Roland Schierholz

Nanodomains in morphotropic lead zirconate titanate ceramics : On the origin of the strong piezoelectric effect

The outstanding piezoelectric properties of lead zirconate titanate (PZT) ceramics with compositions close to the morphotropic phase boundary of the quasibinary phase diagram of lead zirconate and lead titanate are still under debate. A combination of ex situ and in situ transmission electron microscopy and high resolution x-ray diffraction revealed that the extrinsic piezoelectric effect in morphotropic PZT is closely connected to the existence of nanodomains. The in situ transmission electron microscopy investigations with applied electric field show that mainly the nanodomains respond to the electric field while the microdomain structure does not change noticeably in our experiments.

On the origin of differential phase contrast at a locally charged and globally charge-compensated domain boundary in a polar-ordered material

Differential phase contrast (DPC) imaging in the scanning transmission electron microscope is applied to the study of a charged antiphase domain boundary in doped bismuth ferrite. A clear differential signal is seen, which matches the expected direction of the electric field at the boundary. However, further study by scanned diffraction reveals that there is no measurable deflection of the primary diffraction disc and hence no significant free E-field in the material. Instead, the DPC signal arises from a modulation of the intensity profile within the primary diffraction disc in the vicinity of the boundary. Simulations are used to show that this modulation arises purely from the local change in crystallographic structure at the boundary and not from an electric field. This study highlights the care that is required when interpreting signals recorded from ferroelectric materials using both DPC imaging and other phase contrast techniques.

Ferroelectric domains in PZT ceramics at the morphotropic phase boundary. Can the splitting of reflections in SAED patterns be used for the distinction of different pseudo-cubic phases?

Tetragonal, rhombohedral and monoclinic ferroelectric domains can all occur in morphotropic PbZr1−xTixO3 (PZT) ceramics. In this article, the influence of these domains on the splitting of reflections in selected area electron diffraction (SAED) patterns along the main pseudo-cubic zone axes is reported. The orientation of the domain wall in a transmission electron microscopy image with respect to the splitting of reflections in the diffraction pattern has to be considered for the interpretation. The distinction of tetragonal and rhombohedral splitting is achieved for a pronounced splitting except for 〈111〉 with the domain wall edge on. As the monoclinic structure contains tetragonal as well as rhombohedral distortions, the distinction of monoclinic symmetry from tetragonal and rhombohedral based only on the splitting of reflections is not possible. Conceivable models of configurations of monoclinic subdomains inside the existing tetragonal or rhombohedral microdomains are derived from group–subgroup relations. Some experimental observations are given, which can only be explained by these models.

STEM?EELS analysis reveals stable high-density He in nanopores of amorphous silicon coatings deposited by magnetron sputtering

A broad interest has been showed recently on the study of nanostructuring of thin films and surfaces obtained by low-energy He plasma treatments and He incorporation via magnetron sputtering. In this paper spatially resolved electron energy-loss spectroscopy in a scanning transmission electron microscope is used to locate and characterize the He state in nanoporous amorphous silicon coatings deposited by magnetron sputtering. A dedicated MATLAB program was developed to quantify the helium density inside individual pores based on the energy position shift or peak intensity of the He K-edge. A good agreement was observed between the high density (∼35-60 at nm(-3)) and pressure (0.3-1.0 GPa) values obtained in nanoscale analysis and the values derived from macroscopic measurements (the composition obtained by proton backscattering spectroscopy coupled to the macroscopic porosity estimated from ellipsometry). This work provides new insights into these novel porous coatings, providing evidence of high-density He located inside the pores and validating the methodology applied here to characterize the formation of pores filled with the helium process gas during deposition. A similar stabilization of condensed He bubbles has been previously demonstrated by high-energy He ion implantation in metals and is newly demonstrated here using a widely employed methodology, magnetron sputtering, for achieving coatings with a high density of homogeneously distributed pores and He storage capacities as high as 21 at%.

Photogrammetry of the three-dimensional shape and texture of a nanoscale particle using scanning electron microscopy and free software

We apply photogrammetry in a scanning electron microscope (SEM) to study the three-dimensional shape and surface texture of a nanoscale LiTi2(PO4)3 particle. We highlight the fact that the technique can be applied non-invasively in any SEM using free software (freeware) and does not require special sample preparation. Three-dimensional information is obtained in the form of a surface mesh, with the texture of the sample stored as a separate two-dimensional image (referred to as a UV Map). The mesh can be used to measure parameters such as surface area, volume, moment of inertia and center of mass, while the UV map can be used to study the surface texture using conventional image processing techniques. We also illustrate the use of 3D printing to visualize the reconstructed model.

Characterization of V-W and Mo-W Mixed Oxide Catalysts for the Selective Oxidation of Acrolein to Acrylic Acid

The ternary Mo-W and V-W mixed oxides are part of the quaternary V-Mo-W catalyst system which is highly active and selective for the important and widely used selective oxidation of acrolein to acrylic acid. Both subsystems are active and selective catalysts for this reaction, varying with composition. The microstructure of the samples has been identified as homogeneously distributed nanocrystalline grains within the shell of the hollow balls that are typical for the spray-drying process. The crystallite sizes differ between the two sample series and depend on the composition as well. The test for the catalytic performance revealed the highest selectivity and yield in acrylic acid production for the V-W catalyst with 70% tungsten content, although tungsten oxide is known to be chemically inactive. The active components vanadium oxide and molybdenum oxide presumably are the most selective for the reaction of acrolein to acrylic acid when their structure and cluster size is stabilized by a stable and inert WO3 matrix. The heating of the samples during the catalytic performance test caused a visible growth of the nanocrystallites which also protruded from the surface of the hollow balls. The structural rearrangements led to an increase of the catalytic performance.

Influence of microstructure and AlPO4 secondary-phase on the ionic conductivity of Li1.3Al0.3Ti1.7(PO4))3 solid-state electrolyte

A ceramic solid-state electrolyte of lithium aluminum titanium phosphate with the composition of Li1.3Al0.3Ti1.7(PO4)3 (LATP) was synthesized by a sol–gel method using a pre-dissolved Ti-source. The annealed LATP powders were subsequently processed in a binder-free dry forming method and sintered under air for the pellet preparation. Phase purity, density, microstructure as well as ionic conductivity of the specimen were characterized. The highest density (2.77g⋅cm−3) with an ionic conductivity of 1.88×10−4 S⋅cm−1 (at 30∘C) was reached at a sintering temperature of 1100∘C. Conductivity of LATP ceramic electrolyte is believed to be significantly affected by both, the AlPO4 secondary phase content and the ceramic electrolyte microstructure. It has been found that with increasing sintering temperature, the secondary-phase content of AlPO4 increased. For sintering temperatures above 1000∘C, the secondary phase has only a minor impact, and the ionic conductivity is predominantly determined by the microstructur...

Analyzing the defect structure of CuO-Doped PZT and KNN piezoelectrics from electron paramagnetic resonance

The defect structure for copper-doped sodium potassium niobate (KNN) ferroelectrics has been analyzed with respect to its defect structure. In particular, the interplay between the mutually compensating dimeric (CuNb-VO) and trimeric (VO-CuNb-VO) defect complexes with 180° and non-180° domain walls has been analyzed and compared to the effects from (Cu - VO)x× dipoles in CuO-doped lead zirconate titanate (PZT). Attempts are made to relate the rearrangement of defect complexes to macroscopic electromechanical properties.

Observing different modes of mobility in lithium titanate spinel by nuclear magnetic resonance

Lithium titanate (LTO) is a spinel material that is able to reversibly intercalate Li ions with minimal changes of the unit cell dimensions (“zero-strain”), making it an attractive choice as anode material for Li ion batteries. However, the nature of the Li transport in this material is still not fully understood. Here, the Li mobility in Li4+xTi5O12 with x = 0 and x ≈ 1.6 is investigated. By regularized inversion of nuclear magnetic resonance (NMR) relaxation and spin alignment echo (SAE) data and supported by DFT simulations, solid-state NMR spectra were analyzed as a function of the respective relaxation times and correlation time constants. A clear correlation between mobility and NMR spectral features was observed, suggesting the presence of local domains with high Li ion mobility. The long-range mobility is limited by the much slower hopping between such domains and appears to be faster for either larger or less ordered local domains. For x ≈ 1.6, spectral features indicate the formation of separate stoichiometric and overlithiated phases rather than a solid solution, yet no segregation into a fast and a slow component was observed in the relaxation and in the SAE dimension, which points towards an entangling of the two phases on a microscopic scale.

Full solution processed mesostructured optical resonators integrating colloidal semiconductor quantum dots.

Herein we show a solution based synthetic pathway to obtain a resonant optical cavity with embedded colloidal semiconductor quantum dots (CSQDs). The optical cavity pore network, surrounded by two dense Bragg mirrors, was designed ad hoc to selectively host the quantum dots, while uncontrolled infiltration of those in the rest of the layered structure was prevented. Coupling between the optical resonant modes of the host and the natural emission of the embedded nanoparticles gives rise to the fine tuning of the luminescence spectrum extracted from the ensemble. Our approach overcomes, without the need for an encapsulating agent and exclusively by solution processing, the difficulties that arise from the low thermal and chemical stability of the CSQDs. It opens the route to achieving precise control over their location and hence over the spectral properties of light emitted by these widely employed nanomaterials. Furthermore, as the porosity of the cavity is preserved after infiltration, the system remains responsive to environmental changes, which provides an added value to the proposed structure.