Gallium-based Liquid Metal Research Articles

All Solution-Processed Inorganic, Multilevel Memristors Utilizing Liquid Metals Electrodes Suitable for Analog Computing.

Herein, we report a solution-processable memristive device based on bismuth vanadate (BiVO4) and titanium dioxide (TiO2) with gallium-based eutectic gallium-indium (EGaIn) and gallium-indium-tin alloy (GaInSn) liquid metal as the top electrode. Scanning electron microscopy (SEM) shows the formation of a nonporous structure of BiVO4 and TiO2 for efficient resistive switching. Additionally, the gallium-based liquid metal (GLM)-contacted memristors exhibit stable memristor behavior over a wide temperature range from -10 to +90 °C. Gallium atoms in the liquid metal play an important role in the conductive filament formation as well as the device's operation stability as elucidated by I-V characteristics. The synaptic behavior of the GLM-memristors was characterized, with excellent long-term potentiation (LTP) and long-term depression (LTD) linearity. Using the performance of our device in a multilayer perceptron (MLP) network, a ∼90% accuracy in the handwriting recognition of modified national institute of standards and technology database (MNIST) was achieved. Our findings pave a path for solution-processed/GLM-based memristors which can be used in neuromorphic applications on flexible substrates in a harsh environment.

Zwitterionic microgel-functionalized gallium-based liquid-metal nanodroplets as aqueous lubricant additives

Herein, a bioinspired molecule-dopamine n-butenylamide-was self-assembled on Gallium-based liquid metal (GLM) nanodroplets to introduce vinyl group, and the zwitterionic poly (2-methylacryloxyethyl phosphocholine) (PMPC) was grafted onto GLM nanodroplets via surface-initiated free radical polymerization to obtain hybrid PMPC-GLM microgel. The as-prepared PMPC-GLM microgel has good dispersion stability in aqueous solution (chemical stability > 2 months), and shows excellent antibacterial properties due to the hydration of PMPC and antibacterial effect of Ga3+. Moreover, the hydration layer endows PMPC-GLM microgel with satisfactory tribological properties as an aqueous lubricant additive, with the COF and wear rate reduced by 65% and 62%. In addition to hydration lubrication, the improved tribological performance is attributed to the formation of protective film on the worn surface.

Wettability and thermal contact resistance of thermal interface material composited by gallium-based liquid metal on copper foam

As a promising thermal interface material (TIM), gallium-based liquid metal (GBLM) could significantly reduce the thermal contact resistance (TCR) at the solid-solid interfaces, while the leakage of GBLM and the wettability between liquid metal and solid surface restrict further application. In this study, the GBLM was wrapped with a copper foam (CF) skeleton, which prevents leakage and improves the thermal conductivity of the GBLM. Gallium-based liquid metal-copper foam (GBLM-CF) TIM was prepared by the self-wetting of the CF surface through the galvanic corrosion of GBLM. As the concentration of HCl increases from 0 to 5 M, the interfacial tension (IFT) in the HCl solution decreases from 536.80±1.00 mN/m to 481.52±0.49 mN/m, demonstrating that the IFT of the GBLM is inversely proportional to the concentration of HCl. Meanwhile, the decrease of contact angle (CA) by 8° indicates that the presence of CuGa2 on the surface promotes the wettability of GBLM. With the increase of external pressure and the improvement of wettability, the TCR between the GBLM and the copper sheet decreased by 58.28%. The TCR of the sample using GBLM-CF as TIM is 83% lower than that of thermal grease. Moreover, the thermal conductivity of GBLM-CF TIM (51.58±0.01 W/(m·K)) is 212.6% higher than that of GBLM (16.50±0.02 W/(mK)). The GBLM-CF TIM presented in this study can be used for the efficient thermal management of electronic equipment.

Trapping mechanism of metastable β-Ga disclosed by its lattice stability optimization and nucleation behavior exploration

Application of gallium-based liquid metals below 273 K calls for design of novel metastable materials with exceptionally low melting points. It is crucial to develop the thermodynamic database of metastable phases and to unveil their trapping mechanism. In this work, the thermodynamic assessment of metastable β-Ga and its lattice stability were evaluated by integrating experimental and ab-initio thermodynamics. We demonstrated that the lattice stability expression derived from a high-order modified Einstein modeling with anharmonic correction provided the most reliable and the most accurate description hitherto. Thereafter, the critical radius and the work for nucleation were derived by applying our optimized expressions of lattice stability and surface energy. Thereby the capturing of β-Ga at nano sizes and high supercooling were well explained. And it was confirmed that the classical nucleation theory played an important role in trapping metastable β-Ga phase. Furthermore, the significance of surfaces and interfaces in solidification process were highlighted. This work sheds light on designing metastable liquid metals with low melting points. Moreover, it provides a novel workflow for future studying of metastable multicomponent alloys.

Self-healing liquid metal confined in carbon nanofibers/carbon nanotubes paper as a free-standing anode for flexible lithium-ion batteries

• Variation of Young's modulus of Ga in the process of discharge (lithiation) was measured. • Self-healing ability of EGaIn to automatic repair the cracks resulted from charge/discharge. • The CNF/CNT@EGaIn NPs anode is free-standing, flexible and binder-free. Gallium-based liquid metals are considered as potential anode materials for lithium-ion batteries owing to their self-healing, non-poisonous advantages, as well as high theoretical capacity. However, due to the alloying/dealloying reaction mechanism to store lithium, Gallium-based alloys face huge volume change resulted in poor cycle life. Furthermore, the poor wettability of liquid metals on many kinds of substrates (carbon materials, stainless steel, etc.) and their strong ability to form alloys with many metallic current collectors (Cu, Al foil) make them hard to be designed into electrode with excellent electrochemical performance. One objective of this paper is to investigate Young's modulus of Ga discharged to different cut-off potentials, in order to gain a further understanding of mechanical property attenuation of Ga in the process of lithiation. Moreover, a three-dimensional (3D) free-standing electrode is fabricated by confining Ga-based liquid metal, EGaIn, in the matrix of carbon nanofibers/carbon nanotubes paper (CNF/CNT@EGaIn NPs). The network structure provides effective pathways for electrons and ions, as well as enough space to contain the volume expansion of EGaIn. The dense CNT layer plays a further role on preventing EGaIn from shedding of the conductive substrates. Obtained CNF/CNT@EGaIn NPs exhibits a good ionic/electronic conductivity and mechanical stability, delivers capacity of 351 mAh g −1 at 1.6 A g −1 and remains a reversible capacity of ∼420 mAh g −1 after 100 cycles at 800 mA g −1 . This work could provide valuable insights into the development of Ga-based liquid metal anodes.

Fabrication of a Multilayer X-Band Band-Pass Metasurface Using Liquid Metal

Fabrication of a physically flexible multi-layer metasurface composed of gallium-based liquid metal encapsulated in polydimethylsiloxane (PDMS) is reported. The second-order bandpass frequency selective surface (FSS) operating in the X-band is comprised of two modified Jerusalem cross resonator layers separated by an aperture layer. The multi-layer spatial filter has a total thickness of 4.81 mm ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda _{0}$ </tex-math></inline-formula> /7.5). An inverse image mold of the FSS layer patterns is fabricated in silicon, and FSS patterned PDMS is replicated from the silicon mold. The FSS patterned PDMS is spray coated with liquid metal to fabricate a massive array of discrete conductor patterns in PDMS and subsequent encapsulation by an additional layer of PDMS. Multiple versions of the m <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> n metasurfaces have been fabricated and have been repeatedly tested for their bendability, stretchability, twistability, and foldability. The fabricated metasurface in a waveguide environment demonstrated a band-pass center frequency at 8.33 GHz which agreed well with the simulated frequency at 8.31 GHz.

Gallium-Based Liquid Metal Materials for Antimicrobial Applications.

The hazards caused by drug-resistant bacteria are rocketing along with the indiscriminate use of antibiotics. The development of new non-antibiotic antibacterial drugs is urgent. The excellent biocompatibility and diverse multifunctionalities of liquid metal have stimulated the studies of antibacterial application. Several gallium-based antimicrobial agents have been developed based on the mechanism that gallium (a type of liquid metal) ions disorder the normal metabolism of iron ions. Other emerging strategies, such as physical sterilization by directly using LM microparticles to destroy the biofilm of bacteria or thermal destruction via infrared laser irradiation, are gaining increasing attention. Different from traditional antibacterial agents of gallium compounds, the pronounced property of gallium-based liquid metal materials would bring innovation to the antibacterial field. Here, LM-based antimicrobial mechanisms, including iron metabolism disorder, production of reactive oxygen species, thermal injury, and mechanical destruction, are highlighted. Antimicrobial applications of LM-based materials are summarized and divided into five categories, including liquid metal motors, antibacterial fabrics, magnetic field-responsive microparticles, liquid metal films, and liquid metal polymer composites. In addition, future opportunities and challenges towards the development and application of LM-based antimicrobial materials are presented.

Liquid metal embrittlement to boost reactivity and combustion performance of Al in composite propellants

Aluminum (Al) powder has been widely used in solid composite propellants (SCPs) due to its high energy density and appropriate combustion kinetics. However, the presence of a passivation shell (Al2O3) on the surface of Al would reduce its reactivity and heat release. Herein, a facile ‘one-step’ modification method was proposed for the fabrication of micron-sized Al substitutes in SCP, aiming to induce the involvement of Al cores in the reaction by letting the liquid metal embrittlement (LME) damage the homogeneous Al2O3 shell. Oxidation weight gain experiments in the air atmosphere were conducted to investigate the effect of liquid metal content on the breakthrough of the Al2O3 shell to improve ignition and combustion performance. Moreover, the composites with gallium-based liquid metal (Galinstan) addition of 3 wt% exhibited the largest oxidation weight gain and oxidation weight gain rates, which were approximately 6 and 8-fold higher than that of the raw-Al particles, respectively. Furthermore, the resulting Galinstan-modified aluminum-based powder (GLM-Al) was used as a metal fuel for the formulation of SCP to validate its thermal and combustion properties. According to the DSC results, the heat release of SCP-2 containing GLM-Al was 3321 J·g−1, which is 1.42 folds of that from the reference SCP-1(2341 J∙g−1). Furthermore, SCP-2 exhibited a 12 % increase in burning rate under 0.1 MPa air atmosphere and a much higher pressurization rate (5.4 MPa·s−1) than that of SCP-1 (2.6 MPa·s−1) under 1 MPa argon atmosphere. The resulting condensed combustion product (CCP) size of SCP-2 is much smaller than that of SCP-1.

Fabrication of a Flexible Strain Sensor with High‐Aspect‐Ratio Liquid‐Metal Galinstan

AbstractFlexible electronic devices containing gallium‐based liquid metal (GbLM) play an important role in human–computer interaction and the sensing elements of motion monitoring owing to the intrinsic fluidity and metallic conductivity of the GbLM. High‐aspect‐ratio liquid‐metal patterns have 3D structures, which can increase the heat radiating area of the circuits and the stability of the sensors. However, fabricating high‐aspect‐ratio patterns in high resolution is challenging owing to easy oxidation at the surface layer and the high surface tension of the liquid metal. This study proposes a simple method for the efficient and reproducible fabrication of GbLM patterns. A flexible strain sensor was obtained with high‐aspect‐ratio Galinstan (height/width &gt; 5), linearity (R‐square = 0.9975), stability (ΔR/R0 &lt; 0.8 after 10 000 cycles), and low hysteresis. In addition, the application of the strain sensor in the flexible display and motion measurement of soft hand exoskeletons is demonstrated. The high‐aspect‐ratio liquid‐metal patterns in this work have the potential to extend the application of the presented flexible electronics to the fields of wearable systems and rehabilitation devices.

Improving the heat conduction and mechanical properties of thermal interface materials by constructing diphase continuous structure reinforced by liquid metal

Based on the existence of self-gap and bridging effect during powder compaction, chromium-coated diamond particles and gallium-based liquid metal are selected to prepare an inter-connective porous structure, and then polydimethylsiloxane is filled into this porous medium by vacuum infiltration to fabricate co-continuous thermal pads. The results show that the reinforcing phase of diamond particles bridged by liquid metal is responsible for ensuring heat-transfer performance, while the polymer matrix imparts a certain degree of softness, elasticity, and strength. The thermal conductivity of composites with great conformability can go as high as 29 W/(m·K). Furthermore, once thermal pads are applied on substrates under pressure, a small amount of liquid metal will seep out from the composite surfaces, which is beneficial to obtaining a low interface thermal resistance. This work provides a novel approach for fabricating high-performance composites which may further advance their applications in thermal management of electronic industry.

Unique Surface Fluorescence Induced from the Core-Shell Structure of Gallium-Based Liquid Metals Prepared by Thermal Oxidation Processing.

Liquid metals (LMs) have emerged as promising functional materials that combine the properties of both liquid and metal. These characteristics enabled them to find applications in many fields. However, the LMs usually can only display a silver-white physical appearance, which limits their further applications in the fields with the imposition of stringent requirements for color and aesthetics. Herein, we report that the surface of LMs was transformed directly from metal to fluorescent semiconductor layer by an example of eutectic GaInSn (eGaInSn) induced by thermal oxidation. Specifically, a core-shell structure is formed from the fluorescent layer and the LMs. The shell endows the LMs with fluorescence without affecting their interior fluidity and conductivity. In particular, the formation process as well as the degree of crystallization, phase transformation, and light emission of the fluorescent oxide shell on the surface of LMs is regulated by the component content. A thorough analysis of surface morphology, composition, structure, and properties of the fluorescent shell suggests that the Ga2O3 layer is formed on the surface of gallium-based LMs after their immersion in deionized water. Subsequently, thermal oxidation results in the formation of the β-Ga2O3 shell on the surface of liquid metals. Importantly, abundant oxygen vacancies (VO) in β-Ga2O3 as the donors and the gallium vacancies (VGa), gallium-oxygen vacancy pairs (VO-VGa), defect energy levels, and intrinsic defects as the acceptors enabled the light emission. The fluorescent LMs have promising potential for flexible lighting and displays, anticounterfeiting measures, sensing, and chameleon robots.

Imbibition-induced selective wetting of liquid metal

Herein, we present the imbibition-induced, spontaneous, and selective wetting characteristics of gallium-based liquid metal alloys on a metallized surface with micro-scale topographical features. Gallium-based liquid metal alloys are fascinating materials that have enormous surface tension; therefore, they are difficult to pattern into films. The complete wetting of eutectic alloy of gallium and indium is realized on microstructured copper surfaces in the presence of HCl vapor, which removes the native oxide from the liquid metal alloy. This wetting is numerically explained based on the Wenzel’s model and imbibition process, revealing that the dimensions of the microstructures are critical for effective imbibition-driven wetting of the liquid metal. Further, we demonstrate that the spontaneous wetting of the liquid metal can be directed selectively along the microstructured region on the metallic surface to create patterns. This simple process enables the uniform coating and patterning of the liquid metal over large areas without an external force or complex processing. We demonstrate that the liquid metal-patterned substrates maintain electrical connection even in a stretched state and after repetitive stretching cycles.

Direct Current (DC) Electric Field-Enabled Beating Heart of Ga-Based Liquid Metal

Mercury beating heart (MBH) is a well-known demonstration of redox processes. However, the development and application of the MBH system are significantly restricted due to its safety issues. Here, a direct current electric field-enabled gallium-based liquid metal heartbeat in a salt solution or alkaline solution is introduced as a demonstration appropriate for undergraduate general chemistry. The former oscillation occurs on the cathode, the latter on the anode. The objective is to achieve a visually appealing effect in the classroom and motivate students to understand the underlying concepts. Students are encouraged to assemble the instrument and perform the demonstration under supervision. Apart from the gallium-based liquid metal and a helping-hand stand, the demonstration utilizes common laboratory apparatus, and the materials for the solutions can be obtained in an undergraduate general chemistry laboratory.

Towards an artificial peripheral nerve: Liquid metal-based fluidic cuff electrodes for long-term nerve stimulation and recording

Nerve cuff electrodes have been used for decades as peripheral nerve interfacing devices in the fields of neural science, neural disease, and brain-machine interfacing. The currently-used cuff electrode is commonly based on rigid materials whose flexibility and tensile properties are far different from those of biological nervous tissue. Herein, a fluidic cuff electrode using a gallium-based liquid metal (LM) conductor is developed as a prototype artificial peripheral nerve. The results indicate that the LM cuff electrode has high flexibility and maintains outstanding conductivity. After implanted and connected to the sciatic nerve, the LM electrodes within the freely moving rats' bodies survive repeated body stretching and retain their long-term effectiveness in transmitting sciatic nerve signals with a high signal-to-noise ratio during two-week experiments. The LM electrodes are also proven capable of transmitting neural stimuli to the peripheral nerve on a long-term basis by triggering clear event-related potentials (ERPs) in terms of both the cortical potential and sciatic signal. These tests demonstrate that the LM electrodes meet the requirements of peripheral nerve signal recording and stimulation for long-term implantation, and have the potential to become a new generation of artificial peripheral nerve devices to interface with, supplement, or even enhance and replace the real peripheral nerve.

Extreme pressure lubrication between ceramic and bearing steel with liquid metal

The behaviors of extreme pressure (EP) lubrication for tribo-pairs between silicon nitride (Si3N4) and bearing steel (GCr15) with the lubricants of gallium-based liquid metal (GBLM) and Gear Oil (GO) have been studied on the four-ball test (FBT) machine. The experimental results demonstrated that temperature with GBLM directly measured at the bottom of FBT liquid pool was linearly proportional to the applied load and rotating speed, which was consistent with the variation trend of interfacial temperature simulated by COMSOL. These experimental and simulated results have identified that GBLM lubricant could significantly reduce interfacial temperature and was more suitable for EP lubrication due to its high thermal conductivity than that of GO. Moreover, some experimental results showed that the wear scar diameter with GBLM was smaller than that with GO under the same EP working condition. It has been proved that different materials in Si3N4-GCr15 tribo-pairs did not weld together even in a hard EP working condition. This work also indicated that the new ideas for synthesizing materials were urgent as the tribo-pairs and EP lubricants with high thermal conductivity, which could improve the reliability and feasibility of the EP working condition in the industrial field.

Wetting behavior of gallium-based room temperature liquid metal (LM) on nanosecond-laser-structured metal surfaces

Gallium-based room-temperature liquid metal (LM) is a promising emerging functional material in e-skin, soft robots and thermal management systems for its fluidity, conductivity and nontoxicity. However, the intricate wettability of the LM with various surfaces impedes its further development. Knowledge on the gallium-based LM wetting properties especially with structured metal surfaces is relatively insufficient. Herein, wetting behavior of eutectic gallium-indium (EGaIn) on various micro-structured metal surfaces processed by nano-second laser ablation method is systematically studied. Increasing surface roughness, synergized with a high oxygen content, significantly strengthens the LM resistance. Micro-patterns composed of small, discontinuous facets exhibit better EGaIn repellence. Relative humidity (RH) is substantiated to influence the LM wetting behavior as well. Metal surfaces with excellent superlyophobicity (the contact angle> 160°, the adhesion force< 10 μN) are obtained. The LM repellence stability and anti-corrosion properties of the optimized treated metal surfaces are proved via the droplet impacting experiment and the corrosion test. These findings improve the understanding of gallium-based LM wetting behavior on structured metal surfaces, and the obtained LM-repellent metal surfaces will reduce undesired adhesion, blockage or LM corrosion during device preparation and long-term applications, which will facilitate its potential use.

A Liquid Metal-Enhanced Wearable Thermoelectric Generator.

It is a key challenge to continuously power personal wearable health monitoring systems. This paper reports a novel liquid metal-enhanced wearable thermoelectric generator (LM-WTEG that directly converts body heat into electricity for powering the wearable sensor system. The gallium-based liquid metal alloys with room-temperature melting point (24~30 °C) and high latent heat density (about 500 MJ/m3) are used to design a new flexible finned heat sink, which not only absorbs the heat through the solid-liquid phase change of the LM and enhances the heat release to the ambient air due to its high thermal conduction. The LM finned is integrated with WTEG to present high biaxial flexibility, which could be tightly in contact with the skin. The LM-WTEG could achieve a super high output power density of 275 μW/cm2 for the simulated heat source (37 °C) with the natural convective heat transfer condition. The energy management unit, the multi-parameter sensors (including temperature, humidity, and accelerometer), and Bluetooth module with a total energy consumption of about 65 μW are designed, which are fully powered from LM-WTEG through harvesting body heat.

A Supported Palladium on Gallium-based Liquid Metal Catalyst for Enhanced Oxygen Reduction Reaction

A Supported Palladium on Gallium-based Liquid Metal Catalyst for Enhanced Oxygen Reduction Reaction

Chemical Analysis of the Gallium Surface in a Physiologic Buffer

Gallium and its alloys have been regarded as one of the promising materials for flexible bioelectronics due to their liquid-like mechanical properties, excellent electrical property, and low toxicity. Although many studies have fabricated bioelectronics from gallium-based liquid metals, gallium surface chemistry in physiologic conditions is rarely investigated. Here, we investigated the chemical change of the gallium surface in a physiologic buffer at 37 °C over 45 days. The gallium ion concentration and pH measurement indicated that the oxidation and corrosion progressed more rapidly in the physiological buffer than in air. Also, the release of gallium ions and protons followed a square root of time growth. Various spectroscopic techniques were used to measure the chemical composition change on the gallium surface. The FT-IR study indicated that the GaOOH-rich gallium surface produced Ga3+ and OH- ions. The XPS study indicated the oxide layer formation within 5 days, and then the contamination layer was deposited over time, which includes different ions and organic materials derived from the physiologic buffer. This study provides a detailed chemical analysis of the gallium surface in a physiological buffer. These fundamental studies would be a cornerstone for understanding the complex interaction between the gallium surface and the biological environment.

A Universal Mechanochemistry Allows On-Demand Synthesis of Stable and Processable Liquid Metal Composites.

Gallium-based liquid metal (LM) is regarded as one of the most promising candidates for the new-generation jigsaw of stretchable electronics. Nonetheless, the obstacle for the LM application lies in its high surface tension and easy fluidity which leads to great difficulty in handling and processing. Herein, a cross-mechanochemistry between liquid metal and inorganic solid, mediated via the coordination binding between the empty electronic orbits of the former and the lone electron pair of the latter is reported. The mechanism is validated via density functional theory calculation and electron energy loss spectroscopy, and experimentally proven to be universally applicable for various liquid metals and inorganic solids. With the unique mechanochemistry, simple ball milling allows on-demand transformation of the liquid metal into a low-surface-tension liquid, semi-solid paste, or even solid powder. The overcoming of the intrinsic high surface tension of the liquid metal with this approach unleashes the freedom to easily process the liquid metal composites into polymer composites or as direct molding processable paste and printable electronic ink.