NRA-94-OLMSA-06

Technical Description

MICROGRAVITY MATERIALS SCIENCE:
RESEARCH AND FLIGHT EXPERIMENT OPPORTUNITIES

I. INTRODUCTION

A. BACKGROUND

NASA's Microgravity Science and Applications Division (MSAD) conducts a program of basic and applied research in microgravity to improve the understanding of fundamental physical, chemical, and biological processes. In this program, NASA sponsors investigations by university, industry, and Government researchers using both ground-based and space-flight facilities.

The materials science discipline has been an integral part of the microgravity science and applications program since its inception. The Division released a NASA Research Announcement (NRA) for materials science in 1991 and now expects to release NRA's in materials science approximately every two years. Other disciplines with periodically released solicitations are biotechnology, combustion science, and fluid physics. For additional information on research opportunities available through the Microgravity Science and Applications Division, contact:

Dr. Roger K. Crouch
Code UG
National Aeronautics and Space Administration
Washington, DC 20546-0001

Telephone: (202) 358-0818
Telefax: (202) 358-3091

B. RESEARCH ANNOUNCEMENT OBJECTIVES

The materials science program seeks a coordinated research effort involving both space- and ground-based research. Ground-based research forms the foundation of the materials science program providing the necessary experimental and theoretical frameworks for rigorously assessing and, ultimately, quantitatively understanding the phenomena. This research may eventually mature to the point where it can become the focus of a well defined flight experiment. This NRA has the objective of broadening and enhancing the MSAD microgravity materials science program through the solicitation of:

1. Scientific experiments that, through the use of a long duration-microgravity environment, will lead to major advances in the understanding of fundamental aspects of materials science.

2. Scientific experimental and theoretical research that will a) advance research in materials science, b) expand both the scientific scope and the research community associated with the materials science program, c) broaden the understanding of microgravity experiment results, d) clarify objectives for future microgravity experiments, and e) advance materials research and contribute to the national economy by developing enabling technology for U.S. companies.

Further programmatic objectives of this NRA include objectives broadly emphasized by the civil space program, including: the advancement of economically significant technologies; technology infusement into the private sector; and enhancement of the diversity of participation in the space program, and several objectives of specific importance to the microgravity science and applications program. These latter objectives include the support of investigators in early stages of their careers, with the purpose of developing a community of established researchers for International Space Station Alpha and other missions in the next 10-20 years, and the pursuit of microgravity research that shows promise of contributing to economically significant advances in technology.

II. MICROGRAVITY MATERIALS SCIENCE RESEARCH

A. INTRODUCTION

There is abundant practical motivation for advancing materials science; it plays a key role in virtually all aspects of important national economic areas. While the ability to process materials is clearly beneficial to humankind, many areas are still not well understood. Advances in materials science benefit a wide range of applications where materials are important and other areas of research which depend on advances in materials science as a basis for their continued progress. Long-duration microgravity is an important tool for establishing quantitative and predictive cause and effect relationships between the structure, processing, and properties of materials. Establishing, understanding and using these relationships are important elements in achieving increased international competitiveness.

The NASA microgravity materials science program currently supports research in a broad range of areas that can be categorized in two orthogonal ways. The program has previously been described in terms of class behavior. Using this approach, the materials systems being investigated include electronic and photonic materials, glasses and ceramics, metals and alloys, and polymers and nonlinear optical materials. The Materials Science Discipline Working Group (DWG), an advisory body to MSAD, has identified research areas, classified in terms of fundamental physical and chemical phenomena, that it believes would benefit from access to long duration, high quality microgravity conditions. Also included in the recommended research areas are those activities that the DWG believes are required to fully realize the potential of microgravity research (e.g. process modeling, materials characterization, etc.). The recommended research areas are described below. NASA is currently requesting proposals for research in these areas under this announcement. Innovative proposals in areas of materials science not specified in this announcement, that show a clear indication that they will benefit from access to long duration, high quality microgravity conditions and/or support the interpretation of microgravity research results are also invited. A few high-risk, high-payoff-if-successful ideas will be considered for funding at lower than average levels for up to two years if resources are available. Young investigators and investigators new to microgravity materials science research are encouraged to submit proposals.

B. MICROGRAVITY MATERIALS SCIENCE OBJECTIVES AND DESCRIPTION OF PARTICIPATION

Materials science deals with the relationships between the processing, structure, and properties of materials. The goal is to control the processing to yield materials with exceptional properties and enhanced performance. This is being accomplished today in limited cases by applying a fundamental understanding of materials at the atomic, molecular, and macroscopic levels.

The ability of processing to alter the properties of a material is rooted in the understanding that the properties of most materials are dictated by the microstructure of the material, i.e. the morphology, size, spatial distribution, and chemical composition of the material's constituent phases and defects. Thus, if the relationship between processing and microstructural development is well understood, then a first principles design of a material with desired properties can indeed be realized. Although many great strides have been made in understanding this crucial relationship, much more work needs to be done to reach this goal.

Many of the techniques used to process materials are strongly influenced by the presence of a gravitational field. For example, during the formation of a solid phase from a fluid, as is the case during crystal growth and solidification, gravitationally driven convection of the fluid is probable. This fluid flow can alter the spatial distribution of impurities in the liquid and resulting solid, induce structural defects in the crystal, and, due to the complexity of the flows which are possible, make the results of crystal growth and solidification experiments performed on earth difficult to interpret. The presence of a gravitational field also can lead to sedimentation when two phases have different densities and at least one phase is a fluid. This can lead to unwanted coagulation of the minority phase as is the case during phase separation in certain polymer blends and in the colloidal processing of ceramics.

A microgravity environment thus offers new opportunities to develop a deeper understanding of the relationships between many materials processing techniques and the resultant microstructures and materials properties. As the magnitude of the gravitationally induced body force is much lower, the convective flow of fluids can be greatly reduced, thus permitting a more precise control of the phase transformation. In addition, gravitationally induced sedimentation, hydrostatic pressure, and deformation can be greatly reduced. Non-contacting forces such as acoustic, electromagnetic, and electrostatic fields can be used to position specimens and thus reduce the contamination of reactive melts. Finally, experiments performed in a microgravity environment will allow phenomena which are usually masked by the presence of gravity to be rigorously studied.

The processing of materials is evolving from an empirical to a more predictive science. Nevertheless, a fully predictive model of the relationships between the microstructure of a material and the technique used to process the material remains an elusive goal. Microgravity studies offer a unique set of conditions which can be used to extend our present understanding in ways which are not possible in a terrestrial laboratory.

The technological applications of importance to this discipline are quite broad. They range from directional solidification and crystal growth to the production of ceramic powders. The key elements of the supporting scientific knowledge base underpinning these process technologies are listed below in descending priority as recommended by the Materials Science Discipline Working Group:

1. Thermodynamics and kinetics of phase transformations

2. Prediction and control of microstructure including
morphological development and defect formation

3. Heat, mass, and momentum transport

4. Interfacial phenomena

Along with the knowledge base, a data base provided by the quantitative measurement of relevant thermophysical properties is of high priority. These data are of paramount importance for precise modeling and interpretation of experimental phenomena.

The goals of the microgravity materials science research program are 1) to advance the scientific understanding of materials processes affected by gravity, 2) to use low-gravity experiments for insight into the physics and chemistry of materials processes, 3) to provide the scientific knowledge needed to improve these processes, 4) to contribute to the understanding and performance of Earth-based systems that depend on materials science, and 5) to develop unique technologies specifically supporting low-gravity experiments and practical aspects of materials science.

To accomplish these goals, this research announcement is soliciting proposals for all areas of microgravity materials science research. These may be either:

1. Ground-based experimental and theoretical research proposals
or
2. Flight proposals to conduct experimental research in a high quality, long-duration microgravity environment

C. AREAS OF RESEARCH RECOMMENDED BY THE MATERIALS SCIENCE
DISCIPLINE WORKING GROUP

1. Nucleation and Metastable States

In order for a material to transform to a more ordered phase (vapor to liquid or solid, and liquid to solid), it is necessary to first form an aggregate or cluster of molecules above a critical size to initiate the process. Such an aggregate may form on a foreign surface, such as a container wall or a speck of dust (heterogeneous nucleation), or may form spontaneously from random internal fluctuations (homogeneous nucleation). Homogeneous nucleation can occur only if the melt is cooled well below its normal freezing temperature without solidifying. Heterogeneous nucleation will almost always occur first if there are any impurities that can act as nucleation sites. Understanding and being able to control nucleation is extremely important in materials processing.

For example, if it is desired to produce a fine grained casting, one would try to produce a very large number of nuclei and distribute them randomly throughout the melt. Gravity-driven convection plays an important role in this process as was demonstrated in a series of experiments conducted under reduced gravity conditions using SPAR rockets. This is an example of how microgravity experiments may be used to elucidate the essential features of a process and to suggest better control strategies for use on earth, in this case by enhancing convection or artificially stirring the melt.

On the other hand, it is often desirable to suppress nucleation in order to be able to cool a melt to a temperature well below its normal freezing point. Solidification of deeply undercooled melts is usually initiated by a single nucleation site and is very rapid. Rapid solidification can produce an extremely fine microstructure with enhanced mechanical properties. If the solidification is rapid enough, the atoms simply do not have time to arrange themselves in their lower energy or equilibrium configuration and metastable crystalline or amorphous phases can be produced. A metastable phase can have a different crystalline structure which can greatly alter its physical properties. Perhaps the best known example of a metastable crystalline phase is diamond, which is the metastable phase of carbon. Graphite is the equilibrium phase.

An amorphous phase may be thought of as a liquid structure, which lacks long-range crystalline order, frozen in place. Glass is an example of an amorphous solid. Amorphous materials are highly resistant to chemical attack because there are no crystalline grain boundaries, sites which are particularly susceptible to chemical reactions. Similar amorphous structures can be achieved in some metallic alloys by rapid solidification. One example is "Metglas" which consists mainly of iron and boron. The absence of grain structure makes it extremely easy to magnetize and demagnetize with very little hysteresis loss, thus making it very useful in transformer cores where it is 3 times more efficient than the conventional iron-silicon core material. Contrast this with the new iron-boron-neodymium magnet material which is an extremely good permanent magnet because its fine grain structure tends to pin the magnetic domains and prevent demagnetization. These examples demonstrate how crucially the physical properties of a material depend on its microstructure.

The ability to melt and solidify specimens without a container removes a major source of impurities and heterogeneous nucleation sites, thus permitting very large undercooling to be achieved. On Earth this can be accomplished for small samples by levitating and melting them electromagnetically, or by dropping small molten droplets through a long evacuated tube and allowing them to solidify during free-fall.

These techniques have been used extensively but have limitations. It is not possible to control the heating and levitating force independently in an electromagnetic levitator that also employs electromagnetic heating. Use of a quench gas to cool the sample seems to introduce surface nucleation. There is also some evidence that flows in the melt driven by the induced currents may induce 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 droplet as it cools in order to make accurate temperature measurements. Also, the sample size and the amount of undercooling are limited by the time available for the fall.

In reduced gravity, the sample may be positioned by a much lower induced current or may even be allowed to float freely. This decouples the heating from the levitation and reduces the amount of stirring in the melt. The temperature may be measured optically during the cooling and solidification process and thermophysical properties such as specific heat, heat of fusion, and thermal diffusivity can be inferred.

Initial microgravity experiments should focus on factors that limit the degree of undercooling, such as the degree of superheat required to dissolve or evaporate potential nucleation sites, the use of fluxing agents to remove impurities and prevent oxidation, and the effects of stirring on nucleation. Once techniques for obtaining maximum undercooling have been determined, the emphasis of experiments should be on forming metastable and amorphous phases and determining their properties. It may also be possible to perform critical tests to evaluate the various theories of nucleation which have never actually been experimentally verified completely and in fact have been a source of controversy.

Development of a suitable experimental facility for this type of work should be started as soon as possible. Crucial to this development is an accurate contactless technique for continuously measuring the sample temperature that does not require a prior knowledge of the emissivity. Also, methods for obtaining extremely low partial pressures of reactive gases such a oxygen must be developed since oxides and other reaction products will rapidly form on the molten samples and in some systems act as nucleation sites.

Nearly all commercial glass products are formed from high temperature melts (liquids), so it is important to possess both good scientific and technical knowledge for such melts in much the same way as similar knowledge is valuable for metallic melts. Most importantly, the ability to form a glass from a high melting point liquid is controlled by the ability to prevent or control crystal nucleation and growth during cooling. Microgravity offers unique opportunities to investigate the properties and processing of high temperature glass forming melts and their crystallization behavior, for which there is only limited knowledge at this time because of problems directly dependent on gravity. Better understanding of fundamental scientific phenomena such as nucleation/crystallization, mass transport (diffusion), separation of immiscible liquids, and surface tension/segregation forces is possible in microgravity. Microgravity eliminates problems caused by thermally driven convection occurring in fluid (viscosity < 1 Pa.s) melts in one-g that limit our ability to acquire precise and accurate data.

2. Prediction and Control of Microstructure (including pattern formation and morphological stability)

The microstructure developed during the solidification of an alloy plays a key role in determining the properties of a material. Since different microstructures can be developed in a given material by changing processing conditions, it is critical to understand the fundamental phenomena that dictate microstructure formation and how these phenomena are influenced by changes in processing conditions. This knowledge is required for the design of processing conditions to develop a specific microstructure that gives a desired set of properties to the material. A proper design of microstructure involves the selection of stable or metastable phases and the development of an appropriate morphology of the selected phase. For a single phase material, the interface morphology can be planar, cellular or dendritic; whereas for two-phase growth one can obtain either a lamellar/rod eutectic or a layered structure depending upon the growth conditions and the phase diagram. Understanding of the fundamental physics and chemistry that control the selection of phases as well as controls the selection of microstructures is critical to improving our ability to design appropriate microstructures with desired properties.

In some eutectic systems, directional solidification can produce a structure consisting of regularly spaced rods of one phase embedded in the other, or alternating lamellae of the two phases. A potentially important application for these materials, termed "in situ composites", is in turbine blades for high performance jet engines. The aligned structure increases the strength and creep resistance along the axis of the blade. A second potential application is eutectic manganese-bismuth magnets where the magnetic coercivity may be optimized by correlating the eutectic rod diameter to the size of a single elongated magnetic domain.

The processing variables are the growth rate at which the solidification takes place and the temperature profile in the region near the solid/melt interface. The best control of these variables is achieved by the process of directional solidification, in which the material is solidified in one direction under known conditions of growth rate and temperature gradient. Such a directional solidification process is present in many industrial processes such as directional freezing of cast turbine blades, laser welding of alloys, and Bridgman growth of electronic materials. Changes in growth rate and thermal gradient alter the relative importance of thermal/mass transport and interfacial energy effects, and the magnitude of this partitioning of available driving force dictates the formation of specific microstructures. The precise quantitative understanding of this partitioning effect is difficult to evaluate terrestrially since the thermal and mass transport in the liquid occurs by both the diffusive and convective modes of transport. Although there has been significant progress recently in modeling the development of microstructures during solidification, the role of convection is still poorly understood. To a large extent, this is due to the inability of theoretical models to account for convective effects whereas earth-based experiments cannot avoid them. Microgravity experiments provide a unique opportunity to quantitatively understand the fundamental interaction between diffusive transport phenomena and interface energy effects, and thus allow the precise correlation of these interactions with the development of different phases and different microstructures.

Three critical, unsolved theoretical problems in solidification processes are to understand the role of processing conditions in (i) selecting stable or metastable phases, (ii) producing plane front, cellular, dendritic or eutectic growth; and (iii) developing the scaling laws which quantitatively relate the solidified structures' characteristic dimensions to the processing conditions. These characteristic dimensions, for a given phase and a given morphology, are generally related to the mechanical properties of the material. Although significant progress has been made on these problems, the theories generally cannot account for the effects of convection because of the complex phase boundary geometries involved. Convective flows are unavoidable in earth's gravity because of the lateral density gradients that arise due to solute rejection making it impossible to test and refine the existing theoretical models and to quantify the importance of convective effects in ground-based solidification processing.

The solidification conditions in most welding and casting processes give rise to a cellular or dendritic interface which is characterized by a significant segregation of solute in the solidified material. This is due to the solute concentration in the cell or dendrite being much lower than the mean solute composition causing the intercellular or interdendritic region to become richer in solute. The region in which solid cells/dendrites coexist with the surrounding liquid is called the "mushy" zone. When the last liquid finally solidifies, a complex pattern of segregated solute remains. In addition, the solute concentration in the mushy zone can become large enough to form a nonequilibrium phase (for a given composition) which is often an intermetallic compound that can significantly alter the mechanical properties of the material. Through understanding the correlation between microstructure formation and processing conditions, it would be possible to obtain an optimum microstructure from a given processing technique.

When solid and liquid or gaseous phases coexist for some time, microstructural changes can occur such that larger particles will grow at the expense of smaller ones so as to lower the free energy of a system by reducing the total surface energy. This effect, known as Ostwald Ripening or microstructural coarsening, is important in the coarsening of microstructures with time under operating conditions, or in the sintering of fine particles. There is also a large class of industrially important dispersion-hardened alloys in which extremely small particles are either added to or caused to precipitate from the melt during solidification. Since the strengthening effect of the dispersed particles diminishes as their size increases, the coarsening process must be understood and controlled. The process of coarsening requires mass transport which can be influenced by gravitationally induced flow of liquid between particles, or settling of particles in the solid plus liquid or vapor phase. In extreme cases, liquid flow in the two phase zone can lead to transport of solid fragments over large distances. Microgravity experiments provide a unique opportunity to understand the coarsening phenomenon through the suppression of convective and sedimentation effects.

Low-gravity experiments carried out in sounding rockets, in aircraft flying parabolic trajectories, or in orbital flights have illustrated the significance of suppressing convection during solidification processes. Experiments studying dendritic growth have shown evidence that both primary and secondary dendrite arm spacing change as the effects of gravity are reduced. Space experiments with growth of the manganese-bismuth eutectic revealed, surprisingly, that the rod diameter and spacing were considerably smaller than that predicted by the classical theory of eutectic spacing selection which assumes no convection. Growth in strong magnetic fields, which also suppress convective flows, produced results similar to the flight experiments. Paradoxically, control experiments carried out on the ground, in which convective flows were present, agreed very well with the classical theory. European experimenters on Spacelab 1 and the first German Spacelab, D-1, have found similar results with some systems, agreement with the classical theory in other systems, and larger spacings than predicted in still other systems. These disparate results indicate that there is still much to be learned about the role of convection during eutectic solidification.

Many industrial processes in polymer production rely on processing polymer particles or aggregates dispersed in a solvent matrix. Layering or stratification of polymer and solvent occurs on the ground because of gravitational effects. This leads to anisotropic properties in the final product. Conducting experiments on materials such as these in microgravity would eliminate stratification and should lead to a better understanding of fundamental processes involved without the masking effects of gravity. The comparison between the properties of materials processed in 1-g compared to those produced in microgravity would help to establish quantitative relationships between stratification effects and anisotropy.

Most high technology polycrystalline ceramics are sintered or densified assemblages of small single crystal particles. The densification occurs to minimize the high surface energy associated with the small particles. Surface energy is a weak force and both isolated particles and systems of particles tend to adopt configurations that minimize surface energy, e.g., coarse particle size and, in anisotropic systems, faceted particle shape. Both the nature of the minimum energy state and the pathway by which it is achieved are important topics for study. From a technology standpoint, both faceting and coarsening can be viewed as undesirable processes during sintering; that is, they can reduce the amount of energy stored in a powder without causing densification. In contrast, monitoring the evolution of particle size and shape offers the opportunity to gain direct information regarding the mechanism(s) responsible for their change and the relative importance of the different mass transport paths: vapor phase, surface, grain boundary, and volume diffusion.

The study of coarsening and faceting of dispersed or loosely agglomerated particles in microgravity offers the opportunity of obtaining a much better understanding of these two processes that are so central to the technological production of crystalline ceramics. While neither the transport mechanisms nor the driving force involve gravity directly, working in a one-g field can complicate the experimental investigation of such phenomena. In the case of single particles, particles are always in contact with a substrate. The technical issues that arise include chemical reactivity, frictional drag, and morphology of the contact point. In one-g, studying systems of particles effectively means working with powder compacts due to the need for mechanical stability. This restricts the range of variables that are accessible. For example, it may be desirable to work with low number densities of particles that are not fully connected in order to test deviations from mean field models. In addition, the contiguity and connectedness of the particles may strongly influence growth and faceting processes. Furthermore, morphologies produced at particle contacts by a balance of surface energy can obscure that which would otherwise be dominated by surface energy.

The flow of granular media is an important mechanism during handling and processing of ceramic and glass materials. Powders exhibit fluid-like macroscopic behavior in normal gravity and hydrodynamic instabilities can be observed in ground-based experiments on powder. In microgravity environments, van der Walls and electrostatic forces would be expected to play a dominate role in the inertial behavior of granular materials. Experiments and models which illuminate and quantify the cross-over in the behavior of granular materials are needed.

Ceramic processing science has progressed little beyond the needs of traditional ceramics. Yet the emergence of new and improved technologies hinge critically on rational improvements in wet (or suspension) processing of ceramic powders. Previous research has focused on model systems consisting of single phase, monosized ceramic particles. However, it is clear that relevant ceramic systems will either be single or multiphase with engineered particle size distributions (e.g. bimodal or trimodal). Hence, a fundamental understanding of these systems, referred to here as "complex ceramic suspensions", is required. The development of this science base would benefit significantly from research conducted in a microgravity environment since density-driven phase separation and/or particle size-induced segregation effects will be dramatically reduced relative to ambient conditions.

Microgravity conditions offer the possibility of producing composite structures which are not possible or easily obtainable on earth. Of particular interest are ceramic-metal or other composites consisting of phases of widely differing density, and composites which consist of discrete areas of a second phase contained within a major phase. In the first case, approaches to fabricating metal-ceramic composites on earth are restricted by density differences and thus segregation of phases during fabrication. For example, efforts to impregnate ceramic preforms or to mix ceramic and metal powders or other precursors are impeded by problems associated with ceramic preforms floating to the surface and metal phases being non-uniformly distributed. Processing approaches which allow uniform mixtures to be prepared on earth and heat-treated under microgravity conditions should allow the formation of layered ceramic-metal structures, uniform interconnecting two-phase structures, or homogenous particulate-reinforced structures. The removal of gravity's effect on phase segregation will allow wetting interactions between the two or more phases to be more clearly understood.

In the second case, mixing of whiskers or other second phase particles with ceramic powders to produce composites is difficult and can result in non-uniform structures. Mixing powders and whiskers in an aqueous medium in space should allow the preparation of more uniform, novel, or controlled structures because settling due to gravitational forces can be avoided or controlled. For example, mixing can be achieved by the application of mechanical force only without the effects of convection. Although aggregation of particles will occur, these clusters will not settle as in earth's gravity; thus, it should be possible to grow clusters of certain sizes and make layered structures by artificially applying a gravitational force of specified duration (in a centrifuge, for example) or by filtration. A uniform composite containing dispersed clusters or individual particles in a major phase could also be made by filter pressing.

Combustion synthesis of ceramic generates high temperatures at and ahead of the reaction front. These high temperatures generate liquids and possibly gases which are subject to gravity-driven flow. The removal of such gravitational effects is likely to provide increased control of the reaction front with a consequent improvement in control of the microstructure of the synthesized ceramic products. Experiments need to be developed to determine the role that gravity (and lack of gravitational forces) plays in the synthesis process.

3. Phase Separation and Interfacial Phenomena

Another class of polyphase materials of interest to microgravity materials science research is the monotectic systems, characterized by a region of liquid phase immiscibility. For a range of compositions, the melt of such systems will separate into two immiscible liquids as the temperature is lowered. The two liquids may form initially as a fine dispersion of one phase in the other, potentially a good starting point for a useful microstructure. However, when these materials are solidified it is normally found that the liquids have formed much less desirable large agglomerates. If this agglomeration process could be understood and controlled, a range of useful materials might result.

Sedimentation is the most obvious effect causing agglomeration of the dispersed liquids. Since the two liquids of a monotectic system are always of different composition, they will invariably have different densities. In Earth's gravity, the two liquid phases will stratify before they can be frozen, resulting in a highly segregated solid. It was once believed that if this buoyancy-driven sedimentation could be avoided by solidifying such systems in microgravity, a fine dispersion of one phase in the other would be produced. However, attempts to accomplish this have been only partially successful, indicating that effects other than gravity also lead to segregation of the two liquid phases. An extensive series of ground-based experiments, using transparent organic systems that having similar monotectic phase diagrams and which can also be made neutrally buoyant, has revealed the effects which occur when gravitationally-induced segregation is relatively unimportant. These experiments uncovered a rich variety of phenomena driven by interfacial energy.

A clear example of the dominating influence of interfacial energy effects occurs in monotectic alloy systems as the temperature falls below the critical temperature at which liquid phase separation first occurs. Over a limited range of temperature below the critical value, one of the liquids exhibits "perfect wetting" behavior, encapsulating the other liquid and coating any container. If final freezing occurs within this temperature range, massive segregation is produced. Alternatively, if final solidification occurs via the monotectic reaction, below the perfect wetting temperature range, a well aligned two-phase structure with a uniform spacing between phases may be achieved in a directional growth process. Regularity of the structure requires that flows driven by the temperature and composition dependence of surface energy be suppressed. The minority liquid phase may form droplets which migrate in a gradient of temperature or composition. Because larger droplets move faster than smaller ones, the latter will be overtaken and engulfed. Eventually, all of the minority phase is therefore segregated into a few large droplets. In fact, depending on the alloy system, thermal gradients as small as one (1) Kelvin per centimeter in low gravity can have the same effect as earth gravity in causing massive segregation.

The temperature and composition dependence of surface energy may also cause mixing in single phase liquids. Convective flows driven by surface energy forces can be similar to or greater than those induced by gravity. When temperature gradients are large, such as in welding, these flows can be very rapid, and their quantitative understanding is critical for modeling the shape of weld pools.

Surface tension is a function of both temperature and composition, so that a gradient in either will drive flows. Fluid flow driven by variations in the surface tension along a free surface is called Marangoni convection. Usually, Marangoni convection is obscured by density-driven convective flows. Limited experience during space experiments indicates that above a critical Marangoni number (ratio of surface tension force to viscous force), the flow becomes unsteady and temperature fluctuations in the melt are observed. In addition, a detailed series of ground and low-gravity experiments has demonstrated that striations due to Marangoni convection are produced in float-zone grown silicon.

Additional interfacial energy effects are the determining factors in other forms of microgravity materials processing. One example is the formation of potentially unique composite materials by the directional solidification of an alloy melt which initially contains a uniform dispersion of fine solid particles. In this case, the interplay between solidification velocity and particle/liquid interfacial energy is crucial in determining the distribution of the strengthening particles. Some combinations of these processing variables result in the particles being entrapped at the solidification front to give a uniform microstructure, while others result in particles being repelled from the solidification front, producing segregation and poor properties.

Interfacial energy effects are also critical in containerless processing, where the extent of wetting between the solid(s) forming from the liquid controls the degree of undercooling and thus, the resulting microstructure. Brazing, soldering, and welding operations are additional processes which are influenced by interfacial phenomena. Here, understanding of both wetting and surface energy driven flows can be primary factors in achieving success.

In addition to the phenomena which occur at interfaces between two fluids, processes occurring at solid-vapor interfaces can be influenced by gravitationally-induced convective flows. Compound semiconductors are materials whose unique properties are optimized when they are formed as single crystals by a process called physical vapor transport (PVT). Mercurous chloride, an acousto-optic material, mercuric iodide, a gamma ray detector, and silicon carbide, a wide gap semiconductor, represent several examples. In PVT, the material to be crystallized is evaporated from a heated source, transported as vapor, and finally deposited on a seed crystal held at a lower temperature. Crystal properties are influenced by atomic attachment kinetics at the crystal vapor interface, by the rate of arrival of species from the vapor (and hence influenced by convection phenomena), and by the formation of compositionally variant or impurity enriched boundary layers in the vapor near the crystal surface. Sharp, well defined facets are a characteristic feature of high quality PVT crystals grown under reduced gravity which minimizes convective flow in the vapor. These crystals exhibit greater crystallographic homogeneity than their earth grown counterparts, resulting in significantly improved resolution for radioactive detectors fabricated from these crystals. A quantitative explanation for these improvements is still lacking.

It is clear that a consideration of interfacial phenomena is common to numerous materials processing technologies. While limited studies of these interfacial effects are possible on earth using density-matched immiscible systems, even in these systems, perfect density matching is possible only at a single temperature. Therefore a microgravity environment provides a unique opportunity to study and quantify surface energy phenomena in order to promote more effective materials processing both on Earth and under microgravity conditions.

4. Transport Phenomena (including process modeling and thermophysical properties measurement)

All of the important phenomena that determine materials structure and properties during processing are controlled by heat, mass, and momentum transport. For each of these effects, the nature of the transport is determined by the relative importance of convective and diffusive transport. When diffusive effects are dominant, the system response is controlled by materials properties, whereas the system response is controlled by processing conditions when convective effects dominate. Microgravity provides a unique environment in which the relative importance of convective and diffusive transport can be controlled by the experimenter.

Convective flows originate in solidifying systems because the density varies with both composition and temperature. Any density differences interact with gravity to produce buoyancy forces which drive convection. It is often desirable to suppress this convection, producing materials under purely diffusive conditions. Examples include the control of segregation in crystal growth and the testing of theories of microstructure development. To accomplish this requires a comprehensive understanding of the transport processes.

This understanding can be significantly improved through process modeling. It may not be sufficient to simply reduce gravity, even by six orders of magnitude, to eliminate undesirable convection. Alternatively, it may be unnecessary to go to space at all. The goal of process modeling is to represent the experimental system as a set of mathematical equations, whose solution describes the system behavior. Varying system parameters in the model allows exploration of the system, enabling the design and interpretation of experiments, and comparison of theoretical predictions with experimental results. Realistic process models are therefore an invaluable adjunct to experimental investigations.

Most of the process models developed to date have considered only steady accelerations. Additional model development is needed to assess the importance of transient and oscillatory accelerations, known to be present on orbiting spacecraft (g-jitter). Recent developments in mathematical modeling have also made it possible to determine the sensitivity of a given experiment to perturbations or uncertainties in imposed boundary or initial conditions, geometry or thermophysical properties. Further development of these techniques would be of significant benefit to the MSAD program.

A serious deficiency in our ability to model materials processing is the paucity of accurate thermophysical property data for most materials in the molten state. This is a problem not only for scientists modeling microgravity experiments, but for many industrial researchers who are beginning to use process modeling for terrestrial processes. The lack of high temperature thermophysical data is partially due to the extreme difficulty of making accurate measurements on melts in unit gravity. European experiments on Spacelab 1 and the first German Spacelab, D-1, showed that diffusion coefficients for a variety of materials measured in space differ considerably from the accepted values obtained on earth either because of wall effects (such measurements are usually made in capillary tubes) or because of residual convection, which is virtually impossible to avoid. Also, the effect of thermodiffusion (Soret 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 on earth, although it now appears to play a more important role in mass transport in many terrestrial processes than had been realized previously.

The relevant thermophysical properties needed for reliable materials processing models include:

* Electro-optical properties
Emissivity
Electrical conductivity
Optical properties

* Calorimetric properties
Specific heat
Heats of mixing, formation, transformations, etc.

* Transport Coefficients
Thermal conductivity
Viscosity
Diffusion coefficients

* Density

* Thermodynamic Moduli
Thermal expansion coefficients
Compressibility, etc.

* Vapor pressures and activity coefficients

* Surface tension / interfacial energies

* Equations of state

The most time-consuming step in manufacturing common glasses on earth is bubble removal, "fining." Information that leads to faster fining on earth would have enormous economic value. Thermal convection on earth is a major complication in studying bubble motion and the diffusion of gasses from bubbles; convection destroys simple diffusion profiles. An improved understanding of fining of glass would be much more easily obtained without the effects of gravity in a microgravity environment.

Evaporation of volatile components from glasses such as PbO or Na2O can alter the surface composition of glass melts that lead to convective flows, composition variations, and composition and property gradients. Convective flows make analysis of material loss and the resulting composition change difficult if not impossible. Separation of the material loss by convection in the absence of gravity offers the possibility to greatly simplify the process and improve the quality of a large number of commercial glasses.

A fundamental issue of ceramic-metal joining and the fabrication of interpenetrating 3-dimensional ceramic-metal and ceramic-ceramic composites is the wetting of the solid ceramic phase by the molten metal or molten ceramic. Most wetting experiments are carried out under 1g conditions which can influence the spreading and contact of the molten phase with the solid substrate. This gravity-induced contact can lead to increased reaction and spreading of the molten phase and misrepresentation of the real degree of wetting. Infiltration behavior of molten metals and ceramics into porous ceramics to produce composites is inconsistent with measured 1g wetting experiments. Wetting experiments need to be carried out under reduced-gravity conditions to determine the wettability of ceramics by molten metals and other ceramics.

5. Crystal Growth, and Defect Generation and Control

During directional solidification of semiconductors, generation of destabilizing temperature gradients in the melt is unavoidable, resulting in buoyancy-induced convective mixing of the liquid phase. On Earth this convective mixing is intensive, and influences the segregation of melt constituents at the growth front. Nearly all Earth-grown crystals exhibit radial and axial compositional non-uniformities characteristic of growth from well- or nearly well-mixed melts. Although intentional mixing schemes such as the accelerated crucible rotation technique or the application of an oscillatory magnetic field can provide improvements in crystal quality, growing crystals in space provides the opportunity to reduce the convective flow intensity and, for some classes of systems and charge sizes, achieve diffusion-controlled mass transfer during growth. This also is expected to promote the growth of higher quality crystals and can yield information on important materials parameters including the mass-diffusion coefficient and the mass-segregation coefficient.

The early crystal growth experiments of the Apollo-Soyuz Test Project (ASTP) and Skylab have established the potential of microgravity processing to achieve diffusion controlled growth in small diameter charges. However, the reduced-gravity levels achieved during growth in space unmask or introduce phenomena that are not important or present during Earth-based growth. In some materials systems, for example, the normally occurring density variations in the melt provide a stabilizing effect during growth. These same density variations create complications related to the alignment of the residual gravity vector with respect to the growth direction during growth in reduced gravity. Transients in the direction and magnitude of the gravity vector (g-jitter) can introduce additional difficulties. Furthermore, the space experiments have shown that during growth the melt may separate from the non-wetting crucible permitting surface-tension driven convective mixing of the melt.

Development of a comprehensive theory of segregation during directional solidification has been aided recently by the advances in computational power permitting detailed numerical modeling of solidification processes. Because of improvements in growth hardware design providing for accurate quantification of thermal boundary conditions, and because the gravitational field on Earth is one-dimensional and steady, axisymmetric models of crystal growth can currently predict the experimental results well enough to be useful in guiding the selection of experiment parameters. The above complications associated with space processing, however, show that these models cannot be directly used for analysis of space experiments through a simple reduction of the gravity level in the simulations. Indeed, the three dimensional unsteady acceleration field and the possible Marangoni convection in space result in complex flow structures; numerical modeling has only recently begun to assess and quantify these effects. For example, in axisymmetric simulations, it has been shown that the alignment of the gravity vector parallel to the growth surface in space, as opposed to perpendicular to the growth surface, can result in a two order of magnitude increase in the convective intensity at the same gravity level. This phenomenon has been observed experimentally in growth experiments on the first United States Microgravity Laboratory (USML- 1) and the first United States Microgravity Payload (USMP-2) missions in HgZnTe and HgCdTe, respectively. In a detailed analysis of the g-jitter effects, the compositional variations in the melt associated with the transients in residual acceleration have been shown to persist over time periods much longer than those characterizing the g-jitter. Thus, simulation of unsteady transport processes in space is required. These considerations, along with the implications of Marangoni convection if present, indicate that diffusion-controlled growth may be only marginally achievable in space. More importantly, if a crystal grown in space does not exhibit all characteristics of diffusion controlled growth, our ability to interpret the experimental results and identify controlling phenomena is limited.

Magnetic damping of convection in electrically conductive melts can be used to provide a higher degree of control of convection in the melt. In this approach, the motion of an electrically conductive melt in a magnetic field produces an opposing Lorentz force field that reduces the flow velocities in the melt. Modeling results indicate that the superimposed effect of moderate magnetic fields and the microgravity environment of low earth orbit can reduce convective flow intensities to an extent unreachable either by using magnets on Earth, or by microgravity processing alone. Magnetic damping also can potentially reduce the influence of Marangoni convection and high-frequency transients in the melt flow velocities associated with g-jitter. Magnetically stabilized Czochralski crystal growth experiments indicate that intensive turbulent flow perturbations can be suppressed at relatively low magnetic field strengths (of the order of 0.1-0.2 Tesla). This suggests that magnetic fields may significantly reduce the deleterious effects of convective perturbations associated with g-jitter. Fundamental considerations also suggest that magnetic damping may suppress Marangoni convection in microgravity.

The generation and propagation of defects during the fluid to solid phase change is generally understood on only an empirical basis. For example, it is known that defects are generated (or nucleated) at the tri-junction between the container wall, the solid, and the melt during directional solidification and that these defects tend to propagate along a direction perpendicular to the melt-solid interface. The degree of fluid motion is also known to have an influence on defect concentration and distribution as demonstrated by the striations observed in Czochralski-grown materials. The deconvolution of growth rate and fluid flow contributions to defect generation is still unresolved; the details of the generation and propagation of defects remains one of the least understood mechanisms related to the preparation of high quality single crystal materials from the melt or from the vapor This is still true even though defects, whether they are impurity atoms or lattice defects, have a major impact on electronic and optical properties, the very reasons most single crystals are grown.

New characterization techniques, such as atomic force microscopy and synchrotron X-ray topography, are giving a fresh approach to the study of defects in semiconductor crystals. Atomic force microscopy shows that the established technique of etch pit density determination, even when done accurately, does provide a complete representation of the defect structure at the semiconductor surface. Synchrotron X-ray topographs reveal not only twins but a cellular structure of low angle grain boundaries, lines indicating slip planes initiating at the twin boundaries, and dislocations within the subgrains.

There is mounting evidence from flight experiments that fluid flows influence defect generation and propagation, and microgravity experiments should provide an excellent method for learning more about this important topic. For example, there are results from a TEXUS (sounding) rocket flight experiment which show many growth-induced striations in InSb in the earth-grown portion, and no striations in the space-grown portion except for those induced for interface demarcation by electric current pulses. One important experiment that does show promise of providing useful information in this area is the Bridgman growth of Cd0.96Zn0.04Te. The defect generation observed in this material was markedly reduced in two ingots grown aboard the Shuttle during the USML-1 mission compared to ingots grown under otherwise identical conditions on Earth. A quantitative explanation for these results is still being investigated.

Polymer crystal growth is more complex than the growth of inorganic crystals because of the large molecular weights of the individual polymer molecules and their structural complexity that makes molecular attachment to a growing crystal stereochemically complicated. Equally important is that in microgravity it is possible to study the fundamentals of polymer crystal growth. Not only can the effects of temperature and compositional gradients on growth kinetics in the absence of gravity-induced convective effects be studied, but also the effect of the size of the individual polymer units and their interaction with the solvent molecules on the crystal growth be studied.

Many polymeric materials have potential as unique nonlinear optical materials. Thin films of polymers produced by vapor deposition contain high defect concentrations that limit their utility as nonlinear optical materials. Microgravity experiments on inorganic crystal growth by vapor deposition clearly show that materials with lower defect concentrations can be produced. A quantitative explanation for these results is still lacking.

It has been demonstrated that much larger, and defect-free protein crystals can be grown in microgravity compared to those grown on earth. These results suggest that gravity can play a role on the kinetics of rearrangement of large molecules during crystal growth from solution. There are many other polymers, such as nonlinear optical materials, for which the availability of single crystals would be extremely useful for property studies and to determine the effects of crystal defects on properties. Microgravity provides the opportunity to investigate the growth of such crystals.

III. EXPERIMENTAL APPARATUS AND FLIGHT OPPORTUNITIES

Several flight instruments have been developed by or through NASA in order to address the objectives of the materials science discipline described in the preceding section. Appendix B provides brief capability descriptions of the flight hardware and ground-based reduced-gravity testing facilities.

Flight opportunities available through this NRA will be on the appropriate space platform including the Space Shuttle and the International Space Station Alpha. Science protocols should consider the additional benefits that could accrue from skilled crew interaction with experiments available during many of these flight opportunities. Residual acceleration levels of the order of 10[-4]g are available in a Space Shuttle orbiter. Space Shuttle flights are usually of 7-10 days duration, although some flights of 16 days and longer are planned. The acceleration environment aboard the Space Station should be substantially better during limited time periods. The Space Acceleration Measurement System (SAMS) is expected to be available for the measurement and recording of actual accelerations at or near the apparatus during experimentation. Downlink channels enable investigators to monitor their instrument status and science data streams in near real time. An uplink channel enables them to act on that information.

IV. PROPOSAL SUBMISSION INFORMATION

This section gives the requirements for submission of proposals in response to this announcement. The research associated with a typical proposal submitted under this announcement is conducted by a Principal Investigator who is responsible for all research activities and one or more Co-Investigators. Proposers must address all the relevant selection criteria in their proposal as described in Section VI and must format their proposal as described in this section. Additional general information for submission of proposals in response to NASA Research Announcements may be found in Appendix C.

A. LETTER OF INTENT

Organizations planning to submit a proposal in response to this NRA should notify NASA of their intent to propose by sending a Letter of Intent.

All Letters of Intent that are mailed through the U.S. Postal Service by express, first class, registered, or certified mail are to be sent to NASA Headquarters, addressed as follows:

Dr. Michael J. Wargo
Microgravity Science and Applications Division
Code UG
National Aeronautics and Space Administration
Washington, DC 20546-0001

Letters of Intent sent by commercial delivery or courier services (e.g. Federal Express) are to be delivered to the following address between the hours of 8 AM and 4:30 PM:

Dr. Michael J. Wargo
Microgravity Science and Applications Division
Code UG
National Aeronautics and Space Administration
ATTN: Receiving and Inspection (Rear of building)
300 E Street, SW
Washington, DC 20024-3210

NASA cannot receive proposals on Saturdays, Sundays or federal holidays.

The Letter of Intent, which should not exceed two pages in length, must be typewritten in English and must include the following information:

* The potential Principal Investigator (PI ), position, organization, address, telephone, telex, and telefax number.

* A list of potential Co-Investigators (Co-I's), positions, and organizations.

* General scientific/technical objectives of the research.

* The equipment of interest listed in this NRA, if appropriate.

The Letter of Intent should be received at NASA Headquarters no later than January 30, 1995; facsimile transmission (FAX 202-358-3091) is acceptable.

The Letter of Intent is being requested for informational and planning purposes only, and is not binding on the signatories. The Letter of Intent allows NASA to better match expertise in the composition of peer review panels with the response from this solicitation. Investigators may also request more detail on the capabilities of the specific equipment that might be utilized to accomplish their scientific objectives.

B. PROPOSAL

Proposals submitted in response to this Announcement must contain at least the following information in the format shown below:

* Title Page

* Table of Contents

* Executive Summary (replaces abstract) (1-2 pages)

* Research Project Description
Statement of the hypothesis, objective, and value of this research
Review of relevant research
Justification for new or further microgravity research
Description of Experimental or Analytical Method
Data Analysis
References

* Appendices
Management Plan (appropriate for large or complex efforts)
Complete vitae for the PI and all Co-investigators
Current and Pending Support
Facilities and Equipment (see Appendix C, Section 7-h)
Proposed Costs (see Appendix C, Section 7-i)
Signed Certifications (see below)

The title page must clearly identify the research announcement to which the proposal is responding, title of the proposed research, principal investigator, institution, address and telephone number, total proposed cost, proposed duration, and must contain all signatories.

The executive summary should succinctly convey, in broad terms, what the proposer wants to do, how the proposer plans to do it, why it is important, and how it meets the requirements for microgravity relevance. The executive summary replaces the proposal abstract.

The management plan is necessary when the proposed research involves large or complex efforts among numerous individuals or other organizations to insure a coordinated effort.

The proposal should not exceed 20 pages in length, exclusive of appendices and supplementary material, and should be typed on 8-1/2 x 11 inch paper with a 10- or 12-point font. Extensive appendices and ring-bound proposals are discouraged. Reprints and preprints of relevant work will be forwarded to the reviewers if submitted as attachments to the proposal. Proposers should prepare cost estimates by year for a period not greater than four years in preparing budget plans in response to this Research Announcement.

Each proposal requesting financial support should include signed Certifications Regarding Lobbying; Debarment, Suspension and other Responsibility Matters; and Drug-Free Workplace Requirements. Copies of these certifications may be found at the end of this document.

Fifteen copies of the proposal must be received at NASA Headquarters by March 20, 1995, to assure full consideration. Send proposals to Dr. Wargo at the address given below. Treatment of late proposals is described in Appendix C.

All Proposals that are mailed through the U.S. Postal Service by express, first class, registered, or certified mail are to be sent to NASA Headquarters, addressed as follows:

Dr. Michael J. Wargo
Microgravity Science and Applications Division
Code UG
National Aeronautics and Space Administration
Washington, DC 20546-0001

Proposals sent by commercial delivery or courier services (e.g. Federal Express) are to be delivered to the following address between the hours of 8 AM and 4:30 PM:

Dr. Michael J. Wargo
Microgravity Science and Applications Division
Code UG
National Aeronautics and Space Administration
ATTN: Receiving and Inspection (Rear of building)
300 E Street, SW
Washington, DC 20024-3210

NASA cannot receive proposals on Saturdays, Sundays or federal holidays.

V. ADDITIONAL GUIDELINES FOR INTERNATIONAL PARTICIPATION

Non-U.S. proposals and U.S. proposals which include non-U.S. participation, must be endorsed by the appropriate government agency in the country from which the non-U.S. participant is proposing. The letter of endorsement from the sponsoring agency should indicate that 1) the proposal merits careful consideration by NASA, and 2) sufficient funds will be made available to undertake the activity as proposed if the proposal is selected. NASA will not transfer funds to non-U.S. participants.

Proposals from non-U.S. entities should not include a cost plan. All proposals must be in English and must follow all other guidelines and requirements described in the NRA. Non-U.S. proposals will undergo the same evaluation and selection process as those originating in the U.S.

In addition to sending the letter of endorsement and 15 copies of the proposal to Dr. Michael J. Wargo of the Microgravity Science and Applications Division (see page v for Address), 1 copy of the proposal, along with a letter of endorsement from the sponsoring agency, must be provided.

All letters of endorsement and proposals that are mailed through the U.S. Postal Service by express, first class, registered, or certified mail are to be sent to NASA Headquarters, addressed as follows:

Ms. Ruth Rosario
Office of External Relations
Code IH
National Aeronautics and Space Administration
Washington, DC 20546-0001
USA

Letters of endorsement and proposals sent by commercial delivery or courier services (e.g. Federal Express) are to be delivered to the following address between the hours of 8 AM and 4:30 PM:

Ms. Ruth Rosario
Office of External Relations
Code IH
National Aeronautics and Space Administration
ATTN: Receiving and Inspection (Rear of building)
300 E Street, SW
Washington, DC 20024-3210

All proposals must be received before the established closing date; those received after the closing date will be treated in accordance with NASA's provisions for late proposals. Sponsoring government agencies may, in exceptional situations, forward a proposal directly to the above address if review and endorsement is not possible before the announced closing date. In such cases, NASA should be advised when a decision on endorsement can be expected. A letter of endorsement from the sponsoring government agency must be received by NASA before May 1, 1995; if the letter of endorsement is not received prior to this date, the proposal will be deemed non-responsive and will not be reviewed by NASA.

Successful and unsuccessful proposers will be contacted directly by the NASA Program Office (Microgravity Science and Applications Division) coordinating the NRA. Copies of these letters will be sent to the sponsoring government agency. Should a non-U.S. proposal or a U.S. proposal with non-U.S. participation be selected, NASA's Office of External Relations will arrange with the sponsoring agency for the proposed participation on a no-exchange-of-funds basis, in which NASA and the sponsoring agency will each bear the cost of discharging its respective responsibilities. Depending on the nature and extent of the proposed cooperation, these arrangements may entail a letter of notification by NASA, an exchange of letters between NASA and the sponsoring agency, and an agreement or memorandum of understanding between NASA and the sponsoring government agency.

VI. EVALUATION AND SELECTION The following section replaces Section 13 of Appendix C.

A. EVALUATION FACTORS AND PEER REVIEW PROCESS

The principal elements considered in the evaluation of proposals solicited by this NRA are: 1) overall scientific and technical merit of the proposal, 2) relevance to the NASA microgravity program as determined by the investigation's articulated need for a microgravity environment, or articulated support of a microgravity research program, and 3) realism and reasonableness of the proposed cost. Articulation of the need for a microgravity environment in rigorous, quantitative terms will be deemed to have greater microgravity relevance. Qualitative articulation of the need for a microgravity environment will be deemed to have lower microgravity relevance. Intrinsic merit has the greatest weight, followed by relevance to NASA's objectives, which has slightly lesser weight. Both of these elements have greather weight than cost.

The programmatic objectives of this NRA, as discussed in the introduction to this Appendix, will be applied by NASA to enhance program breadth, balance, and diversity.

Evaluation of the intrinsic merit of the proposal includes consideration of the following factors, in decending order of importance:

1. Overall scientific or technical merit, including evidence of unique or innovative methods, approaches, or concepts, and the potential for new discoveries or understanding;

2. Qualifications, capabilities, and experience of the proposed principal investigator, team leader, or key personnel who are critical in achieving the proposal objectives;

3. Institutional resources and experience that are critical in achieving the proposal objectives;

4. Overall standing among similar proposals available for evaluation and/or evaluation against the known state-of-the-art.

Evaluation by NASA of the cost of a proposed effort includes comparison against historical experience with efforts of similar scope and scale, and the relationship of the proposed cost to available funds. Cost is a significant evaluation factor, and NASA may select proposals with an offer of funding below the requested budget.

The evaluation process for this NRA will be based on a peer review of the proposal's intrinsic scientific and technical merit, articulated relevance to the microgravity program, and cost of the research plan. The reviewers will be scientific and technical experts from government, academia, and industry. Each proposal will be reviewed independently by members of the review panel and discussed at a review panel meeting to determine a consensus evaluation for the proposal. All proposals will be evaluated on a merit scale of 1 (worst rating) to 9 (best rating). A rating below 5 is not generally considered for funding. An engineering review of potential flight hardware requirements will be conducted by NASA for proposals that include flight experiments.

The MSAD Director will make the final selection based on science panel and programmatic recommendations. Upon completion of all deliberations, a selection statement will be released notifying each proposer of proposal selection or rejection. Offerers whose proposals are declined will have the opportunity of a verbal debriefing regarding the reasons for this decision. Additional information on the evaluation and selection process is given in Appendix C.

B. SELECTION CATEGORIES, PERIOD OF SUPPORT, AND FLIGHT PROGRAM PROCESS

Proposals selected for support through this NRA will be selected as either ground-based- or flight-definition investigations. Investigators offered support in the ground-based program normally will be required to submit a new proposal for competitive renewal after at most four years of support. Investigators offered flight definition status are expected, in addition to their reseaarch work, to begin preparing detailed flight experiment requirements and concepts for flight development shortly after selection in cooperation with the assigned Project Scientist from a NASA Center.

The Principal Investigator in flight definition must prepare a Science Requirements Document (SRD) early in the development of a flight experiment to guide the design, engineering, and integration effort for the instrument. The SRD describes specific experiment parameters, conditions, background, and justification for flight. Ground-based, normal, and reduced-gravity experimentation, as well as any necessary modeling efforts, may also be required to prepare an adequate Science Requirements Document. The amount of support (technical, scientific, and budgetary) provided to investigators by NASA will be determined by the Program Scientist for Materials Science in collaboration with the Program Manager for Materials Science based upon specific investigator needs and the availability of resources to NASA and MSAD.

These activities are in preparation for a Science Concept Review (SCR) to be held within approximately two years of the beginning of Investigator funding. Investigations not selected for flight because of scientific, technological, or programmatic considerations at the SCR will be placed in the ground-based program and funding will continue until the end of the original four-year period. This Review will be conducted before a scientific peer panel that will be asked to assess:

* The significance of the problem being investigated including the benefits that the experimental and theoretical results would provide to the materials science research community and industry.

* The maturity of the overall scientific investigation.

* The scientific objectives of the proposed flight experiments.

* The need for a microgravity environment to achieve the proposed scientific objectives.

* The priorities of these scientific objectives.

* The rigor with which the proposed flight experiment has been conducted terrestrially. (e.g. influence of gravity, reproducibility and quantification of experimental conditions and results, materials characterization, modeling, application/verification of current and/or developing theoretical framework etc.)

* The scientific specifications for the proposed flight experiments as expressed in the preliminary draft of the Science Requirements Document.

* The conceptual design for the apparatus and whether this design could be expected to deliver a level of performance that allows the scientific objectives to be achieved.

* Technology issues that would prevent a timely, successful achievement of the scientific objectives.

The selected investigations will be required to comply with MSAD policies, including the return of all appropriate information for inclusion in the MSAD archives during the performance of and at the completion of the contract or grant.

Commitment by NASA to proceed from flight definition to the execution of a flight experiment will be made only after several additional engineering and scientific reviews and project milestones have established the feasibility and merit of the proposed experiment. Investigations not selected for flight at these reviews will be funded for a limited period (approximately one year) to allow an orderly termination of the project.

VII. NRA FUNDING

The total amount of funding for this program is subject to the annual NASA budget cycle. The Government's obligation to make awards is contingent upon the availability of appropriated funds from which payment for awards can be made, and the receipt of proposals which the Government determines are acceptable for an award under this NRA.

For the purposes of budget planning, it is assumed that the Microgravity Science and Applications Division will fund as many as 30 to 40 research proposals, at varying levels of funding depending on the objectives of the research. While some proposals may be significantly higher, most are expected to be lower than the anticipated average of $100,000 per year per proposal for ground-based research and $175,000 per year per proposal for flight definition, with correction for inflation. Resources permitting, a few high-risk, high-payoff-if-successful ideas will be considered for funding at an average level of $50,000 per year for up to two years.

Proposed budgets will normally include a portion of the researchers' salaries, travel for the investigators and students to about three science and NASA meetings per year, other expenses (publication costs, computing or workstation costs etc.), burdens, and overhead.