Publications

Micromechanisms of Cyclic and Environmentally-Assisted Subcritical Crack Growth in Ceramic-Matrix Composites

: The future use of ceramics for advanced structural applications represents an important precursor to potential major improvements in design performance for high temperature, corrosion and wear resistance applications. The limited use of such brittle materials to date has primarily been attributed to their inherently low toughness and lack of defect tolerance. Scientific research in the last decade, however, has resulted in major advances in the toughening of ceramics by such mechanisms as whisker reinforcement, transformation and microcrack toughening. However, very recent work has shown that such toughened ceramics and composites, contrary to conventional wisdom, may become susceptible to cyclic fatigue. The intent of this program was to study and model the physics and fundamental micromechanisms of environmentally- assisted and principally cyclic fatigue crack-growth processes in several classes of ceramic-matrix composites, chosen to reflect different primary toughening mechanisms. Based on initial studies, the central hypothesis of this work is that the mechanisms of crack-tip shielding, used to enhance fracture toughness, can lead to degradation in crack-growth resistance under cyclic loading. Cyclic Fatigue, Fracture Toughness, Subcritical Crack Growth, Ceramic Matrix Composites, Small Cracks.

Controlling the Metabolic Flux through the Carotenoid Pathway Using Directed mRNA Processing and Stabilization

The impact of routing policy on Internet paths

The impact of routing policy on Internet paths is poorly understood. In theory, the policy can inflate shortest-router-hop paths. To our knowledge, the extent of this inflation has not been previously examined. Using a simplified model of the routing policy in the Internet, we obtain approximate indications of the impact of policy routing on Internet paths. Our findings suggest that the routing policy does impact the length of Internet paths significantly. For instance, in our model of the routing policy, some 20% of Internet paths are inflated by more than five router-level hops.

LIGO/Virgo S200114f: Gemini Spectra of AT 2020vu, AT 2020vw, AT 2020vy, and AT 2020vz

Magnetoresistance and Anisotropic Spin Dynamics in Antiferromagnetic Semiconductor Eu$_5$Sn$_2$As$_6$

We report on the thermodynamic and transport properties of the rare-earth Zintl compound Eu$_5$Sn$_2$As$_6$, which orders as a canted antiferromagnetic magnetic semiconductor at 10.3~K. The system also displays a complex cascade of magnetic phases arising from geometric and magnetic exchange frustration, with high sensitivity to the application and direction of small magnetic fields. At low temperature, Eu$_5$Sn$_2$As$_6$ exhibits negative colossal magnetoresistance of up to a factor of $6\times10^3$. This may be a lower bound as the conductivity appears to be shunted by an unknown conduction channel, causing the resistivity to saturate. Mechanisms for the low temperature saturation of resistivity are discussed.

Qualitative nonlinear circuit analysis using computers

This paper presents several new algorithms which are used to implement two recently published uniqueness theorems applicable to nonlinear resistive circuits containing independent sources, two-terminal resistors and linear controlled sources. These programs are novel in that they allow a computer to test directly for qualitative behavior. The algorithms and the two programs which use them are described and results are presented for some examples from the original papers.

<i>gem</i>‐Difluoroallylation of Aryl Halides and Pseudo Halides with Difluoroallylboron Reagents in High Regioselectivity

Abstract We report the palladium‐catalyzed gem ‐difluoroallylation of aryl halides and pseudo halides with 3,3‐difluoroallyl boronates in high yield with high regioselectivity, and we report the preparation of the 3,3‐difluoroallyl boronate reactants by a copper‐catalyzed defluorinative borylation of inexpensive gaseous 3,3,3‐trifluoropropene with bis(pinacolato)diboron. The gem ‐difluoroallylation of aryl and heteroaryl bromides proceeds with low catalyst loading (0.1 mol % [Pd]) and tolerates a wide range of functional groups, including primary alcohols, secondary amines, ethers, ketones, esters, amides, aldehydes, nitriles, halides, and nitro groups. This protocol extends to aryl iodides, chlorides, and triflates, as well as substituted difluoroallyl boronates, providing a versatile synthesis of gem ‐difluoroallyl arenes that we show to be valuable intermediates to a series of fluorinated building blocks.

Calcium-dependent copper redistributions in neuronal cells revealed by a fluorescent copper sensor and X-ray fluorescence microscopy

Dynamic fluxes of s-block metals like potassium, sodium, and calcium are of broad importance in cell signaling. In contrast, the concept of mobile transition metals triggered by cell activation remains insufficiently explored, in large part because metals like copper and iron are typically studied as static cellular nutrients and there are a lack of direct, selective methods for monitoring their distributions in living cells. To help meet this need, we now report Coppersensor-3 (CS3), a bright small-molecule fluorescent probe that offers the unique capability to image labile copper pools in living cells at endogenous, basal levels. We use this chemical tool in conjunction with synchotron-based microprobe X-ray fluorescence microscopy (XRFM) to discover that neuronal cells move significant pools of copper from their cell bodies to peripheral processes upon their activation. Moreover, further CS3 and XRFM imaging experiments show that these dynamic copper redistributions are dependent on calcium release, establishing a link between mobile copper and major cell signaling pathways. By providing a small-molecule fluorophore that is selective and sensitive enough to image labile copper pools in living cells under basal conditions, CS3 opens opportunities for discovering and elucidating functions of copper in living systems.

GRB 021004: A Possible Shell Nebula around a Wolf-Rayet Star Gamma-Ray Burst Progenitor

The rapid localization of GRB 021004 by the HETE-2 satellite allowed nearly continuous monitoring of its early optical afterglow decay, as well as high-quality optical spectra that determined a redshift of z3 = 2.328 for its host galaxy, an active starburst galaxy with strong Lyα emission and several absorption lines. Spectral observations show multiple absorbers at z3A = 2.323, z3B = 2.317, and z3C = 2.293 blueshifted by ~450, ~990, and ~3155 km s⁻¹, respectively, relative to the host galaxy Lyα emission. We argue that these correspond to a fragmented shell nebula that has been radiatively accelerated by the gamma-ray burst (GRB) afterglow at a distance gsim0.3 pc from a Wolf-Rayet star GRB progenitor. The chemical abundance ratios indicate that the nebula is overabundant in carbon and silicon. The high level of carbon and silicon is consistent with a swept-up shell nebula gradually enriched by a carbon-rich late-type Wolf-Rayet progenitor wind over the lifetime of the nebula prior to the GRB onset. The detection of statistically significant fluctuations and color changes about the jetlike optical decay further supports this interpretation, since fluctuations must be present at some level as a result of irregularities in a clumpy stellar wind medium or if the progenitor has undergone massive ejection prior to the GRB onset. This evidence suggests that the mass-loss process in a Wolf-Rayet star might lead naturally to an iron core collapse with sufficient angular momentum that could serve as a suitable GRB progenitor. Even though we cannot rule out definitely the alternatives of a dormant QSO, large-scale superwinds, or a several hundred year old supernova remnant responsible for the blueshifted absorbers, these findings point to the likelihood of a signature for a massive-star GRB progenitor.

The effects of titania and alumina overlayers on the hydrogenation of CO over rhodium

Submonolayer quantities of TiOx and AlOx were deposited on a polycrystalline Rh foil. The samples were characterized by Auger electron spectroscopy and then enclosed in an atmospheric-pressure chamber to evaluate their activity for CO hydrogenation. AlOx deposition suppresses the rate of CO hydrogenation in direct proportion to the AlOx coverage, but the product distribution and kinetics are unaffected. These observations indicate that the only effect of AlOx is to block Rh sites. TiOxdeposition has a more complex effect. With increasing TiOx coverage, the methanation rate passes through a maximum at a TiOx coverage of 0.15 monolayer. The maximum rate is three times as large as the rate of clean Rh. The deposited TiOx also affects the catalyst selectivity as well as the kinetics of methane formation. The selectivity for methane reaches a minimum for a TiOx coverage of ca. 0.20 monolayer and in the same range the activation energy for methanation reaches a minimum and the H2 and CO partial pressure dependences reach a maximum. The unusual behaviour of TiOx-promoted Rh is attributed to the occurrence of highly active catalytic sites near the perimeter of the TiOx islands making up the overlayer.

Finite element modeling of the deformation of magnetoelastic film

Le temps du sociologue, le temps du politique : à propos de la question scolaire en Nouvelle Calédonie

Simultaneous Detection and Segmentation

We aim to detect all instances of a category in an image and, for each instance, mark the pixels that belong to it. We call this task Simultaneous Detection and Segmentation (SDS). Unlike classical bounding box detection, SDS requires a segmentation and not just a box. Unlike classical semantic segmentation, we require individual object instances. We build on recent work that uses convolutional neural networks to classify category-independent region proposals (R-CNN [16]), introducing a novel architecture tailored for SDS. We then use category-specific, top- down figure-ground predictions to refine our bottom-up proposals. We show a 7 point boost (16% relative) over our baselines on SDS, a 5 point boost (10% relative) over state-of-the-art on semantic segmentation, and state-of-the-art performance in object detection. Finally, we provide diagnostic tools that unpack performance and provide directions for future work.

Functional engineered cementitious composite preparation via partial substitution of cementing material by reconstructed red mud: Physicochemical and electromagnetic wave absorption properties

A New Perspective and Design Principle for Halide Perovskites: Ionic Octahedron Network (ION)

The metal halide ionic octahedron, [MX₆] (M = metal cation, X = halide anion), is considered to be the fundamental building block and functional unit of metal halide perovskites. By representing the metal halide ionic octahedron in halide perovskites as a super ion/atom, the halide perovskite can be described as an extended ionic octahedron network (ION) charge balanced by selected cations. This new perspective of halide perovskites based on ION enables the prediction of different packing and connectivity of the metal halide octahedra based on different solid-state lattices. In this work, a new halide perovskite Cs₈Au_3.5In_1.5Cl₂₃ was discovered on the basis of a BaTiO₃-lattice ION {[InCl₆][AuCl₅][Au/InCl₄]₃}^8-, which is assembled from three different ionic octahedra [InCl₆], [AuCl₆], and [Au/InCl₆] and balanced by positively charged Cs cations. The success of this ION design concept in the discovery of Cs₈Au_3.5In_1.5Cl₂₃ opens up a new venue for the rational design of new halide perovskite materials.

ChemInform Abstract: Iridium‐Catalyzed Borylation of Primary Benzylic C—H Bonds Without a Directing Group: Scope, Mechanism, and Origins of Selectivity.

Abstract The borylation of methylarenes with a silylborane reagent and a new iridium catalyst containing an electron‐deficient phenanthroline ligand selectively leads to benzylic boronate esters over the corresponding aryl boronate esters.

Evaluation of Bridge Abutment Capacity and Stiffness during Earthquakes

The “actual” capacity and stiffness values of the abutment-soil systems at the US 101/Painter Street Overpass, determined from its earthquake motions, are used to investigate how abutment stiffness varies during earthquakes and to evaluate current modeling procedures. It is found that the “actual” abutment stiffness may be significantly different during different phases of the shaking and decreases significantly as the abutment deformation increases. The CALTRANS modeling procedure leads to a good estimate of the transverse abutment stiffness and capacity. However, this procedure may overestimate the normal abutment stiffness and capacity by a factor of over two, indicating that the assumed value of 7.7 ksf for the ultimate passive resistance of the soil, used in the CALTRANS procedure, may be too high. The AASHTO-83 and ATC-6 procedures lead to an initial estimate of the abutment stiffness that is too high in both directions.

Developing a predictive model of quality of experience for internet video

Improving users' quality of experience (QoE) is crucial for sustaining the advertisement and subscription based revenue models that enable the growth of Internet video. Despite the rich literature on video and QoE measurement, our understanding of Internet video QoE is limited because of the shift from traditional methods of measuring video quality (e.g., Peak Signal-to-Noise Ratio) and user experience (e.g., opinion scores). These have been replaced by new quality metrics (e.g., rate of buffering, bitrate) and new engagement centric measures of user experience (e.g., viewing time and number of visits). The goal of this paper is to develop a predictive model of Internet video QoE. To this end, we identify two key requirements for the QoE model: (1) it has to be tied in to observable user engagement and (2) it should be actionable to guide practical system design decisions. Achieving this goal is challenging because the quality metrics are interdependent, they have complex and counter-intuitive relationships to engagement measures, and there are many external factors that confound the relationship between quality and engagement (e.g., type of video, user connectivity). To address these challenges, we present a data-driven approach to model the metric interdependencies and their complex relationships to engagement, and propose a systematic framework to identify and account for the confounding factors. We show that a delivery infrastructure that uses our proposed model to choose CDN and bitrates can achieve more than 20\% improvement in overall user engagement compared to strawman approaches.