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Abstract  

The vaterite—calcite transition above 630 K has been studied by isothermal and non-isothermal differential scanning calorimetry. Vaterite samples prepared under different conditions were investigated. The transition temperature is strongly dependent on the sample preparation. The observed transition enthalpy H tr is nearly equal for different samples and experimental conditions. From 28 measurements a value of H tr–(3.12±0.11) kJ mol–1 was obtained. The activation energy for the polymorphic transition was calculated from the Arrhenius plot and by use of isoconversional methods, as a function of the degree of conversion. The influence of the kinetic model distortion and experimental uncertainties on the obtained data was discussed. The actual value of the activation energy was assessed at E a=(250±10) kJ mol–1 for nearly all examined samples. Functions, corresponded to the model mechanism of nuclei formation and growth, provide the unambiguous consideration of the transition kinetic for the investigated vaterite samples. Differences in the dynamic behaviour of several samples at the transition are established.

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Abstract  

Effects of metal surfaces, such as aluminium, silver and gold, on the melting behavior and thermal polymerization of long-chain diynoic acids having a diacetylene group at different positions were investigated by thermal analyses using DSC, TG and other methods. The surface effects of metals were significant in the order of Ag, Al and Au. These effects are attributable to the anchoring of carboxyl group on the surface by chemisorption, which leads to unfavorable condition for polymerization of heptadeca-2,4-diynoic acid. In the case of tricosa-10,12-diynoic acid, containing a flexible methylene chain, inserted between COOH and C≡C−C≡C groups, the anchoring of COOH on the metal surface causes rather favorable effect on the polymerization.

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Journal of Thermal Analysis and Calorimetry
Authors: H. Shigematsu, Y. Akishige, T. Matsui, T. Tojo, H. Kawaji, and T. Atake

Summary The phase diagram of the mixed crystal (K1-xRbx)2SeO4 was determined by means of thermal analysis and neutron scattering experiments. The hexagonal to orthorhombic phase transition line exists for any x. The normal-incommensurate phase transition temperature decreases continuously with increasing Rb content. However, the incommensurate-commensurate phase transition was not observed except for K2SeO4. According to the clear softening of the Σ2- Σ 3 phonon branches and the finite frequency at 0 K for x>0.34, an existence of the hypothetical phase transition was confirmed.

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Abstract  

The complex phase transitions involving dielectricity and magnetism on distorted triangular lattice antiferromagnets, "KNiCl3-family" crystals, which have the crystal structures derived from the prototype CsNiCl3, were studied through the heat capacity measurements. These crystals are classified into characteristic three groups from the viewpoint of magnetic and structural properties clarified so far. The results of the dielectric constant measurement as a function of temperature are also presented in detail. The present calorimetric study reveaed that the structural successive phase transitions rather than the magnetic transitions in these crystals are recognized more distinctly as the presence of specific heat anomalies at the respective phase transition points.

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Abstract  

The objective of this study is to determine the influence of partial substitutions of Ti4+ by isovalent Hf4+ in the perovskite-type crystalline structure of PbTiO3. Different samples over the whole composition range (0 ≤ x≤1) in the PbTi1-xHfxO3 family have been prepared. Phase transitions have been determined by thermal analysis (differential scanning calorimeter: DSC) and complex impedance spectroscopy (IS) over a wide temperature range. As a consequence of the cation replacement the changes that take place in the different phase transition temperature are reported. By both techniques, thermal analysis and electrical characterization, it is shown that for all compositions prepared there is only one phase transition in a temperature range between 230 and 460 °C. With these results and the previously known crystalline structure of pure PbTiO3 and PbHfO3 perovskites, the phase diagram of the PbTi1-xHfxO3 family is presented including a morphotropic phase transition at x ~ 0.5.

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Abstract  

Discontinuous and continuous volume phase transitions of organic polymer hydrogels, such as polyacrylamide (PAAm) and poly(N-isopropylamide) (PNIPA) gels, uponpH and temperature were studied by the positron annihilation lifetime measurement, which allows the estimation of size, intensity and size distribution of the free volume. Microscopic changes of physical and chemical interactions between gel network and solvent molecules and among conjugated solvent molecules at volume phase transitions of polyacrylamide gels were discussed.

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Abstract  

Heat capacity measurements were carried out on single-crystalline CuO in the temperature range 130–300 K. Sharp peaks corresponding to the antiferromagnetic transitions were clearly observed at 211 and 227 K. At the low-temperature end, near 160 K, a wide peak in the heat capacity signal was also demonstrated. An electric anomaly was observed in the temperature range 150–160 K, which strongly suggests the possibility of a new low-temperature phase transition in CuO. This study also indicates that DSC measurement is an effective tool to detect magnetic transitions and probe subtle phase transitions in solids.

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The high-temperature phase transition of K2SeO4 was studied by using differential thermal analysis. The Kissinger equation and the Mahadevan approximation were applied to evaluate the effective phase transition activation energy (E). The average value ofE was 12.85 ±0.04 eV.

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