Low-melting-temperature (LMT) metals and alloys based on post-transition metals, frequently dubbed as “liquid metals”, are enigmatic materials. Although they have been around for thousands of years, and we have continuously been fascinated by what they are, our knowledge about them is still rudimentary. Perhaps the reason is that most electromagnetic waves or even electron beams can hardly penetrate through liquid metals and we tend to ignore what we cannot directly observe. Liquid metals’ fluidity and flexibility, capability to alloy and mix with other metals, together with their high thermal and electrical conductivities, are characteristics that do not simultaneously exist in other types of materials. Exploiting these perplexing properties offer certain capabilities within the realms of chemistry and physics for creating functional materials, soft robotics, biosystems, and optoelectronics never-before accessible toward the next generation of advanced technologies.(1−5) Under the context of physical chemistry, liquid metals provide a reaction environment or template which is extraordinary for catalysis and other chemical reactions to benefit a range of areas in engineering, especially metallurgy, mining, and additive manufacturing. Traditionally what are called liquid metals include the elements of mercury (Hg), gallium (Ga), rubidium (Rb), cesium (Cs), and francium (Fr), as they hold a liquid state at below or near room temperature.(6,7) However, only Hg, as a partial post-transition metal, and Ga, as a core member of post-transition metals, are suitable for practical liquid-based applications. It is known that Rb and Cs are explosively reactive and Fr is radioactive, while Hg is highly toxic, which can enter human body via vapor inhalation or skin absorption, leaving us with Ga as the only viable option. Accessing only one element for establishing a new field, namely, the field of “LMT liquid metals”, is very limiting. As such, to meaningfully expand the field, we can include all core post-transition metals in the table of elements (Figure 1), by raising the melting temperature threshold from the ambiguous room temperature definition to 330 °C.(6) This new definition adds indium (In), thallium (Tl), tin (Sn), lead (Pb), and bismuth (Bi) to the family, which is in fact logical and has practical and scientific reasonings behind it. Figure 1. (a) Image of liquid metal droplets, made of EGaIn (eutectic alloy of gallium and indium) dispersed on a substrate. (b) Table of elements highlighting the core post-transition metals (blue) and those sometimes considered as post-transition metals (gray and light orange). (c) Important characteristics and possible strategies for exploiting the potential of LMT liquid metals. Practically, 330 °C is an achievable temperature using commonly affordable gadgets in safe laboratory environments, making handling and investigations easy and at relatively low cost. However, the science behind it is much more interesting. For post-transition elements, the increase in nuclear charge is offset by the enhanced electron presence. As in these elements the electrons are in spatially distributed orbitals, they do not fully screen each successive increase in the nuclear charge and hence their nuclear charges can dominate and atomic radii contract. Consequently, post-transition metals have fewer electrons available for metallic bonding, in comparison to other metals, making them more polarizing and prone to establishing covalent bonds. This means that they can display both metallic and nonmetallic properties, which leads to the emergence of fascinating irregularities including LMT that allow access to liquid metals and alloys, at relatively low applied energies, for a variety of applications. As highly dynamic and stimuli-responsive materials, liquid metals can be tuned for versatile purposes by controlling their intrinsic properties and the choice of the applied external fields (Figure 1c). To explore the world of LMT liquid metals, we have to look deep inside the bulk of these metallic liquids, their interfacial characteristics and also entropy, in order to harness their unusual traits, and to exploit the potential of post-transition metals that as said sometimes behave like metals and at other times like nonmetals. Some nontrivial characteristics of liquid metals are illustrated in Figure 2. We should surpass the classical metallurgy, defined by crude phase diagram interpretations, to discover the mechanisms of surface atomic layering,(8) multiphase systems, forces/interactions at the atomic levels within liquid metals, and undefined elemental solubilities. Figure 2. Conceptualized LMT liquid metal and alloy systems and their unique surface/bulk activities. The spheres represent atoms, and different colors and sizes indicate different elements. (a) Elemental liquid metal featuring disordered atoms. (b) Binary liquid metal alloy. (c) High-entropy liquid alloy. (d) Atomic layering at the surface of a metallic liquid. (e) Surface precipitation and patterning of the dilute phase of a binary liquid alloy. (f) Crystal formation in the bulk of a liquid metal solvent. (g) Formation of surface oxide layer on liquid metals, which can be either crystalline or amorphous depending on the ambient conditions. (h) Carbon–carbon bond formation at the liquid metal surface. (i) Penetration of molecules (CH₄) into a liquid metal and their dehydrogenation to form radicals (indicated with asterisks). Liquid metal constitutes free electrons and disordered ions, which make them different from other liquids. In the presence of any injected or outside charge, electrons in and near the interfacial regions of liquid metals can displace relatively quickly, while the ions, which have several orders of magnitude larger masses in comparison to electrons, take their time to move. While predicting the behaviors of such complicated charge displacements is challenging, their presence defines some of the peculiarities of liquid metals (for instance, surface layering, Figure 2d). We can modulate the electrical conductivity of liquid metals rapidly while ionic conductivity modulations appear with a significant lag. The same analogy also exists for their thermal properties as well as electrical double layer in electrolytes, which are complicated to predict but rewarding to control. The specifications of the boundaries of LMT liquid metals give them some unique physical and chemical properties. While metals such as Hg do not establish an oxide layer in normal ambient environment, the core post-transition metals form an oxide layer that can either be ultrathin like Ga or grow thicker with time like Sn and Bi (Figure 2g). Regardless, these oxide layers, thin or thick, protect the bulk of liquid metal from further oxidation. These oxide layers are generally planar as they are templated onto the ordered and ultrasmooth surface of liquid metals. The ordering of liquid metals, which occur near the interfacial areas, give very special properties to these materials, defining their high surface tension and also governing the solubility of other elements and molecules into the interface and core of liquid metals. Exotic and novel LMT alloys can also expand the concept of liquid metals. Exploring innovative combinations of multiple elements is one of the ways for designing LMT liquid metal alloys with smart functionalities (Figure 2a–c). One of the old examples is the Field’s metal that made of 32.5% Bi, 51% In, and 16.5% Sn (weight ratios) with a melting point of 61.5 °C. Additionally, the possibility of developing high entropy alloys and systems, at the boundaries of multicomponent phase diagrams, should be explored. Boundaries of multicomponent phase diagrams are conventional targets for making alloys. Poly elemental metallic particles play an important role for establishing nano particle suspensions using liquid alloys. The same as their aqueous or organic solvent counterparts, these suspensions can offer exceptional functionalities such as magnetic traits that do not exist in post-transition metals.(9) Additionally, in this context, solidification of LMT liquid alloys is a rich and yet less studied front (Figure 2e,f). We have recently shown how such phase transition and separation on the surface can be different from the bulk for metallic alloys.(10) Phase separations and interactions of liquid metals with substrates(11) and their reactions with nonmetallic materials can also be studied using intricate multiplex systems. Liquid metals can play an extraordinary role in catalysis. There have been a few reports on the catalytic properties of liquid metals,(12−14) and this is just the start of the long, yet rewarding, road that may change the future of catalysis. Catalytic behavior in liquid and solid states are vastly different. The thermodynamics of liquid metals also differs greatly from their solid counterparts. The elemental metallic clusters in liquid forms can move around freely while sharing the dislocated cloud of electrons. The target materials to be catalyzed can penetrate the surface of liquid metals with ease, become catalyzed, and then leave the liquid media freely after they become saturated (Figure 2h). Additionally, secondary metals that are known to be good catalysts in nonoxidized forms can be protected inside post-transition liquid metals. The fluidity and high atom mobility of liquid metal alloys can allow facile and yet protected interactions of precursors with the catalysts. In this regard, the permeability of molecules to be catalyzed in liquid metals should be carefully investigated. This concept offers a fascinating paradigm that creating superefficient liquid catalysts, operating at low temperatures, is a real possibility. Just imagine that a metal like platinum (Pt) keeps its liquid state while surrounded by Hg or Ga atoms for catalysis at room temperature. Like a dream, this allows the creation of the extraordinary concept of room-temperature liquid Pt. The catalysis is only one side of the coin. The concept of liquid metal reaction media can be expanded to other areas of physical chemistry as well. Liquid metal reaction with organic materials can lead to the formation radicals and dehydrogenation at room temperature (Figure 2i), with minimal applied energy. This can potentially revolutionize process engineering industries by drastically reducing the energy input. Liquid metals can also be employed as the precursors for creating high value components for electronics and optics. Novel complexes and crystals can be accessed using the liquid metal reaction media.(15−17) Oxidation or reaction with chalcogenides (synthesis of oxides and chalcogenide compounds), phase separation (dealloying and precipitation), and mixing and the creation of new compounds can occur simultaneously at ease in these metallic liquids. We can use liquid metals as solvents to allow secondary phases to crystallize within and then extract them out (Figure 2f). In this regard, the high surface tension of liquid metals poses a challenge for separating the products that should be fully explored. The surface of liquid metals also offers promising opportunities for creating new materials. It is known that the surface is layered in the chaos of the bulk in such liquid metals. This near surface layering can be subjected to environmental oxides or chalcogenide precursors to create ultrathin products. We have already shown transparent conductive oxides and high hole mobility transparent oxides and sulfides(18,19) and other never-seen-before planar crystals. These ultrathin products are fascinating materials that are transparent and conductive, and at the same time, can be bent at extreme angles without losing their integrity. We have also shown that by replacing the environment with the precursors of sulfide, ultrathin metal chalcogenides can be produced that are great electronic and piezoelectric materials for mechanical energy harvesting.(20) These materials are the future of transparent flexible electronics and optics that we are all longing. Finally, we conclude this Viewpoint by saying that novel smart systems for energy harvesting and storage, high-tech optical and electronic components, new manufacturing approaches, and superefficient functional materials can be created using LMT liquid metals. While not presented here, a variety of biotechnology enabled products can also be developed by taking advantage of the low hazard and right melting point of Ga (29.8 °C) and its alloys with reference to human body. There are great opportunities for establishing viable new industries based on liquid metal technologies that we should efficiently grasp and utilize in the near future. The LMT liquid metal field is certainly enjoying its well-deserving renaissance. This work was supported by the Australian Research Council (ARC) Laureate Fellowship grant (FL180100053) and the ARC Centres of Excellence FLEET (CE170100039). M.A.R. acknowledges the ARC Discovery Early Career Researcher Award (DECRA) grant (DE210101162). This article references 20 other publications.

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低熔点液态金属及其对物理化学的影响

基于后过渡金属的低熔点 (LMT) 金属和合金，通常被称为“液态金属”，是神秘的材料。尽管它们已经存在了数千年，而且我们一直对它们的本质着迷，但我们对它们的了解仍然很初级。或许是因为大部分电磁波甚至电子束都很难穿透液态金属，而我们往往忽略了我们无法直接观察到的东西。液态金属的流动性和柔韧性、与其他金属合金化和混合的能力，以及它们的高导热性和导电性，这些特性在其他类型的材料中是不存在的。利用这些令人困惑的特性，可以在化学和物理领域提供某些功能来创建功能材料，(1-5) 在物理化学的背景下，液态金属提供了一个非常有利于催化和其他化学反应的反应环境或模板工程领域的一系列领域，尤其是冶金、采矿和增材制造。传统上所谓的液态金属包括汞 (Hg)、镓 (Ga)、铷 (Rb)、铯 (Cs) 和钫 (Fr) 等元素，因为它们在低于或接近室温时保持液态。 6,7) 然而，只有作为部分后过渡金属的 Hg 和作为后过渡金属核心成员的 Ga 适合实际的液体基应用。已知 Rb 和 Cs 具有爆炸性反应，而 Fr 具有放射性，而汞是剧毒的，它可以通过蒸气吸入或皮肤吸收进入人体，所以我们只能选择Ga。仅访问一个元素来建立一个新领域，即“LMT液态金属”领域，是非常有限的。因此，为了有意义地扩展该领域，我们可以通过将熔化温度阈值从模糊的室温定义提高到 330 °C，将所有核心后过渡金属包括在元素表中（图 1）。（6）这个新的定义增加了铟（In）、铊（Tl）、锡（Sn）、铅（Pb）和铋（Bi），这实际上是合乎逻辑的，背后有实际和科学的推理。图 1. (a) 液态金属液滴的图像，由分散在基板上的 EGaIn（镓和铟的共晶合金）制成。(b) 突出显示核心后过渡金属（蓝色）和有时被视为过渡后金属（灰色和浅橙色）的元素表。(c) 开发 LMT 液态金属潜力的重要特征和可能的策略。实际上，在安全的实验室环境中使用通常负担得起的小工具可以达到 330 °C 的温度，这使得处理和调查变得容易且成本相对较低。然而，它背后的科学更有趣。对于过渡后元素，核电荷的增加被增强的电子存在抵消。由于在这些元素中，电子在空间分布的轨道中，它们不能完全屏蔽核电荷的每次连续增加，因此它们的核电荷可以占主导地位并且原子半径收缩。最后，与其他金属相比，过渡后金属可用于金属键合的电子较少，这使得它们更极化并易于建立共价键。这意味着它们可以同时显示金属和非金属特性，这导致了包括 LMT 在内的迷人不规则性的出现，这些不规则性允许以相对较低的外加能量获得液态金属和合金，用于各种应用。作为高度动态和刺激响应的材料，液态金属可以通过控制其内在特性和应用的外部场的选择来调整用于多种用途（图 1c）。为了探索 LMT 液态金属的世界，我们必须深入了解这些金属液体的大部分、它们的界面特征和熵，以便利用它们的不同寻常的特性，并利用后过渡金属的潜力，如上​​所述，这些金属有时表现得像金属，有时表现得像非金属。图 2 说明了液态金属的一些重要特征。 我们应该超越由粗相图解释定义的经典冶金学，以发现表面原子分层的机制，（8）多相系统，液体内原子水平的力/相互作用金属和未定义的元素溶解度。图 2. 概念化的 LMT 液态金属和合金系统及其独特的表面/本体活动。球体代表原子，不同的颜色和大小代表不同的元素。(a) 具有无序原子的元素液态金属。(b) 二元液态金属合金。(c) 高熵液态合金。(d) 金属液体表面的原子分层。(e) 二元液态合金稀相的表面析出和图案化。(f) 在大量液态金属溶剂中形成晶体。(g) 在液态金属上形成表面氧化层，根据环境条件可以是晶体或非晶态。(h) 在液态金属表面形成碳-碳键。(i) 分子的渗透（CH₄) 变成液态金属并使其脱氢形成自由基（用星号表示）。液态金属由自由电子和无序离子构成，这使它们与其他液体不同。在存在任何注入或外部电荷的情况下，液态金属界面区域内和附近的电子可以相对较快地位移，而质量比电子大几个数量级的离子则需要时间移动。虽然预测这种复杂电荷位移的行为具有挑战性，但它们的存在定义了液态金属的一些特性（例如，表面分层，图 2d）。我们可以快速调节液态金属的电导率，而离子电导率调节出现明显滞后。它们的热性能以及电解质中的双电层也存在相同的类比，这些特性很难预测，但控制起来却是有益的。LMT 液态金属边界的规格赋予它们一些独特的物理和化学性质。虽然 Hg 等金属在正常周围环境中不会形成氧化层，但核心过渡金属形成的氧化层可以像 Ga 一样超薄，也可以像 Sn 和 Bi 一样随时间变厚（图 2g）。无论如何，这些薄薄的或厚的氧化层可以保护大部分液态金属免于进一步氧化。这些氧化物层通常是平面的，因为它们被模板化到液态金属的有序和超光滑表面上。发生在界面区域附近的液态金属的有序性赋予这些材料非常特殊的特性，定义它们的高表面张力，并控制其他元素和分子在液态金属的界面和核心中的溶解度。异国情调和新颖的 LMT 合金也可以扩展液态金属的概念。探索多种元素的创新组合是设计具有智能功能的 LMT 液态金属合金的方法之一（图 2a-c）。一个古老的例子是 Field 的金属，它由 32.5% 的 Bi、51% 的 In 和 16.5% 的 Sn（重量比）制成，熔点为 61.5 °C。此外，应该探索在多组分相图的边界上开发高熵合金和系统的可能性。多组分相图的边界是制造合金的常规目标。多元素金属颗粒在使用液态合金建立纳米颗粒悬浮液方面发挥着重要作用。与它们的水性或有机溶剂对应物相同，这些悬浮液可以提供特殊的功能，例如后过渡金属中不存在的磁性特征。 (9) 此外，在这种情况下，LMT 液态合金的凝固是丰富的，但较少研究前沿（图2e，f）。我们最近展示了表面上的这种相变和分离如何与金属合金的本体不同。 (10) 液态金属与基材的相分离和相互作用 (11) 及其与非金属材料的反应也可以使用复杂的方法进行研究。多路系统。液态金属可以在催化方面发挥非凡的作用。已经有一些关于液态金属催化性能的报告，(12-14)，这只是可能改变催化未来的漫长而有益的道路的开始。液态和固态的催化行为有很大不同。液态金属的热力学也与固态金属有很大不同。液态元素金属簇可以自由移动，同时共享错位的电子云。待催化的目标材料可以轻松穿透液态金属表面，被催化，然后在饱和后自由离开液态介质（图 2h）。此外，已知为非氧化形式的良好催化剂的二次金属可以在过渡后液态金属中得到保护。液态金属合金的流动性和高原子迁移率可以允许前体与催化剂的轻松且受保护的相互作用。在这方面，应仔细研究在液态金属中催化的分子的渗透性。这个概念提供了一个引人入胜的范例，即创造在低温下运行的超高效液体催化剂是一种真正的可能性。试想像铂 (Pt) 这样的金属在室温下被 Hg 或 Ga 原子包围以进行催化时保持液态。像梦一样，这让创造了室温液体铂的非凡概念。催化作用只是硬币的一面。液态金属反应介质的概念也可以扩展到物理化学的其他领域。液态金属与有机材料的反应可导致在室温下形成自由基和脱氢（图 2i），施加的能量最少。通过大幅减少能源输入，这可能会彻底改变过程工程行业。液态金属也可用作制造高价值电子元件和光学元件的前体。使用液态金属反应介质可以获得新的配合物和晶体。 (15-17) 氧化或与硫属化物反应（氧化物和硫属化物化合物的合成）、相分离（脱合金和沉淀）以及混合和新化合物的产生可以在这些金属液体中同时发生。我们可以使用液态金属作为溶剂，使第二相在其中结晶，然后将其提取出来（图 2f）。在这方面，液态金属的高表面张力对分离应充分探索的产品提出了挑战。液态金属的表面也为创造新材料提供了有希望的机会。众所周知，表面在这种液态金属的大块混沌中分层。这种近表面分层可以经受环境氧化物或硫属化物前体的作用，以制造超薄产品。我们已经展示了透明导电氧化物和高空穴迁移率透明氧化物和硫化物 (18,19) 以及其他前所未见的平面晶体。这些超薄产品是迷人的材料，透明且导电，同时可以以极端角度弯曲而不会失去完整性。我们还表明，通过用硫化物的前体代替环境，可以生产超薄金属硫属化物，它们是用于机械能量收集的出色电子和压电材料。(20) 这些材料是我们都向往的透明柔性电子和光学的未来。最后，我们总结了这一观点，说使用 LMT 液态金属可以创建用于能量收集和存储的新型智能系统、高科技光学和电子元件、新制造方法和超高效功能材料。虽然这里没有介绍，但也可以利用 Ga (29.8 °C) 及其合金的低危害性和正确的熔点，参考人体，开发各种生物技术产品。在不久的将来，我们应该有效地掌握和利用基于液态金属技术的可行的新产业，这是建立可行的新产业的绝佳机会。LMT 液态金属领域无疑正在享受其当之无愧的复兴。这项工作得到了澳大利亚研究委员会 (ARC) 桂冠奖学金 (FL180100053) 和 ARC 卓越中心 FLEET (CE170100039) 的支持。MAR 承认 ARC 发现早期职业研究员奖 (DECRA) 赠款 (DE210101162)。本文引用了 20 篇其他出版物。承认 ARC 发现早期职业研究员奖 (DECRA) 赠款 (DE210101162)。本文引用了 20 篇其他出版物。承认 ARC 发现早期职业研究员奖 (DECRA) 赠款 (DE210101162)。本文引用了 20 篇其他出版物。