Stresses Produced in the BK7 Glass by the K+–Na+ Ion Exchange: Real-Time Process Control Method

Abstract

The paper presents the results of tests on stresses produced by the K+↔Na+ ion exchange method in BK7 glass. Diffusion ion exchange processes were carried out in glass plates with a surface area of a few cm2. The duration of these processes ranged from several hours to several hundred hours; process temperatures from 370 to 402 degree Celsius were used. The area of the glass in which the ion exchange took place shows refractive changes which are also accompanied by stresses. The planar waveguides produced in this way were tested by optical methods (for wavelength = 677 nm) and the refractive index profiles for the Transverse Electric (TE)and Transverse Magnetic (TM) olarization states were determined. On the basis of elasto-optic constants, the resulting stresses were determined. The temperature characteristics of diffusion coefficients of exchanged ions were also determined. Based on them a numerical simulation of real-time diffusion processes was possible, which allowed to predict the stresses arising in the glass. A good agreement between these predictions and the results of measurements was obtained.

Towards a Glass New World: The Role of Ion-Exchange in Modern Technology

“…Conversely, for a high process temperature (i.e., close to the glass T g or above), the position of this maximum is located several microns below the glass surface and the magnitude of the stress rate decreases for the occurrence of surface stress relaxation due to viscous flow of the glass [13]. On the other hand, for a same process temperature, the maximum of the surface compressive stress decreases when the ion-exchange time increases: the depth at which the residual stress equals zero (i.e., the depth of layer, DoL) becomes deeper and, consequently, the flaws that can be involved in the process are larger [110,117,118]. On the other hand, the maximum for the tensile stress is not so easy to identify but, in some cases, it is located immediately below the compressive zone and its value is an order of magnitude lower than that of the compressive stress (~few tens of MPa), depending on the process duration and sample thickness [13].…”

“…On the other hand, A. K. Varshneya, G. Macrelli, and others in their studies modified the Cooper's analysis introducing a new term in the model, related to different relaxation contributions (i.e., the viscoelastic and structural one) together with the network hydrostatic yield strength in order to evaluate the subsurface maximum compressive stress when the process temperature is higher and/or close to the glass transition temperature T g [121][122][123][124]. Also noteworthy is the method, recently proposed by R. Rogozi ński, which allows to control in real time the stresses generated in a glass as a result of the ion-exchange process from the knowledge of some parameters such as the elasto-optical coefficients, the dependence of the diffusion coefficients on the temperature and, finally, the function describing the time relaxation of stresses at the glass surface [118]. Differently from the numerical and analytical approaches mentioned above, N. Terakado and co-workers have developed a very original method for evaluating the compressive stress in a chemically strengthened glass, connecting it directly to the glass structure on an atomic scale.…”

Coupling Light in Ion-Exchanged Waveguides by Silver Nanoparticle-Based Nanogratings: Manipulating the Refractive Index of Waveguides

“…Over the last few decades, many studies in doping metal nanoparticles into a glass substrate investigated the thermal ion-exchange (IE) process. − In this process, the glasses are covered by molten salt, including metal ions, which replace alkali ions in the glass matrix during a thermal process. − Penetration of metal ions such as silver ions (Ag + s) into a glass induces positive stress due to the different ionic radius and polarizability from those of sodium ions (Na + s), accompanied by modification of its refractive index. − Changing the molten salt composition makes it possible to manipulate the refractive index of the glass induced by the IE process …”