SEARCH · Engineering Papers
Results for “controller”
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.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Hydrogen Bonding Controls the Structural Evolution in Perovskite-Related Hybrid Platinum(IV) Iodides
In this paper, we describe the solid-state structural evolution in four hybrid hexaiodoplatinate(IV) compounds, demonstrating the increasingly important role that extended hydrogen bonding plays in directing the structure across the series. The compounds are A 2 PtI 6 , where A is one of the following amines: ammonium, NH 4 + ; methylammonium, CH 3 NH 3 + ; formamidinium, CH(NH 2 ) 2 + ; guanidinium, C(NH 2 ) 3 + . These are closely related in structure and properties to the hybrid halide perovskites of lead(II) that have recently established their prowess in optoelectronics. The first three of these compounds crystallize in the vacancy-ordered double perovskite A 2 Pt$\square$I 6 ($\square$ indicates a vacant site) structure in the K 2 PtCl 6 archetype, despite the relatively large perovskite tolerance factors involved. The last compound, (GUA) 2 PtI 6 , crystallizes in a vacancy-ordered variant of the hexagonal CsNiCl 3 structure: the K 2 MnF 6 structure. A combination of solid-state 195 Pt and 1 H NMR spectroscopy and detailed density functional theory calculations helps to reveal structural trends and establish the hydrogen-bonding tendencies. Finally, the calculations and measured optical properties support the surprising observation in these iodosalt compounds that, for smaller A cations, the conduction bands are considerably disperse, despite lacking extended I–Pt–I connectivity.
Transient Symmetry Controls Photo Dynamics near Conical Intersections
Excited-state chemistry lacks generalised symmetry rules. With many femtochemistry studies focused on individual cases, it is hard to build up the same level of chemical intuition for excited-states as exists for ground states. Here, we unravel the degrees of freedom involved in ultrafast internal conversion (IC) by mapping the vibrational coherence of the initial wavepacket, and the dependence on molecular symmetry in various cyclic tertiary amines. Molecular symmetry plays an important role in the preservation of vibrational coherence in the transit from one electronic state to another. We show here that it is sufficient for the molecule to simply have the possibility of a more symmetric structure to achieve the preservation of vibrational coherence. It can be transient and still lead to preservation. Furthermore, this finding provides an additional angle on how symmetry influences electronic transitions and an additional piece to the puzzle of establishing symmetry-based selection rules for excited-state processes.
Toward Controlled Growth of Helicity-Specific Carbon Nanotubes
Explore the source record for details and available documents.
Manipulating Triplet Yield through Control of Symmetry-Breaking Charge Transfer
The efficiency of an organic solar cell depends on the efficacy of exciton diffusion and dissociation processes, and this can be enhanced by reducing the exciton binding energy and increasing the exciton lifetime. Zinc chlorodipyrrin (ZCl) complexes exhibit reduced exciton binding energy due to ultrafast generation of intramolecular charge transfer (ICT) states via symmetry breaking charge transfer in polar media. This paper explores the fate of the ICT states using nanosecond transient absorption. In cyclohexane, ZCl undergoes intersystem-crossing to produce triplets with ~8 ns time constant (~30% yield) and no ICT states are generated. However, in more polar solvents, triplets are generated within 1 ns via ICT state recombination with ~3 times higher yield than produced via ISC. Furthermore, this high triplet yield in toluene (89%) and acetonitrile (76%) via ICT state recombination is a beneficial pathway to spin-protect the excited state decay for additional charge generation from triplet excited states.
Tunable Polaron Distortions Control the Extent of Halide Demixing in Lead Halide Perovskites
Photoinduced phase separation in mixed halide perovskites emerges from their electro-mechanical properties and high ionic conductivities, resulting in photoinduced I$^-$ rich charge carrier traps that diminish photovoltaic performance. Whether photoinduced phase separation stems from the polycrystalline microstructure or is an intrinsic material property has been an open question. We investigate the nanoscale photoinduced behavior of single-crystal mixed Br$^-$/I$^-$ methylammonium (MA + ) lead halide perovskite (MAPb(Br x I 1- x ) 3 ) nanoplates, eliminating effects from extended structural defects. Even in these nanoplates, we find that phase separation occurs, resulting in I$^-$rich clusters that are nucleated stochastically and stabilized by polarons. Upon lowering the electron-phonon coupling strength by partially exchanging MA + for Cs + , a phase-separated steady state is not reached, nevertheless transient I$^-$ clustering still occurs. Our results, supported by multiscale modeling, demonstrate that photoinduced phase separation is an intrinsic property of mixed halide perovskites, the extent and dynamics of which depends on the electron-phonon coupling strength.
Countercations Control Local Specific Bonding Interactions and Nucleation Mechanisms in Concentrated Water-in-Salt Solutions
One of the continuing challenges presented in aqueous salt solutions is to understand ion association reactions driving dynamic demixing from solvation, complexation and solute clustering. The problems understanding this phenomenon are exacerbated in the highly-concentrated water-in-salt electrolytes, where the deficiency of water leads to a dramatic retardation of solvating water and formation of extended solvent-solute clustering networks. By probing microscopic dynamics of water and prenucleation clusters using quasi-elastic neutron scattering and proton nuclear magnetic resonance spectroscopy, we observed contrasting mechanistic specifics of ion-water mobilities in highly-concentrated Na+- vs. K+-based aluminate solutions (diffusion coefficients of 0.2 vs. 2.6?10-10m2s-1 at 293 K, respectively). The magnitude of the differences is far beyond the kosmotropic (Na+) and chaotropic (K+) effects typically understood at low ionic concentrations. The distinct crystallization mechanisms observed in analogous deuterated systems further imply that different prenucleation cluster dynamics and network packing morphologies can either frustrate or promote crystallization, as described by non-classical nucleation theory.
Temperature-activated PEG surface segregation controls the protein repellency of polymers
Abstract not provided
Bridging-Controlled Network Microstructure and Long-Wavelength Fluctuations in Silica-Poly(2-vinylpyridine) Nanocomposites: Experimental Results and Theoretical Analysis
Explore the source record for details and available documents.
Nickel Bis-Diphosphine Complexes: Controlling the Binding and Heterolysis of H2
Two new Ni(II) complexes, the homoleptic [Ni-(8PCy2NH)2]2+ complex (8PCy 2NH = 3,7-cyclohexyl-1-amino-3,7-diphosphacyclooctane) containing two pendant amines on adjacent ligands, and the heteroleptic [Ni(8PCy2NH)(dppe)]2+ complex (dppe = bis-(diphenylphosphino)ethane) containing only a single pendant amine, have been synthesized, and their electrochemical properties are reported. The [Ni(8PCy2NH)(dppe)]2+ complex is capable of heterolytically cleaving hydrogen, allowing for the first observation of an endoprotonated nickel hydride related to the [Ni(PR2NR'2)2]2+ family of complexes. The [Ni(8PCy2NH)(dppe)]2+ complex did not exhibit electrocatalytic H2 oxidation activity; however, [Ni(8PCy2NH)2]2+ is an active electrocatalyst for H2 oxidation with a maximum turnover frequency (kobs) of 18 s-1 under 1 atm H2 at Ecat/2 = -0.71 V versus the ferrocenium/ferrocene (Cp2Fe+/0) couple. In addition to an analysis of the effect of the number of pendant amines on the rates of electrocatalytic H2 oxidation of compounds related to the [Ni(PR 2NR'2)2]2+ family, the effect of the secondary pendant amine on intermolecular deprotonation is discussed.
Support Induced Control of Surface Composition in Cu–Ni/TiO 2 Catalysts Enables High Yield Co-Conversion of HMF and Furfural to Methylated Furans
5-(hydroxymethyl)furfural (HMF) and furfural (FF) have been confirmed as valuable biomass-derived fuel precursors suitable for catalytic hydrodeoxygenation (HDO) to produce high octane fuel additives such dimethyl furan (DMF) and methyl furan (MF), respectively. In order to realize economically viable production of DMF and MF from biomass, catalytic processes with high yields, low catalyst costs, and process simplicity are needed. In this work, we demonstrate simultaneous co-processing of HMF and FF over Cu-Ni/TiO 2 catalysts, achieving 87.5% yield of DMF from HMF and 88.5% yield of MF from FF in a one pot reaction. The Cu-Ni/TiO 2 catalyst also exhibited improved stability and regeneration compared to Cu/TiO 2 and Cu/Al 2 O 3 catalysts for FF HDO, exhibiting a ~7% loss in FF conversion over 4 sequential recycles, compared to a ~50% loss in FF coversion for Cu/Al 2 O 3 and a ~30% loss in coversion for Cu/TiO 2 . Characterization of the Cu-Ni/TiO 2 catalyst by X-ray Photoelectron Spectroscopy, Scanning Transmission Electron Microscopy, and H 2 -Temperature Programmed Reduction and comparison to monometallic Cu and Ni on Al 2 O 3 and TiO 2 and bimetallic Cu-Ni/Al 2 O 3 catalysts suggest that the unique reactivity and stability of Cu-Ni/TiO 2 derives from support-induced metal segregation in which Cu is selectively enriched at the catalyst surface, while Ni is enriched at the TiO 2 interface. Our findings demonstrate that Cu-Ni/TiO 2 catalysts promise to be a catalyst system capable of integrating directly with a combined HMF and FF product stream from biomass processing to realize lower cost production of liquid fuels from biomass.
Electrostatic Control of Shape Selection and Nanoscale Structure in Chiral Molecular Assemblies
Not Available
Plasma-Catalyst Reactivity Control of Surface Nitrogen Species through Plasma-Temperature-Programmed Hydrogenation to Ammonia
Nonthermal plasma activation of N 2 can facilitate nitrogen adsorption on metal catalysts at low bulk temperatures and atmospheric pressure. Here, we apply a plasma-assisted temperature-programmed reaction (plasma-TPRxn) for ammonia (NH 3 ) synthesis using sequential exposure of a silica-supported metal catalyst to N 2 plasma followed by thermal hydrogen treatment while ramping the temperature to decouple the plasma activation of N 2 from surface catalyzed hydrogenation steps. This approach eliminates the effects from bulk plasma phase reactions, thereby allowing for direct interrogation of plasma activated nitrogen on the active metal surfaces. We confirm previously reported spectroscopic observations that show plasma-generated surface nitrogen can be converted to NH 3 through surface catalyzed pathways. Further, we demonstrate that the ammonia desorption peak temperature is sensitive to metal, with Pt desorbing NH 3 at the lowest temperature. Unsteady state microkinetic models of desorption kinetics as a function of initial N coverage and metal recover observed trends in NH 3 desorption temperatures and confirm that observed results reflect hydrogenation of plasma-induced N accommodation at each surface. In total, we show that the hydrogenation ability of the catalyst after plasma activation of N 2 is responsible for the reactivity trends observed in plasma-assisted NH 3 synthesis.
Hydration Control of the Mechanical and Dynamical Properties of Cellulose
Explore the source record for details and available documents.
Na-Si Clathrates Are High-Pressure Phases: A Melt-Based Route to Control Stoichiometry and Properties
Explore the source record for details and available documents.
Ligand-Controlled Colloidal Synthesis and Electronic Structure Characterization of Cubic Iron Pyrite (FeS2) Nanocrystals
Explore the source record for details and available documents.