Close-up photo of the new piezoelectric sensor developed by University of Houston researchers. It can potentially work in extreme environments. (Image: Courtesy of the researchers)

Extreme environments in several critical industries — aerospace, energy, transportation, and defense — require sensors to measure and monitor numerous factors under harsh conditions to ensure human safety and the integrity of mechanical systems.

In the petrochemical industry, for example, pipeline pressures must be monitored at climates ranging from hot desert heat to near arctic cold. Various nuclear reactors operate at a range of 300-1000 °C, while deep geothermal wells hold temperatures up to 600 °C.

A team of University of Houston researchers has developed a new sensor that was proven to work in temperatures as high as 900 °C (1,650 °F), which is the temperature of mafic volcanic lava, the hottest type of lava on Earth.

“Highly sensitive, reliable and durable sensors that can tolerate such extreme environments are necessary for the efficiency, maintenance, and integrity of these applications,” said Jae-Hyun Ryou, associate professor of mechanical engineering at the University of Houston and corresponding author of a study published in the journal Advanced Functional Materials.

The research team previously developed a III-N piezoelectric pressure sensor using single-crystalline Gallium Nitride (GaN) thin films for harsh-environment applications. However, the sensitivity of the sensor decreases at temperatures higher than 350 °C, which is higher than those of conventional transducers made of lead zirconate titanate (PZT), but only marginally.

The team believed the decrease in sensitivity was due to the bandgap — the minimum energy required to excite an electron and produce electrical conductivity — not being wide enough. To test the hypothesis, they developed a sensor with aluminum nitride (AlN).

“The hypothesis was proven by the sensor operating at about 1000 °C, which is the highest operating temperature among the piezoelectric sensors,” said Nam-In Kim, first author of the article and a post-doctoral student working with the Ryou group.

While both AlN and GaN have unique and excellent properties that are suitable for use in sensors for extreme environments, the researchers were excited to find that AlN offered a wider bandgap and an even higher temperature range. However, the team had to deal with technical challenges involving the synthesis and fabrication of the high-quality, flexible thin film AlN.

“I have always been interested in making devices using different materials, and I love to characterize various materials. Working in the Ryou group, especially on piezoelectric devices and III-N materials, I was able to use the knowledge I learned in my studies,” said Kim, who earned his Ph.D. in materials science and engineering from the University of Houston in 2022. His award-winning dissertation was on flexible piezoelectric sensors for personal health care and extreme environments.

Now that the researchers have successfully demonstrated the potential of the high-temperature piezoelectric sensors with AlN, they will test it further in real-world harsh conditions.

“Our plan is to use the sensor in several harsh scenarios. For example, in nuclear plants for neutron exposure and hydrogen storage to test under high pressure,” Ryou said. “AlN sensors can operate in neutron-exposed atmospheres and at very high-pressure thanks to its stable material properties.”

The flexibility of the sensor offers additional advantages that will make it useful for future applications in the form of wearable sensors in personal healthcare monitoring products and for use in precise-sensing soft robotics.

The researchers look forward to their sensor becoming commercially viable at some point in the future. “It's hard to put a specific date on when that might be, but I think it's our job as engineers to make it happen as soon as possible,” Kim said.