On‐site investigations on concrete resistivity – a parameter of durability calculation of reinforced concrete structures

For determination the electrolytic resistivity of concrete, resistances of realistic designed probes are measured during a one and two year period, respectively. Measurements are made with a commercially available instrument normally used for measuring earth resistances. Comparison to other usually applied methods show not only that the used method GEOHM is qualified for measuring the electrolytic resistance of concrete but also that the magnitude of measured values is realistic. To obtain electrolytic resistivities from these measurements the individual cell constant for the investigated system was derived from experimental research. First results of the in‐situ stored specimens are presented, intended future activities are sketched.     

SCALING IN SITU BIOREMEDIATION PROBLEMS BY APPLICATION OF MULTIPHASE, MULTICOMPONENT TRANSPORT THEORY

A methodology for scaling in situ bioremediation problems is presented. This methodology is based on multiphase, muiticomponent transport theory and employs inspectional analysis and numerical sensitivity studies. A general mathematical model that describes subsurface aerobic biotransformation of organic chemical species in a multiphase setting is first presented. This general model is applied to the specific case of microbial enhanced vapor-vacuum extraction (MEVVE) in a one-dimensional zone of immobile liquids. The resulting simplified MEVVE model considers rate-dependent interphase mass transfer, a flowing gaseous phase, a single hydrocarbon pseudo-component, and either oxygen or hydrocarbon limited microbial activity. By inspectional analysis a set of dimensionless groups are derived that represent the various model parameters. A scries of numerical sensitivity studies are presented that examine the impact of selected dimensionless groups on overall system biotransformation rates. This analysis demonstrates that overall biotranformation rates can be significantly limited not only by insufficient transport of oxygen in the gaseous phase, but also by interfacial mass transfer resistance between nonaqueous phase liquid globules and adjacent fluids. Finally, an examination of the selected dimensionless groups reveals the parameter requirements for properly scaled MEVVE tests. These results indicate the need for further investigation of the importantance of fluid distributions in the pore space and their impact on the design of laboratory and field tests.     

Comprehensive modeling of electrochemical metallization memory cells

This paper reviews our previous theoretical studies on the simulation and modeling of resistively switching electrochemical metallization memory devices. The focus is on the current understanding of the dynamic behavior of this type of memory cells. A wide range of simulation scales is presented ranging from atomistic kinetic Monte Carlo models to 1D compact and analytical models. While these models show consistent results in the mean dynamic behavior, they address different aspects of the device behavior. With the compact and analytical models, the dynamic behavior on timescales ranging from nanoseconds to thousands of seconds can be investigated. The computationally more expensive multidimensional continuum models and kinetic Monte Carlo models, in contrast, give additional information on the detailed filamentary growth and dissolution mechanism, which is the origin of the resistance switching effect in electrochemical metallization memory cells. Using the different models, the different aspects of the resistance switching effect such as switching dynamics, multilevel programming, filament shape, polyfilamentary switching, variability, quantized conduction and polarity-independent filament dissolution are addressed. Moreover, it will be discussed how the models need to be extended in order to address further properties of the switching process.     

Cardiac Arrhythmias and Sleep-Disordered Breathing in Patients with Heart Failure

The relationship between heart failure (HF), sleep-disordered breathing and cardiac arrhythmias is complex and poorly understood. Whereas the frequency of predominantly obstructive sleep apnea in HF patients is low and similar or moderately higher to that observed in the general population, central sleep apnea (CSA) has been observed in approximately 50% of HF patients, depending on the methods used to detect CSA and patient selection. Despite this high prevalence, it is still unclear whether CSA is merely a marker or an independent risk factor for an adverse prognosis in HF patients and whether CSA is associated with an increased risk for supraventricular as well as ventricular arrhythmias in HF patients. The current review focuses on the relationship between CSA and atrial fibrillation as the most common atrial arrhythmia in HF patients, and on the relationship between CSA and ventricular tachycardia and ventricular fibrillation as the most frequent cause of sudden cardiac death in HF patients.     

Redistribution of internal forces in the lateral bracing system / Schnittgrößenumlagerungen bei der Gebäudeaussteifung

The lateral bracing system of a building should ensure the stability of the building, in terms of the usability and the total load capacity. The governing loads for the lateral bracing system are horizontal loads due to wind and earthquake. The horizontal loads are traditionally distributed to the bracing elements according to the uncracked elastic state dependent on the bending stiffness in the elastic (uncracked) state. This method does not allow consideration of non‐linear effects and load redistribution. Within the framework of the PRB Research Cooperation – Improving the Practical Use of Structural Design Standards through pre‐normative work – Subcontract 5: Masonry Construction, Subproject 3: “Large Shear Walls” [7], the cracked or plastic state can be taken into account through a displacement based approach. A matrix formulation for both the linear and cracked states has been derived and implemented.     

Avalanche‐Discharge‐Induced Electrical Forming in Tantalum Oxide‐Based Metal–Insulator–Metal Structures

Oxide‐based metal–insulator–metal structures are of special interest for future resistive random‐access memories. In such cells, redox processes on the nanoscale occur during resistive switching, which are initiated by the reversible movement of native donors, such as oxygen vacancies. The formation of these filaments is mainly attributed to an enhanced oxygen diffusion due to Joule heating in an electric field or due to electrical breakdown. Here, the development of a dendrite‐like structure, which is induced by an avalanche discharge between the top electrode and the Ta₂O_5‐x layer, is presented, which occurs instead of a local breakdown between top and bottom electrode. The dendrite‐like structure evolves primarily at structures with a pronounced interface adsorbate layer. Furthermore, local conductive atomic force microscopy reveals that the entire dendrite region becomes conductive. Via spectromicroscopy it is demonstrated that the subsequent switching is caused by a valence change between Ta⁴⁺ and Ta⁵⁺, which takes place over the entire former Pt/Ta₂O_5‐x interface of the dendrite‐like structure.