Instrumentation and automation of a tube furnace adapted to flash sintering: improvement on sintering process of zirconia

Author(s)

João Vitor Campos

• Document type: Doctoral Thesis
• Press: Pirassununga.
• Institution: Universidade de São Paulo (USP). Faculdade de Zootecnica e Engenharia de Alimentos (FZE/BT)
• Defense date: 2020-12-16

Examining board members

Eliria Maria de Jesus Agnolon Pallone; Carlos Alberto Fortulan; Douglas Gouvêa; João Gustavo Pereira da Silva; Marlu César Steil

• Advisor: Eliria Maria de Jesus Agnolon Pallone; Rafael Vieira de Sousa

Abstract

Flash Sintering (FS) is a sintering technique that stands out for promoting rapid densification and a substantial reduction in the temperature of the sintering environment of a wide range of ceramics, resulting in a reduction in the energy consumption of the process. This innovative technique consists of applying an electric field to the ceramic sample during heating. At a critical temperature, usually much lower than the sintering temperature, the electrical current that passes through the specimen suddenly increases, heating it fast due to the Joule effect. When this occurs, the sample densifies in a few seconds and begins to shine brightly - a phenomenon called Flash Event (FE). Despite all the advantages of FS, this technique still is not used at an industrial level. Some scaling-up challenges need to be solved to make practical use of this technique on a large scale. Also, there is still no consensus on the mechanisms involved in FS, which makes it even harder to solve these problems. With this in mind, the present work had as main goal to point out and propose solutions to the challenges of FS applied to 3mol% yttria-stabilized zirconia (3YSZ). We also sought to find the best combination of FS parameters to sinter 3YSZ, aiming to elucidate possible mechanisms of this technique. To do so, we performed several experiments. First, we built, instrumented, and automated an FS furnace. Then we performed FS in several conditions, looking for the best parameters\' combination to sinter 3YSZ. To understand the difficulties of scaling up FS, we studied the scale (sample size) influence on the microstructure and onset temperature of the FE. Then, we conduct an experiment measuring conductivity and temperature during FS via in-situ X-ray diffraction (XRD) analysis. In short, the results showed that the proposed equipment is capable of sintering 3YSZ via FS up to 99.9% of the theoretical density. With FS, we were able to sinter the samples in a few seconds at temperatures much lower than those of conventional sintering (reduction of up to 900°C). In some FS conditions, we observed a microstructural gradient. We proposed a descending FS technique, called multi-step flash sintering (MSFS). The results obtained with MSFS showed potential in reducing the microstructural gradient. From the study of the influence of the scale (sample size) on the onset temperature of the FE, we concluded that larger samples tend to have a lower onset temperature and to present microstructural heterogeneity. We calculated the activation energy of the conduction mechanism during FE using the conductivity and temperature of 3YSZ samples. The results suggest a possible increase in the conduction of cationic species (Zr4+ and Y3+), which may be responsible for increasing the sintering rate of the material during FE. (AU)

• FAPESP's process: 18/04331-5 - Instrumentation and automation of a furnace adapted to flash sintering
• Grantee: João Vitor Campos
• Support Opportunities: Scholarships in Brazil - Doctorate (Direct)