Room temperature liquid metal and its derivative materials are emerging functional substances that have sprung up in recent years. A series of breakthrough discoveries have been made in this field, leading to the creation and application of many new materials, which is regarded as the second revolution in the use of metals by mankind. This article briefly introduces several typical developments, basic problems and industrial application examples in the field of liquid metal material science, and analyzes the scientific laws behind the phenomenon, including: chip cooling and energy utilization, printed electronics and additive manufacturing (3D printing) , Biomaterials, Flexible Intelligent Machines. On this basis, it discusses the background of the times when the "liquid metal valley" is proposed, the basic approach to develop a new industrial system for liquid metal, and the importance of advancing the research on genetic engineering of liquid metal materials and constructing corresponding databases. As a major scientific, technological and industrial frontier with both basic exploration and practical application value, liquid metal has broad development prospects; corresponding research is expected to promote the progress of human material civilization, optimize social production and lifestyle, and will also profoundly influence China and the world to seek a new generation The process of transformative technology and industry.
1. The rise of normal temperature liquid metal material science
Most people's first impressions of liquid metal come from the science fiction film "Terminator", and the omnipotent robot is made of liquid metal. At present, with the deepening and development of scientific research, a large number of liquid metal technology and advanced equipment made from it are becoming a reality. To a large extent, it can be considered that liquid metal is a cutting-edge functional material between machines and people, which contains a wealth of scientific, technical and application problems. The actual room temperature liquid metal is a large class of emerging functional substances with unique physical and chemical behaviors. Typical types include gallium-based alloys, bismuth-based alloys and their derivative materials; they have many novel characteristics and provide important enlightenment for emerging science and technology. And abundant research space. In recent years, thanks to the pioneering work of domestic and foreign scholars, especially the Chinese research team, in basic exploration and industrialization practice, liquid metal material science has developed from an unpopularity to a major scientific and technological frontier hotspot that has attracted international attention, and its influence is very wide. wide.
Dan Shechtman, the winner of the 2011 Nobel Prize in Chemistry and a professor at the Technion-Israel Institute of Technology, once pointed out: The biggest limitation of technology today comes mainly from the limitation of materials. The emergence of liquid metal and its derivative materials has opened up the bottleneck links of many application technologies and contributed to many subverting traditional industrial applications. Since the beginning of the 21st century, the Chinese research team has played a systematic and pioneering role in this major scientific and technological field, revealing many new scientific phenomena, basic effects and transformative application methods of liquid metal; it has contributed to the formation of a series of high-tech industries , Proposed and promoted the establishment and development of "Liquid Metal Valley" and a new industry of liquid metal. In recent years, some international scientific research institutions have also launched liquid metal exploration and have made gratifying progress. The research and application of liquid metal is getting better, reflecting the formation and evolution of an important technology and industry field.
This article aims to analyze the basic issues and typical developments of liquid metals, and explain the value of this important frontier in catalyzing breakthrough science and industry, focusing on the fields of electronic information, energy, advanced manufacturing, flexible robots, and biomedicine; The role and role of liquid metal material science in opening up new industries, expounding the preliminary practice of building "liquid metal valley" and industrial clusters, and discussing major development opportunities facing us, with a view to providing a frontier reference for the development of my country's new material industry.
2. Classic liquid metal materials and their properties
As the name implies, liquid metal refers to metal that is liquid at room temperature or higher, also known as low melting point metal. Typical gallium-based, bismuth-based metals and their alloys are becoming revolutionary materials that are emerging because of their safety, non-toxicity and excellent performance. Others such as mercury, cesium, sodium potassium alloys, etc., are also in liquid form at room temperature. It is toxic, radioactive or dangerous, and its application is greatly restricted. In contrast to low-melting-point metals, metals or their alloys that can only be liquid at high temperatures (such as 600 to 700 ℃) are called high-melting-point metals and have been widely studied as classic metallurgical materials.
In contrast, in the world, room temperature liquid metal has been neglected for a long time. The subversive discoveries and technological breakthroughs made in recent years are more embodied in revealing the basic scientific phenomena and major applications of liquid metals at room temperature. Liquid metal can flow at room temperature, has strong electrical conductivity, excellent thermal properties, and is easy to achieve solid-liquid conversion. Because of its high boiling point (still in liquid phase when the temperature is as high as 2300 ℃), it will not boil or even explode like water; A single material integrates the advantages of many cutting-edge functional materials, which is expected to break through the application bottleneck of many traditional technologies, and thus open a very broad industrial application space.
The types of normal temperature liquid metals that can be used directly are usually limited. The pure metals in nature that are liquid at room temperature are mainly mercury, gallium, and cesium, with melting points of −38.87 ℃, 29.8 ℃, and 28.65 ℃, respectively. In view of the scarcity of liquid pure metals, liquid alloy materials are generally used in practice, which need to have the following characteristics: ①Excellent physical and chemical properties, such as high thermal conductivity, electrical conductivity, and low viscosity; ②Environmentally friendly, non-toxic and harmless, It is non-flammable and explosive, easy to recycle, with low vapor pressure and volatility; ③The cost should be as low as possible. Because of this, one of the bottlenecks that hinder the rapid development and application of liquid metals is the lack of sufficient optional materials and knowledge of the corresponding material properties. In order to change this situation, the domestic research team proposed the Liquid Metal Material Genome Project, which aims to discover new liquid metal functional materials and solve the shortage of material types. Exploring and discovering more liquid metal composite materials to meet the increasing actual demand is the continuing direction of field research.
3. Creation of new liquid metal materials
(1) Liquid metal materials reinforced or modified based on foreign objects
In the creation of new liquid metal materials, in addition to alloying preparation methods, more new materials can be obtained by means of the compatibility of materials. Experiments have found that metal droplets can swallow surrounding particles into the body with the help of an electric field or chemical substances in a solution environment (see Figure 1), which is similar to the endocytosis effect of cell biology and is extremely efficient. This discovery opens up a quick way to construct high-performance liquid metal materials, which can be made into materials with different physical and chemical properties (such as liquid metals with adjustable electrical, magnetic, thermal, and mechanical properties) according to needs, and even develop a series of materials A metal complex with a state between liquid and solid.
In addition to introducing foreign objects to realize functional materials, liquid metal can also be used as additives to modify other materials. After the concept of liquid metal additives was proposed, a highly thermally conductive and electrically insulating liquid metal material was developed accordingly, integrating the two originally contradictory properties of heat conduction and insulation. This actually became the beginning of the subsequent development of liquid metal composite materials.
(2) Porous liquid metal materials
Although most of the existing liquid metal is in the form of a continuous medium, the liquid metal can also be made into a porous substance, thereby achieving more peculiar functions and behaviors. Research has found that loading iron nanoparticles into liquid metal (such as gallium indium alloy) and introducing chemical reaction mechanisms can quickly produce flexible porous metal materials with good conductivity and magnetism; the pore size of this material can be flexibly adjusted and withstood When heated, it can expand repeatedly (up to 7 times in the limit), and the expanded porous metal can even float on the water with heavy objects.
This type of material was created in nature for the first time, and the discovered mechanism has pushed the research and development of liquid metal smart materials and equipment to a new level. For example, the relevant characteristics can be used to manufacture underwater deformable machines, flexible robotic arms, exoskeletons, flexible intelligent robots, etc.
(3) Liquid metal-based conductor-insulator transition material
High thermal conductivity and electrical insulation, these seemingly contradictory properties can also be adjusted with each other to make conductive insulator transformation (CIT) materials. Some studies have found that the general strategy for preparing wide-temperature-adjustable CIT materials based on liquid metals is that the key lies in the use of inter-substances in composite materials to achieve the material's conductive and insulating transition function. One of the typical cases is the combination of liquid metal with abnormal volume expansion rate and good fluidity simethicone to construct a liquid CIT material. Based on this basic principle, we can develop more liquid CIT material systems that work in a wide temperature range with the help of liquid metals with different melting points and collaborative materials.
The liquid CIT material is formed by mixing liquid metal droplets and solvent based on the interaction mechanism between the internal substances of the composite material (see Figure 4). When the temperature is high, the liquid metal is liquid and separated by the solvent. At this time, the composite material is in an insulating state; when the temperature decreases, the metal droplets solidify and the liquid CIT material becomes conductive; as the temperature changes , The conductivity of liquid CIT materials can differ by 9 orders of magnitude. This process is completely reversible and theoretically can be repeated unlimited times. The transition temperature of liquid CIT materials only depends on the phase transition point of the metal droplets. Therefore, a series of liquid alloys with different phase transition points (such as gallium-based and bismuth-based alloys) can be found to obtain different transition point temperatures.
(4) Lightweight liquid metal materials
Conventional liquid metal usually has a high density, which will result in a larger quality of manufactured devices and equipment, resulting in excessive energy consumption and reduced application flexibility. Based on the basic idea of light liquid metal, represented by eutectic gallium indium alloy and hollow glass beads, a liquid metal composite material with a density of only half that of water and floating on the water can be prepared. This material retains the electrical conductivity, thermal conductivity, mechanical strength, solid-liquid phase transition and other characteristics of pure liquid metal. It also has the characteristics of plasticity, deformability and even magnetism; accordingly, various plane and three-dimensional application scenarios are designed, such as water surface. Circuits and underwater robots can also introduce different packages to control the floating behavior of materials.
The concept of light liquid metal material has basic scientific significance and universal application value, and has created a new way to manufacture new liquid metal functional materials. Combining various liquid metals with corresponding light-weight modified substances (such as plastics, wood, light metals, and magnetic, optical materials, porous substances, etc.) can give the terminal material more target functions; it can combine multiple The functions of cutting-edge materials (such as electricity, magnetism, sound, light, heat, mechanics, fluids, chemistry, etc.) are integrated. This is a performance that existing material systems cannot easily possess and is very useful in many applications.
Fourth, the cutting-edge application of liquid metal materials
So far, the application of liquid metal has penetrated into many fields of natural science and engineering technology, and is defining and creating a new future. The following describes the typical technology and application directions derived from liquid metal from four aspects.
(1) Advanced thermal control and energy technology of liquid metal
With the application and development of micro-nano electronics technology, the thermal barrier caused by highly integrated chips, devices and systems has become a universal problem restricting various high-end applications, and breaking the heat dissipation bottleneck has been raised to an unprecedented level. At the beginning of the 21st century, low-melting alloy fluid heat dissipation technology was introduced in the field of chip cooling. This approach has become an international frontier research hotspot in recent years, and it has become a promising new industry direction in the field of chip cooling.
After nearly 20 years of development, a relatively complete theoretical and application technology system has been established in the field of room temperature liquid metal cooling, mainly involving: liquid metal enhanced heat transfer, phase change and flow theory, electromagnetic, thermoelectric or siphon driven cooling and heat capture , Micro-channel liquid metal heat dissipation, blade heat dissipation, mixed fluid heat dissipation, waterless heat exchanger, low melting point metal solid-liquid phase change heat absorption, high thermal conductivity nano metal fluid and thermal interface materials. In addition to its important application value in high-power density electronic chips, optoelectronic devices, and extreme heat dissipation of defense equipment, liquid metal is gradually expanding to consumer electronics, low-grade thermal energy utilization, photovoltaic power generation, energy storage, smart grids, high-performance batteries, Engine cooling, thermoelectric conversion and other fields. The new technology has broken the traditional model, and a number of achievements have been applied to industrial and commercial fields on a large scale: liquid metal heat sinks for desktop computers, liquid metal thermal interface materials, phase change heat dissipation modules, liquid metal cooling high-power overhead lights and light-emitting diodes ( LED) street lights, ultra-thin liquid metal radiators for notebook computers, liquid metal radiators for high-performance server cooling, etc.
Since the liquid metal chip cooling method was proposed, it has continuously attracted the attention of the industry. The relevant research has won the only best paper award in the 2010-2011 ASME Journal of Electronic Packaging, a representative journal in the field of electronic packaging, and also won the Innovation Award of the China International Industry Fair. Multiple industry awards within.
(2) Liquid metal printing electronics and 3D printing
There are many traditional electronic manufacturing processes, which involve from the preparation of the base material to the thin film deposition, etching, and packaging required for forming interconnections, which consume a lot of raw materials, water, gas and energy. In order to change this situation, Chinese scholars first proposed innovative ideas such as liquid metal printing electronics and room temperature metal 3D printing, established corresponding theories and technical systems, and developed a series of new equipment; printing methods in various flexible, Target circuits, components, integrated circuits and terminal functional devices are directly manufactured on rigid substrates and even human skin. The corresponding breakthrough is considered to be expected to change the manufacturing rules of traditional electronics and integrated circuits, and the "what you see is what you get" electronic direct writing model will accelerate the arrival of the era of inclusive personalized electronics manufacturing.
The establishment of liquid metal printing electronics and room temperature 3D metal printing methods makes it possible to rapidly manufacture personalized functional devices, lowers the threshold for high-end manufacturing, and is expected to be popularized among the general public. Because of this, the related inventions and development equipment have been highly recognized by the domestic and foreign scientific and technological circles. The technological achievements have been reviewed and introduced by scientific journals such as Technology Review, IEEE Spectrum, and Chemistry World, and they have been shortlisted in China's top ten scientific and technological progress news in 2015.
(3) Liquid metal biomedicine and health technology
In the field of biomedicine and health technology, the unique liquid metal has brought about conceptual changes. The Chinese research team took the lead in using liquid metal to solve a series of major biomedical problems and bottlenecks, and systematically proposed and constructed a new field of liquid metal biomedical materials. The research results have triggered an international response. Among them, the liquid metal nerve connection and repair regulation technology is regarded as a "shocking medical breakthrough" because of its originality, and a series of neuroregulation technologies have been derived from it; the liquid metal high-resolution angiography created by Wang et al. , Using relatively simple methods to solve extremely complex problems, and reveal enough details.
In addition, liquid metal embolization of blood vessels to treat tumors, alkali metal fluid thermochemical ablation of tumors, injection-type low-melting metal bone cement, rigid and flexible liquid metal exoskeleton, printed liquid metal flexible radiation protection technology, implanted medical electronics Volume 3D printing and injection electronics, liquid metal skin photothermal conversion and electromagnetics, liquid metal medical sensing technology, etc., have also attracted much attention due to new academic concepts and technological breakthroughs.
(4) Liquid metal flexible intelligent machine
It is the dream of the world's scientific and engineering circles to design a flexible machine that can freely switch between different forms in a controllable manner to replace humans in performing more special and complex tasks. For example, in earthquake relief or special operations, such robots can deform as needed, pass through narrow spaces, and restore their original shape to continue their mission. Academia generally believes that once breakthroughs in software robotics technology, it will significantly affect high-end manufacturing, medical rehabilitation, and defense equipment. Related research and development activities are in full swing. In 2017, the software robot project supported by the National Natural Science Foundation of the United States reached 26 million US dollars.
Starting from the concept of building a new system, the Chinese research team pioneered a technological path different from traditional liquid metal soft robots, and gradually formed corresponding theories and technical systems in terms of materials, devices, and systems. One of the landmark developments is the first disclosure of a series of large-scale deformation, rotation, directional motion, and merging, fracture-recombination behaviors of liquid metal under the control of electric field, which became the beginning of subsequent research on liquid metal deformable robots. Further discovered an unusually unique phenomenon and mechanism, that is, liquid metal can form a self-driving fully flexible machine by swallowing a small amount of aluminum, with a speed of centimeters per second and a running time of several hours, realizing autonomous movement without external electricity. This self-driving liquid metal group of micromotors can form high-speed coordinated motion in an electric field.
The above findings have laid an important theoretical foundation for the development of practical intelligent motors, vascular robots, fluid pumping systems, flexible actuators and even more complex liquid metal robots. Combining liquid metal and rigid materials can also realize a solid-liquid combined machine. The use of electronically controlled deformable and rotating "liquid metal wheels" can drive 3D printed miniature vehicles to achieve travel, acceleration and more complex movements. The study also found that liquid metal can stably exhibit the free shaping ability in any shape on the graphite surface, thereby enabling liquid metal to climb against gravity. If the liquid machine units are programmed in groups, a feasible technical approach for controllable flexible intelligent robots will be established, which will change the technical form of traditional robots.
On the whole, liquid metal has derived a new concept of deformable machines, which will significantly speed up the development of flexible intelligent machines. In view of the breakthrough of these discoveries, the corresponding results have aroused great repercussions in the world. They have been reviewed by Nature, Nature Materials, Science News, New Scientist, Chemistry World, etc., and liquid metal robots are listed as the most promising in the field of robotics. One of the ten major directions.
5. The rise of new liquid metal industry[next page]