Room Temperature Liquid Metal Research Articles

Magnetic flow control in growth and casting of photovoltaic silicon: Numerical and experimental results

A novel, vertical Bridgman-type technique for growing multi-crystalline silicon ingots in an induction furnace is described. In contrast to conventional growth, a modified setup with a cone-shaped crucible and susceptor is used. A detailed numerical simulation of the setup is presented. It includes a global thermal simulation of the furnace and a local simulation of the melt, which aims at the influence of the melt flow on the temperature and concentration fields. Furthermore, seeded growth of cone-shaped Si ingots using either a monocrystalline seed or a seed layer formed by pieces of poly-Si is demonstrated and compared to growth without seeds. The influences of the seed material on the grain structure and the dislocation density of the ingots are discussed. The second part addresses model experiments for the Czochralski technique using the room temperature liquid metal GaInSn. The studies were focused on the influence of a rotating and a horizontally static magnetic field on the melt flow and the related heat transport in crucibles being heated from bottom and/or side, and cooled by a crystal model covering about 1/3 of the upper melt surface.

Surface tension of liquid metal: role, mechanism and application

Surface tension plays a core role in dominating various surface and interface phenomena. For liquid metals with high melting temperature, a profound understanding of the behaviors of surface tension is crucial in industrial processes such as casting, welding, and solidification, etc. Recently, the room temperature liquid metal (RTLM) mainly composed of gallium-based alloys has caused widespread concerns due to its increasingly realized unique virtues. The surface properties of such materials are rather vital in nearly all applications involved from chip cooling, thermal energy harvesting, hydrogen generation, shape changeable soft machines, printed electronics to 3D fabrication, etc. owing to its pretty large surface tension of approximately 700 mN/m. In order to promote the research of surface tension of RTLM, this paper is dedicated to present an overview on the roles and mechanisms of surface tension of liquid metal and summarize the latest progresses on the understanding of the basic knowledge, theories, influencing factors and experimental measurement methods clarified so far. As a practical technique to regulate the surface tension of RTLM, the fundamental principles and applications of electrowetting are also interpreted. Moreover, the unique phenomena of RTLM surface tension issues such as surface tension driven selfactuation, modified wettability on various substrates and the functions of oxides are discussed to give an insight into the acting mechanism of surface tension. Furthermore, future directions worthy of pursuing are pointed out.

Galvanic corrosion couple-induced Marangoni flow of liquid metal.

The Marangoni flow of room temperature liquid metal has recently attracted significant attention in developing advanced flexible drivers. However, most of its induction methods are limited to an external electric field. This study disclosed a new Marangoni flow phenomenon of liquid gallium induced by the gallium-copper galvanic corrosion couple. To better understand this effect, the flow field distribution of liquid gallium was modeled and quantitatively calculated. Then, the intrinsic mechanism of this flow phenomenon was interpreted, during which natural convection and temperature gradient were both excluded and the galvanic corrosion couple was identified as the main reason. In addition, this conclusion was further confirmed by combining the experimental measurement of liquid gallium surface potential and the thermocapillary effect. Moreover, the temperature condition was found to be an indirect factor to the Marangoni flow. This finding broadens the classical understanding of liquid metal surface flow, which also suggests a new way for the application of soft machines.

Recent Advancements in Liquid Metal Flexible Printed Electronics: Properties, Technologies, and Applications.

This article presents an overview on typical properties, technologies, and applications of liquid metal based flexible printed electronics. The core manufacturing material—room-temperature liquid metal, currently mainly represented by gallium and its alloys with the properties of excellent resistivity, enormous bendability, low adhesion, and large surface tension, was focused on in particular. In addition, a series of recently developed printing technologies spanning from personal electronic circuit printing (direct painting or writing, mechanical system printing, mask layer based printing, high-resolution nanoimprinting, etc.) to 3D room temperature liquid metal printing is comprehensively reviewed. Applications of these planar or three-dimensional printing technologies and the related liquid metal alloy inks in making flexible electronics, such as electronical components, health care sensors, and other functional devices were discussed. The significantly different adhesions of liquid metal inks on various substrates under different oxidation degrees, weakness of circuits, difficulty of fabricating high-accuracy devices, and low rate of good product—all of which are challenges faced by current liquid metal flexible printed electronics—are discussed. Prospects for liquid metal flexible printed electronics to develop ending user electronics and more extensive applications in the future are given.

Development of a fast thermal response microfluidic system using liquid metal

Room temperature liquid metal gallium alloy has been widely used in many micro-electromechanical systems applications, such as on-chip electrical microheaters, micro temperature sensors, micro pumps and so on. Injecting liquid metal into microchannels can provide a simple, rapid, low-cost but efficient way to integrate these elements in microfluidic chips with high accuracy. The liquid metal-filled microstructures can be designed in any shape and easily integrated into microfluidic chips. In this paper, an on-chip liquid metal-based thermal microfluidic system is proposed for quick temperature control at the microscale. The micro system utilizes just one microfluidic chip as a basic working platform, which has liquid metal-based on-chip heaters, temperature sensors and electroosmotic flow pumps. Under the comprehensive control of these elements, the micro system can quickly change the temperature of a target fluid in the microfluidic chip. These liquid metal-based on-chip elements are very helpful for the fabrication and miniaturization of the microfluidic chip. In this paper, deionized water is used to test the temperature control performance of the thermal microfluidic system. According to the experimental results, the micro system can efficiently control the temperature of water ranging from 28 °C to 90 °C. The thermal microfluidic system has great potential for use in many microfluidic applications, such as on-chip polymerase chain reaction, temperature gradient focusing, protein crystallization and chemical synthesis.

Electrically driven chip cooling device using hybrid coolants of liquid metal and aqueous solution

Heat dissipation of electronic devices keeps as a tough issue for decades. As the most classical coolant in a convective heat transfer process, water has been widely adopted which however inherits with limited thermal conductivity and relies heavily on mechanical pump. As an alternative, the room temperature liquid metal was increasingly emerging as an important coolant to realize much stronger enhanced heat transfer. However, its thermal capacity is somewhat lower than that of water, which may restrict the overall cooling performance. In addition, the high cost by taking too much amount of liquid metal into the device also turns out to be a big concern for practical purpose. Here, through combining the individual merits from both the liquid metal with high conductivity and water with large heat capacity, we proposed and demonstrated a new conceptual cooling device that integrated hybrid coolants, radiator and annular channel together for chip thermal management. Particularly, the electrically induced actuation effect of liquid metal was introduced as the only flow driving strategy, which significantly simplified the whole system design. This enables the liquid metal sphere and its surrounding aqueous solution to be quickly accelerated to a large speed under only a very low electric voltage. Further experiments demonstrated that the cooling device could effectively maintain the temperature of a hotpot (3.15 W/cm2) below 55oC with an extremely small power consumption rate (0.8 W). Several situations to simulate the practical working of the device were experimentally explored and a theoretical thermal resistance model was established to evaluate its heat transfer performance. The present work suggests an important way to make highly compact chip cooling device, which can be flexibly extended into a wide variety of engineering areas.

Liquid metal spring: oscillating coalescence and ejection of contacting liquid metal droplets

With pretty high surface tension, the room temperature liquid metal may inherit with unexpected behaviors that conventional fluids could not own. Here, we disclosed the coalescence and ejection phenomena of liquid metal droplets via high-speed camera. It was experimentally found that, when gently contacting (rather than colliding) two metal droplets with identical size together in NaOH solution, oscillating coalescence would happen which runs just like a spring after the interface ruptures and forms capillary waves. For two metal droplets with evidently different diameters, the coalescence induces rather unusual ejection phenomena. The large droplet would swallow part of the small one and then eject another much smaller droplet. Such phenomenon provides a direct evidence for the existence of electrical double layer on metal droplets. The dynamics fluid impacting behaviors were quantified through processing images from the recorded movies, and the basic differences between the liquid metal droplets and that of water droplets were clarified. Theoretical mechanisms related to the events were preliminarily interpreted. The present finding refreshes the basic understanding of the liquid metal droplets, which also suggests potential values of applying such fundamental effects to characterize viscosity, surface tension, electrical double layer of the metal fluids and droplet formations.

Liquid Metal Based Stretchable Radiation-Shielding Film

We reported a stretchable and flexible radiation-shielding film based on room-temperature liquid metal. Conceptual experiments showed that the liquid metal based printing technology can achieve an ultrathin flexible radiation-shielding film with a thickness of 0.3 mm. Moreover, the yield strength and ultimate strength of the liquid metal film appear much better than those of a conventional lead-particle-containing radiation-shielding material. In order to evaluate the radiation-shielding performance of the liquid metal material, X-ray radiation experiments to compare the liquid metal film and conventional lead-particle-based shielding material under different stretching conditions were performed. The results indicate that the liquid metal shielding film could achieve a certain radiation-shielding performance. Furthermore, because of the screen-printing properties of liquid metal, a low-cost X-ray mask method using a liquid metal selective radiation-shielding film was also studied, which could serve as a highly efficient and practical method for the medical X-ray shielding applications or semiconductor lithography industry.

Liquid metal actuated ejector vacuum system

An ejector vacuum system using nontoxic room-temperature liquid metal as actuating fluid was fabricated and experimentally demonstrated. With physical merits of high density, fluidity, and ultralow vapor pressure, the liquid metal could serve as a high momentum density carrier fluid. The main performance parameters of the liquid metal actuated ejector were thus found to be significantly improved by orders superior as compared with that of water actuating system. Under oxidation protection, an ultimate vacuum pressure of 170 Pa was achieved and capacity of evacuating a 500 ml nitrogen reservoir from atmospheric pressure to 480 Pa within 96 s was obtained.

Ink Spraying Based Liquid Metal Printed Electronics for Directly Making Smart Home Appliances

The quickly emerging smart home is heavily involved with various information technologies where electronic devices play perhaps the most important roles. However, there is currently a strong lack of an efficient tool for the direct manufacture of electronics as desired. Meanwhile, although printed electronics has aroused much attention in making printed circuit board, radio frequency identification card, sensor and so on, few existing approaches can fulfill the above needs. Among various technologies, liquid metal spraying method owns rather unique merits in printing electronically conductive patterns on nearly all kinds of substrates, either hard or soft, rough or smooth. For this reason, we are dedicated here to extend such method into a generalized strategy of printing electronics for the direct construction of smart home appliances which would generate profound influence on future daily lives. Through adopting the room temperature liquid metal GaIn24.5 as the electronic ink and with various designed masks, a group of typical conductive patterns were directly printed on the surfaces of organic glass, marble windowsill, paper, wood plank of cabinet, building wall and so on, which consist of the most popular materials that could serve as circuit substrates in a home. It was shown that this energy-saving printing method brings about big convenience for equipping future smart home residents. Several new conceptual homemade printed patterns or circuits in providing services such as decoration and entertainment were illustrated. Further, the basic electronic structures of the smart home and the generalized concept of adopting any substrate surface as printed circuit board were explained. The liquid metal spray printing suggests a powerful tool for making consumer electronics and constructing future smart home. It also raised important scientific and technical issues worth of pursuing in the coming time.

Contactless Microstrip Transition for Flexible Microfluidic Circuits and Antennas

This letter presents an ultrawideband contactless microstrip-to-microstrip transition connecting a conventional surface line with an embedded microfluidic line composed of a room-temperature liquid metal. The operational bandwidth of the transition spreads from about 0.8 to 7.9 GHz, which corresponds to the fractional bandwidth of ~ 165%. A detailed parametric study is reported for the transition geometrical parameters and substrate curvature. The performance of the transition is validated successfully via prototyping. The liquid metal used is galinstan, and the substrate is made of PDMS.

Electromigration Induced Break-up Phenomena in Liquid Metal Printed Thin Films

Room temperature liquid metal (RTLM)-related electronics has recently been found increasingly important in a wide variety of emerging areas. In particular, printable liquid metal ink opens the way for direct writing of electronics spanning form microscale to even nanoscale. However, such fluid-like circuits also raised important fundamental as well as practical issues for solving. A big issue facing its future large-scale application is that the failure features of the conductive wires under electrical current densities are not clear. Here, we discovered for the first time that a liquid metal thin film would be broken by the so-called electromigration effect as the applied current increases to a critical magnitude. A theoretical model was established to preliminarily interpret the phenomena and the related effects. The break-up effect in the liquid metal-based circuits could be one of the major hurdles that must be tackled with caution in the research and application of future liquid metal technologies, especially for the printed microelectronics thus enabled.

Splashing phenomena of room temperature liquid metal droplet striking on the pool of the same liquid under ambient air environment

In this article, the fluid dynamics of room temperature liquid metal (RTLM) droplet striking onto a pool of the same liquid in ambient air was systematically investigated. A series of experiments were conducted in order to disclose the influence of the oxidation effect on the impact dynamics. The droplet shape and transient flow behavior were recorded with the aid of a high-speed digital camera. The impact energy stored in the splash structures was estimated via a theoretical model and several morphological parameters obtained from the instantaneous images of the splash. It was observed that the droplet shape and the splashing morphology of RTLM were drastically different from those of water, so was the impact dynamics between room temperature LM pool and high temperature LM pool. According to the energy analysis, it was disclosed that the height of the jet is highly sensitive to the viscosity of the fluid, which is subjected to the oxidation effect and temperature effect simultaneously. These basic findings are important for the application of RTLM in a series of newly emerging technologies such as liquid metal based spray cooling, ink-jet printed electronics, interface material painting and coating, metallurgy, and 3D packages, etc.

Thermally Conductive and Highly Electrically Resistive Grease Through Homogeneously Dispersing Liquid Metal Droplets Inside Methyl Silicone Oil

Thermal grease, as a thermal interface material (TIM), has been extensively applied in electronic packaging areas. Generally, thermal greases consist of highly thermally conductive solid fillers and matrix material that provides good surface wettability and compliance of the material during application. In this study, the room-temperature liquid metal (a gallium, indium and tin eutectic, also called Galinstan) was proposed as a new kind of liquid filler for making high performance TIMs with desired thermal and electrical behaviors. Through directly mixing and stirring in air, liquid metal micron-droplets were accidentally discovered capable to be homogeneously distributed and sealed in the matrix of methyl silicone oil. Along this way, four different volume ratios of the liquid metal poly (LMP) greases were fabricated. The thermal and electrical properties of the LMP greases were experimentally investigated, and the mechanisms were clarified by analyzing their surface morphologies. The experimental results indicate that the original highly electrically conductive liquid metal can be turned into a highly electrically resistive composite, by simply blending with methyl silicone oil. When the filler content comes up to 81.8 vol. %, the thermal conductivity, viscosity and volume resistivity read 5.27 W/(m · °C), 760 Pa · s and 1.07 × 107 Ω m, respectively. Furthermore, the LMP greases presented no obvious corrosion effect, compared with pure liquid metal. This study opens a new approach to flexibly modify the material behaviors of the room-temperature liquid metals. The resulted thermally conductive however highly electrically resistive LMP greases can be significant in a wide variety of electronic packaging applications.

Channelless Fabrication for Large‐Scale Preparation of Room Temperature Liquid Metal Droplets

AbstractThe room‐temperature liquid metal (RTLM), which is far from being fully exploited, is emerging as an ideal material for fabricating micro‐droplets owing to its strong surface tension and easy phase control property. In order to find out a low‐cost and technically simple way for quickly preparing metal particles in large‐scale, here a channelless fabrication method based on stream jetting and self breaking up mechanisms of the RTLM when injected into and interact with the matching solution was proposed. Various typical factors on influencing the droplets generation behavior were experimentally investigated and clarified, such as the effects of the aperture size of the injection needle, the density, viscosity of the liquids and surfactants etc. Further, as an illustration of the flexibility of the present method, the direct construction of a three‐dimensional porous metal block with foam‐structures inside was also demonstrated. This study opened an extremely simple way for large scale fabrication of liquid metal micro‐droplets and particles which has rather important practical values. It also suggests a highly efficient approach for visualizing and investigating the fundamental mechanisms of fluids interactions between RTLM and general solution.

Atomized spraying of liquid metal droplets on desired substrate surfaces as a generalized way for ubiquitous printed electronics

A direct electronics printing technique through atomized spraying for patterning room temperature liquid metal droplets on desired substrate surfaces is proposed and experimentally demonstrated for the first time. This method has generalized purpose and is highly flexible and capable of fabricating electronic components on any desired target objects, with either flat or rough surfaces, made of different materials, or different orientations from 1-D to 3-D geometrical configurations. With a pre-designed mask, the liquid metal ink can be directly deposited on the substrate to form various specific patterns which lead to the rapid prototyping of electronic devices. Further, extended printing strategies were also suggested to illustrate the adaptability of the method such that the natural porous structure can be adopted to offer an alternative way of making transparent conductive film with an optical transmittance of 47% and a sheet resistance of 5.167{\Omega}/O. Different from the former direct writing technology where large surface tension and poor adhesion between the liquid metal and the substrate often impede the flexible printing process, the liquid metal here no longer needs to be pre-oxidized to guarantee its applicability on target substrates. One critical mechanism was found as that the atomized liquid metal microdroplets can be quickly oxidized in the air due to its large specific surface area, resulting in a significant increase of the adhesive capacity and thus firm deposition of the ink to the substrate. This study established a generalized way for pervasively and directly printing electronics on various substrates which are expected to be significant in a wide spectrum of electrical engineering areas.

Liquid metal angiography for mega contrast X-ray visualization of vascular network in reconstructing in-vitro organ anatomy.

Visualization on the anatomical vessel networks plays a vital role in the physiological or pathological investigations. However, so far it still remains a big challenge to identify the fine structures of the smallest capillary vessel networks via conventional imaging ways. Here, the room temperature liquid metal angiography was proposed for the first time to generate mega contrast X-ray images for multiscale vasculature mapping. Particularly, gallium was adopted as the room temperature liquid metal contrast agent and infused into the vessels of in vitro pig hearts and kidneys. We scanned the samples under X-ray and compared the angiograms with those obtained via conventional contrast agent--the iohexol. As quantitatively demonstrated by the grayscale histograms and numerical indexes, the contrast of the vessels to the surrounding tissues in the liquid metal angiograms is orders higher than that of the iohexol enhanced images. And the angiogram has reached detailed enough width of 0.1 mm for the tiny vessels, which indicated that the capillaries can be clearly distinguished under the liquid metal enhanced images. Further, with tomography from the micro-CT, we also managed to reconstruct the 3-D structures of the kidney vessels. Tremendous clarity and efficiency of the method over existing approaches have been experimentally clarified. It was disclosed that the usually invisible capillary networks now become distinctively clear in the gallium angiograms. This basic mechanism has generalized purpose and can be extended to a wide spectrum of 3-D computational tomographic areas. It opens a new soft tool for quickly reconstructing high-resolution spatial channel networks for scientific researches as well as engineering practices where complicated and time-consuming resections are no longer a necessity.

Pervasive liquid metal based direct writing electronics with roller-ball pen

A roller-ball pen enabled direct writing electronics via room temperature liquid metal ink was proposed. With the rolling to print mechanism, the metallic inks were smoothly written on flexible polymer substrate to form conductive tracks and electronic devices. The contact angle analyzer and scanning electron microscope were implemented to disclose several unique inner properties of the obtained electronics. An ever high writing resolution with line width and thickness as 200 μm and 80 μm, respectively was realized. Further, with the administration of external writing pressure, GaIn24.5 droplets embody increasing wettability on polymer which demonstrates the pervasive adaptability of the roller-ball pen electronics.

Liquid metal material genome: Initiation of a new research track towards discovery of advanced energy materials

As the basis of modern industry, the roles materials play are becoming increasingly vital in this day and age. With many superior physical properties over conventional fluids, the low melting point liquid metal material, especially room-temperature liquid metal, is recently found to be uniquely useful in a wide variety of emerging areas from energy, electronics to medical sciences. However, with the coming enormous utilization of such materials, serious issues also arise which urgently need to be addressed. A biggest concern to impede the large scale application of room-temperature liquid metal technologies is that there is currently a strong shortage of the materials and species available to meet the tough requirements such as cost, melting point, electrical and thermal conductivity, etc. Inspired by the Material Genome Initiative as issued in 2011 by the United States of America, a more specific and focused project initiative was proposed in this paper—the liquid metal material genome aimed to discover advanced new functional alloys with low melting point so as to fulfill various increasing needs. The basic schemes and road map for this new research program, which is expected to have a worldwide significance, were outlined. The theoretical strategies and experimental methods in the research and development of liquid metal material genome were introduced. Particularly, the calculation of phase diagram (CALPHAD) approach as a highly effective way for material design was discussed. Further, the first-principles (FP) calculation was suggested to combine with the statistical thermodynamics to calculate the thermodynamic functions so as to enrich the CALPHAD database of liquid metals. When the experimental data are too scarce to perform a regular treatment, the combination of FP calculation, cluster variation method (CVM) or molecular dynamics (MD), and CALPHAD, referred to as the mixed FP-CVMCALPHAD method can be a promising way to solve the problem. Except for the theoretical strategies, several parallel processing experimental methods were also analyzed, which can help improve the efficiency of finding new liquid metal materials and reducing the cost. The liquid metal material genome proposal as initiated in this paper will accelerate the process of finding and utilization of new functional materials.

Liquid–Metal Vertical Interconnects for Flip Chip Assembly of GaAs C-Band Power Amplifiers Onto Micro-Rectangular Coaxial Transmission Lines

Prior work has demonstrated a new process utilizing room-temperature liquid metal, Galinstan, as an interconnect material for flip-chip bonding. This interconnect forms a flexible bond between chips and carriers, and, therefore, a flip-chip assembly using this technology is much less susceptible to thermomechanical stresses. This paper applies this concept to interconnect GaAs MMIC chips to 3-D Polystrata transmission-line structures. Passive assemblies are utilized to model, test, and verify liquid-metal interconnections, giving average losses per liquid-metal transition of about 0.11 dB out to 26.5 GHz, low parasitics per transition, and demonstrated reliability after temperature cycling. A prefabricated GaAs MMIC chip is postprocessed for liquid-metal assembly. Measured results show, over the MMIC's 4.9-8.5-GHz frequency range, the system's overall reduction in gain of the MMIC is 1.4 dB or 0.7 dB per RF transition as compared with direct probing of the MMIC chip.