Formation of an Alkali Diffusion Barrier and Stain‐Resistant Glass Surface

As it is well known, the structures of silica and soda‐lime‐silica glasses are subjected to alkali ion diffusion. This diffusion can lead to staining of the glass surface, thus detrimentally affecting its optical functions. In this work, an effective alkali diffusion barrier layer is created by boron diffusion into the top 10 to 50‐nm thick layer of glass surface. In this layer, alkali mobility is electrochemically arrested by a coordination change of boron. As a result, the glass surface stays pristinely clear, even under sustained exposure to hot and humid conditions. Structural studies indicate that when three‐coordinated boron diffuses into a pre‐existing glass network, it converts nonbridging oxygens, associated with alkali ions, into bridging oxygens by changing its coordination from 3 to 4. This forces alkali ions to change their bond association from nonbridging oxygens to the charge sphere of boron due to the required charge neutrality. This new bond arrangement immobilizes the alkali ions at these sites. As a result of the alkali ion immobilization, and the reduction in nonbridging oxygens in the network, the glass surface becomes resistant to nucleophillic attack, as well as resistant to stain formation, and crystallization.