Helmut Ehrenberg

Giant strain in lead-free piezoceramics Bi0.5Na0.5TiO3–BaTiO3–K0.5Na0.5NbO3 system

Piezoelectric actuators convert electrical into mechanical energy and are implemented for many large-scale applications such as piezoinjectors and ink jet printers. The performance of these devices is governed by the electric-field-induced strain. Here, the authors describe the development of a class of lead-free (0.94−x)Bi0.5Na0.5TiO3–0.06BaTiO3–xK0.5Na0.5NbO3 ceramics. These can deliver a giant strain (0.45%) under both unipolar and bipolar field loadings, which is even higher than the strain obtained with established ferroelectric Pb(Zr,Ti)O3 ceramics and is comparable to strains obtained in Pb-based antiferroelectrics.Piezoelectric actuators convert electrical into mechanical energy and are implemented for many large-scale applications such as piezoinjectors and ink jet printers. The performance of these devices is governed by the electric-field-induced strain. Here, the authors describe the development of a class of lead-free (0.94-x)Bi0.5Na0.5TiO3-0.06BaTiO(3)-xK(0.5)Na(0.5)NbO(3) ceramics. These can deliver a giant strain (0.45%) under both unipolar and bipolar field loadings, which is even higher than the strain obtained with established ferroelectric Pb(Zr,Ti)O-3 ceramics and is comparable to strains obtained in Pb-based antiferroelectrics.

Lead-free piezoceramics with giant strain in the system Bi0.5Na0.5TiO3–BaTiO3–K0.5Na0.5NbO3. II. Temperature dependent properties

The temperature dependence of the dielectric and ferroelectric properties of lead-free piezoceramics of the composition (1−x−y)Bi0.5Na0.5TiO3–xBaTiO3–yK0.5Na0.5NbO3 (0.05⩽x⩽0.07, 0.01⩽y⩽0.03) was investigated. Measurements of the polarization and strain hystereses indicate a transition to predominantly antiferroelectric order when heating from room temperature to 150°C, while for 150<T<200°C both remnant polarization and coercive field increase. Frequency-dependent susceptibility measurements show that the transition is relaxorlike. For some samples, the transition temperature Td is high enough to allow mostly ferroelectric ordering at room temperature. These samples show a drastic increase of the usable strain under an external electric field just after the transition into the antiferroelectric state at high temperatures. For the other samples, Td is so low that they display significant antiferroelectric ordering already at room temperature. In these samples, the usable strain is relatively stable over a...

The synchrotron powder diffractometer at beamline B2 at HASYLAB/DESY: status and capabilities

The synchrotron powder diffraction beamline B2 at HASYLAB/DESY is described. The beamline is capable of high-resolution powder diffraction as well as time-resolved studies and offers several sophisticated ancillary equipments for special applications. A newly developed image-plate system allows for kinetic studies with good resolution in the minutes range. Numerous sample environments allow for various standard applications including structure solution, kinetic studies and in situ observations under flexible and well defined conditions. Representative examples are shown for these setups, which are also supported for experiments of external users.

Field-induced phase transition in Bi1/2Na1/2TiO3- based lead-free piezoelectric ceramics

The origin of the electric field-induced strain in the polycrystalline ceramic 0.92Bi1/2Na1/2TiO3–0.06BaTiO3–0.02K1/2Na1/2NbO3 was investigated using in situ high-resolution X-ray and neutron diffraction techniques. The initially existing tetragonal phase with pseudocubic lattice undergoes a reversible phase transition to a significantly distorted rhombohedral phase under electric field, accompanied by a change in the oxygen octahedral tilting from a0a0c+ to a−a−a− and in the tilting angle. The polarization values for the tetragonal and rhombohedral phases were calculated based on the structural information from Rietveld refinements. The large recoverable electric field-induced strain is a consequence of a reversible electric field-induced phase transition from an almost nonpolar tetragonal phase to a ferroelectrically active rhombohedral phase.

Iron-oxygen vacancy defect centers in PbTiO3 : Newman superposition model analysis and density functional calculations

The iron(III) center in ferroelectric PbTiO3 together with an oxygen vacancy forms a charged defect associate, oriented along the crystallographic c-axis. Its microscopic structure has been analyzed in detail comparing results from a semi-empirical Newman superposition model analysis based on finestructure data and from calculations using density functional theory. Both methods give evidence for a substitution of Fe3+ for Ti4+ as an acceptor center. The position of the iron ion in the ferroelectric phase is found to be similar to the B-site in the paraelectric phase. Partial charge compensation is locally provided by a directly coordinated oxygen vacancy. Using high-resolution synchrotron powder diffraction, it was verified that lead titanate remains tetragonal down to 12 K, exhibiting a c/a-ratio of 1.0721.

Changes in the crystal and electronic structure of LiCoO2 and LiNiO2 upon Li intercalation and de-intercalation

Li(x)CoO(2) and Li(x)NiO(2) (0.5 < x < 1) are used as prototype cathode materials in lithium ion batteries. Both systems show degradation and fatigue when used as cathode material during electrochemical cycling. In order to analyze the change of the structure and the electronic structure of Li(x)CoO(2) and Li(x)NiO(2) as a function of Li content x in detail, we have performed X-ray diffraction studies, photoelectron spectroscopy (PES) investigations and band structure calculations for a series of compounds Li(x)(Co,Ni)O(2) (0 < x < or = 1). The calculated density of states (DOS) are weighted by theoretical photoionization cross sections and compared with the DOS gained from the PES experiments. Consistently, the experimental and calculated DOS show a broadening of the Co/Ni 3d states upon lithium de-intercalation. The change of the shape of the experimental PES curves with decreasing lithium concentration can be interpreted from the calculated partial DOS as an increasing energetic overlap of the Co/Ni 3d and O 2p states and a change in the orbital overlap of Co/Ni and O wave functions.

Thermal Stability of LiCoPO4 Cathodes

The thermal stability of LiCoPO 4 cathodes charged to different lithium contents was studied by synchrotron diffraction and differential thermal analysis. Both olivine-like phases Li Z COPO 4 (z = 0.6) and CoPO 4 appearing during the delithiation of LiCoPO 4 are unstable upon heating, and decompose readily in the range 100-200°C. The decomposition of lithium-poor phases leads to gas evolution and the crystallization of CO 2 P 2 O 7 . The role of carbon present in the electrochemically delithiated samples is discussed. The significantly lower stability of charged LiCoPO 4 in comparison with LiFePO 4 is a serious challenge for the application of this material in rechargeable Li-ion batteries.

Short-range order of Zr62−xTixAl10Cu20Ni8 bulk metallic glasses

Abstract The short-range order and crystallization behavior of slowly cooled Zr 62− x Ti x Al 10 Cu 20 Ni 8 bulk metallic glasses have been investigated in terms of the atomic pair correlation function as a function of Ti content x (2≤ x ≤7.5). The structural parameters point to the presence of chemical short-range order in these bulk glasses. An enhanced local excess free volume around the Ti atoms is concluded from density measurements. The first stage of crystallization in Zr 62− x Ti x Al 10 Cu 20 Ni 8 bulk glasses is related to changes in the medium-range order while the first neighborhood is retained. The atomic pair correlation functions of the first crystallization products are similar for all titanium contents. There is no indication of any special atomic arrangement for the particular alloy forming quasicrystals upon heating ( x =3). In case of Zr 54.5 Ti 7.5 Al 10 Cu 20 Ni 8 an ultrafine microstructure consisting of clusters of 2 nm in size is formed as the first step of crystallization.

Na3V2(PO4)3/C composite as the intercalation-type anode material for sodium-ion batteries with superior rate capability and long-cycle life

A Na3V2(PO4)3/C (NVP/C) composite is successfully synthesized by the sol–gel method and examined as the anode material for sodium-ion batteries (SIBs) by means of galvanostatic charge–discharge profiles, cyclic voltammograms, rate performance and cyclic voltammetry comprehensively. The NVP/C electrode delivers a reversible capacity of about 170 mA h g−1 between 3.0 and 0.01 V at a current density of 20 mA g−1 corresponding to three sodium ions insertion/extraction processes. Besides the voltage plateau at 1.57 V, another novel working platform at around 0.28 V is found for the first time in both charging and discharging profiles, possibly owing to the further reduction of vanadium. NVP/C exhibits an excellent rate capability and long-cycle stability with a capacity retention of 62% after 3000 cycles at a high charge rate of 10 C (2 A g−1). Moreover, the intercalation-type Na-ions storage mechanism is proposed on the basis of ex situ X-ray diffraction and high-resolution transmission electron microscopy. Our findings reveal that the NVP/C sample is a promising anode material for SIBs due to its superior rate capability and long cycle life.

Synthesis, Characterization, and Comparison of Electrochemical Properties of LiM0.5Mn1.5O4 (M = Fe , Co, Ni) at Different Temperatures

LiM 0.5 Mn 1.5 O 4 (M = Fe, Co, Ni) normal spinel oxides were prepared by a citric acid assisted Pechini synthesis with different thermal treatments and compared with respect to their electrochemical performance as cathodes in lithium-ion batteries. Characterization methods include X-ray diffraction, neutron diffraction, inductively coupled plasma optical emission spectroscopy analysis, and scanning electron microscopy. While LiM 0.5 Mn 1.5 O 4 samples crystallize for M = Fe and Co with the 3d cation-disordered cubic spinel-like structure (Fd3m space group), the 600°C annealed LiM 0.5 Mn 1.5 O 4 shows a partially ordered structure (belonging to the P4 3 32 space group). The absolute discharge capacity is slightly higher for the Ni-doped samples in comparison with the Co-and Fe-doped spinels. 1000°C annealed samples show an improved cyclability in comparison with the 600°C annealed samples. At elevated temperatures, Co- and Fe-doped samples show much faster degradation in comparison with the Ni-doped sample. The responsible mechanisms are discussed.