Microwave assisted sol-gel synthesis of bioactive zirconia nanoparticles - Correlation of strength and structure

It is well known that long term stability in zirconia has been a problem because of the structural alteration from stabilized tetragonal zirconia to monoclinic that leads to fracture in implants. Microwave (MW) assisted sol-gel synthesis is employed in the present work to prepare stabilize zirconia nanoparticles. ZrOCl2 center dot 8H(2)O is used as a precursor whereas de-ionized water is used as a solvent. Power of microwave radiations is varied in the range of 100-1000W. Zirconia nanoparticles have been characterized under as-synthesized, 6- and 12-months' room temperature (RT) aged conditions. Metastable phase (MP) of zirconia, appearing under as-synthesized condi-tions, transforms to phase pure tetragonal zirconia (t-ZrO2) after RT aging that was prepared with MW powers of 100, 200 and 700-1000W. Whereas, MP transforms to mixed tetragonal-monoclinic phases at microwave powers of 300-600W after RT aging. XPS results show presence of oxygen-deficient state of ZrO2 lattice along with surface defects contributing towards the tetragonal zirconia phase under all conditions. Value of dielectric constant (i.e. -11-12 at log f = 4.0), hardness (similar to 13 GPa) and fracture toughness observed under all conditions are well in agreement to be used for biological implants. Disks of aged t-ZrO2 nanoparticles are checked for their biodegradation test by dipping in simulated body fluid for several weeks. ZrO2, with 26 weeks of immersion, shows small loss in hardness and weight. Stabilized tetragonal zirconia shows strong anti-oxidant activity. Stabilized ZrO2 nanoparticles presented strong antibacterial activity against both gram positive (S. aureus, Bacillus) and gram negative (E. coli) bacteria. Thus, structural and mechanical stability of zirconia (checked after 6 and 12 months) make this material highly beneficial for long term use in biomedical applications.