The growth of fatigue cracks at elevated temperatures (25–1300°C) is examined under cyclic loading in an in situ toughened, monolithic silicon carbide with Al–B–C additions (termed ABC–SiC), with specific emphasis on the roles of temperature, load ratio, cyclic frequency, and loading mode (static vs cyclic). Extensive crack-growth data are presented, based on measurements from an electrical potential-drop crack-monitoring technique, adapted for use on ceramics at high temperatures. It was found that at equivalent stress-intensity levels, crack velocities under cyclic loads were significantly faster than those under static loads. Fatigue thresholds were found to decrease with increasing temperature up to 1200°C; behavior at 1300°C, however, was similar to that at 1200°C. Moreover, no effect of frequency was detected (between 3 and 1000 Hz), nor evidence of creep cavitation or crack bridging by viscous ligaments or grain-boundary glassy phases in the crack wake. Indeed, fractography and crack-path sectioning revealed a fracture mode at 1200–1300°C that was essentially identical to that at room temperature, i.e. predominantly intergranular cracking with evidence of grain bridging in the crack wake. Such excellent crack-growth resistance is attributed to a process of grain-boundary microstructural evolution at elevated temperatures, specifically involving crystallization of the amorphous grain-boundary films/phases.
We present experimental results on the spontaneous emission from various types of microscopic dielectric resonator structures, which have been designed for the goal of being useful for making a single-mode LED.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>
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The interactions of water with H−ZSM-5, Cu−ZSM-5, and Co−ZSM-5 have been investigated by density functional theory (DFT). Calculations were also performed to determine the thermodynamics of metal removal from the zeolite. For all three forms of ZSM-5, water adsorbs by direct interaction of the O atom with the cation and hydrogen bonding of one of the two H atoms with an oxygen atom in the zeolite framework. The magnitude of the energy of adsorption of one H2O molecule decreases in the order Cu−ZSM-5 (Cu as Cu+) > Co−ZSM-5 (Co as Co2+(OH)-) > H−ZSM-5 > Cu−ZSM-5 (Cu as Cu2+(OH)-). Adsorption of a second H2O molecule occurs with a smaller binding energy, which decreases in the order Cu−ZSM-5 (Cu as Cu+) > Cu−ZSM-5 (Cu as Cu2+(OH)-) > Co−ZSM-5 (Co as Co2+(OH)-) > H−ZSM-5. At 800 K, demetalation to form a gaseous metal hydroxide species is unfavorable thermodynamically but will occur spontaneously if the final product is a metal oxide. Demetalation of Cu is projected to occur much more readily than demetalation of Co, in good agreement with experimental observation.
A FM and sum frequency generation (SFG) vibrational spectroscopy experiments provide complementary information that can be used to relate the morphology and mechanical properties of a polymer interface to the molecular structure of the interface. The application of the two techniques to the study of polymer interface structure is presented, focusing on surface segregation and wetting behavior of polyolefin blends and on the interface structure and mechanical behavior of hydrogels exposed to various hydration conditions.
A qualitative theory of the circuit dynamics of the Josephson-junction device using a simple circuit model is developed. By examining trajectories on a cylindrical phase space and on the surface of a torus, respectively, the main features of thel/V characteristics of the junction excited by D.C. and a.c. are explained, and their properties derived. These results lead to the interpretation of the steps of constant voltage in the a.c. characteristics as a manifestation of the synchronisation phenomenon
Abstract Predictive control over the selectivity outcome of an organic synthetic method is an essential hallmark of reaction success. Electricity‐driven synthesis offers a reemerging approach to facilitate the design of reaction sequences toward increased molecular complexity. In addition to the desirable sustainability features of electroorganic processes, the inherent interfacial nature of electrochemical systems present unique opportunities to tune reaction selectivity. To illustrate this feature, we outline examples of mechanism‐guided interfacial control over CO 2 electroreduction selectivity; a well‐studied and instructive electrochemical process with multiple reduction products that are thermodynamically accessible. These studies reveal how controlled proton delivery to the electrode surface and substrate electrosorption with the electrode dictate reaction selectivity. We describe and compare simple, yet salient, examples from the electroorganic literature, where we postulate that similar effects predominate the observed reactivity. This perspective highlights how the interface serves as a tunable dimension in electrochemical processes and delineates unique tools to study, manipulate, and achieve reaction selectivity in electricity‐driven organic synthesis. KEY POINTS Electricity‐driven synthesis enables chemical and industrial communities to contribute to sustainability goals. Electricity‐driven processes occur at phase boundaries, thus unveiling their molecular‐level roadmaps involves the synergistic study of materials, interfacial, and molecular science. Bridging concepts that transcend the topical nature of two electricity‐driven processes—CO 2 reduction and electroorganic synthesis—reveals tools to manipulate reaction selectivity.
An improved nonlinear circuit model for IMPATT diodes is presented for which each element bears a simple relationship with the physical operating mechanisms inside the device. The model contains lumped nonlinear elements as well as lumped and distributed linear elements. In its most general form it incorporates various second-order effects heretofore neglected in other circuit models. These include the effects due to unequal hole and electron ionization rates, unequal hole and electron drift velocities, and carrier diffusion.
The emergence of Accountable Care Organizations (ACOs), Patient-Centered Medical Homes (PCMHs), and related payment and delivery system innovations provides an unparalleled opportunity to advance the health of the U.S. population. […]
Research Article| July 01, 1994 Tectonics of the Pliocene Loreto basin, Baja California Sur, Mexico, and evolution of the Gulf of California Paul J. Umhoefer; Paul J. Umhoefer 1Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011-4099 Search for other works by this author on: GSW Google Scholar Rebecca J. Dorsey; Rebecca J. Dorsey 1Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011-4099 Search for other works by this author on: GSW Google Scholar Paul Renne Paul Renne 2Institute of Human Origins, 2453 Ridge Road, Berkeley, California 94709 Search for other works by this author on: GSW Google Scholar Geology (1994) 22 (7): 649–652. https://doi.org/10.1130/0091-7613(1994)022<0649:TOTPLB>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Paul J. Umhoefer, Rebecca J. Dorsey, Paul Renne; Tectonics of the Pliocene Loreto basin, Baja California Sur, Mexico, and evolution of the Gulf of California. Geology 1994;; 22 (7): 649–652. doi: https://doi.org/10.1130/0091-7613(1994)022<0649:TOTPLB>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Pliocene Loreto basin is a transtensional basin, exposed just north of Loreto, Baja California Sur, that consists of nonmarine to marine sedimentary rocks and interbedded tuffs. On the basis of stratigraphic study and 40Ar/39Ar dating of tuffs, the southern Loreto basin began to form at ∼3.4 Ma as an alluvial basin with moderate rates of subsidence (<0.4 mm/yr). Between 2.46 and 2.36 Ma, extremely rapid subsidence (5-10 mm/yr) of the basin was accompanied by deposition of vertically stacked Gilbert-type fan deltas. During this period, the southernmost basin began to be cut by an array of dextral-normal faults. The basin returned to a moderate rate of subsidence from ∼2.36 to ∼2.0 Ma, when volcanism increased within the northern part of the basin. The Pacific-North America boundary was first fully located within the Gulf of California beginning at ∼3.5 Ma, at which time the zone of transform-related deformation widened to initiate formation of the Loreto basin. The major change in the Loreto basin at 2.46 Ma may be coeval with the beginning of faulting in southern California on the Elsinore and San Jacinto faults. These widespread events may indicate a minor change in the plate boundary at ∼2.5 Ma. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Reactive polymers play many roles, from supports for solid-phase synthesis or catalysis to media for separations. Although macroporous polymer beads that provide high reactive capacities and excellent solvent tolerance are well established, approaches to monosized beads with optimized pore structures or multiple chemistries segregated within pores of different sizes have expanded their realm of application. Polymer monoliths containing intricate pore networks can be obtained in any desired shape by a simple molding process and provide unique advantages such as fast kinetics, high reactivity, and high throughput. Applications ranging from immobilized enzyme reactors to fast media for the separation of synthetic or biopolymers are presented.
