Photo-Induced Outgassing of Hollow Glass Microspheres

Abstract

All as – received microsphere compositions exhibit large variations in microsphere diameter (10 – 280 μm) and wall thickness (0.90 – 2.78 μm) . The amber and undoped microspheres have true densities around 0.65 g/cm3, whereas the Fe, Ni and Co –doped microspheres have true densities between 0.29 – 0.4 g/cm3. An increase in the amount of gas released from microspheres is observed with a decrease in density for samples of the same mass. Activation energies for permeation of helium lie in the range of 17 – 30 kJ/g–atom, which is consistent with previous values for microspheres, but low compared to bulk glasses of similar composition (49 kJ/g–atom). Compared to thermal outgassing, photo outgassing of doped microspheres is immediate and requires less time to occur. The undoped microspheres do not demonstrate a photo – induced response and release gas only by heating from the infrared source. Photo – outgassing is characterized by switch – like behavior and is maximized at full lamp intensity. Wavelengths in the range of 1500 – 2200 nm are most responsible for the photo – response. Microspheres of all sizes have been successfully filled to pressures as high as 340 atm. The initial rate of photo and thermal outgassing increases significantly as the fill pressure is increased to 340 atm. All compositions exhibit minimal hydrogen losses when stored at room temperature for 5 weeks, with higher losses occurring at 50°C. Photo – ingassing has been demonstrated for doped microspheres and is more efficient than thermal ingassing up to ~300°C. Extended treatments in hydrogen up to 360 hours, results in formation of metallic particles in the nickel and cobalt doped microspheres. Considerably more nickel ions are reduced than cobalt ions during treatment, as metallic nickel can be detected by X – ray diffraction and the Curie temperature of nickel occurring at 352°C can be measured using thermogravimetric analysis under a magnetic field. Photo – outgassing is slightly increased after 335 hours of hydrogen treatment in the nickel and cobalt – doped samples, whereas there is no change in the iron – doped microspheres.