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Microgravity Research Opportunities for the 1990s: Chapter 6
Microgravity Research Opportunities
for the 1990s
PART II—SCIENTIFIC ISSUES
6
Materials Science and Processing
This chapter discusses the areas of microgravity research generally
identified as materials science. As organized in this report, they include metals
and alloys, organic materials and polymers, the growth of inorganic single
crystals, the growth of epitaxial layers on single-crystal substrates, and ceramics
and glasses.
METALS AND ALLOYS
REPORT MENU
NOTICE
BACKGROUND AND INTRODUCTION
MEMBERSHIP
PREFACE
One of the major desires of modern materials scientists and physical
EXECUTIVE SUMMARY
metallurgists is to understand the formation, structure, and properties of materials
PART I
from the atomic and molecular levels (0.1 to 1 nm) up through the mesoscopic
CHAPTER 1
CHAPTER 2 level (0.1 to 100 m). Many physical properties of materials are influenced
PART II directly by this microstructure (i.e., the morphology, size scale, spatial
CHAPTER 3
distribution, interface characteristics, and composition of different phases or
CHAPTER 4
structures that are present within a material). The relationship between the
CHAPTER 5
microstructure of a material and its physical properties and the manipulation of
CHAPTER 6
various processing parameters to obtain a given microstructure constitute key
CHAPTER 7
elements in the modern study of materials.
PART III
CHAPTER 8
APPENDIX A Metallurgy has evolved over the millennia from a purely empirical art to a
APPENDIX B modern branch of materials science, with considerable progress being made in
understanding some of the simpler metallurgical systems and processes from
first principles. A key to this progress has been the improved ability to control the
solidification process, an important scientific feat that has come about through an
improved understanding of the interrelationships between heat and mass
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transport phenomena occurring in the melt and the kinetics of the solidification
interface.
Most metals and alloys are solidified from a fluid state. Since it is often
desirable to mix two or more components to obtain alloys or composites with
improved physical properties, the solidification process becomes complicated by
the fact that different atoms do not generally enter the solid in the same
proportion as they occur in the melt. This chemical segregation produces local
variations in composition in the melt that, in turn, change the temperature at
which various fractions of the solid will form. The situation described here is
complicated further by the presence of convective and advective liquid flows that
also modify the local composition and temperature of the melt. The microgravity
environment, by reducing these gravity-driven phenomena, clearly offers new
opportunities to metallurgists to develop and enhance control of materials
processing.
The microgravity environment provided by an orbiting spacecraft offers
new opportunities in control of the solidification process. Reduction of convective
velocities permits, in some cases, more precise control of the temperature and
composition of the melt. Body force effects such as sedimentation, hydrostatic
pressure, and deformation are similarly reduced. Weak noncontacting forces
derived from acoustic, electromagnetic, or electrostatic fields can be used to
position specimens while they are being processed, thus avoiding contamination
of reactive melts with their containers.
To accomplish the objectives discussed above, it is necessary to conduct
a series of carefully chosen, well-conceived experiments that clearly delineate the
advantages and limitations of microgravity research. These experiments should
be designed to produce new results that could not have been obtained by
terrestrial experiments.
FUNDAMENTAL RESEARCH AREAS-METALS AND ALLOYS
Nucleation and Metastable States
In order for a material to transform to a more ordered phase (vapor to
solid or liquid to solid), it is first necessary to form an aggregate or cluster of
molecules above a critical size to initiate the process. Such an aggregate may (by
heterogeneous nucleation) form on a foreign surface, such as a container wall or
a freely floating mote, or it may form spontaneously (by homogeneous
nucleation) from random internal thermodynamic fluctuations. Homogeneous
nucleation can occur only if the melt is cooled far below its normal freezing
temperature without solidifying. Heterogeneous nucleation almost always occurs
first, provided there are impurities that can act as nucleation sites. Understanding
and controlling phase nucleation are extremely important for control of the overall
solidification process.
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For example, if a fine-grained casting is desired, one would try to produce
a very large number of crystal nuclei and, ideally, distribute them randomly
throughout the melt. Gravity-driven convection plays an important role in this
process, as was demonstrated in a series of early experiments at reduced gravity
using Space Processing Applications Rockets (SPARs) and at increased gravity
using a centrifuge. This research is an example of how microgravity experiments
may be used to elucidate the essential features of a solidification process and to
suggest better control strategies for use on Earth-in this case, by enhancing
convection or mechanically stirring the melt to distribute the crystal nuclei.
It is often desirable to suppress, or even avoid nucleation entirely, in order
to be able to supercool a melt to a temperature well below its normal
thermodynamic freezing point. Solidification of deeply undercooled metallic melts
is usually initiated by a single nucleation site, and freezing tends to be rapid.
These melts generally produce a refined dendritic microstructure, often exhibiting
enhanced mechanical properties. If the solidification is rapid enough, the solute
atoms will not have sufficient time to arrive at positions of lowest energy (i.e.,
near their equilibrium configurations); metastable crystalline or even amorphous
phases can thus be produced. A metastable phase can have a crystalline
structure different from that of the equilibrium phase, which can greatly alter
some of the material's physical properties in technologically significant ways.
Perhaps the best-known metallurgical example of a metastable crystalline phase
is steel, which is hardened by formation of the metastable phase martensite.
Martensite is a nonequilibrium tetragonal phase containing carbon and iron that
forms upon rapidly quenching austenite, which is a cubic, equilibrium phase
consisting of carbon dissolved in the face-centered cubic form of iron.
An amorphous phase may be thought of as a liquid-like structure, frozen
in place, that lacks long-range crystalline order. Ordinary soda-lime silica glass is
a classic example of an amorphous engineering solid. Amorphous materials tend
to be highly resistant to chemical attack because there are no crystalline grain
boundaries on which most chemical reactions occur. Similarly, glassy amorphous
structures can be achieved in some metallic alloys by rapid solidification. One
example is Metglas®—a product developed by Allied Signal Corporation that
consists mainly of iron, silicon, and boron. The absence of grain structure makes
it extremely easy to magnetize and demagnetize Metglas® with little magnetic
hysteresis loss, thus making this material useful in transformer cores, where it
provides greater efficiency than the conventional, crystalline iron-silicon material.
By contrast, a new rapidly solidified iron-boron-neodymium alloy, developed
recently by General Motors Corporation, is an extremely good permanent
magnet, with high coercivity, because its fine grain structure tends to "pin" the
magnetic domains and prevent demagnetization. These examples demonstrate
how crucially the physical properties and performance of a material depend on its
microstructure.
The ability to melt, supercool, and subsequently solidify alloys without a
supporting container or crucible removes a major source of impurities and
heterogeneous nucleation sites, thus permitting very large undercoolings to be
achieved. On Earth this can be accomplished for small metallic samples by
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levitating them on a gas-film or in an electromagnetic field and then releasing the
molten droplets into a long, vertical, evacuated tube and allowing them to solidify
during free-fall.
These techniques have been used extensively over many years but have
severe limitations. It is not possible, for example, to control the heating and
levitating force independently in an electromagnetic levitator. Use of a quench
gas to cool the sample often promotes surface nucleation. There is also some
evidence that flows in the melt driven by the induced alternating currents may
stimulate nucleation and limit the amount of undercooling. These difficulties can
be overcome in a drop tube, but it is not yet possible to observe the falling droplet
closely enough as it cools in order to make accurate temperature measurements.
Also, the sample volume and the amount of undercooling are limited by the time
available for the fall (typically a few seconds). In reduced gravity, the sample may
be positioned by a much lower induced current or may even be allowed to float
freely. This significantly decouples the heating field from the levitation field and
greatly reduces the amount of stirring in the melt. The temperature may then be
measured optically during cooling and the solidification process, and
thermophysical properties such as specific heat, heat of fusion, and thermal
diffusivity can be determined calorimetrically.
Initial microgravity experiments on metals and alloys should focus on
identifying factors that limit the degree of undercooling, such as the amount of
superheat required to dissolve or destroy potential nucleation sites in a melt, the
use of so-called fluxing agents to remove impurities and prevent oxidation, and
the effects of stirring and melt shear on promoting heterogeneous nucleation.
Once techniques for obtaining large undercoolings have been determined for an
alloy, the emphasis of subsequent experiments might be on forming metastable
and amorphous phases from deeply undercooled melts and determining their
properties. It may even be possible to perform critical supercooling experiments
to evaluate various theories of nucleation that have never been verified
experimentally. Convenient methods for obtaining extremely low partial pressures
of reactive gases, such as oxygen, must also be developed since oxides and
other reaction products will form rapidly on molten samples and, in some
systems, act as effective nucleation sites.
Prediction and Control of Microstructure
Microstructure can be manipulated to a significant degree by controlling
the rate at which solidification takes place and the temperature distribution in the
region of the crystal-melt interface. This is often accomplished by the process of
directional solidification, in which heat is extracted unidirectionally either by
placing the bottom of the crucible containing the molten specimen on a chill-block
(gradient freeze method) or by slowly lowering the crucible through a temperature
gradient (Bridgman method).* If the temperature everywhere in the melt is above
the local freezing (liquidus) temperature, a smooth, almost featureless,
solidification front will result. This is desirable for growing single crystals or for
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producing aligned composites. However, in virtually all alloys, the atoms are
incorporated into the solid phase in proportions different from the melt. This leads
to a region ahead of the freezing interface in which the solute concentration is
altered from its mean composition. Within this thin region or boundary layer, the
equilibrium freezing point in the melt can be lower than the freezing point of the
original alloy. If the temperature gradient in the melt is not steep enough, which is
generally the case in a slowly frozen casting, a situation known as "constitutional
supercooling" will arise, in which the melt ahead of the interface is below its
equilibrium freezing temperature. At the onset of constitutional supercooling, the
initially smooth interface becomes unstable and forms a series of protuberances
or cells. At higher degrees of constitutional supercooling, the interface breaks
down completely as these cells and bumps elongate and develop side branches
to form tree-like structures called dendrites. When this happens, partial
solidification proceeds over a substantial distance ahead of the totally frozen
material. The region between the advancing solid and dendrite tip is called the
"mushy" zone because it is composed of a fine, micrometer-length scale mixture
of liquid and solid. The heat and mass transfer processes that take place in this
two-phase "mushy" zone determine the microstructure of the solid.
Although these processes are understood qualitatively, the detailed
quantitative descriptions required to control the microstructure remain poorly
understood. A great deal of theoretical work has been devoted to obtaining
accurate descriptions of dendrite growth rate, shape, and primary and secondary
arm spacing-all as functions of the solidification and materials parameters.1
Generally such fundamental theories ignore the important effects of convection in
order to remain tractable. Convective flows are usually unavoidable in Earth
gravity because of the lateral density gradients that must exist as a result of the
solute rejection described above. In fact, short-duration, low-gravity experiments
carried out on sounding rockets and in aircraft flying parabolic (low-gravity)
trajectories have shown evidence that both the primary and secondary arm
spacings change as the force of gravity is altered.2 Therefore, it was considered
scientifically important to test the first-order fundamental theories that ignore
convection in a microgravity environment before adding modifications to correct
for convective effects that are encountered in terrestrial processes. Such a critical
test, known as the Isothermal Dendritic Growth Experiment (IDGE), was carried
out recently aboard the United States Microgravity (USMP-2) mission launched
March 3, 1994. Preliminary results obtained from the IDGE show that the
terrestrial dendritic growth data used previously to test diffusion-based theories
were, in fact, heavily corrupted by buoyancy-induced convection. As a result,
these fundamental theories, each one purportedly providing the basis of
dynamical pattern selection, now remain in question.3
The discussion, thus far, applies only to alloy systems that have one solid
phase, (i.e., to solid solutions in which the atoms in the solid are completely
miscible or to dilute alloys in which the minor constituent does not exceed the
limit of solid solubility). Most alloy systems of practical interest, however, display
only partial miscibility in the solid state, a situation that produces a more
complicated behavior involving polyphase reactions. An important class of
materials exhibiting this behavior is the eutectics in which a melt consisting of a
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homogeneous solution of the two components solidifies to form a solid with
segregated regions of quite different compositions. A good example of this type
of behavior occurs in cast iron, whose melt consists of a solution of iron and
carbon. On freezing, flakes or nodules of almost pure graphite form in a
crystalline matrix of nearly pure iron. The morphology or form of these graphite
particles largely determines the mechanical properties of the material. Despite
many years of intensive research, all the kinetic and thermodynamic factors that
determine the graphite morphology are still not understood.
By carefully controlling the direction in which heat is extracted (directional
solidification), interesting, controlled, two-phase microstructures can be produced
in a variety of eutectic systems. In some systems, it is possible to grow thin rods
of one composition and crystal structure that are embedded in a matrix of yet
another composition and crystal structure, with both aligned along the direction of
heat flow. In other systems the resulting structure comprises a series of
alternating thin slabs, or lamellae, of the two compositions. These structures are
termed "in situ composites" and often exhibit highly anisotropic properties
because of the aligned microstructure. For example, some turbine blades
manufactured by directional solidification for high-performance jet engines exhibit
this aligned structure and show improved strength and creep resistance. In
another example, the theoretical magnetic coercivity in a manganese-bismuth
eutectic alloy was approached by using directional solidification to match the
eutectic rod diameter to the size of a single elongated magnetic domain.
The classical theory of eutectic spacing selection assumes only diffusive
species transport, because convective effects were thought to be unimportant on
the length scales involved. However, in a series of microgravity experiments for
the manganese-bismuth eutectic, it was found that the rod diameter and spacing
were considerably smaller than predicted from this theory. Interestingly enough,
ground control experiments, which had convective flows, yielded results that
agreed well with the classical theory.4 Growth in strong magnetic fields, on the
other hand, which suppress convective flows, produced results similar to those
from the spaceflight experiments, clearly indicating that convective effects are in
fact important in the process. European experimenters on Spacelab-1 and D-1
have found similar results with other eutectic systems, agreement with the
classical theory in other systems, and even larger spacings than predicted by
classical theory in yet other systems. Obviously, there is still much to be learned
about the science of eutectic solidification.
Another effect that is important in the evolution of the microstructure of an
alloy is the phenomenon of ripening or coarsening. It can be shown from
thermodynamics that the melting point of a solid particle is reduced slightly by the
curvature of its surface (Gibbs-Thomson effect). This effect comes about
because of the interfacial energy change associated with the motion of the
curved surface during melting or freezing. A planar surface (zero curvature)
would not experience a change in energy on melting or freezing. Given a
distribution of solid particles with varying curvatures in a melt, the larger particles
will grow at the expense of smaller particles as the system tends to lower its free
energy by reducing its total surface area. This effect, known as Ostwald ripening
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or phase coarsening, is important in a large class of dispersion-hardened alloys
in which fine particles are either added to or caused to precipitate from the melt
during solidification. Since the strengthening effect of the dispersed particles
diminishes as the particle size increases, the process of coarsening must be
understood and controlled. Similarly, in castings and weldments, dendrite arms
coarsen by the same process, which again affects the final microstructure and
properties.
The classical theory of Ostwald ripening was developed in 1961. An
approximate solution was obtained that is valid in the limit of zero volume fraction
of solid particles. In metallurgical applications, however, the dispersed phase
makes up a significant fraction of the system. Also, it appears that neighboring
particles interact strongly with one another, an effect that is not accounted for in
the approximation used in the classical theory. A number of attempts have been
made recently to modify the classical theory, but so far none of these theories
can predict experimentally observed phenomena. This might be due to the fact
that none of the theories considers convective transport of the continuous liquid
phase (because the fluid channel sizes are small); yet it might be an important
factor, as well as an additional complication.
Well-defined microgravity experiments are needed to confirm the
quantitative aspects of the underlying physics in the theory of Ostwald ripening
under the conditions of pure diffusive transport. Such experiments need to be
performed before the effects of convection and fluid motions can be incorporated
into the theory.
Phase Separating Systems and Interfacial Phenomena
Another class of polyphase materials that is of interest to microgravity
research involves monotectic systems, which are characterized by a
compositional region of liquid-phase immiscibility. At any given temperature
(below the critical temperature) a range of compositions is always encountered
for which the melt will separate into two immiscible liquids. Since the two liquids
are of different composition, they will invariably have different mass densities. In
Earth gravity the two liquid phases will stratify before they can be frozen, resulting
in a highly chemically segregated solid. It was reasoned that this buoyancy-driven
sedimentation could be avoided by solidifying such systems in microgravity.
When actually performed, such experiments have been only partially successful
in producing the expected fine dispersions uniformly distributed throughout the
final solid.5 These experiments indicate that effects other than gravity-induced
buoyancy become important in the solidification of monotectic systems. An
extensive series of ground-based experiments, using systems that have similarly
configured (monotectic) phase diagrams, and which in some instances can be
made neutrally buoyant, has uncovered a rich variety of interfacial energy-driven
effects (such as critical wetting). These monotectic alloys exhibit a variety of
interesting microstructures, which have been scientifically classified through the
help of microgravity research.6
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A clear example of the dominating influence of interfacial energy effects
occurs in monotectic alloy systems below the critical temperature. Over a
temperature range below the critical temperature, one of the liquids always
exhibits perfect (or critical) wetting behavior, so that one fluid phase essentially
encapsulates the other and, even more curiously, the wetting phase will perfectly
coat any container or surface that it contacts. If final freezing occurs within the
temperature range of critical wetting, then massive phase separation will be
produced in the microstructure. Alternatively, if the temperature for the
monotectic reaction, which produces a solid- and liquid-phase pair from a
different liquid composition, falls below the critical range, then useful (dispersed)
microstructure control is possible. A continuous, well-aligned two-phase structure
with uniform spacing between phases can often be achieved in a directional
growth process.7,8 In the latter case, melt flows driven by the temperature and
composition dependence of surface energy can become dominant. The minority
liquid phase can form droplets that will migrate in a thermal gradient
(thermophoresis) as well as in a composition gradient. Larger droplets move
faster than smaller ones, so that the smaller ones will be overtaken and
agglomerated with the bigger ones. Again, massive phase separation will result.
In fact, depending on the alloy system, thermal gradients as small as 1 K/cm in
low gravity can have the same motive effect as Earth gravity in causing massive
segregation.
A second case of dominating interfacial factors occurs in containerless
processing, where the extent of wetting between the solid(s) forming from a melt
controls the degree of undercooling and the resulting microstructures. Brazing,
soldering, and welding operations represent additional technologically important
processes that are influenced greatly by interfacial phenomena. Here both
wetting and surface tension-driven flows can be primary influences in achieving
or not achieving success.
It is clear that interfacial phenomena are common to numerous metals
processing strategies. Although limited studies of interfacial effects are always
possible on Earth using density-matched immiscible systems, density matching
per se under the influence of terrestrial gravity can be accomplished only at a
single temperature. Thus, the microgravity environment provides a unique
opportunity to study and quantify a range of interfacial phenomena in order to
suggest better materials processing strategies on Earth and under microgravity
conditions.
Heat and Mass Transport
The freezing temperature of any alloy is dependent on composition, and a
freezing solid tends to accept one component of a melt more readily than the
other components; thus, the first-to-freeze solid will always have a composition
different from that of the remaining solid. The rejected component sets up a
compositional gradient in the melt at the solidification interface. With gravity
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present, any lateral density gradient resulting from composition or thermal
gradients will produce prompt convective stirring. Convective mixing of the
enriched solute rejected from a growing crystal-melt interface with the bulk melt
causes a spatially varying composition (i.e., segregation) to occur throughout the
solidified specimen. This phenomenon is called macrosegregation. When the
mixing rate varies in time, the temporal variation results in a solid with a profile
that might exhibit bands of composition that vary along its length.
Macrosegregation can be controlled to some extent by extracting heat
unidirectionally from the bottom of the specimen to minimize lateral thermal and
solutal gradients. However, if the rejected component of the melt at the interface
is less dense than the bulk melt, a phenomenon known as a double-diffusive
convection can develop. Channels or plumes of the lighter melt will rise from the
interface, resulting in solidified regions with a grossly different composition
(known as "freckles").
Macrosegregation also plays a significant role in the formation of in situ
composites with aligned microstructure. Even with a well-controlled directional
solidification process such as the Bridgman method, lateral thermal gradients
occur that will produce some convective stirring. Thermosolutal convection
occurs in Earth gravity if the rejected component is less dense than the bulk melt.
Strong magnetic fields can slow these convective flows but cannot eliminate
them. Thus, magnetic fields are generally of limited effectiveness for controlling
macrosegregation. Microgravity offers the opportunity to produce unique
microstructures in a variety of systems (e.g., off-eutectic compositions,
monotectics, and peritectics) that tend to phase-separate in normal gravity, as
well as in solid solutions that are subject to double-diffusive convection. Liquid-
phase sintering of dense tungsten particles with lighter molten transition alloys
(typically Fe-Ni) is a good example of an important terrestrial process that would
be assisted if performed in a microgravity environment.
Experiments in microgravity are being flown by NASA to test theories of
solidification, seeking a more basic understanding of the way in which
microstructures are formed. Since the solidification process involves the transport
of heat and mass and a moving phase boundary, one way to test the theories is
to encode them as a mathematical model and then compare the predicted
thermal and solute fields with those observed experimentally. This allows
evaluation of competing theories using the same experimental data.
Most of the mathematical models that have been developed thus far
consider only steady accelerations. Additional modeling development efforts are
clearly needed to account for transient, random, and oscillatory accelerations
(collectively termed "g-jitter") and their effects on heat and mass transport. The
ability to design and interpret scientific experiments in the microgravity program is
often predicated on the available knowledge of the relevant solute and
temperature fields. Mathematical and numerical models are an indispensable tool
to augment experimental measurements for this purpose. There are two distinct
ways to use the mathematical models that describe the physical transport
processes of interest: (1) to design and then theoretically interpret scientific
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experiments performed in microgravity, and (2) to obtain values for certain
thermophysical properties, which are difficult to measure terrestrially, by using
microgravity experiments, and then employ these data to improve model-based
predictions of terrestrial processes used in industry. In order to improve the
chances of a successful flight experiment and take maximum advantage of the
relatively few flight opportunities that will be available over the next decade,
candidate experiments must be carefully modeled mathematically for both
terrestrial and microgravity conditions. Even for a six-order-of-magnitude
reduction in gravity, significant flows can still result if the residual gravity vector is
perpendicular to a large density gradient. This can easily occur in a directional
solidification experiment unless care is taken to ensure that the residual
acceleration environment is controlled and that the experiment is configured to
minimize its effects, for example, by application of a strong magnetic field.
Thermophysical Properties
Another serious deficiency in our ability to model solidification processes
is a paucity of accurate thermophysical property data for many alloys in the
molten state. This is a problem not only for scientists modeling microgravity
experiments, but also for many industrial researchers who are using the
computational methods made possible by supercomputers to model the
solidification of complex castings. The lack of high-temperature thermophysical
data is due in part to the extreme difficulty of making accurate measurements on
melts under terrestrial gravity. Recent European experiments on Spacelab-1 and
D-1 have shown that diffusion coefficients for a variety of molten alloys measured
in space differ considerably from the accepted values obtained on Earth. This
situation is believed to occur either because of wall effects (such measurements
are often made in capillary tubes) or because of uncompensated convective
transport, which is virtually impossible to avoid in a terrestrial setting. Also, the
effect of thermo-diffusion (Soret-Dufour effect) was found in space experiments to
be as much as an order of magnitude larger than previously estimated. No
accurate measurements had been made of this effect for solidification processes
conducted on Earth. This effect, which causes a mixture of atoms to become
separated in a thermal gradient according to their atomic mass, now appears to
play a more important role in mass transport in many terrestrial processes than
had been realized heretofore.
The following are some thermophysical properties,9 only a few of which
can be advantageously measured in microgravity, that are of interest in the
development of reliable models for metals processing:
Emissivity, electrical conductivity, and optical properties;
Calorimetry including specific heats and heats of mixing, formation,
and transformations;
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Transport coefficients including thermal conductivity, viscosity, and
diffusion constants;
Density data;
Thermodynamic moduli, including thermal expansion coefficients and
compressibility;
Vapor pressures and activity coefficients;
Surface tension and interfacial energies; and
Equations of state.
FUTURE SCIENTIFIC DIRECTIONS—METALS AND ALLOYS
As indicated in the previous section, a number of important scientific
problems in metallurgy are being addressed by experiments currently sponsored
by NASA's microgravity science and applications program. However, other
important areas can be identified that are just beginning to be developed. These
areas include powder metal processing, electrolytic processes, joining,
ultrapurification, ultrahigh-vacuum processing, nanomaterials, and cuprate
superconductors.
Powder metallurgy represents an important technology for processing
many dense refractory metals or materials that tend to phase-separate when
melted in unit gravity. Indeed, some of the gravity-related problems discussed
previously can be avoided by standard powder processing: for example, by
compacting a mixture of the components in the form of a fine powder and heating
it just below the melting point for some time (sintering). Theoretical density can
often be achieved by applying high pressure (hot isostatic pressing) or by raising
the temperature so that one of the components melts (liquid-phase sintering).
Since the powder metal process is designed ab initio to minimize the
effects of gravity, one would not expect to see any large differences between
Earth- and space-processed samples. Rapid particle growth during liquid-phase
sintering is an anomalous processing effect that might arise from sedimentation-
induced convection or from some unidentified interfacial phenomenon that
promotes coalescence. Well-designed microgravity experiments should provide
some valuable insight as to the sensitivity of such powder processes to these
effects.
Little has been reported in the United States concerning
electrodeposition, electropolishing, or corrosion in reduced gravity. Convection
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increasing the fracture toughness, either through elimination of flaws or by
making the ceramic less sensitive to the flaw population (increasing the critical
flaw size or increasing the fracture energy).
Raw materials for ceramic and glass manufacture traditionally are earthy,
oxide materials that are mined in high volume at low cost and are subjected to
relatively little processing. The products made from them are commodity items
such as bricks, tile, and glass windows. Modern technical or engineering
ceramics are higher-value-added materials that have superior properties by virtue
of their more sophisticated processing and tighter control over raw materials. It is
to this latter class of materials that the microgravity environment has the greatest
relevance.
CURRENT ISSUES IN CERAMICS RESEARCH
As discussed in several publications,51-53 the greatest needs and
opportunities in ceramics lie in the areas of synthesis and processing. Specific
recommendations for increased emphasis include:
Interactive research on new materials synthesis that is linked with
characterization and analysis of the product;
Basic research on synthetic solid-state inorganic chemistry to produce
new compounds;
Synthesis of ultrapure materials, for example, fibers with low oxygen or
carbon impurity levels;
Research on techniques for synthesis to net-shape, that is, learning
how to do synthesis, processing, and forming in a single step;
Research on methods for processing ceramic materials far from
equilibrium; and
Research on processing of artificially structured or, as they are
sometimes called, functionally gradient materials.
Whether the availability of a free-fall environment is relevant to these
research areas is an unanswered question.
MICROGRAVITY AS A VARIABLE IN
CERAMICS SYNTHESIS AND PROCESSING
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Evaluating the applicability of the microgravity environment to
opportunities and problems in ceramics R&D requires a good definition of that
environment. Two aspects of this environment seem relevant to ceramics: (1) the
possibility of levitating a specimen for high-temperature containerless processing
and (2) the suppression of buoyancy-driven convection. In both cases, the
applicability is to gas- or liquid-phase phenomena. Thus, for applications of
microgravity to ceramics, one should look to where liquid or gas phases are
important. Some areas of liquid- and gas-phase processing of ceramics are
discussed below.
Melt Synthesis
Ceramics are very seldom made directly by melt processing, mainly
because their melting temperatures are very high (for those that do not
decompose first), the melts react severely with available crucible materials, and
undesirable microstructures are obtained on cooling. Thus, ceramics typically are
made by sintering of powders. Those ceramic powders are produced by a variety
of techniques, including upgrading of mined deposits, solid-state reactions, solid-
liquid reactions, and gas-phase reactions. For some powder compositions, such
as high-purity Al2O3 or SiC, sintering and densification are entirely by solid-state
processes. For other ceramics, a second-phase liquid may be present in sintering
(up to 15% by volume). The unanswered question is what difference microgravity
might make for ceramic melts.
Availability of a general method for contamination-free synthesis using
containerless melting would be of value for producing research specimens of new
high-temperature materials. Lack of promising new techniques is probably one of
the main reasons that melt synthesis of ceramics or inorganic compounds is no
longer a very active research area. Ceramic superconductors, such as
YBa2Cu3Ox, have been proposed for levitation melting to minimize crucible
contamination.54 However, here the problem is not reaction with the container but
that YBa2Cu3Ox melts incongruently and on cooling does not give the stoi-
chiometric compound-a problem not addressed by levitation. Many of the benefits
of microgravity containerless melting are available on Earth using skull or cold-
wall melting. The relative merits of space-based containerless melting and
terrestrial cold-wall melting have not been assessed for ceramic synthesis.
Vaporization is frequently a problem in ceramic and glass synthesis and
processing. For example, Si3N4 vaporizes incongruently at temperatures below
those required for significant solid-state diffusion. Thus, Si3N4 ceramics are made
with additives that lower the sintering temperature (to 1700-1800ºC) by producing
small amounts of liquid phases.
Most commercial glasses are made from high-temperature melts.
Reaction with crucible materials is always a concern for glass melting, but it is
usually dealt with case by case. Glasses crystallize by a nucleation and growth
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mechanism. In some cases the crystallization is desired, as with glass-ceramics.
For glasses, however, crystallization is undesirable and nucleation must be
suppressed. Heterogeneous nucleation occurs at phase interfaces, as in the melt-
crucible wall or on impurities or heterogeneities in the melt. Containerless melting
could be advantageous for some glasses.
Glass melts frequently experience preferential loss of the more volatile
components, changing the composition of the resulting glass. Processing in a
convection-free environment could lead to higher levels of saturation in the
atmosphere at the solid or liquid surface that would suppress evaporation. Lower
rates of evaporation would expand the available processing windows for many
materials and allow shorter processing times.
Optical fibers are the application of glasses that has the most stringent
requirements for purity and homogeneity. Optical fibers are pulled from preforms
made by a CVD process and are not melt synthesized. Their manufacture is
already highly developed and fibers with optical losses less than 1 dB/km in
lengths of 30 km are routinely drawn in a single pull.55 New materials for optical
fibers and signal amplifiers are under development. It is conceivable that
containerless processing could be useful for producing research specimens of
some materials.
Solution Synthesis
A recent trend in ceramic synthesis is use of near-room-temperature
solution techniques to synthesize ceramic precursors. Increasingly, advanced
ceramics are synthesized from highly processed, chemically prepared powders.
Some of the more promising routes to advanced ceramic powders are sol-gel
processing, precipitation from solution, gas-phase synthesis, and powder-surface
modification. The sol-gel method is a familiar example. In that technique,
organometallic reagents in solution are hydrolyzed and condensed to form an
inorganic polymeric gel that, when dried and fired, gives the desired ceramic
composition. These chemical methods can generate controlled size distributions,
extremely reactive precursors, unusually shaped particles, and gels. Solution
methods permit intimate mixing of components, easy dispersion of second
phases, and surface modification of the precursor particles. Liquid precursor
solutions also can be used to make thin films by dipping or spinning, and
because of the high reactivity of the precursor particles, film consolidation occurs
at moderate temperatures. This advantage is being exploited in research on the
synthesis of electronic ceramic films such as PZT (lead zirconate-lead titanate)
and YBa2Cu3O7-x from solution. One potential application might be in epitaxial
growth of films from solution. That is one example of biomimetic synthesis of
ceramics, a proposed approach to ceramic synthesis that mimics biological
processes through the use of self-assembling monolayers. Whether reduction of
convective transport would be useful to biomimetic synthesis should be
determined experimentally.
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Chemical preparation of ceramics is a broad subject. For example,
synthesis of ceramic precursors by polymerization of metal alkoxides using
hydrolysis and condensation reactions is a rapidly expanding area of ceramics.
Ultrafine ceramic particles with enhanced surface reactivity (such as SiO2) can be
synthesized through nucleation or condensation reactions in gas-phase aerosols.
Ceramic powders can be prepared under hydrothermal conditions in which the
solubilities of many oxides are greatly enhanced, permitting direct precipitation of
oxides from solution that is not possible under ambient temperature and
pressure. Hydrothermal growth is well established for growth of quartz crystals,
but the full potential of the technique has not been exploited.
Glass, which is also considered a ceramic, has a lack of crystallinity as its
most distinguishing feature. Glass precursors can be synthesized in the same
ways as ceramics, for example, the process for making the silica boules from
which optical fiber is pulled:
SiCl4 + O2 (C2H2 flame) SiO2 + CO2 + H2O + HCl.
Glasses can also be made by using the alkoxide solution route described above
for sol-gel ceramics.
Powder Synthesis in Microgravity
One of the main objectives in ceramic powder synthesis is to make
particles that are fine, homogeneous, and unagglomerated. Preparation by
precipitation from solution requires rapid introduction and mixing of reagents. One
typical method, used for electronic ceramics such as ZnO varistors and
YBa2Cu3Ox, is to pump the solutions into an ultrasonic mixing cell where reaction
occurs on the order of micro- to milliseconds and the products are carried away
to be removed by filtration. The microgravity environment does not naturally
provide any of those conditions. Microgravity would seem most obviously of
benefit in situations where convection and particle settling must be minimized.
In traditional powder synthesis, the amount of liquid is a small fraction of
the total mass, and capillary effects should be much larger than those due to
convection. Ordinary gas-phase synthesis techniques involve forced flow of the
gases, and it is not clear how large an effect is introduced by convection. It is
possible to design experiments with gas flow rates low enough so that convection
is relatively important. The significant practical problem in gas-phase synthesis is
avoiding agglomeration of very fine particles. Understanding nucleation and
growth of those particles is an important basic science issue.
In summary, gravity may be a significant variable in solution synthesis of
ceramics, but that has yet to be demonstrated. When ceramic powder is the goal,
reactions normally are run very quickly in a flow reactor or with stirring to obtain a
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very fine, homogeneous precipitate. Forced mixing is the rule, and any
convection is a very minor effect. In sol-gel synthesis of bulk objects such as
aerogels, convection may play a more important role, and some gelation
experiments in microgravity may be fruitful.
CERAMIC AND GLASS PROCESSING IN MICROGRAVITY
Processing refers to the steps involved in going from the precursor
powder to the final object. There can be a great deal of overlap between
synthesis and processing because some schemes are more or less continuous
from raw material to final product.
As already discussed, much traditional ceramic processing is by sintering
of powders. Melt processing of ceramics is rare, being used mostly for glasses
and single-crystal growth (e.g., ruby laser crystals). There are several practical
reasons for this, among them the fact that the melting points of many important
ceramics are very high (greater than 2000ºC for Al2O3) and ceramic melts react
with many crucible materials. Other ceramics, such as SiC and Si3N4, cannot be
melt processed because they melt incongruently. Where reaction with crucible
materials is the limiting factor, containerless processing in microgravity may have
significant advantages.
There might be opportunities for microgravity research in solution
processing of ceramics. One example is making sol-gel films by dip coating. Use
of ceramics as films is a rapidly expanding area with applications as diverse as
sensors, microelectronic memory elements, and antireflection coatings. It is
important to determine the physical and chemical factors that control film
formation during dip and spin coating and to develop strategies to tailor film
porosity and microstructure for sensor, membrane, protective, and photonic
applications. A major issue is the balance between gravitational draining, solvent
evaporation, solute particle interactions, and surface tension effects. A key to the
latter is the hydrostatic capillary pressure, which has rarely been measured in a
film or gel.56
Solution deposition is of interest for preparing other kinds of ceramic thin
films because of the low capital investment costs, the ability to closely control
composition, and the relative ease of process integration with other technologies.
One problem with the use of chemical deposition methods to prepare thin films is
lack of a fundamental understanding of the effects of solution chemistry variations
and structural evolution on film properties. For example, the process variables
used in each step of the deposition of PZT ferroelectric thin films (e.g., solution
preparation conditions, heat treatment temperatures, times, and ramp rates)
determine the ferroelectric properties of the film. Reduction of convective flows in
the microgravity environment may be of benefit in the preparation of high-quality
ceramic films, particularly where epitaxy is desired. This would be especially
useful if it led to improved processing on Earth.
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BASIC SCIENCE
There are a few fundamental issues in ceramics, in addition to those
already mentioned, for which the microgravity environment might prove useful.
Perhaps the most obvious is the study of crystal nucleation and growth in glass
melts. This question is important technically because of its relation to glass-
ceramics (where one wants controlled crystallization) and to optical glasses
(where crystallization is to be avoided). This line of research is being pursued in
the NASA containerless melt program.57 There is a large body of work in the
scientific literature from the 1960s and early 1970s on glass crystallization.58 The
challenge to microgravity research is to show how microgravity can be used for
critical experiments that will provide new advances in our understanding of crystal
nucleation and growth.
A second fundamental research opportunity is mass transport or diffusion
studies in glass and ceramic melts. For many systems of interest, the data are
inaccurate or unknown. Diffusion studies in microgravity would not be troubled by
convective mixing, and better data could be obtained in much less time. One area
of application is using diffusion data to test theories that classify glasses as either
strong or fragile, a concept that is related to the degree of polymerization in the
melt.
RECOMMENDATIONS AND CONCLUSIONS
The above discussion identifies a number of areas in which a microgravity
environment might be of benefit to ceramics research and development. The
following is a prioritized listing of those points:
1. The development of a general method for contamination-free synthesis
using containerless melting would provide a capability not available on Earth.
Examples are containerless melting of glasses to suppress heterogeneous
nucleation and containerless processing to produce research specimens of new
glasses for optoelectronic applications.
2. A fundamental study of crystal nucleation and growth in glass melts in
microgravity could provide information difficult to obtain on Earth.
3. Mass transport and diffusion studies of glass and ceramic melts under
microgravity conditions should generate more precise data than the data
available from terrestrial measurements.
4. The suppression of free evaporation from melt surfaces could allow
synthesis at higher temperatures than can be done on Earth.
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5. The epitaxial growth of films from solution, including biomimetic
synthesis (self-assembling monolayers) of ceramics, should be studied.
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