Chapter 13. Microwave-Driven Hydrothermal Synthesis of Oxide Nanopowders for Applications in Optoelectronics

The microwave driven hydrothermal synthesis permits to precisely control the reaction times for this and thus also the grain size of the resulting nanocrystalline powders. An increase of pressure leads to powders with less hydroxide groups comparing to low temperature/pressure synthesis routes. Thus the combination of the two techniques permits to best exploit their advantages: high temperature of the process and fast heating and cooling, in high purity conditions. On the example of ZrO2 doped with Pr it is seen that the luminescence centers in nanocrystalline powders may have a different structure than for bulk materials. Pr3+ ions which would not be stable in a bulk material, are stable on the surface of the powder particles, and influence both their growth rate and luminescence properties. Interaction of the excited states with surfaces leads to very short luminescence decay rates, in the range of 10 nm, which in addition can be controlled by varying the grain size. This opens perspectives for new scintillating materials with short and controlled relaxation times. The nano-powders can be sintered using high pressure techniques and the grain size an be preserved in the nanometer range. The sintered YAG:Nd ceramics displays similar luminescence spectra as single crystals.