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DOE OSTI2021

In 2 O 3 -Ga 2 O 3 Alloys as Potential Buffer Layers in CdTe Thin-Film Solar Cells

The efficiency of state-of-the-art Cd Te solar cells remains limited by the relatively low open-circuit voltage (V OC ). Improving the front interface is key towards realizing a higher V OC after achieving the necessary bulk carrier density and lifetime. Recent efforts in identifying buffer layers beyond Cd S have focused on Mg x Zn 1-x O, which offers tunability of the band offsets, but often suffers from high interfacial defect densities. Ga 2 O 3 -based buffer layers demonstrate tremendous improvements in interfacial defect passivation in crystalline silicon and dye-sensitized solar cells, leading to record high V OC , yet remain largely unexplored in Cd Te-based devices. Here, we perform hybrid density-functional-theory calculations to investigate pure Ga 2 O 3 and InGaO 3 alloys as a window layer in Cd Te photovoltaics. We report calculated band offsets for several pairs of solid-solid interfaces comprising transparent conducting oxide (TCO) and Cd Te heterojunctions. The results support a large conduction band offset spike of 0.67 eV for the Cd Te/Ga 2 O 3 (100) interface, while the offset is reduced to 0.18 eV for the InGaO 3 alloy and matches closely with the preferred optimum value of 0.2 eV. Device-level modeling tests of Cd Te solar cells integrating our results indicate that the highest efficiency is achieved with InGaO 3 acting both as a buffer layer and TCO. Finally, our results suggest that alloys of In 2 O 3 and Ga 2 O 3 may be attractive alternatives to Mg x Zn 1-x O for tailoring optimal conduction-band offsets of the buffer and TCO layers in high-efficiency Cd Te thin-film solar cells.

14 SOLAR ENERGY↗

DOE OSTI2022

Equity in Transactive Energy Systems

Transactive energy (TE) research primarily focuses on efficient and reliable operation of the electricity grid by using economic or market-based constructs to incorporate significant amounts of responsive, demand-side assets. This research examines the literature to evaluate if the design of TE demonstrations incorporates microeconomic principles of equity and fairness. We also consider the extent to which the design and implementation of TE affects energy inequities, and how these inequities could be addressed in future research with specific equity valuation metrics. The design of TE systems can impact energy equity across several dimensions and we provide recommendations for incorporating microeconomic principles of equity and fairness in TE system architecture as well as metrics for improving equitable outcomes in TE system design, implementation, and performance.

Energy, Transactive Energy, Economics↗

DOE OSTI2021

Mapping the combustion modes of a dual-fuel compression ignition engine

Compression-ignition (CI) engines can produce higher thermal efficiency (TE) and thus lower carbon dioxide (CO 2 ) emissions than spark-ignition (SI) engines. Unfortunately, the overall fuel economy of CI engine vehicles is limited by their emissions of nitrogen oxides (NO x ) and soot, which must be mitigated with costly, resource- and energy-intensive aftertreatment. NO x and soot could also be mitigated by adding premixed gasoline to complement the conventional, non-premixed direct injection (DI) of diesel fuel in CI engines. Several such “dual-fuel” combustion modes have been introduced in recent years, but these modes are usually studied individually at discrete conditions. This paper introduces a mapping system for dual-fuel CI modes that links together several previously studied modes across a continuous two-dimensional diagram. This system includes the conventional diesel combustion (CDC) and conventional dual-fuel (CDF) modes; the well-explored advanced combustion modes of HCCI, RCCI, PCCI, and PPCI; and a previously discovered but relatively unexplored combustion mode that is herein titled “Piston-split Dual-Fuel Combustion” or PDFC. Tests show that dual-fuel CI engines can simultaneously increase TE and lower NO x and/or soot emissions at high loads through the use of Partial HCCI (PHCCI). At low loads, PHCCI is not possible, but either PDFC or RCCI can be used to further improve NO x and/or soot emissions, albeit at slightly lower TE. These results lead to a “partial dual-fuel” multi-mode strategy of PHCCI at high loads and CDC at low loads, linked together by PDFC. Drive cycle simulations show that this strategy, when tuned to balance NO x and soot reductions, can reduce engine-out CO 2 emissions by about 1% while reducing NO x and soot by about 20% each with respect to CDC. This increases emissions of unburnt hydrocarbons (UHC), still in a treatable range (2.0 g/kWh) but five times as high as CDC, requiring changes in aftertreatment strategy.

Engineering↗

NASA NTRS1975

Electron temperatures in the wake of an ionospheric satellite

We present measurements of electron temperature (Te) made by a retarding potential analyzer and a Langmuir probe (both flush-mounted on the spin-stabilized satellite Explorer 31) to investigate the variation of Te around the satellite. Most of the time there is a Te variation, which repeats for given ionospheric conditions. The variation is strongly controlled by the angle between the velocity vector and the probe normal, Te usually being enhanced in the near wake of the satellite. Magnetic field control of Te, if it is present, is hidden by the stronger velocity vector control. Our results indicate that the magnitude of the Te enhancement in the wake does not depend on the average ion mass (M), although the electron density depletion in the wake is strongly correlated with M.

Troy, B. E., Jr.↗

NASA NTRS2000

Optical Sensors Based on Single on Arm Thin Film Waveguide Interferometer

Single-arm dual-mode optical waveguide interferometer utilizes interference between two modes of different order. Sensing effect results from the change in propagation conditions of the modes caused by the environment. The waveguide is made as an open asymmetric structure containing a dye-doped polymer film onto a quartz substrate. It is more sensitive to the change of environment than its conventional polarimetric analog using orthogonal modes (TE and TM) of the same order. The sensor still preserves the option of operating in polarimetric regime using a variety of mode combinations such as TE(sub 0)/TM(sub 0) (conventional) TE(sub 0)/TM(sub 1), TE(sub 1)/TM(sub 0), or TE(sub 1)/TM(sub 1) but can also work in nonpolarimetric regime using combinations TE(sub 0)/TE(sub 1) or TM(sub 0)/TM(sub 1). Utilization of different mode combinations simultaneously makes the device more versatile. Application of the sensor to gas sensing is based on doping polymer film with an organic indicator dye targeting a particular gaseous reagent. Change of the optical absorption spectrum of the dye caused by the gaseous pollutant results in change of the reactive index of the dye-doped polymer film that can be detected by the sensor. As indicator dyes we utilize Bromocresol Purple doped into polymer poly(methyl) methacrylate that is sensitive to small concentrations of ammonia. The indicator dye demonstrated an irreversible increase in optical absorption near the peak at 350 nm being exposed to 5% ammonia in pure nitrogen at 600 Torr. The dye also showed reversible growth of the absorption peak near 600 nm after exposure to a vapor of standard medical ammonia spirit (65% alcohol). We have built a breadboard prototype of the sensor with He-Ne laser as a light source and with a single mode fiber input and a multimode fiber output. The prototype showed a sensitivity to temperature change of the order of 2 C per 2pi phase shift. The sensitivity of the sensor to the presence of dTy ammonia is not less than 300 ppm per 2pi phase shift. The proposed sensor can be used as a robust stand-alone instrument for continuous environment pollution monitoring.

Sarkisov, S. S.↗

NASA NTRS2002

Temperature Variations from HST Imagery and Spectroscopy of NGC 7009

We present new HST/WFPC2 imagery and STIS long-slit spectroscopy of the planetary nebula NGC 7009. The primary goal was to obtain high spatial resolution of the intrinsic line ratio [O III] 4364/5008 and thereby evaluate the electron temperature (Te) and the mean-square Te variation (t(sup 2, sub A)) across the nebula. The observations here do not address Te fluctuation along the line of sight. The WFPC2 Te map is rather uniform; almost all values are between 9000 - 11,000 K, with the higher Te's closely coinciding with the inner He(++)-zone. The results indicate very small values - certainly less than 0.01 - for t(sup 2, sub A) throughout. Our STIS data allow an even more direct determination of Te and t(sup 2, sub A), albeit for a much smaller area than with WFPC2. We present results from binning the data along the slit into tiles that are 0.5 in square (matching the slit width). The average [O III] temperature using 45 tiles (excluding the central star and STIS fiducial bars) is 10,146 K; t(sup 2, sub A) is 0.0036. Although we have measured t(sup 2, sub A) in only 2-dimensions, we conclude that temperature fluctuations alone are unlikely to explain for NGC 7009 the large discrepancy between heavy element abundances inferred from emission lines that are collisionally excited compared with those that are due to recombination lines.

Rubin, R. H.↗

NASA NTRS2002

Origin of the Large Bandgap Bowing in Highly Mismatched Semiconductor Alloys

We use a combination of optical and soft x-ray fluorescence techniques to investigate the composition dependence of the band edge energies in ZnSe(1-x)Te(x) and ZnS(1-x)Te(x) alloys. The results reveal entirely different origins of the large bandgap bowing for alloys with small and large Te content. On the Te-rich side, the reduction of the bandgap is well explained by the band anticrossing interaction between the Se or S localized states and the ZnTe conduction band states. On the Se or S-rich side, an interaction between the localized Te states and the degenerate GAMMA valence bands of ZnSe or ZnS is responsible for the bandgap reduction and the rapid increase of the spin-orbit splitting with increasing Te concentration. Results of soft x-ray fluorescence experiments provide direct evidence for the valence band anticrossing interaction. The bandgap bowing over the entire composition range is accounted for by a linear interpolation between the conduction band anticrossing and valence band anticrossing models.

Wu, J.↗

NASA NTRS2004

Effects of Angle of Attack and Velocity on Trailing Edge Noise

Trailing edge (TE) noise measurements for a NACA 63-215 airfoil model are presented, providing benchmark experimental data for a cambered airfoil. The effects of flow Mach number and angle of attack of the airfoil model with different TE bluntnesses are shown. Far-field noise spectra and directivity are obtained using a directional microphone array. Standard and diagonal removal beamforming techniques are evaluated employing tailored weighting functions for quantitatively accounting for the distributed line character of TE noise. Diagonal removal processing is used for the primary database as it successfully removes noise contaminates. Some TE noise predictions are reported to help interpret the data, with respect to flow speed, angle of attack, and TE bluntness on spectral shape and peak levels. Important findings include the validation of a TE noise directivity function for different airfoil angles of attack and the demonstration of the importance of the directivity function s convective amplification terms.

Hutcheson, Florence V.↗

NASA NTRS2006

Effects of Angle of Attack and Velocity on Trailing Edge Noise

Hutcheson, Florence V.↗

NASA NTRS2015

Analysis of Advanced Thermoelectric Materials and Their Functional Limits

The world's demand for energy is increasing dramatically, but the best energy conversion systems operate at approximately 30% efficiency. One way to decrease energy loss is in the recovery of waste heat using thermoelectric (TE) generators. A TE generator is device that generates electricity by exploiting heat flow across a thermal gradient. The efficiency of a TE material for power generation and cooling is determined by the dimensionless Figure of Merit (ZT): ZT = S(exp. 2)sigmaT/κ: where S is the Seebeck coefficient, sigma is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity. The parameters are not physically independent, but intrinsically coupled since they are a function of the transport properties of electrons. Traditional research on TE materials has focused on synthesizing bulk semiconductor-type materials that have low thermal conductivity and high electrical conductivity affording ZT values of 1. The optimization of the σ/κ ratio is difficult to achieve using current material formats, as these material constants are complementary. Recent areas of research are focusing on using nanostructural artifacts that introduce specific dislocations and boundary conditions that scatter the phonons. This disrupts the physical link between thermal (phonon) and electrical (electron) transport. The result is that κ is decreased without decreasing σ. These material formats give ZT values of up to 2 which represent approximately 18% energy gain from waste heat recovery. The next challenge in developing the next generation of TE materials with superior performance is to tailor the interconnected thermoelectric physical parameters of the material system. In order to approach this problem, the fundamental physics of each parameter S, sigma, and κ need to be physically understood in their context of electron/phonon interaction for the construction of new high ZT thermoelectric devices. Is it possible to overcome the physical limit imposed by of the effect of phonon lattice oscillation and energetic electrons towards thermal conductivity? Is the Seebeck coefficient, based on the difference in voltage over temperature gradient ( deltaV/deltaT), an intrinsic parameter of each material? All these parameters were manipulated using nano-bridge and twin-lattice structural concepts at the NASA Langley Research Center. This talk will review the current trend of TE research to optimize the ZT and discuss about new approaches on increasing ZT within functional limits of each parameter.

Kim, Hyun Jung↗

NASA NTRS2016

Development of Multi-Physics Dynamics Models for High-Frequency Large-Amplitude Structural Response Simulation

An analytic approach is demonstrated to reveal potential pyroshock-driven dynamic effects causing power losses in the Thermo-Electric (TE) module bars of the Mars Science Laboratory (MSL) Multi-Mission Radioisotope Thermoelectric Generator (MMRTG). This study utilizes high-fidelity finite element analysis with SIERRA/PRESTO codes to estimate wave propagation effects due to large-amplitude suddenly-applied pyro shock loads in the MMRTG. A high fidelity model of the TE module bar was created with approximately 30 million degrees-of-freedom (DOF). First, a quasi-static preload was applied on top of the TE module bar, then transient tri-axial acceleration inputs were simultaneously applied on the preloaded module. The applied input acceleration signals were measured during MMRTG shock qualification tests performed at the Jet Propulsion Laboratory. An explicit finite element solver in the SIERRA/PRESTO computational environment, along with a 3000 processor parallel super -computing framework at NASA AMES, was used for the simulation. The simulation results were investigated both qualitatively and quantitatively. The predicted shock wave propagation results provide detailed structural responses throughout the TE module bar, and key insights into the dynamic response (i.e., loads, displacements, accelerations) of critical internal spring/piston compression systems, TE materials, and internal component interfaces in the MMRTG TE module bar. They also provide confidence on the viability of this high-fidelity modeling scheme to accurately predict shock wave propagation patterns within complex structures. This analytic approach is envisioned for modeling shock sensitive hardware susceptible to intense shock environments positioned near shock separation devices in modern space vehicles and systems.

Shock Waves↗

DOE OSTI2020

Materials Data on Sc6FeTe2 by Materials Project

Sc6FeTe2 crystallizes in the hexagonal P-62m space group. The structure is three-dimensional. there are two inequivalent Sc sites. In the first Sc site, Sc is bonded in a 4-coordinate geometry to two equivalent Fe and two equivalent Te atoms. Both Sc–Fe bond lengths are 2.62 Å. Both Sc–Te bond lengths are 3.04 Å. In the second Sc site, Sc is bonded in a 5-coordinate geometry to one Fe and four equivalent Te atoms. The Sc–Fe bond length is 2.96 Å. All Sc–Te bond lengths are 3.07 Å. Fe is bonded in a 9-coordinate geometry to nine Sc atoms. Te is bonded in a 9-coordinate geometry to nine Sc atoms.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on Sc6NiTe2 by Materials Project

Sc6NiTe2 crystallizes in the hexagonal P-62m space group. The structure is three-dimensional. there are two inequivalent Sc sites. In the first Sc site, Sc is bonded in a 4-coordinate geometry to two equivalent Ni and two equivalent Te atoms. Both Sc–Ni bond lengths are 2.63 Å. Both Sc–Te bond lengths are 3.03 Å. In the second Sc site, Sc is bonded in a 5-coordinate geometry to one Ni and four equivalent Te atoms. The Sc–Ni bond length is 3.05 Å. All Sc–Te bond lengths are 3.04 Å. Ni is bonded in a 9-coordinate geometry to nine Sc atoms. Te is bonded in a 9-coordinate geometry to nine Sc atoms.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on Dy6FeTe2 by Materials Project

Dy6FeTe2 crystallizes in the hexagonal P-62m space group. The structure is three-dimensional. there are two inequivalent Dy sites. In the first Dy site, Dy is bonded in a 5-coordinate geometry to one Fe and four equivalent Te atoms. The Dy–Fe bond length is 3.31 Å. All Dy–Te bond lengths are 3.23 Å. In the second Dy site, Dy is bonded in a 4-coordinate geometry to two equivalent Fe and two equivalent Te atoms. Both Dy–Fe bond lengths are 2.79 Å. Both Dy–Te bond lengths are 3.26 Å. Fe is bonded in a 9-coordinate geometry to nine Dy atoms. Te is bonded in a 9-coordinate geometry to nine Dy atoms.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on Lu8Te by Materials Project

Lu8Te crystallizes in the hexagonal P-62m space group. The structure is three-dimensional. there are three inequivalent Lu sites. In the first Lu site, Lu is bonded in a 12-coordinate geometry to eight Lu and two equivalent Te atoms. There are a spread of Lu–Lu bond distances ranging from 3.29–3.62 Å. Both Lu–Te bond lengths are 3.12 Å. In the second Lu site, Lu is bonded in a 1-coordinate geometry to twelve Lu and one Te atom. There are four shorter (3.36 Å) and two longer (3.69 Å) Lu–Lu bond lengths. The Lu–Te bond length is 3.49 Å. In the third Lu site, Lu is bonded in a 11-coordinate geometry to eleven Lu atoms. Both Lu–Lu bond lengths are 3.69 Å. Te is bonded in a 9-coordinate geometry to nine Lu atoms.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on Sc6Te2Os by Materials Project

Sc6OsTe2 crystallizes in the hexagonal P-62m space group. The structure is three-dimensional. there are two inequivalent Sc sites. In the first Sc site, Sc is bonded in a 4-coordinate geometry to two equivalent Os and two equivalent Te atoms. Both Sc–Os bond lengths are 2.68 Å. Both Sc–Te bond lengths are 3.01 Å. In the second Sc site, Sc is bonded in a 5-coordinate geometry to one Os and four equivalent Te atoms. The Sc–Os bond length is 2.97 Å. All Sc–Te bond lengths are 3.10 Å. Os is bonded in a 9-coordinate geometry to nine Sc atoms. Te is bonded in a 9-coordinate geometry to nine Sc atoms.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on Ni(TePd)2 by Materials Project

Pd2NiTe2 crystallizes in the orthorhombic Ibam space group. The structure is three-dimensional. Pd is bonded in a 10-coordinate geometry to three equivalent Pd, two equivalent Ni, and five equivalent Te atoms. There are one shorter (2.93 Å) and two longer (3.09 Å) Pd–Pd bond lengths. Both Pd–Ni bond lengths are 2.93 Å. There are a spread of Pd–Te bond distances ranging from 2.67–3.03 Å. Ni is bonded in a 10-coordinate geometry to four equivalent Pd, two equivalent Ni, and four equivalent Te atoms. Both Ni–Ni bond lengths are 2.58 Å. All Ni–Te bond lengths are 2.54 Å. Te is bonded in a 7-coordinate geometry to five equivalent Pd and two equivalent Ni atoms.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on Co(TePd)2 by Materials Project

Pd2CoTe2 crystallizes in the orthorhombic Ibam space group. The structure is three-dimensional. Co is bonded in a 10-coordinate geometry to two equivalent Co, four equivalent Pd, and four equivalent Te atoms. Both Co–Co bond lengths are 2.63 Å. All Co–Pd bond lengths are 2.92 Å. All Co–Te bond lengths are 2.49 Å. Pd is bonded in a 10-coordinate geometry to two equivalent Co, three equivalent Pd, and five equivalent Te atoms. There are one shorter (2.91 Å) and two longer (3.12 Å) Pd–Pd bond lengths. There are a spread of Pd–Te bond distances ranging from 2.68–2.96 Å. Te is bonded in a 7-coordinate geometry to two equivalent Co and five equivalent Pd atoms.

36 MATERIALS SCIENCE↗