NASA Demonstrates Their New Electrostatic Levitator

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!