ORNL'S SMAC FACILITY: IMPROVING ON MOTHER NATURE
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Nature provides us with materials of varying hardness, durability,
conductivity, and a range of other traits. When these materials
don't meet our needs, we can take advantage of natural chemical
interactions to create substances with more desirable
characteristics. However, some specialized technical applications
require materials having combinations of characteristics that don't
occur naturally and can't be created using standard chemical
techniques.
Researchers at ORNL's Surface Modification and Characterization
Collaborative Research Center (SMAC) specialize in circumventing
these limitations by firing beams of ions into or onto the surfaces
of a range of targets, altering their physical properties and
creating materials that Mother Nature never imagined.
Ion-beam processing is calculated to produce or enhance particular
surface characteristics of materials, such as hardness,
conductivity, adhesion, or optical properties. "SMAC is concerned
with surface modifications, not bulk properties" says Steve Withrow
of the Solid State Division. "This type of research enables us to
determine which combination of ion species and target material to
use to tailor a surface having a particular set of
characteristics."
"For instance," says Withrow, "we might want to investigate ways to
prevent unwanted chemical reactions, such as corrosion, or improve
the ability of a material to speed up a chemical reaction, known as
catalysis. Because corrosion and catalysis occur on the surface of
materials, they can be influenced by ion-beam processing.
Similarly, we could try to reduce the friction between a metal ball
bearing and a race (the track the bearing rides in) because
friction depends on the surface characteristics of the bearing and
the race. These are the kinds of problems we can attack."
SMAC researchers rely on a variety of techniques to achieve surface
and near-surface modification of materials. Several of these are
described here.
Ion implantation doping is the process of firing charged atoms, or
ions, of carefully selected masses and energies at a target, with
the goal of embedding these ions in the near-surface layers of the
target material. The higher the energy of the ions, the deeper they
penetrate, disrupting the normal structure of the target material.
Implantation research at SMAC has resulted in a number of
beneficial developments, including a method of increasing the
durability of the wear surface of a titanium alloy artificial hip
joint. Nitrogen-doping of the wear surfaces of the prosthetic joint
reduced corrosion and abrasion by a factor of 1000, extending its
life and presumably reducing the likelihood of subsequent hip
replacement operations. One project this technique is currently
being used for is a study of the structural and electrical
properties of implanted diamond, a material which may have
applications as a thin-film semiconductor.
Ion beam deposition provides a means of building up layers of
isotopically pure thin films on selected surfaces. These films can
be used as research samples to study, for example, the migration of
one isotope into another during heating. These layered films may
ultimately find applications in X-ray optics, semiconductors, and
materials requiring high thermal conductivity.
Ion beam mixing involves bombarding a thin film of material
deposited on a substrate with ions that deposit most of their
energy near the interface between the two materials, causing them
to mix. This results in an intermediate layer of material
possessing new and often desirable properties. This technique may
be used, for instance, to attach a thin metallic film to a
semiconducting or ceramic substrate. Mixing the two materials at
their interface improves their adherence to one another. Such films
could be used for electrical contacts, wear protection, etc.
Ion beam annealing may be used in conjunction with ion implantation
doping to undo structural damage in irradiated materials. Annealing
is achieved by heating the sample with a beam of energetic ions,
thereby allowing internal stresses built up by the implantation
process to be released.
Developing surface modification techniques, however, is only half
of SMAC's mission. The other half is the analysis of modified
surfaces. The results of ion-beam processing done at SMAC or of
other processes that produce near-surface changes in materials can
be studied at SMAC using a range of ion beam and surface analytical
techniques.
Ion scattering analysis provides a nondestructive, depth-sensitive
method of determining the near-surface composition of materials.
This technique involves firing hydrogen or helium ions at a target
and precisely measuring the angles and energies at which they
collide and scatter. The information gained from these collisions
enables researchers to construct a picture of the distribution of
elements in the surface layer before ion-beam processing and to
measure changes in these distributions as a result of
processing.
Nuclear reaction analysis is another ion-scattering technique used
at SMAC. By selecting an ion species that will undergo a nuclear
resonance or possibly a nuclear reaction as a result of a
near-head-on collision with an atom in the target, researchers can
greatly increase the amount of information gathered from the ion
scattering. This technique is useful when normal scattering
techniques do not provide enough quantitative information about a
particular component of the target because, for example, its
concentration is small.
Positive ion channeling, the steering of charged particles between
rows of atoms in a crystalline solid, can be used to gauge the
perfection of crystal structure, determine the extent of damage to
crystal lattice structure caused by ion implantation, locate
impurities, and provide a number of other types of information
about crystalline samples.
Surface analysis techniques employed at SMAC include several
varieties of electron spectroscopy, including low energy electron
diffraction and Auger electron spectroscopy. These techniques are
primarily used for studying reordered surfaces and crystal growth
and for monitoring surface cleanliness.
Computer analysis is also employed to model ion beam interactions
with materials, simulate ion scattering, and predict other
particle-solid interactions.
Withrow encourages outside users, including industrial, academic,
government, or foreign researchers interested in pursuing projects
at the SMAC/RC facility to submit research proposals describing the
scientific problem to be addressed and estimating the time and
support they would require. Nonproprietary use of the facility is
free of charge.
"The SMAC facility is highly versatile," says Withrow. "We can
implant ions of just about every element on the periodic table at
energies from 10 keV to 1 MeV or higher. If there's an element you
want to implant, we can probably do it."
Mother would be so proud.
(keywords: ion implantation, ion beam processing, surface
modification)
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Date Posted: 2/7/94 (ktb)