Ion-exchange-induced formation of glassy electrooptical and nonlinear optical nanomaterial

https://doi.org/10.1016/j.jnoncrysol.2006.11.043Get rights and content

Abstract

Subsurface electrooptical glass-ceramic layers are formed in an alkaline-silica-niobate glass using ion exchange technique. Ways of selection of glass compositions and ion exchange conditions to form such layers are discussed. It is shown that there are two ways to form glass-ceramics; the first one is the glass ceramization during the ion exchange treatment at higher temperatures, and the second one is the glass ceramization in heat treatment of the samples ion exchanged at lower temperatures, at which glass crystallization does not take place. The latter allows producing glass-ceramics layers of controllable thickness, because in this case diffusion process strongly obeys Fick equation.

Introduction

Recently [1], [2], [3], [4], [5], [6], it has been demonstrated that a high-temperature (above glass transition temperature) alkaline–alkaline ion exchange can be used to control formation of two-phase glassy materials. In these studies, the ion exchange in different glass-forming systems brought forth either formation of a new two-phase glassy material (due to crystallization [4], [5] or liquid–liquid phase separation [2]) or homogenization (due to decrystallization [1], [6] or liquid–liquid phase deseparation [3]) of the glassy material, which initially had been a two-phase one. A simple phenomenological description of the processes involved [6] allowed assuming that ion-exchange-induced phase transformations in glassy materials could be used as a powerful technique to form functional composite nanomaterials in a controllable manner. This description predicted that depending on the process temperature it was possible to control the sizes and space distribution of the precipitating crystal grains. This brief report is to demonstrate the possibilities of the proposed technique in the formation of electrooptical glass-ceramics.

In this study, using the proposed technique, we tried to obtain films of electrooptical glass-ceramics like that, which had been earlier developed by the authors and demonstrated a high value of the electrooptical Kerr coefficient (∼6 × 10−13 m/V2 for wavelength 0.63 μm [7]) and enhanced nonlinear optical characteristics (non-linear absorption and refractivity [8], [9], [10], [11]). It should be noted that these glass-ceramics had been successfully used for producing optical waveguides by an ion exchange technique [12], the phase shift of propagating lightwave by π/2 being expected to be provided by the driving voltage of 20 V applied to a channel optical waveguide 3 μm wide and 10 mm long. The compositions of the basic glasses used in glass-ceramics formation had been formalized as 19Na2O–11K2O–2B2O3–2CdO–(66  x)SiO2xNb2O5, where x varied up to 37, and the enhanced electrooptical and nonlinear optical properties of the glass-ceramics produced on the base of these glasses had been due to sodium niobate nanocrystals precipitating in heat treatment. In this report these glasses will be called NaS glasses.

Section snippets

Experimental

To produce such glass-ceramics by the proposed technique, lithium-containing analogues of NaS glasses were used as the initial ones, these glasses being called LiS glasses. Our intention was to replace lithium ions of LiS glass with sodium ions in ion exchange processing, and this was supposed to lead to crystallization of ion exchanged layers with the formation of electrooptical glass-ceramics films containing sodium niobate crystallites. The compositions of LiS glasses can be presented as 19Li

Results

Right after ion exchange processing of LiS25 glass samples for any duration at 600 °C, no changes were revealed, that is, no any crystalline phase in the diffused subsurface layer was visually found. These ion exchanged samples were additionally annealed in air at 625 °C for 2 h. This resulted in the formation of non-transparent subsurface layers, with their thicknesses increasing with a rise in the duration of ion exchange processing. These samples were powdered for XRD analysis. The results of

Discussion

Looking at Fig. 3, one can see that the character of the dependences of the glass-ceramics layer thickness on time at 600 °C and 625 °C differs. In the case of 600 °C this dependence is almost linear function of t1/2 (t is the total processing time, including annealing), and in the case of 625 °C it is not a linear one. This difference is supposed to come from the fact that the diffusion process taking place at 600 °C obeyed the Fickian diffusion laws due to the initial glass at this temperature

Conclusion

One additional glass-forming system suitable for ion exchange formation of subsurface glass-ceramics layers has been found. Glass-ceramics formed is electrooptical one. Ion exchange conditions to form such layers in controllable manner have been found as well, and it has been shown that if ion exchange is performed at low temperatures, at which during the processing glass crystallization does not take place, the thickness of the produced glass-ceramics layers is time-controllable, because in

Acknowledgements

This work was supported by ISTC (Grant #2696) and INTAS (Grant #0818) foundations.

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