This review contains a comparative study of reported fabrication techniques of gallium based liquid metal alloys embedded in elastomers such as polydimethylsiloxane or other rubbers as well as the primary challenges associated with their use. The eutectic gallium–indium binary alloy (EGaIn) and gallium–indium–tin ternary alloy (galinstan) are the most common non-toxic liquid metals in use today. Due to their deformability, non-toxicity and superior electrical conductivity, these alloys have become very popular among researchers for flexible and reconfigurable electronics applications. All the available manufacturing techniques have been grouped into four major classes. Among them, casting by needle injection is the most widely used technique as it is capable of producing features as small as 150 nm width by high-pressure infiltration. One particular fabrication challenge with gallium based liquid metals is that an oxide skin is rapidly formed on the entire exposed surface. This oxide skin increases wettability on many surfaces, which is excellent for keeping patterned metal in position, but is a drawback in applications like reconfigurable circuits, where the position of liquid metal needs to be altered and controlled accurately. The major challenges involved in many applications of liquid metal alloys have also been discussed thoroughly in this article.

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Fabrication methods and applications of microstructured gallium based liquid metal alloys

The galvanic replacement reaction is a highly versatile approach for the creation of a variety of nanostructured materials. However, the majority of reports are limited to the replacement of metallic nanoparticles or metal surfaces. Here we extend this elegant approach and describe the galvanic replacement of the liquid metal alloy galinstan with Ag and Au. This is achieved at a macrosized droplet to create a liquid metal marble that comprises a liquid metal core and a solid metal shell, whereby the morphology of the outer shell is determined by the concentration of metallic ions used in the solution during the galvanic replacement process. In principle, this allows one to recover precious metal ions from solution in their metallic form, which are immobilized on the liquid metal and therefore easy to recover. The reaction is also undertaken at liquid metal microdroplets created via sonication to produce Ag- and Au-based galinstan nanorice particles. These materials are characterized with SEM, XRD, TEM, SA...

Galvanic Replacement of the Liquid Metal Galinstan

Thermal management is a key design aspect of power converters since it determines their reliability as well as their final performance and power density. Cooling technologies have been a research area in electronics since the 1940s and, in the last 15 years, the number of articles related to this field has grown significantly. At present, thermal management is essential in new disciplines and it is a critical enabling technology in the development of power electronic systems. This paper aims at presenting a review of the state-of-the-art technology and provides future design guidelines for high efficiency power electronic converters. The main design trends are focused on the need to develop cooling systems able to manage high local density heat fluxes due to two converging trends: higher power dissipation and smaller module size. Considering the latest advances in thermal management, as well as the huge improvement in power electronics in the last decades, a review and classification of the main thermal management techniques is presented. Besides, they are compared considering important parameters such as peak power dissipation, efficiency, cost/complexity, power density or technical maturity, and a design example for an ultrahigh efficiency converter is presented.

Heat Management in Power Converters: From State of the Art to Future Ultrahigh Efficiency Systems

Liquid metal is a liquid-state metallic material with a low melting point at or around room temperature. Owing to their high thermal or electrical conductivity, low viscosity and superior fluidity, liquid metals are emerging as a highly desirable candidate in a broad array of areas, such as flexible electronics, thermal management, soft machines and biomedical materials. However, bulk liquid metal is not readily utilized because of its high surface tension and large dimension-induced dexterity limitations. To address this challenge, liquid metals have been innovated with micro/nanotechnology to endow the bulk liquid metals with remarkably diversified performances. These new functional materials not only possess the softness of classical liquid metals but also have many outstanding properties, including great thermal conductivity, self-healing ability and stimuli-responsive deformability. Compared to rigid inorganic micro/nanoscale materials, soft liquid metal micro/nanoparticles demonstrate their unconventionally superior flexibility, compliance and tunability. This review is dedicated to summarize the state-of-the-art progress in fabricating methods, highlight unique features, and discuss applications of liquid metal micro/nanoparticles in biomedicine, soft electronics, thermal management and soft motors. Future outlooks, including both opportunities and scientific challenges of liquid metal micro/nanoparticles, are also presented here.

Transformable soft liquid metal micro/nanomaterials

Stretchable electronics and soft robotics have shown unsurpassed features, inheriting remarkable functions from stretchable and soft materials. Electrically conductive and mechanically stretchable materials based on composites have been widely studied for stretchable electronics as electrical conductors using various combinations of materials. However, thermally tunable and stretchable materials, which have high potential in soft and stretchable thermal devices as interface or packaging materials, have not been sufficiently studied. Here, a mechanically stretchable and electrically insulating thermal elastomer composite is demonstrated, which can be easily processed for device fabrication. A liquid alloy is embedded as liquid droplet fillers in an elastomer matrix to achieve softness and stretchability. This new elastomer composite is expected useful to enhance thermal response or efficiency of soft and stretchable thermal devices or systems. The thermal elastomer composites demonstrate advantages such as thermal interface and packaging layers with thermal shrink films in transient and steady-state cases and a stretchable temperature sensor.

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Mechanically Stretchable and Electrically Insulating Thermal Elastomer Composite by Liquid Alloy Droplet Embedment

Galinstan, a gallium, indium, and tin eutectic, may be exploited for enhanced cooling of microelectronics because of its favorable thermophysical properties. A careful evaluation of its cooling potential, however, has not been undertaken. Provided here is a first-order model to compute the total (i.e., caloric plus conjugate conduction and convection) thermal resistance of galinstan-based heat sinks. Geometrically optimized minichannel heat sinks with rectangular channels for surface area enhancement and minigap, i.e., single parallel-plate channel, heat sinks are considered. Direct liquid cooling of a microprocessor die is envisioned. Therefore, the flow channels within the heat sinks are 302- μm tall, the pressure drop prescribed across them is 214 kPa, and their streamwise length is varied from 5 to 20 mm. The calculations suggest that galinstan is a better coolant than water in such configurations, reducing thermal resistance by about 40%.

On the Potential of Galinstan-Based Minichannel and Minigap Cooling

Microchannel heat sinks are a relevant thermal management technology because the combination of surface area enhancement and small length scales results in low wall-to-bulk temperature differences. Previously, a thermal resistance of 0.09 °C/W was achieved when a heat flux of 790 W/cm $^{2}$ was imposed on a 1 cm $\times 1$ cm footprint portion of a 400- $\mu \text{m}$ -thick Si substrate utilizing single-phase water-based microchannel cooling and a 214 kPa pressure difference to drive the flow. Galinstan, a gallium, indium, and tin eutectic, may be utilized for single-phase liquid metal cooling of microelectronics due to its subambient melting temperature and high thermal conductivity. This paper describes the fabrication and assembly of water-based microchannel and Galinstan-based minichannel heat sinks and the flow sheets utilized to characterize them under the aforementioned constraints. The prefix mini rather than micro is used to describe Galinstan-based heat sinks because optimal channel widths are hundreds as opposed to tens of micrometers. The aforementioned thermal resistance of 0.09 °C was experimentally reproduced. Unprecedentedly low thermal resistance and high heat flux in single-phase water-based microchannel cooling, i.e., 0.071 °C/W and 1003 W/cm $^{2}$ , respectively, were achieved. The first experimental data on Galinstan-based minichannel heat sinks are also reported. A thermal resistance as low as 0.077 °C/W was achieved at a heat flux of 1214 W/cm $^{2}$ and a maximum heat flux of 1504 W/cm $^{2}$ was reached.

Water-Based Microchannel and Galinstan-Based Minichannel Cooling Beyond 1 kW/cm $^{2}$ Heat Flux

Galinstan, a gallium, indium and tin eutectic, may be exploited for enhanced cooling of microelectronics due to its unique thermophysical properties. However, a critical evaluation of the cooling potential of galinstan has not been undertaken. Provided here is a first-order optimization model to minimize the total thermal resistance of galinstan-based heat sinks under constraints on form factor and pressure drop. The geometry considered is a minichannel heat sink that includes millimeter-scale rectangular channels to provide surface area enhancement. Calculations, which take into account both caloric and conduction/convection thermal resistances, suggest that galinstan is a better coolant than water in such configurations, reducing thermal resistance by about 30%.

Cooling potential of galinstan-based minichannel heat sinks

Liquid metals, such as Galinstan, a gallium, indium and tin eutectic, may be exploited for enhanced cooling of microelectronics due to their favorable thermophysical properties. Hodes et al. [1] discussed the properties of Galinstan compared with those of water and provided a first-order model for minimizing the total thermal resistance of Galinstan-based heat sinks under form factor and pressure drop constraints. This paper, as a follow up, describes the fabrication and assembly of water-based microchannel and Galinstan-based minichannel heat sinks.1 The authors also discussed the flow loop has been designed and commissioned to characterize the heat sinks and report flow and thermal resistance data and compare them to theory.

Thermo-fluid characteristics of a minichannel heat sink cooled with liquid metal

Robust precision temperature control of photonics components is achieved by mounting them on thermoelectric modules (TEMs) which are in turn mounted on heat sinks. However, the power consumption of TEMs is high because high currents are driven through Bi2 Te3 -based semiconducting materials with high electrical resistivity and finite thermal conductivity. This problem is exacerbated when the ambient temperature surrounding a TEM varies in the usual configuration where the air-cooled heat sink a TEM is mounted to is of specified thermal resistance. Indeed, heat sinks of negligible and relatively high thermal resistances minimize TEM power consumption for sufficiently high and low ambient temperatures, respectively. Optimized TEM-heat sink assemblies reduce the severity of this problem. In the problem considered, total footprint of thermoelectric material in a TEM, thermoelectric material properties, heat load, component operating temperature, relevant component-side thermal resistances and ambient temperature range are prescribed. Provided is an algorithm to compute the unique combination of the height of the pellets in a TEM and the thermal resistance of the heat sink attached to it which minimizes the maximum power consumption of the TEM over the specified ambient temperature range. This optimization maximizes the fraction of the power budget in an optoelectronics circuit pack available for other uses. Implementation of the algorithm is demonstrated through an example for a typical set of conditions.© 2011 ASME

Rui Zhang

Papers

On the Potential of Galinstan-Based Minichannel and Minigap Cooling

Water-Based Microchannel and Galinstan-Based Minichannel Cooling Beyond 1 kW/cm $^{2}$ Heat Flux

Cooling potential of galinstan-based minichannel heat sinks

Thermo-fluid characteristics of a minichannel heat sink cooled with liquid metal

Optimized Thermoelectric Module-Heat Sink Assemblies for Precision Temperature Control