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. ZrOCl₂.8H₂O 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 conditions, 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 (~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.