טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentKeren Ziv
SubjectDensification of Nanocrystalline Yttrium Aluminum
Garnet (YAG) Powders
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Rachman Chaim


Abstract

Yttrium Aluminum Garnet (Y3Al5O12-YAG) is a promising structural and optical ceramic whose technological application range from fibers to solid state lasers. Currently, YAG is grown by the crystal growth methods. Although these methods provide single crystal of application quality, they have many disadvantages which led to development of polycrystalline YAG via powder technology, using hot-pressing, vacuum and reaction sintering.

The present research was focused on investigating the pressureless densification behavior of Nanocrystalline (nc-YAG; 22 m2/gr) and submicron size (m-YAG; 2.3 m2/gr) YAG powders at different temperatures (1500-1800°C) and atmospheres such as low vacuum (1300 Pa, 10 Pa) (mainly carbon monoxide), flowing nitrogen and helium (>101 kPa), high vacuum (<3·10-3 Pa, no carbon residues) and via in-situ shrinkage studies in dilatometer (10 Pa).

First, preparation of the green compacts by dispersion, cold isostatically pressing and silica doping through mechanical milling and chemical sol-gel coating via hydrolysis and condensation of tetraethyl orthosilicate (TEOS) precursor were studied. Second, sintering capabilities of the YAG powders and the influence of its sintering environments as well as silica additives on densification were investigated using full microstructural characterization.

The current research demonstrates that a vacuum environment is preferable for densification of  m-YAG powder which can be enhanced by silica additions. However, the high reactivity of nc-YAG powder hampers attaining high green densities via pressure filtration or CIP compaction, along with abnormal grain growth when silica is added.