Untersuchung und Strukturierung des keramischen Kompositsystems Bariumstrontiumtitanat und Magnesiumborat

Diese Arbeit untersucht das Materialsystem Bariumstrontiumtitanat/Magnesiumborat (BST/MBO), mit einem Fokus auf die Geschehnisse beim Sintervorgang und den dielektrischen Eigenschaften. Hierbei wurden zunächst das Gefüge und die dielektrischen Eigenschaften konventioneller Komposite (homogen vermischt) in Abhängigkeit von Zusammensetzung und Sintertemperatur untersucht. Auffällig war hierbei die Diffusion von Titan aus dem BST in das MBO an den Grenzschichten der Phasen. Diese Prozesse traten bereits bei der üblichen Verdichtungstemperatur des Komposits von 1050 °C auf. Bei höheren Temperaturen kam es zur Ausbildung einer Titan-reichen Fremdphase. Die dielektrischen Eigenschaften zeigten eine Abnahme der relativen Permittivität, des dielektrischen Verlusts und der Steuerbarkeit mit der Zunahme des MBO Anteils. Proben nahe 70Vol.-% MBO Anteil besaßen ein sehr gutes Steuerbarkeit/Verlust-Verhältnis. Es wurden zwei verschiedene Strukturierungen des Komposits realisiert. Zum einen wurden geschichtete Strukturen hergestellt, welche bei geringem MBO-Anteil sehr geringe dielektrische Verluste aufweisen. Deren Umsetzung gestaltete sich kompliziert und das Absinken des Verlusts konnte für dickere MBO-Zwischenschichten nicht erreicht werden. Des Weiteren wurde ein BST-Mikronetzwerk durch die Herstellung Core- Shell-strukturierter Sprühgranulate erzeugt. Die Absicht hierbei war die perkolierende Ausbildung dünner BST-Strukturen bei hohem MBO Anteil, so dass die Steuerbarkeit des Komposits gegen die Verluste optimiert wird. Dies gelang mittels eines zweistufigen Sprühgranulierungsprozesses. Die Steuerbarkeit der strukturierten Komposite waren bei vergleichbaren Verlusten denen der unstrukturierten Komposite stark überlegen und bergen somit großes Potential für die Verwendung in steuerbaren elektrischen Bauteilen

Dielectric Behavior of Thin Polymerized Composite Layers Fabricated by Inkjet-Printing

A detailed study of the dielectric behavior of printed capacitors is given, in which the dielectric consists of a thin (<1 µm) ceramic/polymer composite layer with high permittivities of εr 20–69. The used ink contains surface-modified Ba0.6Sr0.4TiO3 (BST), a polymeric crosslinking agent and a thermal initiator, which allows the immediate polymerization of the ink during printing, leading to homogenous layers. To validate the results of the calculated permittivities, different layer thicknesses of the dielectric are printed and the capacitances, as well as the loss factors, are measured. Afterwards, the exact layer thicknesses are determined with cross sectional SEM images of ion-etched samples. Then, the permittivities are calculated with the known effective area of the capacitors. Furthermore, the ink composition is varied to obtain different ceramic/polymer ratios and thus different permittivities. The packing density of all composites is analyzed via SEM to show possible pores and validate the target ratio, respectively. The correlation between the chosen ratio and the measured permittivity is discussed using models from the literature. In addition, the leakage current of some capacitors is measured and discussed. For that, the dielectric was printed on different bottom electrodes as the nature of the electrode was found to be crucial for the performance

Network-Structured BST/MBO Composites Made from Core-Shell-Structured Granulates

A finite element method (FEM)-based simulation approach to predict the tunability in composite materials was developed and tested with analytical data. These tests showed good prediction capabilities of the simulation for the test data. The simulation model was then used to predict the tunability of a network-structured composite, where the dielectric phase formed clusters in a paraelectric network. This was achieved by simulating a reciprocal core-shell unit cell of said network. The simulation showed a high tunability for this network model, exceeding the tunability of the analytically evaluated layered, columnar, and particulate model. The simulation results were experimentally verified with a Ba0.6Sr0.4TiO3/Mg3B2O6 (BST/MBO) composite, where core-shell granulates were made with a two-step granulation process. These structured samples showed higher tunability and dielectric loss than the unstructured samples made for comparison. Overall, the structured samples showed higher tunability to loss ratios, indicating their potential for use in tunable radio frequency applications, since they may combine high performance with little energy loss

Electromagnetic modeling of tunability of Barium Strontium Titanate and Magnesium Borate composites

A complete tunability electromagnetic simulation model for the Ba$_{0.6}$Sr$_{0.4}$TiO$_3$ (BST), with ≈ 2000, and Mg$_2$B$_2$O$_6$ (MBO), with ≈ 7, composites is proposed here. The model is based on electrostatics, to simulate the effects of bias fields distribution in the composite varactor at the unbiased state to create the biased state for all volumetric mixture compositions. A bulk-ceramic varactor approach is chosen for the fabricated varactors. Varactors are fabricated with different volume compositions of BST and MBO, ranging from 10 to 100 vol-% of BST. Simulated results of the varactor model are then verified with the measured results of the varactor. The simulated and measured tunability shows considerable discrepancy at room temperature, which leads to Curie temperature _ investigation of the fabricated varactors. It has been observed that a shift in _ is directly proportional to the discrepancies in the simulated and measured tunability. After incorporating the _ shifts in the model, the results show close proximity between measured and _ -shifted simulated tunabilities with differences being reduced from around 32% to 2% for 80 vol-% BST varactor

Network-Structured BST/MBO Composites Made from Core-Shell-Structured Granulates

A finite element method (FEM)-based simulation approach to predict the tunability in composite materials was developed and tested with analytical data. These tests showed good prediction capabilities of the simulation for the test data. The simulation model was then used to predict the tunability of a network-structured composite, where the dielectric phase formed clusters in a paraelectric network. This was achieved by simulating a reciprocal core-shell unit cell of said network. The simulation showed a high tunability for this network model, exceeding the tunability of the analytically evaluated layered, columnar, and particulate model. The simulation results were experimentally verified with a Ba₀.₆Sr₀.₄TiO₃/Mg₃B₂O₆ (BST/MBO) composite, where core-shell granulates were made with a two-step granulation process. These structured samples showed higher tunability and dielectric loss than the unstructured samples made for comparison. Overall, the structured samples showed higher tunability to loss ratios, indicating their potential for use in tunable radio frequency applications, since they may combine high performance with little energy loss

Untersuchung und Strukturierung des keramischen Kompositsystems Bariumstrontiumtitanat und Magnesiumborat

Diese Arbeit untersucht das Materialsystem Bariumstrontiumtitanat/Magnesiumborat (BST/MBO), mit einem Fokus auf die Geschehnisse beim Sintervorgang und den dielektrischen Eigenschaften. Hierbei wurden zunächst das Gefüge und die dielektrischen Eigenschaften konventioneller Komposite (homogen vermischt) in Abhängigkeit von Zusammensetzung und Sintertemperatur untersucht. Auffällig war hierbei die Diffusion von Titan aus dem BST in das MBO an den Grenzschichten der Phasen. Diese Prozesse traten bereits bei der üblichen Verdichtungstemperatur des Komposits von 1050 °C auf. Bei höheren Temperaturen kam es zur Ausbildung einer Titan-reichen Fremdphase. Die dielektrischen Eigenschaften zeigten eine Abnahme der relativen Permittivität, des dielektrischen Verlusts und der Steuerbarkeit mit der Zunahme des MBO Anteils. Proben nahe 70Vol.-% MBO Anteil besaßen ein sehr gutes Steuerbarkeit/Verlust-Verhältnis. Es wurden zwei verschiedene Strukturierungen des Komposits realisiert. Zum einen wurden geschichtete Strukturen hergestellt, welche bei geringem MBO-Anteil sehr geringe dielektrische Verluste aufweisen. Deren Umsetzung gestaltete sich kompliziert und das Absinken des Verlusts konnte für dickere MBO-Zwischenschichten nicht erreicht werden. Des Weiteren wurde ein BST-Mikronetzwerk durch die Herstellung Core- Shell-strukturierter Sprühgranulate erzeugt. Die Absicht hierbei war die perkolierende Ausbildung dünner BST-Strukturen bei hohem MBO Anteil, so dass die Steuerbarkeit des Komposits gegen die Verluste optimiert wird. Dies gelang mittels eines zweistufigen Sprühgranulierungsprozesses. Die Steuerbarkeit der strukturierten Komposite waren bei vergleichbaren Verlusten denen der unstrukturierten Komposite stark überlegen und bergen somit großes Potential für die Verwendung in steuerbaren elektrischen Bauteilen

Assessing Ecosystem Services in Rubber Dominated Landscapes in South-East Asia—A Challenge for Biophysical Modeling and Transdisciplinary Valuation

The concept of ecosystem services (ESS) has been increasingly recognized for its potential in decision making processes concerning environmental policy. Multidisciplinary projects on rubber (Hevea brasiliensis) cultivation, integrating research on a variety of ESS, have been few and far between. More than three years of iterative workshops with regional stakeholders resulted in the development of future land use scenarios for our study area in Xishuangbanna, PR China. We used the InVEST (Integrated Valuation of Ecosystem Services and Trade-offs) modeling framework to analyze their impact on sediment retention, water yield, habitat quality, and carbon sequestration and developed a model for assessing rubber yields. We investigated the percentage deviations of integrated ESS indices in each scenario, as compared to the initial state of 2015 and as a novelty used different statistical weighting methods to include rankings for the preference of ESS from three contrasting stakeholder groups. The business-as-usual scenario (BAU, continuous rubber expansions) revealed an increase in rubber yields trading off against all other ESS analyzed. Compared to BAU, the measures introduced in the balanced-trade-offs scenario (reforestation, reduced herbicide application, riverine buffer zones, etc.) reduced the total amount of rubber yield but enhanced habitat quality and regulating ESS. The results show that the integrated indices for the provisioning of ESS would be overestimated without the inclusion of the stakeholder groups. We conclude that policy regulations, if properly assessed with spatial models and integrated stakeholder feedback, have the potential to buffer the typical trade-off between agricultural intensification and environmental protection.Peer Reviewe

Dielectric Behavior of Thin Polymerized Composite Layers Fabricated by Inkjet-Printing

A detailed study of the dielectric behavior of printed capacitors is given, in which the dielectric consists of a thin (&lt;1 &micro;m) ceramic/polymer composite layer with high permittivities of &epsilon;r 20&ndash;69. The used ink contains surface-modified Ba0.6Sr0.4TiO3 (BST), a polymeric crosslinking agent and a thermal initiator, which allows the immediate polymerization of the ink during printing, leading to homogenous layers. To validate the results of the calculated permittivities, different layer thicknesses of the dielectric are printed and the capacitances, as well as the loss factors, are measured. Afterwards, the exact layer thicknesses are determined with cross sectional SEM images of ion-etched samples. Then, the permittivities are calculated with the known effective area of the capacitors. Furthermore, the ink composition is varied to obtain different ceramic/polymer ratios and thus different permittivities. The packing density of all composites is analyzed via SEM to show possible pores and validate the target ratio, respectively. The correlation between the chosen ratio and the measured permittivity is discussed using models from the literature. In addition, the leakage current of some capacitors is measured and discussed. For that, the dielectric was printed on different bottom electrodes as the nature of the electrode was found to be crucial for the performance

Network-Structured BST/MBO Composites Made from Core-Shell-Structured Granulates

A finite element method (FEM)-based simulation approach to predict the tunability in composite materials was developed and tested with analytical data. These tests showed good prediction capabilities of the simulation for the test data. The simulation model was then used to predict the tunability of a network-structured composite, where the dielectric phase formed clusters in a paraelectric network. This was achieved by simulating a reciprocal core-shell unit cell of said network. The simulation showed a high tunability for this network model, exceeding the tunability of the analytically evaluated layered, columnar, and particulate model. The simulation results were experimentally verified with a Ba0.6Sr0.4TiO3/Mg3B2O6 (BST/MBO) composite, where core-shell granulates were made with a two-step granulation process. These structured samples showed higher tunability and dielectric loss than the unstructured samples made for comparison. Overall, the structured samples showed higher tunability to loss ratios, indicating their potential for use in tunable radio frequency applications, since they may combine high performance with little energy loss

Suppression of Acoustic Resonances in BST-Based Bulk-Ceramic Varactors by Addition of Magnesium Borate

This work presents a method for reducing acoustic resonances in ferroelectric barium strontium titanate (BST)-based bulk ceramic varactors, which are capable of operation in high-power matching circuits. Two versions of parallel-plate varactors are manufactured here: one with pure BST and one with 10 vol-% magnesium borate, Mg₃B₂O₆ (MBO). Each varactor includes a 0.85-mm-thick ferroelectric layer. Acoustic resonances that are present in the pure BST varactor are strongly suppressed in the BST-MBO varactor and, hence, the Q-factor is increased over a wide frequency range by the addition of small amounts of a low-dielectric-constant (LDK) MBO. Although the tunability is reduced due to the presence of non-tunable MBO, the increased Q-factor extends the varactor’s availability for low-loss and high-power applications