Nonlinear ac susceptibility studies of high-TcT_c rings: Influence of the structuring method and determination of the flux creep exponent

We have studied the influence of the patterning procedure on the critical current density of high-$T_c$ YBa$_2$Cu$_3$O$_{7-\delta}$ thin rings using the nonlinear ac susceptibility method. At no applied dc magnetic field we have found that laser ablation degrades strongly the critical current density whereas ion beam etching has only a weak influence on it. From the measurements at different frequencies and dc magnetic fields we analyzed the influence of flux creep and obtained the field dependence of the flux creep exponent. Our data reconfirm the recently observed scaling relation for the nonlinear susceptibility response of type-II superconductors.Comment: 10 pages, 12 figure

Untersuchungen zum Einfluss von Cer auf die Primärgefügeentwicklung von Stahl und die Bestimmung der Warmrissneigung mittels thermischer Analyse

Stahl ist der weltweit wichtigste Konstruktionswerkstoff. Nach der Erzeugung wird der flüssige Stahl im Strang-, Form-, oder Blockguss vergossen und erstarrt dabei in Abhängigkeit von den Abkühlbedingungen in der Regel kristallin. Je feiner dieses kristalline Primärgefüge ist, desto höher sind die mechanischen und auch gießtechnologischen Eigenschaften des Stahls. So werden unter anderem Porosität, Warmrisse sowie Seigerungen reduziert. Als metallurgische Lösung hat sich in diesem Zusammenhang die sogenannte Dispersoidmetallurgie entwickelt. Durch die gezielte Erzeugung oder Zugabe von nichtmetallischen Einschlüssen, den sogenannten Dispersoiden, sollen Keimstellen in der Stahlschmelze zur Verfügung gestellt und das Erstarrungsgefüge gefeint werden. Cer bzw. die Verbindungen des Cers haben sich in einigen Untersuchungen als besonders geeignet herausgestellt, da cerhaltige Dispersoide zum einen den Stahl feinen und zum anderen aufgrund ihrer geringen Härte eine geringe Kerbwirkung aufweisen. In den bisherigen Untersuchungen hat sich jedoch neben einer Kornfeinung auch eine Vergröberung des Primärgefüges nach einer Cerzugabe gezeigt. In der angefertigten Dissertation wurden die Mechanismen und Wechselwirkungen bei der Behandlung von Stahlschmelzen mit Cer mit Gießversuchen im Labor- und Industriemaßstab sowie durch thermodynamische Berechnungen und Phasenfeldsimulationen untersucht. Es konnte gezeigt werden, dass Cerverbindungen in primär ferritisch und primär austenitisch erstarrenden Stählen kornfeinend sind. Darüber hinaus wurde gezeigt, dass die Oberflächenspannung des flüssigen Stahls mit dem Anteil endogenen Gefüges korreliert. Die Oberflächenspannung wurde dafür mit einem mathematischen Modell, welches auf empirischen Daten basiert, und mit thermodynamischen Berechnungen bestimmt. Eine der wichtigsten Anforderungen an die gießtechnologischen Eigenschaften im Stahlguss ist eine geringe Warmrissneigung. Diese wird maßgeblich durch das Primärgefüge bestimmt. Jedoch hat sich im Rahmen der Untersuchungen herausgestellt, dass die bekannten Methoden zur Quantifizierung der Warmrissneigung im Stahl dieses nur unzulänglich berücksichtigen. Für Aluminiumlegierungen wird seit mehreren Jahren die thermische Erstarrungsanalyse zur Bestimmung verschiedener gießtechnologischer Eigenschaften verwendet. Unter anderem wird durch die Bestimmung des sogenannten Dendritenkohärenzpunktes eine Vorhersage der Warmrissneigung ermöglicht. Dies konnte bisher jedoch nur bedingt auf Stahl übertragen werden. Aus diesem Grund werden im zweiten Teil der Dissertation Untersuchungen vorgestellt, die auf bekannten experimentellen Versuchsaufbauten zur Bestimmung der Warmrissneigung basieren und mit Ergebnissen aus der thermischen Erstarrungsanalyse kombiniert werden. Es wurde ein Modell entwickelt, dass eine Möglichkeit zur Quantifizierung der Warmrissneigung durch die thermische Analyse in Form eines neu entwickelten CSC (Cracking susceptibility coefficient) aufzeigt.Steel is one of the most important construction materials. After production, the liquid steel is cast continuously in copper molds or batchwise in sand molds or ingots. In general, the liquid steel solidifies crystalline depending on the cooling conditions. The finer the primary structure, the higher the mechanical and casting properties of the steel. In addition, porosity, hot cracks and segregations are reduced. The so-called dispersoid metallurgy has developed in this context as a metallurgical solution. Through the specific formation or addition of non-metallic inclusions, the dispersoids, more nucleation sites in the molten steel are made available, which results in a finer solidification structure. Some investigations have shown that cerium and its compounds are highly suitable. This is because cerium-containing dispersoids produce fine steel on the one hand and have a low notch effect due to their low hardness on the other. In the investigations that have been carried out in the past, however, not only grain refinement but also a coarsening of the primary structure after the addition of cerium has been shown. In the thesis the mechanisms and interactions in the treatment of steel melts with cerium were investigated by casting tests on a laboratory and industrial scale as well as by thermodynamic calculations and phase field simulations. It could be shown that cerium compounds are grain refining in primary ferritic and primary austenitic solidifying steels. In addition, it was shown that the surface tension of the liquid steel correlates with the amount of equiaxed microstructure. The surface tension was determined using a mathematical model based on empirical results and thermodynamic calculations. One of the most important requirements to the casting properties in steel casting is a low hot tearing tendency. This is mainly influenced by the primary structure. However, the investigations have shown that the known methods for quantifying the hot cracking tendency in steel do not adequately take this into account. For aluminium alloys, thermal solidification analysis has been successfully used for several years to determine various casting properties. For example, the determination of the so-called dendrite coherence point enables a prediction of the hot tearing tendency. Up to now, however, its application to steel has been limited. For this reason, the second part of the thesis presents investigations based on known experimental set-ups for determining the hot tearing tendency and combined with results from thermal solidification analysis. A model has been developed which shows a possibility to quantify the hot tearing tendency by thermal analysis in the form of a new developed CSC (Cracking susceptibility coefficient)

Biosynthese und Funktion von Naturstoffen aus Mortierella alpina

In dieser Arbeit wurde anhand des Modellorganismus Mortierella alpina gezeigt, dass basale Pilze wohl zu ausgedehnter Naturstoffbiosyntese befähigt sind. So konnte für die Biosynthese der Malpinine, der Malpicycline, der Malpibaldine sowie das Calpinactam jeweils eine zugehörige Nicht-ribosomale Peptidsynthetase (NRPSen) identifziert werden. Bei den jeweils zugrundeliegenden Biosynthesegenen malA, mpbA, calA und mpcA handelt es sich um pilzliche Gene, die jedoch bakteriellen Ursprungs sind und durch horizontalen Gentransfer auf M. alpina übertragen worden sind. Zusätzlich wurde die biologische Funktion sowie mögliche pharmazeutische Anwendungsmöglichkeiten untersucht

Distribution of the magnetic field and current density in superconducting films of finite thickness

A one-dimensional equation describing the distribution of the effective vector potential $\bar A(y)$ across a film width, which holds for thin ($d\lambda$) films alike, is derived based on the analysis of a 2D Maxwell-Londons equation for superconducting films in a perpendicular magnetic field. The validity of this equation for a finite-thickness film is verified by a numerical analysis. An approximation dependence $\bar A(y)$, finite (with all of its derivatives) across the entire film width, is found for films, being in the Meissner state. The flux-entry field is evaluated for a film of arbitrary thickness. An approximation expression is obtained for the distribution of the sheet current density in the mixed state of a pin-free superconducting film with an edge barrier. The latter approximation allows to estimate magnetic field concentration factor at the film edge as a function of external magnetic field and geometrical parameters of the sample.Comment: 12 pages, 10 figure

Identification of the Kombucha consumer profile in Fortaleza-CE

Made from the fermentation of sweetened tea from the Camellia sinensis herb by a symbiotic culture of bacteria and yeasts, Kombucha has been gaining ground in the Brazilian and international scene for its benefits and its possible ability to assist in human well-being. This fermented beverage has shown significant growth in the beverage market and, because of this, the present work aimed to carry out a data survey on the set of behavioral characteristics of kombucha consumers in the city of Fortaleza-CE and metropolitan region and the relationship of these individuals with the drink; as well as identifying the attributes that influence the purchase decision and prospecting the population's acceptability for fermented beverages such as kombucha. To carry out the research, the Google forms platform was used, and these forms were distributed through social networks following the snowball technique. XLSTAT software, Maxdiff test and Excel were used for data analysis. Thus, it was found that most kombucha consumers are women, young people, with an income between 1 and 4 minimum salary, single, with higher education and residents of the noblest areas of the city. The attributes that were considered decisive for the purchase decision were: health benefits, probiotic, flavor, increase immunity, contain no chemical additives and high antioxidant content. In this way, it is expected that this study will contribute with a direction to kombucha producers for the elaboration of new products directed to a specific public.Made from the fermentation of sweetened tea from the Camellia sinensis herb by a symbiotic culture of bacteria and yeasts, Kombucha has been gaining ground in the Brazilian and international scene for its benefits and its possible ability to assist in human well-being. This fermented beverage has shown significant growth in the beverage market and, because of this, the present work aimed to carry out a data survey on the set of behavioral characteristics of kombucha consumers in the city of Fortaleza-CE and metropolitan region and the relationship of these individuals with the drink; as well as identifying the attributes that influence the purchase decision and prospecting the population's acceptability for fermented beverages such as kombucha. To carry out the research, the Google forms platform was used, and these forms were distributed through social networks following the snowball technique. XLSTAT software, Maxdiff test and Excel were used for data analysis. Thus, it was found that most kombucha consumers are women, young people, with an income between 1 and 4 minimum salary, single, with higher education and residents of the noblest areas of the city. The attributes that were considered decisive for the purchase decision were: health benefits, probiotic, flavor, increase immunity, contain no chemical additives and high antioxidant content. In this way, it is expected that this study will contribute with a direction to kombucha producers for the elaboration of new products directed to a specific public

Higher harmonics of ac voltage response in narrow strips of YBa2Cu3O7 thin films: Evidence for strong thermal fluctuations

We report on measurements of higher harmonics of the ac voltage response in strips of YBa2Cu3O7 thin films as a function of temperature, frequency and ac current amplitude. The third (fifth) harmonic of the local voltage is found to exhibit a negative (positive) peak at the superconducting transition temperature and their amplitudes are closely related to the slope (derivative) of the first (Ohmic) harmonic. The peaks practically do not depend on frequency and no even (second or fourth) harmonics are detected. The observed data can be interpreted in terms of ac current induced thermal modulation of the sample temperature added to strong thermally activated fluctuations in the transition region.Comment: 9 pages, 4 figures (PDF file