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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 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Effects of Solidification Cooling Rates on Microstructures and Physical Properties of Fe-6.5%Si Alloys

Compared to the widely used Fe-3.2wt%Si steel, Fe-6.5wt%Si has superior electric and magnetic properties, including higher electrical resistivity, lower iron loss, higher permeability, and near zero magnetostriction. However, Fe-6.5wt%Si sheet is difficult to produce using traditional manufacturing processes as the high silicon content favors the formation of ordered phases that embrittle the material. Fortunately, these ordered phases can be suppressed if the alloy is cooled fast enough from a high temperature kinetically trapping the disordered solid solution or amorphous state. Planar flow casting is known for its rapid solidification rate. In order to consider it as a viable method to manufacture ductile Fe-6.5wt%Si sheets, the effect of cooling rate on physical properties of Fe-6.5wt%Si alloy are systematically investigated. In this work, various cooling rates are achieved by changing melt-spin wheel speeds, which significantly affect the solidification temperature profile and have profound effects on ordering, microstructures, textures, hardness, and magnetic properties. High cooling rates result in refined grains, reduced ordering, enhanced <100> out of the plane texture, decreased hardness, and increased coercivity. This study demonstrates a critical cooling rate at ~1.7 × 10 5 K/s, corresponding to a tangential wheel speed of 5-7 m/s, below which the hardness significantly increases in agreement with the sudden increase of the ordered phases that causes the material embrittlement.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Heat capacity, entropy, formation energy and spin-fluctuation behavior of U 3 Si 5 from 2.4 to 397.4 K

U-Si intermetallic compounds are of considerable interest for their applications as accident-tolerant nuclear fuels. Here we present low-temperature heat capacity (LTHC) measurements of one of the U-Si phases, U 3 Si 5 , using a Quantum Design Physical Properties Measurement System (PPMS) from 2.4 to 397.4 K. We observed an upturn in C p /T (T) below 10 K and have attributed this behavior to potential spin-fluctuations (SF) with an SF temperature (T sf ) of 27 K. An enhancement of LTHC was also observed, as manifested by a large electronic heat capacity coefficient (γ el ) of 342.9 mJ/mol•K 2 . From the heat capacity data, the following thermodynamic parameters were determined: the characteristic Debye temperature (θ D ) over the temperature range 30 – 397 K is 177 ± 2 K, and the standard entropy ($Δ^{298.15}_0$$S^o$) is 283.3 ± 5.7 J•mol -1 •K -1 (equivalent to 35.4 ± 0.7 J•mol -1 •atom -1 •K -1 ). Combined with our previously measured formation enthalpy ($Δ_fH^°_{el}$) of U 3 Si 5 , the Gibbs free energy of formation of U 3 Si 5 from the elements ($Δ_fG^°_{el}$) was determined to be –45.2 ± 9.0 kJ•mol -1 •atom -1 .

36 MATERIALS SCIENCE↗
DOE OSTI2020

Reactive ion etched, self-aligned, selective area poly-Si/SiO 2 passivated contacts

Front/back poly-Si/SiO 2 contact devices suffer from low short-circuit current density, J sc , due to parasitic optical absorption in the front poly-Si layer. Thin poly-Si (~20 nm) allows for high J sc but is not compatible with screen-printed fire-through contacts. We therefore study the effects of post-deposition etching of a thick poly-Si (200 nm) front layer by reactive ion etching (RIE) using the metal grid lines as a self-aligned mask. We show that passivation is maintained in the device during RIE and that J sc is increased by a gain in the blue quantum efficiency response. However, our specific etching parameters cause non-uniform etching of the poly-Si leading to premature loss of passivation without optimal gain in J sc . Etched, unpassivated layers can be re-passivated with a H-containing dielectric layer leading to a gain in Jsc, open circuit voltage, V oc , Fill-Factor, FF, and efficiency.

14 SOLAR ENERGY↗
DOE OSTI2023

In Situ Raman Mapping of Si Island Electrodes and Stress Modeling as a Function of Lithiation and Size

Si is known for cracking and delamination during electrochemical cycling of a battery due to the large volume change associated with Li insertion and extraction. However, it has been found experimentally that patterned Si island electrodes that are 200 nm thick and less than 7 μm wide can deform in a purely elastic manner. Inspired by this, we performed in situ Raman stress characterization of model poly-crystalline Si island electrodes using an electrochemical cell coupled with an immersion objective lens and designed for a short working distance. A 5 μm wide Si island electrode showed a parabolic stress profile during lithiation, while for a 15 μm Si island electrode, a stress plateau in the center of the electrode was observed. A continuum model with coupled electro-chemo-mechanical (ECM) physics was established to understand the stress measurement. A qualitative agreement was reached between modeling and experimental data, and the critical size effect could be explained by the Li diffusive flux as governed by competition between the Li concentration and hydrostatic stress gradients. Below the critical size, the stress gradient drives Li toward the edges, where the electrode volume is free to expand, while above the critical size, the stress plateau inhibits Li diffusion to the edge and forces destructive stress relief by cracking. Furthermore, this work represents a promising methodology for in situ characterization of ECM coupling in battery electrodes, with suggestions provided for further improvement.

25 ENERGY STORAGE↗
DOE OSTI2025

Characterization of dangling bond defects at the crystalline Si/SiO x interface in a polycrystalline Si passivating contact solar cell at room temperature with electrically detected magnetic resonance spectroscopy

Monocrystalline silicon solar cells can achieve photoconversion efficiencies exceeding 26%; however, performance-limiting defects that trap carriers continue to be a challenge. In this work, we have characterized Si solar cells with tunneling SiO x /polycrystalline-Si (poly-Si) passivating contacts (TOPCon) on As-doped Czochralski Si wafers with electrically detected magnetic resonance (EDMR) spectroscopy. We fabricated 2 × 20 mm 2 TOPCon-like mini solar cells with edge passivation alongside larger 4 cm 2 sister cells and obtained similar device characteristics. We performed EDMR spectroscopy at 300 K on two minicells with different degrees of surface passivation based on the recombination parameter, J o , values of 40 and 310 fA/cm2. We optimized the resolution and the signal-to-noise ratio of the EDMR response of the minicells by varying the forward bias voltage and the magnetic field modulation amplitude. We detect two distinct signals with EDMR spectroscopy, an axial-like signal at g = 2.009, 2.0087, and 2.0015, and an isotropic signal at g = 2.0024, which we attribute to Si dangling bonds (P b0 and P b centers) and boron–oxygen related defects, respectively, at or near the c-Si/SiO x interface. The EDMR signals were lower for the cell with a lower value of J o , while the ratio of the two defect populations was very similar. The EDMR signal increases with forward bias but drops to zero at bias voltages >0.5 V, consistent with interface defects within or near the boron-doped emitter depletion region. Our study demonstrates a method to fabricate minicells that can be characterized with EDMR spectroscopy to detect industrially relevant defects in TOPCon cells.

14 SOLAR ENERGY↗
DOE OSTI2022

Evaluating Contributions of Pitch-Carbon Coating to Improved Stability of Si Anodes Through Voltage-Resolved Multi-Phase Characterization

Silicon nanoparticles have emerged as a promising alternative to graphite to improve the energy density of next-generation lithium-ion battery anodes. Nano-sized Si domains facilitate rapid ion transport and minimize particle-scale mechanical degradation, but also exhibit increased (electro)chemical reactivity with Li-ion electrolyte components due to their high surface area. We have previously demonstrated that surface modification of Si nanoparticles with pitch-carbon is an effective strategy to reduce these parasitic reactions. In the present work, we holistically evaluate the mechanistic contribution of pitch-carbon coating to the observed stability improvement over uncoated Si. We utilize coupled in situ and ex situ methods to probe changes to solid-surface, volatile headspace, and gas-phase chemistry occurring during initial cycling. Measurements taken at targeted potentials associated with electrolyte species reduction enables the decoupling of specific reaction pathways tied to interfacial stability. Further, we demonstrate the non-trivial role of gas reconsumption in dictating the nature of the passivating surface layer evolved on both uncoated and pitch-coated Si. This multi-phase analysis offers insights into the mechanism of effective surface passivation, which may be applied to inform future Si material development.

ENERGY STORAGE↗
DOE OSTI2024

Structural, electronic, and optical-absorption properties of 2D Si thin films

Recent experimental studies highlighted the potential of thin-film crystalline silicon (Si) for high-efficiency solar cells. Using density functional theory, we investigated 2D Si thin films across various orientations, thicknesses, and surface structures to elucidate their structure–property relationships. Through surface-energy calculations and Wulff construction, we determined the crystal habit of Si, which aligns with available experimental observations. Electronic-structure calculations underscored the critical role of valence saturation on surfaces in enabling semiconducting behavior in Si thin films, essential for optical applications. From optical-absorption calculations, we identified the surface index exhibiting the highest absorption coefficients for thin films Si solar cell applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Si by Materials Project

Si is Magnesium structured and crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. Si is bonded to twelve equivalent Si atoms to form a mixture of face, edge, and corner-sharing SiSi12 cuboctahedra. There are six shorter (2.70 Å) and six longer (2.75 Å) Si–Si bond lengths.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Si by Materials Project

Si is diamond structured and crystallizes in the cubic Fd-3m space group. The structure is three-dimensional. Si is bonded to four equivalent Si atoms to form corner-sharing SiSi4 tetrahedra. All Si–Si bond lengths are 2.37 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Si by Materials Project

Si is Lonsdaleite structured and crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. Si is bonded to four equivalent Si atoms to form corner-sharing SiSi4 tetrahedra. There are three shorter (2.36 Å) and one longer (2.38 Å) Si–Si bond lengths.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Si by Materials Project

Si is BC8 structured and crystallizes in the cubic Ia-3 space group. The structure is three-dimensional. Si is bonded to four equivalent Si atoms to form corner-sharing SiSi4 trigonal pyramids. There are one shorter (2.34 Å) and three longer (2.39 Å) Si–Si bond lengths.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Si by Materials Project

Si is Copper structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Si is bonded to twelve equivalent Si atoms to form a mixture of face, edge, and corner-sharing SiSi12 cuboctahedra. All Si–Si bond lengths are 2.74 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Si by Materials Project

Si is Hg_xSn structured and crystallizes in the hexagonal P6/mmm space group. The structure is three-dimensional. Si is bonded to eight equivalent Si atoms to form a mixture of edge and corner-sharing SiSi8 hexagonal bipyramids. There are two shorter (2.48 Å) and six longer (2.65 Å) Si–Si bond lengths.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Si by Materials Project

Si is beta Sn structured and crystallizes in the tetragonal I4_1/amd space group. The structure is three-dimensional. Si is bonded in a 6-coordinate geometry to six equivalent Si atoms. There are four shorter (2.50 Å) and two longer (2.65 Å) Si–Si bond lengths.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Si by Materials Project

Si is Theoretical Carbon Structure structured and crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. Si is bonded to four equivalent Si atoms to form a mixture of edge and corner-sharing SiSi4 tetrahedra. There are two shorter (2.36 Å) and two longer (2.39 Å) Si–Si bond lengths.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Si by Materials Project

Si is Tungsten structured and crystallizes in the cubic Im-3m space group. The structure is three-dimensional. Si is bonded in a body-centered cubic geometry to eight equivalent Si atoms. All Si–Si bond lengths are 2.67 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Si by Materials Project

Si crystallizes in the cubic Im-3m space group. The structure is three-dimensional. Si is bonded to four equivalent Si atoms to form a mixture of distorted edge and corner-sharing SiSi4 tetrahedra. All Si–Si bond lengths are 2.38 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Si by Materials Project

Si crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. Si is bonded to eight equivalent Si atoms to form a mixture of distorted edge, corner, and face-sharing SiSi8 hexagonal bipyramids. There are a spread of Si–Si bond distances ranging from 2.52–2.65 Å.

36 MATERIALS SCIENCE↗