(From left) Univ. of Florida’s Jennifer Forrester and students Cassandra Llano and Tedi-Marie Usher at the Spallation Neutron Source’s VULCAN instrument. Credit: Oak Ridge National Laboratory

Stress, fatigue, and heavy loads aren’t always
negative elements of work—in fact, they are what attracted Jennifer Forrester
to the Spallation Neutron Source at Oak Ridge National Laboratory.

Forrester, a research scientist working in
the research group of professor Jacob Jones at the Univ. of Florida,
came seeking stress at SNS as part of her work on piezoelectric ceramics.
Applying stress to piezoelectric crystals produces an electric field, whereas
applying an electric field to the same material causes a shape change.

These unique properties make piezoelectrics
valuable for diverse applications in such consumer products as cell-phone touch
screens, fuel injection, airbag deployment sensors, fire igniters, and guitar
pickups.

“What we’re trying to do is improve
them—put a higher electric field on them and have more shape change. Or the
other way around—put a higher load on them so that they put out a corresponding
electric field. The more shape change you can get out of them, the more potential
applications you have,” Forrester said.

Traditional piezoelectric materials contain
lead, an element that materials researchers are now trying to avoid because of
environmental concerns. One lead-free alternative, sodium bismuth titanate, or
NBT, was the focus of Forrester’s latest neutron diffraction study at SNS.

Forrester and her students were the first
users of a new load frame at VULCAN, a diffractometer among the suite of
instruments that receive intensely pulsed neutron beams at the SNS facility.

“For applying high mechanical loads,
VULCAN is the perfect instrument,” Forrester said.

Aided by VULCAN’s new equipment, Forrester
and her team applied static loads to different compositions of the NBT samples
to gauge their strength. Initial observations revealed certain compositions
that had fracture stresses about double some of the lead-based materials.

“We’re trying to work out exactly why
two compositions of our NBT were about five times the strength of other
compositions. We want to know why they’re so much better,” Forrester said.

Neutron analysis gives Forrester an extra
edge analyzing subtle differences and improvements in the samples’ performance.

“NBT is an oxide, and neutrons have a
huge advantage when it comes to studying oxides because of the different
scattering factors between x-rays and neutrons,” Forrester said.
“Oxygen has a low scattering factor when using x-rays, and pin-pointing
the oxygen locations is important in structural analyses of these
materials.”

Sample size also plays a role in choosing
neutrons over x-rays, says Forrester.

“Our samples are large, and x-rays can
only penetrate so far into the sample. It would be difficult to obtain bulk
results with x-rays, because they can’t penetrate far below the surface. A
solid surface can behave differently than the bulk material,” Forrester
said.

Forrester is already planning her next
experiment at SNS, where she hopes to use a new instrument, NOMAD, to study how
her samples respond to electric fields.

“After conducting an experiment
at VULCAN, you do realize what a nice setup they have for running these types
of experiments,” Forrester said. “They have a very knowledgeable
group of instrument scientists. After you tell them what you want to do, they
make it happen.”