Cu-Ni diffusion - Electron microprobe study of effects of voids and discontinuities
Cu-Ni diffusion couples voids and edges effect on concentration gradients measurement by electron probe microanalysis
Lewis, B. W.↗
SEARCH · Engineering Papers
Results for “effective diffusivity”
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The Effects of Flame Structure on Extinction of CH4-O2-N2 Diffusion Flames
The effects of flame structure on the extinction limits of CH4-O2-N2 counterflow diffusion flames were investigated experimentally and numerically by varying the stoichiometric mixture fraction Z(sub st), Z(sub st) was varied by varying free-stream concentrations, while the adiabatic flame temperature T(sub ad) was held fixed by maintaining a fixed amount of nitrogen at the flame. Z(sub st) was varied between 0.055 (methane-air flame) and 0.78 (diluted- methane-oxygen flame). The experimental results yielded an extinction strain rate K(sub ext) of 375/s for the methane-air flame, increasing monotonically to 1042/s for the diluted-methane-oxygen flame. Numerical results with a 58-step Cl mechanism yielded 494/s and 1488/s, respectively. The increase in K(sub ext) with Z(sub st) for a fixed T(sub ad) is explained by the shift in the O2 profile toward the region of maximum temperature and the subsequent increase in rates for chain-branching reactions. The flame temperature at extinction reached a minimum at Z(sub st) = 0.65, where it was 200 C lower than that of the methane-air flame. This significant increase in resistance to extinction is seen to correspond to the condition in which the OH and O production zones are centered on the location of maximum temperature.
Du, J.↗
Double-diffusive convection and its effect under reduced gravity
This paper reviews recent results on three aspects of double-diffusive convection under reduced gravity: diffusive instability caused by sideways heating, diffusive instability caused by bottom heating, and the salt-finger convection case. Some of the processes that are likely to occur during solidification are highlighted. Using property values of lead-tin alloy, it is concluded that diffusive instabilities are unlikely to occur at steady reduced gravity levels of 0.01 and 0.0001 g(0). Salt finger convection is likely to be generated, however.
Chen, C. F.↗
Magnetic-field generation and its effect on ablative Rayleigh–Taylor instability in diffusive ablation fronts
The effects of self-generated magnetic fields on the ablative Rayleigh–Taylor (RT) instability are investigated in the linear regime. The main governing parameters are the Froude number (Fr), which stands for the ratio between ablative convection and acceleration of the target, and the Mach number at the ablation front (Ma), assumed to be small (isobaricity). During the development of the RT instability, magnetic fields are generated due to misalignment between pressure and density gradients (Biermann-battery effect). They accumulate at the section of the ablation front where the Nernst and the plasma velocities cancel each other. The magnetic field modifies the dynamics of the instability through the Righi–Leduc term, which acts as a heat source in the energy equation. It is found that the B fields affect perturbations with short wavelengths up to the most unstable wave in the spectrum. Here, the B field plays a destabilizing role for moderate Froude numbers and becomes stabilizing for large Froude numbers. For plastic ablators, the Fr threshold is found to be Fr = 5.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Effect of Carbon on Void Nucleation in Iron
The study reports the significance of carbon presence in affecting void nucleation in Fe. Without carbon, void nucleation rates decrease gradually at high temperatures but remain significantly high and almost saturated at low temperatures. With carbon present, even at 1 atomic parts per million, void nucleation rates show a low-temperature cutoff. With higher carbon levels, the nucleation temperature window becomes narrower, the maximum nucleation rate becomes lower, and the temperature of maximum void nucleation shifts to a higher temperature. Fundamentally, this is caused by the change in effective vacancy diffusivity due to the formation of carbon-vacancy complexes. The high sensitivity of void nucleation to carbon comes from the high sensitivity of void nucleation to the vacancy arrival rate in a void. The void nucleation is calculated by first obtaining the effective vacancy diffusivity considering the carbon effect, then calculating the defect concentration and defect flux change considering both carbon effects and pre-existing dislocations, and finally calculating the void nucleation rate based on the recently corrected homogeneous void nucleation theory. The study is important not only in the fundamental understanding of impurity effects in ion/neutron irradiation but also in alloy engineering for judiciously introducing impurities to increase swelling resistance, as well as in the development of simulation and modeling methodologies applicable to other metals.
36 MATERIALS SCIENCE↗
Determination of diffusion coefficients from viscosity measurements - Effect of higher Chapman-Enskog approximations.
Diffusion coefficients determination from viscosity measurements based on higher Chapman- Enskog approximations
VISCOSITY↗
Highly Effective GeNi Alloy Contact Diffusion Barrier for BiSbTe Long-Term Thermal Exposure
A GeNi alloy diffusion barrier for contacts on bismuth antimony telluride is proposed. Multiple gold contact diffusion barriers were tested at different thermal aging conditions in air and reducing atmospheres. Among all diffusion barriers, the GeNi alloy barrier shows the best performance for bulk samples with no substantial degradation of the contact resistance, no contact color change, and no change of thermoelectric properties. We observed D Au–GeNi = (9.8 ± 2.7) × 10 –20 m 2 /s within the GeNi alloy barrier, which is 4 times smaller than D Au–BiSbTe . The presence of the initial Ge layer also proves to be effective in reducing nickel diffusion yielding D Ni–BiSbTe = (8.57 ± 0.49) × 10 –19 m 2 /s. During GeNi alloy formation, Ge diffusion into BiSbTe produces GeTe, which apparently blocks the van der Waals gaps eliminating Au and Ni fast diffusion pathways. Thermal aging of BiSbTe nanowires shows that Au and Ni diffusion degrades the thermoelectric power factor, whereas the GeNi alloy barrier sample is mostly preserved. The GeNi alloy barrier is a reliable solution to long-term thermal applications of BiTe-based materials.
36 MATERIALS SCIENCE↗
Effective kinetics driven by dynamic concentration gradients under coupled transport and reaction
Biogeochemical reaction kinetics are generally established from batch reactors where concentrations are uniform. In natural systems, many biogeochemical processes are characterized by spatially and temporally variable concentration gradients that often occur at scales which are not resolved by field measurements or biogeochemical and reactive transport models. Yet, it is not clear how these sub-scale chemical gradients affect reaction kinetics compared to batch kinetics. Here we investigate this question by studying the paradigmatic case of localized pulses of solute reacting with a solid or a dissolved species in excess. Additionally, we consider non-linear biogeochemical reactions, representative of mineral dissolution, adsorption and redox reactions, which we quantify using simplified power-law kinetics. The combined effect of diffusion and reaction leads to effective kinetics that differ quantitatively and qualitatively from the batch kinetics. Depending on the nonlinearity (reaction order) of the local kinetics, these effects lead to either enhancement or decrease of the overall reaction rate, and result in a rich variety of reaction dynamics. We derive analytical results for the effective kinetics, which are validated by comparison to direct numerical simulations for a broad range of Damköhler numbers and reaction order. Our findings provide new insights into the interpretation of imperfectly mixed lab experiments, the effective kinetics of field systems characterized by intermittent reactant release and the integration of sub-scale concentration gradients in reactive transport models.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Development and pilot line production of lithium doped silicon solar cells
The work performed over the period of September 1971 to August 1972 to develop production processes for fabrication of lithium doped P/N cells is described. The BCl3 diffusion without 02 was selected as the optimum diffusion process for fabrication of lithium doped cells. An 8-2-7 (warm up - deposition - drive-in time in minutes) diffusion schedule at 1055 C was used for the first two lots (300 cells each) delivered to JPL. Cell efficiencies ranged from 11.0 to 13.7% based on an AMO of 135.3 mW/sq cm. These high efficiencies were obtained using from 10 to 40 cells per boron diffusion; increasing the quantity beyond 40 resulted in lower outputs. At this point, the emphasis was placed on investigation of a BCl3 with 02 diffusion. Through evaluation of the effects of diffusion time and temperature, gas flow rates, and desposition plus drive-in vs. continuous deposition and no drive-in cycles, diffusion parameters were determined which produced short circuit currents of 136 + or - 4 mA for ten cells spaced along 12 in. of the diffusion boat. The quantity was increased to 60, 100, and 150 cell diffusions with no more variation in cell short circuit current than observed with 10 cells.
Payne, P. A.↗
Effect of surface tension on the onset of convection in a double-diffusive layer
The effect of surface tension on the stability of a double-diffusive layer is considered using linear stability analysis. The surface tension is assumed to vary linearly with temperature and solute concentration. The eigenvalue problem is solved by the Galerkin method. Results show that the predicted stability boundary based on Marangoni effects alone is completely altered in the presence of buoyancy effects induced by low gravity levels (about 10 exp -5 g). At reduced gravity levels, salt-finger instability may onset in the overstable mode due to the stabilizing effect of surface tension. Fluid properties in terms of the Prandtl and the Lewis numbers have a profound effect on the stability conditions; opposite stability characteristics are found in salt solutions and in molten metals.
Chen, C. F.↗
THE EFFECT OF AMBIENT DISSOCIATION AND SPECIES DIFFUSION ON NONEQUILIBRIUM SHOCK-LAYERS
Effects of ambient dissociation and species diffusion for nonequilibrium shock layers
INVISCID FLOW↗
Quantitative effects of abrupt deceleration on pulmonary diffusion in man Technical report,
Quantitative effects of abrupt deceleration on pulmonary diffusion in man
DECELERATION↗
Magnetotail Current Sheet Thinning and Magnetic Reconnection Dynamics in Global Modeling of Substorms
Magnetotail current sheet thinning and magnetic reconnection are key elements of magnetospheric substorms. We utilized the global MHD model BATS-R-US with Adaptive Mesh Refinement developed at the University of Michigan to investigate the formation and dynamic evolution of the magnetotail thin current sheet. The BATSRUS adaptive grid structure allows resolving magnetotail regions with increased current density up to ion kinetic scales. We investigated dynamics of magnetotail current sheet thinning in response to southwards IMF turning. Gradual slow current sheet thinning during the early growth phase become exponentially fast during the last few minutes prior to nightside reconnection onset. The later stage of current sheet thinning is accompanied by earthward flows and rapid suppression of normal magnetic field component $B-z$. Current sheet thinning set the stage for near-earth magnetic reconnection. In collisionless magnetospheric plasma, the primary mechanism controlling the dissipation in the vicinity of the reconnection site is non-gyrotropic effects with spatial scales comparable with the particle Larmor radius. One of the major challenges in global MHD modeling of the magnetotail magnetic reconnection is to reproduce fast reconnection rates typically observed in smallscale kinetic simulations. Bursts of fast reconnection cause fast magnetic field reconfiguration typical for magnetospheric substorms. To incorporate nongyritropic effects in diffusion regions we developed an algorithm to search for magnetotail reconnection sites, specifically where the magnetic field components perpendicular to the local current direction approaches zero and form an X-type configuration. Spatial scales of the diffusion region and magnitude of the reconnection electric field are calculated self-consistently using MHD plasma and field parameters in the vicinity of the reconnection site. The location of the reconnection sites and spatial scales of the diffusion region are updated during the simulations. Such an approach allows quantifying the interaction between large-scale global magnetospheric dynamics and microphysical processes in diffusion regions localized near reconnection sites. To clarify the role of smallscale non-MHD effects in diffusion region on the global magnetospheric dynamic and to test different models of dissipation we perform simulations with steady southward IMF driving of the magnetosphere.
Kuznetsova, M. M.↗
Thermal diffusivity of diamond films
A laser pulse technique to measure the thermal diffusivity of diamond films deposited on a silicon substrate is developed. The effective thermal diffusivity of diamond film on silicon was measured by observing the phase and amplitude of the cyclic thermal waves generated by the laser pulses. An analytical model is developed to calculate the effective in-plane (face-parallel) diffusivity of a two layer system. The model is used to reduce the effective thermal diffusivity of the diamond/silicon sample to a value for the thermal diffusivity and conductivity of the diamond film. Phase and amplitude measurements give similar results. The thermal conductivity of the films is found to be better than that of type 1a natural diamond.
Albin, Sacharia↗
An effective description of charge diffusion and energy transport in a charged plasma from holography
We discuss the physics of sound propagation and charge diffusion in a plasma with non-vanishing charge density. Our analysis culminates the program initiated in to construct an open effective field theory of low-lying modes of the stress tensor and charge current in such plasmas. We model the plasma holographically as a Reissner-Nordström-AdS d+1 black hole, and study linearized fluctuations of longitudinally polarized scalar gravitons and photons in this background. We demonstrate that the perturbations can be decoupled and repackaged into the dynamics of two designer scalars, whose gravitational coupling is modulated by a non-trivial dilatonic factor. The holographic analysis allows us to isolate the phonon mode from the charge diffusion mode, and identify the combination of currents that corresponds to each of them. We use these results to obtain the real-time Gaussian effective action, which includes both the retarded response and the associated stochastic (Hawking) fluctuations, accurate to quartic order in gradients.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
An agent-based approach to study the diffusion rate and the effect of policies on joint placement of photovoltaic panels and green roof under climate change uncertainty
As two of the highest trending green technologies, photovoltaic panels and green roofs are proven to be effective practices for energy generation and energy saving. The achievable impact from the widespread installation of such technologies is, however, not clearly established. This is mainly because the degree of this impact highly depends on the inherently uncertain environmental and climate factors, as well as the unknown adoption rates of these technologies, which in turn depend on different characteristics of decision makers and interactions among them. To that end, this study aims to investigate the diffusion rate of these green technologies under uncertainties caused by climate change, characteristics of adopters, and their interactions. An integrated framework is developed to capture the interplay between financial and attitudinal aspects, as well as the uncertainties due to both the stochastic nature of system parameters and the interactions among agents involving human beings. Specifically, this framework consists of a integer programming model to optimize the green roof and/or photovoltaic panel installation settings for a given building under climate change uncertainty, and an agent-based model to factor in the role of human behavior and interactions. A case study for the city of Knoxville, TN, is presented to evaluate the effects of different policies on the diffusion rate of the green technologies of interest. The results show that the affordability of green technologies and public awareness are the key drivers of the adoption of these technologies, which highlight the important role of the decision makers in impacting the diffusion rate.
14 SOLAR ENERGY↗
Effects of Radiative and Diffusive Transport Processes on Premixed Flames near Flammability Limits
A study of the mechanisms of flammability limits and the dynamics of flame extinguishment in premixed gas flames is described, a novel feature of which is the use of diluent gases having a wide range of radiative and diffusive transport properties. This feature enables an assessment of the importance of volumetric heat losses and Lewis number effects on these mechanisms. Additionally, effects of flame dynamics and flame front curvature are studied by employing spherically expanding flames obtained in a microgravity environment whereby natural convection is eliminated. New diagnostics include chamber pressure measurements and the first reported species concentration measurements in a microgravity combustion experiment. The limit mechanisms and extinguishment phenomena are found to be strongly influenced by the combined effects of radiant heat loss, Lewis number and flame curvature. Two new and as yet not well understood phenomena are reported: 'double flames' in rich H2-O2-CO2 mixtures and an 'inverse flammability region' in rich C3H8-O2-CO2 mixtures.
Abbud-Madrid, Angel↗
NASA HECC Geometry and Performance Review Part 2: Geometric Differences Between the as-Manufactured and Design-Intent Impeller Geometry and Their Effects on the Vaneless Diffuser Configuration Performance
An investigation of the NASA High Efficiency Centrifugal Compressor (HECC) vaneless diffuser configuration was performed. This multipart investigation focused on validating the computational model against experimental data. The validated model was then used to explore the effects of observed geometric differences between the as-manufactured and design-intent impellers. Lastly, the validated model was used for comparison against an experimental tip clearance study. Details regarding each of the parts are detailed below: This second part of a multipart investigation of the NASA HECC vaneless diffuser configuration focused on the comparison of the as-manufactured HECC vaneless computational model validated in Part I against a new model that used the same grid and solver settings but replaced the impeller section data with the design-intent sections from NASA/CR-2014-218114/Rev1. The 1D performance values showed that the as-manufactured impeller underperformed compared to the design-intent for all four speedlines that were simulated. Further investigation of radial profiles, loading and contours showed consistent underperformance of the as-manufactured. An investigation into the source of the difference led to the discovery of several geometric differences in the two impellers of magnitudes that could not be explained outside of manufacturing. The differences that were observed can be summarized into four categories: fillets, TE exit radius differences, splitter LE differences and main blade thickness differences. The investigation then shifted to attempting to isolate the geometric differences as best as possible and to understand their contribution to the observed underperformance. The investigation showed that out of the four geometric differences that were explored, the differences in the splitter blade had the largest effect.
centrifugal compressor↗