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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 1,027 records · Page 57
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Effect of Liquid Droplets on Turbulence Structure in a Round Gaseous Jet

A second-order model which predicts the modulation of turbulence in jets laden with uniform size solid particles or liquid droplets is discussed. The approach followed is to start from the separate momentum and continuity equations of each phase and derive two new conservation equations. The first is for the carrier fluid's kinetic energy of turbulence and the second for the dissipation rate of that energy. Closure of the set of transport equations is achieved by modeling the turbulence correlations up to a third order. The coefficients (or constants) appearing in the modeled equations are then evaluated by comparing the predictions with LDA-measurements obtained recently in a turbulent jet laden with 200 microns solid particles. This set of constants is then used to predict the same jet flow but laden with 50 microns solid particles. The agreement with the measurement in this case is very good.

Elghobashi, S. E.↗
DOE OSTI2015

Material requirements for the adoption of unconventional silicon crystal and wafer growth techniques for high-efficiency solar cells

Silicon wafers comprise approximately 40% of crystalline silicon module cost and represent an area of great technological innovation potential. Paradoxically, unconventional wafer-growth techniques have thus far failed to displace multicrystalline and Czochralski silicon, despite four decades of innovation. One of the shortcomings of most unconventional materials has been a persistent carrier lifetime deficit in comparison to established wafer technologies, which limits the device efficiency potential. In this perspective article, we review a defect-management framework that has proven successful in enabling millisecond lifetimes in kerfless and cast materials. Control of dislocations and slowly diffusing metal point defects during growth, coupled to effective control of fast-diffusing species during cell processing, is critical to enable high cell efficiencies. As a result, to accelerate the pace of novel wafer development, we discuss approaches to rapidly evaluate the device efficiency potential of unconventional wafers from injection-dependent lifetime measurements.

14 SOLAR ENERGY↗
NASA NTRS1991

Interleaving Would Enhance Trellis-Coded Modulation

Interleaving of symbols transmitted via trellis-coded modulation, together with deinterleaving at receiver, reduces "radio" or "noisy-reference" loss encountered in reception. This loss increase in bit-error rate (or, equivalently, decrease in effective signal-to-noise ratio), caused by phase error between received signal and locally generated carrier reference signal used to demodulate received signal.

Divsalar, Dariush↗
NASA NTRS1991

Optical phase-locked loop (OPLL) for free-space laser communications with heterodyne detection

Several advantages of coherent free-space optical communications are outlined. Theoretical analysis is formulated for an OPLL disturbed by shot noise, modulation noise, and frequency noise consisting of a white component, a 1/f component, and a 1/f-squared component. Each of the noise components is characterized by its associated power spectral density. It is shown that the effect of modulation depends only on the ratio of loop bandwidth and data rate, and is negligible for an OPLL with loop bandwidth smaller than one fourth the data rate. Total phase error variance as a function of loop bandwidth is displayed for several values of carrier signal to noise ratio. Optimal loop bandwidth is also calculated as a function of carrier signal to noise ratio. An OPLL experiment is performed, where it is shown that the measured phase error variance closely matches the theoretical predictions.

Win, Moe Z.↗
NASA NTRS2009

Reducing Interference in ATC Voice Communication

Three methods have been proposed to be developed to enable reduction of the types of interference that often occur among voice-communication radio signals involved in air-traffic control (ATC). For historical reasons and for compatibility with some navigation systems, control towers and aircraft use amplitude modulation (AM) for voice communication. In the presence of two simultaneous AM transmissions in the same frequency channel, what is heard through a receiver includes not only the audio portions of both transmissions but also an audio heterodyne signal at the difference between the carrier frequencies of the transmissions (as a practical matter, the carrier frequencies almost always differ somewhat). The situation is further complicated by multiple heterodyne signals in the presence of more than two simultaneous transmissions. Even if one of the transmissions does not include AM because of a transmitter malfunction or because a transmitter was inadvertently turned on or left on, the heterodyne signal makes it difficult to understand the audio of the other transmission. The proposed methods would utilize digital signal processing to counteract this type of interference.

Battle, John O.↗
DOE OSTI2015

Two high-field thermodynamically stable conductivity states in photoconductive CdS, one n-type and one p-type

Photoconductive CdS is known to be n-type and develops high-field domains in the range of negative differential conductivities. These domains have been extensively discussed, and when remaining attached to the electrodes have been renamed Böer domains (a broader definition suggested earlier is misleading) [K. Thiessen, Phys. Status Solidi B 248, 2775 (2011)]. They are occurring at high applied voltage in a range at which the current becomes highly non-ohmic that is conventionally described as N-shaped when the conductance decreases with increasing bias or as S-shaped when the current starts to increase again. In this paper only such cases will be discussed in which the current stays below significant Joule heating (no current channel formation), and only for stationary electrode-attached high-field domains. These are the cathode-attached domains that are maintained by field-quenching and are thermodynamically stable. Their finding is summarized in the first segment of this paper. When the applied voltage is increased, an anode-attached hyper-high-field domain develops that is stabilized by a hole blocking anode and will be analyzed in more detail below. It will be shown that they are a thermodynamically stable p-type photoconductive state of CdS. These two new states can be used to determine the carrier densities and mobilities as function of the field and the effective work function in dependence of the spectral distribution of the optical excitation. In a thin slab adjacent to a blocking cathode, the quasi-Fermi levels are spread to a precise amount and are kept there in the entire high-field region. This opens the opportunity to analyze with small modulation of the excitation the trap transition coefficients near these quasi-Fermi levels separately, without broadening interference from other signals. This has already resulted in the discovery of an unusually sharp electron quenching level when the CdS was in a p-type state with an anode adjacent domain. It is also shown that the domains are responsible for the high efficiency of CdTe solar cells when covered with a thin layer of CdS by eliminating junction leakage and increase hole suction into the CdS.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Role of mechanical stress localizations on the radiation hardness of AlGaN/GaN high electron mobility transistors

Multi-material, multi-layered systems such as AlGaN/GaN high electron mobility transistors (HEMTs) contain residual mechanical stresses that arise from sharp contrasts in device geometry and materials parameters. These stresses, which can be either tensile or compressive, are difficult to detect and eliminate because of their highly localized nature. We propose that their high-stored internal energy makes potential sites for defect nucleation sites under radiation, particularly if their locations coincide with the electrically sensitive regions of a transistor. In this study, we validate this hypothesis with molecular dynamic simulation and experiments exposing both pristine and annealed HEMTS to 2.8 MeV Au +3 irradiation. Our unique annealing process uses mechanical momentum of electrons, also known as the electron wind force (EWF) to mitigate the residual stress at room temperature. High-resolution transmission electron microscopy and cathodoluminescence spectra reveal the reduction of point defects and dislocations near the two-dimensional electron gas region of EWF-treated devices compared to pristine devices. The EWF-treated HEMTs showed relatively higher resilience with approximately 10% less degradation of drain saturation current and ON-resistance and 5% less degradation of peak transconductance. Both mobility and carrier concentration of the EWF-treated devices were less impacted compared to the pristine devices. Our results suggest that the lower density of nanoscale stress localization contributed to the improved radiation tolerance of the EWF-treated devices. Intriguingly, the EWF is found to modulate the defect distribution by moving the defects to electrically less sensitive regions in the form of dislocation networks, which act as sinks for the radiation induced defects and this assisted faster dynamic annealing.

AlGaN/GaN HEMTs↗
DOE OSTI2018

Tunable Semiconductors: Control over Carrier States and Excitations in Layered Hybrid Organic-Inorganic Perovskites

Here, for a class of 2D hybrid organic-inorganic perovskite semiconductors based on π-conjugated organic cations, we predict quantitatively how varying the organic and inorganic component allows control over the nature, energy, and localization of carrier states in a quantum-well-like fashion. Our first-principles predictions, based on large-scale hybrid density-functional theory with spin-orbit coupling, show that the interface between the organic and inorganic parts within a single hybrid can be modulated systematically, enabling us to select between different type-I and type-II energy level alignments. Lastly, energy levels, recombination properties, and transport behavior of electrons and holes thus become tunable by choosing specific organic functionalizations and juxtaposing them with suitable inorganic components.

36 MATERIALS SCIENCE↗
NASA NTRS1973

Intelligibility improvement of analog communication systems using an amplitude control technique.

An amplitude control technique has been employed for use with analog voice communication systems, which improves low-level phoneme reception and eliminates the received noise between words and syllables. Tests were conducted on a narrow-band frequency-modulation simplex voice communication channel employing the amplitude control technique. Presented for both the modified rhyme word tests and the phonetically balanced word tests are a series of graphical plots of the tests' score distribution, mean, and standard deviation as a function of received carrier-to-noise power density ratio. At low received carrier-to-noise power density ratios, a significant improvement in the intelligibility was obtained. A voice intelligibility improvement of more than 2 dB was obtained for the modified rhyme test words, and a voice intelligibility improvement in excess of 4 dB was obtained for the phonetically balanced word tests.

Wishna, S.↗
NASA NTRS1980

Performance of convolutionally coded unbalanced QPSK systems

An evaluation is presented of the performance of three representative convolutionally coded unbalanced quadri-phase-shift-keying (UQPSK) systems in the presence of noisy carrier reference and crosstalk. The use of a coded UQPSK system for transmitting two telemetry data streams with different rates and different powers has been proposed for the Venus Orbiting Imaging Radar mission. Analytical expressions for bit error rates in the presence of a noisy carrier phase reference are derived for three representative cases: (1) I and Q channels are coded independently; (2) I channel is coded, Q channel is uncoded; and (3) I and Q channels are coded by a common 1/2 code. For rate 1/2 convolutional codes, QPSK modulation can be used to reduce the bandwidth requirement.

Divsalar, D.↗
NASA NTRS2003

Infrared Avionics Signal Distribution using Wavelength Division Multiplexing

Pilots in the cockpits of aircrafts currently communicate with ground stations using Radio Frequency (RF) signals. Antennas mounted outside the aircraft receive and transmit RF signals from and to the ground stations. The RF signals received at the antennas are sent to the cockpit using coaxial cables. As the number of antennas needed to provide more than one frequency band in aircrafts increases, RF distribution media (such as coaxial cable) adds to the complexity and weight of the cockpit wiring. Concomitantly, the safety and signal to noise ratio also decreases due to the use of RF signals. The University of Oklahoma is collaborating with the National Aeronautics and Space Administration to develop optical fiber based schemes to replace the coaxial cable used for RF signal distribution within an aircraft. The project aims at exploiting emerging Wavelength Division Multiplexing (WDM) techniques to reduce the weight of cabling, and increase the signal to noise ratio and reliability. This will be achieved by wavelength division multiplexing the signals from the various antennas and then demultiplexing the signals to recover the original signals at the cockpit. This paper will show that (i) RF signals can not only be wavelength multiplexed at the end of a fiber, but additional signals can be inserted into the middle of the fiber using WDM technology, and (ii) the signals can also be successfully extracted by tapping into the middle of the fiber. We are currently extending our previous laboratory prototype (which could multiplex signals only at the end of the fiber) to include additional multiplexing and demultiplexing of RF signals from the middle of the optical backbone with a view to validating the proof of concept, and carrying out measurements to determine the effectiveness of Wavelength Division Multiplexing for avionics applications. A test bed to perform measurements of several relevant parameters for various modulation schemes and frequencies (such as VHF, UHF, and L-Band) has been implemented. In particular, results of transmitter and receiver noise, bit-error-rate (BER), effect of cross talk on the quality of the multiplexed signals, and Signal to Noise ratio and Carrier to Noise ratio, obtained using the aforementioned test bed, will be presented.

Atiquzzaman, Mohammed↗
DOE OSTI2018

Elucidating potential-induced degradation in bifacial PERC silicon photovoltaic modules

This paper elucidates the behavior and underlying mechanism of potential-induced degradation (PID) on the rear side of p-type monocrystalline silicon bifacial passivated emitter and rear cell (PERC) photovoltaic modules. At 50 degrees C, 30% relative humidity, and -1000 V bias to the solar cells with aluminium foil on the rear glass surface, the rear-side performance of bifacial PERC modules at standard testing conditions degraded dramatically after 40 hours with a 40.4%, 36.2%, and 7.2% loss in maximum power (Pmpp), short-circuit current (Isc), and open-circuit voltage (Voc), respectively. The front-side standard testing condition performance, on the other hand, showed less degradation; Pmpp, Isc, and Voc dropped by 12.0%, 5.2%, and 5.3%, respectively. However, negligible degradation was observed when the solar cells were positively biased. Based on I-V characteristics, electroluminescence, external quantum efficiency measurements, and the effective minority-carrier lifetime simulation, the efficiency loss is shown to be caused by the surface polarization effect; positive charges are attracted to the passivation/antireflection stack on the rear surface and reduce its field effect passivation performance. Extended PID testing to 100 hours showed an increase in device performances (relative to 40 hours) due to the formation of an inversion layer along the rear surface. In addition, replacing ethylene-vinyl acetate copolymer with polyolefin elastomer films significantly slows down the progression of PID, whereas a glass/transparent backsheet design effectively protects the rear side of bifacial PERC modules from PID. Furthermore, PID on the rear side of bifacial PERC modules is fully recoverable, and light greatly promotes recovery of the observed PID.

14 SOLAR ENERGY↗
NASA NTRS1994

Required Bandwidth, Unwanted Emission and Data Power Efficiency For Residual and Supppressed Carrier Systems

This paper presents a new concept for required bandwidth along with a method for computing this bandwidth and associated unwanted emission for the classes of PCM/PSK/PM,PCM/PM and BPSK signals. The PCM/PSK/PM signals considered here employ either a squarewave or sinwave subcarriers with NRZ date format. Furthermore, the optimum required bandwidth in the presence of noise and the data power efficiency among these modulation schemes will also be investiaged.

Bandwidth Telemetry Signals↗
DOE OSTI2012

Performance Study of Hybrid DS/FFH Spread-Spectrum Systems in the Presence of Frequency-Selective Fading and Multiple-Access Interference

Hybrid spread-spectrum (HSS) systems have recently received considerable interest in commercial, Smart Grid, and military communication systems because they accommodate high data rates with high link integrity, even in the presence of significant multipath effects and interfering signals. A highly useful form of this modulation scheme is the specific code-related combination of standard direct-sequence spread spectrum (DSSS) with fast frequency-hopping (FFH) spread spectrum, denoted hybrid DS/FFH, wherein multiple frequency hops occur within a single data-bit time. In this paper we perform a simulation-based study of the DS/FFH performance as compared to the existing standard DSSS and FHSS wireless networks. The performance metrics are bit-error probability and multiple-access capability. The parameter space of DS/FFH, including the DS spreading rate, frequency hopping rate, carrier frequencies, and numbers of users, is explored to show its performance under frequency-selective Rayleigh fading environments and multiuser interference. Direct digital synthesizers to achieve fast hopping speeds are also considered in our study.

99 GENERAL AND MISCELLANEOUS↗
DOE OSTI2011

Optically Detected Magnetic Resonance Studies on π-conjugated semiconductor systems

Optically Detected Magnetic Resonance (ODMR) techniques were used to investigate the dynamics of excitons and charge carriers in π-conjugated organic semiconductors. Degradation behavior of the negative spin-1/2 electroluminescence-detected magnetic resonance (ELDMR) was observed in Alq3 devices. The increase in the resonance amplitude implies an increasing bipolaron formation during degradation, which might be the result of growth of charge traps in the device. The same behavior of the negative spin-1/2 ELDMR was observed in 2wt% Rubrene doped Tris(8-hydroxyquinolinato)aluminium (Alq3) devices. However, with increasing injection current, a positive spin-1/2 ELDMR, together with positive spin 1 triplet powder patterns at Δm S =±1 and Δm S =±2, emerges. Due to the similarities in the frequency dependences of single and double modulated ELDMR and the photoluminescence-detected magnetic resonance (PLDMR) results in poly[2-methoxy-5-(2 -ethyl-hexyloxy)-1,4-phenyl ene vinylene] (MEH-PPV) films, the mechanism for this positive spin-1/2 ELDMR was assigned to enhanced triplet-polaron quenching under resonance conditions. The ELDMR in rubrene doped Alq3 devices provides a path to investigate charge distribution in the device under operational conditions. Combining the results of several devices with different carrier blocking properties and the results from transient EL, it was concluded trions not only exist near buffer layer but also exist in the electron transport layer. This TPQ model can also be used to explain the positive spin-1/2 PLDMR in poly(3-hexylthiophene) (P3HT) films at low temperature and in MEH-PPV films at various temperatures up to room temperature. Through quantitative analysis, TE-polaron quenching (TPQ) model is shown having the ability to explain most behaviors of the positive spin-1/2 resonance. Photocurrent detected magnetic resonance (PCDMR) studies on MEH-PPV devices revealed a novel transient resonance signal. The signal may originate from the higher concentration of deep traps near cathode. A quantitative analysis based on this assumption was carried out and found to be consistent with the experimental results.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Wafer-Free Crystalline Silicon Solar Cells (CRADA Final Report)

This CRADA project, based on the DOE Solar Energy Technologies Office (SETO) Solar Prize Voucher program, helped Leap Photovoltaics to develop methodologies to immobilize Si particles by permanently attaching them to an Al-coated substrate and thereby forming carrier-selective electrical contacts to the Si particles. The bigger goal was to help Leap Photovoltaics develop these immobilized and contacted particle arrays into relatively efficient, inexpensive, and industrially relevant solar cells. By using Si particles instead of wafers in a solar cell absorber layer, one can avoid costs associated with growing monocrystalline Si ingots, then diamond-sawing them into wafers, then processing wafers into cells – a mainstream practice in today's high-efficiency Si cell and module technology. Monocrystalline or polycrystalline Si particles can be obtained in various ways: for example, Si kerf from wafer sawing is monocrystalline; recycled Si cell wafers can be ball-milled into particles; particles can be grown using various gas-phase techniques (mostly from SiH4). These Si particles can be assembled onto a substrate and serve as an absorber layer for the solar cell, absorbing photons to generate photocarriers. The challenge with this technique is to collect photocarriers from individual Si particles, with separation of photogenerated electrons to the negative cell’s electrode and positive photogenerated holes to the positive electrode. Therefore, each particle must have two isolated, carrier-selective contacts: one for electrons and one for holes. Plus, particles need to be immobilized onto a solid substrate. The goal of this work was focused on the immobilization of Si particles and creating hole-selective contact to them at the same time, using industrially relevant Si photovoltaic (PV) cell technology: screen printing of Al back-surface field electrodes. This is used in the mainstream Propane Education and Research Council (PERC) technology for hole-collecting contacts at the back of the cell. The work performed at NREL consisted of screen printing of Al metal paste on substrates, spreading Si particles onto it, and thermally processing the structures to form hole-collecting contacts. The final structures were investigated by scanning electron microscopy (SEM) after focused ion beam (FIB) cross-sectioning and polishing. The work was done jointly by NREL staff and Leap Photovoltaics (Leap PV) employees stationed at NREL. The samples were then taken to Leap PV for further processing. Training the Leap PV employee on various NREL techniques (laser cutting, screen printing, thermal processing, characterization) was part of the scope.

14 SOLAR ENERGY↗
NASA NTRS2001

The Constellation-X Focal Plane Microcalorimeter Array: An NTD-Germanium Solution

The hallmarks of Neutron Transmutation Doped (NTD) germanium cryogenic thermistors include high reliability, reproducibility, and long term stability of bulk carrier transport properties. Using micro-machined NTD Ge thermistors with integral 'flying' leads, we can now fabricate two-dimensional arrays that are built up from a series of stacked linear arrays. We believe that this modular approach of building, assembling, and perhaps replacing individual modules of detectors is essential to the successful fabrication and testing of large multi-element instruments. Details of construction are presented.

Beeman, J.↗
DOE OSTI2021

High-efficiency perovskite photovoltaic modules achieved via cesium doping

Perovskite solar modules have been attracting increasing attention due to their market potential, yet publications concerned with theintrinsic scale-up potential of different perovskite compositions remain relatively scarce. On the other hand, while great success is being made towards improving the power conversion efficiency (PCE) of perovskite solar cells (PSCs) by cesium cation (Cs + ) doping of the perovskite, more attention is being paid to the perovskite phase stabilization effect of Cs + doping, and less to other properties that are critical to understand and futher improve the PSC's. In this work, moderately-Cs-doped MAPbI 3 was employed as a model perovskite material in order to exclude the phase stabilization effect. Our systematic study revealed the influence of Cs + in organic-inorganic hybrid perovskites on the crystal structure, crystallization process, trap state density, band structure and charge (i.e., ions or photo-carriers) transport. Markedly, it has been observed that Cs + doping can greatly increase the carrier diffusion length in the perovskite films, thus improving the potential to scale-up PSC's.The PCE of small area devices (0.09 cm 2 ) was increased to 21.72% from 19.73%, with decreased hysteresis behavior and increased operational stability (T85 = 1000 h) after Cs + doped, where T85 refers to the retention of 85% of the initial PCE. Moreover, a PCE of 21.08% was obtained for a Cs + -containing perovskite module with an active area > 30 cm 2 , which demonstrates a better "reproducibility" than the reference sample (MAPbI3-based perovskite modules, PCE = 18.26%).

14 SOLAR ENERGY↗