Experimentelle Untersuchungen zu Phasenbeziehungen in Eisen-reichen peralkalinen phonolitischen Schmelzen

The experimental investigation of phase equilibria is a powerful method to study the formation of magmatic rocks. This work presents phase equilibrium experiments investigating an iron-rich peralkaline phonolitic composition with variable fluorine and chlorine contents. The starting composition represents a dyke rock, which is a possible parental melt to the peralkaline Ilímaussaq plutonic complex (South Greenland). The dyke composition is perfectly suited for performing phase equilibrium experiments since, in contrast to the Ilímaussaq plutonic rocks, no major cumulate formation processes occurred. Experiments were performed isobarically (100 MPa) at 1000 - 650 °C with variable H2O concentrations (nominally dry to H2O-saturated) and oxygen fugacities (Δ log FMQ - 4 to +1, where FMQ represents the fayalite-magnetite-quartz oxygen buffer). Experimental conditions were applied using gold capsules and graphite-lined gold capsules in an internally heated argon pressure vessel and rapid-quench cold seal pressure vessels. To cover this large T interval a two-step fractional crystallization strategy was applied where glasses, representing residual melt compositions at 800 °C, were synthesized as starting material for consecutive experiments at lower T. The observed mineral phases coexisting with residual melt are titanomagnetite, fayalitic olivine, clinopyroxene, aenigmatite (Na2Fe5TiSi6O20), alkali feldspar and nepheline ( ±native iron). Above 1.5 wt% F in the coexisting melt, fluorite (CaF2) and hiortdahlite (Ca6Zr2Si4O16F4) are stable in favor of clinopyroxene. Sodalite (Na8Al6Si6O24Cl2) and eudialyte (Na15Ca6Fe3Zr3Si26O73(OH)3Cl2) form at Cl melt concentrations of 0.2 - 0.5 wt% (depending on T) and ZrO2 melt concentrations higher than 0.7 wt%, are additionally needed to stabilize hiortdahlite and eudialyte. The phenocryst assemblage of the dyke rock was reproduced at 850 - 800 °C, nominally dry conditions and Δ log FMQ -2. Except amphibole, all major mineral phases of the groundmass assemblage were reproduced at < 750 °C in H2O-poor experiments at Δ log FMQ -1. Therefore, both the phenocryst assemblage and the groundmass assemblage of the dyke rock may crystallize from one peralkaline melt through fractional crystallization. This may indicate that the Ilímaussaq plutonic rocks evolved from one single magma batch. Geothermometry in such highly evolved peralkaline rocks is often complicated by the absence of Fe-Ti oxides, olivine and clinopyroxene. Erroneous results were obtained when testing several thermometers with experimental data from the present study. Therefore, these experiments were used to calibrate four new geothermometers based on the distribution of Mn between clinopyroxene, aenigmatite, eudialyte and melt, and Na/Ca partitioning between clinoyproxene and melt. Preliminary experiments with REE-doped (Nb, La, Ce, Y and Sr) starting compositions show that phase stabilities and the liquid line of descent are similar to experiments using the same starting composition without trace elements, supporting the significance of the experimental results using simplified starting compositions with respect to nature.Die experimentelle Untersuchung von Phasengleichgewichten ist eine leistungsfähige Methode um die Bildung magmatischer Gesteine zu untersuchen. Diese Arbeit präsentiert Phasengleichgewichtsexperimente an Eisen-reichen peralkalinen phonolitischen Schmelzen mit unterschiedlichen Fluor- und Chlor-Gehalten. Die Ausgangszusammensetzung entspricht einem Ganggestein, das eine mögliche parentale Schmelze für die peralkalinen Plutonite der Ilímaussaq Intrusion (Südgrönland) darstellt. Diese Zusammensetzung ist ideal zur Durchführung von Phasengleichgewichtsexperimenten, weil sie im Gegensatz zu den plutonischen Ilímaussaq-Gesteinen nicht von Kumulat-bildenden Prozessen beeinflusst ist. Die Experimente wurden beim konstantem Druck (100 MPa) bei 1000 - 650 °C und variablen H2O (nominell trocken bis H2Ogesättigt) und variablen Sauerstofffugazitäten durchgeführt (Δ log FMQ -4 bis +1, FMQ entspricht dem Fayalit-Magnetit-Quarz Sauerstoffpuffer). Die Experimente wurden unter Verwendung von Goldkapseln und Gold-Graphit Doppelkapseln in extern beheizten Hydrothermalautoklaven und einer intern beheizten Gasdruckanlage (mit Argon als Druckmedium) durchgeführt. Um das große Temperaturintervall abzudecken wurden neue Ausgangsgläser entsprechend koexistierenden Schmelzen in Experimenten bei 800 °C für Experimente bei niedrigeren Temperaturen synthetisiert. Die synthetisierten Mineralphasen koexistierend mit Restschmelze sind Titanomagnetit, Fayalit-reicher Olivin, Klinopyroxen, Aenigmatit (Na2Fe5TiSi6O20), Alkalifeldspat und Nephelin (±gediegen Eisen). Bei über 1,5 Gew.% F in der koexistierenden Restschmelze wird Klinopyroxen von Fluorit (CaF2) und Hiortdahlit (Ca6Zr2Si4O16F4) ersetzt. Sodalit (Na8Al6Si6O24Cl2) und Eudialyt (Na15Ca6Fe3Zr3Si26O73(OH)3Cl2) benötigen, abhängig von der Temperatur, 0,2 - 0,5 Gew.% Cl in der Restschmelze. Außerdem sind Hiortdahlit und Eudialyt nur stabil, wenn die Restschmelzzusammensetzung mindestens 0,7 Gew.% ZrO2 beinhaltet. Die frühmagmatischen Phänokristalle des untersuchten Ganggesteins wurden bei 850 - 800 °C und nominell H2O-freien und reduzierten Bedingungen (Δ log FMQ -2) experimentell reproduziert. Abgesehen von Amphibol konnten die spätmagmatischen Minerale der Grundmasse bei Temperaturen unter 750 °C, einer H2O-armen koexistierenden Restschmelze und reduzierten Bedingungen (Δ log FMQ -1) reproduziert werden. Folglich können bei fraktionierter Kristallisation sowohl die Phänokristall-Paragenese als auch die Minerale der Grundmasse des Ganggesteins aus einer peralkalinen Schmelze kristallisieren. Das ist ein Hinweis darauf, dass der Ilímaussaq Komplex sich aus einem einzigen Schmelzschub gebildet haben kann. Geothermometrie in solch hochentwickelten peralkalinen Gesteinen ist häufig durch das Fehlen von Fe-Ti Oxiden, Olivin und Ca-reichem Klinopyroxen erschwert. Verschiedene Thermometer wurden anhand der experimentellen Daten dieser Arbeit getestet und erwiesen sich als unzureichend zur Temperaturabschätzung. Daher wurden neue Geothermometer entwickelt, die auf dem Verteilungsverhalten von Mn zwischen Klinopyroxen, Aenigmatit, Eudialyt und Schmelze sowie den Na/Ca Verhältnissen zwischen Klinopyroxen und Schmelze beruhen. Vorläufige Ergebnisse von Experimenten mit Ausgangsgläsern, die zusätzliche Spurenelemente enthalten (Nb, La, Ce, Y und Sr) zeigen, dass Phasenstabilitäten und Restschmelzentwicklung ähnlich den Spurenelement-freien Experimenten sind. Diese Beobachtung unterstützt die Aussagekraft von Experimenten mit vereinfachten synthetischen Ausgangszusammensetzungen bezüglich natürlicher Prozesse

On the State and Stability of Fuel Cell Catalyst Inks

Catalyst layers (CL), as an active component of the catalyst coated membrane (CCM), form the heart of the proton electrolyte membrane fuel cell (PEMFC). For optimum performance of the fuel cell, obtaining suitable structural and functional characteristics for the CL is crucial. Direct tuning of the microstructure and morphology of the CL is non-trivial; hence catalyst inks as CL precursors need to be modulated, which are then applied onto a membrane to form the CCM. Obtaining favorable dispersion characteristics forms an important prerequisite in engineering catalyst inks for large scale manufacturing. In order to facilitate a knowledge-based approach for developing fuel cell inks, this work introduces new tools and methods to study both the dispersion state and stability characteristics, simultaneously. Catalyst inks were prepared using different processing methods, which include stirring and ultrasonication. The proposed tools are used to characterize and elucidate the effects of the processing method. Structural characterization of the dispersed particles and their assemblages was carried out by means of transmission electron microscopy. Analytical centrifugation (AC) was used to study the state and stability of the inks. Herein, we introduce new concepts, S score, and stability trajectory, for a time-resolved assessment of inks in their native state using AC. The findings were validated and rationalized using transmittograms as a direct visualization technique. The flowability of inks was investigated by rheological measurements. It was found that probe sonication only up to an optimum amplitude leads to a highly stable colloidal ink.</p

Effects of dynamic conditions in the corrosion of stainless steel and graphite with molten Al-Si alloy for transport applications

Energy storage systems are key mechanisms for the transition to more sustainable energy sources. In the case of applications that involve heating intermittency, the use of phase change materials (PCMs) as thermal energy storage systems is of relevance. Particular metals and metallic alloys represent a promising alternative for their use as PCMs due to their high latent heat and high density, minimising the storage volume, and high thermal conductivity, allowing fast charge and discharge. However, the high operating temperatures, the reactivity of the molten metals and the periodic melting and solidification cycles promote corrosion of the container materials. Moreover, applications involving relative movement between the components (e.g. sloshing or pumping) can experience erosive-corrosive wear. In this work, a thermal energy storage system using an Al-Si alloy as PCM intended for application in electric vehicle cabin heating is analysed in terms of the compatibility with graphite and 304 stainless steel. The formation of new phases and potential reaction products at a static interface between container and molten material has been studied. Furthermore, the relative motion expected in the application has been simulated using a high temperature rheometer with cup and bob geometry. The type and extent of phases formed under dynamic conditions have been identified, exemplifying the influence of erosive-corrosive interaction as compared to static corrosion. The experimental methods, results and an appraisal of the impact of relative motion on wear in metallic phase change material systems undergoing dynamic conditions will be given

Microstructural effects on hardness and optical transparency of birefringent aluminosilicate nanoceramics

Transparent nanoceramics, synthesized at extreme conditions of high pressure and temperature, are new classes of materials highly attractive for photonic applications, such as optical windows, which require additional increased hardness and toughness. In this study, mechanical properties of transparent polycrystalline nanoceramics consisting of triclinic Al$_2$SiO$_5$ kyanite (~91.4 vol%) and trigonal Al$_2$O$_3$ corundum (~8.6 vol%) fabricated at high pressure (10 GPa) and temperature (1200‐1400°C) were investigated. It is already known that the optical transparency of kyanite‐based nanoceramics increases with decreasing average grain size. The present study shows that the hardness of these ceramics increases with decreasing grain sizes down to ~70 nm according to the Hall‐Petch strengthening. This grain size seems to mark a transition range where an inverse Hall‐Petch effect is indicated due to signs of a moderate hardness decrease at a smaller grain size of ~35 nm. The observed hardness‐grain size relation can fairly be described by an existing composite model, which considers the crystals to be harder than the noncrystalline grain boundaries. Within the range of average grain sizes examined, the kyanite habit changes from more equant to more columnar. This behavior is associated with the observed strong crack deflection by the columnar kyanite grains with aspect (length to diameter) ratios ranging from ~2 to 10 and may positively affect the fracture toughness

Synthesis of Al 2

We report the synthesis of alumina/stishovite nano-nano composite ceramics through a pressure-induced dissociation in Al2SiO5 at a pressure of 15.6 GPa and temperatures of 1300°C-1900°C. Stishovite is a high-pressure polymorph of silica and the hardest known oxide at ambient conditions. The grain size of the composites increases with synthesis temperature from ~15 to ~750 nm. The composite is harder than alumina and the hardness increases with reducing grain size down to ~80 nm following a Hall–Petch relation. The maximum hardness with grain size of 81 nm is 23 ± 1 GPa. A softening with reducing grain size was observed below this grain size down to ~15 nm, which is known as inverse Hall–Petch behavior. The grain size dependence of the hardness might be explained by a composite model with a softer grain-boundary phase