The synthesis of nanocrystalline zirconia based composite and the prevention of agglomeration problem

Zirconia, used in the area of refractory, oxygen sensor, fuel cell and catalyst, must be produced at nanoscale with high purity to exhibit better performance than conventional products. Zirconia based materials are usually prepared by chemical precipitation, hydrothermal synthesis, gas condensation and sol-gel etc. Nanoparticle with narrow size distribution can be produced by controlling synthesis conditions. Powders synthesized under controlled conditions bring about not only high reactivity but also improved mechanical properties after successful sintering process. The one of the main concerns during the synthesis of powders is to prevent them from agglomeration. Nanoparticles having high surface area tend to agglomerate very easily, which is pointed out the one of the biggest technological problems.This study was addressed to agglomeration problem as well as development of intrinsic properties of nanocrystalline tetragonal zirconia and its composites (ZrO2-Al2O3, ZrO2-SiO2, ZrO2-WC). This was successfully achieved by using two powder preparation techniques including (i) tartaric acid gel method -simplified sol-gel process- (ii) synthesis by supercritical fluids as a solvent or anti-solvent.The synthesized ZrO2-Al2O3 nanocomposites by tartaric acid gel method at pH 1 results in high agglomeration degree (N=16433), whereas the nanopowders were prepared at pH 6 revealed lower agglomeration degree (N=102). Furthermore, this N value was decreased down to 1.12 with 104 m2/g by using supercritical CO2 method. The agglomeration degree was found 302.5 at adding Na-PMAA for ZrO2-SiO2 nanocomposites, while it was diminished to 3 by using NH4-PAA (Darvan 821A Ammonium polyacrylate solution). The degree of agglomeration was decreased to 5 for ZrO2-SiO2 synthesized without any additives. Although the hardness of ZrO2-Al2O3 nanocomposites prepared by tartaric acid gel method was varied from 7.6 to 7.9 GPa, same materials by supercritical fluids were 9.7-12.5 GPa. The hardness values of zirconia-silica nanocomposites of gel method were changed from 5.9 to 6.8 GPa as independent from applied load. Their hardness values were increased up to 7.4 GPa. The fracture toughness of ZrO2-Al2O3 nanocomposites was 3.4 MPam1/2 measured by indentation test. Both fracture toughness (2.1-3.8 MPam1/2) and hardness (6.8-7.4 GPa) of ZrO2-SiO2 were showed the highest values at the literature that searched.