SRNL’s George Wicks (left), Leung (Kit) Heung (center) and Ray Schumacher (right) with the flame-former apparatus developed for the formation of hollow glass microspheres.

Hollow glass microspheres have been used for years in lightweight filler material, insulation, abrasives and other uses. Researchers at the U.S. Department of Energy’s (DOE) Savannah River National Laboratory (SRNL) have developed a unique variation, called porous-wall hollow glass microspheres (PWHGMs), which offers the potential for use in targeted drug delivery, hydrogen storage and other applications. What makes SRNL’s patent-pending microspheres unique, and gives them the potential to be useful in so many fields, is a network of interconnected pores in the thin outer shells that allows the tiny “microballoons” to be filled with, hold, and release gases and other materials.

SRNL originally developed the microspheres as a solid-state storage method for the radioactive form of hydrogen; they have been successfully demonstrated to store and release the gas. Subsequent work has shown potential in other uses, including battery and medical applications.

SRNL is involved in about a half-dozen different programs involving PWHGMs, many in collaboration with academic institutions and industrial partners. As part of a program with Toyota, for example, SRNL investigated filling these microspheres with special hydrogen absorbents to develop safe hydrogen gas storage systems. A licensing agreement between SRNL and Mo-Sci Corp., a specialty glass provider located in Rolla, Mo., will provide SRNL with a cost-effective supply of the microspheres to continue the research and development of additional applications.

Glass R&D

Enlarge this picture Porosity of SRNL PWHGM walls.

SRNL, the applied research and development national laboratory located at the DOE’s Savannah River Site (SRS) near Aiken, S.C., has long been recognized for its expertise in the science and engineering of vitreous or glass-based systems, especially in the waste management field. For example, the laboratory developed the flowsheets and methods used in the SRS’ Defense Waste Processing Facility, which incorporates and immobilizes high-level radioactive wastes into an inert and stable glass form.

As a part of this glass experience and expertise, SRNL has also developed a number of niches in the glass arena, one of which is the development of porous glass systems for a variety of applications. These porous glass systems include sol-gel systems and phase-separated glass compositions that can be subsequently treated to produce unique types of porosity within the glass forms. The porous glass systems can be fabricated into a variety of forms, including coatings, plates, fibers, beads and—among the most interesting—hollow microspheres.

Each PWHGM is about 50 microns in diameter, about half the width of a human hair. A PWHGM’s walls, which are only 10,000 angstroms thick (an angstrom is one-tenth of one-billionth of a meter), feature pores that range from 100 to 1000 angstroms. This porosity results in interesting properties, including the ability to use these channels to fill the microballoons with special absorbents and other materials, thus providing a contained environment for even reactive substances. Gases can enter the microspheres and be retained on the absorbents. Also, the porosity can be altered and controlled in various ways, and even used to filter mixed gas streams within this system.

Microsphere Fabrication

SRNL glass microsphere filled with palladium, a material used for storing hydrogen; the top of the microsphere has been removed to show the inside.

The PWHGMs are fabricated using a flame-former apparatus developed by SRNL that heats glass powders in a hot zone formed by a controlled gas-air flame. As the glass particles pass through the zone, the flame softens the glass and forms a spherical particle. The glass contains a latent blowing agent that becomes unstable as the glass is heated and forms a gas nucleus or bubble. The bubble expands as the glass is heated and forms the hollow spheres.

These original hollow glass microspheres are converted into PWHGMs through heat-treating to induce phase separation. This process produces two different glass phases, one rich in silica and the other rich in sodium borate. The sodium borate phase is an interconnected, worm-like morphology. When it is removed by an acid leaching process, similar to the making of commercial Vycor^® glass,* it leaves behind interconnected pores or channels that extend from the outside of the shell to the inside.

These pores are used to fill the microspheres. Because the glass spheres provide a protective environment, or cocoon, for their contents, they can be used to hold reactive or flammable absorbents or stored materials, including solids, liquids, or gases. This has the potential to provide a safe method of handling, storing or transporting a variety of difficult materials.

*Vycor is a product of Corning Inc.

Future Possibilities

During the collaboration with Toyota, it was discovered that very effective, but reactive, absorbents could be filled and subsequently protected within the microballoons or cocoons. Bundles of nano-filaments were produced inside as well. When chemically analyzed, these structures do not appear to be any of the predicted known phases. This means that, in addition to new nanostructures produced by the porosity of the microsphere walls, new phases can possibly result. Further work is needed to clarify this interesting finding.

SRNL puts science to work to support the DOE and the nation in the areas of environmental management, national and homeland security, and energy security. The management and operating contractor for SRS and SRNL is Savannah River Nuclear Solutions, LLC. For additional information regarding glass microspheres, contact Eric Frickey of SRNL at Bldg. 773-41A, Savannah River Site, Aiken, SC 29808; (803) 725-0406; e-mail eric.frickey@srnl.doe.gov; or visit srnl.doe.gov.