856 research outputs found
Estudos cinéticos de uma reação de Heck intramolecular em meio aquoso catalisadas por nanopartículas de paládio suportadas em polietilenoimina ramificada
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina, Centro de Ciências Físicas e Matemáticas, Programa de Pós-Graduação em Química, Florianópolis, 2013Reações de acoplamento Carbono-Carbono do tipo Heck, em meio aquoso, tem se destacado como uma nova forma de realizar síntese orgânica, com uma metodologia mais barata, segura e confiável para a construção de diferentes moléculas. Até o momento poucos trabalhos foram dedicados ao estudo mecanístico destas reações. Neste sentido, este trabalho envolve a preparação, caracterização e o emprego de colóides de paládio, estabilizados pelo polímero comercial polietilenoimina, como catalisador em uma Reação de Heck Intramolecular, na presença de base, usando água como solvente e sem controle da atmosfera. O estudo dos diversos aspectos que influenciaram a velocidade da reação, assim como também a conversão ao produto, foi realizado por uma nova metodologia cinética. Análise Cinética do Progresso da Reação foi aplicada e pelos resultados obtidos foi observado que a reação é de primeira-ordem em relação ao catalisador e ordens em relação aos outros reagentes. Além disso, identificou-se uma forte relação, tanto na velocidade da reação quanto na conversão ao produto, com as concentrações iniciais da base. A análise destes resultados, aliados à estudos morfológicos das nanopartículas de paládio por microscopia eletrônica de transmissão, indicam a participação da superfície da nanopartícula e a alteração de seu tamanho durante o progresso da reação. Ainda, nas condições reacionais estudadas foi possível obter até 90% de conversão a produto após 24 horas, sem uso de atmosfera inerte e em condições brandas de temperatura.Abstract : Carbon-Carbon Heck coupling reactions, in aqueous media, have become a new way to make organic synthesis, as a cheaper, safer and more reliable methodology to build different molecules. So far only a few works have been dedicated to the mechanistic studies of these reactions. Through this way, the present work involves the preparation, characterization and the use of palladium colloids, stabilized by the commercial polymer polyethyleneimine, in an Intramolecular Heck Reaction, in the presence of a base, using water as the reaction solvent and without control of the atmosphere. The study of the different aspects that influenced the reaction rate, as well as the product conversion, was performed by a new kinetic methodology. Reaction Progress Kinetic Analysis was applied and from the results it was observed that the reaction is first-order in respect to the catalyst and have complex orders in respect to other reactants. Besides, it has been identified a strong relationship of the reaction rate, and product conversion, with the initial concentrations of the base. Analysis of these results, allied to the palladium nanoparticles morphological studies by transmission electron microscopy, indicated a participation of the nanoparticle#s surface and its size shifting during the reaction progress. Moreover, in the reaction conditions studied it was possible to obtain up to 90% of conversion to product in 24 hours without inert atmosphere and under mild conditions of temperature
Zwischen Boykott und Benchmarks : das Abgeordnetenhaus in Bagdad und aktuelle parlamentarische Arbeit im Irak
No full textvon Michael Bröning ; Dominik Heck und Matthias S. KleinElectronic ed.: Amman ; Bonn : FES, 2008. - Title only available onlin
Bayesian Estimation and Comparison of Idiographic Network Models
No full textContains supplementary materials and code for the preprint "Bayesian Estimation and Comparison of Idiographic Network Models" (Siepe & Heck, 2023). Both Preprint and Supplement can be found in the folder "manuscript", where "v2" contains the updated version of the preprint and supplement after a first round of revisions. During the revision, Matthias Kloft was added as a co-author
Bayesian Estimation and Comparison of Idiographic Network Models
No full textContains supplementary materials and code for the preprint "Bayesian Estimation and Comparison of Idiographic Network Models" (Siepe & Heck, 2023). Both Preprint and Supplement can be found in the folder "manuscript", where "v2" contains the updated version of the preprint and supplement after a first round of revisions. During the revision, Matthias Kloft was added as a co-author
Hanns Hubach, Matthias Grünewald. Der Aschaffenburger Maria Schnee- Altar. Geschichte-Rekonstruktion-Ikonographie. Mayence, Gesellschaft für mittelrheinische Kirchengeschichte, "Quellen und Abhandlungen zur mittelrheinischen Kirchengeschichte, 77",1996. 344 p., 16 pl. coul., 108 ill. n. et bl.
No full textHeck Christian. Hanns Hubach, Matthias Grünewald. Der Aschaffenburger Maria Schnee- Altar. Geschichte-Rekonstruktion-Ikonographie. Mayence, Gesellschaft für mittelrheinische Kirchengeschichte, "Quellen und Abhandlungen zur mittelrheinischen Kirchengeschichte, 77",1996. 344 p., 16 pl. coul., 108 ill. n. et bl.. In: Revue de l'Art, 1997, n°118. p. 81
Diblock Copolymers with Similar Glass Transition Temperatures in Both Blocks for Comparing Shear Orientation Processes with DPD Computer Simulations
No full textAbstract The purpose of this study is the development of phase‐separating diblock copolymer model systems suitable for the comparison between the orientation process under shear and computer simulations of the same process. To do so, the polymer systems have to fulfill certain requirements like similar dynamics of both polymer blocks, which are associated with comparable glass transition temperatures. The development of suitable diblock copolymer systems such as poly(ethyl methacrylate)‐ b ‐poly(2‐vinylpyridine) or the tuning of already established polymer systems like polystyrene‐ b ‐poly(2‐vinylpyridine) via copolymerization with 4‐vinylpyridine is essential. Anionic polymerization is used for the sample synthesis to achieve this goal with the lowest possible polydispersity. Soft, coarse‐grained models are used to study the block dynamics in computer simulations. Within the dissipative particle dynamics methods the remaining differences in the block mobility are addressed. The experimentally determined zero‐shear viscosity η 0 is used to match the simulation results to the synthesized polymer melts
Transitions between Lamellar Orientations in Shear Flow
No full textShear flow is a versatile strategy to align microphase-separated morphologies of diblock copolymers over macroscopic scales. Details of the local mechanism of reorientation toward the steady, nonequilibrium state, however, are only incompletely understood. Using large scale molecular simulation as well as experiments, we study the shear-alignment mechanism using lamella-forming, symmetric, unentangled diblock copolymers in steady and oscillatory shear flow. First we study homogeneously oriented systems and investigate the stability of different orientations with respect to the shear flow by the Rayleighian. Second, we investigate the process of reorienting an unstable grain with parallel orientation embedded in a matrix of stable, perpendicularly oriented lamellae. We observe two different reorientation mechanisms as a function of the shear rate: a fast transition, which is comparable to experimental conditions in large-amplitude oscillatory shear (LAOS) tests, and a slower transition occurring at lower shear rates. We show that for high shear rates the long-range orientational order of the lamellae inside the unstable grain disintegrates, while the grain remains spatially structured with the same characteristic length scale (similar to a microemulsion). At lower shear rates, however, we observe a shrinking of the unstable grain, i.e., a directed movement of the grain boundaries. Additionally, we compare the results of steady shear with oscillatory shear
Transparency of AI Systems (Practitioner Track)
No full textArtificial Intelligence (AI) has now established itself as a tool for both private and professional use and is omnipresent. The number of available AI systems is constantly increasing and the underlying technologies are evolving rapidly. On an abstract level, most of these systems are operating in a black box manner: only the inputs to and the outputs of the system are visible from outside. Moreover, system outputs often lack explainability, which makes them difficult to verify without expert knowledge. The increasing complexity of AI systems and poor or missing information about the system make an assessment by eye as well as assessing the system’s trustworthiness difficult. The goal is to empower stakeholders in assessing the suitability of an AI system according to their needs and aims. The definition of the term transparency in the context of AI systems represents a first step in this direction. Transparency starts with the disclosure and provision of information and is embedded in the broad field of trustworthy AI systems. Within the scope of this paper, the Federal Office for Information Security (BSI) defines transparency of AI systems for different stakeholders. In order to keep pace with the technical progress and to avoid continuous renewals and adaptions of the definition to the current state of technology, this paper presents a technology-neutral and future-proof definition of transparency. Furthermore, the presented definition follows a holistic approach and thus also takes into account information about the ecosystem of an AI system. In this paper, we discuss our approach and proceeding as well as the opportunities and risks of transparent AI systems. The full version of the paper includes the connection to the transparency requirements in the EU AI Act of the European Parliament and council
Impedance change and capacity fade of lithium nickel manganese cobalt oxide-based batteries during calendar aging
No full textThe calendar aging of commercial 18650 lithium-ion batteries with lithium nickel manganese cobalt oxide cathode and graphite anode is studied by regular electrochemical characterization of batteries stored at defined conditions. The cell capacity is found to decrease linearly with time and shows a faster decrease at higher storage temperatures. From current pulse tests, it is determined that both higher temperature and higher state of charge (SOC) cause accelerated resistance increase with storage time. Changes in different battery parameters during storage are also quantified by analyzing electrochemical impedance spectroscopy (EIS) spectra. The cell degradation causes a gradual increase of the ohmic and the total polarization resistance with storage duration, where the latter one is found to be the main contributor to the increased cell impedance. An increase in the mean relaxation time constant and changes in the porous structure for the electrode processes are observed from EIS analysis. Resistance for this cell chemistry is found to be current independent by comparing the cell resistance calculated from the current pulse method after 1s and from the EIS analysis at 1 Hz. Furthermore, it is seen that the additional charge throughput due to the periodic electrochemical characterization induces significant cell degradation effects
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