In-depth study of the evolving thermal runaway and thermal gradient in the dog bone sample during flash sintering using finite element analysis

Abstract

Flash Sintering (FS) techniques are widely being researched due to its potential in reducing the energy cost in ceramic processing. However, the mechanism of flash sintering is still unclear. Additionally, the majority of the flash sintering study uses dog bone geometry. But till now, different aspects of dog bone sample geometry have not been studied. This work is geared towards understanding the role of Joule heating and thermal runaway in the densification of different ceramics during FS and the intricacies of the dog bone sample geometry. Four distinct ceramic oxides namely 3 mol% Yttria stabilized zirconia (3YSZ), 8 mol% Yttria Stabilized Zirconia (8YSZ), Zinc Oxide (ZnO) and Titania (TiO2) are chosen in this study due to their contrasting electrical and thermal properties. In this study finite element analysis is performed to compute the temperature, heating rate and thermal gradient in the sample. The result shows that thermal runaway occurs in the sample with four orders of magnitude higher heating rate compared to conventional sintering. However, the thermal runaway is limited as the heating rate decreases with time very rapidly, and thus the temperature gets stabilized very fast. Our results also show that a significant thermal gradient develops in the sample during FS because of the dog bone geometry. This can lead to crack propagation in the sample which also has been observed experimentally. The extent of the thermal gradient also varies significantly depending on the thermal and electrical conductivity of the material involved. In this work, the dog bone sample geometry is optimized with respect to reference dog bone sample geometry to reduce the thermal gradient. Understanding and minimizing the thermal gradient are of critical importance for commercialization of FS technique.