Characteristics of the Weathered Surfaces of Phosphate Glasses

1. Characteristics of the Weathered Surfaces of Phosphate Glasses Brad Tischendorf* & Richard Brow University of Missouri-Rolla USA XX International Congress on Glass Oct. 1, 2004 Kyoto, Japan

2. Phosphate glasses are prone to weathering degradation Example of a badly weathered glass surface Scattered light due to weathering XX ICG : Kyoto, Japan

3. Surface corrosion is due to precipitation of hydrated orthophosphate crystals XX ICG : Kyoto, Japan

4. Weathering depends on the interactions between water and the glass surface Diffusion of water and hydration of chains and modifiers Humid Aqueous Glass Dissolution with chain and cation removal into solution Weathering reactions with surface hydrolysis and crystallization XX ICG : Kyoto, Japan

5. Glass compositions • These glass compositions have been chosen to act as models for commercial phosphate glass amplifiers such as those used at the National Ignition Facility XX ICG : Kyoto, Japan

6. Techniques used to measure the different reaction mechanisms • Diffusion and Hydration • Weight gain of samples during weathering • Elastic Recoil Detection (ERD) for H depth profiles • Surface Hydrolysis • MAS NMR • High Pressure Liquid Chromatography (HPLC) • Surface Crystallization • Thin Film X-ray Diffraction (TF-XRD) XX ICG : Kyoto, Japan

7. This slide won’t be included, it’s to add commentary for you • The first step was to measure how much water is getting into the glass surfaces. • Initially we simply measured the gain in weight as a function of weathering time. • This was then followed by measurement on H depth profiles with ERD • The two results seem to qualitatively agree with the systems we have looked at. XX ICG : Kyoto, Japan

8. Following the weight gain at 50ºC/80%RH shows MgO containing glasses reacting faster XX ICG : Kyoto, Japan

9. The same effect is observed at 40ºC/80% RH XX ICG : Kyoto, Japan

10. At 25ºC/80%RH there is no measurable weight gain until after 14 days XX ICG : Kyoto, Japan

11. ERD shows equal amounts of water present on the KMAP and KBAP surfaces after 7 days XX ICG : Kyoto, Japan

12. This data agrees with the weight gain data: after 7 days, KMAP and KBAP are near the same XX ICG : Kyoto, Japan

13. NMR indicates compositional differences in the rates of reactivity of the glasses 7 days 50oC 80%RH XX ICG : Kyoto, Japan

14. This slide won’t be included, it’s to add commentary for you • Now that we know how much water was getting in we wanted to measure the structural changes occurring due to this water. • WE began with NMR which shows a rate of reactivity where KAP>KMAP>KBAP. • Downside is that NMR probes entire sample and must be a powdered sample,not necessarily representative of a real system. • So we looked into HPLC where we could take a polished piece of glass and dissolve the surface away to measure its structure. XX ICG : Kyoto, Japan

15. HPLC provides a method to probe the structure of phosphate glass surfaces • A solution containing the phosphate glass is loaded into the injection valve. • Once injected, the gradient pump ramps the solution from 0.05 to 0.5M NaCl. • After separation, the phosphate species are reacted at 170ºC in 1.8M H2SO4 with ammonium molybdate • The resultant “blue” species concentration is measured with the UV-VIS XX ICG : Kyoto, Japan

16. HPLC experimental procedure • Glass plates were polished to 1mm finish • Plates were measured and weighed then placed into a controlled humidity temperature environment • Samples periodically removed and weighed for weight gain, then surface removed in a solution of EDTA • Weight loss was measured • Solution was run through HPLC system • Prior to and after runs, all solutions were refrigerated for storage • Nomenclature: • P1 = PO4 • P3m = P3O9 ring species XX ICG : Kyoto, Japan

17. Standard solutions of known phosphate species are used to calibrate species and concentrations • P1 – 26% • P2 – 26% • P3 – 25% • P3m – 23% XX ICG : Kyoto, Japan

18. Runs of stored samples show they are stable for at least 3 weeks when refrigerated • This plot shows LHG-8 weathered for 4 days at 25ºC and 80%RH immediately after surface removal, and after sitting for three weeks XX ICG : Kyoto, Japan

19. HPLC shows that KAP reacts rapidly at 50ºC and 80%RH • P1 – 62% • P2 – 32% • P3 – 4% XX ICG : Kyoto, Japan

20. Increased production of PO4 and P2O7 is seen in the MgO containing glass at 50ºC/80%RH XX ICG : Kyoto, Japan

21. As is seen in the weight gain data, this trend continues at lower temperatures of 40 and 25ºC XX ICG : Kyoto, Japan

22. The ratio’s of P1+P2 show the same dependence with MgO glasses more reactive XX ICG : Kyoto, Japan

23. Weathered surface removal from LG-770 trends with either absolute or relative humidity Not sure, may dump this section XX ICG : Kyoto, Japan

24. Activation Energies for water diffusion and chain hydration/hydrolysis • Rough calculations from both ERD profiles and weight gain data show that KMAP exhibits a diffusion activation energy of ~1 kJ/mole • Problem is that these were two point fits • Calculations of the activation energy of chain hydration/hydrolysis gives numbers near 100 kJ/mole whether weight loss is measured directly by surface removal or by the results of HPLC concentration conversions XX ICG : Kyoto, Japan

25. Ongoing work • Adding additional humidity's to the study • Compare the rates of reactivity of poly and pyro-phosphate glasses to the meta-phosphate species already studied • What is the effect of surface preparation on the weathering rate? XX ICG : Kyoto, Japan

26. The type of sample studied may affect the results of the chromatography experiment XX ICG : Kyoto, Japan

27. Conclusions • The rates of penetration of water into glass surfaces has been measured • The effect of this water on the structure of the glass surface was studied • The dependence of these reactions on glass composition was studied. XX ICG : Kyoto, Japan

28. Acknowledgements • This work is funded by Lawrence Livermore National Laboratory. Work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. Special Thanks to: • Joe Hayden of Schott Glass Technologies for Glass Cullet used to produce weathering samples • Barney Doyle of Sandia National Laboratories for running the ERD spectra used in this study • Todd Alam and Brian Cherry also of Sandia National Laboratories for their aid in performing the NMR experiments • Fred Stover of Asteris for HPLC advice and providing phosphate standards XX ICG : Kyoto, Japan