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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS2001

A Hydrodynamic Theory for Spatially Inhomogeneous Semiconductor Lasers: Microscopic Approach

Starting from the microscopic semiconductor Bloch equations (SBEs) including the Boltzmann transport terms in the distribution function equations for electrons and holes, we derived a closed set of diffusion equations for carrier densities and temperatures with self-consistent coupling to Maxwell's equation and to an effective optical polarization equation. The coherent many-body effects are included within the screened Hartree-Fock approximation, while scatterings are treated within the second Born approximation including both the in- and out-scatterings. Microscopic expressions for electron-hole (e-h) and carrier-LO (c-LO) phonon scatterings are directly used to derive the momentum and energy relaxation rates. These rates expressed as functions of temperatures and densities lead to microscopic expressions for self- and mutual-diffusion coefficients in the coupled density-temperature diffusion equations. Approximations for reducing the general two-component description of the electron-hole plasma (EHP) to a single-component one are discussed. In particular, we show that a special single-component reduction is possible when e-h scattering dominates over c-LO phonon scattering. The ambipolar diffusion approximation is also discussed and we show that the ambipolar diffusion coefficients are independent of e-h scattering, even though the diffusion coefficients of individual components depend sensitively on the e-h scattering rates. Our discussions lead to new perspectives into the roles played in the single-component reduction by the electron-hole correlation in momentum space induced by scatterings and the electron-hole correlation in real space via internal static electrical field. Finally, the theory is completed by coupling the diffusion equations to the lattice temperature equation and to the effective optical polarization which in turn couples to the laser field.

Li, Jianzhong↗
NASA NTRS2002

Microscopic Foundation and Simulation of Coupled Carrier-Temperature Diffusions in Semiconductor Lasers

A typical semiconductor-based optoelectronic device, such as a diode laser, consists of three subsystems: an optical field, an electron-hole plasma (EHP), and a host crystal lattice. The physics of such a device involves the interplay of optical, electrical and thermal processes. A proper description of such a device requires that all three processes are treated on equal footing and in a self-consistent fashion. Furthermore, since a semiconductor laser has intrinsic spatial inhomogeneity, such a self-consistency naturally leads to a set of partial differential equations in space and time. There is a significant lacking of research interest and results on the transport aspects of optical devices in the literature with only a few exceptions. Even the most important carrier diffusion coefficient has not been properly derived and studied so far for optically excited plasma, while most of the work adopted results from electronics community where heavily doped semiconductors with mainly one type of carriers are dealt with. The corresponding transport equation for plasma energy or temperature has received even less attention. In this talk we describe our recent results on such a self-consistent derivation of temperature and carrier-density diffusion equations coupled with the lasing process. Starting from the microscopic semiconductor Bloch equations (SBEs) including the Boltzmann transport terms in the distribution function equations for electrons and holes, we derived a closed set of diffusion equations for carrier densities and temperatures with self-consistent coupling to Maxwell's equation and to an effective optical polarization equation. The coherent many-body effects are included within the screened Hartree-Fock approximation, while scatterings are treated within the second Born approximation including both the in- and out-scatterings. Microscopic expressions for electron-hole (e-h) and carrier-LO (c-LO) phonon scatterings are directly used to derive the momentum and energy relaxation rates. These rates expressed as functions of temperatures and densities lead to microscopic expressions for self- and mutual-diffusion coefficients in the coupled density-temperature diffusion equations. Approximations for reducing the general two-component description of the electron-hole plasma (EHP) to a single-component one are discussed. In particular, we show that a special single-component reduction is possible when e-h scattering dominates over c-LO phonon scattering. The ambipolar diffusion approximation is also discussed and we show that the ambipolar diffusion coefficients are independent of e-h scattering, even though the diffusion coefficients of individual components depend sensitively on the e-h scattering rates. Our discussions lead to new perspectives into the roles played in the single-component reduction by the electron-hole correlation in momentum space induced by scatterings and the electron-hole correlation in real space via internal static electrical field. Finally, the theory is completed by coupling the diffusion equations to the lattice temperature equation and to the effective optical polarization which in turn couples to the laser field. The equations derived above are implemented in various limiting cases to a typical diode laser to study the consequences of nonlinear diffusion and the cross diffusion terms on laser behavior, especially the dynamic behavior of a diode laser under modulation. Detailed results will be presented by comparing with the standard rate equation results.

Li, J.↗
NASA NTRS1990

InAs/Ga(1-x)In(x)Sb superlattices for infrared detector applications

The successful growth of InAs/Ga(1-x)In(x)Sb superlattices and their optical and structural characterization is discussed. Samples were grown by molecular beam epitaxy at fairly low substrate temperatures (less than 400 C). Structural quality was assessed by reflection high energy electron difrraction, transmission electron microscopy, and x ray diffraction. Excellent structures were achieved for growth on thick, strain relaxed GaSb buffer layers on GaAs substrates, despite a residual threading dislocation density of 10(exp 9)cm(exp -2) originating at the GaSb/GaAs interface. Despite a lattice mismatch of 1.7 percent, InAs/Ga(0.75)In(0.25)Sb superlattices are observed to be free of misfit dislocations at the thicknesses examined here, owing to the close lattice match between the superlattice and GaSb, which evenly distributes compressive and tensile stresses between the InAs and Ga(0.75)In(0.25)Sb layers. Photoluminescence and photoconductivity measurements indicate that the energy gaps of the strain-layer superlattices are smaller than those of InAs/GaSb superlattices with the same layer thicknesses, and are in agreement with the theoretical predictions of Smith and Mailhiot. Energy gaps of 80 to 250 meV (15 to 5 microns) have been measured for InAs/Ga(0.75)In(0.25)Sb superlattices with 45 to 25 A/25 A layer thickness. Results demonstrate that far-infrared cutoff wavelengths are compatible with the thin superlattice layers required for strong optical absorption in type-II superlattices.

Miles, Richard H.↗
NASA NTRS1991

A new technique to study transient conductivity under pulsed monochromatic light in Cr-doped GaAs using acoustoelectric voltage measurement

The transient conductivity of high-resistivity Bridgman-grown Cr-doped GaAs under pulsed monochromatic light is monitored using transverse acoustoelectric voltage (TAV) at 83 K. Keeping the photon flux constant, the height and transient time constant at the TAV are used to calculate the energy dependence of the trap density and its cross section, respectively. Two prominent trap profiles with peak trap densities of approximately 10 to the 17th/cu cm eV near the valence and the conduction bands are detected. These traps have very small capture cross sections in the range of 10 to the -23 to 10 to the -21st cm sq. A phenomenon similar to the persistent photoconductivity with transient time constants in excess of a few seconds in high-resistivity GaAs at T = 83 K is also detected using this technique. These long relaxation times are readily explained by the spatial separation of the photo-excited electron-hole pairs and the small capture cross section and large density of trap distribution near the conduction band.

Tabib-Azar, Massood↗
NASA NTRS2012

Multiscale Modeling of Grain Boundaries in ZrB2: Structure, Energetics, and Thermal Resistance

A combination of ab initio, atomistic and finite element methods (FEM) were used to investigate the structures, energetics and lattice thermal conductance of grain boundaries for the ultra high temperature ceramic ZrB2. Atomic models of idealized boundaries were relaxed using density functional theory. Information about bonding across the interfaces was determined from the electron localization function. The Kapitza conductance of larger scale versions of the boundary models were computed using non-equilibrium molecular dynamics. The interfacial thermal parameters together with single crystal thermal conductivities were used as parameters in microstructural computations. FEM meshes were constructed on top of microstructural images. From these computations, the effective thermal conductivity of the polycrystalline structure was determined.

Lawson, John W.↗
NASA NTRS1972

Interrelated structures of the transport shock and collisional relaxation layer in a multitemperature, multilevel ionized gas

The gas dynamic structures of the transport shock and the downstream collisional relaxation layer are evaluated for partially ionized monatomic gases. Elastic and inelastic collisional nonequilibrium effects are taken into consideration. Three electronic levels are accounted for in the microscopic model of the atom. Nonequilibrium processes with respect to population of levels and species plus temperature are considered. By using an asymptotic technique the shock morphology is found on a continuum flow basis. The asymptotic procedure gives two distinct layers in which the nonequilibrium effects to be considered are different. A transport shock appears as the inner solution to an outer collisional relaxation layer in which the gas reaches local equilibrium. A family of numerical examples is displayed for different flow regimes. Argon and helium models are used in these examples.

Vinolo, A. R.↗
NASA NTRS1973

Interrelated structures of the transport shock and collisional relaxation layer in a multitemperature, multilevel ionized gas

The gas dynamic structures of the transport shock and the downstream collisional relaxation layer are evaluated for partially ionized monatomic gases. Elastic and inelastic collisional nonequilibrium effects are taken into consideration. In the microscopic model of the atom, three electronic levels are accounted for. By using an asymptotic technique, the shock morphology is found on a continuum flow basis. This procedure gives two distinct layers in which the nonequilibrium effects to be considered are different. A transport shock appears as the inner solution to an outer collisional relaxation layer. The results show four main interesting points: (1) on structuring the transport shock, ionization and excitation rates must be included in the formulation, since the flow is not frozen with respect to the population of the different electronic levels; (2) an electron temperature precursor appears at the beginning of the transport shock; (3) the collisional layer is rationally reduced to quadrature for special initial conditions, which (4) are obtained from new Rankine-Hugoniot relations for the inner shock.

Vinolo, A. R.↗
NASA NTRS1995

Electronic properties of Y-Ba-Cu-O superconductors as seen by Cu and O NMR/NQR

Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) allow the investigation of electronic properties at the atomic level. We will report on such studies of typical members of the the Y-Ba-Cu-O family such as YBa2Cu30(6 + x) (1-2-3-(6 + x)), YBa2Cu4O8 (1-2-4) and Y2Ba4Cu7015 (2-4-7) with many examples of recent work performed in our laboratory. In particular, we will deal with Knight shift and relaxation studies of copper and oxygen. We will discuss important issues of current studies such as: (1) Existence of a common electronic spin-susceptibility in the planes (and perhaps in the chains) of 1-2-4; (2) Strong evidence for the existence of a pseudo spin-gap of the antiferromagnetic fluctuations in 1-2-4 and 2-4-7; (3) Evidence for d-wave pairing in 1-2-4; (4) Strong coupling of inequivalent Cu-O planes in 2-4-7 and possible origin for the high Tc value of this compound; and (5) The possibility to describe NMR data in the framework of a charge-excitation picture.

Brinkmann, D.↗
NASA NTRS1999

Inspection of Space Station Cold Plate Using Visual and Automated Holographic Techniques

Real-time holography has been used to confirm the presence of non-uniformity in the construction of an International Space Station cold plate. Ultrasonic C-scans have previously shown suspected areas of cooling fin disbonds. But both neural-net processed and visual holography did not evidence any progressive permanent changes resulting from 3000 pressurization and relaxation cycles of a Dash 8 cold plate. Neural-net and visual inspections were performed of characteristic patterns generated from electronic time-average holograms of the vibrating cold plate. Normal modes of vibration were excited at very low amplitudes for this purpose, The neural nets were trained to flag very small changes in the mode shapes as encoded in the characteristic patterns. Both the whole cold plate and a zoomed region were inspected. The inspections were conducted before, after, and during pressurization and relaxation cycles of the cold plate. A water-filled cold plate was pressurized to 120 psig (827 kPa) and relaxed for each cycle. Each cycle required 5 seconds. Both the artificial neural networks and the inspectors were unable to detect changes in the mode shapes of the relaxed cold plate. The cold plate was also inspected visually using real-time holography and double-exposure holography. Regions of non-uniformity correlating with the C-scans were apparent, but the interference patterns did not change after 3000 pressurization and relaxation cycles. These tests constituted the first practical application of a neural-net inspection technique developed originally with support from the Director's Discretionary Fund at the Glenn Research Center at Lewis Field.

Decker, Arthur J.↗
NASA NTRS2003

Study of Dislocation-Ordered In(x)Ga(1-x)As/GaAs Quantum Dots

A report describes an experimental study of dislocation-induced spatial ordering of quantum dots (QDs) comprising nanometer-sized In(x)Ga(1-x)As islands surrounded by GaAs. Metastable heteroepitaxial structures were grown by molecular-beam epitaxy of In(x)Ga(1-x)As onto n+ GaAs and semi-insulating GaAs substrates. Then the structures were relaxed during a post-growth annealing/self-organizing process leading to the formation of surface undulations that acted as preferential sites for the nucleation of QDs. Structural effects of annealing times and temperatures on the strain-relaxed In(x)Ga(1-x)As/GaAs and the subsequent spatial ordering of the QDs were analyzed by atomic-force microscopy and transmission electron microscopy. Continuous-wave spectral and time-resolved photoluminescence (PL) measurements were performed to study the effects, upon optical properties, of increased QD positional ordering, increased QD uniformity, and proximity of QDs to arrays of dislocations. PL spectral peaks of ordered QD structures formed on strain-relaxed In(x)Ga(1-x)As/GaAs layers were found to be narrower than those of structures not so formed and ordered. Rise and decay times of time-resolved PL were found to be lower at lower temperatures -- apparently as a consequence of decreased carrier-transport times within the barriers surrounding the QDs.

Leon, Rose↗
NASA NTRS1995

Development work for a superconducting linear collider

For future linear e(+)e(-) colliders in the TeV range several alternatives are under discussion. The TESLA approach is based on the advantages of superconductivity. High Q values of the accelerator structures give high efficiency for converting RF power into beam power. A low resonance frequency for the RF structures can be chosen to obtain a large number of electrons (positrons) per bunch. For a given luminosity the beam dimensions can be chosen conservatively which leads to relaxed beam emittance and tolerances at the final focus. Each individual superconducting accelerator component (resonator cavity) of this linear collider has to deliver an energy gain of 25 MeV/m to the beam. Today s.c. resonators are in use at CEBAF/USA, at DESY/Germany, Darmstadt/Germany KEK/Japan and CERN/Geneva. They show acceleration gradients between 5 MV/m and 10 MV/m. Encouraging experiments at CEA Saclay and Cornell University showed acceleration gradients of 20 MV/m and 25 MV/m in single and multicell structures. In an activity centered at DESY in Hamburg/Germany the TESLA collaboration is constructing a 500 MeV superconducting accelerator test facility (TTF) to demonstrate that a linear collider based on this technique can be built in a cost effective manner and that the necessary acceleration gradients of more than 15 MeV/m can be reached reproducibly. The test facility built at DESY covers an area of 3.000 m2 and is divided into 3 major activity areas: (1) The testlinac, where the performance ofthe modular components with an electron beam passing the 40 m long acceleration section can be demonstrated. (2) The test area, where all individual resonators are tested before installation into a module. (3) The preparation and assembly area, where assembly of cavities and modules take place. We report here on the design work to reach a reduction of costs compared to actual existing superconducting accelerator structures and on the facility set up to reach high acceleration gradients in a reproducible way.

Matheisen, Axel↗
NASA NTRS2017

Crack Opening Displacement Behavior in Ceramic Matrix Composites

Ceramic Matrix Composites (CMC) modeling and life prediction strongly depend on oxidation, and therefore require a thorough understanding of when matrix cracks occur, the extent of cracking for given conditions (time-temperature-environment-stress), and the interactions of matrix cracks with fibers and interfaces. In this work, the evolution of matrix cracks in a melt-infiltrated Silicon Carbide/Silicon Carbide (SiC/SiC) CMC under uniaxial tension was examined using scanning electron microscopy (SEM) combined with digital image correlation (DIC) and manual crack opening displacement (COD) measurements. Strain relaxation due to matrix cracking, the relationship between COD's and applied stress, and damage evolution at stresses below the proportional limit were assessed. Direct experimental observation of strain relaxation adjacent to regions of matrix cracking is presented and discussed. Additionally, crack openings were found to increase linearly with increasing applied stress, and no crack was found to pass fully through the gage cross-section. This observation is discussed in the context of the assumption of through-cracks for all loading conditions and fiber architectures in oxidation modeling. Finally, the combination of SEM with DIC is demonstrated throughout to be a powerful means for damage identification and quantification in CMC's at stresses well below the proportional limit.

acoustic emission↗
NASA NTRS1989

Spin-lattice relaxation and the calculation of gain, pump power, and noise temperature in ruby

The use of a quantitative analysis of the dominant source of relaxation in ruby spin systems to make predictions of key maser amplifier parameters is described. The spin-lattice Hamiltonian which describes the interaction of the electron spins with the thermal vibrations of the surrounding lattice is obtained from the literature. Taking into account the vibrational anisotropy of ruby, Fermi's rule is used to calculate the spin transition rates between the maser energy levels. The spin population rate equations are solved for the spin transition relaxation times, and a comparison with previous calculations is made. Predictions of ruby gain, inversion ratio, and noise temperature as a function of physical temperature are made for 8.4-GHz and 32-GHz maser pumping schemes. The theory predicts that ruby oriented at 90 deg will have approximately 50 percent higher gain in dB and slightly lower noise temperature than a 54.7-deg ruby at 32 GHz (assuming pump saturation). A specific calculation relating pump power to inversion ratio is given for a single channel of the 32-GHz reflected wave maser.

Lyons, J. R.↗
NASA NTRS1983

Studies in electron phenomena in MOS structures: The pulsed C-V method

The pulse hysteresis capacitance voltage (C-V) provides a straight forward technique for measuring the change of various charges in MOS structures and a tool for investigating the kinetics of various electron phenomena is developed and described. The method can be used for measuring the energy distribution and kinetics of surface states with the resolution of about 1/5 x 10 to the -9 power cm eV. Some transients in an MOS structure, particularly, the thermal generation of minority charge carriers via surface states and the relaxation of minority charge carriers supplied from the inversion layer outside the MOS structure are theoretically investigated. Analytical expressions which clearly present the physics of those electron phenomena are derived.

Kaplan, G.↗
NASA NTRS1974

Magnetic environment of hydrogen in Fe from muon precession measurements

Polarized positive muon radiation was stopped in an ellipsoidal iron target and its precession was observed in a transverse magnetic field. Results indicate that the conduction electron polarization in the 77 K-Fe Curie point region is less than expected, and that the relaxation time of the muon polarization is dominated by the static inhomogeneity to 900 K, at which point magnetization fluctuations become important.

Heiman, N.↗
NASA NTRS1978

Relativistic radiationless transitions in atoms

The perturbing interaction appropriate for the Dirac-Fock formalism is used to formulate relativistic radiationless transitions. The transition rate between two jj-coupled many-electron configurations is expressed as a linear combination of radial integrals, suitable for numerical computation. The correlation and relaxation effects and the generalisation of the present work are discussed briefly.

Huang, K.-N.↗
NASA NTRS1986

Nonradiative transition dynamics in alexandrite

The first direct picosecond time-resolved measurement of the nonradiative transition dynamics between the excited 4T2 pump band and the metastable 2E storage level of the trivalent chromium ion in alexandrite is reported. The nonradiative relaxation times of 17 ps for intra-4T2 vibrational transitions, and 27 ps for 4T2-2E electronic transition are obtained. The thermal repopulation rate of the 4T2 state from the metastable 2E level is of the order 3.5 x 10 to the 9th per s.

Gayen, S. K.↗