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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 1,441 records · Page 80
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

A molecular model for ice nucleation and growth, attachment 2

The central field empirical pair potential model is applied to studying the effects of kinks, ledges, and vacancies on the absorption of water molecules from the vapor. Molecular dynamics simulations indicate that cluster and/or surface modes play a primary role in the absorption process, the flexibility of the hydrogen bond serves to decrease the energy required for structural interconversion, and the rapid distribution of added energy in a hydrogen bonded system lead to aggregate stability which greatly exceeds that predicted by static energy calculations.

Hale, B. N.↗
NASA NTRS1982

Experimental program on nucleation and structure in undercooled melts

Undercooling and structural refinements in droplets of molten metal levitated in an induction field and/or by dispersion in a fluid carrier were studied. Nickel base and lower melting point alloys levitated in molten carrier fluids are considered. The dispersion of molten alloy droplets in a high temperature fluid following the procedures developed by Perepezko and co-workers for lower melting point alloys; obtaining a similar dispersion by room temperature mechanical mixing of particles of the metal and solidified liquid carrier; and solidification of single relatively large droplets in a transparent fluid carrier, enabling high-speed temperature measurement of the recalescence and subsequent cooling behavior are described.

Source record↗
NASA NTRS1982

Contact ice nucleation by submicron atmospheric aerosols

An apparatus designed to measure the concentrations of submicron contact ice nuclei is described. Here, natural forces transfer nuclei to supercooled sample drops suspended in an aerosol stream. Experimental measurements of the scavenging rate of the sample drops for several humidities and aerosol sizes are found to be in agreement with theory to within a factor of two. This fact, together with the statistical tests showing a difference between the data and control samples, is seen as indicating that a reliable measurement of the concentrations of submicron contact ice nuclei has been effected. A figure is included showing the ice nucleus concentrations as a function of temperature and assumed aerosol radius. For a 0.01 micron radius, the average is 1/liter at -15 C and 3/liter at -18 C. It is noted that the measurements are in fair agreement with ice crystal concentrations in stable winter clouds measured over Elk Mountain, WY (Vali et al., 1982).

Deshler, T.↗
NASA NTRS1983

A potential energy scaling Monte Carlo simulation of thin film nucleation and growth

The initial growth of thin Ge fims on the (100) surface of an Fe substrate is investigated theoretically by means of Monte Carlo simulations based on a potential-energy-scaling technique. The substrate is modeled as a 20 x 20-square array with periodic boundary conditions, as described by Heinbockel et al. (1983), and the movement of surface atoms under the influence of the substrate interaction potential and the lateral interaction of neighboring atoms is explored via continuous updating (on the time scale of single events) of the potential energy at each site in the array. Results for the clustering of nine dispersed atoms over 1.0 s at 600 K and for deposition at 5 x 10 to the -14th/sq cm s over 2.0 s at 500 K are presented graphically.

Outlaw, R. A.↗
NASA NTRS1986

Stability of surface nucleation

The growth and decomposition rates of nuclei on a surface are expressed in microscopic terms. A stability factor is defined and employed for the derivation of a criterion for nucleus stability. Simulation results indicate that the stability factor can be used as a measure of the system stability.

Salik, J.↗
NASA NTRS1988

On the theory of the conversion of metallic mixed phases. 5: Fluctuations and nucleation in sub-cooled phases

A comparison is made between fluctuation-theoretical calculations and empirical results. Concentration fluctuations and the resulting separation, and the distribution function of the reversible fluctuations are approximately calculated. The relation of this distribution to the concentration-dependence of the free energy is discussed. Possible effects of fluctuations are discussed, and other attempts to explain the sub-cooling are described.

Borelius, G.↗
NASA NTRS1988

Nucleate pool boiling: High gravity to reduced gravity; liquid metals to cryogens

Requirements for the proper functioning of equipment and personnel in reduced gravity associated with space platforms and future space station modules introduce unique problems in temperature control; power generation; energy dissipation; the storage, transfer, control and conditioning of fluids; and liquid-vapor separation. The phase change of boiling is significant in all of these. Although both pool and flow boiling would be involved, research results to date include only pool boiling because buoyancy effects are maximized for this case. The effective application of forced convection boiling heat transfer in the microgravity of space will require a well grounded and cogent understanding of the mechanisms involved. Experimental results are presented for pool boiling from a single geometrical configuration, a flat surface, covering a wide range of body forces from a/g = 20 to 1 to a/g = 0 to -1 for a cryogenic liquid, and from a/g = 20 to 1 for water and a liquid metal. Similarities in behavior are noted for these three fluids at the higher gravity levels, and may reasonably be expected to continue at reduced gravity levels.

Merte, Herman, Jr.↗
NASA NTRS1988

Grain boundary oxidation and an analysis of the effects of oxidation on fatigue crack nucleation life

The effects of preoxidation on subsequent fatigue life were studied. Surface oxidation and grain boundary oxidation of a nickel-base superalloy (TAZ-8A) were studied at 600 to 1000 C for 10 to 1000 hours in air. Surface oxides were identified and the kinetics of surface oxidation was discussed. Grain boundary oxide penetration and morphology were studied. Pancake type grain boundary oxide penetrates deeper and its size is larger, therefore, it is more detrimental to fatigue life than cone-type grain boundary oxide. Oxide penetration depth, a (sub m), is related to oxidation temperature, T, and exposure time, t, by an empirical relation of the Arrhenius type. Effects of T and t on statistical variation of a (sub m) were analyzed according to the Weibull distribution function. Once the oxide is cracked, it serves as a fatigue crack nucleus. Statistical variation of the remaining fatigue life, after the formation of an oxide crack of a critical length, is related directly to the statistical variation of grain boundary oxide penetration depth.

Oshida, Y.↗
NASA NTRS1989

Protein crystal nucleation kinetics using relative light-scattering techniques: Center director's discretionary fund

Light-scattering intensity measurements are a sensitive method for following changes in the hydrodynamic radius of particles in solution. The approach used in this report utilizes the light-scattering intensity ratios of a polydisperse to a monodisperse system. By numerically modeling the process, and fitting the model curves to the data, estimates have been obtained for the dimerization equilibrium constant, the dimer + dimer yields tetramer equilibrium constant, and the association rate constant for the dimerization process.

Pusey, Marc Lee↗
NASA NTRS1988

Bulk undercooling, nucleation, and macrosegregation of Pb-Sn alloys

Preliminary ground-based studies on the undercooling behavior of large samples (23 g) of lead-tin alloys are presented. Evidence of gravity-related segregation effects is found, and a possible area for future microgravity experimentation is thus identified. Detailed descriptions of the experimental procedure used to achieve bulk undercoolings of between 0.5 and 34 K, depending on composition, are given. The bulk undercoolings obtained in this study are comparable with those found in small droplets. The large size of the present samples enabled the observation of sedimentation and other macrosegregation processes.

Degroh, H. C., III↗
NASA NTRS1989

Neptune's visual albedo variations over a solar cycle - A pre-Voyager look at ion-induced nucleation and cloud formation in Neptune's troposphere

A model of Neptune's troposphere and stratosphere is used to estimate the cosmic-ray-induced ionization on Neptune at sunspot maximum and minimum for a range of possible planetary magnetic field strengths. The impact of this ionization on aerosol formation and the effect of the aerosol layer on the planet's albedo are examined. The results suggest that the 'white haze' theory may explain observations of the variation of Neptune's visible albedo with solar cycle (Lockwood and Thompson, 1986).

Moses, Julianne I.↗
NASA NTRS1990

Aerosol nucleation in the winter Arctic and Antarctic stratospheres

The formation rate of sulfuric-acid-water aerosol particles is calculated as a function of altitude for the conditions of the winter Arctic and Antarctic stratospheres. The theoretical results indicate that sulfate particle formation can occur in the polar winter stratosphere. Conditions for new particle formation are increasingly favorable as the altitude increases between 20 and 30 km because of the decrease in surface area of preexisting particles and increasing sulfuric-acid vapor supply. The theoretical predictions are consistent with observations of a high-altitude CN layer over Antarctica in the spring. Available vapor-pressure data indicate that ternary particles composed of sulfuric acid, nitric acid, and water are not thermodynamically stable under winter stratospheric conditions.

Hamill, Patrick↗
NASA NTRS1990

Nucleation and growth control in protein crystallization

The five topics summarized in this final report are as follows: (1) a technique for the expedient, semi-automated determination of protein solubilities as a function of temperature and application of this technique to proteins other than lysozyme; (2) a small solution cell with adjustable temperature gradients for the growth of proteins at a predetermined location through temperature programming; (3) a microscopy system with image storage and processing capability for high resolution optical studies of temperature controlled protein growth and etching kinetics; (4) growth experiments with lysozyme in thermosyphon flow ; and (5) a mathematical model for the evolution of evaporation/diffusion induced concentration gradients in the hanging drop protein crystallization technique.

Rosenberger, Franz↗
NASA NTRS1991

The influence of Ag+Mg additions on the nucleation of strengthening precipitates in a non-cold-worked Al-Cu-Li alloy

Aluminum-copper-lithium alloys generally require cold work to attain their highest strengths in artificially aged tempers. These alloys are usually strengthened by a combination of the metastable delta prime (Al3Li) and theta prime (Al2Cu) phases and the equilibrium T sub 1 (Al2CuLi) phase, and where the T sub 1 phase is a more potent strengthener than the delta prime. Various investigators have shown that the high strengths obtained after artificial aging associated with cold work result from the heterogeneous precipitation of T sub 1 on matrix dislocations. The objective here is to elucidate the mechanism by which the Ag+Mg additions stimulate the precipitation of T sub 1 type precipitates without cold work. To accomplish this, the microstructure of an Al-6.3Cu-1.3Li-0.14Zr model alloy was evaluated in a T6 type temper with and without the Ag+Mg addition.

Source record↗
NASA NTRS1991

A fundamental study of nucleate pool boiling under microgravity

An experimental study of incipient boiling in short-term microgravity and with a/g = +/- 1 for pool boiling was performed. Calibrated thin gold films sputtered on a smoothly polished quartz surface were used simultaneously for thermal resistance measurements and heating of the boiling surface. The gold films were used for both transient and quasi-steady heating surface temperature measurements. Two test vessels were constructed for precise measurement and control of fluid temperature and pressure: a laboratory pool boiling vessel for the a/g = +/- experiments and a pool boiling vessel designed for the 131 m free-fall in the NASA Lewis Research Center Microgravity Research Facility for the microgravity tests. Measurements included the heater surface temperature, the pressure near the heating surface, and the bulk liquid temperatures. High speed photography was used in the experiments. With high quality microgravity and the measured initial temperature of the quiescent test fluid, R113, the temperature distribution in the liquid at the moment of boiling inception resulting from an imposed step in heat flux is known with a certainty not possible previously. The types of boiling propagation across the large flat heating surface are categorized; the conditions necessary for their occurrence are described. Explosive boiling propagation with a striking pattern of small scale protuberances over the entire vapor mass periphery not observed previously at low heat flux levels is described. For the heater surface with a/g = -1, a step in the heater surface temperature of short duration was imposed. The resulting liquid temperature distribution at the moment of boiling inception was different from that obtained with a step in heat flux.

Ervin, Jamie S.↗
NASA NTRS1992

Mercury iodide nucleation and crystal growth in vapor phase (4-IML-1)

The objectives of this experiment are to grow simultaneously three single crystals of mercuric iodide (HgI2) in an imposed temperature profile and to assess the advantages of growth in microgravity on the HgI2 crystal quality. Growth in microgravity should reduce fluctuations in HgI2 concentrations and thus decrease the resultant crystal defects. In order to test this hypothesis, a seeded growth of HgI2 crystals will be performed on International Microgravity Lab. (IML-1).

Cadoret, Robert↗