Don't forget to visit the CSLM STS-83
Results Page, including plans for reflight
on STS-94.
The Coarsening in Solid-Liquid Mixtures (CSLM) experiment is a materials science space flight experiment whose purpose is to investigate the kinetics of competitive particle growth within a liquid matrix. During coarsening, small particles shrink by losing atoms to larger particles, causing the larger particles to grow. In this experiment solid particles of tin will grow (coarsen) within a liquid lead-tin eutectic matrix. Figures 1a, 1b, and 1c show the coarsening of tin particles in a lead-tin eutectic as a function of time. By conducting this experiment in a microgravity environment, a greater range of solid volume fractions can be studied, and the effects of convection present in terrestrial experiments will be negligible. The CSLM experiment is slated to fly on board the space shuttle Columbia as part of the Spacelab mission Microgravity Science Laboratory (MSL-1) scheduled for launch April 1, 1997 on STS-83. The Experiment will be run in the Glovebox installed in the Spacelab module.
The coarsening of particles within a matrix is a phenomenon that occurs
in many metallurgical and other systems. For example, the second-phase particles
in high temperature turbine blade materials undergo coarsening at the operating
temperature of the turbine. The coarsening process degrades the strength
of the blade because turbine alloys containing a few large particles are
weaker than those containing many small ones. Coarsening occurs in liquid-phase
sintered materials such as tungsten carbide - cobalt, iron - copper, dental
amalgam for fillings, and porcelain. An example of liquid-liquid coarsening
is the growth and coalescence of quartz droplets in molten steel during
deoxidation with ferrosilicon. The growth of liquid droplets in a vapor
phase that occurs inside rain clouds (particularly near the equator, where
the vapor pressure of water is high) is a commonplace example of the coarsening
phenomenon. The CSLM study will help define the mechanisms and rates of
coarsening that govern all of these systems.
 |
Figure 1a: Sn-Pb microstructure coarsened (on earth) for 375 seconds (6.25 min). Click here for a larger image (65k JPEG). |
 |
Figure 1b: Sn-Pb microstructure coarsened (on earth) for 2340 seconds (39 min). Click here for a larger image (48k JPEG). |
 |
Figure 1c: Sn-Pb microstructure coarsened (on earth) for 14600 seconds (4 hrs). Click here for a larger image (37k JPEG). |
The coarsening of second phase particles is also known as Ostwald ripening. The driving force for the coarsening process is a reduction in the total interfacial area, which translates to a reduction in the total system energy. The kinetics of the process are controlled by diffusional mass flow through the matrix phase. The classical work of Lifshitz, Sloyozov, and Wagner predicts that the coarsening rate will be proportional to the cube of the average particle radius, but their theories are only valid for near-zero particle volume fractions. More recent theories predict that the coarsening rate and particle size distributions are functions of the particle volume fraction. Generally the theories assume a spatially random particle distribution. However, on earth the differences in density between the particles and the matrix cause sedimentation. In the lead-tin system employed for this experiment, tin particles float to the top of the sample, resulting in a high concentration of tin particles at the top and a zone of Pb-Sn eutectic liquid at the bottom. To avoid sedimentation on earth, large volume fractions (greater than 0.55) of the coarsening phase must be employed so that a solid skeletal structure forms.
There are several reasons why this experiment will be worthwhile only if it is conducted in the microgravity conditions present about the Space Shuttle. The motion of solid particles in a 10 micro-g environment due to the density difference between solid and liquid is very small, so the solid tin particles should be uniformly distributed over very long time periods. The absence of particle sedimentation will also allow for experiments with a wide range of solid volume fractions in different specimens. Systems with low solid volume fractions have never been accurately studied on earth without using adjustable parameters. With negligible convection in the liquid under microgravity conditions, the coarsening process will be entirely diffusion controlled. The absence of gravity may also alter the formation of the solid skeletal structure for the high solid volume fraction specimens.
The lead-tin system is ideal for coarsening experiments for a variety of reasons. The low melting temperature means that diffusion and coarsening occur fairly rapidly. Also, the design of the isothermal furnace is more straightforward for low temperature service (185C coarsening temperature). The nature of the interfacial energies is such that the liquid Pb-Sn eutectic penetrates the tin grain boundaries, insuring that particle contact will not induce coalescence. Thermophysical properties such as diffusion coefficients and interfacial energies are also well known for this system. For the space experiments pure tin bicrystals in contact with Pb-Sn eutectic will be flown with the other specimens. The grain boundary grooves that grow in the bicrystals during the soak at 185C will later be measured to help define thermophysical properties under microgravity conditions.
The flight hardware consists of two separable pieces of equipment, the sample processing unit (SPU) and the electronic control unit (ECU). The SPU incorporates a small electric sample heater with a water quench system. The electric heater consists of a rectangular sample holder plate sandwiched by two plates with embedded heater wires. The holder plate has nine cylindrical sample holes plus five thermistors for temperature monitoring and control. One of the five thermistors will be implanted inside a Sn-Pb sample for accurate monitoring of specimen temperature. The heater assembly with PID control algorithm can provide temperatures to ± 0.2C across the width of the heater, and record temperatures to an absolute accuracy of ±0.05C. Aluminum was used for the heater and holder plates because of its high thermal conductivity (which facilitates rapid heatup and quench), and because molten lead-tin does not "wet" the surface of aluminum.
Figure 2a: View of SPU with endcap assembly off. The sample holder - heater assembly is on the right, and the spray heads are visible inside the cylindrical body. Click here for a larger image (66k JPEG).
The quenching system has been designed to provide for a rapid quench within the small enclosure of the SPU, and without translating the samples from the heater to a quench bath. When the quench system is activated, a solenoid valve releases pressurized nitrogen, which forces water in a reservoir through rupture discs. The water flows into two spray heads, one on either side and parallel to the sample heater, and sprays through numerous small holes directly onto the heater plates. This system can quench the Sn-Pb specimens from 185C to under 120C in less than 6 seconds.
The ECU contains the power supply, electrical control, and data storage components. There are three toggle switches on the front of the ECU that allow the astronaut to power up the unit, activate the experiment run, and abort the run with quench. There are also three indicator lights and an LCD display that shows the status of the experiment and the temperatures of the thermistors in the sample holder. The temperature-time data from the experiment run is stored on a hard disk located in the ECU. Temperature data sampled at low rates before, during, and after flight is recorded by a data logger mounted in the SPU.
Figure 3: Interior view of the ECU. Click here for a larger image (112k JPEG).
There will be eight separate experimental runs (possibly a ninth) to
be conducted on STS-83.
The coarsening times will be 0, 150, 375, 940, 2340, 5860, 14600, 36600,
and possibly 86400 seconds at 185.0C. Each coarsening run of a given length
will have a separate SPU. Each SPU has an identical set of nine specimens,
of which the primary seven have solid tin volume fractions (at 185.0 C)
of 0.05, 0.1, 0.2, 0.3, 0.5, 0.7, and 0.8. The eighth sample is a 0.3 solid
volume fraction sample with an embedded thermistor, and the ninth sample
is a tin bicrystal for grain boundary grooving measurements. The Sn-Pb samples
were manufactured by casting a cylindrical ingot, swaging the ingot down,
and axially cold-working cut lengths of the rod. These procedures result
in a microstructure that is uniform in all directions. The samples were
machined down to 10.00 mm diameter by 5.00 mm height, and fit tightly into
the cylindrical holes in the sample holder so there will be no significant
free volume at the coarsening temperature.
The SPU and ECU will be installed inside the Glovebox,
and the two units will be connected manually with a patch cable (Figure
4). Each run will be activated simply by flipping the “experiment on”
switch, and the ECU will identify which SPU is connected by the configuration
(resistance) of jumpers that are unique to each SPU. The run will progress
automatically through initialization, heatup, soak, and quench phases of
the experiment, after which the run can be deactivated by a toggle switch.
The run can also be aborted with quench by a separate toggle switch should
the need arise. The ninth SPU will have an additional switch whereby the
astronaut can select any of the other eight run times, or a ninth 24 hour
run.
Figure 4: The ECU and SPU connected in their flight configuration. Click here for a larger image (83k JPEG).
The CSLM experiment runs do not need to be attended by an astronaut after activation. There is no need for orbit-to-ground telemetry directly from the experimental apparatus. A videocamera mounted outside the Glovebox will be aimed at the LCD screen on the ECU so that scientists on earth can view the progress of the experiment. The only data collected during flight is the time-temperature data that will be retrieved from the ECU after landing.
After the specimens are returned to earth, extensive microstructural analyses will be performed at Northwestern University. Microstructural features that will be studied are tin particle size distribution, particle morphology, skeletal structure and coalescence (if any), and particle crystallographic orientation. Sample sections for 2-D microstructural analyses will be prepared using metallographic polishing and/or micromilling. A micromiller can prepare a scratch-free parallel surface at a predetermined depth with a vertical resolution of 2µm. The 2-D images will be digitally stacked to generate 3-D reconstructions of sample morphology. These reconstructions of the skeletal structure will provide 3-D images of the microstructure that will illustrate the physical processes occurring during coarsening. All of the prepared sections will be etched, and the images will be digitized for analysis. Each image will be stored on a write-once compact disc to insure that the information will not be accidentally altered or deleted.
In order to characterize the coarsening kinetics, many stereological measurements will need to be made. On each section approximately 10,000 intercept lengths and section areas of the particles will be measured. From these measurements the particle size distributions as functions of the coarsening times will be produced. The morphology of the individual particles and agglomerates will be examined by measuring the roundness of the particles and the agglomerates, plus the average number of particles per agglomerate exposed in a section. These stereological measurements will also reveal differences in morphology of the solid-liquid mixtures processed in space and on the ground.
Lastly, the crystallographic orientation of the tin particles will be analyzed via electron diffraction techniques. By using a computer automated system, hundreds of grains and particles will be sampled, and orientation distributions can be generated. From these distributions it will be determined whether the grain boundary misorientations evolve with time or vary with volume fraction.
Don't forget to visit the CSLM STS-83
Results Page, including plans for reflight
on STS-94.
The CSLM experimental techniques, methods of analyses, and flight hardware were developed by a dedicated team of people at Northwestern University and NASA Lewis Research Center. Acknowledgement must also be made to the numerous others who assisted in the design, fabrication, and testing of the engineering and flight hardware.
Principal Investigator:
Dr. Peter W. Voorhees
Dept. Of Materials Science and
Engineering
Northwestern University
Evanston, IL 60208
Tel: (847) 491-7815
Project Manager:
John J. Caruso
Microgravity Science Division
NASA Lewis Research Center
Cleveland, OH 44135
Tel: (216) 433-3324
Program Scientist:
Dr. Michael J. Wargo
Office of Biological and Physical
Research
NASA Headquarters
Washington, DC 20546
Return to the MSD
Experiment Page
Return to the MSD Home Page
Go to the NASA Glenn Home Page
This Web page was created by Mark Assel , with material supplied
by Peter Voorhees and the rest of the CSLM
team.
This page last updated 12/08/97.
The information contained in this document may be outdated.
If any of the above links are not working, contact
Dawn Jenkins, Fluid Physics
Web Administrator. Also check the
Fluid Physics Contact Page if attempts
to contact the above listed personnel fail.
