Caffeine has been found to display a low-temperatureβ- and a high-temperatureα-modification. By quantitative DTA the following data were determined: transformation temperature 141±2°; enthalpy of transition 4.03±0.1 kJ·mole−1; enthalpy of fusion 21.6±0.5 kJ·mole−1; molar heat capacity
in good accord with drop-calorimetric data. For the constants of the equation log (p/Pa)=−A/T+B, static vapour pressure measurements on liquid and solidα-caffeine, and effusion measurements on solidβ-caffeine yielded:
Authors:Y. Arita, T. Ogawa, B. Tsuchiya, and T. Matsui
Heat capacities, electrical conductivities and phase transition temperature of hafnium hydrides, HfHx (0.99≤x≤1.83), were studied using a direct heating pulse calorimeter and a differential scanning calorimeter from room temperature
to above 500 K. The heat capacity of HfH1.83 was larger than that of pure hafnium and showed no anomaly of heat capacity. In contrast, there were λ-type peaks for the
heat capacity and DSC curves for HfHx (1.1≤x≤1.6) near 385 and 356 K. The anomalies of heat capacity and electrical conductivity of HfHx (1.1≤x≤1.6) were considered the result of phase transition and order-disorder phase transition for hydrogen in the hafnium hydride
lattice for HfHx (1.1≤x≤1.3).
The modeling of magnetocaloric effect (MCE) based on quasiparticle formalism was used to show a possibility to increase our
knowledge of diffusion and phase transitions by a transdisciplinary scientific approach. Generalized understanding of diffusion
and of some phase transitions was suggested. New relations between MCE and elastic parameters of materials, obtained from
modeling procedure, were presented. The paper contains also the short discussion of a necessity to use hypercomplex mathematics
in modeling of magnetic processes and specifically in a modeling of MCE. Some remarks concerning nanomaterials are added.
Phase transitions of hexatriacontane (C36) and octacosane (C28), both as the solution grown single crystal (SGC) and polycrystalline aggregates (MCC) prepared by cooling at 1 K min–1 from the isotropic liquid state, were measured by the simultaneous DSC-FTIR method. MCC of C36 showed the freezing of the high temperature stable phase, which had a slight lower order of the lattice vibration mode comparing with the room temperature stable phase. MCC of C28 demonstrated thermo-reversible phase transition, however, had a binomial distribution of crystal stability of the monoclinic phase.
Authors:M. Bouchaour, A. Ould-Abbas, N. Diaf, and N. Chabane Sari
The phase transition of a fluid - in particular water - confined in the pores of silicon during drying is studied. The influence
of this process on surface size and porosity is discussed. Methods of air drying, supercritical drying and freeze drying are
The effect of mechanical treatment on phase transitions of calcium borate-colemanite was studied. The decrease in the particle size of the original material and the increase in its internal structural disorder affect the thermal stability, characteristic temperatures and thermodynamic parameters of phase transitions such as structural relaxation and crystallization, occurring during the heating of this amorphous material. With increasing degree of amorphization of colemanite a change is observed in the structural mechanism of its crystallization.
In this paper we try to perform a thermodynamic analysis of the temperature-induced transition from the molten globule to the unfolded state of globular proteins. A series of calorimetric investigations showed that this process is not associated with an excess heat capacity absorption peak, and cannot be regarded as a first-order phase transition. This result contrasts with the well-established conclusion that the thermal unfolding of the native tertiary structure of globular proteins is a first-order phase transition. First, the theoretical approach developed by Ikegami is outlined to emphasize that a second-order or gradual transition induced by temperature is expected for globular proteins when the various secondary structure elements do not interact cooperatively. Secondly, a simple thermodynamic model is presented which, taking into account the independence of the secondary structure elements among each other, is able to rationalize the shape of the experimental DSC profiles.
Authors:Zhaohe Yang, Qingcai Guan, Jingqian Wei, and Jiyang Wang
The thermal properties of KTN crystal were investigated at low temperatures by DSC and TM. The phase transition enthalpies
and the average specific heats of the crystal were measured. Results are analyzed and discussed.
Authors:Y. Akishige, H. Shigematsu, T. Tojo, H. Kawaji, and T. Atake
Summary Specific heats on the single crystals of Sr2Nb2O7, Sr2Ta2O7 and (Sr1-xBax)2Nb2O7 were measured in a wide temperature range of 2-600 K. Heat anomalies of a λ-type were observed at the incommensurate phase transition of TINC (=495 K) on Sr2Nb2O7 and at the super-lattice phase transition of TSL (=443 K) on Sr2Ta2O7; the transition enthalpies and the transition entropies were estimated. Furthermore, a small heat anomaly was observed at the low temperature ferroelectric phase transition of TLOW (=95 K) on Sr2Nb2O7. The transition temperature TLOW decreases with increasing Ba content x and it vanishes for samples of x>2%.