Low-temperature exoemission accompanying phase transitions in fullerite

Low-temperature (77–570 K) emission of negative charges (exoemission) has been revealed and studied during cooling followed by heating of crystalline C₆₀ and the C₆₀–C₇₀ binary system. Peaks of emission current appear at the phase transition temperatures. Intense weakly decaying electron emission caused by the relaxation of strained structures containing weakly bound electrons occurs in the C₆₀–C₇₀ binary system at 77 K.     

Self-organization of macromolecules and liquid-crystalline phase transitions in solutions of cellulose esters

For hydroxypropyl cellulose-ethanol, hydroxyethyl cellulose-water, and cyanoethyl cellulose-DMAA systems, phase diagrams are constructed and the regions of existence of isotropic and anisotropic phases and the dimensions of macromolecules and supramacromolecular particles in a wide composition range are determined through the use of the cloud-point method, polarization microscopy, the turbidityspectrum method, dynamic light-scattering measurements, and a polarization optical apparatus. It is shown that the formation of LC phases with an increase in the polymer concentration is associated with a significant enlargement of supramolecular particles.     

Thermoluminescence characteristics of different phase transitions from nanocrystalline alumina

Journal of Radioanalytical and Nuclear Chemistry - Nanocrystalline boehmite material was synthesized using the hydrothermal method. Different annealing temperatures have been used to transform...     

Self-organization of lactates in the gas phase

Chiral recognition and subsequent selective self-organisation into hydrogen-bonded n-mers is observed in supersonic methyl lactate expansions. The nu(OH) and nu(C=O)-vibrations are investigated by ragout-jet FTIR-spectroscopy and lead to the assignment of homo- and heterochiral clusters of at least three different cluster sizes. Whereas homo- and heterochiral dimers are formed in similar amounts in the racemic mixture, prominent absorptions due to different homochiral and heterochiral lactate trimers and tetramers indicate highly specific chiral self-recognition beyond molecular pairs. Chemical modification of the ester-group (methyl-, ethyl- and isopropyl-lactate) and argon admixture to the helium expansion contribute importantly to an understanding of the cluster spectra and topology.     

Radiation-induced defects and processes on the surface of radiation detector oxides according to exoelectron emission data

Exoelectron emission from materials for high-sensitivity thermoluminescence (TL) and exoelectron emission (EE) dosimeters based on α-Al₂O₃, BeO, and LiF was examined, a scheme for the formation of TL and EE centers induced by low-penetrating (β, α) radiation was proposed, and their relation to electron and hole (F, V _K) centers and to the emergence of dosimetric peaks during dose readout (thermal stimulation) was revealed. A unified approach to the interpretation of the phenomenon of thermally stimulated emission (TSEE) from these materials in terms of valence transformations of oxygen that is weakly bound to the surface was developed. The difficulties in the manufacture of hybrid high-sensitivity TL-TSEE dosimeters are explained from the standpoint of the physicochemical mechanism of exoelectron emission.     

Melting temperature: from nanocrystalline to amorphous phase

By extrapolating the mean grain size of nanocrystal to an infinitesimal value, an amorphous phase has been obtained from the Voronoi construction. The molecular dynamics simulations indicated that for nanocrystal, the grain size variation of melting temperature exhibits two characteristic regions. As mean grain size above about 4 nm for Ag, the melting temperatures decrease with decreasing grain size. However, with grain size further shrinking, the melting temperatures almost keep a constant. This is because the dominant factor on the melting temperature of nanocrystal shifts from grain phase to grain boundary. As a result of fundamental difference in structure, the amorphous phase has a much lower solid-to-liquid transformation temperature than that of nanocrystal.     

Model for phase coexistence in phase transitions

We propose a general model for describing the phenomena of phase coexistence in relation to pressure induced phase transformations by means of the T–P distribution in statistical thermodynamics. Using the well‐known B1–B2 transition in NaCl as a prototype, we demonstrate how phase coexistence gives rise to the changes in the bulk modulus and the equation‐of‐state across the transition. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Phase and thermal stability of nanocrystalline hydroxyapatite prepared via microwave heating

A simple method to prepare nanocrystalline hydroxyapatite (nHAP) is performed using a precipitation method assisted with microwave heating method. This method can be reported notably with high reproducibility and productivity. The received ceramic powder possesses characteristic of needle-shaped nanocrystals with dimension about 50 nm in diameter and 200 nm in length. The particle size distribution has been confirmed being in the range of 28-159 nm. Thermal analyses revealed that nHAP has at least three thermal events influenced by elevated temperatures. Phase stability and microstructure evolution of the nHAP calcined at temperatures range between 700 and 1200 °C are discussed in terms of the formation of secondary phases, the decomposition of HAP releasing carbonate and water. Various experimental techniques have been employed in this work, including powder X-ray diffraction, IR spectroscopy, DSC and TGA thermal analyses, dynamic light scattering and scanning electron microscopy.     

Toluene-induced phase transitions in blue phase liquid crystals

ABSTRACT

We report phase transitions in blue phase-forming liquid crystals (LCs) that are triggered by exposure to toluene vapours. Specifically, we reveal that room-temperature cholesteric phase mixtures of MLC-2142 and S-811 form blue phases (BP I, II and III) with increasing vapour pressure of toluene. To probe the mechanism underlying this observation, we investigated the phase behaviour of mixtures of BP-forming LCs containing a range of non-volatile aromatic compounds (e.g. pyrene). We interpret our observations to indicate that the principal effect of small aromatic compounds is to decrease the energy penalty associated with the formation of disclination lines in BPs. We also conclude that the absorption of toluene into the BP-forming LCs lowers the energy required for the formation of disclination cores in the BP phase, thus allowing the elastically favoured double-twist cylinders to form at lower temperatures. We demonstrate that BP-forming LCs containing pyrene can be used to detect toluene at concentrations below 200 ppm at room temperature. Overall, these results guide the design of LC-based materials that respond to VOCs at concentrations relevant to occupational settings.     

Surface phase transitions in foams and emulsions

Surface phase transitions in surfactant adsorption layers are known to affect the dynamic properties of foams and to induce surface nucleation in freezing emulsion drops. Recently, these transitions were found to play a role in several other phenomena, opening new opportunities for controlling foam and emulsion properties. This review presents a brief outlook of the emerging opportunities in this area. Three topics are emphasized: (1) the use of surfactant mixtures for inducing phase transitions on bubble surfaces in foams; (2) the peculiar properties of natural surfactants saponins, which form extremely viscoelastic surface layers; and (3) the main phenomena in emulsions, for which the surface phase transitions are important. The overall conclusion from the reviewed literature is that surface phase transitions could be used as a powerful tool to control many foam and emulsion properties, but we need deeper understanding of the underlying phenomena to fully explore these opportunities.     

Interactions and phase transitions in protein solutions

Understanding interactions and phase transitions in protein solutions is essential in order to develop systematic protein crystallisation strategies. Recent studies show that proteins near crystallisation behave as particles interacting via a very short-range attractive potential. The microscopic origin of this attractive term, and its strong dependence on the nature of the crystallisation agent, is, however, still obscure. The interplay of crystal nucleation with formation of weakly bonded amorphous aggregates also deserves further analysis.     

Magnetic phase transitions in nanoclusters and nanostructures

The review includes theoretical models and experimental results on magnetic phase transitions in magnetic nanoclusters and nanostructures. It is shown, that nanoclusters with the sizes of a few or tens nanometers possess the critical sizes (similarly to critical temperatures Curie or Neel), less of which cluster or cluster nanostructure loses magnetic ordering due to the first order magnetic phase transitions (by the jump). Change of the character of magnetic phase transitions (first-to-second order), the reduction of Curie or Neel points and the finding of critical sizes of nanoclusters has been observed on ferric oxides and hydroxides in solid state and matrix nanostructures.     

Positron annihilation and phase transitions in n-octadecanol

The positronium formation probability and the mean lifetime in n-octadecanol-1 were observed as a function of temperature. The changes of that probability are discussed in the terms of the spur reaction model, suggesting the residual effect of dielectric constant increase in the rotational phase.     

Magnetic phase transitions in nanoclusters and nanostructures

Theoretical models and experimental data on magnetic phase transitions in magnetic nanoclusters and nanostructures were reviewed. It was shown that nanoclusters measuring from several nanometers to several tens of nanometers possess a critical size (which can be likened to the critical Curie or Neel temperatures). At undercritical cluster sizes the magnetic order in cluster and cluster nanostructure vanishes by first-order magnetic phase transitions (abruptly). In this context, a change-over from the first- to second-order magnetic phase transition, a decrease or increase in the Neel and Curie temperatures, and critical nanocluster size calculations were accomplished for a number of nanoobjects. These include ferric oxides and hydroxides in matrix nanostructures comprised of isolated nanoclusters, as well as in nanostructures including strongly interacting or organized clusters and in nanostructures induced by shear stress under high pressure loading.     

Activity coefficients of butanol isomers in liquid crystalline cholesteryl tridecylate according to data of reverse phase gas chromatography and tensiometry

The reverse phase gas chromatography technique was used to determine the activity coefficients of butanol isomers at infinite dilution in the smectic, cholesteric and isotropic phases of cholesteryl tridecylate. Experimental parameters with which the chromatographic process may be viewed as balanced and proceeding within the bulk of the liquid crystal were defined. Alcohol activity coefficients in binary systems with cholesteryl tridecylate determined by the gas chromatographic and tensiometric techniques were found to be in good agreement. A conclusion is reached on the suitability of the reversed-phase gas chromatographic technique for studying smectic and cholesteric liquid crystalline phases.     

Size-dependent phase transformation kinetics in nanocrystalline ZnS

Nanocrystalline ZnS was coarsened under hydrothermal conditions to investigate the effect of particle size on phase transformation kinetics. Although bulk wurtzite is metastable relative to sphalerite below 1020 degrees C at low pressure, sphalerite transforms to wurtzite at 225 degrees C in the hydrothermal experiments. This indicates that nanocrystalline wurtzite is stable at low temperature. High-resolution transmission electron microscope data indicate there are no pure wurtzite particles in the coarsened samples and that wurtzite only grows on the surface of coarsened sphalerite particles. Crystal growth of wurtzite stops when the diameter of the sphalerite-wurtzite interface reaches approximately 22 nm. We infer that crystal growth of wurtzite is kinetically controlled by the radius of the sphalerite-wurtzite interface. A new phase transformation kinetic model based on collective movement of atoms across the interface is developed to explain the experimental results.     

High-pressure phase transitions in liquid crystals

Simultaneous measurements of the rate of heat evolution and changes of the mechanical variable of a transformation such as volume or pressure, performed in a p-V-T controlled scanning calorimeter have been applied to investigations of phase transitions in liquid crystals. In the instrument, the phase transitions can be induced by a controlled change of pressure, volume or temperature under isothermal, isobaric or isochoric conditions respectively. The present investigations have ben performed on 4-n-penthyl-penthylthiol-4-decycloxybenzoate which demonstrates in the liquid crystal state a nematic and three smectic phases

---

Zusammenfassung In einem p-V-T-kontrollierten Scanning Kalorimeter wurden zur Untersuchung der Phasenumwandlungen in Flüssigkeitskristallen simultane Messungen der Geschwindigkeit der Wärmeentwicklung und der Veränderungen von mechanischen Größen von Umwandlungen, wie z.B. von Volumen oder Druck durchgeführt. In diesem Gerät kann die Phasenumwandlung durch eine kontrollierte Veränderung von Druck, Volumen oder Temperatur unter isothermen, isobaren oder isochoren Bedingungen ausgelöst werden. Vorliegende Untersuchungen wurden an 4-n-Pentyl-phenylthiol-4-dezyloxybenzoat durchgeführt, welches im Flüssigkristallzustand eine nematische und drei smektische Phasen aufweist.