טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentTwersky Anton Raoul
SubjectSintering and Microstructural Evolution in the
Nanocrystalline Alumina-Glass Ceramics
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Rachman Chaim


Abstract

This research deals with densification and microstructural evolution during sintering of nanocrystalline meta-stable transition alumina powders coated with a thin layer of borate glass containing alkali cations (Ba, Sr and Cs).

Alumina powders were coated via a Sol-Gel method using aqueous solution of appropriate nitrate or carbonate salt with a mixture of TEB (Tri-Ethyl Borate) and ethanol. Ammonium hydroxide was used for the gelation process; the mixture was aged, dried and calcined at 700ºC.

Cold isostatically pressed samples were sintered in the temperature range 1000ºC-1650ºC for 5 hr in air, in dilatometer (constant heating rate) up to 1700ºC, and isothermal treatments at 1350ºC and 1550ºC for various durations.

The coatings exhibited uniform thickness and homogeneous distribution; their thickness was between 5 to 8 nm. The cation additives increased the transformation temperature to a-alumina according to: barium borate to 1160°C, strontium borate to 1155°C, cesium borate to 1020°C, pure borate to 1080°C, compared to 1075°C for pure alumina. The activation energies of 395±1.7 KJ/mol for pure alumina, and 229±1.3 KJ/mol for alumina coated with barium borate were determined. Isothermal dilatometry resulted in shrinkage rate constants of n @ 0.3 for both samples at low temperatures (1050°C-1150°C). The rate constant and activation energies were characteristic for grain boundary controlled diffusion in pure alumina, and for liquid-phase sintering, controlled by cation diffusion in barium borate coated alumina. Grain growth rate constants were determined as 2.5 and 2 for pure alumina at the temperatures 1350°C and 1550°C, and 7 for barium borate coated alumina at 1550°C.

The original nanostructure of the alumina coated with borate glass was not persevered during the phase transformation and sintering. Throughout these processes, a glassy phase remained at the grain boundaries and contributed to the shrinkage and grain growth by liquid phase sintering.