Scientific publications

A high-throughput screening method for the exploration of optimal curing parameters and resistance to solvents of NANOMER (R) coating materials based on the temperature-dependent swellability was developed. The screening method was first tested using a model sol made of pre-hydrolyzed (3-glycidoxypropyl)triethoxysilane (GPTES), tetraethoxysilane (TEOS), (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethoxysilane (FTS) and zirconium complex (prepared of zirconium-tert-butoxide complexed with acetylacetonate) charged with reactive diluent trimethylolpropan-triglycidether and defined amounts of fluorescein and cured at different temperatures. Afterwards, fluorescein was extracted with sodium hydroxide solution and the optical density of the supernatant of all samples was measured at 490 nm which is sensitive to the dye concentration. The optical density (OD) correlates with the degree of curing. According to this screening a temperature >= 140°C is necessary for proper curing. The time dependence of extraction reveals information on resistance against sodium hydroxide solution, i. e. alkali resistance. The time dependent extraction of fluorescein at 60°C of coatings cured at 100 and 140°C, respectively, shows a better resistance against 0.1 M sodium hydroxide solution for the one cured at 140°C, especially in the time range 10 – 60 min. The whole process – sol preparation, mixing of sols with dye, extraction, and optical spectroscopy-can be performed automatically. Further testes were made to proof the usability of this process. 96 hybrid materials were screened in regard to their alkali resistance and finally, a total number of 14 clear organic-inorganic hybrid coating systems with improved stability against sodium hydroxide solution were derived from this study.

Transparent nanocrystalline pure anatase titania (nano-TiO2) was synthesized by hydrothermal process at 200°C. Photocatalytic activity of the nano-TiO2 as in the form of sol was tested for degradation of Methylene Blue (MB) and Reactive Red 120 (RR-120) in aqueous solutions. Structural and physico-chemical properties of the nano-TiO2 were characterized using powder XRD, SEM, BET, FTIR and elemental analyses. Complete photodegradation of RR-120 was successfully achieved by aid of the nano-TiO2 whereas NIB was not degraded, maybe because of reversible color change in nano-TiO2 sol/MB mixture after the LTV irradiation was stopped. Potocatalytic activity of the synthesized the nano-TiO2 for degradation of RR-120 was compared with Degussa P-25 at optimum conditions determined for RR-120. It was found that the nano-TiO2 can be repeatedly used with higher photocatalytic activity than Degussa P-25.

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.