Enhanced Photoconductivity at Dislocations in SrTiO₃

Dislocations are 1D crystallographic line defects and are usually seen as detrimental to the functional properties of classic semiconductors. It is shown here that this not necessarily accounts for oxide semiconductors in which dislocations are capable of boosting the photoconductivity. Strontium titanate single crystals are controllably deformed to generate a high density of ordered dislocations of two slip systems possessing different mesoscopic arrangements. For both slip systems, nanoscale conductive atomic force microscope investigations reveal a strong enhancement of the photoconductivity around the dislocation cores. Macroscopic in-plane measurements indicate that the two dislocation systems result in different global photoconductivity behavior despite the similar local enhancement. Depending on the arrangement, the global photoresponse can be increased by orders of magnitude. Additionally, indications for a bulk photovoltaic effect enabled by dislocation-surrounding strain fields are observed for the first time. This proves that dislocations in oxide semiconductors can be of large interest for tailoring photoelectric functionalities. Direct evidence that electronic transport is confined to the dislocation core points to a new emerging research field

Requirements for the transfer of lead-free piezoceramics into application

The recent review for the Restriction of Hazardous Substances Directive (RoHS) by the expert committee, appointed by the European Union, stated that the replacement of PZT “… may be scientifically and technologically practical to a certain degree …”, although replacement “… is scientifically and technically still impractical in the majority of applications.” Thus, two decades of sustained research and development may be approaching fruition, at first limited to a minority of applications. Therefore, it is of paramount importance to assess the viability of lead-free piezoceramics over a broad range of application-relevant properties. These are identified and discussed in turn: 1. Cost, 2. Reproducibility, 3. Mechanical and Thermal Properties, 4. Electrical Conductivity, and 5. Lifetime. It is suggested that the worldwide efforts into the development of lead-free piezoceramics now require a broader perspective to bring the work to the next stage of development by supporting implementation into real devices. Guidelines about pertinent research requirements into a wide range of secondary properties, measurement techniques, and salient literature are provided

Microstructure and conductivity of blacklight‐sintered TiO₂, YSZ, and Li₀.₃₃La₀.₅₇TiO₃

Rapid densification of ceramics has been realized and its merits were demonstrated through multiple approaches out of which UHS and flash sintering attract recent attention. So far, however, scalability remains difficult. A rise in throughput and scalability is enabled by the introduction of blacklight sintering powered by novel light source technology. Intense illumination with photon energy above the bandgap (blacklight) allows high absorption efficiency and, hence, very rapid, contactless heating for all ceramics. While heating the ceramic directly with light without any furnace promises scalability, it simultaneously offers highly accurate process control. For the technology transfer to industry, attainable material quality needs to be assured. Here, we demonstrate the excellent microstructure quality of blacklight‐sintered ceramics observed with ultrahigh voltage electron microscopy revealing an option to tune nanoporosity. Moreover, we confirm that electronic, electron, oxygen, and lithium‐ion conductivities are equal to conventionally sintered ceramics. This gives the prospect of transmitting the merits of rapid densification to the scale of industrial kilns

Room‐temperature dislocation plasticity in SrTiO₃ tuned by defect chemistry

Dislocations have been identified to modify both the functional and mechanical properties of some ceramic materials. Succinct control of dislocation-based plasticity in ceramics will also demand knowledge about dislocation interaction with point defects. Here, we propose an experimental approach to modulate the dislocation-based plasticity in single-crystal SrTiO₃ based on the concept of defect chemistry engineering, for example, by increasing the oxygen vacancy concentration via reduction treatment. With nanoindentation and bulk compression tests, we find that the dislocation-governed plasticity is significantly modified at the nano-/microscale, compared to the bulk scale. The increase in oxygen vacancy concentration after reduction treatment was assessed by impedance spectroscopy and is found to favor dislocation nucleation but impede dislocation motion as rationalized by the nanoindentation pop-in and nanoindentation creep tests

Dislocation‐tuned electrical conductivity in solid electrolytes (9YSZ): A micro‐mechanical approach

Tailoring the electrical conductivity of functional ceramics by introducing dislocations is a comparatively recent research focus, and its merits were demonstrated through mechanical means. Especially bulk deformation at high temperatures is suggested to be a promising method to introduce a high dislocation density. So far, however, controlling dislocation generation and their annihilation remains difficult. Although deforming ceramics generate dislocations on multiple length scales, dislocation annihilation at the same time appears to be the bottleneck to use the full potential of dislocations‐tailoring the electrical conductivity. Here, we demonstrate the control over these aspects using a micromechanical approach on yttria‐stabilized zirconia ‐ YSZ. Targeted indentation well below the dislocation annihilation temperature resulted in extremely dense dislocation networks, visualized by chemical etching and electron channeling contrast imaging. Microcontact‐impedance measurements helped evaluate the electrical response of operating individual slip systems. A significant conductivity enhancement is revealed in dislocation‐rich regions compared to pristine ones in fully stabilized YSZ. This enhancement is mainly attributed to oxygen ionic conductivity. Thus, the possibility of increasing the conductivity is illustrated and provides a prospect to transfer the merits of dislocation‐tuned electrical conductivity to solid oxygen electrolytes

High temperature creep‐mediated functionality in polycrystalline barium titanate

Dislocations in oxides can be described as charged line defects and means for one-dimensional doping, which can tune electrical and thermal properties. Furthermore, theoretically it was shown that dislocations can pin ferroelectric domain walls. Broader application of this concept hinges on the development of a methodology to avail this approach to polycrystalline ceramics. To this end, we use different creep mechanisms as a method to introduce multidimensional defects and quantify structural changes. A deformation map for fine-grained barium titanate is provided and the influences of the defects and creep regimes are correlated in this first study to modifications of electrical conductivity, dielectric, ferroelectric, and piezoelectric properties. A plastic deformation of 1.29% resulted in an increase in the Curie temperature by 5°C and a decrease in electromechanical strain by 30%, pointing toward electromechanical hardening by dislocations

Oxide ion transport in donor doped lead zirconate titanate

Zsfassung in dt. SpracheDegradations- und Ermüdungsmechanismen in Pb([Zr tief x][Ti tief 1-x])[O tief 3] (PZT) werden oft mit Prozes-sen, in denen Sauerstoffleerstellen eine Rolle spielen, in Verbindung gebracht. Um Informationen über die Defektchemie von PZT zu erhalten, wurden Isotopenaustauschversuche mit [hoch 18][O tief 2] durchgeführt und mit time-of-flight(ToF) Sekundärionenmassenspektrometrie (SIMS) untersucht.Damit wurden Hinweise auf eine Raumladungsschicht in der Nähe der Oberfläche ermittelt. In dieser Schicht sind Sauerstoffleerstellen angereichert, wodurch sich kastenförmige Konzentrationstiefenprofile des [hoch 18]O-Isotops entwickeln. Ab 650°C verändern sich die Profile allerdings, was durch eine Abreichung der Leerstellen in der Nähe der Oberfläche beschrieben werden kann. Es wird also von einer Änderung der Raumladung bei dieser Temperatur ausgegangen. Es hat sich ebenfalls herausgestellt, dass sich die Profile schon bei kleinsten Modifikationen der Oberfläche verändern, wie beispielsweise durch Ätzen oder Tempern bei höheren Temperaturen. Die Vorbehandlung des PZT-Materials hat generell den größten Einfluss auf die Isotopenprofile. Durch Anlegen von Spannung wird an der Kathode und im Oberflächenbereich zwischen den Elektroden der Einbau des Isotops erhöht. Auch wurde erhöhte Korngrenzmigration festgestellt. Es konnte allerdings ermittelt werden, dass der Beitrag der ionische Leitfähigkeit vergleichsweise klein ist bei Temperaturen um die 400°C. PZT zeigt bei Temperaturen um die 650°C schnelle Korngrenzdiffusion.Dies konnte anhand von [hoch 18]O-Intensitätsbildern von Tiefenprofilen bestimmt werden. Besonders bei donordotiertem PZT, das mit Ag/Pd-Innenelektroden gesintert wurde, konnte im Querschliff hohe [hoch 18]O-Intensität in Korngrenzen als Folge schneller Korngrenzdiffusion gemessen werden. Dies ist ein Hinweis darauf, dass ein gewisser Ag-Gehalt die Korngrenzeigenschaften beeinflusst. Zu Vergleichszwecken wurden Isotopenauschtauschsexperimente mit (La) donordotieren Bariumtitanat (Mn co-Dotierung) durchgeführt. Auch hier konnten Hinweise auf eine ähnliche Raumladungszone an der Oberfläche gefunden werden, da sich zu den PZT-Ergebnissen vergleichbare kastenförmige Tiefenprofile im Temperaturbereich von 600°C bis 900°C ergaben. Ebenfalls konnte auch für das Bariumtitanat erhöhte Korngrenzdiffusion ermittelt werden.Degradation and fatigue mechanisms in Pb([Zr tief x][Ti tief 1-x])[O tief 3] (PZT) are often attributed to oxygen vacancy related processes. To obtain further information on oxygen vacancies in donor doped PZT material experiments on the oxygen tracer diffusion in PZT were conducted and investigated by time-of-flight(ToF)-secondary ion mass spectrometry (SIMS). Evidence for a space charge layer near the surface is provided. In this layer an enrichment of oxygen vacancies is found at temperatures up to about 600°C resulting in a box-shaped tracer diffusion profile. At 650°C and above, however, the profile shape is modified and can only be explained by a depletion of oxygen vacancies.Accordingly, a change in surface charge can be assumed at such temperatures.Furthermore, the tracer diffusion profiles are very sensitive to surface modifications due to etching and annealing at higher temperatures. The strongtest effects, however, result from the sintering procedure of the PZT material. An application of an external field during the tracer experiments leads to an increased incorporation of oxygen at the cathode and the free surface area between the electrodes. Additionally, evidence for an enhanced grain boundary tracer migration due to the field was found.However, estimates of the ionic particle flux led to the conclusion that electronic conductivity dominates the PZT conduction properties at temperatures around 400°C.PZT also shows fast grain boundary diffusion at temperatures around 650°C. [hoch 18]O intensity images from depth profiles clearly coincide with maps of grain boundaries. For donor doped PZT sintered with Ag/Pd-inner electrodes high [hoch 18]O intensity could even be observed in the cross section images as a result of very fast tracer grain boundary diffusion. Therefore, it can be assumed that Ag doping influences the grain boundary properties. For the sake of comparison oxygen tracer diffusion experiments were conducted on (La) donor and (Mn) acceptor co-doped barium titante. Also in this case evidence for a near-surface space charge layer could be found. Additionally, the material showed fast tracer grain boundary diffusion in the temperature range from 750°C to 900°C.16

Sinterability of sodium bismuth titanate-based electroceramics at low temperatures

The demand for robust multilayer ceramic capacitors with high-temperature and high-power capabilities is surging. Cost-effective production is a key challenge, often linked to precious metal electrodes like Pt, Pd, and Ag in multilayer ceramic capacitors (MLCCs). To address this, cost-efficient base metal electrodes such as Cu and Ni are sought. Promising lead-free options are Na0.5Bi0.5TiO3 (NBT)-based materials. However, their sintering temperatures exceed Cu's melting point (1085 °C), necessitating sintering aids for compatibility. We investigated several low-temperature sintering aids (Li2O, Na2O, Bi2O3, B2O3, ZnO, and V2O5) to lower the sintering temperature of Na0.5Bi0.5TiO3-BaTiO3-CaZrO3-BiAlO3 solid solutions. Results reveal NBT-based materials' sensitivity to acceptor dopant incorporation from sintering aids, impacting electrical properties. Yet, we successfully reduced the sintering temperature to 975 °C, suitable for Cu co-sintering, with minimal adverse effects on capacitor properties using well-tuned sintering aids