20005 research outputs found
Sort by
Tuning the crystallization temperature of titanium dioxide thin films by incorporating silicon dioxide via supercycle atomic layer deposition
No full textTitanium oxide-based nanomaterials are nowadays of great interest in various application fields such as optics, sensing, photocatalysis, and solar cells. Tuning their physical properties by tailoring the geometry or combining them with different materials further expands their applicability and even allows for the generation of new functionalities. The materials’ crystalline phase also determines its properties and the crystallization behavior can be modified by doping or multilayering thin films with various materials. For instance, the combination of TiO2 with silicon dioxide (SiO2) renders these composites ideal candidates for coatings applied in harsh environments based on the high chemical and mechanical stability of both materials. Applying such coatings in optics, sensing, or photocatalysis require accurate prediction of the evolution of their properties and crystalline phase during heat treatments within the fabrication and application. Herein, we present the fabrication of SiO2-incorporated TiO2 thin films by supercycle atomic layer deposition (ALD). Specifically, TiO2-SiO2 multilayers with varying material ratios, TiO2 thicknesses, and individual layer numbers as well as SiO2-doped TiO2 thin films are prepared. Their crystallization behavior is studied by in situ X-ray diffraction during thermal annealing. The structural properties of the composite materials are assessed by X-ray reflectivity, spectroscopic ellipsometry, and transmission electron microscopy before and after annealing. TiO2-SiO2 multilayers show increasing crystallization temperatures from amorphous TiO2 to anatase with decreasing TiO2 layer thickness from 50 nm to 4 nm and with increasing number of TiO2 layers. Their layered structure is retained during annealing while the interfaces roughen slightly. SiO2-doped TiO2 thin films demonstrate increasing crystallization temperatures with increasing SiO2 contents up to 10 %. The refractive index of these doped structures is tailored by the SiO2 content. Detailed characterization of ALD deposited SiO2-containing TiO2 thin films could further expand their application in the future by precisely adjusting the fabrication process for the desired material properties and target application
Approximate dynamic programming with feasibility guarantees
No full textSafe and economic operation of networked systems is challenging. Optimization-based schemes are frequently considered, since they achieve near-optimality while ensuring safety via the explicit consideration of constraints. In applications, these schemes often require solving large-scale optimization problems. Iterative techniques from distributed optimization are frequently proposed for complexity reduction. Yet, they achieve feasibility only asymptotically, which induces a substantial computational burden. This work presents an approximate dynamic programming scheme, which is guaranteed to deliver a feasible solution in "one shot", i.e., in one backward-forward iteration over all subproblems provided they are coupled by a tree structure. Our approach generalizes methods from seemingly disconnected domains such as power systems and optimal control. We demonstrate its efficacy for problems with nonconvex constraints via numerical examples from both domains
Predicting novel terrorism: media coverage as early-warning system of novelty in terror attacks
No full textIn this article, we argue that the process of predicting terrorist attacks needs to integrate the evolving dynamic of terrorism and we make a case for novelty as crucial feature to encompass terrorism’s changing nature. To predict when and how terrorist organizations will conduct their next attack, and whether it will have a novel approach, we base our analysis on media coverage. As media continuously covers political, economic, and societal analyses on a national and international scale, it provides rich information that can fuel early-warning systems for terror attacks. We analyze the content of 2,173,544 newspaper articles, reporting on 42,252 terror attacks by 1,121 organizations. Our analyses show that content of media coverage relates to the interval until the following attack from the same terror organization as well as whether they will conduct a novel and even more devastating terror attack. Hence, our approach and findings can contribute to building early-warning systems
A model template for reachability-based containment checking of imprecise observations in timed automata
No full textVerifying safety requirements by model checking becomes increasingly important for safety-critical applications. For the validity of such proof in practice, the model needs to capture the actual behavior of the real system, which could be tested by containment checks of real observation traces. Basic equivalence checks, however, are not applicable if the system is only partially or imprecisely observable, if the model abstracts from explicit states with symbolic semantics, or if the checks are not expressible in the logics supported by a model checker. In this article, we solve the problem of observation containment checking in timed automata via reachability checking on tester systems. We introduce the logic SRL (sequence reachability logic) to express observations as sequences of delayed reachability properties. Through SBLL (introduced by Aceto et al.) as intermediate logic, we synthesize a set of matcher model templates for partial and imprecise observations and further extend these templates for the case of limited state accessibility in a model. For the obtained matching traces, we define the back-transformation into the original model domain and formally prove the correctness of the transformation. We implemented the observation matching approach, and apply it to a set of 7 demo and 3 case study models with different levels of observability. The results show that all positive and negative observations are correctly classified, and that the most advanced matcher model instance still offers average run times between 0.1 and 1 s in all but 3 scenarios
Efficiency and process development for microbial biomass production using oxic bioelectrosynthesis
No full textAutotrophic microbial electrosynthesis (MES) processes are mainly based on organisms that rely on carbon dioxide (CO2) as an electron acceptor and typically have low biomass yields. However, there are few data on the process and efficiencies of oxic MES (OMES). In this study, we used the knallgas bacterium Kyrpidia spormannii to investigate biomass formation and energy efficiency of cathode-dependent growth. The study revealed that the process can be carried out with the same electron efficiency as conventional gas fermentation, but overcomes disadvantages, such as the use of explosive gas mixtures. When accounting only for the electron input via electrical energy, a solar energy demand of 67.89 kWh kg–1 dry biomass was determined. While anaerobic MES is ideally suited to produce methane, short-chain alcohols, and carboxylic acids, its aerobic counterpart could extend this important range of applications to not only protein for use in the food and feed sector, but also further complex product
Efficient numerical methods for the Maxey-Riley-Gatignol equations with Basset history term
No full textThe Maxey-Riley-Gatignol equations (MRGE) describe the motion of a finite-sized, spherical particle in a fluid. Because of wake effects, the force acting on a particle depends on its past trajectory. This is modeled by an integral term in the MRGE, also called Basset force, that makes its numerical solution challenging and memory intensive. A recent approach proposed by Prasath et al. (2019) [9] exploits connections between the integral term and fractional derivatives to reformulate the MRGE as a time-dependent partial differential equation on a semi-infinite pseudo-space. They also propose a numerical algorithm based on polynomial expansions. This paper develops a numerical approach based on finite difference instead, by adopting techniques by Koleva (2005) [35] and Fazio and Jannelli (2014) [37] to cope with the issues of having an unbounded spatial domain. We compare convergence order and computational efficiency for particles of varying size and density of the polynomial expansion by Prasath et al., our finite difference schemes and a direct integrator for the MRGE based on multi-step methods proposed by Daitche (2013) [29]. While all methods achieve their theoretical convergence order for neutrally buoyant particles with zero initial relative velocity, they suffer from various degrees of order reduction if the initial relative velocity is non-zero or the particle has a different density than the fluid
Region-Specific Coarse Quantization with Check Node Awareness in 5G-LDPC Decoding
No full textThis paper presents novel techniques for improving the error correction performance and reducing the complexity of coarsely quantized 5G LDPC decoders. The proposed decoder design supports arbitrary message-passing schedules on a base-matrix level by modeling exchanged messages with entry-specific discrete random variables. Variable nodes (VNs) and check nodes (CNs) involve compression operations designed using the information bottleneck method to maximize preserved mutual information between code bits and quantized messages. We introduce alignment regions that assign the messages to groups with aligned reliability levels to decrease the number of individual design parameters. Group compositions with degree-specific separation of messages improve performance by up to 0.4 dB. Further, we generalize our recently proposed CN-aware quantizer design to irregular LDPC codes and layered schedules. The method optimizes the VN quantizer to maximize preserved mutual information at the output of the subsequent CN update, enhancing performance by up to 0.2 dB. A schedule optimization modifies the order of layer updates, reducing the average iteration count by up to 35 %. We integrate all new techniques in a rate-compatible decoder design by extending the alignment regions along a rate-dimension. Our complexity analysis shows that 2-bit decoding can double the area efficiency over 4-bit decoding at comparable performance
Modeling cellular self-organization in strain-stiffening hydrogels
No full textWe derive a three-dimensional hydrogel model as a two-phase system of a fibre network and liquid solvent, where the nonlinear elastic network accounts for the strain-stiffening properties typically encountered in biological gels. We use this model to formulate free boundary value problems for a hydrogel layer that allows for swelling or contraction. We derive two-dimensional plain-strain and plain-stress approximations for thick and thin layers respectively, that are subject to external loads and serve as a minimal model for scaffolds for cell attachment and growth. For the collective evolution of the cells as they mechanically interact with the hydrogel layer, we couple it to an agent-based model that also accounts for the traction force exerted by each cell on the hydrogel sheet and other cells during migration. We develop a numerical algorithm for the coupled system and present results on the influence of strain-stiffening, layer geometry, external load and solvent in/outflux on the shape of the layers and on the cell patterns. In particular, we discuss alignment of cells and chain formation under varying conditions
Perturbation properties of the generalized spectral radius
No full textLet n∈N, K∈{R,C} and Mn(K) be the set of all n-by-n matrices with entries in K. We investigate the sensitivity of the generalized spectral radius ρK(A):=max{|λ|:λ∈Kand|Ax|=|λx|foranx∈Kn∖{0}}of A∈Mn(K) under perturbations, i.e. we ask how ρK(A) is related to ρK(A+E) for perturbations E∈Mn(K). For example and somewhat surprisingly, Elsner's famous bound on the spectral variation of two complex square matrices fully translates to |ρC(A)−ρC(B)|⩽∥A+B∥1−1n2∥A−B∥1n2for all A,B∈Mn(C), where ∥⋅∥2 is the matrix 2-norm. For ρR this result holds true locally
Wissenschaft verstehen, Universität erleben, Horizonte erweitern – das ist die Mission der Hamburg Open Online University an der TU Hamburg seit 10 Jahren.
No full textWas treibt Menschen an, die Wissenschaft und Lehre neu denken wollen? Diese Jubiläumsbroschüre präsentiert in einer Zeitreise durch zehn Jahre HOOU an der TU Hamburg Teams und Projekte, die die Universität zu einem innovativen Ort der Wissenschaftsvermittlung gemacht haben. Erfahren Sie, wie eine Wissenschaftlerin Mechanik mit Alltagsbeispielen unkompliziert vermittelt. Verfolgen Sie ein internationales Team, das mit dem „RUVIVAL“-Projekt die ökologische Wiederbelebung ländlicher Räume vorantreibt. Entdecken Sie, wie Mikrobiologie im virtuellen Labor lebendig wird und wie Science-Fiction-Filme zu Katalysatoren für technologische Innovation werden. Diese und viele weitere Einblicke in ein Jahrzehnt HOOU an der TU Hamburg porträtieren Menschen als Innovationstreibende, die Wissenschaft aus den Hörsälen in die Gesellschaft tragen – sei es in Open-Air-Kinos, Biogasanlagen oder digitalen Experimentierräumen. Sie zeigen Menschen, die mit Leidenschaft und Expertise die Zukunft der Wissenschaftsvermittlung und Wissenschaftskommunikation gestalten. Dabei beweisen sie eins: Hier entsteht Innovation, weil Begeisterung auf Kompetenz trifft