Glass microspheres are spheres of glass technically manufactured with a diameter in the micrometer range (from 1 to 1000 (microns))although the term is also used for a wider range of 100 nanometres to 5 millimetres. Hollow glass microspheres, sometimes termed microballoons, have diameters ranging from 10 to 300 micrometers (microns)[http://www.emerson.com.

Selected uses of solid spheres are: calibrating measuring systems, sieve and filter calibration, reflective markings, abrasives, binder spacings. Hollow spheres have uses ranging from storage and slow release of pharmaceuticals and radioactive tracers to research in controlled storage and release of hydrogen.

Hollow spheres are also used as a lightweight filler in composite materials such as syntactic foam. Microballoons give syntactic foam its light weight, low thermal conductivity, and a resistance to compressive stress that far exceeds that of other foams These properties are exploited in the hulls of submersibles and deep-sea oil drilling equipment, where other types of foam would implode. Hollow spheres of other materials create syntatic foams with different properties, for example ceramic balloons can make a light syntatic aluminium foam.[http://www.memagazine.org/backissues/january99/features/foams/foams.html (Foams on the Cutting Edge)

Glass microspheres can be made by heating tiny droplets of dissolved water glass in a process known as ultrasonic spray pyrolysis, and properties can be improved somewhat by using an acid treatment to remove some of the sodium ^*.

References

1. Syntactic foam properties & applications
2. Microballoon synthesis & thermal properties.
3. Syntactic foam buoyancy applications