The /D/ matrix for multicomponent diffusion
Multicomponent diffusion process calculation using eigenvalues of Onsager diffusion matrix
Cooper, A. R., Jr.↗
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Prediction and evaluation of eddy-viscosity models for free mixing
Analysis for the turbulent mixing of free jets is presented in this paper and compared to recent experimental results. A turbulent mass diffusion model is presented and is based on the concentration potential core. The model yielded good results when compared with the experimental results except for low-speed flows where few experimental data are available. A review of recent experimental results verifies again that the three diffusion processes in turbulent mixing are interrelated; however, no single diffusion model may be used for all three processes. This is especially true when pressure gradients are present in the flow field. It is shown that even though momentum diffusion is significantly affected by pressure gradients, mass diffusion is not. It is further indicated that the mass diffusion model has been derived and is based on the accurate correlations of experimental results obtained for the concentration potential core. Similar techniques may be used in deriving an expression for the momentum and thermal diffusion coefficients. These expressions would be more complicated since they would have to take care of the boundary layer at the start of the mixing region. Finally, a comparison of the analyses, using this particular model and Ferri's model, with available experimental results is made.
Zakkay, V.↗
Physical electronics and surface physics
Current analytical and experimental investigations of the influence of surface processes on the rate of permeation of gases through solids are summarized. A brief description is presented of an analytical model that provides an approximate prediction of the dependence of the permeation rate on the rates of surface processes relative to the rate of the diffusion process. Results of experiments indicate that the desorption process of hydrogen from various materials is influenced by surface impurities to the degree that substantial changes in the spatial distribution of the desorbed molecules are observed when the impurities are varied. Except possibly for H-Nb, the permeation measurements indicate that the desorption process is not influenced to the degree that detectable changes in the permeation rates are observed when the impurities and temperatures are varied. On the basis of analysis, it is unlikely that the desorption process will be the rate-controlling step under the conditions of the present experiments.
Stickney, R. E.↗
Baffle aperture design study of hollow cathode equipped ion thrusters
A simple theoretical model which can be used as an aid in the design of the baffle aperture region of a hollow cathode equipped ion thruster was developed. An analysis of the ion and electron currents in both the main and cathode discharge chambers is presented. From this analysis a model of current flow through the aperture, which is required as an input to the design model, was developed. This model was verified experimentally. The dominant force driving electrons through the aperture was the force due to the electrical potential gradient. The diffusion process was modeled according to the Bolm diffusion theory. A number of simplifications were made to limit the amount of detailed plasma information required as input to the model to facilitate the use of the model in thruster design. This simplified model gave remarkably consistant results with experimental results obtained with a given thruster geometry over substantial changes in operating conditions. The model was uncertain to about a factor of two for different thruster cathode region geometries. The design usefulness was limited by this factor of two uncertainty and by the accuracy to which the plasma parameters required as inputs to the model were specified.
Brophy, J. R., Jr.↗
Coupled low-energy - ring current plasma diffusion in the Jovian magnetosphere
The outwardly diffusing Iogenic plasma and the simultaneously inwardly diffusing ring current plasma in the Jovian magnetosphere are described using a coupled diffusion model which incorporates the effects of the pressure gradient of the ring current into the cross-L diffusion coefficient. The coupled diffusion coefficient is derived by calculating the total energy available to drive the diffusion process. The condition is imposed that the diffusion coefficient takes on a local minimum value at some point in the region L = 7-8, at which point the gradient of the Io plasma density is specified as ramp value given by Siscoe et al. (1981). The hypothesis that the pressure gradient of the ring current causes the diminution of radial plasma transport is tested, and solution profiles for the Iogenic and ring current plasma densities are obtained which imply that the Io plasma ramp is caused by a high-density, low-energy component of the ring current hitherto unobserved directly.
Summers, D.↗
Diffusion mechanisms in Ir-coated Re for high-temperature, radiation-cooled rocket thrusters
Materials used for radiation-cooled rocket thrusters must be capable of surviving under extreme conditions of high temperatures and oxidizing environments. Thruster chambers were developed using chemical-vapor-deposited (CVD) Re coated with CVD Ir on the inside surface which is exposed to hot combustion gases. Ir serves as an oxidation barrier protecting the Re which maintains structural integrity at high temperatures. In order to predict and extend the performance limits of these Ir-coated Re thrusters, the diffusion kinetics of CVD materials at temperature are studied. Thruster end ring sections were examined using electron microprobe analysis both before and after exposure to high temperature vacuum environments. The resulting elemental maps for Re, Ir, and Mo in the near-surface region allow identification of diffusion mechanisms operating at these temperatures. Line scans for Ir and Re were fit using a diffusion model to extract relevant diffusion constants. The fastest diffusion process is seen to be grain boundary diffusion with Re diffusing down grain boundaries in the Ir overlayer. The measured dependence of the diffusion rate on temperature will allow prediction of operating lifetimes for these thrusters.
Hamilton, J. C.↗
Relativistic electron precipitation enhancements near the outer edge of the radiation belt
Characteristics of relativistic electron precipitation bursts observed by the Heavy Ion Large Telescope (HILT) experiment onboard the Solar, Anomalous, and Magnetospheric Partical Explorer (SAMPEX) satellite were examined. Relatively narrow, persitent, latitudinal bands of precipitation with time scales of 10 to approximately 30 sec near the outer edge of the radiation belt which develop and decay with a time scale of a few hours are reported. Acceleration processes more effective than the usual radial diffusion process or scattering process would be needed to explain this strong precipitation band phenomenon. Another prominent signature is microbursts with a time scale down to a few hundred milliseconds. It is suggested that these microbursts are due to wave-particle interaction involving a relaxation-oscillator type of mechanism.
Nakamura, R.↗
Effects of Experiment Location and Orbiter Attitude on the Residual Acceleration On-Board STS-73 (USML-2)
A knowledge of the quasi-steady acceleration environment on the NASA Space Shuttle Orbiter is of particular importance for materials processing experiments which are limited by slow diffusive processes. The quasi-steady (less than 1 HZ) acceleration environment on STS-73 (USML-2) was measured using the Orbital Acceleration Research Experiment (OARE) accelerometer. One of the facilities flown on USML-2 was the Crystal Growth Furnace (CGF), which was used by several Principal Investigators (PIS) to grow crystals. In this paper the OARE data mapped to the sample melt location within this furnace is presented. The ratio of the axial to radial components of the quasi-steady acceleration at the melt site is presented. Effects of Orbiter attitude on the acceleration data is discussed.
Hakimzadeh, Roshanak↗
Direct Visualization of Defect‐Controlled Diffusion in van der Waals Gaps
Abstract Diffusion processes govern fundamental phenomena such as phase transformations, doping, and intercalation in van der Waals (vdW) bonded materials. Here, the diffusion dynamics of W atoms by visualizing the motion of individual atoms at three different vdW interfaces: hexagonal boron nitride (BN)/vacuum, BN/BN, and BN/WSe 2 , by recording scanning transmission electron microscopy movies is quantified. Supported by density functional theory (DFT) calculations, it is inferred that in all cases diffusion is governed by intermittent trapping at electron beam‐generated defect sites. This leads to diffusion properties that depend strongly on the number of defects. These results suggest that diffusion and intercalation processes in vdW materials are highly tunable and sensitive to crystal quality. The demonstration of imaging, with high spatial and temporal resolution, of layers and individual atoms inside vdW heterostructures offers possibilities for direct visualization of diffusion and atomic interactions, as well as for experiments exploring atomic structures, their in situ modification, and electrical property measurements of active devices combined with atomic resolution imaging.
Chemistry↗
Development of an improved membrane for a vapor diffusion water recovery process
Recovery of potable water from urine on manned space missions of extended duration was the objective of work aimed at the improvement of membrane performance for the vapor diffusion process (VDR). Kynar, Teflon, PVC, and polysulfone candidate membranes were evaluated from chemical, thermal, mechanical, and fabricating standpoints to determine their suitability for operation in the VDR pervaporation module. Pervaporation rates and other performance characteristics were determined in a breadboard pervaporator test rig. Kynar and Teflon membranes were demonstrated to be chemically stable at pervaporation temperatures in urine pretreated with chromic acid bactericide. The separation of the pervaporator and condenser modules, the use of a recirculating sweep gas to conduct pervaporate to the condenser, and the selection of a hollow fiber membrane configuration for pervaporator module design is recommended as a result of the investigation.
Rich, T. R.↗
Pioneer 11 observations of trapped particle absorption by Amalthea
The discovery of a microsignatury of trapped radiation in the Amalthea orbit as detected by Pioneer 11 in a flyby of Juptier in 1974 is reported and its implications for the radial diffusion coefficient in Jupiter's inner magnetosphere are discussed. A low energy telescope registered the absorption of low-energy protons as a function of the magnetic L shell durig inbound and outbound trajectories. Drift velocities of the 1 MeV particles were calculated. No correspondingly heightened effects were observed from high-energy electrons or heavier nuclei. Further analysis of the 0.5-8.7 MeV protons showed data to be consistent erosion of the particle drift shadows by a diffusion process. A limit was calculated for the highest diffusion coefficient value for the 1 MeV protons at the Amalthea orbit. The results indicate that the diffusion is driven by fluctuating electric or magnetic fields.
Mckibben, R. B.↗
Radiation-reduced diffusion of Nd in bcc Fe
Here, this study combines ab initio calculations and kinetic Monte Carlo simulations to investigate the bulk diffusion of Nd, an exemplar lanthanide element, in bcc Fe. Contrary to the conventional belief that irradiation accelerates diffusion processes, our study reveals that irradiation can unexpectedly reduce the diffusion of Nd due to the strong binding between substitutional Nd atoms and vacancies in bcc Fe. Combined with the high concentration of nonequilibrium vacancies, this promotes the abundant formation of Nd+3Va defect complexes containing a single Nd substitutional atom and three vacancies, accompanied by the depletion of Nd-vacancy pairs that mediate the diffusion of Nd.
Density functional theory (DFT)↗
Liapunov criteria for weak stochastic stability.
Sufficient conditions for recurrence and positivity of diffusion process defined by stochastic differential equation, using Liapunov function
DIFFERENTIAL EQUATION↗
Composition waves in the thermosphere
Neutral-composition waves excited by auroral heat sources are investigated. For horizontal wavelengths of the order of 1000 km, it is concluded that diffusion processes (1) play a significant role such that deviations from diffusive equilibrium prevail for He throughout the thermosphere; (2) produce phase differences of about 220 deg (or -140 deg) between He and N2, and (3) account for He amplitudes comparable to those of N2. These results basically explain recent AE-C satellite measurements which have revealed an anticorrelation between the heavier and lighter constituents of the thermosphere. The calculations also indicate that temperature and N2 concentration are generally out of phase by about 100 deg.
Mayr, H. G.↗
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.↗
Design model for the baffle aperture region of a hollow cathode thruster
A current balance on the main discharge chamber of an electron bombardment thruster is carried out. It is shown that the discharge current is the sum of the net cathode emission current, the beam current, and the ion current to cathode potential surfaces. Measurements of ion currents outside the virtual anode surface in a SERT II divergent magnetic field thruster show that relatively few ions cross this surface. This implies that ions are directed preferentially toward the anode and cathode pole pieces. A simple theoretical model useful as an aid in the design of the baffle aperture region of a hollow cathode equipped thruster is developed. The electron diffusion process through the aperture is modeled in accordance with the Bohm diffusion theory. The model is shown to yield consistent results for a given thruster geometry over substantial changes in operating conditions. The model's design usefulness is limited by a factor of two uncertainty observed over different thruster/cathode region geometries and by the accuracy to which the plasma parameters required as inputs to the model can be specified.
Brophy, J. R.↗
Diffusion of Siderophile Elements in Fe Metal: Application to Zoned Metal Grains in Chondrites
The distribution of highly siderophile elements (HSE) in planetary materials is controlled mainly by metal. Diffusion processes can control the distribution or re-distribution of these elements within metals, yet there is little systematic or appropriate diffusion data that can be used to interpret HSE concentrations in such metals. Because our understanding of isotope chronometry, redox processes, kamacite/taenite-based cooling rates, and metal grain zoning would be enhanced with diffusion data, we have measured diffusion coefficients for Ni, Co, Ga, Ge, Ru, Pd, Ir and Au in Fe metal from 1200 to 1400 C and 1 bar and 10 kbar. These new data on refractory and volatile siderophile elements are used to evaluate the role of diffusional processes in controlling zoning patterns in metal-rich chondrites.
Righter, K.↗
Development and pilot line production of lithium doped silicon solar cells
Scaling up the BCl3 without O2 diffusion beyond 30 to 40 cells was investigated by using a 100 cell capacity diffusion boat which held the cells vertically. Sheet resistances and I-V curves were uniform with 10 to 20 cells spaced along the entire boat, so the quantity was increased to 40 and then 60 cells per diffusion. There was no change in cell output and uniformity going from 20 to 40 cells per diffusion; however only half the lithium cells fabricated from slices diffused in the 60 cell diffusion had efficiencies of 11% or better. Although uniform sheet resistances and I-V characteristic curves were obtained with up to 60 cells in the BCl3 with O2 diffusion, the short circuit currents were approximately 15% lower than the anticipated 135 to 140 mA. Consequently, work on this diffusion process has been aimed solely at increasing the short circuit current. The diffusion temperature was lowered from 1055 to 1000 and 950 C, and at each of these temperatures variations in diffusion time were investigated. At 1000 C short circuit currents were approximately 10 mA higher, 130 rather than 120 mA average.
Payne, P. A.↗