טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentShlayer Amit
SubjectDensification of Nanocrystalline Ceramic Powders by Spark
Plasma Sintering (SPS)
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Rachman Chaim
Full Thesis textFull thesis text - English Version


Abstract

Densification of nanocrystalline (nc) yttrium oxide (yttria - Y2O3) by spark plasma sintering (SPS) was investigated at different SPS conditions. Yttria specimens were fabricated by SPS at different heating rates (50-180 ºC/min), temperature (1100-1600°C), pressure (50, 100 MPa), pressure application regime and duration (5 min). The yttria nano-powder was characterized for phase content and particle size. The nano-powder particle size was 18 ± 8 nm with cubic crystal structure. The density of the specimens after SPS were measured, and their structure and microstructure were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the increase in heating rate increased both density and the final grain size. The heating rate of 180°C/min was used for further investigations. SPS was performed at the temperature range 1100-1600°C for 5 min duration using either 50 or 100 MPa pressure. Investigation of the pressure application regime showed that the application of pressure in the beginning of the SPS process (before reaching the SPS temperature) compared to pressure applied after reaching the SPS temperature, and enhanced the shrinkage rate profile although no density improvement was found. The highest density received was 98.5 % at 1400°C at 100 MPa. The two pressure values displayed different densification behaviors. At 100 MPa the grain size and density increased up to 1400°C above which the grain size continued to increase while the corresponding density decreased. However, at 50 MPa pressure, increase in temperature caused an increase in both grain size and density. This difference was explained in terms of competition between the two processes of densification and grain growth at high SPS temperatures. Kinetic analysis of the grain growth during the SPS was performed and resulted in activation energy of 150 KJ/mol. Comparison of this activation energy to the data from the literature showed that the diffusion mechanism associated with SPS is mixed grain boundary and surface diffusion.