The crystallization sequence of TiO2-nucleated lithium-magnesium-aluminosilicate glasses based on the composition of cordierite and keatite solid solution

Diese kumulative Doktorarbeit wurde als ein Screening des Kristallisationsablaufes potentiell interessanter neuartiger glaskeramischer Materialien konzipiert, deren Zusammensetzung eine Mischung der Grundsysteme Li2O-Al2O3-SiO2 (LAS) und MgO-Al2O3-SiO2 (MAS) (bzw. der kristallinen Phasen: Keatit-Mischkristall und Cordierit) entsprach. TiO2 wurde den Glasproben als Keimbildner zugegeben. Im MAS-System wurde der TiO2-Gehalt variiert (0–8 mol%), um damit den Einfluss der keimbildenden Kristallphase (Mg-Al-Titanat) auf die gesamte Kristallisationssequenz zu untersuchen. Eine hohe TiO2-Dotierung induzierte die Volumenkristallisation von einer Hochquarz-Mischkristallphase und verzögerte seine beim weiteren Tempern stattfindende Umwandlung in Indialit. Diese Verzögerung wurde in Verbindung mit einer durch die Bildung der Mg-Al-Titanat-Kristalle hervorgerufenen kompositionellen Änderung des anfänglichen stöchiometrischen Grünglases gebracht. Darüber hinaus wurde die Entwicklung des Al-Si-Ordnungsindexes von Indialit bis 1300 °C verfolgt. Dagegen wurden die Untersuchungen des LAS-systems auf die während der Temperung zwischen dem Glasübergang und der Volumenkristallisation auftretenden Keimbildungsvorgänge fokussiert. Nach der Entmischung einer TiO2-reichen amorphen Phase war TiO2(B) (in der Vergangenheit fälschlich als Pseudobrookit-Mischkristall bezeichnet) unveränderlich die erste nachweisbare kristalline Phase. Eine SiO2-Anreicherung des Grünglases verschob den Existenzbereich von TiO2(B) zu höheren Temperaturen, wobei er sich immer vollständig vor der Volumenkristallisation des Hochquarz-Mischkristalles in Anatas umwandelte. Al-reiche Schichten wurden während der TEM-Charakterisierung um die TiO2(B)-Keime festgestellt. Abschließend dienten diese Erkenntnisse der Untersuchung des kompositionell gemischten LAS-MAS-Dreieck. Alle 16 analysierten Proben wiesen nach einer ersten Temperung Quarz-Mischkristall auf, wobei eine zufriedenstellende Keimbildungseffizienz bei dem festgelegten TiO2-Gehalt (4 mol%) und dem gewählten Keramisierungsprogramm nur mit Li/Mg-Verhältnissen höher als 1 erreicht wurde. In den SiO2-reicheren LAS-Proben koexistierte ein Keatit-Mischkristall mit dem Hochquarz-Mischkristall schon während der ersten Temperungsschritte. Ausnahmslos destabilisierten höhere Keramisierungstemperaturen den Quarz-Mischkristall, obwohl er bei gemischten Zusammensetzungen eine längere Beständigkeit aufweist. Demgegenüber bildeten sich Indialit und/oder Keatit-Mischkristall in den Glaskeramiken. Ihre Kristallisationssequenz und ihr Anteil hingen direkt von der Stöchiometrie der anfänglichen Grüngläser ab. Die erhältlichen Mikrostrukturen waren ebenfalls sehr divers und manifestieren das große Optimierungspotential des untersuchten kompositionellen Glaskeramik-Systems.This cumulative PhD thesis was designed as a screening of the crystallization sequence in potentially interesting novel glass-ceramic materials, possessing still limitedly explored chemical compositions between the Lithium aluminosilicate (LAS) and magnesium aluminosilicate (MAS) endmembers, respectively in the stoichiometry of keatite solid solution and cordierite. To ensure a certain degree of volume crystallization, TiO2 was added to all compositions as nucleating agent. In the MAS system, analysis of samples doped with TiO2 amounts ranging from 0 to 8 mol% allowed to appreciate the strong influence played by the nucleating phase, Mg-Al-titanate, on the overall crystallization sequence. Especially at high doping content, it did not only induce effective volume crystallization of quartz solid solution (Qss), but it also appeared to slow down and retard its transformation into indialite upon further annealing. This was attributed to a compositional control exerted by Mg-Al-titanate after its precipitation from the parent glass, taking up part of Mg and Al and therefore hindering easy precipitation of a stoichiometric indialite in the following stages. The evolution of the apparent Al-Si ordering index of indialite could be additionally monitored up to 1300 °C and related again to the TiO2 content of the samples. On the other hand, the LAS samples, containing 4 mol% TiO2, were tested to elucidate the very first steps of nucleation, occurring during annealing after the glass transition and before the onset of volume crystallization. It was possible to observe that, after phase separation of a Ti-rich phase had occurred, the first detectable crystalline phase was invariably TiO2(B), a monoclinic polymorph up to now never identified in bulk glass and previously mistakenly interpreted as a pseudobrookite-like solid solution. The persistence of this phase was longer in the Si-richer samples of the triplet, but it eventually underwent complete transformation into anatase before the appearance of Qss. Al-enriched layers could be noticed during TEM examination around TiO2(B) nuclei, in agreement with previous literature sources. Finally, the collected fundamental knowledge was employed for the examination of the target Kss – cordierite compositional triangle. Allsixteen investigated samples were able to crystallize Qss, but nucleation efficiency appeared satisfactory only at Li/Mg ratios higher than 1 (at the set TiO2 amount of 4 mol% and chosen temperature-time program). At Si-rich and Mg-free compositions, non-negligible amounts of Kss were coexisting with Qss during these early annealing stages. At higher temperature, this latter phase underwent destabilization into Kss and/or indialite at all compositions, showing longer persistence in the center of the system, at Li/Mg equal to 1. The sequence by which Kss and cordierite were forming and their relative amounts were also dependent on the stoichiometry of the base glass: the first phase was predominant and appeared earlier in Li-richer samples, and vice versa. Obtainable microstructures mirrored these relations and were strikingly diverse, promising high potential for a future optimization of functional glass-ceramic materials based on these compositions

Mg-bearing quartz solid solutions as structural intermediates between low and high quartz

Glass powder samples of cordierite composition (doped with 8 mol% TiO2) were heat-treated to produce a series of increasingly SiO2-enriched Mg-bearing quartz solid solutions (Qss). The obtained materials were then analyzed by X-ray diffraction: Rietveld structural refinements revealed that Mg-bearing Qss phases possess trigonal symmetry and a compositionally dependent intermediate structural arrangement between those of low and high quartz. High-temperature diffraction measurements were performed up to 700°C to characterize the thermal expansion behavior of the crystals. At SiO 2-rich compositions, a reversible high-to-intermediate inversion of the quartz structure is observed, which shifts with increasing stuffing to lower temperatures than the conventional 573°C for pure quartz. Similarities and differences to the better-established Li-bearing Qss are discussed in the text

Spray-dried sol-gel glass-ceramic powders based on the tunable thermal expansion of quartz and keatite solid solutions

Lithium aluminosilicate glass-ceramic powders were synthesized by the heat treatment of spray-dried sol-gel glassy nanobeads, obtaining quartz solid solution (Qss) and keatite solid solution (Kss) crystals. Their composition ranged between 75 mol% SiO2 and pure silica along the spodumene join. The metastable crystals displayed tunable coefficients of thermal expansion ranging from +30 × 10−6 to −2.7 × 10−6 K−1 at room temperature, as obtained from their crystallographic characterization. The solid solution boundaries of Kss could be extended to 85 mol% SiO2. Concurrently, X-ray diffraction measurements performed in situ at high temperature and at cryogenic conditions confirmed the known linear shift of the high-low quartz inversion temperature upon increasing Al+Li doping. The obtained results qualify aerosol synthesis as a very versatile method for the production of glass-ceramic powders in the LAS system

Spray-dried TiO2(B)-containing photocatalytic glass-ceramic nanobeads

Glass-ceramic nanospheres of molar composition 0.83 SiO2 · 0.17 TiO2 are produced by the sol-gel spray-drying method followed by controlled heat treatments up to 1200 °C. TiO 2 (B) and anatase nanocrystals are precipitated in the glassy matrix: the latter phase gradually predominates with increasing ceramization temperature and time, in parallel to an overall increase in crystal sizes. The nanospheres exhibit evident photocatalytic activity under UV-A irradiation, especially at annealing stages involving a comparatively higher amount of TiO 2(B) and smaller crystals. The occurrence of TiO2(B) in this simplified binary glass-ceramic material underlines the key role of this phase in the dynamics of crystallizing TiO 2-bearing silicate melts

Neotropical freshwater fisheries : A dataset of occurrence and abundance of freshwater fishes in the Neotropics

The Neotropical region hosts 4225 freshwater fish species, ranking first among the world's most diverse regions for freshwater fishes. Our NEOTROPICAL FRESHWATER FISHES data set is the first to produce a large-scale Neotropical freshwater fish inventory, covering the entire Neotropical region from Mexico and the Caribbean in the north to the southern limits in Argentina, Paraguay, Chile, and Uruguay. We compiled 185,787 distribution records, with unique georeferenced coordinates, for the 4225 species, represented by occurrence and abundance data. The number of species for the most numerous orders are as follows: Characiformes (1289), Siluriformes (1384), Cichliformes (354), Cyprinodontiformes (245), and Gymnotiformes (135). The most recorded species was the characid Astyanax fasciatus (4696 records). We registered 116,802 distribution records for native species, compared to 1802 distribution records for nonnative species. The main aim of the NEOTROPICAL FRESHWATER FISHES data set was to make these occurrence and abundance data accessible for international researchers to develop ecological and macroecological studies, from local to regional scales, with focal fish species, families, or orders. We anticipate that the NEOTROPICAL FRESHWATER FISHES data set will be valuable for studies on a wide range of ecological processes, such as trophic cascades, fishery pressure, the effects of habitat loss and fragmentation, and the impacts of species invasion and climate change. There are no copyright restrictions on the data, and please cite this data paper when using the data in publications