A model is proposed for AC calorimetry (ACC) at the first order phase transition point. The model is compared with the results of ACC around the melting point of ann-paraffin (C20H42). The observed frequency dependence of ACC is consistent with the model. A harmonic component of the temperature modulation with a frequency equal to twice the heating frequency was observed at the phase transition point. It is shown that the harmonic component can be explained on the basis of the proposed model.
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
Htr is nearly equal for different samples and experimental conditions. From 28 measurements a value of
Htr–(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 Ea=(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.
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.
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.
Authors:K. Yamanaka, Y. Nishiwaki, K. Iio, T. Kato, T. Mitsui, T. Tojo, and T. Atake
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.
Authors:M. de la Rubia, R. Alonso, J. de Frutos, and A. López-Garcia
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.
Authors:K. Ito, Y. Ujihira, T. Yamashita, and K. Horie
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.
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.