J. Vázquez

A theoretical method for determining the crystallized fraction and kinetic parameters by DSC, using non-isothermal techniques

A procedure has been developed for determining expressions for the volume fraction crystallized and for the kinetic parameters in non-isothermal reactions in solid systems involving the formation and growth of nuclei. This method makes use of an equation for the evolution with time of the volume fraction crystallized. This equation has been integrated under non-isothermal conditions and assuming an Arrhenian temperature dependence of the nucleation frequency and of the crystal growth rate, thus obtaining a general expression for the volume fraction crystallized for each value of the related parameter with the dimensionality of the crystal. The kinetic parameters have been deduced, obtaining the maximum crystallization rate, bearing in mind the fact that, in the non-isothermal processes, the reaction rate constant is a time function through its Arrhenian temperature dependence. Finally, the theoretical expressions of the kinetic parameters have been applied to the experimental data corresponding to a set of glassy alloys, quoted in the literature, thus obtaining mean values that agree very satisfactorily with the published data. This fact shows the reliability of the theoretical method developed.

Glass transition and crystallization kinetics in Sb0.18As0.34Se0.48 glassy alloy by using non-isothermal techniques

Abstract A study of the crystallization kinetics of glassy alloy, Sb0.18As0.34Se0.48, was made using the formal theory of transformations for heterogeneous nucleation, case referred to as “site saturation”. The kinetic parameters were deduced, bearing in mind the dependence of the reaction rate constant on time, through temperature. The procedure was applied to the experimental data obtained by differential scanning calorimetry (DSC), using continuous-heating techniques. In addition, from the heating rate dependence of the glass transition temperature, the glass transition activation energy was derived. The kinetic parameters determined have made it possible to postulate the type of crystal growth exhibited in the crystallization process. The phases at which the alloy crystallizes after the thermal process have been identified by X-ray diffraction. The diffractogram of the transformed material indicates the presence of microcrystallites of Sb2Se3 and AsSe, remaining an additional amorphous matrix.

Generalization of the Avrami equation for the analysis of non-isothermal transformation kinetics. Application to the crystallization of the Cu0.20As0.30Se0.50 alloy

Abstract Non-isothermal differential scanning calorimetry is frequently employed to study the kinetics of the transformation reactions and, in particular, the crystallization of the glassy alloys. Such data are analyzed by the Kissinger method, which was originally derived for the study of homogeneous reactions. The consensus in the literature, in several decades, was that such applications (i.e. to heterogeneous solid state transformations) of the Kissinger method are not valid. In the present work the principal objections to these applications are addressed and alternative derivations of theoretical results are provided. These results demonstrate that the Kissinger method is valid for heterogeneous reactions of the type described by Johnson–Mehl–Avrami equation in the isothermal case. Isothermal and non-isothermal data on crystallization of the Cu 0.20 As 0.30 Se 0.50 glassy alloy are presented. These experimental results and the discussions presented here help to clarify the effects of incubation times in non-isothermal transformation kinetic and provide a further demonstration of validity of the generalized Johnson–Mehl–Avrami theory for the description of heterogeneous solid state transformations.

A study of the crystallization kinetics of some Cu-As-Te glasses

The crystallization kinetics of two amorphous alloys in the Cu-As-Te system was studied by differential scanning calorimetry, using continuous heating methods, and applying a new analysis procedure in order to calculate the kinetic parameters which define the crystallization reactions, in the Johnson-Mehl-Avrami model. In this analysis, the crystallized fraction interval at which the characteristic function of said model is constant was taken into account. The values obtained for these parameters made it possible to discuss the glass-forming ability of the compounds under study, and the types of crystalline growths in the alloys.

On the crystallization kinetics of glassy alloys in the Cu-As-Se system

Abstract A study of the crystallization kinetics of amorphous alloys in the Cu-As-Se system was made using a method in which the crystallization rate is deduced bearing in mind the dependence of the reaction rate constant on time, through temperature. The method was applied to the experimental data obtained by differential scanning calorimetry, using continuous-heating techniques. The kinetic parameters determined have made it possible to discuss the glass-forming ability, as well as the different types of nucleation and crystal growth exhibited by the alloys studied.

A theoretical method for deducing the evolution with time of the fraction crystallized and obtaining the kinetic parameters by DSC, using non-isothermal techniques

Abstract A procedure has been developed for analyzing the evolution with time of the volume fraction crystallized and for calculating the kinetic parameters at non-isothermal reactions in materials involving formation and growth of nuclei. By means of this method, and considering the assumptions of extended volume and random nucleation, a general expression of the fraction crystallized has been obtained, as a function of time. In the quoted expression one considers that the crystal growth rate is anisotropic. In addition, the particular case of isotropic growth rate has been studied, and the obtained equation has been integrated for the important case of nucleation frequency and growth rate independent of time, resulting an expression that may be taken as a detailed specific case of the Johnson-Mehl-Avrami relation. The kinetic parameters have been deduced, fitting a theoretical function, obtained from the JMA model to the experimental data, temperature and volume fraction crystallized. A least-squares method has been used, bearing in mind the fact that, in most non-isothermal processes, the reaction rate constant exhibits an Arrhenian temperature dependence. Finally, the theoretical derivations of the kinetic parameters have been applied to the experimental data corresponding to a set of glassy alloys, quoted in the literature, thus obtaining mean values that agree very satisfactorily with the bibliographical data. This fact shows the reliability of the developed theoretical method.

Evaluation of the glass forming ability of some alloys in the Sb–As–Se system by differential scanning calorimetry

Abstract The glass formation and devitrification of alloys in the Sb–As–Se system were studied by differential scanning calorimetry. A comparison of various simple quantitative methods to assess the level of stability of the glassy materials in the above mentioned system is presented. All of these methods are based on characteristic temperatures, such as the glass transition temperature, Tg, the onset temperature of crystallization, Tin, the temperature corresponding to the maximum crystallization rate, Tp, or the melting temperature, Tm. In this work the parameter Kr(T) is added to the stability criteria. The thermal stability of some ternary compounds of the SbxAs0.52−xSe0.48 type has been evaluated experimentally and correlated with the activation energies of crystallization by this kinetic criterion and compared with those evaluated by other criteria.

A study on glass transition and crystallization kinetics in Sb0.12As0.36Se0.52 glassy alloy by using non-isothermal techniques

Abstract Crystallization kinetics of the glassy alloy Sb 0.12 As 0.36 Se 0.52 was studied by made using a method in which the kinetic parameters are deduced bearing in mind the dependence of the reaction rate constant on time, through temperature. The method was applied to the experimental data obtained by differential scanning calorimetry, using continuous-heating techniques. In addition, two approaches are used to analyze the dependence of glass transition temperature, T g , on the heating rate, β . One is empirical linear relationship between T g and β . The other approach is the use of straight line ln( T 2 g / β ) vs. 1/ T g for evaluation of the activation energy for glass transition. The kinetic parameters determined have made it possible to find a bulk nucleation mechanism with decreasing nucleation rate and diffusion controlled growth for the crystallization process. The phases at which the alloy crystallizes after the thermal process have been identified by X-ray diffraction. The diffractogram of the transformed material suggests the presence of microcrystallites of Sb 2 Se 3 and AsSe, in the residual amorphous matrix.

Determination of kinetic parameters of crystallization and study of glass forming ability for alloys in the GeAsTe amorphous system

Abstract A mathematical method which makes it possible to calculate the kinetic parameters of crystallization, activation energy, E , reaction order, n , and frequency factor, K 0 , was developed. The procedure was applied to chalcogenide glasses Ge 0.05 As 0.20 Te 0.75 , Ge 0.10 As 0.20 Te 0.70 and Ge 0.14 As 0.43 Te 0.43 , confirming the validity of the method. A criterion is discussed for analyzing the glass forming ability (GFA) of these alloys, from the thermal events observed on heating the glasses, and evaluated in terms of the reaction rate constant of each alloy.

Structural characteristics and some concepts related to switching properties in As0.45Se0.10Te0.45 glassy alloy

A structural model of the bulk Aso.4sSeo.roTeo.4s amorphous alloy has been built by the random Monte Carlo technique. The model is in good agreement with the atomic radial distribution function obtained by X-ray diffraction. The mean coordination number obtained was 2.3, thus allowing the sample to be characterized as a bistable material. Chalcogenide glasses are studied mainly because they present a switching phenomenon and a memory effect, and therefore can be used in the making of a great number of electronic devices [l]. The main interest in the analysis of the glass system As-Se-Te [2,3], amongst other reasons, is the dependence of the electrical conductivity on sample composition, as well as presenting memory effect together with the switching process. For these reasons, a structural analysis as well as an electrical study of the glassy alloy As0.45Se0.10Te0.45 has been undertaken, given that the establishment of the structural units, the distribution of chemical bonds between atoms, the radii of the coordination spheres and the mean values of the bonds, besides being of intrinsic interest, allows the process of electrical conductivity to be explained, as it is closely linked to the type and amount of chemical bonds existing in the material. The ‘4s0.45Se0.10Te0.45 composition was prepared so that the influence of the amount of Te could be observed, previous studies having been carried out with a smaller atomic fraction of Te [2,3]. Electrical resistance measurements of the composition with a greater amount of Te indicate that it reduces electrical resistance, due to the dislocation of covalent bonds as a result of the metallization of the chemical bond. Analogous behaviour has been reported in the literature [41. The number of bonds between each of the elements,