Materials Science

CONTENTS

Electronic Materials	Polymers
Glasses and Ceramics	Microgravity Research and Exploration
Metals and Alloys

Materials science is an extremely broad field that encompasses the study of all materials. Materials scientists seek to understand the formation, structure, and properties of materials on various scales, ranging from the atomic to microscopic to macroscopic (large enough to be visible). Establishing quantitative and predictive relationships between the way a material is produced (processing), its structure (how the atoms are arranged), and its properties is fundamental to the study of materials. Materials exist in two forms: solids and fluids. Solids can be subdivided into two categorie--crystalline and noncrystalline (amorphous)- based on the internal arrangement of their atoms or molecules. Metals (such as copper, steel and lead), ceramics (such as aluminum oxide and magnesium oxide), and semiconductors (such as silicon and gallium arsenide) are all crystalline solids because their atoms form an ordered internal structure. Most polymers (such as plastics) and glasses are amorphous solids, which means that they have no long range specifically ordered atomic or molecular arrangement. One principal objective of microgravity materials science research is to gain a better understanding of how gravity-driven phenomena affect the solidification and crystal growth of materials. Buoyancy-driven convection, sedimentation, and hydrostatic pressure can create defects (irregularities) in the internal structure of materials, which in turn alter their properties.

Many materials scientists use a triangle such as this to describe the relationship between structure, processing, and properties. Microgravity can play an important role in establishing the relationships in a quantitative and predictive manner.

The virtual absence of gravity-dependent phenomena in microgravity allows researchers to study underlying events that are normally obscured by the effects of gravity and which are therefore difficult or impossible to study quantitatively on Earth. For example, in microgravity, where buoyancy-driven convection is greatly reduced, scientists can carefully and quantitatively study segregation, a phenomenon that influences the distribution of a solid's components as it forms from a liquid or gas. Microgravity also supports an alternative approach to studying materials called containerless processing. Containerless processing has an advantage over normal processing in that containers can contaminate the materials being processed inside them. In addition, there are some cases in which there are no containers that will withstand the very high temperatures and corrosive environments needed to work with certain materials. Containerless processing, in which acoustic, electromagnetic, or electrostatic forces are used to position and manipulate a sample, thereby eliminating the need for a container, is an attractive solution to these problems. Furthermore, microgravity requires much smaller forces to control the position of containerless samples, so the materials being studied are not disturbed as much as they would be if they were levitated on Earth.

Schematic of the Electromagnetic Containerless Processing Facility (TEMPUS) used on Shuttle missions STS-65 and STS-83.

Materials science research in microgravity leads to a better understanding of how materials are formed and how the properties of materials are influenced by their formation. Researchers are particularly interested in increasing their fundamental knowledge of the physics and chemistry of phase changes (when a material changes from liquid to solid, gas to solid, etc.). This knowledge is applied to designing better process-control strategies and production facilities in laboratories on Earth. In addition, microgravity experimentation will eventually enable the production of limited quantities of high-quality materials and of materials that exhibit unique properties for use as benchmarks. Microgravity researchers are interested in studying various methods of crystallization, including solidification (like freezing water to make ice cubes), crystallization from solution (the way rock candy is made from a solution of sugar and water), and crystal growth from the vapor (like frost forming in a freezer). These processes all involve fluids, which are the materials that are most influenced by gravitational effects. Examining these methods of transforming liquids or gases into a solid in microgravity gives researchers insight into other influential processes at work in the crystallization process. Electronic Materials Electronic materials play an important role in the operation of computers, medical instruments, power systems, and communications systems. Semiconductors are well-known examples of electronic materials and are a main target of microgravity materials science research. Applications include creating crystals for use in X-ray, gamma-ray, and infrared detectors, lasers, computer chips, and solar cells. Each of these devices epends on the ability to manipulate the crystalline and chemical structure (perfection) of the material, which can be strongly influenced by gravity as crystals are formed.

Diagram of a multizone furnace used to grow semiconductor materials on the Shuttle. A mechanism moves an existing c crystal through the temperature zones, melting the sample then cooling it so that it solidifies. In other furnace designs, the heating mechanism moves and the sample is stationary. What are the advantages and disadvantages of each approach?

The properties of electronic materials are directly related to the degree of chemical and crystalline perfection present in the materials. However, perfect crystals are not normally the ultimate goal. For example, the presence of just a few impurities in some electronic materials can change their ability to conduct electricity by over a million times. By carefully controlling crystalline defects and the introduction of desirable impurities to the crystals, scientists and engineers can design better electronic devices with a wide range of applications.

Glasses and Ceramics
A glass is any material that is formed without a long range ordered arrangement of atoms. Some materials that usually take crystalline forms, like metals, can also be forced to form as glasses by rapidly cooling molten materials to a temperature far below their normal solidification point. When the material solidifies, it freezes so quickly that its atoms or molecules do not have time to arrange themselves systematically.

Ceramics are inorganic nonmetallic materials that can be extraordinarily strong at very high temperatures, performing far better than metallic systems under certain circumstances. They will have many more applications when important fundamental problems can be solved. If a ceramic turbine blade, for example, could operate at high temperatures while maintaining its strength, it would provide overall thermodynamic efficiencies and fuel efficiencies that would revolutionize transportation. The problem with ceramics is that when they fail, they fail catastrophically, breaking in an irreparable manner.

Glasses and ceramics are generally unable to absorb the impacts that metals can; instead, they crack under great force or stress (whereas metals generally bend before they break). An important part of ceramics and glass research in microgravity involves controlling the minute flaws that govern how these materials fail. From information obtained through microgravity research, scientists hope to be able to control the processing of ceramics so that they can, during processing, prevent the formation of imperfections that lead to catastrophic failure.

Applications for knowledge obtained through research in these areas include improving glass fibers used in telecommunications and creating high-strength, abrasion-resistant crystalline ceramics used for gas turbines, fuel-efficient internal combustion engines, and bioceramic artificial bones, joints, and teeth.

Metals and Alloys Metals and alloys constitute an important category of engineered materials. These materials include structural materials, many types of composites, electrical conductors, and magnetic materials. Research in this area is primarily concerned with advancing the understanding of metals and alloys processing so that structure and, ultimately, properties, can be controlled as the materials are originally formed. By removing the influence of gravity, scientists can more closely observe influential processes in structure formation that occurs during solidification. The properties of metals and alloys are linked to their crystalline and chemical structure; for example, the mechanical strength and corrosion resistance of an alloy are determined by its internal arrangement of atoms, which develops as the metal or alloy solidifies from its molten state. One aspect of the solidification of metals and alloys that influences their microstructures is the shape of the boundary, or interface, that exists between a liquid and a solid in a solidifying material. During the solidification process, as the rate of solidification increases under the same thermal conditions, the shape of the solidifying interface has been shown to go through a series of transitions.

Magnification of a sample of an aluminum-indium alloy. When the sample is melted then controllably solidifies in the AGHF; the indium forms in cylindrical fibers within a solid aluminum matrix.

At low rates of growths the interface is planar (flat or smoothly curved on a macroscopic scale). As the rate of growth increases, the interface develops a corrugated texture until three dimensional cells (similar in shape to the cells in a beehive but much smaller) form in the solid. A further increase in the rate of growth causes the formation of dendrites. The development of these different interface shapes and the transition from one shape to another is controlled by the morphological stability (shape stability) of the interface. This stability is influenced by many factors. Gravity plays an important role in a number of them. In particular, buoyancy-driven convection can influence the stability and, thus, the shape of the solidifying interface. Data obtained about the conditions under which certain types of solidification boundaries appear can help to explain the formation of the crystalline structure of a material.

Another area of interest in metals and alloys research in microgravity is multiphase solidification. Certain materials, which are known as eutectics and monotectics, transform from a single phase liquid to substances of more than one phase when they are solidified. When these materials are processed on Earth, the resultant substances have a structure that was influenced by gravity either due to buoyancy-driven convection or sedimentation. But when processed in microgravity, theory predicts that the end product should consist of an evenly dispersed, multiphase structure.

Eutectic solidification is when one liquid, of uniform composition, forms with two distinct solid phases. An example of such a material is the alloy manganese-bismuth. Solidifying liquid Mn-Bi results in two different solids, each of which has a chemical composition that differs from the liquid. One solid (the minor phase) is distributed as rods, particles, or layers throughout the other solid (a continuous matrix, or major phase).

Monotectics are similar to eutectics, except that a monotectic liquid solidifies to form a solid and a liquid (both of which are different in composition from the original liquid). Al-In is a monotectic that starts out as indium dissolved completely in aluminum, but when the alloy is solidified under the appropriate conditions, it forms a solid aluminum matrix with long thin "rods" of liquid indium inside it. As the system cools, the rods of liquid indium freeze into solid rods. The indium rods are dispersed within the structure of the solidified material.

Polymers
Polymers are macromolecules (very large molecules) made up of numerous small repeating molecular units called monomers. They appear naturally in wool, silk, and rubber and are manufactured as acrylic, nylon, polyester, and plastic. Polymers are typically composed of long chains of monomers, appearing on the molecular scale as if they had a spine of particular elements such as carbon and nitrogen. The bonding between individual polymer molecules affects the material's physical properties such as surface tension, miscibility, and solubility. Manipulation of these bonds under microgravity conditions may lead to the development of processes to produce polymers with more uniform and controlled specific properties. Important optoelectronic and photonic applications are emerging for polymers, and many of the properties needed are affected by the polymers' crystallinity. This crystallinity, which is the extent to which chains of molecules line up with each other when the polymer is formed, may be more easily understood and controlled when removed from the influence of gravity.

Growing polymer crystals is more difficult than growing inorganic crystals (such as metals and alloys) because the individual polymer molecules weigh more and are more structurally complex, which hinders their ability to attach to a growing crystal in the correct position. Yet in microgravity, the process of polymer crystal growth can be studied in a fundamental way, with special attention to the effects of such variables as temperature, compositional gradients, and the size of individual polymer units on crystal growth. In addition, just as microgravity enables the growth of larger protein crystals, it may allow researchers to grow single, large polymer crystals for use in studying properties of polymers and determining the effects of crystal defects on those properties.

Microgravity Research and Exploration
There is one endeavor for which microgravity research is essential. That is the goal of exploring new frontiers of space and using the Moon and Mars as stepping stones on our journey. To achieve these goals, we must design effective life support systems, habitation structures, and transportation vehicles. To come up with workable designs, we must have a thorough understanding of how the liquids and gases that we need to sustain human, plant, and animal life can be obtained, transported, and maintained; of how structural materials can be formed in-situ (on site); and of what types of fuels and fuel delivery systems would allow us to get around most efficiently. Microgravity research can provide the insight needed to get us on our way. The ability to use extraterrestrial resources is a key element in the exploration of the solar system. We believe that we can use the Moon as a research base to develop and improve processes for obtaining gases and water for human life support and plant growth; for creating building materials; and for producing propellants and other products for transportation and power generation. Oxygen extracted from lunar rocks and soils will be used for life support and liquid oxygen fuel. A byproduct of the extraction of oxygen from lunar minerals may be metals and semiconductors such as magnesium, iron, and silicon. Metals produced on the Moon and material mined from the surface will then be used for construction of habitats, successive processing plants, and solar cells.

Current research in the areas of microgravity science will guide our path as we develop the means to use the Moon as a stepping stone to Mars. Research into how granular materials behave under reduced gravity conditions will be important when we design equipment to mine and move large amounts of lunar material. The ability to extract gases and metals from minerals requires an understanding of how gases, liquids, and solids of different densities interact in lunar gravity. Building blocks for habitats and other structures can be made from the lunar regolith. Research into sedimentation and sintering under reduced gravity conditions will lead to appropriate manufacturing procedures. Experiments have already been performed on the Space Shuttle to determine how concrete and mortar mixes and cures in microgravity. An understanding of fluid flow and combustion processes is vital for all the materials and gas production facilities that will be used on the Moon and beyond.

NASA's Enterprise for the Human Exploration and Development of Space The goals of this Enterprise are to Increase human knowledge of nature's processes using the space environment, Explore and settle the Solar System, Achieve routine space travel, Enrich life on Earth through people living and working in space. Microgravity research will contribute to the areas of cryogenic fuel management, spacecraft systems, in-situ resource utilization, power generation and storage, life support, fire safety, space structures, and science exploration.

Elemental Percent Weight on Earth and Moon    Earth's Crust  Lunar Highland Soils O 47 45 Fe 5 5 Si 28 21 Mg 2 4 Ca 4 11 Al 8 13 Na 3 0 K 3 0

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