Electronic structure of fully epitaxial Co2TiSn thin films

In this article we report on the properties of thin films of the full Heusler compound Co2TiSn prepared by DC magnetron co-sputtering. Fully epitaxial, stoichiometric films were obtained by deposition on MgO (001) substrates at substrate temperatures above 600{\deg}C. The films are well ordered in the L21 structure, and the Curie temperature exceeds slightly the bulk value. They show a significant, isotropic magnetoresistance and the resistivity becomes strongly anomalous in the paramagnetic state. The films are weakly ferrimagnetic, with nearly 1 \mu_B on the Co atoms, and a small antiparallel Ti moment, in agreement with theoretical expectations. From comparison of x-ray absorption spectra on the Co L3/L2 edges, including circular and linear magnetic dichroism, with ab initio calculations of the x-ray absorption and circular dichroism spectra we infer that the electronic structure of Co2TiSn has essentially non-localized character. Spectral features that have not been explained in detail before, are explained here in terms of the final state band structure.Comment: 11 pages, 8 figure

Comparison of the electrical conductivity of bulk and film Ce1–xZrxO2–d in oxygen-depleted atmospheres at high temperatures

Featuring high levels of achievable oxygen non-stoichiometry d, Ce1-xZrxO2-d solid solutions (CZO) are crucial for application as oxygen storage materials in, for example, automotive three-way catalytic converters (TWC). The use of CZO in form of films combined with simple manufacturing methods is beneficial in view of device miniaturization and reducing of TWC manufacturing costs. In this study, a comparative microstructural and electrochemical characterization of film and conventional bulk CZO is performed using X-ray diffractometry, scanning electron microscopy, and impedance spectroscopy. The films were composed of grains with dimensions of 100 nm or less, and the bulk samples had about 1 lm large grains. The electrical behavior of nanostructured films and coarse-grained bulk CZO (x [ 0) was qualitatively similar at high temperatures and under reducing atmospheres. This is explained by dominating effect of Zr addition, which masks microstructural effects on electrical conductivity, enhances the reducibility, and favors strongly electronic conductivity of CZO at temperatures even 200 K lower than those for pure ceria. The nanostructured CeO 2 films had much higher electrical conductivity with different trends in dependence on temperature and reducing atmospheres than their bulk counterparts. For the latter, the conductivity was dominantly electronic, and microstructural effects were significant at T \ 700 °C. Nanostructural peculiarities of CeO 2 films are assumed to induce their more pronounced ionic conduction at medium oxygen partial pressures and relatively low temperatures. The defect interactions in bulk and film CZO under reducing conditions are discussed in the framework of conventional defect models for ceria

Linking the electrical conductivity and non-stoichiometry of thin film Ce1−xZrxO2−δ by a resonant nanobalance approach

Bulk ceria-zirconia solid solutions (Ce1−xZrxO2−δ, CZO) are highly suited for application as oxygen storage materials in automotive three-way catalytic converters (TWC) due to the high levels of achievable oxygen non-stoichiometry δ. In thin film CZO, the oxygen storage properties are expected to be further enhanced. The present study addresses this aspect. CZO thin films with 0 ≤ x ≤ 1 were investigated. A unique nano-thermogravimetric method for thin films that is based on the resonant nanobalance approach for high-temperature characterization of oxygen non-stoichiometry in CZO was implemented. The high-temperature electrical conductivity and the non-stoichiometry δ of CZO were measured under oxygen partial pressures pO2 in the range of 10−24–0.2 bar. Markedly enhanced reducibility and electronic conductivity of CeO2-ZrO2 as compared to CeO2−δ and ZrO2 were observed. A comparison of temperature- and pO2-dependences of the non-stoichiometry of thin films with literature data for bulk Ce1−xZrxO2−δ shows enhanced reducibility in the former. The maximum conductivity was found for Ce0.8Zr0.2O2−δ, whereas Ce0.5Zr0.5O2-δ showed the highest non-stoichiometry, yielding δ = 0.16 at 900 °C and pO2 of 10−14 bar. The defect interactions in Ce1−xZrxO2−δ are analyzed in the framework of defect models for ceria and zirconia

Thin-film calorimetry: analytical tool for in-situ characterization of lithium ion batteries

Thin-Film Calorimetry (TFC) as presented in this work is a novel analytical tool to determine phase transformation temperatures and enthalpies of thin films and thin-film sequences. The key component is a high-temperature stable piezoelectric langasite (La3Ga5SiO14) resonator serving as a highly sensitive planar temperature sensor. Deviations in its frequency are related to temperature fluctuations caused by phase transformations and used to calculate the related enthalpies. Temperature ramps from room temperature up to 1000°C are applied to perform calorimetric thin-film investigations. Thereby, the atmosphere can be controlled. To the best of our knowledge, the presented TFC is the only existing technique combining the aspects “thin films” and “high-temperature calorimetry.” The first part of this article describes the newly developed TFC system. The second part presents TFC investigations on lithium manganese oxide (LMO) thin films. Measurements are carried out in ambient air and in 0.5 %H2/Ar. In air three phase transformations appear (at 330, 410 and 600°C) while in 0.5 %H2/Ar four phase transformations are observed (at 389, 471, 730 and 758°C). Their progression and related enthalpies are discussed. To determine the associated crystallographic phases, X-ray diffraction and Raman spectroscopy are performed

Hochtemperatur-Dünnschichtkalorimetrie auf der Basis piezoelektrischer Resonatoren

Die vorliegende Arbeit beschreibt den Aufbau und die Entwicklung eines neuen Messsystems zur Kalorimetrie an dünnen Schichten für den Hochtemperaturbereich. Dünne Schichten nehmen einen wichtigen Platz in technisch relevanten Systemen und in der aktuellen Forschung ein. Der genauen Kenntnis der entsprechenden thermodynamischen Daten kommt eine Schlüsselrolle beim Verständnis und der Entwicklung neuer Materialien und Anwendungen zu. Der Großteil der bekannten Materialdaten beruht auf kalorimetrischen Messungen an Volumenproben. Die Daten dünner Schichten können hiervon allerdings signifikant abweichen. Es besteht daher ein Bedarf an kalorimetrischen Messverfahren, die speziell für die Charakterisierung dünner Schichten geeignet sind. Zu Beginn der Dissertation existierte kein Verfahren, welches für den Einsatz oberhalb von 500 °C geeignet ist. Das hier vorgestellte Hochtemperatur- Dünnschichtkalorimeter ist, soweit bekannt, das derzeit einzige System, welches Kalorimetrie an dünnen Schichten bis in den Hochtemperaturbereich von ca. 1000 °C ermöglicht. Der erste Teil der Dissertation beschäftigt sich mit der Systementwicklung und stellt ein Modell zur Datenauswertung vor. Die zentrale Komponente des Messsystems bilden piezoelektrische Resonatoren aus Langasit-Einkristallen (LGS, La3Ga5SiO14). Auf diese werden die zu untersuchenden Aktivschichten abgeschieden. Bei LGS handelt es sich um einen hochtemperaturstabilen Quarzisomorph, der bei elektrischer Anregung Volumenscherschwingungen ausführt. Die Resonanzfrequenz ist stark temperaturabhängig und zeigt einen stetigen Verlauf über der Temperatur. Tritt in der Aktivschicht eine Phasenumwandlung auf, so wird die freigesetzte Energie an den Resonator übertragen (exotherm) oder die zur Umwandlung benötigte Energie wird dem Resonator entzogen (endotherme Reaktion). In beiden Fällen ändert sich die Temperatur des Resonators und manifestiert sich als Abweichung vom ansonsten ungestörten Frequenzgang. Diese Frequenzänderungen können über den Temperaturkoeffizienten des Resonators in Temperaturänderungen umgerechnet werden. Gegenwärtig können auf diese Weise kleine Wärmemengen ab 1,2 mJ detektiert werden. Die detektierten Temperaturänderungen werden hiernach in Enthalpien umgerechnet. Der zweite Teil präsentiert Messungen an den Referenzmaterialien Sn und Al, um die Reproduzierbarkeit des Systems zu demonstrieren. Aktuelle Anwendungsbeispiele stammen aus dem Bereich der Li-Ionenbatterien. Derartige dünne Schichten finden Verwendung in miniaturisierten Batteriezellen (z. B. Medizin, Biotechnologie ...) oder als Modellsysteme um Degradation und Grenzschichteffekte zu untersuchen. Die Elektrodenmaterialien Lithium-Mangan-Oxid, Lithium-Metall-Oxide (Metall = Ni, Co, Mn, Al) und Molybdändisulfid sowie der Festkörperelektrolyt Lithium-Vanadium-Silikat werden über einen weiten Temperaturbereich untersucht. Phasenumwandlungstemperaturen und Enthalpien werden ermittelt, um Stabilitätsbereiche zu identifizieren. Im isothermen Betriebsmodus ermöglicht das vorgestellte System In-situ-Kalorimetrie an Festkörper-Dünnschichtbatterien während der Ladung bzw. Entladung. Zyklovoltammetrie wird bei Raumtemperatur und bei 50 °C betrieben. Die hierdurch induzierten kalorimetrischen Effekte in den Zellen werden diskutiert

High-temperature stable piezoelectric transducers using epitaxially grown electrodes

Piezoelectric resonators are of great importance for application in high-precision transducers. However, at elevated temperatures, the degradation of commonly used metal electrodes may affect the performance of oxide electrodes of piezoelectric transducers; with sufficiently high electrical conductivity they are expected to overcome this deficit. In the latter case, the stable operation of piezoelectric transducers could be further enhanced if the resonator and electrodes would consist of identical or at least very similar materials; thus, nearly monolithic resonators are created. The present work focuses on two major aspects: the growth of high-quality langasite (La3Ga5SiO14; LGS) and doped LGS thin-film electrode layers by pulsed laser ablation and the characterization of the developed resonator devices. To obtain epitaxial films of the correct stoichiometry, the deposition on heated substrates is performed in oxygen atmosphere in the range from 10−3 to 10 Pa. Another requirement for adjusting the stoichiometry is an increased Ga content in the sputter targets with respect to LGS to account for Ga evaporation during film deposition. Additional doping with Sr increases the electrode film conductivity; thus combined with the use of low-conductivity single-crystalline catangasite (Ca3TaGa3Si2O14; CTGS) substrates the ratio between the electrical conductivities of the substrate and the film is increased, enabling the preparation of nearly monolithic resonators. The properties of these nearly monolithic resonators are characterized in the temperature range of 600 to 1000°C and compared to those of CTGS resonator blanks without electrodes. Particular attention is paid to the reproducibility of resonator properties, the electrode orientation and the quality factor. The created nearly monolithic resonator demonstrates stable operation in the temperature range from 600 to 1000°C

Preparation and characterization of c-LiMn2O4 thin films prepared by pulsed laser deposition for lithium-ion batteries

In this work, lithium manganese oxide (LMO) thin films are prepared using pulsed laser deposition (PLD) at room temperature. The as‐prepared films are amorphous and require a subsequent annealing step to achieve dense films of c‐spinel LMO (LiMn2O4). We applied different annealing temperatures under an argon atmosphere to investigate the thermodynamics of the films and to find the minimum crystallization temperature. Thereby, a simple film deposition process with only one subsequent annealing step is developed to prepare crystalline films. The samples are characterized using scanning electron microscopy (SEM), secondary ion mass spectrometry (SIMS), X‐ray diffraction (XRD), thin‐film‐calorimetry, impedance spectroscopy, and electrochemical methods. The results indicate that a narrow temperature range around 700 °C is suitable for the preparation of the spinel phase. Using this preparation route, no further crystalline phases could be identified by XRD. The electrochemical properties of the films are investigated and compared to electrodes made of commercially available LMO powders. The electrochemical characterization shows a capacity of 95 mAh g−1 for the commercial powder and 110 mAh g−1 for the thin‐film samples

In situ analysis of hydration and ionic conductivity of sulfonated poly(ether ether ketone) thin films using an interdigitated electrode array and a nanobalance

Proton-conducting polymers, such as sulfonated poly(ether ether ketone) (SPEEK), are of great industrial interest. Such proton exchange membranes show high tendencies for water and water vapor uptake. The incorporation of water not only leads to mass and dimensional changes, but also to changes in conductivity by several orders of magnitude. Both properties highly impact the potential application of the materials and, therefore, have to be known precisely. As hydration is diffusion controlled, thin films may behave differently to bulk specimens. However, the determination of small mass changes occurring in thin-film samples is very challenging. In this work, a new measurement setup is presented to simultaneously characterize the mass change and the conductivity of thin polymer films. The mass change is measured by resonant piezoelectric spectroscopy (RPS) with a nanobalance, which is based on high-precision piezoelectric resonators operating in thickness-shear mode (TSM). The mass resolution of this nanobalance is ±7.9 ng. Electrochemical impedance spectroscopy and an interdigitated electrode array are used for conductivity measurements. The approach is validated by comparing two SPEEK films with different degrees of sulfonation (DS). The relative humidity (RH) in the measurement setup was changed stepwise within the range ∼ 2 % < RH < ∼ 85 %. For both material compositions, DS = 0.5 and DS = 0.9, the mass uptake, the hydration number and the proton conductivity are presented and discussed depending on RH. This newly designed experimental setup allows for in situ characterization of the properties mentioned above; it can monitor not only the data for the stationary state, but also the dynamics of the hydration. To the authors' knowledge this is the first simultaneous and in situ measurement device for simultaneously sensing mass and conductivity change due to hydration of polymeric thin-film materials

Thin-film chemical expansion of ceria based solid solutions: laser vibrometry study

Abstract The chemical expansion of Pr0.1Ce0.9O2–δ (PCO) and CeO2–δ thin films is investigated in the temperature range between 600 °C and 800 °C by laser Doppler vibrometry (LDV). It enables non-contact determination of nanometer scale changes in film thickness at high temperatures. The present study is the first systematic and detailed investigation of chemical expansion of doped and undoped ceria thin films at temperatures above 650 °C. The thin films were deposited on yttria stabilized zirconia substrates (YSZ), operated as an electrochemical oxygen pump, to periodically adjust the oxygen activity in the films, leading to reversible expansion and contraction of the film. This further leads to stresses in the underlying YSZ substrates, accompanied by bending of the overall devices. Film thickness changes and sample bending are found to reach up to 10 and several hundred nanometers, respectively, at excitation frequencies from 0.1 to 10 Hz and applied voltages from 0–0.75 V for PCO and 0–1 V for ceria. At low frequencies, equilibrium conditions are approached. As a consequence maximum thin-film expansion of PCO is expected due to full reduction of the Pr ions. The lower detection limit for displacements is found to be in the subnanometer range. At 800 °C and an excitation frequency of 1 Hz, the LDV shows a remarkable resolution of 0.3 nm which allows, for example, the characterization of materials with small levels of expansion, such as undoped ceria at high oxygen partial pressure. As the correlation between film expansion and sample bending is obtained through this study, a dimensional change of a free body consisting of the same material can be calculated using the high resolution characteristics of this system. A minimum detectable dimensional change of 5 pm is estimated even under challenging high-temperature conditions at 800 °C opening up opportunities to investigate electro-chemo-mechanical phenomena heretofore impossible to investigate. The expansion data are correlated with previous results on the oxygen nonstoichiometry of PCO thin films, and a defect model for bulk ceria solid solutions is adopted to calculate the cation and anion radii changes in the constrained films during chemical expansion. The constrained films exhibit anisotropic volume expansion with displacements perpendicular to the substrate plane nearly double that of bulk samples. The PCO films used here generate high total displacements of several 100 nm’s with high reproducibility. Consequently, PCO films are identified to be a potential core component of high-temperature actuators. They benefit not only from high displacements at temperatures where most piezoelectric materials no longer operate while exhibiting, low voltage operation and low energy consumption.</jats:p