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the in-situ monitoring of crystal growth
using Mephisto experiments is a
collaborative American and French
investigation of the fundamentals of
crystal growth the french space agency
cnes and the french nuclear energy
agency ceng designed and built Mephisto
the crystal growth hardware for this
experiment project management and
support functions of the experiments are
provided by personnel of the NASA Lewis
Research Center in Cleveland Ohio the
experiment will fly as part of the
second United States microgravity
payload us mp2 aboard the space shuttle
Columbia the United States principal
investigator for this new Fisto flight
is professor Raisa and las tunas chair
of the Department of material science
and engineering at the University of
Florida the data from this
french-american collaboration will be
analyzed and the scientific information
will be exchanged the knowledge gained
from these experiments will be of
fundamental importance to the
semiconductor industry for the upcoming
US mp2 mission dr. Bastian and his
research team have chosen bismuth with
small amounts of tin to study
solidification and crystal growth
researchers have found that crystals
grown from this alloy possess a
well-behaved Seebeck coefficient this
property of the bismuth tin crystal
allows a non-intrusive real-time
measurement of the solid liquid
interface temperature the measurement of
temperature and other local conditions
at the solid-liquid interface or
boundary between solid and liquid
material are important to scientists in
understanding the structural and
chemical composition of the crystals
that are produced
to illustrate a solid-liquid interface
consider the point at which the solid
wax of a burning candle turns to liquid
the solid liquid interface continues to
move downward as the candle continues to
burn the hardware is mounted on a bridge
like carrier in the space shuttles cargo
bay the french-built mephisto furnace is
designed to grow crystals under well
controlled conditions of temperature and
velocity as the experiment begins the
sample rods are melted the thermal
condition is identical for all three
samples as illustrated here the red
components represent the furnaces while
the blue components represent the heat
sinks or cooling sections the furnace on
the left is stationary while the furnace
on the right is mounted on a sliding
mechanism as the experiment continues
the melted sample rod is slowly
solidified the boundary between the
liquid and solid material or solid
liquid interface occurs between the red
and blue components of the Mephisto
apparatus as the mobile furnace moves
toward the fixed furnace the solid
liquid interface moves also this causes
the temperature of the liquid material
to fall slightly below its freezing
point before solidifying this process
known is supercooling provides a means
of controlling the solidification of the
crystal this is a key variable in
determining the structural composition
of the crystal super cooling is used to
describe the temperature of the
interface when this interface is moving
if the interface is not moving in other
words if you are not growing crystals
the interface temperature is exactly at
the melting point however when the
interface moves its temperature is
slightly below the melting point and the
difference between the actual interface
temperature and the melting point of
that interface is called super cooling
it is actually the driving
force that causes the interface to move
although mephisto processes all three
samples simultaneously different
measurements are taken of each sample
the Seebeck technique is a non intrusive
way of measuring the temperature of the
solid liquid interface a voltage across
the sample is created by a potential
difference between the stationary solid
and liquid interface and the moving
solid and liquid interface the level of
super cooling at the interface in the
moving furnace is determined from this
voltage difference another measurement
is pelletier marking these markings are
created by pulsing an electric current
through the sample this current pulse
causes a momentary change in the local
chemical composition that outlines the
shape of the solid liquid interface by
examining the shape of this interface
scientists can determine the growth
conditions necessary to produce crystals
with fewer imperfections and more
uniform composition at the conclusion of
the experiment the final two centimeters
of one of the melted sample rods is
gassed quenched the quenching
instantaneously freezes the tin in the
liquid the quenched sample can then be
analyzed to determine the at temperature
concentration of tin in the liquid all
this experiment information is
transmitted to the payload operations
control center in real time as it is
happening the principal investigator can
issue commands to adjust or repeat any
segment of the experiment such as
rerunning some of the melting rates when
these experience we hope to learn the
fundamentals that are involved in Salta
fication of crystals you hope to
understand what is happening at the
solid liquid interface and use this
understanding in improving our
scientific and technological basis for
crystal growth so how is all this
experimentation with crystal growth
going to help us here on earth well
since the inception of the
first I'm so sad when people were amazed
by the miracle of wireless
communications the development and
ever-increasing news of crystals has
generated the search for improvement in
crystal quality on earth gravity makes
the growing process more complex non
uniformity defects and voids are common
and results in adverse effects on the
properties of the crystal by growing
crystals in space these earthbound
effects are eliminated we'll have a
better understanding of the growth
process which will enable us to optimize
techniques for growing crystals here on
earth this information translates into
practical uses such as the development
of better semiconductors electrical
circuits are built into the silicon
crystals used in today's solid-state
devices in industry complex machining
operations robotic assembly and
automated inspection are all controlled
by crystal equipped mechanisms in the
medical field new techniques in the
operating rooms are the result of
advances in crystal technology computers
are increasingly faster and more
efficient because of better quality
crystals and today's communications have
come a long way from the first crystal
set this experiment will help to advance
the fundamental knowledge of
solidification and crystal growth it is
a collaborative international exercise
in which the scientific data will be
shared and exchanged for the benefit of
all of us
you