the others, sufficiently removing impurities as described above, and mixing the raw materials. Further, similar impurities were sufficiently removed in the following examples as well.

As a parent material, S r VO! , l
I chose this substance. At that time, since the valence of V is trivalent, the firing was performed in a reducing atmosphere. Calcination conditions include relatively high temperatures, e.g. 1250 °C, to facilitate reduction.
Firing was carried out at 0.degree. C. by flowing a mixed gas of CO:COt=too:t. The raw materials were weighed using an electronic microbalance so as to have a stoichiometric molar ratio, mixed using a ball mill, and then pressed into pellets and fired. The firing time was usually about 4 hours, but if an unreacted portion was found by XRD, the firing time was further extended. In this way, we obtained the parent material.

Next, the obtained S r V O2,6 and B t t 0
2 and 2 were mixed at a molar ratio of 1=1, formed into pellets, and then heated at 800° C. for 4 hours by flowing a gas diluted with Ar so that the H2 concentration was 4%. To determine whether a reaction is occurring, 5rVOz before calcination,
This was done by comparing the XRD of the mixture of S and BtzOs and the XRD after firing.

Although the peak of the raw material powder remained, it was confirmed that the reaction was progressing.

When the temperature dependence of the resistivity of the sample thus obtained was investigated, it was found that superconducting properties were obtained, with the critical temperature TC indicating superconductivity being 130 at onset and about 85 at offset. Figure 2 shows the resistivity and temperature characteristics at this time.

The amount of impurities in the obtained superconducting material was determined by ICP or atomic absorption spectrometry, and the amount was 0.05% or less for sodium and 0.01% for potassium among the alkali metal elements.
Below, the halogen element was 0.02% or less.

(f) Example 2 As Example 2, an example in which Ce is used as A will be shown. As the raw material, the same materials as in Example 1 were used except for Ce oxide. The parent material also has the same 5rVO2,s
The parent material and Ce oxide were mixed at a molar ratio of 1:1, and fired under the same firing conditions as in Example 1.

I was able to get about 35K with a 1 in 50 Tc onset offset.

The amount of impurities in the superconducting material obtained was determined by ICP or atomic absorption spectrometry, and the amount was 0.06% or less for sodium and 0.02% for potassium among the alkali metal elements.
Below, the halogen element was 0.01% or less.

(g) Example 3 As Example 3, an example is shown in which T1 is used as A and Ba is used as B. As a parent material, B
I chose a V O2. The firing conditions and the like are the same as in Example 1. Next, TI oxide was mixed with the parent material at a molar ratio of 1:1, formed into pellets, wrapped in a gold sheet, and fired. The firing conditions here are also the same as in Example 1. The obtained sample was able to obtain a Tc onset of 150K and a Tc offset of about 135K.

The amount of impurities in the superconducting material obtained was determined by ICP or atomic absorption spectrometry, and the amount was 0.06% or less for sodium and 0.02% for potassium among the alkali metal elements.
Below, the halogen element was 0.01% or less.

(h) Example 4 As Example 4, an example in which La is used as A and Ca is used as B is shown. As a parent material, Ca
I chose VO2. The firing conditions and the like are the same as in Example 1. Next, an oxide of La was mixed with the parent material at a molar ratio of 1:1, and the mixture was shaped into pellets and fired. The firing conditions here are also the same as in Example 1. The obtained sample was able to obtain a Tc onset of 155 and an offset of about 145K.

The amount of impurities in the obtained superconducting material was determined by ICP or atomic absorption spectrometry, and the amount was 0.04% or less for sodium and 0.02% for potassium among the alkali metal elements.
Below, the halogen element was 0.01% or less.

In addition, when we measured the superconducting properties of this sample after holding it at 50°C and 80% humidity for about 100 hours, the value changed by only 2% compared to the initial value, and it was stable with good reproducibility. We were able to develop superconducting properties.

(i) Effects of the Invention The present invention has made it possible to produce superconducting ceramics having a critical temperature Tc that exhibits much higher superconductivity than conventional ceramics. This does not simply mean an increase in the critical temperature at which superconductivity is exhibited. In other words, it means that there is a very large margin in the temperature of liquid nitrogen, the refrigerant that will be used in practical applications, and that it is possible to dramatically improve the critical current density Jc and to achieve superconductivity even in a relatively large external magnetic field. This is because it is easily expected to hold.

Further, according to the present invention, stable superconducting properties were obtained.

The present invention has made it possible to produce superconducting ceramics with practically sufficient properties.

[Brief explanation of drawings]

FIG. 1 schematically shows a perovskite structure. FIG. 2 shows a characteristic diagram of resistivity and temperature of a superconducting material. Diagram

Claims (3)

[Claims] 1. General formula (A_1_-_xB_x)_yV_zO_w
, (where x=0 to 1.0, y=1.0 to 3.0, z=1
．． 0 to 3.0, w=2.0 to 10.0), and in the above formula, A is Y (yttrium), La (lanthanum), Ce (
Cerium), Pr (praseodymium), Nd (neodymium)
, Pm (promethium), Sm (samarium), Eu(
europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho (holmium)
, Er (erbium), Tm (thulium), Yb (ytterbium), Lu (lutetium), Ag (silver), Cd
(cadmium), In (indium), Sn (tin),
Hg (mercury), Tl (thallium), Pb (lead), Bi (
B is composed of one type or a combination of two or more elements selected from Be (beryllium),
Li (lithium) and Na (sodium) in superconducting ceramics consisting of one or a combination of two or more elements selected from Mg (magnesium), Ca (calcium), Sr (strontium), Ba (barium), etc. A superconducting ceramic characterized in that the content of alkali metal elements such as , K (potassium), etc. is 0.2% by weight or less. 2. General formula (A_1_-_xB_x)_yV_xO_w
, (where x=0 to 1.0, y=1.0 to 3.0, z=1
．． 0 to 3.0, w = 2.0 to 10.0), and in the above formula, A is Y (yttrium), La (lanthanum), Ce
(cerium), Pr (praseodymium), Nd (neodymium), Pm (promethium), Sm (samarium), Eu
(europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Yb (ytterbium), Lu (lutetium), Ag (silver), C
d (cadmium), In (indium), Sn (tin)
, Hg (mercury), Tl (thallium), Pb (lead), Bi
(Bismuth), one type or a combination of two or more types of elements, B is Be (Beryllium)
, Mg (magnesium), Ca (calcium), Sr (
Strontium), Ba (barium), 1 selected from
1. A superconducting ceramic comprising a type or a combination of two or more types of elements, and the content of a halogen element in the superconducting ceramic is 0.2% by weight or less. 3. General formula (A_1_-_xB_x)_yV_zO_w
, (where x=0 to 1.0, y=1.0 to 3.0, z=1
．． 0 to 3.0, w=2.0 to 10.0), and in the above formula, A is Y (yttrium), La (lanthanum), Ce (
Cerium), Pr (praseodymium), Nd (neodymium)
, Pm (promethium), Sm (samarium), Eu(
europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho (holmium)
, Er (erbium), Tm (thulium), Yb (ytterbium), Lu (lutetium), Ag (silver), Cd
(cadmium), ln (indium), Sn (tin),
Hg (mercury), Tl (thallium), Pb (lead), Bi (
B is composed of one type or a combination of two or more elements selected from Be (beryllium),
Li (lithium) and Na (sodium) in superconducting ceramics consisting of one or a combination of two or more elements selected from Mg (magnesium), Ca (calcium), Sr (strontium), Ba (barium), etc. , the content of alkali metal elements such as K (potassium) is 0.2% by weight or less, and the content of halogen elements is 0.2% by weight or less.
A superconducting ceramic characterized by having a content of 2% by weight or less.