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Organisationseinheit der BAM
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.
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.
Zn-K transmission EXAFS measurements were performed on vitrified ashes from municipal waste incinerator facilities, which contain low amounts of Zn (up to 3 wt%) and sulphur (up to 0.5 wt%) in a ZnS phase. From the Fourier transform it can be concluded that the zinc ions are fourfold co-ordinated by oxygen or sulphur ions. The amount of zinc ions of sulphur environment, e.g. the concentration of ZnS, was determined by Fourier filtering and subsequent parameter fitting. A quantitative analysis of low ZnS concentrations up to 1 wt% has been performed for samples with a molar ratio Zn/S < 5.
A comprehensive analysis of composition and structure is essential for the development of new materials with tailored properties. We will demonstrate the application of SEM, XPS and XRD for the development of thin films containing nano-scaled electrical conductive particles for application in hybrid resistors.
The films were prepared using the sol-gel process and precursor solutions were synthesized from an acid hydrolysis of a mixture of TEOS, water and ethanol. Alcoholic solutions of ruthenium chloride were then allocated to obtain up to 30 vol% filler in the films. The films were precipitated by dip-coating on silicon, silica or glass sheets and heat treated at up to 600°C in air or nitrogen atmospheres.
A nano-structured morphology could be identified consisting of homogeneously dispersed crystalline RuO2 particles of 10 to 180 nm in size, in an amorphous SiO2 matrix. Film morphology can be varied using the synthesis process. The resistivity of the films was measured by an I-V measurement system. It changes drastically from 109·cm to 10-2·cm according to the amount of dispersed conductive particles.
The results show that composition-structure-property relations can be described by use of sophisticated analytical methods.
The electrical properties of solgel-derived films can be tailored by embedding conductive particles of ruthenium dioxide or carbon black in an insulating amorphous SiO2 silica matrix. The preparation process included an acid hydrolysis of tetraethoxysilane and methyltrimethoxysilane. Then alcohol solutions of ruthenium chloride or carbon black were added. Films of filler concentration up to 60 vol.% were prepared by dip coating and then dried and heat-treated at various temperatures up to 600_°C. The D.C. resistance of the films can be varied within the range of 109 to 102 OHgr sdot cm. A non-linear dependence on filler composition in the films was observed for both systems, which is explained by a modified percolation theory. A percolation threshold of 5.5 vol.% for SiO2-RuO2 or 50 vol.% for SiO2-C films, whereby the resistance drastically decreases, was determined. Moreover the temperature dependency of resistance and the current-voltage characteristics of the films can also be explained by this geometric model.
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.
The preparation of sol-gel derived silica-based nanomaterials containing electrical conductive carbon fillers in an extensive composition range is described and their electrical properties are presented. Nanomaterials of carbon filler concentrations up to 60% (v/v) were obtained by dip coating or screen-printing from precursors of hydrolysed alkoxysilanes. Nanostructured morphology could be identified to consist of homogeneously dispersed carbon black particles or carbon fibres of 30 to 500 nm in size in a modified silica matrix. The electrical resistivity of the films changes drastically from 1010 to 10?1 O?cm, according to the amount of dispersed conductive particles. A threshold between 5 and 50% (v/v), at which the resistance abruptly decreases, was determined. A geometrical model related to percolation theory explains this non-linear dependence on the filler composition in the materials. Moreover the temperature dependence of resistance and the current-voltage characteristics of the nanomaterials can also be illustrated using this geometric model.
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.
Aluminium oxide films were prepared by a dip-coating procedure from solutions of hydrolysed aluminium-tri-sec-butoxide in methoxyethanol on austenitic steel as well as hard metal tools. The heat treatment was performed in an argon atmosphere or vacuum at temperatures up to 1100 °C. Characterisation of the coatings by SEM, showed nearly crack free coatings with a dense morphology which completely cover the substrate. The chemical composition of the coatings was investigated by ESCA, GD-OES and XRD measurements. This analysis detected a multi-layered arrangement of spinel phases, ?- and ?-alumina in the coatings. The scratch test showed sufficient adhesion of the coatings comparable to CVD layers. Pin-on-disc and oscillating sliding tests were carried out on solgel coated samples and also on uncovered reference samples. The determined wear rates and friction coefficients showed significant improvement in wear resistance of solgel coatings containing a corundum phase. The strong adhesion to the substrates is due to a layered assembly formation and gradient composition of the coating caused by an interdiffusion of metal cations and oxygen in the metal/oxide interface during heat treatment.
Silica coatings filled with nanoscaled inorganic fullerene-like tungsten disulphide (IF-WS2) have been prepared through a solgel process on stainless steel substrates, and the structure and mechanical properties have been investigated. The precursor was prepared from a mixture of colloidal silica, 3-glycidoxypropyltrimethoxysilane (GLYMO), water and ethanol, adjusted to pH 4 with HNO3. In this solution WS2 is dispersed and in some cases immediately before coating ethylenediamine (ED) is added. The stainless steel substrates are dip-coated, dried in air and heat-treated in the temperature range of either 150360 °C in air or up to 900 °C in vacuum. The solidification process is followed by differential thermal analysis (DTA). The resulting brown coloured coatings have a thickness of 1.54 µm. Scanning electron microscopy investigations (SEM) show that the WS2 nanoparticles are embedded as small aggregates in a hybrid silica matrix. X-ray diffraction (XRD) measurements prove that most of the tungsten disulphide embedded in the matrix can be protected against oxidation even after curing the samples at temperatures up to 900 °C. Hardness and modulus of the hybrid silica films were measured through an instrumented indentation test. Increasing the temperature of the heat treatment yields an increase of hardness from 0.3 to 1 GPa and of modulus from 3 to 17 GPa. The amount of up to 10 wt.% WS2 in the coatings has no remarkable influence on hardness and modulus of the samples.
Traces of microbial volatile organic compounds (MVOCs) in air can indicate the presence of growth of moulds in the indoor environment. Ion-mobility spectrometry is a very promising method for detection of these MVOCs, because of its high sensitivity. For development of an in-situ method for detection of MVOCs, a portable ion-mobility spectrometer (IMS) was used and test gases of 14 MVOCs and their respective mixtures were investigated. IMS spectra were recorded as a function of concentration of MVOCs in air. Drift time and mobility of reactant ions formed in positive polarity mode were determined and correlated with the mass-to-charge ratio (m/z) of the MVOCs investigated. The estimated detection limit has a specific value for each MVOC and is in the range 3 to 96 µg m-3 (1 to 52 ppbV). Indoor trials show that IMS can indicate hidden mould growth.
The solgel process was applied to enhance properties of pine sapwood. For this purpose wood prisms were soaked in nanoscaled precursor solutions prepared from titanium(IV) n-butoxide and titanium(IV) iso-propoxide, respectively, using vacuum impregnation technique. The wet composites were cured by special program with final heat treatment at 103 °C. Weight percent gains (WPG) of the wood specimen in the range of 19-25% were obtained due to these procedures. SEM investigations show that precursor solutions penetrate into the whole wood body and the titania formed after heat treatment in the composites is deposited in the pores (lumen) and partly in the cell walls of the wooden matrix. The moisture sorption was investigated in long term tests for a period of some months by storage at 20-23 °C in humid air (relative humidity of 99%) and ambient atmosphere (relative humidity 40-60%), respectively. For untreated reference samples the moisture sorption results in increasing of mass and volume according to saturation values of 24 and 13%, respectively, after about 15 days. The incorporation of titania reduces the saturation values of the moisture sorption by up to 12% in mass and by up to 5% in volume at a relative humidity of 99%. Thus, an enhancement of the dimensional stability of about 60% is obtained at best. The results demonstrate that modification of wood with sol-gel derived precursors can enhance its dimensional stability, which prevents the formation of cracks. Because of that reduced moisture sorption biological attacks should be delimited. Additionally, thermal analyses show a retarded combustion of the wood matrix due to titania infiltration.