Editing Glass microsphere (section)

{{Short description|Microscopic spheres}}
[[Image:Glass microsphere in concrete.jpg|thumb|right|[[Scanning electron microscope|SEM]] micrograph of a glass microsphere in [[concrete]]]]

'''Glass microspheres''' are [[microscopic]] spheres of [[glass]] manufactured for a wide variety of uses in [[research]], [[medicine]], [[consumer goods]] and various industries. Glass microspheres are usually between 1 and 1000 micrometers in diameter, although the sizes can range from 100 nanometers to 5 millimeters in diameter. Hollow glass microspheres, sometimes termed '''microballoons''' or '''glass bubbles''', have diameters ranging from 10 to 300 [[micrometre|micrometer]]s.

Hollow spheres are used as a lightweight [[aggregate (composite)|filler]] in [[composite material]]s such as [[syntactic foam]] and [[lightweight concrete]].<ref>[http://www.ces.clemson.edu/earnest/0699pages/16.htm "Whatever Floats Your Boat, Clemson Student Chapter of the American Society of Civil Engineers"] {{webarchive|url=https://web.archive.org/web/20090131035333/http://www.ces.clemson.edu/earnest/0699pages/16.htm |date=2009-01-31 }}. ces.clemson.edu</ref> Microballoons give syntactic foam its light weight, low [[thermal conductivity]], and a resistance to [[compressive stress]] that far exceeds that of other foams.<ref>Common Microballoons have a density of 0.15 to 0.20 g/cm<sup>3</sup>, with an isostatic crush strength of 300 to 500 psi. Denser, high strength forms offer 0.38 g/cm<sup>3</sup> with 5500 psi strength, and 0.6 g/cm<sup>3</sup> (still offering considerable flotation) with 18,000 psi crush pressure.</ref>  These properties are exploited in the hulls of [[submersible]]s and deep-sea oil drilling equipment, where other types of foam would [[Implosion (mechanical process)|implode]].  Hollow spheres of other materials create syntactic foams with different properties: ceramic balloons e.g. can make a light syntactic [[aluminium]] foam.<ref>Ray Erikson  (1 January 1999). [https://web.archive.org/web/20050604075250/http://www.memagazine.org/backissues/january99/features/foams/foams.html Foams on the Cutting Edge]. Mechanical Engineering-CIME</ref>

Hollow spheres also have uses ranging from storage and slow release of [[pharmaceutical]]s and [[radioactive tracer]]s to research in controlled [[hydrogen storage|storage]] and release of [[hydrogen]].<ref>J.E. Shelby, M.M. Hall, and F.C. Raszewski (2007). [http://www.osti.gov/bridge/product.biblio.jsp?query_id=0&page=0&osti_id=918691 A radically new method for hydrogen storage in hollow glass microspheres] {{webarchive|url=https://web.archive.org/web/20110604151559/http://www.osti.gov/bridge/product.biblio.jsp?query_id=0&page=0&osti_id=918691 |date=2011-06-04 }}.  DOE Technical Report FG26-04NT42170.</ref> Microspheres are also used in composites to fill polymer resins for specific characteristics such as weight, sandability and sealing surfaces. When making surfboards for example, shapers seal the [[Polystyrene#Expanded polystyrene (EPS)|EPS]] foam blanks with epoxy and microballoons to create an impermeable and easily sanded surface upon which fiberglass laminates are applied.

Glass microspheres can be made by heating tiny droplets of dissolved [[sodium silicate|water glass]] in a process known as [[Ultrasonic nozzle|ultrasonic spray pyrolysis]] (USP), and properties can be improved somewhat by using a chemical treatment to remove some of the [[sodium]].<ref>{{cite journal|doi=10.1557/JMR.1996.0368|bibcode=1996JMatR..11.2908I|title=Characterization of hydrated silicate glass microballoons|date=2011|last1=Isobe|first1=Hiroshi|last2=Tokunaga|first2=Ichiro|last3=Nagai|first3=Noriyoshi|last4=Kaneko|first4=Katsumi|journal=Journal of Materials Research|volume=11|issue=11|pages=2908|s2cid=136718493 }}</ref>  Sodium depletion has also allowed hollow glass microspheres to be used in chemically sensitive resin systems, such as long [[wikt:pot life|pot life]] epoxies or non-blown polyurethane composites.

Additional functionalities, such as silane coatings, are commonly added to the surface of hollow glass microspheres to increase the matrix/microspheres interfacial strength (the common failure point when stressed in a tensile manner).

Microspheres made of high quality optical glass, can be produced for research on the field of [[Optical microcavity|optical resonators or cavities]].<ref>[[Optical resonator]]</ref>

Glass microspheres are also produced as waste product in [[coal-fired power station]]s. In this case the product would be generally termed "[[cenosphere]]" and carry an aluminosilicate chemistry (as opposed to the sodium silica chemistry of engineered spheres). Small amounts of silica in the coal are melted and as they rise up the chimneystack, expand and form small hollow spheres. These spheres are collected together with the ash, which is pumped in a water mixture to the resident ash dam. Some of the particles do not become hollow and sink in the ash dams, while the hollow ones float on the surface of the dams. They become a nuisance, especially when they dry, as they become airborne and blow over into surrounding areas.