Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 327
resulted in lower at. % of cationic constituents of the glass at the polished surfaces. Semi-
quantitative analysis of cleaved surfaces showed good agreement with batched at. %
values, with only fluorine differing significantly due to melt volatilisation.
NaOH, H2SO4, HCl, and nitric acid were poor etchants for the TeO2-Na2O-ZnO and
TeO2-Na2O-ZnF2 glasses due to severe hydrolysis at the glass surface. HF, acetic and
phosphoric acid did not produce this poor visible surface layer on TeO2-Na2O-ZnF2
glasses, however infrared spectra of 4% HF glasses showed a significant increase in OH
bands, which may have an undesirable effect on fibre drawing of preforms treated this
way. A hydrolysed layer could potentially result in stresses, crystal nucleation sites, and
contrasting viscosity to the bulk glass in a preform. After these initial studies HF seems
the best etchant for fluorotellurite glasses, however any water containing solution will
introduce some degree of OH to the glass surface; therefore exposure to high
concentration acids for short times would be recommended.
Ag + /Na + ion-exchange of TeO2-Na2O-ZnO glasses by two methods (immersion in
molten salt and heat treated Ag-layer) produced measurable silver content (by EDX) in
the glasses, with the molten salt incorporating higher silver content, and the heat
treatment lower content, but with no chemical erosion of he treated surface. These Ag +
concentration profiles were modelled using erfc function, and diffusion parameters were
obtained.
7.5. References
[1] T. Findakly, "Glass waveguides by ion exchange: a review," Optical Engineering,
vol. 24, pp. 244-250, 1985.