| Summary: | 碩士 === 國立臺灣大學 === 生物環境系統工程學研究所 === 107 === Apart from artistic ceramics in early stage, advanced ceramics has been widely used in different applications according to their functions along with development of ceramics industry. Thus, comparing to conventional sintering method of ceramics, lots of rapid sintering methods had been proposed like microwave sintering and spark plasma sintering. However, some development of these rapid sintering methods were limited by categories of materials or complicated sintering structure. Until flash sintering was first reported in 2012, it had potential for commercial manufacturing ceramics among these rapid sintering with lowering sintering temperature and sintering time by only assisting external electric field.
There are three stages during flash sintering. Stage I is for constant voltage period with only tiny current through specimen, and also called as incubation period to induce flash event. Then depending on different materials, after seconds to several minutes, current would abruptly surge to the pre-setting upper limit with flash event and these called stage II. In this short period, specimen was almost densified instantaneously. Then, output of electric field would turn to constant current mode and maintain the stable dissipation, and finally finish the flash sintering.
Due to lower sintering temperature and shorter sintering time, specimen would be densified with smaller grain size, comparing to conventional sintering in higher sintering temperature resulted in higher diffusion rate and grain growth rate. Most dental material use zirconia. However, low temperature degradation is the main issue for zirconia in the human mouth of humid environment. In this work, we manufacture YSZ for dental application by flash sintering to prevent larger grain size occurred and increase mechanical strength simultaneously. In addition, the specimen under flash sintering method would be conducted with aging test to prevent from low temperature degradation without phase transformation and this specimen would be expected using for longer life.
There were lots of literatures worked on mechanism of flash sintering from it was first reported and expect to commercialize flash sintering method in the future. So far, there were three main explanation in flash sintering. One is Joule heating from external electric field output and higher diffusion rate was resulted from higher temperature of specimen. But Joule heating was only attributed during stage II and III. Another explanation is higher concentration of extrinsic defect was nucleated by electric field and then it could induce transient state of flash event. The other explanation is according to experience results of asymmetric grain size distribution between two electrodes. This phenomenon was considered to electrochemical reduction related and resulted in higher diffusion rate of reduced cations and pores produced by accumulation of oxygen vacancies. In this work, we conducted flash sintering in vacuum environment and showed external oxygen is not necessary during flash sintering. In addition, we find some small grains were precipitated and blackening phenomenon on the sample surface of partial reduction zirconia. Therefore, we consider electrochemical redox reaction was involved in flash sintering. Stage one is for incubation of partial reduction zirconia. Until partial reduction zirconia penetrated the core of specimen, its high conductivity could offer a path for steady current during stage II and III in flash sintering. Through high conductivity of specimen, larger output energy of electric field could densified specimen rapidly with higher diffusion rate.
|
|---|
