Zur Rekonstruktion einer Typologie jugendlichen Medienhandelns gemäß dem Leitbild der Triangulation

Die im Folgenden dargestellten Ergebnisse sind im Rahmen des von der DFG geförderten Forschungsprojekts „Eine Untersuchung zum Mediennutzungsverhalten 12- bis 20-Jähriger und zur Entwicklung von Medienkompetenz im Jugendalter“ entstanden, das gemeinsam von Klaus Peter Treumann, Uwe Sander und Dorothee Meister geleitet wird. Das Forschungsprojekt untersucht das Medienhandeln Jugendlicher sowohl hinsichtlich Neuer als auch alter Medien. Zum einen fragen wir dabei nach den Ausprägungen von Medienkompetenz in verschiedenen Dimensionen und zum anderen konzentrieren wir uns auf die Entwicklung einer empirisch fundierten Typologie jugendlichen Medienhandelns. Methodologisch ist die Untersuchung an dem Leitbild der Triangulation orientiert und kombiniert qualitative und quantitative Zugänge zum Forschungsfeld in Form von Gruppendiskussionen, leitfadengestützten Einzelinterviews und einer Repräsentativerhebung

Hauptkomponentenanalytische Untersuchungen zum Medienhandeln Jugendlicher

Die vorliegende Arbeit ist im Rahmen des von der DFG geförderten Forschungsprojekts „Eine Untersuchung zum Mediennutzungsverhalten 12-20jähriger und zur Entwicklung von Medienkompetenz im Jugendalter“ entstanden, welches von Klaus Peter Treumann, Uwe Sander und Dorothee Meister geleitet wird. Es bezieht sich auf die Bundesländer Mecklenburg-Vorpommern, Nordrhein-Westfalen und Sachsen-Anhalt mit den drei Projektstandorten Bielefeld, Halle a.d. Saale und Rostock. Das Forschungsvorhaben kombiniert qualitative und quantitative Zugänge zum Forschungsfeld in Form von Gruppendiskussionen, leitfadengesteuerten Interviews und einer standardisierten Befragung von Jugendlichen. Im Folgenden wird über einige Befunde quantitativer Analysen berichtet

Morphological transitions in Al-Ge alloys observed with in-situ X-radiography

In-situ X-radiography and X-ray tomoscopy is used to investigate the evolution of dendrites in Al-Ge alloys towards the final grain morphology. The focus was drawn on the investigation of the globulitic to dendritic transition (GDT) with respect to destabilization of the solid-liquid interface, cooling rate and grain density. Furthermore, the dendritic to globulitic transition (DGT) was investigated during advanced stages of solidification. As in most solidification cases the solid-liquid interface of a growing grain destabilizes at an early stage of the solidification process. A nucleus first grows with a spherical shape into the undercooled melt. Dendritic arms are formed due to perturbations of the solid-liquid interface. Synchrotron high speed tomoscopy was employed to get a deeper insight into this phenomenon. For the first time, the GDT was directly observed and the critical radius at which the GDT takes place was measured. Comparison between the measured radii from the experiment and the model predictions by Dantzig and Rappaz showed that the model underestimates the critical stable radius for globulitic growth in the Al-Ge system. To avoid destabilization of the solid-liquid interface the cooling rate and the grain density play an important role. The cooling rate has a direct impact on the nucleation undercooling which is needed for the nucleation process. A higher cooling rate leads to higher nucleation undercooling, which triggers more inoculants to nucleate. The thermal gradient between nuclei and melt causes perturbations of the solid-liquid interface. A smaller and more homogeneous nucleation undercooling was thought to prevent destabilization of the solid-liquid interface. A higher grain density leads to an early interaction of the growing dendrites. Solutal fields due to enrichment of the melt by germanium, rejected by the growing Al-dendrite or impingement of the grains can also promote globulitic grain morphology. More grains impinge earlier and suppress the formation of dendritic branches. The impact of cooling rates from 0.0016 Ks-1 to 1.5 Ks-1 was in-situ investigated with X-radiography. Grain refiner was used to vary the grain density. The Al-Ge alloy compositions of Al-10 wt.%Ge, Al-20 wt.%Ge, Al-33 wt.%Ge and Al-46 wt.%Ge were analyzed. The near-isothermal, X-ray transparent furnace developed at DLR (ITF1) enables the measurement of the solutal fields around a nucleating and growing grain in an isothermal field. The X-ray transparent Bridgman type gradient furnace (XRMON-GF), at NTNU, was operated in isothermal mode and used to achieve larger cooling rates. One challenge in increasing the grain density of Al-Ge alloys was to find a suitable grain refiner. For the standard grain refiner for Al-alloys, Al-5Ti-1B, a poisoning effect was observed, getting stronger with increasing germanium or titanium concentration. Germanium and titanium form stable interlayers around the grain refiner particle, which prevents the epitactical growth of the Al-matrix on the particle and reduces the efficiency of the grain refiner. SEM and TEM analysis techniques were employed to characterize those stable Ti-Ge layers. A new, niobium-based grain refiner showed promising results for application in the Al-Ge system. In this work, Al-4.6Nb-0.5B was tested the first time as a grain refiner for Al-Ge alloys. Stable and reproducible results in Al-Ge alloys were found and an overall better performance compared to Al-5Ti-1B was observed. Comparing the industrial grain refiner AlB3 with Al-4.6Nb-0.5B the performance was found to be similar. Only for small amounts of germanium, the niobium-based grain refiner showed better results. Both grain refiners Al-5Ti-1B and Al-4.6Nb-0.5B were used for nucleation studies in the system Al-Ge and were compared with the “solute segregation nucleation stifling model”) by Li and Mathiesen et al. (model 1) and the “interdependence theory” of St.John et al. (model 2). Model 1 supposes recalescence-free solidification and a nucleation stifling caused by solute segregation around the nucleated and growing grain. Modeling data were compared with the experimental results obtained at the XRMON-GF furnace at NTNU. Due to rapid heat extraction, good heat transfer between the sample, furnace and environment and constantly applied cooling rates, a recalescence-free solidification process could be assumed. For sample inoculated with Al-5Ti-1B grain refiner a deviation of the prediction of the grain density and the final average grain size predicted by model 1 and experimental results was found. Only a few and large grains were growing. The results can be attributed to poisoning of the grain refiner particles in the Al-Ge system, whereby less grain refiner particles were active in the melt. Inoculation with Al-4.6Nb-0.5B grain refiner showed conformity with the model results for high cooling rates and well grain refined samples. For low cooling rates and a poorly grain refined melt, the model parameters need further adaption. The experimental result from the near-isothermal furnace (ITF1) developed at the DLR were compared with the “interdependence theory” of St.John et al. (model 2). Assuming a recalescence event after the nucleation takes place, the model can be used to predict the final grain size in the cast and a resulting nucleation free zone (NFZ) due to recalescence. Measurements of the enrichment of the solute around the growing grain from the in-situ experiments with Al-33 wt.%Ge inoculated with Al-5Ti-1B grain refiner were compared with model 2. The comparison showed an underestimation of the size of the NFZ and the final grain size by model 2. Key determinants for the deviation between the prediction by the model and the experimental length of the NFZ were: i) the poisoned performance of the Al-5Ti-1B grain refiner particles. Fewer dendrites nucleate and have more time to grow. ii) the treatment of the dendritic growth velocity by the “interdependence theory”. Model 2 assumes a steady-state growth velocity of the dendritic tip, while the experimental results show that the dendritic tip velocity decrease over time, before a steady-state growth velocity is reached. iii) in addition, the influence of the calculation of the solute supersaturation in front of the growing dendrite were found to have a major impact. Different approaches to calculate the germanium concentration in front of the dendritic tip were made and compared with experimental values. A more precise examination of the germanium concentration in front of the dendritic tip is proposed. Besides the investigation of the GDT the transition from dendritic to globulitic morphology (DGT) was investigated. Here, two aspects were found to be the determining factor: the coarsening of the dendrites and globulitic growth from a semi-solid state. For the investigation of the DGT during advanced stages of solidification, a new, X-ray transparent, isothermal, furnace setup (ITF2) for long term solidification experiments was designed and successfully tested. A new heater concept enabled a constant heat input parallel to the sample surface. Temperature gradient zone melting (TGZM), caused by a thermal gradient could be prevented with this new furnace setup. Improvement of the temperature measurement inside the furnace additionally enables a more precise determination of the temperature in the melt

Datenbeschreibung

Strotmann M, Wegener C. Datenbeschreibung. In: Mikos L, Wegener C, eds. Qualitative Medienforschung. Ein Handbuch. Konstanz: UVK-Verlag; 2005: 395-403

Isothermal furnace for long-term in-situ and real-time X-radiography solidification experiments

A new x-ray isothermal furnace has been developed, suitable for in situ observations of semi-solid processes including the transition from dendritic to globulitic grain morphology and grain coarsening in metallic samples. A homogeneous, isothermal temperature field is achieved using a novel heater concept. The furnace structure is sandwich-like with heating elements positioned in the beam line and parallel to the sample. Planar heat transfer to the sample enables measurements with low cooling rates and a minimized temperature gradient. Cooling rates from 0.1 to 15 K min-1 can be controlled in the temperature range 1170-670 K. The furnace setup is integrable in the existing rotatable laboratory x-ray facility (X-RISE) at the German Aerospace Center (DLR). In this setup, an effective pixel size of 3 mum and a field of view of 8 mm in diameter can be achieved. Preliminary solidification and semi-solid experiments in the hypo-eutectic alloy systems Al-Ge and Al-Cu, inoculated with Al-5Ti-1B grain refiner, are presented. They indicate a very uniform temperature distribution in the sample

XRISE-M: X-radiography facility for solidification and diffusion studies of alloys aboard sounding rockets.

A compact fully protected microfocus X-radiography facility (XRISE-M) is presented for the study of microstructure evolution during the solidification of thin liquid alloy samples and chemical diffusion in liquid binary alloys in situ and in real-time aboard a sounding rocket. XRISE-M presently enables the simultaneous processing of either two near-isothermal solidification furnaces or a combination of a linear-shear cell diffusion furnace and a near-isothermal solidification furnace. For optimal detector calibration shortly before flight, the furnaces can be rotated around the central beam axis and calibration images can be recorded. The facility allows preheating the samples into the liquid state prior to lift-off without leakage during the ascent phase at accelerations of up to 27 g. Macrosegregation on remelting of thin metal samples for microstructure evolution investigations is prevented by an inclinable furnace metric. The use of ion-getter pumps for vacuum generation enables us to exploit the entire available time of reduced gravity for image recording and data acquisition. With the device and currently available sample environments, microstructure formation upon solidification and chemical diffusion under purely diffusive conditions in alloys can be investigated. The facility can be used equally for other investigations such as granular matter dynamics or metal foaming, provided that suitable experiment inserts are developed in the future

Hauptkomponentenanalytische Untersuchungen zum Medienhandeln Jugendlicher

Treumann KP, Burkatzki E, Strotmann M, Wegener C. Hauptkomponentenanalytische Untersuchungen zum Medienhandeln Jugendlicher. Jahrbuch Medienpädagogik. 2005;4:145-168