Publikationen

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.

Four-point bending creep behavior of mullite ceramics with monomodal and bimodal distribution of grain sizes was studied in the temperature range of 1320-1400 °C under the stresses between 40 and 160 MPa. Mullite ceramic with bimodal grain size distribution was prepared using aluminum nitrate nonahydrate as alumina precursor. When γ-Al2O3 or boehmite were used as alumina precursors, mullite grains are equiaxial with mean particle size of 0.6 µm for the former and 1.3 µm for the latter alumina precursor. The highest creep rate exhibited the sample with monomodal morphology and grains in size of 0.6 µm, which is about one order of magnitude greater thin) that for the monomodal morphology but with grains in size of 1.3 µm. The highest activation energy for creep (Q = 742 +/- 33 kJ/mol) exhibits mullite with equiaxial grains of 1.3 µm, whereas for sample with smaller equiaxial grains the activation energy is much smaller and similar to mullite ceramics with bimodal grain morphology. Intergranular fracture is predominant near the tension Surface, while transgranular more planar fracture is predominant near the compression surface zone.

Pure and doped niobium oxide (Nb2O5) layers are electrochromic (EC) materials which change their color by insertion of Li+ ions from transparent to brown, grey or blue depending on the crystallinity of the layer. EC-devices with the configuration K-glass/EC-layer/composite electrolyte/ion-storage (IS) layer/K-glass, were produced using different Nb2O5 EC-layers, a (CeO2)x(TiO2)1-x (x = 0.45) IS-layer and an inorganic-organic composite electrolyte to which a small amount of water (up to 3 wt.%) was added. The grey coloring all-solid-state sol-gel devices fabricated with Nb2O5:MO coatings show a high reversible coloration (∆OD = 0.3) and a long-term stability of more than 55000 switching cycles. Large area EC-devices (30 x 40 cm) show a transmittance change between 60% and 25% at 550 nm after galvanostatic coloration and bleaching for 3 min and a coloration efficiency of 27 cm2/C. The results obtained with blue and brown coloring Nb2O5 EC-layers and a comparison with blue coloring WO3 layers are also presented.

Two-and four-probe electrical measurements on individual tin oxide (SnO2) nanowires were performed to evaluate their conductivity and contact resistance. Electrical contacts between the nanowires and the microelectrodes were achieved with the help of an electron-and ion-beam-assisted direct-write nanolithography process. High contact resistance values and the nonlinear current-bias (I-V) characteristics of some of these devices observed in two-probe measurements can be explained by the existence of back-to-back Schottky barriers arising from the platinum-nanowire contacts. The nanoscale devices described herein were characterized using impedance spectroscopy, enabling the development of an equivalent circuit. The proposed methodology of nanocontacting and measurements can be easily applied to other nanowires and nanometre-sized materials.

Transparent conducting coatings of sol-gel ATO (antimony-doped tin oxide) were used to improve surface smoothness of commercial sputter-deposited ITO (indium tin oxide) coatings for application as display electrodes. In order to overcome the deteriorating evaporation-cooling during dip coating, the coating solution was heated moderately to 25°C thus providing the substrate with the required heat. This way, the surface roughness of the ITO could be reduced with an only 45 nm thick ATO coating to R-pv = 3.8 nm (R-a = 0.4 nm) compared to 31 nm (3.8 nm) for the ITO substrate. Another benefit of such additional coating is the possibility to tailor surface properties of the electrodes in wide ranges. This was used to increase the work function of the ITO substrate from initially 4.3-4.6 eV to about 4.8-5.2 eV by the ATO coating.

This paper describes the manufacturing and spectroelectrochemical characterization of all solid electrochromic devices (ECD). Both electrochromic (WO3) and storage ion (CeO2-TiO2) thin films were obtained by sol-gel method and deposited by dip-coating technique. The electrolytes were obtained by plasticization of starch with glycerol and addition of LiClO4 salt. The spectroelectrochemical measurements were performed with the complete devices as a function of the applied potential. The obtained results revealed that the color/bleaching process was reversible and the inserted/extracted charge was about 4.6 mC/cm2 for the applied potential of -2.0 V and increased up to 5.3 mC/cm2 for -2.3 V. This value was stable up to -2.5 V applied. The transmittance change at 630 nm was about 30% for 2.3 V applied and the optical density was about 0.25. The memory tests showed that the colored device bleached completely in open circuit in about 500 min. All the obtained results show that the presented devices are very good candidates to be tested as smart windows for architectural applications.