1. Introduction
Chemical strengthening, achieved by ion exchange through immersing an alkali-containing glass into a molten salt bath, generates high compressive stress in thin or irregular shaped glass objects without measurable optical distortion, making it one of the most important techniques of glass strengthening methods.1 Sodium aluminosilicate glasses (NAS) are suitable for ion exchange strengthening and currently receive significant interest not only because of their excellent mechanical properties and chemical durability, leading to wide applications in touch-screen displays,2 windshields,3,4 and architectures,5,6 but also because they can serve as a good model system to introduce a wide range of oxides for tailoring their properties.7-11 Among these oxide additions, P2O5 has been recently studied and found to be able to increase the hardness, compressive stress and depth of ion-exchange layer by causing depolymerized silicate anions.12
Phosphorus pentoxide (P2O5), even with a very small addition, is known to play an important role in aluminosilicate glass system and can impact the physical and chemical properties.13,14 The immiscibility of phosphate-rich and silicate-rich phases caused by P2O5 addition is also observed,15,16 accounting for crystallization after further thermal treatment. The 31P MAS NMR study of the P2O5 containing NAS glasses with a molar ratio of Al/Na ranging from 0 to about 1.27 shows that in peralkaline region P2O5 is partly present as phosphate tetrahedra attached to the aluminosilicate network charge balanced by sodium, while in peraluminous region it integrated into the glass former network as monomeric tetrahedra for which all three available single bonded oxygens are connected to tetrahedrally coordinated Al.17 In lithium aluminosilicate system, the P, Al and Si local environment investigations also show that P is mainly presented as orthophosphate and pyrophosphate species with a low amount of Al2O3 and gradually enters the Si and Al based glass forming network with increasing molar ratio of Al2O3/Li2O.18 While in metaluminous sodium aluminosilicate glasses, P is found to mainly integrate into the glass network via P-O-Al bonding because of the alternative charge compensation scheme for [AlO4]− and [PO4]+ units, and the formation of Si-O-P bonding played only a minor role.19 Although these experimental results provide valuable information for distinguishing the topology of the glassy states with phosphorus, fully understanding the structural change aluminosilicate glasses due to P2O5 and associated ion exchange strengthening still remains a grand challenge.
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In the past few decades, the structure models for oxide glasses have been developed. The basic concept was first established in the 1930s,20 and then a modified random network (MRN) model was proposed by Greaves21 in mid-1980s, in which the oxide glass is described as partially covalent random networks made up of corner-shared glass former-oxide tetrahedra with glass modifiers in the inter-networking regions. Recently, a new modified model has been proposed to explain the prevention of the alumina-avoidance principle in term of aluminosilicate glass system,22 in which the network structure region composed of well-separated homonuclear framework containing only silicon-oxygen connection and heteronuclear framework containing both silicon and aluminum tetrahedra, and the inter-networking region composed of non-bridging oxygens and non-framework cations. Considering the fact that the ionic diffusion mechanisms are mostly vacancy-based rather than interstitial-based,23 and the possibility that the inter-networking region formed due to phosphorus addition, such modified random network models are instructive to understand the ion diffusion in P2O5-Na2O-Al2O3-SiO2 glass system.
Molecular dynamics (MD) simulations is an effective method to study the atomic structures and structure-property relationship of glass materials,24 and has been successfully applied to the investigation of structural heterogeneity in borosilicate25 and oxyfluoride glasses.26 In the present work, to explore the detailed structural information of P2O5-bearing sodium aluminosilicate glass, compositions with various P2O5 content and various Al/Na ratios were investigated by using MD simulations. Short- and medium-range structural features, such as pair distribution function (PDF), total correlation function (TDF), coordination number (CN), oxygen speciation, linkage between different network former cations (T-O-T connections, T = Si, Al, P in this work), network former species (Qxn), and network connectivity (NC), were analyzed. Based on the structural information from simulations, a modified random network model for P2O5-bearing sodium aluminosilicate glass is proposed, considering the relations between different network formers and percolated channels with sodium clustering behaviors. This structural model is then used to better understand ion-exchange efficiency acceleration of phosphorus addition in sodium aluminosilicate glass from the perspective of the glass network topology.
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