Materials Science - Why? Studying the Growth of Dendrites Studying Semiconductor Crystal Growth Studying Solidification of Metal Alloys More Metals Research!

We perform materials science research in space to improve things that are made on Earth.  It is no accident that the terms Stone Age, Iron Age, and Bronze Age, each referring to specific materials, are used to describe particular periods in human history. It is the availability of materials, and our ability to use them to our advantage, that drives the progress of civilization.

The situation today is no different than in the past. As we enter the 21st century and the arrival of the Space Age, the materials of the 20th century - titanium aircraft parts, silicon-based semiconductors, lightweight aluminum alloys, and many others - simply cannot meet the needs of progress.

New materials must be researched, designed, and understood in order to meet these technical challenges. The materials science research activities within NASA use the unique laboratory environment of space in order to perform research into these new materials that cannot be done on the ground.

Studying the Growth of Dendrites

Many of the products we encounter on a daily basis, such as aluminum foil, soda cans, automobiles, and jet engines, are made from metals and alloys. When these products are created, many are formed from a liquid or molten state, that freezes to form a solid part or piece of metal. When you examine a freshly solidified metallic alloy with a strong magnfying glass, you'll see that the surface is not uniform, but is made up of many tiny individual crystalline grains. When examined even more closely with a microscope, you'll see that each of these metallic grains consist of a structure that llooks like tiny metallic pine trees crowding and growing into each other. These metallic tree-like crystals are called dendrites.

Dendrites are extremely important to study and understand. In many materials, the final material properties, such as resistance to corrosion, toughness, and strength are dependent on the original dendritic microstructure that is present in the material at the time of solidification from the liquid state. Therefore, in order to achieve certain desired physical properties in metallic alloy materials, it is important to understand and control the dendritic growth as these materials solidify.

Over the years, scientists have learned that the growth of dendrites is controlled by the transport of heat and/or solute from the moving solid-liquid interface into the supercooled melt. Under terrestrial conditions, gravity-induced convection, which transports heat and energy through the solidifying system, increases the dendrite growth speeds by nearly a factor of two compared to the microgravity environment experienced by Isothermal Dendritic Growth Experiment, or IDGE, for example, on USMP-2. Furthermore, the manner in which dendrites form in the near-absence of gravity is different than the conventional mathematical theory of dendrite growth would predict, in both the speed of dendrite growth, as well as in the spatial dimensions of the tip of the dendrite. Understanding the differences between theory and experiment is a hallmark of the reason for scientific investigation.

In the long term, scientists and engineers would like to be able to develop computational methods, such as computer programs and algorithms, that reveal both the microstructure and chemical segragation at the microscopic level, and the macroscopic end-product that is made up of the agglomeration of millions of tiny dendridic grains. This is a formidable challenge, which, when met, will allow the production of better, less expensive, and more reliable metal products. The full understanding of the behavior of a single, isolated dendrite, such as those to be examined on USMP-4, represents an important step in achieving better understanding and control of the final properties of materials that are dendritically solidified.

IDGE does not actually use metals on-orbit to study dendritic growth, but instead will use a surrogate material. On its first three missions, IDGE used a liquid - succinonitrile, or SCN - which grows crystals by forming dendritic, treelike shapes. This is a pattern followed by many metals and alloys. SCN forms a body-centered cubic crystal as it grows. Elements such as sodium, potassium, chromium, iron, cesium, and barium, all exhibit this type of structure.

For this fourth flight, IDGE will use pivalic acid, another transparent liquid, which prefers to grow along the stem, or the long axis, of the dendrite. It forms a face-centered cubic crystal. Metals including aluminum, copper, nickel, silver, platinum, lead, and gold are all elements with this structure. The use of a new material provides a much greater understanding of how dendrites form.

Studying Semiconductor Crystal Growth

How many things do you encounter on a daily basis that function through the use of semiconductors? Hundreds of daily-life examples, the computer in your car, your hand-held calculator, the computer you are using to read this WWW page, cellular phones, satellites, the grocery-store check out machine, and virtually every other piece of electronic equipment you use today has semiconductor material inside.

Most of the semiconductor material in use today is based around the element silicon. However as the performance requirements for semiconductors increase - faster switching times, smaller-scale circuitry, larger environmental operating ranges, higher precision instrumentation - the ability to utilize new, innovative, and unique materials outside the traditional silicon family becomes increasingly more important.

But before we can use these new materials, we must first understand both their physical properties, and how to manufacture them to the specifications required for use. On USMP-4, the Advanced Automated Directional Solidification Furnace (AADSF) will process two types of semiconductor materials which hold promise for a variety of high-tech applications; mercury cadmium telluride and lead tin telluride.

Space is a critical laboratory for conducting this research. Most semiconductor components on Earth are formed by melting and then resolidifying the material into the desired form. Any perturbation, impurity, or irregularity in the solidified material can lead to a direct degradation of the performance and utility of the material. These impurities or irregularities are often the result of gravity-driven flows within the melted material, or other gravity-induced effects. The picture below shows just what kinds of effects gravity can have on the uniformity of a sample. These data represent slices through two samples of mercury cadmium telluride. The sample on the left was formed on the ground in a 1-g environment, while the sample on the right was formed in microgravity. The higher uniformity of the space-grown sample compared to the ground-based sample is evident.

The formation of the solidified semiconductor material is extremely sensitive to the presence of gravity or other external forces. In fact on previous flights, investigators have found that characteristics of the samples' growth were noticeably different, depending on factors like the orientation of the experiment with respect to the shuttle's orbit around the Earth. In other words, even the tiny residual gravitational forces due to the shuttle itself, the non-spherical shape of the Earth, movement by the crew and equipment inside the orbiter, thruster firings, and other effects produce noticeable manifestations in the solidified semiconductor material.

For USMP-4, the AADSF will be operated with the Shuttle pointed in different directions relative to its orbit for each sample as scientists look for the best conditions for growth.

Pictured below is one AADSF ampoule, constructed of fused silica, which contains the semiconductor sample materials which will be grown through heating by the furnace.

---

Studying Solidification of Metal Alloys

Semiconductors are not the only materials we use in daily life that are formed from the molten state into the solidified state. A significant fraction of the metallic or alloy components of automobiles, buildings, bridges, railroad cars, and a host of other things are made by first melting the metal or alloy, then solidifying it into the part or component that is desired. Like the semiconductor material above, the chemical composition, structure, and properties of these metallic alloys - such as the strength of a turbine blade in an aircraft engine - are all partly dependent on how the materials solidify.

The detailed structure of a material is greatly affected by gravity-induced convection: flows which are caused by lighter portions of the material rising and denser portions sinking. This can disturb the ordering of atoms and distribution of chemical elements as the material crystallizes, causing defects and often diminishing the performance. MEPHISTO (the French acronym for "Apparatus for Studying Interesting Solidification Phenomena on Earth and in Space") will study the process of solidification of bismuth (a metal), with small amounts of tin (another metal) added.

While you can't see through metal alloys as they melt and freeze, you can sense what is happening inside by passing an electrical current through the samples. MEPHISTO contains three identical samples of bismuth with traces of tin. When in operation, these cylindrical samples will be solid at both ends, and liquid in the middle, providing two different solid-liquid interfaces. Two of the cartridges are equipped for different measurements of an electrical current passing through the samples. These measurements in effect become a probe of the regions where the samples freeze. The third sample will be quick-frozen to preserve the nature of the solid-liquid interface, that might otherwise be distorted or modified during a slow cool-down. This can then be studied after the flight.

More Metals Research!

Composite materials play critical roles in industries such as transportation, where light weight and high strength are essential properties. When these materials are processed, they can generally involve the mixture of hard, ceramic particles within a melted metal. To get a high-quality material with uniform strength, it is often important to make sure that the ceramic particles are distributed homogeneously throughout the solidified metal.

However, as the metal solidifies, the boundary between the liquid portion of the metal and the solidified portion, can often move particles around, adversely effecting their distribution. Particles can either be pushed by the moving interface, conceptually like snow in front of a snow plow, or particles can be engulfed into the solidifying metal, depending on the conditions. Therefore the interaction between the solid/liquid interface and these particles can have deterministic effects on the overall bulk properties of the final material.

The Particle Engulfment and Pushing by a Solid/Liquid Interface (PEP) experiment will be conducted in the glovebox facility to study the interaction between particles and the solid/liquid interface, without the additional gravity-generated convection flows that can wash out the ability to study the important, yet more subtle interactions of interest.

Some metallic alloys involve the mixture of components that are hard to combine (conceptually like oil and vinegar in Italian salad dressing) in the liquid state. These alloys, called immiscibles have many useful properties, but also have several drawbacks. The inability of the components to mix is often due to a significant density difference between the components, causing the heavier component to sink to the bottom when processed on the ground. The resulting non-uniform structure can have detrimental effects on the final properties of the material.

This gravity-driven separation can be overcome by processing in a microgravity envrionment, but curiously enough, separation is observed to take place through a process in which one material begins to collect on the walls of its container. This process, called "wetting," again causes a non-uniform distribution of components in the alloy.

The Wetting Characteristics of Immiscibles (WCI) investigation will examine potential methods to control this wetting behavior in order to produce more uniform alloys. The knowledge gained from processing these materials in space may then be translated to efforts on the ground that may lead to higher-quality immiscible alloys of more uniform composition.

return to top of page
Go to the USMP-4 Science Home Page

last updated October 29, 1997

Author: Dave Dooling, Dr. John Horack
Curator: Linda Porter
NASA Official: Dr. Greg Wilson