Search results for "Phase Transition"

Recent progress in high pressure X-ray absorption spectroscopy studies at the ODE beamline

2020

I.J. and A.K. are grateful to the Latvian Council of Science project no. lzp-2018/2-0353 for financial support. The research leading to these results has been partially supported by the project CALIPSOplus under the Grant Agreement No. 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020.

Temperature Dependence of Electronic Transitions of Single-Wall Carbon Nanotubes:  Observation of an Abrupt Blueshift in Near-Infrared Absorption

2007

Near-infrared (NIR) absorption spectra of single-wall carbon nanotube (SWNT) films are studied between 10 and 293 K. The most prominent effect is the shift of bands with temperature. Some nanotubes show a redshift of transition upon increasing temperature while some show blueshift and others show no shift. The shift is interpreted to originate mainly from the effect of strain induced in the tubes because of interaction with the environment. In particular, at temperatures T = 175−225 K, for some bands, there is an abrupt large blueshift, which is interpreted to originate from interaction of the nanotubes with water. Two models could be considered to explain the effect:  (1) strain induced by…

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

2014

Crystal structure transformations induced by surface stresses in BaTiO3and BaTiO3@SiO2nanoparticles and ceramics

2015

International audience; Lattice structure transformations in nanopowders of ferroelectric BaTiO3 and BaTiO3@SiO2 core-shell nanostructured ceramics were studied by nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) at the temperatures 120–450 K and particle size of 300 and 500 nm. NMR spectra of all studied samples in the paraelectric phase are identical to the spectra in bulk material indicating their perfect perovskite structure without visible influence of particle surface. However, we have found that surface of particles essentially influence the ferroelectric phase transitions detected by both NMR and EPR techniques. The strongest changes as compared to bulk mat…

In-situ high-pressure Raman scattering studies in PbWO4 up to 48 GPa

2016

The effect of pressure on the Raman spectrum of PbWO4 has been investigated up to 48 GPa in a diamond-anvil cell using neon as pressure-transmitting medium. Changes are detected in the Raman spectrum at 6.8 GPa as a consequence of a structural phase transition from the tetragonal scheelite structure to the monoclinic PbWO4-III structure. Two additional phase transitions are detected at 15.5 and 21.2 GPa to the previously unknown crystalline phases IV and V. The last one remains stable up to 43.3 GPa. At 47.7 GPa all Raman modes disappear, which could be caused by a pressure-induced amorphization. All structural changes are reversible, being the scheelite phase recovered at ambient pressure.…

DIELECTRIC PROPERTIES OF LEAD-FREE [(1-x)(Na0.5Bi0.5)-xBa]Zr1−yTiyO3CERAMICS (x = 0.01, 0.06, 0.085, 0.09, 0.1 AND y = 0.96, 0.97)

2008

ABSTRACT Lead-free ceramics based on (Na0.5Bi0.5TiO3, NBT)-(Ba(Ti, Zr)O3, BTZ) were prepared by solid phase synthesis and hot pressing sintering process and their dielectric properties have been studied. The obtained results were compared with these for pure NBT. The obtained samples show pure perovskite structure. Low frequency (100 Hz-100 kHz) investigations reveal the diffuse phase transitions and relaxorlike behaviour. The obtained results are discussed in terms of inhomogeneity of ions distribution, their ferro- and antiferroactivity and internal stresses. The NBT-BTZ system is expected to be a new promising candidate for lead-free electronic ceramics.

Formation of Layers of Diverse Stoichiometric and Phase Composition in Lithium Tantalate Crystals at Treatment by Vapour Transport Equilibration

2012

Layers from tens to hundreds of microns thick of a different phase composition and stoichiometry are shown to form in lithium tantalate crystals subjected to treatment by vapor transport equilibration in lithium vapour. The 500 microns thick stoichiometric (Li/Ta ≈ 1) layers have ten times lower values of the coercive field compared with congruent lithium tantalate. As electrical measurements suggest, a new polar structure of phase transition at ∼120°C is possibly formed within a thin (30 μm) surface layer of the lithium tantalate sample after vapour transport equilibration treatment.

Guanine Assemblies on the Au(111) Surface: A Theoretical Study

2013

It was found previously [Small2009, 5, 1952 and Angew. Chem., Int. Ed.2005, 44, 2270] that guanine molecules when assembled in a vacuum on the Au(111) surface at room temperature form a hydrogen-bonded network consisting of guanine quartets of the same chirality; this was supported by ground-state density functional theory (DFT) gas-phase calculations. In this Article, we re-examine this system and show that many more (almost equally stable) both homo- and heterochiral structures are possible; however, the homochiral structure observed experimentally becomes definitely the most favorable only if the vibrational contribution to the free energy is accounted for. Interaction with the gold surf…

Direct observation of strain-induced non-equilibrium grain boundaries

2015

Abstract The grain boundary (GB) wetting by the melt and second solid phase has been studied in the Sn–Pb system both in equilibrium conditions and during continuous strain. The percentage of Sn/Sn GBs completely wetted by the melt increases from 80% at eutectic temperature Te=183 °C to 100% at 220 °C. The percentage of Pb/Pb GBs completely wetted by the melt increases from 0% at Te to 100% at 270 °C. Below Te only incomplete wetting of Pb/Pb GBs by solid Sn and Sn/Sn GBs by solid Pb has been observed after long annealing. However, during strain the lattice dislocations are continuously absorbed by GBs increasing the GB energy. As a result, the complete wetting of such non-equlibrium Sn/Sn …

Core-Shell Microgels with Switchable Elasticity at Constant Interfacial Interaction.

2016

Hydrogels based on poly(N-isopropylacrylamide) (pNIPAAm) exhibit a thermo-reversible volume phase transition from swollen to deswollen states. This change of the hydrogel volume is accompanied by changes of the hydrogel elastic and Young's moduli and of the hydrogel interfacial interactions. To decouple these parameters from one another, we present a class of submillimeter sized hydrogel particles that consist of a thermosensitive pNIPAAm core wrapped by a nonthermosensitive polyacrylamide (pAAm) shell, each templated by droplet-based microfluidics. When the microgel core deswells upon increase of the temperature to above 34 °C, the shell is stretched and dragged to follow this deswelling i…