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  • Author or Editor: K. Paraskevopoulos x
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Summary The effect of sintering on the maximum capture efficiency of CO2 is studied, using a carbonation/calcination cycle for a series of samples with different stoichiometries of dolomite and calcite. For the materials that belong to the categories of limestone and dolomitic limestone, sintering decreases the extent of carbonation significantly at the two different highest temperatures studied. The extent of carbonation for the same maximum heating temperature depends mainly on the percentage of dolomite. Sintering is negligible in the dolomitic rocks, especially at the maximum heating temperature of 1005°C. The composition of the carrier gas does not seem to play a significant role. The reduction of the extent of carbonation at the second heating /cooling cycle in limestone, and the durability after enough successive cycles of calcination/carbonation in the dolomitic rocks, does not seem to be affected by the maximum temperatures of calcination that were used at the experiments.

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The thermal effect accompanying the transition of Cu2–xSe into a superionic conduction state was studied by non-isothermal measurements, at different heating and cooling rates (β=1, 2.5, 5, 10 and 20C min–1). During heating the peak temperature (T p) remains almost stable for all values of β, (136.80.4C for Cu2Se and 133.00.3C for Cu1.99Se). A gradual shift of the initiation of the transformation towards lower temperatures is observed, as the heating rate increases. During cooling there is a significant shift in the position of the peak maximum (T p) towards lower temperatures with the increase of the cooling rate. A small hysteresis is observed, which increases with the increase of the cooling rate, β. The mean value of transformation enthalpy was found to be 30.30.8 J g–1 for Cu2Se and 28.90.9 J g–1 for Cu1.99Se. The transformation can be described kinetically by the model f(ǯ)=(1–ǯ)n(1+kcatX), with activation energy E=175 kJ mol–1, exponent value n equal to 0.2, logA=20 and log(k cat)= 0.5.

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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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Urea-formaldehyde (UF) resins are the most used polycondensation resins today, in manufacturing particleboards. UF resins possess some advantages such as fast curing, good performance in the panel, water solubility and low price. However, the main chemical bonds of the UF resins macromolecules are hydrolysis sensitive. This causes low water and mositure resistance performance and subsequent formaldehyde release from the UF-bonded panels. A multitude of pathways have been explored for the improvement of UF resins’ behavior relating either to their synthesis procedure or application parameters during panel manufacture. In this study, two UF resins (a conventional and an innovative one produced at very low pH and temperature conditions) were analyzed for their specifications and characterized with TG-DTA technique in dynamic heating conditions and FTIR measurements both in their pre-polymer and cured state.

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Phase changes of iron containing solid wastes from steel mill pickling lines after thermal treatments were investigated aiming the determination of the appropriate conditions for its transformation to be useful for industrial raw materials. Above 275°C, the thermally treated wastes contain a mixture of α-Fe2O3 (hematite) and γ-Fe2O3 (maghemite) in different proportions, depending on the maximum heating temperature of the thermal treatment. Increasing the maximum temperature the maghemite participation is decreased through its transformation to hematite. Above 850°C hematite is the main constituent, suggesting that thermal treatment of the wastes in this temperature will give a product that could be used as red iron pigment.

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Abstract

Nanocomposites of poly(l-lactic acid) (PLLA) containing 2.5 wt% of fumed silica nanoparticles (SiO2) and organically modified montmorillonite (OMMT) were prepared by solved evaporation method. From SEM micrographs it was observed that both nanoparticles were well dispersed into PLLA matrix. All nanocomposites exhibited higher mechanical properties compared to neat PLLA, except elongation at break, indicating that nanoparticles can act as efficient reinforcing agents. Nanoparticles affect, also, the thermal properties of PLLA and especially the crystallization rate, which in all nanocomposites is faster than that of neat PLLA. From the thermogravimetric curves it can be seen that neat PLLA nanocomposites present a relatively better thermostability than PLLA, and this was also verified from the calculation of activation energy (E). From the variation of E with increasing degree of conversion it was found that PLLA/nanocomposites decomposition takes place with a complex reaction mechanism, with the participation of two different mechanisms. The combination of models, nth order and nth order with autocatalysis (Fn–Cn), for PLLA and PLLA/OMMT as well as the combination of Fn–Fn for PLLA/SiO2 give the better results. For the PLLA/OMMT the values of the E for both mechanisms are higher than neat PLLA. For the PLLA/SiO2 nanocomposite the value of the E is higher than the corresponding value for PLLA, for the first area of mass loss, while the E of the second mechanism has a lower value.

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Journal of Thermal Analysis and Calorimetry
Authors: M. Anastasiou, Th. Hasapis, T. Zorba, E. Pavlidou, K. Chrissafis, and K. Paraskevopoulos

Abstract  

Historic plasters from wall paintings of Byzantine and post-Byzantine churches situated in the Balkan region were studied. All wall paintings were made with fresco technique and are dated from IX - XVI century. Plaster samples were followed from room temperature to 1000C by Thermogravimetric (TG) and Differential Thermal Analysis (DTA), and one or two significant temperature regions, corresponding to thermal decomposition mechanisms were observed. The analysis of the plaster samples and the composition characterization was carried out using also, Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). Although the main components are calcite and quartz (from sand) in different proportions, there are differences between them such as the presence of gypsum being either as a constituent element or due to environmental pollution. The results are examined comparatively taking into account the creation time and place of the paintings.

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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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Abstract  

Bioactive glasses have attracted considerable interest in recent years, due to their technological application, especially in biomaterials research. Differential scanning calorimetry (DSC) has been used in the study of the crystallization mechanism in the SiO2–Na2O–CaO–P2O5 glass system, as a function of particle size. The curve of the bulk glass presents a slightly asymmetric crystallization peak that could be deconvoluted into two separate peaks, their separation being followed in the form of powder glasses. Also, a shift of the crystallization peaks to lower temperatures was observed with the decrease of the particle size. FTIR studies – that are confirmed by XRD measurements – proved that the different peaks could be attributed to different crystallization mechanisms. Moreover, it is presented the bioactive behavior of the specific glass as a function of particle size. The study of bioactivity is performed through the process of its immersion in simulated human blood plasma (simulated body fluid, SBF) and the subsequent examination of the development of carbonate-containing hydroxyapatite layer on the surface of the particles. The bioactive response is improved with the increase of the particle size of powders up to 80 μm and remains almost unchanged for further increase, following the specific surface to volume ratio decrease.

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Journal of Thermal Analysis and Calorimetry
Authors: C. Dagounaki, K. Chrissafis, A. Kassoli-Fournaraki, A. Tsirambides, C. Sikalidis, and K. Paraskevopoulos

Abstract  

Representative carbonate rock samples collected from the broader area of Kozani (NW Macedonia, Greece) are examined. The participation of constituents in the formations was determined combining three different methods of analysis, atomic absorption spectroscopy (AAS), X-ray diffraction analysis (XRD) and thermogravimetry (TG). The three methods are used in a complimentary way in order to specify the exact composition of the samples. Although the question about the composition can be answered by AAS, the percentage of the included calcite and dolomite can be determined with accuracy, only through TG analysis, under the appropriate conditions, regarding the gas carrier. The results of the analysis lead to a more complete view of the geological conditions that predominated in the studied area.

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