Alloys and Nanocomposites –
The Future of Microdevices
The future is here, at LSU's Chemical Engineering department. Science-fiction
tales of micro-miniature devices being used on an everyday basis are close
to becoming a reality, thanks to the work of Dr. Elizabeth Podlaha. Dr. Podlaha
currently holds the Clarence M. Eidt, Jr. Professional Development Professor.
After receiving her Ph.D. from Columbia University in 1992, Dr. Podlaha performed
post-doctorate work as a Research Collaborator at the Ecole Polytechnique
Fédérale de Lausanne in Switzerland, before coming to LSU.
Her work with alloys and microfabrication have earned her Lead Principle
Investigator status on several prestigious collaborative projects, including
"Chemical Analysis for the Development of Microfabricated Structures," awarded
over $47,000 from the State of Louisiana Board of Regents' Enhancement program
and "Acquisition of Instrumentation for Microsystems Research and Development"
awarded nearly $240,000 from the National Science Foundation/MRI program.
Shown in Figure 1 is the X-ray Fluorescence system that was acquired from
these grants. [picture of student and equipment, student.jpg. Caption: Figure
1: Regina Bergeron works on the X-ray Fluorescence system, purchased with
grants from the Board of Regents and the NSF.] A recent grant in the amount
of $109,000 was awarded for "Electrodeposition of Ternary Alloys for
Microfabrication," from the State of Louisiana Board of Regents' Research
and Development program. She is also a recipient of the esteemed NSF CAREER
award. Dr. Podlaha currently advises eight graduate and two undergraduate
students.
Dr. Podlaha's research is currently focusing on three major areas:
nanocomposites, alloy coatings, and microdevices, all converging in
electrodeposition technology.
Electrodeposition of Nickel Alloys for
Microdevices
Electrodeposited nickel alloys are being investigated as improved materials
for MEMS. This project is a collaboration between Dr. Podlaha and professors
from LSU's Mechanical Engineering department. Two model alloy systems were
considered: nickel-copper and nickel-tungsten. The nickel-copper system
represents a codeposition mechanism that is usually characterized by independent,
metal reduction reaction rates. In contrast, the nickel-tungsten system is
an example of the induced codeposition mechanism; nickel's reaction rate
enhances the codeposition of tungsten. The challenges that have been encountered
are those related to gas evolving side reactions (H2) and diffusional
limitations. Gas bubbles that cling to the deposit surface result in nonuniform
deposit growth, and moderate pH rises can influence the deposition mechanisms,
resulting in compositional changes.
One way to circumvent problems associated with gas evolving side reactions
is to optimize the plating conditions, avoiding conditions where these reactions
occur if possible. Dr. Podlaha's research takes a different approach. Pulse
current and pulse potential schemes have been considered to manage the gas
evolution reactions, instead of trying to eliminate them, which aims to
generalize conditions for different alloy systems. Pulses longer than
conventional pulse plating are used. According to Dr. Podlaha, this is "due
to the longer diffusion relaxation times associated with deep recesses,"
in this case, recesses 500 microns deep.
Electrodeposition of
Nanocomposites
Dr. Podlaha has been instrumental in developing pulse-reverse plating, shown
to be a promising avenue for controlling and increasing the amount of particle
incorporation in metal matrix composites. She describes the process as
"selectively incorporating nanoparticles to make nanocomposites." Dr. Podlaha
received a $200,000 award from the National Science Foundation to support
work on nanocomposites over the next four years. In this process, particles
are first embedded into the growing metal during the reduction cycle. Part
of the metal is subsequently dissolved under an applied anodic current.
Thereafter, the plating and dissolution are continued, resulting in an enrichment
of the particle concentration in the composite. By controlling the net thickness
per cycle in the nanometer range only nanometric particles are retained in
the deposit, even when larger particle sizes are present in the electrolyte
(Figure 3a). Successive layering thus results in a nanocomposite. This
pulse-reverse plating technique has to-date been used to fabricate thin films
of copper-(-alumina nanocomposites as shown in Figure 3b. Dr. Podlaha presented
her findings at this year's meeting of The Electrochemical Society.
Electrodeposition of Ternary
Alloys
Ternary electrodeposited nickel-cobalt-iron alloys have received recent attention
because of their unique magnetic properties in cobalt-rich alloys, and
thermophysical properties in iron-rich alloys. They are also of interest
as materials for micro-electro-mechanical systems (MEMS). Dr. Podlaha, with
others, has been investigating the electrodeposition mechanism in the
Electrochemical Engineering Laboratory. Shown in Figure 4a are the reaction
rates of each metal reduction. As expected, the nickel deposition rate was
inhibited. Concurrently, the iron rate was accelerated and the cobalt rate
exhibited both inhibition and acceleration, which has never been reported
before for this ternary system. Dr Podlaha and the graduate student working
on this project have recently submitted their results to the Journal of
Electrochemical Society. "We are exploring this new phenomena experimentally
with a view towards developing a mathematical model for process control
applications," says Dr. Podlaha.
Another ternary electrodeposition system that is being studied is
nickel-tungsten-iron based. Dr. Podlaha and her research group theorize that
tungsten codeposition occurs through a mixed-metal intermediate requiring
the inducing elements. If the tungstate ion concentration is kept smaller
than both nickel or iron ion concentration, then the deposition rate of tungsten
should not be influenced by the codepositing metal. This is illustrated,
for the first time, in Figure 4b. Dr. Podlaha and the undergraduate student
working on the project plan to present their findings at the next meeting
of the Electrochemical Society.
Electrodeposition of Rare-Earth
Alloys
According to Dr. Podlaha, bulk Terfenol, TbFe2 (Tb: 58.7 wt%, Fe: 41.3 wt%),
exhibit giant magnetostriction at room temperature; in other words, they
deform under an applied magnetic field. Instead of standard non-electrochemical
deposition techniques, Dr. Podlaha and her colleagues fabricated these materials
by electrodeposition from aqueous solutions. Her work with nickel-cobalt-iron
alloys crossed over into this research. "We have found that the presence
of the iron ion facilitates electrodeposition of the terbium rare-earth,"
she says. Figure 5 shows an example of the weight percent terbium measured
in a terbium-iron alloy electrodeposited in a citrate and a tartrate electrolyte.
The current was varied to determine the influence of the current density
on alloy composition, and a high terbium concentration was found with the
tartrate solution at low current densities.
Electrodeposition of Oxide
Sorbents
This groundbreaking project recently received funding of nearly $200,000
from the Department of Energy, as part of the University Coal Research (UCR)
Core Program. Dr. Podlaha collaborated with fellow Chemical Engineering professor
Dr. Douglas Harrison to generate ceria-zirconia sorbents electrochemically
to permit the desulfurization of coal gas. These sorbents have been manufactured
by other methods (for example, chemical precipitation), but have yet to be
fabricated by electrodeposition. Other researchers have shown that the
electrogeneration of ceria and zirconia is possible by applying a cathodic
current at an electrode surface from two separate electrolytes. Drs. Podlaha
and Harrison devised a method where both ceria and zirconia are deposited
together from a single electrolyte in order to provide intimate mixing of
the two oxides. Figure 6 shows how the current density decreases as the electrode
surface is progressively covered with the oxide sorbent.
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