V. M. Egorov

Anomalies of Glass Transition: Manifestation in fullerene core polystyrene stars

Segmental dynamics around Tg in the 4- and 6-arm fullerene (C60) core star-like polystyrenes with different preset arm lengths was studied by DSC as compared to that in the linear PS and PS/C60 blend. The ‘anomalies’ in glass transition behavior were found for the stars including both suppression and facilitation of segmental motion, and pronounced dynamic heterogeneity within a transition range. The results are interpreted in terms of breakdown of intermolecular cooperativity of segmental motions and PS-C60 interactions.

Calorimetric study of phase transitions in the perovskite BaCeO3

Differential scanning calorimetry was used to study phase transitions (PT) in the perovskite BaCeO3. It is shown that its phase state is determined by a second-order λ transition at Ttr=520–540 K and a first-order δ transition at Ttr=600–670 K. Differences in PT parameters between ceramic and fused BaCeO3 have been established.

DSC INVESTIGATION OF OXYGEN SUBSYSTEM IN Ba-Y-Cu-O SUPERCONDUCTORS

DSC investigations have been performed for a series of compounds Ba2YCu3Oy with the oxygen content varying in the rangey=6.0...6.9 by means of various heat treatments at 800–1200 K followed by quenching, or through the chemical extraction of oxygen by placing the sample in dihydrogen at 470–490 K. The sample preserving a constant oxygen content during heating in nitrogen exhibited exothermal effects between 450 and 850 K. It has been shown that the ΔH vs. y function reaches maximum aty ≈ 6.5. Kinetic measurements have shown that the diffusive mobility of oxygen atoms in the lattice is responsible for these effects, viz. the Arrhenius and cooperative processes of reorganization in the non-equilibrium oxygen subsystem of the bulk.ZusammenfassungFür eine Reihe von Verbindungen der allgemeinen Formel Ba2YCu3Oy und einem Sauerstoffgehalt vony=6.0....6.9 wurden mittels verschiedener Wärmebehandlungen bei 800–1200 K, gefolgt durch Abschrecken oder beim chemischen Sauerstoffentzug durch Einbringen der Probe in Diwasserstoff bei 470–490 K DSC-Untersuchungen durchgeführt. Wird die Probe in Stickstoff erhitzt, so behält sie ihren Sauerstoffgehalt bei und zeigt bei 450–850 K einen exothermen Effekt. Es wurde gezeigt, daß die Funktion ΔH(y) bei etwa y=6.5 ein Maximum erreicht. Kinetische Messungen zeigen, daß für diesen Effekt die diffusive Beweglichkeit der Sauerstoffatome im Gitter verantwortlich ist.

Hydrogen-induced phase transition in barium cerate

The action of water vapor on nominally pure BaCeO3 perovskite at 850 K causes hydrogen to be incorporated into the lattice and a thermally metastable state to form. The lattice symmetry changes as a result of an orthorhombic-pseudocubic transition which takes place at a temperature substantially lower than the known thermally initiated rhombohedral-cubic transition at 1173 K.

Electrical conductivity of hydrogenated barium-yttrium cuprate and of oxidized hydrocuprate in the 300–450 K interval

The electrical conductivity of ceramic samples with a general formula HxBa2YCu3O7+z has been measured in the course of hydrogenation (formation of the hydrocuprate) and subsequent oxidation (formation of the oxyhydrocuprate). The hydrocuprate (x>0, −0.1<z<0) was synthesized by exposing the parent Ba2YCu3O6.9 ceramic to hydrogen at a temperature of 450 K and a pressure of 50 kPa. The oxyhydrocuprate (x=2, z⩽1.5) was prepared by treating the preliminarily hydrogenated ceramic of composition H2Ba2YCu3O6.9 with oxygen at 470 K at a pressure of 70 kPa. The four-probe dc method was used in the electrophysical measurements. The measurements were performed in the temperature interval 300–450 K and within the compositional ranges 0<x<2.1, −0.1<z<1.5. The resistivity of the samples increases by nearly six orders of magnitude with hydrogenation from x=0 to x=2, and decreases by three orders of magnitude with oxidation, changing from z=−0.1 to z=0.8 for x=2. The electrode polarization effect is observed, which is apparently caused by ionic processes occurring at the electrode-ceramic interface. Water vapor is experimentally found to exert considerable effect on the electrophysical parameters of the hydrocuprate-copper electrodes system.

Kinetics of “quenching” effect in C60 crystals

The thermodynamic transition in the C60 crystals subjected to uniaxial compression at different temperatures and heat treatment has been investigated by differential scanning calorimetry (DSC). The kinetic parameters of the irreversible endothermic “quenching” effect are determined. It is revealed that the process is accompanied by the cooperative molecular motion, which is characteristic of the high-elasticity state of organic glasses.

Inelastic deformation of C60 single crystals in the temperature range 80–300 K

The rate spectrum of small inelastic strains of the C60 single crystal in the temperature range 80–300 K has been obtained with a precision laser interferometer. It is revealed that the spectrum exhibits two large peaks in the glass formation (90–100 K) and phase transition (250–260 K) ranges. A small strain acceleration is also observed at ∼220 and 240 K. The first two maxima are attributed to the changes in strain resistance upon transitions, and the strain acceleration at 220 and 240 K is associated with the annealing of the defects formed upon rapid cooling of the crystal. It is demonstrated that the peak at 250–260 K broadens with an increase in the stress. The spectrum of strain rates is compared with the calorimetric curve for the same single crystal.

Small-inelastic-strain rate spectrum of C60 single crystals near the phase transition at 250–260 K

The temperature spectrum of small-inelastic-strain rates in a vapor-phase-grown C60 single crystal has been measured within the 200–290 K interval with a high-precision strain-rate meter based on a laser interferometer. The spectrum exhibits a strong peak in the region of the phase transition at 250–260 K and a slight strain acceleration at ∼240 K, which correlates well with the calorimetric curve. The first maximum is associated with strain that developes more easily in an fcc than in a primitive cubic lattice, and the second, to the effect on the strain rate of annealing of the defects created with fast crystal heating.

Effect of low-temperature compression on the parameters of the phase transition at 250–260 K in C60 single crystals

A thermodynamic transition in C60 crystals, subjected beforehand to uniaxial compression at various temperatures and to thermal action, was investigated by differential scanning calorimetry. It was established that compression of the crystal at temperatures below the phase transition has a much smaller effect on the transition itself than a similar or even much weaker action at a temperature above the phase transition. A “quenching” effect was also found. This effect is probably due to the orientational order of individual fullerene molecules. A correlation was established between the magnitude of the effect and the preliminary deformation of C60 crystals.

Calorimetric studies of C70S48 crystals

A comparative study of the thermodynamic properties of the C70 fullerene, sulfur, and C70S48 crystals has been made by differential scanning calorimetry. It is shown that only C70S48 has an endothermic δ phase transition with a peak at 430 K, which lies in the temperature region exhibiting conductivity and dielectric anomalies. A correlation between the behavior of the sulfur sublattice in the C70S48 crystals and the thermodynamic parameters of the transition has been established.