The chemical preparation of ceramic materials has been widely studied over the past few decades, and provides the potential for excellent control over the microstructure and properties of the final product. This control is dependent on a comprehensive understanding of the microstructure and physical/chemical processes that occur at each stage. Aqueous routes have much potential for adoption by industry, but in many cases a comprehensive understanding of the microstructure and chemistry is lacking, partly due to the complicated aqueous chemistry of many transition-metals.
This investigation has focussed on a specific inorganic, aqueous, sol-gel route for the preparation of pure zirconia (Zr02). Zirconia is a ceramic with a wide range of current and potential applications, such as catalysis, fuel-cells, coatings and biomaterials. The emphasis has been placed on the characterisation of the structure at each stage of the route, leading to an understanding of the various mechanisms that are at work. This project has also provided an opportunity to investigate broader issues concerning the solution-based processing of zirconia, particularly those involving the 'metastable' tetragonal phase. This phase is frequently observed to be formed by non-equilibrium methods, but the mechanisms of formation and de-stabilisation are not properly understood.
The studied route consists of a number of stages: the preparation of an aqueous sol of 'zirconium hydroxide' particles by forced hydrolysis of a zirconyl nitrate solution; the conversion of the sol to a gel by removal of the aqueous phase; the conversion of the gel to a crystalline tetragonal zirconia powder by heating; and transformation of the tetragonal phase to the stable monoclinic phase with further treatment. At each stage of processing a number of aspects of the material structure have been investigated, including the short-range order, crystalline lattice parameters, particle packing, porosity, and speciation of the nitrate anion. This has required a wide range of complementary characterisation techniques, including Raman spectroscopy, XRD, TEM, DTA/TGA, SAXS, dynamic light scattering, EXAFS, NMR, and nitrogen sorption. The importance of techniques that allow changes in structure to be characterised in-situ during heating has been emphasised.
The particles in the sol and gel are plate-shaped, approximately 0.5 nm thick and 3 - 4 nm across. They are composed of up to several stacked `sheets' of zirconium hydroxide, each of which is composed of zirconium atoms arranged in a regular square lattice, joined by double hydroxy-bridges. Detailed evidence for this structure has not been previously reported.
The stages of decomposition of the precursor have been elucidated, including the stages at which oxolation and loss of nitrate occur. The complex crystallisation process at 450°C has been investigated, and a structural mechanism for crystallisation of the 'metastable' tetragonal phase proposed, based on similarities between the tetragonal crystal structure and the disordered sheet structure in the amorphous material just prior to crystallisation. The crystalline material consists of nano-sized crystals, containing unusual intracrystalline mesopores.
The lattice parameters of the tetragonal phase change with increasing heat treatment, with the unit-cell tetragonality (c/a) increasing from 1.017 to 1.020. This is a previously-unreported phenomenon which may be associated with the stability of the phase. The tetragonal phase transforms to the monoclinic phase after heating to a 'critical temperature' between 900 and 950°C; this temperature is associated with the loss of residual surface nitrate species and/or a substantial increase in the mass diffusion rate. The crystal size and surface area has little influence on the tetragonal-to-monoclinic transformation, a result which is contrary to much previously-published work and that has significant implications for certain theories explaining the stability of the tetragonal phase. The transformation itself occurs during cooling, over a range between 400 and 100°C, and has been studied in-situ by time-resolved Raman spectroscopy.
The conclusions of this investigation contribute not only to the understanding of this particular route for processing zirconia, but also to a broader understanding of aqueous zirconium systems, the chemical processing of zirconia, and the tetragonal-to-monoclinic zirconia transformation mechanisms.
University of Technology, Sydney. Department of Chemistry, Materials & Forensic Science
OPUS (Open Publications of UTS Scholars) is the UTS institutional repository. It showcases the research of UTS staff and postgraduate students to a global audience. For you, as a researcher, OPUS increases the visibility and accessibility of your research by making it openly available regardless of where you choose to publish.
Items in OPUS are enhanced with high quality metadata and seeded to search engines such as Google Scholar as well as being linked to your UTS research profile, increasing discoverability and opportunities for citation of your work and collaboration. In addition, works in OPUS are preserved for long-term access and discovery.
The UTS Open Access Policy requires UTS research outputs to be openly available via OPUS. Depositing your work in OPUS also assists you in complying with ARC, NHMRC and other funder Open Access policies. Providing Open Access to your research outputs through OPUS not only ensures you comply with these important policies, but increases opportunities for other researchers to cite and build upon your work.
OPUS archives UTS research submitted for Higher Education Research Data Collection (HERDC) and Excellence in Research for Australia (ERA). It also stores digital theses and forms of scholarship that do not usually see formal publication.
How can you deposit works in OPUS?
When you claim (or enter) your research in Symplectic Elements, simply upload a copy of your work which can be made openly available. Symplectic provides information on which version of your work to upload. If you are unsure, please supply a copy of the Accepted Manuscript version. Ensure you check the box to "agree to the OPUS license terms".
Once uploaded, your works are automatically sent to OPUS and placed temporarily in Closed Access until reviewed by UTS Library staff.
Library staff check to ensure compliance with copyright and publisher agreements
If the version you supplied cannot be hosted in OPUS you may receive an email requesting a copy of the Accepted Manuscript
Once items are cleared of copyright constraints and/or publisher embargoes, your work is moved to Open Access and made accessible to the public.
Instructions are available from the Symplectic User Guide or contact firstname.lastname@example.org for further information.