Publikationen

Polymers were grafted on aluminum surfaces in order to modify the chemical and physical properties of the interface. The properly cleaned and activated surface of the aluminum substrate was first "silanized" either with 3-(trimethoxysilyl)propylamine or allyltrimethoxysilane. The grafting was carried out following two methods: (i) by the reaction of preformed poly(methyl vinyl ketone) with the aminosilane-modified surface; and (ii) by polymerization of methyl vinyl ketone with the vinylsilane-modified surface. The modified aluminum surfaces were characterized by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy. The new surfaces were examined by contact-angle measurements, and determinations of the Lewis basicity.

Iron oxide nanotubes of 50-150 nm outer diameter and 2-20 nm wall thickness are prepared in ordered arrays. Atomic layer deposition (ALD) of Fe2O3 from the precursor iron(III) tert-butoxide at 130-180 °C yields very smooth coverage of the pore walls of anodic alumina templates, with thickness growth of 0.26(+/- 0.04) A per cycle. The reduced Fe3O4 tubes are hard ferromagnets, and variations of the wall thickness d(w) have marked consequences on the magnetic response of the tube arrays. For 50 nm outer diameter, tubes of d(w) = 13 nm yield the largest coercive field (H-c > 750 Oe), whereas lower coercivities are observed on both the thinner and thicker sides of this optimum.

A gelatin-based electrolyte has been developed and characterized by impedance spectroscopy, X-ray diffraction, UV-vis-NIR spectroscopy and atomic force microscopy (AFM). The heat treatment temperature was found the key factor affecting its ionic conductivity that increases from 1.5 × 10-5 S/cm to 4.9 × 10-4 S/cm by heating from room temperature up to 80 °C. The temperature dependence of the ionic conductivity exhibits an Arrhenius behavior. EC-devices with the configuration K-glass/Nb2O5:Mo EC-layer/gelatin-based electrolyte/(CeO2)x(TiO2)1-x ion-storage (IS) layer/K-glass, have been assembled and characterized. They show a good long time cyclic stability, but the change of the optical density measured at 550 nm after 25 000 cycles was only 0.13.

An electrochemical quartz crystal microbalance was used as a sensitive detector to analyse the mass changes occurring during the coloration/bleaching processes of sol-gel NiO-TiO2 electrochromic layers. Double layers were deposited on gold-coated quartz crystal electrode and sintered at 300°C in air. The electrochemical process was studied in KOH electrolyte in the potential range -0.4 to + 0.57 V vs. SCE during 650 CV cycles. The current density, charge and mass were found to increase with cycling. The shape of the mass spectrum is rather complex and changes continuously by cycling. The mass of the layer increases after each cycle slowly up to about the 150th cycle then it increases strongly after about the 250th cycle. It passes through a maximum around the 570th cycle with high amplitude variation within each cycle and then decreases fast without drastic change of the cathodic charge. Finally, a complete breakdown occurs around the 650th CV cycle impeding to record any further mass variation. The study is divided in two typical regions where the mechanism of coloration is found to change by cycling. During the first 150 CV cycles, the reversible change of the mass with the charge (increase in the anodic range for V > 0.35 V vs. SCE and decrease in the cathodic range for V < -0.2 V) was related to exchange of OH- groups that involves a change of the oxidation state of Ni from 2+ to 3+ and vice versa leading to coloration and bleaching processes, respectively. For further cycling a model is proposed taking into account the incorporation of K(H2O)n+ ions in the hydrated structure of the layer. The irreversible increase of both the mass and charge exchanged during each cycle is interpreted as due to an increase of the amount of Ni(OH)2.

Conductive coatings made from very thin functionalized (-NH2) multi-walled carbon nanotubes dispersed in a TiO2-based sol were irradiated with CO2 laser using different intensities. It was observed that the electrical and optical properties of the film have changed. Laser-treated films present lower resistivity than the original non-irradiated coating. We attribute this behavior to a better densification of the nanotubes with the assist of the laser. Films irradiated with CO2 presented a higher refractive index than non-irradiated coatings, showing the susceptibility of the films in optic-electronic devices.

Caries and periodontitis, the most wide-spread oral diseases around the world, are caused by bacterial adherence and biofilm formation onto the natural as well as restored tooth surface. One possible way to prevent the pathogenic consequences of intraoral biofilm formation might be the modification of the tooth surface by application of an anti-adhesive coating that interferes with the bacterial attachment and subsequent bacterial accumulation. The objective of this study was to investigate the effect of an experimental, low surface free energy nano-composite coating material on biofilm formation in situ. For this purpose, an organic/inorganic nano-composite coating (NANOMER (R), INM, Saarbrucken, Germany) with a surface free energy of 18-20 mJ/m2 was applied to enamel as well as titanium specimens. The nano-composite coated specimens and un-coated controls were attached to removable intraoral splints and carried by volunteers over 24 h in the oral cavity. After intraoral exposure, specimens were processed for transmission electron microscopic analysis. On non-coated enamel and titanium control samples a multi-layer of adherent bacteria was found. In contrast, on nano-composite coated specimens strongly reduced biofilm formation was observed. In most areas of the surface-coated specimens only a 10-20 nm thick electron dense layer of adsorbed salivary proteins with adherent protein agglomerates of 20-80 nm diameter could be detected. In addition, detachment of the adsorbed biofilm from the nano-composite coated surfaces was evident in electron microscopic micrographs. The present investigation provides ultrastructural evidence that it is possible to cover enamel as well as titanium with a nano-composite coating revealing easy-to-clean surface properties that cause reduced biofilm formation and accelerated removal of adherent biofilms under oral conditions.

Individual SnO2 nanowires were integrated in suspended micromembrane-based bottom-up devices. Electrical contacts between the nanowires and the electrodes were achieved with the help of electron- and ion-beam-assisted direct-write nanolithography processes. The stability of these nanomaterials was evaluated as function of time and applied current, showing that stable and reliable devices were obtained. Furthermore, the possibility of modulating their temperature using the integrated microheater placed in the membrane was also demonstrated, enabling these devices to be used in gas sensing procedures. We present a methodology and general strategy for the fabrication and characterization of portable and reliable nanowire-based devices.

Nanosized BaTiO3 powders with a specific surface area of 60-75 m2/g have been prepared by precipitation of a titanium ester with Ba(OH)2 solution at temperatures less than 100 °C. The effects of the Ba(OH)2 concentration, isopropanol mixing with water as a solvent, the Ba:Ti ratio and surface modifiers on the surface area, the particle size, the crystalline phase, the agglomeration and aggregation degree of the synthesized powders as well as dielectric properties of sintered pellets have been investigated. The properties of the obtained powders have been characterized with XRD, BET, TG-DTA, ICP-AES, HRTEM and dilatometer. A high concentration of Ba(OH)2 can increase the agglomeration and aggregation degree of the particles while the addition of isopropanol in water is beneficial for lowering it. To obtain stoichiometrical barium titanate, the ratio of Ba:Ti should be 1.1. The leaching of barium ions during processing can be limited by washing the powder with ammonia solution at pH 10.2. A BaTiO3 ceramic (95.8% of the theoretic density) has been fabricated by sintering the powders at 1250 °C for 2 h.

Nanocrystalline BaSnO3 with a primary particle size of 40-60 nm was prepared through hydrolysis of a barium tin isopropoxide and following crystallization. The thermal decomposition, the crystallization and the microstructure of the obtained powders were investigated with the help of TG-DTA, IR, XRD, HRSEM and HRTEM. The organic rest groups in the as-prepared powder decompose thermally at 350 °C, which is accompanied by the building of BaCO3 that disappear again at 600 °C. The crystallization of BaSnO3 takes place at 500-600 °C. Single-phase BaSnO3 powders have been obtained at a temperature as low as 600 °C. The amorphous as-prepared powder shows a cluster structure. Nucleation of BaSnO3 beginning at 350 °C was observed under HRTEM, and the spherical nano-particles of BaSnO3 calcined at 760 °C crystallize well and are strongly aggregated. The presented results indicate a heterogeneous nucleation and growth mechanism by the formation of BaSnO3.

BaSnO3 powders have been prepared from the tin oxide hydrate gel and the Ba(OH)2 solution via hydrothermal synthesis route. The influence of the process parameters on the characteristics of BaSnO3 has been studied. A powder with the single-phase of BaSnO3 can be obtained only when the concentration of Ba(OH)2 solution is no less than 0.2 M and the ratio of Ba:Sn lies between 1.0 and 1.2. At a hydrothermal temperature of 330°C or higher, uniform BaSnO3 powders can be directly prepared through hydrothermal reaction. When the hydrothermal temperature is lower than 250°C, the as-prepared powder consists of BaSn(OH)6 that transforms through an amorphous phase into BaSnO3 by calcination at 260°C. In the hydrothermal temperature range of 130-250°C, a higher temperature can promote the crystallization of BaSnO3, increases its specific surface area and decreases the average particle size. The duration of the hydrothermal reaction affects the morphology of the powder particles. The effects of the nonaqueous solvents on the properties of powders have also been investigated.