B. Domengès

Orthorhombic superconducting YBa2Cu3O7±ϵ, HREM study: I. Structure of the ordered oxygen-deficient perovskite and problems of twins and domains

Abstract Study of the orthorombic high-Tc superconductor YBa2Cu3O7±ϵ was carried out by electron microscopy in an attempt to understand the influence of its non-stoichiometry on its superconducting properties. Observed and calculated high-resolution images of regular crystals of YBa2CU3O7 were acquired in four favorable orientations. Most crystals exhibit numerous domains which can be identified as either twinning domains or oriented domains. Both phenomena are interpreted in terms of change in the orientation of the CuO4 groups; different models are proposed. Misorientation between thin oriented domains is a typical feature, besides strains and moire patterns.

The monophosphate tungsten bronzes with pentagonal tunnels (MPTBP, P4O8(WO3)2m: A high-resolution electron microscopy study

Abstract A series of monophosphate tungsten bronzes of composition P 4 O 8 (WO 3 ) 2 m with pentagonal tunnels, MPTB P , have been investigated by high resolution electron microscopy. Most of the micrographs of the integral m members exhibit an asymmetrical contrast which could not be explained qualitatively by the structural models derived from X-ray diffraction studies of some members of the series. Image calculations were thus performed on the m = 4 member (P 4 W 8 O 32 ), which showed that the unexpected symmetry does not result from a structural anomaly, but could be due to a titled electron beam. The observations revealed that ordered crystals can be obtained up to m = 16. The investigation of the nonintegral m compositions showed two sorts of intergrowths: disordered intergrowths of different m members belonging to the MPTB P series and ordered intergrowths of the MPTB P structure with a related phosphate tungsten bronze structure based upon hexagonal tunnels.

BaMn1−xFexO3−δ, an oxygen-deficient 6H′ oxide: Electron microscopy, powder neutron diffraction, and Mössbauer study

Abstract A rather rare six-layer form of “hexagonal perovskite” 6H′, was isolated for BaMn 1− x Fe x O 3− δ (0 ≤ x ≤ 0.3). X-ray powder diffraction and HREM studies have shown that the stacking sequence of the BaO 3 layers is hhhchc . The distributions of oxygen vacancies and octahedral cations were determined by powder neutron diffraction for x = 0.233. The coordination of iron ions was determined by Mossbauer spectroscopy.

Potassium monophosphate tungsten bronzes with hexagonal tunnels, Kx(PO2)4(WO3)2m: X-ray diffraction and HREM study

Abstract The series Kx(PO2)4(WO3)2m has been investigated by X-ray diffraction and electron microscopy. Five compositions have been isolated as pure phases for x = 2 and m = 4, 6, 7, 8, and 10. These bronzes are built up from WO3 slabs connected through PO4 groups forming hexagonal tunnels. High-resolution lattice images allow a correlation between the image contrast and the structure to be established. Three groups of defects are observed in the crystals which deal with (i) the distribution of the phosphate planes with respect to the ReO3-type slabs, (ii) the distribution of the K+ ions in the tunnels, and (iii) the uneven arrangements of the phosphate planes. From this study, it appears that the mechanism of nonstoichiometry is different from that observed in the other phosphate tungsten bronze families according to the existence range and the specific defects which generally involve large parts of the bulk.

Formation of pentagonal tunnels bordered with P2O7 groups in diphosphate tungsten bronzes (P2O4)2(WO3)2m: a HREM investigation

Abstract During the investigation of the phosphate bronzes (PO 2 ) 4 (WO 3 ) 2 m [MPTB P ] and K x (P 2 O 4 ) 2 WO 3 ) 2 m [DPTB H ] crystals of a new type were observed. HREM images of these crystals showed twinned ReO 3 -type slabs the junction of which was parallel to the (102) ReO 3 plane. The proposed model identified the twin boundary as built from P 2 O 7 groups involving the formation of pentagonal tunnels. The structure of this new type of extended defects is quite original: it corresponds to a new structural type named “diphosphate tungsten bronzes with pentagonal tunnels” [DPTB P ], for which no regular member could be synthesized. Image calculations were performed to confirm the junction model. Apart from the disordered stacking of the ReO 3 -type slabs, very few defects were observed and shear planes were only obtained in reduced samples. This new structural type takes its place in the large family of phosphate tungsten bronzes where all members (DPTB H , MPTB H , MPTB P ) are very closely related.

Direct insights on flax fiber structure by focused ion beam microscopy.

In this article, it is shown that focused ion beam (FIB) systems can be used to study the inner structure of flax fibers, the use of which as a reinforcing material in polymer composites still draws much interest from multiple disciplines. This technique requires none of the specific preparations necessary for scanning electron microscopy or transmission electron microscopy studies. Irradiation experiments performed on FIB prepared cross sections with very low Ga+ ion beam currents revealed the softer material components of fibers. Thus, it confirmed the presence of pectin-rich layers at the interfaces between the fibers of a bundle, but also allowed the precise localization of such layers within the secondary cell wall. Furthermore, it suggested new insights on the transition modes between the sublayers of the secondary cell wall.

Cationic and Anionic Disorder in CZTSSe Kesterite Compounds: A Chemical Crystallography Study

The cationic and anionic disorder in the Cu2ZnSnSe4-Cu2ZnSnS4 (CZTSe-CZTS) system has been investigated through a chemical crystallography approach including X-ray diffraction (in conventional and resonant setup), 119Sn and 77Se NMR spectroscopy, and high-resolution transmission electron microscopy (HRTEM) techniques. Single-crystal XRD analysis demonstrates that the studied compounds behave as a solid solution with the kesterite crystal structure in the whole S/(S + Se) composition range. As previously reported for pure sulfide and pure selenide compounds, the 119Sn NMR spectroscopy study gives clear evidence that the level of Cu/Zn disorder in mixed S/Se compounds depends on the thermal history of the samples (slow cooled or quenched). This conclusion is also supported by the investigation of the 77Se NMR spectra. The resonant single-crystal XRD technique shows that regardless of the duration of annealing step below the order-disorder critical temperature the ordering is not a long-range phenomenon. Finally, for the very first time, HREM images of pure selenide and mixed S/Se crystals clearly show that these compounds have different microstructures. Indeed, only the mixed S/Se compound exhibits a mosaic-type contrast which could be the sign of short-range anionic order. Calculated images corroborate that HRTEM contrast is highly dependent on the nature of the anion as well as on the local anionic order.

Analysis of Nuclei in a Heavy-Section Nodular Iron Casting

The microstructure of heavy section nodular graphite cast irons often presents a bimodal distribution of nodule size associated with so-called primary and secondary graphite nucleation. It has been found that the nuclei in both types of nodules consist mainly in magnesium sulphide. However, nuclei in primary nodules contain some traces of calcium and are thus related with the inoculation treatment. On the contrary, nuclei in secondary nodules do not contain any element that could be associated to inoculation. It is suggested they form in the late stage of the eutectic reaction as a result of microsegregation build-up in magnesium and sulphur.

Thermomechanical Ageing of Copper-Clad Aluminum Wires Submitted to Creep Test Conditions

Creep tests were performed on copper-clad aluminum wires at 423 K and different stresses to cover potential operating load ranges in automotive industry. The lifetime of the wires is strongly dependent on the existence of an initial heat treatment and on the applied stress. It can be correlated with the formation of the three intermetallics Al2Cu, AlCu and Al4Cu9 identified by TEM diffraction. All results are discussed to understand mechanisms that could lead to the embrittlement of copper-clad aluminum wires by creep.