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Spotlight - Taking the Heat

If you think keeping your car running at peak performance can be expensive and time-consuming, imagine the level of maintenance required to make the extraordinarily complex engines in military aircraft operate without a hitch.

A typical gas turbine jet engine can reach temperatures of more than 2,000 degrees Fahrenheit, with red-hot metal parts spinning at a nearly unfathomable rate. Determining when a critical component might be nearing the end of its useful life under such extreme conditions is difficult, though, which means that mechanics typically must tear apart the engine and replace parts to ensure an aircraft’s readiness and safety at all times.

The Department of Defense recently decided that all of its new aeronautical and aerospace systems should be monitored continuously, using sensor technology that can automatically assess the health of the components and reduce costly manual inspections. The problem is that the high temperatures a jet engine generates can break down the diagnostic sensors, rendering them ineffective when things really get hot in flight.

But now researchers in the University of Maine Laboratory for Surface Science and Technology (LASST) believe they have developed the first sensor that truly can take the heat, and the U.S. Air Force is excited about the possibilities.

The high-temperature acoustic wave sensor, which is a few millimeters in size, is made of new materials that allow it to function at about 1,000 degrees Celsius (1,832 degrees F) and possibly much higher, thereby significantly surpassing the effective operating range of similar devices before it.

“The sensors will be targeting temperature, pressure, vibration and corrosion in the engines, and determining the probability of failure of the parts over time so they could be used longer,” says Mauricio Pereira da Cunha, an associate professor of electrical and computer engineering and member of LASST. “The Air Force is very interested in this technology because it would potentially help save more than $1.6 billion in engine maintenance costs, and free up Air Force money to renovate the fleet.”

Acoustic wave devices have been used commercially for more than 60 years, and can be found today in computers, quartz watches, cell phones, garage door openers, pagers and other modern electronics.

Applying voltages to the metallic electrodes on the device generates sound, or acoustic, waves that propagate along the surface. Any change at the surface of the device affects the wave propagation, which can be accurately gauged by measuring the frequency response. The wave speed’s extreme sensitivity to environmental conditions allows the versatile devices to act as sensors that can precisely monitor such variables as temperature, pressure, vibration and corrosive gases.

Until now, however, the devices had never been used successfully at very high temperatures because of the limits of their materials. Pereira da Cunha and Robert Lad, physics professor and director of LASST, are confident that the new materials used in their sensors will change all that.

A decade ago, Pereira da Cunha began researching the high-temperature behavior of a crystalline material called langasite, first grown in Russia in the 1980s, to determine its potential applications for acoustic wave devices. In 2001, shortly after coming to UMaine, he received a NASA-funded Maine Space Grant Consortium seed grant to test the feasibility of using acoustic wave devices made with langasite as sensors in aerospace vehicles to detect leaks of explosive hydrogen gas.

Pereira da Cunha’s NASA research demonstrated that langasite works reliably at 750 degrees C in environments such as those found in gas and oil drilling operations, whereas other traditional acoustic wave materials degrade above a few hundred degrees C.

The improved high-temperature device also was tested for use as a hydrocarbon sensor to monitor the fuel-burning efficiency of combustion engines. Hydrocarbon in exhaust, the result of inefficient burning of fuel, pollutes the air and decreases the distance a jet, or any other vehicle, can travel.

With a nearly $392,000 grant from the Defense Department’s Experimental Program to Stimulate Competitive Research (DEPSCoR), and additional funding from the Air Force Research Laboratory, Pereira da Cunha and Lad are now pushing the new sensor technology to operate at 1,000 degrees C and more for use in Air Force jet engines.
To further enhance the sensor’s high-temperature capabilities, the UMaine team has developed a new configuration of precious metals to help electrodes maintain their integrity. Also created were very thin ceramic coatings to protect the devices from heat, abrasion and damaging particulates whirling around inside the jet engines.

In March, the researchers will hand over three of the prototype sensors to the Wright-Patterson Air Force Research Laboratory in Ohio. The Air Force will put the devices in prototype turbine engines and run some diagnostics.

“Measuring the condition of components in the Air Force environment will help transition our sensors from lab prototypes into routine devices in the field,” says Lad.

The UMaine team also is hoping to eventually make the devices wireless, Lad says, so that they can be put on certain moving parts where the wired versions cannot function at this point.
Two UMaine patents are pending on the sensors, which are fabricated and tested on campus. Researchers have developed the capacity to cut the crystal substrate material into wafers just 500 microns thick (a human hair is about 100 microns in diameter), and have the machines to align, grind and polish them. The sensors are then equipped with patterned electrodes and thin film layers in the clean room microfabrication facility.

With UMaine chemical engineer and LASST member Paul Millard, Pereira da Cunha also is adapting the devices to rapidly and reliably detect potential bioterrorism-linked microbial pathogens in water supplies. With chemical engineer Clay Wheeler and Bruce Segee in electrical and computer engineering, Pereira da Cunha is helping develop sensors that can sniff out hydrogen fluoride, a potentially harmful gas found in solvents, refrigerants and herbicides that could be unleashed in industrial accidents or in breaches of homeland security.

In addition, Pereira da Cunha also is researching wireless sensors with UMaine electrical engineers Ali Abedi and Don Hummels.

As for the new high-temperature turbine engine sensor, Pereira da Cunha and Lad believe the devices could be of enormous benefit to the commercial aviation industry as well the military.

“We know that the sensors work, but we need to do more research and development to improve their operation,” Pereira da Cunha says. “We would like to keep developing them here and possibly spin off a private company.”

by Tom Weber
March-April, 2008

 


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