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Journal of Thermal Analysis and Calorimetry
Authors: M. Daviti, K. Chrissafis, K. Paraskevopoulos, E. Polychroniadis, and T. Spassov

Abstract  

The kinetics of the α-β phase transition of HgI2 were investigated by isothermal and non-isothermal differential scanning calorimetry. The effective activation energy of the transition, 41540 kJ mol-1, was determined applying the methods of Kissinger and Ozawa. The transition kinetics were described by the Johnson-Mehl-Avrami model and the value of the Avrami exponent n was found to range from high values (n>3) at the early stages to lower values at later stages of the transformation, with an average value of 2.

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Journal of Thermal Analysis and Calorimetry
Authors: K. Chrissafis, K. Efthimiadis, E. Polychroniadis, and S. Chadjivasiliou

Abstract  

In this work we study the influence of Mo admixtures on the crystallization process of amorphous Fe78-xMoxSi9B13 (x=1, 2, 3 and 4) alloys by measurements of differential scanning calorimetry and on the soft ferromagnetic properties of the alloys by magnetic measurements. The addition of Mo by replacing Fe, results in magnetic hardening of materials. In DSC curves two peaks appear which are distinct when the concentration of Mo is 1 at.% and partly overlap when the Mo content is 2 at.%. Further increase in the Mo content leads to the appearance of just one peak. The activation energy was calculated both with Kissinger's and isoconversional Flynn, Wall and Ozawa methods.

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A differential scanning calorimetric study on phase transformations of HgI2

II. The dependence of stoichiometry on the α toβ transformation temperature

Journal of Thermal Analysis and Calorimetry
Authors: S. N. Toubektsis, E. K. Polychroniadis, and C. A. Alexiades

The influence of stoichiometry on the mercuric iodideα toβ transformation temperature was studied in a limited region around the exact stoichiometry. It is concluded that although this influence is small, the transformation temperature has its highest value at the exact stoichiometry composition.

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Journal of Thermal Analysis and Calorimetry
Authors: K. Chrissafis, M. Ozer, E. Vinga, E. Polychroniadis, X. Chatzistavrou, and K. M. Paraskevopoulos

Abstract  

TlSbSe2 monocrystals were grown using the modified Bridgman–Stockbarger method and were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Reflectivity spectra have been registered in the range 50 to 4000 cm–1 for E parallel to a and E parallel to b polarizations, on the cleavage plane. A remarkable anisotropy at two directions was verified. With regard to previous observations, additional peaks were discriminated and the fundamental phonon parameters were determined using classical dispersion relations. The material presents a complex phase transformation – with two thermal effects – that was examined using differential scanning calorimetry (DSC). Non-isothermal measurements, at different heating and cooling rates (β), were used to study the thermal phenomena. The main effect is attributed to a structural displacement and the second one to a cation exchange procedure. The phase transformation temperature depends strongly on the cooling rate and the peaks are shifted by 30 K with the increase of this rate, on the contrary to the increase of the heating rate that has a smaller effect. Phenomena related with the influence of the previous, repeated heating and cooling cycles on the transformation are also examined and analytically discussed.

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Journal of Thermal Analysis and Calorimetry
Authors: X. Chatzistavrou, K. Chrissafis, E. Polychroniadis, E. Kontonasaki, P. Koidis, and K. Paraskevopoulos

Abstract  

Dental materials restore morphology and function of lost or destroyed teeth, but cannot completely rebuild the structural relationship with soft periodontal tissues. The induction of bioactivity on classic dental porcelain can be achieved through the addition of bioactive glass. The aim of this study was to investigate the effect of Bioglass® on the thermal properties of dental porcelain in order to correlate the proportions of mixtures with the changes in thermal properties. Differential thermal analysis was performed in order to determine the characteristic temperatures of the mixtures. The increase of bioactive glass concentration in mixtures induces a shift to lower temperatures of the melting point temperature. This observation is attributed to the substitution of silicon ions by aluminium ions and to the formation of Al–O bond, which is weaker than Si–O bond. Mixtures heated at 950°C were examined also by the transmission electron microscopy (TEM) in order to be studied the microstructure of these samples at this critical temperature. The observed microstructural changes, confirm the process of substitution of Si 4+ ions by Al 3+ ions.

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Journal of Thermal Analysis and Calorimetry
Authors: N. Pistofidis, G. Vourlias, E. Pavlidou, K. Chrissafis, G. Stergioudis, E. K. Polychroniadis, and D. Tsipa

Abstract  

The effect of Pb and Sn on the structure of zinc hot-dip galvanized coatings on steel wires is examined. The coating quality is often low because of the Sandelin effect. An improvement could be expected if 1.0 mass% Pb is added to the Zn bath. In this case the η phase formation is enhanced, while the coating thickness is reduced. The Sn addition at the same concentration promotes the formation of the η phase but it does not affect the thickness.

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Journal of Thermal Analysis and Calorimetry
Authors: N. Pistofidis, G. Vourlias, D. Chaliampalias, E. Pavlidou, K. Chrissafis, G. Stergioudis, E. Polychroniadis, and D. Tsipas

Abstract  

Zn pack coating formation takes place in three steps as differential scanning calorimetry shows. The initial step (at 193.9C) is endothermic and involves the transformation of α-NH4Cl to β-NH4Cl and the NH4Cl decomposition to NH3 and HCl. During the second step (at 248.6C), which is exothermic, Zn2+ salts are formed and most probably ZnCl2. Finally at 264.1C (endothermic reaction) it seems that ZnCl2 is decomposed to form Zn that is deposited on the ferrous substrate. The as-cast Zn diffuses in the iron substrate forming the gamma and delta phase of the Fe–Zn phase diagram. Al2O3 is not involved in the above-mentioned mechanism and acts only as filler.

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