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This dissertation is devoted to explicitly investigate the suitability of titania and silica based precursors for wood modification.
Novel TiO2 / SiO2 wood inorganic composites were prepared by two-step process. In first step, freshly prepared precursor solutions of silicon and titanium alkoxides were vacuum impregnated (one or more cycles) to oven dried pine sapwood (Pinius sylvestris L.). Precursor solutions with nano-scaled species and particulates (TiO2 and SiO2) were soaked by the entire wood matrix during this step. In (second) subsequent curing step, these penetrating liquids transformed into respective gel layers and depositions therein by sol-gel processing (in-situ hydrolysis-condensation).
Sol-gel based depositions were evenly covered on the cell walls in the form of thin layers. These gel films were not crack-free however; very few cracks were visible in the gel coatings present within the wood matrix of composites prepared with precursors of low alkoxide content. In addition, the gels were randomly distributed, found to be mainly deposited in the wood cell lumen and also localized in the cell walls as ESEM-EDX mapping revealed.
The impregnated samples show an increase in mass and volume expressed as weight percent gain (WPG) and bulking (B). Both of the parameters are decisive for tailoring a number of physical properties of the resultant composites. In general, moisture and water sorption capacities were decreased by 43 to 50 % while anti-swelling efficiency (ASE) and bending strength (MOE) of the precursor-modified wood (composite) were increased up to 34 % and 41 % respectively, when compared to unmodified wood. These improvements are attributed mainly to gel depositions in the wood structure that physically interact with the wood matrix consequently narrowing the main flow passages there and reinforcing strengths of the fiber.
These materials show superior resistance against fire of different scenarios. In small scale fire, their flame was retarded markedly (up to 78 % in optimum case) in comparison to wood controls as studied by oxygen index test (LOI). Cone calorimetric investigations revealed their better fire retardancy in terms of time resolved heat release rates (HRR) in the developing fire scenario. A remarkable reduction of 40 % in the second peak of HRR was achieved through sol-gel treatment. Furthermore, fire hazards such as CO and total smoke production were considerably lowered for these materials. Beside these improvements, reductions in first peak HRR and in fire load (total heat evolved) were not worth considering in compliance to fire retardancy principles. In relation to the mechanism of protection, it can be concluded that stable gel layer (independence of material loading) inside the wood retards markedly the proceeding combustion processes (oxidation) after first pyrolysis by improving the overall protection properties of the fire residue.
These materials show excellent resistance against bio-deterioration. Almost full decay protection was achieved against wood destroying brown rot fungi (Coniophora puteana and Poria placenta) in 10 weeks laboratory trials only with 1 mass% loadings (WPG). Mass loss in prolonged test (16 weeks) was similar to 10 week lasted test indicating that protection is permanent. In addition, composites imparted moderate resistances against soft rot fungi and very minor one against surface blue stain. The lowering in moisture content that discourages fungal colonization, gel layer that provides better shielding against fungal exposure and un-hydrolyzed alkoxides (organics) that probably induce biocidal effects are the main reasons for protection against biological attack in these materials.
Composites are utmost leach resistant because of the insolubility of gel depositions in water as well as due to their fixture into the wood matrix. In addition to this intrinsic property, gel precipitates can also reduce substantially (70 to 80 %) the release of hazardous active agents (e.g. CuCl2) of wood preservatives into the environment. This anti-leaching effect is supposed to be due to the reduced ion mobility of active agents inside the wood by their encapsulations into the gel matrix, lowering of water penetration into the wood depths as well as due to the internal covering action of the gel layer.
In short, it can be assessed that TiO2 / SiO2 based sol-gel precursor treatment of wood has a possibility to be commercialize in near future due to easy handling and non-toxic nature of the precursor used. Resultant end products (composites) are also novel and promising multifunctional materials that can be used for multi-purposes in relation to use class 3 (outdoor but without ground contact according to EN 335 2006) environments.
Generally accepted quality criteria for the comparison of immunoassays are still missing and assay conditions vary greatly between different laboratories. To address this problem, the influence of different parameters on the overall assay performance was assessed, specifically for different enzyme immunoassays (EIAs) for the anthropogenic markers caffeine (CAF) and carbamazepine (CBZ). Special emphasis was dedicated to the parameters temperature, assay format and enzyme-substrate combination. The temperature parameter was systematically studied for all incubation steps of the direct EIA formats employing the photometric horseradish peroxidase (HRP) substrate 3,3’,5,5’- tetramethylbenzidine (TMB) and the fluorometric HRP substrate 3-(4-hydroxyphenyl)propionic acid for both analytes. A temperature decrease only during the competition step led to an increase in assay sensitivity by a factor of 10 to 15 for CBZ and CAF, respectively, independent of the enzyme substrate used. Room temperature experiments yielded the smallest coefficients of variations, minimizing the edge effect. The influence of the assay format on different performance parameters was studied with the determination of CAF in consumer products. In addition to the HRP substrates, the enzyme alkaline phosphate (AP) and its chromogenic substrate para-nitrophenyl phosphate and a fluorescent substrate, 4-methylumbelliferyl phosphate, were employed. Seven quality criteria were defined and validated to compare these immunoassays. The evaluation of the four criteria (sensitivity, measurement range, relative dynamic range and goodness of fit) for the standard curves revealed that the direct format is superior to the indirect format, with the HRP TMB format showing the best performance. Three additional criteria for an applicationdriven analysis of real samples, in this case CAF-containing beverages and cosmetics, confirmed this result in terms of accuracy as well as intra- and inter-plate precision. The enzyme-substrate combination was investigated when several direct CBZ assays were applied to the analysis of water samples; here, three HRP assays and four AP assays were studied, along with luminescence detection. The HRP assays reached better sensitivities and lower quantifiable concentrations compared to the AP assays. Only the HRP assays and the chemiluminescent AP juice assay fulfilled the requirements for the four criteria applied to standard curves; all other AP assays were not considered for application to real samples based on these criteria. The AP juice assay can only be employed for influent samples whereas all HRP assays are applicable to influent and effluent wastewater samples according to intra- and inter-plate precision. Furthermore, the HRP assays alone are suitable for surface water analysis; here, the chromogenic HRP TMB assay yielded the best results, as any type of water sample can be quantified with high precision. Whether these quality criteria, derived here for standard curves as well as their application to real samples, can be transferred to other immunoassay formats for quality assurance remains to be shown.
Due to the growing environmental awareness worldwide, containment provisions for CO2 emissions in mobility systems and increasing performance requirements the demands on mechanical systems and their materials continuously rise. These high demands require the implementation of new technical approaches, for example of light-weight strategies in automotive powertrains, and directly raise questions about the suitability of the most promising technical solution. Two basic parameters, the surface hardness of the tooth flanks and the core fatigue strength of the tooth root, illustrate exemplarily increasing demands on material grades used for gear wheels in automotive powertrains. In addition to light-weight strategies, a reduction in friction and an increase of the fatigue lifetime are two other major development directions to strive the mentioned targets. It is clear that any kind of solution must show an equal application profile, preferably an improvement, compared to the state-of- the-art solutions. For tribological systems, the following paths may offer lower friction and higher load carrying capabilities: 1. Alternative base oils and additives (such as esters, polyglycols), 2. Thin film coatings (e.g. DLC) and/or 3. Novel steel metallurgies. In previous investigations on the slip-rolling resistance of thin film coatings (a-C, ta-C, Zr(C,N)) the substrates were mainly made of the bearing steels 100Cr6H and Cronidur 30. Applying contact pressures of up to P0max = 2.9 GPa (FN = 2,000 N), the samples were tested up to 10 million load cycles in endurance tests. The aim of the present work is to broaden the research by varying the input parameters. Newly developed engine oil mixtures, high performance thin film coatings and alternative steel solutions are intensively investigated in highly stressed slip-rolling contacts at lubricant temperatures of 120°C. Specifically, in using new steel metallurgies, i.e. the high toughness and high strength steels V300 and NC310YW (Aubert & Duval) as well as CSS-42L (Latrobe Specialty Steel Company), in combination with thin film coatings, even if they compete in the uncoated state, the Hertzian contact pressures could be increased up to P0max = 4.2 GPa (FN = 5,000 N) without any surface failures of coating or substrate. It was shown that selected thin film coatings can minimize the wear rates down to nearly ‘zero-wear’ in highly stressed contacts [Woy08] [Woy11]. In addition, the studies revealed not only the high potential in slip-rolling resistance, but also a possible friction reduction down to 0.047 by use of uncoated steels with increased toughness. Compared to steels like 100Cr6H and Cronidur 30 this means a reduction in friction of approximately 40% under identical testing conditions. Different test series with newly developed base oil-additive formulations were investigated with specific emphasis on the frictional behavior of selected bio-no-tox EP/AW additives and friction modifiers. Additional influencing factors like the structural and surface conditions of the steels/coatings before and after the tests were analyzed by means of REM, EDX, XRD and TEM.
In der vorliegenden Arbeit wird das Schädigungsverhalten eines kurzglasfaserverstärkten Polyamids analysiert. Die Mikrorissschädigung wird mittels Röntgenrefraktionsanalyse für verschiedene mechanische Betriebsbelastungen untersucht. Dabei werden Möglichkeiten und Grenzen der Röntgenrefraktionsanalyse für kurzglasfaserverstärkte Polyamide herausgearbeitet. Insbesondere werden der Einfluss der Faserorientierungsverteilung und der Einfluss der Schädigungsmechanismen auf die Röntgenrefraktionsanalyse und deren Ergebnisinterpretation betrachtet. Die Methode ermöglicht eine quantitative und phänomenologisch basierte Mikrorissschädigungscharakterisierung. Für den untersuchten Werkstoff tritt Mikrorissschädigung in Form von Faser-Matrix-Ablösung und Matrixrissbildung auf. Die Schädigungshöhe korreliert linear mit dem nichtlinear elastischen Dehnungsanteil und ist abhängig von der Art der Betriebsbelastung. Eine Feuchtigkeitsaufnahme des Werkstoffs kann das Mikrorissschädigungsverhalten deutlich beeinflussen. Die Schädigung durch Mikrorissbildung tritt vorrangig unter Zugbelastungen auf. Die Makroschädigung durch Einzelrissfortschritt wird in Abhängigkeit der Faserorientierung und des Feuchtegehalts in einem automatisierten Versuch charakterisiert. Aufgrund der gewonnenen Erkenntnisse wird ein empirisches Berechnungsmodell erstellt. Die Untersuchungen des Mikro- und Makroschädigungsverhaltens werden fraktografisch begleitet, um die in den Modellvorstellungen vorausgesetzten Schädigungs- und Versagensmechanismen abzusichern.
Characterization of Polymer Nanocomposites based on Layered Double Hydroxide and Carbon Nanotubes
(2013)
Polymer based nanocomposites by melt blending of synthesized ZnAl-Layered Double Hydroxide (ZnAl-LDH) and Polyolefines [Polypropylene (PP) and Polyethylene (PE)] and also Polylactide (PLA) with MgAl-LDH and multi-walled Carbon Nanotubes (MWCNT) were investigated. The LDH was organically modified by using a surfactant sodium dodecylbenzene sulfonate (SDBS) to increase the interlayer spacing of the LDH, so that polymer chains can intercalate the inter layer galleries. Some amount of maleic anhydride grafted PP and PE were incorporated in the nanocomposites based on PP and PE respectively to enable the interaction of the non-polar polymers (PP and PE) with the LDH. The resulting morphology was investigated by a combination of Differential Scanning Calorimeter (DSC), Small and Wide-angle X-ray scattering (SAXS and WAXS) and broadband dielectric relaxation spectroscopy (BDS). In case of LDH based nanocomposites (PP, PE and PLA), the homogeneity of the nanocomposites and the average number of stack size (4 – 7 layers) were determined using scanning micro focus SAXS (BESSY II). DSC investigations of PP and PE based LDH nanocomposites showed a linear decrease in crystallinity as a function of filler concentration. The extrapolation of this decreasing dependence to zero estimates a limiting concentration of 40 wt% and 45 wt% respectively. Above this amount of LDH the crystallinity of the polymers is completely suppressed. This finding is in agreement with WAXS investigations where the area below the crystalline reflections and amorphous halo were calculated and used to estimate the degree of crystallinity. PLA/LDH nanocomposites presented a little different behavior, the crystallinity of the polymer at first increases and then decreases as a function of LDH concentration. In this case the crystallinity will be suppressed at around 15 wt%. The dielectric spectra of the nanocomposites based on PP/LDH and PE/LDH show several relaxation processes which are discussed in detail. The intensity of the dynamic glass transition increases with the concentration of LDH. This is attributed to the increasing concentration of the exchanged anion dodecylbenzene sulfonate (SDBS) which is adsorbed at the LDH layers. Therefore, a detailed analysis of the β-relaxation provides information about the structure and the molecular dynamics in the interfacial region between the LDH layers and the polymer matrix which is otherwise dielectrically invisible (low dipole moment, non-polar). In case of PLA/LDH, three relaxation processes related to dynamic glass transition and one localized fluctuations were identified and analyzed in detail to understand the morphology. For this system, one dynamic glass transition process originates from the fluctuations of the interfacial molecules, second from the PLA matrix (polar polymer, C=O in the main chain) and the third from segments confined between the intercalated LDH sheets. Additional thermal investigations were carried out for PP/LDH and PLA/LDH samples. The increase in the rigid amorphous fraction (RAF) was observed in both the cases. This is attributed to the polymer molecules which are in close proximity to LDH sheets, as they hinder their mobility. This is analyzed in detail and related to the BDS results. PLA based MWCNT nanocomposites were investigated by BDS as initial result. The findings showed that between 0.5 and 1 wt% of CNT, a percolating network of the nanotubes is formed which leads to DC conductivity. This is due to the high aspect ratio of the CNTs and also the van der Waals interaction between the nanotubes which forms a network leading to conductivity.
The reduced size of nanoparticles (diameter < 100 nm) confers them high specific surface areas and permeability through many biological pathways resulting in high interaction with biological systems. Therefore, in the recent years, nanoparticles (NPs) have increasingly found many applications in biomedical research. Herein, silica-based NPs are among the most promising candidates for biomedical studies due to their relative low toxicity and the possibility of functional variability. The main focus of this thesis work has been the synthesis and characterisation of novel hybrid NPs with enhanced properties for biomedical studies. More specifically, suppression of protein adsorption and achievement of highly fluorescent NPs in serum-rich media are well focused. First, a chemical strategy for the preparation of highly fluorescent silica nanoparticles by covalent attachment of Alexa dyes and subsequent shielding by an additional pure silica shell is well presented. These nanoparticles were investigated by Dynamic light scattering (DLS), Transmission electron microscopy (TEM) and fluorescence spectroscopy, the latter includes determination of absolute fluorescence quantum yields of such scattering suspensions with an integrating sphere setup and the assignment of fluorescence intensity values. At low shelling extension core-shell fluorescent silica nanoparticles show smooth surfaces and high quantum yields, even comparable to those for free dyes. However, by increasing the amount of shell precursor, nanoparticle surfaces show raspberry morphologies and decay of the quantum yields. Secondly, two different types of novel silica-poly(ethylene glycol) hybrid nanoparticles (H- SiO2-PEG and G- SiO2@PEG) have been synthesized by use of the same polymer precursor: Here the influence of concentration of the polymer precursor poly(ethylene glycol) methyl ether-3-(triethoxysilyl) propyl urethane (mPEG-IPTES) on the particle properties was scrutinised. For polymer grafted NPs, the concentration of polymer precursor increases the PEG density and the hydrophobicity of the NPs surface. On the other hand, for condensated NPs, the polymer precursor influences the size, but not the density of polymer chains on the NPs surface, which indicates that PEG on the surface of the NPs effectively reduces the adsorption of Bovine serum albumin (BSA). Finally, the influence of polymer length on the ability to repel BSA adsorption onto nanoparticles is reported. SNPs@PEG with different molecular weights (mPEG: 350, 2000 and 5000 g/mol) were synthesized by nucleophilic substitution of tosylated mPEG to aminated silica nanoparticles (chemical grafting). The resulted hybrid nanoparticles were consistently characterized by DLS, TEM, Fourier transform infrared spectroscopy (FTIR), Thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). BSA at different concentrations were used as a model protein to study the protein-corona formation after adsorption onto the pristine and modified nanoparticles (SNPs@PEG). For pristine SNPs and SNPs@PEG (MW = 350 g/mol), zeta potential at different incubation times (0, 24 and 48 h) show a dynamic evolution of the nanoparticle-protein corona. Conversely, for SNPs@PEG with MW ≥ 2000 g/mol, a significant suppression of corona formation and time evolution was observed. In resume, protein corona is strongly influenced by the adsorption inhibition of PEG surfaces.
TRIP Stähle realisieren durch eine unter mechanischer Beanspruchung hervorgerufene Phasenumwandlung von metastabilen Austenit in Martensit (TRIP-Effekt) hohe Festigkeiten bei gleichzeitig guten Verformungseigenschaften. Während der Verarbeitung dieser Werkstoffe mittels Widerstandspunktschweißen können unter den prozessspezifischen extremen Aufheiz- und Abkühlraten Modifikationen des Austenitanteils hervorgerufen werden die als Folge lokal zu einer Änderung der mechanisch-technologischen Eigenschaften führen. Wesentliche Zielsetzung der vorliegenden Arbeit war daher die Analyse der Gefügestruktur im Bereich einer Punktschweißverbindung hinsichtlich der Änderung des Austenitanteils und den daraus resultierenden Änderungen der lokalen mechanischen Eigenschaften unter besonderer Berücksichtigung des TRIP-Effekts. Durch in-situ Beugungsuntersuchungen mittels hochenergetischer Synchrotronstrahlung erfolgte zunächst die Quantifizierung des metastabilen Austenitanteils im unbehandelten Grundwerkstoff. Darauf aufbauend wurden unter definierten Temperaturprofilen in Ofenversuchen die grundlegenden Aspekte der thermisch bedingten Austenitumwandlung im Aufheiz- und Abkühlprozess untersucht. Durch Gleeble-Versuche und Ofenexperimente wurden weiterführend verschiedene Temperaturprofile mit unterschiedlichen im Punktschweißprozess lokal vorliegende Spitzentemperaturen genutzt um eine systematische Bewertung des Einflusses der Temperatur und der Aufheiz- sowie der Abkühlbedingungen auf den Austenitgehalt unter realen Bedingungen zu ermöglichen. Durch Korrelationsuntersuchungen zwischen den mechanischen Kennwerten thermisch präparierter Zugproben und den mittels Metallographisch wie auch röntgenographisch ermittelten Austenitgehalten konnten die metastabilen, d.h. umwandlungsfähigen Austenitanteile bestimmt werden. Schließlich erfolgte eine Bewertung hinschlich der Übertragbarkeit der Ergebnisse auf reale Widerstandspunktschweißverbindungen. Es wurde dabei der Nachweis erbracht, dass die Austenit-Martensit-Phasenumwandlung lediglich in einem lokal eng begrenzen Werkstoffbereich am Übergang WEZ/Grundwerkstoff wirksam werden kann. Demzufolge führt der TRIP-Effekt zu keiner signifikanten Beeinflussung von Festigkeits- und Verformungseigenschaften im Fügebereich von widerstandspunktgeschweißten TRIP Stählen. Die ermittelten mechanischen Kennwerte wurden abschließend als Eingangsdaten für die numerische Simulation des Verformungs- und Festigkeitsverhaltens von Punkschweißverbindungen genutzt. Dabei wurde insbesondere die Notwendigkeit der Implementierung von mechanischen Kenndaten der Wärmeeinflusszone für eine realitätsnahe Modellierung von Scherzugversuchen untersucht.
Eine Vielzahl von Schwergasunfällen der Vergangenheit verdeutlichen das Gefahrenpotential, das von brennbaren und vor allem toxischen Schwergasen ausgeht. Zur Bestimmung des Gefährdungsbereichs müssen in diesem Fall Ausbreitungsabschätzungen angestellt werden. Während der Gefährdungsbereich für brennbare Gase auf das Erreichen der unteren Explosionsgrenze (c > 1 Vol.-%) beschränkt ist, so ergibt sich bei der Freisetzung toxischer Gase ein weitaus größerer Gefahrenbereich. Toxische Gase haben auch in sehr geringen Konzentrationen (c < 3000 ppm) noch ein hohes Schadenspotenzial. Zur Abschätzung der Schwergasausbreitung kann die VDI-Richtlinie 3783 verwendet werden. Diese Richtlinie entspricht zurzeit dem Stand des Wissens in Deutschland. Die VDI 3783 macht keine Angaben zur Höhe und Breite der Schwergaswolke, welche sowohl im Rahmen von quantitativen Risikoanalysen als auch evtl. für die Kopplung eines Lagrangeschen-Partikelmodells mit dem Schwergasmodell erforderlich sind. In der vorliegenden Arbeit wurden deshalb Berechnungen mit verschiedenen Modellen zur Erweiterung der VDI 3783 durchgeführt. Die Berechnungsergebnisse wurden mit experimentellen Untersuchungen zur Länge, Breite und Höhe des Schwer- und Neutralgasbereichs verglichen und bewertet. Darüber hinaus wurde der Einfluss der Quellhöhe auf die Schwergasausbreitung untersucht. Hierbei wurde festgestellt, dass sich die Länge des Schwergasbereichs mit zunehmender Quellhöhe verringert und ab einer kritischen Höhe kein Schwergasbereich am Boden mehr vorliegt. Für Freisetzungshöhen über der kritischen Quellhöhe ist der Schwergaseffekt vernachlässigbar und es ist keine Berechnung nach der VDI 3783, Blatt 2 erforderlich. Die Freisetzung des Schwergases kann hierbei sowohl aus einer verfahrenstechnischen Anlage als auch aus handelsüblichen Gasflaschen erfolgen. Für die Freisetzung aus handelsüblichen Gasflaschen wurde ein mathematisches Modell zur Vorhersage des zeitabhängigen Massenstroms weiterentwickelt. Dieses Modell beinhaltet die Berechnung der instationären Wärmeleitung in der Flaschenwand, eine komplexe Beschreibung der Energieund Massenbilanzen sowie die Berücksichtigung einer Zweiphasenströmung. Zur Validierung des Modells wurden experimentelle Untersuchungen mit verschiedenen Gasen, Gasflaschengrößen und –typen durchgeführt. Experimentelle Ergebnisse und Berechnungen zeigen eine gute Übereinstimmung.