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At least 1,261 records · Page 70

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NASA NTRS1986

Hugoniot equation of state of anorthite glass and lunar anorthosite

Twenty-one Hugoniot experiments were conducted on an amorphous material of anorthite composition, in the pressure range 8-120 GPa, using both routine and new methods. Two Hugoniot measurements at about 120 GPa were made on lunar gabbroic anorthosite (Apollo 15,418). Theoretical Hugoniots are constructed for both materials assuming they are disproportionate to their component oxides. These accurately predict the P-p behavior of the lunar anorthosite Hugoniot at 120 GPa and the anorthite glass Hugoniot above 50 GPa, but overestimate the shock temperatures of anorthite glass. The mixed oxide model fails to predict the release paths of either material. It is concluded that the mixed oxide model is a good description of the bulk properties of the high-pressure phases of anorthite, but does not represent the actual phases. A significant enrichment of calcic refractory material in the earth's lower mantle is not precluded by the bulk properties of the anorthite high-pressure phases.

Boslough, M. B.↗

NASA NTRS1987

Glass-to-Metal Seal Against Liquid Helium

Simple compression joint with indium gasket forms demountable seal for superfluids. Seal developed for metal lid on glass jar used in experiments on liquid helium. Glass container allows contents to be viewed for such purposes as calibration of liquid-level detectors and adjustments of displacement plungers. Seal contains liquid helium even when temperature drops below 2.19K. Made from inexpensive, commercially available materials and parts.

Watkins, John L.↗

NASA NTRS1986

Effect of OH content on phase separation behavior of soda-silica glasses

The kinetics of phase separation in a gel and ordinary soda-silica glass are followed by use of small angle X-ray scattering. Also, the influence of OH on the phase separation behavior is studied. It is found that OH accelerates the growth of the secondary phase, and that gel and ordinary glasses of similar composition and OH concentration exhibit similar phase separation characteristics.

Neilson, G. F.↗

NASA NTRS1987

Chaotic nature of the spin-glass phase

The microscopic structure of the ordered phase of spin glasses is investigated theoretically in the framework of the T = 0 fixed-point model (McMillan, 1984; Fisher and Huse, 1986; and Bray and Moore, 1986). The sensitivity of the ground state to changes in the interaction strengths at T = 0 is explored, and it is found that for sufficiently large length scales the ground state is unstable against arbitrarily weak perturbations to the bonds. Explicit results are derived for d = 1, and the implications for d = 2 and d = 3 are considered in detail. It is concluded that there is no hidden order pattern for spin glasses at all T less than T(C), the ordered-phase spin correlations being chaotic functions of spin separation at fixed temperature or of temperature (for a given pair of spins) at scale lengths L greater than (T delta T) exp -1/zeta, where zeta = d(s)/2 - y, d(s) is the interfacial fractal dimension, and -y is the thermal eigenvalue at T = 0.

Bray, A. J.↗

NASA NTRS1987

Mean-field theory of spin-glasses with finite coordination number

The mean-field theory of dilute spin-glasses is studied in the limit where the average coordination number is finite. The zero-temperature phase diagram is calculated and the relationship between the spin-glass phase and the percolation transition is discussed. The present formalism is applicable also to graph optimization problems.

Kanter, I.↗

NASA NTRS1987

Qualification of S-glass/epoxy thermal isolator bands

Unidirectional fiberglass reinforced epoxy structures have been evaluated as thermal isolator tension straps for the charge-coupled devices on the Hubble Space Telescope's Wide Field/Planetary Camera. Mechanical and thermal properties are reported for filament-wound S-2 glass in a generic epoxy resin and compared to S-901 glass bands used on a previous camera. Measurements were performed on very small paper clip-shaped bands. Probably the smallest in length and have a load carrying mean cross sectional area of only 0.00024 square inches.

Bauer, Jerry↗

NASA NTRS1984

Glass formation and properties in the gallia-calcia system

The critical cooling rate for glass formation was measured for five compositions in the Ga2O3-CaO system and varied from a low of (315 + or - 85) C/s for a eutectic melt containing 37.5 mol pct Ga2O3 to a high of (840 + or - 60) C/s for a melt containing 52 mol pct Ga2O3. The density and refractive index both increased with increasing Ga2O3 content, but the crystallization temperature and microhardness varied only slightly. The IR spectra of these glasses suggest that both GaO4 tetrahedra and GaO6 octahedra are present.

Whichard, G.↗

NASA NTRS1984

Crystallization of fluorozirconate glasses

The crystallization of a number of glasses of the fluorozirconate family has been studied (using powder X-ray diffraction and DSC) as a function of time and temperature of heating. The main crystalline phases were beta BaZrF6 and beta BaZr2F10. Stable and metastble transformations to the low-temperature alpha phases were also investigated. The size of crystallites in fully devitrified glasses was calculated (from line broadening of the X-ray diffraction peaks) to be about 60 nm.

Bansal, Narottam P.↗

NASA NTRS1986

Annealing of electron damage in mid-IR transmitting fluoride glass

Damage in ZrF4-BaF2-LaF3 glass induced by high-energy electrons was studied by ESR and optical spectroscopy. An optical absorption band at 314 nm in the irradiated glass annealed rapidly above about 50 C, probably by a second-order reaction at room temperature; the ESR lines annealed very slowly at room temperature.

Bansal, Narottam P.↗

NASA NTRS1988

Long-Lived Glass Mirrors For Outer Space

Paper summarizes available knowledge about glass mirrors for use in outer space. Strengths and weaknesses of various types of first and second reflective surfaces identified. Second-surface glass mirrors used in outer space designed to different criteria more stringent for terrestrial mirrors. Protons, electrons, cosmic rays, meteorites, and orbiting space debris affect longevities of components. Contamination also factor in space.

Bouquet, Frank L.↗

NASA NTRS1987

Fracture study of windshield glass panes

The major stresses which cause crack propagation in windshield glass panes are induced by bending moments which result from the pressure differentials across the panes. Hence the stress intensity factors for the finite plate with the semi-elliptical surface flaw and edge crack under the bending moments are examined. The results show that the crack growth will be upperbound if it is computed by using the stress intensity factor for the finite plate with the edge crack subjected to pure bending moments. Furthermore, if the ratio of crack depth to plate thickness, a/t, is within 0.3, the stress intensity factor can be conservatively assumed to be constant at the value of a/t equal to zero. A simplified equation to predict the structural life of glass panes is derived based on constant stress intensity factor. The accuracy of structural life is mainly dependent on how close the empirical parameter, m, can be estimated.

Yeh, H. Y.↗

NASA NTRS1988

Noncontact temperature measurement in glass and other transparent materials

The relationship between the optical properties of glass and temperature measurements in it by radiation pyrometry are described. Equations for the calculation of emissivity are presented and the transmittance, surface reflection and absorption characteristics of glass are defined. Recommendations are given regarding the selection of pyrometer wavelength sensitivity and the use of a blackbody radiator.

Doremus, Robert H.↗

NASA NTRS1987

Comparison of the structure of harmonic aqueous glasses and liquid water

Glassy structures of water were generated by rapidly quenching configurations of 64 and 343 molecules of liquid water; and the potential energy was then expanded through quadratic order around local minima generated in this way, and the properties of the resulting harmonic system were calculated. The results obtained were used to test the degree to which the structure of liquid water is similar to that of a harmonic aqueous glass. The radial distribution functions for the glass were found to be remarkably similar to those of the liquid. Translational modes were found to be almost entirely responsible for the broadening of the oxygen-oxygen radial distribution function of the quenched configuration, and also primarily responsible for the broadening of other radial distribution functions.

Pohorille, Andrew↗

NASA NTRS1988

Apparatus for rapid thermal analysis studies of reluctant glass formers

An apparatus is described for performing rapid thermal analysis of milligram quantities of reluctant glass formers. Nucleation and crystallization kinetic analysis on samples cooled at very rapid cooling rates of 10 to 2500 C/s is described. The apparatus utilizes focused heating of a sample contained on a fine thermocouple. The measured times and temperatures for nucleation of crystallization at various cooling rates and experimental conditions permit the construction of continuous cooling crystallization boundaries and the determination of critical cooling rates for glass formation.

Ethridge, Edwin C.↗

NASA NTRS1987

PbO reduction and crucible reactions of 70 wt pct PbO-30 wt pct B2O3 glass

NASA has conducted an investigation of PbO-B2O3 glass with a view to PbO reduction and subsequent crucible reactions as a function of temperature and oxygen partial pressure, in order to establish the optimum processing conditions for subcentimetric glass hollow spheres applicable to inertial confinement fusion targets. The results obtained support the selection of appropriate crucible materials and oxygen partial pressure-temperature combinations that avoid phase separation from PbO reduction and/or crucible reactions.

Schilling, Christopher H.↗

NASA NTRS1988

The oxidative stability of carbon fibre reinforced glass-matrix composites

The environmental stability of carbon fibre reinforced glass-matrix composites is assessed. Loss of composite strength due to oxidative exposure at elevated temperatures under no load, static load and cyclic fatigue as well as due to thermal cycling are all examined. It is determined that strength loss is gradual and predictable based on the oxidation of carbon fibres. The glass matrix was not found to prevent this degradation but simply to limit it to a gradual process progressing from the composite surfaces inward.

Prewo, K. M.↗

NASA NTRS1989

Glass Coats For Hot Isostatic Pressing

Surface voids sealed from pressurizing gas. Coating technique enables healing of surface defects by hot isostatic pressing (HIP). Internal pores readily closed by HIP, but surface voids like cracks and pores in contact with pressurizing gas not healed. Applied to casting or weldment as thick slurry of two glass powders: one melts at temperature slightly lower than used for HIP, and another melts at higher temperature. For example, powder is glass of 75 percent SiO2 and 25 percent Na2O, while other powder SiO2. Liquid component of slurry fugitive organic binder; for example, mixture of cellulose acetate and acetone. Easy to apply, separates voids from surrounding gas, would not react with metal part under treatment, and easy to remove after pressing.

Ecer, Gunes M.↗

NASA NTRS1988

Noble gases released by vacuum crushing of EETA 79001 glass

An EETA 79001 glass sample was crushed in a vacuum to observe the gases released. About 15 pct of the total gas concentrations were a mixture of a small amount of SPB-type gas with larger proportions of another air-like component. Less than 5 pct of the SPB gas was released by crushing, while 36-40 pct of the EETV (indigenous) gas was crush-released. The results are consistent with a siting of the EETV component in 10-100 micron vesicles seen in the glass. It is suggested that the SPB component is either in vesicles less than 6 microns in diameter or is primarily sited elsewhere.

Wiens, R. C.↗