2014 Fiscal Year Annual Research Report
酸化亜鉛［０００１］傾角粒界の原子配列および偏析挙動
| Project/Area Number |
14J10865
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| Research Institution | The University of Tokyo |
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Principal Investigator |
ROH Jiyoung 東京大学, 工学系研究科, 特別研究員(DC2)
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| Project Period (FY) |
2014-04-25 – 2016-03-31
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| Keywords | 粒界 / 酸化亜鉛 / 透過電子顕微鏡 / プラセオジム |
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| Outline of Annual Research Achievements |
[0001] symmetric tilt GBs are systematically investigated based on the coincidence site lattice (CSL) theory in order to determine detailed atomic structures. Tilt angles (2θ) were controlled from 0° to 60° to cover various types of GBs. For this purpose, a series of single GBs were fabricated using bicrystal methods, and scanning transmission electron microscopy observation and first-principles calculation have performed.
It was found that the GBs are described by periodic arrangement of a several low-energy configurations, which are often called structure units (SUs). GBs of 2θ below 27.8° (Σ13) are described by SUs of α and/or β types that are the same as those for undoped GBs. On the other hand, GBs of the 2θ near 30° are described by new type of SU γ, which has not been found in undoped GBs. Moreover, GBs with the 2θ higher than 32.2° (Σ13) were described by SU α in zig-zag arrangements. Each type of SU has specific number of Pr at certain Zn substitution sites.
Non-low-Σ CSL GBs were observed in order to reveal relevance between those unique arrangements of SUs. The GBs were described by SUs those are a combination of SUs of neighbored CSL GBs, called GB delimitation. Based on these results, a concept of phase diagram for ZnO GB, which describes the stable GB structure as a function of 2θ can be proposed. In addition, distribution of Pr-concentration as a function of 2θ can be assumed as well.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
プラセオジムが添加された酸化亜鉛は、電圧量に応じて抵抗値が変化する特性を持っており、この特性は酸化亜鉛界面へのプラセオロジウムの偏析によって左右されると知られている。しかし、この材料の界面構造や偏析挙動に関する研究が非常に不足している。従って、本研究員は粒界における酸化亜鉛粒界の原子レベル構造及び不純物のプラセオジムの粒界偏析挙動について研究を行った。バイクリスタル方法でプラセオジムドープ酸化亜鉛単一粒界を作製、電子顕微鏡と理論計算を用いて粒界構造と偏析を原子レベルでくわしく解析が行われた。また、結晶オリエンテーションを変化させながら、いくつかの種類の特殊単一粒界（シグマ(S)粒界）を観察することによって、特集な粒界に限らず、より一般的な酸化亜鉛の粒界挙動が理解できるようになった。示された電子顕微鏡と理論計算を融合した実験アプローチは、粒界を根本的に理解する有効な手法であり、これらの結果を基に得られた結論は材料科学的にも価値が高いと判断できると期待されている。
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| Strategy for Future Research Activity |
By knowing the atomic structure, it is possible to speculate the concentration of segregated Pr at each GB. It has been reported that the varistor performance (non-linear I-V (current-voltage)coefficient (α))is determined by segregated Pr, since the Pr promotes formation of native defect such as Zn vacancies that acts like a electron trap at the GB interface state. In addition, the voltage gradient (non-linear coefficient, alpha) may also increase as concentration of Zn vacancy increases. Therefore, it would be possible to find the most effective GB orientation that accompanies the highest α value. On-going experiments suggests that 2θ = 30° tilt GB has the highest Pr concentration among 2θ[0001]GBs. By measuring I-V curve from a several GBs, a relationship between Pr-concentration and the α value can be revealed. The result would be valuable for optimizing ZnO-based varistor with minimal size but high performance.
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