Synthesis Temperature Effect on the stabilization of Tetragonal Zirconia Nanopowders

Abstract

The present work is an appraisal of the effectiveness of sodium borohydride in the stabilization tetragonal zirconia nanopowders, synthesized at different temperatures.
Zirconia ceramics have found broad applications in a variety of energy and biomedical applications because of their unusual combination of strength, fracture toughness, ionic conductivity, and low thermal conductivity. These attractive characteristics are largely associated with the stabilization of the tetragonal and cubic phases through alloying with aliovalent ions. The high fracture toughness exhibited by many of zirconia ceramics is attributed to the constraint of the tetragonal-to-monoclinic phase transformation and its release during crack propagation. In other zirconia ceramics containing the tetragonal phase, the high fracture toughness is associated with ferroelastic domain switching.
Our objectives in the present work include the preparation of zirconia nanopowders using sodium borohydride, the study of the effect of synthesis temperature on the properties of zirconia nanopowders, and the stabilization of tetragonal zirconia at room temperature.
In the first part of the work, the effect of temperature and solvent in the size reduction and hence stabilization of the tetragonal phase at room temperature was studied. It was revealed that with the increase in the synthesis temperature, the fraction of tetragonal phase in the calcined zirconia samples increased. Calcined powders at 700ºC were shown to exhibit as low as 11 nm particle size as revealed by TEM and BET specific surface area measurement. The fraction of tetragonal phase was inevitably linked to the particle or crystallite size: smaller crystallites were essential for the stabilization of t-ZrO2 at room temperature.
The results of the first part of our work motivated us to carry on the stabilization of t-ZrO2 with minimal amount of yttria as additive. The second part of the work dealt with the stabilization of zirconia in the presence of yttria. This stabilization is not possible because of the formation of yttrium orthoborate, as revealed by the X-Ray diffraction patterns, which separates out as a different phase, and prevents the stabilization of tetragonal zirconia.