Effect of pH-induced nanopowder deagglomeration on sintering, microstructure and dielectric properties of Ba0.77Ca0.23TiO3 ceramics

Ba0.77Ca0.23TiO3 ceramics were produced in this work starting from nanopowders synthesized via a polymeric precursor method. By adjusting the pH values of the precursor solutions above 7, it was possible to prepare powders weakly aggregated and with a smaller particle size, both facts which traduced into an enhanced nanopowders' sintering process at comparatively lower temperatures. Irrespective of the initial pH value, highly-dense and second phase-free ceramics were obtained following optimal sintering parameters (temperature and time) extracted from dilatometric and density measurements. By considering these and other sintering conditions, moreover, polycrystalline materials with an average grain size varying from 0.35 to 8 mm were produced, the grain growth process involving liquid phase-assisted sintering for heat treatments achieved at 1320 °C. The study of grain size effects on the ferroelectric properties of these materials was conducted, the results being discussed in the light of previous debates, including grain size-dependent degree of tetragonal distortion in such materials, as verified in this work.

Preparation and characterisation of refractory whiskers and selected alumina composites

A whisker is a common name of single crystalline inorganic fibre of small dimensions, typically 0.5-1 μm in diameter and 20-50 μm in length. Whiskers are mainly used as reinforcement of ceramics. This work describes the synthesis and characterisation of new whisker types. Ti0.33Ta0.33Nb0.33CxN1-x, TiB2, B4C, and LaxCe1-xB6 have been prepared by carbothermal vapour–liquid–solid (CTR-VLS) growth mechanisms in the temperature range 900-1800°C, in argon or nitrogen. Generally, carbon and different suitable oxides were used as whisker precursors. The oxides reacted via a carbothermal reduction process. A halogenide salt was added to form gaseous metal halogenides or oxohalogenides and small amount of a transition metal was added to catalyse the whisker growth. In this mechanism, the whisker constituents are dissolved into the catalyst, in liquid phase, which becomes supersaturated. Then a whisker could nucleate and grow out under continuous feed of constituents. The syntheses of TiC, TiB2, and B4C were followed at ordinary synthesis conditions by means of mass spectrometry (MS), thermogravimetry (TG), differential thermal analysis (DTA) and quenching. The main reaction starting temperatures and reaction time for the different mixtures was revealed, and it was found that the temperature inside the crucible during the reactions was up to 100°C below the furnace set-point, due to endothermic nature of the reactions. Quench experiments showed that whiskers were formed already when reaching the temperature plateau, but the yield increased fast with the holding time and reached a maximum after about 20-30 minutes. Growth models for whisker formation have been proposed. Alumina based composites reinforced by (2-5 vol.%) TiCnano and TiNnano and 25 vol.% of carbide, and boride phases (whiskers and particulates of TiC, TiN, TaC, NbC, (Ti,Ta)C, (Ti,Ta,Nb)C, SiC, TiB2 and B4C) have been prepared by a developed aqueous colloidal processing route followed by hot pressing for 90 min at 1700°C, 28 MPa or SPS sintering for 5 minutes at 1200-1600°C and 75 MPa. Vickers indentation measurements showed that the lowest possible sintering temperature is to prefer from mechanical properties point of view. In the TiNnano composites the fracture mode was typically intergranular, while it was transgranular in the SiCnano composites. The whisker and particulate composites have been compared in terms of e.g. microstructure and mechanical properties. Generally, additions of whiskers yielded higher fracture toughness compared to particulates. Composites of commercially available SiC whiskers showed best mechanical properties with a low spread but all the other whisker phases, especially TiB2, exhibited a great potential as reinforcement materials.

Spark Plasma Sintering of Si3N4-based Ceramics : Sintering mechanism-Tailoring microstructure-Evaluationg properties

Spark Plasma Sintering (SPS) is a promising rapid consolidation technique that allows a better understanding and manipulating of sintering kinetics and therefore makes it possible to obtain Si3N4-based ceramics with tailored microstructures, consisting of grains with either equiaxed or elongated morphology. The presence of an extra liquid phase is necessary for forming tough interlocking microstructures in Yb/Y-stabilised α-sialon by HP. The liquid is introduced by a new method, namely by increasing the O/N ratio in the general formula RExSi12-(3x+n)Al3x+nOnN16-n while keeping the cation ratios of RE, Si and Al constant. Monophasic α-sialon ceramics with tailored microstructures, consisting of either fine equiaxed or elongated grains, have been obtained by using SPS, whether or not such an extra liquid phase is involved. The three processes, namely densification, phase transformation and grain growth, which usually occur simultaneously during conventional HP consolidation of Si3N4-based ceramics, have been precisely followed and separately investigated in the SPS process. The enhanced densification is attributed to the non-equilibrium nature of the liquid phase formed during heating. The dominating mechanism during densification is the enhanced grain boundary sliding accompanied by diffusion- and/or reaction-controlled processes. The rapid grain growth is ascribed to a dynamic ripening mechanism based on the formation of a liquid phase that is grossly out of equilibrium, which in turn generates an extra chemical driving force for mass transfer. Monophasic α-sialon ceramics with interlocking microstructures exhibit improved damage tolerance. Y/Yb- stabilised monophasic α-sialon ceramics containing approximately 3 vol% liquid with refined interlocking microstructures have excellent thermal-shock resistance, comparable to the best β-sialon ceramics with 20 vol% additional liquid phase prepared by HP. The obtained sialon ceramics with fine-grained microstructure show formidably improved superplasticity in the presence of an electric field. The compressive strain rate reaches the order of 10-2 s-1 at temperatures above 1500oC, that is, two orders of magnitude higher than that has been realised so far by any other conventional approaches. The high deformation rate recorded in this work opens up possibilities for making ceramic components with complex shapes through super-plastic forming.

De Novo Design of secondary structures: Synthesis and conformational studies

Chemists have long sought to extrapolate the power of biological catalysis and recognition to synthetic systems. These efforts have focused largely on low molecular weight catalysts and receptors; however, biological systems themselves rely almost exclusively on polymers, proteins and RNA, to perform complex chemical functions. Proteins and RNA are unique in their ability to adopt compact, well-ordered conformations, and specific folding provides precise spatial orientation of the functional groups that comprise the “active site”. These features suggest that identification of new polymer backbones with discrete and predictable folding propensities (“foldamers”) will provide a basis for design of molecular machines with unique capabilities. The foldamer approach complements current efforts to design unnatural properties into polypeptides and polynucleotides. The aim of this thesis is the synthesis and conformational studies of new classes of foldamers, using a peptidomimetic approach. Moreover their attitude to be utilized as ionophores, catalysts, and nanobiomaterials were analyzed in solution and in the solid state. This thesis is divided in thematically chapters that are reported below. It begins with a very general introduction (page 4) which is useful, but not strictly necessary, to the expert reader. It is worth mentioning that paragraph I.3 (page 22) is the starting point of this work and paragraph I.5 (page 32) isrequired to better understand the results of chapters 4 and 5. In chapter 1 (page 39) is reported the synthesis and conformational analysis of a novel class of foldamers containing (S)-β3-homophenylglycine [(S)-β3-hPhg] and D- 4-carboxy-oxazolidin-2-one (D-Oxd) residues in alternate order is reported. The experimental conformational analysis performed in solution by IR, 1HNMR, and CD spectroscopy unambiguously proved that these oligomers fold into ordered structures with increasing sequence length. Theoretical calculations employing ab initio MO theory suggest a helix with 11-membered hydrogenbonded rings as the preferred secondary structure type. The novel structures enrich the field of peptidic foldamers and might be useful in the mimicry of native peptides. In chapter 2 cyclo-(L-Ala-D-Oxd)3 and cyclo-(L-Ala-DOxd) 4 were prepared in the liquid phase with good overall yields and were utilized for bivalent ions chelation (Ca2+, Mg2+, Cu2+, Zn2+ and Hg2+); their chelation skill was analyzed with ESI-MS, CD and 1HNMR techniques and the best results were obtained with cyclo-(L-Ala-D-Oxd)3 and Mg2+ or Ca2+. Chapter 3 describes an application of oligopeptides as catalysts for aldol reactions. Paragraph 3.1 concerns the use of prolinamides as catalysts of the cross aldol addition of hydroxyacetone to aromatic aldeydes, whereas paragraphs 3.2 and 3.3 are about the catalyzed aldol addition of acetone to isatins. By means of DFT and AIM calculations, the steric and stereoelectronic effects that control the enantioselectivity in the cross-aldol addition of acetone to isatin catalysed by L-proline have been studied, also in the presence of small quantities of water. In chapter 4 is reported the synthesis and the analysis of a new fiber-like material, obtained from the selfaggregation of the dipeptide Boc-L-Phe-D-Oxd-OBn, which spontaneously forms uniform fibers consisting of parallel infinite linear chains arising from singleintermolecular N-H···O=C hydrogen bonds. This is the absolute borderline case of a parallel β-sheet structure. Longer oligomers of the same series with general formula Boc-(L-Phe-D-Oxd)n-OBn (where n = 2-5), are described in chapter 5. Their properties in solution and in the solid state were analyzed, in correlation with their attitude to form intramolecular hydrogen bond. In chapter 6 is reported the synthesis of imidazolidin-2- one-4-carboxylate and (tetrahydro)-pyrimidin-2-one-5- carboxylate, via an efficient modification of the Hofmann rearrangement. The reaction affords the desired compounds from protected asparagine or glutamine in good to high yield, using PhI(OAc)2 as source of iodine(III).

Two fluid numerical model for coastal applications

Wave breaking is an important coastal process, influencing hydro-morphodynamic processes such as turbulence generation and wave energy dissipation, run-up on the beach and overtopping of coastal defence structures. During breaking, waves are complex mixtures of air and water (“white water”) whose properties affect velocity and pressure fields in the vicinity of the free surface and, depending on the breaker characteristics, different mechanisms for air entrainment are usually observed. Several laboratory experiments have been performed to investigate the role of air bubbles in the wave breaking process (Chanson & Cummings, 1994, among others) and in wave loading on vertical wall (Oumeraci et al., 2001; Peregrine et al., 2006, among others), showing that the air phase is not negligible since the turbulent energy dissipation involves air-water mixture. The recent advancement of numerical models has given valuable insights in the knowledge of wave transformation and interaction with coastal structures. Among these models, some solve the RANS equations coupled with a free-surface tracking algorithm and describe velocity, pressure, turbulence and vorticity fields (Lara et al. 2006 a-b, Clementi et al., 2007). The single-phase numerical model, in which the constitutive equations are solved only for the liquid phase, neglects effects induced by air movement and trapped air bubbles in water. Numerical approximations at the free surface may induce errors in predicting breaking point and wave height and moreover, entrapped air bubbles and water splash in air are not properly represented. The aim of the present thesis is to develop a new two-phase model called COBRAS2 (stands for Cornell Breaking waves And Structures 2 phases), that is the enhancement of the single-phase code COBRAS0, originally developed at Cornell University (Lin & Liu, 1998). In the first part of the work, both fluids are considered as incompressible, while the second part will treat air compressibility modelling. The mathematical formulation and the numerical resolution of the governing equations of COBRAS2 are derived and some model-experiment comparisons are shown. In particular, validation tests are performed in order to prove model stability and accuracy. The simulation of the rising of a large air bubble in an otherwise quiescent water pool reveals the model capability to reproduce the process physics in a realistic way. Analytical solutions for stationary and internal waves are compared with corresponding numerical results, in order to test processes involving wide range of density difference. Waves induced by dam-break in different scenarios (on dry and wet beds, as well as on a ramp) are studied, focusing on the role of air as the medium in which the water wave propagates and on the numerical representation of bubble dynamics. Simulations of solitary and regular waves, characterized by both spilling and plunging breakers, are analyzed with comparisons with experimental data and other numerical model in order to investigate air influence on wave breaking mechanisms and underline model capability and accuracy. Finally, modelling of air compressibility is included in the new developed model and is validated, revealing an accurate reproduction of processes. Some preliminary tests on wave impact on vertical walls are performed: since air flow modelling allows to have a more realistic reproduction of breaking wave propagation, the dependence of wave breaker shapes and aeration characteristics on impact pressure values is studied and, on the basis of a qualitative comparison with experimental observations, the numerical simulations achieve good results.

Materiali Ceramici per Edilizia con Funzionalità Fotocatalitica

This thesis wad aimed at the study and application of titanium dioxide photocatalytic activity on ceramic materials. As a matter of fact, photocatalysis is a very promising method to face most of the problems connected with the increasing environmental pollution. Furthermore, titanium dioxide, in its anatase crystallographic phase, is one of the most investigated photocatalytic material and results to be perfectly compatible with silicate body mixes. That goal was pursued by two different strategies: 1. the addition to a body mix used for heavy clay products of several titania powders, with different mean crystallite size, surface area, morphology and anatase/rutile ratio and a titania nanosuspension as well. The titania addition followed two procedures: bulk and spray addition over the ceramic samples surface. Titania was added in two different percentages: 2.5 and 7.5 wt.% in both of the methods. The ceramic samples were then fired at three maximum temperatures: 900, 950 and 1000 °C. Afterwards, the photocatalytic activity of the prepared ceramic samples was evaluated by following the degradation of an organic compound in aqueous medium, under UV radiation. The influence of titania morphological characteristics on the photoactivity of the fired materials was studied by means of XRD and SEM observations. The ceramic samples, sprayed with a slip containing 7.5 wt.% of titania powder and fired at 900 °C, have the best photoactivity, with a complete photo-decomposition of the organic compound. At 1000 °C no sample acted as a photocatalyst due to the anatase-to-rutile phase transformation and to the reaction between titania and calcium and iron oxides in the raw materials. 2. The second one foresaw the synthesis of TiO2-SiO2 solid solutions, using the following stoichiometry: Ti1-xSixO2 where x = 0, 0.1, 0.3 and 0.5 atoms per formula unit (apfu). The mixtures were then fired following two thermal cycles, each with three maximum temperatures. The effect of SiO2 addition into the TiO2 crystal structure and, consequently, on its photocatalytic activity when fired at high temperature, was thoroughly investigated by means of XRD, XPS, FE-SEM, TEM and BET analysis. The photoactivity of the prepared powders was assessed both in gas and liquid phase. Subsequently, the TiO2-SiO2 solid solutions, previously fired at 900 °C, were sprayed over the ceramic samples surface in the percentage of 7.5 wt.%. The prepared ceramic samples were fired at 900 and 1000 °C. The photocatalytic activity of the ceramic samples was evaluated in liquid phase. Unfortunately, that samples did not show any appreciable photoactivity. In fact, samples fired at 900 °C showed a pretty low photoactivity, while the one fired at 1000 °C showed no photoactivity at all. This was explained by the excessive coarsening of titania particles. To summarise, titania particle size, more than its crystalline phase, seems to have a relevant role in the photocatalytic activity of the ceramic samples.

Protonenleitung in imidazolhaltigen Materialien als Modellsysteme für wasserfreie Brennstoffzellenmembranen

Es wird ein neues Konzept für ein protonenleitendes Polymer vorgestellt, das ohne eine zweite, flüssige Phase auskommt. Es beruht darauf, basische Gruppen (Imidazol) über flexible Spacer kovalent an ein Polymerrückgrat zu binden und durch Dotierung mit einer geringen Menge Säure Ladungsträger (Protonen) in dieses System einzubringen.Um die für die Leitfähigkeit und ihren Mechanismus verantwortlichen Größen zu identifizieren, wurde ein Satz von niedermolekularen Modellverbindungen definierter Struktur und hoher Reinheit synthetisiert und im reinen Zustand sowie nach Dotierung mit geringen Mengen Säure umfassend charakterisiert. Untersucht wurden die thermischen Eigenschaften, die Leitfähigkeit, die Diffusion der jeweiligen Modellverbindung sowie ggf. der zugesetzten Säure, das Protonierungsgleichgewicht und die dielektrischen Eigenschaften. Insbesondere wurden durch den Vergleich von Leitfähigkeits- und Diffusionsdaten unter Anwendung der Nernst-Einstein-Beziehung Rückschlüsse auf den Leitmechanismus gezogen.Es wurden Leitfähigkeiten von bis zu 6.5E-3 S/cm bei 120°C erreicht. Der Anteil der Strukturdiffusion (vergleichbar mit dem Grotthus-Mechanismus in Wasser) an der protonischen Leitfähigkeit betrug bis zu über 90%. Als entscheidende Faktoren für die Leitfähigkeit wurden die Glastemperatur und, mit geringerer Priorität, der Imidazolgehalt des Materials identifiziert. Die Temperaturabhängigkeit aller untersuchten Transportgrößen ließ sich durch die Vogel-Tamman-Fulcher-Gleichung exzellent beschreiben.Die vorgestellten Daten bilden die Grundlage für den Entwurf eines entsprechenden Polymers.

Zu Problemen der Flüssigszintillationsspektroskopie bei Transactiniden-Experimenten

Die vorliegende Arbeit beschäftigt sich vorwiegend mit Detektionsproblemen, die bei Experimenten zur Chemie der Transactiniden mit dem schnellen Flüssig-Flüssig-Extraktionssystem SISAK auftraten. Bei diesen Experimenten wird als Detektionsmethode die Flüssigszintillationsspektroskopie (LSC) eingesetzt. Es werden Szintillationspulse registriert, die für das verursachende Teilchen charakteristische Formen zeigen, die unterschieden werden müssen. Am Beispiel der Auswertung des SISAK-Experimentes zur Chemie des Rutherfordiums vom November 1998 wurde gezeigt, dass es mit den herkömmlichen Verfahren zur Pulsformdiskriminierung nicht möglich ist, die aus dem Zerfall der Transactiniden stammenden alpha-Ereignisse herauszufiltern. Ursache dafür ist ein hoher Untergrund, der in erster Linie von beta/gamma-Teilchen, Spaltfragmenten und pile ups verursacht wird. Durch die Verfügbarkeit von Transientenrecordern ergeben sich neue Möglichkeiten für eine digitale Pulsformdiskriminierung. In dieser Arbeit wird erstmals die Methode der digitalen Pulsformdiskriminierung mit künstlichen neuronalen Netzen (PSD-NN) vorgestellt. Es wurde im Zuge der Auswertung des SISAK-Experimentes vom Februar 2000 gezeigt, dass neuronale Netze in der Lage sind, Pulsformen automatisch richtig zu klassifizieren. Es ergeben sich nahezu untergrundfreie alpha-Flüssigszintillationsspektren. Es werden Vor- und Nachteile der neuen Methode diskutiert. Es ist dadurch möglich geworden, in SISAK-Experimenten Transactinidenatome anhand ihres Zerfalls eindeutig zu charakterisieren. Das SISAK-System kann somit bei Experimenten zum Studium des chemischen Verhaltens von Transactiniden in flüssiger Phase eingesetzt werden.____

Wasserfreie, polymere Protonenleiter für Brennstoffzellen durch Immobilisierung von Imidazol

Es werden neuartige, polymere Protonenleiter vorgestellt die nach dem 'Konzept des polymergebunden Protonensolvens' realisiert wurden. Sie zeigen protonische Leitfähigkeit als intrinsische Eigenschaft, sodass keine zweite, flüssige Phase zur Protonenleitung nötig ist. Verwirklicht wurde das Konzept anhand von kammartigen Siloxanoligomeren und -polymeren, wobei Imidazol als Protonensolvens durch flexible Spacer kovalent an das Rückgrat gebunden ist. Durch Pfropfung mit imidazoltragenden Spacereinheiten wurden ferner Kieselgelnanopartikel oberflächenmodifiziert. Um die Auswirkungen der Immobilisierung von Imidazol auf die Leitfähigkeit zu untersuchen, wurden neben unterschiedlichen Molekulargewichten, die Verbindungen auch jeweils mit verschiedenen Spacerlängen synthetisiert. Die Materialien wurden umfassend charakterisiert und auf ihr thermisches Verhalten, Stabilität, Leitfähigkeit, Diffusion und dielektrisches Verhalten sowie auch nach Dotierung mit Säure untersucht. Thermisch stabil sind die Materialien bis ca. 200°C. Die Leitfähigkeiten betragen bis zu 1,5E-3 S/cm bei 160°C, welche aufgrund der Immobilisierung des Imidazols ausschließlich auf Strukturdiffusion zurückzuführen sind. Die Strukturdiffusion ist vergleichbar mit dem Grotthus-Mechanismus in Wasser und wird durch die lokale Mobilität der Imidazolmoleküle, d.h. durch die Glasübergangstemperatur des Systems bestimmt. Entsprechend wird das für Glasbildner typische Vogel-Tamman-Fulcher-Verhalten für alle untersuchten Transportprozesse gefunden. Die mit abnehmender Glasübergangstemperatur abnehmende mechanische Stabilität der Materialien kann, wie gezeigt ist, durch Compoundierung mit Kieselgelnanopartikeln entscheidend verbessert werden, was eine kostengünstige und aussichtsreiche Möglichkeit zur Herstellung von Membranen für Brennstoffzellen darstellt.

The grain-scale distribution and behaviour of melt and fluid in crystalline analogue systems

The production, segregation and migration of melt and aqueous fluids (henceforth called liquid) plays an important role for the transport of mass and energy within the mantle and the crust of the Earth. Many properties of large-scale liquid migration processes such as the permeability of a rock matrix or the initial segregation of newly formed liquid from the host-rock depends on the grain-scale distribution and behaviour of liquid. Although the general mechanisms of liquid distribution at the grain-scale are well understood, the influence of possibly important modifying processes such as static recrystallization, deformation, and chemical disequilibrium on the liquid distribution is not well constrained. For this thesis analogue experiments were used that allowed to investigate the interplay of these different mechanisms in-situ. In high-temperature environments where melts are produced, the grain-scale distribution in “equilibrium” is fully determined by the liquid fraction and the ratio between the solid-solid and the solid-liquid surface energy. The latter is commonly expressed as the dihedral or wetting angle between two grains and the liquid phase (Chapter 2). The interplay of this “equilibrium” liquid distribution with ongoing surface energy driven recrystallization is investigated in Chapter 4 and 5 with experiments using norcamphor plus ethanol liquid. Ethanol in contact with norcamphor forms a wetting angle of about 25°, which is similar to reported angles of rock-forming minerals in contact with silicate melt. The experiments in Chapter 4 show that previously reported disequilibrium features such as trapped liquid lenses, fully-wetted grain boundaries, and large liquid pockets can be explained by the interplay of the liquid with ongoing recrystallization. Closer inspection of dihedral angles in Chapter 5 reveals that the wetting angles are themselves modified by grain coarsening. Ongoing recrystallization constantly moves liquid-filled triple junctions, thereby altering the wetting angles dynamically as a function of the triple junction velocity. A polycrystalline aggregate will therefore always display a range of equilibrium and dynamic wetting angles at raised temperature, rather than a single wetting angle as previously thought. For the deformation experiments partially molten KNO3–LiNO3 experiments were used in addition to norcamphor–ethanol experiments (Chapter 6). Three deformation regimes were observed. At a high bulk liquid fraction >10 vol.% the aggregate deformed by compaction and granular flow. At a “moderate” liquid fraction, the aggregate deformed mainly by grain boundary sliding (GBS) that was localized into conjugate shear zones. At a low liquid fraction, the grains of the aggregate formed a supporting framework that deformed internally by crystal plastic deformation or diffusion creep. Liquid segregation was most efficient during framework deformation, while GBS lead to slow liquid segregation or even liquid dispersion in the deforming areas.

Molekulardynamik-Computersimulation einer amorph-kristallinen SiO 2 Grenzschicht

In dieser Arbeit werden Molekulardynamik-Computersimulationen zur Untersuchung der statischen und dynamischen Eigenschaften einer amorph/kristallinen Siliziumdioxid(SiO2)-Grenzschicht durchgefuehrt.Die Grenzflaeche wird von der [100]-Ebene des beta-Kristobalit-Kristalls und der fluessigen SiO2-Phase gebildet und in einem Temperaturbereich zwischen 2900K und 3100K im Zustand eines metastabilen Gleichgewichts untersucht. Als Modellpotential zur Beschreibung der mikroskopischen Wechselwirkungen zwischen den Teilchen wird ein einfaches Paarpotential aus der Literatur verwendet, das sowohl die Struktur der kristallinen Phase als auch die der fluessigen Phase gut reproduziert. Bezogen auf die Dichte und die potentielle Energie der Teilchen erstreckt sich der Uebergang von der fluessigen in die kristalline Phase ueber 3-5 Atomlagen. Ein Layering-Effekt der Dichte in der fluessigen Phase in der Naehe der Grenzschicht wird nicht beobachtet. Der Einfluss der Grenzschicht auf statische Groessen, welche das System auf einer mittelreichweitigen Laengenskala beschreiben (z. B. Koordinationszahlverteilung und Ringverteilung) reicht im Vergleich dazu weiter in die fluessige Phase hinein und manifestiert sich in Defektstrukturen, wie z. B. der Erhoehung der Wahrscheinlichkeit fuer das Auftreten von 5-fach koordiniertem Silizium und der vermehrten Bildung von 2er-Ringen in der Fluessigkeit. Dies beguenstigt das Aufbrechen und Umklappen von Si-O-Bindungen und fuehrt zu einer Beschleunigung der Dynamik und einer Erhoehung der Diffusionsgeschwindigkeit in der Fluessigkeit. Im weiteren wird die Hochfrequenzdynamik der reinen SiO2-Fluessigkeit untersucht. Dazu berechnen wir die vibratorische Zustandsdichte in harmonischer Naeherung aus der inhaerenten Struktur. Wir finden einen stark ausgepraegten Peak bei einer Frequenz von 0.6 THz. Dieser Peak kann der niederenergetischsten transversalen akustischen Mode zugeordnet werden, die auch als Scherschwingung des Systems direkt sichtbar ist.

Wasserfreie Protonenleiter für Brennstoffzellen auf der Basis von Imidazol und Benzimidazol

Mit Hilfe von Brennstoffzellen wird eine effiziente Energieumwandlung von chemischer in elektrische Energie möglich. Die kommerziellen PEM-Brennstoffzellen benutzen Membra-nen, die zum Erreichen hoher Leitfähigkeiten eine wässrige Phase erfordern, in der der Proto-nentransport stattfindet. Somit wird die Betriebstemperatur durch den Siedepunkt des Wassers limitiert. Die verwendeten Pt-Katalysatoren zeigen bei niedrigen Temperaturen eine höhere Empfindlichkeit gegenüber CO, dass im Reformierungsprozess bei der Erzeugung von Was-serstoff entsteht. Austausch der wässrigen Phase gegen Heterozyklen, die ein zu Wasser ver-gleichbares Wasserstoffbrückennetzwerk aufbauen, in dem der Protonentransport stattfinden kann, ermöglicht eine höhere Betriebstemperatur. Durch das im Laufe des Brennstoffzellen-betriebs gebildete Wasser, können die Heterozyklen verdünnt bzw. komplett aus der Memb-ran ausgewaschen werden. Daher ist es erforderlich, die Ladungsträger an ein Polymerrück-grat zu binden, so dass sie eine hohe Beweglichkeit und Konzentration, die denen in der flüs-sigen Phase einer konventionellen Membran entsprechen, aufweisen. Diese Arbeit beschreibt die Synthese und Charakterisierung von Protonenleitern, die ohne eine flüssige Phase auskommen, da sie bereits protonische Leitfähigkeit als intrinsische Ei-genschaft zeigen. Es wurden verschiedene imidazol- bzw. benzimidazolhaltige Dimere und Polythiophene, in denen Benzimidazol in der Seitenkette über verschieden flexible Spacer mit dem Polymerrückgrat verbunden ist, synthetisiert. Die Materialien wurden in undotierten Zu-stand und nach Dotierung mit geringen Mengen Phosphorsäure umfassend charakterisiert und auf thermisches Verhalten, Stabilität und Leitfähigkeit untersucht. Die benzimidazolhaltigen Dimere weisen mit 250 °C die höchsten Zersetzungstemperaturen auf. Mit zunehmender Temperatur kann in allen Fällen eine Erhöhung der Leitfähigkeit beobachtet werden, die sich in der Arrhenius-Auftragung durch eine Gerade anpassen lässt, somit kann der Protonentrans-port durch einen Protonen-hüpfmechanismus beschrieben werden. Die höchste beobachtete Leitfähigkeit liegt im Bereich von 10-6 S/cm bei 160 °C. Durch Zusatz von Phosphorsäure kann die Leitfähigkeit z.T. um einige Größenordnungen gesteigert werden. Eine Ausnahme bilden die Polythiophene, die sowohl protonische als auch elektronische Leitfähigkeit besit-zen. Hier führt die Säure zu einer Lokalisierung der Ladungsträger, so dass die elektronische Leitfähigkeit eingeschränkt wird.

Computer-Simulationen zur Strukturbildung von einzelnen Polymerketten

In dieser Arbeit wurden die Phasenübergänge einer einzelnen Polymerkette mit Hilfe der Monte Carlo Methode untersucht. Das Bondfluktuationsmodell wurde zur Simulation benutzt, wobei ein attraktives Kastenpotential zwischen allen Monomeren der Polymerkette gewirkt hat. Drei Arten von Bewegungen sind eingeführt worden, um die Polymerkette richtig zu relaxieren. Diese sind die Hüpfbewegung, die Reptationsbewegung und die Pivotbewegung. Um die Volumenausschlußwechselwirkung zu prüfen und um die Anzahl der Nachbarn jedes Monomers zu bestimmen ist ein hierarchischer Suchalgorithmus eingeführt worden. Die Zustandsdichte des Modells ist mittels des Wang-Landau Algorithmus bestimmt worden. Damit sind thermodynamische Größen berechnet worden, um die Phasenübergänge der einzelnen Polymerkette zu studieren. Wir haben zuerst eine freie Polymerkette untersucht. Der Knäuel-Kügelchen Übergang zeigt sich als ein kontinuierlicher Übergang, bei dem der Knäuel zum Kügelchen zusammenfällt. Der Kügelchen-Kügelchen Übergang bei niedrigeren Temperaturen ist ein Phasenübergang der ersten Ordnung, mit einer Koexistenz des flüssigen und festen Kügelchens, das eine kristalline Struktur hat. Im thermodynamischen Limes sind die Übergangstemperaturen identisch. Das entspricht einem Verschwinden der flüssigen Phase. In zwei Dimensionen zeigt das Modell einen kontinuierlichen Knäuel-Kügelchen Übergang mit einer lokal geordneten Struktur. Wir haben ferner einen Polymermushroom, das ist eine verankerte Polymerkette, zwischen zwei repulsiven Wänden im Abstand D untersucht. Das Phasenverhalten der Polymerkette zeigt einen dimensionalen crossover. Sowohl die Verankerung als auch die Beschränkung fördern den Knäuel-Kügelchen Übergang, wobei es eine Symmetriebrechung gibt, da die Ausdehnung der Polymerkette parallel zu den Wänden schneller schrumpft als die senkrecht zu den Wänden. Die Beschränkung hindert den Kügelchen-Kügelchen Übergang, wobei die Verankerung keinen Einfluss zu haben scheint. Die Übergangstemperaturen im thermodynamischen Limes sind wiederum identisch im Rahmen des Fehlers. Die spezifische Wärme des gleichen Modells aber mit einem abstoßendem Kastenpotential zeigt eine Schottky Anomalie, typisch für ein Zwei-Niveau System.

Advances in understanding of osmotic dehydration and vacuum impregnation of fruits

Osmotic Dehydration and Vacuum Impregnation are interesting operations in the food industry with applications in minimal fruit processing and/or freezing, allowing to develop new products with specific innovative characteristics. Osmotic dehydration is widely used for the partial removal of water from cellular tissue by immersion in hypertonic (osmotic) solution. The driving force for the diffusion of water from the tissue is provided by the differences in water chemical potential between the external solution and the internal liquid phase of the cells. Vacuum Impregnation of porous products immersed in a liquid phase consist of reduction of pressure in a solid-liquid system (vacuum step) followed by the restoration of atmospheric pressure (atmospheric step). During the vacuum step the internal gas in the product pores is expanded and partially flows out while during the atmospheric step, there is a compression of residual gas and the external liquid flows into the pores (Fito, 1994). This process is also a very useful unit operation in food engineering as it allows to introduce specific solutes in the tissue which can play different functions (antioxidants, pH regulators, preservatives, cryoprotectants etc.). The present study attempts to enhance our understanding and knowledge of fruit as living organism, interacting dynamically with the environment, and to explore metabolic, structural, physico-chemical changes during fruit processing. The use of innovative approaches and/or technologies such as SAFES (Systematic Approach to Food Engineering System), LF-NMR (Low Frequency Nuclear Magnetic Resonance), GASMAS (Gas in Scattering Media Absorption Spectroscopy) are very promising to deeply study these phenomena. SAFES methodology was applied in order to study irreversibility of the structural changes of kiwifruit during short time of osmotic treatment. The results showed that the deformed tissue can recover its initial state 300 min after osmotic dehydration at 25 °C. The LF-NMR resulted very useful in water status and compartmentalization study, permitting to separate observation of three different water population presented in vacuole, cytoplasm plus extracellular space and cell wall. GASMAS techniques was able to study the pressure equilibration after Vacuum Impregnation showing that after restoration of atmospheric pressure in the solid-liquid system, there was a reminding internal low pressure in the apple tissue that slowly increases until reaching the atmospheric pressure, in a time scale that depends on the vacuum applied during the vacuum step. The physiological response of apple tissue on Vacuum Impregnation process was studied indicating the possibility of vesicular transport within the cells. Finally, the possibility to extend the freezing tolerance of strawberry fruits impregnated with cryoprotectants was proven.