High School: Lahainaluna High School
Institute when accepted: Univ. of Pennsylvania
Melting Borosilicate Glass for Mirror Substrates
Project Site: HNu Photonics LLC
Mentor: David Kim, Jarrod Latoya & Richard Puga
Borosilicate glasses, because of their superior thermal properties, are used in numerous applications. One of the most common is their utilization as the substrate for mirrors in many telescopes. To assess the possibility of molding large chucks of borosilicate for mirrors in-house at HNu Photonics, the reconstruction of an old kiln was commissioned. Borosilicate glass has a working point of approx. 2300 degrees F, but the temperature specifications of this particular kiln were unknown. The broken firebrick interior was repaired using refractory mortar, and a new programmable temperature controller and thermocouple were installed on the kiln. Since detailed information on borosilicate heat treatments is not widely available due to its proprietary nature, a rough treatment based on available information was devised and performed. First, borosilicate frit was created in-house using scrap borosilicate and a variety of methods. This frit was placed into a simple graphite mold, and the temperature was slowly ramped up to the glass’s working point, then held constant until all air bubbles had escaped. Next, the glass was very slowly cooled to its annealing temperature, 1050 degrees F, and soaked at that temperature for a few hours to allow any stresses developed from cooling to dissipate. Finally, the glass was cooled very slowly to its strain point at 800 degrees F, after which the cooling rate was increased until the kiln returned to room temperature. Based the on the quality of the glass in the first run, different cooling rates and more complex molds will be attempted.