Authors:Alfredo Román-Tejeda and Heriberto Pfeiffer
phasetransition reversed when the process was performed at 500 °C or higher temperatures. Of course, a part of the ceramic reacted to produce lithium carbonate. These findings are all in good agreement with the literature, where it has been shown that α
The aim of this study was to correlate the results of experimental data using DTA method and predictions of artificial neural
network (ANN) and multivariate linear regression (MLR). Thermal decomposition of polymers was analyzed by simultaneous DTA
method, and kinetic parameters (critical points, the change of enthalpy and entropy) of polymers were investigated. A computer
model based on multilayer feed forwarding back propagation and multilayer linear regression model were used for the prediction
of critical points, phase transitions of low-density polyethylene (LDPE) and mid-density polyethylene. As a result of our
study, we concluded that ANN model is more suitable than MLR about prediction of experimental data.
The influence of the temperature program parameters of an ODSC experiment on the calculated “reversing” and “kinetic” signals has been studied. Mixed orthophosphate salts of KMPO4 (where M=Ni2+, Co2+ and Fe2+) which present at least one structural phase transition have been used for this purpose. On these crystalline compounds we have shown that the non reversing heat flow is partly associated with the formation and disappearance of ferroelastic and ferroelectric domain walls. However a proper choice of the temperature program parameters is important so that the calculated “reversing” and “kinetic” curves have the supposed physical meaning according to the assumptions made for the calculations.
Molar enthalpies of solid-solid and solid-liquid phase transitions of the LaBr3, K2LaBr5, Rb2LaBr5, Rb3LaBr6 and Cs3LaBr6 compounds were determined by differential scanning calorimetry. K2LaBr5 and Rb2LaBr5 exist at ambient temperature and melt congruently at 875 and 864 K, respectively, with corresponding enthalpies of 81.5 and
77.2 kJ mol-1. Rb3LaBr6 and Cs3LaBr6 are the only 3:1 compounds existing in the investigated systems. The first one forms from RbBr and Rb2LaBr5 at 700 K with an enthalpy of 44.0 kJ mol-1 and melts congruently at 940 K with an enthalpy of 46.7 kJ mol-1. The second one exists at room temperature, undergoes a solid-solid phase transition at 725 K with an enthalpy of 9.0 kJ
mol-1 and melts congruently at 1013 K with an enthalpy of 57.6 kJ mol-1. Two other compounds existing in the CsBr-based systems (Cs2LaBr5 and CsLa2Br7) decompose peritectically at 765 and 828 K, respectively.
The heat capacities of the above compounds in the solid as well as in the liquid phase were determined by differential scanning
calorimetry. A special method - 'step method' developed by SETARAM was applied in these measurements. The heat capacity experimental
data were fitted by a polynomial temperature dependence.
Authors:J. Morales, L. Hernan, L. V. Flores, and A. Ortega
Differential thermal analysis and differential scanning calorimetry techniques have been used to study the kinetics of phase transitions. The aragonite/calcite transformation was chosen as test reaction.
Authors:G. Graziano, G. Barone, F. Catanzano, and A. Riccio
A general feature of temperature-induced reversible denaturation of small globular proteins is its all-or-none character.
This strong cooperativity leads to think that protein molecules, possessing only two accessible thermodynamic states, the
native and the denatured one, resemble ‘crystal molecules’ that melt at raising temperature. An analysis, grounded on mean
field theory, allows to conclude that the two-state transition is a first-order phase transition. The implication of this
conclusion are briefly discussed.
Authors:A. V. Volkov, M. A. Moscvina, O. V. Arzhakova, A. L. Volynskii, and N. F. Bakeev
Orientation, phase composition and phase transitions of a series of long chain low molecular weight compounds (LMC), such as heneicosane, cetyl alcohol, normal fatty acids, introduced into porous structure (crazes) of polymeric matrices oriented in liquid medium have been studied by means of DSC and SAXS techniques. Different types of LMC crystallites orientation in crazes of polymeric matrices have been observed. LMC phase state in crazes is shown to be characterized by higher stability of high-temperature polymer midifications. LMC melting temperature in crazes usually decreases as well as melting enthalpy (heat) and entropy. The origin of LMC properties changes observed is high dispersity (40–100nm) of LMC particles in crazes resulting in a marked growth of polymer/LMC interface influence on principal thermodynamic parameters of the systems studied.
Authors:E. Mikuli, B. Grad, K. Zaremba, and S. Wróbel
Fourier transform far- and mid-infrared (FT-FIR and FT-MIR) and Fourier transform Raman scattering (FT-RS) spectra of [Fe(H2O)6](ClO4)3 and [Cr(H2O)6](ClO4)3 indicate that these compounds are ionic molecular crystals built from complex cations and complex anions of octahedral (Th) and tetrahedral (Td) symmetry, respectively. The thermodynamic parameters for two phase transitions in polycrystalline [Fe(H2O)6](ClO4)3 and [Cr(H2O)6](ClO4)3 were determined by differential scanning calorimetry (DSC): melting of the crystals (at Tm=359.2 and 363.1 K) and solid-solid phase transition (at TC1=126.5 and 139.4 K), respectively.
The molar enthalpies of the solid–solid and solid–liquid phase transitions were determined by differential scanning calorimetry
for pure TbCl3 and KTb2Cl7, RbTb2Cl7, CsTb2Cl7, K3TbCl6, Rb3TbCl6 and Cs3TbCl6 compounds. Both types of compounds, i.e. M3TbCl6 and MTb2Cl7 (M=K, Rb, Cs) melt congruently and show additionally a solid–solid phase transition with a corresponding enthalpy ΔtrsH0 of 6.1, 7.6 and 7.0 kJ mol–1 for potassium, rubidium and caesium M3TbCl6 compounds andΔtrsH0 of 17.1 (rubidium) and of 12.1 and 10.9 kJ mol–1 (caesium) for MTb2Cl7 compounds, respectively. The enthalpies of fusion were measured for all the above compounds with the exception of Rb3TbCl6 and Cs3TbCl6. The heat capacities of the solid and liquid compounds have been determined by differential scanning calorimetry (DSC) in
the temperature range 300–1100 K. The experimental heat capacity strongly increases in the vicinity of a phase transition,
but varies smoothly in the temperature ranges excluding these transformations. Cp data were fitted by an equation, which provided a satisfactory representation up to the temperatures of Cp discontinuity. The measured heat capacities were checked for consistency by calculating the enthalpy of formation of the
liquid phase, which had been previously measured. The results obtained agreed satisfactorily with these experimental data.
Authors:H. Honda, A. Oshima, H. Hinode, and M. Wakihara
Enthalpy increment HT-H289K measurements have been made on iron Chevrel phase sulphide Fe2Mo6S7.8, in the temperature range 300 to 500 K by the drop method using a hightemperature Calvet-type twin calorimeter. The first-order phase transition of this sulphide from a triclinic (low-temperature phase) to a rhombohedral (high-temperature phase) occurred at 375 K, and the enthalpy was evaluated to be 6.0 kJ/mol. The heat capacities of iron Chevrel phase sulphide Fe2Mo6S7.8 were also calculated before and after the phase transition.