Power/Performance Bits: April 24

Waste heat to power
Engineers at the University of California, Berkeley, developed a thin-film system that can be applied to electronics to turn waste heat into energy. The thin-film system uses pyroelectric energy conversion, which is well suited for tapping into waste-heat energy supplies below 100 degrees Celsius, called low-quality waste heat.

In particular, the technology might be particularly attractive for installing on and harvesting waste heat from high-speed electronics but could have an array of applications. For fluctuating heat sources, the study reports that the thin film can turn waste heat into useable energy with higher energy density, power density and efficiency levels than other forms of pyroelectric energy conversion.

An illustration of how the thin film device converts waste heat into energy. (Source: Shishir Pandya)

A main goal of the research was to improve the understanding of pyroelectric physics.

The team synthesized thin-film versions of materials 50-100 nanometers thick and then fabricated and tested the pyroelectric-device structures based on these films. These structures allow the team to simultaneously measure the temperature and electrical currents created, and source heat to test the device’s power generation capabilities.

“By creating a thin-film device, we can get the heat into and out of this system quickly, allowing us to access pyroelectric power at unprecedented levels for heat sources that fluctuate over time,” said Lane Martin, associate professor of materials science and engineering at UC Berkeley. “All we’re doing is sourcing heat and applying electric fields to this system, and we can extract energy.”

This study reports new records for pyroelectric energy conversion energy density (1.06 Joules per cubic centimeter), power density (526 Watts per cubic centimeter) and efficiency (19% of Carnot efficiency, the standard unit of measurement for the efficiency of a heat engine).

The next steps in will be to better optimize the thin-film materials to specific waste heat streams and temperatures.

Diamond power conversion
Researchers from Japan’s National Institute for Materials Science fabricated a key circuit for power conversion systems using hydrogenated diamond (H-diamond) that can function at temperatures as high as 300 degrees Celsius. These circuits could be used in diamond-based electronic devices that are smaller, lighter and more efficient for power conversion than silicon-based devices.

Silicon’s material properties make it a poor choice for circuits in high-power, high-temperature and high-frequency electronic devices, according to Jiangwei Liu, a researcher at National Institute for Materials Science. “For the high-power generators, diamond is more suitable for fabricating power conversion systems with a small size and low power loss.”

The researchers tested the stability at high temperatures of an H-diamond NOR logic circuit consisting of two MOSFETs. When the researchers heated the circuit to 300 degrees Celsius, it functioned correctly, but failed at 400 degrees. They suspect that the higher temperature caused the MOSFETs to breakdown. Higher temperatures may be achievable however, as another group reported successful operation of a similar H-diamond MOSFET at 400 degrees Celsius. For comparison, the maximum operation temperature for silicon-based electronic devices is about 150 degrees.

The view of the H-diamond MOSFET NOR logic circuit from above (left), and the operation of the NOR logic circuits, showing that the circuit only produces voltage when both inputs are at zero. (Source: Liu et al. / AIP)

In the future, the researchers plan to improve the circuit’s stability at high temperatures by altering the oxide insulators and modifying the fabrication process. They hope to construct H-diamond MOSFET logic circuits that can operate above 500 degrees Celsius and at 2.0 kilovolts.

“Diamond is one of the candidate semiconductor materials for next-generation electronics, specifically for improving energy savings,” said Yasuo Koide, a director at the National Institute for Materials Science. “Of course, in order to achieve industrialization, it is essential to develop inch-sized single-crystal diamond wafers and other diamond-based integrated circuits.”

Sensing with TENGs
Researchers from KAUST and the Georgia Institute of Technology are developing small, self-powered photodetectors and wearables for health monitoring by combining triboelectric nanogenerators, supercapacitors, and sensors.

Triboelectric nanogenerators (TENGs) capture mechanical energy from movement, such as vibration, finger tapping, or hand motions, and convert it into electricity. Frictional contact between materials of different polarity creates oppositely charged surfaces. Repeated friction causes electrons to hop between these surfaces, resulting in electric voltage.

Initially, the researchers developed a self-powered photodetector by coupling the silicone-based polymer polydimethylsiloxane (PDMS) as a TENG with an organometallic halide perovskite. To streamline their design and eliminate the need for a motion actuator, the team fabricated the photodetector using two multilayered polymer-based sheets separated by a small gap. One sheet comprised the perovskite ultrathin film while the other contained a PDMS layer. The gap allowed the team to harness the triboelectric effect when the device was activated by finger tapping.

“The self-powered device showed excellent responsiveness to incident light, especially when exposed to light of low intensity,” said Mark Leung of KAUST, the lead author of the photodetector study. Because of its flexible and transparent polymer components, it also retained its performance after being bent 1,000 times and regardless of the orientation of the incident light.

The researchers also created a wearable self-powered bracelet that can store the converted mechanical energy by combining a carbon-fiber-embedded silicone nanogenerator with MXene microsupercapacitors.

The wearable power bracelet can capture and transform energy from human motion into electricity and store it in MXene supercapacitors to drive different sensors. (Source: KAUST)

They incorporated a nanogenerator and miniaturized electrochemical capacitors into a single monolithic device encased in silicone rubber. The leak-proof and stretchable shell provided a flexible and soft bracelet that fully conformed to the body. Fluctuations in the skin-silicone separation altered the charge balance between electrodes, causing the electrons to flow back and forth across the TENG and the microsupercapacitor to charge up.

In addition to exhibiting longer cycle life and short charging time, MXene microsupercapacitors can accumulate more energy in a given area than thin-film and micro-batteries, offering faster and more effective small-scale energy storage units for TENG-generated electricity, according to the team. When active, the bracelet can use the stored energy to operate various electronic devices, such as watches and thermometers.

“Our ultimate goal is to develop a self-powered sensor platform for personalized health monitoring,” said Qiu Jiang, Ph.D. student at KAUST and the lead author of the self-charging band project. The team is now planning to introduce sensors into the system to detect biomarkers in human sweat.