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This paper describes the concept of a low melting temperature alloy deployment mechanism, U.S. Patent 4,842,106. It begins with a brief history of conventional dimethyl-silicone fluid damped mechanisms. Design fundamentals of the new melting alloy mechanism are then introduced. Benefits of the new over the old are compared and contrasted. Recent experiments and lessons learned complete this paper.
Recent isotopic and mineralogical studies have demonstrated a temporal and chemical link between the Chicxulub multiring impact basin and ejecta at the Cretaceous-Tertiary boundary. A fundamental problem yet to be resolved, however, is identification of the projectile responsible for this cataclysmic event. Drill core samples of impact melt rock from the Chichxulub structure contain Ir and Os abundances and Re-Os isotopic ratios indicating the presence of up to approx. 3 percent meteoritic material. We have used a technique involving microdrilling and high sensitivity instrumental neutron activation analysis (INAA) in conjunction with electron microprobe analysis to characterize further the distribution of siderophile and other trace elements among phases within the C1-N10 melt rock.
Pressure-assisted infiltration with a 2014 Al alloy of plain and Cu-coated single crystal platelets of alpha silicon carbide was used to study particulate wettability under dynamic conditions relevant to pressure casting of metal-matrix composites. The total penetration length of infiltrant metal in porous compacts was measured at the conclusion of solidification as a function of pressure, infiltration time, and SiC size for both plain and Cu-coated SiC. The experimental data were analyzed to obtain a threshold pressure for the effect of melt intrusion through SiC compacts. The threshold pressure was taken either directly as a measure of wettability or converted to an effective wetting angle using the Young-Laplace capillary equation. Cu coating resulted in partial but beneficial improvements in wettability as a result of its dissolution in the melt, compared to uncoated SiC.
This experiment is the first to melt and freeze a high temperature thermal energy storage (TES) material under an extended duration of microgravity. It is one of a series to validate an analytical computer program that predicts void behavior of substances undergoing phase change under microgravity. Two flight experiments were launched in STS-62. The first, TES-1, containing lithium fluoride in an annular volume, performed flawlessly in the 22 hours of its operation. Results are reported in this paper. A software failure in TES-2 caused its shutdown after 4 seconds. A computer program, TESSIM, for thermal energy storage simulation is being developed to analyze the phenomena occurring within the TES containment vessel. The first order effects, particularly the surface tension forces, have been incorporated into TESSIM. TESSIM validation is based on two types of results. First is the temperature history of various points of the containment structure, and second, upon return from flight, the distribution of the TES material within the containment vessel following the last freeze cycle. The temperature data over the four cycles showed a repetition of results over the third and fourth cycles. This result is a confirmation that any initial conditions prior to the first cycle had been damped out by the third cycle. The TESSIM simulation showed a close comparison with the flight data. The solidified TES material distribution within the containment vessel was obtained by a tomography imaging process. The frozen material was concentrated toward the colder end of the annular volume. The TESSIM prediction showed the same pattern. With the general agreement of TESSIM and the data, a computerized visual representation can be shown which accurately shows the movement and behavior of the void during the entire freezing and melting cycles.
Studies of the reactive melt infiltration of silicon-niobium alloys in microporous carbon preforms prepared by the pyrolysis of a polymer precursor have been carried out using modeling, Differential Thermal Analysis (DTA), and melt infiltration. Mercury porosimetry results indicate a very narrow pore size distribution with virtually all the porosity within the carbon preforms open to infiltrants. The morphology and amount of the residual phases (niobium disilicide and silicon) in the infiltrated material can be tailored according to requirements by careful control of the properties (pore size and pore volume) of the porous carbon preforms and alloy composition. The average room temperature four-point flexural strength of a reaction-formed silicon carbide material (made by the infiltration of medium pore size carbon preform with Si - 5 at. % Nb alloy) is 290 +/- 40 MPa (42 +/- 6 ksi) and the fracture toughness is 3.7 +/- 0.3 MPa square root of m. The flexural strength decreases at high temperatures due to relaxation of residual thermal stresses and the presence of free silicon in the material.
A rift over 6000 km in total length runs along the border of Lada Terra, a highland in the southern hemisphere of Venus, and Lavinia Planitia, a basin that has been interpreted as a site of early-stage mantle downwelling. Along the length of the rift are a number of volcanic centers of widely varying morphology and volcanic output. These include coronae, radially fractured domes, and large flow fields similar in scale to terrestrial flood basalts. We develop a model for the origin of extension related to passive rifting in response to stresses created by the adjacent downwelling. Volcanism and extension at other rifts on Venus, such as Devana Chasma, have been attributed to deep-seated mantle plume activity. In contrast, we interpret the origin of extension and volcanism along the Lada rift to be linked to upwelling and decompression melting of mantle material due to rifting and, possibly, to counterflow associated with downwelling. Extension occurred generally prior to the formation of volcanic centers and the eruption of large-scale flow fields, although most of the volcanic centers have been fractured by continued extension along the rift. Current debate over the formation of terrestrial flood basalts centers on the necessity of preexisting extension and stretched and thinned lithosphere to produce enhanced decompression melting within a large plume head or mantle thermal anomaly. Our studies of large-scale flow fields associated with the Lada rift and coronae on Venus indicate that extension is a prerequisite for the formation of the majority of large-scale flow units on Venus.
Report discusses preparation of thermoelectric materials from melts. First stage in preparation of thermoelectric material is one of alloying or synthesis, in which high-purity elemental constituents melted together in stoichiometric or nonstoichiometric mixture.
In this article, the solidus temperatures of the Hg(sub 1-x) Zn(sub x)Te pseudobinary phase diagram for several compositions in the low x region were measured by differential thermal analysis and the HgTe-ZnTe pseudobinary phase diagram was constructed. The densities of two HgZnTe melts, x = 0.10 and 0.16, were determined by an in situ pycnometric technique in a transparent furnace over, respectively, 110 and 50 C ranges of temperature. The thermodynamic properties of the melts, such as the heat capacity and enthalpy of mixing, were calculated for temperatures between the liquidus and 1500 C by assuming an associated solution model for the liquid phase.
An oscillating-cup viscometer was developed to measure viscosity of molten HgZnTe ternary semiconductor alloys. Data were collected for the pseudobinary Hg(0.84)Zn(0.16)Te melt between 770 and 850 C. The kinematic viscosity was found to vary from approximately 1.1 to 1.4 x 10(sup -3)sq cm/s. A slow relaxation phenomena was also observed for temperatures from the melting point of 770 to approx. 800 C. Possible mechanisms for this effect are discussed.
The processes and mechanisms of melting and their applications to chondrule formation are discussed A model for the kinetics of congruent melting is developed and used to place constraints on the duration and maximum temperature experienced by the interiors of relict-bearing chondrules. Specifically, chondrules containing relict forsteritic olivine or enstatitic pyroxene cannot have been heated in excess of 1901 C or 1577 C, respectively, for more than a few seconds.
An approximate theoretical analysis was made of the shielding mechanism whereby the rate of heat transfer to the forward stagnation point of blunt bodies is reduced by melting and evaporation. General qualitative results are given and a numerical example, the melting and evaporation of ice, is presented and discussed in detail.
A computational model is presented for the study of the solidification and melting of a pure substance and of a binary alloy. The enthalpy method has been used, and incorporated into a commercial CFD code. Three examples of the use of the model are described: the three-dimensional solidification of a pure substance (succinonitrile), the results of which are compared with experiment; an example of the solidification of a bismuth-tin alloy; and a simulation of a solidification and melting experiment done in space known as the MEPHISTO program.
The solidification of a dilute bismuth-tin alloy under Bridgman crystal growth conditions is investigated in support of NASA's MEPHISTO space shuttle flight experiment. Computations are performed in two-dimensions with a uniform grid. The simulation includes the species-concentration, temperature and flow fields, as well as conduction in the ampoule. Fully transient simulations have been performed; no simplifying steady state approximations are used. Results are obtained under microgravity conditions for pure bismuth, and Bismuth-0.1 at.% Sn and Bi-1.0 at.% Sn alloys. The concentration dependence of the melting temperature is neglected; the solid/liquid interface temperature is assumed to be the melting temperature of pure bismuth for all cases studied. For the Bi-1.0 at.% Sn case the results indicate that a secondary convective cell, driven by solutal gradients, forms near the interface. The magnitude of the velocities in this cell increases with time; this causes increasing solute segregation at the liquid/solid interface.
Experimental data on Al-0.8Au horizontal solidification of a 1 mm thick specimen in a BN crucible shows the effect of growth rate on the solidification interface shape. For translation rates below 0.5 micron/s the interface maintains a plain and flat shape. When the translation rate is 3 to 5 micron/s or more, the interface appearance changes to two planar zones, with the zone closer to the bottom having higher inclination. The interface shapes were measured by first quenching in place during growth. X-ray microscopy shows the interface shape within the quenched sample by viewing through the side of the specimen. In order to provide theoretical explanation of the phenomena, numerical modeling was undertaken using finite element code FIDAP. Double diffusion convection in Al-0.8Au melt and crystal-melt interface curvature during directional solidification was analyzed numerically. Actual thermophysical properties of Al-0.8Au including the binary Al-Au phase diagram were used. Although convection in the sample is weak, for the slower translation rate convection and diffusion is sufficient for the redistribution of initial compositional stratification caused by gravity. When translation rate is raised, neither convection nor diffusion can provide proper mixing so that solidification temperatures differ significantly near the bottom within the bulk of the sample. As a result, the solid-liquid interface appears to have two planar zones with different inclination.
Bulk single crystals of triethylphosphine sulfide (C2H5)3P(S), a potential nonlinear optical organic material has been grown from melt using Bridgman- Stockbarger method. Commercially available material triethylphosphine sulfide procured from Johnson Matthey was purified by physical vapor transport using low pressure sublimaton (about 30 mTorr). Modified Bridgman-Stockbarger technique is characterized by the smallest possible quantity of product in the melt for a short period of time. The temperature gradient was chosen to be about 10 C/cm and the ampoule was lowering rate was chosen to be 0.2-0.3 mm/h. For various growth conditions, several single crystal of triethylphosphine sulfide have been grown with sizes 10X10X15mm(exp 3). The second harmonic generation (SHG) efficiency was measured and has been found to be comparable to phase matched potassium dihydrogen phosphate crystals. Further characterization of the grown single crystals is in progress and the results will be presented at the symposium.
Results of a study of the effectiveness of using static magnetic fields to reduce convection in Ge-Si melts will be presented. Lenz's law causes a retardation of convection when a static magnetic field is applied to an electrically conducting liquid. However, during the solidification of a solid-solution system such as Ge-Si, the interface is neither isothermal nor isoconcentrational. The variation of temperature and chemical composition along the interface causes thermoelectric currents to be generated within the solidifying material (and the container if it is electrically conductive). These currents, in the presence of a magnetic field, can cause movement (stirring, convection) in the melt which can exceed convection induced by normal thermosolutal mechanisms. Crystals have been grown by both the Bridgman and floating-zone methods. Clear evidence for the existence of this thermoelectromagnetic convection, especially in the case of Si floating-zone growth, will be presented.
During Bridgman growth of semiconductors detachment of the crystal and the melt meniscus has occasionally been observed, mainly under microgravity (microg) conditions. An important factor for detached growth is the wetting angle of the melt with the crucible material. High contact angles are more likely to result in detachment of the growing crystal from the ampoule wall. In order to achieve detached growth of germanium (Ge) and germanium-silicon (GeSi) crystals under 1g and microg conditions, sessile drop measurements were performed to determine the most suitable ampoule material as well as temperature dependence of the surface tension for GeSi. Sapphire, fused quartz, glassy carbon, graphite, SiC, pyrolytic Boron Nitride (pBN), AIN, and diamond were used as substrates. Furthermore, different cleaning procedures and surface treatments (etching, sandblasting, etc.) of the same substrate material and their effect on the wetting behavior were studied during these experiments. pBN and AIN substrates exhibited the highest contact angles with values around 170 deg.
An axially traveling magnetic wave induces a meridional basic flow in a molten cylindrical zone. This flow can be beneficial for crystal growth applications. In particular, it can be used to effectively stir the melt in long cylindrical columns. Also, it can beneficially modify the thermal and species concentration fields in the melt and the interface shape of the growing crystal. The basic theory of such an application is developed and preliminary data from a mercury column experiment are also presented.