High-Temperature Diffusion of Potassium in Silica

Abstract

This work focuses on determining the diffusion coefficient of potassium in silica at high temperatures. Understanding this diffusion helps predict the final profile of optical fiber. This work also investigates several variables that may affect the potassium diffusion rate: viscous flow, oxygen diffusion, and water diffusion. The glass used in this experiment was HSQ300, a silica glass from Heraeus. The core of the glass cane was doped with ~1.2mol% potassium oxide. The cane underwent thermal cycling, then gradient index measurements were taken to trace the movement of potassium. Four thermal cycles were made on a glass-working lathe with a traversing set of hydrogen-oxygen burners that heated the cane for 31.5 minutes per cycle. The cane's surface was heated to 2256°C, and the equivalent temperature for diffusion in the core was calculated to be 1865°C. After four thermal cycles, the gradient index data showed a diffusion distance of 1.70mm. This resulted in a diffusion coefficient for potassium in silica D = (1.23 ± 0.54) x 10^-6 cm^2 /second. Further analysis indicated that potassium diffusion rate may have been influenced by water which diffused into the glass from the hydrogen-oxygen burner.