Ulm, Univ., Diss., 2007.

Zugl.: Aachen, Techn. Hochsch., Diss., 2008

Universiẗat, Diss., 1999--München.

Techn. Hochsch., Diss., 2002--Aachen.

Techn. Universiẗat, Diss., 2006--Darmstadt.

This master thesis deals with the development of electrophoretic methods for determination of tantalum in recycled coolants used in the production of tantalum capacitors, as well as the separation of tantalum from chemically similar niobium. Developed methods are based on the formation of negatively charged complexes of tantalum and niobium with a number of ligands added to the separation electrolyte, especially tartaric and citric acid and 4-(2-pyridylazo)resorcinol. The separations were performed in a fused silica capillary of the inner diameter of 50 µm and the effective length of 36.5 cm, a negative voltage of -25 kV was applied to the capillary. Using a separation electrolyte containing 0.1 mol·l-1 of tartaric and citric acid at pH = 2, separation of niobium and tantalum was achieved in less than 3 minutes; the absorption spectrometric detection was carried out in the ultraviolet region at a wavelength of 200 nm, the detection limit was 2.8 ppm of tantalum, and the quantification limit was 9.2 ppm of tantalum. Addition of 4-(2-pyridylazo)resorcinol reduced the detection limit to 0.8 ppm of tantalum and the quantification limit to 2.6 ppm of tantalum; the electrolyte contained 0.1 mmol·l-1 of 4-(2-pyridylazo)resorcinol, 10 mmol·l-1 of tartaric acid, 10 mmol·l-1 of citric acid, and 25 mmol·l-1...

Die vorliegende Arbeit entstand im Zeitraum 12/2014 bis 02/2018 im erweiterten Rahmen des BMWi-Projekts „NeoKarII“ in Kooperation mit der Umicore AG & Co. KG. Das Projekt befasste sich mit der Suche nach neuartigen, oxidischen Elektrodenmaterialien für Polymerelektrolytmembranbrennstoffzellen (PEM-FC). Im Rahmen dieser Arbeit wurden Niob- und Tantal-dotierte Zinnoxide mit verschiedenen Dotiergraden (hauptsächlich 1 bis 10 %) über Sol-Gel Synthesen, Imprägnierungen und Co-Fällung hergestellt. Für die Co-Fällungen wurde eine MicroJet-Reaktor Anlage entwickelt und aufgebaut. Die Materialien wurden anschließend röntgenografisch untersucht und auf ihre Eignung für die Anwendung als Katalysatorträgermaterial in PEM-FC geprüft. Als Zielgrößen dienten die BET-Oberfläche und spezifische Leitfähigkeit, welche mit einem eigens entwickelten Leitfähigkeitsmessstand ermittelt wurde.:1 Einleitung 1.1 Brennstoffzellen 1.2 Methoden zur Synthese der Mischoxide 1.3 Eigenschaften von Zinnoxiden 1.4 Zielsetzung 2 Ergebnisse und Diskussion 2.1 Synthesen und Beobachtungen 2.2 Röntgenfluoreszenzanalyse 2.3 Pulver-Röntgendiffraktometrie 2.4 BET-Oberfläche 2.5 Leitfähigkeit 2.6 Röntgenphotoelektronenspektroskopie 3 Experimenteller Teil 3.1 Synthesen 3.2 Analytische Methoden 4 Zusammenfassung und Ausblick 4.1 Zusammenfassung 4.2 Ausblick 5 Anhang / This thesis was developed between 12/2014 and 02/2018 in an extended framework of BMWi project „NeoKarII “ in cooperation with Umicore AG & Co. KG. The project was concerned with the search for novel oxidic electrode materials for polymer electrolyte fuel cells (PEM-FC). In this work we prepared niobium- and tantalum-doped tin oxides with different doping levels (mainly 1 to 10 %) by sol-gel synthesis, impregnation and co-precipitation. For co-precipitation we developed and built a MicroJet reactor plant. We analysed the materials by X-ray diffraction and tested them for suitability for use as catalyst support material in PEM-FC. Target values were BET surface area and specific conductivity, which was determined using a specially developed conductivity measurement setup.:1 Einleitung 1.1 Brennstoffzellen 1.2 Methoden zur Synthese der Mischoxide 1.3 Eigenschaften von Zinnoxiden 1.4 Zielsetzung 2 Ergebnisse und Diskussion 2.1 Synthesen und Beobachtungen 2.2 Röntgenfluoreszenzanalyse 2.3 Pulver-Röntgendiffraktometrie 2.4 BET-Oberfläche 2.5 Leitfähigkeit 2.6 Röntgenphotoelektronenspektroskopie 3 Experimenteller Teil 3.1 Synthesen 3.2 Analytische Methoden 4 Zusammenfassung und Ausblick 4.1 Zusammenfassung 4.2 Ausblick 5 Anhang

Niobium (Nb) and Rare Earth Elements (REE’s) have in recent years received considerable attention because of their importance to the modern technical industry, and more specifically the enhanced sustainability that comes with them. The main source for Nb and REE’s on Earth are carbonatites and associated alkaline silicate rocks. This report examines the magnetic properties of rocks from the Oldoinyo Dili carbonatite complex in northern Tanzania. Previous workers have suggested a link between the Fe-bearing mineralogy and the formation of Nb-mineralizations at Oldoinyo Dili. This hypothesis is further examined in this report by combining detailed petrographic observations and withnew measurements of magnetic susceptibility. The aim is to see if any correlation exists between occurrence of Nb-mineralizations and the types of Fe-minerals present at Oldoinyo Dili. Based on the magnetic susceptibility measurements, at least two different species of Fe-minerals arefound in the examined samples. These are characterized by different magnetic trends during heating/cooling and also by their separate Curie temperatures (Tc). In combination with the petrographic observations these minerals are interpreted to be magnetite (Fe2O4) with Tc ~580°C, and a mineral that most likely represents a solid solution between ilmenite (FeTiO3) and hematite (Fe2O3) with Tc ~300°C. Here, no clear link between the type of opaque mineral(s) present and the total Nb content of the carbonatites can be conclusively determined based on the petrography and the magnetic measurements alone. Although the results of this report provide an important first step towards understanding the relationship between Nb-mineralizations and the magnetic mineralogy at Oldoinyo Dili, more detailed analyses of the mineral chemistry is a necessity to fully understand their complex relations and the specific conditions under which they formed. / Niob (Nb) och sällsynta jordartsmetaller (REE’s) har på senare år fått stor uppmärksamhet för sin betydelse för den moderna tekniska industrin, och specifikt för den förhöjda hållbarhet som de bidrar med. Den huvudsakliga källan till Nb och REE’s på jorden är karbonatiter och associerade alkalisilikater. Denna rapport undersöker de magnetiska egenskaperna för karbonatit-komplexet Oldoinyo Dili i norra Tanzania. Forskare har tidigare anat att det finns en koppling mellan Fe-bärande mineralogi och bildandet av Nb-mineraliseringar vid Oldoinyo Dili. Denna hypotes undersöks vidare i denna rapport genom att kombinera detaljerade petrografiska observationer med nya mätningar av magnetisk susceptibilitet. Syftet är att undersöka om det finns någon korrelation mellan förekomst av Nb-mineraliseringar och de typer av järnmineral som finns vid Oldoinyo Dili. Baserat på de genomförda magnetiska susceptibilitets-mätningarna så finns det åtminstone två olika sorters järnmineral i de undersökta proverna. De karaktäriseras av olika magnetiska trender vid upphettning/nedkylning och även av sina olika Curietemperaturer (Tc). Kombinerat med petrografiska observationer uttolkas att dessa mineral är magnetit (Fe2O4) med Tc ~580°C, samt en mineral som troligen är en solid solution av ilmenit (FeTiO3) och hematit (Fe2O3) med Tc ~300°C. Det går inte att senågon tydlig koppling mellan förekommande opaka mineral och det totala Nb-innehållet i karbonatiterna med säkerhet enbart utifrån petrografin och de genomförda magnetiska mätningarna. Resultaten av denna rapport utgör ett bra första steg mot att förstå relationen mellan Nb-mineraliseringar och den magnetiska mineralogin för Oldoinyo Dili, men mer detaljerade analyser av mineralkemin är nödvändigt för att till fullo förstå de komplexa förhållanden som råder vid bildning av dessa.

The aim of the present work is to characterize a metastable hard magnetic phase referred to as "A1" in Nd-Fe alloys, which forms as a part of the fine eutectic depending on the composition and cooling rate. In order to define the range of composition for the formation of A1, Nd100-xFex (x = 20, 25, 40) alloys are cooled at about 150 K/s. The results indicate that for a cooling rate of 150 K/s, the hypereutectic Nd100-xFex (x = 20) alloys solidify into hard magnetic A1 whilst the hypoeutectic alloys (x = 40) show the formation of Nd2Fe17 crystallites. However, no sample cooled at 150 K/s shows the peaks of Nd5Fe17 as expected from the equilibrium Nd-Fe phase diagram. The effect of cooling rate on the formation of hard magnetic A1 is studied by investigating the Nd80Fe20 alloys cooled at different rates. The microstructure of hard magnetic Nd80Fe20 alloys displays a fine eutectic-like matrix consisting of Nd-richer and Fe-richer regions. The Nd-richer regions are identified as dhcp Nd and fcc Nd-Fe solid solution. However, the Fe-richer regions also referred to as A1, are diffuse and give an average composition of Nd56Fe44. These regions yield complex electron diffraction patterns, which do not match with any known Nd-Fe phase. HRTEM images of the Fe-richer regions reveal the presence of 5-10 nm crystallites embedded in an amorphous phase. Thus the Fe-richer regions of the hard magnetic Nd80Fe20 specimens are not a single homogeneous phase rather they are mixture of finely dispersed nanocrystallites in an amorphous phase. The demagnetization curves the hard magnetic Nd80Fe20 measured at temperatures above 30 K are typical of a hard magnetic material. The coercivity increases from 0.48 to 4.4 T with the temperature decreasing from 300 to 55 K. The demagnetization curves change from single to two-phase type when the temperature approaches 29 K, ordering temperature of fcc Nd-Fe solid solution. The measurements of initial magnetization, field dependence of coercivity, and temperature dependence of coercivity suggest the Stoner-Wohlfarth type magnetization reversal process for the hard magnetic A1. The values of anisotropy constant are estimated by fitting the magnetization data to the law-of-approach to saturation. The temperature dependence of anisotropy constant and the coercivity indicate that the origin of coercivity is magnetic anisotropy. A cluster model with sperimagnetic arrangement of Nd and Fe spins is used to explain the hard magnetic behavior of the mold-cast Nd80Fe20. Structural and magnetic properties of multicomponent Nd60Co30-xFexAl10 (0 < x < 30) alloys are compared with the binary Nd-Fe alloys. Magnetic measurements of the multicomponent alloys show that the magnetic properties are controlled by the fraction of the Fe content. The coercivity of the Nd60Co30-xFexAl10 mold-cast rods does not vary much with the Fe-content for more than 10 at.% Fe but the remanence and the maximum magnetization increase linearly with the Fe content. The temperature dependence of coercivity, effective anisotropy constant, and anisotropy field are identical to those for the binary Nd80Fe20 mold-cast rod. These results clearly suggest that the binary Nd80Fe20 and the multicomponent Nd60Co30-xFexAl10 (x > 5) mold-cast rods are magnetically identical.

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