１ はじめに
私たちは昨年8月に、京都大学院理学研究科の金相学研究室を訪れ、大学における研究活動を体験した。その時に、高温超伝導体を二通りの方法で作製し、作製方法の違いによる超伝導転移温度、結晶構造の違いについて研究してきたので、その内容を発表したい。
２ 目的
原料は同じだが異なる熱処理過程を経た、2種類の高温超伝導体YBa2Cu3Oyを作成し、粉末Ｘ線回折測定、マイスナー効果の確認、電気抵抗測定等を行った。実験の結果から物性の違い、構造の違いを確認し考察する。
1 Introduction
We visited the Solid State Chemistry and Physics Laboratory at Kyoto University Graduate School of Science in August last year and experienced research at a university lab. We prepared high-temperature superconductors in two different methods and studied their effects on the transition temperature and the crystal structure, which we would like to present here.
2 Objective
Two different high-temperature superconductors YBa2Cu3Oy were prepared from the same starting materials using different heat treatment procedures, and their properties were studied by X-ray powder diffraction, verification of the Meissner effect, and electric resistance measurement. We discuss the differences in physical property and structure confirmed by the experiments.
高い転移温度で超伝導が起こる物質のこと。ここでいう「高温」は通常人間が熱いと感じるレベルではなく、－200℃～－100℃程度を指す。超伝導帯は次の２つの性質を示す。
■ゼロ抵抗・・・電気抵抗がある時は、電圧をかけなければ電流は流れない。しかし超伝導状態では、電気抵抗がないので電圧をかけなくても電流が流れる。また、抵抗がないので、永久電流が流れる。
■マイスナー効果（完全反磁性）・・・永久電流の磁場が外部磁場に重なり合って、超伝導体内部の磁束密度が常にゼロである現象が見られる。
Materials that become superconductors at high transition temperatures. "High-temperature" in this case does not mean a temperature that we humans normally feel hot, but refers to a temperature range between -200 to -100 degrees Celsius. Superconductors exhibit the following two properties.
■Zero resistance: When there is electric resistance, current does not flow unless a voltage is applied. However, in the superconducting state, electric current flows without applied voltage. Moreover, the current flows indefinitely because there is no resistance.
■The Meissner effect (perfect diamagnetism): The magnetic field from the persistent current and the applied magnetic field overlap and cancel each other such that the magnetic flux inside the superconductor becomes zero all the time.
４ 材料と方法
試薬Y2O3、BaO3、CuOのモル比が１：４：６となるよう秤量し、乳鉢で混合した後、直径８mmの円盤状にプレス形成（8~10MPa程度）して試料（ペレット）を作製する。作製したペレットを900℃のボックス型電気炉で8時間仮焼きする。
仮焼き後、自然冷却させた試料を再び乳鉢で粉砕、混合し半量ずつ一度目と同じ方法でプレス成形する。1,5mm~2mmの厚さの試料を2つ得る。
この後、片方の試料（試料A）は900℃のボックス型電気炉で8時間加熱した後、急冷する。
4 Materials and methods
Reagents Y2O3, BaO3, and CuO were weighed and mixed in a mortar at a molar ratio of 1:4:6, and pressed into 8 mm discs (approximately 8-10 MPa) to prepare the samples (pellets). The prepared pellets were pre-baked for 8 hours at 900 degrees Celsius in an electric box furnace. The pre-baked samples were naturally cooled, and powdered and mixed again in a mortar. The sample was divided into two halves and pressed as described above into two 1.5-2 mm thick discs. Subsequently, one sample (sample A) was baked at 900 degrees Celsius for 8 hours in an electric box furnace and rapidly cooled.
また、もう片方の試料は温度プログラム付き電気炉に入れ2hで900℃まで上昇、8hキープ、その後10hかけてゆっくりと冷却というプログラムを実行し酸素を十分に含ませた。
□マイスナー効果（液体窒素温度）
試料Aではマイスナー効果が見られなかったが試料Bではマイスナー効果を確認することができた。
□結晶構造解析
結晶構造において、試料Aについては酸素の欠損により
a軸＝b軸となり正方晶を示した。熱処理時に時間をかけ
冷却した試料Bは酸素を多く含み、a軸とb軸が非等価で
ある斜方晶を示した。
The other sample was baked in a temperature programmable furnace and the sample was programmed to be heated to 900 degrees Celsius over 2 hours and kept at the temperature for 8 hours, and slowly cooled back to ambient temperature over 10 hours, in order to allow ample oxygen to be absorbed.
□The Meissner effect (at liquid nitrogen temperature)
Sample A did not show the Meissner effect. However, we observe the Meissner effect with sample B.
□Crystal structure analysis
In the crystal structure, sample A was shown to be a cubic crystal system with the axes a=b due to lack of oxygen. Sample B which was cooled over a long period of time during the heat treatment contained more oxygen and was shown to be orthrhombic with nonequivalent a and b axes.
□電気抵抗測定
試料Aは電気抵抗が消えなかったが、試料Bは94.5［Ｋ］で大きく抵抗が下がり始め93.0［K］で抵抗が限りなく0に近づき、超伝導状態を確認することができた。
６ 考察
結晶構造の解析結果から、試料Aについては酸素が欠損することによりa軸＝b軸となり、正方晶を示した。結晶構造において、熱処理時に時間をかけて冷却した試料Bは酸素を多く含みa軸とb軸が非等価である斜方晶を示した。
□Electric resistance measurement
Although the electric resistance did not disappear with sample A, sample B showed a sudden decrease in resistance at 94.5 [K] and it became infinitely close to 0 at 93.0 [K], confirming superconductivity.
6 Discussion
Based on the crystal structure analysis, sample A was indicated to be a cubic crystal system with a axis = b axis due to lack of oxygen. The crystal structure of sample B which was cooled over a longer period of time during heat treatment contained more oxygen and was indicated to be orthrombic with nonequivalent a and b axes.
試料Aで見られなかったマイスナー効果や電気抵抗ゼロの状態が、試料Bでは見られたことにより、超伝導帯の形成において物質内に含まれる酸素量の違い（結晶構造の違い＝斜方晶であること）が重要であることがわかった。
７ 謝辞
今回の体験実習で講義、実験指導していただきました、京都大学大学院理学研究科金相学研究室 教授 吉村一良先生、助教 道岡千城先生、研究室の大学院生の皆様に深く感謝申し上げます。
Based on our results that the Meissner effect and zero electric resistance were observed with sample B but not with sample A, we discovered that the oxygen content (difference in crystal structure = orthrombic) is important for the formation of superconductors.
7 Acknowledgement
We would like to express our deepest gratitude to Professor Kazuyoshi Yoshimura, Assistant Professor Chishiro Michioka, and the graduate students in the Solid State Chemistry and Physics Laboratory at Kyoto University Graduate School of Science who supported us through the lectures and laboratory research.
