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Support for the astronomically calibrated geomagnetic polarity time scale is provided by new 40 Ar/ 39 Ar dating and magnetostratigraphic data from the Hadar Formation, Ethiopia. Three tuffs and a basalt flow at Hadar span the Mammoth subchron of the Gauss chron, and range in age from 3.18±0.01 to 3.40 ±0.03 Ma. Interpolation of sediment accumulation rates in two magnetostratigraphic sections yields an estimated age range of 3.30±0.02 to 3.21±0.01 Ma for the Mammoth subchron, closely coinciding with orbitally based predictions of 3.33 to 3.22 Ma rather than with previous estimates of 3.15 to 3.05 Ma.
Self-assembled polyester dendritic bow-ties with various peripheral groups were prepared, and their association constants were measured by (1)H NMR in CDCl(3). The two complementary dendrons were prepared by attachment of either a bis(adamantylurea) or a glycinylurea to the focal point of the dendron. The parent self-assembled system with benzylidene acetals on one periphery and isopropylidene acetals on the other had an association constant of 520 M(-)(1). Upon deprotection of one dendron, the association constant is increased by more than an order of magnitude as the solubility of the hydroxyl-terminated dendron in CDCl(3) is decreased. In contrast, attachment of tri(ethylene oxide) units to the periphery of one dendron lowers the association constant by almost an order of magnitude. The causes of these relatively large changes in complex strength are discussed in terms of solubility, steric effects, competitive hydrogen bonding, and the structure of the dendritic scaffold.
The growth of fatigue cracks under (tension-tension) cyclic loading is unequivocally demonstrated for ceramic materials, based on experiments using compact-tension specimens of a MgO partially-stabilized zirconia (PSZ), heat treated to vary the fracture toughness K{sub c} from {approximately}3 MPa{radical}m (overaged condition) to 16 MPa{radical}m (peak-toughness condition) and tested in inert and moist environments. Analogous to behavior in metals, cyclic fatigue-crack growth rates (over the range 10{sup {minus}11} to 10{sup {minus}5} m/cycle) are found to be a function of the stress-intensity range, environment, fracture toughness and load ratio, and to show evidence of crack closure. Similarly under variable-amplitude cyclic loading conditions, crack-growth rates show transient accelerations following low-high block overloads and transient retardations following high-low block overloads or single tensile overloads, again analogous to behavior commonly observed in ductile meals. Cyclic crack-growth rates are observed at stress intensities as low as 50% of K{sub c}, and are typically some 7 orders of magnitude faster than corresponding stress-corrosion crack-growth rates under sustained-loading conditions. 23 refs., 6 figs.
The cycloisomerization of 1,5-enynes catalyzed by cationic triphenylphosphinegold(I) complexes produces bicyclo[3.1.0]hexenes. Substitution at all positions of the 1,5-enyne is tolerated, leading to a wide range of bicyclo[3.1.0]hexane structures, including those containing quaternary carbons. Substrates containing a 1,2-disubstituted olefin undergo stereospecific cycloisomerization (cis-olefin produces cis-cyclopropane, and trans-olefin gives trans-cyclopropane). Additionally, enantioenriched bicyclo[3.1.0]hexenes can be obtained from the gold(I)-catalyzed cycloisomerization of enantioenriched 1,5-enynes with excellent chirality transfer. The preparation of tricyclic systems is accomplished through a gold(I)-catalyzed tandem cycloisomerization-ring enlargement reaction.
Studies on the palladium-catalyzed formation of aryl amines, aryl ethers and a-aryl carbonyl compounds from aryl halides are reported.These studies range from synthetic methodology, to detailed mechanistic analysis, to new methods one can use to screen for catalytic covalent bond formation.Improved methods for formation of aryl ethers and room temperature amination chemistry have resulted from a mechanistic understanding of the reaction.Over the past 20 years, palladium-catalyzed methods for aromatic substitution have provided useful routes to biaryl, alkynyl-aryl and vinyl-aryl compounds [1,2], The palladium-catalyzed formation of aromatic carbon-heteroatom bonds has become a useful synthetic tool in the past few years [3].The new direct formation of a-aryl carbonyl compounds provides a method to form sp 3 -aromatic carbon-carbon bonds [4±7], The purpose of this presentation is to provide examples of C±N, C±O and C±C bond formation that have been developed recently in the author's laboratory and to provide mechanistic information that allows one to understand why one type of catalyst is more effective than another for a particular class of transformation (Scheme 1).
We prove analytically that 2-element memristive circuits consisting of a passive linear inductor in parallel with a passive memristor, or an active memristive device, can be described explicitly by a Hamiltonian equation, whose solutions can be periodic or damped, and can be represented analytically by the constants of the motion along the circuit Hamiltonian. Generalizations to 3-element and 2N-element memristive Hamiltonian circuits are also presented where complex bifurcation phenomena including chaos, abound.
ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMechanism of the carbon-carbon cleavage of acetone by the ruthenium benzyne complex (PMe3)4Ru(.eta.2-C6H4): formation and reactivity of an oxametallacyclobutane complexJohn F. Hartwig, Robert G. Bergman, and Richard A. AndersenCite this: J. Am. Chem. Soc. 1990, 112, 8, 3234–3236Publication Date (Print):April 1, 1990Publication History Published online1 May 2002Published inissue 1 April 1990https://pubs.acs.org/doi/10.1021/ja00164a067https://doi.org/10.1021/ja00164a067research-articleACS PublicationsRequest reuse permissionsArticle Views464Altmetric-Citations30LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts
Heat-Assisted Magnetic Recording (HAMR) has promise to allow for data writing in hard disks of beyond 1 Tb/in2 areal density, by temporarily heating the area of a single datum to its Curie temperature while simultaneously applying a magnetic field from a conventional electromagnet. However, the metallic optical antenna or near-field transducer (NFT) used to apply the nano-scale heating to the media may self-heat by several hundreds of degrees. With the NFT reaching such extreme temperatures, demonstrations of HAMR technology observe write-head lifetimes that are orders of magnitude less than that required for commercial product. Hence, thermal reliability of the NFT is of upmost importance. In this paper, we first derive fundamental limits on the self-heating of the NFT to drive design choices for low temperature operation. Next, we employ Inverse Electromagnetic Design software, which provides deterministic gradient-based optimization of electromagnetic structures with thousands of degrees of freedom using the adjoint method. The Inverse Design software solves for unintuitive solutions to Maxwells equations, and we present computationally generated structures for HAMR write-heads that offer a 40 percent or 170 C reduction in NFT self-heating compared to typical industry designs.
A highly reflective rear mirror can increase the efficiency of thermophotovoltaics through regeneration of unused photons. Based on this concept, we recently demonstrated a record 29.1% thermophotovoltaic device efficiency. We have also identified the challenges as we aim towards 50% thermophotovoltaic efficiency.
