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Eingeladener Vortrag
- nein (51)
27Al nuclear magnetic resonance spectra of polycrystalline aluminium borate 9Al2O3 · 2B2O3 have been measured in double rotation at 11.7 and 7.0 T and high-speed magic-angle spinning at 7.0, 9.4, 11.7, 14.1 and 17.6 T. Spinning sidebands from the satellite transitions were observed at 7.0 and 14.1 T. Each of the four structural aluminium sites [Al(IV), Al(V)(1), Al(V)(2) and Al(VI)] are observed, characterised and assigned in the spectra. The obtained parameter set gives a fully consistent interpretation in agreement with the crystal structure of the compound.
EXAFS investigation of an intermediate crystalline phase in TiO2-rich strontium borate glasses
(1995)
Glasses in the systems Li2OB2O3V2O5 and ZnOB2O3V2O5 as well as crystalline reference substances were investigated by V-K-EXAFS. The glasses have a high modifier content ([Li2O, ZnO]/[B2O3] ratio>1) and contain 1050 mol% V2O5. The VO interatomic distance, coordination number and DebyeWaller factor were determined from the analysis of the measured absorption spectra. In the glasses investigated (and in binary lithium or zinc vanadate glasses too) VO4 tetrahedra are the predominant coordination polyhedra for the vanadium ions. The symmetry of the tetrahedra is more or less distorted in comparison to crystalline substances. The VO4 tetrahedra can be either directly connected to each other or incorporated into borate BO3/BO4 groupings. Structural elements which are comparable to those in crystalline substances can be discussed. In glasses of the metaborate composition, vanadium can be incorporated in chain or ring [BØ2O-]n metaborate groupings. With increasing V2O5 content the vanadium ions can extensively replace boron ions and form distorted tetrahedra in VØ2O2- metavanadate groups. In glasses of high modifier content and low vanadium amount the vanadium ions create nearly regular tetrahedra of isolated VO43- ortho- or V2ØO64- pyrovanadate groups in the presence of B2ØO44- pyroborate, BO33- orthoborate and BØ4- groups.
Humidity-Sensing Materials
(1999)
The structure of glasses in the Li2OB2O3V2O5and ZnOB2O3V2O5systems of a high modifier content ([Li2O, ZnO]/[B2O3] ratio > 1) and 10 to 50 mol % V2O5are investigated by V-KEXAFS and Raman spectroscopy. In the glasses investigated, VO4tetrahedra are the predominant coordination polyhedra for the vanadium ions. The symmetry of the tetrahedra is more or less distorted in comparison with crystalline substances. The VO4tetrahedra can be either directly connected to each other or incorporated into the borate groupings of the BO3and BO4units. An increase in the vanadium content in the glasses reduces the interconnection of structural units and, therefore, the glass transition and crystallization temperatures.
The paper demonstrates the scope of the differential thermal analysis coupled with the thermal gravimetry as an express method for estimation of thin film protecting ability and oxidation kinetics. Oxidation of an iron foil protected by the PVD titanium nitride thin film coating was investigated as well as non-isothermal oxidation of PVD thin films of TiN (two types of the preparation procedure), (Ti,Nb)N and (Ti,Al)N isolated from a substrate by dissolving them in an acid. Additions of aluminium and niobium are expected to improve chemical, thermal and biomedical properties of thin films for their technical application. It was established that the addition of niobium decreases, whereas the addition of aluminium not only increases the thermal stability of TiN coatings, but also changes the oxidation mechanism. It was demonstrated that the TiN film oxidation is a many-staged process depending on the heating rate. An opportunity of a quite formal treatment for getting Kissinger's estimation of kinetics parameters of different oxidation stages is also demonstrated.
Es wurden Gläser im System Li2O-B2O3-V2O5 und ZnO-B2O3-V2O5 wie auch kristalline Referenzmaterialien mit Hilfe der Röntgenabsorptions- (V-K-EXAFS, -XANES) und der RAMAN-Spektroskopie untersucht. Zur Diskussion der Ergebnisse wird angenommen, dass in den Gläsern in Abhängigkeit von der chemischen Zusammensetzung diskrete Sauerstoffkoordinationspolyeder der Kationen in unterschiedlicher Verknüpfung vorliegen.
1. Die Gläser enthalten vor allem BO3- bzw. BO4- sowie VO4-Polyeder.
2. Die VO4-Tetraeder besitzen abhängig von der Glaszusammensetzung eine unterschiedliche Verzerrung. Sie verknüpfen sich zu Struktureinheiten ähnlich denen der kristallinen Vanadate oder sie werden in Boratstrukturen eingebaut.
3. In Gläsern mit einem hohen Netzwerkwandler-Gehalt und geringer Vanadiumoxid-Konzentration treten reguläre Tetraeder von isolierten Ortho- oder Pyrovanadat-Gruppen in Gegenwart von Pyroborat-, Orthoborat- und BO4-Gruppen auf. Mit steigendem V2O5-Gehalt werden zunehmend verzerrte Tetraeder gebildet, die sich z. B. zu Metavanadat-Gruppen verknüpfen oder in Metaborat-Baugruppen eingebaut werden.
The aluminium oxide films on austenitic steel are prepared from sols of re-dispersed boehmite nano powders in water. After dip-coating of the sol, a heat treatment including drying, calcination and annealing in vacuum at temperatures up to 1100°C is performed to obtain crack-free coatings of a thickness up to 6 µm. XRD measurements detect α- and γ-alumina, a TiOx-phase at the metal/coating interface and a gradient of phase formation in the coating. The strong adhesion on the substrates is due to the layered assembly and gradient composition of the coating caused by an inter-diffusion of metal cations and oxygen in the metal/oxide interface during heat treatment. Residual stress measurements by X-rays result in compressive stresses of 24 GPa in the alumina coatings. The pin-on-disc test shows a remarkable improvement of wear resistance obtained by sol-gel coatings. The α-alumina content and the compressive stress of the coatings correlate with wear resistance of the coatings.
CrN film coatings were prepared by magnetron sputter deposition at different nitrogen partial pressures. The film characterisation by XRD and DTG gives average bulk compositions of Cr2N and CrN for the coatings. Highly sensitive XPS investigations were performed and the chemical and phase compositions of a film surface range of about 10 nm thickness was estimated quantitatively from the deconvoluted peak intensities. It is demonstrated that the composition of the surface of chromium nitride thin films differs from the core and is more complex in constitution. Not only chromium nitrides (Cr2N and CrN) but also chromium oxynitrides and chromium oxides (CrOx and CrOxHy) were detected. Metallic chromium was also found in films prepared at higher nitrogen flow. The concentration of the estimated phases shows dependence on film preparation and additional heat treatment.
Es wird ein Sensorsystem (HUMItrace III) zur Spurenfeuchtebestimmung in der Prozessmesstechnik vorgestellt. Die Bestimmung der Spurenfeuchte erfolgt mit neuentwickelten planaren Sensoren (HUMItrace-Sensoren) nach dem coulometrischen Messprinzip im Frostpunkttemperaturbereich von -80 °C bis -30 °C mit der Kalibriereinrichtung THG11 durch Mischen von trockenen und befeuchteten Teilgasströmen kalibriert. Die Gasfeuchte von Luft kann in diesem Bereich mit den HUMItrace-Sensoren mit einer erweiterten Unsicherheit von 2 K bestimmt werden. Die Sensoren können aber auch zur Gasfeuchtemessung in Wasserstoff, Stickstoff und Distickstoffmonoxid eingesetzt werden. Eine weitere Kalibriereinrichtung, THG-P12, generiert Prüfgase nach der Permeationsmethode; sie kann in das HUMItrace III Sensorsystem implementiert und zur vor-Ort-Kalibrierung der HUMItrace-Sensoren nach dem Standardadditionsverfahren genutzt werden.
A silicon MIS-structure prepared with thin layers of the super-ionic conductor LaF3 and Pd is used to detect hydrogen. The sensor is able to detect hydrogen in a concentration range from below one ppm to the lower flammable limit in air which is 4 vol.-%. The chip operates at room temperature which results in an extremely low energy consumption compared to other hydrogen sensors. Only once per day a short heating pulse is necessary to reactivate the sensor chip. In cooperation with the BAM, a long-term experiment was carried out. The duration of the measurement was about three months, with a hydrogen exposure every two weeks. The sensor signal remained stable over the measured period.
A new planar sensor element for continuous coulometric trace humidity measurements in industrial gases has been developed. In order to ensure precise measurements a calibration facility including a precision dew point hygrometer as a reference device was developed. The sensor can measure the humidity in the frost point temperature range of -20 °C to -80 °C and has an expanded uncertainty of 2 K, a fast reaction time and a settling time of the entire system from 15 to 30 min.
Carbon nanofibers (CNFs) were functionalized by a multistage process including oxidation, reduction and silanization. The chemical modifications were examined by Fourier transform infrared spectroscopy, X-ray photoelectron spectrometry, Raman spectroscopy and thermogravimetric analysis. The silanized CNFs were then added into an epoxy resin (EPON 828) to study the effect of the surface modification of CNFs on the properties of nanocomposites. For comparison, nanocomposites containing original unmodified CNFs were also investigated. Scanning electron microscopy indicates better dispersion of modified fibers in the epoxy polymer matrix; the mechanical and thermal properties of composites are also improved; the electrical conductivity of the composites is reduced.
It has been shown that tetraethoxysilane-derived precursors are suitable solutions for impregnating pine sapwood to improve its dimensional stability. Tailored sol–gel syntheses result in precursors with nano-scaled silica species which are able to penetrate into the cell walls of wood. The physical fixation of those species inside the cell walls was verified by ESEM/EDX investigations. There is evidence that the silica species are chemically bonded to wood components. Nonreacted alkoxy groups can exist in the wood composites after impregnation. The amount of these organic residues depends on the composition of the precursors, especially their condensation degree and reactivity. Treatments for finishing the composites after the impregnation step to get clean products and stable properties are discussed in this context. An explanation of the complex relationship between solids content in the impregnation solutions and percentage weight gain and bulking of the composites is given. It can be demonstrated that high WPG values (>20 %) are pre-conditions for an effective protection of the resulting composites but they only guarantee efficient improvements if connected with the incorporation of the inorganic component inside the cell walls. Therefore, the sol–gel syntheses have to be carried out in such a manner that very small (<2 nm) as well as reactive species are sufficiently available. This demands syntheses using sub-stoichiometric water contents (H2O/TEOS <4, better <2).