IMPACT OF COMPOSITION AND HEAT TREATMENT ON PORE SIZE IN POROUS WALLED HOLLOW GLASS MICROSPHERES
The Savannah River National Laboratory (SRNL) developed a new geometric form: hollow glass microspheres (HGMs), with unique porous walls. The new geometric form combines the existing technology of HGMs with basic glass science knowledge in the realm of glass-in-glass phase separation. Conceptually, the development of a HGM with porous walls (referred to as a PWHGM) provides a unique system in which various media or filling agents can be incorporated into the PWHGM (via transport through the porous walls) and ultimately has the capacity to serve as a functional delivery system in various industrial applications. Applications of these types of systems could range from hydrogen storage, molecular sieves, drug and bioactive delivery systems, to environmental, chemical and biological indicators, relevant to Energy, Environmental Processing and Homeland Security fields. As a specific example, previous studies at SRNL have introduced materials capable of hydrogen storage (as well as other materials) into the interior of the PWHGMs. The goal of this project was to determine if the microstructure (i.e., pore size and pore size distribution) of a PWHGM could be altered or tailored by varying composition and/or heat treatment (time and/or temperature) conditions. The ability to tailor the microstructure through composition or heat treatments could provide the opportunity to design the PWHGM system to accommodate different additives or fill agents. To meet this objective, HGMs of various alkali borosilicate compositions were fabricated using a flame forming apparatus installed at the Aiken County Technical Laboratory (ACTL). HGMs were treated under various heat treatment conditions to induce and/or enhance glass in glass phase separation. Heat treatment temperatures ranged from 580 C to 620 C, while heat treatment times were either 8 or 24 hours. Of the two primary variables assessed in this study, heat treatment temperature was determined to be most effective in changing the porosity of PWHGMs. Pore diameter in a non-heat treated baseline sample is approximately 100 {angstrom} and with heat treatment at 600 C for 8 hours, the diameter is approximately 1000 {angstrom}; an increase of a factor of 10. The results of this study also indicate significant microstructural differences with only a 20 C difference in heat treatment temperature (580 C and 600 C) for constant times. The microstructural changes observed via electron microscopy as a function of heat treatment temperature were confirmed by mercury porosimetry measurements, where considerable increases in pore volume were measured. Under constant heat treatment conditions, composition may impose a secondary effect on the resulting microstructure as micrographs indicate variations in the degree of porosity. Although microstructural differences were observed among the compositions assessed, the magnitude of the impact (i.e., difference in pore size or pore volume) appears to be smaller than that associated with heat treatment temperature. With respect to heat treatment time, the results suggest that the change in the degree of porosity is minimal for samples heat treated between 8 and 24 hours (it should be noted that the assessment of the impact of time on the resulting microstructure was limited to two compositions). The minimal impact of heat treatment time (on the two glasses evaluated) was confirmed by mercury porosimetry measurements indicating that there was a very slight shift in pore diameter and very little increase in pore volume in the baseline sample. Another important parameter, which will need to be considered under manufacturing or operational conditions, is the yield of the HGM and/or PWHGM and the characteristics of the final product (i.e., not only microstructure characteristics, but perhaps strength of the PWHGM for use under certain applications). In this report, yield is defined as the percentage of feed material converted to HGMs or the percentage of HGMs converted to PWHGMs. The yield of HGM formation was found to be a strong function of composition. As the SiO{sub 2} and B{sub 2}O{sub 3} contents are increased or decreased from the baseline composition, the yield is reduced. PWHGM yield doesn't appear to be influenced by composition, but there is a noticeable increase in yield when comparing the non-heat treated samples to those that were heat treated prior to acid leaching. The results of this study suggest that PWHGM microstructures (pore size and pore volume) can be tailored or specifically designed to meet different end-user needs within certain limitations. The primary effect on the resulting microstructure is heat treatment temperature, which can produce a significant shift in pore size or volume even with a very small difference in heat treatment temperature (20 C). The ability to control the microstructure of PWHGMs provides the opportunity to design the PWHGM system to accommodate different additives or fill agents as required.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- DE-AC09-96SR18500
- OSTI ID:
- 921683
- Report Number(s):
- WSRC-STI-2007-00605; TRN: US200806%%127
- Country of Publication:
- United States
- Language:
- English
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