Die polare Mikrostruktur und das nanoskalige elektromechanische Verhalten von bleifreien piezoelektrischen Keramiken

In der Fülle an Energiewandlern werden piezoelektrische Materialien besonders geschätzt, da sie die Eigenschaft besitzen, mechanische und elektrische Energie linear ineinander umzuwandeln. Die Piezoelektrizität dieser Materialien wird in zahlreichen Hochtechnologie-Anwendungen eingesetzt, unter anderem in Aktoren, Einspritzventilen, Ultraschallwandlern, piezoelektrischen Motoren oder Mikro- und Nanopositionierungssystemen. Um die nachteiligen Effekte des weitverbreiteten Hochleistungswerkstoffs Blei-Zirkonat-Titanat auf die menschliche Gesundheit und die Umwelt zu minimieren, wurden weitreichende Forschungsbestrebungen initiiert, die zu einer Verbesserung bereits bestehender und der Entwicklung gänzlich neuer ungiftiger Piezoelektrika führen sollen. Die vorliegende Arbeit konzentriert sich auf die Untersuchung der herausragenden Eigenschaften von KNN- und BNT-basierten piezoelektrischen Materialien auf submikroskopischen Längenskalen. Die zugrundeliegenden physikalischen Mechanismen wurden mithilfe der Piezoantwort-Kraft-Mikroskopie für drei unterschiedliche Materialklassen untersucht: Erstens, ein Ferroelektrikum, stellvertreten durch 0.95(Na0.49K0.49Li0.02)(Nb0.8Ta0.2)O3-0.05CaZrO3; zweitens, ein Relaxor-Ferroelektrikum, namentlich Bi1/2Na1/2TiO3-0.19Bi1/2K1/2TiO3-yBiZn1/2Ti1/2O3; drittens, ein Kompositmaterial, das sowohl einen nichtergodischen Relaxorphasenanteil Bi1/2Na1/2TiO3-0.07BaTiO3 als auch einen ergodischen Relaxorphasenanteil, Bi1/2Na1/2TiO3-0.06BaTiO3-0.02K0.5Na0.5NbO3, enthält. Dieser ganzheitliche Ansatz beinhaltet auch den Einsatz verschiedener Charakterisierungsmethoden zur Untersuchung des makroskopischen Verhaltens, um einen Vergleich mit den mittels PFM ermittelten submikroskpischen Eigenschaften zu ermöglichen. Die strukturellen, mikrostrukturellen und elektrischen Eigenschaften als auch Ihre gegenseitige Wechselbeziehungen werden als Funktion von Zusammensetzung, Temperatur und elektrischem Feld untersucht. Die durchgeführte Forschungsarbeit basiert damit auf den verknüpften Beobachtungen des materialspezifischen elektromechanischen Verhaltens auf verschiedenen Größenordnungen, vom Submikroskopischem bis hin zum Makroskopischem. Ersteres wird im Rahmen dieser Arbeit durch die Piezoantwort-Kraft-Mikroskopie erreicht, einer hochmodernen Variante der Rasterkraftmikroskopie. Der direkte Vergleich der Eigenschaften auf verschiedenen Längenskalen erlaubt einen tiefen Einblick in die grundlegenden Mechanismen für die teils überragenden Eigenschaften der untersuchten Materialsysteme. Neuartige, fortschrittliche Datenanalysemethoden werden eingeführt und vorgestellt, um eine quantitative Beschreibung der komplexen Domänenstruktur zu erlauben, die in diesen Materialien beobachtet werden. Darüber hinaus wird dadurch eine Differenzierung des lokalen Schaltverhaltens ermöglicht.Within the plethora of energy converters, piezoelectric materials are highly recognized due to their ability of linear, bidirectional translation of mechanical and electric energy. The piezoelectricity of these materials is, as such, employed for countless high-tech applications from piezoelectric actuators, fuel injectors, transducers to piezoelectric motors, micro- and nanopositioning systems, and many others. In order to mitigate the adverse effect of prevalent, high-performance lead zirconate titanate and its compounds on human health and environment, a rapid growth of research efforts has been encouraged in the field of lead-free piezoelectric materials, resulting in an improvement of already existing and the development of a variety of new non-toxic piezoelectric materials. The present work focuses on the investigation of the salient properties of KNN- and BNT-based piezoelectric materials, performed on the sub-micron scale. The underlying, fundamental physical mechanisms were assessed by means of piezoresponse force microscopy for three distinct material classes: First, a ferroelectric, represented by 0.95(Na0.49K0.49Li0.02)(Nb0.8Ta0.2)O3-0.05CaZrO3; second, a relaxor ferroelectric, namely Bi1/2Na1/2TiO3-0.19Bi1/2K1/2TiO3-yBiZn1/2Ti1/2O3; third, a composite material that comprises nonergodic phase fraction of Bi1/2Na1/2TiO3-0.07BaTiO3 along with an ergodic phase Bi1/2Na1/2TiO3-0.06BaTiO3-0.02K0.5Na0.5NbO3. This comprehensive approach includes the investigation of the macroscopic constitutive behavior by means of various electric characterization methods, contrasted against the sub-microscopic investigations performed via piezoresponse force microscopy. Their structural, microstructural, and electrical properties, as well as their mutual interrelation are examined as a function of composition, temperature, and electric field. The conducted research is based on the interrelated observations of the materials’ electromechanical behavior on different length scales, ranging from a submicroscopic to a macroscopic perspective. Within the framework of this work, the former one is achieved by means of piezoresponse force microscopy, a state of-the-art scanning probe microscopy technique. The direct comparison of properties on multiple length scales affords deep insight into the fundamental mechanisms responsible for the enhanced electromechanical behavior of the investigated material systems. Novel, advanced data analysis methods are introduced, aiming at a quantitative description of the complex domain microstructures witnessed in the materials in question. Moreover, a distinction of local polarization switching character is sought

A piezoresponse force microscopy study of CaxBa1-xNb2O6 single crystals

Polar structures of CaxBa1-xNb2O6 (CBN100x) single crystals were investigated using piezoresponse force microscopy. Increasing Ca content results in decreasing domain size and enhancement of the polar disorder. For the composition with x = 0.32 the characteristic domain size is similar to that reported for relaxor Sr0.61Ba0.39Nb2O6 (SBN61). However, decay of an artificial macroscopic domain in CBN32 takes place below the macroscopic transition temperature, contrary to SBN61, where random fields stabilize it above the transition temperature. We can conclude that CBN with 0.26 ≤ x ≤ 0.32 does not display classical relaxor behavior and might be considered as a disordered ferroelectric

Ergodicity reflected in macroscopic and microscopic field-dependent behavior of BNT-based relaxors

The effect of heterovalent B-site doping on ergodicity of relaxor ferroelectrics is studied using (1 - y)(0.81Bi(1/2)Na(1/2)TiO(3)-0.19Bi(1/2)K(1/2)TiO(3))-yBiZn(1/2)Ti(1/2)O(3) (BNT-BKT-BZT) with y - {0.02;0.03;0.04} as a model system. Both the large- and small-signal parameters are studied as a function of electric field. The crystal structure is assessed by means of neutron diffraction in the initial state and after exposure to a high electric field. In order to measure ferroelastic domain textures, diffraction patterns of the poled samples are collected as a function of sample rotation angle. Piezoresponse force microscopy (PFM) is employed to probe the microstructure for polar regions at a nanoscopic scale. For low electric fields E < 2 kV.mm(-1), large- and small-signal constitutive behavior do not change with composition. At high electric fields, however, drastic differences are observed due to a field-induced phase transition into a long-range ordered state. It is hypothesized that increasing BZT content decreases the degree of non-ergodicity; thus, the formation of long-range order is impeded. It is suggested that frozen and dynamic polar nano regions exist to a different degree, depending on the BZT content. This image is supported by PFM measurements. Moreover, PFM measurements suggest that the relaxation mechanism after removal of the bias field is influenced by surface chargesopen2

Macroscopic and Nanoscopic Polarization Relaxation Kinetics in Lead-Free Relaxors Bi1/2Na1/2TiO3-Bi1/2K1/2TiO3-BiZn1/2Ti1/2O3

The stability of the field-induced ferroelectric (FE) state was studied in relaxor lead-free ceramics (1 − y)[0.81Bi1/2Na1/2TiO3–0.19Bi1/2K1/2TiO3]–yBiZn1/2Ti1/2O3 both macroscopically and microscopically. A strong dc electric field results in the formation of a stable FE state with a large piezoelectric coefficient for compositions with a small amount of Bi(Zn1/2Ti1/2)O3, which are in the non-ergodic relaxor state at room temperature. Increasing temperature promotes ergodic relaxor behavior, which is accompanied by the rapid destabilization of the induced state, that is, small relaxation times. Based on the obtained data, it is proposed that the depolarization is a two-step process consisting of an initial realignment of the FE domains and their subsequent breakup into polar nanoregions. The ergodic relaxor behavior is also promoted by increasing the Bi(Zn1/2Ti1/2)O3 content. The related charge disorder results in an enhancement of random electric fields and consequently a stable FE state cannot be induced even at room temperature

Temperature dependence of the local piezoresponse in (K,Na)NbO3-based ceramics with large electromechanical strain

We report on temperature dependence of local electromechanical properties of lead-free ( K,Na)NbO3-based ceramics that macroscopically manifests a large temperature-insensitive strain. Piezoresponse force microscopy reveals the particular role of the orthorhombic-tetragonal phase transition, where a reconstruction of the domain structure occurs and local piezoelectric response shows a peak value. A good quantitative agreement between temperature dependences of the local and previously reported macroscopic small-signal piezoelectric coefficients is observed. An influence of the polymorphic phase transition on polarization switching kinetics was revealed

Nanoscale mapping of heterogeneity of the polarization reversal in lead-free relaxor–ferroelectric ceramic composites

Relaxor/ferroelectric ceramic/ceramic composites have shown to be promising in generating large electromechanical strain at moderate electric fields. Nonetheless, the mechanisms of polarization and strain coupling between grains of different nature in the composites remain unclear. To rationalize the coupling mechanisms we performed advanced piezoresponse force microscopy (PFM) studies of 0.92BNT–0.06BT–0.02KNN/0.93BNT–0.07BT (ergodic/non-ergodic relaxor) composites. PFM is able to distinguish grains of different phases by characteristic domain patterns. Polarization switching has been probed locally, on a sub-grain scale. k-Means clustering analysis applied to arrays of local hysteresis loops reveals variations of polarization switching characteristics between the ergodic and non-ergodic relaxor grains. We report a different set of switching parameters for grains in the composites as opposed to the pure phase samples. Our results confirm ceramic/ceramic composites to be a viable approach to tailor the piezoelectric properties and optimize the macroscopic electromechanical characteristics

Electronic transduction of proton translocations in nanoassembled lamellae of bacteriorhodopsin

An organic field-effect transistor (OFET) integrating bacteriorhodopsin (bR) nanoassembled lamellae is proposed for an in-depth study of the proton translocation processes occurring as the bioelectronic device is exposed either to light or to low concentrations of general anesthetic vapors. The study involves the morphological, structural, electrical, and spectroscopic characterizations necessary to assess the functional properties of the device as well as the bR biological activity once integrated into the functional biointerlayer (FBI)-OFET structure. The electronic transduction of the protons phototranslocation is shown as a current increase in the p-type channel only when the device is irradiated with photons known to trigger the bR photocycle, while Raman spectroscopy reveals an associated C=C isomer switch. Notably, higher energy photons bring the cis isomer back to its trans form, switching the proton pumping process off. The investigation is extended also to the study of a PM FBI-OFET exposed to volatile general anesthetics such as halothane. In this case an electronic current increase is seen upon exposure to low, clinically relevant, concentrations of anesthetics, while no evidence of isomer-switching is observed. The study of the direct electronic detection of the two different externally triggered proton translocation effects allows gathering insights into the underpinning of different bR molecular switching processes. © 2014 American Chemical Society