Search results for "Roca"

Dielectric investigations of pure and mixed fluorocarbons in their condensed phases

1991

We review recent work and present new results on the liquid and solid fluocarbons CMF3 with M = H, F, Cl, Br, CH3, and CF3. The pure compounds as well as several series of binary mixtures were investigated using dielectric measurements. The permittivities of the non-hydrogenatcd non-polar and polar liquids could be described using the Clausius-Mosotti and the Onsager equation, respectively. In CHF3 and C2H3F3 hydrogen bridging leads to deviations from this simple behavior. The trends in the miscibility of four series of fluorocarbon mixtures are in qualitative accord with regular solution theory. The geometrical shapes of the compounds dominate the degrees of order below their melting point…

Phase Transitions and Electrocaloric Effect in Ca-Modified Na1/2Bi1/2TiO3–SrTiO3–PbTiO3Solid Solutions

2014

Sugar-induced stabilization of the monoolein Pn3m bicontinuous cubic phase during dehydration

2001

To explore the molecular mechanism of the protective function of sugars on cubic lipidic systems, the mesomorphic properties of the monoolein-water system, dehydrated in the presence of a series of sugars, have been studied by osmotic stress experiments. Two bicontinuous inverse cubic structures $(Pn3m$ and $\mathrm{Ia}3d)$ and a lamellar ${L}_{\ensuremath{\alpha}}$ phase form under dehydration in pure water. In sugar solutions, the $\mathrm{Pn}3m$ phase shows an extraordinary stability: as a function of sugar concentration, the lattice parameter decreases to very low values, but no phase transitions occur. Instead, the $\mathrm{Pn}3m$ to $\mathrm{Ia}3d$ phase transition is obtained by equi…

Electrocaloric Effect in Na1/2Bi1/2TiO3-SrTiO3-PbTiO3Solid Solutions

2012

Electrocaloric effect is studied in a group of triple solid solutions 0.4Na1/2Bi1/2TiO3-(0.6-y)SrTiO3-yPbTiO3. Contribution of electric field-induced phase transition in the total value of temperature change, caused by electric field, is evaluated. The highest value of electrocaloric effect is observed for composition 0.4Na1/2Bi1/2TiO3-0.4SrTiO3-0.2PbTiO3, in which the depolarization temperature, characteristic for relaxor ferroelectrics, merges with the temperature of maximum of dielectric permittivity. Applicability of thermodynamic description is discussed.

High Electrocaloric Effect in Ferroelectrics

2010

The existing situation in prospects of application of electrocaloric effect (ECE), related to finding ferroelectrics exhibiting high values of ECE, is discussed. Besides 1st order ferroelectric-paraelectric phase transitions, which were most promising till recent time with respect to high values of ECE, there are other approaches, related to ECE in high field range in relaxors and polymers. The restrictions of application of traditional indirect evaluation of ECE in ferroelectrics are discussed.

Two-state protein-like folding of a homopolymer chain

2010

Many small proteins fold via a first-order "all-or-none" transition directly from an expanded coil to a compact native state. Here we study an analogous direct freezing transition from an expanded coil to a compact crystallite for a simple flexible homopolymer. Wang-Landau sampling is used to construct the 1D density of states for square-well chains of length 128. Analysis within both the micro-canonical and canonical ensembles shows that, for a chain with sufficiently short-range interactions, the usual polymer collapse transition is preempted by a direct freezing or "folding" transition. A 2D free-energy landscape, built via subsequent multi-canonical sampling, reveals a dominant folding …

Isomorphous ion substitutions and order-disorder phenomena in highly electrocaloric lead-scandium tantalate solid solutions

1996

Abstract The authors discuss the physical background of the electrocaloric effect in PST ceramics and its solid solutions. The most important contribution to the electrocaloric effect is due to field-induced Fm3m→R3m phase transition in the case of high long range ordering degree of B-ions in the perovskite structure ABO3.

Interpretation of the Electrocaloric Effect in Na1/2Bi1/2TiO3-SrTiO3-PbTiO3Solid Solutions

2015

Results of direct measurements of electrocaloric effect and polarization are mutually compared in solid solutions 0.4Na1/2Bi1/2TiO3-(0.6-x)SrTiO3-xPbTiO3. Three temperature regions are distinguished in respect of a phase transition temperature. Above the phase transition, electrocaloric effect is determined by orientation of polar nanoregions by electric field. In wide temperature regions above and below the phase transition temperature, the difference between results, obtained from direct electrocaloric effect measurements and from polarization hysteresis loops, is explained by temperature dependence of concentration of polar nanoregions, oriented by electric field. Existence of a common Δ…

A reactor for photoelectrocatalytic oxidation of aromatic alcohols

2008

Blue lasing at room temperature in high quality factor GaN/AlInN microdisks with InGaN quantum wells

2007

The authors report on the achievement of optically pumped III-V nitride blue microdisk lasers operating at room temperature. Controlled wet chemical etching of an AlInN interlayer lattice matched to GaN allows forming inverted cone pedestals. Whispering gallery modes are observed in the photoluminescence spectra of InGaN/GaN quantum wells embedded in the GaN microdisks. Typical quality factors of several thousands are found (Q>4000). Laser action at similar to 420 nm is achieved under pulsed excitation at room temperature for a peak power density of 400 kW/cm(2). The lasing emission linewidth is down to 0.033 nm.