In this work, the reaction mechanism used in the preparation of fluorine-free superconducting YBa(2)Cu(3)O(7-delta) (YBCO) was investigated. To determine which precursor interactions are dominant, a comprehensive thermal analysis (thermogravimetric analysis-differential thermal analysis) study was performed. The results suggest that a three step reaction mechanism, with a predominant role for BaCO(3), is responsible for the conversion of the initial state to the superconducting phase. In the presence of CuO, the decarboxylation of BaCO(3) is kinetically favored with the formation of BaCuO(2) as a result. BaCuO(2) reacts with the remaining CuO to form a liquid which ultimately reacts with Y(2)O(3) in a last step to form YBCO. High temperature X-ray diffraction experiments confirm that these results are applicable for thin film synthesis prepared from an aqueous fluorine-free sol-gel precursor.

In this work, a ceramic composite of ZrW 2 O 8 and ZrO 2 was synthesized, in order to investigate the possibility of compensating the positive thermal expansion of ZrO 2 with the negative thermal expansion (NTE) compound ZrW 2 O 8 , tailoring the thermal expansion of these composites. The NTE material was mixed with varying amounts of ZrO 2 . The thermal expansion coefficients of this series of composites decrease with increasing amounts of ZrW 2 O 8 . Nevertheless, a negative deviation from the values expected by the rule of mixtures was found to be most pronounced in the middle of the compositional region.

No abstract

Colorimetric sensors for monitoring and visual reporting of acidic environments both in water and air are highly valuable in various fields, such as safety and technical textiles. Until now solgel based colorimetric sensors are usually non-flexible bulk glass or thin film sensors. Large area, flexible sensors usable in strong acidic environments are not available. Therefore, in this study organically modified silicon oxide nanofibrous membranes are produced by combining electrospinning and sol-gel technology. Two pH-indicator dyes are immobilized in the nanofibrous membranes: Methyl Yellow via doping, Methyl Red via both doping and covalent bonding. This resulted in sensor materials with a fast response time and high sensitivity for pHchange in water. The covalent bond between dye and the sol-gel network showed to be essential to obtain a reusable pH-sensor in aqueous environment. Also a high sensitivity was obtained for sensing of HCl and NH3 vapors, including a memory function allowing visual read-out up to 20 minutes after exposure. These fast and reversible, large area flexible nanofibrous colorimetric sensors are highly interesting for use in multiple applications such as protective clothing and equipment. Moreover, the sensitivity to biogenic amines was demonstrated, offering potential for control and monitoring of food quality.

Chemical solution deposition (CSD) of YBa2Cu3O7−δ (YBCO) nanocomposites from colloidal precursor solutions containing double metal oxide preformed nanocrystals is a promising, costeffective and reproducible approach to produce superconducting films with high critical current density (Jc) and enhanced pinning. Here, the influence of the preformed nanocrystal composition on the microstructure and superconducting properties of the YBCO nanocomposite films is studied, with a focus on establishing a simple and scalable process to grow nanocomposites that can be transferred to grow nano-added coated conductors. Colloidal stable BaZrO3, BaHfO3, BaTiO3 and SrZrO3 nanocrystals (3-6 nm in diameter) were synthesized and added to an environment-friendly low-fluorine YBCO precursor solution. High-quality superconducting layers were grown on LaAlO3 single-crystal substrates from these four nanocomposite precursor solutions in a single deposition process, without the need of a seed layer, yielding Jc of 4-5 MA/cm² at 77 K in self-field. The different YBCO microstructures produced by the four types of nanocrystals and the resulting microstrain of the films are compared and related with the magnetic-field and angular dependence of Jc. We demonstrate the BaHfO3-containing nanocomposite as the best-performing with a homogeneous distribution of nanoparticles with 7 nm in average diameter and a high density of stacking faults, which leads to some of the best superconducting properties ever achieved via low-fluorine CSD. The Jc exhibits a much smoother decay in applied magnetic fields and a much more isotropic behaviour for non-parallel magnetic fields, and the pinning force is increased by a factor of 3.5 at 77 K and 1 T with respect to the pristine film.