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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 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Ultrafast Shock-Induced Reactions in Pentaerythritol Tetranitrate Thin Films

The chemical and physical processes involved in the shock-to-detonation transition of energetic solids are not fully understood due to difficulties in probing the fast dynamics involved in initiation. Here, we employ shock interferometry experiments with sub-20-ps time resolution to study highly textured (110) pentaerythritol tetranitrate (PETN) thin films during the early stages of shock compression using ultrafast laser-driven shock wave methods. We observe evidence of rapid exothermic chemical reactions in the PETN thin films for interface particle velocities above ~1.05 km/s as indicated by shock velocities and pressures well above the unreacted Hugoniot. The time scale of our experiment suggests that exothermic reactions begin less than 50 ps behind the shock front for these high-density PETN thin films. Thermochemical calculations for partially reacted Hugoniots also support this interpretation. Finally, the experimentally observed time scale of reactivity could be used to narrow possible initiation mechanisms.

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

Quantifying the Ligand-Induced Triplet Energy Transfer Barrier in a Quantum Dot-Based Upconversion System

During photon upconversion, quantum dots (QDs) transfer energy to molecules in solution through a long ligand shell. This insulating ligand shell imparts colloidal stability at the expense of efficient photosensitization. For the first time, we quantify the barrier these aliphatic ligands pose for triplet energy transfer in solution. Using transient absorption spectroscopy, we experimentally measure a small damping coefficient of 0.027 Å −1 for a ligand exceeding 10 carbons in length. The dynamic nature of ligands in solution lowers the barrier to charge or energy transfer compared to organic thin films. In addition, we show that surface ligands shorter than 8 carbons in length allow direct energy transfer from the QD, bypassing the need for a transmitter ligand to mediate energy transfer, leading to a 6.9% upconversion quantum yield compared with 0.01% for ligands with 18 carbons. Furthermore, this experimentally derived insight will enable the design of efficient QD-based photosensitizers for catalysis and energy conversion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

In Situ Probing Potassium-ion Intercalation-induced Amorphization in Crystalline Iron Phosphate Cathode Materials

Na-ion and K-ion batteries are promising alternatives for large-scale energy storage applications due to their abundance and low cost. Intercalation of these large ions could cause irreversible structural deformation and partial to complete amorphization in the crystalline electrodes. The designing of new amorphous electrodes is another route to develop electrodes to store these ions reversibly. Lack of understanding of dynamic changes in the amorphous nanostructures during battery operation is the bottleneck for further developments. Here, we report the utilization of in situ digital image correlation and in-operando X-ray diffraction (XRD) techniques to probe dynamic changes in the amorphous phase of iron phosphate during potassium intercalation. In-operando XRD demonstrates amorphization in the electrode’s nanostructure during the first charge/discharge cycle. Additionally, the ex-situ high-resolution transmission electron microscopy further confirms the amorphization after potassium insertion. In situ strain analysis detects the reversible deformation associated with redox reactions in the amorphous phases. Our approach offers new insights on the mechanisms of ion intercalation in the amorphous nanostructures which are highly potent for development of next-generation batteries.

Ozdogru, Bertan↗
DOE OSTI2016

Interfacial Coupling-Induced Ferromagnetic Insulator Phase in Manganite Film

Interfaces with subtle differences in atomic and electronic structures in perovskite ABO 3 heterostructures often yield intriguingly different properties, yet their exact roles remain elusive. Here, we report an integrated study of unusual transport, magnetic, and structural properties of Pr 0.67 Sr 0.33 MnO 3 film on SrTiO 3 substrate. The variations in the out-of-plane lattice constant and BO 6 octahedral rotation across the Pr 0.67 Sr 0.33 MnO 3 /SrTiO 3 interface strongly depend on the thickness of the Pr 0.67 Sr 0.33 MnO 3 film. In the 12-nm film, a new interface-sensitive ferromagnetic polaronic insulator (FI') phase is formed during the cubic-to-tetragonal phase transition of SrTiO 3 , apparently due to the enhanced electron–phonon interaction and atomic disorder in the film. The transport properties of the FI' phase in the 30-nm film are masked because of the reduced interfacial coupling and smaller interface-to-volume ratio. In conclusion, this work demonstrates how thickness-dependent interfacial coupling leads to the formation of a theoretically predicted ferromagnetic–polaronic insulator, as illustrated in a new phase diagram, that is otherwise ferromagnetic metal (FM) in bulk form.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Reduced Recombination and Improved Performance of CdSe/CdTe Solar Cells due to Cu Migration Induced by Light Soaking

The performance of CdTe solar cells has advanced impressively in recent years with the incorporation of Se. Instabilities associated with light soaking and copper reorganization have been extensively examined for the previous generation of CdS/CdTe solar cells, but instabilities in Cu-doped Se-alloyed CdTe devices remain relatively unexplored. In this work, we fabricated a range of CdSe/CdTe solar cells by sputtering CdSe layers with thicknesses of 100, 120, 150, 180, and 200 nm on transparent oxide-coated glass and then depositing CdTe by close-spaced sublimation. After CdCl 2 annealing, Cu-doping, and back metal deposition, a variety of analyses were performed both before and after light soaking to understand the changes in device performance. The device efficiency was degraded with light soaking in most cases, but devices fabricated with a CdSe layer thickness of 120 nm showed reasonably good efficiency initially (13.5%) and a dramatic improvement with light soaking (16.5%). Here, the efficiency improvement is examined within the context of Cu ion reorganization that is well known for CdS/CdTe devices. Low-temperature photoluminescence data and V oc versus temperature measurements indicate a reduction in nonradiative recombination due to the passivation of defects and defect complexes in the graded CdSe x Te 1–x layer.

CdSexTe1−x↗
DOE OSTI2019

SO 2 -Induced Selectivity Change in CO 2 Electroreduction

Electrochemical conversion of carbon dioxide (CO 2 ) to value-added chemicals has attracted much attention in recent years as a potential alternative to fossil resources. Although significant works have studied the influence of impurities in the electrolyte (e.g. metal ions), few studies have been performed to understand the influence of gaseous impurities in CO 2 electroreduction. Herein, we study the effects of sulfur dioxide (SO 2 ) on Ag-, Sn-, and Cu-catalyzed CO 2 electrolysis in a flow-cell electrolyzer in near-neutral electrolyte, representing a broad range of CO 2 reduction catalysts. We show that the presence of SO 2 impurity reduces the efficiency of converting CO 2 due to the preferential reduction of SO 2 . In the case of Ag and Sn, the effect of SO 2 impurity was reversible and the catalytic activities of both catalysts were recovered. On the contrary, a shift in selectivity toward formate accompanied with a suppression of multi-carbon (C 2+ ) products was observed on Cu catalyst, demonstrating that Cu is highly sensitive to SO 2 impurity. Furthermore, our results suggest that CO 2 obtained from direct air capture technologies or biorefineries could be more suitable for Cu-catalyzed CO 2 electrolysis as these CO 2 sources would be relatively cleaner (SO 2 -free) than fossil-derived sources such as power plants and can be directly coupled with distributed renewable energy sources such as wind and solar.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗