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Simultaneous volume and enthalpy relaxation

The effect of experimental conditions

Journal of Thermal Analysis and Calorimetry
Authors: M. Liška and M. Chromčíková
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Abstract  

Temperature dependence of viscosity of title glasses (x=0, 2, 4, 6, 8, 10, abbreviated as M0, M2, M4, M6, M8, and M10, respectively) was measured by rotational viscometry (high temperature region: 102−106.5 dPas) and thermomechanical analysis (low temperature region: 108.5−1011.5 dPas) and described by the Vogel-Fulcher-Tammann equation. The MgO/CaO equimolar substitution (i.e. the increasing x value) smoothly shifts the high temperature viscosity to higher values. In the low temperature region the mixed alkali effect is demonstrated, and the highest viscosities are observed for the glasses M0 and M10. In the low temperature range the activation energy of viscous flow linearly decreases with the increasing x value (E act/kJ mol−1=479−9.0x). No significant dependence of activation energy on x was found in the high temperature range (E act/kJ mol−1=238.1±4.2). The structural relaxation was measured by thermomechanical experiment and theoretically interpreted in the frame of Tool-Narayanaswamy-Mazurin’s model. The broadening of the relaxation time spectrum was observed for the calcium-magnesium glasses in comparison with the pure calcium or magnesium glasses.

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Abstract  

A new method of calculation of parameters of enthalpy relaxation models is proposed. Regression analysis treatment compares the experimental and calculated values of relaxation enthalpy. The experimental values of relaxation enthalpy are obtained by numerical integration of the difference between the two DSC curves. Contrary to the overall shape of the DSC curve the integral values are not affected by particular heat flow conditions during the DSC experiment. The Narayanaswamy's numerical model based on the Kohlrausch—William—Watts relaxation function was used to calculate the theoretical values of relaxation enthalpy. The application of the proposed method on the DSC experimental data of enthalpy relaxation of As2Se3 is shown.

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Abstract  

The results of the StepScan DSC obtained for 15Na2O⋅xMgO⋅(10–x)CaO⋅75SiO2 glasses were described in the frame of the commonly accepted theory of the glass transition. A new simplified model of the reversible part of StepScan DSC record was developed on the basis of the Tool Narayanaswamy Moynihan relaxation theory. Equivalence between the formal activation energy of enthalpy relaxation process on one side, and the viscous flow activation enthalpy on the other side, was found.

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Abstract  

A simple mathematical model of thermal polishing of rough glass surface as observed by light beam thermal analysis (LBTA) is presented. The rough surface is represented by a triangular profile used for calculation of the light beam intensity attenuation and an equivalent rectangular profile is used for the modeling of the time course of the thermal polishing. Computational results obtained for the NBS 711 viscosity standard glass showed that the characteristic temperature, obtained from the series of LBTA experiments after extrapolation to zero starting surface roughness, does not represent the glass transition temperature, but it corresponds to the viscosity value of about 109 dPa.s. The validity of the proposed model was confirmed by the comparison of simultaneous LBTA and viscosity measurements of CaO-Y2O3-Al2O3-SiO2 glasses.

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Abstract  

The enthalpic relaxation of the title glasses, studied by differential scanning calorimetry, is well described by a mathematical model based on the stretched exponential relaxation function with the relaxation time proportional to the actual viscosity. The dependence of viscosity on temperature and the fictive temperature was expressed by Mazurin's approximation. The relaxation parameters obtained correlated significantly with the glass composition, indicating the changes in the structural of the TiO2 role near a TiO2 content of 3–4 mol%.

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Abstract

Structural relaxation of scintillating Ce-doped Na–Gd phosphate glass with a nominal composition of Ce:NaGd(PO3)4 was experimentally studied using non-isothermal thermo-mechanical analysis, and the relaxation process was described by the Tool–Narayanaswamy–Mazurin model. The distribution of relaxation times was expressed by the empirical Kohlrausch–Williams–Watts relaxation function with relaxation time directly proportional to dynamic viscosity. The model parameters and material constants were obtained by the nonlinear regression analysis of thermo-mechanical data. It has been concluded that the model used of structural relaxation correctly describes relaxation processes in studied Ce-doped NaGd(PO3)4 glass.

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Solidification of molten zinc chloride

Experimental and theoretical studies

Journal of Thermal Analysis and Calorimetry
Authors: M. Rodová, M. Liška, K. Nitsch and Z. Kožíšek

Abstract  

Solidification of molten zinc chloride was studied both experimentally and theoretically. By isothermal thermal analysis the time needed for the melt to crystallize at a given temperature ranging between 453 and 553 K was determined and the data obtained were compared with those of a calculated TTT curve. The extremum coordinates (temperature T N, time t N) of the curve, critical cooling rate v CR, interfacial energy σ, and an additional parameter of kinetics barrier for nucleation ɛ were determined as T N=508 K, t N=7.29 s, v CR=11.38 K s−1, σ=0.11956 J m−2 and ɛ=0.5712. By non-isothermal method the critical cooling rate of glass formation was determined as 1.25 K s−1.

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Journal of Thermal Analysis and Calorimetry
Authors: S. Ľalíková, M. Pajtášová, M. Chromčíková, M. Liška, V. Šutinská, M. Olšovský, D. Ondrušová and S. C. Mojumdar

Abstract

This paper is devoted to the investigation of the properties of the natural rubber composites prepared using the cation exchanged-montmorillonite fillers. The characteristics of the montmorillonite fillers were studied by Fourier transform infrared spectroscopy (FTIR) and thermogravimetry (TG). These characterized fillers were used to preparation of the natural rubber composites, which were submitted to measurements of dynamic-mechanical thermal analysis (DMTA) and vulcanizing characteristics (M H, M L, t s, t c(90), R v) as well as physico-mechanical properties (tensile strength, modulus at 300 elongation—M 300, tensibility).

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