[en] Document available in extended abstract form only. Full text of publication follows: We investigated the role of microbial activity on overpack steel corrosion under geological disposal conditions. In particular we studied the role of temperature increase in disposal site following sealing of galleries on microbiological viability. The temperature is expected to reach about 90 deg. C a few decades after site closure due to radioactive decay of heat generating waste such as ¹³⁷Cs. Batch experiments using COX clay and synthetic groundwater with steel coupons were conducted at 30 deg. C and 90 deg. C for 1, 3 and 6 months. In addition, some experiments at 90 deg. C were continued at 30 deg. C to test microbial growth. Control experiments were conducted with sterilized systems. Iron corrosion kinetics and secondary mineral formation have been studied by XRD and μ-Raman measurements. Chemical analyses have been performed by ICP-MS and ionic chromatography. At 30 deg. C non-sterilized experiments showed the formation of magnetite and traces of mackinawite, while in control sterilized systems magnetite was identified as a the only corrosion product. At 90 deg. C in all experiments a continuous mackinawite layer was identified. We can then conclude that temperature increase does not affect microbial viability and at 90 deg. C chemical reduction of sulphate by hydrogen cannot be excluded. Hence, at site temperature (30-35 deg. C) microbial activity of sulphate-reducing bacteria may enhance the steel corrosion via sulphide production. (authors)

[en] Structural and magnetic properties of γ-Fe₂O₃ nanoparticles of 4 nm diameter, dispersed into silica SiO₂ matrix with a wide range value of volume fraction (0.05 to 1), were investigated. Produced γ-Fe₂O₃@SiO₂ nanocomposites consist of an assembly of very small single domain magnetic object (<10 nm) with a random distribution of both inter-particle distance and direction of particle magnetic moment. We focused on the determination of a magnetic percolation threshold, defined as the magnetic particles concentration value above which the magnetic properties of isolated particles vanished in favor of a magnetic collective behavior induced by magnetic interactions. A percolation threshold value of 0.63 was obtained by a local probe technique such as ⁵⁷Fe Mössbauer spectrometry and confirmed by global magnetic measurements through zero-field cooled, field-cooled, and ac susceptibility data. Below this threshold, dynamic ac magnetization measurements show a thermally activated Arrhenius dependence of the blocking temperature of superparamagnetic nanoparticles and above this critical value, one observes a slowing down of their dynamic properties, which lead toward the establishment of a spin-glass like state

[en] We report an x-ray diffraction (XRD) and a Raman-scattering investigation of ferroelectric/paraelectric superlattices [BaTiO₃] _(1−x)Λ/[BaZrO₃]_xΛ for which the composition varied, 0.15 ≤ x ≤ 0.85, while the superlattice (SL) modulation period Λ was kept constant at about 100 Å. The samples were epitaxially grown by pulsed laser deposition on MgO substrates buffered with La_0.5Sr_0.5CoO₃. Based on the XRD analysis and on polarized Raman spectra, we have showed that the large strain in SLs induced ferroelectricity in BaZrO₃ (BZ) for all SLs, a material that is paraelectric in the bulk form at any temperature and in the single film. The induced polar axis in BZ layers is perpendicular to the plane of substrate while BaTiO₃ (BT) layers exhibit in-plane polar orientation. Raman spectroscopy revealed a lattice ordering in SLs due to the misfit strain generated by the large lattice mismatch between the alternating BZ and BT layers. This strain induced a huge upward frequency of the lowest E(1TO) soft mode from 60 cm⁻¹ in the BT single film to 215 cm⁻¹ in the SL with x = 0.85. These results show that in spite of relatively large periodicity of SLs, they are highly constrained and the variation of BZ ratio allowed modifying strains between layers. The temperature dependence of the Raman spectra for BT_0.3Λ/BZ_0.7Λ and BT_0.7Λ/BZ_0.3Λ samples revealed giant shift of the ferroelectric phase transition. The phase transition temperature was found to be upshifted by about 300 °C with respect to BT single crystal.

[en] Graphical abstract: The carbon steel corrosion under simulated geological conditions has been investigated and the results show the formation of iron sulphide on steel surface due to microbial corrosion at 30 °C and to the reduction by hydrogen of pyrite originating from claystone into iron monosulphide and hydrogen sulphide at 90 °C. - Highlights: • The role of temperature and microbial activity on steel corrosion was investigated. • At 30 °C, steel developed iron sulphide surface due to microbial activity. • At 90 °C, the microbial activity was inhibited. • At 90 °C, H₂S was produced via pyrite reduction by H₂. • Sulphide production may occur at high temperature. - Abstract: We investigated the role of temperature on the carbon steel corrosion under simulated geological conditions. To simulate the effect of temperature increase due to radioactive decay, we conducted batch experiments using Callovo-Oxfordian (COx) claystone and synthetic water formation with steel coupons at 30 °C and 90 °C for 6 months. The corrosion products have been studied by scanning electron microscope/energy dispersive X-ray spectroscopy, X-ray diffraction and micro-Raman spectroscopy. At 30 °C, experiments showed the formation of magnetite and iron sulphide, indicating the activation of sulphate-reducing bacteria. At 90 °C a continuous iron sulphide layer was identified on steel surface due to the reduction by hydrogen of pyrite originating from claystone into pyrrhotite and hydrogen sulphide. Thus, sulphide production may occur even in the absence of microbial activity at high temperature and must be taken into consideration regarding the near-field geochemical evolution