90° domain dynamics and relaxation in thin ferroelectric/ferroelastic films

We investigated the dynamics and relaxation of 90° domains in 60-nm-thick lead-zirconium titanate (PbZr0.3 T0.7 O3) films, with enhanced piezoresponse force microscopy. We show that under opposite electric fie ld, ferroelectric domains are reversibly switched while ferroelastic domains reorganize in a nonreversible way. Moreover, we show that the relaxation-time constant of 90° domains is two orders of magnitude shorter than for the previously reported 180° domains relaxation. Furthermore, we demonstrate the influence of geometry and scale on the relaxation process. Finally, we propose a relaxation mechanism for ferroelastic-ferroelectric systems, with implications for devices based on these materials. © 2010 The American Physical Society.

Gain-enhanced optical coherence in a high optical gain semiconductor

The propagation of ultrashort pulses in a traveling wave semiconductor amplifier is considered. It is demonstrated that the effective polarization relaxation time, which determines the coherence of the interaction of pulses within the medium, strongly depends on its optical gain. As a result, it is shown that at large optical gains the coherence time can exceed the transverse relaxation time T2 by an order of magnitude, this accounting for the strong femtosecond superradiant pulse generation commonly observed in semiconductor laser structures. © 2012 Elsevier B.V. All rights reserved.

Validation of a lattice Boltzmann model for gas-solid reactions with experiments

A lattice Boltzmann method is used to model gas-solid reactions where the composition of both the gas and solid phase changes with time, while the boundary between phases remains fixed. The flow of the bulk gas phase is treated using a multiple relaxation time MRT D3Q19 model; the dilute reactant is treated as a passive scalar using a single relaxation time BGK D3Q7 model with distinct inter- and intraparticle diffusivities. A first-order reaction is incorporated by modifying the method of Sullivan et al. [13] to include the conversion of a solid reactant. The detailed computational model is able to capture the multiscale physics encountered in reactor systems. Specifically, the model reproduced steady state analytical solutions for the reaction of a porous catalyst sphere (pore scale) and empirical solutions for mass transfer to the surface of a sphere at Re=10 (particle scale). Excellent quantitative agreement between the model and experiments for the transient reduction of a single, porous sphere of Fe 2O 3 to Fe 3O 4 in CO at 1023K and 10 5Pa is demonstrated. Model solutions for the reduction of a packed bed of Fe 2O 3 (reactor scale) at identical conditions approached those of experiments after 25 s, but required prohibitively long processor times. The presented lattice Boltzmann model resolved successfully mass transport at the pore, particle and reactor scales and highlights the relevance of LB methods for modelling convection, diffusion and reaction physics. © 2012 Elsevier Inc.

Near-ultraviolet zinc oxide nanowire sensor using low temperature hydrothermal growth.

Two near-ultraviolet (UV) sensors based on solution-grown zinc oxide (ZnO) nanowires (NWs) which are only sensitive to photo-excitation at or below 400 nm wavelength have been fabricated and characterized. Both devices keep all processing steps, including nanowire growth, under 100 °C for compatibility with a wide variety of substrates. The first device type uses a single optical lithography step process to allow simultaneous in situ horizontal NW growth from solution and creation of symmetric ohmic contacts to the nanowires. The second device type uses a two-mask optical lithography process to create asymmetric ohmic and Schottky contacts. For the symmetric ohmic contacts, at a voltage bias of 1 V across the device, we observed a 29-fold increase in current in comparison to dark current when the NWs were photo-excited by a 400 nm light-emitting diode (LED) at 0.15 mW cm(-2) with a relaxation time constant (τ) ranging from 50 to 555 s. For the asymmetric ohmic and Schottky contacts under 400 nm excitation, τ is measured between 0.5 and 1.4 s over varying time internals, which is ~2 orders of magnitude faster than the devices using symmetric ohmic contacts.

Role of a high gain of the medium in superradiance generation and in observation of coherent effects in semiconductor lasers

A theoretical model of superradiant pulse generation in semiconductor laser structures is developed. It is shown that a high optical gain of the medium can overcome phase relaxation and results in a built-up superradiant state (macroscopic dipole) in an assembly of electron - hole pairs on a time scale much longer than the characteristic polarisation relaxation time T2. A criterion of the superradiance generation is the condition acmT2 > 1, where α is the gain coefficient and cm is the speed of light in the medium. The theoretical model describes both qualitatively and quantitatively the author's own experimental results.

Electrical properties of LiNbO3 (electrolyte)/Cu (anode) bi-layers

In this work specific film structures of Li-Nb-O/Li/Li-Nb-O are investigated by AC Impedance Spectroscopy measurements at different temperatures. This gives the opportunity to investigate properties of the material itself and, at the same time, to consider the influence of the grain boundaries on the ionic behavior of the polycrystalline Lithium Niobate. On the other hand, LiNbO3/Li/Cu multi-layers are studied as electrolyte/anode bi-layers and potential parts of "Li-free" microbatteries. The Li deficiency in the as deposited Li-Nb-O films is cured by forming a "sandwich" of Li-Nb-O/Li/Li-Nb-O, which after annealing becomes ionic conductor. The electrical behavior of an annealed film depends on two sources. The first is due to properties of the material itself and the second is based on the network of the grain boundaries. The average size of the grains is strongly influenced by the structure of the ohmic-contact/substrate. The electrical behavior of the electrolyte/anode interface of the "Li-free" structure LiNbO3/Li/Cu/Au is very similar to the impedance measurements of the single LiNbO3 single films. The whole multilayer structure, though, presents a third relaxation time which is consistent of a small resistance. This resistance is independent of temperature and it seems that is due to the metallic interface Li/Cu/Au. © 2010 Elsevier B.V. All rights reserved.

On kinematic relaxation and deposition of water droplets in the last stages of low pressure steam turbines

In the first part of the paper steady two-phase flow predictions have been performed for the last stage of a model steam turbine to examine the influence of drag between condensed fog droplets and the continuous vapour phase. In general, droplets due to homogeneous condensation are small and thus kinematic relaxation provides only a minor contribution to the wetness losses. Different droplet size distributions have been investigated to estimate at which size inter-phase friction becomes more important. The second part of the paper deals with the deposition of fog droplets on stator blades. Results from several references are repeated to introduce the two main deposition mechanisms which are inertia and turbulent diffusion. Extensive postprocessing routines have been programmed to calculate droplet deposition due to these effects for a last stage stator blade in three-dimensions. In principle the method to determine droplet deposition by turbulent diffusion equates to that of Yau and Young [1] and the advantages and disadvantages of this relatively simple method are discussed. The investigation includes the influence of different droplet sizes on droplet deposition rates and shows that for small fog droplets turbulent diffusion is the main deposition mechanism. If the droplets size is increased inertial effects become more and more important and for droplets around 1 μm inertial deposition dominates. Assuming realistic droplet sizes the overall deposition equates to about 1% to 3% of the incoming wetness for the investigated guide vane at normal operating conditions. Copyright © 2013 by Solar Turbines Incorporated.