Strukturelle und chemische Relaxationsprozesse in aerosol-abgeschiedenen Bariumtitanat-Dickschichten

The aerosol deposition method (ADM) represents a novel process to fabricate ceramic thick films at room temperature. The low temperature nature of the process makes it attractive for application in energy conversion and energy storage systems, implementing material combinations that were considered impossible to achieve due to thermal constraints of the substrate materials. ADM allows for direct consolidation of a raw powder into a highly dense film. In contrast to conventional sintered ceramics, the microstructure and dielectric properties of AD films vastly differ from bulk ceramics. AD films feature a nano-grained microstructure with significant internal residual stress that is hypothesized to reduce the dielectric permittivity compared to a conventionally sintered material. Additionally, the consolidation of powder into a film involves kinetic particle fracture in a low partial pressure environment, promoting the formation of point defects and hence, increasing the electronic conductivity of the film. The attained grain size and conductivity in as-deposited state may therefore be considered unfavourable for the desired application. Thermal annealing is an established technique attempting to mitigate these deficiencies. However, owing to the occurrence of a thermal expansion mismatch between the film and the substrate, as well as a given temperature-dependent corrosion resistance of the latter, a careful selection of annealing temperature and annealing atmosphere must be conducted to achieve optimum results within the economics of a low-temperature process. This work aims to draw a clear correlation between AD-process intrinsic and post-treatment induced influences on the functional properties of AD-films. In this study, BaTiO3 (BT) is selected as a model material due to its well understood dielectric properties and defect chemistry, in addition it is anticipated to serve as a lead-free ferroelectric material for capacitor applications using the AD-process. In Chapter 3.1, the basic observations during thermal treatment of aerosol deposited BT thick films are reported. In order to understand the extrinsic and intrinsic contributions of point defects such as oxygen vacancies and internal stress release during thermal treatment, the evolution of the crystallographic unit cell volume is of particular interest, requiring to isolate the film from the substrate. Therefore, a novel method to generate free-standing AD-films without prior heat treatment was developed based on a water soluble, sacrificial layer. In Chapter 3.2, the thermal expansion of freestanding BT-AD structures is investigated combining a tailored defect chemistry by aliovalent doping of the raw powder with different annealing atmospheres. In Chapter 3.3 this view is expanded by including a different material system and modifying the deposition atmosphere, therefore providing a holistic view of the extrinsic and intrinsic modulated unit-cell volume of AD-films. In addition, the mechanism of internal residual stress release and densification in AD-films is addressed. The experiments are subsequently concluded in Chapter 3.4 where the role of oxygen partial pressure during deposition and annealing on the dielectric properties of BT-films is investigated.Das Aerosol-Deposition Verfahren stellt einen neuartigen Prozess zur Herstellung keramischer Dickschichten dar. Der Vorteil des Verfahrens ist die Prozessführung bei Raumtemperatur. Somit können Kombinationen von Werkstoffen realisiert werden, die aufgrund thermischer Beschränkungen des Substratmaterials bisher als nicht realisierbar angesehen wurden. Insbesondere für Anwendungen im Bereich der Energieumwandlung und Energiespeicherung ist dies von besonderem Interesse. Das Verfahren erlaubt hierbei die direkte Generierung einer dichten Beschichtung ausgehend vom Rohpulver. Allerdings unterscheiden sich die Mikrostruktur und die dielektrischen Eigenschaften von AD-Filmen signifikant von konventionellen, durch Sintern hergestellten Keramiken. AD-Filme weisen eine nanoskalige Mikrostruktur auf, dies bedingt prozessinduzierte Eigenspannungen, die als Ursache für die im Vergleich zu einem konventionell gesinterten Material deutlich reduzierte dielektrische Permittivität angesehen werden. Ferner basiert die Erzeugung der Filme auf kinetisch induzierter Fraktur der zugeführten Partikel in einem Unterdrucksystem. Dies begünstigt die Entstehung von kristallographischen Punktdefekten und erhöht damit potentiell die elektrische Leitfähigkeit. Die im Ausgangszustand der Filme vorliegende Korngröße und elektrische Leitfähigkeit kann sich daher unvorteilhaft auf den gewünschten Anwendungszweck auswirken, eine subsequente Wärmebehandlung wird daher als imperativ betrachtet. Hierbei ergibt sich allerdings die Problemstellung, dass sowohl der unterschiedliche thermische Ausdehnungskoeffizient zwischen Film und Substrat, als auch temperaturbedingte Korrosionserscheinungen des Substrats eine klare Abwägung bei der Wahl der Auslagerungstemperatur und Atmosphäre getroffen werden muss. Die vorliegende Studie versucht, eine klaren Zusammenhang zwischen AD-Film spezifischen, intrinsischen und prozessinduzierten Einflussfaktoren auf die funktionalen Eigenschaften von AD-Filmen herzustellen. Aufgrund seiner bekannten dielektrischen Eigenschaften und Defektchemie sowie der potentiellen Verwendung als bleifreies Ferroelektrikum in Energiespeichern wird hierzu BaTiO3 (BT) als Modellsystem herangezogen. Kapitel 3.1 befasst sich mit den grundlegenden Beobachtungen bei der Wärmebehandlung von aerosol-abgeschiedenem BT. Um die extrinsischen sowie intrinsischen Einflüsse von Punktdefekten (z.B. Sauerstoffleerstellen) und die Eigenspannungsreduktion während der Wärmebehandlung auf das Materialverhalten zu ergründen, ist das Ausdehnungsverhalten der Kristallstruktur von entscheidender Bedeutung. Dies erfordert eine unabhängige Betrachtung von Film und Substrat. Hierzu wurde ein Verfahren entwickelt, dass freistehende AD-Filme unter Verwendung einer wasserlöslichen Opferschicht ohne weitere Temperatureinflüsse generiert. In Kapitel 3.2 wird das Ausdehnungsverhalten dieser Strukturen anhand verschiedener Dotierungen des Ausgangsmaterials sowie deren Temperaturabhängigkeit dargestellt. Die dabei gewonnen Erkenntnisse werden in Kapitel 3.3 um die Prozessatmosphäre während der Filmabscheidung sowie andere Materialsysteme erweitert und die intrinsischen sowie extrinsischen Einflussfaktoren auf die Kristallstruktur definiert. Darüber hinaus wird auch die Eigenspannungsreduktion und Dichtemodulation während der Wärmebehandlung betrachtet. In Kapitel 3.4 wird abschließend der Zusammenhang zwischen dem Sauerstoffpartialdruck während der Filmabscheidung und der nachfolgenden Wärmebehandlung auf die dielektrischen Eigenschaften der untersuchen BT-Filme hergestellt

Feasibility of Powder‐Based High‐Throughput Synthesis for Ceramics Development: Case Study on the Influence of Calcination Temperature in BiFeO3‐BaTiO3

Recent advances in machine learning capabilities have increased interest in materials research to improve the efficiency of materials discovery and optimization as well as to better understand the underlying phenomena responsible for the observed physical properties. While combinatorial chemistry and compositional engineering is well established in the development of pharmaceutical and chemical products, its use in the field of bulk functional polycrystalline ceramics is far from mature. In this work, a critical review of a high‐throughput powder‐based dispensing system is provided and the challenges involved with the transition from a conventional, human resources intensive workflow to a fully automated process are highlighted. Based on the lead‐free piezoelectric BiFeO3–BaTiO3 binary system, the applicability and robustness of high‐throughput engineering is investigated to increase data point density in phase diagrams, including the composition variations of the resulting materials. This work presents 16 different BiFeO3–BaTiO3 compositions at four different calcination temperatures, both demonstrating the potential of the system. This is coupled to automated crystal structure analysis, which will be used to investigate the role of calcination temperature on the resulting compositions at room temperature.Recent advances in machine learning capabilities have spawned considerable interest in materials science to automate labour‐intensive and repetitive processes in research. In the field of functional ceramics, automatic powder dispensing systems are a recent development. In this work the robustness and reliability of a commercially available powder dispensing system in a high‐throughput solid‐state synthesis workflow is investigated and proved. image © 2024 WILEY‐VCH GmbHDeutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/50110000165

Fabrication of porous thick films using room‐temperature aerosol deposition

Abstract A novel technique for the rapid room‐temperature deposition of porous ceramic, glass, or metal thick films using the aerosol deposition (AD) method is presented. The process is based on the co‐deposition of the desired film material and a second water‐soluble constituent, resulting in a ceramic‐ceramic composite. Following the subsequent removal of water‐soluble end member, a network of pores is retained. To demonstrate the process, porous BaTiO3 thick films were fabricated through co‐deposition with NaCl. Microstructural images show the clear development of a porous structure, which was found to enhance the dielectric properties over dense thick films, possibly related to the lower extent of internal residual stress. This simple but highly effective porous structure fabrication can be applied to any film and substrate material stable in water and is promising for the application of AD‐processed films in gas sensors, solid oxide fuel cells, and humidity sensors

Multifunctional energy storage and piezoelectric properties of 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 thick films on stainless-steel substrates

AbstractThe miniaturization of electronic devices and power systems requires the fabrication of functional components in the form of micrometer-sized thick films. A major challenge is the integration of functional ceramics with metals, which are considered incompatible with high-temperature ceramic processing. To overcome the integration barrier, an aerosol deposition (AD) spray-coating method based on room temperature deposition can be used. By employing the AD method, we were able to deposit relaxor-ferroelectric 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 ceramic thick films on low-cost stainless-steel substrates. The as-deposited films were dense, with ∼97% of the theoretical density. Moreover, the post-deposition annealing at 500 °C did not result in any microstructural changes. Compared to the as-deposited films, the annealed films exhibit improved energy storage and electromechanical properties. The annealed thick films achieve a recoverable energy density of 15.1 J⋅cm−3 at an electric field of 1350 kV⋅cm−1 and an electric-field cycling stability of 5 million cycles. A piezoelectric response was detected through the entire film thickness by piezoelectric force microscopy. Macroscopic displacement measurements revealed a maximum relative strain of 0.38% at 1000 kV⋅cm−1, corresponding to inverse effective piezoelectric coefficient of ∼40 pm⋅V−1. In this study, we overcame the integration challenges and demonstrated the multifunctionalization of future ceramic-metal structures, as the deposited thick films on stainless steel exhibit energy storage capability and piezoelectric properties

MatriGrid® based biological morphologies: tools for 3D cell culturing

Recent trends in 3D cell culturing has placed organotypic tissue models at another level. Now, not only is the microenvironment at the cynosure of this research, but rather, microscopic geometrical parameters are also decisive for mimicking a tissue model. Over the years, technologies such as micromachining, 3D printing, and hydrogels are making the foundation of this field. However, mimicking the topography of a particular tissue-relevant substrate can be achieved relatively simply with so-called template or morphology transfer techniques. Over the last 15 years, in one such research venture, we have been investigating a micro thermoforming technique as a facile tool for generating bioinspired topographies. We call them MatriGrid ® s. In this research account, we summarize our learning outcome from this technique in terms of the influence of 3D micro morphologies on different cell cultures that we have tested in our laboratory. An integral part of this research is the evolution of unavoidable aspects such as possible label-free sensing and fluidic automatization. The development in the research field is also documented in this account

Influence of stress on the electromechanical properties and the phase transitions of lead-free (1 − x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3

The influence of stress on the phase boundaries of polycrystalline lead-free perovskite (1 − x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 (x = 0.4, 0.5, and 0.6) was characterized through the temperature- and stress-dependent small-signal dielectric and piezoelectric response from − 150 to 200 °C under uniaxial compressive stress up to − 75 MPa. For all three compositions, the phase transition temperatures separating the rhombohedral, orthorhombic, tetragonal, and cubic phases were shifted to higher temperatures with an increase in the uniaxial mechanical loading, corresponding to a significant decrease in the dielectric and piezoelectric responses. Additional stress-dependent relative permittivity measurements up to − 260 MPa were conducted at four different constant temperatures (− 10, 10, 25, and 40 °C), revealing significant increases in the dielectric response, making these materials interesting for tunable dielectric applications. Furthermore, the stress-induced shift in phase transition temperatures was confirmed by in situ combined temperature- and stress-dependent Raman spectroscopy measurements under different constant uniaxial loads within the temperature range from 30 to 130 °C. Graphical abstractOpen Access funding enabled and organized by Projekt DEAL.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Friedrich-Alexander-Universität Erlangen-Nürnberg (1041

Temperature‐ and stress‐dependent electromechanical properties of phase‐boundary‐engineered KNN‐based piezoceramics

Abstract The influence of stress on the small‐signal dielectric permittivity and piezoelectric coefficient of polycrystalline lead‐free perovskite 0.92(Na1/2K1/2)NbO3–(0.08 − x)Bi1/2Li1/2TiO3–xBaZrO3 (x = 0, 0.02, 0.04, 0.06, and 0.07) was characterized under different constant uniaxial stress up to −200 MPa within a temperature range of −150 to 450°C, revealing stress‐induced suppression of the electromechanical response as well as shifts in the phase boundaries. For all compositions, the interferroelectric and ferroelectric–paraelectric phase transitions were shifted to higher temperatures under the uniaxial compressive stress. Interestingly, the sensitivity to the applied stress was found to increase with increasing BZ/BLT ratio in the system. The origin of a different extent of stress‐sensitivity with BZ/BLT ratio is suggested to be related to the change in the crystal structure. Additionally, at temperatures below −50°C, the relative permittivity showed a significant increase under applied compressive stress

Predictive Value of Multiparametric MRI for Response to Single-Cycle Induction Chemo-Immunotherapy in Locally Advanced Head and Neck Squamous Cell Carcinoma

Objectives To assess the predictive value of multiparametric MRI for treatment response evaluation of induction chemo-immunotherapy in locally advanced head and neck squamous cell carcinoma. Methods Twenty-two patients with locally advanced, histologically confirmed head and neck squamous cell carcinoma who were enrolled in the prospective multicenter phase II CheckRad-CD8 trial were included in the current analysis. In this unplanned secondary single-center analysis, all patients who received contrast-enhanced MRI at baseline and in week 4 after single-cycle induction therapy with cisplatin/docetaxel combined with the immune checkpoint inhibitors tremelimumab and durvalumab were included. In week 4, endoscopy with representative re-biopsy was performed to assess tumor response. All lesions were segmented in the baseline and restaging multiparametric MRI, including the primary tumor and lymph node metastases. The volume of interest of the respective lesions was volumetrically measured, and time-resolved mean intensities of the golden-angle radial sparse parallel-volume-interpolated gradient-echo perfusion (GRASP-VIBE) sequence were extracted. Additional quantitative parameters including the T1 ratio, short-TI inversion recovery ratio, apparent diffusion coefficient, and dynamic contrast-enhanced (DCE) values were measured. A model based on parallel random forests incorporating the MRI parameters from the baseline MRI was used to predict tumor response to therapy. Receiver operating characteristic (ROC) curves were used to evaluate the prognostic performance. Results Fifteen patients (68.2%) showed pathologic complete response in the re-biopsy, while seven patients had a residual tumor (31.8%). In all patients, the MRI-based primary tumor volume was significantly lower after treatment. The baseline DCE parameters of time to peak and wash-out were significantly different between the pathologic complete response group and the residual tumor group (p < 0.05). The developed model, based on parallel random forests and DCE parameters, was able to predict therapy response with a sensitivity of 78.7% (95% CI 71.24–84.93) and a specificity of 78.6% (95% CI 67.13–87.48). The model had an area under the ROC curve of 0.866 (95% CI 0.819–0.914). Conclusions DCE parameters indicated treatment response at follow-up, and a random forest machine learning algorithm based on DCE parameters was able to predict treatment response to induction chemo-immunotherapy