Device could be integrated into clothing, harvest body heat to power gadgets.
The No. 1 nuisance with smartphones and smartwatches is that we need to charge them every day. As warm-blooded creatures, however, we generate heat all the time, and that heat can be converted into electricity for some of the electronic gadgetry we carry.
Flexible thermoelectric devices, or F-TEDs, can convert thermal energy into electric power. The problem is that F-TEDs weren’t actually flexible enough to comfortably wear or efficient enough to power even a smartwatch. They were also very expensive to make.
But now, a team of Australian researchers thinks they finally achieved a breakthrough that might take F-TEDs off the ground.
“The power generated by the flexible thermoelectric film we have created would not be enough to charge a smartphone but should be enough to keep a smartwatch going,” said Zhi-Gang Chen, a professor at Queensland University of Technology in Brisbane, Australia. Does that mean we have reached a point where it would be possible to make a thermoelectric Apple Watch band that could keep the watch charged all the time? “It would take some industrial engineering and optimization, but we can definitely achieve a smartwatch band like that,” Chen said.
Manufacturing heaven
Thermoelectric generators producing enough power to run something like an Apple Watch were, so far, made with rigid bulk materials. The obvious problem with them was that nobody would want to wear a metal slab on their wrist or run a power cable from anywhere else to their watch. Flexible thermoelectric devices, on the other hand, were perfectly wearable but offered efficiencies that made them good for low-power health-monitoring electronics rather than more power-hungry hardware like smartwatches.
Back in 2021, generating 35 microwatts per square centimeter in a wristband worn during a typical walk outside was impressive enough to land your research paper in Nature. Today, Chen and his colleagues made a flexible thermoelectric device that performed over 34 times better at room temperature. “To the best of our knowledge, we hold a current record in this field,” Chen says.
What’s more, their thermoelectric film was made using bismuth telluride, a semiconductor with a fairly simple manufacturing process. “We wanted to go for the most cost-effective method, so each step did not require too much time or energy,” says Xiao-Lei Shi, a Queensland Technical University researcher and co-author of the study. To make the film, the team used a technique called screen printing, which is widely used in manufacturing printed circuit boards. The process began with synthesizing bismuth telluride nanoplatelets and tellurium nanorods in an autoclave under high temperature and pressure. Then, the two compounds were mixed to produce an ink.
Next, the ink was used to soak a screen that was then pressed onto a very thin polyamide substrate. Finally, the substrates with the deposited ink were fused together by applying high pulsed current and pressure at the same time in a process called spark plasma sintering. “The process is easy to perform and easy to scale up,” Chen claims. The resulting flexible thermoelectric film was only one micron thick, and yet it worked like a charm.
Bending over
To test what the film could do, the team manually cut a small sample out of an A4 size sheet of the material and fitted it with silver paste electrodes connected to measuring equipment. The generator achieved a power output of 1.2 milliwatts per square centimeter using a temperature difference between the skin and air-facing sides measured at 20 Kelvin. This means a performance like that should be more or less achievable on a walk when it’s around 16° Celsius outside—so neither too hot nor too chilly.
While bismuth telluride was used to produce flexible thermoelectric films before, the key to record-breaking performance of Chen’s film was the addition of tellurium nanorods. It turned out that having 7.5 percent by weight of those rods in the ink allowed them to fill in the pores in the bismuth telluride layer, making it denser, and connecting tellurium nanorods.
What’s more, the ink had very little effect on flexibility. Chen’s team bent the film a thousand times and found that the strain was reduced only by 2 percent compared to polyamide substrate without any ink deposited on it. Repeated bending also had a limited impact on performance, which went down by 2 percent during these tests.
But serving as a potential material for power-generating smartwatch bands is just one side of the thermoelectric coin. The other is thermoelectric cooling, which is basically the energy-harvesting process running in reverse—electric current is used to decrease temperature. And Chen’s team has that in their sights, too.
Staying cool
“We demonstrated our device could achieve up to 11.7 Kelvin temperature drop off without any heat sink [and] with a very tiny input current,” Chen says. For cooling tests, the team used the same device it used for generating power. It took just 84.2 milliamps to produce that 11.7 Kelvin difference between the two sides of the material. “Because the film’s thickness is just one micrometer, it is possible to integrate it with silicon chips in the future. For us this would be a new research direction,” Chen claims.
He suggests that thermoelectric cooling films are going to be particularly useful in processors with small feature sizes, like the three nanometer architectures used in the Apple M3 family of chips. The team argues in their study that ultra-thin thermoelectric films could be applied directly onto the chips to provide cooling and harvest power at the same time. This, the researchers claim, should not require radical alterations in the chip manufacturing because screen-printing is used during the manufacturing of processors anyway.
There are some improvements to work on before that could happen, though. Chen would like to improve the material’s flexibility—his goal is for the film to bend 10,000 or even 1,000,000 times before taking a performance hit. “The third challenge is integration. How do we integrate flexible thermoelectric devices with silicon chips,” Chen says. Integrating thermoelectric films with silicon chips would require redesigning thermal and power management of those chips to work with thermoelectric cooling and establishing processes to manufacture those films at a truly massive scale. “This would take scientists and engineers from multiple disciplines working together to achieve this,” Chen adds. And what his team believes should bring all those scientists and engineers together is the simplicity of their design.
“Other research on flexible thermoelectric films is complicated—the mechanisms involved are very hard to reproduce,” Shi claims. “Our work is unique in that it is very easy to reproduce and use for practical applications.”
Science, 2024. DOI: 10.1126/science.ads5868
Jacek Krywko is a freelance science and technology writer who covers space exploration, artificial intelligence research, computer science, and all sorts of engineering wizardry.
