For some, the mere mention of liquid metal could conjure up visions of T-1000: the shapeshifter and almost invincible villain who puts the pressure on the future savior of mankind in “Terminator 2”.
But for Eric Markvicka and his colleagues at the University of Nebraska-Lincoln, the droplets of this product appear to be a protagonist in the quest to dissipate heat – and prevent overheating – in wearable technology, soft robotics and others. microelectronic applications.
“As computing power increases, heat dissipation becomes an increasingly important factor,” said Markvicka, assistant professor of mechanical and materials engineering.
Are you not helping? The fact that many wearable devices and other smart technologies incorporate malleable and elastic materials that reduce weight and increase comfort, but also trap heat. To solve the problem, Markvicka and other engineers have tried charging the insulating materials with liquid metal droplets which naturally conduct heat and can therefore move it away from the microelectronics that generate it.
The approach has worked, to a point. Yet at that point a realization came that although liquid metal droplets improve thermal conductivity, their density – and the number needed to really improve that conductivity – can also add an impractical amount of weight.
This tug of war between thermal conductivity and density had left engineers at a certain impasse. But in a new study, Markvicka’s team has shown that incorporating a silicone material with gallium-based droplets – and, most importantly, incorporating those droplets with microscopic hollow glass spheres – can mainly keep the increase in heat dissipation without sacrificing the light flexibility of the material.
“It’s still soft and rubbery, but has thermal conductivity approaching (that of) certain rigid metals, like titanium or stainless steel, with about half the density of those metals,” Markvicka said. “This combination of properties makes the material unique and interesting.
By experimenting with the glass microspheres, the researchers tested versions of the silicone whose liquid metal droplets harbored varying volumes of hollow glass, from 0% to 50%. The 50% increase in volume resulted in a 35% decrease in overall material density and a only 14% decrease in thermal conductivity, the latter already starting from a higher baseline than in the silicone free of liquid metal.
This achievement, in itself, was enough to make engineers rejoice. Markvicka, doctoral student Ethan Krings and their colleagues weren’t done, however. Using previous modeling work, the Nebraska team then developed what Markvicka described as a “contour map” to guide future customization of soft materials that rely on the Russian nesting doll approach of the team.
In developing the map, the team formalized what their experiments revealed: that careful engineering can unravel the normally intertwined properties of a polymer, granting unparalleled mastery over the material’s performance.
An axis of the map represents the volume of liquid metal droplets in a material; the other axis quantifies the volume of glass microspheres in the droplets. Changing the volume of the glass microspheres alone, the map showed, can change the density of the material while leaving thermal conductivity almost completely unchanged. Changing the ratios of glass to liquid metal, on the other hand, can change thermal conductivity without affecting density.
“So we were able to show that we can now independently control the thermal conductivity and density of these composites, which has never been demonstrated before,” said Markvicka, whose team detailed their proof of concept in the review. Small.
The researchers further demonstrated this control by making several silicone versions of the university’s Nebraska N. Each version had a different density, as evidenced by the fact that the denser sank to the bottom of a liquid-filled cylinder, the less dense floated at the top, and a moderately dense version floated in between. Despite their varying densities, the Ns dissipated heat at about the same rate when electricity was conducted through a heating element implanted in each.
Markvicka sees a myriad of ways that a flexible but thermally conductive material could benefit emerging technology. For starters, he said, it could help alleviate the computing power limitations of microelectronics that are squeezed in wearable technology, easing the path to faster devices with more functionality.
Larger-scale digital technology engineers, including computers and game consoles, might also find this useful when fabricating so-called interface materials that carry substantial amounts of heat from processors, for example, to devices. coolants. The PlayStation 5 console, for example, already uses liquid metal for this purpose.
Beyond that are the obvious applications in thermoregulatory clothing, said Markvicka, which can monitor a wearer’s skin temperature and then provide or remove heat accordingly.
“A lot of big tooling companies have these heated jackets and gear to help workers stay warm in cold environments,” he said. “This material could function as a passive heat sink to achieve more even heating throughout an entire jacket and eliminate hot spots, without restricting the wearer’s movement.
“Whatever the human body interacts with, there could be applications for this material. “
Markvicka and Krings wrote the study with Sangjin Ryu of Nebraska, Jeffrey Shield, Haipeng Zhang, and Suchit Sarin. The team received support in part from Nasa and the Nebraska Research Initiative.