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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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Five Rules for Measuring Biomass Pyrolysis Rates: Pulse-Heated Analysis of Solid Reaction Kinetics of Lignocellulosic Biomass
Pyrolytic conversion of lignocellulosic biomass utilizes high temperatures to thermally fragment biopolymers to volatile organic compounds. The complexity of the degradation process includes thousands of reactions through multiple phases occurring in less than a second. In this work, the requirements are established for measuring the reaction kinetics of high temperature (>400 °C) biomass pyrolysis in the absence of heat and mass transfer limitations. Additionally, experimental techniques must heat and cool biomass samples sufficiently fast to elucidate the evolution of reaction products with time while also eliminating a substantial reaction during the heating and cooling phases, preferably by measuring the temperature of the reacting biomass sample directly. Furthermore, these requirements are described with the PHASR (pulse-heated analysis of solid reactions) technique and demonstrated by measuring the time-resolved evolution of six major chemical products from loblolly pine pyrolysis over a temperature range of 400 to 500 °C. Differential kinetics of loblolly pine pyrolysis are measured to determine the apparent activation energy for the formation of six major product compounds including levoglucosan, furfural, and 2-methoxyphenol.
Defining Rules for the Shape Evolution of Gold Nanoparticles
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Tuning the Redox Activity of Metal–Organic Frameworks for Enhanced, Selective O 2 Binding: Design Rules and Ambient Temperature O 2 Chemisorption in a Cobalt–Triazolate Framework
Metal–organic frameworks (MOFs) with coordinatively unsaturated metal sites are appealing as adsorbent materials due to their tunable functionality and ability to selectively bind small molecules. Through the use of computational screening methods based on periodic density functional theory, we investigate O 2 and N 2 adsorption at the coordinatively unsaturated metal sites of several MOF families. A variety of design handles are identified that can be used to modify the redox activity of the metal centers, including changing the functionalization of the linkers (replacing oxido donors with sulfido donors), anion exchange of bridging ligands (considering μ-Br – , μ-Cl – , μ-F – , μ-SH – , or μ-OH – groups), and altering the formal oxidation state of the metal. As a result, we show that it is possible to tune the O 2 affinity at the open metal sites of MOFs for applications involving the strong and/or selective binding of O 2 . In contrast with O 2 adsorption, N 2 adsorption at open metal sites is predicted to be relatively weak across the MOF dataset, with the exception of MOFs containing synthetically elusive V 2+ open metal sites. As one example from the screening study, we predicted that exchanging the μ-Cl – ligands of M 2 Cl 2 (BBTA) (H 2 BBTA = 1H,5H-benzo(1,2-d:4,5-d')bistriazole) with μ-OH – groups would significantly enhance the strength of O 2 adsorption at the open metal sites without a corresponding increase in the N 2 affinity. Experimental investigation of Co 2 Cl 2 (BBTA) and Co 2 (OH) 2 (BBTA) confirms that the former exhibits weak physisorption of both N 2 and O 2 , whereas the latter is capable of chemisorbing O 2 at room temperature in a highly selective manner. The O 2 chemisorption behavior is attributed to the greater electron-donating character of the μ-OH – ligands and the presence of H-bonding interactions between the μ-OH – bridging ligands and the reduced O 2 adsorbate.
Topology-Informed Design Rules for Deconstructable Thermoset Copolymer Networks
Existing models of thermoset deconstruction facilitated by incorporating cleavable comonomers rely on a mean-field reverse gel point paradigm, which predicts network dissolution once cleavable bonds reach a critical stoichiometric threshold, but does not account for where those bonds reside within the network architecture. Using reactive coarse-grained molecular dynamics simulations coupled with graph-theoretic analysis, we extend this stoichiometric picture to show that deconstructability is governed by the curing-imprinted network topology rather than stoichiometry alone. This topological organization is hierarchical: at the local scale, the elastic effectiveness of cross-link junctions determines which cross-links constitute the load-bearing scaffold; at the mesoscale, the cross-linking rate kinetically templates that scaffold into topologically modular communities─densely cross-linked clusters connected by sparse bridging strands that sustain network connectivity. Using betweenness centrality to identify nodes that disproportionately lie on intercommunity shortest paths, we demonstrate that effective deconstruction of the network into macromolecular fragments requires cleavable comonomers to intercept these high-centrality bridging strands. We further find that under uniform, disassortative comonomer incorporation, this topological requirement provides a mechanistic basis for extending the reverse gel point to incorporate network topology. We also show that modularity imposes a fundamental limit on fragment uniformity that persists even when the centrality requirement is met. Finally, we demonstrate that chain stiffness provides a nearly independent lever to suppress mechanically redundant cross-links and raise the glass transition temperature without significantly altering the deconstruction outcome. Together, these findings reframe the thermoset design space around network topology and provide actionable guidelines for engineering thermoset copolymers with predictable deconstructability and targeted thermomechanical performance.
Avoiding Steric Congestion in Dendrimer Growth through Proportionate Branching. A Twist on da Vinci's Rule of Tree Branching
In making defect-free macromolecules, the challenge occurs during chemical synthesis. This challenge is especially pronounced in dendrimer synthesis where exponential growth quickly leads to steric congestion. To overcome this difficulty, proportionate branching in dendrimer growth is proposed. In proportionate branching, both the number and the length of branches increase exponentially but in opposite directions to mimic tree growth. The effectiveness of this strategy is demonstrated through the synthesis of a fluorocarbon dendron containing 243 chemically identical fluorine atoms with a MW of 9082 Da. Monodispersity is confirmed by nuclear magnetic resonance spectroscopy, mass spectrometry, and small-angle X-ray scattering. Moreover, growing different parts proportionately, as nature does, could be a general strategy to achieve defect-free synthesis of macromolecules.
Calculation from First Principles of Intramolecular Golden-Rule Rate Constants for Photo-Induced Electron Transfer in Molecular Donor–Acceptor Systems
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One Stomatal Model to Rule Them All? Toward Improved Representation of Carbon and Water Exchange in Global Models
Abstract Stomatal conductance schemes that optimize with respect to photosynthetic and hydraulic functions have been proposed to address biases in land‐surface model (LSM) simulations during drought. However, systematic evaluations of both optimality‐based and alternative empirical formulations for coupling carbon and water fluxes are lacking. Here, we embed 12 empirical and optimization approaches within a LSM framework. We use theoretical model experiments to explore parameter identifiability and understand how model behaviors differ in response to abiotic changes. We also evaluate the models against leaf‐level observations of gas‐exchange and hydraulic variables, from xeric to wet forest/woody species spanning a mean annual precipitation range of 361–3,286 mm yr −1 . We find that models differ in how easily parameterized they are, due to: (a) poorly constrained optimality criteria (i.e., resulting in multiple solutions), (b) low influence parameters, (c) sensitivities to environmental drivers. In both the idealized experiments and compared to observations, sensitivities to variability in environmental drivers do not agree among models. Marked differences arise in sensitivities to soil moisture (soil water potential) and vapor pressure deficit. For example, stomatal closure rates at high vapor pressure deficit range between −45% and +70% of those observed. Although over half the new generation of stomatal schemes perform to a similar standard compared to observations of leaf‐gas exchange, two models do so through large biases in simulated leaf water potential (up to 11 MPa). Our results provide guidance for LSM development, by highlighting key areas in need for additional experimentation and theory, and by constraining currently viable stomatal hypotheses.
Realizing a SnO2-Based Ultraviolet Light-Emitting Diode via Breaking the Dipole-Forbidden Rule
Abstract not provided
Quantitative kinetic rules for plastic strain-induced α - ω phase transformation in Zr under high pressure
Plastic strain-induced phase transformations (PTs) and chemical reactions under high pressure are broadly spread in modern technologies, friction and wear, geophysics, and astrogeology. However, because of very heterogeneous fields of plastic strain $E$ p and stress σ tensors and volume fraction c of phases in a sample compressed in a diamond anvil cell (DAC) and impossibility of measurements of σ and $E$ p , there are no strict kinetic equations for them. Here, we develop a kinetic model, finite element method (FEM) approach, and combined FEM-experimental approaches to determine all fields in strongly plastically predeformed Zr compressed in DAC, and specific kinetic equation for α-ω PT consistent with experimental data for the entire sample. Since all fields in the sample are very heterogeneous, data are obtained for numerous complex 7D paths in the space of 3 components of the plastic strain tensor and 4 components of the stress tensor. Kinetic equation depends on accumulated plastic strain (instead of time) and pressure and is independent of plastic strain and deviatoric stress tensors, i.e., it can be applied for various above processes. Our results initiate kinetic studies of strain-induced PTs and provide efforts toward more comprehensive understanding of material behavior in extreme conditions.
Universal chemomechanical design rules for solid-ion conductors to prevent dendrite formation in lithium metal batteries
Dendrite formation during electrodeposition while charging lithium metal batteries compromises their safety. Although high-shear-modulus ( G s ) solid-ion conductors (SICs) have been prioritized to resolve the pressure-driven instabilities that lead to dendrite propagation and cell shorting, it is unclear whether these or alternatives are needed to guide uniform lithium electrodeposition, which is intrinsically density-driven. Here, we show that SICs can be designed within a universal chemomechanical paradigm to access either pressure-driven dendrite-blocking or density-driven dendrite-suppressing properties, but not both. This dichotomy reflects the competing influence of the SIC’s mechanical properties and the partial molar volume of Li + $(V_{\mathrm{Li}^+})$ relative to those of the lithium anode ( G Li and V Li ) on plating outcomes. Within this paradigm, we explore SICs in a previously unrecognized dendrite-suppressing regime that are concomitantly ‘soft’, as is typical of polymer electrolytes, but feature an atypically low $(V_{\mathrm{Li}^+})$ that is more reminiscent of ‘hard’ ceramics. Li plating (1 mA cm -2 ; T = 20 °C) mediated by these SICs is uniform, as revealed using synchrotron hard X-ray microtomography. As a result, cell cycle life is extended, even when assembled with thin Li anodes (~30 µm) and either high-voltage NMC-622 cathodes (1.44 mAh cm -2 ) or high-capacity sulfur cathodes (3.02 mAh cm -2 ).
Molecular basis of GID4-mediated recognition of degrons for the Pro/N-end rule pathway
Abstract not provided
Data mining with molecular design rules identifies new class of dyes for dye-sensitised solar cells
Big data science informs energy research: large-scale screening of crystal structures identifies unforeseen class of dyes for dye-sensitised solar cells.
Rules for the design of aza-glycine stabilized triple-helical collagen peptides
Guidelines for incorporating aza-glycine residues in collagen peptides are presented, detailing their effects on triple-helical thermal stability.
Metallo-boranes: a class of unconventional superhalogens defying electron counting rules
A class of unconventional superhalogens by doping closo -boranes with selected metal atoms.