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Abstract  

Low-temperature heat capacity of two polymorphs of glycine (α and γ) was measured from 5.5 to 304 K and thermodynamic functions were calculated. Difference in heat capacity between polymorphs ranges from +26% at 10 K to -3% at 300 K. The difference indicates the contribution into the heat capacity of piezoelectric γ polymorph, probably connected with phase transition and ferroelectricity. Thermodynamic evaluations show that at ambient conditions γ polymorph is stable and α polymorph is metastable.

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Abstract  

Dielectric experiments are often performed in non-isothermal conditions. Thus, there is a difference between the temperature of the sample and the sensor temperature. In this work we propose and compare three temperature calibration methods based on the detection of transitions or relaxations: i) the melting of high-purity metallic standards (indium and tin), ii) the 2nd order phase transition of a ferroelectric crystal (TGS); iii) the -relaxation of an amorphous polymer (poly(carbonate)). The results obtained from the three different methods were used to construct a calibration curve for a given heating rate.

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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.

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Abstract  

Because of unique dielectric, piezoelectric, thermoelectric, optical and ferroelectric properties of titanates of alkaline earth metals, they have become an object of many scientific research. This article is concerned with mechanochemical synthesis of calcium titanate as an alternative technique to hydrothermal, sol–gel, thermal methods. The aim of this study was to verify the mechanochemical conditions of CaTiO3 formation with the use of three calcium oxide precursors—CaO, CaCO3 and Ca(OH)2. The differences in processes of calcium titanate synthesis are presented.

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Abstract  

The ultra-slow modes with a relaxation time much longer than 1 ns are discovered by a time-resolved spectroscopy in uniaxial ferroelectric (NH2CH2COOH)3H2SO4 (TGS) and in glass-former HOCH2(CHOH)4CH2OH (D-sorbitol). A mode discovered in TGS is a thermal relaxational mode and is proved to be the physical origin of the central peak found by Brillouin scattering. Two modes are discovered in D-sorbitol. One is a thermal relaxational mode. The other is a mode characterized by the Kohlraush-Williams-Watts function with a relaxation time R. A remarkable result is that R indicates a critical behavior not at the liquid-glass transition temperature T g =–7°C but at the ergodic to nonergodic transition temperature T c=33°C.

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Aluminate sodalite ceramics, (Sr8Al12O24)(CrO4)2) — SACR, have been prepared from solution spray-dried precursors. The spray drying of a mixed aqueous nitrate solution gave an anhydrous powder which was thermally decomposed in air at 1350 °C to give a crystalline SACR powder with a median volume diameter of 4–6 μm. The SACR powder was dry pressed and sintered to >90% density. The yellow SACR powder showed two phase transitions between 10 and 45 °C with a 25 ° intermediate phase region indicative of a low defect concentration in the crystalline structure. A colour change observed in the sintered ceramic (changing to a mixed green/yellow) and a narrowing of the phase transition region (15–40 °C) are discussed. Dielectric measurements were consistent with the presence of two extrinsic ferroelectric transitions at 17 and 35 °C.

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Journal of Thermal Analysis and Calorimetry
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 T INC (=495 K) on Sr2Nb2O7 and at the super-lattice phase transition of T SL (=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 T LOW (=95 K) on Sr2Nb2O7. The transition temperature T LOW decreases with increasing Ba content x and it vanishes for samples of x>2%.

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Thermal analysis of binary liquid crystalline mixtures

System of bent core and calamitic molecules

Journal of Thermal Analysis and Calorimetry
Authors: A. Bubnov, Věra Hamplová, M. Kašpar, Anikó Vajda, Maja Stojanović, Dusanka Obadović, N. Éber, and Katalin Fodor-Csorba

Abstract  

Two series of binary mixtures composed of bent shaped and rod like molecules are reported. The first star shaped bent core molecules were synthesized and used as a component of binary mixtures. The chiral rod like compounds having commensurable length with the arms of the bent core compounds have been chosen for these mixtures. The resulted compositions show various thermotropic liquid crystalline phases that are characteristic to both types of liquid crystalline materials. In case of mixing the rod like molecules to the bent core compound the B2, B7 and induced B1 phases have been observed. While using the star-shaped bent core and chiral rod like compounds in mixture, the paraelectric smectic A, ferroelectric smectic C* and orthogonal hexatic smectic B phases were preferred. The appearing mesophases were investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction methods.

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Abstract  

We have designed, built and calibrated a microcomputer-controller automated a.c. calorimeter for liquid crystals and solid samples, in the temperature range from 30 to 250°C, in which we enhance several features of previous ones reported in the literature. We have incorporate the lock-in amplifier model 5302 from EG&G in our set-up, which permits to extend the frequency range to 1 mHz. This allows the performance of suitable frequency sweeps in order to choose the optimum operating frequency for low thermal conductivity materials. The calorimeter has been calibrated with the 8OCB liquid crystal compound. The resolution obtained forC p values was better than 0.1% and the absolute erroer around 5%. As applications, we show the ferroelectric transition of the triglicerine sulphate (TGS), the Nematic-Smetic A transition of the 8OCB and a study in a binary system of ferroelectric liquid crystals, SCE9-SCE10.

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JPC - Journal of Planar Chromatography - Modern TLC
Authors: Virginia Coman, Ştefan Kreibik, Cristian Tudoran, Ocsana Opriş, and Florina Copaciu

Ferroelectrics). Scrisul Românesc Publishing House, Craiova, 1982, p. 98. Nicula A. Dielectrici şi feroelectrici (Dielectrics and Ferroelectrics

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