Thermoelectric device switches between power generation and heat-flow sensing

A research group led by Professor Masayuki Takashiri of the Department of Applied Chemistry, School of Engineering at Tokai University, developed a thermoelectric element utilizing single-walled carbon nanotubes (SWCNTs) that can be switched between a "power source" and a "heat flux sensor" with the same thin-film element simply by changing the substrate. The findings were published in Advanced Science.

Conventional thermoelectric elements required keeping one side at a high temperature and the other side at a low temperature to convert heat into electricity. Because cooling plates or fans were required to maintain the low-temperature side, installation options and applications were limited. In addition, substrates were largely regarded as mere supports for thin films, and their role in determining device performance remained poorly understood.

The research group fabricated SWCNT thin-film devices that operate simply by uniformly heating the entire structure and compared their performance using three different substrate materials. They found that even when the thin films, circuit design, and fabrication methods were identical, changing only the substrate altered the device's function. On a substrate that transmits infrared radiation (COP), the device generated a stable voltage and functioned as a power source. On a substrate that absorbs infrared radiation (PEN), it produced a transient reverse voltage only while the temperature was changing, allowing it to function as a heat-flux sensor that detects changes in heat flow. The researchers further showed that device behavior is governed by two key substrate properties: its infrared absorption characteristics and its resistance to temperature change. Because a temperature difference of 1 to 3℃ naturally develops within the device regardless of the substrate used, there is no need for the one-sided cooling systems required in conventional designs.

The element developed this time has a structure connecting two regions with different electricity-conducting properties within a single thin film. Because the direction in which electricity flows changes depending on whether the central portion of the element becomes higher in temperature than surroundings, the direction of generated voltage also changes. This difference generates two functions: power sources and heat flux sensors.

The key factor is the substrate. Although SWCNT thin films absorb nearly all incident infrared radiation, the amount of infrared radiation absorbed by the substrate varies depending on its material and thickness. COP substrates transmit 45.8% of infrared radiation. Because the radiation passes through the substrate and directly heats the thin film, the center of the device becomes hotter than its surroundings, enabling it to function as a power source. In contrast, PEN substrates, which readily absorb infrared radiation (transmittance: 0.2%), heat up before the thin film does. As a result, the center of the device becomes relatively cooler than the surrounding areas. However, this temperature distribution disappears once the entire device reaches thermal equilibrium. Consequently, a voltage is generated only while the temperature is changing.

In addition, the length of time the voltage is generated depends on the substrate's resistance to heating. Because PEN is thicker than PI and has a larger specific heat capacity, it takes longer for its temperature to become uniform, allowing a larger voltage to be generated over a longer period. The researchers therefore concluded that the device's function is determined by two substrate properties: the degree of infrared absorption, which determines the direction of the voltage, and resistance to heating, which determines the voltage's magnitude and duration.

At present, the device produces an output voltage of 0.38 mV and an output power of 0.47 nW. To drive wireless communication devices and other electronics, approximately 100 device pairs would need to be connected in series and combined with a voltage-boosting circuit. In this study, the researchers demonstrated that a single device can be switched between power generation and thermal sensing simply by changing the substrate, eliminating the need for cooling systems. However, further improvements in performance will be required for practical applications.

Going forward, the team plans to increase the device's output, while also improving long-term reliability through encapsulation technologies and evaluating performance under repeated heating and cooling cycles that simulate real-world operating conditions.

Takashiri said, "When the direction of voltage reversed, I suspected a measurement error at first. However, thin films and circuits were identical, and the cause could only be substrates. We realized that substrates, previously considered merely as bases supporting thin films, actually played an important role in determining functions of elements. Thermoelectric elements had constraints requiring cooling sides, limiting installation locations. If cooling devices become unnecessary, it means locations where they can be installed are unlimited. Simply attaching them to surfaces of factory piping or battery packs enables detection of thermal anomalies. We wish to connect to such elements."

Journal Information
Publication: Advanced Science
Title: Substrate Engineering of SWCNT p-n Junctions for Dual-Mode Power Generation and Heat-Flux Sensing
DOI: 10.1002/advs.76889