Peshcherova N.A., Yerysh L.A.

Donetsk National University of Economics and Trade named after Mykhaylo Tugan-Baranovsky, Ukraine

Peculiarities of formation of the surface structure of oxide nanoparticles ZrO₂

One of the most popular nanomaterials is currently a highly dispersed zirconia. Variety of properties determines its application to many areas of industry and medicine. The possession of such properties as high corrosion resistance and radioactive resistance, ionic conductivity and good biocompatibility of nanoparticles causes the use of ZrO₂ in the fields of industry, such as in catalysis, production of technical ceramics, artificial dentures, solid oxide fuel cells, etc [1]. The important task is to get the nanopowder with predetermined properties. This can be achieved by controlling the synthesis process of zirconium dioxide and intentional influence on the structure and condition of the product surface.

The formation of the surface structure of nano-sized zirconia is a complex process that occurs throughout the synthesis. It depends on the method and mode of synthesis as well as the nature of the precursors used. The most convenient in terms of management of structural characteristics are wet methods for obtaining such co-precipitation or sol-gel method [2]. So when zirconia nanopowder is obtained by co-precipitation or sol-gel method, the surface structure is formed at all stages of a powder on the structure of solutions or sol precursors to crystallization and polymorphic transformations during annealing of ZrO₂ at high temperatures.

The gel is an important stage in the sol-gel-oxide process. Studying the gel structure gives us a range of information on the structure and properties of the sol particles, and the processes that take place during gelation. A knowledge of the structure of the gel also gives us a starting point for understanding the thermal decomposition and crystallisation reactions.

Two main kinds of structural information are studied in the work [3]. Firstly, the short-range structure of the particles has been investigated. Since the structure of the particles is not affected by the physical gelation process, it is much more convenient to study their structure in the ‘solid state’ rather than in the sol. Secondly, the arrangement of the particles in the gel has been studied.

A detailed study of the structure of the gel requires a range of special techniques. Elemental analysis is useful for confirming the composition of the gel. X-ray absorption spectroscopy, in particular the extended X-ray absorption fine structure technique, plays a critical role in determining the short-range structure of the gel. Raman spectroscopy also provides information on the shortrange structure, and is particularly useful for comparing the gel with the sol and examining the coordination of the nitrate groups. X-ray diffraction and small-angle scattering provide further information on the structure of the gel, covering a length scale from several Ångstroms to tens of nanometres.

Once a zirconia-precursor has been prepared from a sol or solution, the conventional method to convert it to a pure oxide is by heating, during which crystallisation also usually occurs. The decomposition reactions are important to understand for two reasons. Firstly, they provide information on the original structure of the gel; secondly, decomposition reactions may be able to be controlled through processing conditions to obtain certain structures, control crystallisation, promote the loss of volitile components, etc. Process decomposition the semi-ordered structure in the gel and losing mass with heating, and also the crystallisation of the amorphous oxide into the ‘metastable’ tetragonal phase at approximately 450°C are considered in the work [3].

The crystallisation of the tetragonal phase is of particular interest. Over the past 30 years there have been many theories proposed to explain the crystallisation and metastability of the tetragonal phase, and its eventual transformation to the monoclinic phase. There has been a wide range of suggested mechanisms for the nucleation and stabilisation of the tetragonal phase at low temperature, such as lattice defects, non-stoichiometry, structural similarity with the amorphous precursor, retention of various anions or water, strain, surface energy, etc. Although the low-temperature tetragonal zirconia phase is generally referred to as ‘metastable’, these mechanisms may rely on both thermodynamic and kinetic factors, and a clear distinction between them is not usually made. There may well be a number of mechanisms, each of which may dominate under different conditions.

Very few of the theories attempt to account for all of the observations; rather they suggest mechanisms for either the formation of the tetragonal phase at low temperature, its stabilisation, or various aspects of the tetragonal-to-monoclinic transformation. The ‘surface area’ theory comes close to a full explanation, but this is because it takes into account many difficult-to-measure properties, such as surface area, microstrain, etc, and is difficult to assess quantitatively.

Until recent in-situ studies were carried out, researchers have been able to observe only the phase structure after cooling, and have been unable to separate crystallisation behaviour from transformation. To complicate matters, characteristics of the t± m transformation have been found to be heavily dependent on processing conditions.

The particular interest is the microstructural changes that occur in the crystalline zirconia during heating to temperatures between 450 and 1100°C. The microstructure of the oxide is important, as it determines the processing properties such as the ability to sinter. It is also of interest to investigate the relative stability of the tetragonal and monoclinic phases in the nanocrystalline zirconia. The structural features of most interest in this study are phase composition, crystal size and morphology, and porosity.

Thus, obtaining nanoparticles of zirconia with a given surface structure is important to consider all the peculiarities of the structure and condition of the surface. Moreover, special attention should be paid to the initial stages of gel formation, since they largely determine the mechanism of formation of the surface structure.

References:

1. Hosokawa M., Nogi K., Naito M., Yokoyama T. Nanoparticle Technology Handbook, Netherlands (2007).- 622 p.

2. Gogotsi Y. Nanomaterials Handbook, Costa-Rica (2006). – 780p.

3. Southon, P. Structural evolution during the preparation and heating of nanophase zirconia gels, University of Technology, Sydney (2000). – 300p.