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Results for “order-disorder”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Mg/2+/-Fe/2+/ order-disorder in cummingtonite, /Mg, Fe/7Si8O22/OH/2 - A new geothermometer.

Determination of the temperature dependence of the Mg(2+)-Fe(2+) distribution between the M4 site and the M1, M2 and M3 sites in magnesium-rich cummingtonites by Fe-57 Mossbauer resonance spectroscopy. For the basic reversible ion-exchange reaction involved, the Gibbs free energy is estimated to be 3.6 to 4.1 kcal/mole, assuming an ideal solution model at each set of sites. The crystallization temperatures of two cummingtonite-bearing metamorphosed ultrabasic rocks are estimated to be 265 and 290 deg C. The Mossbauer resonance spectra are found to be an effective basis for detecting changes in site occupancies as a function of temperature.

Ghose, S.↗
NASA NTRS1972

Ordering Effects in NbC and TaC

By means of transmission electron microscopy and electron diffraction, evidence has been obtained for the existence of long range carbon atom ordering in single-crystal niobium carbide that has a carbon-to-metal ratio close to the integral composition Nb6C5. The ordering, which gives rise to superlattice and domain structures similar to those observed in V6C5, appears, however, only in samples that have been cooled slowly through the order-disorder temperature of 1025 C. In TaC of similar composition, the ordering, although present, remains very imperfect even after the crystals are subjected to the same thermal treatment. The results are interpreted in terms of the electronic structure of the transition metal carbides as it is currently understood, and their relevance to the mechanical properties of NbC and TaC are discussed.

Venables, J. D.↗
NASA NTRS1976

Adsorption of oxygen on W/100/ - Adsorption kinetics and structure

The adsorption of oxygen on W(100) single-crystal surfaces is studied by Auger electron spectroscopy (AES), flash desorption, low-energy electron diffraction (LEED), and retarding-field work-function measurements. The AES results reveal stepwise changes in the sticking coefficients in the coverage range 0 to 1 and activated adsorption at higher coverages. Upon room-temperature adsorption, a series of complex LEED patterns is observed. In layers adsorbed at 1050 K and cooled to room temperature, the p(2 x 1) structure is the first ordered structure observed. This structure shows a reversible order-disorder transition between 700 and 1000 K and is characterized by a work function which is lower than that of the clean surface. Heating room-temperature adsorbates changes their structure irreversibly. At temperatures below 750 K, some new structures are observed.

Bauer, E.↗
NASA NTRS1976

Scaled equation of state parameters for gases in the critical region

In the light of recent theoretical developments, the paper presents an accurate characterization of anomalous thermodynamic behavior of xenon, helium 4, helium 3, carbon dioxide, steam and oxygen in the critical region. This behavior is associated with long range fluctuations in the system and the physical properties depend primarily on a single variable, namely, the correlation length. A description of the thermodynamic behavior of fluids in terms of scaling laws is formulated, and the two successfully used scaled equations of state (NBS equation and Linear Model parametric equation) are compared. Methods for fitting both equations to experimental equation of state data are developed and formulated, and the optimum fit for each of the two scaled equations of the above gases are presented and the results are compared. By extending the experimental data for the above one-component fluids to partially miscible binary liquids, superfluid liquid helium, ferromagnets and solids exhibiting order-disorder transitions, the principle of universality is concluded. Finally by using this principle, the critical regions for nine additional fluids are described.

Sengers, J. M. H. L.↗
NASA NTRS1980

Phase transitions in the ordered compound TiAl and in alloys having it as a base

The combined effect of alloying and heat treatment on the order-disorder transition in TiAl of stoichiometric composition was investigated. It is concluded from the data obtained that the TiAl 1 to TiAl 2 transition is a phase transition of the second kind. A phase transition was observed in TiAl of equiatomic composition in the 450-550 C temperature range. The introduction of niobium into the ordered compound TiAl of stoichiometric composition, instead of titanium or aluminum atoms, leads to partial softening. In this case, the effect of niobium atoms on softening of the alloys was greater when the niobium was introduced in place of aluminum atoms.

Zelenkov, I. A.↗
NASA NTRS1980

Two-dimensional O2 adsorbed on graphite

Properties of two-dimensional O2 adsorbed on graphite are calculated in the extremely low-coverage delta region and for monolayers, with use of pattern-recognition optimization and Monte Carlo techniques. Equilibrium configurations and orientations, orientational order-disorder, melting, and dissociation transitions are predicted at various O2 densities. Phase characteristics, including a plastic crystallite phase, are compared with experiment.

Etters, R. D.↗
NASA NTRS1981

Thermodynamic properties and phase transitions in CO2 molecular clusters

The thermodynamic properties of (CO2)N molecular aggregates of size N between 2 and 13 have been investigated. These crystallites exhibit well defined orientational order-disorder rotational transitions accompanied by a structural transition into a plastic crystallite phase. In addition, they exhibit melting and disassociation transitions. It is shown that the interpretation of experimental data, based upon dimer properties, depends crucially on these results. Equilibrium structures and orientations are also given.

Etters, R. D.↗
NASA NTRS1982

Chemical and physical studies of type 3 chondrites. I - Metamorphism related studies of Antarctic and other type 3 ordinary chondrites

New thermoluminescence (TL) sensitivity measurements on 17 finds and one fall are presented, and petrologic assignments are made on the basis of TL sensitivity and silicate heterogeneity. The correlation that exists between TL sensitivity and glow curve shape is discussed, with the difference in the glow curve shapes of meteorite types 3.4 and less and type 3.5 and greater being tentatively attributed to an order-disorder transition in plagioclase, the TL phosphor. A correlation between TL sensitivity and heteogeneity of the silicate composition is found, but it tends to break down in the most heterogeneous meteorites. For Antarctic meteorites, the TL data show a much more restricted range in TL sensitivity than in silicate heterogeneity. The Quinyambie meteorite is the only one not consistent with the observed trend. Several explanations are offered, none of them totally satisfactory.

Sears, D. W.↗
NASA NTRS1986

Thermoluminescence and Antarctic meteorites

The level of natural thermoluminescence (TL) in meteorites is the result of competition between build-up, due to exposure to cosmic radiation, and thermal decay. Antarctic meteorites tend to have lower natural TL than non-Antarctic meteorites because of their generally larger terrestrial ages. However, since a few observed falls have low TL due to a recent heating event, such as passage within approximately 0.7 astronomical units of the Sun, this could also be the case for some Antarctic meteorites. Dose rate variations due to shielding, heating during atmospheric passage, and anomalous fading also cause natural TL variations, but the effects are either relatively small, occur infrequently, or can be experimentally circumvented. The TL sensitivity of meteorites reflects the abundance and nature of the feldspar. Thus intense shock, which destroys feldspar, causes the TL sensitivity to decrease by 1 to 2 orders of magnitude, while metamorphism, which generates feldspar through the devitrification of glass, causes TL sensitivity to increase by a factor of approximately 10000. The TL-metamorphism relationship is particularly strong for the lowest levels of metamorphism. The order-disorder transformation in feldspar also affect the TL emission characteristics and thus TL provides a means of paleothermometry.

Sears, D. W. G.↗