Investigations of the non-isothermal crystallization of Bi4Ge3O12 (2:3) glasses

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Abstract

Amorphous Bi4Ge3O12 glass samples were produced by melt quenching procedure stating with Bi4Ge3O12 (BGO) powder, obtained by solid state reaction between oxides. The kinetics of non-isothermal crystallization of BGO nano-crystals has been investigated. Differential Thermal Analysis (DTA) can give the main parameters of crystallization with an exothermic peak from 813 K to 851 K depending on the heating rate, which was assigned to the crystallization of cubic BGO in the amorphous matrix and compared with the X-Ray Diffraction (XRD) patterns. The nano-crystal dimensions were calculated from the XRD patterns by using the Debye–Scherrer method and were compared with Transmission Electron Microscopy (TEM) images. It was shown that the Ozawa model is most suitable for describing the behavior of non-isothermal crystallization of BGO nano-crystals within the glass matrix. Experimental results suggest a disk-shape type growth mechanism for the Bi4Ge3O12 nano-crystallites. The Flynn–Wall–Ozawa method has shown that the average activated energy value is 385 ± 14 kJ/mol which was computed within the same model and agrees very well with the activation energy of the crystallization.

Highlights

► Kinetics of non-isothermal crystallization of Bi4Ge3O12 (2:3) glasses. ► Nucleation rates are described by the disk-shape mechanism. ► Differential Thermal Analysis (DTA) compared with the TEM and XRD measurements. ► Ozawa model describes the non-isothermal crystallization of BGO nano-crystals. ► Flynn–Wall–Ozawa method gives the average activated energy value of 385 ± 14 kJ/mol.

Introduction

Amorphous materials exhibit different behaviors between homogenous and heterogeneous crystallization, especially when they have more than one intermediate phase. Glass ceramic materials are obtained by controlled nucleation in which the glass precursor is thermally transformed, firstly in nano-crystalline dispersed particles in the whole glass [1]. As a general rule, the control of crystallization processes requires control of either nucleation or growth, which must be determined in order to see which one is dominant phase [2]. It is generally agreed that the nucleation processes can be hardly controlled, implying a number of parameters which must be controlled, especially nucleation rate and number of nano-crystals generated before growth predominates. The growth process in the glass matrix is mainly based on the expansion from nucleation centers which also can be the same compound as the final crystallite (homogenous crystallization) or completely different compound (heterogeneous crystallization).

In the case of extrinsic crystallization, the Ozawa model is most suitable for describing the behavior of non-isothermal crystallization within the glass matrix [3]. The mean value of the Avrami exponent was found 1.16 which indicates the diffusion-controlled growth process of nano-particles in the glass system with zero nucleation rates as the single operative mechanism.

Apparently, bismuth germanate materials should act as a unique cubic phase of Bi4Ge3O12 at the end of crystallization, which is widely used for the scintillating properties [4]. However, an intermediate phase, Bi2GeO5, is the most important one because is an orthorhombic phase having ferroelectric behaviors [5]. Bismuth germanate materials can be obtained by solid state reaction between Bi2O3 and GeO2 at temperatures between 973 and 1073 K with careful attention at high temperatures because of the 1096 K melting point of Bi2O3.

Several methods have been proposed in order to obtain the parameters characterizing the kinetics of non-isothermal crystallization, Ozawa method [6], modified Ozawa method [7], modified Avrami method [8] being some of the most popular ones. Moreover, experimental methods, as Transmission Electron Microscopy (TEM), X-ray diffraction (XRD) and absorption spectroscopy are well-known and widely used in the isothermal crystallization experiments of the glass–ceramics.

The aim of the paper is to investigate the kinetics of non-isothermal crystallization of bismuth germanate glasses by correlation with additional data resulted from XRD and TEM measurements during isothermal crystallization. We focused our calculation for the stable cubic phase of Bi4Ge3O12 (BGO), but the interfered phase of Bi2GeO5 was evidenced and takes it into account due to their coexistence in a limited range of temperatures.

Section snippets

Experimental

Glasses with the composition of Bi2O3:GeO2 (2:3) were prepared from high purity materials [9]. Mixtures of 40% Bi2O3 and 60% GeO2 (in molar percents) have been prepared wet-mixed in acetone, dried at 373 K and transferred to an Al2O3 crucible, and then heated at 973 K (700 °C) for 24 h. The mixtures have been rapidly heated to 1328 K and the molten glass was kept for 5–10 min. The resulting melts have been poured onto a preheated graphite plate at different temperatures, between 623 K and 773 K,

Glass transition

With increasing temperature, a thermal fluctuation induces broken bonds in the inorganic glasses leading towards clustering processes. The red-brown color of the obtained samples, due to the turbidity effects of the light scattering processes (Mie or Rayleigh) on the BGO clusters [10], leading to one broad peak centered at 500 nm. After the annealing at 833 K for 200 min with 10 K/min, the sample become transparent (is reduced with 20%) and the broad peak disappears, suggesting the rearranging of

Results

After the cooling procedure of the melted glasses, the first step of analysis is the X-ray diffraction method, in order to investigate the crystallinity of the obtained samples. The as-obtained sample (glass sample) shows a broad peak centered at 28o, without any shape peaks, which indicates a complete amorphous structure (Fig. 1).

After step-by-step annealing, with 2 K/min, at defined temperatures obtained from Differential Scanning Calorimetry (DSC), shape peaks appear, proving the

Discussion

In order to obtain the parameters characterizing the kinetics of non-isothermal crystallization we have applied both Ozawa model and Avrami model [6], [7], [8]. In terms of Ozawa model (i.e. Eq. (1), the plots of ln[− ln(1  α)] versus ln |β| at different temperatures should be linear at a given temperature.

As can be seen in the Fig. 5 and Table 2 (r2 values) a clear linear relationship holds between ln[− ln(1  α)] and ln |β| at 1, 2 and 5 K/min heating rate, which indicates the modified Ozawa method

Conclusion

The kinetic crystallization behaviors of BGO (2:3) glasses have been investigated by using non-isothermal procedure. From the presented results it can be concluded that the Ozawa model is most suitable for describing the behavior of non-isothermal crystallization.

Experimental results suggest a disk-shape type for the Bi4Ge3O12 nanocrystallites. Flynn–Wall–Ozawa method has shown that the average activated energy value is 385 ± 14 kJ/mol. The activation energy of the nano-crystals growth was

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