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I

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Oh

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the in-situ monitoring of crystal growth

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using Mephisto experiments is a

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collaborative American and French

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investigation of the fundamentals of

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crystal growth the french space agency

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cnes and the french nuclear energy

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agency ceng designed and built Mephisto

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the crystal growth hardware for this

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experiment project management and

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support functions of the experiments are

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provided by personnel of the NASA Lewis

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Research Center in Cleveland Ohio the

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experiment will fly as part of the

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second United States microgravity

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payload us mp2 aboard the space shuttle

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Columbia the United States principal

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investigator for this new Fisto flight

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is professor Raisa and las tunas chair

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of the Department of material science

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and engineering at the University of

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Florida the data from this

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french-american collaboration will be

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analyzed and the scientific information

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will be exchanged the knowledge gained

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from these experiments will be of

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fundamental importance to the

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semiconductor industry for the upcoming

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US mp2 mission dr. Bastian and his

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research team have chosen bismuth with

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small amounts of tin to study

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solidification and crystal growth

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researchers have found that crystals

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grown from this alloy possess a

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well-behaved Seebeck coefficient this

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property of the bismuth tin crystal

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allows a non-intrusive real-time

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measurement of the solid liquid

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interface temperature the measurement of

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temperature and other local conditions

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at the solid-liquid interface or

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boundary between solid and liquid

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material are important to scientists in

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understanding the structural and

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chemical composition of the crystals

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that are produced

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to illustrate a solid-liquid interface

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consider the point at which the solid

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wax of a burning candle turns to liquid

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the solid liquid interface continues to

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move downward as the candle continues to

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burn the hardware is mounted on a bridge

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like carrier in the space shuttles cargo

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bay the french-built mephisto furnace is

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designed to grow crystals under well

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controlled conditions of temperature and

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velocity as the experiment begins the

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sample rods are melted the thermal

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condition is identical for all three

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samples as illustrated here the red

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components represent the furnaces while

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the blue components represent the heat

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sinks or cooling sections the furnace on

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the left is stationary while the furnace

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on the right is mounted on a sliding

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mechanism as the experiment continues

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the melted sample rod is slowly

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solidified the boundary between the

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liquid and solid material or solid

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liquid interface occurs between the red

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and blue components of the Mephisto

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apparatus as the mobile furnace moves

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toward the fixed furnace the solid

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liquid interface moves also this causes

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the temperature of the liquid material

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to fall slightly below its freezing

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point before solidifying this process

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known is supercooling provides a means

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of controlling the solidification of the

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crystal this is a key variable in

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determining the structural composition

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of the crystal super cooling is used to

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describe the temperature of the

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interface when this interface is moving

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if the interface is not moving in other

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words if you are not growing crystals

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the interface temperature is exactly at

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the melting point however when the

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interface moves its temperature is

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slightly below the melting point and the

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difference between the actual interface

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temperature and the melting point of

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that interface is called super cooling

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it is actually the driving

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force that causes the interface to move

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although mephisto processes all three

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samples simultaneously different

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measurements are taken of each sample

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the Seebeck technique is a non intrusive

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way of measuring the temperature of the

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solid liquid interface a voltage across

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the sample is created by a potential

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difference between the stationary solid

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and liquid interface and the moving

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solid and liquid interface the level of

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super cooling at the interface in the

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moving furnace is determined from this

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voltage difference another measurement

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is pelletier marking these markings are

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created by pulsing an electric current

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through the sample this current pulse

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causes a momentary change in the local

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chemical composition that outlines the

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shape of the solid liquid interface by

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examining the shape of this interface

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scientists can determine the growth

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conditions necessary to produce crystals

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with fewer imperfections and more

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uniform composition at the conclusion of

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the experiment the final two centimeters

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of one of the melted sample rods is

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gassed quenched the quenching

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instantaneously freezes the tin in the

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liquid the quenched sample can then be

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analyzed to determine the at temperature

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concentration of tin in the liquid all

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this experiment information is

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transmitted to the payload operations

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control center in real time as it is

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happening the principal investigator can

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issue commands to adjust or repeat any

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segment of the experiment such as

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rerunning some of the melting rates when

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these experience we hope to learn the

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fundamentals that are involved in Salta

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fication of crystals you hope to

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understand what is happening at the

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solid liquid interface and use this

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understanding in improving our

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scientific and technological basis for

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crystal growth so how is all this

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experimentation with crystal growth

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going to help us here on earth well

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since the inception of the

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first I'm so sad when people were amazed

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by the miracle of wireless

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communications the development and

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ever-increasing news of crystals has

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generated the search for improvement in

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crystal quality on earth gravity makes

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the growing process more complex non

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uniformity defects and voids are common

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and results in adverse effects on the

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properties of the crystal by growing

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crystals in space these earthbound

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effects are eliminated we'll have a

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better understanding of the growth

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process which will enable us to optimize

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techniques for growing crystals here on

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earth this information translates into

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practical uses such as the development

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of better semiconductors electrical

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circuits are built into the silicon

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crystals used in today's solid-state

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devices in industry complex machining

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operations robotic assembly and

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automated inspection are all controlled

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by crystal equipped mechanisms in the

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medical field new techniques in the

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operating rooms are the result of

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advances in crystal technology computers

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are increasingly faster and more

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efficient because of better quality

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crystals and today's communications have

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come a long way from the first crystal

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set this experiment will help to advance

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the fundamental knowledge of

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solidification and crystal growth it is

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a collaborative international exercise

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in which the scientific data will be