E2 Network Formators, Modifiers and Intermediates

Topic

The chemical composition of this glass is organized around the distinct roles played by its constituent oxides; the first of these is the network former. A network former is an oxide capable of independently constructing the continuous vitreous network of tetrahedra—a structure described by the model characterizing the glass's amorphous nature. By acting as a network former, the oxide establishes the continuous connectivity between its own tetrahedra that sustains the entire network.

Contrasting with the continuous network built by the network former is the role of the network modifier. A network modifier is an oxide that, rather than joining the continuous network, disrupts it; it introduces a cation that is not part of any tetrahedron, thereby converting the oxygen atom that previously linked two neighboring tetrahedra into a non-bridging oxygen. As the network modifier breaks more bridging bonds, it generates more non-bridging oxygens, fragmenting the continuous network into smaller, less interconnected units.

Between these two opposing roles lies the intermediate oxide, whose behavior is not fixed beforehand but depends on the compositional context of the rest of the glass. Given its amphoteric nature, aluminum oxide can act as a network former—joining the continuous network—when the rest of the composition allows it, or as a network modifier—breaking that same network—when conditions do not permit the former role. Because this intermediate oxide is amphoteric rather than fixed, its role within the network depends on the composition of the surrounding glass, rather than solely on its own chemical nature. Thus, in this model—characterized by the network former and network modifier with which it was introduced—the ratio of the amount of network modifier to the amount of network former collectively determines the macroscopic behavior of the glass: a higher ratio results in a network that is more fragmented by non-bridging oxygen (a quantity also measured by this same ratio in the model describing the glass's amorphous structure) and leads to a lower approximate melting temperature; conversely, a lower ratio results in a more connected network and a higher approximate melting temperature. Furthermore, this same modifier-to-former ratio is used by the Appen model to estimate the glass's density based on its composition, and is also utilized by a different model within this set—the one describing the glass's network connectivity—to detail how that connectivity is distributed throughout the network.

ID:4518