December 1, 1997
United States Microgravity Payload-4 - Flight
Day 12
Boiling water may seem about as far as you can get from the super-insulated dewar of liquid helium now aboard the fourth U.S. Microgravity Payload (USMP-4) mission. Yet the two are closely related because they represent matter making a phase transition.
Nailing down the conditions where that transition occurs will help us understand how to design better materials in the future. That was that goal of a predecessor of USMP-4's Confined Helium Experiment, the Lambda Point Experiment on USMP-1 in October 1992. Indeed, CHeX is built from hardware used on LPE (in turn, that was built partially from the helium dewar carried on the Spacelab 2 mission in 1985).
"Lambda point is a fairly esoteric transition,"
said Dr.
John Lipa of Stanford
University. Lipa is the principal investigator
for both LPE and CHeX (in the white cylinder
marked JPL, right). "Its beauty is that you can study it with fairly
high precision" in the low-gravity environment of space.
Helium is the second most abundant element in the universe: it's the "ash" from hydrogen fusion in stars. It's also the gas of choice for blimps and balloons. Because it's an inert gas, helium does not ordinarily combine with other atoms, nor does it easily form a solid.
At 2.177 K (4 deg. F above absolute zero, -459 deg. F), though, it becomes helium II, an ordinary name for a superfluid.
"Above superfluid conditions," Lipa said, "liquid helium acts like a normal liquid. Below the superfluid transition, its properties are different."
Helium goes from being an ordinary conductor of heat to an almost infinite conductor. This does not mean it will withstand a blowtorch or even a match. It does mean that superfluid helium does a great job of carrying away miniscule quantities of heat, say, from a sensor that you want to cool so you can see dark stars.
"The heat diffuses like a wave motion," Lipa said. "It's just like a sound wave." Superfluid helium also flows without resistance through pores in almost anything as compared to ordinary liquid helium dragging its way through small openings.
"It goes almost as if it were a huge blob," Lipa said, recalling the 1950s science fiction movie, "The Blob."
While few of us will ever come close to helium II, understanding its life in this liquid-solid nether region may make better life for us in the universe of condensed matter - solids, liquids, and even gases.
As matter changes forms, from solid to liquid to gas and back, it undergoes phase transitions, like water boiling or freezing. Understanding the details of these changes can help us in understanding how to manipulate matter to make better products.
Phase transitions have been an important pursuit of physicists. An important theory in the area was developed in the 1970s by Dr. Kenneth G. Wilson of Cornell University. That work earned him the 1982 Nobel Prize in physics.
The Lambda Phase Experiment was conceived as one of the most rigorous tests of second-order transitions. These are important in making magnetic materials, superconductors, and other advanced materials.
Lipa explained that LPE measured helium's heat capacity, how much heat it would absorb to raise its temperature by a certain amount. On Earth, the experiment is limited by convection. To achieve the high resolutions needed to test math models of phase transitions, LPE was designed for space where compression from the fluid's own weight can be eliminated.
"This is frontier science," Lipa said. "We're trying to find out how far we can push some of the aspects of condensed matter, to find the limits."
With LPE he did just that. A series of measurements were made which gauged the heat capacity with changes as small as 2 nanokelvin (2 billionths of a degree K) above the transition point.
"We got orders of magnitude improvements in the parameters predicted by theory," he said. "We are now four to five times more accurate than theoretical models. Now we've turned the tables on the model builders."
As a result, Lipa has set a benchmark that will be used in guiding future research in this area.
He also set the stage for this mission's experiments which take physics to the edge of Flatland in a quest to understand just how small microcircuits can be.
Check
out the daily
Mission Status Reports prepared by Marshall's Public Affairs Office.
Author: Dave
Dooling
Curator: Linda Porter
NASA Official: Gregory S.
Wilson