JP2558383B2 - Halogenated fluorine treatment method for oxide high temperature superconductor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a treatment method with a fluorine halide for increasing the superconducting transition start temperature of an oxide superconductor having oxygen defects.

[Conventional technology and problems to be solved] Since Bednorz and Mueller created an oxide with a superconducting transition initiation of 30K in the La-Ba-Cu-O system, the oxide superconductor containing Cu suddenly attracted attention. It was started, and many researchers decided to concentrate on this field.

In February and March 1987, a substance with a superconducting transition temperature Tc (hereinafter abbreviated as Tc) of 90K was independently created around the world, and a high-temperature superconductor with a liquid nitrogen temperature or higher was produced. Existence is established. The crystal structure of this high-temperature superconductor was also immediately analyzed by synthesizing a single crystal, and its chemical formula was YBa ₂ Cu ₃ O.
It was found to be an oxygen-deficient triple perovskite structure of _x (x = 7−δ) [hereinafter abbreviated as YBCO].

Furthermore, research has been repeated on the x value, and usually x is 6.
It was found that the values varied from 0 to 7.0, and various properties including Tc changed correspondingly.

Generally, Tc decreases as the x value decreases, but x
The specific numerical relationship between Tc and Tc varies depending on each researcher, and is not always a fixed value. However, it is known that those at least around 6.5 no longer exhibit their superconducting properties at liquid nitrogen temperatures.

The x value changes depending on the temperature of the sintered body and the partial pressure of oxygen in the surroundings, and in order to obtain a high x value, it is necessary to perform so-called annealing, in which it takes a long time after firing to cool while absorbing oxygen. The difficulty in controlling the oxygen content as described above is one of the problems in producing an oxide superconductor.

[Specific Means for Solving the Problem] Therefore, the inventors of the present invention investigated the oxide superconductor having a low x value by any means to increase the Tc, and found that x Tc due to low value
The present invention has been found by finding that Tc is increased by treating a sintered body with a fluorine halide, even if the value is considerably decreased.

That is, the present invention provides a method for treating an oxide superconductor with a halogen fluoride, which is characterized in that an oxide superconductor having an oxygen defect is treated with a fluorine halide at a temperature of 200 to 400 ° C. To do. The oxide superconductor targeted by the present invention is YBa ₂ Cu ₃ O _x (where x is
A copper-based oxide superconductor having oxygen defects having a pseudo-perovskite structure represented by the structure of (6.53 ≦ x ≦ 6.88).

The method for producing the superconductor is the same method as that for producing general ceramics, that is, so-called solid phase sintering, in which oxides of the elements of the respective components are sufficiently mixed in a predetermined molar ratio and then fired in air. In addition to the method described above, it can be produced by firing various raw materials such as a method using oxalate as a raw material and a method using alkoxide as a raw material. Oxygen defects are easily generated in the structure, and it is necessary to anneal after firing in an oxidizing atmosphere.

The sintered body that is not sufficiently annealed has a lower oxygen content than the sintered body that has been subjected to the above-mentioned treatment, and therefore Tc also decreases.

For example, in the case of a YBCO-based high-temperature oxide superconductor, yttrium oxide, barium carbonate and cupric oxide are mixed at a predetermined molar ratio and then reacted at about 900 ° C for several hours, and then allowed to reach room temperature for about 10 hours or more. The one obtained by annealing for about x = 6.9 has a superconducting start temperature of about 94K.
However, if the annealing is not performed sufficiently, the superconducting start temperature will be lowered.

According to the present invention, the superconducting starting temperature is lowered as described above.
Increasing the superconducting start temperature by subjecting a YBCO-based oxide superconductor to a fluorine halide treatment,
That is, it can be converted into an oxide in a high Tc state.

Fluorine halide used in the treatment of the present invention is ClF, Cl
F ₃ , ClF ₅ , BrF, BrF ₃ , BrF _{5 and the} like, among which ClF ₃ is preferable.

When performing the halogenated fluorine treatment, the system in which the sintered body of the normal raw material is present is evacuated to a vacuum or sufficiently replaced with an inert gas such as nitrogen or argon, and then the halogenated fluorine gas or the inert gas is used. The diluted halogenated fluorine gas may be introduced into the system, kept at a temperature of 200 ° C. to 400 ° C. as it is, and treated for at most 3 hours.

The reason why the superconducting transition start temperature is increased by the above-mentioned method is that the second one in the unit cell of the perovskite structure is used.
F or Cl at the oxygen vacancy sites around the copper atoms in the layer
It is presumed that -F was introduced, and as a result, the volume fraction of the superconducting phase having a high superconducting onset temperature was increased.

The crystal containing F or Cl-F once introduced exists stably and maintains its structure without decomposition even at a temperature of about 450 ° C.

On the other hand, when the treatment is performed with fluorine gas or chlorine gas alone, the crystal structure is different. That is, in the fluorine gas treatment, a broad peak, which is considered to be a heterogeneous phase, appears around 2θ = 25 °, while in the chlorine gas treatment, the YBCO structure is amorphized, which is different from the case of the treatment with fluorine halide.

As described above, even if the sintered body has a low oxygen content and is insufficiently annealed, the superconducting start temperature of the sintered body is increased by the treatment with the fluorine fluoride gas, and the annealing is sufficiently performed. The same effect can be obtained.

[Examples] Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited to the following Examples.

Example 1 An oxide high temperature superconductor having a superconducting starting temperature of 90K was manufactured. Raw material powders of yttrium oxide, barium carbonate, and cupric oxide were weighed so that the molar ratio was 0.5: 2: 3, sufficiently mixed with a ball mill, dried, and then compacted in a high-purity alumina boat to 920 ° C. The solid phase reaction was carried out in air for 6 hours. Then, it cooled to room temperature in the furnace over 10 hours. By this method, orthorhombic single-phase YBCO (x = 6.88) having a superconducting onset temperature of 94K was obtained.

The crystal structure of the product was confirmed by powder X-ray diffractometry. The superconducting property was measured by the DC 4-terminal method.

Example 2 YBCO produced in Example 1 was used as a raw material and
By annealing at 0 to 500 ° C. in an argon stream for different treatment times, x values of 6.70, 6.61 and 6.53 were obtained.

Next, including those having an x value of 6.88 obtained in Example 1,
The sample was treated with ClF ₃ . First, a Ni dish is further placed in the nickel reaction tube, and the compound is weighed and spread thinly. Furthermore, the inside of the reaction tube is 1 × 10-2 mmHg at room temperature.
Evacuate to the following temperature and introduce 100 mmHg of ClF ₃ gas.

The temperature was raised to about 3 ° C./min to each set temperature shown in each table below, held for 2 hours, and then rapidly cooled to room temperature.

Here, the fluorine ion concentration was analyzed using a fluorine ion electrode after dissolving the produced compound in a 1N aqueous hydrochloric acid solution.

Moreover, it was confirmed that fluorine and chlorine were introduced into the YBCO crystal, including the results of X-ray diffraction and ESCA, as well as the above measurement. Next, the average valence of copper was measured by the iodine titration method, and the x value was obtained from this. Furthermore, the relative amount of the superconducting phase contained in the sample was obtained from the magnitude of the Meissner effect. As the measuring device, a Meissner effect evaluation device developed by the present inventors (hereinafter abbreviated as MEED) was used.

For this device, see [M.Takashima, et al., JJA
P., 28, 2631 (1988)], but a brief explanation is given. First, the entire device is installed in a glass cylinder that can be evacuated, and a copper sample holder having a flat surface at an angle of 20 ° from the horizontal is installed in the lower part of the cylinder. This sample holder has a rod-shaped lower part, and by contacting it with liquid nitrogen,
Control the temperature of the sample.

Just above the sample holder, parallel to it
There is a disc-shaped magnet made of Sm-Co suspended by a stainless steel pipe to a width of 2 mm, and the magnet is supported so that it can move freely. Since the magnet is repulsed and laterally displaced, the diamagnetic effect can be measured by accurately measuring this displacement with an electrical measuring device.

Therefore, here, the repulsion by the magnet is shown in Table 1 as the value of lateral displacement per 1 g (mm / g).

FIG. 1 shows each set temperature and the weight increase of the sample at that time. As can be seen from this figure, the maximum weight increase is near 300 ° C., and the weight decreases above 300 ° C.

Next, for x = 6.61, untreated, 200 ° C treated, 300
Figure 1 shows the result of X-ray diffraction at ℃. As the reaction temperature rises, three peaks (tetragonal crystal) appear due to the strain due to oxygen defects near 2θ = 47 ° and 58 °. ) Is orthorhombic single phase and becomes two, but other changes are not observed, and no peak due to different phase appears, and further, the x value is small as shown in FIG. Since the weight increase is so large, it is considered that ClF ₃ acts on the second phase copper while maintaining the structure of YBCO.

Also, the average valence of copper, the superconducting onset temperature, and the MEED measurement value change at each temperature in the above treatment, which is x = 6.
First about the average valence of copper per 88 and 6.61 respectively
Tables and 2 show the superconducting onset temperatures in Tables 3 and 4.
The results of the MEED measurement are shown in Tables 5 and 6 together with the comparative examples described below.

As can be seen from these results, x = 6.6 treated at 300 ° C.
The value of 1 is higher than the value measured by MEED, the average valence of copper is also high, and the superconducting start temperature is also high, which is the same value as x = 6.88, and the result is remarkable. .

Comparative Examples 1 and 2 The results of treating with chlorine and fluorine in the same manner as in Example 2 are shown in Tables 1 to 6 as Comparative Example 1 for fluorine and Comparative Example 2 for chlorine.

As a result, although there is some effect on fluorine and chlorine, the proportion of the superconducting phase is small as can be seen from the results of MEED, and when X-ray diffraction is measured, a heterogeneous phase is generated and it can be obtained as a single phase. The results are quite poor as only ClF ₃ treatment is used.

[Advantages of the Invention] The method for treating an oxide superconductor with a fluorine halide of the present invention is a method in which YBa ₂ Cu ₃ O _x (where x is 6.53 ≤ x ≤ x) that does not undergo annealing for a long time and has a low oxygen content. 6.88) can raise the superconducting onset temperature of the oxide superconductor, which makes it possible to produce an oxide high-temperature superconductor with a high Tc without the need for inefficient operation during production. Since the obtained halogenated fluorine-treated oxide high temperature superconductor of the present invention is also stable, it can be applied particularly to the treatment of YBCO thin films and is extremely useful.

[Brief description of drawings]

FIG. 1 is a diagram showing a weight change of YBCO depending on each reaction (treatment) temperature, and FIG. 2 is an X-ray diffraction diagram of a compound produced by each reaction (treatment) temperature.

Claims (1)

(57) [Claims] 1. YBa ₂ Cu ₃ O _x having oxygen defects (provided that x
Is 6.53 ≤ x ≤ 6.88).
A method for treating a halogenated fluoride of an oxide superconductor, which is characterized in that the halogenated fluoride is treated at a temperature of 0 to 400 ° C.