Gallium-based Liquid Metal Research Articles

Low-melting-point liquid metal convective heat transfer: A review

Low-melting-point liquid metal convection is rapidly emerging as a high-performance heat transfer technology in electronics thermal management and energy fields. The advantages of gallium-based and bismuth-based liquid metals, such as low melting point, high thermal conductivity, nonflammability, and nontoxic characteristic, make liquid metals highly attractive for high heat flux density applications at high temperatures. Their convective heat transfer behavior, driving techniques, and application systems are different from those of conventional liquids, such as water, oil, sodium-potassium, and lead-bismuth. Although their significance in both academia and industry has gradually increased, to the best of our knowledge, a systematic description of gallium-based and bismuth-based liquid metal convection and its applications has not been reported thus far. Therefore, in this paper, we present a thorough review of low-melting-point liquid metal convective heat transfer technologies. Specifically, liquid metal fluids and their convection mechanisms are introduced first, followed by the description of typical driving techniques of liquid metals based on electromagnetic, thermal, electrical and magnetic methods. Subsequently, applications of typical liquid metal convection methods in industrial heat transfer and energy fields are presented. Finally, extended liquid metal convection enhancement techniques based on microchannel and jet impingement are interpreted. Both the fundamental mechanisms and recent application research are elaborated, and critical issues are discussed. The scientific and technical challenges, along with future developments in these areas, are also highlighted in the paper.

Method to Reduce the Contact Resistivity between Galinstan and a Copper Electrode for Electrical Connection in Flexible Devices.

This study demonstrates a method to mount electronic components using gallium-based liquid metals (LMs) with reduced contact resistivity between the LM and a copper (Cu) electrode. Gallium-based LMs have low volume resistivity and low melting points, and they are used as electronic components such as interconnects and sensors of stretchable electronic devices. However, the high contact resistivity of the oxide layer on the surface of the Ga-based LMs becomes a problem when the Ga-based LMs are used in contact with rigid electronic components. To overcome this problem, we studied herein the effect of the oxide layer on contact resistivity via the contact methods of the Ga-based LM (galinstan) and the Cu film. Through the placement of galinstan after the placement of the Cu film and application of vacuum to reduce the effect of the oxide layer, the contact resistivity was reduced to 0.59 × 10-7 Ωm2, which was 90% lower than that in the case where the Cu film was placed on galinstan on which the oxide layer grew (5.7 × 10-7 Ωm2) (day 1). Additionally, it was found that the contact resistivity decreased in the same order (10-8 Ωm2) over time regardless of the methods in which galinstan was applied (day 103). Furthermore, alloy formation on the Cu film surface was confirmed via elemental analysis. Finally, the mounting method using galinstan was demonstrated, which enabled the change in contact resistance to be maintained as low as 7.2% during 100% stretching deformation repeated 100 times (day 1 and day 130). Our results show that low and stable contact resistance with a high stretch tolerance can be achieved via the mounting method using galinstan based on our contact methods. This mounting method, therefore, expands the range of materials suitable for use as substrates and provides new opportunities for the development of stretchable electronics.

Improving tribological behaviors of gallium-based liquid metal by h-BN nano-additive

Gallium-based liquid metal is a potential lubricant on account of its high load capacity and thermal stability. However, the lubrication performance of gallium-based liquid metal (LM) is limited by its metallic characteristics. In this work, hexagonal boron nitride (h-BN) nanosheet with good lubricity is utilized as additive to enhance lubrication behaviors of LM. The bonding between LM and h-BN particles is reversible. In alkaline solution, LM is able to be separated from h-BN particles and agglomerate to bulk liquid metal. Moreover, our frictional tests demonstrate that when using h-BN as additive in LM, it holds enhanced tribological performance including reduction of the friction coefficient and wear rate in contrast to the pure LM. Moreover, the study of the lubrication mechanisms suggests that the adhesion of LM and deposition of h-BN are the critical factors for improving lubricity. This work proposed a facile method to improve the tribological behaviours of LM and promoted the understanding of the lubricating principles of LM and h-BN.

A double inclusion model for liquid metal polymer composites

Liquid metal elastomer composites have gained significant attention in advanced technologies including wearable electronics, soft robotics, and human-computer interactions. This is due to the combination of metallic conductivity and fluidic properties of liquid metal (LM) inclusions in addition to their facile fabrication process. With the emergence of gallium-based liquid metal nanocomposites and advances in synthesis and integration of LM nanoparticles in a variety of polymer matrices, there is a pressing need for a materials design tool to accelerate the development of these multifunctional composites. Here, we introduce a double inclusion (DI) model capable of predicting the properties of polymer composites with core-shell liquid metal droplets. The size-dependent elasticity of LM inclusions is modeled by considering the solid gallium oxide interphase between the liquid metal core and the solid polymer matrix. As the size of inclusions reduces from tens of microns to tens of nanometers, the role of the oxide interface (shell) becomes more dominant. The results of the DI model show excellent agreement with finite element analysis and experimental results for a wide range of droplet sizes and volume fractions. This model provides a design framework for the synthesis of LM composites with tailored multifunctional properties.

The Design and Manufacturing Process of an Electrolyte-Free Liquid Metal Frequency-Reconfigurable Antenna.

This communication provides an integrated process route of smelting gallium-based liquid metal (GBLM) in a high vacuum, and injecting GBLM into the antenna channel in high-pressure protective gas, which avoids the oxidation of GBLM during smelting and filling. Then, a frequency-reconfigurable antenna, utilizing the thermal expansion characteristic of GBLM, is proposed. To drive GBLM into an air-proof space, the thermal expansion characteristics of GBLM are required. The dimensions of the radiating element of the liquid metal antenna can be adjusted at different temperatures, resulting in the reconfigurability of the operating frequency. To validate the proposed concept, an L-band antenna prototype was fabricated and measured. Experimental results demonstrate that the GBLM in the antenna was well filled, and the GBLM was not oxidized. Due to the GBLM being in an air-proof channel, the designed liquid metal antenna without electrolytes could be used in an air environment for a long time. The antenna is able to achieve an effective bandwidth of over 1.25–2.00 GHz between 25 °C and 100 °C. The maximum radiation efficiency and gain in the tunable range are 94% and 2.9 dBi, respectively. The designed antenna also provides a new approach to the fabrication of a temperature sensor that detects temperature in some situations that are challenging for conventional temperature sensing technology.

Gallium-based liquid metal alloy incorporating oxide-free copper nanoparticle clusters for high-performance thermal interface materials

Thermal interface materials (TIMs) with high thermal conductivity, fluidic characteristics, and surface wettability are crucial for the effective thermal management of high-power electronics. Gallium-based liquid metal (LM) can be a candidate for obtaining the purposes due to their thermo-physical properties. Several studies attempted to further increase the thermal conductivity of composites by adding conductive fillers. However, these efforts have been limited due to the oxidation and solidification issues at high vol% of fillers. In this work, we apply three strategies to obtain the enhanced thermal conductivity while maintaining the fluidity: 1) suppressing Ga-induced oxidation, 2) reducing the matrix-fillers interfacial resistance, and 3) forming additional heat-pathways within the LM matrix. An oxide-free ultrasonication-assisted particle internalization method has been developed, in which the copper nanoparticles (Cu NPs) are incorporated into the gallium-indium-tin (GaInSn) LM matrix. The fabricated composite shows ~180% enhancement of thermal conductivity (~64.8 Wm−1K−1) with only 4 vol% of nano-fillers compared to the untreated GaInSn. In the droplet impacting test, the synthesized GaInSn/Cu NPs composite presents almost identical spreading diameters with the untreated one, which confirms the maintained fluidity. The predicted thermal conductivity using the theoretical model, calculating the measured cluster fraction, is well-matched to the experimental data, which clarifies that the nanoparticle clusters created effective heat-pathways. A high-quality interface with a silicon substrate is confirmed by the X-ray photoelectron spectroscopy, demonstrating the presence of a thin-film Ga2O3 adhesion layer. In the cooling performance test, the developed TIM provides over 20% lower hot-spot temperature than that of the grease-type TIM at high heat flux regime (>150 W/cm2), showing excellent thermal stability over 230 cycles of acceleration test. This work will help develop high-performance TIMs, especially for high power density semiconductor applications.

Study on the Performance of Liquid Metal Lubricated V-Groove Bearing Considering Turbulence

To study the application of liquid metal (LM) in the field of practical lubrication, the viscosity of gallium based liquid metal (GBLM) was measured initially, and the relationship between viscosity and temperature was fitted to obtain the viscosity under high temperature. Under the simulated high temperature and vacuum working environment of computed tomography tube (CTT) machines, considering the influence of turbulence, the changes, with eccentricity, of bearing capacity, discharge, friction power consumption, temperature rise, stiffness, and critical mass of the GBLM lubricated V-groove bearing (V-g B) were analyzed. Due to the special structure of V-g B, the coordinate transformation was carried out and the turbulent Reynolds equation was solved by using the finite difference method and the local integral method. The bearing film thickness and pressure distribution under the two coordinate systems were analyzed and compared and the pressure distribution of V-g B under small eccentricity and large eccentricity was studied, respectively. The performance of GBLM lubricated V-g B was studied, which provides theoretical guidance and an analytical method for LM bearing of high-performance CT equipment.

High thermal conductivity in indium-based metal/diamond composites by good wettability of diamond with indium

Low melting point metal (LMPM) has potential application value in the field of thermal management. Indium-based LMPM/diamond composites were manufactured using sintering technique. The thermal conductivity of Bi-In-Sn/diamond composites was improved by pre-adding indium particles fabricated using slice technique. Using in-situ imaging and particle dipping experiment, the wetting behavior of diamond microparticle with pure indium, indium-based and gallium-based liquid metal (LM) was investigated. The diamond microparticle was well wetted by molten indium. The wettability of diamond with gallium can be improved by alloying gallium with indium. Oxide film of LM would hinder the wetting of LM on diamond. The highest thermal conductivity of Bi-In-Sn/diamond composites and indium/diamond composites obtained in this work was up to 157 W m−1 K−1 and 211 W m−1 K−1, respectively.

Surface modification of liquid metal as an effective approach for deformable electronics and energy devices.

The fields of flexible or stretchable electronics and energy devices, reconfigurable and compliant soft robotics, wearable e-textiles or health-care devices have paid significant attention to the need of deformable conductive electrodes due to its critical role in device performances. Gallium-based liquid metals, such as the eutectic gallium–indium (EGaIn) being an electrically conductive liquid phase at room temperature, have attracted immense interests as a promising candidate for deformable conductor. However, in the case of bulk liquid metal, there are several limitations such as the need of encapsulation, dispersion in a polymer matrix, or accurate patterning. For these reasons, the preparation of liquid metal particles in harnessing the properties in a non-bulk form and surface modification is crucial for the success of incorporating liquid metal into functional devices. Herein, we discuss the current progress in chemical surface modification and interfacial manipulations of liquid metal particles. The physical and chemical properties of the surface modification-assisted liquid metal polymer composite are also reviewed. Lastly, the applications of the surface-modified liquid metal particles such as flexible electrode, soft robotics, energy storage or harvester, thermal conductor, dielectric sensor, and bioelectronics are discussed, and the corresponding perspectives of deformable electronics and energy devices are provided. In particular, we focus on the functionalization method or requirement of liquid metal particles in each application. The challenging issues and outlook on the applications of surface-modified liquid metal particles are also discussed.

Small universal mechanical module driven by a liquid metal droplet.

Gallium-based liquid metal droplets (LMDs) from micro-electromechanical systems (MEMS) have gained much attention due to their precise and sensitive controllability under an electric field. Considerable research progress has been made in the field of actuators by taking advantage of the continuous electrowetting (CEW) present within the solution. However, the motion generated is confined within the specific liquid environment and is lacking a way to transmit its motion outwardly, which undoubtedly serves as the greatest obstacle restricting any further development. Therefore, a driving module is proposed to generate rotational motion outside the solution for universality. Its performance can be easily tuned by adjusting the applied voltage. As an example of further application, the module is designed in the form of a pump that realizes the continuous/intermittent propulsion to mimic the veins/arteries of the human body without the problem in the previous LMD-based pumps. The feasibility of this pump in the on-chip in vitro analysis is proved by preparing a dynamic cell culture to simulate the movement of biofluids within human bodies. This study proposes an optional solution with an LMD-based motor for generating rotational motion and to expand current research on soft materials in actuators.

Gallium-Based Liquid Metal Substrate Integrated Waveguide Switches

This letter presents a new approach for reconfiguring substrate integrated waveguide (SIW) devices. It involves allowing or prohibiting the wave propagation by using a removable wall which is built from a series of drill holes. When the wall is required, the holes are filled with gallium-based liquid metal (LM), thus forming vias. When the wall is no longer required, the holes are emptied of LM. The method has been applied to a single-pole single-throw (SPST) SIW switch as well as a single-pole double-throw (SPDT) switch. Both switches perform well over a wide frequency bandwidth, approaching one octave. The SPST switch performs well from 2.4 to 4.3 GHz. In the ON-state (i.e., wall removed), the insertion loss (IL) is ≤0.5 dB. In the OFF-state (i.e., wall inserted), the isolation is 30 dB. The SPDT switch operates effectively from 4.7 to 7.2 GHz. The IL, between two connected ports, is 0.7 dB. The isolation between unconnected ports is 40 dB. The proposed approach will be applicable within a wide range of different reconfigurable microwave devices.

All-in-One ENERGISER design: Smart liquid metal-air battery

Self-powered integration is an effective way of promoting efficiency and miniaturizing in portable intelligent electronic systems. Conventional battery design focuses on the improvement of charge–discharge property, and energy storage units usually design independently from energy consumption units including sensing or transmitting device. Here, an unusual battery design in which the batteries possess the function of energy storage, sensing, and signal transducer, called all-in-one ENERGISER, is integrated for the first time, which is realized by fabricating a gallium-based liquid metal-air battery (LMAB) without any other extra electronic components. Gallium-based liquid metals (LMs) wetted on the coppered-coated carbon fibers act as anode and air electrode acts as the cathode, the chemical activity of LMs and the sensitivity of gallium oxide layer are utilized for sensing function. For the discharge performance of LMABs, the energy density of LMABs changes from 1.52 to 0.79 mWh/cm2 while the corresponding power densities range from 0.36 to 2.05 mW/cm2. Additionally, the function of sensing environmental parameters achieves selectively detecting main components of air and distinguishing liquid type. Remarkably, the physical signals, such as humidity, is converted into low and high-potential signals, and further encoded into digital signals. Eventually, the all-in-one battery strategy is validated and shows excellent energy storage capacity, marvelous sensing function, and superb signal transmitting activity. The all-in-one battery design concept may open up a new direction for the exploitation of state-of-the-art functional and highly integrated battery.

Liquid metal motor.

SummaryLiquid metal has demonstrated an enormous potential for developing soft functional devices and machines. However, current liquid metal enabled machines suffer from several issues, such as the requirement of a liquid environment, generation of weak actuating forces, and insufficient maneuverability. To overcome these restrictions, here, a motor is developed based on the electrical actuation of liquid metal droplets without the need for conventional electromagnets. The approach is distinguished by (1) the encapsulation of electrolyte and multiple liquid metal droplets within an enclosed system, and (2) the creation of stable and continuous torque outside a liquid environment. In addition, a liquid metal electrical brush is introduced to operate the motor with low friction and low wear. The unique driving mechanism endows the motor with several advantages, including low friction, no sparking, low noise, versatile working environment, and being built from soft materials that could offer new opportunities for developing soft robotics.

Fabrication of a Flexible Photodetector Based on a Liquid Eutectic Gallium Indium.

A fluidic gallium-based liquid metal (LM) is an interesting material for producing flexible and stretchable electronics. A simple and reliable method developed to facilitate the fabrication of a photodetector based on an LM is presented. A large and thin conductive eutectic gallium indium (EGaIn) film can be fabricated with compressed EGaIn microdroplets. A solution of LM microdroplets can be synthesized by ultrasonication after mixing with EGaIn and ethanol and then dried on a PDMS substrate. In this study, a conductive LM film was obtained after pressing with another substrate. The film was sufficiently conductive and stretchable, and its electrical conductivity was 2.2 × 106 S/m. The thin film was patterned by a fiber laser marker, and the minimum line width of the pattern was approximately 20 μm. Using a sticky PDMS film, a Ga2O3 photo-responsive layer was exfoliated from the fabricated LM film. With the patterned LM electrode and the transparent photo-responsive film, a flexible photodetector was fabricated, which yielded photo-response-current ratios of 30.3%, 14.7%, and 16.1% under 254 nm ultraviolet, 365 nm ultraviolet, and visible light, respectively.

An In Situ Fabrication of CuGa2 Film on Copper Surface With Improved Tribological Properties

Abstract Interface interaction between gallium-based liquid metal and copper-based materials results in the formation of intermetallic CuGa2 grains. In particular, CuGa2 grains are able to produce a uniform film and the newly formed CuGa2 film holds peculiar characteristics, which have not been fully explored up to now. In this study, we present an electrochemical fabrication method of an in situ CuGa2 film on copper surface. Surface morphology and chemical composition of this film are confirmed. Tribological experiments demonstrate that the CuGa2 film enables good antifriction and antiwear abilities. Furthermore, the lubrication mechanisms of the CuGa2 film are revealed.

Oxidation of Gallium-based Liquid Metal Alloys by Water.

Gallium alloys with other low melting point metals, such as indium or tin, to form room-temperature liquid eutectic systems. The gallium in the alloys rapidly forms a thin surface oxide when exposed to ambient oxygen. This surface oxide has been previously exploited for self-stabilization of liquid metal nanoparticles, retention of metastable shapes, and imparting stimuli-responsive behavior to the alloy surface. In this work, we study the effect of water as an oxidant and its role in defining the alloy surface chemistry. We identify several pathways that can lead to the formation of gallium oxide hydroxide (GaOOH) crystallites, which may be undesirable in many applications. Furthermore, we find that some crystallite formation pathways can be reinforced by typical top-down particle synthesis techniques like sonication. This improved understanding of interfacial interactions provides critical insight for process design and implementation of advanced devices that utilize the unique coupling of flexibility and conductivity offered by these gallium-based liquid metal alloys.

Controlled Assembly of Liquid Metal Inclusions as a General Approach for Multifunctional Composites.

Soft composites that use droplets of gallium-based liquid metal (LM) as the dispersion phase have the potential for transformative impact in multifunctional material engineering. However, it is unclear whether percolation pathways of LM can support high electrical conductivity in a wide range of matrix materials. This issue is addressed through an approach to LM composite synthesis that focuses on the interrelated effects of matrix curing/solidification and droplet formation. The combined influence of LM concentration, particle size, and sedimentation is explored. By developing this approach, the functionalities that have been demonstrated with LM composites can be generalized to other matrix materials that impart additional functionality. Specifically, composites are synthesized using a biodegradable/reprocessable plastic (polycaprolactone), a hydrogel (poly(vinyl alcohol)), and a processable rubber (a styrene-ethylene-butylene-styrene derivative) to demonstrate wide applicability. This method enables synthesis of composites: i) with high stretchability and negligible electromechanical coupling (>600% strain); ii) with Joule-heated healing and reprocessability; iii) with electrical and mechanical self-healing; and iv) that can be printed. This approach to controlled assembly represents a widely applicable technique for creating new classes of LM composites with unprecedented multifunctionality.

Resistive Switching Observation in a Gallium-Based Liquid Metal/Graphene Junction

Resistive switching effect is observed for a gallium–indium/gallium oxide/graphene junction. The use of a gallium-based liquid metal (LM) alloy, in this case, the eutectic gallium–indium with its n...

Corrosion-Resistant Functional Diamond Coatings for Reliable Interfacing of Liquid Metals with Solid Metals.

Gallium-based liquid metals (GLMs) exist as atypical liquid-phase metals at and near room temperature while being electrically and thermally conductive, enabling copious applications in soft electronics and thermal management systems. Yet, solid metals are affected by interfacing with GLMs, resulting in liquid metal embrittlement and device failure. To avert this issue, mechanically durable and electrically tunable diffusion barriers for long-term reliable liquid metal-solid metal interfacing based on the deposition of various diamond coatings are designed and synthesized, as they feature high chemical inertness and extraordinary mechanical resistance. The diamond coatings show superlyophobicity (GLM contact angle ≥ 155°) and are nonstick toward GLMs, thereby achieving high mobility of GLM droplets (sliding angle 8-12°). The excellent barrier and anti-adhesion performance of the diamond coatings are proven in long-term experiments (3 weeks) of coated titanium alloy (Ti) samples in contact with GLMs. The electrical performance of the conductive diamond coating deposited on Ti is reliable and stable over a period of 50 h. As proof-of-concept applications a switch and a thermal management device based on liquid metals are demonstrated, signifying that coating diamond films on metals is a potent means to achieve stable integration of solid metals with GLMs.

Magnetic Field-Induced Recoverable Dynamic Morphological Change of Gallium-Based Liquid Metal

We report magnetic field-driven dynamic morphological change of a liquid metal droplet: deformation and recovery; separation and merge. Low melting, non-toxic gallium-based liquid metal has been widely investigated due to its material properties and thus utilized to various applications. Morphological change of the liquid metal in required to tune characteristics of application. In order to enable the liquid metal to be non-wettable and magnetically controllable, we coated the surface with ferromagnetic iron (Fe) particles using hydrochloric acid (HCl) solution resulting in a magnetic liquid metal marble. With and without magnetic field, Fe particles-coated magnetic liquid metal marble was deformed and recovered readily. Against $\sim 8~\mu \text{L}$ magnetic liquid metal marble, depending on the coated Fe particles mass, the shape deformation under magnetic field was changed: conventional (Fe $\geqq ~4$ mg) wetting shape. Accordingly, the deformability and recovery time of the magnetic liquid metal marble were varied. With an applied magnetic field, one marble was readily separated into 2, 3, and 4 marbles. Due to re-oxidation of the magnetic liquid metal marble surface, we treated the marble with HCl solution and vapor. Split and merge occurred depending on treatment time of HCl solution or vapor. Finally, we demonstrated magnetic field-based dynamic behavior which is reversible. [2020-0218]