Structure and composition of surface depletion layers in poled aluminosilicate glasses

Abstract

Thermal poling processes can be used to form modified surface layers on glass that, under ion‐blocking electrode conditions, are depleted of virtually all network‐modifying cations relative to the network‐forming species. During this process, many outstanding questions remain as to the structure of these layers and how it may vary between glasses of different “parent” composition, with important implications for resultant surface properties and industrial applications of this technology. This phenomenon of depleting modifiers is particularly difficult to rationalize in aluminosilicate glass compositions, where—in the parent glass—aluminum ions are predominantly present as cation‐charge‐compensated [AlO4]− tetrahedra prior to poling. Here, we present results of a detailed investigation into the surface depletion layers formed across a wide range of ternary sodium aluminosilicate (NAS) glasses, applying a host of surface‐sensitive spectroscopy methods to directly interrogate the resulting composition and structure within the Na‐depleted, anode‐side surface layers. The desired depletion layers were successfully formed on all of the NAS glasses attempted, all showing (a) near‐complete depletion of alkali within 300‐500 nm‐thick layers on the anode‐side surfaces, (b) thin zones of Al depletion with the Na‐depleted layer, and (c) the absence of injected H+ ions that could serve as an alternative charge‐compensation mechanism. These data essentially confirmed a true binary Al2O3–SiO2 composition inside the depletion layers. However, no significant structural dependence was found as a function of parent glass, where initial compositions ranged from peralkaline to charge‐balanced. Importantly, TEM imaging showed the depletion layers to be fully amorphous and homogeneous (not phase‐separated) at the nanoscale, despite final compositions in the range of 5‐33 mol% Al2O3—a composition space notoriously prone to phase‐separation if prepared by conventional melting. Within the depletion layers, ELNES and TEY‐XANES evidence is shown for retention of Al in a 4‐coordinated state, along with XPS data indicating elimination of non‐bridging oxygen. Taken as a whole, our results indicate a highly‐connected aluminosilicate network, most likely with a relatively high concentration of 3‐coordinated oxygen—or O “triclusters”—as a plausible means of charge‐compensating 4‐coordinated Al in the absence of Na+ or H+. The combined results of this work provide convincing new evidence for unique glass structures within the depletion layers not achievable through analogous melt pathways, with important implications for surface properties.

Athermal electric field‐induced restructuring of glass during poling

“…Post‐poling, the anode had a 96% BO signal while the cathode had 58% BO and 42% NBO. These ratios of BO to NBO in Table 2 support the conclusion that electrolysis has occurred at the anode/glass interface surface during poling, consistent with prior literature 6 …”

“…These ratios of BO to NBO in Table 2 support the conclusion that electrolysis has occurred at the anode/glass interface surface during poling, consistent with prior literature. 6 Present results confirm that power density added to a sample during poling and resulting Joule heating does increase sample temperature locally, but its magnitude is relatively small, keeping the sample temperature well below T g . SIMS and XPS results reveal compositional changes, which must have occurred well below the T g .…”

“…Anion migration and electrolysis appear to be significant enough at the anode/glass interface that oxygen ions were observed to neutralize by reacting with electrode material or through the formation of gaseous diatomic oxygen molecules 6,18 . Structural rearrangements in poled soda lime glasses have also been reported as irreversible polymerization of the glass located near the anode.…”

“…However, such layers not only show pronounced compositional changes, but evidence has been also presented that indicates significant structural alterations to the parent glass network, including elimination of anionic oxygen species and repolymerization of the network. 2,[4][5][6] Such observations contradict the general understanding that structural transformation of the network should not occur to any observable extent when the glass is so far below its glass transition temperature (T g ), wherein the expected timescales for (thermally induced) structural transformation are significantly longer than observation timescales. [7][8][9] A plausible explanation of this discrepancy has been suggested that due to the passage of a finite amount of electrical current during…”

Enhancement of mechanical properties and chemical durability of Soda‐lime silicate glasses treated by DC gas discharges

“…The process induces cations to migrate, leaving a depletion layer beneath the anode surface associated with a reorganization of the glass network 8,10 . Such approach allows the development of innovative multilayer glass structures with new compositions previously not realized using classical techniques 11 . In the recent years thermal poling has also shown great potential for glass surface structuring, 7,12,13 paving the way to the fabrication of innovative architectures on glass surfaces.…”