JP3167316B2 - Method for producing Tl-based oxide superconductor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a thallium (Tl) -based oxide superconductor.

[Problems to be Solved by the Related Art and the Invention] In recent years, Tl-Ba-Ca-Cu-O-based oxide superconductors have been
Since the publication of the high superconducting transition temperature (critical temperature), studies on Tl-based oxide superconductors have been actively conducted. However, in the case of Tl-based oxide superconductors, there have been many reports of compounds having substantially different critical temperatures even though they have the same composition. In particular, for compounds reported to have a composition of Tl ₂ Ba ₂ CuO ₆ , various reports have been made from those having a critical temperature of 80 K to those having no superconductivity even at 0 K.

However, in Tl-based oxide superconductors, the relationship between the manufacturing method and the critical temperature has not yet been grasped, and it is possible to reliably produce a Tl-based oxide superconductor having a critical temperature within a desired range. There is a problem that can not be.

The present invention has been made in view of such circumstances, and in each composition, it is possible to reliably manufacture a Tl-based oxide superconductor having a critical temperature within a desired range, respectively. It is an object of the present invention to provide a method for manufacturing a superconductor.

[Means and Actions for Solving the Problems] The method for producing a Tl-based oxide superconductor according to the present invention comprises:
Baking a mixed raw material in which a metal element constituting the base oxide superconductor is prepared at a predetermined ratio in an oxygen-containing atmosphere, and gradually cooling the sintered body obtained in this step in an oxygen-containing atmosphere And after the sintered body reaches a predetermined temperature in the slow cooling step, or after the sintered body reaches a predetermined temperature by reheating after the slow cooling step, the sintered body is vacuumed or Cooling in an inert gas atmosphere. In this case, the reheating is preferably performed in a vacuum or an inert gas atmosphere, and the reheating is preferably performed while measuring the thermogravimetric change of the sintered body.

The present inventors have previously described in Japanese Patent Application No. 1-257805 that the variation in the critical temperature described above is based on the difference in the amount of oxygen taken into the crystal structure due to the difference in the cooling rate after the sintering of the raw material. When reheating is performed after synthesizing the Tl-based oxide of the above, oxygen is released, and the release amount increases as the temperature rises, and it shows good superconductivity characteristics when an appropriate amount of oxygen release occurs If re-heating is followed by slow cooling, re-uptake of oxygen will occur and superconductivity will decrease, and such re-uptake of oxygen will be eliminated by rapid cooling after re-heating. showed that.

However, the present inventors have an atmosphere that does not take up oxygen cooling after reheating for oxygen evolution, i.e. by performing an inert gas atmosphere ₂ such or in N vacuum, oxygen even without quenching It was found that reuptake did not occur. The present invention has been made based on such findings.

According to the present invention, it is possible to provide a stable Tl-based oxide superconductor having a high critical temperature without quenching for preventing oxygen re-incorporation. There is an advantage that is.

In addition, when reheating is performed while measuring the thermogravimetric change of the sintered body, the amount of oxygen released from the sintered body can be grasped more accurately. Accurate recognition can be performed, and the critical temperature of the manufactured oxide superconductor can be controlled more precisely.

The present invention can be applied to all Tl-based superconducting oxides, but Tl _m AE ₂ Can _n-1 Cu _n O _{2n + p} (m is 1 or 2, AE is Ba or Sr, n Is an integer of 1 or more, and p is a value which becomes 3 when m is 1 and becomes 4 when m is 1.

Tl _m AE ₂ C a _n-1 C _n O _{2n + p} , m = 2, Tl ₂ Ba ₂ CuO ₆ , T
By performing the heat treatment of the present invention on Tl ₂ Ba ₂ Ca _n-1 Cu _n O _{2n + 4} represented by l ₂ Ba ₂ Ca ₂ Cu ₃ O ₁₀ , Tl ₂ Ba ₂ Ca _n-1 Cu _n O
_{2n + 4-y} (y is in the range of 0 <y <1), and an oxide superconductor having a high critical temperature can be effectively produced. Substances represented by _{Tl 2 Ba 2 Ca n-1} Cu n O 2n + 4 , since the amount of holes is large amount, it is possible to obtain a high critical temperature by oxygen amount decreases.

_{Tl m AE 2 Ca n-1} Cu n O 2n + TlBa 2 Ca of m = 1 of _{p n-1} Cu _n O
For the _{2n + 3,} it is represented by _{TlBa 2 Ca n-1 Cu n} O 2n + 3-y, TlBa 2 C
it is possible to obtain an oxide superconductor having a critical temperature higher than _{a n-1 Cu n O 2n} + 3. That is, TlBa ₂ Ca _n-1 Cu _n O _{2n + 3}
By depleting oxygen from, an oxide superconductor having a higher critical temperature can be obtained. This is because when the applicant's applicant has partially substituted the Ca site of the oxide superconductor TlBa ₂ CaCu ₂ O ₇ described in Japanese Patent Application No. 1-236174 previously filed with an appropriate amount of Y. This is based on the substantially same action as the phenomenon in which the critical temperature rises. That is, Ca
Substituted by Y sites, none of the effect of lowering the average valence of Cu in the material, this effect _{TlBa 2 Ca n-1 Cu n} O
It is also achieved by depleting oxygen from _{2n + 3} . Note that when n is 2, that is, TlBa ₂ CaCu ₂
In the structure of O _7-y , the value of y at which the critical temperature becomes maximum is stoichiometrically about 0.2 at which the average valence of Cu becomes about 2.3.

 Hereinafter, the present invention will be described in detail.

As the mixed raw material, it is desirable to use a mixture of oxides of the respective metal elements constituting the Tl-based oxide superconductor, and the mixing ratio is such that the ratio of each metal is the desired oxidation ratio in terms of the atomic ratio. Tl is made to be substantially the same as the composition ratio of the oxide superconductor, or in view of the fact that Tl is likely to evaporate during sintering, the Tl is excessively larger than the intended oxide superconductor composition. I do.

The firing is performed in an oxygen-containing atmosphere as described above.
In this case, since the sintering temperature is as high as 800 to 950 ° C., the sintering time is desirably short, and the range of several minutes to tens of minutes is appropriate. After firing, the sintered body is gradually cooled in an oxygen-containing atmosphere.

Slow cooling of the sintered body in a vacuum or an inert gas atmosphere,
When the temperature of the sintered body reaches a predetermined temperature in the slow cooling step in an oxygen-containing atmosphere following the firing step, the atmosphere may be switched to vacuum or an inert gas, and may be continuously performed. It is desirable to reheat to a predetermined temperature after the completion of the cooling step.

The reheating of the sintered body is a process for releasing oxygen from the crystal structure of the sintered body, and its atmosphere is not particularly limited. Since the process is performed in an atmosphere, it is preferable to perform the process in a vacuum or an inert gas atmosphere such as nitrogen. More preferably,
This is performed by increasing the gas pressure of an inert gas atmosphere made of nitrogen or the like. The reason is that as the gas pressure increases, the evaporation of Tl can be suppressed and the temperature at which the crystal decomposes can be shifted to a higher temperature side, so that more oxygen is released from the crystal structure Because you can do it. The reheating temperature and the holding time are appropriately determined according to the composition of the Tl-based oxide superconductor. Since the amount of oxygen released from the crystal structure of the sintered body varies depending on the temperature, the critical temperature is almost determined by this.

After reheating, the sintered body is gradually cooled in a vacuum or an inert gas atmosphere. This slow cooling is performed to prevent oxygen from being re-incorporated into the sintered body. Slow cooling here means
This means that cooling is performed without being introduced into a liquid refrigerant such as liquid nitrogen. When the reheating is performed in a vacuum or an inert gas atmosphere, the furnace may be cooled while maintaining the same atmosphere in the furnace.

Examples Examples of the present invention will be described below.

Example 1 Fine powders of Tl ₂ O ₃ , BaO ₂ and CuO were mixed as starting materials to prepare a mixed powder raw material having an atomic ratio of Tl: Ba: Cu = 2: 2: 1. In this case, since Tl was toxic, these operations were performed in a glove box.

Next, such a mixed powder raw material was molded at a pressure of about 200 kg / cm ² , and six pellet-shaped samples having a diameter of 10 mm and a thickness of 1 to 1.5 mm were prepared.

After that, in consideration of the high reactivity of Tl, the sample was loosely wrapped in gold foil that is difficult to react with Tl, and due to the toxicity of Tl, a double trap was further attached in a quartz tube with an oxygen flow of 120 ml / min. C. for 5 minutes, then cooled at a rate of 10.degree. C./min.

As a result, an oxide having a composition of Tl ₂ Ba ₂ CuO ₆ was synthesized.

The synthesized sample was placed in a nitrogen stream at a flow rate of 120 ml / min.
The temperature rises at the rate of 200 minutes, 300 ° C, 400 ° C, 500 ° C,
When the temperature reached 600 ° C. and 700 ° C., the heater power of the heating furnace was turned off, and the samples were cooled. As a result, the sample heated to 200 ° C. and then cooled in the furnace did not show superconductivity, but the other samples showed superconductivity. That is, Tl ₂ Ba ₂
CuO _6-y superconducting oxide was formed.

1 to 5 show the results of measuring the temperature change of the resistivity of each sample by the four-terminal method. Figure 1 is 30
FIG. 2 shows a furnace heated to 400 ° C. and then cooled, FIG. 3 shows a furnace heated to 500 ° C. and then cooled, and FIG. 4 shows a furnace cooled to 600 ° C. FIG. 5 shows a furnace cooled to 700 ° C. and then cooled in a furnace.

The critical temperature of each sample was determined from the resistivity change curves in FIGS. 1 to 5 as follows. First, the linear part of the resistivity change curve is extended, and the extended part is the OK axis (vertical axis).
The 50% point is plotted on the OK axis based on the value of the point intersecting with. Next, a 50% point is plotted on the 100K axis with reference to an appropriate temperature axis, for example, a point at which the 100K axis intersects with the resistivity curve in the linear region of the resistivity change curve. Then, the temperature at the point where the straight line connecting the point plotted on the OK axis and the point plotted on the 100K axis intersects the resistivity change curve is determined as the midpoint, and this is defined as the critical temperature Tc. The same work is performed 90% of points and
A 10% point was also performed, which was defined as an onset point and an end point, respectively.

FIG. 6 shows the relationship between the critical temperature of each sample and the heating temperature (the temperature reached at the time of reheating) determined in this manner. Sixth
In the figure, solid circles indicate midpoints, and bars above and below indicate onset points and end points, respectively. As shown in this figure, the critical temperature of each sample was 28 K
(Onset point 31K, end point 27K, the same applies in parentheses below), 63K (64K, 61K), 77K (80K, 74K), 83
K (86K, 82K) and 78K (83K, 73K). That is,
It was confirmed that a superconductor having a critical temperature of about 30K to 85K can be synthesized when heated to a temperature of 300 ° C. or higher and then cooled in a furnace.

The oxide having the composition of Tl ₂ Ba ₂ CuO ₆ described above was supplied at a flow rate of 12
The thermogravimetric change when the temperature is increased at a rate of 5 ° C./min in a nitrogen stream of 0 ml / min is as shown in FIG. As shown in this figure, the weight loss of Tl ₂ Ba ₂ CuO ₆ starts at 200 ° C. and ends at 600 ° C.
Up to ° C. is proportional to the temperature change. This weight loss corresponds to the release of oxygen. From around 600 ° C., the weight loss rate has increased, and it is estimated that Tl evaporation has occurred from around this temperature, but as shown in the above-described Example, 700 ° C.
It has been confirmed that the sample reheated in the above was also an oxide superconductor having the composition of Tl ₂ Ba ₂ CuO _6-y , and the amount of Tl evaporation around 700 ° C did not significantly affect the crystal structure. Absent. The crystals decomposed at around 770 ° C.

In a temperature range where Tl does not evaporate, the heating temperature and the amount of released oxygen correspond to 1: 1.By measuring the thermogravimetric change of the sample in a vacuum or in an inert gas atmosphere, The slow cooling start temperature can be grasped. Therefore, at the time of manufacture, when the annealing temperature in the oxygen-containing atmosphere reaches the annealing start temperature, or when the reheating temperature reaches the annealing starting temperature, the vacuum or inert gas atmosphere is used. What is necessary is just to start slow cooling. If reheating is performed while measuring the thermogravimetric change of the sample, the weight loss of the sample, that is, the amount of released oxygen can be directly grasped, so the critical temperature of the oxide superconductor to be manufactured can be controlled more precisely. can do.

Example 2 In the same manner as in Example 1, three similar pellet-shaped samples were produced. Thereafter, these samples were fired under the same conditions as in Example 1 to synthesize an oxide having a composition of Tl ₂ Ba ₂ CuO ₆ .

Each of the three synthesized samples was 480 each at a rate of 10 ° C./min.
C., 500.degree. C., and 540.degree. C., and annealed in an Ar gas atmosphere. The annealing time at this time is 4 at 480 ° C.
Time, 10 hours at 500 ° C and 540 ° C. afterwards,
Each sample was cooled at a rate of 10 ° C./min. All of the samples thus formed exhibited superconducting properties, and it was confirmed that a superconductor was generated.

8 to 10 are diagrams showing the results of measuring the temperature change of the magnetic susceptibility of each sample by the AC susceptibility method. FIG. 8 shows a case where the temperature was raised to 480 ° C. and annealed for 4 hours, FIG. 9 shows a case where the temperature was raised to 500 ° C. and annealed for 10 hours, and FIG. The results are shown. From these figures, the critical temperature of each sample was 40
K, 50K and 55K were confirmed.

[Effects of the Invention] According to the present invention, Tl whose critical temperature is within a desired range
A system-based oxide superconductor can be easily manufactured.

The oxide superconductor manufactured according to the present invention is expected to be applied to a Josephson device having a Josephson junction, a SQUID (superconducting quantum interferometer), and a superconducting generator, and has a small energy loss. It is expected to contribute to the practical use of superconducting equipment in various fields, such as power storage and power transmission cables with low energy loss.

[Brief description of the drawings]

1 to 5 are diagrams showing the temperature change of the resistivity of a sample prepared by the method according to the first embodiment of the present invention by the four-terminal method, and FIG. 6 is obtained from the results of FIGS. 1 to 5. FIG. 7 shows the relationship between the critical temperature and the heating temperature before slow cooling, and FIG. 7 shows Tl ₂ B
FIGS. 8 to 10 show thermogravimetric changes when a ₂ CuO ₆ is reheated in a nitrogen atmosphere. FIGS. 8 to 10 show the magnetic susceptibility of a sample prepared by the method according to the second embodiment of the present invention by the AC susceptibility method. FIG. 5 is a diagram showing a temperature change of the radiator.

────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Masae Kikuchi 2-11-5 Migamimine, Taishiro-ku, Sendai-city, Miyagi Prefecture (72) Inventor Yasuhiko Shono 3--12-12 Yoshinari, Aoba-ku, Aoba-ku, Sendai City, Miyagi Prefecture (72) Inventor Norio Kobayashi 2-6-11 Nagamigaoka, Izumi-ku, Sendai-shi, Miyagi Examiner Hitoshi Misaki (56) References JP-A-3-199157 (JP, A) Patent 2820480 (JP, B2) Patent 3006071 (JP, B2) ) Patent 3121001 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) C01G 1 / 00,15 / 00 C04B 35/64

Claims (3)

(57) [Claims] 1. A process in which a mixed material prepared by mixing metal elements constituting a Tl-based oxide superconductor at a predetermined ratio is fired in an oxygen-containing atmosphere, and the sintered body obtained in this process is oxygen-containing. A step of gradually cooling in an atmosphere, and after the sintered body reaches a predetermined temperature in the slow cooling step, or after reheating after the slow cooling step and the sintered body reaches a predetermined temperature, Gradually cooling the compact in a vacuum or an inert gas atmosphere. A method for producing a Tl-based oxide superconductor, comprising: 2. The method for producing a Tl-based oxide superconductor according to claim 1, wherein the reheating is performed in a vacuum or in an inert gas atmosphere. 3. The method according to claim 1, wherein the reheating is performed while measuring a thermogravimetric change of the sintered body.
3. The method for producing a Tl-based oxide superconductor according to 1.).