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
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
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,
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
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 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
- Explore and settle the Solar System,
- Achieve routine space travel,
- Enrich life on Earth through people living and working
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