Glass - Back to the Future!

Presenting Author:
Kicheol Kim

article posted 6 April 2016

Kicheol Kim has completed his MS in February 2016 at Kunsan National University Korea and is working as a research assistant in Lab. for Glass Chemistry & Technology in the same university. He is interested in the characterization of glass melt in conjunction with electrochemical approach to redox behaviour of multivalent elements and recycling of display waste glasses. He has published four papers related with recycling of LCD waste glass and redox interaction by spectroscopy in SCI Journal. Now he is looking for PhD position in Europe or America.

Voltammetry and Oxygen Activity of Glass Melts in Fused Silica Crucible

Kicheol Kim*, Kiseok Cheon & Kidong Kim
Dept. Mat. Sci. & Eng. Kunsan National University, Kunsan Korea

Voltammetry, especially square wave voltammetry (SWV) using platinum crucible is a powerful method, which makes it possible to trace the redox reaction of multivalent element (M) in-situ in melt state electrochemically. In the last three decades a lot of studies on redox behaviour of various single multivalent elements in silicate or borosilicate melts was performed by SWV from the viewpoint of thermodynamics and kinetics for the reduction of
M(x+n)+ to Mx+ (This is hereafter designated M(x+n)+/Mx+). However, the recent some patents have reported that the platinum contacted with glass melts prepared from H3BO3-containing batches allows so called permeation of hydrogen derived from the dissociation of H2O which results in oxygen bubbles in the melts contacted with platinum.

Figure 1. Voltammograms at 50 Hz of glass melts measured in (a) platinum and (b) fused silica crucible

In the present work, voltammetry experiments for alkali free alumino-borosilicate melts doped with Sn were performed in a crucible of platinum and fused silica, respectively. Oxygen activity was determined in a crucible of sintered alumina and fused silica, respectively. Figure 1 shows voltammograms of the melts measured in two kinds of crucible. It is observed that the peak potential and peak current due to Sn4+/Sn2+ at constant temperature are different in both crucibles. In Figure 2 the corresponding peak potentials are described as a function of temperature. This shows a considerable difference in peak potential for both cases at 50 Hz. The peak is shifted to positive direction with change of crucible from platinum to fused silica. In Figure 3 the oxygen activity in sintered alumina and fused silica crucible is described as a function of temperature and Table 1 shows detailed oxygen activity at each crucible. Oxygen activity in fused silica crucible shows smaller value than alumina crucible. The results in voltammetry are in good agreement with those in oxygen activity. Comparing with the peak potential in both crucibles, the lower oxygen equilibrium pressure of melts in fused silica crucible causes a shift of Sn4+/Sn2+ peak to the positive or reduction direction. This difference in voltammogram or oxygen activity depending on crucible implies that something unexpected such as foregoing hydrogen permeation occurs at the surface of crucible.

Figure 2. Peak potential due to Sn4+/Sn2+ at 50 Hz in platinum (filled triangle) and fused silica crucible (filled square)

There is a great difference in microstructure between fused silica and platinum including sintered alumina. The fused silica consists of glass phase and thus has no grain boundary. On the other hand, both platinum and sintered alumina has grain boundary and even pore. This grain boundary could offer a path for gas permeation. Moreover the microstructure of platinum depends on its thermal history. The longer the platinum is exposed at high temperature, the larger the grain boundary gap is. According to some literatures, the permeability of H2 for platinum is quite larger than for fused silica and the partial pressure of H2 in glass melts is proportional to the concentration of water content.

Figure 3. Oxygen activity of glass melts in sintered alumina (filled circle) and fused silica crucible (filled square)

In order to clarify the influence of platinum crucible on voltammetry in relation to hydrogen permeation, some additional experiments must be performed for glass batches without hydrous compounds.

Table 1. Oxygen activity (logPO2 in bar)