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All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

Differential Thermal Analysis of Hg(1-x)Mn(x)Te Alloys in the X=0 to 0.3 Range

Understanding the experimental conditions necessary for the development of radial and axial compositional homogeneity in directionally solidified Hg(0.89)Mn(0.11)Te(MMT) crystals has been difficult due to the lack of segregation coefficient data on the Hg(1-x)Mn(x)Te alloy system in the X = 0 to 0.3 composition range. Determining segregation coefficient data from the available Hg(1-x)Mn(x)Te alloy phase equilibria data is not practical due to discrepancies in the shape of the reported solidus and liquidus curves in the X = 0 to 0.3 range. To resolve these discrepancies and to obtain segregation coefficient data which can be used to understand homogeneity in directionally solidified MMT crystals, the solidus and liquidus temperatures of seven Hg(1-x)Mn(x)Te alloys in the X = 0 to 0.3 range were determined using differential thermal analysis (DTA). The Hg(1-x)Mn(x)Te phase diagram constructed for the X = 0 to 0.3 range of this alloy system from the DTA measurements clarifies the shape of the solidus and liquidus curves in this range. The segregation coefficient for the Hg(1-x)Mn(x)Te system was found to vary from 5 to 4.4 as the solidus composition increased from 0-30 atomic percent MnTe. This information will be useful in the analysis of axial and radial homogeneity of directionally solidified MMT crystals.

Price, M. W.↗
NASA NTRS1998

3-D Modeling of Directional Solidification of a Non-Dilute Alloy with Temperature and Concentration Fields Coupling via Materials Properties Dependence and via Double Diffusive Convection

Numerical simulation of the HgCdTe growth by the vertical Bridgman method was performed using FIDAP finite element code. Double-diffusive melt convection is analyzed, as the primary factor at controls inhomogeneity of the solidified material. Temperature and concentration fields in the model are also coupled via material properties, such as thermal and solutal expansion coefficients with the dependence on both temperature and concentration, and melting temperature evaluation from pseudobinary CdTe-HgTe phase diagram. Experimental measurements were used to obtain temperature boundary conditions. Parametric study of the melt convection dependence on the gravity conditions was undertaken. It was found, that the maximum convection velocity in the melt can be reduced under certain conditions. Optimal conditions to obtain a near flat solidified interface are discussed. The predicted interface shape is in agreement with one obtained experimentally by quenching. The results of 3-D calculations are compared with previous 2- D findings. A video film featuring 3-D melt convection will be presented.

Bune, Andris V.↗
NASA NTRS1999

Effect of a Nonplanar Melt-Solid Interface on Lateral Compositional Distribution during Unidirectional Solidification of a Binary Alloy with a Constant Growth Velocity V: Theory - Part 1

Infrared detector materials, such as Hg(1-x)Cd(x)Te, Hg(1-x)Zn(x)Te have energy gaps almost linearly proportional to its composition. Due to the wide separation of liquidus and solidus curves of their phase diagram, there are compositional segregations in both of axial and radial directions of these crystals grown in the Bridgman system unidirectionally with constant growth rate. It is important to understand the mechanisms which affect lateral segregation such that large uniform radial composition crystal is possible. Following Coriell, etc's treatment, we have developed a theory to study the effect of a curved melt-solid interface shape on the lateral composition distribution. The system is considered to be cylindrical system with azimuthal symmetric with a curved melt-solid interface shape which can be expressed as a linear combination of a series of Bessell's functions. The results show that melt-solid interface shape has a dominate effect on lateral composition distribution of these systems. For small values of b, the solute concentration at the melt-solid interface scales linearly with interface shape with a proportional constant of the product of b and (1 - k), where b = VR/D, with V as growth velocity, R as sample radius, D as diffusion constant and k as distribution constant. A detailed theory will be presented. A computer code has been developed and simulations have been performed and compared with experimental results. These will be published in another paper.

Wang, Jai-Ching↗
NASA NTRS1998

Measurement of Interfacial Undercooling in a Dilute Pb-Sn Alloy Near the Regime of Morphological Instability

The solid/liquid (s/1) interfacial Seebeck technique was used to measure interfacial undercooling in a Pb-1 wt.% Sn alloy. For plane front solidification an undercooling in the range of 0.54-0.57C was recorded. However, for growth velocities between 6 to 15 micron/s the undercooling, increased with increase in velocity. 'Tbis is in contradiction to the theoretical predictions based on growth at the extremum condition. A modified analytical expression was developed based on the maro,,inal stability criterion to predict undercooling for growth velocities immediately over the limit of morphological stability. Between 6 to 15 microns/s the trend predicted by this modified expression is in qualitative agreement with the experimental results. This model is more consistent with the analysis of Venugopal and Kirkaldy which states that the extremum condition criterion is not applicable in the marginal stability regime. At higher growth velocities between 20 and 50 microns/s where stable cellular growth is expected the undercooling, decreased with increase in velocity. The predictions of Burden and Hunt based on extremum criterion agree reasonably well with the experimental results in this velocity range. Finally, the importance of non-equilibrium phase diagram parameters such as partition coefficient and liquidus slope were also evaluated using the experimental undercooling results obtained in this investigation.

Sen, S.↗
NASA NTRS1999

Theory of Dust Voids in Plasmas

Dusty plasmas in a gas discharge often feature a stable void, i.e., a dust-free region inside the dust cloud. This occurs under conditions relevant to both plasma processing discharges and plasma crystal experiments. The void results from a balance of the electrostatic and ion drag forces on a dust particle. The ion drag force is driven by a flow of ions outward from an ionization source and toward the surrounding dust cloud, which has a negative space charge. In equilibrium the force balance for dust particles requires that the boundary with the dust cloud be sharp, provided that the particles are cold and monodispersive. Numerical solutions of the one-dimensional nonlinear fluid equations are carried out including dust charging and dust-neutral collisions, but not ion-neutral collisions. The regions of parameter space that allow stable void equilibria are identified. There is a minimum ionization rate that can sustain a void. Spatial profiles of plasma parameters in the void are reported. In the absence of ion-neutral collisions, the ion flow enters the dust cloud's edge at Mach number M = 1. Phase diagrams for expanding or contracting voids reveal a stationary point corresponding to a single stable equilibrium void size, provided the ionization rate is constant. Large voids contract and small voids expand until they attain this stationary void size. On the other hand, if the ionization rate is not constant, the void size can oscillate. Results are compared to recent laboratory and microgravity experiments.

Goree, J.↗
NASA NTRS2000

The Scientific Case for Sample Return from a Primitive Near-Earth Asteroid

This paper is predicated on the recent experimental findings that the smallest structural ferromagnesiosilica entities in collected chondritic aggregate interplanetary dust particles (IDPs) have a predictable metastable eutectic composition. Kinetically controlled gas to solid condensation of MgFe-SiO-O2-H2 vapors does NOT produce stoichiometric crystalline solids (i.e. minerals) such as predicted by equilibrium condensation models but instead yields amorphous solids that are chemically ordered at metastable eutectics in the binary phase diagrams. Therefore these condensed dust grains will have a considerable amount of 'internal free energy' that will make them highly responsive to changes in their environments. Their inherent high energy-content 'buys' time and energy for mineralogical modification and chemical readjustment in response to changing environmental conditions. The activation energy barrier for reactions in metastable eutectic solids will be lower than for crystalline solids. As a result the reactions can take place at much lower temperatures and on much shorter time scales. That is, they will also occur much earlier in the evolution of a parent body wherein heat-producing sources may either be immature or inefficient. The ensuing reaction chains from metastable equilibrium to full thermodynamic equilibration with the local environment will be a chaotic 'Ostwald cascade' but from a well defined starting point to a predictable end result. Here we will discuss the implications of metastable eutectic dust as we see them for dust properties in icy and ice-free parent bodies wherein circumstellar dust is still recognizable because of the primitive nature of these parent bodies. We point to potential engineering constraints on sample acquisition and storage during Earth transit. Among the scientific goals of a primitive Near Earth Asteroid (NEA) sample return mission will be verification of the nature of dust forming and modification processes during hierarchical accretion in the earliest protoplanets. We propose sampling an infrared P- or D-class NEA or an object showing cometary activity such as 2201 Oljato that could be an asteroid or evolved comet. Additional information is contained in the original extended abstract.

Rietmeijer, Frans J. M.↗
NASA NTRS2000

A Remote Sensing/Geographic Information Systems Approach in the Selection of Mars Sites of Biological Interest

The search for extinct or extant life on Mars is the search for past or present liquid water, respectively. There are numerous signs of past liquid water on Mars in the form of dry river valleys, paleolakes, and their associated flow and sediment patterns. While some of these features are recent (Amazonian, 1.8 billion years ago to present), there is no evidence that any are currently flowing. Liquid water on the surface would only be possible at those sites with sufficiently high temperatures and pressure. The key to the selection of sites on Mars to search for evidence of life is the search for the presence of water. An approach to this problem is the use of remotely sensed data incorporated in a geographic information system (GIS). A GIS is a computer-based system capable of assembling, storing, manipulating, and displaying geographically referenced information, i.e., data identified according to their locations. In planetary studies these data are acquired from remote sensing (RS) platforms (orbiters). These data are co-registered layers and, through the use of GIS analysis functions, areas on these layers can be selected as a function of the information desired. Our work used existing data layers from the Viking and Mars Global Surveyor missions to determine where water could be possible in liquid form on the Martian surface, based on the phase diagram for water.

Lobitz, B. M.↗
NASA NTRS2001

Impact and Collisional Processes in the Solar System

In the past year, we have successfully developed the techniques necessary to conduct impact experiments on ice at very low temperatures. We employ the method of embedding gauges within a target to measure the shock wave and material properties. This means that our data are not model dependent; we directly measure the essential parameters needed for numerical simulations of impact cratering. Since then we have developed a new method for temperature control of icy targets that ensures temperature equilibrium throughout a porous target. Graduate student, Sarah Stewart-Mukhopadhyay, is leading the work on ices and porous materials as the main thrust of her thesis research. Our previous work has focused on icy materials with no porosity, and we propose to extend our research to include porous ice and porous ice-silicate mixtures. There is little shockwave data for porous ice, and none of the data was acquired under conditions applicable to the outer solar system. The solid ice Hugoniot is only defined for initial temperatures above -20 C. Our program uniquely measures the properties of ice at temperatures directly applicable to the solar system. Previous experiments were conducted at ambient temperatures soon after removing the target from a cold environment, usually just below freezing, or in a room just below freezing. Since ice has an extremely complicated phase diagram, it is important to conduct experiments at lower temperatures to determine the true outcome of impacts in the outer solar system. This research is complementary to other programs on icy materials. Our work focuses on the inherent material properties by measuring the shock wave directly; this complements the macroscopic observations and immediately provides the parameters necessary to extend this research to the gravity regime. Our numerical simulations of impacts in porous ice under very low gravity conditions, such as found on comets, show that the final crater size and shape is very dependent on the dynamic strength of the material.

Ahrens, Thomas J.↗
NASA NTRS2001

Thermodynamic Database for the NdO(1.5)-YO(1.5)-YbO(1.5)-ScO(1.5)-ZrO2 System

A database for YO(1.5)-NdO(1.5)-YbO(1.5)-ScO(1.5)-ZrO2 for ThermoCalc (ThermoCalc AB, Stockholm, Sweden) has been developed. The basis of this work is the YO(1.5)-ZrO2 assessment by Y. Du, Z. Jin, and P. Huang, 'Thermodynamic Assessment of the ZrO2-YO(1.5) System'. Experimentally only the YO(1.5)-ZrO2 system has been well-studied. All other systems are only approximately known. The major simplification in this work is the treatment of each single cation unit as a component. The pure liquid oxides are taken as reference states and two term lattice stability descriptions are used for each of the components. The limited experimental phase diagrams are reproduced.

Jacobson, Nathan S.↗
NASA NTRS2001

Calculation of Phase Equilibria in the Y2O3-Yb2O3-ZrO2 System

Rare earth oxide stabilized zirconias find a wide range of applications. An understanding of phase equilibria is essential to all applications. In this study, the available phase boundary data and thermodynamic data is collected and assessed. Calphad-type databases are developed to completely describe the Y2O3-ZrO2, Yb2O3-ZrO2, and Y2O3-Yb2O3 systems. The oxide units are treated as components and regular and subregular solution models are used. The resultant calculated phase diagrams show good agreement with the experimental data. Then the binaries are combined to form the database for the Y2O3-Yb2O3-ZrO2 psuedo-ternary.

Jacobson, Nathan S.↗
NASA NTRS2001

Thermodynamic Activity Measurements with Knudsen Cell Mass Spectrometry

Coupling the Knudsen effusion method with mass spectrometry has proven to be one of the most useful experimental techniques for studying the equilibrium between condensed phases and complex vapors. The Knudsen effusion method involves placing a condensed sample in a Knudsen cell, a small "enclosure", that is uniformly heated and held until equilibrium is attained between the condensed and vapor phases. The vapor is continuously sampled by effusion through a small orifice in the cell. A molecular beam is formed from the effusing vapor and directed into a mass spectrometer for identification and pressure measurement of the species in the vapor phase. Knudsen cell mass spectrometry (KCMS) has been used for nearly fifty years now and continues to be a leading technique for obtaining thermodynamic data. Indeed, much of the well-established vapor specie data in the JANAF tables has been obtained from this technique. This is due to the extreme versatility of the technique. All classes of materials can be studied and all constituents of the vapor phase can be measured over a wide range of pressures (approximately 10(exp -4) to 10(exp -11) bar) and temperatures (500-2800 K). The ability to selectively measure different vapor species makes KCMS a very powerful tool for the measurement of component activities in metallic and ceramic solutions. Today several groups are applying KCMS to measure thermodynamic functions in multicomponent metallic and ceramic systems. Thermodynamic functions, especially component activities, are extremely important in the development of CALPHAD (Calculation of Phase Diagrams) type thermodynamic descriptions. These descriptions, in turn, are useful for modeling materials processing and predicting reactions such as oxide formation and fiber/matrix interactions. The leading experimental methods for measuring activities are the Galvanic cell or electro-motive force (EMF) technique and the KCMS technique. Each has specific advantages, depending on material and conditions. The EMF technique is suitable for lower temperature measurements, provided a suitable cell can be constructed. KCMS is useful for higher temperature measurements in a system with volatile components. In this paper, we briefly review the KCMS technique and identify the major experimental issues that must be addressed for precise measurements. These issues include temperature measurements, cell material and cell design and absolute pressure calibration. The resolution of these issues are discussed together with some recent examples of measured thermodynamic data.

Copland, Evan H.↗
NASA NTRS2002

Thermophysical Property Measurements in the MSFC ESL

Electrostatic Levitation (ESL) is an advanced technique for containerless processing of metals, ceramics, and semiconductors. Because no container is required, there is no contamination from reaction with a crucible, allowing processing of high temperature, highly reactive melts. The high vacuum processing environment further reduces possible contamination of the samples. Finally, there is no container to provide heterogeneous nucleation sites, so the undercooled range is also accessible for many materials. For these reasons, ESL provides a unique environment for measuring thermophysical properties of liquid materials. The properties that can be measured in ESL include density, surface tension, viscosity, electrical and thermal conductivity, specific heat, phase diagram, TTT- and CCT- curves, and other thermodynamic properties. In this paper, we present data on surface tension and viscosity, measured by the oscillating drop technique, and density, measured by an automated photographic technique, measured in the ESL at NASA Marshall Space Flight Center.

Hyers, R. W.↗
NASA NTRS2002

Theoretical Studies of Liquid He-4 Near the Superfluid Transition

We performed theoretical studies of liquid helium by applying state of the art simulation and finite-size scaling techniques. We calculated universal scaling functions for the specific heat and superfluid density for various confining geometries relevant for experiments such as the confined helium experiment and other ground based studies. We also studied microscopically how the substrate imposes a boundary condition on the superfluid order parameter as the superfluid film grows layer by layer. Using path-integral Monte Carlo, a quantum Monte Carlo simulation method, we investigated the rich phase diagram of helium monolayer, bilayer and multilayer on a substrate such as graphite. We find excellent agreement with the experimental results using no free parameters. Finally, we carried out preliminary calculations of transport coefficients such as the thermal conductivity for bulk or confined helium systems and of their scaling properties. All our studies provide theoretical support for various experimental studies in microgravity.

Manousakis, Efstratios↗
NASA NTRS2004

Nucleation of the Widmanstatten Pattern in Iron Meteorites

The Widmanstatten pattern develops at low temperatures during the evolution of the asteroids. We have studied the origin of the Widmanstatten pattern in order to obtain metallographic cooling rates in the temperature range (approx. 700 to 300 deg C). This paper summarizes our recent evaluation of the various mechanisms for the formation of the Widmanstatten pattern. All chemical groups of the iron meteorites are considered. We also propose a new mechanism for the formation of the Widmanstatten pattern in the low P metal phase of iron, stony-iron and stony meteorites. The results of this evaluation enables us to more accurately determine metallographic cooling rates particularly when incorporated with other recent advances in Fe-Ni and Fe-Ni (P saturated) phase diagrams and interdiffusion coefficients.

Yang, J.↗
NASA NTRS2004

Lunar and Planetary Science XXXV: Impacts: Modeling and Observations

This document covers the following topics: Cratering on Titan: Projectiles, Craters and Impact Melt; The Cratering Database: Making Code Jockeys Honest; Popigai Impact Structure Modeling: Morphology and Worldwide Ejecta; Anhydrite EOS and Phase Diagram in Relation to Shock Decomposition; Computational Investigations of the Chesapeake Bay Impact Structure; Hydrocode Simulations of the Chesapeake Bay Impact; Lockne Crater as a Result of Oblique Impact; The Influence of a Deep Shelf Sea on the Excavation and Modification of a Marine-Target Crater, the Lockne Crater, Central Sweden; Pre-Drilling Investigation of the Lake Bosumtwi Impact Crater: Constraints from Geophysics and Numerical Modelling; Central Uplift Formation at the Middlesboro Impact Structure, Kentucky, USA; A SRTM Investigation of Serra da Cangalho Impact Structure, Brazil; Brazilian Impact Craters: A Review; Flynn Creek Impact Structure: New Insights from Breccias, Melt Features, Shatter Cones, and Remote Sensing; The Howell Structure, Lincoln County, Tennessee: A Review of Past and Current Research; After the Chicxulub Impact: Control on Depositional and Diagenetic History of the Cenozoic Carbonate Formations of the Northwestern Yucatan Peninsula, Mexico; Ni Contents by Non-Destructive In-Situ XRF Method of Takamatsu-Kagawa Crater District in Japan; and Akiyoshi Limestone Blocks Transported by the P/T Boundary Event to Japan Islands.

Source record↗
NASA NTRS2003

Studies of Nucleation and Growth, Specific Heat and Viscosity of Undercooled Melts of Quasicrystals and Polytetrehedral-Phase-Forming Alloys

By investigating the properties of quasicrystals and quasicrystal-forming liquid alloys, we may determine the role of ordering of the liquid phase in the formation of quasicrystals, leading to a better fundamental understanding of both the quasicrystal and the liquid. A quasicrystal is solid characterized by a symmetric but non-periodic arrangement of atoms, usually in the form of an icosahedron (12 atoms, 20 triangular faces). It is theorized that the short-range order in liquids takes this same form. The degree of ordering depends on the temperature of the liquid, and affects many of the liquid s properties, including specific heat, viscosity, and electrical resistivity. The MSFC role in this project includes solidification studies, phase diagram determination, and thermophysical property measurements on the liquid quasicrystal-forming alloys, all by electrostatic levitation (ESL). The viscosity of liquid quasicrystal-forming alloys is measured by the oscillating drop method, both in the stable and undercooled liquid state. The specific heat of solid, undercooled liquid, and stable liquid are measured by the radiative cooling rate of the droplets.

Source record↗
NASA NTRS2005

Proper Criteria of Nonlinear Optical Crystals for Space Laser Systems and the Possible Causes for Space Laser Failures

NASA is striving to develop a scientific understanding of the universe and the Earth-Sun System and its response to natural or human-induced changes. Space lasers are vital tools for NASA's missions to advance our understanding of space research and improving our prediction capability for climate, weather, and natural hazards. Unfortunately, several past space missions that utilized lasers proved to be short-lived and unreliable. In this paper, we are reporting the results of our investigations on several nonlinear optical crystals, which are vital components in space lasers. Examples of these investigations are: The correlation of the phase diagrams of nonlinear crystals and its durability, the effect of radiating these crystals by high-energy beams of protons and gamma on their second harmonic efficiency, and measurements of the high-energy and low-energy thresholds for each crystal before and after irradiation. A set of proper criteria for these crystals will be presented. We will also discuss the possible causes of failures in a space laser and propose a solution to a contamination problem in all future space lasers.

Abdeldayem, Hossin A.↗
NASA NTRS2005

Beamline Electrostatic Levitator (BESL) for in-situ High Energy K-Ray Diffraction Studies of Levitated Solids and Liquids at High Temperature

Determinations of the phase formation sequence, the crystal structures and the thermodynamic properties of materials at high temperatures are difficult because of contamination from the sample container and environment. Containerless processing techniques, such as electrostatic (ESL), electromagnetic (EML), aerodynamic, and acoustic levitation, are most suitable these studies. An adaptation of ESL for in-situ structural studies of a wide range of materials, including metals, semiconductors, insulators using high energy (125 keV) synchrotron x-rays is described here. This beamline ESL (BESL) allows the in-situ determination of the atomic structures of equilibrium solid and liquid phases, including undercooled liquids, as well as real-time studies of solid-solid and liquid-solid phase transformations. The use of image plate (MAR345) or GE-Angio detectors enables fast (30 ms - 1s) acquisition of complete diffraction patterns over a wide q-range (4 - 140/mm). The wide temperature range (300 - 2500 K), containerless processing under high vacuum (10(exp -7) - 10(exp -8) torr), and fast data acquisition, make BESL particularly suitable for phase diagram studies of high temperature materials. An additional, critically important, feature of BESL is the ability to also make simultaneous measurement of a host of thermo-physical properties, including the specific heat, enthalpy of transformation, solidus and liquidus temperatures, density, viscosity, and surface tension; all on the same sample and simultaneous with the structural measurements.

Gangopadhyay, A. K.↗