It floatsNew tool levitates molten materials
It floats New MSFC tool levitates molten materials
A 3 mm drop of nickel-zirconium, heated to incandescence, hovers between electrically charged plates inside the Electrostatic Levitator.
March 9, 1998: A new lab tool doesn't exactly defy gravity, but it does hold it at bay so scientists can run materials experiments that could be spoiled if the samples touched a container wall.
"It's what I consider to be the next generation in containerless processing," said Dr. Mike Robinson of the Electrostatic Levitator (ESL) recently installed at NASA's Marshall Space Flight Center.
The ESL uses static electricity to suspend an object inside a vacuum chamber. While that happens, a laser heats the sample until it melts, so scientists can record a wide range of physical properties without contact with the container.
To make improved alloys and other compounds we must understand the physical properties that govern ingredients behave. These include:
- Surface tension, the same effect that lets small bugs walk on water
- Viscosity, how "thick" a liquid is
- Heat capacity, how slowly heat is absorbed or released
- Undercooling and nucleation, how far below freezing it will stay liquid
Determining these properties precisely is difficult, because anything that handles or contains a molten sample will alter the results. It can dampen vibrations or rapidly cool the sample. In some cases, the metal is reactive enough to damage its container.
One answer to this problem is "hands off" processing using static electricity to levitate a small sample. This is possible with the ESL, developed by Loral Space Systems of Palo Alto, California, and recently donated to NASA's Marshall Space Flight Center in Huntsville, Alabama.
"They wanted to give it to a national laboratory where it would be available to the scientific community," Robinson said of Loral's generosity. "We're the only national laboratory doing containerless processing."
The ESL will complement research with NASA/Marshall's Drop Tube Facility and with flight facilities such as the TEMPUS electromagnetic levitation furnace flown last year on the MSL-1 Spacelab mission. ESL holds the samples in full view of the detectors for several minutes at a time, but they are still under the effects of gravity. Drop Tube experiments are truly weightless, but for just 4.3 seconds, and the samples quickly fall past detectors. TEMPUS and other orbital facilities allow weightless experiments for long periods, but must be scheduled years in advance on Spacelab missions.
The ESL suspends liquid samples, including metals, without the sample touching a container and without the scientists handling equipment that might alter measurements. This makes ESL a premier tool for investigating fundamental physical properties of advanced materials, including undercooling and metallic glasses.
"It's a very quiet, controllable environment with completely independent heating and positioning systems," Robinson said.
|The heart of the ESL is the vacuum chamber (left) containing a pair of electrostatic plates and four electrodes that position the sample being processed (right). 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.|
The ESL uses the same effect that makes freshly dried socks push away from each other, although the application is controlled and precise. Two large, horizontal electrode plates electrically charge the sample and repel it upward until it balances between the two plates. Two smaller pairs of electrodes position the sample horizontally. A high-power deuterium arc lamp shines on the sample to replace the electrical charge the sample loses as it emits electrons while hot.
To keep the sample centered, a sophisticated three-dimensional digital feedback system controls the electrode charges. Two lasers (operating at different wavelengths or colors and at right angles to each other) shine through the vacuum chamber to cast a shadow on position sensors on the outside. The ESL computer, in turn, uses those positions to calculate subtle changes in the electrode charges, recenter the sample, and to aim the heating laser. The power of the electrostatic levitators is limited, so samples can be no more than 3 mm (0.12 in) in diameter.
Most experiments require melting a sample so scientists can record viscosity, surface tension, volume, and undercooling. A 50-watt laser, controlled from 0 to 100 percent and with a spot size from 10 mm down to 0.5 mm, heats samples, or controls cooling by supplying slightly less energy than the sample radiates. Experiments can also be run without laser heating.
The whole system is held in place by a special optical bench mounted atop an air-bearing table to isolate the system from vibration.
Right: ESL team members - Larry Savage, project scientist Jan Rogers, and Michael Robinson of NASA, and Doug Huie of Mevatec - check a test in progress. Robinson is peering through the long-distance microscope which is includes a TV camera. The vacuum chamber is just in front of Rogers. Positioning lasers are in the foreground, and the heating laser is mounted under the table.
... and shine
To make "hands off" measurements, the ESL employs several sensors looking in through viewports in the vacuum chamber wall. A CCD video camera behind a long-range microscope provides a magnified view of the sample illuminated by a conventional lamp until the sample is hot enough to glow.
Images can be recorded as standard and high-speed video, and digitized for analyses. A pyrometer, viewing through an infrared filter, measures the heat radiated by the sample. The images collected by the position sensors can also be used in scientific analysis. Scientists using the ESL may also supply their own instruments.
While the samples "float" in the ESL, they are not in a microgravity environment and convective flow effects can occur. The ESL is one of a suite of instruments for studying the physical properties. Others which NASA employs include drop towers, in which a sample is in free fall for 2-3 seconds and passes quickly through an instrument's field of view, and a variety of space-based facilities in which samples are under microgravity for extended periods.
Optical ports ring the ESL vacuum chamber to admit light from the heating laser (the beam passes through the at left), positioning lasers (one port is at center), and lamps (such as the deuterium arc lamp at right), and to allow diagnostic instruments to view the sample.
A large number of studies - primarily in undercooling research - will use the ESL. Measurements will include thermophysical properties such as heat capacity, viscosity, surface tension, and thermal conductivity. The ESL can also accommodate measurements of nucleation temperatures and rates, and solidification velocities.
Print quality JPG copies of these and other ESL images, and a Quicktime movie are available.
The ESL is available for use by scientists who are competitively selected through NASA Research Announcements (NRA), which solicit guest investigators to use NASA facilities and resources.
For additional information, check NASA Research Announcements (NRA) or the Microgravity Program Office, or e-mail the Microgravity Program Office. An NRA on materials science, including the ESL, is to be released later this year. Scientists can also propose to use the ESL through Advanced Technology Development agreements, which are reserved for promising, high-risk research.
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