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  • Author or Editor: J. Málek x
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Abstract  

Crystallization processes of GexS1-x (0.322≤x≤0.44) glasses have been studied by thermal analysis and a new simple method of kinetic analysis is proposed. This method allows the definition of an appropriate model characterizing the crystallization process of glass, as well as calculation of reliable kinetic parameters. Results of kinetic analysis allow definition of a thermal stability criterion which has a general applicability for any glass-forming system.

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Abstract  

Structural relaxation for simple and more complex thermal histories is described by a phenomenological model based on a non-exponential relaxation function, the reduced-time concept and the nonlinear structural contribution to the relaxation time. The history, development of experimental techniques and data analysis is described. It is shown that the volume and enthalpy relaxation response can conveniently be compared on the basis of a fictive relaxation rate, R f. A simple equation relating R f and the parameters of the phenomenological model is given. The calculated data for moderate departures from equilibrium are in good agreement with our experiments and data previously reported in the literature.

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Abstract  

The volume and enthalpy relaxation rate of inorganic glasses and organic polymeric materials subjected to temperature jump T has been analyzed. It is shown that the relaxation behavior in isothermal conditions can be compared on the basis of the fictive relaxation rate defined as R f=(dT f/dlogt)i. No significant difference between volume and enthalpy relaxation rate has been found for all materials examined. A simple equation relating the R f and parameters of Tool-Naraynaswamy-Moynihan (TNM) phenomenological model has been derived. This equation predicts increasing R f with the magnitude of temperature jump. It seems that correct determination of TNM parameters might be problematic for slowly relaxing polymers as the effect of these parameters becomes comparable with experimental uncertainty.

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For experimental investigation of the temperature-dependences of specific heat and thermal conductivity in the range 4–300 K a continuous-flow helium cryostat has been developed. Its adaptation for low-temperature calorimetry and its use for measurement of the temperature-dependences of the specific heats of bulk samples of metals and insulators are described in this note. The phase transition from the normal to the superconductive state has been measured on NbTi and its critical temperature determined. Two methods of determination of the temperature-dependences of the specific heats of metals and insulators have been developed. The inaccuracy of specific heat determination did not exceed 2 % with metal materials and 5 % with insulating materials.

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The crystallization processes of GexS1−x glasses were studied by means of heat flux DSC. It was shown that germanium disulphide crystallizes atx⩽1/3. On the other hand, a higher germanium content (x>1/3) led to the crystallization of both GeS2 and GeS. Kinetic analysis of the processes of crystallization of the studied glasses was performed using the Arrhenius rate constant and the Šesták-Berggren kinetic model. The kinetic parameters were calculated through non-linear regression of the experimental DSC curves.

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Abstract  

The structural relaxation and viscosity behavior of Ge38S62 glass has been studied by thermomechanical analysis. The relaxation response to any thermal history is well described by the Tool-Naraynaswamy-Moynihan model. The apparent activation energy of structural relaxation is very close to the activation energy of viscous flow (Eη=47812 kJ mol-1). However, the activation energy of crystal growth obtained by optical microscopy is about one half of this value. Similar result has been obtained from isothermal DSC measurement (Ea=22020 kJ mol-1). The kinetic analysis of these data reveals interface controlled crystal growth with zero nucleation rate.

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