Application of gel-casting to the fabrication of 1-3 piezoelectric ceramic-polymer composites for high-frequency ultrasound devices

A modified gel-casting technique was used to fabricate a 1-3 piezoelectric ceramic/polymer composite substrate formed by irregular-shaped pillar arrays of small dimensions and kerfs. This technique involves the polymerization of aqueous piezoelectric (PZT) suspensions with added water-soluble epoxy resin and polyamine-based hardener that lead to high strength, high density and resilient ceramic bodies. Soft micromoulding was used to shape the ceramic segments, and micropillars with lateral features down to 4 m and height-to-width aspect ratios of ∼10 were achieved. The composite exhibited a clear thickness resonance mode at approximately 70 MHz and a k eff ∼ 0.51, demonstrating that the ceramic micropillars possess good electrical properties. Furthermore, gel-casting allows the fabrication of ceramic structures with non-conventional shapes; hence, device design is not limited by the standard fabrication methods. This is of particular benefit for high-frequency transducers where the critical design dimensions are reduced. © 2012 IOP Publishing Ltd.

Liquid crystalline chromophores for photonic band-edge laser devices

We present results on laser action from liquid crystal compounds whereby one sub-unit of the molecular structure consists of the cyano-substituted chromophore, {phenylene-bis (2-cyanopropene)}, similar to the basic unit of the semiconducting polymer structure poly(cyanoterephthalylidene). These compounds were found to exhibit nematic liquid crystal phases. In addition, by virtue of the liquid crystalline properties, the compounds were found to be highly miscible in wide temperature range commercial nematogen mixtures. When optically excited at λ = 355 nm, laser emission was observed in the blue/green region of the visible spectrum (480-530 nm) and at larger concentrations by weight than is achievable using conventional laser dyes. Upon increasing the concentration of dye from 2 to 5 wt.% the threshold was found to increase from Eth = 0.42 ± 0.02 μJ/pulse (≈20 mJ/cm2) to Eth = 0.66 ± 0.03 μJ/pulse (≈34 mJ/cm2). Laser emission was also observed at concentrations of 10 wt.% but was less stable than that observed for lower concentrations of the chromophore. © 2012 Elsevier B.V. All rights reserved.

Magnetocaloric effect and magnetic properties of Tb0.9 Sn0.1 MnO3

The magnetocaloric effect in magnetic materials is of great interest nowadays. In this article we present an investigation about the magnetic properties near the magnetic transition in a polycrystalline sample of a manganite Tb0.9 Sn0.1 MnO3. Particularly, we are interested in describing the nature of the magnetic interactions and the magnetocaloric effect in this compound. The temperature dependence of the magnetization was measured to determine the characteristics of the magnetic transition and the magnetic entropy change was calculated from magnetization curves at different temperatures. The magnetic solid is paramagnetic at high temperatures. We observe a dominant antiferromagnetic interaction below Tn =38 K for low applied magnetic fields; the presence of Sn doping in this compound decreases the Ńel temperature of the pure TbMnO3 system. A drastic increase in the magnetization as a function of temperature near the magnetic transition suggests a strong magnetocaloric effect. We found a large magnetic entropy change Δ SM (T) of about -4 J/kg K at H=3 T. We believe that the magnetic entropy change is associated with the magnetic transition and we interpret it as due to the coupling between the magnetic field and the spin ordering. This relatively large value and broad temperature interval (about 35 K) of the magnetocaloric effect make the present compound a promising candidate for magnetic refrigerators at low temperatures. © 2007 American Institute of Physics.

Anchors, scales and the relative coding of value in the brain.

People are alarmingly susceptible to manipulations that change both their expectations and experience of the value of goods. Recent studies in behavioral economics suggest such variability reflects more than mere caprice. People commonly judge options and prices in relative terms, rather than absolutely, and display strong sensitivity to exemplar and price anchors. We propose that these findings elucidate important principles about reward processing in the brain. In particular, relative valuation may be a natural consequence of adaptive coding of neuronal firing to optimise sensitivity across large ranges of value. Furthermore, the initial apparent arbitrariness of value may reflect the brains' attempts to optimally integrate diverse sources of value-relevant information in the face of perceived uncertainty. Recent findings in neuroscience support both accounts, and implicate regions in the orbitofrontal cortex, striatum, and ventromedial prefrontal cortex in the construction of value.

Detailed design of242mAm breeding in pressurized water reactors

There is growing interest in the use of 242mAm as a nuclear fuel. Because of its very high thermal fission cross section and its large number of neutrons released per fission, it can be used for various unique applications, such as space propulsion, medical applications, and compact energy sources. Since the thermal absorption cross section of 242mAm is very high, the best way to obtain 242mAm is by the capture of fast or epithermal neutrons in 241Am. However, fast spectrum reactors are not readily available. In this paper, we explore the possibility of producing 242mAm in existing pressurized water reactors (PWRs) with minimal interference in reactor performance. As suggested in previous studies on the subject, the 242mAm breeding targets are shielded with strong thermal absorbers in order to suppress the thermal neutron flux that causes 242mAm destruction. Since 242mAm enrichment within the Am target mainly depends on the neutron energy distribution, which in turn depends on the Am target thickness and on the neutron filter cutoff energy (thermal absorber type), this unique Am target design was developed. In our study, Cd, Sm, and Gd were considered as thermal neutron filters, as suggested by Cesana et al. The most favorable results were obtained by irradiating Am targets covered either with Gd or Cd. In these cases, up to 8.65% enrichment of 242mAm is obtained after 4.5 yr (three successive PWR fuel cycles) of irradiation. It was also found that significant quantities [up to 1.3 kg/GW (electric)-yr] of 242mAm can be obtained in PWR reactors without notable interference with reactor performance. However, in order to maintain the original fuel cycle length, the enrichment of the driver (UO2) fuel must be increased by ∼1%, raised from the conventional 4.5 to 5.5%, depending on the thermal neutron filter used. The most important reactivity feedback coefficients for fuel assemblies containing the 242mAm breeding targets were evaluated and found to be close to those of a standard PWR. Another product of neutron capture in the 241Am reaction is 238Pu. It was found that in a typical 1000 MW (electric) PWR core with one-third of the fuel assemblies containing 241Am targets, up to 15.1 kg of 238Pu enriched to 80% can be produced per year.

A printable optical time-temperature integrator based on shape memory in a chiral nematic polymer network

An optical and irreversible temperature sensor (e.g., a time-temperature integrator) is reported based on a mechanically embossed chiral-nematic polymer network. The polymer consists of a chemical and a physical (hydrogen-bonded) network and has a reflection band in the visible wavelength range. The sensors are produced by mechanical embossing at elevated temperatures. A relative large compressive deformation (up to 10%) is obtained inducing a shift to shorter wavelength of the reflection band (>30 nm). After embossing, a temperature sensor is obtained that exhibits an irreversible optical response. A permanent color shift to longer wavelengths (red) is observed upon heating of the polymer material to temperatures above the glass transition temperature. It is illustrated that the observed permanent color shift is related to shape memory in the polymer material. The films can be printed on a foil, thus showing that these sensors are potentially interesting as time-temperature integrators for applications in food and pharmaceutical products. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A survey of alternative once-through fast reactor core designs

Reprocessing of Light Water Reactor (LWR) spent fuel to recover plutonium or transuranics for use in Sodium cooled Fast Reactors (SFRs) is a distant prospect in the U.S.A. This has motivated our evaluation of potentially cost-effective operation of uranium startup fast reactors (USFRs) in a once-through mode. This review goes beyond findings reported earlier based on a UC fueled MgO reflected SFR to describe a broader parametric study of options. Cores were evaluated for a variety of fuel/coolant/reflector combinations: UC/UZr/UO 2/UN;Na/Pb; MgO/SS/Zr. The challenge is achieving high burnup while minimizing enrichment and respecting both cladding fluence/dpa and reactivity lifetime limits. These parametric studies show that while UC fuel is still the leading contender, UO 2 fuel and ZrH 1.7 moderated metallic fuel are also attractive if UC proves to be otherwise inadequate. Overall, these findings support the conclusion that a competitive fuel cycle cost and uranium utilization compared to LWRs is possible for SFRs operated on a once-through uranium fueled fuel cycle. In addition, eventual transition to TRU recycle mode is studied, as is a small test reactor to demonstrate key features.

Spontaneous emission from radiative chiral nematic liquid crystals at the photonic band-gap edge: An investigation into the role of the density of photon states near resonance

In this article, we investigate the spontaneous emission properties of radiating molecules embedded in a chiral nematic liquid crystal, under the assumption that the electronic transition frequency is close to the photonic edge mode of the structure, i.e., at resonance. We take into account the transition broadening and the decay of electromagnetic field modes supported by the so-called "mirrorless"cavity. We employ the Jaynes-Cummings Hamiltonian to describe the electron interaction with the electromagnetic field, focusing on the mode with the diffracting polarization in the chiral nematic layer. As known in these structures, the density of photon states, calculated via the Wigner method, has distinct peaks on either side of the photonic band gap, which manifests itself as a considerable modification of the emission spectrum. We demonstrate that, near resonance, there are notable differences between the behavior of the density of states and the spontaneous emission profile of these structures. In addition, we examine in some detail the case of the logarithmic peak exhibited in the density of states in two-dimensional photonic structures and obtain analytic relations for the Lamb shift and the broadening of the atomic transition in the emission spectrum. The dynamical behavior of the atom-field system is described by a system of two first-order differential equations, solved using the Green's-function method and the Fourier transform. The emission spectra are then calculated and compared with experimental data. © 2013 American Physical Society.

Spontaneous emission from radiative chiral nematic liquid crystals at the photonic band-gap edge: an investigation into the role of the density of photon states near resonance.

In this article, we investigate the spontaneous emission properties of radiating molecules embedded in a chiral nematic liquid crystal, under the assumption that the electronic transition frequency is close to the photonic edge mode of the structure, i.e., at resonance. We take into account the transition broadening and the decay of electromagnetic field modes supported by the so-called "mirrorless"cavity. We employ the Jaynes-Cummings Hamiltonian to describe the electron interaction with the electromagnetic field, focusing on the mode with the diffracting polarization in the chiral nematic layer. As known in these structures, the density of photon states, calculated via the Wigner method, has distinct peaks on either side of the photonic band gap, which manifests itself as a considerable modification of the emission spectrum. We demonstrate that, near resonance, there are notable differences between the behavior of the density of states and the spontaneous emission profile of these structures. In addition, we examine in some detail the case of the logarithmic peak exhibited in the density of states in two-dimensional photonic structures and obtain analytic relations for the Lamb shift and the broadening of the atomic transition in the emission spectrum. The dynamical behavior of the atom-field system is described by a system of two first-order differential equations, solved using the Green's-function method and the Fourier transform. The emission spectra are then calculated and compared with experimental data.

A nano-patterned photonic crystal laser with a dye-doped liquid crystal

Covering a nano-patterned titanium dioxide photonic crystal (PC) within a well-oriented film of dye-doped liquid crystal (LC), a distributed feedback laser is constructed whereby the emission characteristics can be manipulated in-situ using an electric field. This hybrid organic-inorganic structure permits simultaneous selectivity of both the beam pattern and laser wavelength by electrical addressing of the LC director. In addition, laser emission is obtained both in the plane and normal to the PC. Along with experimental data, a theoretical model is presented that is based upon an approximate calculation of the band structure of this birefringent, tuneable laser device. © 2013 AIP Publishing LLC.

Optically activated shutter using a photo-tunable short-pitch chiral nematic liquid crystal

We report the demonstration of an optically activated shutter based upon a short-pitch chiral nematic liquid crystal (LC) device sandwiched between crossed polarizers. This LC is comprised of photo-active chiral dopants. In the trans-state, the LC appears dark between crossed polarizers due to the very short pitch. As the pitch is extended through exposure to ultraviolet light, the device becomes transmissive reaching a maximum for a particular value of the pitch. As a result, it is possible to switch between the light and dark states by subjecting the device to visible light so as to cause a cis-trans photo-isomerisation. © 2013 AIP Publishing LLC.

Adaptive holographic pumping of thin-film organic lasers

In this Letter, we use a reconfigurable hologram to dynamically control the position of incidence of the pump beam onto a liquid-crystal dye-based laser. The results show that there is an increase in the stability of the laser output with time and the average power when compared with the output of the same laser when it is optically excited using a static pump beam. This technique also provides additional functionality, such as wavelength tuning and spatial shaping of the pump beam, both of which are demonstrated here. © 2013 Optical Society of America.