Residual internal stress in partially crystallized photothermorefractive glass: Evaluation by nuclear magnetic resonance spectroscopy and first principles calculations

Abstract

In some circumstances, the mechanical and optical properties of multiphase brittle materials strongly depend on the level of residual micromechanical stresses that arise upon cooling due to thermal and elastic mismatch between the constituent phases. Here we study the residual internal stress in a partially crystallized oxyfluoride glass, best known as photothermorefractive ͑PTR͒ glass. This material is composed of a glass matrix with embedded nanosize sodium fluoride ͑NaF͒ crystals. Using both the Selsing model and solid-state nuclear magnetic resonance in combination with first principles calculations we found that the crystals are under a tensile stress field of approximately 610-800 MPa. For this stress level the estimated critical crystal diameter for spontaneous cracking is about 2300-1900 nm, which greatly exceeds the observed diameters of 7-35 nm. Hence no spontaneous cracking is expected for the PTR glasses. First principles calculations indicate that the stress induced change of the refractive index of the NaF crystals is about −0.08%, which agrees with the observed refractive index changes.

Structure of fluoride-containing bioactive glasses

“…The origin of this narrow signal is not clear and could also arise from the surface hydration. Comparison of these deconvolution results with an NMR study on bioactive glasses by Lockyer et al 33 indicates that the broad site perhaps dominates in the low field spectra, giving a nearly -10 ppm value for MAS shift at 8.45 T. In the glass with the highest fluorine content (glass G) the signal becomes broader, but does show at least two lines one of which has the larger quadrupolar constant (site I), as shown in Structural role of phosphate 31 P MAS NMR spectra (Fig. 12) , which result in displacement of charge from the oxygen and hence from the P-O bond.…”

“…In glass ceramics, the 23 Na MAS NMR signal from NaF varies between 5 and 7 ppm depending on the crystal size. 31 Therefore, the appearance of a small shoulder at ca. 7 ppm can be explained by complexing of sodium with fluorine with increasing fluorine concentrations.…”

“…29 Si NMR experiments on crystallised samples were performed using 36.5 µs echo delay and 50 Hz exponential filter was applied to these data. 31 P MAS NMR spectra were acquired at 81.0 MHz in the 4 mm rotor spinning at 4.5 kHz. 64 transients of a single pulse experiment with 2.5 µs π/2 pulse and 49 s recycle delay were collected for each sample.…”

“…64 transients of a single pulse experiment with 2.5 µs π/2 pulse and 49 s recycle delay were collected for each sample. 31 P chemical shift was referenced to the signal of 85% H 3 PO 4 . 23 Na NMR was performed at 158.7 MHz in 2.5 mm and 4 mm rotors and spinning speeds of about 20-25 kHz and 15 kHz, respectively.…”

Directly photoinscribed refractive index change and Bragg gratings in Ohara WMS-15 glass ceramic

“…Unfortunately, we do not know the exact physical and chemical composition of WMS-15 (which is a trade secret), so it was not possible for us to attribute the effects of UV exposure to specific structures or constituents in the manner of studies such as those of Livingston et al [19] or the stress studies of Zwanziger et al [33]. However, analysis of the net change in absorption spectrum caused by 248 nm exposure followed by annealing at 200°C does reveal a broad feature, centered at about 360 nm, which is apparent only for exposures between 0.01 and 0:3 kJ=cm 2 .…”