Ermüdung von Pb(Zr,Ti)O3 für unterschiedliche elektrische Belastungsformen

In der vorliegenden Arbeit wurden die Auswirkungen von unipolarer, sesquipolarer und bipolarer Zyklierung auf Pb(Zr,Ti)O3 (PZT) Keramiken untersucht und Modelle zu ihrer Beschreibung und Erklärung erarbeitet. Im Fall der bipolaren Ermüdung wurden unterschiedliche Elektrodenmateralien und Dotierungen untersucht. Zusätzlich wurde der Einfluss einer Gleichspannungsbelastung auf die Materialparameter gemessen. Die Gleichfeldbelastung und die Ermüdung mit unipolaren elektrischen Feldern führen zur Entwicklung von asymmetrischen und verschobenen ferroelektrischen Hysteresen. Diese Veränderungen können durch Trennung von Ladungsträgern im Material erklärt werden, die ein Offsetfeld und eine Offsetpolarisation aufbauen. Bei der bipolaren Ermüdung treten mehrere Ermüdungsmechanismen gleichzeitig auf. Ist der elektrische Kontakt zwischen Probe und Elektrode inhomogen oder sind die Korngrenzen mechanische Schwachpunkte (Mikrorissbildung), so tritt eine Veränderung der Probe in Bereichen nahe den Elektroden auf. Diese Bereiche heben sich farblich vom übrigen Probenvolumen ab, und die Mikrostruktur erscheint aufgeschmolzen. Als Ursache hierfür wird die Zersetzung der Probe bzw. die Bildung von PbO durch elektrische Überschläge und Temperaturerhöhungen in der Probe angenommen. Bei einem guten elektrischen Kontakt und keiner oder wenig Mikrorissbildung ermüden die Proben homogen im Volumen, und es entstehen durch die Ermüdung Raumladungen, die Einfluss auf die Polarisationsorientierung haben und Domänen bei den angelegten Feldern festhalten können. PFM-Messungen (Piezoresponse Force Microscopy) zeigen, dass es sich hierbei um positive Raumladungen handelt. Einen weiteren Einfluss auf die bipolare Ermüdung hat das mittels PFM-Messungen bestimmte lokale Schaltverhalten der Probe, welches auch durch die bipolare Ermüdung nicht verändert wird. Ist die Verteilung der Koerzitivspannungen schmal, so ermüdet die Probe weniger als bei einer breiten Verteilung. Bei letzterer treten mehr mechanische Spannungen zwischen Körner auf, weil die Schaltspannungen von benachbarten Körnern unterschiedlich sein können. Zusätzlich wurde neben dem rein unipolaren oder bipolaren Fall wurden auch Ermüdungsuntersuchungen mit sesquipolaren Signalen durchgeführt. Durch das sesquipolare Signal werden die Domänen in der Probe in positiver Feldrichtung komplett und in negativer Feldrichtung teilweise geschaltet. Dieses Signal stellt also ein Zwischenzustand zwischen unipolaren und bipolaren Signalen dar. Die sesquipolare Zyklierung führt zu ähnlichen Veränderungen der Materialparameter wie die unipolare Ermüdung oder die Gleichfeldbelastung, jedoch tritt ein Übergang zu bipolaren Ermüdungsmechanismen ein, wenn die negativen Felder so groß sind, dass signifikante Anteile der Probe in positive und negative Feldrichtung geschaltet werden

Improved fluorescent phytochromes for in situ imaging

Modern biology investigations on phytochromes as near-infrared fluorescent pigments pave the way for the development of new biosensors, as well as for optogenetics and in vivo imaging tools. Recently, near-infrared fluorescent proteins (NIR-FPs) engineered from biliverdin-binding bacteriophytochromes and cyanobacteriochromes, and from phycocyanobilin-binding cyanobacterial phytochromes have become promising probes for fluorescence microscopy and in vivo imaging. However, current NIR-FPs typically suffer from low fluorescence quantum yields and short fluorescence lifetimes. Here, we applied the rational approach of combining mutations known to enhance fluorescence in the cyanobacterial phytochrome Cph1 to derive a series of highly fluorescent variants with fluorescence quantum yield exceeding 15%. These variants were characterised by biochemical and spectroscopic methods, including time-resolved fluorescence spectroscopy. We show that these new NIR-FPs exhibit high fluorescence quantum yields and long fluorescence lifetimes, contributing to their bright fluorescence, and provide fluorescence lifetime imaging measurements in E.coli cells

In situ tracking of the nanoscale expansion of porous carbon electrodes

Electrochemical double layer capacitors (EDLC) are rapidly emerging as a promising energy storage technology offering extremely large power densities. Despite significant experimental progress, nanoscale operation mechanisms of the EDLCs remain poorly understood and it is difficult to separate processes at multiple time and length scales involved in operation including that of double layer charging and ionic mass transport. Here we explore the functionality of EDLC microporous carbon electrodes using a combination of classical electrochemical measurements and scanning probe microscopy based dilatometry, thus separating individual stages in charge/discharge processes based on strain generation. These methods allowed us to observe two distinct modes of EDLC charging, one fast charging of the double layer unassociated with strain, and another much slower mass transport related charging exhibiting significant sample volume changes. These studies open the pathway for the exploration of electrochemical systems with multiple processes involved in the charge and discharge, and investigation of the kinetics of those processes

Room-temperature multiferroic hexagonal LuFeO3_3 films

The crystal and magnetic structures of single-crystalline hexagonal LuFeO$_3$ films have been studied using x-ray, electron and neutron diffraction methods. The polar structure of these films are found to persist up to 1050 K; and the switchability of the polar behavior is observed at room temperature, indicating ferroelectricity. An antiferromagnetic order was shown to occur below 440 K, followed by a spin reorientation resulting in a weak ferromagnetic order below 130 K. This observation of coexisting multiple ferroic orders demonstrates that hexagonal LuFeO$_3$ films are room-temperature multiferroics

Giant thermally-enhanced electrostriction and polar surface phase in La2Mo2O9 oxygen ion conductors

Ferroelectrics possess spontaneous electric polarization at macroscopic scales which nonetheless imposes strict limitations on the material classes. Recent discoveries of untraditional symmetry-breaking phenomena in reduced material dimensions have indicated feasibilities to extend polar properties to broader types of materials, potentially opening up the freedom for designing materials with hybrid functionalities. Here, we report the unusual electromechanical properties of La2Mo2O9 (LAMOX) oxygen ion conductors, systematically investigated at both bulk and surface length levels. We first observed giant electrostriction effects in La2Mo2O9 bulk ceramics that are thermally enhanced in concert with their low-energy oxygen-vacancy hopping dynamics. Moreover, while no clear bulk polarization was detected, the surface phases of LAMOX were found to be manifestly polar, likely originating from the coupling between the intrinsic structural flexibilities with strain gradients (i.e., flexoelectricity) and/or chemical heterogeneities present in the materials. These findings identify La2Mo2O9 as a promising electromechanical material system and suggest that the flexible structural and chemical configurations in ionically active materials could enable fundamentally different venues to accommodate electric polarization.Q.L. and H.W. were supported by the US Department of Energy, Office of Science, Materials Science and Engineering Division. T.L. and Y.L. acknowledge the support of the Australian Research Council (ARC) in the form of Discovery Projects (DP160104780). N.L. was supported by the Eugene P. Wigner Fellowship program at ORNL (No. DE-AC05-00OR22725). The PFM experiments were performed at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility at Oak Ridge National Laboratory (ORNL). The use of Advanced Photon Source was supported by the US DOE, Basic Energy Science under Contract No. DE-AC02-06CH11357

Giant thermally-enhanced electrostriction and polar surface phase in La2Mo2O9 oxygen ion conductors

Ferroelectrics possess spontaneous electric polarization at macroscopic scales which nonetheless imposes strict limitations on the material classes. Recent discoveries of untraditional symmetry-breaking phenomena in reduced material dimensions have indicated feasibilities to extend polar properties to broader types of materials, potentially opening up the freedom for designing materials with hybrid functionalities. Here, we report the unusual electromechanical properties of La2Mo2O9 (LAMOX) oxygen ion conductors, systematically investigated at both bulk and surface length levels. We first observed giant electrostriction effects in La2Mo2O9 bulk ceramics that are thermally enhanced in concert with their low-energy oxygen-vacancy hopping dynamics. Moreover, while no clear bulk polarization was detected, the surface phases of LAMOX were found to be manifestly polar, likely originating from the coupling between the intrinsic structural flexibilities with strain gradients (i.e., flexoelectricity) and/or chemical heterogeneities present in the materials. These findings identify La2Mo2O9 as a promising electromechanical material system and suggest that the flexible structural and chemical configurations in ionically active materials could enable fundamentally different venues to accommodate electric polarization