Gallium and several of its alloys are liquid metals at or near room temperature. Gallium has low toxicity, essentially no vapor pressure, and a low viscosity. Despite these desirable properties, applications calling for liquid metal often use toxic mercury because gallium forms a thin oxide layer on its surface. The oxide interferes with electrochemical measurements, alters the physicochemical properties of the surface, and changes the fluid dynamic behavior of the metal in a way that has, until recently, been considered a nuisance. Here, we show that this solid oxide “skin” enables many new applications for liquid metals including soft electrodes and sensors, functional microcomponents for microfluidic devices, self-healing circuits, shape-reconfigurable conductors, and stretchable antennas, wires, and interconnects.

Emerging Applications of Liquid Metals Featuring Surface Oxides

High temperature power electronics has become possible with the recent availability of silicon carbide devices. This material, as other wide-bandgap semiconductors, can operate at temperatures above 500 °C, whereas silicon is limited to 150-200 °C. Applications such as transportation or a deep oil and gas wells drilling can benefit. A few converters operating above 200 °C have been demonstrated, but work is still ongoing to design and build a power system able to operate in harsh environment (high temperature and deep thermal cycling).

/pdf/state-of-the-art-of-high-temperature-power-electronics-3qyx36fnso.pdf

State of the art of high temperature power electronics

This highlight describes emerging methods to pattern metals that are liquid at room temperature. The ability to pattern liquid metals is important for fabricating metallic components that are soft, stretchable, conformal, and in some cases, shape-reconfigurable. Applications include electrodes, antennas, micro-mirrors, plasmonic structures, sensors, switches, and interconnects. Gallium (Ga) and its liquid metal alloys are attractive alternatives to toxic mercury. This family of alloys spontaneously forms a surface oxide that dominates the rheological and wetting properties of the metal. These properties pose challenges using conventional fabrication methods, but present new opportunities for patterning innovations. For example, Ga-based liquid metals may be injected, imprinted, or 3D printed on either soft or hard substrates. The use of a liquid metal also enables rapid and facile room temperature processing. The patterning techniques organize into four categories: (i) patterning enabled by lithography, (ii) injection, (iii) subtractive techniques, and (iv) additive techniques. Although many of these approaches take advantage of the surface oxide that forms on Ga and its alloys, some of the approaches may also be suitable for patterning other soft-conductors (e.g., conductive inks, pastes, elastomeric composites).

Methods to pattern liquid metals

Gallium-based liquid metals are of interest for a variety of applications including flexible electronics, soft robotics, and biomedical devices. Still, nano- to microscale device fabrication with these materials is challenging because, despite having surface tension 10 times higher than water, they strongly adhere to a majority of substrates. This unusually high adhesion is attributed to the formation of a thin oxide shell; however, its role in the adhesion process has not yet been established. In this work, we demonstrate that, dependent on dynamics of formation and resulting morphology of the liquid metal–substrate interface, GaInSn adhesion can occur in two modes. The first mode occurs when the oxide shell is not ruptured as it makes contact with the substrate. Because of the nanoscale topology of the oxide surface, this mode results in minimal adhesion between the liquid metal and most solids, regardless of substrate’s surface energy or texture. In the second mode, the formation of the GaInSn–substrat...

Different shades of oxide: from nanoscale wetting mechanisms to contact printing of gallium-based liquid metals.

The current state of wide bandgap device technology is reviewed and its impact on power electronic system miniaturization for a wide variety of voltage levels is described. A synopsis of recent complementary technological developments in passives, integrated driver, and protection circuitry and electronic packaging are described, followed by an outline of the applications that stand to be impacted. A glimpse into the future based on the current technological trends is offered.

Wide Bandgap Technologies and Their Implications on Miniaturizing Power Electronic Systems

A number of emerging applications of low-temperature co-fired ceramic (LTCC) require embedded fluidic structure within the co-fired ceramic and or precise external dimensional tolerances. These structures enable the control of fluids for cooling, sensing, and biomedical applications, and variations in their geometry from the design can have a significant impact on the overall performance of the devices. One example of this type of application is a multilayer cooler developed recently by the authors for cooling laser diode bars. In many laser systems, laser diodes are the primary emitters, or assemblies of these diode bars are used to pump traditional laser crystals such as Nd:YLF. Assemblies of these diodes require large amounts of electrical current for proper operation, and the device operating temperature must be carefully controlled in order to avoid a shift in the output wavelength. These diodes are packaged into water-cooled assemblies and by their nature dissipate enormous amounts of heat, with waste heat fluxes on the order of 2000 W/cm2. The traditional solution to this problem has been the development of copper multilayer coolers. Assemblies of laser diodes are then formed by stacking these diode bars and coolers. Several problems exist with this approach including the erosion of the copper coolers by the coolant, a requirement for the use of deionized water within the system, and a significant CTE mismatch between the diode bar and the metal cooler. Diodes are bonded to these metal structures and liquid coolant is circulated through the metal layers in order to cool the diode bar. In contrast, the coolers developed by the authors utilize fluid channels and jets formed within LTCC as well as embedded cavity structures to control the flow of a high-velocity liquid and actively cool the laser diode bars mounted on the surface of the LTCC.† The dimensional tolerances of these cooler assemblies and complex shapes that are used to control the fluid can have a significant impact on the overall performance of the laser system. This paper describes the fabrication process used to create the precise channel and jet structures used in these LTCC-based coolers, as well as some of the challenges associated with these processes.

Fabrication of Precise Fluidic Structures in LTCC

This paper reports on the measured dc characteristics of a SiC JFET device from room temperature up to 450°C in order to evaluate the device's capability for high-temperature operation. The authors packaged SiC JFET bare die into a dedicated high-temperature package to be able to perform experiments under extremely high ambient temperatures. The experimental results show that the device can operate at 450°C, which is impossible for conventional Si devices, but the current capability of the SiC JFET diminishes with rising temperatures. For example, the saturation current becomes 20% at 450°C with respect to the value at the room temperature.

/pdf/sic-jfet-dc-characteristics-under-extremely-high-ambient-4ihhj1w34q.pdf

SiC JFET dc characteristics under extremely high ambient temperatures

A next-generation microchannel cooler has been developed for packaging laser diode arrays th at eliminates many of the problems associated with typical copper-based cooling designs.  The coolers are built on well-established Low-Temperature Cofired Ceramic technology and provide excellent thermal performance. They do not require the use of deionized water. This work highlights the strengths of the new cooler technology. The results of a long-term, high-flow-rate test which demonstrates the excellent erosion resistance of these coolers are presented. Three devices have been tested for 2500 hours at a flow rate of 0.25 GPM and show minimal signs of erosion. This data is compared to a similar test conducted with copper coolers. Several design parameters are also addressed for the ceramic coolers. The available form and fit characteristics are addressed, as is the custom-configurable nature of the devices. Keywords: laser diode, microchannel, cooling, diode array, LTCC, ceramic, deionized water, erosion

Next-generation microchannel coolers

We report on the development of a novel, ultra-low thermal resistance active heat sink (AHS) for thermal management of high-power laser diodes (HPLD) and other electronic and photonic components. AHS uses a liquid metal coolant flowing at high speed in a miniature closed and sealed loop. The liquid metal coolant receives waste heat from an HPLD at high flux and transfers it at much reduced flux to environment, primary coolant fluid, heat pipe, or structure. Liquid metal flow is maintained electromagnetically without any moving parts. Velocity of liquid metal flow can be controlled electronically, thus allowing for temperature control of HPLD wavelength. This feature also enables operation at a stable wavelength over a broad range of ambient conditions. Results from testing an HPLD cooled by AHS are presented.

Liquid metal heat sink for high-power laser diodes

This paper reports on SiC devices operating in a dc-dc buck converter under extremely high ambient temperatures. To this end, the authors packaged SiC JFET and Schottky diodes in thermally stable packages and built a high-temperature inductor. The converter was tested at ambient temperatures up to 400°C. Although the conduction loss of the SiC JFET increases slightly with increasing temperatures, the SiC JFET and Schottky diode continue normal operation because their switching characteristics show minimal change with temperature. This work further demonstrates the suitability of the SiC devices for high-temperature power converter applications.

Jeremy Junghans

Papers

Fabrication of Precise Fluidic Structures in LTCC

SiC JFET dc characteristics under extremely high ambient temperatures

Next-generation microchannel coolers

Liquid metal heat sink for high-power laser diodes

Switching characteristics of SiC JFET and Schottky diode in high-temperature dc-dc power converters