Nanostructured oxide coatings via emulsion precipitation
Prof. Dr. Ir. H. Verweij
Dr. W.F.C. Sager
In the last two decades nanoscale science- and engineering- based research has taken a big step forward and new challenges are found in developing methods for the production of nanosized materials. The demand for nanostructured ceramic materials has increased tremendously, since the large surface-to-volume ratio of nanoscale materials gives them unique physical and chemical properties not observed for bulk-sized materials. Furthermore, the grain size affects sintering kinetics, i.e., reducing the grain size of a material from microns to nanometers can enhance sintering rates by 9 orders of magnitude, which means that processing temperatures can often substantially be reduced. Although the kinetics of sintering are different, a nanostructured ceramic compact generally follows the same sintering paths as the conventional powder system.
The work described in this thesis is concerned with the preparation of well-defined dense nanostructured oxide coatings, consisting of grains <20 nm, at significantly lower temperatures than for the bulk materials. Given the proper processing techniques, such coatings could be applied as protective ceramic materials on a variety of materials, such as metals or even plastic. As an example, dense nanostructured zirconia coatings are of special interest for several high-performance applications, due to their chemical inertness, high stability and superior mechanical properties. To prepare coatings via deposition of nanoparticles, it is imperative to deal with agglomerate-free particles with an average size below 10 nm.
The modified emulsion precipitation route adjusted and further developed in this thesis, enables the preparation of non-agglomerated ZrO2, Fe2O3, BaTiO3 and Al2O3 precursor particles, by employing thermally stable water-in-oil emulsion droplets with sizes of 300-600 nm. The average precursor particle size is, regardless of the type of the material, approximately 5 nm.
The particle dispersions obtained from the modified emulsion precipitation route were deposited onto silicon wafers. After drying at room temperature, the coatings were treated at moderate temperatures to remove organic residues. The crystallisation and densification of the ZrO2 and Fe2O3 thin films were performed at 500-600°C. For BaTiO3 and Al2O3, a temperature of at least 800°C and 1000°C, respectively, was needed to obtain a crystalline dense layer.The zirconia coatings have a large number of technologically interesting applications and the possibility to prepare composites with other functional oxides, such as Fe2O3 en Al2O3, offers many perspectives.