How Does Thermal Poling Produce Interstitial Molecular Oxygen in Silicate Glasses?

Abstract

Thermal poling of glasses induces structural and compositional modification and breaks central symmetry of these initially isotropic media. In spite of numerous experimental data accumulated, little is known about the processes occurring in soft glasses under this processing. We use micro-Raman technique to study the formation of interstitial molecular oxygen and structural modification of the subsurface layer of a soda-lime silicate glass in the course of thermal poling. The presence of O 2 is demonstrated in cation-depleted subanodic region of the glass, the thickness of which depends on the charge passed during the poling procedure and, in our experiments, reaches one micron. O 2 concentration in this layer is independent on the charge passed and is of the order of 3⋅10 20 cm -3 being maximal in anodic surface craters arising because of poling current non-uniformities. O 2 generation is accompanied by an increase in the concentration of three-membered Si-O rings in the modified region of the glass matrix. The ways of non-bridging oxygen recombination are discussed and proposed as the main mechanism of the interstitial O 2 formation.

Chemical structure and mechanical properties of soda lime silica glass surfaces treated by thermal poling in inert and reactive ambient gases

“…Note that these are the samples exhibited a larger blue shift in the ν Si‐O‐Si band (from 1070 cm −1 to ≥1094 cm −1 ) and a larger decrease in the NBO band at ~940 cm −1 in SR‐IR (Figure ) and more collapsed pits in the surface topographay (Figure ). The trapped molecular O 2 has also been observed in the cases where thermal poling was conducted with a blocking electrode configuration . When the gas environment during thermal poling contains 1.3% H 2 O, then the molecular O 2 peak decreases substantially; when the H 2 O concentration is increased to 2.9%, the molecular O 2 peak is completely suppressed.…”

“…In the area poled in Ar and N 2 , many craters with depths ranging from ~20 nm to ~80 nm can be seen frequently. The sodium depletion which accompanied the conversion of most NBOs to BOs must cause local deformation or aggregation of void space in the silicate network . Such processes can create craters observable in high‐resolution optical profilometry.…”

“…When the amount of reactive species entering the glass is not sufficient enough to compensate or consume all NBO sites left, emission of electrons and extinction of anions can take place . One such process could be the condensation of NBOs forming molecular O 2 and a new bridging oxygen (BO) . These reactions could lead to restructuring of the silicate network even though thermal poling is carried out far below the glass transition temperature (T g ).…”

“…17,19 One such process could be the condensation of NBOs forming molecular O 2 and a new bridging oxygen (BO). 16,20 These reactions could lead to restructuring of the silicate network even though thermal poling is carried out far below the glass transition temperature (T g ). It was also reported that NO 2 and N 2 O 4 species can be formed in a borosilicate glass during thermal poling in air.…”

Effect of heat treatment on the surface chemical structure of glass: Oxygen speciation from in situ XPS analysis

“…This may occur via NBO*–NBO* reaction to evolve O 2 (Scheme A in Figure ) or conversion of NBO* to SiOH (from residual water content in UHV environment) followed by SiOH‐SiOH condensation resulting in BO and evolution of H 2 O (Scheme B in Figure ) . The current method cannot confirm the occurrence of either schemes but other researchers have posited the possibility of restructuring of the glass network due to Scheme A upon exposure to elevated temperatures in the presence of electric fields or inert atmospheres on multi‐component silicate glasses …”