Effect of heat treatment conditions and particles size on the devitrification of high purity fused silica

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Zachi Shmueli, MSc candidate

Zachi Shmueli, MSc candidate

Department of Materials Science and Engineering, Technion – Israel Institute of Technology

High purity fused silica is amorphous and serves as a structural raw material in many applications due to its outstanding qualities. Its excellent thermal shock resistance, low dielectric constant, loss tangent constant, and high mechanical stability at elevated temperatures, have made it a popular raw material for manufacturing of missiles’ radomes (radar-domes). The radome, which is attached to the front-end of a missile, is subjected to thermal and mechanical loads during flight. One of silica’s common devitrification products is cristobalite, a polymorph of silica. The presence of the cristobalite phase in an amorphous silica matrix might encourage the formation of micro-cracks, which are destructive during a missile flight.

In the present study, we investigated how surface, atmosphere and particle size affect the crystallization of high purity silica. Longer heat treatments, higher temperatures and free oxygen atmospheres all induce the crystallization of amorphous silica. Samples comprised of different particles size were heated at different temperatures and durations, at a static air atmosphere. In addition, heating samples at 1250oC within a reducing atmosphere of forming gas in a constant flow was performed. The latter was compared to the same heat treatments in oxidizing (medical air) and inert (N2) atmospheres.

Formed cristobalite phase’s morphology was investigated utilizing electron microscopy. Particles’ surface-to-volume ratio was determined by BET measurements. The content of cristobalite formed in amorphous silica was determined by means of XRD measurements. We also investigated the a to b cristobalite phase transition using XRD and DSC measurements.

We observed a significant decrease in crystallization of cristobalite for particles exhibiting lower surface-to-volume ratios as compared to those exhibiting higher ratios. In addition, it was found that when applying a reducing atmosphere smaller amounts of cristobalite were formed as compared to the other atmospheres.

Applying the knowledge acquired in this research will help fabricating sintered fused silica products with minimum cristobalite content and with better operational performance.


Supervisor: Prof. Boaz Pokroy