Impact of Composition and Heat Treatment on Pore Size in Borosilicate Glass Microspheres

1. Impact of Composition and Heat Treatment on Pore Size in Borosilicate Glass Microspheres Fabienne C. Raszewski, Erich K. Hansen, Ray F. Schumacher, David K. Peeler, Scott W. Gaylord*, Nathan A. Carlie*, Laeticia Petit* and Kathleen A. Richardson* David A. Crowley, Manager Laboratory Directed Research and Development Process Science and Engineering Section February 26, 2008 Materials Innovations in an Emerging Hydrogen Economy, Cocoa Beach, FL, February 24 – 28, 2008

2. Background – Hollow Glass Microspheres (HGMs) • Glass bubbles with a solid shell • Typical compositions • Soda lime silica (container glass) • Borosilicate (Pyrex® - like) • Traditional applications • Low density fillers for foams, composites and concrete • Paints and coatings • Modern applications • Hydrogen storage • Gas Separation (10 – 200+μm) Matthew M. Hall, Alfred University

3. Background – Phase Separation • Phase separation in bulk glass • Oil in water concept • Formation of two completely separate phases • Example • Borosilicate glasses • Structures • Droplets • Interconnected • Phases • Silica rich • B2O3/alkali rich Depends on temperature and composition B.R. Wheaton and A.G. Clare, J. Non-Cryst. Solids (2007).

4. Background – Porous Bulk Glasses • Formed from phase separated glasses • Interconnected structure • Dissolve non – durable B2O3/alkali rich phase • Durable SiO2 rich phase remains • Sponge – like structure • “Passageways” throughout material • Example • VycorTM (aka “Thirsty Glass”) • Excellent absorbing properties • Filtration 3D Representation R. Pellenq, B. Rousseau and P. E. Levitz. Phys. Chem. Chem. Phys. 3. 1207-1212 (2001).

5. Fabricate HGMs Treat in acid to dissolve B2O3/alkali rich phase Heat Treatment Porous Walled HGMs VycorTM type composition Interconnected microstructure PWHGMs

6. Potential Applications of PWHGMs • Hydrogen storage • Molecular sieves • Drug/bioactive delivery systems • Indicators • Environmental • Biological • Chemical

7. Research Objectives • Porosity could govern potential use and/or performance of material under certain environmental conditions • Determine if porosity changes with • Composition • Heat treatment conditions • Temperature • Time • Develop a “compositional road map” • Tailor microstructure for specific applications

8. Surface of PWHGM Many pores Glass shell Small passageways connecting exterior to interior of HGM Change these features by either heat treatment temperature or time or composition Concept: Heat Treatment and Composition Effects PWHGM Glass shell

9. Research Objectives • Porosity governs potential use and/or performance of material under certain environmental conditions • Determine if porosity changes with • Composition • Heat treatment conditions • Temperature • Time • Develop a “compositional road map” • Tailor microstructure for specific applications

10. Defining the Immiscibility Dome Regions 1 and 3 3 1 B 2 Region 2 • Unknowns • Location of baseline composition • Size and shape of immiscibility dome • Challenge to define compositional changes to base composition

11. Defining the Immiscibility Dome Regions 1 and 3 3 1 B 2 Region 2 • Unknowns • Location of baseline composition • Size and shape of immiscibility dome • Challenge to define compositional changes to base composition

12. Glass Selection • Baseline composition from previous work • Based on the Na2O – B2O3 – SiO2 system • Known to produce both HGMs and PWHGMs for specific application • Change two critical parameters to compositionally map this region • SiO2 concentration (wt%) • Constant B/R molar ratio • B2O3/alkali (B/R) molar ratio • Constant SiO2 B/R +0.5 -3 SiO2 +3 SiO2 B -6 SiO2 +6 SiO2 B/R -0.5

13. Process Melt glass Microscopy – Clemson University Porosity Measurements - Micromeritics Microscopy Make Frit Size Frit Microscopy Microscopy HGMs Flame Process PWHGMs Heat Treat* Acid Leach Porosity * For comparison some HGMs were acid leached without any heat treatment

14. Path to a PWHGM Heat Treatment Acid Treatment HGM 580°C PWHGM 600°C

15. Impact of Heat Treatment Baseline composition 8 hours 600°C Non heat treated 150 nm 600 nm • Pore size is extremely small in sample with no heat treatment • At 200,000X pores are barely detectable (Pore diameter: ~100 Ǻ) • Heat treatment enhances the formation of the interconnected microstructure • Pores are clearly visible at only 50,000X (Pore diameter: ~1000 Ǻ)

16. Impact of Heat Treatment Baseline composition – Mercury Porosimetry Data • Considerable increase in pore volume with heat treatment • Pore diameter increases from ~100 Ǻ to ~1000 Ǻ

17. Impact of Heat Treatment Temperature Baseline composition – Same Magnification 8 hours 600°C 8 hours at 580°C • Microstructure is strongly influenced by temperature • Only a 20°C difference in temperature • Mercury porosimetry results are inconclusive for 8 hours at 580°C • Sample treated at 600°C for 8 hours has a pore diameter of ~1000 Ǻ 600 nm 600 nm

18. Impact of Heat Treatment Time Baseline composition 24 hours 580°C 8 hours at 580°C Apparent “cracking” is due to sample preparation • Variation in microstructure is minimal for heat treatment times of 8 – 24 hours • Heat treatment time is not as effective as heat treatment temperature

19. Impact of Heat Treatment Time Baseline composition – Mercury Porosimetry Data • Very little (if any) increase in pore volume • No noticeable shift in pore diameter

20. Heat treatment for 8 hours at 600°C Impact of Composition B/R +0.5 Images taken at same magnification -3 SiO2 +3 SiO2 Base +6 SiO2 -6 SiO2 B/R -0.5 Similar microstructures….

21. Impact of Composition • All compositions yield interconnected morphology • Possible influence of composition on microstructure • Varying degrees of porosity • Mercury porosimetry data is inconclusive

22. Heat treatment for 8 hours at 600°C Impact of Composition B/R +0.5 Images taken at same magnification -3 SiO2 +3 SiO2 Base +6 SiO2 -6 SiO2 B/R -0.5 Similar microstructures….

23. Impact of Composition • All compositions yield interconnected morphology • Possible influence of composition on microstructure • Varying degrees of porosity • Mercury porosimetry data was inconclusive

24. Conclusions • Task Objectives • Determine the impact of heat treatment time and temperature and composition on porosity • TEMPERATURE – PRIMARY EFFECT 580°C 8 hrs. 600°C 8 hrs. No HT ~1000 Ǻ ~100 Ǻ Increase in the degree of phase separation/porosity with increasing heat treatment temperature

25. Conclusions • COMPOSITION – SECONDARY EFFECT* • Micrographs indicate variations in the degree of porosity • *Assuming no confounding effects of HGM diameter/wall thickness • HEAT TREATMENT TIME – NO EFFECT (8 – 24 hours) 580°C 8 hrs. 580°C 24 hrs. No change with heat treatment time

26. Acknowledgements • LDRD for funding this work • George Wicks and Leung K. Heung for their technical insight • Frances Williams, Irene Reamer, Phyllis Workman and Debbie Marsh for laboratory and technical assistance • Clemson team for all microscopy work (Kathleen Richardson, Laeticia Petit, Scott Gaylord and Nathan Carlie) • Micromeritics for conducting the mercury porosimetry analyses on all samples • Don Blankenship for assistance with the analysis of the mercury porosimetry data