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Caltech Authors (147.820 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 1.643

  1. Outstanding hydrogen evolution reaction catalyzed by porous nickel diselenide electrocatalysts

    Zhou, Haiqing; Yu, Fang; Liu, Yuanyue; Sun, Jingying; Zhu, Zhuan; He, Ran; Bao, Jiming; Goddard, William A., III; Chen, Shuo; Ren, Zhifeng
    To relieve our strong reliance on fossil fuels and to reduce greenhouse effects, there is an ever-growing interest in using electrocatalytic water splitting to produce green, renewable, and environment-benign hydrogen fuel via the hydrogen evolution reaction. For commercially feasible water electrolysis, it is imperative to develop electrocatalysts that perform as efficiently as Pt but using only earth-abundant commercial materials. However, the highest performance current catalysts consist of nanostructures made by using complex methods. Here we report a porous nickel diselenide (NiSe_2) catalyst that is superior for water electrolysis, exhibiting much better catalytic performance than most first-row transition metal dichalcogenide-based catalysts,...

  2. Pulsed laser-deposited n-Si/NiO_x photoanodes for stable and efficient photoelectrochemical water splitting

    He, Lingyun; Zhou, Wu; Cai, Dongping; Mao, Samuel S.; Sun, Ke; Shen, Shaohua
    An electrocatalytic and stable nickel oxide (NiO_x) thin layer was successfully deposited on an n-Si (100) substrate by pulsed laser deposition (PLD), acting as a photoanode for efficient photo-oxidation of water under solar illumination. It was revealed that the formed n-Si/NiO_x heterojunction with good Schottky contact could improve photogenerated charge separation, and thus n-Si photoanodes deposited with a 105 nm-thick NiO_x electrocatalytic layer exhibited a photovoltage of ∼350 mV, leading to greatly improved photoelectrochemical performances for water oxidation. The stability of the photoanode was significantly enhanced with the increasing thickness of NiO_x protective layers. This study demonstrates a simple and...

  3. Representation of functions on big data associated with directed graphs

    Chui, Charles K.; Mhaskar, H. N.; Zhuang, Xiaosheng
    This paper is an extension of the previous work of Chui et al. (2015) [4], not only from numeric data to include non-numeric data as in that paper, but also from undirected graphs to directed graphs (called digraphs, for simplicity). Besides theoretical development, this paper introduces effective mathematical tools in terms of certain data-dependent orthogonal systems for function representation and analysis directly on the digraphs. In addition, this paper also includes algorithmic development and discussion of various experimental results on such data-sets as CORA, Proposition, and Wiki-votes.

  4. 3D nano-architected metallic glass: Size effect suppresses catastrophic failure

    Liontas, Rachel; Greer, Julia R.
    We investigate the mechanical behavior of 3D periodically architected metallic glass nanolattices, constructed from hollow beams of sputtered Zr-Ni-Al metallic glass. Nanolattices composed of beams with different wall thicknesses are fabricated by varying the sputter deposition time, resulting in nanolattices with median wall thicknesses of ∼88 nm, ∼57 nm, ∼38 nm, ∼30 nm, ∼20 nm, and ∼10 nm. Uniaxial compression experiments conducted inside a scanning electron microscope reveal a transition from brittle, catastrophic failure in thicker-walled nanolattices (median wall thicknesses of ∼88 and ∼57 nm) to deformable, gradual, layer-by-layer collapse in thinner-walled nanolattices (median wall thicknesses of ∼38 nm and...

  5. Time Resolved Study of Hydroxyl Radical Oxidation of Oleic Acid at the Air-Water Interface

    Zhang, Xinxing; Barraza, Kevin M.; Upton, Kathleen T.; Beauchamp, J. L.
    The ubiquity of oleic acid (OA) renders it a poster child for laboratory investigations of environmental oxidation chemistry. In the current study, mechanistic details of the oxidation of OA by hydroxyl radicals at the air-water interface are investigated using field-induced droplet ionization mass spectrometry (FIDI-MS). Products from OH oxidation of both unsaturated and saturated carbon atoms are identified, and mechanisms for both types of oxidation processes are proposed. Uptake of oxygen in the interfacial layer increases linearly with time, consistent with Langmuir-Hinshelwood reaction kinetics. These results provide fundamental knowledge relating to OH initiated degradation of fatty acids in atmospheric aerosols.

  6. Rotational energy transfer in collisions between CO and Ar at temperatures from 293 to 30 K

    Mertens, Laura A.; Labiad, Hamza; Denis-Alpizar, Otoniel; Fournier, Martin; Carty, David; Le Picard, Sébastien D.; Stoecklin, Thierry; Sims, Ian R.
    Experimental measurements and theoretical calculations are reported for rotational energy transfer in the Ar-CO system. Experiments were performed in cold uniform supersonic flows of Ar, using an infrared – vacuum ultraviolet double resonance technique to measure absolute state-to-state rate constants and total relaxation cross sections for rotational energy transfer within the (v = 2) vibrational state of CO in collision with Ar at temperatures from 30.5 to 293 K. Close-coupled calculations were also performed using a recent potential energy surface (Y. Sumiyoshi and Y. Endo, J. Chem. Phys. 142 (2015) 024314). Very good agreement is obtained between measured and calculated...

  7. Quantum Mechanics Reactive Dynamics Study of Solid Li-Electrode/Li_6PS_5Cl-Electrolyte Interface

    Cheng, Tao; Merinov, Boris V.; Morozov, Sergey I.; Goddard, William A., III
    Abstract We have performed a theoretical study of the Li/Li_6PS_5Cl interface and showed the ability of the applied computational methods to properly describe the chemical processes that occurs at the interface. After a 500 ps ab initio molecular dynamics simulation, we find that the Li/Li_6PS_5Cl interface decomposes with formation of multiple phases and the main decomposition products are Li_2S, Li_3P, LiCl, and possibly LiP. These findings are in good agreement with reported experimental data. The observed quick decomposition is attributed to the weak bonding between P and S. Basing on this and earlier obtained experimental results, we conclude that the...

  8. Conservation of vibrational coherence in ultrafast electronic relaxation: The case of diplatinum complexes in solution

    Monni, Roberto; Auböck, Gerald; Kinschel, Dominik; Aziz-Lange, Kathrin M.; Gray, Harry B.; Vlček, Antonín; Chergui, Majed
    We report the results of ultrafast transient absorption studies of tetrakis(μ-pyrophosphito)diplatinate(II), [Pt_2(μ-P_2O_5H_2)_4]^(4−) (Pt(pop)) and its perfluoroborated derivative [Pt_2(μ-P_2O_5(BF_2)_4]^(4−) (Pt(pop-BF_2)) in water and acetonitrile upon excitation of high lying (<300 nm) UV absorption bands. We observe an ultrafast relaxation channel from high lying states to the lowest triplet state that partly (Pt(pop) in H_2O, Pt(pop-BF_2)) or fully (Pt(pop) in MeCN) bypasses the lowest singlet excited state. As a consequence, vibrational wave packets are detected in the lowest triplet state and/or the lowest excited singlet of both complexes, even though the electronic relaxation cascade spans ca. 2 and 1.3 eV, respectively. In...

  9. Measured Anxiety Affects Choices in Experimental "Clock" Games

    Kang, Min Jeong; Camerer, Colin
    We measure anxiety by skin conductance response (SCR) in an economic setting. In “clock” games, six agents receive private signals when an asset's price exceeds its fundamental value. They can sell for immediate value or wait to sell at a higher value. Waiting is risky because the price crashes to a lower value when three agents sell. Anxiety could lead people to sell too quickly when the game is played dynamically over time, compared to a static version with precommitted selling. Empirically, delays are shorter in dynamic games than in payoff-equivalent static games, and are associated with anxiety as measured.

  10. Meteoritic minerals and their origins

    Rubin, Alan E.; Ma, Chi
    About 435 mineral species have been identified in meteorites including native elements, metals and metallic alloys, carbides, nitrides and oxynitrides, phosphides, silicides, sulfides and hydroxysulfides, tellurides, arsenides and sulfarsenides, halides, oxides, hydroxides, carbonates, sulfates, molybdates, tungstates, phosphates and silico phosphates, oxalates, and silicates from all six structural groups. The minerals in meteorites can be categorized as having formed by a myriad of processes that are not all mutually distinct: (1) condensation in gaseous envelopes around evolved stars (presolar grains), (2) condensation in the solar nebula, (3) crystallization in CAI and AOI melts, (4) crystallization in chondrule melts, (5) exsolution during...

  11. Relating geologic units and mobility system kinematics contributing to Curiosity wheel damage at Gale Crater, Mars

    Arvidson, R. E.; DeGrosse, P.; Grotzinger, J. P.; Heverly, M. C.; Shechet, J.; Moreland, S. J.; Newby, M. A.; Stein, N.; Steffy, A. C.; Zhou, F.; Zastrow, A. M.; Vasavada, A. R.; Fraeman, A. A.; Stilly, E. K.
    Curiosity landed on plains to the north of Mount Sharp in August 2012. By June 2016 the rover had traversed 12.9 km to the southwest, encountering extensive strata that were deposited in a fluvial-deltaic-lacustrine system. Initial drives across sharp sandstone outcrops initiated an unacceptably high rate of punctures and cracks in the thin aluminum wheel skin structures. Initial damage was found to be related to the drive control mode of the six wheel drive actuators and the kinematics of the rocker-bogie suspension. Wheels leading a suspension pivot were forced onto sharp, immobile surfaces by the other wheels as they maintained...

  12. Reply to comment on "Geochronology and geochemistry of rhyolites from Hormuz Island, southern Iran: A new Cadomian arc magmatism in the Hormuz Formation" by Atapour, H. and Aftabi, A

    Faramarzi, Narges Sadat; Amini, Sadraddin; Borg, Gregor; Schmitt, Axel Karl; Hassanzadeh, Jamshid; McKeegan, Kevin; Mortazavi, Seyed Mohsen; Jamshidibadr, Mahboobeh; Razavi, Seyed Mohammad Hosein
    We appreciate the discussion by Atapour and Aftabi (2016) and would like to reply to their comments in detail. Atapour and Aftabi (2016) do not dispute our key findings concerning zircon geochronology, geochemistry, and thermometry. In fact, their criticism is focused on two topics: 1) our geological mapping, which they claim is not robust; 2) that the Hormuz rhyolites are not of the I-type, formed from subduction-related magmatism in an active continental margin setting but rather claim that these rocks are A-type rhyolites formed in a rift-related or within plate setting.

  13. Joint Source-Channel Coding With Feedback

    Kostina, Victoria; Polyanskiy, Yury; Verdú, Sergio
    This paper quantifies the fundamental limits of variable-length transmission of a general (possibly analog) source over a memoryless channel with noiseless feedback, under a distortion constraint. We consider excess distortion, average distortion and guaranteed distortion (d-semifaithful codes). In contrast to the asymptotic fundamental limit, a general conclusion is that allowing variable-length codes and feedback leads to a sizable improvement in the fundamental delay-distortion tradeoff. In addition, we investigate the minimum energy required to reproduce k source samples with a given fidelity after transmission over a memoryless Gaussian channel, and we show that the required minimum energy is reduced with feedback...

  14. Opportunities for Price Manipulation by Aggregators in Electricity Markets

    Ruhi, Navid Azizan; Dvijotham, Krishnamurthy; Chen, Niangjun; Wierman, Adam
    Aggregators of distributed generation are playing an increasingly crucial role in the integration of renewable energy in power systems. However, the intermittent nature of renewable generation makes market interactions of aggregators difficult to monitor and regulate, raising concerns about potential market manipulation by aggregators. In this paper, we study this issue by quantifying the profit an aggregator can obtain through strategic curtailment of generation in an electricity market. We show that, while the problem of maximizing the benefit from curtailment is hard in general, efficient algorithms exist when the topology of the network is radial (acyclic). Further, we highlight that...

  15. Application of Bottlebrush Block Copolymers as Photonic Crystals

    Liberman-Martin, Allegra L.; Chu, Crystal K.; Grubbs, Robert H.
    Brush block copolymers are a class of comb polymers that feature polymeric side chains densely grafted to a linear backbone. These polymers display interesting properties due to their dense functionality, low entanglement, and ability to rapidly self-assemble to highly ordered nanostructures. The ability to prepare brush polymers with precise structures has been enabled by advancements in controlled polymerization techniques. This Feature Article highlights the development of brush block copolymers as photonic crystals that can reflect visible to near-infrared wavelengths of light. Fabrication of these materials relies on polymer self-assembly processes to achieve nanoscale ordering, which allows for the rapid preparation...

  16. Learning-Based Abstractions for Nonlinear Constraint Solving

    Dathathri, Sumanth; Aréchiga, Nikos; Gao, Sicun; Murray, Richard M.
    We propose a new abstraction refinement procedure based on machine learning to improve the performance of nonlinear constraint solving algorithms on large-scale problems. The proposed approach decomposes the original set of constraints into smaller subsets, and uses learning algorithms to propose sequences of abstractions that take the form of conjunctions of classifiers. The core procedure is a refinement loop that keeps improving the learned results based on counterexamples that are obtained from partial constraints that are easy to solve. Experiments show that the proposed techniques significantly improved the performance of state-of-the-art constraint solvers on many challenging benchmarks. The mechanism is...

  17. From Rousettus aegyptiacus (bat) Landing to Robotic Landing: Regulation of CG-CP Distance Using a Nonlinear Closed-Loop Feedback

    Syed, Usman A.; Ramezani, Alireza; Chung, Soon-Jo; Hutchinson, Seth
    Bats are unique in that they can achieve unrivaled agile maneuvers due to their functionally versatile wing conformations. Among these maneuvers, roosting (landing) has captured attentions because bats perform this acrobatic maneuver with a great composure. This work attempts to reconstruct bat landing maneuvers with a Micro Aerial Vehicle (MAV) called Allice. Allice is capable of adjusting the position of its Center of Gravity (CG) with respect to the Center of Pressure (CP) using a nonlinear closed-loop feedback. This nonlinear control law, which is based on the method of input-output feedback linearization, enables attitude regulations through variations in CG-CP distance....

  18. Co-Design of Strain-Actuated Solar Arrays for Spacecraft Precision Pointing and Jitter Reduction

    Chilan, Christian M.; Herber, Daniel R.; Nakka, Yashwanth Kumar; Chung, Soon-Jo; Allison, James T.; Aldrich, Jack B.; Alvarez-Salazar, Oscar S.
    This work presents a novel spacecraft attitude control architecture using strain-actuated solar arrays (SASA) that does not require the use of conventional attitude control hardware. SASA enables attitude slewing maneuvers and precision-pointing (image acquisition) stares, while simultaneously suppressing structural vibrations. Distributed piezoelectric actuators help achieve higher precision, higher bandwidth, and quieter operation than reaction wheels. To understand the design tradeoffs for this architecture, a framework for the integrated design of distributed structural geometry and distributed control is presented. The physical properties of the array are modeled and designed with respect to a piecewise linear distributed thickness profile. The distributed control...

  19. Reactive Molecular Dynamics Simulations to Understand Mechanical Response of Thaumasite under Temperature and Strain Rate Effects

    Hajilar, Shahin; Shafei, Behrouz; Cheng, Tao; Jaramillo-Botero, Andres
    Understanding the structural, thermal, and mechanical properties of thaumasite is of great interest to the cement industry, mainly because it is the phase responsible for the aging and deterioration of civil infrastructures made of cementitious materials attacked by external sources of sulfate. Despite its importance, the effects of temperature and strain rate on the mechanical response of thaumasite have remained unexplored to date. In this study, the mechanical properties of thaumasite are fully characterized using the reactive molecular dynamics (RMD) method. With employing a first-principles based reactive force field, the RMD simulations enable the description of bond dissociation and formation...

  20. Atomic-scale Structural and Chemical Characterization of Hexagonal Boron Nitride Layers Synthesized at the Wafer-Scale with Monolayer Thickness Control

    Lin, Wei-Hsiang; Brar, Victor W.; Jariwala, Deep; Sherrott, Michelle C.; Tseng, Wei-Shiuan; Wu, Chih-I; Yeh, Nai-Chang; Atwater, Harry A.
    Hexagonal boron nitride (h-BN) is a promising two-dimensional insulator with a large band gap and low density of charged impurities that is isostructural and isoelectronic with graphene. Here we report the chemical and atomic-scale structure of CVD-grown wafer-scale (~25 cm2) h-BN sheets ranging in thickness from 1-20 monolayers. Atomic-scale images of h-BN on Au and graphene/Au substrates obtained by scanning tunneling microscopy (STM) reveal high h-BN crystalline quality in monolayer samples. Further characterization of 1-20 monolayer samples indicates uniform thickness for wafer-scale areas; this thickness control is a result of precise control of the precursor flow rate, deposition temperature and...

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