Glass - Back to the Future!

Presenting Author:
Lenka Müller

article posted 20 April 2016

Lenka Müller received her PhD in Engineering of inorganic materials on ICT Prag in 2001. She joined the Biomaterials group at the Institute of Glass and Ceramics of the University of Erlangen-Nuremberg in 2002 and studied biomimetic calcium phosphate growth on inorganic materials as well as designing of biomorphous and porous ceramics. Since 2015 she has been a member of Laboratory of Glass Science at the University Jena and working on sol-gel processing of zirconia. She has a track record of more than 60 publications in international peer-reviewed scientific journals.

Sol-gel- derived ZrO2 coatings - preparation and characterization

Lenka Müller1*, Kerstin Simon1, René Limbach1, Jan Dellith2 & Lothar Wondraczek1.
1Otto Schott Institute of Materials Research, University of Jena, Fraunhoferstrasse 6, 07743 Jena, Germany
2Leibniz Institute of Photonic Technology IPHT, Albert-Einstein-Straße 9, 07745 Jena, Germany

Over the past years, with its extensive applicability in industry, there has been enormous interest in new preparation techniques of zirconia depending on its targeted application. Furthermore, there has been increasing demand on nanostructured zirconia for their lower sintering temperatures and improved mechanical properties. In that case, the sol–gel method is one of the most advantageous techniques due to its excellent chemical purity and homogeneity that can be achieved by fixing the dispersion state in a gel using hydrolysis and condensation reactions. Metal alkoxides are the most applied precursors in sol-gel synthesis of zirconia. However, their high cost led to their replacement by commercially more favourable inorganic salts (e.g. zirconyl chloride) and water as solvent.

In the present study zirconia layers on glass (monoclinic or tetragonal phase) were prepared via the sol-gel route using zirconyl dichloride octahydrate and yttrium (III) nitrate hexahydrate as Zr-precursors and Y-dopant, respectively. Urotropin and oxalic acid were found to be suitable reactants to produce stable and transparent sols, which were analysed in the meaning of pH, density and dynamic viscosity. The gel point ranged from 2 hours, using oxalic acid, up to 8 hours in the case of urotropin, but was not apparently influenced by the presence of yttrium. The as-obtained gels were further calcinated at 500 °C or 800°C and characterized using XRD. Tetragonal ZrO2 (t-ZrO2) with crystallite size ranging between 5 nm and 12 nm was detected in all samples. Upon heating at 800 °C the phase transformation to monoclinic zirconia (m-ZrO2) occurred and crystallite size grew up to 35 nm in the undoped samples. On the other hand, only t-ZrO2 with a crystallite size of 25 nm was detected in the samples containing 1.5% yttria, due to its stabilisation mechanism.

The dip-coated zirconia layers produced by means of oxalic acid had a thickness of 22 nm and 46 nm by withdrawal speeds of 10 mm/min and 100 mm/min, respectively. The films showed an effective refractive index of 1.94 (± 0.02). The doping with yttrium had no effect on the optical properties. However, the elastic modulus measured through nanoindentation increased by up to 8 % for the yttrium coating containing (1.5% Y2O3) probably as a result of the stabilization of the t-ZrO2 phase.

The zirconia layers described in this work from aqueous solutions could be of particular interest as refractive coatings or protective barriers due to their optical and mechanical properties.