Elastic and structural properties of low silica calcium aluminosilicate glasses from molecular dynamics simulations
Introduction
The mechanical and structural properties of glass are considered as one of the fundamental problems of condensed matter physics and chemistry because they can help to understand how macroscopic properties emerge from microscopic characteristics [[1], [2], [3], [4]]. It is known that certain glassy compositions can be obtained from CaO–Al2O3 without the existence of any strong network formers in glass state networks such as B2O3, SiO2 or P2O5 [5]. Pioneer works [6,7] have reported that the addition of a high content of SiO2 to the calcium aluminate glasses enhances their glass-forming ability but also deteriorate the optical properties owing to the stretching motion of the SiO bonds. Adding a low amount of silica to calcium aluminate leads to low silica calcium aluminosilicate (LSCAS) glasses, CaO-Al2O3-SiO2, which present a high potential for technological applications [8,9].This chemical composition achieves easily a glassy state by the conventional quenching technique where the CaO content is substantially broadened (around 60%–70% of CaO) [9]. It is important to note that the tellurite [10], phosphate [11], and aluminosilicate [9] glasses system can dissolve a relatively large modifier content compared to pure silica glass [12]. Ternary CaO–Al2O3–SiO2 glasses are used in many applications such as: glass fibers, gems and printed circuit boards [13,14]. Furthermore, the chemical durability, thermo-mechanical properties and infrared transparencies of LSCAS glasses make them very attractive for near- and mid-infrared laser applications [15,16].
As far as the structure of alkaline earth-aluminosilicate glasses is concerned, it has long been known that the three-dimensional structure consists of the interpenetration of the two networks of formers, consisting respectively of SiO4 and AlO4 tetrahedra, in which the SiO and AlO bonds are highly covalent.
Extra framework cations such as Ca can act either as charge compensator and/or modifier, depending on the ratio [CaO]/[Al2O3] and silica amount. The geological glasses are a good example of multicomponent systems with a significant extent of topological and chemical disorder where the ratio [CaO]/[Al2O3] plays an important role [9]. Despite the important number of studies devoted to aluminosilicate glasses in this field [9] and references therein, the detailed mechanisms at the molecular scale as well as their connection with the macroscopic properties have not been well understood yet.
The presence of a modifier such as calcium together with the formers of the LSCAS glasses drives the transformation of the bridging oxygens (BO) into nonbridging oxygens (NBOs) increasing the extent of depolymerization. MD has led to a better understanding of the relationship between chemical composition and mechanical properties in silicate glasses (see [1]). In LSCAS glasses, Hwa [17] showed that the nature of chemical bonds plays an essential role in the elastic properties of glasses, and that ionic bonds have a decisive effect on the structure of these systems. For that, the main subject of the present work is to calculate the elastic constants of LSCAS to correlate them with the structural properties. Modeling and simulation techniques can be used to understand how glasses properties are linked to the microscopic scale. MD simulations are generally applied in the field of glasses to get structural and mechanical properties with a very good accuracy [18,19]. To investigate the elastic constants of LSCAS glasses, we have used two methods; the minimization energy at zero temperature (Zero-T) and a second one that allows to calculate the elastic constants at finite temperature (FT) using a script treating the output from the LAMMPS package [20].The reliability of the two methods to calculate these parameters has been evaluated comparing our results with those measured using Brillouin light scattering (BLS) spectroscopy [21]. Our second objective is to understand the structural changes in LSCAS glasses with the evolution of the silica content, and to compare our results with available experimental data.
This paper is structured as follows. Section II presents a brief description of the interatomic potential used together with the simulation technique and explains the methods for preparing the glass samples and for determining the elastic constants. Section III.A shows our detailed results on elastic constants determined by the two methods Zero-T and FT. In Section III.B, we present the effects of the addition of silica on the LSCAS glass structures. The results will be then discussed in Section IV, especially the relationship between the structural and elastic properties for various loading of SiO2 in the LSCAS, in connection with the literature. The last section summarizes the findings of this work.
Section snippets
Simulation procedure
The quality of molecular dynamics results is strongly dependent on the choice of the interatomic potential form and on the successful fitting of the parameters to the aimed systems [22]. The Born− Mayer−Huggins potential [23] is quite adapted to investigate the glassy states because it contains a so-called repulsive term in addition to the Coulomb and dispersive terms. Silicate [1] and aluminosilicate [18,19] glasses properties have been successfully recovered using this potential formula:
Elastic Constants
In a first step, we must mention that the choice of the interaction potential in MD simulations has a critical effect on the determination of mechanical properties, as demonstrated by Bauchy [18]. Indeed, using three potentials to simulate structural and mechanical properties of high silica calcium aluminosilicate glass (composition (SiO2)60(Al2O3)10(CaO)30), Bauchy found that the Matsui's potential [29] enhanced by Jakse's research group, namely Bouhadja et al. potential [19], appears to offer
Discussion
In this work, we focus on the mechanical properties of the LSCAS glasses with a low content of silica (5–25 mol%) and [CaO]/[AlO2] = 2. One main issue of the present work was to compare computed elastic constants by two methods, FT and Zero-T, with experimental data from literature [21]. We have found that in almost all cases the elastic constant values C11 and C44 measured using BLS spectroscopy are lower than those computed by Zero-T and are higher than those computed by FT method. Overall,
Conclusion
The mechanical properties of LSCAS glasses have been computed and the structure has been simulated using molecular dynamics simulations. Overall, our MD simulations offer a good agreement with experiments for the elastic constants and structural features. The elastic constants have been computed at zero and finite temperature. Globally, the two methods follow the experimental trends. From a quantitative point of view, the FT method offers the best agreement with the experimental values. Both
Acknowledgements
We thank the PMMS (Pôle Messin de Modélisation et de Simulation) and GENCI-CCRT/CINES (Grant No. x2017-085106) for providing us HPC resources.
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