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International audience; In this work, structural and dielectric properties of BaTi(1-x)ZrxO3 (BTZ) ceramics prepared by Spark Plasma Sintering (SPS) from powders obtained via Self-propagating High-temperature Synthesis (SHS) are shown to be strongly affected by the sintering temperature. In addition, a post-annealing treatment in air of the as-prepared ceramics leads to a transition from the hexagonal to the tetragonal and cubic phases. The SPS ceramics corresponding to compositions 0.05 ≤ x ≤ 0.20 and obtained at a sintering temperature of 1200 °C exhibit a standard ferroelectric behavior. In contrast, a diffuse phase transition is observed for the case of ceramics sintered at higher temperatures. Finally, the BTZ ceramic containing 5 at.% of Zr displays the best dielectric permittivity and piezoelectric properties as compared to the other compositions taken into account.

International audience; Herein we investigate the technologically relevant blend of the ferroelectric polymer poly(vinylidene fluorideco-trifluoroethylene), P(VDF-co-TrFE), with the semiconducting polymer poly(3-hexylthiophene), P3HT, by means of a combination of Scanning Probe Microscopy techniques, namely Atomic Force Microscopy, Conductive Force Microscopy, Kelvin Probe Force Microscopy and Piezoresponse Force Microscopy. This combination proves to be a powerful tool for the non-destructive morphological reconstruction of multifunctional nano-structured thin films, as those under study. Each modality allows discerning the two blend constituents based on their functionality, and, additionally, probes layers of different thickness with respect to the films surface. The depth-dependent information that is collected allows a qualitative reconstruction of the blend's composition and morphology both in-plane and out-of-plane of the film. We demonstrate that P3HT exhibits the tendency to reside the film surface at an almost constant composition of 15%, independent of blend's composition. Increasing the P3HT content in the blend results in the segregation of P3HT at the upper layers of the films, partially buried below a P(VDF-co-TrFE) superficial layer. The depletion of P3HT from the substrate/film interface is reflected by the poor existence of conducting pathways that connect the top and bottom planes of the film. The three-dimensional morphology of this polymer blend that is revealed thanks to the employed techniques deviates substantially from the ideal morphology proposed for the efficient performance of the targeted memory devices.

In this paper, we revisit the high-pressure behavior of BaZrO3 by a combination of first-principle calculations, Raman spectroscopy, and x-ray diffraction under high pressure. We confirm experimentally the cubic-to-tetragonal transition at 10 GPa and find no evidence for any other phase transition up to 45 GPa, the highest pressures investigated, at variance with past reports. We re-investigate phase stability with density functional theory considering not only the known tetragonal (I4/mcm) phase but also other potential antiferrodistortive candidates. This shows that the tetragonal phase becomes progressively more stable upon increasing pressure as compared to phases with more complex tilt systems. The possibility for a second transition to another tilted phase at higher pressures, and in particular to the very common orthorhombic Pnma structure, is therefore ruled out.

Tunable ferroelectric capacitors, which exhibit a decrease in the dielectric permittivity under an electric field, are widely used in electronics for RF tunable applications. Current devices use barium strontium titanate (BST) as the tunable dielectric, but new applications call for tunable materials with specific performance improvements. It is then of crucial importance to dispose of a large panel of electrically characterized materials to identify the most suited compound for a given set of device specifications. Here, we report on the dielectric tuning properties of Ba1−xCaxTi1−yZryO3 (BCTZ) thin films libraries (0 ≤ x ≤ 30% and 0 ≤ y ≤ 28.5%) synthesized by combinatorial pulsed laser deposition (CPLD). An original CPLD approach allowing reliable and statistical ternary phase diagrams exploration is reported. The effects of Ca and Zr content on tunability, breakdown voltage and dielectric losses are explicated and shown to be beneficial up to a certain amount. Compounds close to (Ba0.84Ca0.16)(Ti0.8Zr0.2)O3 exhibit the highest figures of merit, while a zone with compositions around (Ba0.91Ca0.09)(Ti0.81Zr0.19)O3 show the best compromise between tuning ratio and figure of merit. These results highlight the potential of BCTZ thin films for electrically tunable applications.

International audience; BaTiO3-based crystal fibres with mm-sized grains were grown by the micro-pulling down technique from the BaTiO3–CaTiO3–BaZrO3 solid solution with pulling velocities of about 6, 9 and 15 mm h−1. The natural growth direction was identified as (001)pc. For the pulling velocities of about 15 mm h−1 and 9 mm h−1, effective partition coefficients have been calculated from Castaing micro-probe measurements, and gave, respectively, 1.3 and 2 for Zr, and 0.95 and 0.9 for Ca. Laser-induced breakdown spectroscopy measurements reveal a strong inhomogeneity and variations of Zr contents while Ca contents show an opposite variation trend with a more steady distribution. Coexistence of two crystallized perovskite solid solutions is suggested. Most efficient polycrystals with mm-sized grains and 0.5 mol% Zr and 11 mol% Ca as average contents exhibit Curie temperatures higher than 113 °C, electromechanical coupling factors kt up to 41% and piezoelectric charge coefficients d33 up to 242 pC N−1 at room temperature. These values are similar to piezoelectric coefficients reported in the literature for oriented flux-grown single crystals with close compositions. Both chemical and physical results obtained in the BCTZ system make the μ-PD technique a promising way to improve the piezoelectric response of lead-free solid solution-based single crystals.

Piezoelectric thick films are of real interest for devices such as ceramic Micro-ElectroMechanical Systems (MEMS) because they bridge the gap between thin films and bulk ceramics. The basic design of MEMS includes electrodes, a functional material, and a substrate, and efforts are currently focused on simplified processes. In this respect, screen-printing combined with a sacrificial layer approach is attractive due to its low cost and the wide range of targeted materials. Both the role and the nature of the sacrificial layer, usually a carbon or mineral type, depend on the process and the final device. First, a sacrificial layer method dedicated to screen-printed thick-film ceramic and LTCC MEMS is presented. Second, the recent processing of piezoelectric thick-film ceramic MEMS using spark plasma sintering combined with a protective layer approach is introduced. Whatever the approach, the focus is on the interdependent effects of the microstructure, chemistry, and strain/stress, which need to be controlled to ensure reliable and performant properties of the multilayer electroceramics. Here the goal is to highlight the benefits and the large perspectives of using sacrificial/protective layers, with an emphasis on the pros and cons of such a strategy when targeting a complex piezoelectric MEMS design.

International audience; Les matériaux ferroélectriques trouvent de nombreuses applications sous forme de condensateurs en électronique, de générateurs et de capteurs piézoélectriques, de cristaux pour l’optique non linéaire. Ces applications résultent de leurs propriétés très spécifiques rappelées en début d'article. Les problématiques actuelles autour de leur synthèse et de leur mise en forme résultent principalement de l’intégration attendue et toujours plus poussée. Cette dernière implique la réduction de la taille des matériaux et donc des conséquences très importantes sur les fonctionnalités des ferroélectriques.

International audience; The design and processing of vibrational energy harvester based on screen-printed piezoelectric lead zirconate titanate (Pb(ZrxTi1-x)O3 (PZT)) are here described. Two different structures, a simple cantilever and a complex zigzag geometry made of PZT layer sandwiched between gold electrodes and supported on a metallic stainless steel substrate have been successfully fabricated by screen printing thick film technique. Compared to bulk PZT ceramics, the main limiting features at different scales are porosity, interfaces and bending issues. The microstructural analysis of the interfaces in the cantilever has highlighted the formation of an interface between the substrate and the bottom electrode which ensures cohesion of the structure but can limit its dynamic. Bending has shown to be dependent on the thickness of the active piezoelectric layer. Dielectric and electromechanical characterizations performed on multilayers, bulk ceramics, and free-standing screen-printed disks are compared and discussed on the basis of interface issues.

International audience; Interfaces are a major issue when designing ferroelectric nanostructured materials with tailored properties. In a context of integration and multifunctionality in the field of electronics, several strategies have been developed to control the microstructure and defect chemistry of interfaces that strongly impact the macroscopic properties. The suitability of the core-shell approaches that allow a subtle tuning of interface phenomena at different scales has been widely demonstrated. We focus here on the flexibility of the core-shell approach devoted to the processing of nanostructured ferroelectric composites. Our strategy relies on the use of advanced synthesis processes to design ferroelectric grains coated with shells of different nature, morphology and crystallinity. Typical examples will be reviewed with a specific attention on their impact on both microstructure and dielectric properties. Our approach, based also on the use of fast sintering technique, provides a guidance to design 3D bulk nanostructured ferroelectrics while controlling and/or exploiting size, interface and defects chemistry. The contribution of specific spectroscopies to probe interfacial chemistry and defects is underlined. The high density of interfaces in core-shell materials is obviously an advantage to target additional functionality such as magneto-electric coupling. This is illustrated in 3D composites and one dimensional nanostructures that coaxially combine electric and magnetic materials. The core-shell approach described here could be transferred to a much broader range of materials covering many functionalities provided a deeper understanding of the interfaces at the atomic scale is achieved and a further development of low temperature processing is reached.

${\mathrm{BaZrO}}_{3}$ is a perovskite that remains in the simple cubic phase at all temperatures, hence with no first-order Raman-active phonon mode allowed by symmetry. Yet, it exhibits an intense Raman spectrum with sharp and well-defined features. Here, we report the evolution of the Raman spectrum of ${\mathrm{BaZrO}}_{3}$ single crystals in a broad temperature range (4--1200 K) and discuss its origin with the support of detailed first-principle calculations of the lattice dynamics. Phonon calculations are performed not only for the cubic phase of ${\mathrm{BaZrO}}_{3}$, but also for the low-symmetry phases with octahedra tilts that have been suspected to exist at the nanoscale. We show that the Raman spectrum shows no direct evidence for these nanodomains, but can instead be explained by classical second-order Raman scattering. We provide an assignment of the dominant features to phonon mode combinations. In particular, we show that the high frequency range of the spectrum is dominated by overtones and shows an image of the phonon density of states corresponding to the stretching modes of the oxygen octahedra.

International audience; We report the growth of BaZrO3 single crystals by the optical floating zone technique and the investigation on its flux growth using BaB2O4 as a solvent. 6 mm long colorless and transparent single crystals were obtained with a mirror furnace without the need for post-treatment annealing. Its properties are determined and compared with those of two commercial crystals grown by the tri-arc Czochralski method. The chemical composition was investigated using glow discharge mass spectrometry (GDMS) and secondary ion mass spectrometry (SIMS), which indicate minor impurities of Sr, Hf, Ca and Ti, with maximal concentrations for Sr and Hf in the range of 0.3–0.5% at. The optical band gap determined by UV-visible spectroscopy is found to be ∼4.8 eV and indicates the high quality of the BaZrO3 crystals grown by the optical floating zone technique. Raman spectroscopy at ambient conditions and at low temperatures down to 4.2 K reveals a relatively sharp second-order spectrum and does not reveal any structural phase transition. Prospective high-temperature solution growth using BaB2O4 self-flux was investigated and led to 150–200 μm BaZrO3 crystals. This solvent opens the way to grow BaZrO3 at half its melting point by the flux method.

From DFT based calculations establishing energy-volume equations of state and electron localization mapping, the electronic structure and crystal chemistry changes from Sn2TiO4 to Sn2TiO6 by oxidation are rationalized; the key effect being the destabilization of divalent tin SnII towards tetravalent state SnIV leading to rutile Sn2TiO6 as experimentally observed. The subsequent electronic structure change is highlighted in the relative change of the electronic band gap which increases from ~1eV up to 2.2 eV and the 1.5 times increase of the bulk modulus assigned to the change from covalently SnII based compound to the more ionic SnIV one. Such trends are also confronted with the relevant properties of black SnIIO characterized by very small band gap., Comment: 21 pages; 5 Figs, 1 table

Monocrystalline titanium dioxide (TiO2) micro-spheres support two orthogonal magnetic dipole modes at terahertz (THz) frequencies due to strong dielectric anisotropy. For the first time, we experimentally detected the splitting of the first Mie mode in spheres of radii inline imagem through near-field time-domain THz spectroscopy. By fitting the Fano lineshape model to the experimentally obtained spectra of the electric field detected by the sub-wavelength aperture probe, we found that the magnetic dipole resonances in TiO2 spheres have narrow linewidths of only tens of gigahertz. Anisotropic TiO2 micro-resonators can be used to enhance the interplay of magnetic and electric dipole resonances in the emerging THz all-dielectric metamaterial technology.

International audience; Polycrystals and centimeter-sized BaTiO3-based single crystals were grown by top seeded solution growth from the BaTiO3–CaTiO3–BaZrO3 system. High effective partition coefficients of Zr ranging from 15 to 6 with small Zr content have been calculated from Castaing microprobe measurements, whereas those of Ca increase slightly from 0.45 to 0.7. A spinodal decomposition mechanism is emphasized during the growth leading to the emergence of two phases with close compositions. Chemical analysis displayed periodical Zr and Ca content fluctuations within the whole boules, and Rietveld measurements highlighted two phases belonging to perovskite structures with tetragonal P4mm and orthorhombic Amm2 space groups. Samples with various calcium and zirconium contents were characterized by means of dielectric and piezoelectric measurements. Most efficient samples are indistinctly polycrystals or oriented single crystals where electromechanical performances are compositional-dependent. Polycrystalline samples and single crystals oriented along (001)pc and (110)pc displayed Curie temperatures ranging from 50 to 111 °C. Electromechanical coupling factor up to 58% and piezoelectric charge coefficient d33 = 496 pC·N–1 were obtained at room temperature. The miscibility gap between the two perovskite solid solutions as well as Ca and Zr element content variation in single crystals lower crystal piezoelectric response that remains nonetheless of the same efficiency compared to that of ceramics of the same composition.

International audience; (K,Na)NbO 3 -based ferroelectric single crystals have recently undergone a substantial development, resulting in improved crystal quality and large piezoelectric coefficients, exceeding 700 pC/N, over a broad temperature range. However, further development necessitates a detailed understanding of the mechanisms defining the domain structure and its temperature evolution. This paper presents the investigation into the crystallographic structure and domain configurations of a (K,Na,Li)(Ta,Nb)O 3 single crystal over a broad temperature range. The crystal was grown by the submerged-seed solution growth technique and investigated using in situ transmission electron microscopy, X-ray diffraction, dielectric measurements, and polarized light microscopy. The lattice distortion, structural phase transitions, and domain configurations are reported. A transition from the lamellar orthorhombic to the rectangular tetragonal domain structure is observed upon heating. Moreover, the milky optical appearance of the crystal was investigated and found to result from the presence of regions with different domain configurations and domain sizes. The formation of these regions is related to the growth defects, which govern the domain formation when cooling below the Curie temperature.

The functional properties of P(VDF-co-TrFE) are strongly dependent on its structure, which, in turn, depends on processing conditions applied. In this work we investigate the P(VDF-co-TrFE) processing structure-function relationships, in order to find the thermal conditions that result in optimum ferroelectric performance in thin film configuration. Our results show that annealing temperature affects mostly the remnant polarization value, P, while annealing time has a severe effect on the coercive field, E-c, An optimized ferroelectric functionality, in terms of high P-r of about 90 mC/m(2) and low E-c of 50 MV/m, is achieved and rationalized through structural analysis by means of GIWAXS. The best performing structure exhibits a high degree of crystallinity, a preferential orientation of the crystallites with the polymer chains parallel to the substrate and the occurrence of three ferroelectric phases. A deconvolution study demonstrates the presence of a moderately unstable ferroelectric phase that is designated to facilitate ferroelectric switching. Our findings show that a single step of 15 min annealing at 135 degrees C leads to high performance P(VDF-co-TrFE) structure, proving that the 2 h-long annealing step that is traditionally applied is not necessary. (C) 2018 Elsevier Ltd. All rights reserved.

International audience; The dielectric properties of Ba1-xCaxTi1-xZrxO3 (BCTZ) thin films are explored by varying x from 0 to 0.18 through an RF magnetron co-sputtering process. Thin films are analyzed by Rutherford Backscattering Spectroscopy, X-Ray Diffraction and dielectric measurements on parallel plate capacitors as a function of temperature (from 100 to 420 K), frequency (from 100 Hz to 1 MHz) and static electric field (from 0 to 250 kV cm−1). The evolutions of their dielectric properties with composition are systematically compared to bulk ceramic counterparts. In both cases, a continuous crossover from classical-to-relaxor ferroelectric behavior along with a decrease of the temperature of the permittivity maximum are observed as x increases. Besides, a phase convergence region (PCR) between cubic, tetragonal, orthorhombic and rhombohedral symmetries is evidenced for compositions close to x = 0.12. This is the first time that such a PCR is observed in thin films. At this point in the phase diagram, all of the dielectric parameters undergo specific variations. In particular, the enhanced permittivity dependence to external electric field for these Ca and Zr contents is attributed to an emerging relaxor behavior together with its vicinity of the PCR. These results make BCTZ thin films potential candidates to achieve electrically tunable devices.

International audience; Ferroelectric single crystals exhibit the largest known piezoelectric constants and therefore show high potential for application in electronic devices, including sensors, actuators, and transducers. Their large properties, however, can only be fully exploited if the behavior is not influenced by lattice defects, formed during high-temperature processing. Such defects can inhibit domain-wall movement, increase leakage currents, and are predominantly responsible for the poor performance of the emerging ferroelectric crystals. Here, an approach to considerably enhance the piezoelectric and ferroelectric properties of (K,Na,Li)(Nb,Ta,Sb)O3 crystals by oxygen annealing is investigated. As compared to the non-annealed crystals, polarization, strain, and piezoelectric constant of the annealed sample were enhanced by a factor of two, resulting in an outstanding room-temperature value of piezoelectric constant d33 = 732 pC/N and a peak value of d33 = 1431 pm/V at the orthorhombic-tetragonal transition. The behavior was analyzed using electromechanical measurements, Mössbauer spectroscopy, and impedance spectroscopy, revealing decreased concentrations of both oxygen vacancies and Sb3+ ions after annealing. The findings indicate that the control of the defect chemistry enables these ferroelectric (K,Na)NbO3-based single crystals to outperform their polycrystalline counterparts and reach the values of lead-containing compositions. Moreover, the present strategy enables further detailed studies of the inherent crystal anisotropy and the possibilities for domain engineering.

International audience; The re-oxidation kinetics of BaTiO3 ceramics sintered by Spark Plasma Sintering (SPS) was investigated using in-situ impedance spectroscopy. Thanks to the flexibility of the SPS process, the grain size of the dense ceramics was tuned from 0.5 μm to 10 μm. The re-oxidation kinetics are found to be very fast regardless of the grain size and a full re-oxidation of the ceramics are achieved after 20 h of exposure to an ambient environment at only 600 °C. The residual density of charge carriers is reduced when using finer starting powders. SPS ceramics made with micrometer size grains demonstrate a residual charge-carrier density that is one tenth that of ceramics made from 10 μm particles. Grain-boundary conduction is dominant through fine-grain SPS ceramics. This latter feature is similar to BaTiO3 sintered using the conventional route with 10 μm size grain. Finally, the critical grain size for optimal dielectric permittivity is found to shift from 0.7 μm in standard ceramics to 1.5 μm in SPS ceramics.

We show that the type-II multiferroic properties of bulk MnWO4 are kept in nanostructured ceramics of 50 nm grain size prepared via spark plasma sintering (SPS). This means that ferroelectric polarization is robust against downsizing, which is at variance with standard ferroelectrics like BaTiO3. We ascribe this stability to the spin-driven nature of the ferroe-lectric correlations in type-II multiferroics while it is resulting from lattice distortion in other cases. This may open the way for persistent type-II multiferroicity with no need for external stabilization like substrate-generated strain.

International audience; In the framework of the crystal chemical investigation of Tetragonal Tungsten Bronze (TTB) niobates, the Ba2NdFeNb4−xTaxO15 solid solution has been investigated in order to gain insights on the relaxor to ferroelectric crossover and its features. Single-phase ceramics were synthesised by conventional two-step firing and sintering and they underwent extended chemical, structural, microstructural and physical characterizations. For Nb-rich compositions (x ≤ 0.5), the temperature dependence of the dielectric permittivity, the pyroelectric coefficient, and also the ferroelectric hysteresis loops revealed a nonconventional relaxor to ferroelectric crossover, which was confirmed by piezoelectric measurements for x ≤ 0.1. A thermal hysteresis of the ferroelectric transition revealed the existence of a metastable ferroelectric phase in these TTB niobates. An accurate and reliable dielectric phase diagram is established from these observations, which allows discussing the original sequence of dielectric phase transitions encountered in this solid solution.

This paper addresses for the first time two of the main challenges in the field of electroceramics. The first one is the development of a continuous, fast and reliable synthesis method for producing sub-20 nm barium titanate zirconate (BaTi1-yZryO3 with 0 ≤ y ≤ 1–BTZ) nanocrystals over the whole solid solution, while keeping a narrow size distribution. The second one concerns the processing of dense and nanostructured lead free electroceramics highlighting the size effect on the crossover from ferroelectric to relaxor, in the case of grains smaller than 100 nm. This was achieved combining the supercritical fluid technology to produce in continuous and at moderate temperature (400 °C) BTZ nanocrystals, with Spark Plasma Sintering (SPS) to prevent grain growth during the sintering. Keywords: Barium titanate zirconate (BTZ), Supercritical fluid, Nanocrystal, Nanostructured ceramic, Lead free, Ferroelectric, Relaxor

International audience; Dielectric response of Ba2NdFeNb4-xTaxO15 (x = 0.3, 0.6 and 2) solid solutions with Tetragonal Tungsten Bronze structure has been investigated by means of broadband dielectric spectroscopy in the frequency range from 20 Hz to 37 GHz. Pure Ba2NdFeNb4O15 compound displays ferroelectric behavior. Substitution of Nb5+ by Ta5+ considerably alters dielectric response of this system. Ferroelectric state was observed only for x = 0.3 compound with slightly lowered ferroelectric phase transition temperature. Further increase of x causes formation of relaxor state in these solid solutions and considerably lowers corresponding dielectric permittivity dispersion temperature region.

Based on advanced industrial fabrication processes and on specialized design strategies, a new class of ferroelectric metal–insulator–metal (MIM) capacitors with high tunability and high quality factor ( $Q$ -factor) is here presented. Modeled by means of lumped element equivalent circuits and experimentally validated up to 67 GHz, a maximum tunability of 81% (0–20-V bias), and a $Q$ -factor improvement up to 30% (at 0 V) could be demonstrated at 1 GHz. These varactors have been exploited in the design of reconfigurable filters, with a center frequency at around 1 GHz that can be tuned up to 112%, and a figure of merit (FoM) per applied bias better than ${\hbox{31.5}}~{\hbox{dB}}^{-1} /{\hbox{kV}}$ . Owing to their promising features, these materials have been exploited to design small-size capacitors suitable for millimeter-wave frequencies. The results demonstrate tunabilities and FoMs superior to the state-of-the-art. Based on this, two 60-GHz tunable phase shifters are proposed. They represent the first example of such devices based on MIM $({\hbox{Ba,Sr}}){\hbox{TiO}}_{3}$ (BST) capacitors. In terms of insertion loss, size, FoM, and FoM per bias they show a remarkable improvement with respect to the state-of-the-art of ferroelectric-based devices, thus proving that the BST represents a promising candidate to operate into the millimeter frequency band.

International audience; Lattice structure transformations in nanopowders of ferroelectric BaTiO3 and BaTiO3@SiO2 core-shell nanostructured ceramics were studied by nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) at the temperatures 120–450 K and particle size of 300 and 500 nm. NMR spectra of all studied samples in the paraelectric phase are identical to the spectra in bulk material indicating their perfect perovskite structure without visible influence of particle surface. However, we have found that surface of particles essentially influence the ferroelectric phase transitions detected by both NMR and EPR techniques. The strongest changes as compared to bulk material were observed in BaTiO3@SiO2 core-shell ceramics. Thorough analysis of NMR spectra suggests that the orthorhombic-like symmetry phase coexists here with other polar phases up to the Curie temperature. Depending on temperature, its relative volume varies from 25% to 100%. We assume that the orthorhombic-like symmetry phase is stabilized by anisotropic components of surface stresses which increase also global stability of polar state in nanoceramics to the temperature in bulk material. We summarize our results in a phase diagram.

International audience; A reliable and flexible synthesis route was used for processing high density Ni–BaTiO3 nanocable arrays based on wet chemical impregnation and subsequent electrodeposition within a highly ordered unidirectional porous alumina membrane. The core–shell structure was carefully investigated by bright field scanning transmission electronic microscopy coupled with energy dispersive X-ray spectroscopy. The strength of the dipolar interaction arising from the packing density of the magnetic nanowires was correlated with the BaTiO3 wall thickness through magnetometry and ferromagnetic resonance measurements. Our approach opens a pathway to obtain optimized nanostructured multiferroic composites exhibiting tunable magnetic properties.

A new strategy to elaborate (1-3) type multiferroic nanocomposites with controlled dimensions and vertical alignment is presented. The process involves a supported nanoporous alumina layer as a template for growth of free-standing and vertically aligned CoFe

International audience; A new strategy to elaborate (1-3) type multiferroic nanocomposites with controlled dimensions and vertical alignment is presented. The process involves a supported nanoporous alumina layer as a template for growth of free-standing and vertically aligned CoFe2 nanopillars using a room temperature pulsed electrodeposition process. Ba0.70Sr0.30TiO3–CoFe2O4 multiferroic nanocomposites were grown through direct deposition of Ba0.7Sr0.3TiO3 films by radio-frequency sputtering on the top surface of the pillar structure, with in situ simultaneous oxidation of CoFe2 nanopillars. The vertically aligned multiferroic nanocomposites were characterized using various techniques for their structural and physical properties. The large interfacial area between the ferrimagnetic and ferroelectric phases leads to a magnetoelectric voltage coefficient as large as ~320 mV cm−1 Oe−1 at room temperature, reaching the highest values reported so far for vertically architectured nanocomposite systems. This simple method has great potential for large-scale synthesis of many other hybrid vertically aligned multiferroic heterostructures.

International audience; The present work aims to obtain Hydroxyapatite (HA) – Barium Titanate (BT) piezocomposites with improved electrical properties by retaining OH-ion and tuning BT grain size. Dense piezocomposites of HA-BT were obtained using single step Spark Plasma Sintering (SPS). The presence of OH-ions in the sintered piezocomposites of HA-BT was confirmed by high-temperature X-ray diffraction and Fourier transform infrared transmittance. No decomposition/ interaction between HA and BT phases were observed in the complete range of temperature studied. The dielectric and pyroelectric properties were studied as a function of temperature (100 K to 450 K) and frequency (100 Hz to 2 MHz). The influence of the ferroelectric grain size (300 and 500 nm) on the electrical properties of HA-BT has been analyzed. The dielectric permittivity of HA-BT ceramics can be enhanced with appropriate ferroelectric grain size and volume fraction of BT (

International audience; Ferroelectrics materials have been tremendously attractive since the 40 s with the discovery of ferroelectricity in metal oxide perovskite materials and more precisely in barium titanate. Due to their high potential for industrial applications, intensive research has been carried out to better understand their behavior and develop processes to produce them. Trying to face the down scaling demand of high quality particles towards the nanometer range, some conventional methods such as the solid state one reach their limits. The development of other processes are thus required and the synthesis in supercritical fluids can be considered as a promising alternative. This technology exhibits very interesting characteristics such as fast continuous synthesis (few seconds) of high quality nanoparticles (well crystallized nanoparticles with narrow size distribution) with controlled composition (Ba1−xSrxTiO3 with 0 ⩽ x ⩽ 1) at intermediate synthesis temperatures (

The energy band alignment at interfaces between different materials is a key factor, which determines the function of electronic devices. While the energy band alignment of conventional semiconductors is quite well understood, systematic experimental studies on oxides are still missing. This work presents an extensive study on the intrinsic energy band alignment of a wide range of functional oxides using photoelectron spectroscopy with in-situ sample preparation. The studied materials have particular technological importance in diverse fields as solar cells, piezotronics, multiferroics, photoelectrochemistry and oxide electronics. Particular efforts have been made to verify the validity of transitivity, in order to confirm the intrinsic nature of the obtained band alignment and to understand the underlying principles. Valence band offsets up to 1.6 eV are observed. The large variation of valence band maximum energy can be explained by the different orbital contributions to the density of states in the valence band. The framework provided by this work enables the general understanding and prediction of energy band alignment at oxide interfaces, and furthermore the tailoring of energy level matching for charge transfer in functional oxides. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

International audience; Centimeter-sized single crystals of Ba2LaFeNb4O15 were grown from a high-temperature solution by using LiBO2 flux and a sealed platinum assembly. The obtained single crystals display the same physical properties as their ceramic counterparts. A frequency-dependent dielectric permittivity maximum was found (Tm = 100 K at 5 kHz), which indicates relaxor behavior. Magnetic susceptibility measurements revealed purely paramagnetic behavior between 10 and 350 K. X-ray diffraction measurements of Ba2LaFeNb4O15 single crystals revealed an incommensurate structure at room temperature with a bidimensional modulation characterized by vectors q1 = (α, α, 1/2) and q2 = (α, -α, 1/2) with α = 0.295(1). This crystal growth method offers a promising elaboration route to centimeter-sized crystals of niobate-based compounds, which may not be grown from the pure liquid phase, especially those with a tetragonal tungsten bronze (TTB) structure.

We have grown by flux method centimeter-sized single crystals from pseudo-hexanary Li2O–Na2O–K2O–Nb2O5–Ta2O5–Sb2O3 system. Based on chemical analysis, crystals of compositions (Li0.023Na0.583K0.394)(Nb0.925Ta0.037Sb0.038)O3 and (Li0.034Na0.609K0.357)(Nb0.896Ta0.047Sb0.057)O3 were characterized by X-rays diffraction which revealed a tetragonal structure. The dielectric analysis confirmed that the ferroelectric behavior of these crystals is very sensitive to little changes in composition as previously observed on ceramics. Such high flexibility of the ferroelectric properties in crystals opens the way towards improved understanding of the relations between structure and polarization in solid solutions which may be an alternative to the lead-based materials.

International audience; Orientation and temperature dependence of dielectric and electromechanical properties of Mn-doped (Li,Na,K)(Nb,Ta)O3 single crystals were investigated. Samples exhibited very low dielectric losses, with tanδ between 0.03 and 0.05 over a broad temperature range between room temperature and 480 °C. Influences of the crystallographic structure and external electric field on polarization and strain parameters are discussed. The orientation-dependent electrical properties were ascribed to the anisotropic polarization rotation in the crystals. Higher maximum polarization, coercive field, and negative strain were achieved when the electric field was oriented along one of the spontaneous polarization directions. The highest maximum unipolar strain of 0.42% (at 3 kV/mm) and the normalized strain d 33 * of 1391 pm/V were obtained in the [001]PC-oriented sample at 100 °C, which was much higher than the values obtained for the [110]PC-oriented sample. Further insight of the phase transition behavior is given by comparing the temperature-dependence of the small- and large-signal dielectric and piezoelectric properties. The observed changes are rationalized by the different increase rates of the dielectric permittivity and piezoelectric coefficients with temperature.

A series of centimeter-sized lead-free piezoelectric Li+- and Ta5+-modified (Na,K)NbO3 single crystals with an ABO3 perovskite structure was successfully grown by the top-seeded solution growth method. Effective segregation of the elements was considered in order to further develop the growth of Li+- and Ta5+-modified (Na,K)NbO3 single crystals. The crystallographic structure and electrical behaviour along the [001]PC orientation were studied. X-ray diffraction, Raman spectroscopy and dielectric studies reveal that the increase in Ta5+ content reduces the orthorhombic–tetragonal phase transition. Bipolar and unipolar strain curves were investigated at 2 kV mm−1. The highest bipolar and unipolar strains associated with a large-signal piezoelectric constant of 368 pm V−1 were achieved for Li0.02Na0.626K0.354Nb0.808Ta0.192O3. The asymmetric bipolar strain curves were observed and related to the existence of internal bias fields induced by defect dipoles, which were created during the crystal growth process.

International audience; All-oxide composites are increasingly investigated in the field of microelectronics and telecommunications because of their multifunctionalities. Advanced control of interfaces is required to fully tune the macroscopic properties of the composites. Processing ferroelectric Ba0.65Sr0.35TiO3 (BST65/35)-TiO2 composites by Spark Plasma Sintering (SPS), we show that the SPS electric current promotes an interphase growth whose thickness depends on the BST/TiO2 stacking and on the current direction. In all cases, the interface is composed of Ba1.14(2) Ti8O16-δ a highly defective, but stable phase which can be used to tune the overall dielectric properties of the composites.

International audience; The production of BaTi1–yZryO3 (0 ≤ y ≤ 1, BTZ) nanocrystals is known to be challenging due the low reactivity of zirconium precursors. Here we have successfully studied the impact of zirconium on the BTZ particle formation in sub- and supercritical fluid conditions along the entire solid solution. In situ synchrotron wide angle X-ray scattering (WAXS) analyses were conducted in batch at 150 and 400 °C to follow, in real time, the BTZ crystallite synthesis. This approach revealed the complexity behind the nucleation and growth mechanisms of ABO3 nanocrystals, especially toward high zirconium content (more than 50 atomic %). This type of substitution induces, among other things, microstrain within the structure. Moreover, for the cases of BaTi0.4Zr0.6O3 and BaTi0.2Zr0.8O3, the experiments showed the apparition of two crystallite size populations. In the BaTi0.4Zr0.6O3 case, at 400 °C, these two size populations merged into a single one after at least 8 min; in contrast to what was observed for the case of BaTi0.2Zr0.8O3. This is a manifestation of how with increasing the zirconium content the particles become more refractory and in these cases the temperature is not high enough to enable their ripening. It is important to note that this behavior was not observed for particles produced at 400 °C using a flow synthesis method, with a residence time of only 50 s. There, the particles presented a single size population close to the one obtained after 8 min in batch. This thus suggests that for batch syntheses a longer time is required to achieve a similar product quality to the one obtained with a flow process.

International audience; A major challenge in the development of efficient magnetoelectric nanocomposites is the adequate control of the interfaces, in order to avoid the formation of undesirable interphases and to ensure an optimal strain mediated coupling. In this work we used a combination of low-cost impregnation and electrodeposition processes within the ordered nanochannels of a porous anodic alumina template, to elaborate CoFe2O4–PbZr0.52Ti0.48O3 core/shell nanocable arrays with high interfacial areas between the ferroelectric and the magnetic phases. We have shown in this way that the thermal annealing steps required for phase's crystallization and oxidation of metallic CoFe2 into spinel CoFe2O4 are critical with respect to interdiffusion phenomena through the interfaces. The impact of the processing temperature on the morphological and structural features of the nanocables was then discussed. We demonstrated that an optimization of process variables in the synthesis of CoFe2O4–PbZr0.52Ti0.48O3 nanocables allows a significant improvement in their microstructure and ensures the chemical integrity of the two-phase materials, an important concern when seeking for enhanced multiferroic properties.

Ba 2 NdFeNb 4 O 15 tetragonal tungsten bronze (TTB)/BaFe 12 O 19 (BaM) hexaferrite bilayers have been grown by RF magnetron sputtering on Pt/TiO 2 /SiO 2 /Si (PtS) substrates. The BaM layer is textured along (0 0 1) while the TTB layer is multioriented regardless of the PtS or BaM/PtS substrate. Dielectric properties of TTB films are similar to those of bulk, i.e. , e ∼ 150 and a magnetic hysteresis loop is obtained from TTB/BaM bilayers, thanks to the BaM component. This demonstrates the possibility of transferring to 2 dimensional structures the composite multiferroic system TTB/BaM previously identified in 3 dimensional bulk ceramics.

International audience; The influence of magnetic field on the dielectric properties of several Mn1-x(Mg,Zn)xWO4 multiferroic ceramics is investigated, and confirms an additional phase transition below the ferroelectric Curie temperature previously detected in this system in zero magnetic field. In highly doped samples an original plateau of the capacitance is observed between T2 and this additional transition. Pyroelectric measurements in zero magnetic field for a Mn0.85Mg0.15WO4 ceramic confirm the existence of this unexpected low temperature phase transition. The magnetic or structural origin of this transition is discussed.

Usually, resonating cantilevers come from silicon technology and are generally activated with pure bending mode. In this work, we suggest to combine high sensitive cantilever structure with full self-actuated piezoelectric thick-film for high electrical-mechanical coupling. This cantilever is realized through the screen-printing deposition associated with a sacrificial layer. The beam part of the cantilever is composed of a PZT layer between two gold electrodes. Optimum performances of ferroelectric ceramics strongly rely on their composition and microstructure which imply generally the use of mechanical pressure and very high sintering temperature. These conditions are not compatible with the screen-printing process. As already shown with ceramics, addition of eutectic composition Li2CO3-CuO-Bi2O3 or borosilicate glass-frit to PZT powder and application of isostatic pressure, the sintering temperature of cantilevers may be decreased. Screen-printed piezoelectric samples are thus fired at 850°C, 900°C and 950°C. SEM analyses show the densification of the microstructure while increasing the sintering temperature. Electrical and mechanical characterizations are then achieved after poling the self-actuated cantilevers of different dimensions. In addition to the bending mode observed with a vibrometer, we pointed out other resonances related to the piezoelectric properties of the PZT cantilevers. Indeed, the three first orders of “in-plane” 31-longitudinal vibration mode and the first order “out-of-plane” 33-thickness resonance mode are revealed. The resonance frequencies deduced from analytical modeling are linked to the cantilever dimensions and thus confirm the experimental results. The piezoelectric parameters of PZT cantilevers approach those of ceramics. Quality factors of 300 to 400 associated to the unusual 31-longitudinal mode make self-actuated screen-printed PZT cantilevers good candidates for detection in liquid and gaseous media.

International audience; New ferroelectric ceramics of ABO3 perovskite type were synthesized in the Ba(Ni1/3Nb2/3)xTi1−xO3 (0 ≤ x ≤ 0.2) system by solid state reaction technique. The effect of the replacement of titanium by nickel and niobium in the B site on structural and dielectric properties was investigated. For this, we have use X-ray diffraction and dielectric characterizations. Samples in cubic phase were generally obtained with a sintering temperature of 1350 °C. Dielectric measurements revealed that the very small change in composition by the substitution of titanium by nickel and niobium in the BaTiO3 has a strong influence on the dielectric proprieties: fast decreasing of TC or Tm (for BaTiO3 TC = 340 K, for x = 0.06, TC = 270 K and for x = 0.16, Tm = 150 K). The evolution from classic ferroelectric behavior for (0 ≤ x x ≤ 0.2) is emphasized.

Investigation of crystal structure, dielectric, magnetic and local ferroelectric properties of the diamagnetically substituted Bi 1− x A x FeO 3− x /2 (A=Ca, Sr, Pb, Ba; x =0.2, 0.3) polycrystalline samples has been carried out. It has been shown that the heterovalent A 2+ substitution result in the formation of oxygen vacancies in the host lattice. The solid solutions have been found to possess a rhombohedrally distorted perovskite structure described by the space group R 3 c . Piezoresponse force microscopy has revealed signs of existence of the ferroelectric polarization in the samples at room temperature. Magnetization measurements have shown that the magnetic state of these compounds is determined by the ionic radius of the substituting elements. A-site substitution with the biggest ionic radius ions has been found to suppress the spiral spin structure of BiFeO 3 giving rise to the appearance of room-temperature weak ferromagnetism.