Kinetics of Phase Transformations and Thermal Stability of Se₅₈Ge_42-xPb_x (x = 15, 18 & 20) Glasses

Deepika  , Kuldeep Singh Rathore, Narendra Sahai Saxena

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DOI: 10.4236/njgc.2012.21005                

Abstract

This paper presents the results of kinetic studies of glass transition and crystallization in Se₅₈Ge_42-xPb_x (x = 15, 18 & 20) glasses using differential scanning calorimetry (DSC). It has been observed that these glassy systems exhibit single glass transition and double crystallization on heating. The crystalline phases have been identified by annealing the samples at temperatures intermediate between the first and second crystallization peaks. The structural characterization of the as-prepared and annealed glassy alloy has been done through X-ray diffraction (XRD). The activation energy for glass transition as well as crystallization region has been calculated using various theoretical models. In addition, the effect of annealing on various kinetic parameters of transformations has been studied. On the basis of the experimental results on phase transformations in these glasses, thermal stability of the samples under investigation has been ascertained. It was found that the thermal stability is profoundly affected by annealing since the glass transition as well as crystallization temperatures are strongly influenced by annealing the samples. The phase transformation study reveals that the thermal stability of the samples increases with the increase in lead content in the samples.

Keywords

Activation Energy; Differential Scanning Calorimetry (DSC); Thermal Stability

1. Introduction

The nature of glassy state and the mechanism of glass transition are considered as the deepest and most interesting unsolved problems in solid state theory. In addition to the fundamental scientific interest of the problem, the practical aspects are of vital importance to the glass industries as the properties of the glass product depend strongly on both its composition and thermal history. The material system in which glass forming kinetics are observed requires supercooling below the melting point, where the relaxation processes of the supercooled liquids are much slower than the cooling rate. This leads eventually to the glass transition where the material is frozen in an amorphous rather than crystalline state. This amorphous state is thermodynamically unstable and crystallizes at elevated temperatures. The transition of amorphous state into crystalline state proceeds by nucleation and growth reactions. Considering that glass formation is a competing process against crystallization, a systematic study on the kinetics of the crystal phase remains one of the most interesting aspects of condense matter physics.

The kinetic behavior associated with a structural change from an amorphous state to crystalline state is of great importance to establish the thermal stability and glass forming ability (GFA) of glasses and ultimately to determine the useful range of operating temperatures for a specific technological application before the eventual crystallization takes place. The critical issue for the potential applications of these glasses is their thermal stability against crystallization. They should be stable against thermal aging during their application. Therefore, it is very important to investigate the thermal stability of glasses against crystallization, when they are subjected to reheating during the fabrication of glass ceramics.

Thermal analysis tools, in particular differential scanning calorimetry (DSC) have been successfully employed [1-4] in studying phase transformations involving nucleation and growth and continuous grain growth of pre-existing nuclei and for investigating the crystallization kinetics of glass forming liquids. Kinetic data on first order transformations are often obtained from this technique in either isothermal or non-isothermal mode. The isothermal analysis is more definitive, in most cases, it has been shown that the non-isothermal technique also have several advantages, in particular that experiments can be performed quite rapidly. Additionally, many phase transformations occur too rapidly to be measured under isothermal conditions because of transients associated with the experimental apparatus. For this reason, nonisothermal methods are frequently used for studying the kinetics of phase transformations of glasses.

Kinetics of glass transition has been studied [5-8] widely from the viewpoint of understanding various structural and thermodynamic properties in the glass transition region. Also the structural relaxation due to sub-T_g annealing of the glass can be studied through the investigation of kinetics of this region. Moreover, activation energy of glass transition is a kinetic parameter which can throw light on the thermal stability of glass and can be determined through the knowledge of glass transition temperatures at different heating rates.

Many authors used the so-called Kissinger model [9] or Ozawa model [10] directly to examine the kinetics of crystallization of amorphous materials. These methods, however can not be directly applied to the crystallization of amorphous materials and the physical meaning of the activation energies thus obtained are obscure because the crystallization is advanced not by the n^th order reaction but by the nucleation and  growth processes. On the other hand, some authors [11-15] have applied the Johnson-Mehl-Avrami (JMA) equation to the non-isothermal process. Although sometimes they appeared to get reasonable activation energies, this procedure is not appropriate because the JMA equation was derived for isothermal crystallization [16]. Matusita and Sakka [17-19] have proposed method for analyzing the non-isothermal crystallization kinetics on the basis of nucleation and growth processes, and emphasized that the crystallization mechanism such as bulk crystallization or surface crystallization should be taken into account for obtaining the meaningful activation energy.

Authors [20] have studied the thermodynamic properties of this system and confirmed that the stability of the system increases with the increase in lead content. In the present paper, kinetics of phase transformation as well as thermal stability of Se₅₈Ge_42-xPb_x (x = 15, 18 & 20) glasses under non-isothermal conditions has been studied. The Se-Ge-Pb system is of interest and has been investigated because very few attempts have been made to study chalcogenide glasses with Pb as one of the component. It is due to the fact that lead is the last element in radioactive series, which is more stable, or lead is one with which it is difficult to form a glass. Moreover, the charge reversal from usual p-type to n-type [21,22] has created an additional interest in the thermal properties of these glasses.

In view of this, kinetics of phase transformations and related thermal properties of Se₅₈Ge_42-xPb_x (x = 15, 18 & 20) glasses have been studied using Differential Scanning Calorimetry (DSC). The results obtained from DSC have been interpreted using many theoretical models. These models were used to extract the kinetic parameters of the phase transformation. The effect of annealing on the phase transformation of the system under investigation has also been reported in this paper. Besides these, thermal stability of the samples has been evaluated using the kinetic parameters. Effect of increasing lead content on the crystallization and thermal stability of these glasses has also been investigated in detail in this paper.

2. Experimental Details

Glassy alloys of Se₅₈Ge_42-xPb_x (x = 15, 18 & 20) have been prepared by melt-quenching technique discussed elsewhere [20]. The amorphous nature of the alloys was ascertained through X-ray diffraction pattern of the samples using Bragg-Brentanno geometry on Panalytical X’- pert Pro differactometer in 2θ range of 20˚ - 90˚ with CuKα radiation source (λ = 1.5406 Å). The X-ray tube was operated at 45 kV and 40 mA.

Rigaku Model 8230 of DSC is used to measure the caloric manifestation of the phase transformation and to study the crystallization kinetics under non-isothermal condition. The accuracy of heat flow measurement is ±0.01 mW and the temperature precision, as determined by the microprocessor of the thermal analyzer, is ±0.1 K. DSC runs have been taken at five different heating rates, i.e. 10, 15, 20, 25, 30 K/min on accurately weighed samples taken in aluminum pans under non-isothermal conditions. The temperature range covered in DSC is from room temperature (300 K) to 753 K.

3. Results and Discussion

3.1. Structural and Thermal Analysis

Figure 1 shows the X-ray diffraction patterns of as-prepared Se₅₈Ge_42-xPb_x (x = 15, 18 & 20) glassy alloys. The absence of any sharp peak in the diffraction patterns confirms the amorphous nature of these samples. Figure 2 shows the DSC curves of as-prepared Se₅₈Ge_42-xPb_x (x = 15, 18 & 20) samples at a heating rate of 20 K/min. The glassy alloys under consideration show an endothermic step corresponding to its glass transition temperature (T_g) and two exothermic peaks corresponding to crystallization. The double stage crystallization observed in Se₅₈Ge_42-xPb_x

Conflicts of Interest

The authors declare no conflicts of interest.

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