Optimization of nanostructured oxide-based powders by surface modification

Abstract

This thesis is divided into two parts. The first part deals with the study of surface modification of a transition alumina nanopowder and the second part is devoted to the production and mechanical characterization at room- and high-temperature of Alumina 5 vol. % YAG nanocomposites. In order to study, the effect of the dispersion on a transition alumina several physico-chemical characterization techniques were employed such as the DTA-TG analysis, XRD, BET Specific Surface Area, HR-TEM and the FT-IR spectroscopy. In particular, the transition aluminas present metastable phases which suffer transformations during sintering and induce the formation of a vermicular microstructure, consisting of a network of large pores. As a consequence, the final density and the microstructure have been improved thanks to the dispersion which allowed to achieved higher densities and promotes the transformation into alpha-phase. Moreover , the influence of the dispersion on the transformation kinetics (Kissinger Method), as well as, on the sintering kinetics (SID Method) has been evaluated. In the second part, it is presented the development of Alumina/YAG nanocomposites from two commercial nanopowders naturally sintered and sintered by non-conventional methods, such as HP and SPS. The mechanical characterization at room temperature (Hardness, Toughness, Elastic modulus) has been correlated to a microstructural study (ESEM). Interesting values regarding hardness and toughness have been measured in samples sintering by SPS and HP, around 20GPA and 7 MPA.m1/2, respectively. For the characterization at the high temperature, creep tests were carried in a 4-point bending fixture at 1200C and an applied stress of 100 Mpa. The results show that mechenical properties depend on the second phase distribution into the alumina matrix. In all cases, the obtains results were interestingVILLEURBANNE-DOC'INSA LYON (692662301) / SudocSudocFranceItalyFRI