Publikationen

Transparent TiO2 films were produced via sol-gel spin and dip-coating techniques. Soda-lime glass (SLG) and SiO2 precoated glass were used as substrates. The thin films were characterized by X-ray diffraction (XRD), X-ray reflectometry (XRR), optical profilometer and glow discharge optical emission spectroscopy (GD-OES). Na migration was detected in the amorphous TiO2 films which are deposited on SLG substrates. In order to prevent sodium migration a barrier layer was introduced between TiO2 film and glass. The beneficial role of this barrier layer on alkali migration is verified and the mechanism of prevention of migration is proposed relying on the results of GD-OES depth profile measurements.

The dilithium silylamide [(Me2SiNtBuLi)2O]2 shows in the temperature range between ambient temperature and 150 K three solid state phases with transition points at 253 K and 163 K which were characterised by means of X-ray diffraction and solid state NMR spectroscopy. In the high temperature phase the dimeric molecules of [(Me2SiNtBuLi)2O]2 crystallize in the tetragonal space group P42/nmc. It can be deducted from X-ray diffraction studies that, at room temperature, the four lithium atoms in [(Me2SiNtBuLi)2O]2 are statistically distributed on two split positions at a distance of 0.89(1) Å. By cooling below 253 K the crystal transforms into an intermediate phase with space group P42/n. The symmetry of the molecule is reduced from point symmetry (4) over bar 2m (D2d) to (4) over bar (S4). The split positions of the lithium atoms are found in different planes and their distance is slightly reduced from 0.89 Å to 0.81 Å. Solid state NMR spectroscopic measurements and powder diffraction analysis reveal the existence of a monoclinic low temperature phase below 163 K. From 7Li MAS and 13C CP/MAS NMR spectroscopic measurements follows furthermore that the disorder of the lithium atoms in the two higher temperature phases (above 163 K) is due to a local dynamic exchange of the four lithium atoms over eight possible positions in the molecule. 7Li NMR measurements at variable MAS rates reveal a very small activation energy of 11 KJ/mol for the dynamic process with ex hange rates of the lithium atoms in the order of 5 to 15 K Hz. With respect to the lithium atoms in the order of 5 to 15 KHz. With respect to the lithium motions, the dynamics are intramolecular with an intermolecular coupling process due to van-der-Waals forces.

The new lithiumtitanates (py)2Li[(py)2Ti(OPh)4] (1) and (py)2Li[(py)Ti(OPh)5] (2) have been obtained from the reaction of titaniumtrichloride (respectively titaniumtetrachloride 2) with LiOPh in the presence of the base pyridine (py). The crystal structures of both compounds show that the titanium atoms are in the centres of distorted octahedral coordination figures. In compound 1, four oxygen and two nitrogen atoms (in cis orientation) are bonded to titanium, whereas in 2, five oxygen and one nitrogen atom form the coordination polyeder around titanium. In both compounds, the lithium atoms are attached through phenolate bridges to the octahedra. The titanate (py)2Li[(py)2Ti(OPh)4] (1) has a single absorption band in the visible region of the UV-spectrum showing a shoulder shifted to the bathochromic region, due to the Jahn-Teller-effect for d1-systems.

The selective synthesis of new types of alumosiloxanes can be achieved by the reaction of the bicyclic compound Al2[(OPh2Si)2O]3·2 Et2O (3) with water in donor solvents like acetone or THF, leading to the polycyclic THF adduct [(Ph2SiO)8(AlO1,5)4]·2 THF, (7), and to the polycyclic alumosiloxane [(Ph2SiO)8(AlO(OH))4]·4 Me2C{double bond, long}O (8), respectively. In the presence of the base triethylamine, the starting compound 3 gives rise to two other condensation products. At medium temperatures (50–60 °C) in toluene the dispiro "alumosilicate" [(Ph2SiO)2O]2{Al[(Ph2SiO)2OH·NEt3]}2 (9) is formed and at higher temperatures under reflux in toluene, the product [(Ph2SiO)2O]Al[(Ph2SiO)2OH·NEt3] (10) is generated. The spirocyclic compounds 9 and 10 present the first isolated "alumosilicates" with six-membered O(SiO)2Al cycles. These new types of alumosiloxanes and "alumosilicates" have been characterised by spectroscopic methods as well as by single crystal X-ray analyses.

The subsolidus phase relations of the ternary system CuO-TiO2-CaO sintered at 950 °C in air have been determined by powder X-ray diffraction method. Only one ternary compound CaCu3Ti4O12 was found in this system. From room-temperature dielectric property mapping at 10 kHz, a giant dielectric constant (εr>104) was observed for most of the ceramic composites in the CuO-rich region and in the region along the CaO-CuO binary line. The composites in the CaCu3Ti4O12-rich region were found to give a comparable giant dielectric constant when sintered at 1050 °C. The particular microstructure of larger grains with predominant phase surrounded by smaller grains with the secondary phases was found in such composites with a high dielectric constant. The relations between structures and dielectric properties were investigated. An internal barrier layer capacitance effect is the most probable mechanism to explain this particular dielectric behavior.

The synthesis, structural characterization, and NMR studies of new tin(II) alkoxides with thiophene-based substituents are presented. The reaction of ligand HO-C(C4H3S)3 (1) with bis(hexamethyldisilazyl)tin {Sn[N(SiMe3)2]2} in the ratio 2:1 in toluene yields the dinuclear compound {Sn[OC(C4H3S)3]2}2 ∙ 2 toluene (2). The same reaction in tetrahydrofuran allows the isolation of two compounds: a dinuclear {Sn[OC(C4H3S)3]2}2 ∙ 2 thf (3) and a mononuclear Sn[OC(C4H3S)3]2(thf) (4) molecule. The molecular structures of these tin(II) methoxides containing thienyl substituents 2-4 were investigated by single-crystal X-ray crystallography. The NMR spectra of solutions of Sn[OC(C4H3S)3]2}2 ∙ 2 toluene (2) or {Sn[OC(C4H3S)3]2}2 ∙ 2 thf (3) and {Sn[OC(C4H3S)3]2}2(thf) (4) are almost identical, showing only variations of the molar ratio of the dimeric and monomeric compounds.

Hybrid organic-inorganic sol-gel-matrices, with up to 20 wt.% incorporated ceria nanoparticles, have been employed as coatings for an AA2024-T3 aluminium alloy. The morphology of the coatings and associated nanoparticles has been examined by conventional and high-resolution transmission electron microscopy, revealing a relatively uniform distribution of 5 nm size nanoparticles across the coating thickness. Electrochemical studies indicate a general beneficial effect of incorporation of ceria nanoparticles, although the performance of the coated alloy depends on the nanoparticle content. Electrochemical polarisation behaviour revealed that the coating decreased the anodic current density by about seven orders of magnitude compared with the uncoated alloy, with high breakdown potentials in chloride-containing solution. Accelerated salt spray testing showed that corrosion in an artificial scratch is blocked most efficiently by high ceria contents, whereas general corrosion is inhibited effectively with comparatively low ceria contents. Electrochemical impedance spectroscopy indicated degradation of the barrier properties of coatings with increased amounts of incorporated nanoparticles. Assessment of the abrasion and scratch resistance, and hydrophobicity also revealed additional beneficial functional properties of the coatings containing nanoparticles.

Bias current applied to conductometric gas sensors consisting of individual metal oxide nanowires can be used to heat them up to the temperature necessary for sensing. This approach in combination with the good sensitivity and stability of metal-oxide nanowires, can be used to develop prototypes with low power requirements (few tens of microwatts). Here, we present new sensors devices based on this approach that display fast dynamic performance only limited by the gas-solid interaction kinetics.

Dynamics of gas-surface interactions determine the limits of the fastest response times of sensors based on metal oxides. Here, the kinetics of adsorption and desorption of gaseous molecules onto the surface of metal oxide nanowires was analyzed through pulsed self-heating assisted conductometric measurements. This approach overcomes gas diffusion, which is typical of conventional porous film based devices, and provides thermal response times fast enough to evaluate the fundamental gas-surface reactions kinetics. Experimental response and recovery times of individual SnO2 nanowires toward oxidizing and reducing gases obtained with the here-proposed methodology were related to the reaction barriers predicted by theoretical models and other experimental techniques.

In this paper, the authors present an effective experimental method to estimate the temperature of individual metal oxide nanowires that can be used to quantify the heating produced in conductometric or other operating conditions. The here-proposed method is based on the analysis of the recovery time of the nanowire's resistance after exposure to a gas pulse (0.5 ppm of NO2 in dry air). It is reproducible with different devices always with uncertainties below +/- 20°C in the temperature range (70-300°C) studied herein. The exploration of alternative gases and nanolithography techniques may help to extend its operating range and its applicability to other materials. In any case, the opportunity to probe temperatures at the nanoscale opens the door to a number of fundamental and applied advancements in the field of nanotechnology.