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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Pressure-Dependent Magnetic Properties of Quasi-2D Cr 2 Si 2 Te 6 and Mn 3 Si 2 Te 6

Recently, pressure has been used to induce structural and magnetic phase transitions in many layered quantum materials whose layers are linked by van der Waals forces. Materials with such weakly held layers allow for relatively easy manipulation of the superexchange mechanism, which can give rise to novel magnetic behavior. Here, using hydrostatic pressure as a disorderless means to manipulate the interlayer coupling, we applied pressure on two quasi-2D sister compounds, namely, Cr 2 Si 2 Te 6 (CST) and Mn 3 Si 2 Te 6 (MST), up to ~1 GPa. Magnetic property measurements with the application of pressure revealed that the ferromagnetic transition temperature decreases in CST, while the opposite occurs for the ferrimagnetic MST. In MST, magnetization decreases with the increase in pressure, and such a trend is not clearly observed within the pressure range studied for CST. The overall pressure effect on magnetic characteristics such as exchange couplings and magnetic anisotropy energies is also examined theoretically using density functional theory. Exchange coupling in MST is strongly frustrated, and the first nearest neighbor interaction is the most dominant of the components with the strongest pressure dependence. In CST, the exchange coupling parameters exhibit very little dependence on pressure. This combined experimental and theoretical work has the potential to expand to other relevant quantum materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Fabrication of Poly-Si on Locally Etched SiOx as Passivating Contacts for c-Si Solar Cells

Polysilicon on silicon oxide (poly-Si/SiO x ) passivating contacts with predominant charge-carrier transport via pinholes were prepared with room temperature metal-assisted chemical etching. Pinhole areal densities in the range of 2.8 x 10^4 to 4.5 x 10^7 cm^-2 were imaged by SEM. Contact resistivity of 32 m-ohm-cm^2 and implied open circuit voltage of 729 mV were obtained for symmetric n+ poly-Si/SiOx grown onto randomly textured n-Cz. We also show preliminary data pertaining boron-doped polysilicon on nitride/oxide passivating contacts, with which we achieved implied open circuit voltage above 730 mV and recombination current of 0.2 fA/cm^2.

amorphous Si↗
DOE OSTI2022

A performance comparison between GaInP-on-Si and GaAs-on-Si 3-terminal tandem solar cells

The pursuit of ever-higher solar cell efficiencies has focused heavily on multijunction technologies. In tandem cells, subcells are typically either contacted via two terminals (2T) or four terminals (4T). Simulations show that the less-common three-terminal (3T) design may be comparable to 4T tandem cells in its compatibility with a range of materials, operating conditions, and methods for subcell integration, yet the 3T design circumvents shading losses of the 4T intermediate conductive layers. This study analyzes the performance of two superstrate 3T III-V-on-Si (III-V//Si) tandem cells: One has slightly greater current contribution from the Si bottom cell (GaInP//Si) and the other has substantially greater current contribution from the GaAs top cell (GaAs//Si). Our results show that both tandem cells exhibit the same efficiency (21.3%), thereby demonstrating that the third terminal allows for flexibility in the selection of the top cell material, similar to the 4T design.

14 SOLAR ENERGY↗
DOE OSTI2024

Evolution of the nuclear spin-orbit splitting explored via the 32 Si( d,p ) 33 Si reaction using SOLARIS

The spin-orbit splitting between neutron 1p orbitals at 33 Si has been deduced using the single-neutron-adding (d,p) reaction in inverse kinematics with a beam of 32 Si, a long-lived radioisotope. Reaction products were analyzed by the newly implemented SOLARIS spectrometer at the reaccelerated-beam facility at the National Superconducting Cyclotron Laboratory. The measurements show reasonable agreement with shell-model calculations that incorporate modern cross-shell interactions, but they contradict the prediction of proton density depletion based on relativistic mean-field theory. The evolution of the neutron 1p-shell orbitals is systematically studied using the present and existing data in the isotonic chains of N = 17, 19, and 21. In each case, a smooth decrease in the separation of the 1p 3/2 - 1p 1/2 orbitals is seen as the respective p-orbitals approach zero binding, suggesting that the finite nuclear potential strongly influences the evolution of nuclear structure in this region.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Reaction of Hydrazine with Solution- and Vacuum-Prepared Selectively Terminated Si(100) Surfaces: Pathways to the Formation of Direct Si–N Bonds

Here, the reactivity of liquid hydrazine (N 2 H 4 ) with respect to H-, Cl-, and Br-terminated Si(100) surfaces was investigated to uncover the principles of nitrogen incorporation into the interface. This process has important implications in a wide variety of applications, including semiconductor surface passivation and functionalization, nitride growth, and many others. The use of hydrazine as a precursor allows for reactions that exclude carbon and oxygen, the primary sources of contamination in processing. In this work, the reactivity of N 2 H 4 with H- and Cl-terminated surfaces prepared by traditional solvent-based methods and with a Br-terminated Si(100) prepared in ultrahigh vacuum was compared. The reactions were studied with X-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy, and the observations were supported by computational investigations. The H-terminated surface led to the highest level of nitrogen incorporation; however, the process proceeds with increasing surface roughness, suggesting possible etching or replacement reactions. In the case of Cl-terminated (predominantly dichloride) and Br-terminated (monobromide) surfaces, the amount of nitrogen incorporation on both surfaces after the reaction with hydrazine was very similar despite the differences in preparation, initial structure, and chemical composition. Density functional theory was used to propose the possible surface structures and to analyze surface reactivity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Surface Immobilization of a Re(I) Tricarbonyl Phenanthroline Complex to Si(111) through Sonochemical Hydrosilylation

A sonochemical-based hydrosilylation method was employed to covalently attach a rhenium tricarbonyl phenanthroline complex to silicon(111). fac-Re(5-(p-Styrene)-phen)(CO) 3 Cl (5-(p-styrene)-phen = 5-(4-vinylphenyl)-1,10-phenanthroline) was reacted with hydrogen-terminated silicon(111) in an ultrasonic bath to generate a hybrid photoelectrode. Subsequent reaction with 1-hexene enabled functionalization of remaining atop Si sites. Attenuated total reflectance–Fourier transform infrared spectroscopy confirms attachment of the organometallic complex to silicon without degradation of the organometallic core, supporting hydrosilylation as a strategy for installing coordination complexes that retain their molecular integrity. Detection of Re(I) and nitrogen by X-ray photoelectron spectroscopy (XPS) further support immobilization of fac-Re(5-(p-styrene)-phen)(CO) 3 Cl. Cyclic voltammetry and electrochemical impedance spectroscopy under white light illumination indicate that fac-Re(5-(p-styrene)-phen)(CO) 3 Cl undergoes two electron reductions. Mott–Schottky analysis indicates that the flat band potential is 239 mV more positive for p-Si(111) co-functionalized with both fac-Re(5-(p-styrene)-phen)(CO) 3 Cl and 1-hexene than when functionalized with 1-hexene alone. XPS, ultraviolet photoelectron spectroscopy, and Mott–Schottky analysis show that functionalization with fac-Re(5-(p-styrene)-phen)(CO) 3 Cl and 1-hexene introduces a negative interfacial dipole, facilitating reductive photoelectrochemistry.

X-ray photoelectron spectroscopy↗
DOE OSTI2022

Revisiting the K-edge X-ray absorption fine structure of Si, Ge–Si alloys, and the isoelectronic series: CuBr, ZnSe, GaAs, and Ge

Extended X-ray absorption fine structure (EXAFS) has evolved into an unprecedented local-structure technique that is routinely used to study materials’ problems in the biological, chemical, and physical sciences. Like many other experimental techniques, EXAFS also requires that several key atomic parameters must be known a priori before structural information can be quantitatively determined. Utilizing current analytical methods, we revisit the isoelectronic series CuBr, ZnSe, GaAs, and Ge originally studied by Stern et al. during the early development of EXAFS. We demonstrate that the ab initio EXAFS code FEFF accurately predicts the atomic phase shifts and backscattering amplitudes that are primarily functions of the sum of atomic numbers Z along an EXAFS scattering path. We also investigate quantitative fitting and first- and second-shell phase transferability together with problems that arise if a backscattering atom is identified incorrectly in an EXAFS fitting model. Features in the near-edge region, on the other hand, are shown to require a comprehensive treatment of the band structure and density-of-states, including effects of the screened Coulomb interaction between the photoelectron and core hole. Here, we demonstrate that the Bethe–Salpeter equation (BSE) accurately captures the NEXAFS (or XANES) portion of the spectrum for the isoelectronic series in addition to Si and Ge–Si alloys, including within a few eV of the absorption edge, where band structure and excitonic effects are most important.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Low-Frequency Noise and Deep Level Transient Spectroscopy in n-p-n Si Bipolar Junction Transistors Irradiated with Si Ions

The properties of defects in n-p-n Si bipolar junction transistors (BJTs) caused by 17 MeV Si ions are investigated via current-voltage, low-frequency noise, and deep-level transient spectroscopy (DLTS) measurements. Four prominent radiation-induced defects in the base-collector junction of these transistors are identified via DLTS. At least two defect levels are observed in temperature-dependent low-frequency 1/ f noise measurements, one that is similar to a prominent defect in DLTS and another that is not. Defect microstructures are discussed. Here our results show that DLTS and 1/ f noise measurements can provide complementary information about defects in linear bipolar devices.

42 ENGINEERING↗
DOE OSTI2021

Performance of direct injected propane and gasoline in a high stroke-to-bore ratio SI engine: Pathways to diesel efficiency parity with ultra low soot

This work explores pathways to achieve diesel-like, high-efficiency combustion with stoichiometric 3-way catalyst compatible combustion in a single-cylinder spark ignition (SI) research engine. A unique high stroke-to-bore engine design (1.5:1) with cooled exhaust gas recirculation (EGR) and high compression ratio ( r c ) was used to improve engine efficiency by up to 30% compared with a production turbocharged gasoline direct injection spark ignition engine. Engine experiments were conducted with both 91 RON E10 gasoline and liquified petroleum gas (LPG) (i.e. autogas) and were compared to legacy gasoline data on the production engine. Geometric compression ratio ( r c ) of 13.3:1 was used for both fuels with additional experiments at 16.8:1 for LPG only. Measurements of exhaust soot particle size and number concentrations were made with both fuels. Significant reduction in soot particles across the whole particle size range were achieved with LPG due to the elimination of in-cylinder liquid films. The effects of EGR, late intake valve closing (IVC) and fuel characteristics were investigated through their effects on efficiency, combustion stability and soot production. Results of 47% gross thermal efficiency, and 45% net thermal efficiency at stoichiometric engine operation, at up to 17 bar IMEP and 2000 r/min with 16.8:1 r c were achieved with LPG. Estimated brake efficiency values were compared to a contemporary medium duty diesel engine illustrating the benefits of the chosen path for achieving diesel efficiency parity.

33 ADVANCED PROPULSION SYSTEMS↗
NASA NTRS1978

Limb-brightening observations from the OSO-7 satellite. III - Comparison of EUV line intensities of Fe XII, Fe XI, Fe XV, Si X and S XII, Si IX and S XI with predictions

Continuing a study of heliocentric dependence of EUV emission line intensities observed by the Goddard OSO-7 spectroheliograph in 1972, the variation of lines of the ions Fe XII, Fe XI, Fe XV, Si X and S XII, Si IX and S XI is compared with the results of individual calculations for these ions, including theoretical intensities presented for Fe XII and Fe XI. Agreement is found to be good for Fe XII and reasonable for some of the lines of the other ions which in general are weaker in intensity. Several apparent anomalies are found however which may be due to unknown line components near the wavelengths observed.

Kastner, S. O.↗
NASA NTRS1985

New adatom model for Si(11) 7X7 and Si(111)Ge 5X5 reconstructed surfaces

A new adatom model differing from the conventional model by a reconstruction of the substrate is proposed. The new adatom structure provides an explanation for the 7x7 and 5x5 size of the unit cells seen on annealed Si(111) and Si(111)-Ge surfaces, respectively. The model is consistent with structural information from vacuum-tunneling microscopy. It also provides simple explanations for stacking-fault-type features expected from Rutherford backscattering experiments and for similarities in the LEED and photoemission spectra of 2x1 and 7x7 surfaces.

Chadi, D. J.↗
NASA NTRS1990

A technique for determining Urbach edge, midgap states and electric field in a-Si:H and a-(Si,Ge):H devices

A technique for measuring the Urbach energy of valence band tail states and midgap defect densities in a-Si:H and a-(Si,Ge):H devices is described. The Urbach energy is determined by measuring the quantum efficiency (QE) of delocalized holes in the devices, whereas the midgap state density (DOS) is estimated by measuring the QE of localized holes. The distinction between delocalized and localized holes is obtained from the behavior of the QE upon the application of reverse bias to the device. The QE of holes localized in midgap states increases significantly upon the application of reverse bias because of Frenkel-Poole tunneling, whereas the QE of holes in tail states does not show such an increase. It is shown that upon light soaking the Urbach edge does not change, but the midgap DOS does increase significantly. A primary consequence of the increase in DOS is a decrease in electric field in the low-field middle i region of the p-i-n cell. The decrease in electric field is experimentally estimated by fitting the increase in the reverse bias QE to Frenkel-Poole tunneling.

Dalal, Vikram L.↗
NASA NTRS1991

Sputtered Ta-Si-N diffusion barriers in Cu metallizations for Si

Electrical measurements on shallow Si n+-p junction diodes with a 30-nm TiSi2 contacting layer demonstrate that an 80-nm-thick amorphous Ta36Si14N50 film prepared by reactive RF sputtering of a Ta5Si3 target in an Ar/N2 plasma very effectively prevents the interaction between the Si substrate with the TiSi2 contacting layer and a 500-nm Cu overlayer. The Ta36Si14N50 diffusion barrier maintains the integrity of the I-V characteristics up to 900 C for 30-min annealing in vacuum. It is concluded that the amorphous Ta36Si14N50 alloy is not only a material with a very low reactivity for copper, titanium, and silicon, but must have a small diffusivity for copper as well.

Kolawa, E.↗
NASA NTRS1992

Bose-Einstein correlations in Si + Al and Si + Au collisions at 14.6A GeV/c

The E802 Spectrometer at the Brookhaven Alternating Gradient Synchrotron has been used to measure the correlation in relative momentum between like-sign pions emitted in central Si + Al and Si + Au collisions at 14.6A GeV/c. Data are presented in terms of the correlation function for both identified pi(-) and pi(+) pairs near the nucleon-nucleon center-of-mass rapidity. All parametrizations of the correlation function are consistent with a spherically symmetric source of rms radius 3.5 +/- 0.4 fm and lifetime fm/c.

Abbott, T.↗
NASA NTRS1994

Pourous Si(x)Ge(1-x) Layers Within Single Crystals Of Si

Layers of porous Si(x)Ge(1-x) buried within single crystals of Si formed by epitaxial growth of Si/Si(x)Ge(1-x)/Si structures followed by etching in solutions of HF:HNO3:H2O. Electroluminescence from these layers utilized in novel optoelectronic devices.

Fathauer, Robert W.↗
DOE OSTI2021

Pinhole Formation in Poly-Si/SiOx Passivating Contacts on Si(111)-Oriented Textures

Poly-Si/SiOx passivating contacts were grown on inverted pyramid and random pyramid textured wafers to investigate the hypothesis that pinhole formation is most susceptible at the valleys and edges of a textured surface. Tetramethylammonium hydroxide (TMAH) etching and electron beam induced current (EBIC) were used to assess pinhole formation and charge-carrier transport. Scanning electron microscopy revealed that TMAH etching can expose pinhole locations on textured surfaces. We show that pinholes may preferentially form at the vertices of inverted pyramids. Our TMAH and EBIC analyses on random pyramids show no preferential oxide breakup at the pyramid valleys.

41 EE - Solar Energy Technologies Office (EE-4S)↗
NASA NTRS1993

An investigation on the rheological behavior of metallic semi-solid slurries of Al-6.5 pct Si and semi-solid composite slurries of SiC particulates in an Al-6.5 pct Si alloy matrix

The rheology of SiC particulate/Al-6.5 pct Si composite slurries was explored. The rheological behavior of the composite slurries shows both thixotropic and pseudoplastic behaviors. Isostructural experiments on the composite slurries revealed a Newtonian behavior beyond a high shear rate limit. The rheology of fully molten composite slurries over the low to high shear rate range indicates the existence of a low shear rate Newtonian region, an intermediate pseudoplastic region and a high shear rate Newtonian region. The isostructural studies indicate that the viscosity of a composite slurry depends upon the shearing history of a given volume of material. An unexpected shear thinning was noted for SiC particulate + alpha slurries as compared to semi-solid metallic slurries at the same fraction solid. The implications of these findings for the processing of slurries into cast components is discussed.

Moon, H.-K.↗