JP4101930B2

JP4101930B2 - Oxide bulk superconductor

Info

Publication number: JP4101930B2
Application number: JP12663198A
Authority: JP
Inventors: 辰雄藤本
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1998-04-22
Filing date: 1998-04-22
Publication date: 2008-06-18
Anticipated expiration: 2018-04-22
Also published as: JPH11306877A

Description

【０００１】
【発明の属する技術分野】
本発明は、ＲＥＢａ₂ Ｃｕ₃ Ｏ_7-x 型の酸化物超電導体相を有するバルク超電導材料に関するものである。
【０００２】
【従来の技術】
ＲＥＢａ₂ Ｃｕ₃ Ｏ_7-x 型の酸化物バルク超電導体は超強力マグネットや磁気浮上などの応用に有望な材料の一つと目されているが、その特性を最大限に引き出すためにはバルク体試料の単結晶化を図る必要がある。これを実現する方法の一つとしてＱＭＧ法（特開昭６３−２６１６０７）が開発され、多結晶バルク体に特有の結晶粒界が基本的に皆無な、ＲＥＢａ₂ Ｃｕ₃ Ｏ_7-x 型酸化物バルク超電導体が比較的容易に得られるようになった。
【０００３】
ＱＭＧ法とは、通常、予め所定の配合比になるように秤量されたＲＥ、Ｂａ、Ｃｕの複合酸化物の混合粉末を加圧成形した後、半溶融状態になるように加熱し、適度な温度勾配を保ちながら徐冷することによって一方向結晶成長を誘起し、その結果ＲＥＢａ₂ Ｃｕ₃ Ｏ_7-x 型酸化物超電導体の単結晶バルク体を得るというものである。また、試料周辺部において発生しやすい未配向結晶粒の生成を抑制し、より確実な単結晶化を行う目的から、成形体内部に１２３相の結晶生成温度の異なる成分層あるいは組成層を成形体の温度勾配に沿って配置するなどの発明がこれまでになされており（特開平５−１７０５９８号公報（特願平３−３５４４６９号））、直径１００ｍｍを越える大型円盤状単結晶が既に作製可能な現況にある。
【０００４】
このようにして得られた単結晶材は、基本的には１２３相のマトリックス内に微細な２１１相が分散した組織を有しており、近年特に後者の２１１相については、Ｐｔ、Ｒｈ、あるいはＣｅＯ₂ などの微量添加により約１μｍ程度まで微細化が可能になっている。微細な２１１相は超電導体内に侵入した磁束線を強固にピン止めする作用があるため、同種の無添加従来材に比べて臨界電流密度を格段に向上させることに成功している。近年では、液体窒素温度下で強磁場を印加することによって磁束を捕捉させ、試料を超強力マグネットとして用いるなど、高臨界電流密度の特徴を生かした様々な用途に供試されつつある。
【０００５】
【発明が解決しようとする課題】
前項で述べたように、本系の半溶融体に適当な結晶化開始起点を与えると、適度な温度環境下では、一般的には｛１００｝面の晶癖面で覆われた１２３相結晶が成長し、最終的に試料全体の単結晶化が完了する。円盤状の半溶融体の場合について例示するならば、通常は種結晶と呼ばれる、半溶融体の１２３相結晶生成温度よりも高い１２３相結晶生成温度を有する成分系の、大きさ約数ｍｍ角程度の微小単結晶片を結晶化開始起点として用い、円盤の底面に向かって適当な温度勾配を付与しながら徐冷を行うことにより単結晶化を行う方法が一般的に行われている。このとき、種結晶をそのｃ面が円盤状半溶融成形体の上面部中央に接触するように静置すると、１２３相のｃ軸（［００１］軸）が円盤の中心軸に平行な単結晶が得られる。
【０００６】
このような円盤状単結晶に液体窒素温度下で中心軸方向に十分に大きな強磁場（通常は１．５Ｔ以上）を印加し、しかる後に上面あるいは底面の表面近傍の磁束密度の強度分布を測定すると、試料内の１２３相が完全な単結晶状態にあるならば、円盤の中心軸上に最大値を有する同心円状の磁束密度分布（等値曲線）が得られる筈である。臨界電流密度の磁場依存性が問題とならない条件下では、試料直径が大きい程、中心軸上の磁束密度の最大値も大きくなり、このため、マグネットとしてより強力な磁力、あるいはより強力な磁気浮上力を要求されるような応用局面においては、試料の直径を大きくする大型化が有効な方法の一つとなる。従来までの知見では、直径が約４５ｍｍの円盤状単結晶（厚さ約１５ｍｍ）では、ほぼ同心円状の磁束密度分布が得られている。
【０００７】
しかしながら、円盤状単結晶の直径がこれを越えて大型化するにつれて次第に磁束密度分布の乱れが大きくなるという問題が指摘されていた。その典型例を図１ないし図３に示す。図中の３試料はいずれも未配向結晶粒の無い単結晶体である。なお、液体窒素温度下での最大外部印加磁場は１．７Ｔとした。発明者らの解析によれば、結晶化開始起点を含む試料上面の表面磁束密度分布を測定すると、円盤の直径が大きくなるに従い（１００）あるいは（０１０）晶癖面のほぼ中心を結ぶ軸上（面内の［１００］および［０１０］軸方向）の試料周辺付近における磁束密度強度が小さく、その結果、磁束密度分布としてはこの方向の等値曲線が円盤中心部に向かって屈曲した、全体としてほぼ四回対称に近い形状を呈することが判明している。このような現象は、図３に顕著に見られるように、特に直径が７０ｍｍを越える場合に著しい。かような状況では、不均一な磁束密度分布の影響を受けて中心軸上の最大磁束密度が低下したり、永久磁石体との磁気反発力、すなわち磁気浮上力が弱められてしまう。また他方で、バルク試料を加工して各種用途に供試する場合に超電導特性の不均一性が顕現し、動作特性に問題が生じる。すなわち、試料が本来有している超電導特性が大型試料において十分に発現できなくなり、大型化によるメリットが失われるという問題が生じてしまう。
【０００８】
本発明は、ＲＥＢａ₂ Ｃｕ₃ Ｏ_7-x 型の酸化物超電導相を有するバルク超電導体において、試料の大型化に伴って発生する磁束密度分布の不均一化の問題を改善し、大型試料においても、磁束密度分布に、バルク体形状以外の要因に起因する歪曲を有しない、超電導特性の極めて均一な大型バルク超電導材料を提供することを目的としてなされたものである。
【０００９】
【課題を解決するための手段】
表面磁束密度分布の均一度を表す指標を以下のように定義する。
バルク体の上面あるいは底面の中心付近の磁束密度最大位置から、面内の［１００］あるいは［０１０］あるいはそれらと等価な任意の１２３相結晶方向に、最大磁束密度値Ｂ_m に対してｘ×Ｂ_m （０．０≦ｘ≦１．０）の磁束密度値を与える点までの距離をＴA、また同様に面内の［１１０］あるいは［−１１０］あるいはそれらと等価な任意の１２３相結晶方向についての距離をＴBとした場合に、比ＴB／ＴAを、図１ないし図３において示した３つの試料について計算した。図４にその結果を示す。なお図において横軸は上記のｘ、すなわち最大磁束密度値Ｂ_m を１と規格化した場合の各点における磁束密度値を表す。
【００１０】
バルク体が大型化すると磁束密度分布に四回対称に近い乱れが生じる結果、比ＴB／ＴAが増加しており、特に直径７５ｍｍのバルク体の場合では、その値はｘ＝０．６において最大１．６３に及ぶことが判る。このように、比ＴB／ＴA はバルク体大型化に伴う表面磁束密度分布の乱れを表し得るパラメーターの一つであり、その変化が最も大きいｘ＝０．６における値を、以下の本文では均一度と定義する。すなわち、バルク体の上面あるいは底面の表面磁束密度分布について、中心付近の表面最大磁束密度値Ｂ_m に対して０．６×Ｂ_m の磁束密度値に対応する等値曲線と磁束密度最大位置までの距離のうち、面内の［１００］あるいは［０１０］あるいはそれらと等価な１２３相結晶方向の値の中の最小値をｔA、また面内の［１１０］あるいは［−１１０］あるいはそれらと等価な１２３相結晶方向の最大値をｔBとした場合、均一度をｔB／ｔAと定義する。図５に本定義式を説明する模式図を示す。
【００１１】
本発明は、通常の方法により作製された、円あるいはそれに類する形状の断面を有する酸化物バルク超電導体において、その大型化に伴って発生する前記のような磁束密度分布の乱れが抑制され、断面円の直径が５０ｍｍ以上７０ｍｍ未満の円盤あるいは円柱であるバルク超電導体の場合、均一度が１．１以下、また断面円の直径が７０ｍｍ以上の円盤あるいは円柱であるバルク超電導体の場合、１．３以下であることを特徴とする、ＲＥＢａ₂ Ｃｕ₃ Ｏ_7-x 型の酸化物超電導相を有するバルク超電導材料に関するものである。本発明のバルク超電導材料の具体的な構成要件の詳細について以下に述べる。
【００１２】
【発明の実施の形態】
本系の結晶成長は超電導相である１２３相の｛１００｝晶癖面の前進によって進行することは前項にて述べた。ところで詳細な結晶学的解析によれば、一方向結晶成長によって得られた１２３単結晶中には、小傾角粒界などの下部構造が併存しており無欠陥な完全単結晶ではないこと、またｃ晶癖面（（００１）面）の前進によって形成された結晶領域と、ａあるいはｂ晶癖面（（１００）あるいは（０１０）面）の前進によって形成された結晶領域とでは、その下部構造が著しく異なっていることが判明している。図６に前項にて取り上げた円柱状バルク体の例について、そのような異なる結晶領域の存在部分の概略を示す。このような二つの結晶領域を以下ではそれぞれ簡単にｃ結晶領域、ａ／ｂ結晶領域と略記することとする。
【００１３】
発明者らによる解析の結果、通常の方法にて作製された大型バルク材において、磁束密度分布を劣化させている原因について、目下のところ主として以下に述べる点を指摘するに至っている。すなわち、
（１）試料が大型化すると、１２３相の結晶成長過程中に、液相の一部あるいは２１１相などの非超電導相が、成長末端部の、特にａ／ｂ晶癖面の前面部に偏析して孤立化しやすい傾向があり、それらの非超電導相が結晶成長の過程で１２３相結晶中に取り込まれることにより、結晶成長後のバルク体内部での超電導電流の通路に迂回が生じ、結果的に磁束密度分布に乱れが生じる。
（２）バルク体が大型化した場合に、１２３相中の下部構造による影響が指摘され、特にａ／ｂ結晶領域中に存在する小傾角粒界において、超電導弱結合が発生しやすく、このため超電導特性が劣化することにより磁束密度分布の形状に乱れが生じる。
【００１４】
上記二点の観点より鋭意検討した結果、発明者らは、特に断面円の直径が５０ｍｍを越える大型バルク材において、ほぼ同心円状の表面磁束密度分布形状が実現される、ＲＥＢａ₂ Ｃｕ₃ Ｏ_7-x 型の酸化物超電導体相を有するバルク超電導材料を見出すに至った。以下に本発明の酸化物バルク超電導体の概要と、そのバルク体において均一な表面磁束密度分布形を発現しているメカニズムについて、具体的製造方法の一例を例示しながら述べる。
【００１５】
円盤状バルク体の場合、先ずバルク体本体の１２３相結晶化開始温度よりも高い結晶化開始温度を持つ組成の成形体を別途作製し、これをバルク体本体上部に配置する。この時の成形体の形状（直径および厚さ等々）は炉内温度分布や試料内温度勾配などの成長条件を考慮して決定する。しかる後、通常法と同様に結晶成長を行うが、微小種結晶は上部成形体の上面中央に静置し、炉内温度を降下させながら成形体全体を順次結晶化させる。このような場合、まず１２３相結晶化開始温度の高い上部成形体が完全に結晶化する。このとき試料の厚さを予め適当に指定することによりバルク体本体と接している上部成形体下部面の大半がｃ結晶領域で構成されるようにすることが出来る。しかる後にさらに温度を降下、徐冷することにより上部成形体下部面のｃ結晶領域を成長起点としてバルク体本体の結晶化が進行し、最終的にバルク体本体内部の結晶化が完了する。
【００１６】
図７に、本法を適用した場合のｃ、およびａ／ｂ結晶領域の存在部分の概念図を示す。このようにして作製した試料では、バルク体本体内の、ａ／ｂ結晶領域のバルク体全体積に対する体積占有率が小さく、その結果直径４５ｍｍの円盤状単結晶のような小型バルク試料のそれに近づけることが可能になり、非超電導相の偏析・孤立化を抑制して、前記（１）が解決されるに及んだと考えられる。
【００１７】
また前記（２）についてであるが、Ｗ．Ｌｏらによって指摘されているように、小傾角粒界を挟んで、亜結晶粒のｃ軸方向に数度の偏差が存在することが知られている［Ｊ.Ｍａｔｅｒ.Ｒｅｓ.，１２（１９９７）ｐ.２８８９］。１２３相のようなコヒーレンス長が極端に短い超電導物質の場合、角度偏差が約５度を越えると超電導弱結合が発生するといわれ［Ｐｈｙｓ．Ｒｅｖ．Ｂ，４１（１９９０）ｐ.４０３８］、このような場合にはａ／ｂ結晶領域中での磁束捕捉力が低下し、磁束密度分布の形状を乱してしまう。発明者らの知見によれば、円盤状単結晶の場合、その直径が４５ｍｍを越えて大きくなると、ａ／ｂ結晶領域中の小傾角粒界における角度偏差に比較的大きなものが現れ易く、特に直径６５ｍｍのバルク体では、上記の超電導弱結合が起こる臨界角度偏差に及ぶものが生成する確率が大きくなる。
【００１８】
本発明の酸化物バルク超電導体は、ａ／ｂ結晶領域の体積占有率を小さくすることによって、上述の角度偏差が比較的大きな小傾角粒界の発生を抑制し、その結果として大型バルク材の下部構造を、上述の原因（２）が特性上問題にならない程度に抑えられている、直径４５ｍｍの円盤状単結晶のような小サイズのバルク体（その表面磁束密度分布が図１に示されている）の下部構造に近づけることを可能にすることによってもたらされているものと推測される。
【００１９】
なお、特開平５−１７０５９８号公報（特願平３−３５４４６９号）において、上記と類似の手法によって大型単結晶試料を得るという製造方法が提案されているが、そこで述べられている解決手段の目的は、未配向結晶粒の無い完全な単結晶試料を得る点にある。未配向結晶粒とは、結晶成長過程において結晶成長中の晶癖面端部とは異なる部分より独立に生成する、種結晶により成長した主結晶部分とは基本的に結晶配向に相関を持たない結晶粒を指すものであり、バルク体の表面磁束密度分布をはじめとする各種の超電導特性を著しく劣化させる原因となるものである。しかしながら、そのような未配向結晶粒が皆無なバルク体試料においても、単結晶中の微細組織においては、小傾角粒界等々の下部構造や異相は不可避なものとして通常必ず存在する。
【００２０】
本発明は、前項にて述べた通常の溶融・一方向結晶成長法で作製された酸化物バルク超電導体において、これらの下部構造等を含むもののうち、ａ／ｂ結晶領域の試料全体積に対する体積占有率が低減された大型酸化物バルク超電導体の表面磁束密度分布が著しく改善されていることを主旨とするものであり、この点前記の発明の主旨とは根本的に異なるものである。また、特開平７―４１３９４において述べられているような、未配向結晶粒の発生を抑制することによって再現性を向上させることを目的とする発明とも、上記同様、目的とする効果自体が本発明と基本的に異なることも明白である。
【００２１】
【実施例】
以下、本発明の実施例について詳細に説明する。
（実施例１）
モル比でＹＢａ₂ Ｃｕ₃ Ｏ_7-x ：Ｙ₂ ＢａＣｕＯ₅ ＝０．７５：０．２５となるようにＹ₂ Ｏ₃ 、ＢａＯ₂ 、及びＣｕＯ原料粉を混合した混合粉Ａ、及びモル比でＤｙＢａ₂ Ｃｕ₃ Ｏ_7-x ：Ｄｙ₂ ＢａＣｕＯ₅ ＝０．７５：０．２５となるようにＤｙ₂ Ｏ₃ 、ＢａＯ₂ 、及びＣｕＯ原料粉を混合した混合粉Ｂを作製した後、約９００℃で８時間仮焼した。これらの仮焼粉を解砕し、ＣＩＰ成形により直径１００ｍｍ、厚さ３５ｍｍのＡ混合粉圧縮成形体、及び直径６０ｍｍ、厚さ２０ｍｍのＢ混合粉圧縮成形体を作製した。
【００２２】
次いで、Ｂ混合粉圧縮成形体を、図７に示すようにＡ混合粉圧縮成形体上部に配置し、１１８０℃で１時間保持した。しかる後、予め作製しておいた約２ｍｍ角の（Ｎｄ０．５Ｓｍ０．５）Ｂａ₂ Ｃｕ₃ Ｏ_7-x 種結晶を、そのｃ面がＢ混合粉圧縮成形体上部面中央に接触するように静置し、引き続いて９５０℃まで平均０．２℃／ｈｒで徐冷して単結晶化させた。成長完了後、一度室温まで冷却し、さらに酸素雰囲気中にて４５０℃１００時間の酸素富化アニール処理を行った。
【００２３】
Ａ混合粉圧縮成形体（Ｙ系超電導体部分）部分の単結晶体は直径７５ｍｍであった。これから厚さ１５ｍｍの円盤状試料を切りだし、液体窒素中に浸漬した後、円盤の中心軸方向に平行に１．７Ｔの磁場を印加した。液体窒素に浸漬した状態で円盤試料の表面磁束密度をホール素子を用いて測定した。その結果を図８に示す。図３に示す従来の酸化物バルク超電導体と比較した場合、目視によっても明らかなように、本発明の酸化物バルク超電導体において表面磁束密度分布の形状が大幅に改善され、ほぼ同心円状になっていることが判る。
【００２４】
図８および図３に示す試料について、図４と同様に比ＴB／ＴAのｘ依存性を計算した。その結果を図９に示す。両試料の均一度ｔB／ｔAは、それぞれ１．０１（本発明）および１．６２（従来法＝比較例）であり、表面磁束密度分布の大幅な改善が本結果によって裏付けられていることが判る。
【００２５】
（実施例２）
実施例１と同様な方法により、直径８５ｍｍ、厚さ３０ｍｍのＡ混合粉圧縮成形体、及び直径４０ｍｍ、厚さ１８ｍｍのＢ混合粉圧縮成形体を作製し、引き続いて同様の結晶成長処理および酸素富化処理を行った。得られた単結晶体は直径６５ｍｍであった。なお、Ｂ混合粉圧縮成形体を用いない従来法によって、同様サイズの比較用試料を作製した。実施例１と同様に、上記の二試料から直径約６５ｍｍの円盤状バルク体を切り出し、しかる後に液体窒素中で１．７Ｔの磁場を印加後、表面磁束密度を測定した。本発明の試料および比較試料の均一度ｔB／ｔAは、それぞれ１．０３および１．１８であった。
【００２６】
（実施例３）
実施例１と同様な方法により、直径１３０ｍｍのＡ混合粉圧縮成形体、及び直径６０ｍｍのＢ混合粉圧縮成形体を作製し、引き続いて同様の結晶成長処理および酸素富化処理を行った。本体部分から得られた単結晶体の直径は約１００ｍｍ、厚さは２０ｍｍである。比較用試料として、Ｂ混合粉圧縮成形体を用いない従来法により同じサイズのバルク体を作製した。実施例１と同様に、液体窒素中で１．７Ｔの磁場を印加後、表面磁束密度を測定した。本発明の試料および比較試料の均一度ｔB／ｔAは、それぞれ１．２６、１．９４であった。
【００２７】
これらの試料を液体窒素温度に冷却し、しかる後に円盤垂直上方から永久磁石を漸次近づけ、その時の永久磁石が超電導試料より受ける反発力、すなわち磁気浮上力を測定した。ここで使用した磁石は直径９０ｍｍ、高さ７０ｍｍのネオジウム・鉄・ほう素系永久磁石である。測定の結果、本発明材料の場合１５４ｋｇｆ、比較試料の場合で１２７ｋｇｆであり、表面磁束密度分布形状が改善された本発明のバルク試料において磁気浮上力が大幅に改善されていることが判る。
【００２８】
【発明の効果】
本発明により、大型試料においても、表面磁束密度分布にバルク体形状以外の要因に起因する歪曲が無く、基本的にはほぼ同心円状の磁束密度分布を有する、超電導特性の極めて均一なバルク超電導材料が作製可能になった。バルク体本体のマグネット応用や浮上力応用は勿論のこと、バルク体を加工することによって作製された試料についても超電導特性が極めて均一であるがゆえに、電流リード等の応用にあたっても、格段に素子の動作特性や信頼性を向上することが可能になった。
【図面の簡単な説明】
【図１】直径が４５ｍｍの円盤状バルク体試料の表面磁束密度分布
【図２】直径が６５ｍｍの円盤状バルク体試料の表面磁束密度分布
【図３】直径が７５ｍｍの円盤状バルク体試料の表面磁束密度分布
【図４】図１〜３に示す３試料の表面磁束密度分布について計算の結果、得られたＴA ／ＴBのｘ依存性
【図５】円盤形状バルク体（中心付近の最大表面磁束密度値＝１．０Ｔを仮定）の場合の均一度ｔB／ｔAを説明する模式図
【図６】円柱状試料におけるａ／ｂ結晶領域およびｃ結晶領域の存在部分を示す模式図
【図７】ａ／ｂ結晶領域の体積占有率が小さい酸化物バルク超電導体を作製する製造方法の概念図
【図８】図７に示す方法により作製された、本発明の直径７５ｍｍの円形状断面を有する円盤状バルク体試料の表面磁束密度分布の測定結果
【図９】図８に示す本発明の試料、および図３に示す従来法による試料の表面磁束密度分布について計算の結果、得られたＴA／ＴBのｘ依存性
【符号の説明】
４１種結晶
４２ａ／ｂ結晶領域
４３ｃ結晶領域
５１種結晶
５２中間体
５３本体結晶
５４ａ／ｂ結晶領域
５５ｃ結晶領域[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a bulk superconducting material having a REBa ₂ Cu ₃ O _7-x type oxide superconductor phase.
[0002]
[Prior art]
The REBa ₂ Cu ₃ O _7-x type oxide bulk superconductor is regarded as one of the promising materials for applications such as super strong magnets and magnetic levitation. It is necessary to achieve single crystallization of the sample. As one of the methods for realizing this, the QMG method (Japanese Patent Laid-Open No. 63-261607) has been developed, and the REBa ₂ Cu ₃ O _7-x type oxidation is essentially free of crystal grain boundaries peculiar to polycrystalline bulk bodies. Bulk bulk superconductors can be obtained relatively easily.
[0003]
In the QMG method, a mixed powder of RE, Ba, and Cu complex oxides weighed so as to have a predetermined blending ratio is usually pressure-molded and then heated so as to be in a semi-molten state. One-way crystal growth is induced by slow cooling while maintaining a temperature gradient, and as a result, a single crystal bulk body of a REBa ₂ Cu ₃ O _7-x type oxide superconductor is obtained. In addition, for the purpose of suppressing the formation of unoriented crystal grains that are likely to occur in the periphery of the sample and performing more reliable single crystallization, a component layer or a composition layer having a different 123-phase crystal formation temperature is formed in the molded body. Have been made so far (Japanese Patent Application Laid-Open No. 5-170598 (Japanese Patent Application No. 3-354469)), and a large disk-shaped single crystal having a diameter of more than 100 mm can already be produced. The current situation.
[0004]
The single crystal material thus obtained basically has a structure in which fine 211 phases are dispersed in a 123-phase matrix. Recently, particularly the latter 211 phases have Pt, Rh, or Miniaturization to about 1 μm is possible by adding a small amount of CeO ₂ or the like. Since the fine 211 phase has the effect of firmly pinning the magnetic flux lines that have penetrated into the superconductor, it has succeeded in remarkably improving the critical current density as compared with the conventional additive-free material. In recent years, a magnetic field is captured by applying a strong magnetic field at a liquid nitrogen temperature, and a sample is used as an ultra-strong magnet. For example, it is being used for various applications that take advantage of the characteristics of high critical current density.
[0005]
[Problems to be solved by the invention]
As described in the previous section, when a suitable crystallization start point is given to the semi-melt of the present system, a 123 phase crystal generally covered with a {100} crystal habit plane under an appropriate temperature environment. Grows, and finally single crystallization of the entire sample is completed. To illustrate the case of a disk-shaped semi-melt, a component system having a 123-phase crystal formation temperature higher than the 123-phase crystal formation temperature of the semi-melt, usually referred to as a seed crystal, is about several millimeters square. A method of performing single crystallization by using a small single crystal piece of a certain degree as a starting point of crystallization and performing slow cooling while applying an appropriate temperature gradient toward the bottom surface of the disk is generally performed. At this time, when the seed crystal is allowed to stand so that its c-plane is in contact with the center of the upper surface portion of the disc-shaped semi-molten molded body, a single crystal whose 123-phase c-axis ([001] axis) is parallel to the central axis of the disc Is obtained.
[0006]
A sufficiently strong magnetic field (usually 1.5T or more) is applied to such a disk-shaped single crystal in the central axis direction at liquid nitrogen temperature, and then the magnetic flux density intensity distribution near the top or bottom surface is measured. Then, if the 123 phase in the sample is in a complete single crystal state, a concentric magnetic flux density distribution (equal curve) having a maximum value on the central axis of the disk should be obtained. Under conditions in which the dependence of the critical current density on the magnetic field does not matter, the larger the sample diameter, the larger the maximum value of the magnetic flux density on the central axis. Therefore, the magnet has a stronger magnetic force or a stronger magnetic levitation. In applications where force is required, increasing the diameter of the sample is an effective method. According to the conventional knowledge, a substantially concentric magnetic flux density distribution is obtained in a disk-like single crystal having a diameter of about 45 mm (thickness: about 15 mm).
[0007]
However, a problem has been pointed out that the disturbance of the magnetic flux density distribution gradually increases as the diameter of the disk-shaped single crystal increases beyond this. A typical example is shown in FIGS. All three samples in the figure are single crystal bodies having no unoriented crystal grains. Note that the maximum externally applied magnetic field under liquid nitrogen temperature was 1.7T. According to the analysis by the inventors, when the surface magnetic flux density distribution on the upper surface of the sample including the crystallization start point is measured, as the disc diameter becomes larger, the axis is connected to the approximate center of the (100) or (010) crystal habit plane. The magnetic flux density strength in the vicinity of the sample (in the [100] and [010] axial directions in the plane) is small, and as a result, the isoelectric curve in this direction is bent toward the center of the disk as the magnetic flux density distribution. It has been found that it has a shape that is almost quaternary symmetrical. Such a phenomenon is remarkable particularly when the diameter exceeds 70 mm, as can be seen in FIG. In such a situation, the maximum magnetic flux density on the central axis decreases due to the influence of the non-uniform magnetic flux density distribution, or the magnetic repulsion force with the permanent magnet body, that is, the magnetic levitation force is weakened. On the other hand, when the bulk sample is processed and used for various purposes, the non-uniformity of the superconducting characteristics is manifested, causing problems in the operating characteristics. In other words, the superconducting characteristics inherent to the sample cannot be sufficiently expressed in a large sample, and a problem arises in that the merit due to the increase in size is lost.
[0008]
In the bulk superconductor having the REBa ₂ Cu ₃ O _7-x type oxide superconducting phase, the present invention improves the problem of non-uniformity of the magnetic flux density distribution generated with the increase in size of the sample. The present invention has been made for the purpose of providing a large bulk superconducting material having extremely uniform superconducting characteristics and having no distortion caused by factors other than the bulk shape in the magnetic flux density distribution.
[0009]
[Means for Solving the Problems]
An index representing the uniformity of the surface magnetic flux density distribution is defined as follows.
From the maximum magnetic flux density position near the center of the top surface or the bottom surface of the bulk body, in the in-plane [100] or [010] or any equivalent 123 phase crystal direction, xx with respect to the maximum magnetic flux density value B _m The distance to the point giving the magnetic flux density value of B _m (0.0 ≦ x ≦ 1.0) is TA, and similarly [110] or [−110] in the plane or any equivalent 123 phase crystal The ratio TB / TA was calculated for the three samples shown in FIGS. 1-3, where TB is the distance in the direction. FIG. 4 shows the result. In the figure, the horizontal axis represents the above-mentioned x, that is, the magnetic flux density value at each point when the maximum magnetic flux density value B _m is normalized to 1.
[0010]
When the bulk body is enlarged, the magnetic flux density distribution is disturbed in a nearly four-fold symmetry. As a result, the ratio TB / TA increases. In particular, in the case of a bulk body with a diameter of 75 mm, the value is maximum at x = 0.6. It can be seen that it reaches 1.63. Thus, the ratio TB / TA is one of the parameters that can represent the disturbance of the surface magnetic flux density distribution accompanying the increase in bulk size, and the value at x = 0.6 with the largest change is expressed in the following text. Define once. That is, the surface magnetic flux density distribution of the top or bottom surface of the bulk body, until equality curve and the magnetic flux density up to the position corresponding to the magnetic flux density value of 0.6 × B _m to the surface the maximum magnetic flux density value B _m near the center Among the distances of [100] or [010] in the plane, or the equivalent 123 phase crystal direction value, tA, and [110] or [−110] in the plane or equivalent to them. When the maximum value in the 123 phase crystal direction is tB, the uniformity is defined as tB / tA. FIG. 5 is a schematic diagram for explaining the definition formula.
[0011]
In the present invention, an oxide bulk superconductor having a circular or similar cross-section manufactured by a normal method, the disturbance of magnetic flux density distribution as described above, which occurs with the increase in size, is suppressed, and the cross-section In the case of a bulk superconductor that is a disk or cylinder whose diameter is 50 mm or more and less than 70 mm, the uniformity is 1.1 or less, and in the case of a bulk superconductor that is a disk or cylinder whose cross-sectional diameter is 70 mm or more. The present invention relates to a bulk superconducting material having a REBa ₂ Cu ₃ O _7-x type oxide superconducting phase, characterized by being 3 or less. Details of specific constituent requirements of the bulk superconducting material of the present invention will be described below.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
As described in the previous section, the crystal growth of this system proceeds by the advance of the {100} crystal habit plane of the 123 phase, which is a superconducting phase. By the way, according to the detailed crystallographic analysis, the 123 single crystal obtained by the unidirectional crystal growth is not a defect-free perfect single crystal because there are substructures such as low-angle grain boundaries. The crystal structure formed by the advancement of the crystal habit plane ((001) plane) and the crystal area formed by the advancement of the a or b crystal habit plane ((100) or (010) plane) are substructures thereof. Are found to be significantly different. FIG. 6 shows an outline of the existence of such different crystal regions in the example of the cylindrical bulk body taken up in the previous section. Hereinafter, these two crystal regions will be simply referred to as a c crystal region and an a / b crystal region, respectively.
[0013]
As a result of the analysis by the inventors, at present, the following points are mainly pointed out as to the cause of the deterioration of the magnetic flux density distribution in the large bulk material produced by the ordinary method. That is,
(1) When the sample is enlarged, a part of the liquid phase or a non-superconducting phase such as the 211 phase segregates at the growth end, particularly the front part of the a / b crystal habit plane, during the 123 phase crystal growth process. These non-superconducting phases tend to be isolated, and the non-superconducting phase is incorporated into the 123-phase crystal in the course of crystal growth, resulting in a detour in the path of the superconducting current inside the bulk body after crystal growth. The magnetic flux density distribution is disturbed.
(2) When the bulk body is enlarged, the influence of the substructure in the 123 phase is pointed out, and superconducting weak coupling is likely to occur especially at the low-angle grain boundaries present in the a / b crystal region. As the superconducting characteristics deteriorate, the shape of the magnetic flux density distribution is disturbed.
[0014]
As a result of intensive studies from the above two viewpoints, the inventors have realized a substantially concentric surface magnetic flux density distribution shape, particularly in a large bulk material having a cross-sectional circle diameter of more than 50 mm, REBA ₂ Cu ₃ O _7. We have found a bulk superconducting material with _-x type oxide superconductor phase. Hereinafter, the outline of the oxide bulk superconductor of the present invention and the mechanism that expresses a uniform surface magnetic flux density distribution in the bulk body will be described with reference to an example of a specific manufacturing method.
[0015]
In the case of a disc-shaped bulk body, first, a molded body having a composition having a crystallization start temperature higher than the 123 phase crystallization start temperature of the bulk body main body is separately prepared, and this is disposed on the bulk body main body. The shape (diameter, thickness, etc.) of the molded body at this time is determined in consideration of growth conditions such as furnace temperature distribution and sample temperature gradient. Thereafter, crystal growth is performed in the same manner as in the normal method, but the micro seed crystal is allowed to stand at the center of the upper surface of the upper molded body, and the entire molded body is sequentially crystallized while lowering the furnace temperature. In such a case, the upper molded body having a high 123 phase crystallization start temperature is first completely crystallized. At this time, by appropriately specifying the thickness of the sample in advance, most of the lower surface of the upper molded body in contact with the bulk body main body can be constituted by the c crystal region. Thereafter, the temperature is further lowered and gradually cooled, so that the crystallization of the bulk body proceeds from the c crystal region on the lower surface of the upper molded body as the growth starting point, and finally the crystallization inside the bulk body is completed.
[0016]
FIG. 7 shows a conceptual diagram of c and a / b crystal region existing portions when this method is applied. The sample produced in this way has a small volume occupancy ratio with respect to the entire bulk body of the a / b crystal region in the bulk body, and as a result approaches that of a small bulk sample such as a disk-shaped single crystal having a diameter of 45 mm. It is considered that the above (1) has been solved by suppressing the segregation and isolation of the non-superconducting phase.
[0017]
As for the above (2), W.W. As pointed out by Lo et al., It is known that there is a deviation of several degrees in the c-axis direction of subgrains across a small-angle grain boundary [J. Mater. Res., 12 (1997). ) P.2889]. In the case of a superconducting material having an extremely short coherence length such as 123 phase, it is said that superconducting weak coupling occurs when the angular deviation exceeds about 5 degrees [Phys. Rev. B, 41 (1990) p. 4038], in such a case, the magnetic flux trapping force in the a / b crystal region is reduced, and the shape of the magnetic flux density distribution is disturbed. According to the knowledge of the inventors, in the case of a disk-shaped single crystal, when the diameter is larger than 45 mm, a relatively large angle deviation tends to appear in the small-angle grain boundary in the a / b crystal region. In a bulk body having a diameter of 65 mm, there is a high probability that a material having a critical angle deviation in which the superconducting weak coupling occurs will be generated.
[0018]
The oxide bulk superconductor of the present invention suppresses the occurrence of a low-angle grain boundary having a relatively large angular deviation by reducing the volume occupancy of the a / b crystal region. The substructure is suppressed to such a degree that the above-mentioned cause (2) does not cause a problem in terms of characteristics, and a small-sized bulk body such as a disk-shaped single crystal having a diameter of 45 mm (the surface magnetic flux density distribution is shown in FIG. It is presumed that it is brought about by making it possible to approach the substructure of
[0019]
In JP-A-5-170598 (Japanese Patent Application No. 3-354469), a manufacturing method for obtaining a large single crystal sample by a method similar to the above is proposed. The purpose is to obtain a complete single crystal sample free of unoriented grains. Unoriented crystal grains are formed independently of the crystal habit plane edge that is different from the crystal habit plane edge during crystal growth, and have no correlation with the main crystal part grown by the seed crystal. It refers to crystal grains, and causes a significant deterioration of various superconducting characteristics including the surface magnetic flux density distribution of the bulk body. However, even in such a bulk sample having no unoriented crystal grains, in the fine structure in the single crystal, substructures and heterogeneous phases such as small-angle grain boundaries are usually necessarily present.
[0020]
The present invention relates to an oxide bulk superconductor manufactured by the usual melting / unidirectional crystal growth method described in the previous section, and includes a volume of the a / b crystal region relative to the total volume of the sample, including these substructures. The main purpose is that the surface magnetic flux density distribution of the large oxide bulk superconductor with reduced occupancy is remarkably improved. This point is fundamentally different from the main point of the present invention. Further, as described in Japanese Patent Laid-Open No. 7-41394, the present invention aims at improving the reproducibility by suppressing the occurrence of unoriented crystal grains, and the target effect itself is the same as described above. It is also obvious that it is basically different.
[0021]
【Example】
Examples of the present invention will be described in detail below.
Example 1
Mixed powder A in which Y ₂ O ₃ , BaO ₂ , and CuO raw material powder are mixed so that the molar ratio is YBa ₂ Cu ₃ O _7-x : Y ₂ BaCuO ₅ = 0.75: 0.25, and the molar ratio After preparing mixed powder B in which Dy ₂ O ₃ , BaO ₂ and CuO raw material powder were mixed so that DyBa ₂ Cu ₃ O _7-x : Dy ₂ BaCuO ₅ = 0.75: 0.25 Calcination was performed at 900 ° C. for 8 hours. These calcined powders were crushed, and a C mixed powder compression molded body having a diameter of 100 mm and a thickness of 35 mm and a B mixed powder compression molded body having a diameter of 60 mm and a thickness of 20 mm were produced by CIP molding.
[0022]
Next, the B mixed powder compression molding was placed on the A mixed powder compression molding as shown in FIG. 7 and held at 1180 ° C. for 1 hour. After that, about 2 mm square (Nd0.5Sm0.5) Ba ₂ Cu ₃ O _7-x seed crystal prepared in advance is placed so that its c-plane is in contact with the center of the upper surface of the B mixed powder compression molding. The mixture was allowed to stand, and then gradually cooled to 950 ° C. at an average of 0.2 ° C./hr to be single crystallized. After completion of the growth, the sample was once cooled to room temperature, and further subjected to oxygen enrichment annealing treatment at 450 ° C. for 100 hours in an oxygen atmosphere.
[0023]
The single crystal body of the A mixed powder compression molded body (Y-based superconductor portion) portion had a diameter of 75 mm. From this, a disk-shaped sample having a thickness of 15 mm was cut out and immersed in liquid nitrogen, and then a 1.7 T magnetic field was applied in parallel to the central axis direction of the disk. The surface magnetic flux density of the disk sample was measured using a Hall element while immersed in liquid nitrogen. The result is shown in FIG. When compared with the conventional oxide bulk superconductor shown in FIG. 3, the shape of the surface magnetic flux density distribution in the oxide bulk superconductor of the present invention is greatly improved and becomes substantially concentric as is apparent from the visual inspection. You can see that
[0024]
For the samples shown in FIGS. 8 and 3, the x dependence of the ratio TB / TA was calculated in the same manner as in FIG. The result is shown in FIG. The uniformity tB / tA of both samples is 1.01 (invention) and 1.62 (conventional method = comparative example), respectively, and this result supports a significant improvement in the surface magnetic flux density distribution. I understand.
[0025]
(Example 2)
In the same manner as in Example 1, an A mixed powder compression molded body having a diameter of 85 mm and a thickness of 30 mm, and a B mixed powder compression molded body having a diameter of 40 mm and a thickness of 18 mm were prepared. Enrichment treatment was performed. The obtained single crystal had a diameter of 65 mm. In addition, the comparative sample of the same size was produced by the conventional method which does not use B mixed-powder compression molding. In the same manner as in Example 1, a disc-shaped bulk body having a diameter of about 65 mm was cut out from the above two samples, and a surface magnetic flux density was measured after applying a 1.7 T magnetic field in liquid nitrogen. The uniformity tB / tA of the sample of the present invention and the comparative sample were 1.03 and 1.18, respectively.
[0026]
(Example 3)
A mixed powder compression molded body having a diameter of 130 mm and a B mixed powder compression molded body having a diameter of 60 mm were produced by the same method as in Example 1, and subsequently, the same crystal growth treatment and oxygen enrichment treatment were performed. The diameter of the single crystal obtained from the main body portion is about 100 mm and the thickness is 20 mm. As a comparative sample, a bulk body of the same size was produced by a conventional method not using a B mixed powder compression molded body. Similarly to Example 1, after applying a 1.7 T magnetic field in liquid nitrogen, the surface magnetic flux density was measured. The uniformity tB / tA of the sample of the present invention and the comparative sample were 1.26 and 1.94, respectively.
[0027]
These samples were cooled to liquid nitrogen temperature, and then the permanent magnet was gradually approached from above the disk vertically, and the repulsive force that the permanent magnet at that time received from the superconducting sample, that is, the magnetic levitation force was measured. The magnet used here is a neodymium / iron / boron permanent magnet having a diameter of 90 mm and a height of 70 mm. As a result of the measurement, it is 154 kgf in the case of the material of the present invention and 127 kgf in the case of the comparative sample, and it can be seen that the magnetic levitation force is greatly improved in the bulk sample of the present invention in which the surface magnetic flux density distribution shape is improved.
[0028]
【The invention's effect】
According to the present invention, even in a large sample, there is no distortion caused by factors other than the shape of the bulk body in the surface magnetic flux density distribution, and basically a bulk superconducting material with extremely uniform superconducting characteristics having a substantially concentric magnetic flux density distribution. Can be made. The superconducting properties of the sample made by processing the bulk body as well as the bulk body magnet application and levitation force application are extremely uniform. It has become possible to improve operating characteristics and reliability.
[Brief description of the drawings]
[Fig. 1] Surface magnetic flux density distribution of a disk-shaped bulk sample having a diameter of 45 mm [Fig. 2] Surface magnetic flux density distribution of a disk-shaped bulk sample having a diameter of 65 mm [Fig. 3] Surface magnetic flux density distribution [Fig.4] The x-dependence of TA / TB obtained as a result of the calculation of the surface magnetic flux density distribution of the three samples shown in Figs. 1-3 [Fig.5] Disc-shaped bulk body (maximum surface near the center) FIG. 6 is a schematic diagram for explaining the uniformity tB / tA in the case of magnetic flux density value = 1.0 T. FIG. 6 is a schematic diagram showing the a / b crystal region and the c crystal region existing in a cylindrical sample. FIG. 8 is a conceptual diagram of a manufacturing method for manufacturing an oxide bulk superconductor having a small volume occupancy in the a / b crystal region. FIG. 8 has a circular cross section having a diameter of 75 mm according to the present invention manufactured by the method shown in FIG. Of the surface magnetic flux density distribution of a disk-shaped bulk sample Constant results [9] Samples of the present invention shown in FIG. 8, and the surface magnetic flux density distribution of the sample according to the conventional method shown in FIG. 3 of the calculation result, x dependency of the obtained TA / TB [Description of symbols]
41 seed crystal 42 a / b crystal region 43 c crystal region 51 seed crystal 52 intermediate 53 main body crystal 54 a / b crystal region 55 c crystal region

Claims

In a single crystal-like REBa ₂ Cu ₃ O _7-x phase (123 phase) (RE is a rare earth element including Y and a combination thereof), a fine RE ₂ BaCuO ₅ phase (211 phase) is dispersed, In the crystallization process, a molded body having a composition having a crystallization start temperature higher than the 123 phase crystallization start temperature of the bulk body is placed at the top of the bulk body, and the seed crystal is allowed to stand at the center of the upper surface of the molded body. Then, it is a bulk body that is slowly cooled at 0.2 ° C./hr or less and has a disk or columnar shape with a cross-sectional diameter of 50 mm or more and less than 70 mm, and a c-phase of 123 phase crystal in the central axis direction. An oxide bulk superconductor composed of a complex oxide of RE, Ba, and Cu with parallel axes ([001] axis), the surface magnetic flux of either the top surface or the bottom surface measured after applying a strong magnetic field at liquid nitrogen temperature About density distribution Of the distance between the iso-curve corresponding to the magnetic flux density value 0.6 times the maximum magnetic flux density near the center of the plane and the maximum magnetic flux density position, [100] or [010] in the plane or 123 phase equivalent to them When the minimum value of the distance in the crystal direction is tA and the maximum value of the distance in the in-plane [110] or [−110] or equivalent 123 phase crystal direction is tB, tB / tA is 1.1. An oxide bulk superconductor characterized by:

In a single crystal-like REBa ₂ Cu ₃ O _7-x phase (123 phase) (RE is a rare earth element including Y and a combination thereof), a fine RE ₂ BaCuO ₅ phase (211 phase) is dispersed, In the crystallization process, a molded body having a composition having a crystallization start temperature higher than the 123 phase crystallization start temperature of the bulk body is placed at the top of the bulk body, and the seed crystal is allowed to stand at the center of the upper surface of the molded body. Then, it is a bulk body that is gradually cooled at 0.2 ° C./hr or less and has a disk or columnar shape with a cross-sectional diameter of 70 mm or more, and the c-axis ( [001] Axis) is an oxide bulk superconductor composed of a complex oxide of RE, Ba, and Cu with parallel surfaces, and the surface magnetic flux density distribution on either the top surface or the bottom surface measured after applying a strong magnetic field at liquid nitrogen temperature About the center of the plane Of the distance between the iso-curve corresponding to the magnetic flux density value 0.6 times the maximum magnetic flux density and the maximum magnetic flux density position, the distance in the in-plane [100] or [010] or equivalent 123 phase crystal direction TB / tA is 1.3 or less, where tA is the minimum value of t and the maximum distance in the in-plane [110] or [−110] or equivalent 123 phase crystal direction is tB. Oxide bulk superconductor characterized by