Bernard Lesage

Sr diffusion in undoped and La-doped SrTiO3 single crystals under oxidizing conditions

Strontium titanate SrTiO3(100), (110), and (111) single crystals, undoped or donor doped with up to 1 at% La, were isothermally equilibrated at temperatures between 1523 and 1773 K in synthetic air followed by two different methods of Sr tracer deposition: ion implantation of 87Sr and chemical solution deposition of a thin 86SrTiO3 layer. Subsequently, the samples were diffusion annealed under the same conditions as before. The initial and final depth profiles were measured by SIMS. For strong La-doping both tracer deposition methods yield similar Sr diffusion coefficients, whereas for weak doping the tracer seems to be immobile in the case of ion implantation. The Sr diffusivity does not depend on the crystal orientation, but shows strong dependency on the dopant concentration supporting the defect chemical model that under oxidizing conditions the donor is compensated by Sr vacancies. A comparison with literature data on Sr vacancy, Ti, and La diffusion in this system confirms the concept that all cations move via Sr vacancies. Cation diffusion is several orders of magnitude slower than oxygen diffusion.

Cation transport in yttria stabilized cubic zirconia: 96Zr tracer diffusion in (ZrxY1–x)O2–x/2 single crystals with 0.15⩽x⩽0.48

Abstract For a wide range of stabilizer concentrations in yttria stabilized cubic zirconia (YSZ), Zr diffusion data extracted from published creep data and dislocation loop shrinkage data are discussed together with published Zr tracer diffusion data and our own data on Zr tracer diffusion in order to identify the most probable point defect responsible for Zr diffusion. From this evaluation, complex defects can be ruled out, as the single vacancy, VZr4′, fits best.

Oxygen Diffusion in Alumina. Application to Synthetic and Thermally Grown Al2O3

S. Chevalier, B. Lesage, C. Legros, G. Borchardt, G. Strehl, M. Kilo Laboratoire de Recherches sur la Reactivite des Solides, CNRS UMR 5613, Universite de Bourgogne, F-21078 Dijon, France Laboratoire d’Etudes des Materiaux Hors Equilibre, CNRS UMR 8647, Universite Paris XI, F-91405 Orsay, France. Institut fur Metallurgie, TU Clausthal, Robert Koch Strasse 42, D38678 Clausthal-Zellerfeld, Germany. * sebastien.chevalier@u-bourgogne.fr

SIMS study of transition metal transport in single crystalline yttria stabilised zirconiaPresented at the 85th Bunsen Colloquium on ?Atomic Transport in Solids: Theory and Experiments?, Gieen, Germany, October 31, 2003.

The diffusion of Co, Fe and Ni in single crystalline yttria stabilized zirconia (YSZ) containing 9.5 mol% Y2O3 was studied in the temperature range between 1373 and 1673 K using secondary ion mass spectroscopy. Two different types of diffusion sources were used: thin oxide layers made by spin coating with a thickness of about 150 nm containing all three transition metals (Fe, Co and Ni) on YSZ single crystals and YSZ single crystals implanted with Ni (3 × 1016 ions cm−2, 100 keV) at a mean depth of 45 nm. The determined diffusivities varied in the order D(Fe) < D(Co) < D(Ni). Activation energies for the diffusion of the elements were determined to be 2.7 ± 0.4 eV, 3.9 ± 0.3 eV and 3.8 ± 0.3 eV for Fe, Co and Ni (3.6 ± 0.5 eV for implanted Ni), respectively. For the latter ion, the value of the activation energy was practically independent of the type of Ni source. The values for all elements were lower by 1–2 eV than for the host cation (Y and Zr) diffusion.

Nitrogen diffusion in nitrogen-doped yttria stabilised zirconiaPresented at the 85th International Bunsen Colloquium on ?Atomic Transport in Solids: Theory and Experiments? Rauischholzhausen, Germany, October 31, 2003.

Nitrogen self-diffusion was measured in single crystalline nitrogen-doped yttria-stabilised zirconia (YZrON) containing 10 mol% yttrium oxide. Samples containing two different nitrogen contents (5 and 6 mol% N on the anion sublattice) were investigated as a function of temperature (650–1000 K) using implanted 15N as a stable tracer. For a given temperature, the nitrogen diffusivity was nearly independent of the nitrogen content in the nitrogen-doped yttria-stabilised zirconia, which can be only partially understood using defect chemistry. The activation enthalpy of nitrogen diffusion was between 2 and 2.5 eV with a preexponential factor of the order of 100 cm2 s−1, which corresponds to a migration entropy of 5 kB. The surface exchange reaction between nitrogen and the oxonitride surface was investigated at 1073 K using 200 mbar gaseous 15N2 and was found to be slow but considerable. Decreasing the oxygen content in the gas phase can enhance the nitrogen incorporation into the oxonitrides.

Cation diffusion in calcia stabilized zirconia (CSZ)

Abstract Tracer diffusivities of both cations in single crystalline calcia stabilized cubic zirconia (11 and 17 mol% CaO) were measured in the temperature range 1130°C < T < 1460°C. The calcium tracer diffusion is more than 10 times faster than the zirconium tracer diffusion. For both elements, the activation energy of tracer diffusion varies between 5.1 and 5.6 eV. The pre-exponential factors are higher than in the case of yttria stabilized zirconia (102−103cm2s−1 as compared to 1–10 cm2s−1). There is no significant dependency of the cation diffusivity on the stabilizer content, which indicates a diffusion mechanism via vacancy clusters (V4 г, 2V2∗o).

Diffusion Study of Cerium and Gadolinium in Single- and Polycrystalline Yttria-Stabilized Zirconia

Yttria-stabilized zirconia (YSZ) ceramic is considered as an attractive matrix for nuclear applications, such as inert matrix for the destruction of excess plutonium or good host material for nuclear waste storage. Some actinide elements in high-level radioactive wastes can be simulated by cerium as tetravalent actinide, and gadolinium as trivalent actinide or neutron absorber. The present work is focused on the diffusion study of Ce and Gd in YSZ single crystal and high density polycrystals. A thin film of Ce or Gd was deposited either by spin-coating method or by physical vapour deposition on the surface of polished samples. The diffusion experiments were performed from 1173 to 1673 K under air. The Ce or Gd diffusion profiles were determined by secondary ion mass spectrometry. The experiments led to the determination of effective diffusion coefficient, Deff, bulk and grain boundary diffusion coefficients, DB and DGB. The dependence of these diffusion coefficients on temperature is described by means of Arrhenius equations and the diffusivity is compared with literature.

Lanthanide Diffusion in Single Crystalline and Polycrystalline Pure or Yttrium Doped Alpha-Alumina

In order to bring a contribution to cationic diffusion in alpha-alumina, results on diffusion of 14 lanthanides and Y are presented. Samples are either single crystals with a known orientation or polycrystals with various grain-sizes elaborated from Y-doped alumina powders (100 ppm or 1000 ppm) or from pure alumina powder. After pre-heating in air at the diffusion temperature, small drops of a solution containing these elements are deposited at the surface of the sample. After the diffusion treatment (1200°C, 48 h or 1300°C, 21h), the diffusion profiles are obtained by a CAMECA IMS 4F SIMS device. The isotopes which are taken in account are: 45Sc, 89Y, 139La, 140Ce, 141Pr, 146Nd, 147Sm, 153Eu, 158Gd, 159Tb, 163Dy, 165Ho, 166Er, 169Tm, 174Yb and 175Lu which are the most suitable for this analysis. At first, it can be observed that the diffusion profiles are quite the same for all lanthanides. Diffusion profiles obtained for single crystals allow to calculate the lattice diffusion coefficients, which are necessary to determine the diffusion coefficients values in grain boundaries. Lattice and grain-boundary diffusion coefficients values are compared with cationic diffusion coefficients determined earlier.

Cation Transport and Surface Reconstruction in Lanthanum Doped Strontium Titanate at High Temperatures

Tracer diffusion experiments were carried out in synthetic air at 1573 K in SrTiO 3 (100) and (110) single crystals, which were either undoped or doped with up to 1 at.% La, respectively. Tracer sources of 139 La and 142 Nd were applied by ion implantation. The resulting depth profiles were measured by SIMS. The reconstruction of the surface was monitored ex-situ using microscopic and spectroscopic methods including SEM, EPMA, and AFM. The measured tracer diffusivities show no dependency on orientation. The tracer diffusion takes place via cation vacancies. Under oxidizing conditions the dopant is compensated by Sr vacancies. Hence the diffusion is increasing strongly with La concentration. The observed time dependency of the diffusivities may be related to a space charge layer postulated by the current defect chemistry model for donor doped SrTiO 3 . At high dopant concentrations annealing leads to segregation of bulk La to the surface. La is not significantly incorporated into the secondary crystallites at the surface which consist almost entirely of Sr and O.