Isabelle Monot-Laffez

Evidence for intra-unit-cell magnetic order in Bi2Sr2CaCu2O8+δ

Polarized elastic neutron scattering measurements have been performed in the bilayer copper oxide system Bi2Sr2CaCu2O8+d, providing evidence for an intra unit cell (IUC) magnetic order inside the pseudo-gap state. That shows time reversal symmetry breaking in that state as already reported in Bi2Sr2CaCu2O8+d through dichroism in circularly polarized photoemission experiments. The magnetic order displays the same characteristic features as the one previously reported for monolayer HgBa2CuO4+d and bilayer YBa2Cu3O6+x, demonstrating that this genuine phase is ubiquitous of the pseudo-gap of high temperature copper oxide materials.

Synthesis, microstructure and electromechanical properties of undoped (K0·5Na0·5)NbO3

Abstract Conventional solid state mixed oxide route using manual and ball milling is investigated for the preparation of K0·5Na0·5NbO3 (KNN) ceramics. Microstructure engineering was made using two milling methods and sintering techniques, and the crystal growth; then electromechanical properties were investigated as a function of sintering temperature, densification and grain size. The sintering conditions were set at 920°C/5 min for spark plasma sintering and 1090–1120°C/10 and 48 h for classical sintering. KNN crystal was grown using floating zone technique under nitrogen gas, where the translation and rotation speeds were fixed at 3 mm h−1 and 20 rev min−1 respectively. Piezoelectric and dielectric performances were measured and related to the microstructure. High kt (33 to 48%), kp of 18 to 48% and d33 of 127–140 pC N−1 were reached for relative densities of 84 to 96%. KNN ceramics are now available for the design of ultrasonic sensors.

Investigation of the domain structure and hierarchy in potassium–sodium niobate lead-free piezoelectric single crystals

Recently, many techniques have been used to grow large K0.5Na0.5NbO3 (KNN) based single crystals. However, most of them required the use of a crucible, fluxes in the melt at high temperature and long process times that could lead to alkali volatilization or inclusion of impurities in the crystals. In this study, the floating zone method, which is especially suitable for compounds that melt incongruently or present volatile elements, is employed to study the microstructure of [011] oriented KNN crystals. Then, the domain structure and their relationships with piezoelectric properties in KNN are investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and piezo force microscopy (PFM). This study reveals self-organized and hierarchized domain structures on various length scales ranging from micrometer to nanometer scale in KNN crystals. First, parallel stripe-like microdomains of 6–7 μm wide, which contain themselves parallel stripes (1–4 μm wide), are observed using SEM. These domains have been confirmed by TEM. TEM observations have also shown the presence of additional zigzag sub-microdomains with 90 and 120° orientations, which are arranged inside these observed stripes. Moreover, high resolution TEM highlighted the presence of regular antiphase boundaries at the location of the domain walls, which are caused by the small misfits between the parameters of the different structures. Split spots on the FFT image attests of these phenomena. Additionally, PFM images showed also parallel stripe-like microdomains (3–4 μm wide), which contain sub-microdomains. These sub-microdomains consist of parallel stripes of 200–400 nm and zigzag nanodomains with 60, 90, 120 and 180° orientations. The local polarization switching test by PFM emphasized that the observed domain structures correspond well with ferroelectric domains, confirming through microstructural observations the ferroelectricity of KNN. The combination of SEM, TEM and PFM observations of orthorhombic KNN crystals have shown the correlation between microstructure, sub-microstructure and domain structures at different scales. The [011] oriented KNN crystal exhibited interesting piezoelectric properties such as kt of 0.47, d33 ∼ 60–80 pm V−1, eSr,33 of 123, dielectric losses (δe) of 0.07 but also high mechanical losses (δm) of 0.21 that can be induced by 90° domain walls moving under the influence of an electric field during poling.

Synthesis and transport properties of oxygen controlled melt growth textured(MixedRE)Ba2Cu3Oy (RE = Nd,Sm, Gd, Eu)

((RE)0.33Eu0.33Gd0.33)Ba2Cu3Oy bulk superconductors (RE = xa0Sm, Nd) were textured by the top seeded melt textured growth method in a reduced oxygen atmosphere, so-called oxygen controlled melt growth (OCMG). Large single domains of diameter 16xa0mm were successfully grown. Their microstructure and their magnetic and transport properties were investigated and compared. The resistive transition was found around 88xa0K, which is not the optimal Tc of 95xa0K that could be obtained on a small re-oxygenated sample cut out from the bulk. Transport measurements were also performed on these samples at various temperature from 77 to 88xa0K. In the case of Nd0.33Eu0.33Gd0.33Ba2Cu3Oyxa0(NEG) composition, the angular dependence of the critical current densities along the (a,b) planes as a function of magnetic field was also measured. Transport critical current densities higher than 50u2009000xa0Axa0cm−2 are reached at 77xa0K and up to 2xa0T, corresponding to the nominal critical currents of 90xa0A injected through sections less than 0.25xa0mm2 reproducibly. This confirms the high quality of the single domain obtained with a well controlled process, evidencing that mixing of rare earth is a very promising composition for transport and high-field applications.

Properties of the (Sm0.33Eu0.33Gd0.33)Ba2Cu3Oy superconductor prepared by different processes in air

Bars and pellets of the (Sm0.33Eu0.33Gd0.33)Ba2Cu3Ox superconductor were processed in air, using the floating zone method and the top-seeded melt-textured growth method, respectively. The samples were prepared using different experimental conditions, i.e. maximal processing temperature, translation rate or cooling rate. Their physical properties and their microstructure were studied. All the samples exhibit a satisfying superconducting transition whereas the critical current density greatly depends on the processing parameters. The sample prepared by the floating zone method at 1070 °C with a translation rate of 2 mm h−1 exhibits a very high Jc value of 70000 A cm−2 in the self-field and more than 30000 A cm−2 at 1.7 T. The pellet processed at 1080 °C with a cooling rate of 2 °C h−1 has a high Jc reaching about 56000 A cm−2 in the self-field and more than 32000 A cm−2 under 1.5 T and still 20000 A cm−2 at 2.5 T. This rare-earth mixture is promising for the application of this material under a high magnetic field.

Piezoelectric performances of undoped (K,Na)NbO3 ceramics and crystals for transducers applications

The aim of this work is to take control on the grain size and densification of undoped K0.5Na0.5NbO3 (KNN), from micrometer to millimeter grain sizes. For this purpose, ceramics are prepared by spark plasma sintering (SPS) and conventional sintering. Their piezoelectric properties are comparatively studied. After full structural and microstructural characterizations, high electromechanical properties (kt = 45 %, kp = 30 %, Z = 20 MRay) for conventional sintering and (kt = 45 %, kp = 48 %, Z = 30 MRay) for SPS sintering are obtained. Otherwise, millimeter grains are produced by the floating zone method used for crystal growth, and their electromechanical properties are determined and compared to their homologous ceramics. Moreover, using the characteristics of the three materials, simulated electroacoustic responses of single-element transducers are compared. The results show that KNN is suitable for transducer applications.

Electromechanical Properties and Microstructure of Undoped K0.5Na0.5NbO3 Ceramics and KNbO3-NaNbO3 Crystals

K0.5Na0.5NbO3 (KNN) was manufactured by spark plasma sintering (SPS), which is a fast sintering method allowing to control the grain growth. Different samples of KNN are sintered with SPS at 920°C under 50 MPa for 5 minutes. High densities over than 97% are achieved. In order to make domain engineering, KNN crystals are grown by floating zone method. Stable molten zone is reached when oxygen or nitrogen gas flux is used, leading up to 50 mm length of crystals. High electromechanical coupling factor kt about 46 %, kp around 45 % and ε33S/ε0 of 253 are achieved for KNN ceramics poled at optimum electric field about 3 kV / mm. KNN crystal boule exhibits kt about 40 % against 34 % for KNN ceramic, both poled at 1 kV / mm. These results are promising to replace PZT for transducers applications.