Florence Boulc'h

Structural changes of rare-earth-doped, nanostructured zirconia solid solution

A systematic study of the influence of the rare-earth (Sc, Yb, Y, Gd, Sm) dopant content and the ionic radius on the structural changes was undertaken on nanocrystalline-doped zirconia ceramics, which present the same microstructure. Tetragonal single-phased ceramics which were prepared by sintering of nano-powders obtained by spray pyrolysis at 1500 °C for 2 h in air, are characterized by an average grain size as low as 60 nm. Their densities are about 96% of the theoretical values. A good homogeneity in composition and in grain size was obtained for all samples. This exceptionally low grain growth during the sintering was related to the high purity of the materials and also to the morphology of the powders. Tetragonal solid solution domains were determined from Differential Thermal Analysis (DTA), X-ray Diffraction (XRD), and Raman spectrometry measurements. The size of the solid solution domain depends on the dopant ionic radius and on the grain size.

Delay of tetragonal-to-monoclinic transition in water vapour due to nanostructural effect

The ageing behaviour of ultrafine yttria-doped single-phased tetragonal zirconia was investigated at 400 °C in dry air and in water vapour by complex impedance analysis and Raman spectroscopy. Pellets sintered at 1500 °C for 2 h in air were characterized by a 60 nm average grain size, a unimodal distribution of 300 nm average aggregate size, a density of 97% of theoretical one and pore sizes lower than 20 nm. No degradation of the electrical properties of dense pellets was detected in both gaseous environments during 1000 h at 400 °C. Simultaneously, no monoclinic signature was detected by Raman spectroscopy. Furthermore, tests were investigated in conditions close to those of IT-SOFC: 700 °C in water vapour during 1000 h. In these conditions, a conductivity decrease of 5% was recorded and less than 2 mol% of monoclinic zirconia was observed by Raman spectroscopy.

Dopant size effect on structural and transport properties of nanometric and single-phased TZP

AbstractElectrical properties of doped nanocrystalline ceramics of xR 2 O 3 –ZrO 2 with 1Vx mol% final contentV3.5 of Sc 2 O 3 ,Yb 2 O 3 ,Y 2 O 3 ,Gd 2 O 3 and Sm 2 O 3 were studied. Rare-earth oxide dopants were selected for their lower valence (+3) compared to Zr 4+ and for an increase in their ionic radii ranging from 0.087 to 0.109 nm, respectively, from Sc 3+ to Sm . This systematic study isfocused on compositions defined as the minimal dopant concentration at which the tetragonal single-phase is stabilized. Theseceramics are characterized by a good purity (quantity of Si less than 0.1 wt.%) and are found to be perfectly stable intemperature, keeping their microstructure constant. Impedance spectroscopy was used to determine the tetragonal zirconiamatrix electrical contribution and the internal interface blocking effect. Both specific and blocking conductivities decrease whenthe dopant ionic radius is increased. The influence of space-charge layers on the increasing blocking effect was suggested.D 2002 Elsevier Science B.V. All rights reserved.

Physico-chemical characterization of highly pure nanocrystalline doped TZP

Abstract Yttria and scandia doped nanocrystalline tetragonal zirconia ceramics have been prepared from sintering of spray-pyrolysis powders. The average primary crystallite size after sintering has been found equal to 70 nm from X-ray diffraction line broadening while aggregated particle size of 300 nm has been characterized by scanning electron microscopy (SEM). No trace of segregated Si has been detected in the grain boundaries using wavelength dispersion scanning (WDS). Impedance diagrams show two well-resolved components in the 300–700°C range. Small size of crystallites and existence of nanoporosity give rise to a large grain boundary contribution compared to microcrystallized zirconia.

Cold isostatic and explosive isodynamic compaction of Y-TZP nanoparticles

Yttria-doped zirconia compacts (2.5 mol%) of spray-pyrolysed powders were prepared by two routes: cold isostatic pressing (CIP) and explosive isodynamic compaction (EXIC). Starting powders of 6 nm mean primary crystallite size are tetragonal single-phased with good homogeneity in chemical composition and with a narrow crystallite size distribution. This paper reports on a comparative study of the microstructure of these pellets in terms of grain size and density. CIP and sintering at 1500 °C led to nanocrystalline ceramics consisting of an average primary grain size of 60 nm and characterized by a density of 96% of the theoretical value with an unimodal distribution of 300 nm average aggregate size. Ceramics with 6 nm average grain size were successfully obtained by EXIC but with densities as low as 65% of the theoretical one. Both ceramics were still tetragonal single-phased, characterized by a good purity: only 0.1 wt.% Si and some traces of Hf.