Scientific publications

The subsolidus phase relations of the ternary system CuO-TiO2-CaO sintered at 950 °C in air have been determined by powder X-ray diffraction method. Only one ternary compound CaCu3Ti4O12 was found in this system. From room-temperature dielectric property mapping at 10 kHz, a giant dielectric constant (εr>104) was observed for most of the ceramic composites in the CuO-rich region and in the region along the CaO-CuO binary line. The composites in the CaCu3Ti4O12-rich region were found to give a comparable giant dielectric constant when sintered at 1050 °C. The particular microstructure of larger grains with predominant phase surrounded by smaller grains with the secondary phases was found in such composites with a high dielectric constant. The relations between structures and dielectric properties were investigated. An internal barrier layer capacitance effect is the most probable mechanism to explain this particular dielectric behavior.

The synthesis, structural characterization, and NMR studies of new tin(II) alkoxides with thiophene-based substituents are presented. The reaction of ligand HO-C(C4H3S)3 (1) with bis(hexamethyldisilazyl)tin {Sn[N(SiMe3)2]2} in the ratio 2:1 in toluene yields the dinuclear compound {Sn[OC(C4H3S)3]2}2 ∙ 2 toluene (2). The same reaction in tetrahydrofuran allows the isolation of two compounds: a dinuclear {Sn[OC(C4H3S)3]2}2 ∙ 2 thf (3) and a mononuclear Sn[OC(C4H3S)3]2(thf) (4) molecule. The molecular structures of these tin(II) methoxides containing thienyl substituents 2-4 were investigated by single-crystal X-ray crystallography. The NMR spectra of solutions of Sn[OC(C4H3S)3]2}2 ∙ 2 toluene (2) or {Sn[OC(C4H3S)3]2}2 ∙ 2 thf (3) and {Sn[OC(C4H3S)3]2}2(thf) (4) are almost identical, showing only variations of the molar ratio of the dimeric and monomeric compounds.

Hybrid organic-inorganic sol-gel-matrices, with up to 20 wt.% incorporated ceria nanoparticles, have been employed as coatings for an AA2024-T3 aluminium alloy. The morphology of the coatings and associated nanoparticles has been examined by conventional and high-resolution transmission electron microscopy, revealing a relatively uniform distribution of 5 nm size nanoparticles across the coating thickness. Electrochemical studies indicate a general beneficial effect of incorporation of ceria nanoparticles, although the performance of the coated alloy depends on the nanoparticle content. Electrochemical polarisation behaviour revealed that the coating decreased the anodic current density by about seven orders of magnitude compared with the uncoated alloy, with high breakdown potentials in chloride-containing solution. Accelerated salt spray testing showed that corrosion in an artificial scratch is blocked most efficiently by high ceria contents, whereas general corrosion is inhibited effectively with comparatively low ceria contents. Electrochemical impedance spectroscopy indicated degradation of the barrier properties of coatings with increased amounts of incorporated nanoparticles. Assessment of the abrasion and scratch resistance, and hydrophobicity also revealed additional beneficial functional properties of the coatings containing nanoparticles.

Bias current applied to conductometric gas sensors consisting of individual metal oxide nanowires can be used to heat them up to the temperature necessary for sensing. This approach in combination with the good sensitivity and stability of metal-oxide nanowires, can be used to develop prototypes with low power requirements (few tens of microwatts). Here, we present new sensors devices based on this approach that display fast dynamic performance only limited by the gas-solid interaction kinetics.

Dynamics of gas-surface interactions determine the limits of the fastest response times of sensors based on metal oxides. Here, the kinetics of adsorption and desorption of gaseous molecules onto the surface of metal oxide nanowires was analyzed through pulsed self-heating assisted conductometric measurements. This approach overcomes gas diffusion, which is typical of conventional porous film based devices, and provides thermal response times fast enough to evaluate the fundamental gas-surface reactions kinetics. Experimental response and recovery times of individual SnO2 nanowires toward oxidizing and reducing gases obtained with the here-proposed methodology were related to the reaction barriers predicted by theoretical models and other experimental techniques.

In this paper, the authors present an effective experimental method to estimate the temperature of individual metal oxide nanowires that can be used to quantify the heating produced in conductometric or other operating conditions. The here-proposed method is based on the analysis of the recovery time of the nanowire's resistance after exposure to a gas pulse (0.5 ppm of NO2 in dry air). It is reproducible with different devices always with uncertainties below +/- 20°C in the temperature range (70-300°C) studied herein. The exploration of alternative gases and nanolithography techniques may help to extend its operating range and its applicability to other materials. In any case, the opportunity to probe temperatures at the nanoscale opens the door to a number of fundamental and applied advancements in the field of nanotechnology.

The injection of water beneath liquid diethylene triamine in a glass cuvette leads to an unexpected phase evolution behavior of the two liquids. The space and time dependent developments of the molecular structure and the underlying transport associated with mixing of the two liquids are monitored by optical imaging and scanning Brillouin microscopy. Apparently, results obtained by either experimental technique lead to disparate interpretations. Whereas optical imaging suggests the existence of a two phase structure, which disappears within a few hours, acoustic microscopy indicates the evolution of a more gradually evolving and longer-lived three phase structure. According to molecular acoustics, the transport of diethylene triamine into water and vice versa behaves strongly asymmetric in time. An attempt is made to reconcile the observed optical and acoustic manifestations of the mixing process on the basis of molecular complex formation.

Metal oxide nanostructures offer interesting possibilities to design functional surfaces for biosensing applications, for instance, through higher surface area leading to enhanced immobilization of biomolecules, which increases the detection limit. Herein, an amplified electrochemical sensing method has been presented for the detection of DNA based on the readout resulting from chemical oxidation of guanine on nanoscaled metal oxides (TiO2, SnO2 and Fe3O4) obtained by chemical vapor deposition (CVD) onto pencil graphite electrode (PGE) as electrochemical transducer. The proposed strategy is suitable to produce cost-effective disposable sensor elements enabling quantitative detection of nanomolar concentrations of DNA. When preparing these metal oxide surfaces by CVD onto PGEs, the various experimental conditions; such as, the effect of different concentrations of 20 mer-bases DNA oligonucleotide (ODN20), and the surface pretreatment steps were Studied to obtain better surface properties for DNA immobilization. The detection limit estimated for signal-to-noise ratios >3 corresponds to 21.3, 519 and 45.8 nmole/ml ODN20 concentrations for PGEs modified with TiO2, SnO2 and Fe3O4 films, respectively. The electrochemical detection of DNA onto metal oxide@PGEs is discussed together with the application potential.

Highly crystalline CoFe2O4 nanoparticles (<d> ~ 5.6 nm) were synthesized by thermal decomposition of (i) single molecular precursor CoFe2(OtBu)8 and (ii) stoichiometric mixture of individual metal precursors. The precursors were thermolyzed in a high-boiling solvent (Docosane, bp. 369 °C) to produce uniformly dispersed cobalt ferrite nanoparticles in the case of molecular pre-cursor, whereas bimodal distribution of nanoparticles was obtained in the multi-source synthesis. TG/DSC and HR-TEM reveraled single- and multi-step decomposition profiles and differential nucleation steps in the two cases, which influence size dispersion and phase purity.

Increasing requirements concerning the powder quality for fine-grained ferroelectric ceramics or for low temperature coating technologies provoke further improvements in particle characterization methods. So far, a correct quantitative measurement of the ferroelectric properties for as synthesized particles is lacking and only already sintered ceramics can be measured precisely. We report on a measurement setup to characterize films made of as synthesized ferroelectric particles. The chosen measurement setup enables the definite determination of ferroelectricity in such nondensified particle-based structures. A centrifugally casted BaTiO3 particulate film with 50% porosity is used for characterization. The correct value of the remanent polarization for this porous film obtained by the new method is almost half the value estimated by conventional measurement setups. Compared to a sintered BaTiO3 ceramic the achieved remanent polarization is much lower (10%). At the same time a higher maximum field has to be applied to the sample as lower fields act on the individual particles because of the porosity of this film.