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NASA Space Science News
Using a force field to float molten test samples
precisely in mid-air, NASA's Electrostatic Levitator creates
a unique environment for space-age materials processing.
Right
before your eyes, floating in mid-air with no visible means
of support or containment, is a molten glob of glowing metal.
Scientists gather around, talking excitedly, observing and
recording their measurements. Unlike the hovering sample,
however, the onlookers aren't floating. Gravity holds them
firmly to the ground.
What
makes the metal float? Have these scientists converged on
a magic show?
Above: A 3 mm droplet of nickel-zirconium,
heated to incandescence, hovers between electrically charged
plates inside the vacuum chamber of the Electrostatic Levitator.
No,
it's not hocus-pocus. This is the Electrostatic Levitator
(ESL) at NASA's Marshall Space Flight Center (MSFC) in Huntsville,
AL -- a unique facility for experiments with space-age materials.
The
ESL provides a unique way to test substances without touching
a container or crucible that would contaminate the sample.
It is used to examine metals, ceramics and glass compounds
in a hot or cool liquid state. The answers that Dr. Jan
Rogers, ESL Project Scientist, and her colleagues in MSFC's
Microgravity Materials Science Group derive from their tests
teach fundamental facts about many kinds of materials.
The
ESL "containerless processing" laboratory also
produces information that leads to the creation of entirely
new materials, such as metallic glasses and metal-ceramic
compounds. Uses for these new substances range from new
optical materials and superconductors to unbreakable glass,
materials with memory, and even "liquid metal"
golf clubs that are longer-hitting.
"To
create new materials for both earthbound and space technologies,
we must understand the physical properties that govern how
substances and their ingredients behave," Rogers explains.
"Studies being done on metal and ceramic materials
in a liquid state in the ESL cover a wide range of characteristics.
We need to gather data on the stability of their molten
surface, the density and flow of the liquid materials, their
capacity to absorb and release heat, and how they solidify.
We study some quite special phenomena such as undercooling
and nucleation."
Undercooling
describes how far below its normal freezing point a material
will stay liquid -- and whether or not it forms nuclei and
crystallizes when it cools. Some metals stay in an amorphous
non-crystalline form when they re-solidify and become a
highly useful new kind of material called metallic glass.
"Precise
determination of all these properties is very difficult
when materials are tested in a container," notes Rogers,
"because anything that holds a molten sample will alter
the results and observations by interacting with it physically
or chemically."
The
ESL uses static electricity to suspend an object inside
a vacuum chamber. While the sample is levitated, a laser
beam heats the sample until it melts so scientists can measure
its physical properties without any interference from a
container.
Above: The heart of the ESL is
the vacuum chamber (right) containing a pair of electrostatic
plates and four electrodes that position the sample being
processed (left). The sample's position is determined from
the shadows cast on detectors as two lasers shine at right
angles through the vacuum chamber onto the sample.
Two
horizontal electrode plates electrically charge the sample
and repel it upward until it balances between them. The
sample is positioned horizontally by two smaller pairs of
electrodes. A high-power deuterium arc lamp shines on the
sample to replace the electrical charge that the sample
loses as it emits electrons while hot. Digital feedback
controls the system, using lasers to calculate subtle changes
in the electrode charges and recentering the sample so it
remains in the path of the heating laser -- although experiments
can also be run without laser heating.
At present
the power of the electrostatic levitators is limited, so
samples can be no more than 3 mm (0.12 in) in diameter and
weigh 30-40 mg. However, according to Rogers, several investigators
are using the electrostatic levitation data to develop processes
and devices to handle bulk quantities of materials.
What
is "Undercooling"?
Undercooling
is a fascinating process. If done right, the temperature
of a liquid can be lowered below the normal freezing point
while its remains unfrozen, or unsolidified, and still in
the liquid state. This is undercooling.
The
undercooling phenomenon occurs in pure, undisturbed substances
that are slowly cooled. So long as no molecules join to
form a solid nucleus (called "nucleation"), the
sample remains liquid. Once a solid does form, it spreads
rapidly through the sample, and another contradiction occurs.
The temperature rises rapidly as the latent heat of fusion
is released in an effect called "recalescence"
-- and a flash of light often is seen. Then the sample cools
as it completes the change from liquid to solid.
Right:
The temperature profile as a sample is heated and cooled,
and during recalescence, is shown in the graph at right.
This profile is important in understanding the physical
properties of different alloys.
What's
even more interesting is that when the undercooled liquid
is allowed to freeze, or solidify, it forms a kind of material
that is very different from the "normally" frozen
material -- as is the case with the new material forms that
arise from solidifying an undercooled metal substance.
Almost
everyone has probably seen an undercooled, then rapidly-solidified,
liquid many times -- the one called snow! Snowflakes occur
when undercooled water falls through the atmosphere, eventually
striking another drop of water or piece of dust in the air
that causes it to rapidly solidify into a beautiful crystalline
snowflake structure.
The
snowflake structure is very different from regular ice,
although both are frozen water, because ice freezes from
a normal state, and snow solidifies from the undercooled
state. But if you leave snowflakes alone, they eventually
turn into regular ice -- because the "snowflake state"
is only partially stable, or metastable.
If a
metal suspended in a vacuum, not touching any container,
is heated and allowed to cool, its temperature will drop
below its freezing point while it stays liquid. It is possible
to have liquid metals that are at temperatures several hundred
degrees below the normal solidification or "freezing"
point of the metal.
When
the metal eventually does freeze, it happens in just a fraction
of a second, so fast that its energy emits a pulse of light.
The kinds of metallic solids that we get out of this process
are very different than one can obtain in any other way.
Some
of the new metal compounds formed from the undercooled state
remain un-crystallized and amorphous in their molecular
structure. They are called metallic glass, or by the commercial
name of "liquid metal" -- including materials
such as the Liquidmetal(tm) Alloy blend of titanium, zirconium,
nickel, copper and beryllium now being used in unique golf
clubs.
The
ESL is used by the NASA team and also by university and
corporation investigators selected through the NASA Research
Announcement (NRA) program, which solicits guest investigators
to use NASA facilities and resources. NRAs announce research
interests in support of NASA's programs and identify proposals
to be funded from among competitive project ideas conceived
by the respondents. NRA selections may result in grants,
contracts or cooperative agreements. NASA supports ESL experiments
on Earth in gravity conditions and also as Flight Definition
Projects to be carried out on the Space Shuttle and International
Space Station.
Right: The logo of MSFC's ESL
Facility implies the magical quality of levitating samples
for observation.
Scientists
can also propose to use the ESL through Advanced Technology
Development agreements, which are reserved for promising
high-risk research.
"Our
ESL containerless processing facility is the only national
laboratory available for selected investigators to run such
experiments with a variety of metal and ceramic compounds,"
Rogers states. "Our goal is to provide a place where
investigators from both the public and private sectors can
focus on creating new materials and new applications based
on the materials data we have developed."
The
Marshall Space Flight Center hosts visiting investigators
during their ESL processing sessions. Typical visits last
from one to two weeks. Investigators provide material samples,
known data on sample properties, an experiment plan and
occasionally some of their own data-gathering instrumentation.
The ESL facility staff works with investigators as experiments
are performed.
In the
year 2000, NASA selected 65 researchers to receive grants
in the field of Microgravity Materials Science -- totaling
approximately $22 million for earthbound and space experiments
over a four-year period.
In addition
to the many NASA-funded ground-based ESL research projects,
there are now four ESL Flight Definition Projects in progress
with three universities and one corporation:
Massachusetts
Institute of Technology, Cambridge, MA -- Prof. Merton C.
Flemings: "The Role of Convection and Growth Competition
on Phase Selection in Microgravity" -- which addresses
how movements in a suspended liquid sample and the growth
of competing crystals within it affect each phase of a material's
changes from liquid to solid state in the low-gravity of
space.
California
Institute of Technology, Pasadena, CA -- Prof. William L.
Johnson: "Physical Properties and Processing of Metallic
Glass Undercooled Alloys" -- which looks at many aspects
of metallic glass, or "liquid metal", alloys formed
in microgravity.
Washington
University at St. Louis, St. Louis, MO -- Prof. Kenneth
F. Kelton: "Studies of Nucleation and Growth, Specific
Heat and Viscosity of Undercooled Melts of Quasicrystals
and Polytetrahedral-Phase-Forming Alloys" -- a study
of several characteristics of undercooled solid alloys created
by specific kinds of crystal formation.
Containerless
Research Inc., Evanston, IL -- Dr. Richard Weber: "Microgravity
Studies of Liquid-liquid Phase Transitions in Undercooled
Alumina-Yttria Melts" -- experiments in how certain
substances form from undercooling of melted glass and oxide
materials.
NASA's
marvelous Electrostatic Levitator may not actually be magic,
but the discoveries that come from its use in these many
diverse studies are certain to be continually amazing!
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