Impact of Defect Structure on ’Bulk’ and Nano-Scale Ferroelectrics

Ferroelectric materials offer a wide range of dedicated physical properties such as high dielectric constant, spontaneous polarisation, pyroelectric and piezoelectric effects which can be applied in thin-film non-volatile memories or ‘bulk’ actuators, multi-layer capacitors, thermal sensors and transducers (1–3). In that respect, desiredmaterials properties for specific applicationsmay be tailored by controlling the defect structure bymeans of aliovalent doping, rendering so-termed ’hard’ or ’soft’ piezoelectric materials (4–6). Another important impact on ferroelectric properties results from the confined size in nano-scale architectures (7). At the nanometer scale physical and chemical properties are expected to differ markedly from those of the ’bulk’ material. Owing to a size-driven phase transition, a critical particle size exists below which ferroelectricity does no longer occur (8). In this chapter, we will first outline the nature of the size-driven para-to-ferroelectric phase transition, as well as the concepts of defect chemistry. On that basis, the interplay between confined size at the nano-regime and the development of defect structure will be characterized. The here studied ferroelectric lead titanate nano-powders may be considered as a model system for more complex ferroelectric nano architectures (1; 2). Furthermore, the results discussed here may be transferred to large extent to other important perovskite oxides with divalent A- and tetravalent B-site, such as BaTiO3 or Pb[Zr,Ti]O3 (PZT). The defect chemistry of ferroelectric perovskite oxideswith monovalent A- and pentavalent B-site, such as the [K,Na]NbO3 (KNN) solid solution system, however has shown some important deviations from the defect structure characterized for PZT compounds (9; 10)

Rhetorical Witnessing: Recognizing Genocide in Guatemala

The article explores the rhetorical dimensions of witnessing. We concentrate, in particular, on two groups: 1) university students at the University of San Carlos, Quetzaltenango, whose murals are dramatic reminders of the massacres that resulted in the deaths of over 200,000 indigenous people in the 1980s and early 90s and of the corrupt governmental leaders responsible for them, and 2) U.S. accompaniers sponsored by an organization within our own community, the Copper Country Guatemala Accompaniment Project (CCGAP)

Priors in quantum Bayesian inference

In quantum Bayesian inference problems, any conclusions drawn from a finite number of measurements depend not only on the outcomes of the measurements but also on a prior. Here we show that, in general, the prior remains important even in the limit of an infinite number of measurements. We illustrate this point with several examples where two priors lead to very different conclusions given the same measurement data.Comment: 7 pages; published in AIP Conference Proceedings 1101: Foundations of Probability and Physics 5, edited by L. Accardi et al, p. 255 (2009

Modular CO₂‑to-CO Electrolysis Short-Stack DesignImpact of Temperature Gradients and Insights into Position-Dependent Cell Behavior

The performance of flow cells for aqueous CO2-to-CO electrolysis at ambient conditions is reportedly close to meeting industrially applicable rates when operating with membrane electrode assemblies (MEAs) based on anion exchange membranes. However, the challenges of the stacking of these cells are underrepresented in the literature, despite being a major milestone for scaling the technology. Therefore, we report a modular short-stack design for MEA cells and demonstrate its operation with three cells. The short stack replicates the performance of its respective single cell at current densities between 100 and 200 mA/cm2. At higher current densities (300–400 mA/cm2), the short stack surprisingly outperforms the single cell showing a lower stack voltage and varying cell voltage depending on the cell’s position in the short stack. The temperature distribution affected the membrane conductivity and activation energies for the reactions at the electrodes, which was verified by electrochemical impedance spectroscopy. It could be demonstrated that the temperature distribution is the leading cause of the observed position dependency of the individual cell voltages in the short stack. We show that the inherent asymmetry of the cells results in an asymmetric temperature distribution in the short stack. Taking advantage of the modular stack design, we designed a quasi-symmetric cell eliminating the problem. In this cell, we observed a much smaller voltage variation at 400 mA/cm2, caused by shunt current, which is well-known from alkaline water electrolysis. In the CO2 electrolysis short stack, however, their effect was found negligible

Toward a Stackable CO₂‑to-CO Electrolyzer Cell DesignImpact of Media Flow Optimization

Aqueous CO2-to-CO electrolysis is a promising technology for closing the carbon cycle and defossilizing industrial processes. Considering the technological readiness, consensus has been achieved about using silver as a stable and selective electrocatalyst for the CO2-to-CO reduction reaction in aqueous electrolyte. On the other hand, challenges such as media flow management, component stability, and force distribution are still associated with improving the process performance and developing a stackable cell concept to meet industrially relevant levels. We therefore report on a promising stack concept with continuous flowcells operated with gas diffusion electrodes (GDEs). To enhance the CO2-to-CO conversion efficiency, dedicated media flow chambers were developed on two levels. In the gas chamber, which touches the GDE from the far side of the anode, the feed gas flow and distribution over the GDE were controlled by introducing various gas path architectures in a modular flowcell. In addition, an ionically conductive spacer was implemented in the catholyte chamber, which is adjacent to the opposite side of the GDE. The effect of these modifications on the cell voltage, selectivity, and overall conversion was investigated at 100 mA/cm2 with varying CO2 feed gas flow and concentration. Noteworthy, an optimized feed gas distribution generated an increase of the Faraday efficiency for CO under reduced CO2 supply. Furthermore, the implementation of the spacer enhanced the process stability by suppressing gas-bubble-induced noise in the cell voltage measurements. By functioning as support structures to the GDE, the combined modifications provided the cell with mechanical integrity and allowed an ionic and electric contact over the full active cell area, which is required for both stacking and upscaling of the cell. The corresponding performance was demonstrated by a two-cell short-stack

Modeling 3D-Deposition of TiO₂ Using a Monte Carlo Chemical Kinetics Approach

3D microbatteries are indispensable to cope with the increasing energy demand of autonomous smart devices. To synthesize 3D microbatteries, step-conformal deposition of thin films into 3D-substrates is vital, and low pressure chemical vapor deposition (LPCVD) is a technique that is capable of achieving this goal. In the present work, the 3D-deposition of TiO₂ is investigated. It is shown that the growth of anatase TiO₂ can be characterized by two rate-determining processes. In the diffusion-controlled temperature region, the TiO₂ films deposited into 3D-substrates lack step-conformity. In contrast, in the kinetically controlled temperature region, uniform films were deposited inside these microstructures. To understand and improve the LPCVD deposition process, the experimental results were simulated using a Monte Carlo chemical kinetics (MCCK) model. Good agreement between the model and experiments was achieved in all cases. It was found that the deposition probability is low in the kinetically controlled deposition region, while this probability was found to be high in the diffusion-controlled region. It is also shown that the reflections of precursor molecules inside the trenches play an important role in achieving homogeneous 3D deposition. To show the strength of the MCCK model, the optimized deposition parameters are applied to predict the film thickness profiles in narrower microstructures

Monolithic All-Phosphate Solid-State Lithium-Ion Battery with Improved Interfacial Compatibility

High interfacial resistance between solid electrolyte and electrode of ceramic all-solid-state batteries is a major reason for the reduced performance of these batteries. A solid-state battery using a monolithic all-phosphate concept based on screen printed thick LiTi₂(PO₄)₃ anode and Li₃V₂(PO₄)₃ cathode composite layers on a densely sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ solid electrolyte has been realized with competitive cycling performance. The choice of materials was primarily based on the (electro-)­chemical and mechanical matching of the components instead of solely focusing on high-performance of individual components. Thus, the battery utilized a phosphate backbone in combination with tailored morphology of the electrode materials to ensure good interfacial matching for a durable mechanical stability. Moreover, the operating voltage range of the active materials matches with the intrinsic electrochemical window of the electrolyte which resulted in high electrochemical stability. A highly competitive discharge capacity of 63.5 mAh g^–1 at 0.39 C after 500 cycles, corresponding to 84% of the initial discharge capacity, was achieved. The analysis of interfacial charge transfer kinetics confirmed the structural and electrical properties of the electrodes and their interfaces with the electrolyte, as evidenced by the excellent cycling performance of the all-phosphate solid-state battery. These interfaces have been studied via impedance analysis with subsequent distribution of relaxation times analysis. Moreover, the prepared solid-state battery could be processed and operated in air atmosphere owing to the low oxygen sensitivity of the phosphate materials. The analysis of electrolyte/electrode interfaces after cycling demonstrates that the interfaces remained stable during cycling

Spectroscopic Evidence of Reversible Disassembly of the [FeFe] Hydrogenase Active Site

[FeFe] hydrogenases are extremely active and efficient H₂-converting biocatalysts. Their active site comprises a unique [2Fe] subcluster bonded to a canonical [4Fe–4S] cluster. The [2Fe] subsite can be introduced into hydrogenases lacking an assembled H-cluster through incubation with a synthesized [2Fe]_H precursor, which initially produces the CO-inhibited state of the enzyme. We present FTIR spectroelectrochemical studies on the CO-inhibited state of the [FeFe] hydrogenase from <i>Desulfovibrio desulfuricans</i>, <i>Dd</i>HydAB. At very negative potentials, disassembly of the H-cluster and dissociation of the [2Fe] subcluster is observed. Subsequently raising the potential allows cofactor rebinding and H-cluster reassembly. This demonstrates how the stability of the [2Fe]–[4Fe–4S] intercluster bond depends on the applied potential and the presence of an inhibiting CO ligand on the [2Fe] subcluster. These results provide insight into the mechanisms of CO inhibition and H-cluster assembly in [FeFe] hydrogenases. A fundamental understanding of these properties will provide clues for designing better H₂-converting catalysts