Contents lists available at ScienceDirect Thermochimica Acta journal homepage: www.elsevier.com/locate/tca Crystallization kinetics study of silver-doped germanium glasses Roger Gomes Fernandes ⁎ , Paula Squinca Valle, Douglas Faza Franco, Marcelo Nalin São Paulo State University (Unesp), Institute of Chemistry, Araraquara, Brazil ARTICLE INFO Keywords: Non-isothermal crystallization DSC Oxide glasses Photonics ABSTRACT A series of germanium glasses in the systems (100-x)[40GeO 2 -30Bi 2 O 3 -20PbO-10TiO 2 ] – (x) AgCl with x = 0, 1, 3 and 5 (mol%) was prepared using the melt-quenching method and then characterized taking into account their crystallization kinetics mechanisms and microstructural analysis. Through Diferential Scanning Calorimetry (DSC) performed in diferent linear heating rate programs, the experimental data obtained were based on non- isothermal models of phase transformation, since their versatility allows us to calculate kinetic parameters, for example, activation energy for crystallization (E c ) and Avrami index n (). Thus, it is possible to obtain in- formation on the crystallization mechanism of these glasses. Independent of the model, the values of the Avrami index for each composition are the same, considering the uncertainty attributed to them. It is safe to afrm that bulk and surface nucleation contribute to the crystallization of compositions containing 0 and 1 mol% of AgCl, whereas internal and surface crystallization with one-dimensional growth occur in samples with 3 and 5 mol% of AgCl, respectively. These results were supported by a microstructural analysis from bulk samples that underwent non-isothermal heat treatment at diferent temperatures according to DSC crystallization peak. 1. Introduction Many eforts are concentrated on the synthesis of optical glasses either in bulk or optical fber shapes whose crystallization mechanism in the volume is predominant over the surface. Among the most studied glasses, Heavy Metal Oxide Glasses (HMOG) are widely investigated because of their recognized potential for photonic and optoelectronic applications [1,2]. They are formed by oxygen atoms and cations with high atomic mass and low bonding energy between them. This low bonding energy provides a long wavelength (infrared region) light transmission and low phonon energy when compared to silicate glasses. Besides, a general rule states that glasses exhibiting a high refractive index are more likely to have signifcant non-linear optical properties [3,4]. Since heavy metals (Pb, Bi, Te, Sb) are highly polarizable and, consequently, have a high refractive index, glasses containing high concentrations of such metals must have satisfactory non-linear optical properties. The inconvenience that arises when working with glasses for optical applications refers to the intrinsic characteristics of the spectroscopic properties of the optically active species in this medium. As the material is amorphous, there is a random distribution of sites for these ions. Consequently, the absorption and emission lines are broader, de- creasing the emission efciency at specifc wavelengths. This negative aspect is minimized if the ion is located within a distinct crystalline phase [5,6]. For this reason, obtaining dispersed nanocrystals in a glassy phase is ideal to obtain a transparent material with efcient optical and non-linear properties. However, a homogeneous distribu- tion of these nanoparticles into the matrix as well as their controlled growth for glass-ceramic preparation is quite challenging, and con- stitutes a prerequisite to achieve the optimal properties of such appli- cations. In practice, crystallization in most glass systems begins with het- erogeneous nucleation [7]. Only certain vitreous compositions, when subjected to times and temperatures of specifc heat treatments, present nucleation in the volume [8]. In other systems, additives called nucle- ating agents are introduced together with the raw material before glass melting to induce the crystallization in the volume [9–11]. In general, it is necessary to fnd a specifc nucleating agent for each composition, which is not always an easy task. In this work, a series of HMOG glasses based on the glass former GeO 2 and doped with silver chloride was prepared. Non-isothermal approximation methods based on Diferential Scanning Calorimetry performed in diferent linear heating rate programs were used to in- vestigate the crystallization kinetics of these compositions. We corro- borate the values of the Avrami index, n, from each of these models with a detailed analysis of the glass-ceramic microstructure of the compositions that underwent non-isothermal heat treatment at dif- ferent temperatures according to DSC crystallization peak. https://doi.org/10.1016/j.tca.2019.01.013 Received 15 October 2018; Received in revised form 28 December 2018; Accepted 14 January 2019 ⁎ Corresponding author. E-mail address: rgfernandes@usp.br (R. Gomes Fernandes). Thermochimica Acta 673 (2019) 40–52 Available online 14 January 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved. T