4,425 research outputs found
Juncture stress fields in multicellular shell structures. Vol. IX - Summary of results and recommendations Final report
Juncture stress fields in multicellular shell structures - bending and bucking analyse
A Multivariate GARCH Model with Time-Varying correlations
In this paper we propose a new multivariate GARCH model with time- varying correlations. We adopt the vech representation based on the conditional variances and the conditional correlations. While each conditional-variance term is assumed to follow a univariate GARCH formulation, the conditional-correlation matrix is postulated to follow an autoregressive moving average type of analogue. By imposing some suitable restrictions on the conditional-correlation-matrix equation, we construct a MGARCH model in which the conditional-correlation matrix is guaranteed to be positive definite during the optimisation. Thus, our new model retains the intuition and interpretation of the univariate GARCH model and yet satisfies the positive-definite condition as found in the constant-correlation and BEKK models. We report some Monte Carlo results on the finite-sample distributions of the MLE of the varying- correlation MGARCH model. The new model is applied to some real data sets. It is found that extending the constant-correlation model to allow for time-varying correlations provides some interesting time histories that are not available in a constant-correlation model.BEKK model, constant correlation, Monte Carlo method, multivariate GARCH model, maximum likelihood estimate, varying correlation
Attenuation of stress waves in single and multi-layered structures
Analytical and experimental studies were made of the attenuation of the stress waves during passage through single and multilayer structures. The investigation included studies on elastic and plastic stress wave propagation in the composites and those on shock mitigating material characteristics such as dynamic stress-strain relations and energy absorbing properties. The results of the studies are applied to methods for reducing the stresses imposed on a spacecraft during planetary or ocean landings
Optimum design of structures of composite materials in response to aerodynamic noise and noise transmission
Elastic wave propagation and attenuation in a model fiber matrix was investigated. Damping characteristics in graphite epoxy composite materials were measured. A sound transmission test facility suitable to incorporate into NASA Ames wind tunnel for measurement of transmission loss due to sound generation in boundary layers was constructed. Measurement of transmission loss of graphite epoxy composite panels was also included
Field-Induced Resistive Switching in Metal-Oxide Interfaces
We investigate the polarity-dependent field-induced resistive switching
phenomenon driven by electric pulses in perovskite oxides. Our data show that
the switching is a common occurrence restricted to an interfacial layer between
a deposited metal electrode and the oxide. We determine through impedance
spectroscopy that the interfacial layer is no thicker than 10 nm and that the
switch is accompanied by a small capacitance increase associated with charge
accumulation. Based on interfacial I-V characterization and measurement of the
temperature dependence of the resistance, we propose that a field-created
crystalline defect mechanism, which is controllable for devices, drives the
switch.Comment: 4 pages, 3 figure
Plasma-Like Negative Capacitance in Nano-Colloids
A negative capacitance has been observed in a nano-colloid between 0.1 and
10^-5 Hz. The response is linear over a broad range of conditions. The
low-omega dispersions of both the resistance and capacitance are consistent
with the free-carrier plasma model, while the transient behavior demonstrates
an unusual energy storage mechanism. A collective excitation, therefore, is
suggested.Comment: 3 pages, 3 figure
Field-Driven Hysteretic and Reversible Resistive Switch at the Ag-Pr0.7Ca0.3MnO3 Interface
The hysteretic and reversible polarity-dependent resistive switch driven by
electric pulses is studied in both Ag/Pr0.7Ca0.3MnO3/YBa2Cu3O7 sandwiches and
single-layer Pr0.7Ca0.3MnO3 strips. The data demonstrate that the switch takes
place at the Ag-Pr0.7Ca0.3MnO3 interface. A model, which describes the data
well, is proposed. We further suggest that electrochemical diffusion is the
cause for the switch.Comment: 4 pages, 3 figures; submitted to Appl. Phys. Lett. (December 17,
2002
Juncture stress fields in multicellular shell structures. Volume III - Stresses and deformations on fixed-edge segmental conical shells Final report
Equations for thin elastic conical shells and digital program for analysis of stress and deformation on fixed edge segmental conical shells - solution by finite difference techniqu
Juncture stress fields in multicellular shell structures. Volume IV - Stresses and deformations of fixed-edge segmental spherical shells Final report
Equations for thin elastic spherical shells and digital program for analysis of stresses and deformation of fixed edge segmental spherical shells - solution by finite difference techniqu
Carbon dioxide cleavage by a Ni_2 complex supported by a binucleating bis(N-heterocyclic carbene) framework
A binucleating bis(N-heterocyclic carbene) ligand was designed as a means to coordinate and proximally constrain two transition metal centers. Using an imidazopyridine-based NHC afforded a framework structurally related to previously reported para-terphenyl diphosphines. Bimetallic copper, cobalt, and nickel complexes supported by this framework were synthesized and structurally characterized. Strong interactions between the metal centers and the central arene were observed in all nickel complexes. Dinickel(0) complexes of this ligand framework were found to react with CO_2 to form a dicarbonyl-bridged dinickel(0) product, demonstrating facile CO_2 reduction
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