Céline Viazzi

Ice shaping properties, similar to that of antifreeze proteins, of a zirconium acetate complex.

The control of the growth morphologies of ice crystals is a critical issue in fields as diverse as biomineralization, medicine, biology, civil or food engineering. Such control can be achieved through the ice-shaping properties of specific compounds. The development of synthetic ice-shaping compounds is inspired by the natural occurrence of such properties exhibited by antifreeze proteins. We reveal how a particular zirconium acetate complex is exhibiting ice-shaping properties very similar to that of antifreeze proteins, albeit being a radically different compound. We use these properties as a bioinspired approach to template unique faceted pores in cellular materials. These results suggest that ice-structuring properties are not exclusive to long organic molecules and should broaden the field of investigations and applications of such substances.

Ice-Structuring Mechanism for Zirconium Acetate

The control of ice nucleation and growth is critical in many natural and engineering situations. However, very few compounds are able to interact directly with the surface of ice crystals. Ice-structuring proteins, found in certain fish, plants, and insects, bind to the surface of ice, thereby controlling their growth. We recently revealed the ice-structuring properties of zirconium acetate, which are similar to those of ice-structuring proteins. Because zirconium acetate is a salt and therefore different from proteins having ice-structuring properties, its ice-structuring mechanism remains unelucidated. Here we investigate this ice-structuring mechanism through the role of the concentration of zirconium acetate and the ice crystal growth velocity. We then explore other compounds presenting similar functional groups (acetate, hydroxyl, or carboxylic groups). On the basis of these results, we propose that zirconium acetate adopts a hydroxy-bridged polymer structure that can bind to the surface of the ice crystals through hydrogen bonding, thereby slowing down the ice crystal growth.

High specific surface area YSZ powders from a supercritical CO2 process as catalytic supports for NOx storage–reduction reaction

Nanometric yttria-stabilized zirconia (3 mol% yttria, SC-3YSZ) was synthesized using an innovative supercritical carbon dioxide-aided sol–gel process and used as a support for NOx storage catalysts. Wash-coats composed of noble metals (Pt and Rh) dispersed on 3YSZ were deposited inside the porosity of mini-diesel particulate filters. Commercial and SC-3YSZ powders were used as supports independently and mixed. NOx storage/reduction performances were compared under cycling conditions according to the YSZ support surface area, the washcoat localization inside the DPF and the metallic dispersions. All the results emphasize that the presence of high specific surface area SC-3YSZ inside the DPF improves the NOx conversion, the N2 selectivity and the NOx storage capacity. This enhancement of the catalytic properties was linked with the increase in both the metallic dispersion and the number of storage sites on the catalyst surface.

Erosion and High Temperature Oxidation Resistance of New Coatings Fabricated by a Sol-Gel Route for a TBC Application

This paper examines the erosion and cyclic oxidation performance of novel thermal barrier coatings produced via the sol-gel route. The ceramic top coat, with a thickness of 5-80 m, was deposited via a sol-gel route onto standard MCrAlY and PtAl bond coats. In both the erosion and the cyclic oxidation tests it was found that the bond coat had a profound affect on the results. The erosion of the sol-gel coatings were compared to standard EB PVD and PS TBCs and were found to be significantly higher. The effect of aging (100 h at 1100°C) on the erosion rates was also evaluated and was found to increase the erosion rates. The information obtained from the erosion and cyclic oxidation tests have highlighted the need to develop and optimise the parameters for producing thicker coatings

Synthesis of stoichiometric Y2Si2O7 powders by sonohydrolysis-assisted sol–gel route

Stoichiometric compounds Y2Si2O7 were synthesised by an intensified sonohydrolysis–condensation reaction using hydrate yttrium nitrate and tetraethyl orthosilicate as starting materials. The resulting powders were characterized by means of thermo gravimetric–differential thermal analysis, high temperature X-ray diffraction, electron probe microanalysis, scanning electron microscopy, laser scattering particle size analyzer, N2 adsorption–desorption isotherms measurements and specific surface area analysis. We found that the phase formation and texture were very dependent on the sol–gel process parameters such as starting compounds, catalyst, water content, molar ratios of Y3+/Si4+ and other experiment conditions. The combined effects of polyethylene glycol and acetic acid on the prepared powders have been discussed. The investigation on thermal stability of the obtained disilicate is also presented for potential high temperature membrane or thermal barrier/environmental barrier coating application.

Elaboration of Sol-Gel Coatings from Aerogels and Xerogels of Doped Zirconia for TBC Applications

Thermal Barrier Coatings (TBCs) are used as insulators on hot section components to reduce operating temperatures in aircraft engines and industrial gas turbine. The TBC system consists of two layers: the ceramic top coat traditionally Yttria Stabilized Zirconia (YSZ) with a low conductivity, and the bond coat generally MCrAlY, M=Ni and/or Cr or Co or Pd or Pt modified aluminides. In the industry, two dry-route processes used to deposit TBCs give quite different microstructures of coatings. In one hand, coatings resulted by plasma spraying (PS) present a lamellar microstruture with a low thermal conductivity in the range from 0.7 to 0.9 Wm−1K−1. In the other hand, Electron Beam Physical Vapour Deposition (EBPVD) coatings with columnar microstruture coatings present the best mechanical performances but perpendicular orientation of the columns makes their thermal conductivity twice higher compared to PS coatings. The present study proposes the elaboration of zirconia coatings via the sol-gel route combined with dip-coating process. It is a versatile process able to produce either thin ceramic coatings or thick deposits. The main advantage of this method is to decrease the crystallization temperature, much lower than conventional processes. Moreover, the sol-gel process is a nondirectional deposition technique, which is very different to the physical methods described above. Doped zirconia have been chosen to constitute isolating multilayers coatings. Sol formulation, slurries stability but also dip-coating conditions have been optimized in order to obtain homogeneous layers on nickel based superalloys substrates.

Advances in YSZ Coatings Prepared by Sol-Gel Route. Applications to Fuel Cells or Thermal Barrier Coatings

The objective of this research is to develop a novel process for SOFC electrolytes and fully ceramic TBC to prepare yttria stabilized zirconia (YSZ) by both a sol-gel derived route and a dip-coating method. The research focuses on the development of a process compatible with industrial requirements in terms of cost and easiness to processing. The microstructure of dipcoated thick films is studied by varying the characteristics of an organic YSZ suspension (concentration, rheology) used as coating bath. The suspension was obtained by addition of a polymeric matrix in a stable suspension of YSZ powders dispersed in an azeotropic MEK-EtOH mixture. The obtained films are continuous, homogeneous and films thicknesses vary between 1 to 100 μm. Different microstructures are obtained depending on the synthesis parameters of the slurry composition. The microstructure of the films is found to depend on the YSZ precursor powders content and the polymeric sol /dispersant solution (EtOH-MEK) ratio. Finally, the dip-coated suspensions are used to prepare dense or porous layers according to the applications. The coating microstructure is engineered to decrease sintering ability, to be gas-tight for fuel cells electrolytes or to enhance thermal shock resistance for Thermal Barrier applications. Introduction Various deposition methods have been used in the fabrication of electrolytes for intermediate temperature SOFC or thermal barrier coatings (TBC) such as electrochemical vapor deposition (EVD) [1], physical vapor deposition (PVD) [2], vacuum plasma spraying [3], tape calendaring [4], screen-printing [5] and spray pyrolisis [6]. However, these technologies require costly investment and complicated operation and do not allow a control of the coatings microstructure. On the other hand, the sol-gel process has very seldom been applied for SOFC and TBC technologies. In fact, the dip-coating of sol or gel is a coating technology that has the potential to produce thin films (0.1 μm to 1 μm) but it is difficult to prepare thicker films even by using multiple layers technology [7-13]. In this work, a suspension constituted by sol and particles was investigated in order to increase the film thickness. To our knowledge, only few studies on the use of suspensions with the dip-coating technique have been presented to prepare thick films such as SOFCs and/or TBCs with controlled thickness. In this paper, we propose a novel approach to prepare YSZ thick films with controlled morphology (thickness, porosity). The key issue of this work is in the formulation of the dip-coated suspensions. During polymerization, there is a mechanism of metallic chelation which guarantees a good adhesion on the substrate. Our strategy is based on both the slurries composition for tape Key Engineering Materials Online: 2006-08-15 ISSN: 1662-9795, Vols. 317-318, pp 529-532 doi:10.4028/www.scientific.net/KEM.317-318.529