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