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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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12 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

X-ray fluorescence and XANES spectroscopy revealed diverse potassium chemistries and colocalization with phosphorus in the ectomycorrhizal fungus Paxillus ammoniavirescens

Ectomycorrhizal (ECM) fungi play a major role in forest ecosystems and managed tree plantations. Particularly, they facilitate mineral weathering and nutrient transfer towards colonized roots. Among nutrients provided by these fungi, potassium (K) has been understudied compared to phosphorus (P) or nitrogen (N). The ECM fungus Paxillus ammoniavirescens is a generalist species that interacts with the root of many trees and can directly transfer K to them, including loblolly pine. However, the forms of K that ECM fungi can store is still unknown. Here, we used synchrotron potassium X-ray fluorescence (XRF) and K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy on P. ammoniavirescens growing in axenic conditions to investigate the K chemistries accumulating in the center and the edge of the mycelium. We observed that various K forms accumulated in different part of the mycelium, including K-nitrate (KNO 3 ), K-C-O compounds (such as K-tartrate K 2 (C 4 H 4 O 6 ) and K-oxalate (K 2 C 2 O 4 )), K-S and K-P compounds. Saprotrophic fungi have been shown to excrete carboxylic acids, which in turn play a role in soil mineral weathering. Our finding of several K counter-ions to carboxylic acids may suggest that, besides their direct transfer to colonized roots, K ions can also be involved in the production of compounds necessary for sourcing nutrients from their surrounding environment by ECM fungi. Additionally, this work reveals that XANES spectroscopy can be used to identify the various forms of K accumulating in biological systems.

Ectomycorrhizal symbiosis↗
NASA NTRS1985

Structure variation of the index of refraction of GaAs-AlAs superlattices and multiple quantum wells

A detailed calculation of the index refraction of various GaAs-AlAs superlattices is presented for the first time. The calculation is performed by using a hybrid approach which combines the k-p method with the pseudopotential technique. Appropriate quantization conditions account for the influence of the superstructures on the electronic properties of the systems. The results of the model are in very good agreement with the experimental data. In comparison with the index of refraction of the corresponding AlGaAs alloy, characterized by the same average mole fraction of Al, the results indicate that the superlattice index of refraction values attain maxima at the various quantized transition energies. For certain structures the difference can be as large as 2 percent. These results suggest that the waveguiding and dispersion relation properties of optoelectronic devices can be tailored to design for specific optical application by an appropriate choice of the superlattice structure parameters.

Kahen, K. B.↗
NASA NTRS1994

Numerical study of the wave-vector dependence of the electron interband impact ionization rate in bulk GaAs

Ensemble Monte Carlo calculations of the electron interband impact ionization rate in bulk GaAs are presented using a wave-vector (k)-dependent formulation of the ionization transition rate. The transition rate is evaluated through the use of numerically generated wavefunctions determined via a k-p calculation within the first two conduction bands at numerous points within a finely spaced three-dimensional grid in k space. The transition rate is determined to be greatest for states within the second conduction band. Is is found that the interband impact ionization transition rate in bulk GaAs is best characterized as having an exceedingly soft threshold energy. As a consequence, the dead space, defined as the distance over which the ionization probability for a given carrier is assumed to be zero, is estimated to be much larger than that estimated using a harder threshold. These results have importance in the design of the multiquantum-well avalanche photodiodes.

Wang, Yang↗
NASA NTRS1994

Semiclassical Study of the Wave Vector Dependence of the Interband Impact Ionization Rate in Bulk Silicon

We present calculations of the interband impact ionization rate calculated using a wave vector dependent (k-dependent) semiclassical formulation of the transition rate. The transition rate is determined using Fermi's golden rule from a two-body screened Coulomb interaction assuming energy and momentum conservation. The transition rate is calculated for the first two conduction bands of silicon by numerically integrating over the full Brillouin zone. The overlap integrals in the expression for the transition rate are determined numerically using a 15 band k-p calculation. It is found that the transition rate depends strongly on the initiating electron wave vector (k vector) and that the transition rate is greatest for electrons originating within the second conduction band than the first conduction band. An ensemble Monte Carlo simulation, which includes the numerically determined ionization transition rate as well as the full details of the first two conduction bands, is used to calculate the total impact ionization rate in bulk silicon. Good agreement with the experimentally determined electron ionization rate data is obtained.

Wang, Yang↗
NASA NTRS1994

Numerical Study of the Wave-Vector Dependence of the Electron Interband Impact Ionization Rate in Bulk GaAs

Ensemble Monte Carlo calculations of the electron interband impact ionization rate in bulk GaAs are presented using a wave-vector (k)-dependent formulation of the ionization transition rate. The transition rate is evaluated through use of numerically generated wavefunctions determined via a k-p calculation within the first two conduction bonds at numerous points within a finely spaced three-dimensional grid in k space. The transition rate is determined to be greatest for states within the second conduction band. It is found that the interband impact ionization transition rate in bulk GaAs is best characterized as having an exceedingly "soft" threshold energy. As a consequence, the dead space, defined as the distance over which the ionization probability for a given carrier is assumed to be zero, is estimated to be-much larger than that estimated using a "harder" threshold. These results have importance in the design of multiquantum-well avalanche photodiodes.

Wang, Yang↗
NASA NTRS2003

Intersubband Transitions in InAs/AlSb Quantum Wells

We have studied intersubband transitions in InAs/AlSb quantum wells experimentally and theoretically. Experimentally, we performed polarization-resolved infrared absorption spectroscopy to measure intersubband absorption peak frequencies and linewidths as functions of temperature (from 4 K to room temperature) and quantum well width (from a few nm to 10 nm). To understand experimental results, we performed a self-consistent 8-band k-p band-structure calculation including spatial charge separation. Based on the calculated band structure, we developed a set of density matrix equations to compute TE and TM optical transitions self-consistently, including both interband and intersubband channels. This density matrix formalism is also ideal for the inclusion of various many-body effects, which are known to be important for intersubband transitions. Detailed comparison between experimental data and theoretical simulations is presented.

Li, J.↗
DOE OSTI2020

Materials Data on K4P3 by Materials Project

K4P3 crystallizes in the orthorhombic Cmcm space group. The structure is three-dimensional. there are three inequivalent K1+ sites. In the first K1+ site, K1+ is bonded in a 6-coordinate geometry to six P+1.33- atoms. There are a spread of K–P bond distances ranging from 3.34–3.68 Å. In the second K1+ site, K1+ is bonded in a 7-coordinate geometry to seven P+1.33- atoms. There are a spread of K–P bond distances ranging from 3.32–3.50 Å. In the third K1+ site, K1+ is bonded in a square co-planar geometry to four equivalent P+1.33- atoms. All K–P bond lengths are 3.40 Å. There are two inequivalent P+1.33- sites. In the first P+1.33- site, P+1.33- is bonded in a 9-coordinate geometry to eight K1+ and one P+1.33- atom. The P–P bond length is 2.19 Å. In the second P+1.33- site, P+1.33- is bonded in a 9-coordinate geometry to seven K1+ and two equivalent P+1.33- atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K2P3 by Materials Project

K2P3 crystallizes in the orthorhombic Fddd space group. The structure is three-dimensional. there are two inequivalent K1+ sites. In the first K1+ site, K1+ is bonded to eight P+0.67- atoms to form a mixture of face, edge, and corner-sharing KP8 hexagonal bipyramids. There are a spread of K–P bond distances ranging from 3.46–3.53 Å. In the second K1+ site, K1+ is bonded in a 10-coordinate geometry to ten P+0.67- atoms. There are a spread of K–P bond distances ranging from 3.35–3.44 Å. There are two inequivalent P+0.67- sites. In the first P+0.67- site, P+0.67- is bonded in a 8-coordinate geometry to six K1+ and two equivalent P+0.67- atoms. Both P–P bond lengths are 2.17 Å. In the second P+0.67- site, P+0.67- is bonded in a 8-coordinate geometry to six K1+ and two P+0.67- atoms. The P–P bond length is 2.17 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on KP by Materials Project

PK1 is Magnesium tetraboride-like structured and crystallizes in the orthorhombic P2_12_12_1 space group. The structure is three-dimensional. there are two inequivalent K1+ sites. In the first K1+ site, K1+ is bonded in a 6-coordinate geometry to six P1- atoms. There are a spread of K–P bond distances ranging from 3.24–3.67 Å. In the second K1+ site, K1+ is bonded in a 5-coordinate geometry to five P1- atoms. There are a spread of K–P bond distances ranging from 3.20–3.34 Å. There are two inequivalent P1- sites. In the first P1- site, P1- is bonded in a 7-coordinate geometry to five K1+ and two equivalent P1- atoms. There are one shorter (2.26 Å) and one longer (2.28 Å) P–P bond lengths. In the second P1- site, P1- is bonded in a 8-coordinate geometry to six K1+ and two equivalent P1- atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K2P3 by Materials Project

K2P3 crystallizes in the orthorhombic Fmmm space group. The structure is three-dimensional. there are two inequivalent K1+ sites. In the first K1+ site, K1+ is bonded to eight P+0.67- atoms to form a mixture of distorted edge, face, and corner-sharing KP8 hexagonal bipyramids. There are four shorter (3.47 Å) and four longer (3.51 Å) K–P bond lengths. In the second K1+ site, K1+ is bonded in a 10-coordinate geometry to ten P+0.67- atoms. There are a spread of K–P bond distances ranging from 3.36–3.46 Å. There are two inequivalent P+0.67- sites. In the first P+0.67- site, P+0.67- is bonded in a 8-coordinate geometry to six K1+ and two P+0.67- atoms. Both P–P bond lengths are 2.17 Å. In the second P+0.67- site, P+0.67- is bonded in a 8-coordinate geometry to six K1+ and two equivalent P+0.67- atoms.

36 MATERIALS SCIENCE↗
NASA NTRS2013

Pre-Study Walkthrough with a Commercial Pilot for a Preliminary Single Pilot Operations Experiment

The number of crew members in commercial flights has decreased to two members, down from the five-member crew required 50 years ago. One question of interest is whether the crew should be reduced to one pilot. In order to determine the critical factors involved in safely transitioning to a single pilot, research must examine whether any performance deficits arise with the loss of a crew member. With a concrete understanding of the cognitive and behavioral role of a co-pilot, aeronautical technologies and procedures can be developed that make up for the removal of the second aircrew member. The current project describes a pre-study walkthrough process that can be used to help in the development of scenarios for testing future concepts and technologies for single pilot operations. Qualitative information regarding the tasks performed by the pilots can be extracted with this technique and adapted for future investigations of single pilot operations.

Single Pilot Operations↗