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- Herausgeber: Wiley-VCH GmbH
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- Sprache: Englisch
An up-to-date exploration of the properties and most recent applications of liquid metals
In Liquid Metal: Properties, Mechanisms, and Applications, a pair of distinguished researchers delivers a comprehensive exploration of liquid metals with a strong focus on their structure and physicochemical properties, preparation methods, and tuning strategies. The book also illustrates the applications of liquid metals in fields as varied as mediated synthesis, 3D printing, flexible electronics, biomedicine, energy storage, and energy conversion.
The authors include coverage of reactive mediums for synthesizing and assembling nanomaterials and direct-writing electronics, and the book offers access to supplementary video materials to highlight the concepts discussed within. Recent advancements in the field of liquid metals are also discussed, as are new opportunities for research and development in this rapidly developing area.
The book also includes:
- A thorough introduction to the fundamentals of liquid metal, including a history of its discovery, its structure and physical properties, and its preparation
- Comprehensive explorations of the external field tuning of liquid metal, including electrical, magnetic, and chemical tuning
- Practical discussions of liquid metal as a new reaction medium, including nanomaterial synthesis and alloy preparation
- In-depth examinations of constructing techniques of liquid metal-based architectures, including injection, imprinting, and mask-assisted depositing
Perfect for materials scientists, electrochemists, and catalytic chemists, Liquid Metal: Properties, Mechanisms, and Applications also belongs in the libraries of inorganic chemists, electronics engineers, and biochemists.
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Veröffentlichungsjahr: 2022
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Table of Contents
Cover
Title Page
Copyright
Preface
Part I: Fundamentals of Liquid Metal
1 Introduction
1.1 The Discovery and Development History of Liquid Metal
1.2 Liquid Metal Family
1.3 Overview of Liquid Metal
References
2 Structure and Physical Properties of Liquid Metal
2.1 Structure of Liquid Metal
2.2 Low Melting Point of Liquid Metal
2.3 Rheological Properties of Liquid Metal
2.4 Electrical Conductivity of Liquid Metal
2.5 Thermal Property of Liquid Metal
2.6 Surface Properties of Liquid Metal
2.7 Liquid Metal in Other Dimensions
2.8 Conclusion
References
3 Preparation of Liquid Metal
3.1 Introduction
3.2 Preparation of Liquid Metal Droplets
3.3 Preparation of Liquid Metal Nanowires
3.4 Preparation of Liquid Metal Nanosheets
3.5 Conclusion and Prospect
References
4 External Field Tuning of Liquid Metal
4.1 Introduction
4.2 Electrical Field Tuning
4.3 Magnetic Field Tuning
4.4 Chemical Tuning
4.5 Force Field Tuning
4.6 Others
4.7 Conclusion and Prospect
References
Part II: Emergent Applications of Liquid Metal
5 Liquid Metal as a New Reaction Medium
5.1 Introduction
5.2 Nanomaterial Synthesis
5.3 Alloy Preparation
5.4 Conclusion and Prospect
References
6 Constructing Techniques of Liquid Metal‐Based Architectures
6.1 Introduction
6.2 Injection
6.3 Imprinting
6.4 Mask‐Assisted Deposition
6.5 Direct Writing
6.6 Laser Ablation
6.7 External Field Guiding
6.8 Others
6.9 Conclusion and Prospect
References
7 Liquid Metal Toward Flexible Electronics
7.1 Introduction
7.2 Traditional Materials for Flexible Electronics
7.3 Advantages of Liquid Metals
7.4 Construction Strategies for Liquid Metal‐Based Flexible Electronics
7.5 Typical Flexible Electronics Based on Liquid Metals
7.6 Conclusion and Prospect
References
8 Liquid Metal for Biomedicine
8.1 Introduction
8.2 Therapeutics
8.3 Biomedical Imaging
8.4 Biomimetics
8.5 Biodevices
8.6 Conclusion and Prospect
References
9 Liquid Metal for Energy
9.1 Introduction
9.2 Liquid Metal as Electrodes
9.3 Liquid Metal as Coolants
9.4 Liquid Metal as Catalysis
9.5 Conclusion and Prospect
References
Index
Wiley End User License Agreement
List of Tables
Chapter 1
Table 1.1 Atomic number and melting point of liquid metals.
Table 1.2 The composition and melting point of several eutectic binary alloy...
Chapter 2
Table 2.1 The electrical resistance of metals both in solid and liquid state...
Table 2.2 The data of changing in volume of some metals as they melt.
Table 2.3 Recommended reference thermal conductivity values of liquid metals...
Table 2.4 Summary of surface tension and temperature coefficient.
Table 2.5 Contact angle of liquid metal on substrate.
Chapter 3
Table 3.1 Methods for preparation of LM droplets.
Table 3.2 Methods for preparation of LM NWs.
Table 3.3 Methods for preparation of LM NSs.
Chapter 6
Table 6.1 The comparison of different patterning techniques of liquid metal‐...
Chapter 9
Table 9.1 Properties of water and typical liquid metals [68].
List of Illustrations
Chapter 1
Figure 1.1 Liquid metal base elements.[10].
Figure 1.2 The application of liquid metals [11].
Figure 1.3 Schematic of liquid metals (or alloys) and gallium‐based LMs for ...
Figure 1.4 (a) Stretchable materials based on liquid metals [19].(b) The...
Figure 1.5 Prospective designs and applications of the liquid‐metal‐based el...
Figure 1.6 Candidate materials needed to satisfy the space physical environm...
Chapter 2
Figure 2.1 The structure of liquid alkali metals. (a)
S
(
q
) and (b)
g
(
r
) near...
Figure 2.2 The measurement setup and local structure of liquid Pb. (a) Schem...
Figure 2.3 Phase diagram of Ga and structure of liquid Ga. (a) Phase diagram...
Figure 2.4 Structures of liquid Ga, EGaIn, and EGaInSn. (a) Predicted
g
(
r
) o...
Figure 2.5 The structure of biphasic liquid metal. (a–d) The structure evolu...
Figure 2.6 (a) Crystal structure of α‐Ga. (b) Crystal structure of α‐Ga show...
Figure 2.7 (a) Relationship between the torque and the rotational rate of li...
Figure 2.8 Rheological behavior of liquid metal. (a) Rheological property of...
Figure 2.9 (a–d) The pair contribution function of liquid Ga, Ge, Bi, and Sb...
Figure 2.10 Thermal conductivity of liquid metal (a) Thermal conductivity te...
Figure 2.11 (a) Scheme of electrowetting, where
θ
v
<
θ
0
. There is ...
Figure 2.12 (a) Grinding amalgamation process of liquid metal to fabricate f...
Figure 2.13 (a) Mott model of the ions passing through the oxidation layer....
Figure 2.14 (a–c) X‐ray reflectivity of the Ga, Sn, and K, which clearly sho...
Figure 2.15 (a) Ga nanowire thermometer.(b) The length variation of ther...
Figure 2.16 (a) Diameter‐dependent optical property of In.(b) The superc...
Figure 2.17 (a–c, f
)
the atomic structure of 2D Ga, Pb, Bi, and In.(e) T...
Chapter 3
Figure 3.1 Liquid‐phase preparation of LM NPs. (a) The TEM image and the cry...
Figure 3.2 Vapor‐phase preparation of LM NPs. (a) A schematic of the synthes...
Figure 3.3 Other preparation methods of LM NPs. (a) Schematic illustration o...
Figure 3.4 Synthesis of LM NWs. (a) Se seeded growth of Sn NWs.(b) (Colo...
Figure 3.5 Synthesis of LM NSs. (a) Atomic structure of hydrogen decorated t...
Chapter 4
Figure 4.1 Continuous‐electrowetting‐induced locomotion of liquid metals. (a...
Figure 4.2 Liquid metal‐enabled moving robots. (a) Sequential snapshots of t...
Figure 4.3 Electromigration‐induced locomotion of the liquid metals. (a) Seq...
Figure 4.4 The locomotion of the liquid metals based on other driving forces...
Figure 4.5 (a) The schematic of the side view of the surfing liquid metal dr...
Figure 4.6 Electrocapillarity‐induced deformation of liquid metal. (a) Seque...
Figure 4.7 (a) The electrochemically induced withdrawal of EGaIn in microflu...
Figure 4.8 Applications based on the electrical‐field‐induced deformation of...
Figure 4.9 (a) Schematic of an EGaIn droplet under an AMF. (b) AMF‐induced d...
Figure 4.10 (a) Sequential snapshots of the locomotion behaviors of the EGaI...
Figure 4.11 (a) The different morphology of the liquid metal bath under the ...
Figure 4.12 (a) Schematic of the experimental method used for magnetic manip...
Figure 4.13 (a) Schematic of the fabrication process of the magnetic liquid ...
Figure 4.14 (a) Schematic of the inchworm‐like motion of the EGaIn/NdFeB‐coa...
Figure 4.15 (a) Inductive heat effect with and without EGaIn on the pigskin ...
Figure 4.16 (a) The viscosity variation of the GaInSn/Fe mixture under magne...
Figure 4.17 (a) The dynamic change of the voltage between the GaInSn and gra...
Figure 4.18 (a) Spontaneous dispersion behavior of the liquid metal triggere...
Figure 4.19 (a) Schematic of the formed EDL on the liquid metal droplet afte...
Figure 4.20 (a) Sequential snapshots of the locomotion of a liquid metal dro...
Figure 4.21 (a) Schematic of the Pt‐coated GaInSn motor and self‐propulsion ...
Figure 4.22 (a) Height profile in AFM image of the DPA on Ga oxide.(b) C...
Figure 4.23 (a) An EGaIn‐injected stretchable fiber and the insets is the cr...
Figure 4.24 Deformation induced by pressure. (a) The schematic of structural...
Figure 4.25 Thermal‐field tuning of the liquid metals. (a) The schematic of ...
Figure 4.26 Light tuning for liquid metals. (a) Schematic of the production ...
Figure 4.27 Humidity tuning for liquid metals. Volume expansion phenomenon o...
Chapter 5
Figure 5.1 Liquid metal‐assisted synthesis of 0D nanomaterials. (a) The sche...
Figure 5.2 Liquid metal‐assisted synthesis of 1D nanomaterials. (a) Schemati...
Figure 5.3 Liquid metal‐assisted synthesis of 1D nanomaterials. (a) Schemati...
Figure 5.4 Liquid Cu‐assisted self‐limited growth of monolayer graphene. (a)...
Figure 5.5 Liquid Ga‐assisted self‐limited growth of monolayer graphene. (a)...
Figure 5.6 The mechanism of liquid metal‐assisted self‐limited growth of mon...
Figure 5.7 Liquid alloy‐assisted self‐limited growth of monolayer graphene. ...
Figure 5.8 Liquid alloy‐assisted self‐limited growth of monolayer graphene. ...
Figure 5.9 Liquid alloy‐assisted self‐limited growth of bilayer graphene. (a...
Figure 5.10 Liquid metal substrate‐assisted synthesis of the 2D materials. (...
Figure 5.11 The surficial layers of the liquid metal substrate acting as the...
Figure 5.12 Liquid metal printing technique for exfoliating its films. (a) G...
Figure 5.13 Oxide skins of the liquid metal‐assisted the synthesis of 2D mat...
Figure 5.14 Liquid metal vapor‐assisted synthesis of the 2D materials and it...
Figure 5.15 Liquid Cu‐assisted rapid growth of graphene. (a) Schematic illus...
Figure 5.16 Liquid metal‐assisted morphology control of graphene. (a–c) Tran...
Figure 5.17 Oriented stitching control of graphene grains on the liquid meta...
Figure 5.18 The formation of twinned graphene polycrystals on the liquid Cu ...
Figure 5.19 Schematic illustration for the growth of SC‐
h
‐BN films on liquid...
Figure 5.20 Ordered assembly control toward the 2DSOS. (a) Illustrations of ...
Figure 5.21 Self‐Assembly of metal oxide nanoparticles in liquid metal towar...
Figure 5.22 The self‐symmetrical etching growth of the
h
‐BN‐G CSA on the liq...
Figure 5.23 Liquid metal‐assisted heterogeneous stacking control. (a) Schema...
Figure 5.24 Liquid metal‐assisted heterogeneous stacking control. (a) Schema...
Figure 5.25 Liquid metal‐assisted heterogeneous stacking control. (a) Schema...
Figure 5.26 Liquid metal‐assisted sliding transfer process. (a) Synthetic di...
Figure 5.27 Microstructure characterization of solid‐solution alloys formed ...
Figure 5.28 Microstructure characterization of ordered phase alloys formed w...
Figure 5.29 Liquid metal‐assisted alloy shape formation with DC current impr...
Figure 5.30 Description of dealloying process and characterization of nanopo...
Chapter 6
Figure 6.1 (a) Schematic illustrations of narrow and wide junction channels ...
Figure 6.2 (a) Schematic illustration of vacuum filling. (b) Optical microgr...
Figure 6.3 (a) Schematic illustration of the device with a filter paper and ...
Figure 6.4 (a) Schematic description of the liquid metal imprinting process....
Figure 6.5 (a) A selectively amphiphilic stamp. (b) Illustration and photogr...
Figure 6.6 (a) The scheme of photolithography‐based liquid metal patterning ...
Figure 6.7 (a) Schematic illustration of atomized spray deposition of liquid...
Figure 6.8 (a) SEM images of paper surface and PMA glue, and schematic illus...
Figure 6.9 (a) Schematic of inkjet printing techniques. (b) Schematic of mic...
Figure 6.10 (a, b) TEM images of EGaInSn nanodroplets via a simple ultrasoni...
Figure 6.11 (a) Schematic of direct writing system. (b) OM of traces from no...
Figure 6.12 (a) Schematic of the brush‐like porous pinhead. (b) OM of printe...
Figure 6.13 (a) Schematic of the 4‐DOF direct writing system
.
(b) Complicate...
Figure 6.14 (a) Sequential images of direct‐writing free‐standing liquid met...
Figure 6.15 (a) Schematic of a 3D printing apparatus. (b) SEM images of prin...
Figure 6.16 (a) Schematic of direct CO
2
laser ablation process for preparing...
Figure 6.17 (a) Schematic of the direct patterning of liquid metals using ex...
Figure 6.18 (a) Schematic of the direct patterning of liquid metals using ex...
Figure 6.19 (a) Schematic of continuous electrowetting using an EGaIn plug. ...
Figure 6.20 (a) Schematic of preparing liquid metals channels.(b) Schema...
Figure 6.21 Direct mixing of liquid metals with elastomer, polymer, hydrogel...
Chapter 7
Figure 7.1 Microstructures designed for stretchable devices. (a) Schematic i...
Figure 7.2 Strategies for stretchable and soft conductors. (a) (i) Solid yet...
Figure 7.3 Schematic of the stretchable electronic devices. (a) Thermoplasti...
Figure 7.4 Liquid metal in elastomer can connect and disconnect by bringing ...
Figure 7.5 Schematic of the process of liquid interconnects. (a) The idea fo...
Figure 7.6 Electrical stability of 3D stretchable conductor and its performa...
Figure 7.7 Integrated LEDs are on under different conditions [84].
Figure 7.8 Liquid metals can form stretchable circuit elements for resistors...
Figure 7.9 Soft, functional circuit elements using liquid metal. (a) A memri...
Figure 7.10 Self‐healing stretchable wires for reconfigurable circuit wiring...
Figure 7.11 Liquid metals can form excellent stretchable electronics for cir...
Figure 7.12 Liquid metals build excellent antennas owing to metallic conduct...
Figure 7.13 The liquid metal‐based keypad and corresponding application, tes...
Figure 7.14 Soft‐matter printed circuit board. (a) Top and bottom of a circu...
Figure 7.15 Microfluidic haptic diaphragm pressure sensor. (a) Picture of a ...
Figure 7.16 Heart‐rate monitoring. (a–c) Schematic of how the PDMS sensor wr...
Figure 7.17 Application demonstration of a wearable sensor based on hydrogel...
Figure 7.18 The biphasic gold–gallium thin films and the sensors based on th...
Figure 7.19 Tactile sensing glove. (a) Optical image of handshaking wearing ...
Figure 7.20 The mechanism and applications of LM‐TENG (a) Structure of LM‐TE...
Figure 7.21 Schematic of performance and application of SH‐LM‐TENG. (a) Sche...
Chapter 8
Figure 8.1 pH‐responsive drug delivery based on LMs. (a) Preparation diagram...
Figure 8.2 Laser‐responsive drug delivery based on LMs. (a) Routes for prepa...
Figure 8.3 Magnetic hyperthermia based on LMs. (a) Schematic illustration of...
Figure 8.4 Photothermal therapy based on LMs. (a) Scheme of the PTT of Ga. (...
Figure 8.5 The shape control of LMs for PTT. (a) Schematic of LM nanodroplet...
Figure 8.6 The surface engineering of LMs for combined therapy. (a) Schemati...
Figure 8.7 Radiofrequency hyperthermia based on LMs. (a) Schematic for LM ba...
Figure 8.8 Microwave dynamic therapy based on LMs. (a) Diagram of MDT and th...
Figure 8.9 LMs for electrochemical therapy and embolotherapy. (a) Schematic ...
Figure 8.10 CT scans of various tissues and organs filled with LMs. (a, b) T...
Figure 8.11 PA imaging based on LM nanocapsules. (a) TEM image of LM nanocap...
Figure 8.12 LMs bone cement. (a) Schematic of filling and removal process of...
Figure 8.13 Repair of PNI based on LMs. (a) Schematic of the broken sciatic ...
Figure 8.14 In vivo neurological signal detection based on the Ga‐reconnecte...
Figure 8.15 The restoration of the movement of bullfrog limb with LMs as con...
Figure 8.16 Movement monitoring of biosensors based on LMs. Top views of the...
Figure 8.17 (a) Schematic of smart glove. (b) The image of a smart glove com...
Figure 8.18 Biological signals analysis of biosensors based on LMs. (a) The ...
Figure 8.19 Simulation and in vivo experiments of implantable electrodes bas...
Figure 8.20 Characterization of fluid motion in microfluidic pump based on L...
Figure 8.21 The artificial heart based on LMs
.
(a) Schematic of an experimen...
Chapter 9
Figure 9.1 (a) SEM image and schematic of the damaged site healed by release...
Figure 9.2 (a) Conventional configuration of liquid metal consists of two li...
Figure 9.3 The influence factors of wetting behavior of liquid metal. (a) Co...
Figure 9.4 (a) Mechanism of self‐healing Li–Bi liquid metal system during di...
Figure 9.5 (a) TEM revealing the real‐time morphological evolution of Ga dro...
Figure 9.6 (a) Adsorption of Na–K alloy in a porous carbon paper by heating ...
Figure 9.7 (a) Preparation process of CFC/Na–K electrode at room temperature...
Figure 9.8 (a) Mechanism of touch‐ablation process when Li plated. (b) XPS d...
Figure 9.9 Appearance of a prototype device of optical near‐field lithograph...
Figure 9.10 An integrated cooling system with hybrid coolants. (a) Exploded ...
Figure 9.11 Heat dissipation process of the integrated cooling device. (a) A...
Figure 9.12 Nanofluids based on liquid metals. (a) Photographs of pure liqui...
Figure 9.13 Self‐driven cooling system via thermosiphon effect. (a) The corr...
Figure 9.14 (a) SEM image of Pd–Ga alloy. (b) The AR‐XPS of Pd 3d in the mix...
Figure 9.15 Mechanism of Ga–Ce alloy catalyst in catalytic conversion of CO
2
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
Wiley End User License Agreement
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Liquid Metals
Properties, Mechanisms, and Applications
Lei FuMengqi Zeng
Author
Prof. Lei FuWuhan UniversityCollege of Chemistry and Molecular SciencesNo. 299, Bayi RoadWuchang District430072 WuhanChina
Prof. Mengqi ZengWuhan UniversityCollege of Chemistry and Molecular SciencesNo. 299, Bayi RoadWuchang District430072 WuhanChina
Cover Image: © Oxygen/Getty images
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Print ISBN: 978‐3‐527‐82818‐0ePDF ISBN: 978‐3‐527‐82877‐7ePub ISBN: 978‐3‐527‐82879‐1oBook ISBN: 978‐3‐527‐82878‐4
Preface
Liquid metals can flow freely like water at relatively low temperatures, thus changing the perception of metals as solid. Taking gallium (Ga) as an example, hold it in your hand for a while and, interestingly, it melts and turns into a silvery drop that rolls around in your hand. More interestingly, it can be encapsulated in carbon tubes to form the smallest thermometer in the world. This kind of fascinating materials bring people into a new world. Nowadays, liquid metals are emerging as a new cutting‐edge material in the frontiers of science and technology and nurturing a revolution in both fundamental discovery and unconventional applications.
Combining the characteristics of liquid (with deformable and injectable characteristics) with metal (with electrically and thermally conductive properties), liquid metals are favorable for future flexible electronics such as high‐resolution conductors, keypads, pressure sensors, triboelectric nanogenerator, and other wearable sensors. For precisely constructed wearable electronics, higher integrated resolution is always in pursuit. Liquid metal inks can enable high‐resolution, reconfigurable three‐dimensional (3D) printing, as they are intrinsically stretchable and exhibit low toxicity, negligible volatility, and superb electrical conductivity.
Like in the movies of Terminator, liquid metals can deform to any shape and are capable of autonomous healing after suffering damage. They can exhibit more biological‐like behaviors, such as breathing, eating, self‐moving, and leaping. They could even bestow a soft robot with a sense of touch. Furthermore, to meet the requirement of invasive surgery or drug delivery in human body, microrobots are in urgent need. More recently, the core parts of robots, soft electromagnetic actuators, were formed by replacing solid metal coils with liquid metal channels. Thus, advanced microrobots for medical applications were enabled. In this way, the classic movie scene from Fantastic Voyage, a miniaturized submarine destroys a blood clot and saves the patient's life may come true one day. Nowadays, many researchers from different disciplines are entering into the field of liquid metals. It is believed that this kind of interdisciplinary crossing will certainly give liquid metal robots more new life.
On account of high deformability, high conductivity, and superior electrochemical properties, liquid metals are also promising as electrodes and they can endow rechargeable batteries with high theoretical specific capacity, and meanwhile intrinsically avoid uneven dendrites issues. Compared to traditional Na electrode, the emerging room temperature liquid metals are considered as electrode materials with more promising features, because they can keep the liquid advantages without external energy input. Liquid metals also hold enormous potential as routes to catalysts for many industries and applications, owing to the catalytic ability given by their metal nature and the liquid characteristic, offering a low‐energy way to reduce atmospheric carbon and help combat climate change.
In addition, unlike solids, liquids can facilitate reactions below the surface, where a sea of ions and electrons can be exploited. Due to their wide melting range, reducing atmosphere, and electron‐rich environment, liquid metals can dissolve different metals and are recently emerging as medium for chemical reactions.
The field of liquid metals owns great and limitless potentiality, providing a solid foundation for future discoveries and developments. Over the last half century, a large number of research and review articles on the properties and applications of liquid metals have been reported. Even so, accurate and reliable information of their structures and physicochemical properties are still not plentiful. Therefore, it is urgent to unveil the mystery of liquid metals. And that's what I and Professor Zeng try to do in the book. After several years of accumulating knowledge in the liquid metal field, we would like to share our understanding about this unique material family in a more professional and widespread way. The contributors of this book are all the members of our lab, Laboratory of Advanced Nanomaterials of Wuhan University.
To give a thorough understanding of liquid metals and promote their further researches and rapid applications, this book is divided into two parts, namely, fundamental properties and emergent applications of liquid metals. The first part of this book is the fundamentals of liquid metals. The book first gives a brief introduction of liquid metals in Chapter 1 (contributors: Yao Wang, Linyi Li, Shasha Zhao). Then it describes the structures of liquid metals and discusses their physicochemical properties, as dictated by the structures in Chapter 2 (contributors: Yiran Ding, Chenyang Wang). Based on the understanding of the atomic structure of liquid metals, we move toward describing the properties of liquid metals: rheological, electrical, thermal, and surface properties. It is these amazing properties that have generated the exuberant fascination in liquid metals. Then we have included Chapter 3 (contributors: Linyang Li, Xuejiao Chang) as a summary of the preparation methods of liquid metals. It covers preparation of liquid metals from zero‐dimensional to two‐dimensional. Then Chapter 4 (contributors: Jiaqian Zhang, Jinxin Liu, Jia Lei) gives a description of external field tuning of liquid metals, such as electrical, magnetic, chemical, force field, and others. The other part including Chapters 5–9 gives an overview of the representative emergent applications of liquid metals: new reaction medium (Chapter 5, contributors: Kena Yang, Yunxu Chen, Guanghui Cao), constructing architectures (Chapter 6, contributors: Jingjing Si, Feifei Cao, Jingjing Liang), stretchable electronics (Chapter 7, contributors: Yao Xiao, Jinglu Liu, Mengyue Zhou), biomedicine (Chapter 8, contributors: Luyang Wang, Yu Ding, Xian Sun), and energy (Chapter 9, contributors: Wenjie Wang, Xiaohui Zhu, Fangyun Lu). Finally, the advancements achieved recently will be summarized, and perspectives for future development in the applications of liquid metals will be outlined. As technology advances and fundamental discoveries are made in this direction, liquid metals are promising to offer more fantastic opportunities as powerful new generation functional materials.
Over the past few years, my group and collaborators have devoted great efforts to shaping the fields of liquid metals. We would like to thank those people for their professional contribution. Meanwhile, we also wish to express our gratitude toward the editors for their invitation and advices for completing this book.
We trust that the book will serve as an introduction to those who want to take part in the exciting developments of this field. This book can help readers understand the technical advances and fundamental discoveries at the leading edge of liquid metals and solve the puzzles of various phenomena about liquid metals. It is believed that the intriguing and inspiring findings presented in this book will lead to more activities in the research of liquid metals and provide further opportunities for advancing liquid metal science.
January 2021
Lei Fu
Part IFundamentals of Liquid Metal
1Introduction
The seeds of science are grown for the harvest of the people.
Source: Dmitri Mendeleev (1834–1907). Public Domain.
1.1 The Discovery and Development History of Liquid Metal
In the world of metals, the general “members” are solid. Generally, only when the metals are heated to a relatively high temperature, the metals will exhibit a reddish liquid state. In fact, even if there are no severe high‐temperature conditions, we can observe the flowing liquid metal. The most common liquid metal is mercury, which is found in the thermometers we use every day. In ancient China, mercury was also endowed with mysterious colors. Emperor Qin Shi Huang, the first emperor of China, built a model of a river infused with mercury in his tomb, representing his ruling power over the land and his expectation of immortality [1]. In Europe, it is well‐known that the philosophy of alchemy laid the foundation for the principles and rules of modern chemistry, and mercury is the core of alchemy research. European alchemists believed that mercury is the main component of all metals and can be combined with other metals to become gold [2]. Although not as expected by ancient alchemists, mercury can indeed dissolve almost all metals to form a soft alloy, that is, amalgam, which is regarded as a metal solvent and has a wide range of applications. Unlike mercury, which always appears in a liquid state in a general environment, the melting point of gallium is just around room temperature. We can easily switch the state of gallium between solid and liquid. For example, if we put gallium on the palm, we can observe its transition from solid to liquid (different from mercury). This may be the reason why people are fascinated by gallium.
As early as the nineteenth century, the special property of gallium became the subject of a classic prank among chemists. One popular trick is “a disappearing spoon.” Since gallium looks like aluminum and molds easily, is to fashion gallium spoons and serve the unsuspecting guests with tea. Upon stirring the tea, the gallium spoon melts rapidly. And watch your guests' surprised looks when their tea “eats” their spoons [3]. More in practical, its low‐melting point and high boiling point mean that gallium could exhibit metallic properties in the liquid phase across a range of useful, desirable, and accessible temperature.
Recently, scientists have played a new trick with gallium. They have written the smallest book in the world with the help of focused gallium ion beam. Teeny Ted from Turnip Town (2007), published by Robert Chaplin, is certified by Guinness World Records as the world's smallest reproduction of a printed book. The book, costing an enormous $15 000 (around £10 000), yet made is a 30 micro‐tablet book carved on a pure crystalline silicon page by using a focused ion beam, is measured to be just 0.07 mm by 0.10 mm, according to Simon Fraser University. As we know, gallium is typically used as a liquid metal ion source for a focused ion beam. It is gallium as the liquid metal ion source used in the focused ion beam that created the smallest book in the world. The book even has its own ISBN reference, ISBN‐978‐1‐894897‐17‐4, though the readers will require a scanning electron microscope to read the story of a turnip contest [4].
Liquid metals can bring us much more than that. In fact, they have become a part of our daily lives. Liquid metal may exist as a dental implant in our teeth, as fusible metal in automatic fire‐fighting devices, or it could exist as a central processing unit (CPU) coolant in our computers. In the future, liquid metal could also be found in our foldable smartphones. Now, it's time to enter the world of liquid metals and learn more about this fascinating substance.
1.1.1 A Vindication for Mendeleev's Prediction
Gallium has only a short history. It was first discovered only a little over a 100 years ago. Particularly, gallium was the first chemical element in the history of chemistry that was predicted theoretically and then verified in nature in 1875.
The periodic table is undoubtedly the most commonly used tool in chemistry. In fact, it greatly facilitates scientific research and the memory and analysis of elements. Before Dmitri Mendeleev arranged the elements as the periodic tables that we are familiar with now in 1869, there were various periodic tables. However, in contrast to the previous scientists, Mendeleev's method of dividing the elements is more concise and beautiful, which is widely recognized and has been used ever since. He reasoned that there should be an as‐yet‐undiscovered element. Based on the position of the element in the table, he predicted some of their characteristics and properties. The main reason that Mendeleev's periodic table is so convincing may come from the fact that he initially left gaps in it to predict the existence of elements and then to be confirmed later.
As early as 1875, the French chemist Lecoq de Boisbaudran discovered gallium and successfully extracted and purified this new metal. The measured properties of this new element were surprisingly in agreement with Mendeleev's prediction and confirmed his placement of the eka‐elements in the periodic table. At present, the industry of gallium production and purification technology has been highly mature, and the purity of gallium can reach 99.9999% [3,5].
1.1.2 Gallium's Applications in the Semiconductor Industry
With the discovery of gallium arsenide (GaAs) compounds as semiconductor materials, in the early 1960s, gallium alloy began to attract the attention of various researchers. Perhaps alloying with various metals easily is the most crucial characteristic of gallium. And the low‐melting point property of gallium can be conferred to the final gallium alloy, making the resulting material more easily workable, stable, and cost‐effective. In the semiconductor industry, its alloying with other elements and phase transition properties are highly valuable. Thus, a lot of gallium applications are concentrated in the semiconductor industry. GaAs, as gallium's most common alloy and a new type of high‐quality semiconductor, has launched a research boom, which is widely used in preamplifiers and high‐speed logic chips in mobile phones, while AlGaAs and InGaAs are often applied as the light‐emitting materials. Blue light‐emitting diodes (LEDs) were successfully developed in the early 1990s, and the development of white LEDs also followed. Since then, a “lighting revolution“ have begun. With gallium consumption soaring, coupled by commercial speculation, the price of gallium rose substantially. After nearly 20 years of development, white LED lighting technology has made great achievements. Compared with the traditional lighting technology, LED lighting technology has the advantages of high‐efficiency, energy‐saving, long life, green environmental protection, and high light efficiency, which has been supported by governments all over the world. At present, the research and production of GaAs have been mostly turned to the LED industry [3].
1.1.3 Tackling Fundamental Problems in Fuel in Energy Science
In addition to its contribution to the semiconductor field, gallium chemistry has also addressed many fundamental questions in energy science. Early studies have shown that the zeolite doped with gallium can effectively catalyze methylcyclohexane [6]. The ring is opened and broken to form short alkanes, which is used to recover gasoline pyrolysis products. Moreover, gallium zeolites can also catalyze the aromatization of n‐decane [7]. Emerging studies have shown that GaN nanowires can even catalyze the formation of benzene from methane [8]. Breaking the CH bond of methane is not easy, which will contribute to these processes mentioned above. The relative inactive methane exhalations and undesired by‐products of gasoline production can be converted into useful petrochemicals. These reactions are crucial for the conversion, which are of great significance for fine chemical production and fuel storage.
1.2 Liquid Metal Family
It is common knowledge that the existing form (solid, liquid, and gas) of all elements and chemical compounds rely on conditions of temperature and pressure. But when it comes to metal, the first thing we think about is bulky objects and high melting point. It is hard to believe that some metals exist in liquid form at room temperature. One of the most intriguing properties of gallium is its liquid state at room temperature. When you cut off a piece of solid gallium with a knife and put it in your hand for a closer look, an interesting thing happens: It melts and turns into a silvery drop that rolls around in your hand like a drop of water on a lotus leaf. The reason for this curious phenomenon is that the melting point of gallium is only 29.8 °C, below the human body temperature, so it will melt in the hand. In addition, although gallium's melting point is low, the boiling point can be as high as 2200 °C. In other words, from about 30 °C to about 2000 °C, gallium would remain in liquid state, while mercury would boil at 360 °C. Therefore, gallium can be used to make high‐temperature thermometers that can measure temperatures below 1500 °C [3].
Herein, we define metals and alloys with melting point between room temperature and 300 °C as liquid metals. Single‐component liquid metals are predominantly composed of post‐transition metals (Ga, In, Tl, Sn, Pb, Al, and Bi), zinc‐group metals (Zn, Cd, and Hg), and alkali metals (Li and Na). We note that alloys possess much lower melting point than the pure metal. For example, the melting point of eutectic gallium and indium alloy (EGaIn) is only 16 °C [9]. So there are much more liquid metal alloys. Tables 1.1 and 1.2 list the melting point of several liquid metals and alloys. To enrich the content of this book, we will also introduce some molten metals (such as copper, gold, etc.), which have similar properties compared to liquid metals [10] (Figure 1.1).
Table 1.1 Atomic number and melting point of liquid metals.
Elements
Atomic number
Name (abb.)
Melting point (°C)
Lithium
3
Li
180.5
Sodium
11
Na
97.8
Aluminum
13
Al
660.3
Cadmium
48
Cd
321.1
Mercury
80
Hg
−38.8
Zinc
30
Zn
419.5
Gallium
31
Ga
29.8
Indium
49
In
156.6
Thallium
81
Tl
304.0
Tin (white)
50
Sn
231.9
Lead
82
Pb
327.5
Bismuth
83
Bi
271.4
Table 1.2 The composition and melting point of several eutectic binary alloys, ternary alloys, quaternary alloys, and quinary alloys.
Alloyed elements
Element A (at%)
Element B (at%)
Element C (at%)
Element D (at%)
Element E (at%)
Melting point ( °C)
Ga/In(EGaIn)
85.8
14.2
0
0
0
15.4
Ga/Sn
91.7
8.3
0
0
0
21.0
Ga/Bi
38.3
61.7
0
0
0
222.0
In/Sn
52.7
47.3
0
0
0
116.9
In/Bi
79.2
20.8
0
0
0
72.5
Pb/Au
84.6
15.4
0
0
0
215.0
Sn/Tl
56.6
43.4
0
0
0
170.2
Ga/In/Sn (Galinstan)
78.3
14.9
6.8
0
0
13.2
In/Sn/Bi (Field's alloy)
60.1
18.8
21.1
0
0
62.0
Bi/Pb/Sn (Rose's alloy)
43.1
23.5
33.4
0
0
95.0
Bi/Pb/Sn/Cd (Wood's alloy)
41.5
20.9
18.3
19.3
0
70.0
Bi/Pb/Sn/Cd/In (French's alloy)
31.5
17.1
14.4
11.7
25.3
46.9
Figure 1.1 Liquid metal base elements.[10].
Source: Daeneke et al. [10]. Reproduced with permission of Royal Society of Chemistry.
In recent decades, low‐melting point liquid metals have been used in many different fields. Gallium base liquid metal at room temperature has unique surface properties and physicochemical properties. It can achieve a variety of morphological changes such as deformation, movement, separation, and fusion through a variety of energy fields such as electric, magnetic, and concentration gradient fields or surface modification. Liquid metals show a promising application in microfluidics, biomedicine, and robotics, and other fields.
At present, numerous studies devoted to liquid metals are dispersed and need to be unified. All the emerging applications in the fields of catalysis, synthesis, microfluidics, soft electronics, sensor, and therapy exploit the same phenomena that occur in liquids and at metal interfaces. Integrating these seemingly unrelated research directions into a single field of liquid metal is quite challenging. But it will facilitate the development of innovative applications that take full advantage of the unique properties of liquid metals and its alloys [10].
1.3 Overview of Liquid Metal
What is well‐known is that mercury can dissolve almost all metals to form an amalgam and is regarded as a metal solvent with considerable application. Similarly, liquid metals can also dissolve some metallic elements and nonmetallic elements. And unlike conventional molecular liquid and ionic liquid, liquid metals possess some unique properties (high electrical conductivity, etc.). So liquid metals can serve as a new reaction medium to synthesis different dimensions, compositions, and morphology materials. Liquid metals can spontaneously form smooth, soft, defect‐free, stress‐free interfaces, which provide an ideal substrate for crystal growth. There are also self‐limiting metal oxide films on the liquid metal surface, which are naturally available high‐quality two‐dimensional films and can serve as precursors to synthesis other two‐dimensional materials. Moreover, many two‐dimensional atomic crystals have been predicted to grow on liquid metals [11] (Figure 1.2).
Figure 1.2 The application of liquid metals [11].
Source: Zavabeti et al. [11].
In the Terminator series of science fiction movies, the fact that the T‐1000 is the most amazing terminator is indisputable. He can transform into anyone and anything, and can heal wounds back to their original state as quickly as a fluid when attacked. It was the presence of liquid metal that endow the killer magical ability to transform and repair himself, making him an unbreakable killer. Thanks to this cool sci‐fi fantasy about liquid metal robots, it brought liquid metals into the public eye and aroused the interest of scientists in the study of liquid metals. Compared to conventional rigid robotics, liquid metals hold huge merits, such as low‐melting point, high electrical conductivity and thermal conductivity, and liquidity, which broaden their utilization in stretchable and wearable electronics, such as flexible pressure sensors, a stretchable electromagnetic actuator, stretchable wireless power transfer, and stretchable loudspeaker. Recently, a series of magical phenomena have been discovered successively, such as the movement and deformation of liquid metal through electric field control or magnetic field, the repair of biological fractured nerve with liquid metal, and the autonomous movement of liquid metal without external power after swallowing a small number of substances [12–14]. Liquid metals can also be applied to manufacture a new artificial bioinspired optoelectronic sensorimotor system, which is similar to the neuron system [15]. Besides, the color of liquid metals, which is generally shiny silver‐white, can be changed by applying an electrical field, such as pink, purple, and blue. This provides a new route to create kaleidoscopic and colorful liquid metals and will expand their applications in biomimetic intelligent soft robots. These findings lay the theoretical and technical foundation for complex liquid metal robots in the future. So, will robots like Terminator become a reality in the future? Let's wait and see!
Although the pure liquid gallium has no biological activity, gallium ions can influence the reaction in our body, similar to iron ions. Several gallium salts have been used in pharmaceutical and radiopharmaceutical industries. For example, gallium nitrate has been used to treat hypercalcemia associated with tumor metastasis to bones. Besides, owing to the nontoxic and extraordinary flexibility, liquid metals can also be applied in biomedical applications, such as drug delivery, molecular, cancer therapy, and biomedical devices. The nanomachines prepared by liquid metals can be injected into cells to treat some cell diseases [16]. Due to its self‐driven ability, the liquid metals robot containing medicine can be automatically delivered into targeted tissues to release medicines to cure diseases [17]. As for the biocompatibility problem of theses liquid metal microrobots, it can be solved by the functionalization of liquid metal (Figure 1.3).
Figure 1.3 Schematic of liquid metals (or alloys) and gallium‐based LMs for biomedical applications [18].
Source: Yan et al. [18]. Reproduced with permission of Royal Society of Chemistry.
Recently, various technology giants have started the launch boom of foldable smartphones. The emergence of foldable phones (mobile phones with folding screen) has provided a new application opportunity for liquid metals and attracts people's attention to liquid metals again. Going back to the problem of screens, folding screens is not as easy as it seems. How to solve this technical problem? The liquid metal gives the answer. The liquid metal itself has excellent shaping ability and excellent dimensional stability. Therefore, it can meet the requirements of thickness, strength, and precision of hinge support parts, which makes liquid metal one of the main solutions for support. In this case, the prospects for liquid metals with outstanding performance advantages are clearly limitless. In addition, liquid metals combine the advantages of traditional metal conductivity with the flexibility of nonmetallic materials. At present, liquid metals have been widely used to make flexible electrons, such as stretchable wire, interconnects, and antennas. Liquid metals can also be applied in memory devices, diodes, electrodes, and capacitors. In principle, the deformation ability of liquid metals is not limited. In addition, liquid metals can form self‐healing and shape‐reconfigurable circuits, and it can be utilized in soft sensors, such as strain sensing, pressure sensor, and touch sensing, because their resistance or capacitance will correspondingly change when deformed. Liquid metals can also be utilized as electronics to sense the change of relative humidity, oxygen, and temperature. Compared with the traditional rigid electron, the flexible electron has great bendability and stretchability, leading to promising applications in the fields of information, energy, medical treatment, and national defense (Figure 1.4).
Figure 1.4 (a) Stretchable materials based on liquid metals [19].
Source: Pan et al.[19].
(b) The shape‐reconfigurable circuits [20].
Source: Park et al. [20].
(c) The self‐healing circuits [21].
Source: Blaiszik et al. [21]. Reproduced with permission of John Wiley and Sons.
(d) The sensor based on liquid metals [22].
Source: Rui et al. [22]. Reproduced with permission of Springer Nature.
Energy is the basis of human survival and development. How to use energy efficiently has been extensively studied? Considering the intermittent and nonuniformity of energy, energy storage and conversion is very important. At present, batteries, capacitors, and inductors have been widely used in energy storage and conversion. The battery is widely used in people's life because of high efficiency and convenience. But the major challenges of traditional lithium batteries lie in dendrite growth. Liquid metals and alloys, which possess inherent deformability, high electronic conductivity, self‐healing, and excellent electrochemical properties, have attracted considerable attention. Utilizing liquid metals can realize dendrite‐free, self‐healing, low reduction potential, high capacity and energy density, and fast mass/charge transport [23] (Figure 1.5).
Figure 1.5 Prospective designs and applications of the liquid‐metal‐based electrodes in various energy storage strategies [23].
Source: Guo et al. [23]. Reproduced with permission of Royal Society of Chemistry.
Liquid metals usually exhibit outstanding thermal conductivity. For example, the thermal conductivity of water is 0.6 W (m K)−1, which is seriously lower than those of gallium (29.6 W m−1 K−1) at 50 °C [24]. So it is a good coolant. In fact, the application of gallium is particularly prominent in the field of heat dissipation [25]. As a liquid metal, it is inherently more conductive than air, water, and many nonmetallic media. Sapphire Technology, a famous video card manufacturer, displayed a graphics card with liquid metals at the E3 game show and Taipei International Computer Show in 2005. The data showed liquid metal cooled the working temperature of the strongest video card at that time, the Radeon X850 XT PE, to 12 °C, and its heat conduction efficiency was 65 times or faster than that of water at that time. At the beginning of 2019, liquid metals cooling systems appeared in high‐end gaming laptops, such as ASUS, HP, and MACHENIKE. There is no doubt that the price will increase, but it can give you a very enjoyable experience. Making good use of liquid metals as a radiator of laptops would bring rapid advances in technology.
In recent years, people focus on exploring, developing, and utilizing the universe, so new space materials with a strong performance is essential to overcome the evil environment, such as zero gravity, vacuum, larger temperature, and strong radiation. Up till now, space materials mainly include carbon‐based materials, porous metals, polyimide, and fiber. Compared to the above materials, liquid metals possess a lot of distinguished properties, such as high electric conductivity, high thermal conductivity, fluidity, and low volatility, which make them a good choice for utilization in space science and technology. It has also been introduced that liquid metal has very good heat dissipation capacity. For space exploration, a liquid metal cooling system has been proved to be the most effective space nuclear electric heating management method [26]. In addition, liquid metal can be used as a thermal interface material for heat conduction. Solar energy is the only energy source in spacecraft operation, so how to store solar energy is very important. Usually, solar energy is converted into heat energy, and then heat energy is converted into electrical energy for storage. Compared with the common thermal storage materials (fluorine), liquid metal has high thermal conductivity, high latent heat of phase change, and low thermal expansion rate, which can be used as a new type for energy storage material. Three‐dimensional printing technology of liquid metals provides a very simple method for rapid manufacturing and maintaining electronic circuits. More importantly, there is a large amount of electromagnetic radiation in the universe, which is not conducive to the operation of devices. The flexible liquid metal electromagnetic shielding material has good wettability and plasticity, which can meet the requirements of complex shape and effectively protect the equipment from radiation. Moreover, the liquid metals usage in science and technology is still in its infancy and we need to keep exploring and look forward to your joining (Figure 1.6).
Figure 1.6 Candidate materials needed to satisfy the space physical environment [27].
Source: Zhang and Liu [27]. Reproduced with permission of Springer Nature.
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Zhang, J., Guo, R., and Liu, J. (2016). Self‐propelled liquid metal motors steered by a magnetic or electrical field for drug delivery.
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2Structure and Physical Properties of Liquid Metal
All of the reasons must come from observation and experiments.
Source: Galileo (1564–1642). Public Domain
2.1 Structure of Liquid Metal
The structure of liquid metal (LM) is different from periodic crystalline solid and total disorder feature of gas; it features a short‐range order and long‐range disorder structures. The structure of liquid metal can be characterized by static structure factor (SSF, S(q)) and pair distribution function (PDF, g(r)), which can be obtained by experiment or theoretical simulation. The structure of liquid metal is also affected by temperature and pressure. Exploring the structure of liquid metal and alloy is of great significance in the field of solid metals and condensed matter physics. We classify liquid metal into single‐component liquid metal, monophasic liquid metal, and biphasic liquid metal mix based on their component and phase, and their corresponding liquid structures will be introduced in this section. Moreover, monocomponent liquid metal contains hard‐sphere‐like liquid metal atoms, merely distorted structure, and strongly anormal structure. The local structure of monophasic liquid metal alloy is more disordered than that of monocomponent liquid metal, and it can be described as the mixture of several types of local structures. As for biphasicliquid metal mix, it contains liquid phase and solid phase at the same time, and can exist stably for a long time if the size of particles is appropriate. Besides, the composition of biphasic liquid metal mix can be affected by the temperature and time. Up to now, the structure of liquid metal is controversial. To analyze the structure of liquid metal, it is necessary to combine the advanced experimental technology and theoretical simulation.
2.1.1 Monocomponent Liquid Metal
Unlike the long‐range order structure of crystalline solid and total disorder feature of gas, the atomic arrangement in liquid is a compromise between the two, which is commonly considered as short‐range order and long‐range disorder. When the metal is in crystal solid phase, its structure can be well explained by three‐dimensional (3D) crystallography. However, the short‐range‐order structure of liquid metal cannot be directly “seen.” For one thing, it is invalid to characterize liquid structure by experimental techniques based on crystallography. For another, crystallography theory based on 3D periodicity is not applicable to explain different liquid structures. Generally, SSF (S(q)) and PDF (g(r)) are used to characterize the structure of liquid metal. SSF is a mathematical description of how a material scatters incident radiation. PDF is a statistic approach to describe the atomic arrangement around a given central atom, generated from the sine Fourier transformation of the normalized scattering function SSF. We can get structure information of liquid metal at statistic average by X‐ray diffraction and neutron diffraction. Moreover, taking advantages of computer simulation, we can obtain more detailed information of liquid metal, such as 3D atomic configuration, dynamic evolution of atomic structures with the temperature or time, and even thermodynamic or kinetic quantities and their dynamic evolutions. The combination of X‐ray and neutron diffraction in experiments and numerical simulations in theory provides an effective method to study the structure of liquid metal and alloy, and some progresses have been made [1]. Nonetheless, there is still a lack of comprehensive theory and model to describe the structure of liquid metal. Understanding the local structure of liquid metal plays an important role in the specialized alloy design and multiple property analysis.
Based on Waseda's classification, the structure of single‐component liquid metal can be essentially classified into three types, which is mainly dependent on the relative relation of positions of peaks in S(q) and g(r). Q1 and Q2 represent the first‐peak and second‐peak positions of S(q), and R1 and R2 are the first‐peak and second‐peak positions of g(r), respectively. These three types of structures have the following characteristics.
(1) The first type of liquid structure (hard‐sphere‐like structure) has the following features:
(i) The first peaks of both the
S
(
q
) and
g
(
r
) are symmetrical.
(ii)
Q
2
/
Q
l
= 1.86 and
R
2
/
R
1
= 1.91.
(iii) The coordination number (
N
c) is 9.0–11.0.
Liquid IA and IIA elements, Al and Pb, are classified into this class.
(2) The second type of liquid structure (merely distorted structure) is characterized by the following features:
(i) The first peaks in
S
(
q
) and
g
(
r
) are asymmetric.
(ii) The ratios
Q
2
/
Q
l
and
R
2
/
R
1
are slightly nonideal.
(iii) The coordination numbers are essentially metallic, that is
Nc
= 10.0–11.0.
Liquid IIB elements, In and Tl, belong to these “distorted structures.”
(3) The third type of liquid structure (strong anomaly structure) has the following characteristics:
(i) There is a shoulder on the high‐
q
side of the first peak in
S
(
q
), and subsidiary maxima shoulder in
g
(
r
) located in between the two maxima.
(ii) The peak positions are strongly nonideal, in which
Q
2
/
Q
1
is 1.96–2.1 and
R
2
/
R
1
is 2.0–2.3.
(iii) The coordination number is exceptionally low (
Nc
= 6–7), suggesting a relatively open local structure.
