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Hydrogen trapping and diffusion in polycrystalline nickel:The spectrum of grain boundary segregation

Journal of Materials science.& technology 2024年第6期173卷 225-236页

作者： Yu Ding Haiyang Yu Meichao Lin Michael Ortiz Senbo Xiao Jianying He Zhiliang Zhang Department of Structural Engineering Norwegian University of Science and Technology(NTNU)Trondheim 7491Norway Division of Applied Mechanics Department of Materials Science and EngineeringUppsala UniversityUppsala 75121Sweden Graduate Aerospace Laboratories California Institute of Technology1200 E.California Blvd.PasadenaCA 91125USA

Hydrogen as an interstitial solute at grain boundaries(GBs)can have a catastrophic impact on the mechanical properties of many *** the global research effort,the underlying hydrogen-GB interactions in polycrystals remain inadequately *** this study,using Voronoi tessellations and atomistic simulations,we elucidate the hydrogen segregation energy spectrum at the GBs of polycrystalline nickel by exploring all the topologically favorable segregation *** distinct peaks in the energy spectrum are identified,corresponding to different structural *** first peak(-0.205 eV)represents the most favorable segregation sites at GB core,while the second and third peaks account for the sites at GB *** incorporating a thermodynamic model,the spectrum enables the determination of the equilibrium hydrogen concentrations at GBs,unveiling a remarkable two to three orders of magnitude increase compared to the bulk hydrogen concentration reported in experimental *** identified structures from the GB spectrum exhibit vastly different behaviors in hydrogen segregation and diffusion,with the low-barrier channels inside GB core contributing to short-circuit diffusion,while the high energy gaps between GB and neighboring lattice serving as on-plane diffusion *** square displacement analysis further confirms the findings,and shows that the calculated GB diffusion coefficient is three orders of magnitude greater than that of *** present study has a significant implication for practical applications since it offers a tool to bridge the gap between atomic-scale interactions and macroscopic behaviors in engineering materials.

关键词： Hydrogen embrittlement Grain boundaries Impurity segregation Polycrystal Atomistic simulations

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Mitigating Stealth Attacks via Game-Theoretic Switching in Multi Spacecraft Systems 57

Mitigating Stealth Attacks via Game-Theoretic Switching in M...

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57th IAA Symposium on Safety, Quality and Knowledge Management in Space Activities at the 75th International Astronautical Congress, IAC 2024

作者： Ragan, James Ibrahim, Joshua Hadaegh, Fred Y. Chung, Soon-Jo Division of Engineering and Applied Science California Institute of Technology PasadenaCA91125 United States

ISBN: (纸本)9798331312275

Due to limited sensing or the inability to monitor every agent at once, spacecraft swarm networks are vulnerable stealthy attacks. These attacks not only seek to disrupt the spacecraft formation, but to also prevent defenses from being mounted by escaping detection. Switching between sensing topologies can reveal stealthy attacks and provides a viable solution even when no configuration can observe all of the state space simultaneously. However, solving for an optimal switching strategy in a general setting is an open question. By formulating a zero-sum game between the stealthy attacker and the sensing topology of the defender, we develop an online tree search algorithm which plans the best sequence of sensing topologies to limit the stealthy disruption of attacks over a horizon. We validate our method in an angles-only relative navigation setting through quantitative and qualitative numerical experiments. Despite limited sensing at each spacecraft, we demonstrate stealthy attacks are revealed or their influence is successfully bounded. Copyright © 2024 by the International Astronautical Federation (IAF).

关键词： Trees (mathematics)

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Power-frequency relationship of wave dynamics in fluid-filled compliant tubes

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Physical Review Fluids 2025年第3期10卷 033102-033102页

作者： Arian Aghilinejad Faisal Amlani Morteza Gharib Division of Engineering and Applied Science California Institute of Technology Pasadena California 91125 USA

The human heart pumps in a pulsatile fashion that generates pressure and flow waves in the cardiovascular system. These waves propagate and reflect through a fractal tree network composed of fluid-filled compliant tubes. The resulting complex pressure and flow waves create a pulsatile workload (an important contributor to the pumping efficiency of the heart). Although such load has been shown to be dependent on inflow wave frequency, the fundamental relationship between power and frequency has not been directly explored. Hence the general idea of this work is to address this by modeling wave propagation in a generic pulsatile system through a fluid-structure interaction solver (based on a reduced-order Navier-Stokes formulation) applied to a fluid-filled compliant tube terminated by a controlled reflection site. We analyze the impact of reflection on such pulse waves and their associated power-frequency relationships. Our analysis considers moderate frequencies and large wavelengths as inspired by cardiovascular flows. Examining pressure-flow dynamics reveals two distinct regimes as a function of inflow frequency: a pressure-leading regime and a flow-leading regime. We observe that the latter transitions into the former at particular “optimum frequencies” that minimize the pulsatile load on the inlet and create nonlinearity in power-frequency patterns. Results suggest that the values of these frequencies strongly depend on the reflection coefficient of the tube. Our findings highlight the significance of reflection as a key determinant in design that can provide control of optimum inflow frequencies toward minimizing destructive pulsatile loads.

关键词： Fluid structure interaction

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Motor Imagery Teleoperation of a Mobile Robot Using a Low-Cost Brain-Computer Interface for Multi-Day Validation 1

Motor Imagery Teleoperation of a Mobile Robot Using a Low-Co...

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1st IEEE Conference on Telepresence, Telepresence 2024

作者： An, Yujin Mitchell, Daniel Lathrop, John Flynn, David Chung, Soon-Jo California Institute of Technology Division of Engineering and Applied Science United States University of Glasgow Autonomous Systems and Connectivity Division Glasgow United Kingdom

ISBN: (纸本)9798331528249

Brain-computer interfaces (BCI) have the potential to provide transformative control in prosthetics, assistive technologies (wheelchairs), robotics, and human-computer interfaces. While Motor Imagery (MI) offers an intuitive approach to BCI control, its practical implementation is often limited by the requirement for expensive devices, extensive training data, and complex algorithms, leading to user fatigue and reduced accessibility. In this paper, we demonstrate that effective MI-BCI control of a mobile robot in real-world settings can be achieved using a fine-tuned Deep Neural Network (DNN) with a sliding window, eliminating the need for complex feature extractions for real-time robot control. The fine-tuning process optimizes the convolutional and attention layers of the DNN to adapt to each user's daily MI data streams, reducing training data by 70% and minimizing user fatigue from extended data collection. Using a low-cost (∼$3k), 16-channel, non-invasive, open-source electroencephalogram (EEG) device, four users teleoperated a quadruped robot over three days. The system achieved 78% accuracy on a single-day validation dataset and maintained a 75% validation accuracy over three days without extensive retraining from day-to-day. For real-world robot command classification, we achieved an average of 62% accuracy. By providing empirical evidence that MI-BCI systems can maintain performance over multiple days with reduced training data to DNN and a low-cost EEG device, our work enhances the practicality and accessibility of BCI technology. This advancement makes BCI applications more feasible for real-world scenarios, particularly in controlling robotic systems. © 2024 IEEE.

关键词： Deep neural networks

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Investigation of drain noise in InP pHEMTs using cryogenic on-wafer characterization

Investigation of drain noise in InP pHEMTs using cryogenic o...

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2023 IEEE/MTT-S International Microwave Symposium, IMS 2023

作者： Gabritchidze, Bekari Cleary, Kieran Readhead, Anthony Minnich, Austin J. California Institute of Technology Cahill Radio Astronomy Lab United States California Institute of Technology Division of Engineering and Applied Science United States

ISBN: (纸本)9798350347647

We present on-wafer measurements of microwave noise temperature and S-parameters of InP pseudomorphic high electron mobility transistors (pHEMTs) over various drain-source voltages (VDS) and physical temperatures (Tph). From these data, we extract the small signal model (SSM) and drain noise temperature Td at each VDS and Tph. We find that Td follows a non-linear trend with both VDS and Tph. The observed trends are consistent with a recent drain noise model [1], [2], where Td originates from the sum of a thermal noise, attributed to physical temperature of electrons (Tel) in the channel;and real-space transfer (RST) noise, attributed to thermionic emission of electrons from the channel to the barrier. Using this model, we find that at the bias that minimizes the noise temperature, RST accounts for ~ 50% of Td at 300 K and ~ 30 % at 40 K. Possible improvements in noise performance in pHEMTs if RST were suppressed are discussed. © 2023 IEEE.

关键词： Low noise amplifiers

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Tensor Network Computations That Capture Strict Variationality, Volume Law Behavior, and the Efficient Representation of Neural Network States

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Physical Review Letters 2024年第26期133卷 260404-260404页

作者： Wen-Yuan Liu Si-Jing Du Ruojing Peng Johnnie Gray Garnet Kin-Lic Chan Division of Chemistry and Chemical Engineering California Institute of Technology Pasadena California 91125 USA Division of Engineering and Applied Science California Institute of Technology Pasadena California 91125 USA

We introduce a change of perspective on tensor network states that is defined by the computational graph of the contraction of an amplitude. The resulting class of states, which we refer to as tensor network functions, inherit the conceptual advantages of tensor network states while removing computational restrictions arising from the need to converge approximate contractions. We use tensor network functions to compute strict variational estimates of the energy on loopy graphs, analyze their expressive power for ground states, show that we can capture aspects of volume law time evolution, and provide a mapping of general feed-forward neural nets onto efficient tensor network functions. Our work expands the realm of computable tensor networks to ones where accurate contraction methods are not available, and opens up new avenues to use tensor networks.

关键词： Tensors

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Progress towards on-chip entangled photon spectroscopy and bioimaging 20

Progress towards on-chip entangled photon spectroscopy and b...

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Quantum Sensing and Nano Electronics and Photonics XX 2024

作者： He, Manni Hwang, Emily Harper, Nathan Cushing, Scott K. Division of Chemistry and Chemical Engineering California Institute of Technology PasadenaCA91125 United States Department of Applied Physics and Materials Science California Institute of Technology PasadenaCA91125 United States

ISBN: (纸本)9781510670501

Entangled photons could translate complex, ultrafast laser-based spectroscopy and imaging to a chip-sized or distributable platform. The inherent temporal and energy correlations between the two entangled photons created in spontaneous parametric down conversion (SPDC) allow a continuous-wave laser diode source to replicate ultrafast, two-pulse experiments that are usually performed in table-top scale setups. Although entangled photons were originally proposed to enhance multiphoton or nonlinear processes, few entangled experiments to date have outperformed classical photon sources from a full systems perspective. The low pump power criteria and periodically poled nonlinear sources, however, do make entangled photon experiments natural candidates for on-chip photonic circuits and spectroscopy. In our talk, we will discuss experiments that prove that the miniaturization of ultrafast experiments is a key area where entangled photons can compete with or prove superior to classical photon experiments. We will first discuss our progress in creating entangled photon sources in the visible to deep ultraviolet (UV) wavelengths as needed for spectroscopy and imaging, the integration of the subsequent photonic circuitry needed to create on-chip spectrometers, and then experimental results proving that on-chip entangled photon sources can replicate ultrafast pump-probe or be used for fluorescence lifetime imaging microscopy (FLIM) experiments. We will also discuss our increasingly null experiments on topics like entangled two-photon absorption (ETPA) which are equally important for the progressing the field. © 2024 SPIE. All rights reserved.

关键词： Continuous wave lasers

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Valleytronics and negative differential resistance in cubic boron nitride: A first-principles study

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Physical Review Materials 2024年第10期8卷 104602页

作者： Benjamin Hatanpää Austin J. Minnich Division of Engineering and Applied Science California Institute of Technology Pasadena CA United States

Cubic boron nitride (c-BN) is an ultrawide-bandgap semiconductor of significant interest for high-frequency and high-power electronics applications owing to its high saturation drift velocity and high electric breakdown field. Beyond transistors, devices exploiting the valley degree of freedom or negative differential resistance are of keen interest. While diamond has been found to have potential for these applications, c-BN has not been considered owing to a lack of knowledge of the relevant charge transport properties. Here, we report a study of the high-field transport and noise properties of c-BN using first-principles calculations. We find that c-BN exhibits an abrupt region of negative differential resistance (NDR) below 140 K, despite the lack of multivalley band structure typically associated with NDR. This feature is found to arise from a strong energy dependence of the scattering rates associated with optical phonon emission. The high optical phonon energy also leads to an intervalley scattering time rivaling that of diamond. The negative differential resistance and long intervalley scattering time indicate the potential of c-BN for transferred-electron and valleytronic devices, respectively.

关键词： Electrical conductivity Electron-phonon coupling Electronic structure of atoms & molecules First-principles calculations Valley degrees of freedom

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Coherence Awareness in Diffractive Neural Networks

Coherence Awareness in Diffractive Neural Networks

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CLEO: Fundamental science. CLEO:FS 2024 - Part of Conference on Lasers and Electro-Optics, CLEO 2024

作者： Kleiner, Matan Michaeli, Lior Michaeli, Tomer Faculty of Electrical and Computer Engineering Technion Haifa32000 Israel Division of Engineering and Applied Science California Institute of Technology 1200 E. California Avenue PasadenaCA91125 United States

We demonstrate the significant i nfluence of th e il lumination co herence on diffractive networks, and propose a framework for network optimization with any prescribed degree of spatial and temporal coherence. We ana... 详细信息

ISBN: (纸本)9781957171395

关键词： Diffractive optics

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Hot electron diffusion, microwave noise, and piezoresistivity in Si from first principles

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Physical Review B 2024年第23期109卷 235201-235201页

作者： Benjamin Hatanpää Austin J. Minnich Division of Engineering and Applied Science California Institute of Technology Pasadena 91125 CA United States

Ab initio calculations of electron-phonon interactions in materials without adjustable parameters have provided microscopic insights into their charge-transport properties. Other transport properties such as the diffusion coefficient provide additional microscopic information and are readily accessible experimentally, but few ab initio calculations of these properties have been performed. Here, we report first-principles calculations of the hot electron diffusion coefficient in Si and its dependence on electric field over temperatures from 77–300 K. While qualitative agreement in trends such as anisotropy at high electric fields is obtained, the quantitative agreement that is routinely achieved for low-field mobility is lacking. We examine whether the discrepancy can be attributed to an inaccurate description of f-type intervalley scattering by computing the microwave-frequency noise spectrum and piezoresistivity. These calculations indicate that any error in the strength of f-type scattering is insufficient to explain the diffusion coefficient discrepancies. Our findings suggest that the measured diffusion coefficient is influenced by factors such as space-charge effects, which are not included in ab initio calculations, impacting the interpretation of experimental measurements in terms of microscopic charge-transport processes.

关键词： Electrical conductivity Electron-phonon coupling First-principles calculations Noise Elemental semiconductors

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