Publications

Fatigue crack propagation in oil environments— II. A model for crack closure induced by viscous fluids

Several mechanisms have recently been identified for fatigue crack closure. The current work examines the concept of crack closure induced by the hydrodynamic wedging action of a viscous fluid within an advancing crack during cyclic loading. Unlike previous analyses, the model considers both the hydrodynamics of the pressure distribution within the crack and the kinetics of the penetration of the fluid into the crack. In addition, the results are presented in fracture mechanics terminology, and expressed as an effective (near tip) stress intensity range. Analyses involving both “full-” and “partial-penetration” of the viscous fluid inside the crack are utilized to rationalize the influence of viscosity on fatigue crack propagation in dehumidified oil environments, described in Part I of this paper. The roles of stress intensity range, crack size and frequency on the development of such fluid-induced crack closure are also examined.

All-Optically Controlled Memristor

Memristors have emerged as key candidates for beyond-von-Neumann neuromorphic or in-memory computing owing to the feasibility of their ultrahigh-density three-dimensional integration and their ultralow energy consumption. A memristor is generally a two-terminal electronic element with conductance that varies nonlinearly with external electric stimuli and can be remembered when the electric power is turned off. As an alternative, light can be used to tune the memconductance and endow a memristor with a combination of the advantages of both photonics and electronics. Both increases and decreases in optically induced memconductance have been realized in different memristors; however, the reversible tuning of memconductance with light in the same device remains a considerable challenge that severely restricts the development of optoelectronic memristors. Here we describe an all-optically controlled (AOC) analog memristor with memconductance that is reversibly tunable over a continuous range by varying only the wavelength of the controlling light. Our memristor is based on the relatively mature semiconductor material InGaZnO (IGZO) and a memconductance tuning mechanism of light-induced electron trapping and detrapping. We demonstrate that spike-timing-dependent plasticity (STDP) learning can be realized in our device, indicating its potential applications in AOC spiking neural networks (SNNs) for highly efficient optoelectronic neuromorphic computing.

ChemInform Abstract: Influence of Support Composition on the Reduction of Nitric Oxide Over Rhodium Catalysts.

Abstract The activity of Rh supported on SiO 2 , Al 2 O 3 , MgO, La 2 O 3 , and TiO 2 for NO reduction by CO and H 2 has been investigated in regard to automotive emission.

Faculty Opinions recommendation of Soil respiration in perennial grass and shrub ecosystems: Linking environmental controls with plant and microbial sources on seasonal and diel timescales.

Read the full review for this Faculty Opinions recommended article: Soil respiration in perennial grass and shrub ecosystems: Linking environmental controls with plant and microbial sources on seasonal and diel timescales.

A zoo of strange attractors from the canonical Chua's circuits

By adding a linear resistor R/sub 0/ in Chua's circuit, an immensely richer bifurcation landscape can be obtained, including an endowment of more than 20 new distinct strange attractors which were suppressed when R/sub 0/ to 0. The author interprets this augmented circuit as a global unfolding of Chua's circuit because its basic mechanism is similar to the local unfolding theory in nonlinear mathematics. This augmented Chua's circuit, which has only seven parameters, is canonical in the sense that it is capable of duplicating all qualitative behaviors of a 21-parameter family C of ordinary differential equations in R/sup 3/. Explicit formulas are given for calculating the seven circuit parameters of the augmented Chua's circuit so that it is topologically conjugate to any member of this 21-parameter family of third-order piecewise-linear circuits; namely, the Chua's circuit family. A gallery of selected strange attractors from this canonical circuit is presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Synthesis and Stoichiometry of Different Layered Sodium Cobalt Oxides

Layered Na–metal oxides can form in different crystal structures, each with different electrochemical behavior. As a prototype system to better understand how each phase can be formed, we present the conditions under which different layered phases of NaxCoO2 can be stabilized in solid-state synthesis. Using a novel combination of ex situ XRD on as-synthesized samples, with in situ XRD to monitor the relation between Na content and lattice parameters, we are able to construct a phase diagram of NaxCoO2 between 450 to 750 °C in air and for Na:Co sample ratios ranging from 0.60 to 1.05. Four single phase domains of O3, O′3, P′3, and P2 are revealed based on the XRD analysis. In contrast to previous reports it is found that pure O3, O′3 and P′3 phase can only form at a fixed stoichiometry of x = 1.00, 0.83, and 0.67, respectively, while the P2 phase forms in a slightly larger composition range from 0.68 to 0.76. Galvanostatic charging of O3–Na1.00CoO2 shows several flat and sloping regions on the voltage profile, which follows the sequence of O3–O′3–P′3–P3–P′3, with increasing interslab distances. Our results indicate that the electrochemically important P2 structure is likely stabilized by entropy.

Preparation of hydrophobic poly(isobutylene) star polymers with hydrophilic poly(propylene imine) dendritic cores

Considering the Complexities of U-Pb Geochronology at the Millennial Scale

Germanium-on-SOI photo-detector based on an FET structure

An integrated Ge-on-SOI photodetector with an FET structure, based on secondary photoconductivity is demonstrated. The Ge gate absorbs ~100 nW of 1.55μm light, thereby modulating the conductance of the Silicon channel by a factor of 25.

What are the roles of antibodies versus a durable, high quality T-cell response in protective immunity against SARS-CoV-2?

Correction to: Production efficiency of the bacterial non-ribosomal peptide indigoidine relies on the respiratory metabolic state in S. cerevisiae

Following publication of the original article [1], the authors have noted that the standard curve in Additional file 1: Figure S7 is incorrect.

Chord: a scalable peer-to-peer lookup protocol for internet applications

A fundamental problem that confronts peer-to-peer applications is the efficient location of the node that stores a desired data item. This paper presents Chord, a distributed lookup protocol that addresses this problem. Chord provides support for just one operation: given a key, it maps the key onto a node. Data location can be easily implemented on top of Chord by associating a key with each data item, and storing the key/data pair at the node to which the key maps. Chord adapts efficiently as nodes join and leave the system, and can answer queries even if the system is continuously changing. Results from theoretical analysis and simulations show that Chord is scalable: Communication cost and the state maintained by each node scale logarithmically with the number of Chord nodes.

Mechanism of fatigue in micron-scale films of polycrystalline silicon for microelectromechanical systems

Reported nearly a decade ago, cyclic fatigue failure in silicon thin films has remained a mystery. Silicon does not display the room-temperature plasticity or extrinsic toughening mechanisms necessary to cause fatigue in either ductile (e.g., metals) or brittle (e.g., ceramics and ordered intermetallics) materials. This letter presents experimental evidence for the cyclic fatigue of silicon via a conceptually different mechanism termed reaction-layer fatigue. Based on mechanical testing, electron microscopy, and self-assembled monolayers, we present direct observation of fatigue-crack initiation in polycrystalline silicon, the mechanism of crack initiation, and a method for altering fatigue damage accumulation.

Generation of Axially Chiral Fluoroallenes through a Copper-Catalyzed Enantioselective β-Fluoride Elimination

Herein we report the copper-catalyzed silylation of propargylic difluorides to generate axially chiral, tetrasubstituted monofluoroallenes in both good yields (27 examples >80%) and enantioselectivities (82-98% ee). Compared to previously reported synthetic routes to axially chiral allenes (ACAs) from prochiral substrates, a mechanistically distinct reaction has been developed: the enantiodiscrimination between enantiotopic fluorides to set an axial stereocenter. DFT calculations and vibrational circular dichroism (VCD) suggest that β-fluoride elimination from an alkenyl copper intermediate likely proceeds through a <i>syn</i>-β-fluoride elimination pathway rather than an <i>anti</i>-elimination pathway. The effects of the C1-symmetric Josiphos-derived ligand on reactivity and enantioselectivity were investigated. Not only does this report showcase that alkenyl copper species (like their alkyl counterparts) can undergo β-fluoride elimination, but this elimination can be achieved in an enantioselective fashion.

On the Molecular Origin of Charge Separation at the Donor–Acceptor Interface

Abstract Fullerene‐based acceptors have dominated organic solar cells for almost two decades. It is only within the last few years that alternative acceptors rival their dominance, introducing much more flexibility in the optoelectronic properties of these material blends. However, a fundamental physical understanding of the processes that drive charge separation at organic heterojunctions is still missing, but urgently needed to direct further material improvements. Here a combined experimental and theoretical approach is used to understand the intimate mechanisms by which molecular structure contributes to exciton dissociation, charge separation, and charge recombination at the donor–acceptor (D–A) interface. Model systems comprised of polythiophene‐based donor and rylene diimide‐based acceptor polymers are used and a detailed density functional theory (DFT) investigation is performed. The results point to the roles that geometric deformations and direct‐contact intermolecular polarization play in establishing a driving force (energy gradient) for the optoelectronic processes taking place at the interface. A substantial impact for this driving force is found to stem from polymer deformations at the interface, a finding that can clearly lead to new design approaches in the development of the next generation of conjugated polymers and small molecules.

Fluxes through the prokaryotic and eukaryotic pathways of lipid synthesis in the ‘16:3’ plant <i>Arabidopsis thaliana</i>

The kinetics of [1-14C]acetate incorporation in Arabidopsis thaliana L. (Heyn) showed almost equal labelling of phosphatidylcholine (PC) and diacylgalactosylglycerol (DGG) at early times and the transfer of radioactivity from PC to DGG and diacyldigalactosylglycerol (DDG) at longer times. These kinetics demonstrated the parallel operation of the prokaryotic and eukaryotic pathways of lipid synthesis [Roughan &amp; Slack (1982) Annu. Rev. Plant Physiol. 33, 97-132] in this tissue. At 2 h after the application of [1-14C]acetate, more than 85% of the radioactivity at the sn-2 position of each chloroplast lipid was in 16-carbon fatty acids. However, after 60 h, molecular species containing labelled C18 fatty acids at position sn-2 and presumably derived from microsomal PC made a large contribution (20-70%) to each chloroplast lipid except phosphatidylglycerol. These findings are consistent with the contention that the chain length of the fatty acid at the sn-2 position of glycerol is an accurate predictor of whether a particular lipid molecule has been synthesized by the prokaryotic or eukaryotic pathway. At 30 min after the start of [1-14C]acetate labelling, only 12.3% of the radioactivity in PC was in saturated fatty acids, but the proportion increased steadily to 24.3% after 142 h. It is suggested that steps involved in the conversion of PC to chloroplast lipids on the eukaryotic pathway discriminate against palmitate-containing species. The step involved does not appear to be transfer of PC to the chloroplast because extrachloroplastic and chloroplast membranes purified from Arabidopsis mesophyll protoplasts each contained PC with a fatty acid composition similar to that of the same lipid from leaves. Positional analysis of unlabelled lipids, together with the information summarized above, is used to construct a quantitative scheme of the fluxes through the prokaryotic and eukaryotic pathways during lipid synthesis in Arabidopsis. This scheme shows that 38% of the fatty acids synthesized de novo in the chloroplast enter the prokaryotic pathway in the chloroplast envelope. Of the 62% which are exported as acyl-CoA species to enter the eukaryotic pathway, 56% (34% of the total) are returned to complete synthesis of the chloroplast's complement of glycerolipids.

CCDC 249634: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The Art of Adaptation: Globalization and the European Model

However well the European economy may have performed in the past, there are reasons to worry about whether it can keep pace in a world of accelerating technical change and global competition led by the rise of India and China.