Industries like aerospace, energy, transportation, and defense require sensors to measure and monitor various factors under harsh conditions to ensure human safety and the integrity of mechanical systems. In the petrochemical industry, for instance, pipeline pressures must be monitored at temperatures ranging from hot desert heat to near-arctic cold. Various nuclear reactors operate at a temperature range of 300-1000 degrees Celsius, while deep geothermal wells hold temperatures up to 600 degrees Celsius.

A research team at the University of Houston has developed a new sensor that can work in temperatures as high as 900 degrees Celsius or 1650 degrees Fahrenheit. This is the temperature at which mafic volcanic lava, the hottest type of lava on Earth, erupts. The researchers believed that highly sensitive, reliable, and durable sensors that can withstand such extreme environments are necessary for the efficiency, maintenance, and integrity of these applications.

The UH research team had previously developed III-N piezoelectric pressure sensors using single-crystalline Gallium Nitride (GaN) thin films for harsh-environment applications. However, the sensitivity of the sensor decreased at temperatures higher than 350 degrees Celsius, which is higher than those of conventional transducers made of lead zirconate titanate (PZT), but only marginally. The team believed that the decrease in sensitivity was due to the bandgap, which is the minimum energy required to excite an electron and supply electrical conductivity, not being wide enough.

Aluminum Nitride Used to Develop the Sensor

To test their hypothesis, the team developed a sensor with aluminum nitride (AlN), which has a wider bandgap than GaN. While both AlN and GaN have unique and excellent properties 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 high-quality, flexible thin-film AlN.

Nam-In Kim, a post-doctoral student working with the Ryou group, said that it was interesting to see the process leading to the actual results, and they solved the technical challenges during the development and demonstration of the sensor. Kim, who earned his Ph.D. in materials science and engineering from UH in 2022, was able to use the knowledge he learned in his studies while working in the Ryou group, especially on piezoelectric devices and III-N materials. 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 plan to 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,” said Jae-Hyun Ryou, associate professor of mechanical engineering at UH and corresponding author of the study published in the journal Advanced Functional Materials. “AlN sensors can operate in neutron-exposed atmospheres and at very high-pressure ranges 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 health care monitoring products and for use in precise-sensing soft robotics. The researchers are looking forward to their sensor being 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.

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