Synthesis and characterization of nanostructured y-tzp-al2o3 granule composite microspheres

SYNTHESIS AND CHARACTERIZATION OF NANOSTRUCTURED Y-TZP-Al2O3 GRANULE COMPOSITE MICROSPHERES Muhterem KOÇ Materials Science and Engineering, Ph.D Thesis, 2017 Thesis Advisor: Prof. Dr. Osman ŞAN, Assistant Thesis Advisor: Assoc. Prof. Dr. Cem ÖZGÜR SUMMARY In this study, yttria stabilized zirconia (Y-TZP)/alumina (Al2O3) nano composite granule powders were synthesized by ultrasonic spray pyrolysis (USP) technique from a mixture of yttria-zirconia sol and alumina solution. The effect of different reactor temperatures (600°, 800° and 1000°C) and the amount of Al2O3 content (5-20 mol%) on the powder properties was determined and the sintering behaviors of the powders were investigated in powder form and under pressure in high frequency induction (YFI) system. Powder sintering was carried out in a furnace at different temperatures (1000°-1250°C) and produced different sintering times (1-24 hours). The sintering behavior of the powders with the YFI system was investigated at different temperatures (1400°-1600°C) and sintering times (60-600 sec) at two different compression pressures (10 and 20 MPa). The grain size, surface area, surface morphology and phase structures of the granular powders vary with the amount of Al2O3 added and the applied reactor temperature. The powders produced at low reactor temperature are amorphous, and tetragonal zirconia crystallization begins when the reactor temperature is 1000°C. The boehmite structures formed by the Al2O3 admixture increase the surface area from 2 m2/g to 137 m2/g and the crystallization temperature from 513°C to 817°C. Activation energy varies with alumina contribution. The activation energies of alumina-free, 10 mol% and 20 mol% Al2O3 added powders are 43.93 kJ/mol, 31 kJ/mol and 27.69 kJ/mol, respectively. The alumina additive reduced the activation energy by preventing grain growth and the crystal size of the powders was found to be 67 nm, 45 nm and 38 nm, respectively. Sintered powders at lower activation energies are low in crystal size; wherein the crystal dimensions were measured as 67 nm, 45 nm and 38 nm, respectively. In alumina-free powders, the average grain growth is 426 nm in primary granules, while it remains 225 nm with 10 mole% of alumina addition. In the process of sintering with YFI, the powders lose their granular structure and are packed as primary granules. In the sintering of the powders with YFI, the amount of Al2O3 in the grain growth prevention is effective, but when the sintering time is more than 300 seconds, this effect is decreased due to the transport of Al2O3, the Al2O3-rich regions are formed in the microstructure and the homogeneous distribution of the second phase is disturbed. The Y-TZP/Al2O3 composite material produced with the YFI system at a temperature of 1600°C and a sintering time of less than 300 seconds has a density of 97%, a crystal size of 68 nm and a grain size of 300 nm. Key words: Al2O3, granule, nanocomposite, grain growth, USP, Y-TZP, YFI.