Effect of (external) electric fields on heterogeneous solid state reactions – Special role of grain boundary diffusion

During ope­ra­tion ceramic materials are of­ten ex­posed to high elec­tri­cal fields, which create a se­cond dri­ving force for mobile com­po­nents in ad­di­tion to the chemical po­ten­tial gra­dient. Due to the ongoing miniaturisation in modern appli­ca­tions, interfaces gain in importance for the materials pro­per­ties. So­lid state reac­tions, neg­li­gi­ble on ma­cro­scopic length sca­les, be­come more im­portant on the nanoscale and thus be­come a frequent source of materials degra­da­tion In this contribution the influence of an electric field on the kinetics and the morphological evo­lution of (hetero­ge­ne­ous) solid state model reactions will be highlighted [1-4]. Experiments on the reac­tion couples MgO + In2O3 (for­ming one product: MgIn2O4) and Al2O3 + Y2O3 (forming three pro­ducts:YAG, YAM and YAP) were per­formed in thin film technique. Using linear tran­sport theory, a time in­de­pen­dent growth rate for the product layer(s) is ex­pec­ted, de­pen­ding on the magnitude and the direction of the ionic current through the electrochemical cell and the dif­fe­rence of the ionic transference numbers in the product phase. This is generally different com­pa­red to solely diffu­sion control­led reactions without electric fields, where the reaction rate decreases with increasing pro­duct layer thickness. In a spinel forming reaction an en­han­ced growth rate for the product layer is predicted, when the di­va­lent cations are more mobile and the tri­va­lent oxide is attached to the catho­de side (Fig. 1). The role of grain boun­da­ries as fast dif­fu­sion paths is highly em­pha­si­sed. The mor­pho­logy of the pro­duct layer is sig­ni­fi­cant­ly dif­fe­rent compared to a non-field-driven reaction. An analysis shows that the ionic transference num­bers in (large angle) grain boun­daries differs significantly from the bulk phase, cau­sing locally different growth rates of the product layer. In case of a spinel forming reaction the divalent oxide tends to grow along grain boundaries through the complete product layer, reaching the trivalent oxide. In solid states reactions forming more than one product layer, applying an additional electric field opens a pos­si­bi­lity to control the product formation. Driven by an electric field, the growth kinetics of the product layer depends on the difference of the ionic transference numbers, without field on the Nernst-Planck coupled conductivities. In case of the reaction between Al2O3 + Y2O3 (three pro­ducts:YAG, YAM and YAP)the formation of the perovskite phase (YAP) can be selectively en­han­ced when connecting the Y2O3 layer to the cathode side (Fig. 2)

The magnetoresistance of homogeneous and heterogeneous silver-rich silver selenide

The magnetoresistance (MR) effect of the low-temperature phase of silver selenide (-Ag2 + Se) is measured as a function of composition. Very small composition variations in the order of = 10–6 are achieved by coulometric titration and can be performed simultaneously during the MR measurement. A homogeneous Ag2 + Se shows an ordinary magnetoresistance (OMR) effect, which can be well described by the two-band model. For silver selenide with a heterogenous silver excess, we found quite a different MR behavior. Up to a minor silver excess of 1×10–4 10–2) shows again an OMR effect

Zur Wechselwirkung von Ionentransport und Mikrostruktur in inneren Grenzflächen : Untersuchungen an oxidischen Dünnschichten

In vielen technologisch wichtigen Anwendungen findet man heute keramische Materialien in Form von dunnen Einzelschichten, mehrphasigen Multischichten oder Kompositen. In integrierten Schaltungen erfüllen sie die Funktion von Isolationsschichten. In miniaturisierten Kondensatoren und Metall-Oxid-Halbleiter-Feldeffekttransistoren (MOSFET) findet man sie als Dielektrikum. Strukturierte ferroelektrische Schichten werden als mögliche Speicherelemente (FeRAM) für permanente Speicherbausteine diskutiert. Piezoelektrische Materialien finden Anwendung in mechanischen Aktuatoren. Keramische Dünnschichten sind ebenfalls Bestandteil vieler Anwendungen aus der chemischen und physikalischen Sensorik. In Gassensoren findet man z.B. Schichten halbleitender Oxide und in GMR-, TMR- und SQUID-Magnetfeldsensoren Schichten ferrimagnetischer Oxide und keramischer Hochtemperatursupraleiter. In Hochtemperaturbrennstoffzellen (SOFC) werden spezielle Keramiken mit ionen- und halbleitenden Eigenschaften als gemischtleitende Kathodenmaterialien genutzt. Die in vielen der aufgezählten Anwendungen anzutreffenden miniaturisierten Strukturen erhöhen die Bedeutung äußerer und innerer Grenzflächen wie Oberflächen, Korn- und Phasengrenzen in Hinblick auf die Funktionseigenschaften. Die Grenzflächen eines Festkörpers unterscheiden aber sich hinsichtlich der mikroskopischen Struktur und den physikalisch-chemischen Eigenschaften deutlich von der Volumenphase. In vielen experimentellen Untersuchungen an ionischen und metallischen Materialien zeigen sich Grenzflächen als Pfade für schnellen Massen- und Ladungstransport. In ionischen Materialien ist im Gegensatz zum Volumentransport der Transport entlang von Grenzflächen und seine Abhängigkeit von der Struktur und der lokalen Zusammensetzung erst unzureichend verstanden. Zur Charakterisierung der lokalen Umgebung von Gitteratome und -defekte in einer Grenzfläche sind mehr Parameter nötig als in der Volumenphase. Die meisten in der Literatur verfügbaren experimentellen und theoretischen Arbeiten zum Korngrenz- und Phasengrenztransport in ionischen Materialien erklären die besonderen Transporteigenschaften der Grenzflächen mit dem Aufbau von Raumladungszonen, in denen eine Anreicherung oder eine Verarmung von beweglichen Gitterdefekten auftritt. In metallischen Materialien ist aufgrund der hohen Dichte mobiler Ladungsträger der Aufbau von Raumladungszonen ausgeschlossen. Die besonderen Transporteigenschaften entlang von Metallkorngrenzen werden daher auf die vom Volumen abweichende lokale Gitterstruktur zurückgeführt. Wichtige diskutierte Einflussgrößen sind die lokale Packungsdichte und die Ausbildung von Versetzungsnetzwerken. Im Vergleich zu ionischen Materialien findet man zum Korngrenztransport in Metallen und Metalllegierungen deutlich mehr experimentelle und theoretische Studien. Die Frage, ob und wie weit strukturelle Parameter auch einen Einfluss auf die Transporteigenschaften entlang von Phasen- und Korngrenzen in Systemen aus ionischen Materialien haben, wird deutlich weniger diskutiert. Insbesondere für Systeme aus stark extrinsischen bzw. hochdotierten ionischen Materialien mit kleinen Raumladungszonen ist ein Verhalten analog zu den Systemen aus metallischen Materialien anzunehmen. In dieser Arbeit wird die Frage experimentell und theoretisch untersucht. Für den Fall des schnellen Phasengrenztransports soll herausgefunden werden, ob und welche strukturellen Parameter wie Fehlpassungen, elastische Verspannungen und Versetzungsdichten einen Einfluss haben

Is Reduced Strontium Titanate a Semiconductor or a Metal?

In recent decades, the behavior of SrTiO3 upon annealing in reducing conditions has been under intense academic scrutiny. Classically, its conductivity can be described using point defect chemistry and predicting n-type or p-type semiconducting behavior depending on oxygen activity. In contrast, many examples of metallic behavior induced by thermal reduction have recently appeared in the literature, challenging this established understanding. In this study, we aim to resolve this contradiction by demonstrating that an initially insulating, as-received SrTiO3 single crystal can indeed be reduced to a metallic state, and is even stable against room temperature reoxidation. However, once the sample has been oxidized at a high temperature, subsequent reduction can no longer be used to induce metallic behavior, but semiconducting behavior in agreement with the predictions of point defect chemistry is observed. Our results indicate that the dislocation-rich surface layer plays a decisive role and that its local chemical composition can be changed depending on annealing conditions. This reveals that the prediction of the macroscopic electronic properties of SrTiO3 is a highly complex task, and not only the current temperature and oxygen activity but also the redox history play an important role

A physical method for investigating defect chemistry in solid metal oxides

The investigation of the defect chemistry of solid oxides is of central importance for the understanding of redox processes. This can be performed by measuring conductivity as a function of the oxygen partial pressure, which is conventionally established by using buffer gas mixtures or oxygen pumps based on zirconia. However, this approach has some limitations, such as difficulty regulating oxygen partial pressure in some intermediate-pressure regions or the possibility of influencing the redox process by gases that can also be incorporated into the oxide or react with the surface via heterogeneous catalysis. Herein, we present an alternative physical method in which the oxygen partial pressure is controlled by dosing pure oxygen inside an ultra-high vacuum chamber. To monitor the conductivity of the oxide under investigation, we employ a dedicated four-probe measurement system that relies on the application of a very small AC voltage, in combination with lock-in data acquisition using highly sensitive electrometers, minimizing the electrochemical polarization or electro-reduction and degradation effects. By analyzing the model material SrTiO3, we demonstrate that its characteristic redox behavior can be reproduced in good agreement with the theory when performing simultaneous electrical conductivity relaxation (ECR) and high-temperature equilibrium conductivity (HTEC) measurements. We show that the use of pure oxygen allows for a direct analysis of the characteristic oxygen dose, which opens up various perspectives for a detailed analysis of the surface chemistry of redox processes.Comment: 25 page

Is reduced strontium titanate a semiconductor or a metal?

In recent decades, the behavior of SrTiO3 upon annealing in reducing conditions has been under intense academic scrutiny. Classically, its conductivity can be described using point defect chemistry and predicting n-type or p-type semiconducting behavior depending on oxygen activity. In contrast, many examples of metallic behavior induced by thermal reduction have recently appeared in the literature, challenging this established understanding. In this study, we aim to resolve this contradiction by demonstrating that an initially insulating, as-received SrTiO3 single crystal can indeed be reduced to a metallic state, and is even stable against room temperature reoxidation. However, once the sample has been oxidized at a high temperature, subsequent reduction can no longer be used to induce metallic behavior, but semiconducting behavior in agreement with the predictions of point defect chemistry is observed. Our results indicate that the dislocation-rich surface layer plays a decisive role and that its local chemical composition can be changed depending on annealing conditions. This reveals that the prediction of the macroscopic electronic properties of SrTiO3 is a highly complex task, and not only the current temperature and oxygen activity but also the redox history play an important role

Электропривод прокатной клети непрерывно -заготовочного стана

Был выполнен выбор основного и вспомогательного электрооборудования. Регулировка тока, скорости. Переходные процессы двигательной системы постоянного тока.The choice of basic and auxiliary electrical equipment was carried out. Adjustment of the current, speed. The transient processes of the DC motor system