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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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Tuning Phonon Energies in Lanthanide‐doped Potassium Lead Halide Nanocrystals for Enhanced Nonlinearity and Upconversion

Abstract Optical applications of lanthanide‐doped nanoparticles require materials with low phonon energies to minimize nonradiative relaxation and promote nonlinear processes like upconversion. Heavy halide hosts offer low phonon energies but are challenging to synthesize as nanocrystals. Here, we demonstrate the size‐controlled synthesis of low‐phonon‐energy KPb 2 X 5 (X=Cl, Br) nanoparticles and the ability to tune nanocrystal phonon energies as low as 128 cm −1 . KPb 2 Cl 5 nanoparticles are moisture resistant and can be efficiently doped with lighter lanthanides. The low phonon energies of KPb 2 X 5 nanoparticles promote upconversion luminescence from higher lanthanide excited states and enable highly nonlinear, avalanche‐like emission from KPb 2 Cl 5 : Nd 3+ nanoparticles. The realization of nanoparticles with tunable, ultra‐low phonon energies facilitates the discovery of nanomaterials with phonon‐dependent properties, precisely engineered for applications in nanoscale imaging, sensing, luminescence thermometry and energy conversion.

Zhang, Zhuolei↗
DOE OSTI2019

Interplay between Short and Long-Ranged Forces Leading to the Formation of Ag Nanoparticle Superlattice

Nanoparticle (NP) superlattices have attracted increasing attention due to their unique properties. However, key questions remain for predicting the nanoparticle behavior from interparticle forces, especially the role of hydration force (Fhyd) and its correlation to other forces, regarding the transient and final superlattice configurations. Here we investigated the self-assembly of Ag NPs in aqueous solution by employing in situ liquid cell transmission electron microscopy, combined with atomic force microscopy based force measurements and theoretical calculations. In spite of instantaneous Brownian motion of NPs, a correlation between velocity and van der Waals force (FvdW) on average was observed during the later self-assembly process. In the post-assembly state, a delicate balance between Fhyd and FvdW gives rise to ~ 0.6~0.9 nm separation be-tween NPs in the superlattice. Our study demonstrates pivotal roles of interparticle forces, enabling the control of the separation which is critical for tailoring the properties of superlattices.

Lee, Jaewon↗
DOE OSTI2024

Intermittent Criticality Multi‐Scale Processes Leading to Large Slip Events on Rough Laboratory Faults

Abstract We discuss data of three laboratory stick‐slip experiments on Westerly Granite samples performed at elevated confining pressure and constant displacement rate on rough fracture surfaces. The experiments produced complex slip patterns including fast and slow ruptures with large and small fault slips, as well as failure events on the fault surface producing acoustic emission bursts without externally‐detectable stress drop. Preparatory processes leading to large slips were tracked with an ensemble of ten seismo‐mechanical and statistical parameters characterizing local and global damage and stress evolution, localization and clustering processes, as well as event interactions. We decompose complex spatio‐temporal trends in the lab‐quake characteristics and identify persistent effects of evolving fault roughness and damage at different length scales, and local stress evolution approaching large events. The observed trends highlight labquake localization processes on different spatial and temporal scales. The preparatory process of large slip events includes smaller events marked by confined bursts of acoustic emission activity that collectively prepare the fault surface for a system‐wide failure by conditioning the large‐scale stress field. Our results are consistent overall with an evolving process of intermittent criticality leading to large failure events, and may contribute to improved forecasting of large natural earthquakes.

Geochemistry & Geophysics↗
DOE OSTI2018

Low cycle fatigue behavior of AlSi10Mg(Cu) alloy at high temperature

Highlights: • The cyclic stress response of AlSi10Mg(Cu) alloy in LCF regime is temperature-dependent. • Precipitates transitions were observed after high temperature cycle loading. • Changes in the interaction mechanism between dislocations and nanoparticles lead to different cyclic stress responses. - Abstract: Low cycle fatigue (LCF) behavior of AlSi10Mg(Cu) alloy was investigated at 100 °C and 250 °C over a range of strain amplitude from 0.25% to 0.40%. Cycle stress response, hysteresis loops, fatigue life, as well as post-fatigue microstructure were studied in detail. Except 0.25%, the alloy performed cycle hardening behavior at 100 °C and softening behavior at 250 °C at any given strain amplitude, which corresponded well with the observed phenomena in hysteresis loops. At any given temperature, cyclic stress increases while fatigue life decreases with the increasing strain amplitude. The fatigue life of the studied alloy at 100 °C was higher than that at 250 °C under the same strain amplitude. Fractography indicated that most fatigue crack initiated from casting defects. The fatigue crack propagation was mainly characterized by fatigue striations and tear ridges in any conditions. Section inspection shows that the number of cracked eutectic Si increases with decreasing the temperature or increasing strain amplitude. Post-mortem transmission electron microscopy (TEM) investigations indicated that cyclic hardening behavior at 100 °C was due to the pinning effect of coherent precipitates (β″-Mg{sub 2}Si and GP zones). Most of tiny coherency precipitates and clusters transformed to large semi-coherency Q′ and β′-Mg{sub 2}Si precipitates after cyclic loading at 250 °C, leading to the sharp decrease of pinning number and dislocation density, which is a key reason for the cyclic softening. Another factor is that more dislocations can be activated and dislocation climb is easier at higher temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Methyl formate oxidation kinetics up to 100 atm

Methyl formate (MF, CH3OCHO), the simplest ester, is a representative oxygenated fuel with high oxygen content, and low sooting tendency. However, its oxidation behavior under high-pressure and intermediate-temperature conditions remains insufficiently understood, especially where low-temperature peroxy radical chemistry, methanol chemistry, and pressure-dependent reaction pathways play a critical role. In this study, MF oxidation experiments were conducted in the Princeton supercritical-pressure jet-stirred reactor (SP-JSR) at 20 and 100 atm over the temperature range of 400–950 K under both fuel-lean and fuel-rich conditions. Based on the experimental results, an updated HP-Mech was developed by incorporating previous MF sub-mechanisms, expanded low-temperature peroxy pathways, and evaluated pressure-dependent decomposition kinetics. The newly updated HP-Mech shows greatly improved performance in predicting the onset temperature, the key intermediate species fractions, methanol formation, and the progression of MF oxidation across all the experimental conditions. Path flux analysis indicates that MF consumption at the onset stage is dominated by H-abstraction at the methyl site, forming CH2OCHO radicals that lead to the formation and isomerization of O2CH2OCHO, driving low-temperature chain propagation. Moreover, H-abstraction at the formate site forms CH3OCO radicals that preferentially decompose to CH3, initiating the methanol formation pathway linked to CH3O2 and HO2 chemistry. At the same time, HO2 formation is strongly coupled to MF oxidation through multiple MF-derived radical pathways. HCO originates from MF oxidation and acts as a key coupling species linking fuel consumption to HO2 buildup, especially under high-pressure and intermediate-temperature conditions. In addition to this dominant channel, supplementary HO2 formation pathways involving CH3, CH3O, CH2OH, and CH3O2 reacting with O2 further connect methanol chemistry and oxygenated radical chemistry to the HO2 pool, indicating the central role of HO2 in governing MF oxidation. Sensitivity analysis identifies MF with OH/HO2/CH3O2 reactions and the HO2/H2O2/OH sequence as the key factors controlling reactivity in the high-pressure and intermediate-temperature regime. MF directly reacts with OH/HO2/CH3O2 to consume the fuel and produce reactive radicals like CH2OCHO and CH3OCO that undergo subsequent oxidation pathways. Moreover, HO2 recombination suppresses oxidation at lower temperatures, while thermal decomposition of H2O2 accelerates OH production and promotes fuel consumption as temperature increases. The direct formation of active OH from HO2 radicals further completes the mechanism, improving its prediction especially during the oxidation onset stage.

Low-temperature Chemistry↗