JP3668510B2

JP3668510B2 - Method for producing Bi-based oxide superconductor

Info

Publication number: JP3668510B2
Application number: JP23805594A
Authority: JP
Inventors: 隆代長谷川; 祐北村
Original assignee: 昭和電線電纜株式会社
Priority date: 1994-09-30
Filing date: 1994-09-30
Publication date: 2005-07-06
Anticipated expiration: 2020-07-06
Also published as: JPH08104521A

Description

【０００１】
【産業上の利用分野】
本発明は超電導体の製造方法に係り、特に熱処理条件を改善することにより、長尺の膜体を容易に形成することのできるＢｉ系酸化物超電導体の製造方法の改良に関する。
【０００２】
【従来の技術】
Ｂｉ系（Ｂｉ−Ｓｒ−Ｃａ−Ｃｕ−Ｏ系）の超電導体は、その臨界温度（Ｔｃ）が高く、Ｙ−Ｂａ−Ｃｕ−Ｏ系（Ｙ系）の超電導体に比較して安定性および加工性に優れるため、酸化物超電導体の実用材料として期待されている。
このＢｉ系の超電導体には、その組成により３種のＴｃを有する相が存在するが、特に８０Ｋ級の（２２１２）相（Ｂｉ：Ｓｒ：Ｃａ：Ｃｕのモル比＝２：２：１：２、以下同様。）と１１０Ｋ級の（２２２３）相は、液体窒素温度以上で使用し得る材料として実用化へ向けての検討が進められている。
【０００３】
上記のＢｉ系超電導体の応用分野としては、まず線材や素子が挙げられるが、この他に磁気シールドや共振器等の大面積で異形状の成型体への応用が検討されている。
この、特に後者へ応用するための方法としては、（イ）酸化物超電導粉末にバインダーを混合して所定の粘度に調整したスラリーを基体上に塗布し、乾燥後、焼成する方法が一般に行われており、この他（ロ）金属有機酸塩を溶解した溶液を基体上に塗布した後、熱分解する方法も検討されている。
【０００４】
上記の（２２１２）相は、基材上に超電導前駆体の膜を形成し、これを部分溶融点（８７０〜８８０℃）以上に加熱した後、（２２１２）相の凝固点以下まで徐冷することにより、配向性に優れた、即ち電気的特性に優れた膜体を得ることができることが知られている。
また（２２１２）相は、その結晶中に含まれる酸素の数によりＴｃが変化することが知られている。通常大気中で焼成した場合には、Ｔｃは約７５〜８０Ｋとなり、液体窒素温度では殆ど超電導特性を示さない。このため、不活性なガス中で熱処理を施して脱酸素処理を行いＴｃを約９５℃程度まで上昇させることが行われる。
【０００５】
【発明が解決しようとする課題】
しかしながら、上記の（２２１２）相の超電導膜の形成方法においては、熱処理の最高温度が８９０〜９００℃とかなり高く、基材として銀を用いたときの銀の融点（９６０℃）に近い。また、その後（２２１２）相の凝固点（８５０〜８６０℃）近傍まで２〜５℃／ｈ程度の冷却速度で徐冷する必要があり、冷却に長時間を要する。
【０００６】
その結果、熱処理中に基材が軟化し、長尺の膜体を製造する場合に、基材の変形や破断を生ずる。このため、（２２１２）相の熱処理温度を低下させる必要がある。
また、上述したように、（２２１２）相の結晶中に含まれる酸素数を低減させるために、不活性なガス中で脱酸素処理、即ちポストアニールを施す必要があるが、この熱処理が長尺化や大面積化する上での難点となっている。
【０００７】
本発明は以上の問題を解決するためになされたもので、熱処理条件を改善することにより、部分溶融点以上の加熱後、凝固点近傍までの徐冷により超電導体を製造する方法において、熱処理温度を低下させ、かつポストアニールを不要とするＢｉ系酸化物超電導体の製造方法を提供することをその目的とする。
【０００８】
【課題を解決するための手段】
上記目的を達成するために、本発明に係るBi系酸化物超電導体の製造方法は、基材上に、Bi、S r、CaまたはCuを２：２：１：２のモル比で含む超電導前駆体の膜を形成した複合膜に熱処理を施して超電導体を製造する方法において、酸素分圧が5モル％以下の雰囲気中で、かつ基板の軟化温度以下で超電導前駆体の部分溶融点以上に加熱した後、凝固点近傍まで徐冷する熱処理を施し、Bi（２２１２）相超電導体の生成と脱酸素処理を同時に行うようにしたものである。
【０００９】
本発明における超電導前駆体の組成は、Ｂｉ：Ｓｒ：Ｃａ：Ｃｕ＝（１．９〜２．１）：（１．７〜２．３）：（０．７〜１．３）：（１．７〜２．３）の範囲とすることが好ましい。これらの範囲外の組成の場合には非超電導相が生成し易くなり、その特性が低下する。
基材としては、Ａｇ、Ａｕ、Ｐｔまたはこれらの合金等の金属やＳｒＴｉＯ3 、ＭｇＯ，ＹＳＺ等のセラミックス等で熱処理温度範囲内において耐酸化性に優れ、かつ超電導体との反応を生じないもの、あるいは超電導体との反応を生じても超電導性を低下させないものが好ましい。
【００１０】
ペーストの基材上への塗布方法は、一般に塗料のコーティングに用いられている方法を使用することができ、例えばドクターブレード、ディップコーティング、アプリケーターコーティング等をあげることができる。
本発明における超電導前駆体の膜は、Ｂｉ、Ｓｒ、ＣａまたはＣｕを含む金属有機酸塩あるいは有機金属化合物を所定の金属モル比で有機溶媒中に溶解した溶液を基材上に塗布した後、熱処理を施すことにより、またＢｉ、Ｓｒ、ＣａまたはＣｕを含む酸化物粉末を、有機溶媒中に溶解した溶液を基材上に塗布した後、熱処理を施すことにより形成することができる。
【００１１】
前者の溶液としては、例えばオクチル酸、ネオデカン酸、ナフテン酸等の金属有機酸塩あるいは金属アルコキシド、金属アセチルアセトナート等の有機金属化合物で溶媒に可溶であるものを、炭化水素系、エーテル系、アルコール系等の有機溶剤や有機溶剤と水等を混合した溶媒中に完全に溶解させたものを用いることができる。
【００１２】
また、後者の溶液としては、例えば、固相法、共沈法あるいはゾルーゲル法等により作製した超電導組成を有する粉末または超電導粉末を有機溶媒中に溶解した溶液を用いることができる。勿論これ等の混合溶液を使用できることは言うまでもない。
本発明における熱処理は、酸素分圧が５モル％以下の雰囲気中で行われるが、この雰囲気ガスは酸素ガスと窒素やアルゴン等の不活性なガスとを混合したものを用い、特に酸素分圧が０．１〜５モル％の混合ガスを用いることが好ましい。酸素分圧が５モル％を越えると部分溶融点の低下の効果が小さく、また０．１モル％部分溶融後、凝固までの反応形態が変化し、（２２１２）相の生成が困難となる。
【００１３】
熱処理の最高温度は、酸素分圧によって変化するため、部分溶融点より５〜３０℃程度高い温度に設定することが好ましい。
本発明のＢｉ系酸化物超電導体の製造方法は、Ｂｉ系の（２２１２）相からなる膜体の製造に適し、この場合の最良の方法は、銀又は銀合金からなる基材上に、Ｂｉ、Ｓｒ、ＣａまたはＣｕを約Ｂｉ：Ｓｒ：Ｃａ：Ｃｕ＝２：２：１：２の組成で含む超電導前駆体の膜を形成した複合膜に熱処理を施して超電導体を製造する際に、酸素分圧が０．１〜５モル％の雰囲気中で、かつ基板の軟化温度以下で超電導前駆体の部分溶融点以上に加熱した後、凝固点近傍まで徐冷する熱処理を施すものである。
【００１４】
【作用】
本発明においては、熱処理の雰囲気を５モル％以下の酸素分圧とすることにより、部分溶融点を低下させることが可能となり、これにより低い熱処理温度で超電導体が生成する。また脱酸素処理も不要となる。
【００１５】
【実施例】
以下本発明の実施例および比較例について説明する。
実施例
Ｂｉ、Ｓｒ、ＣａおよびＣｕの各オクチル酸塩を、その金属分がＢｉ：Ｓｒ：Ｃａ：Ｃｕ＝２：２：１：２のモル比を有するようにキシレン中に５０ｗｔ％の濃度で溶解し、この溶液中に酸化物粉末を添加してペーストを作製した。
【００１６】
上記の酸化物粉末は、Ｂｉ：Ｓｒ：Ｃａ：Ｃｕ＝２：２：１：２のモル比を有する共沈粉を、８５０℃で２０時間焼成して作製した。
このペーストをＡｇ基板上にアプリケーターコート法により厚さ２５μｍに塗布し、５００℃で熱分解させた後、酸素分圧１モル％の雰囲気中で、部分溶融点以上の温度に加熱し、次いで所定の冷却速度で凝固点以下まで徐冷して膜体を形成した。この部分溶融点は８４０℃であった。
【００１７】
このようにして得られた超電導膜について、その臨界温度（Ｔｃ）及び臨界電流値（Ｉｃ：７７Ｋ、０Ｔ）を測定した。
その結果、
（イ）熱処理の最高温度８７０℃、冷却速度２℃／ｈの条件で、Ｔｃ＝９３Ｋ、
Ｉｃ＝１２Ａ、
（ロ）熱処理の最高温度８７０℃、冷却速度４℃／ｈの条件で、Ｔｃ＝９３Ｋ、
Ｉｃ＝１０Ａ、
（ハ）熱処理の最高温度８６０℃、冷却速度２℃／ｈの条件で、Ｔｃ＝９３Ｋ、
Ｉｃ＝１８Ａ、
（ニ）熱処理の最高温度８５０℃、冷却速度２℃／ｈの条件で、Ｔｃ＝９３Ｋ、
Ｉｃ＝８Ａの結果が得られた。
【００１８】
比較例１
実施例と同様にしてＡｇ基板上に膜体を形成し、５００℃で熱分解させた後、大気中で、部分溶融点以上の温度に加熱し、次いで所定の冷却速度で凝固点以下まで徐冷して膜体を形成した。この部分溶融点は８８０℃であった。
このようにして得られた超電導膜について、その臨界温度（Ｔｃ）及び臨界電流値（Ｉｃ）を測定した。
【００１９】
その結果、熱処理の最高温度８９０℃、冷却速度３℃／ｈの条件で、Ｔｃ＝７９Ｋ、Ｉｃ＝１Ａの結果が得られた。
比較例２
実施例と同様にしてＡｇ基板上に膜体を形成し、５００℃で熱分解させた後、８モル％の酸素分圧の雰囲気中で部分溶融点以上の温度に加熱し、次いで所定の冷却速度で凝固点以下まで徐冷して膜体を形成した。この部分溶融点は８６０℃であった。
【００２０】
このようにして得られた超電導膜について、その臨界温度（Ｔｃ）及び臨界電流値（Ｉｃ）を測定した。
その結果、熱処理の最高温度８９０℃、冷却速度３℃／ｈの条件で、Ｔｃ＝８３Ｋ、Ｉｃ＝５Ａの結果が得られた。
比較例３
実施例と同様にしてＡｇ基板上に膜体を形成し、５００℃で熱分解させた後、窒素ガス雰囲気中で部分溶融点以上の温度に加熱し、次いで所定の冷却速度で凝固点以下まで徐冷して膜体を形成した。この部分溶融点は８００℃であった。
【００２１】
このようにして得られた超電導膜について、その臨界温度（Ｔｃ）及び臨界電流値（Ｉｃ）を測定した。
その結果、熱処理の最高温度８５０℃、冷却速度３℃／ｈの条件ではＴｃを示さなかった。
【００２２】
【発明の効果】
以上述べたように本発明によれば、部分溶融点以上の加熱後、凝固点近傍までの徐冷により超電導体を製造する方法において、熱処理の雰囲気を５モル％以下の酸素分圧とすることにより、部分溶融点を低下させることが可能となるため、これにより低い熱処理温度で超電導体を製造することができ、かつポストアニールも不要となり、膜体の長尺化や大面積化を容易に実現することができる。[0001]
[Industrial application fields]
The present invention relates to a method for manufacturing a superconductor, and more particularly to an improvement in a method for manufacturing a Bi-based oxide superconductor capable of easily forming a long film body by improving heat treatment conditions.
[0002]
[Prior art]
Bi-based (Bi-Sr-Ca-Cu-O-based) superconductors have a high critical temperature (Tc) and are more stable and stable than Y-Ba-Cu-O-based (Y-based) superconductors. Because of its excellent workability, it is expected as a practical material for oxide superconductors.
In this Bi-based superconductor, there are three types of phases having Tc depending on the composition thereof. In particular, the 80K class (2212) phase (Bi: Sr: Ca: Cu molar ratio = 2: 2: 1: 2, the same shall apply hereinafter) and the 110K-class (2223) phase are being studied for practical use as materials that can be used at liquid nitrogen temperatures or higher.
[0003]
As an application field of the Bi-based superconductor described above, firstly, a wire rod and an element can be mentioned. In addition to this, the application to a molded article having a large area and an irregular shape such as a magnetic shield and a resonator is being studied.
As a method for application to the latter, in particular, (a) a slurry prepared by mixing a binder with oxide superconducting powder and adjusting the viscosity to a predetermined viscosity is applied onto a substrate, dried and then fired. In addition, a method in which a solution in which (b) a metal organic acid salt is dissolved is applied on a substrate and then thermally decomposed is also being studied.
[0004]
In the (2212) phase, a superconducting precursor film is formed on the substrate, heated to a temperature equal to or higher than the partial melting point (870 to 880 ° C.), and then gradually cooled to the temperature below the freezing point of the (2212) phase. Thus, it is known that a film body having excellent orientation, that is, excellent electrical characteristics can be obtained.
Further, it is known that the Tc of the (2212) phase changes depending on the number of oxygen contained in the crystal. When fired in the normal air, Tc is about 75-80 K, and hardly exhibits superconducting properties at liquid nitrogen temperature. For this reason, heat treatment is performed in an inert gas to perform deoxygenation, and Tc is increased to about 95 ° C.
[0005]
[Problems to be solved by the invention]
However, in the method for forming the (2212) phase superconducting film, the maximum temperature of the heat treatment is as high as 890 to 900 ° C., which is close to the melting point of silver (960 ° C.) when silver is used as the base material. Further, it is necessary to gradually cool at a cooling rate of about 2 to 5 ° C./h until the freezing point (850 to 860 ° C.) of the (2212) phase thereafter, and it takes a long time for cooling.
[0006]
As a result, the base material softens during the heat treatment, and when the long film body is manufactured, the base material is deformed or broken. For this reason, it is necessary to lower the heat treatment temperature of the (2212) phase.
Further, as described above, in order to reduce the number of oxygen contained in the (2212) phase crystal, it is necessary to perform deoxygenation treatment, that is, post-annealing in an inert gas. This is a difficulty in increasing the size and area.
[0007]
The present invention has been made to solve the above problems, and in a method for producing a superconductor by gradually cooling to the vicinity of the freezing point after heating above the partial melting point by improving the heat treatment conditions, the heat treatment temperature is reduced. It is an object of the present invention to provide a method for producing a Bi-based oxide superconductor that can be reduced and does not require post-annealing.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, a method for producing a Bi-based oxide superconductor according to the present invention includes a superconductivity containing Bi, Sr, Ca or Cu in a molar ratio of 2: 2: 1: 2 on a substrate. In a method of manufacturing a superconductor by performing a heat treatment on a composite film in which a precursor film is formed, in an atmosphere having an oxygen partial pressure of 5 mol% or less and at or below the softening temperature of the substrate, the melting point is equal to or higher than the partial melting point of the superconducting precursor. Then, heat treatment is performed to gradually cool to the vicinity of the freezing point, and Bi (2212) phase superconductor is generated and deoxygenated at the same time.
[0009]
The composition of the superconducting precursor in the present invention is as follows: Bi: Sr: Ca: Cu = (1.9 to 2.1) :( 1.7 to 2.3) :( 0.7 to 1.3) :( 1 .7 to 2.3). In the case of a composition outside these ranges, a non-superconducting phase is likely to be generated, and its characteristics are deteriorated.
As the substrate, a metal such as Ag, Au, Pt or an alloy thereof or a ceramic such as SrTiO3, MgO, YSZ, etc., which has excellent oxidation resistance within a heat treatment temperature range and does not cause a reaction with a superconductor, Or the thing which does not reduce superconductivity even if it reacts with a superconductor is preferable.
[0010]
As a method of applying the paste onto the substrate, a method generally used for coating a paint can be used, and examples thereof include a doctor blade, dip coating, applicator coating, and the like.
The film of the superconducting precursor in the present invention is obtained by applying a metal organic acid salt containing Bi, Sr, Ca or Cu or an organic metal compound dissolved in an organic solvent at a predetermined metal molar ratio on a substrate, It can be formed by applying a heat treatment, and after applying a solution in which an oxide powder containing Bi, Sr, Ca or Cu is dissolved in an organic solvent on a substrate, the substrate is subjected to a heat treatment.
[0011]
As the former solution, for example, a metal organic acid salt such as octylic acid, neodecanoic acid, naphthenic acid or an organic metal compound such as metal alkoxide and metal acetylacetonate, which is soluble in a solvent, a hydrocarbon type, an ether type In addition, an alcoholic organic solvent or a solvent in which an organic solvent and water are mixed can be used.
[0012]
As the latter solution, for example, a powder having a superconducting composition produced by a solid phase method, a coprecipitation method, a sol-gel method or the like, or a solution in which a superconducting powder is dissolved in an organic solvent can be used. Of course, it goes without saying that these mixed solutions can be used.
The heat treatment in the present invention is carried out in an atmosphere having an oxygen partial pressure of 5 mol% or less, and this atmospheric gas is a mixture of oxygen gas and an inert gas such as nitrogen or argon, and particularly oxygen partial pressure. It is preferable to use a mixed gas of 0.1 to 5 mol%. When the oxygen partial pressure exceeds 5 mol%, the effect of lowering the partial melting point is small, and after 0.1 mol% partial melting, the reaction form until solidification changes, making it difficult to produce the (2212) phase.
[0013]
Since the maximum temperature of the heat treatment varies depending on the oxygen partial pressure, it is preferably set to a temperature about 5 to 30 ° C. higher than the partial melting point.
The method for producing a Bi-based oxide superconductor of the present invention is suitable for the production of a film body composed of a Bi-based (2212) phase. In this case, the best method is to form Bi on a substrate composed of silver or a silver alloy. When a superconductor is manufactured by heat-treating a composite film formed with a superconducting precursor film containing Sr, Ca or Cu at a composition of about Bi: Sr: Ca: Cu = 2: 2: 1: 2, Heating is performed in an atmosphere having an oxygen partial pressure of 0.1 to 5 mol% and below the softening temperature of the substrate above the partial melting point of the superconducting precursor and then gradually cooled to near the freezing point.
[0014]
[Action]
In the present invention, by setting the heat treatment atmosphere to an oxygen partial pressure of 5 mol% or less, it becomes possible to lower the partial melting point, thereby producing a superconductor at a low heat treatment temperature. Further, deoxygenation treatment is not necessary.
[0015]
【Example】
Examples of the present invention and comparative examples will be described below.
Example Bi, Sr, Ca and Cu octylates were added at a concentration of 50 wt% in xylene such that the metal content had a molar ratio of Bi: Sr: Ca: Cu = 2: 2: 1: 2. It melt | dissolved and added the oxide powder in this solution, and produced the paste.
[0016]
The above oxide powder was produced by firing a coprecipitated powder having a molar ratio of Bi: Sr: Ca: Cu = 2: 2: 1: 2 at 850 ° C. for 20 hours.
This paste is applied on an Ag substrate to a thickness of 25 μm by an applicator coating method, thermally decomposed at 500 ° C., heated to a temperature equal to or higher than the partial melting point in an atmosphere having an oxygen partial pressure of 1 mol%, and then predetermined. The film body was formed by gradually cooling to below the freezing point at a cooling rate of. This partial melting point was 840 ° C.
[0017]
The superconducting film thus obtained was measured for its critical temperature (Tc) and critical current value (Ic: 77K, 0T).
as a result,
(A) Tc = 93K under conditions of a maximum heat treatment temperature of 870 ° C. and a cooling rate of 2 ° C./h.
Ic = 12A,
(B) Tc = 93K under the conditions of a maximum heat treatment temperature of 870 ° C. and a cooling rate of 4 ° C./h.
Ic = 10A,
(C) Tc = 93K under the conditions of a maximum heat treatment temperature of 860 ° C. and a cooling rate of 2 ° C./h.
Ic = 18A,
(D) Tc = 93K under conditions of a maximum heat treatment temperature of 850 ° C. and a cooling rate of 2 ° C./h.
A result of Ic = 8A was obtained.
[0018]
Comparative Example 1
In the same manner as in the examples, a film body was formed on an Ag substrate, thermally decomposed at 500 ° C., heated in the atmosphere to a temperature above the partial melting point, and then gradually cooled to a temperature below the freezing point at a predetermined cooling rate. Thus, a film body was formed. This partial melting point was 880 ° C.
The critical temperature (Tc) and critical current value (Ic) of the superconducting film thus obtained were measured.
[0019]
As a result, the results of Tc = 79K and Ic = 1A were obtained under the conditions of the maximum heat treatment temperature of 890 ° C. and the cooling rate of 3 ° C./h.
Comparative Example 2
In the same manner as in the examples, a film body was formed on an Ag substrate, thermally decomposed at 500 ° C., heated to a temperature equal to or higher than the partial melting point in an atmosphere having an oxygen partial pressure of 8 mol%, and then subjected to predetermined cooling. The film body was formed by slow cooling to the temperature below the freezing point. This partial melting point was 860 ° C.
[0020]
The critical temperature (Tc) and critical current value (Ic) of the superconducting film thus obtained were measured.
As a result, the results of Tc = 83K and Ic = 5A were obtained under the conditions of the maximum heat treatment temperature of 890 ° C. and the cooling rate of 3 ° C./h.
Comparative Example 3
In the same manner as in the examples, a film body was formed on an Ag substrate, thermally decomposed at 500 ° C., heated to a temperature equal to or higher than the partial melting point in a nitrogen gas atmosphere, and then gradually reduced to the freezing point or lower at a predetermined cooling rate. The film body was formed by cooling. This partial melting point was 800 ° C.
[0021]
The critical temperature (Tc) and critical current value (Ic) of the superconducting film thus obtained were measured.
As a result, Tc was not exhibited under the conditions of a maximum heat treatment temperature of 850 ° C. and a cooling rate of 3 ° C./h.
[0022]
【The invention's effect】
As described above, according to the present invention, in the method for producing a superconductor by annealing to the vicinity of the freezing point after heating at or above the partial melting point, the heat treatment atmosphere is set to an oxygen partial pressure of 5 mol% or less. This makes it possible to lower the partial melting point, which makes it possible to manufacture superconductors at a low heat treatment temperature and eliminates the need for post-annealing, making it easy to increase the film length and area. can do.

Claims

In the method for producing a superconductor by performing heat treatment on a composite film in which a superconducting precursor film containing Bi, Sr, Ca or Cu in a molar ratio of 2: 2: 1: 2 is formed on a substrate, Bi (2212) is obtained by heating the heat treatment in an atmosphere having an oxygen partial pressure of 5 mol% or less and at or below the softening temperature of the substrate to a temperature higher than or equal to the partial melting point of the superconducting precursor and then gradually cooling to near the freezing point. A method for producing a Bi-based oxide superconductor, wherein the generation of the phase superconductor and the deoxygenation treatment are performed simultaneously.

2. The superconducting precursor film is formed by applying a solution obtained by dissolving a metal organic acid salt or an organic metal compound in an organic solvent on a base material, and then performing a heat treatment. A method for producing a Bi-based oxide superconductor.

2. The Bi-based oxide superconductor according to claim 1, wherein the superconducting precursor film is formed by applying a solution obtained by dissolving an oxide powder in an organic solvent on a base material, and then performing a heat treatment. Body manufacturing method.

Superconductivity is obtained by applying heat treatment to a composite film in which a superconducting precursor film containing Bi, Sr, Ca or Cu in a molar ratio of 2: 2: 1: 2 is formed on a base material made of silver or a silver alloy. In the method for producing a body, the heat treatment is performed in an atmosphere having an oxygen partial pressure of 0.1 to 5 mol% and after being heated above the softening temperature of the substrate above the partial melting point of the superconducting precursor, and then at a freezing point. A method for producing a Bi-based oxide superconductor, wherein the Bi (2212) phase superconductor is generated and deoxygenated at the same time by being gradually cooled .