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Schematic of a piezoelectric energy harvester that transforms mechanical vibrations to electrical energy. Credit: Elizabeth Floresgomez Murray/ Penn State

By Jamie Oberdick

Piezoelectric materials hold great promise as sensors and as energy harvesters but are normally much less effective at high temperatures, limiting their use in environments such as engines or space exploration. However, a new piezoelectric device developed by a team of researchers from Penn State and QorTek remains highly effective at elevated temperatures.

Clive Randall, director of Penn State’s Materials Research Institute (MRI), developed the material in partnership with researchers from QorTek, a State College, Pennsylvania-based company specializing in smart material devices and high-density power electronics. The research was partially funded by the National Aeronautics and Space Administration’s (NASA) Small Business Technology Transfer Program and was published in The Journal of Applied Physics.

“NASA’s need was how to power electronics in remote locations where batteries are difficult to access for changing,” Randall said. “They also wanted self-powering sensors that monitor systems such as engine stabilities and have these devices work during rocket launches and other high-temperature situations where current piezoelectrics fail due to the heat.”

Piezoelectric materials generate power via conversion of kinetic energy into electricity. Certain crystals used in these materials produce an electric current when rapidly compressed due to mechanical force during vibrations or motion, such as in machinery or an engine. This effect can also serve as a sensor to measure changes such as in pressure, temperature, strain or acceleration. Therefore, piezoelectrics hold promise for uses ranging from powering personal electronics like wristband devices to bridge stability sensors.

The material the team developed was integrated into a bimorph version of piezoelectric energy harvester technology. A bimorph is a cantilever that consists of one or more active layers. This enables the piezoelectric device to act either as a sensor, an energy harvester or an actuator. In actuators, the one layer contracts while the other expands if voltage is applied. In sensors and energy harvesters, bending the bimorph can either produce an electrical signal for measurement or act as a power source.

Unfortunately, these functions work less-effectively in high-temperature environments, as current state-of-the-art energy piezoelectric harvesters are normally limited to a maximum effective operating temperature range of 80°C to 120°C.

“A fundamental problem with piezoelectric materials is their performance starts to drop pretty significantly at temperatures above 120°C, to the point where above 200°C their performance is negligible,” Gareth Knowles, chief technical officer of QorTek, said. “Our research demonstrates a possible solution for that for NASA.”

However, the new piezoelectric material composition developed by the researchers showed a near-constant efficient performance at temperatures up to 250°C. In addition, while there was a gradual drop-off in performance above 250°C, the team’s research showed their material remained effective as an energy harvester or sensor at temperatures to well-above 300°C.

“The compositions performing just as well at these high temperatures as they do at room temperature is a first, as no one has ever managed piezoelectric materials that effectively operate at such high temperatures,” Knowles said.

Another benefit of the research team’s piezoelectric material was an unexpectedly high level of electricity production. While at present, piezoelectric energy harvesters are not yet at the level of more efficient power producers such as solar cells, the new piezoelectric material’s performance was strong enough to open possibilities for other applications, according to Randall.

“The energy production part of this was very impressive, the material had piezoelectric properties at levels that we had not seen before,” Randall said. “Even the best energy harvesters are nothing like solar, but this would potentially enable a continuous, battery-free power supply in dark or concealed environments such as inside an automotive system or even the human body.”

Another benefit of the team’s piezoelectric material is found in its application design. Piezoelectric devices are coupled with a mechanical structure to be attached to the rest of the system, such as an engine or electronic device, as well as to act an intermediate medium between the piezoelectric element and the mechanical source. Conventionally, epoxy materials are utilized for bonding piezoelectric materials to external mechanical structure, however, they have either operating temperature ceilings or limited functionality at elevated temperatures.

Instead of epoxy, the team coupled the high temperature operating piezoelectric material to a multi-layer, cantilever-type mechanical structure.  This bonding technique also enables solder-less wiring since the wires are now mechanically held in place.

“This novel wireless design is important because one of the issues of creating high temperature piezoelectric systems is wire connectivity becomes quite challenging, as wire connections have a tendency to degrade or even melt at high temperatures,” Knowles said.

Both Randall and Knowles noted that the partnership between Penn State and QorTek, which goes back over 20 years, enabled development of the new, improved piezoelectric material by complementing each other’s resources.

“In general, a big benefit of a partnership like this is you can tap into the large knowledge reservoir in the field that MRI and Penn State has and that small companies like ours sometimes do not,” Knowles said. “Another benefit is often universities have physical resources such as equipment that again, you won't ordinarily find inside a small company.”

Randall noted that since QorTek has many employees who are Penn State alumni, there is a familiarity with both the research subject and the people involved.

“One of my post-doctoral researchers and first author on the paper, Wei-Ting Chen, was hired by QorTek so there was a transfer of expertise in that case,” Randall said. “Also, the skillsets offered by QorTek such as mechanical engineering, device design and measurement expertise pushed development at a much faster rate than would be possible given the budget we were given. So the partnership enabled a really fruitful amplification of the project.”

Along with Randall and Chen, other authors on the paper include from Penn State, Xiao-Ming Chen, MRI research assistant, and from QorTek, Safakcan Tuncdemir, vice president of materials and devices; Ahmet Erkan Gurdal, materials group manager; and Josh Gambal, mechanical design engineer. Along with the NASA support, additional funding was provided by the National Science Foundation.