液体金属は環境浄化の隠し味

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(Liquid metals the secret ingredients to clean up environment)

2019/10/14 オーストラリア連邦ニューサウスウェールズ大学 (UNSW)

 

・ UNSW は、少量のエネルギーで化学反応を加速する液体金属とその触媒としての役割を解明。
・ レンジやオーブン内で 300℃以下で融解するガリウム、インジウム、ビスマスやスズ合金等の液体金属を組合せて、CO2 変換や浄水に使用できる触媒を、一般的な家庭の台所にあるシェーカーとレンジ台で誰でも作製できる。
・ ビスマスとスズを組合せた合金では、各金属を単体で加熱するよりもかなり低い温度で融解する。このような挙動を呈する物質は、共晶と呼ばれる。
・ 共晶合金は、特定の金属の組合せにより最低温度で融解する金属の混合物。例えば、ビスマスを 57%、スズを 43%で混合すると 139℃で融解するが、ビスマスもスズもそれぞれ融点は 200℃を超える。
・ 共晶物質では、特定の比率での混合で、ナノレベルで自然に発生する大規模な欠陥によりその融点が下がる。また、その逆も可能で、液体の共晶物質は、混合された各金属の通常の凝固点よりも低い温度で固化する。液体金属を固化する際に材料内に自然に形成された欠陥が、「触媒的」活性を大幅に増強させる。
・ 液体金属触媒の作り方の手順は、次のとおり。


1. 共晶合金を鍋にいれて強火にかける。
2. 金属が融けたら、それをボトルに入った水の中に慎重に注入し、ボトルのキャップをしめる。
3. ボトルをよく振って液体金属と水を混ぜ合わせ、水中に液体金属の小滴を作る(オイルと酢を混ぜたドレッシング中のオイルの小滴のように)。
4. それらの小滴を固化して粉状にすると、CO2 の電気化学的な変換に使用できる触媒が完成。


・ 液体合金は、環境の汚染物質の除去や中和、また、CO2 からの炭素回収に利用できる。スズ、ガリウムやビスマスは液体で、CO2 を有益な副生物に変換する電極として利用できる。また、液体金属を加熱して作製した酸化物は、水中の汚染物質を分解する光エネルギーの吸収に使用できる。
・ 液体金属が環境問題解決に魅力的な材料である理由は、低エネルギー、ローテクな環境で安価に作製可能な点。
・ 本研究の主任研究員の一人である、Kalantar-Zadeh 教授は、Australian Research Council (ARC) より Laureate Fellowship を受賞、液体金属に関する研究に今後 4 年間資金を提供する予定。
URL: https://newsroom.unsw.edu.au/news/science-tech/liquid-metals-secret-ingredients-cleanenvironment

(関連情報) Nature Communications 掲載論文(フルテキスト)
Advantages of eutectic alloys for creating catalysts in the realm of nanotechnology-enabled metallurgy
URL: https://www.nature.com/articles/s41467-019-12615-6

<NEDO海外技術情報より>

Advantages of eutectic alloys for creating catalysts in the realm of nanotechnology-enabled metallurgy

Jianbo Tang,Rahman Daiyan,Mohammad B. Ghasemian,Shuhada A. Idrus-Saidi,
Ali Zavabeti,Torben Daeneke,Jiong Yang,Pramod Koshy,Soshan Cheong,
Richard D. Tilley,Richard B. Kaner,Rose Amal &Kourosh Kalantar-Zadeh

Nature Communications volume 10, Article number: 4645 (2019) Cite this article

Abstract

The nascent field of nanotechnology-enabled metallurgy has great potential. However, the role of eutectic alloys and the nature of alloy solidification in this field are still largely unknown. To demonstrate one of the promises of liquid metals in the field, we explore a model system of catalytically active Bi-Sn nano-alloys produced using a liquid-phase ultrasonication technique and investigate their phase separation, surface oxidation, and nucleation. The Bi-Sn ratio determines the grain boundary properties and the emergence of dislocations within the nano-alloys. The eutectic system gives rise to the smallest grain dimensions among all Bi-Sn ratios along with more pronounced dislocation formation within the nano-alloys. Using electrochemical CO2 reduction and photocatalysis, we demonstrate that the structural peculiarity of the eutectic nano-alloys offers the highest catalytic activity in comparison with their non-eutectic counterparts. The fundamentals of nano-alloy formation revealed here may establish the groundwork for creating bimetallic and multimetallic nano-alloys.

Introduction

It is an ancient wisdom that alloys produced by mixing different metals together, such as bronze, brass, and pewter, can offer more desirable properties than their single-metal counterparts1. Interests in alloys have not subsided in present time but rather significantly expanded as combinatoric metallurgy in pursuit of well-tailored alloys. These efforts have led to the discovery of metallic glasses2, ultrahigh strength super alloys3, high-entropy alloys4, and many more5,6,7.

Interesting behaviours are seen in liquid metal alloys. It has been known for long that mixing different metals can lead to a decrease in both melting and freezing temperatures8. At an exact mixing ratio, the temperature drop becomes most significant, leading to the state referred to as the eutectic, from the Greek word meaning ‘easily melting’. The single-phase-like transition behaviour of eutectic systems has been shown to be advantageous for many technologically important applications, such as heat exchange and electronic switches9,10,11. While these applications mostly concern liquid metals in their bulk form, in recent years more knowledge regarding the properties in low dimensions, has been gained, thanks to advancements in electron microscopy and other spectroscopic capabilities12.

To explore the fundamentals of liquid alloys across different scales, both their surface and core should be studied. To date, such studies on bulk systems have resulted in intriguing observations. For instance, despite the fact that liquid metals and some of their alloys are defined by their disordered condensed state within their bulk, X-ray reflectivity analyses have revealed that their surfaces are ordered at atomic level13,14,15, a unique virtue of liquid alloys which has led to new methods for creating two-dimensional materials16,17. Moreover, in a liquid metal bulk, adding extra metallic entities have been found to improve their catalysis performance18. Similarly, it is perceived that many other distinct traits of liquid metals can be observed in nanoscales.

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