Scientific publications

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.

Galvanised and galvannealed steels are widely used due to their good corrosion resistance in aqueous solutions. However, when additional protection is required, organic coatings, corrosion inhibitors or conversion coatings are used to improve their corrosion protection. In this work, sol-gel coating was used to improve the corrosion behaviour of these two materials. This paper analyses the final protective properties of a sol-gel coating prepared by basic catalysis and its dependence on the sintering temperature and time of treatment. The influence of the sintering conditions on the galvanised and galvannealed substrates is a decisive factor for the coating quality and for the barrier affect against the aggressive media. While heat treatment time is the controlling factor for the galvannealed steels, the temperature is determining in the case of the galvanised. Corrosion mechanisms for sol-gel galvanised steels did not changed with respect to the uncoated steel. However for galvannealed steel, after coating the mechanism is not purely cathodic.

Some mixed 1,3-dithia-2-stibacyclopentane derivatives with phosphorus based dithiolato ligands of the types (SCH2CH2SS)bS(S) (POGO) {where G = -CH(Me)-CH(Me)- and -C(Me)2-C(Me)2-} and (SCH2SS)bS(S)P(OR)2 {where R = Prn, Bun and Ph} have been synthesized by the reaction of 2-chloro-1,3-dithia-2-stibacyclopentane and the ammonium/sodium salt of the corresponding phosphorus based ligands in an equimolar ratio in anhydrous benzene solution. These yellow crystalline solid/semi-solid derivatives have been characterized by elemental analysis (C, H, S and Sb), molecular weights, melting point as well as spectral [UV, IR and NMR (1H, 13C and 31P)] studies. Single crystal X-ray diffraction analyses of 1,3-dithia-2-stibacyclopentane 2,3-butylenedithiophosphate revealed a monodentate mode of bonding of the dithiophosphate ligand in the complex. The free ligands and their antimony(III) complexes have also been screened for their antibacterial and antifungal activities.

An unusual almost flat broadband plasmonic absorption, ranging from 400 nm to well beyond 2500 nm, was observed in a 150 nm thin film of Ag nanoparticles embedded in a Teflon AF (R) matrix. The nanocomposites were synthesized by a simple single-step vapor-phase codeposition method. The Ag nanoparticles of various sizes and shapes, and thus various resonance frequencies, form a fractal percolating network. The broadband absorption, attributed to plasmon excitations within the nanoparticles, could be useful for multicolor applications in the visible and infrared wavelengths region. (c) 2006 American Institute of Physics.

Transparent conducting In2O3:Sn (ITO) layers have been deposited by spin coating on glass substrates using two different solutions, an ethylene glycol solution of indium and tin salts (sol) and a suspension of crystalline ITO nanoparticles redispersed in ethanol. The coatings have been sintered in air at 550°C for 30 min. and then post annealed in a reducing atmosphere at 300°C for 30 min. The electrical, optical, morphological and mechanical properties of both types of coatings are compared. The use of the suspension leads to thick 500 nm single layers with a specific resistivity ρ=1.6×10-2 Ωcm but only very thin ones, about 12 nm for a single layer, with ρ = 1.8 x 10-3 Ωcm (a factor of 10 smaller) are obtained using the sol. The resistivity still decreases down to a minimum ρ= 6 x 10-4 Ωcm for multilayer coatings, a value quite close to that obtained by PVD or CVD processes. These differences originate from the different morphologies of the coatings. The conventional sol-gel layers exhibit a columnar structure with a low porosity (28%) while the others have a granular structure with a high porosity (51%). Correspondingly the electron mobility µ is 14 and 1.1 cm2/V s, respectively. The visible transmission of both types of layers is high (T>85%). The influence of the sintering temperature is also discussed.

A novel approach was developed for the synthesis of high-dispersed anatase nano-TiO2 by a hydrothermal process without solvent at 200°C in 1 h. It was characterized using XRD, TEM, BET and elemental analysis. Nanoparticle-TiO2 was used as a photocatalyst by considering complete degradation of Rhodamine B (RB) dye. The photocatalytic reaction parameters such as photocatalyst amount, irradiation time and dye concentration were optimized and it was found that 0.125 wt% catalyst in 30 mg/L of RB aqueous solution is adequate for full degradation of RB in 50 min with 770 W/m2 irradiation power. Photocatalytic activity of the nanoparticle-TiO2 was compared with Degussa P-25 at optimum catalysis conditions determined for the nanoparticle-TiO2. It was concluded that when compared to Degussa P-25, the nanoparticle-TiO2 can be repeatedly used with increasing photocatalytic activity. The results revealed that the photodegradation of RB proceeds by pseudo first-order reaction kinetics in which the rate constant of the degradation is 0.132 min-1.