1,331 Matching Results

Search Results

Advanced search parameters have been applied.

ANOMALY STRUCTURE OF SUPERGRAVITY AND ANOMALY CANCELLATION

Description: We display the full anomaly structure of supergravity, including new D-term contributions to the conformal anomaly. This expression has the super-Weyl and chiral U(1){sub K} transformation properties that are required for implementation of the Green-Schwarz mechanism for anomaly cancellation. We outline the procedure for full anomaly cancellation. Our results have implications for effective supergravity theories from the weakly coupled heterotic string theory.
Date: June 10, 2009
Creator: Butter, Daniel & Gaillard, Mary K.
Partner: UNT Libraries Government Documents Department

Asymmetric Synthesis of (-)-Incarvillateine Employing an Intramolecular Alkylation via Rh-Catalyzed Olefinic C-H Bond Activation

Description: An asymmetric total synthesis of (-)-incarvillateine, a natural product having potent analgesic properties, has been achieved in 11 steps and 15.4% overall yield. The key step is a rhodium-catalyzed intramolecular alkylation of an olefinic C-H bond to set two stereocenters. Additionally, this transformation produces an exocyclic, tetrasubstituted alkene through which the bicyclic piperidine moiety can readily be accessed.
Date: February 18, 2008
Creator: Tsai, Andy; Bergman, Robert & Ellman, Jonathan
Partner: UNT Libraries Government Documents Department

Optimal information transmission in organizations: search and congestion

Description: We propose a stylized model of a problem-solving organization whose internal communication structure is given by a fixed network. Problems arrive randomly anywhere in this network and must find their way to their respective specialized solvers by relying on local information alone. The organization handles multiple problems simultaneously. For this reason, the process may be subject to congestion. We provide a characterization of the threshold of collapse of the network and of the stock of floating problems (or average delay) that prevails below that threshold. We build upon this characterization to address a design problem: the determination of what kind of network architecture optimizes performance for any given problem arrival rate. We conclude that, for low arrival rates, the optimal network is very polarized (i.e. star-like or centralized), whereas it is largely homogeneous (or decentralized) for high arrival rates. These observations are in line with a common transformation experienced by information-intensive organizations as their work flow has risen in recent years.
Date: January 1, 2008
Creator: Arenas, A.; Cabrales, A.; Danon, L.; Diaz-Guilera, A.; Guimera, R. & Vega-Redondo, F.
Partner: UNT Libraries Government Documents Department

String Field Equations from Generalized Sigma Model

Description: We propose a new approach for deriving the string field equations from a general sigma model on the world-sheet. This approach leads to an equation which combines some of the attractive features of both the renormalization group method and the covariant beta function treatment of the massless excitations. It has the advantage of being covariant under a very general set of both local and non-local transformations in the field space. We apply it to the tachyon, massless and first massive level, and show that the resulting field equations reproduce the correct spectrum of a left-right symmetric closed bosonic string.
Date: January 29, 1997
Creator: Bardakci, K. & Bernardo, L.M.
Partner: UNT Libraries Government Documents Department

Automatic Transformations in the Inference Process

Description: A technique for incorporating automatic transformations into processes such as the application of inference rules, subsumption, and demodulation provides a mechanism for improving search strategies for theorem proving problems arising from the field of program verification. The incorporation of automatic transformations into the inference process can alter the search space for a given problem, and is particularly useful for problems having broad rather than deep proofs. The technique can also be used to permit the generation of inferences that might otherwise be blocked and to build some commutativity or associativity into the unification process. Appropriate choice of transformations, and new literal clashing and unification algorithms for applying them, showed significant improvement on several real problems according to several distinct criteria.
Date: July 1980
Creator: Veroff, Robert L.
Partner: UNT Libraries Government Documents Department

A Methodology for Algorithm Development Through Schema Transformations

Description: A programming methodology based on schema transformations is presented. Such an approach is a logical outcome of recent developments in program manipulating systems. Concurrent development of algorithms and their proofs of correctness is a significant feature of the proposed methodology. As the development process begins with an abstract schema, it is often possible to derive several related end algorithms in a single development process. This has implications in both the economics of software development and the understanding and teaching of algorithms. The initial schematic specification (a skeleton algorithm schema), the intermediate and final algorithm schemata are all expressed in Darlington's first-order recursion equation language exploiting set-theoretic constructs. A set of transformation rules together with a set of reduction rules for set expressions is then used to successively transform the schematic specification into different algorithm schemata. Most of the transformations are applications of a small number of common rewriting rules.
Date: July 1982
Creator: Muralidharan, M. N.
Partner: UNT Libraries Government Documents Department

Computation of Confidence Limits for Linear Functions of the Normal Mean and Variance

Description: A program is described that calculates exact and optimal (uniformly most accurate unbiased) confidence limits for linear functions of the normal mean and variance. The program can therefore also be used to calculate confidence limits for monotone transformations of such functions (e.g., lognormal means). The accuracy of the program has been thoroughly evaluated in terms of coverage probabilities for a wide range of parameter values.
Date: September 1, 1999
Creator: Land, C.E. & Lyon, B.F.
Partner: UNT Libraries Government Documents Department

In Situ Spectroscopic Observation of Activation and Transformation of Tantalum Suboxides

Description: Using ambient pressure X-ray Photoelectron Spectroscopy (AP-XPS), we were able to observe the process of oxidation of tantalum with different morphological parameters. Being able to trace surface evolution during oxidation, we evaluated activation energy of oxidation under the influence of strain and grain boundaries. It was found that the metal oxidized through three different stages and there was a transition stage where the phase transformation from suboxides to the equilibrium state of pentoxide. The applied stress and surface defects reduced the activation energy oxidation.
Date: December 16, 2009
Creator: Wang, Ke; Liu, Zhi; Cruz, Tirma Herranz; Salmeron, Miquel & Liang, Hong
Partner: UNT Libraries Government Documents Department

Burst Martensitic Transformations in a Steel and in a Pu-Ga Alloy

Description: Upon cooling a Pu-2.0 at% Ga alloy from the ambient temperature, the metastable delta phase partially transforms martensitically to the alpha-prime phase. Because this transformation involves a 25% volume contraction, plastic accommodation by the delta matrix must occur. When the material is isochronally heated or isothermally annealed above ambient temperatures, the reversion of alpha-prime to delta is likely to occur by the alpha-prime/delta interface moving to consume the alpha-prime particles. This reversion exhibits a burst martensitic mode and is observed as sharp spikes in differential scanning calorimetry data and as steps in resistometry data. These large bursts appear to be the result of an interplay between the autocatalytically driven transformation of individual alpha-prime particles and self-quenching caused by small changes in temperature and/or stress accompanying each burst. The behavior of this Pu-Ga alloy is compared to that of a steel referred to as a ''burst martensite'' in the literature, which also exhibits bursts during both thermal cycling and isothermal holds.
Date: June 14, 2005
Creator: Blobaum, K; Krenn, C; Wall, M & Schwartz, A
Partner: UNT Libraries Government Documents Department

Near-equilibrium polymorphic phase transformations in Praseodymium under dynamic compression

Description: We report the first experimental observation of sequential, multiple polymorphic phase transformations occurring in Praseodymium dynamically compressed using a ramp wave. The experiments also display the signatures of reverse transformations occuring upon pressure release and reveal the presence of small hysteresys loops. The results are in very good agreement with equilibrium hydrodynamic calculations performed using a thermodynamically consistent, multi-phase equation of state for Praseodymium, suggesting a near-equilibrium transformation behavior.
Date: February 12, 2007
Creator: Bastea, M & Reisman, D
Partner: UNT Libraries Government Documents Department

On the Micromechanisms of Shock-Induced Martensitic Transformation in Tantalum

Description: Shock-induced twinning and martensitic transformation in tantalum, which exhibits no solid-state phase transformation under hydrostatic pressures up to 100 GPa, have been further investigated. Since the volume fraction and size of twin and phase domains are small in scale, they are considered foming by heterogeneous nucleation that is catalyzed by high density lattice dislocations. A dynamic dislocation mechanism is accordingly proposed based upon the observation of dense dislocation clustering within shock-recovered tantalum. The dense dislocation clustering can cause a significant increase of strain energy in local regions of {beta} (bcc) matrix, which renders mechanical instability and initiates the nucleation of twin and phase domains through the spontaneous reactions of dislocation dissociation within the dislocation clusters. That is, twin domains can be nucleated within the clusters through the homogeneous dissociation of 1/2<111> dislocations into 1/6<111> partial dislocations, and {omega} phase domains can be nucleated within the closters through the inhomogeneous dissociation of 1/2<111> dislocations into 1/12<111>, 1/3<111> and 1/12<111> partial dislocations.
Date: December 7, 2005
Creator: Hsiung, L L
Partner: UNT Libraries Government Documents Department

Spinodal Decomposition and Ordering Transformation in U-6 wt% Nb

Description: Phase stability and aging mechanisms in a water-quenched (WQ) U-6wt% Nb (WQ-U6Nb) alloy artificially aged at 200 C (16 hours) and naturally aged at room temperature for 15 years have been investigated. Age hardening/softening phenomenon is recorded from the artificially aged samples by microhardness measurement. The age hardening can be readily rationalized by the occurrence of spinodal decomposition (or fine-scaled Nb segregation), which results in the formation of a modulated structure in the aged samples. Prolonged aging leads to age softening of the alloy by coarsening of the modulated structure. Disorder-order or chemical ordering transformation is found within the naturally aged alloy according to TEM observations of antiphase domain boundaries (APBs) and superlattice diffraction patterns. The formation of a partially ordered phase in the naturally aged alloy is proposed and identified.
Date: August 15, 2005
Creator: Hsiung, L M
Partner: UNT Libraries Government Documents Department

Structure, chemistry, and properties of mineral nanoparticles

Description: Nanoparticle properties can depart markedly from their bulk analog materials, including large differences in chemical reactivity, molecular and electronic structure, and mechanical behavior. The greatest changes are expected at the smallest sizes, e.g. 10 nm and below, where surface effects are expected to dominate bonding, shape and energy considerations. The precise chemistry at nanoparticle interfaces can have a profound effect on structure, phase transformations, strain, and reactivity. Certain phases may exist only as nanoparticles, requiring transformations in chemistry, stoichiometry and structure with evolution to larger sizes. In general, mineralogical nanoparticles have been little studied.
Date: December 2, 2008
Creator: Waychunas, G.A.; Zhang, H. & Gilbert, B.
Partner: UNT Libraries Government Documents Department

Spontaneous Generation of Voltage in Single-Crystal Gd5Si2Ge2 During Magnetostructural Phase Transformations

Description: The spontaneous generation of voltage (SGV) in single-crystal and polycrystalline Gd{sub 5}Si{sub 2}Ge{sub 2} during the coupled magnetostructural transformation has been examined. Our experiments show reversible, measurable, and repeatable SGV responses of the materials to the temperature and magnetic field. The parameters of the response and the magnitude of the signal are anisotropic and rate dependent. The magnitude of the SGV signal and the critical temperatures and critical magnetic fields at which the SGV occurs vary with the rate of temperature and magnetic-field changes.
Date: April 19, 2006
Creator: Zou, M.; Tang, H.; Schlagel, D.L.; Lograsso, T.A.; K.A. Gschneidner,jr. & Pecharsky, V.K.
Partner: UNT Libraries Government Documents Department

Neutron Scattering Studies of Pre-Transitional Effects in Solid-Solid Phase Transformations

Description: Neutron scattering studies have played a fundamental role in understanding solid-solid phase transformations, particularly in studying the lattice dynamical behavior associated with precursor effects. A review of the studies performed on solids exhibiting Martensitic transformations is given below. The mode softening and associated elastic diffuse scattering, previously observed in NiAl alloys, will be discussed as well as more recent work on Ni{sub 2}MnGa, a system exhibiting magnetic order as well as a Martensitic transformation. Also, new results on the precursor effects in ordered and disordered FePt alloys will be presented.
Date: June 30, 1999
Creator: Shapiro, S. M.
Partner: UNT Libraries Government Documents Department