JP2004111203A

JP2004111203A - MANUFACTURING METHOD FOR MgB2 SERIES SUPERCONDUCTIVE WIRE

Info

Publication number: JP2004111203A
Application number: JP2002271763A
Authority: JP
Inventors: Takahito Machi; 町　敬人; Satoshi Shimura; 志村　聡; Naoki Koshizuka; 腰塚　直己; Masahito Murakami; 村上　雅人
Original assignee: International Superconductivity Technology Center
Current assignee: International Superconductivity Technology Center
Priority date: 2002-09-18
Filing date: 2002-09-18
Publication date: 2004-04-08
Anticipated expiration: 2022-09-18
Also published as: JP4014149B2

Abstract

<P>PROBLEM TO BE SOLVED: To establish a means capable of stably providing MgB<SB>2</SB>series superconductive wires excellent in the superconductive characteristics. <P>SOLUTION: The MgB<SB>2</SB>series superconductive wire is fabricated through such processes that powder-form Mg and B and eventually further Ti are put in a metal tube of copper etc. and subjected to an area reduction processing such as strip rolling, swaging, rolling, a drawing using a die, etc. and a heating process at 600°C or over. At this time, the heating process is conducted in an inert gas (Ar gas etc.) pressurized atmosphere with 2 atom. or more, and the area reduction processing and the heating process are conducted alternately in at least two cycles. It is preferable that the metal tube is of copper and the temperature of the heating process is 600-900°C. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
この発明は、ＭｇＢ_２　系超電導線材の作製方法に関し、電力用ケ−ブル，マグネット，モ−タ，発電機等に適用するための超電導線材を低コストで安定供給できる道を開くものである。
【０００２】
【従来の技術】
強磁界マグネット等に適用されている超電導線材としてはＮｂＴｉや　Ｎｂ_３Ｓｎ等の金属系超電導材料が主流をなしているが、これらの材料は臨界温度Ｔｃが低いのでその使用は液体ヘリウム温度領域に限られ、そのため超電導クエンチの問題が大きかった。
【０００３】
そこで、最近になって超電導特性を示すことが見出されたＭｇＢ_２　が注目されるようになり、このマグネシウムのホウ化物を超電導材料として利用すべく様々な検討がなされている。
しかし、ＭｇＢ_２　は臨界温度Ｔｃが３９Ｋと比較的高くてクエンチの点で有利であるだけでなく、従来の金属間化合物超電導体よりも高い２０Ｋ程度まで使用温度が拡大すると期待されているものの、非常に硬い材料であるＭｇＢ_２　の線材化手段が見出されておらず、これがＭｇＢ_２　を超電導材料として利用する上での大きな障害となっていた。
【０００４】
ところで、従来より、金属間化合物等の如き硬くて脆い材料の線材化にＰＩＴ法（Ｐｏｗｄｅｒ　ｉｎ　ｔｕｂｅ　ｐｒｏｓｅｓｓ）の適用がなされている。このＰＩＴ法は、金属間化合物等の粉末を銅や銀等の管状金属に装入してから加熱・焼結し、この管状金属と共に延伸加工に付して線材化する手法である。
そこで、多くの研究機関では、「合成したＭｇＢ_２　を粉末状にして管状金属に装入し、　これを加熱・焼結したものを延伸加工してＭｇＢ_２　線材を製作する手法」が種々検討されてきた。
しかし、未だに超電導線材として利用し得る高い臨界電流密度を有したＭｇＢ_２　線材を安定生産する技術が確立されるには至っていない。
【０００５】
このようなことから、本発明の目的は、超電導特性に優れたＭｇＢ_２　系超電導線材を安定提供できる手段を確立することに置かれた。
【０００６】
【課題を解決するための手段】
本発明者らは上記目的を達成すべく鋭意研究を行った結果、次のことが明らかとなった。
即ち、ＭｇＢ_２　線材の製造を目指してＰＩＴ法を適用する場合、これまで多くの研究機関で試みられてきた“粉末状のＭｇＢ_２　を管状金属に装入して焼結・延伸加工を施す手法”によるとＭｇＢ_２　が極めて硬質であるが故に超電導特性を備えた細径長尺材とすることは困難であったが、前記管状金属内にＭｇＢ_２　の合成原料である粉末状のＭｇとＢとの混合物を装入し、これに引抜き等の断面減少加工を施して線状となしてから加熱処理にてＭｇＢ_２　の合成を行うと、細径長尺化したＭｇＢ_２　線材を比較的容易に得ることが可能である。しかし、上述のように管状金属内に粉末状のＭｇとＢとの混合物を装入した後これに断面減少加工を施し、次いでＭｇＢ_２　合成のための加熱処理を行って得られるＭｇＢ_２　線材は、超電導特性を有することは認められるものの臨界電流密度が著しく低く、そのため超電導線材として用いることが困難である。
【０００７】
そこで、臨界電流密度がより高いＭｇＢ_２　超電導線材の作製手段を求めて更に研究を続けた結果、次の　ａ）　〜　ｄ）　項に示す如き知見を得ることができた。
ａ）　ＭｇとＢとの混合物を加熱処理してＭｇＢ_２　の合成を行うと、反応後の体積は反応前のそれの約半分にまで減少してしまう。そのため、管状金属内に粉末状のＭｇとＢとの混合物を装入してからこれに断面減少加工を施し、その後にＭｇＢ_２　合成のための加熱処理を行う方法によってＭｇＢ_２　線材を製造すると、反応により生成したＭｇＢ_２　の粒同士や当該ＭｇＢ_２　粒と管状金属外皮（シ−ス金属）とが十分に密着しなくなって電気的接触が不足したり、ＭｇとＢとの反応が十分に進まずに未反応物が少なからず残留したりするのを如何ともし難い。そして、これが臨界電流密度の低い原因となっている。
【０００８】
ｂ）　しかるに、断面減少加工と特定温度以上での加熱処理を少なくとも交互に２回以上行うと、生成するＭｇＢ_２　粒同士の電気的接触やＭｇＢ_２　粒と管状金属外皮との電気的接触が十分となる上、未反応のＭｇやＢの残留も少なくなり、臨界電流密度の高いＭｇＢ_２　超電導線材が得られる。
ｃ）　ただ、この場合、加熱処理は２気圧以上の不活性ガス加圧雰囲気中で実施する必要があり、加熱処理をこのような不活性ガス加圧雰囲気中で行わない場合には、原料の酸化（ＭｇＯの生成）が生じたり、昇華するＭｇの高蒸気圧により管状金属内の装入物が吹き出す現象が起きて線材の作製が不可能となる。
【０００９】
ｄ）　また、原料のＭｇとＢとを混合する際に適量の金属Ｔｉを添加しておくと、焼結材の粒組織が微細化して非常に緻密なものとなり、かつ最も臨界電流密度の向上に寄与する粒界が著しく増加することとなって、より高い臨界電流密度を示すＭｇＢ_２　系超電導線材を得ることが可能になる。
【００１０】
本発明は、上記知見事項等を基に成されたものであって、次の▲１▼項乃至▲３▼項に示すＭｇＢ_２　系超電導線材の作製方法を提供するものである。
▲１▼　管状金属内に粉末状のＭｇとＢとを装入し、これに断面減少加工と６００℃以上の温度域での加熱処理とを施してＭｇＢ_２　系超電導線材を作製するに際して、前記加熱処理を２気圧以上の不活性ガス加圧雰囲気中で実施すると共に、前記断面減少加工と当該加熱処理とを少なくとも交互に２回以上施すことを特徴とする、ＭｇＢ_２　系超電導線材の作製方法。
▲２▼　管状金属内に粉末状のＭｇとＢとＴｉとを装入し、これに断面減少加工と６００℃以上の温度域での加熱処理とを施してＭｇＢ_２　系超電導線材を作製するに際して、前記加熱処理を２気圧以上の不活性ガス加圧雰囲気中で実施すると共に、前記断面減少加工と当該加熱処理とを少なくとも交互に２回以上施すことを特徴とする、ＭｇＢ_２　系超電導線材の作製方法。
▲３▼　管状金属として銅チュ−ブを用い、かつ加熱処理の温度範囲を６００〜９００℃とする、前記▲１▼項又は▲２▼項に記載のＭｇＢ_２　系超電導線材の作製方法。
【００１１】
【発明の実施の形態】
上述のように、本発明に係るＭｇＢ_２　系超電導線材の作製方法は、「粉末状のＭｇとＢあるいは更にＴｉとの混合物を金属チュ−ブ等の管状金属の中に装入し、　次いでこれに“断面減少加工”と“２気圧以上の不活性ガス加圧雰囲気中での６００℃以上の温度域での加熱処理”とを少なくとも交互に２回以上施す」ことを特徴としているが、管状金属の中に装入する粉末状原料混合物中のＭｇとＢとの割合はＭｇＢ_２　の組成比（原子比で１：２）とされることは言うまでもない。
【００１２】
なお、本発明法では原料混合物中に必要に応じてＴｉの添加がなされるが、前述したように、適量のＴｉ添加は焼結材の緻密性向上作用を通じて得られるＭｇＢ_２　系超電導線材の臨界電流密度を改善する効果をもたらす。この場合のＴｉ添加量は、質量％で２〜２０％程度が適当である。なぜなら、Ｔｉ添加量が２％に満たないと得られるＭｇＢ_２　系超電導線材に対する臨界電流密度改善効果が十分でなく、また２０％を超えるＴｉ添加量では第２相の析出による臨界電流密度Ｊｃの低下が懸念されるようになるからである。
【００１３】
本発明法に適用される管状金属としては、基本的にはその材質が限定されるものではなく種々の金属や合金の管状体（例えば銅，銅合金，ニッケル，鉄，ステンレス鋼等のチュ−ブ）を使用することができる。ただ、この管状金属については、伸線工程を経てＭｇＢ_２　系超電導線材が作製された後でもそのまま金属外皮として残して通電補償材（補償金属）の役割を担わせることもできるので、極力導電性の高い材質のものが好ましいとも言える。
従って、これらの点や価格面を考慮すれば、前記管状金属としては銅製のチュ−ブが適当である。
【００１４】
なお、管状金属内への粉末状混合原料の装入は非酸化性雰囲気中で実施されるべきである。なぜなら、原料粉の１つであるＭｇは非常に酸化しやすい金属であって、酸化によってＭｇＯが生成すると得られる線材の超電導特性に悪影響が及ぶからである。
【００１５】
粉末状混合原料を装入した管状金属には断面減少加工が施されるが、この“断面減少加工”とは線材化のための塑性加工であり、圧延，スエ−ジング加工，ロ−リング加工，引抜きダイス等による線引き加工等を意味している。
なお、この断面減少加工の前後に管状金属（金属管製外皮）の軟化焼鈍｛銅チュ−ブの場合には４００℃以下（例えば３５０℃）の温度での焼鈍｝を施し、管状金属（金属管製外皮）に割れが生じるのを防止する手だてを講じることが推奨される。
【００１６】
粉末状混合原料を装入した管状金属には、また、“２気圧以上の不活性ガス加圧雰囲気中での６００℃以上の温度域での加熱処理”も施される。この加熱処理は、ＭｇとＢとを反応させてＭｇＢ_２　を合成したり、生成したＭｇＢ_２　粒同士を焼結するために実施されるものである。即ち、ＭｇとＢとの反応は６００℃程度から進行するので、ＭｇＢ_２　を生成させるためには６００℃以上の温度域での加熱処理を欠くことはできない。
なお、工業的な生産性等を考慮した場合には、管状金属として銅チュ−ブを用いると共に銅の融点をも考慮した６８０〜９００℃程度の温度域で加熱処理を実施することが推奨される。
【００１７】
ただ、この加熱処理は２気圧以上の加圧不活性ガス雰囲気中で実施する必要がある。なぜなら、管状金属内に装入したＭｇとＢとの混合原料を上記高温域で加熱すると、Ｍｇの昇華によって生じる高い蒸気圧によって管状金属内の原料が外に吹き出すという特異な現象が起き、ＭｇＢ_２　系超電導線材の安定製造が叶わなくなるためである。また、上記加熱処理が非酸化性雰囲気中で実施されないと、原料混合物中のＭｇの酸化が生じ、得られる線材の超電導特性に悪影響が及ぶ。
しかしながら、上記加熱処理を“２気圧以上の加圧不活性ガス（Ａｒガス等）雰囲気中”で実施した場合には、これらの不都合が解消されてＭｇＢ_２　の合成反応が速やかに進行する。
【００１８】
ところで、前記“断面減少加工”と“２気圧以上の不活性ガス加圧雰囲気中での６００℃以上の温度域での加熱処理”とは少なくとも交互に２回以上施す必要がある。
先に説明したように、ＭｇとＢとの混合物を加熱処理してＭｇＢ_２　の合成を行うと反応後の体積は反応前のそれの約半分にまで減少してしまう。そのため、粉末状のＭｇとＢとの混合物を装入した管状金属に断面減少加工を施し、次いでＭｇＢ_２　合成のための加熱処理を施すと、体積収縮のために反応により生成したＭｇＢ_２　の粒同士や当該ＭｇＢ_２　粒と管状金属外皮とが十分に密着しなくなって電気的接触が不足したり、ＭｇとＢとの反応が十分に進まずに未反応物が少なからず残留したりする。
【００１９】
しかし、断面減少加工と加熱処理とを交互に少なくとも２回以上行うと、加熱処理によってＭｇＢ_２　の合成が進んだ後に更なる断面減少加工が施される結果となり、従って生成したＭｇＢ_２　粒同士やＭｇＢ_２　粒と管状金属外皮との密着が十分となる上、未反応のＭｇやＢも十分に密着するので続く更なる加熱処理によりＭｇＢ_２　の合成がより十分に進んで未反応物が極力減少する。そのため、臨界電流密度の高いＭｇＢ_２　系超電導線材を安定して作製することが可能になる。
なお、本発明法によれば、断面減少加工と加熱処理との順序を特に問わなくても超電導特性の良好なＭｇＢ_２　系超電導線材を得ることができる。
【００２０】
ところで、図１は、本発明に係るＭｇＢ_２　系超電導線材の作成例についてその作成過程における材料の状態を説明した概念図であり、　（ａ）は管状金属内に粉末状のＭｇとＢとが装入されている状態（断面減少加工歴の有無を問わない）を、　（ｂ）は上記　（ａ）の状態のものにＭｇＢ_２　を合成するための加熱処理が施された状態を、また　（ｃ）は上記加熱処理後に断面減少加工が施された状態を、そして　（ｄ）は再度の加熱処理（この加熱処理ではＭｇＢ_２　粒の焼結と未反応残留物が存在しておればその未反応物からのＭｇＢ_２　の合成がなされる）が施された状態を、それぞれ示している。
【００２１】
前述したように、管状金属に装入されたＭｇとＢ｛図１の　（ａ）の状態｝を加熱処理してＭｇＢ_２の合成を行うと、処理後の体積は図１の　（ｂ）に示した通り反応前の約半分にまで減少してしまい、生成したＭｇＢ_２　粒同士やＭｇＢ_２　粒と管状金属外皮との密着不良が生じる。
しかし、これに断面減少加工を施すと図１の　（ｃ）に示したようにＭｇＢ_２　粒同士やＭｇＢ_２　粒と管状金属外皮との密着が進むので、この状態のものに再度の加熱処理を施してＭｇＢ_２　粒の焼結を図ると、図１の　（ｄ）の如くにＭｇＢ_２　粒同士やＭｇＢ_２　粒と管状金属外皮との焼結が十分になされるだけでなく、未反応のまま残留したＭｇやＢの反応も十分に進んで電流の流路が確保・拡大され、臨界電流密度の高いＭｇＢ_２　系超電導線材が得られるようになる。
【００２２】
以下、本発明を実施例によって説明する。
【実施例】
まず、Ａｒガス雰囲気中にて、純度が９９％で粒度が３００メッシュのＭｇ粉末とＢ粉末（アモルファス粉末）とをＭｇＢ_２　の組成比（原子比で１：２）で混合し、直径６ｍｍ，厚さ６ｍｍの複数のタブレットを加圧成形した。
【００２３】
また、これとは別に、Ａｒガス雰囲気中にて、純度が９９％で粒度が３００メッシュのＭｇ粉末とＢ粉末（アモルファス粉末）とをＭｇＢ_２　の組成比（原子比で１：２）となるように秤量し、これに純度が９９％で粒度が３００メッシュのＴｉ粉末を質量％で１０％添加して混合し、同じく直径６ｍｍ，厚さ６ｍｍの複数のタブレットを加圧成形した。
【００２４】
次いで、Ａｒガス雰囲気中で上記各タブレットのそれぞれを複数個ずつ外径１２．０ｍｍ，内径６．２ｍｍ　の銅チュ−ブに装入してから、それぞれの銅チュ−ブにつき、スウェ−ジングにて外径　３．０ｍｍまで断面減少加工した後、更に丸ダイスを用いて外径　１．０ｍｍまで伸線加工した。
【００２５】
次に、得られたそれぞれの線材につき、３気圧の加圧Ａｒガス雰囲気中で７００℃，５時間の加熱処理を施した後、圧延によって外径　０．５ｍｍにまで断面減少加工した。その後、更に、３気圧の加圧Ａｒガス雰囲気中で７００℃，５時間の加熱処理を施してから圧延によって外径　０．４ｍｍにまで断面減少加工した。そして、引続き、３気圧の加圧Ａｒガス雰囲気中で７００℃，５時間の加熱処理を施した。
このようにして得られた各ＭｇＢ_２　系超電導線材について液体ヘリウム温度における電流−電圧（Ｉ−Ｖ）特性を測定したが、この測定結果を図１に示す。
【００２６】
図１に示される結果からも明らかなように、本発明法によって得られたＴｉ非添加のＭｇＢ_２　線材（図中の□印で示したもの）は電流が７６Ａの時点でクェンチしたが、これに基づいて計算した臨界電流密度は、超電導部分の断面積で計算した値が約１７００００Ａ／ｃｍ^２で、金属部分まで含めた線径での値は約４００００Ａ／ｃｍ^２であった。また、この時に発生した磁場は０．１４Ｔであった。
なお、この電流−電圧（Ｉ−Ｖ）特性は、線径が　０．４ｍｍの上記Ｔｉ非添加ＭｇＢ_２　超電導線材をステンレス鋼製ボビン（直径２７ｍｍ）に５０タ−ン巻きつけてコイルとし、このコイルによって測定した。
【００２７】
一方、本発明法により得られたＴｉを１０％添加したＭｇＢ_２　線材（図中の●印で示したもの）は電流が９５Ａの時点でクェンチを起こした。これに基づいて計算した臨界電流密度は、超電導部分の断面積で計算した値が約２１００００Ａ／ｃｍ^２で、金属部分まで含めた線径での値は約５００００Ａ／ｃｍ^２であった。また、この時に発生した磁場は０．２１Ｔであった。
なお、この電流−電圧（Ｉ−Ｖ）特性は、線径が　０．４ｍｍの上記Ｔｉ添加ＭｇＢ_２　系超電導線材をステンレス鋼製ボビン（直径２７ｍｍ）に３５タ−ン巻きつけてコイルとし、このコイルによって測定した。
【００２８】
【発明の効果】
以上に説明した如く、この発明によれば、これまで作製が困難であった良好な超電導特性を有するＭｇＢ_２　系超電導線材を安定して提供することが可能となり、電力用ケ−ブル，マグネット，モ−タ，発電機等の更なる性能向上に寄与できるなど、産業上有用な効果がもたらされる。
【図面の簡単な説明】
【図１】本発明に係るＭｇＢ_２　系超電導線材の作成例につき、その作成過程における材料の状態を説明した概念図である。
【図２】本発明法に従って作製されたＭｇＢ_２　系超電導線材についての電流−電圧（Ｉ−Ｖ）特性の測定結果を示した図面である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing a MgB ₂ -based superconducting wire, and opens the way to provide a low-cost and stable supply of a superconducting wire for use in power cables, magnets, motors, generators, and the like.
[0002]
[Prior art]
Although strong metal-based superconducting material such as NbTi and Nb 3 _Sn as superconducting wire which is applied to the magnetic field magnet or the like is the mainstream, since these materials have a low critical temperature Tc its use in a liquid helium temperature region Therefore, the problem of superconducting quench was serious.
[0003]
Accordingly, MgB ₂ , which has recently been found to exhibit superconducting properties, has attracted attention, and various studies have been made to use this boride of magnesium as a superconducting material.
However, although MgB ₂ is not only advantageous in terms of quenching because its critical temperature Tc is relatively high at 39K, it is expected that the use temperature will be extended to about 20K, which is higher than that of the conventional intermetallic compound superconductor. No means for converting MgB ₂ , which is a very hard material, into a wire has been found, and this has been a major obstacle in using MgB ₂ as a superconducting material.
[0004]
By the way, conventionally, the PIT method (Powder in tube process) has been applied to wire forming of hard and brittle materials such as intermetallic compounds. The PIT method is a method in which a powder of an intermetallic compound or the like is charged into a tubular metal such as copper or silver, heated and sintered, and then stretched together with the tubular metal to form a wire.
Therefore, many research institutions, "approach the synthesized MgB ₂ in the powder form was charged to a tubular metal fabricating a stretching process to MgB ₂ wire material those heated and sintering the" it has been studied Have been.
However, a technique for stably producing an MgB ₂ wire having a high critical current density that can be used as a superconducting wire has not yet been established.
[0005]
In view of the above, an object of the present invention was to establish means for stably providing an MgB ₂ -based superconducting wire having excellent superconducting properties.
[0006]
[Means for Solving the Problems]
The present inventors have conducted intensive studies to achieve the above object, and as a result, the following has become clear.
That is, when the PIT method is applied for the purpose of manufacturing MgB ₂ wire rod, a method of charging powdered MgB ₂ into a tubular metal and performing sintering / stretching, which has been tried by many research institutions, has been tried. According to the above, it was difficult to obtain a thin and long material having superconducting properties because MgB ₂ was extremely hard. However, powdered Mg and B, which are raw materials for synthesizing MgB ₂ , were contained in the tubular metal. When the mixture with MgB ₂ is subjected to cross-section reduction processing such as drawing and the like, the mixture is made linear, and then MgB ₂ is synthesized by heat treatment, the MgB ₂ wire having a small diameter and length becomes relatively easy. It is possible to obtain. However, this applies sectional reduction process was charged a mixture of powdered Mg and B in the tubular metal as described above, then MgB ₂ wire obtained by performing heat treatment for MgB ₂ synthesis Although it is recognized that it has superconductivity, the critical current density is remarkably low, so that it is difficult to use it as a superconducting wire.
[0007]
Thus, as a result of further research on a method of manufacturing a MgB ₂ superconducting wire having a higher critical current density, the following findings a) to d) were obtained.
a) When a mixture of Mg and B is heat-treated to synthesize MgB ₂ , the volume after the reaction is reduced to about half that before the reaction. Therefore, when a mixture of powdered Mg and B is charged into a tubular metal, a cross-section reduction process is performed on the mixture, and then a MgB ₂ wire is manufactured by a method of performing a heat treatment for MgB ₂ synthesis. The MgB ₂ particles generated by the reaction or the MgB ₂ particles and the tubular metal shell (sheet metal) do not sufficiently adhere to each other, resulting in insufficient electrical contact or sufficient reaction between Mg and B. First of all, it is hardly possible for any unreacted material to remain. And this is a cause of low critical current density.
[0008]
b) However, when the section reduction processing and the heat treatment at a specific temperature or more are performed at least alternately at least twice, the electrical contact between the generated MgB ₂ particles and the electrical contact between the MgB ₂ particles and the tubular metal outer cover are sufficient. In addition, unreacted Mg and B residues are reduced, and a MgB ₂ superconducting wire having a high critical current density is obtained.
c) However, in this case, it is necessary to perform the heat treatment in an inert gas pressurized atmosphere of 2 atm or more. If the heat treatment is not performed in such an inert gas pressurized atmosphere, Oxidation (generation of MgO) occurs, and a phenomenon in which the charge in the tubular metal blows out due to the high vapor pressure of subliming Mg occurs, making it impossible to produce a wire.
[0009]
d) Further, if an appropriate amount of metal Ti is added when mixing the raw materials Mg and B, the grain structure of the sintered material becomes finer and very dense, and the most important improvement in critical current density is obtained. This significantly increases the grain boundaries contributing to the above, making it possible to obtain an MgB ₂ -based superconducting wire exhibiting a higher critical current density.
[0010]
The present invention has been made based on the above findings and the like, and provides a method for producing an MgB ₂ -based superconducting wire as described in the following items (1) to (3).
{Circle around (1)} When powdered Mg and B are charged into a tubular metal and subjected to a cross-section reduction process and a heat treatment in a temperature range of 600 ° C. or more to produce an MgB ₂ -based superconducting wire, A method for producing a MgB ₂ -based superconducting wire, wherein the heat treatment is performed in an inert gas pressurized atmosphere of 2 atm or more, and the cross-section reduction processing and the heat treatment are performed at least alternately at least twice. .
{Circle around (2)} When powdered Mg, B, and Ti are charged into a tubular metal and subjected to cross-section reduction processing and heat treatment in a temperature range of 600 ° C. or more to produce a MgB ₂ -based superconducting wire. Wherein the heat treatment is performed in an inert gas pressurized atmosphere of 2 atm or more, and the cross-section reduction processing and the heat treatment are performed at least alternately twice or more, wherein the MgB ₂ -based superconducting wire is Production method.
{Circle around (3)} The method for producing an MgB ₂ -based superconducting wire according to the above item [1] or [2], wherein a copper tube is used as the tubular metal and the temperature range of the heat treatment is 600 to 900 ° C.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
As described above, the manufacturing method of the MgB ₂ -based superconducting wire according to the present invention is as follows: “A mixture of powdered Mg and B or further Ti is charged into a tubular metal such as a metal tube; The method is characterized in that a “section reduction processing” and “a heat treatment in a temperature range of 600 ° C. or more in an inert gas pressurized atmosphere of 2 atm or more” are performed at least alternately at least twice. It goes without saying that the ratio of Mg and B in the powdery raw material mixture charged into the metal is the composition ratio of MgB ₂ (atomic ratio of 1: 2).
[0012]
In the method of the present invention, Ti is added to the raw material mixture as necessary, but as described above, an appropriate amount of Ti is added to the criticality of the MgB ₂ -based superconducting wire obtained through the action of improving the denseness of the sintered material. This has the effect of improving the current density. In this case, the addition amount of Ti is appropriately about 2 to 20% by mass%. This is because the critical current density improvement effect on the MgB ₂ -based superconducting wire obtained when the Ti addition amount is less than 2% is not sufficient, and when the Ti addition amount exceeds 20%, the critical current density Jc due to the precipitation of the second phase is reduced. This is because there is a concern about a decrease.
[0013]
The material of the tubular metal applied to the method of the present invention is not basically limited, and various metals and metal alloys (for example, tubes of copper, copper alloy, nickel, iron, stainless steel, etc.) can be used. B) can be used. However, even when the MgB ₂ -based superconducting wire is manufactured through the drawing process, the tubular metal can be left as a metal sheath as it is and can serve as a current compensating material (compensating metal). Can be said to be preferable.
Therefore, in consideration of these points and cost, a copper tube is suitable as the tubular metal.
[0014]
The charging of the powdery mixed raw material into the tubular metal should be performed in a non-oxidizing atmosphere. This is because Mg, which is one of the raw material powders, is a metal that is very easily oxidized, and the generation of MgO by oxidation adversely affects the superconducting properties of the obtained wire.
[0015]
Cross-section reduction processing is applied to the tubular metal charged with the powdered mixed raw material. This “cross-section reduction processing” is plastic processing for turning into a wire rod, and is performed by rolling, swaging, and rolling. , Wire drawing with a drawing die or the like.
Before and after this cross-section reduction processing, softening annealing of the tubular metal (skin made of metal tube) (in the case of a copper tube, annealing at a temperature of 400 ° C. or less (for example, 350 ° C.)) is performed. It is recommended that measures be taken to prevent cracking of the tubular shell).
[0016]
The tubular metal charged with the powdery mixed raw material is also subjected to "a heat treatment in a temperature range of 600 ° C. or more in an inert gas pressurized atmosphere of 2 atm or more". This heat treatment is performed to react Mg and B to synthesize MgB ₂ or to sinter the generated MgB ₂ particles. That is, since the reaction between Mg and B proceeds from about 600 ° C., heat treatment in a temperature range of 600 ° C. or more is indispensable to generate MgB ₂ .
In consideration of industrial productivity, it is recommended to use a copper tube as the tubular metal and to perform the heat treatment in a temperature range of about 680 to 900 ° C. in consideration of the melting point of copper. You.
[0017]
However, this heat treatment needs to be performed in a pressurized inert gas atmosphere of 2 atm or more. This is because, when the mixed raw material of Mg and B charged in the tubular metal is heated in the high temperature range, a unique phenomenon that the raw material in the tubular metal blows out due to a high vapor pressure generated by sublimation of Mg occurs, and MgB _This is because stable production of the _second superconducting wire cannot be realized. Further, if the heat treatment is not performed in a non-oxidizing atmosphere, Mg in the raw material mixture is oxidized, which adversely affects the superconductivity of the obtained wire.
However, when the above heat treatment is performed in an atmosphere of a pressurized inert gas (Ar gas or the like at 2 atm or more), these inconveniences are resolved and the synthesis reaction of MgB ₂ proceeds promptly.
[0018]
By the way, the "section reduction processing" and the "heat treatment in a temperature range of 600 ° C. or more in an inert gas pressurized atmosphere of 2 atm or more" must be performed at least alternately at least twice.
As described above, when the mixture of Mg and B is heat-treated to synthesize MgB ₂ , the volume after the reaction is reduced to about half of that before the reaction. Therefore, subjecting the mixture of the reduction process in a tubular metal was charged with powdered Mg and B, then when subjected to a heat treatment for MgB ₂ synthesis, MgB ₂ grains produced by the reaction for volume shrinkage The _two or the MgB ₂ particles and the tubular metal skin do not adhere sufficiently to each other, resulting in insufficient electrical contact, or the reaction between Mg and B does not proceed sufficiently, and some unreacted material remains.
[0019]
However, when the cross-section reduction processing and the heat treatment are alternately performed at least twice or more, the result is that a further cross-section reduction processing is performed after the synthesis of MgB ₂ is advanced by the heat treatment, and therefore, the generated MgB ₂ particles are The adhesion between the MgB ₂ particles and the tubular metal shell is sufficient, and the unreacted Mg and B also adhere sufficiently, so that the further heating treatment will further promote the synthesis of MgB ₂ and reduce the unreacted materials as much as possible. I do. Therefore, it is possible to stably produce an MgB ₂ -based superconducting wire having a high critical current density.
According to the method of the present invention, it is possible to obtain an MgB ₂ -based superconducting wire having good superconducting properties, regardless of the order of the cross-section reduction processing and the heat treatment.
[0020]
Incidentally, FIG. 1 is a conceptual diagram illustrating the state of the materials in the preparation process of the preparation example of the MgB ₂ -based superconducting wire according to the present invention, and FIG. (B) shows a state in which a heat treatment for synthesizing MgB ₂ is applied to the state in (a), and c) shows a state in which the cross-section reduction processing has been performed after the above-mentioned heat treatment, and (d) shows another heat treatment (in this heat treatment, sintering of MgB ₂ grains and unreacted residue, if any) are present. (The synthesis of MgB ₂ from the reaction product is performed).
[0021]
As described above, when Mg and B charged in the tubular metal are heat-treated to synthesize MgB ₂ (the state of FIG. 1A), the volume after the treatment becomes as shown in FIG. 1B. As shown, the amount is reduced to about half of that before the reaction, and poor adhesion between the _two generated MgB grains or between the _two MgB grains and the tubular metal outer skin occurs.
However, if the cross-section reduction processing is performed on this, as shown in FIG. 1 (c), the adhesion between the _two MgB grains or between the _two MgB grains and the tubular metal outer skin progresses. When achieve sintering of MgB ₂ grains subjected, not only sintering of MgB ₂ grains or between the MgB ₂ grains and the tubular metal outer skin is made sufficiently in as in FIG. 1 (d), the remains unreacted The reaction of the remaining Mg and B also proceeds sufficiently, and the current flow path is secured and expanded, so that a MgB ₂ -based superconducting wire having a high critical current density can be obtained.
[0022]
Hereinafter, the present invention will be described with reference to examples.
【Example】
First, in an Ar gas atmosphere, an Mg powder having a purity of 99% and a particle size of 300 mesh and a B powder (amorphous powder) were mixed at a composition ratio of MgB ₂ (atomic ratio of 1: 2), and the diameter was 6 mm. A plurality of tablets having a thickness of 6 mm were pressure molded.
[0023]
Separately, in an Ar gas atmosphere, a Mg powder having a purity of 99% and a particle size of 300 mesh and a B powder (amorphous powder) have a composition ratio of MgB ₂ (atomic ratio of 1: 2). Then, 10% by mass of Ti powder having a purity of 99% and a particle size of 300 mesh was added thereto and mixed, and a plurality of tablets having a diameter of 6 mm and a thickness of 6 mm were formed under pressure.
[0024]
Next, in an Ar gas atmosphere, a plurality of each of the above-mentioned tablets were charged into a copper tube having an outer diameter of 12.0 mm and an inner diameter of 6.2 mm, and then each copper tube was swaged. After reducing the cross section to an outer diameter of 3.0 mm, the wire was further drawn to an outer diameter of 1.0 mm using a round die.
[0025]
Next, each of the obtained wire rods was subjected to a heat treatment at 700 ° C. for 5 hours in a pressurized Ar gas atmosphere at 3 atm, and then processed to reduce the cross section to an outer diameter of 0.5 mm by rolling. Thereafter, a heat treatment was further performed at 700 ° C. for 5 hours in a pressurized Ar gas atmosphere at 3 atm, and then the cross-section was reduced to an outer diameter of 0.4 mm by rolling. Then, a heat treatment was performed at 700 ° C. for 5 hours in a pressurized Ar gas atmosphere of 3 atm.
The current-voltage (IV) characteristics at the temperature of liquid helium were measured for each of the MgB ₂ -based superconducting wires thus obtained, and the measurement results are shown in FIG.
[0026]
As is clear from the results shown in FIG. 1, the Ti-free MgB ₂ wire obtained by the method of the present invention (indicated by □ in the figure) quenched when the current was 76 A. the critical current density was calculated based on the value calculated by the cross-sectional area of the superconducting portion of about 170000A / cm ^2, the value of a line diameter, including to the metal part was about 40000A / cm ^2. The magnetic field generated at this time was 0.14T.
The current-voltage (IV) characteristics were obtained by winding the above-mentioned non-Ti-added MgB ₂ superconducting wire having a wire diameter of 0.4 mm around a stainless steel bobbin (diameter 27 mm) to form a coil. Measured by coil.
[0027]
On the other hand, MgB ₂ wire material of Ti obtained by the process of the present invention was added 10% (shown by the mark ● in the drawing) current is caused to quenched at the time of 95A. The critical current density was calculated based on this, the value calculated by the cross-sectional area of the superconducting portion about 210000A / cm ^2, the value of a line diameter, including to the metal part was about 50000A / cm ^2. The magnetic field generated at this time was 0.21T.
Incidentally, the current - voltage (I-V) characteristics are wire diameter of the Ti added MgB ₂ superconducting wire of 0.4 mm 35 data into a stainless steel bobbin (diameter 27 mm) - the coil put down winding, this Measured by coil.
[0028]
【The invention's effect】
As described above, according to the present invention, it is possible to stably provide an MgB ₂ -based superconducting wire having good superconducting properties, which has been difficult to fabricate so far. Industrially useful effects such as a further improvement in the performance of motors, generators and the like are brought about.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram illustrating a state of a material in a production process of an example of producing an MgB ₂ -based superconducting wire according to the present invention.
FIG. 2 is a drawing showing the measurement results of current-voltage (IV) characteristics of an MgB ₂ -based superconducting wire produced according to the method of the present invention.

Claims

管状金属内に粉末状のＭｇとＢとを装入し、これに断面減少加工と６００℃以上の温度域での加熱処理とを施してＭｇＢ_２　系超電導線材を作製するに際して、前記加熱処理を２気圧以上の不活性ガス加圧雰囲気中で実施すると共に、前記断面減少加工と当該加熱処理とを少なくとも交互に２回以上施すことを特徴とする、ＭｇＢ_２　系超電導線材の作製方法。When powdered Mg and B are charged into a tubular metal and subjected to a cross-section reduction process and a heat treatment in a temperature range of 600 ° C. or more to produce an MgB ₂ -based superconducting wire, the heat treatment is performed. A method for producing a MgB ₂ -based superconducting wire, characterized in that the method is performed in an inert gas pressurized atmosphere of 2 atm or more, and the cross-section reduction processing and the heat treatment are performed at least alternately at least twice.

管状金属内に粉末状のＭｇとＢとＴｉとを装入し、これに断面減少加工と６００℃以上の温度域での加熱処理とを施してＭｇＢ_２　系超電導線材を作製するに際して、前記加熱処理を２気圧以上の不活性ガス加圧雰囲気中で実施すると共に、前記断面減少加工と当該加熱処理とを少なくとも交互に２回以上施すことを特徴とする、ＭｇＢ_２　系超電導線材の作製方法。When powdered Mg, B, and Ti are charged into a tubular metal and subjected to a cross-section reduction process and a heat treatment in a temperature range of 600 ° C. or more, the above-described heating is performed when preparing an MgB ₂ -based superconducting wire. processes as well as in an inert gas pressurized atmosphere of 2 atm or more, and characterized by applying more than once and the area reduction working and the heat treatment at least alternately, a method for manufacturing a MgB ₂ superconducting wire.

管状金属として銅チュ−ブを用い、かつ加熱処理の温度範囲を６００〜９００℃とする、請求項１又は２に記載のＭｇＢ_２　系超電導線材の作製方法。Copper Ju as a tubular metal - with blanking, and the temperature range of heat treatment to 600 to 900 ° C., a method for manufacturing a MgB ₂ superconducting wire according to claim 1 or 2.