Abstract
Stabilized zirconia in nanoparticles in c-ZrO2 polymorph has been synthesized with a small quantity of (5 mol%) additive of Cr3+ and Cr4+ cations by thermomechanical attrition (MA) process. The MA process allows the chromium cations to dissolve in the precursor in an amorphous state in order to obtain a stabilized phase in c-ZrO2. This is not so easily possible by conventional methods. The synthesis process consists of a controlled Cr3+/Cr4+:c-ZrO2 nucleation and growth from an energized amorphous precursor of the metal cations. It involves a reactive milling of dispersed Cr6+ cations by (NH4)2Cr2O7 in an amorphous precipitate of Zr4+ oxyhydroxide. During milling, the Cr6+ reacts with Zr4+ in producing a high-energy solid solution in an amorphous structure. The occurrence in the change from oxidation state is promoted by a plastic deformation and mixing between the two types of cations during the milling. The resulting precursor powder undergoes a self-controlled reconstructive thermal decomposition at 500°C and yields Cr3+/Cr4+ stabilized c-ZrO2 nanoparticles. Average crystallite size is estimated from x-ray diffraction peak-widths in sample stabilized by 5 mol% Cr3+/Cr4+ in 20 hr of milling. The D value decreases from 18 to 15 nm by extending the milling time to 50 hr. The stabilized nanoparticles are found to exist in the c-phase up to a temperature as high as 1000°C. The results are analyzed and discussed in terms of x-ray diffraction, thermal analysis, scanning electron microscopy microstructure, and x-ray photoelectron and optical spectra.
ACKNOWLEDGMENTS
This work has been financially supported by the Council of Scientific and Industrial Research (CSIR), Government of India.