Publikationen

Polyimide-silica hybrid materials have been prepared through the sol-gel process by mixing various proportions of tetraethoxysilane (TEOS) with polyamic acids (PAAs). Two types of PPAs were employed. The first was obtained by reacting an equimolar mixture of oxydianiline (ODA) and pyromellitic dianhydride (PMDA) in dimethylactamide (DMAc) as solvent. The second was prepared using a mixture of ODA and 2,2-Bis(3-amino-4-hydroxyphenyl) hexafluoropropane (6F-OHDA) in molar ratio 9: 1, respectively and reacting with a stoichiometric amount of PMDA in DMAc. Polyamic acids were converted to polyimides and a sol-gel reaction proceeded simultaneously by heating the hybrid films to 300 degrees C. The hydroxyl groups from 6F-OHDA allows the secondary bonding between the polyimide and growing silica phase and thus retard the gross phase separation. Only the 10 mol% addition of 6F-OHDA in the polyimide chain resulted in a drastically different microstructure for the resulting hybrids. SEM, stress-strain analysis, temperature variation of storage and loss modulus, and thermal stability were used to characterize the hybrid materials. Properties of both types of hybrids have been compared and related to the two different types of structures of polyimides used in the preparation of the hybrids.

Nanostructured films of iron, tin and titanium oxides were grown on glass and quartz by gas phase decomposition of [Fe(OtBu)3]2, [Sn(OtBu)4] and [Ti(OiPr)4], respectively. Films obtained exhibit good adhesion and homogeneous morphology with structural features in the nanometer range. The influence of microstructure and phase on optical (α-Fe2O3 and Fe3O4), gas sensing (SnO2) and photocatalytic (TiO2) properties was systematically studied to explore the potential of these coatings for technological applications.

Yttrium aluminum garnet (YAG) nanopowders doped with high neodymium (Nd) content (3 at.%) were synthesized by the sol-gel processing of (I) alkoxide precursors and (II) metal chelates formed by complexing the cations with polyethylene glycol. A stoichiometric YAG composition was obtained following both procedures; however, the agglomeration of particles was significantly higher in glycolate synthesis, which shielded residual organics from oxidation (elemental analyses). Distribution of Nd3+ ions in the YAG matrix, as shown by the absorption of pump energy and photoluminescence spectra of Nd:YAG ceramics, was more homogeneous in alkoxide-derived powders. The segregation of Nd centers in the glycolate-derived sample was supported by the precipitation of a crystalline Nd2O3 phase (X-ray diffraction) during sintering. High-resolution absorption spectra (4I9/2(1)-> 4F9/2(1)) of the powders showed that a higher absorption cross-section of glycolate-derived powders is due to Nd3+-Nd3+ ion pairing, which leads to the quenching of photoluminescence. Owing to the better dispersion of optically active centers, the photoluminescence signal was found to be substantially enhanced in alkoxide-derived Nd:YAG ceramics.

Deposition of titanium oxide (TiO2) films has been investigated in a pulsed DC plasma assisted CVD process (PACVD) using titanium isopropoxide, Ti(OPri)4, as the metal-organic source. The coatings were compared with TiO2 films obtained by a thermally activated decomposition of Ti(OPri)4 in a LPCVD process and characterized with respect to their morphology, microstructure, chemical composition and mechanical properties by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), photoelectron spectroscopy (XPS) and micro-indentation studies, respectively. While nanostructured deposits of crystalline TiO2 could be obtained in the LPCVD process at temperatures as low as 350 °C, the PACVD films were identified to be amorphous under different deposition conditions. On the other hand, the plasma assisted CVD produced dense and mechanically stable TiO2 films at temperatures below 200 °C showing less residual carbon contamination when compared to LPCVD coatings. Despite the similar chemical composition, the precursor fragmentation and surface reactions (diffusion, nucleation and grain growth) were different in the two cases and found to be sensitive to the energy mode. Whereas high substrate temperatures forced nucleation and crystallization in thermally activated process, incomplete decomposition of organic ligands caused high carbon contamination in the films. The fragmentation of precursor was found to be more effective in PACVD process as indicated by low residual carbon contamination in the films. Nevertheless, the PACVD films possessed an amorphous character and a post-annealing step was necessary to obtain crystalline deposits, which showed hydrophilic behaviour stimulated by UV irradiation. While the deposition temperature was found to be a key parameter for the LPCVD process which affects morphology and microstructure, the chemical and mechanical properties in the PACVD process were strongly influenced by plasma power variations, e.g. discharge voltage and gas composition.

Differential mobility analyzers (DMAs) are sometimes used to characterize aerosols that contain aggregates of low fractal dimension. However, these instruments are normally calibrated for spherical particles and the calibrations are not directly applicable to aggregates. A method proposed by Lall and Friedlander [(2006). On-line measurement of ultrafine aggregate surface area and volume distributions by electrical mobility analysis, I: Theoretical analysis. Journal of Aerosol Science, in press] for characterizing ultrafine aggregate number, surface area and volume distributions by electrical mobility measurements was tested experimentally. The method is best applied to idealized aggregates composed of uniform primary particles smaller than the mean free path of the gas. It relates the number and size of the primary particles that compose the aggregate to the mobility diameter of a spherical particle. Aggregate number distributions were obtained by calculations based on aggregate drag and aggregate charging efficiency; surface area and volume were obtained by summing over the primary particles that compose the aggregate. The theory was tested experimentally using silver aggregates generated by an evaporation-condensation method. Primary particle diameter was. To obtain distributions with respect to particle volume, aggregates were sintered to form spheres. It was assumed that the aggregate volume does not change upon sintering and coagulation was neglected. Thus the number of aggregates in a given volume range (number distribution, vs. v) should not change after sintering. Agreement between aggregate number distribution based on idealized aggregates and the values measured for spheres of sintered aggregates was good. The agreement also indicates that the aggregate volumes based on idealized aggregates were accurate. The aggregate number distribution and volume based on the conventional calibration for spheres were significantly overpredicted. A separate experimental test of the theory was made using literature data for diesel aggregates. Primary particle diameter was. Aggregate volumes calculated from theory agreed well with aggregate volumes measured by transmission electron microscope analysis.

Transparent conducting coatings of doped SnO2 and Zn2SnO4 have been deposited by the sol-gel technique on borosilicate and alumosilicate glass substrates, respectively. As films of SnO2:Sb (ATO) show some intrinsic drawbacks, which are discussed in this report, alternative dopants for SnO2 such as Ta, Nb and W, were examined concerning the electrical and optical properties of the deposited coatings. In this way, coatings with resistivities on the order of 3 x 10-2 Ωcm characterized by much higher charge carrier mobilities than for SnO2:Sb and a transmission of >85% were realized. In the search for ternary compounds (AxByOz), coatings in the system ZnO-SnO2 were examined, focusing on the fabrication of meta zinc stannate (Zn2SnO4). Coatings made from tin alkoxides yielded monophasic materials with high crystallinity and good morphological properties. By post-annealing, a resistivity of 0.4 Ωcm, with charge carrier mobilities of up to 7 cm2/Vs were obtained. Therefore, the development of the coating solution is as crucial as the elaboration of the heat treatment procedure of as-deposited films in order to obtain pure Zn2SnO4 phase coatings without any impurity phases.

Transmission electron microscopy (TEM), small angle X-ray (SAXS) and dynamical mechanical thermal analysis (DMTA) were used to characterize the morphology and thermo-mechanical properties of hybrid organic inorganic materials. These materials were based on polyimide (PI) and tetraethoxysilane (TEOS). Polyimide polymer is prepared from 4,4'-oxydianiline (ODA) 2,2-Bis(3-amino-4-hydroxyphenyl) hexafluoro-propane (6F-OHDA) pyromellitic dianhydride (PMDA) polyamic polymer. In one family of hybrid materials 3-isocyanatopropyltriethoxysilane (ICTS) is used as coupling agent in order to enhance the interfacial interaction between polyimide and silica. It was possible to modulate the morphology as well as the optical and thermo-mechanical properties of these hybrid materials depending on the formulation used. TEM and SAXS analysis indicated that silica domains on the nanoscale level are obtained when coupling agent is used in the formulation. Additionally the TEM and SAXS analysis indicated that miscibility of the organic and the inorganic phases on the molecular scale is obtained in the hybrid films when ICTS as coupling agent is added to the polyamic acid. These techniques show a fractal structure of the hybrid materials with coupling agent. This was confirmed with DMTA analysis which shows very high temperature relaxation (more than 450 °C). From this result it could be derived that the addition of ICTS causes a morphological transformation from discrete particulate microstructure to fine interpenetrated or co-continuous phases. The intimate miscibility of the phases is accompanied at the same time by the amelioration of thermo-mechanical properties of the hybrid films.