JP3845729B2 - Method and apparatus for measuring current and voltage characteristics of superconductors - Google Patents
Method and apparatus for measuring current and voltage characteristics of superconductors Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は、大面積超伝導膜、長尺超伝導テープ線材、および大型超伝導バルク材などの大面積・大型の超伝導体における、電流と電圧の関係（電流・電圧特性）を、非破壊的かつ非接触で評価する測定方法に関する。
【０００２】
【従来の技術】
大面積超伝導膜、長尺超伝導テープ線材および大型超伝導バルク材などの大面積・大型の超伝導体は、限流器、超伝導マグネット、軸受け等、さまざまな電力機器への応用が期待されている。超伝導体の電流・電圧特性は、これらの電力機器に応用したときの超伝導体の性能を決める重要な特性であり、特に、高温超伝導酸化物においては、電流・電圧特性がなだらかであることが知られているため、さまざまな応用において、重要な役割を果たす。
【０００３】
限流器においては、系統事故時の常伝導転移挙動を左右するため、機器設計に必要であり、超伝導マグネットおよび軸受けにおいては、磁束クリープによる超伝導電流の減衰を決めることから、マグネットにおける永久電流モード運転および軸受けにおける長期運転のために必要不可欠である。これらの大面積・大型の超伝導体を、さまざまな電力機器に応用するため、超伝導体の電流・電圧特性とその分布を非破壊的に非接触で、かつ簡便に評価する方法の開発が望まれていた。
【０００４】
また、超伝導体の電流・電圧特性は、超伝導試料の均一性を表わすパラメーターであり、試料が均一であればあるほど、電流・電圧特性が急峻であることが知られている。従って、超伝導テープ線材や限流器用の導体等の製造工程において、その電流・電圧特性を評価すれば、プロセスの評価が可能となり、製造技術へのフィードバックをかけることができる。
【０００５】
（１）超伝導体の電流・電圧特性を評価するのに最もよく用いられている方法の一つは、超伝導体に電流端子と電圧端子の４つの電極を付け、通電電流を流して電圧を測定する４端子法である。しかし、この方法を用いるためには、超伝導体を加工する必要があり、そのときの超伝導特性の劣化が問題となる。また、臨界電流以上に通電電流を流す必要があるため、急激な熱の発生等で超伝導体を壊してしまう恐れもある。
【０００６】
電流・電圧特性を非破壊的に評価するのに最もよく用いられている方法の一つは、直流磁化を測定する方法であるが、これは、小さな超伝導体全体の平均的な電流・電圧特性しか評価することができない。
【０００７】
（２）超伝導膜の局所的な臨界電流密度の分布を評価する方法として、超伝導膜直上に配置したコイルに流す交流電流Ｉ＝Ｉ０ｃｏｓ２πｆｔ（ｆは交流電流の周波数、ｔは時間）とコイルに生じる第３高調波誘導電圧（の振幅）Ｖ３を測定する方法がすでに提案されている。これは、Ｉ０をゼロから増加して行き、Ｖ３が大きく生じ始めるときの交流電流値Ｉ０＝Ｉｔｈから臨界電流密度Ｊｃを評価する方法である（下記非特許文献１参照）。
【０００８】
同様の方法として、コイルに生じる第３高調波誘導電圧を測定するかわりに、駆動コイルと超伝導膜を挟んで対称の位置に検出コイルを配置し、駆動コイルに流す交流電流値Ｉ０と検出コイルに誘起される基本波誘導電圧を測定する方法もある（下記非特許文献２参照）。なお、検出コイルを駆動コイルと共巻きにして、基本波誘導電圧の変化を測定することでも、臨界電流密度を評価することができる（下記非特許文献３参照）。
【０００９】
また、同様な方法で、超伝導厚膜、バルク材について、Ｉ０とＶ３の関係を詳細に解析して臨界電流密度を測定する方法がすでに提案されている（下記非特許文献４参照）。
【００１０】
しかし、これらの方法では、超伝導体の臨界電流密度を測定できるのみで、電流・電圧特性までは評価できなかった。
【００１１】
【非特許文献１】
J. H. Claassen, M. E. Reeves and R. J. Soulen, Jr., "A contactless method for measurement of the critical current density and critical temperature of superconducting films", Rev. Sci. Instrum. 62, 996 (1991).
【非特許文献２】
H. Hochmuth and M. Lorenz, "Inductive determination of the critical current density of superconducting thin films without lateral structuring", Physica C 220, 209 (1994).
【非特許文献３】
H. Hochmuth and M. Lorenz, "Side selective and non-destructive determination of the critical current density of double-sided superconducting thin films", Physica C 265, 335 (1996).
【非特許文献４】
馬渡康徳、山崎裕文、中川愛彦、「超伝導薄膜，バルク材における臨界電流密度と第三高調波誘導電圧」、第６６回２００２年度春季低温工学・超電導学会講演概要集
【００１２】
【発明が解決しようとする課題】
本発明の課題は、大面積超伝導膜、長尺超伝導テープ線材、および大型超伝導バルク材などの大面積・大型の超伝導体の電流・電圧特性とその分布を、いかに非破壊的かつ非接触で測定するかである。
【００１３】
【課題を解決するための手段】
本願発明は、上記課題を達成するために以下の解決手段を採用する。
（１）超伝導膜の直上に小さなコイルを配置し、該コイルに交流電流を流し、このコイルに流れる電流Ｉ０と、この電流によりコイルに誘起される第３高調波誘導電圧Ｖ３とを測定する。Ｉ０をゼロから増加して行き、Ｖ３が大きく生じ始めるときの交流電流値Ｉｔｈから臨界電流密度Ｊｃを評価するのは、上記非特許文献１と同様である。
【００１４】
（２）別法として、超伝導膜の直上に小さな駆動コイルを配置し、該駆動コイルに交流電流を流し、このコイルに流れる電流Ｉ０と、この電流により駆動コイルと超伝導膜を挟んで対称の位置に配置された検出コイルに誘起される基本波誘導電圧Ｖ1を測定する。Ｉ０をゼロから増加して行き、Ｖ1が大きく生じ始めるときの交流電流値Ｉｔｈから臨界電流密度Ｊｃを評価するのは、上記非特許文献２と同様である。また、検出コイルを駆動コイルと共巻きにし、Ｖ1の変化が生じるときの交流電流値Ｉｔｈから臨界電流密度Ｊｃを評価することもできる（上記非特許文献３参照）。
【００１５】
（３）コイルの電流値がＩｔｈ付近のとき、超伝導体に誘起される電界（の振幅）は、コイルの作る交流磁界をＨ０ｃｏｓ２πｆｔ、超伝導膜の厚さをｄ、真空の透磁率をμ０とすると、ほぼＥ０＝４μ０Ｈ０ｆｄで与えられる。即ち、電界Ｅ０で駆動させたときに電流密度Ｊｃの超伝導電流が流れることがわかる。従って、周波数ｆを変化させ、Ｅ０を変化させて、臨界電流密度Ｊｃを複数回測定し、Ｊｃの周波数依存性を測定することによって、電流密度と電界の関係（電流・電圧特性）を評価することができる。このことは、第３高調波誘導電圧を用いる方法（上記（１））と基本波誘導電圧を用いる方法（上記（２））の両者に当てはまる。
【００１６】
【実施の態様】
（１）超伝導膜に流れる電流密度が、臨界電流密度Ｊｃに等しいか、より小さいときに抵抗ゼロ、それより大きいときに抵抗無限大という臨界状態モデル（電流・電圧特性が無限に急峻）によれば、第３高調波誘導電圧は、Ｖ３＝μ０ｆＩｔｈＧ（Ｉ０／Ｉｔｈ） と表わされる。ここに、Ｇは、コイルの形状と巻き数、超伝導膜に対する配置のみで決まるスケール関数である。従って、周波数ｆを変化させて、Ｖ３／ｆをＩ０に対して測定すると、周波数によらず、全く同じ曲線が得られるはずであり、Ｉｔｈから計算される臨界電流密度Ｊｃは、全く同じになるはずである。
【００１７】
（２）しかし、実際の超伝導体においては、超伝導膜に流れる電流密度が臨界電流密度Ｊｃより小さいときでも微小な電圧が発生しており、また、電流密度がＪｃより少し大きくなっても、すぐに非常に大きな抵抗（電圧）が発生するわけではなく、電流・電圧特性は、臨界状態モデルよりもなだらかである。このため、超伝導膜に流れる電流密度は駆動する電界によって決まり、通常、臨界電流密度Ｊｃを電界の関数として定義している。
【００１８】
（３）第３高調波誘導電圧測定法において、Ｖ３／ｆ対Ｉ０曲線を周波数ｆを変化させて複数回測定した場合、全く同じ曲線が得られるわけではなく、超伝導体の電流・電圧特性を反映した変化が生ずる。そして、Ｉｔｈから計算される臨界電流密度Ｊｃも電流・電圧特性を反映して変化する。このため、Ｖ３／ｆをＩ０に対して測定し、ＩｔｈからＪｃを測定することを、周波数ｆを変化させ、駆動する電界Ｅ０＝４μ０Ｈ０ｆｄを変化させて、複数回行なえば、電流・電圧特性を評価することができる。
【００１９】
（４）ここでは、第３高調波誘導電圧の測定から超伝導膜の電流・電圧特性を測定する方法を示したが、駆動コイルの近傍に配置された検出コイルに誘起される基本波誘導電圧の測定から超伝導膜の電流・電圧特性を測定することもできる。また、厚さの厚いバルク材等においても、上記特許文献４に記載の方法による臨界電流密度の測定を周波数を変えて複数回行えば、同様に、電流・電圧特性を評価することができる。
【００２０】
【実施例】
図１は、コイルに流した交流電流（実効値）Ｉ０／√２に対する、コイルに生じた第３高調波誘導電圧（実効値）Ｖ３／√２を周波数ｆで割った値の特性図であり、周波数を２００Ｈｚから２０ｋＨｚまで変化させて複数回測定している。この図から、Ｉｔｈの周波数依存性を得ることができる。なお、この特性図のデータは、１センチ角、厚さｄ＝５５０ｎｍの超伝導ＹＢａ２Ｃｕ３Ｏ７−ｄ膜において、液体窒素温度７７．３Ｋで測定したものである。
【００２１】
この実験に用いたコイルでは、交流磁界Ｈ０（Ａ／ｍ）＝６２，０００×Ｉ０（Ａ）で既知であるため、超伝導体にかかる電界Ｅ０＝４μ０Ｈ０ｆｄは、各周波数において簡単に計算することができる。また、臨界電流密度Ｊｃは、Ｉｔｈに比例し、その比例係数は、実験または計算により容易に求めることができる。
【００２２】
図１のデータにおいて、（第３高調波誘導電圧／周波数）＝０．０５μＶ secとなる点で、各々の周波数に対してＩｔｈを決め、それからＪｃを計算した。各周波数に対して、電界Ｅ０＝４μ０Ｈ０ｆｄを計算し、この両者から求めた電流・電圧特性（電流密度と電界の関係）を図１の挿入図に示す。高温超伝導酸化物においてよく観測される、冪乗の電流・電圧特性（Ｅ〜Ｊｎ）に近い特性が得られ、ｎ＝２８と計算されたが、細かく見ると冪乗からのずれが観測された。このずれの原因について調べたところ、第３高調波誘導電圧を測定する際のノイズに起因することがわかったため、そのノイズを低減する工夫を行なった。
【００２３】
図２に、ノイズ低減用のキャンセルコイルを含む測定回路の模式図を示す。キャンセルコイルは、試料コイルと同一仕様で製作したコイルであり、臨界電流密度及び臨界電流の大きい（試料薄膜とは別の）超伝導薄膜の直上に配置されているため、試料コイルとほぼ等しい電気抵抗、インダクタンスを有する。発振器及び電力増幅器を用いて、試料コイル及びそれと直列に接続したキャンセルコイルに周波数ｆの交流電流を流し、それらのコイルに発生した第３高調波誘導電圧を測定するわけであるが、図に示すように、Ａ点の電圧からＢ点の電圧の２倍を減じた電圧を測定すれば、発振器及び電力増幅器に起因する高調波ノイズ電圧を有効に除去することができ、試料コイルの直下に配置された試料薄膜に起因する信号電圧Ｖ３のみを正確に測定することができる。
【００２４】
図３に、図２の測定回路を用いて測定した、Ｉ０／√２対Ｖ３／√２ｆの特性図を示すが、この図では、周波数を１００Ｈｚから２０ｋＨｚまで変化させて複数回測定している。なお、この特性図のデータは、１センチ角、厚さｄ＝２５０ｎｍの超伝導ＹＢａ２Ｃｕ３Ｏ７−ｄ膜において、液体窒素温度７７．３Ｋで測定したものである。図３のデータにおいて、Ｖ３／√２ｆ＝０．０５μＶ sec となる点で、各々の周波数に対してＩｔｈを決め、それからＪｃを計算した。各周波数に対して、電界Ｅ０＝４μ０Ｈ０ｆｄを計算し、この両者から求めた電流・電圧特性（電流密度と電界の関係）を図４に示す。ノイズ低減の効果で、ほぼ冪乗の電流・電圧特性（Ｅ〜Ｊｎ）が観測され、ｎ＝２０．８と計算された。なお、図４の挿入図には、同じ電流・電圧特性を線形スケールで示している。
【００２５】
【発明の効果】
本願発明によれば、コイルに流す交流電流Ｉ０とコイルに生じる第３高調波誘導電圧Ｖ３の関係を、周波数ｆを変化させて複数回測定し、Ｖ３が大きく生じ始めるときの交流電流値Ｉｔｈの周波数依存性を調べることによって、局所的な電流・電圧特性を評価することが可能になる。こうして、大面積超伝導膜や超伝導テープ線材を電力機器に応用する上で重要な特性である電流・電圧特性を正しく評価することができる。
【００２６】
また、超伝導バルク材のように厚さが厚い場合でも、Ｉ０とＶ３の関係を、周波数ｆを変化させて複数回測定することによって、電流・電圧特性を評価することが可能になる。
超伝導体の上でコイルを走査させて測定を行えば、電流・電圧特性の分布を評価することが可能である。
【図面の簡単な説明】
【図１】 超伝導薄膜において、コイルに流した交流電流Ｉ０／√２に対する、コイルに生じた第３高調波誘導電圧Ｖ３／√２を周波数ｆで割った値の特性図である。周波数を２００Ｈｚから２０ｋＨｚまで変化させて複数回測定している。挿入図は、このデータから計算された電流・電圧特性を示す。
【図２】 第３高調波誘導電圧のノイズ低減用のキャンセルコイルを含む測定回路の模式図。
【図３】 図２の測定回路を用いて測定した、Ｉ０／√２対Ｖ３／√２ｆの特性図。周波数を１００Ｈｚから２０ｋＨｚまで変化させて複数回測定している。
【図４】 本発明によって評価された、超伝導薄膜の電流・電圧特性。本図はログスケール、挿入図は線形スケールで表示している。[0001]
BACKGROUND OF THE INVENTION
The present invention provides a non-destructive relationship between current and voltage (current / voltage characteristics) in large-area and large-sized superconductors such as large-area superconducting films, long superconducting tape wires, and large superconducting bulk materials. The present invention relates to a measurement method that evaluates automatically and non-contactly.
[0002]
[Prior art]
Large-area and large-sized superconductors such as large-area superconducting films, long superconducting tape wires, and large superconducting bulk materials are expected to be applied to various power devices such as current limiters, superconducting magnets, and bearings. Has been. The current / voltage characteristics of superconductors are important characteristics that determine the performance of superconductors when applied to these power devices. Especially in high-temperature superconducting oxides, the current / voltage characteristics are gentle. As it is known, it plays an important role in various applications.
[0003]
In the case of a current limiter, it is necessary for equipment design because it affects the normal conduction transition behavior at the time of a system fault.For superconducting magnets and bearings, the attenuation of superconducting current due to magnetic flux creep is determined. Essential for current mode operation and long term operation in bearings. In order to apply these large-area and large-sized superconductors to various power devices, there is a development of a non-destructive, non-contact and simple method for evaluating the current / voltage characteristics and distribution of superconductors. It was desired.
[0004]
The current / voltage characteristics of a superconductor are parameters that represent the uniformity of a superconducting sample. It is known that the more uniform the sample, the sharper the current / voltage characteristics. Therefore, if the current / voltage characteristics are evaluated in the manufacturing process of a superconducting tape wire or a current limiter conductor, the process can be evaluated and feedback to the manufacturing technology can be applied.
[0005]
(1) One of the most commonly used methods for evaluating the current / voltage characteristics of a superconductor is to attach four electrodes, a current terminal and a voltage terminal, to the superconductor and pass an energizing current to the voltage. Is a four-terminal method. However, in order to use this method, it is necessary to process the superconductor, and deterioration of the superconducting characteristics at that time becomes a problem. In addition, since it is necessary to pass an energizing current higher than the critical current, there is a possibility that the superconductor may be broken due to sudden heat generation.
[0006]
One of the most commonly used methods to non-destructively evaluate current / voltage characteristics is to measure DC magnetization, which is the average current / voltage of a small superconductor. Only properties can be evaluated.
[0007]
(2) As a method for evaluating the distribution of the local critical current density of the superconducting film, the alternating current I = I 0 cos2πft flowing through the coil disposed immediately above the superconducting film (f is the frequency of the alternating current, and t is the time) And a method of measuring the third harmonic induced voltage (amplitude) V 3 generated in the coil has already been proposed. This is a method of evaluating the critical current density Jc from the alternating current value I 0 = I th when I 0 is increased from zero and V 3 starts to greatly occur (see Non-Patent Document 1 below).
[0008]
As a similar method, instead of measuring the third harmonic induced voltage generated in the coil, a detection coil is arranged at a symmetrical position across the drive coil and the superconducting film, and the AC current value I 0 flowing through the drive coil is detected. There is also a method of measuring the fundamental wave induced voltage induced in the coil (see Non-Patent
[0009]
In addition, a method for measuring the critical current density by analyzing the relationship between I 0 and V 3 in detail for a superconducting thick film and a bulk material by a similar method has already been proposed (see Non-Patent Document 4 below). .
[0010]
However, these methods can only measure the critical current density of the superconductor and cannot evaluate the current / voltage characteristics.
[0011]
[Non-Patent Document 1]
JH Claassen, ME Reeves and RJ Soulen, Jr., "A contactless method for measurement of the critical current density and critical temperature of superconducting films", Rev. Sci. Instrum. 62, 996 (1991).
[Non-Patent Document 2]
H. Hochmuth and M. Lorenz, "Inductive determination of the critical current density of superconducting thin films without lateral structuring", Physica C 220, 209 (1994).
[Non-Patent Document 3]
H. Hochmuth and M. Lorenz, "Side selective and non-destructive determination of the critical current density of double-sided superconducting thin films", Physica C 265, 335 (1996).
[Non-Patent Document 4]
Yasunori Mawatari, Hirofumi Yamazaki, Aihiko Nakagawa, “Critical Current Density and Third Harmonic Induced Voltage in Superconducting Thin Films and Bulk Materials”, 66th Annual Spring Cryogenic Engineering and Superconductivity Society Presentations [0012]
[Problems to be solved by the invention]
The object of the present invention is to determine how non-destructive the current / voltage characteristics and distribution of large-area / large-sized superconductors such as large-area superconducting films, long superconducting tape wires, and large superconducting bulk materials are. Whether to measure without contact.
[0013]
[Means for Solving the Problems]
The present invention employs the following means for solving the above-mentioned problems.
(1) A small coil is disposed immediately above the superconducting film, an alternating current is passed through the coil, a current I 0 flowing through the coil, and a third harmonic induction voltage V 3 induced in the coil by the current are obtained. taking measurement. Continue to increase the I 0 from zero, to evaluate the critical current density Jc from the AC current value I th at which V 3 begins to occur largely is similar to Non-Patent Document 1.
[0014]
(2) As an alternative method, a small drive coil is arranged immediately above the superconducting film, an alternating current is passed through the driving coil, a current I 0 flowing through the coil, and the current is sandwiched between the driving coil and the superconducting film. A fundamental wave induced voltage V 1 induced in a detection coil arranged at a symmetric position is measured. Continue to increase the I 0 from zero, to evaluate the critical current density Jc from the AC current value I th at which V 1 is begins to occur largely is similar to Non-Patent
[0015]
(3) when the current value of the coil in the vicinity of I th, the electric field induced in the superconductor (amplitude) is, H 0 cos2πft an alternating magnetic field generated by the coil, the thickness of the superconducting film d, vacuum magnetic When the magnetic permeability and mu 0, is given by approximately E 0 = 4μ 0 H 0 fd . That is, it can be seen that the superconducting current density Jc flows when driven by an electric field E 0. Therefore, the relationship between the current density and the electric field (current / voltage characteristics) is evaluated by changing the frequency f, changing E 0 , measuring the critical current density Jc multiple times, and measuring the frequency dependence of Jc. can do. This applies to both the method using the third harmonic induced voltage (above (1)) and the method using the fundamental induced voltage (above (2)).
[0016]
Embodiment
(1) A critical state model (current / voltage characteristics are infinitely steep) in which the current density flowing in the superconducting film is equal to or smaller than the critical current density Jc, the resistance being zero, and the larger current density being infinite. Accordingly, the third harmonic induced voltage is expressed as V 3 = μ 0 fI th G (I 0 / I th ). Here, G is a scale function determined only by the shape and number of turns of the coil and the arrangement with respect to the superconducting film. Therefore, if the frequency f is changed and V 3 / f is measured with respect to I 0 , the same curve should be obtained regardless of the frequency, and the critical current density Jc calculated from I th is Should be the same.
[0017]
(2) However, in an actual superconductor, a minute voltage is generated even when the current density flowing in the superconducting film is smaller than the critical current density Jc, and even if the current density is slightly higher than Jc. However, a very large resistance (voltage) is not generated immediately, and the current / voltage characteristics are gentler than the critical state model. For this reason, the current density flowing in the superconducting film is determined by the electric field to be driven, and usually the critical current density Jc is defined as a function of the electric field.
[0018]
(3) In the third harmonic induced voltage measurement method, when the V 3 / f vs. I 0 curve is measured a plurality of times while changing the frequency f, the exact same curve is not obtained, and the current of the superconductor Changes that reflect the voltage characteristics occur. Then, the critical current density Jc, which is calculated from the I th also changes to reflect the current-voltage characteristics. For this reason, measuring V 3 / f with respect to I 0 and measuring Jc from I th is performed several times by changing the frequency f and changing the driving electric field E 0 = 4 μ 0 H 0 fd. If so, current / voltage characteristics can be evaluated.
[0019]
(4) Although the method of measuring the current / voltage characteristics of the superconducting film from the measurement of the third harmonic induced voltage is shown here, the fundamental wave induced voltage induced in the detection coil arranged in the vicinity of the drive coil From this measurement, the current / voltage characteristics of the superconducting film can be measured. Further, even in a thick bulk material or the like, if the measurement of the critical current density by the method described in Patent Document 4 is performed a plurality of times at different frequencies, the current / voltage characteristics can be similarly evaluated.
[0020]
【Example】
FIG. 1 is a characteristic diagram of a value obtained by dividing a third harmonic induced voltage (effective value) V 3 / √2 generated in a coil by a frequency f with respect to an alternating current (effective value) I 0 / √2 passed through the coil. The frequency is changed from 200 Hz to 20 kHz, and the measurement is performed a plurality of times. From this figure, the frequency dependence of I th can be obtained. The data in this characteristic diagram was measured at a liquid nitrogen temperature of 77.3 K in a superconducting YBa 2 Cu 3 O 7-d film having a 1 cm square and a thickness d = 550 nm.
[0021]
Since the coil used in this experiment is known with an alternating magnetic field H 0 (A / m) = 62,000 × I 0 (A), the electric field E 0 = 4 μ 0 H 0 fd applied to the superconductor is It can be easily calculated in frequency. The critical current density Jc is proportional to I th, the proportionality factor can be determined easily by experiment or calculation.
[0022]
In the data of FIG. 1, I th was determined for each frequency at a point where (third harmonic induction voltage / frequency) = 0.05 μV sec, and Jc was calculated therefrom. The electric field E 0 = 4 μ 0 H 0 fd is calculated for each frequency, and the current / voltage characteristics (relationship between current density and electric field) obtained from both are shown in the inset of FIG. A characteristic close to the power / voltage characteristics (E to J n ) of the power, which is often observed in high-temperature superconducting oxides, was obtained and calculated to be n = 28. It was done. When the cause of this deviation was examined, it was found that it was caused by noise when measuring the third harmonic induced voltage. Therefore, a device for reducing the noise was devised.
[0023]
FIG. 2 is a schematic diagram of a measurement circuit including a cancel coil for noise reduction. The cancel coil is a coil manufactured with the same specifications as the sample coil, and is disposed directly on the superconducting thin film (separate from the sample thin film) having a large critical current density and critical current. Has resistance and inductance. Using an oscillator and a power amplifier, an AC current having a frequency f is passed through a sample coil and a cancel coil connected in series with the sample coil, and the third harmonic induced voltage generated in these coils is measured. Thus, if the voltage obtained by subtracting twice the voltage at the B point from the voltage at the A point is measured, the harmonic noise voltage caused by the oscillator and the power amplifier can be effectively removed, and the voltage is arranged immediately below the sample coil. have been only the signal voltage V 3 resulting from the thin film sample can be measured accurately.
[0024]
FIG. 3 shows a characteristic diagram of I 0 / √2 vs. V 3 / √2f measured using the measurement circuit of FIG. 2. In this figure, the frequency was changed from 100 Hz to 20 kHz and measured several times. ing. The data in this characteristic diagram was measured at a liquid nitrogen temperature of 77.3 K in a superconducting YBa 2 Cu 3 O 7-d film having a 1 cm square and a thickness d = 250 nm. In the data of FIG. 3, I th was determined for each frequency at a point where V 3 /√2f=0.05 μV sec, and Jc was calculated therefrom. For each frequency, the electric field E 0 = 4 μ 0 H 0 fd is calculated, and the current / voltage characteristics (relationship between current density and electric field) obtained from both are shown in FIG. Due to the effect of noise reduction, a nearly square current-voltage characteristic (E to J n ) was observed, and n = 20.8 was calculated. In the inset of FIG. 4, the same current / voltage characteristics are shown on a linear scale.
[0025]
【The invention's effect】
According to the present invention, the relationship between the AC current I 0 flowing through the coil and the third harmonic induced voltage V 3 generated in the coil is measured a plurality of times while changing the frequency f, and the AC current when V 3 starts to be greatly generated. by examining the frequency dependence of the value I th, it is possible to evaluate the local current-voltage characteristic. In this way, current / voltage characteristics, which are important characteristics in applying large-area superconducting films and superconducting tape wires to power equipment, can be evaluated correctly.
[0026]
Further, even when the thickness is large like a superconducting bulk material, it is possible to evaluate the current / voltage characteristics by measuring the relationship between I 0 and V 3 a plurality of times while changing the frequency f. .
If measurement is performed by scanning a coil on a superconductor, it is possible to evaluate the distribution of current / voltage characteristics.
[Brief description of the drawings]
FIG. 1 is a characteristic diagram of a value obtained by dividing a third harmonic induction voltage V 3 / √2 generated in a coil by a frequency f with respect to an alternating current I 0 / √2 passed through the coil in a superconducting thin film. The frequency is measured several times while changing from 200 Hz to 20 kHz. The inset shows the current-voltage characteristics calculated from this data.
FIG. 2 is a schematic diagram of a measurement circuit including a cancel coil for noise reduction of a third harmonic induced voltage.
FIG. 3 is a characteristic diagram of I 0 / √2 vs. V 3 / √2f measured using the measurement circuit of FIG. The frequency is changed from 100 Hz to 20 kHz and is measured a plurality of times.
FIG. 4 shows current-voltage characteristics of a superconducting thin film evaluated according to the present invention. This figure shows the log scale, and the inset shows the linear scale.
Claims (8)
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