Abstract A nonlinear lumped circuit model for Gunn diodes which includes the effects due to domain extinction and nucleation phenomena is presented. The model is based upon physical principles and allows an arbitrary nonlinear drift velocity curve v ( E ) and a nonlinear diffusion curve D(E) to be specified by the user. It is valid for simulating arbitrary Gunn‐diode circuits operating in any matured high‐field domain mode, or in the LSA mode. Under additional assumptions, the model simplifies to other existing models. Several computer‐simulated examples of Gunn‐diode circuits operating under both steady‐state and transient regimes are presented. Finally, a rigorous definition of a ‘DC’ I‐V curve for Gunn diodes is offered and shown to be rather useful for predicting the qualitative behaviour of Gunn‐diode circuits during all time intervals where a matured domain exists.
Using first-principles calculations, we study the effect of cation substitution on the transition-metal sublattice in phospho-olivines, with special attention given to the Li[subscript x](Fe[subscript 1−y]Mn[subscript y])PO[subscript 4] system. We use a cluster expansion model derived from first-principles with Monte Carlo simulations to calculate finite-T phase diagrams, voltage curves, and solubility limits of the system. The phase diagram of Li[subscript x](Fe[subscript 1−y]Mn[subscript y])PO[subscript 4] shows two low-temperature miscibility gaps separated by a solid solution phase centered at Li composition x≈y, which corresponds to a state where most Fe ions are oxidized and most Mn are not. This intermediate low-T solid solution is stabilized by the dilution of phase-separating interactions caused by the disorder of redox potentials on the transition-metal sites. The calculated voltage curves show two plateaus at ~4–4.2 V and ~3.5–3.7 V, corresponding to the Mn(3+)/Mn(2+) and Fe(3+)/Fe(2+) redox couples, respectively, with an extended sloping region in between corresponding to the low-T solid solution phase. In agreement with experiment, we find that the Mn(3+)/Mn(2+) (Fe(3+)/Fe(2+)) voltage is decreased (increased) by Fe (Mn) substitution. We explain this by considering the energy of the solid solution which is the discharged (charged) state for these redox couples and argue that such changes are generic to all mixed olivine systems. We also find reduced phase transformation polarization on both plateaus which we attribute to the decreased composition difference between the oxidized and reduced state for each redox couple.
We present a probabilistic model for a document corpus that combines many of the desirable features of previous models. The model is called "GaP" for Gamma-Poisson, the distributions of the first and last random variable. GaP is a factor model, that is it gives an approximate factorization of the document-term matrix into a product of matrices Λ and X. These factors have strictly non-negative terms. GaP is a generative probabilistic model that assigns finite probabilities to documents in a corpus. It can be computed with an efficient and simple EM recurrence. For a suitable choice of parameters, the GaP factorization maximizes independence between the factors. So it can be used as an independent-component algorithm adapted to document data. The form of the GaP model is empirically as well as analytically motivated. It gives very accurate results as a probabilistic model (measured via perplexity) and as a retrieval model. The GaP model projects documents and terms into a low-dimensional space of "themes," and models texts as "passages" of terms on the same theme.
We describe a new sample of Seyfert nuclei discovered during the course of an optical spectroscopic survey of nearby galaxies. The majority of the objects, many recognized for the first time, have luminosities much lower than those of classical Seyferts and populate the faint end of the AGN luminosity function. A significant fraction of the nuclei emit broad H-alpha emission qualitatively similar to the broad lines seen in classical Seyfert 1 nuclei and QSOs.
This document provides a description and technical summary of the Second International Engineering Foundation Conference/Workshop on Small Fatigue Cracks, held in Santa Barbara, California, 5-10 January, 1986. It provides a current assessment of the pertinent issues with respect to definition, differences in behavior compared to long cracks, environmental effects, driving forces for small crack extension, intrinsic thresholds, and the application of small crack methodology to life prediction and alloy design. A listing of the conference participants and the technical program are appended to the report.
Abstract not Available.
Aluminum alloys containing small amounts of Zn, Bi, Te, ln, Ga, Pb, and TI, either individually or in various combinations, have been prepared and investigated as possible fuels for alkaline aluminum-air batteries. The binary alloys, containing generally <1% of the alloying elements, exhibit corrosion rates in 4 M KOH at 50 C that are higher than that for pure aluminum in the same medium. Addition of two or more alloying elements can result in a sharp reduction in the open circuit corrosion rate to well below that for aluminum, and several alloys containing Ga, ln, TI, and P that appear promising as high performance fuels were identified. Mechanical processing [cold working (CW)] was found to have little reproducible effect on the corrosion rate, except for Al-0.5% Ga in which case the corrosion rate was found to pass through a minimum at 50% CW. The corrosion rate data also demonstrate that very low concentrations of the alloying elements are sufficient to achieve inhibition and that little is gained by exceeding the critical levels.
The opportunity exists to apply spread spectrum concepts into the enormous bandwidth of optical fibers. We introduce a new optical CDMA network architecture based on passive linear unitary filtering of the optical carrier signals.
Iridium is blind to C: Highly regio- and enantioselective, iridium-catalyzed N-allylations of indoles, which complement the more common reactivity of indoles as carbon nucleophiles, have been developed (see scheme). These reactions form highly enantioenriched N-allylindoles, which are readily transformed into enantioenriched 3-(1H-indol-1-yl)-N-methyl-3-aryl propan-1-amines, dihydropyrrolo[1,2-a]indoles, and indol-1-yl propanoic acids.
Transition metal ions as a template method has been widely used in the field of supramolecular chemistry. The metal complexation is advantageous in making complex supramocluar architectures because it pre-organizes the ligands into a desirable orientation which faciliatates the following ring-closing reaction, with shorter synthetic steps and generally higher yield. In a similar fashion, this synthetic strategy has recently been adopted to make extended materials by linking the metal-coordinated building blocks with design principles of reticular synthesis. Individual building units are stitched together through strong covalent bond formation to yield long covalent molecular threads that are woven two- or three-dimensionally (2D or 3D), at regular intervals templated by the metal ions. For example, by linking functionalized tetrahedrally-shaped metal complexes with linear links through reversible imine bond formation, crystalline 3D covalent organic frameworks with diamond topology, COF-505 and COF-112, have been constructed by design. In particular, the metal templates can be post-synthetically removed so that the threads have high degrees of freedom to move in respect to each other, which leads to unusal mechanical properties of the woven materials.
