Els Bruneel

Evaluation of the phase composition of BPSCCO bulk samples by XRD- and susceptibility analysis

Abstract This paper describes the comparison of the phase purity analysis, using XRD and AC-susceptibility measurements, on a number of BPSCCO samples, containing different ratios of the Bi-2212 and Bi-2223 phase. The differences observed in the results of both techniques were correlated with the growth mechanism of the Bi-2223 phase. The higher values for the percentage of the Bi-2223 phase observed in AC-susceptibility analysis, could be explained by the assumption that there is a specific distribution of the Bi-2212 and Bi-2223 phase during particle growth, resulting in shielding effects.

Virus Removal by Biogenic Cerium

The rare earth element cerium has been known to exert antifungal and antibacterial properties in the oxidation states +III and +IV. This study reports on an innovative strategy for virus removal in drinking water by the combination of Ce(III) on a bacterial carrier matrix. The biogenic cerium (bio-Ce) was produced by addition of aqueous Ce(III) to actively growing cultures of either freshwater manganese-oxidizing bacteria (MOB) Leptothrix discophora or Pseudomonas putida MnB29. X-ray absorption spectroscopy results indicated that Ce remained in its trivalent state on the bacterial surface. The spectra were consistent with Ce(III) ions associated with the phosphoryl groups of the bacterial cell wall. In disinfection assays using a bacteriophage as model, it was demonstrated that bio-Ce exhibited antiviral properties. A 4.4 log decrease of the phage was observed after 2 h of contact with 50 mg L(-1) bio-Ce. Given the fact that virus removal with 50 mg L(-1) Ce(III) as CeNO(3) was lower, the presence of the bacterial carrier matrix in bio-Ce significantly enhanced virus removal.

Bimetallic–Organic Framework as a Zero‐Leaching Catalyst in the Aerobic Oxidation of Cyclohexene

A gallium 2,2′‐bipyridine‐5,5′‐dicarboxylate metal–organic framework (MOF), denoted as COMOC‐4, has been synthesized by solvothermal synthesis. This MOF exhibits the same topology as MOF‐253. CuCl2 was incorporated into COMOC‐4 by a post‐synthetic modification (PSM). The spectroscopic absorption properties of the MOF framework before and after PSM were compared with theoretical data obtained by employing molecular dynamics combined with time‐dependent DFT calculations on both the as‐synthesized and functionalized linker. The catalytic behavior of the resulting Cu2+@COMOC‐4 material was evaluated in the aerobic oxidation of cyclohexene with isobutyraldehyde as a co‐oxidant. In addition, the catalytic performance of Cu2+@COMOC‐4 was compared with that of the commercially available Cu‐BTC (BTC=benzene‐1,3,5‐tricarboxylate) MOF. Cu2+@COMOC‐4 exhibits a good cyclohexene conversion and an excellent selectivity towards cyclohexene oxide in comparison to the Cu‐based reference catalyst. Furthermore, no leaching of the active Cu sites was observed during at least four consecutive runs.

Structure and phase transition of Sn-substituted Zr(1−x)SnxW2O8

A conventional solid state reaction between ZrO2, SnO2 and WO3 was used to prepare the negative thermal expansion material Zr(1−x)SnxW2O8. The strong negative thermal expansion over a broad temperature range, which is well known for the pure zirconium tungstate compound, is also demonstrated in this substituted material. However, the order–disorder phase transition of the cubic materials was shown to shift towards lower temperatures, dependent on the degree of Sn4+-substitution, by dilatometry and temperature variable X-ray diffraction. This is attributed to the lower bond strength of the Sn–O bond in comparison to the Zr–O bond. The unit cell parameters of the material are significantly smaller due to the insertion of smaller Sn4+-cations on the Zr4+-position in the structure. For one composition (x = 0.3), the structure of Zr(1−x)SnxW2O8 was studied by neutron diffraction at two temperatures, 293 K and 473 K, corresponding to respectively the low temperature α-, and high temperature β-polymorph of Zr(1−x)SnxW2O8. The refined structures were found to be similar to that of ZrW2O8 at the same temperatures. Variable temperature X-ray diffraction of the same sample was used to establish the phase transition temperature, by refining the fractional occupancy of the possible tungstate orientations with temperature.

Chemical Approach to the Deposition of Textured CeO2 Buffer Layers Based on Sol Gel Dip Coating

The widespread use of vacuum techniques for the development of coated conductors, in which buffer and superconducting (REBa2Cu3O7-delta) layers are deposited epitaxially on a substrate, is well established in the research environment. However, obtaining uninterrupted deposition at high speed, increasing flexibility in composition and in film thickness and attaining independence of geometric constraints are areas in which many vacuum techniques will need sustained development in order to answer industrial demands. This work describes the deposition of textured CeO2 buffer layers based on sol gel dip coating under atmospheric environment and from aqueous precursor materials. Research has been performed towards the deposition of CeO2-buffer layers using the amorphous citrate method on sapphire substrates and Ni-W foils. Coating is performed using the dip-coating technique, which allows extension to a continuous system. The withdrawal speed and the thermal treatment have been optimised in order to obtain highly oriented (001) layers exhibiting a smooth and crack-free morphology both on ceramic and metallic substrates, From the results it was concluded that sintering atmosphere and sintering temperature play a crucial role in the growth mechanism. This study describes the structural and morphological analysis of the thin layer with special attention to the difference between ceramic and metallic substrates.

Deposition of CeO2 Buffer Layers and YBa2 Cu3O7-δ Superconducting Layers Using an Aqueous Sol-Gel Method

Recently, the sol-gel method has been developed as a potential alternative to the conventional vacuum techniques (e.g. pulsed laser deposition and magnetron sputtering) towards the synthesis of a coated conductor. The advantages of the sol-gel method include a more simple and faster way to deposit films, deposition at a lower cost and the possibility to incorporate it in a continuous system. Possible applications of these coated conductors are long lengths of superconducting cables, strong magnets, fault current limiters,... In Our work, the coated conductor consists of a CeO2 buffer layers and a YBa2Cu3O7-delta superconducting film, coated on a textured substrate. Two kinds of substrates have been used in this work: ceramic R-cut (1102) sapphire substrate and metallic (001) Ni-alloy tape. For both Substrates, a cleaning procedure was developed to improve the wettability. An almost complete wettability is necessary as an aqueous sol-gel method is used in this work. Dip-coating the cleaned substrates in the aqueous metal-citrate solution leaves a wet film on the substrate which can be converted to the ceramic oxide by thermal treatment. The dip-coating technique and thermal treatment were first optimized onto the sapphire substrate, as in the case of Ni-alloy, special attention is needed to avoid oxidation of the substrate. Due to the larger lattice mismatch, no biaxially textured CeO2 could be obtained on R-cut sapphire substrate. However, on Ni-alloy tape, perfectly (001)-oriented CeO2 is formed when the thermal treatment is performed in an Ar-H-2 atmosphere to avoid oxidation of the substrate. The results of microscopic and structural study of both CeO2 and YBa2Cu3O7-delta will be presented. Special attention will be given to the XRD and pole figure results, as these results give a good indication of the texture of the layers. Texture is of huge importance in the case of coated conductors, as in this way high critical current densities might be obtained.

X-ray photoelectron spectroscopy (XPS) depth profiling for evaluation of La2Zr2O7 buffer layer capacity

Lanthanum zirconate (LZO) films from water-based precursors were deposited on Ni-5%W tape by chemical solution deposition. The buffer capacity of these layers includes the prevention of Ni oxidation of the substrate and Ni penetration towards the YBCO film which is detrimental for the superconducting properties. X-ray Photoelectron Spectroscopy depth profiling was used to study the barrier efficiency before and after an additional oxygen annealing step, which simulates the thermal treatment for YBCO thin film synthesis. Measurements revealed that the thermal treatment in presence of oxygen could severely increase Ni diffusion. Nonetheless it was shown that from the water-based precursors’ buffer layers with sufficient barrier capacity towards Ni penetration could be synthesized if the layers meet a certain critical thickness and density.

Processing effects on the microstructure observed during densification of the NTE-compound ZrW2O8

Abstract The crystal structure of zirconium tungstate exhibits large isotropic negative thermal expansion. As a result, zirconium tungstate might find use in applications which require tailored thermal expansion properties. In this work, the sintering ability of commercially available ZrW2O8 and the reactive sintering of an oxide mixture was studied. The microstructure obtained when sintering oxide mixtures on porous alumina based refractory fibre-boards is characterised by large pores and clear grain boundaries, arising from a liquid phase sintering mechanism due to chemical interaction of the oxide mixture with the fibre-board. Although this interaction is avoided when using commercially available zirconium tungstate its sintering ability is very poor. When reactive sintering of oxide mixtures was performed in sealed quartz tubes in air, dense zirconium tungstate was obtained. The reactivity of the oxide mixtures using quartz tubes as support, is dependent on the homogeneity and particle size of the precursor oxide mixture.

Mechanical and superconducting properties of BiPbSrCaCuO-PE and BiPbSrCaCuO-MgO composites

In this work it is shown that the inherently weak mechanical properties of monolithic high-temperature superconductors can be improved by processing the superconductor into a composite. Both polyethylene (PE) and MgO were used to prepare a composite material with BiPbSrCaCuO (BSCCO) superconductors. Three-point bending tests were conducted on monolithic BSCCO, 50 vol% BSCCO-PE and 50 vol% BSCCO-MgO particulate composites. As expected, the PE composites exhibited an increased ductility in comparison with the monolithic material and a decreased flexural modulus, while the flexural strength remains comparable with that of monolithic BSCCO. Although the presence of a resistive percolation path could be demonstrated at and below room temperature, zero resistivity at was not observed and the magnetic expulsion force diminished after processing. Magnetic measurements show that this is due to poor intergrain coupling. In contrast to the PE composites, the BSCCO-MgO composites can be re-sintered after mixing. This provides an opportunity for the material to regain the intergrain couplings and results in the recovery of zero resistivity at 110 K and in magnetic expulsion due to intergrain currents. Mechanical measurement on the MgO composites showed a decrease in ductility, an increased flexural modulus and a comparable flexural strength.

Steps Towards a Textile-Based Transistor: Development of the Gate and Insulating Layer

During recent years, intensive research has been carried out in the area of electronic textiles. There is an emerging trend to create garments that host electronic components embedded in the textile substrate, as well as electronic textiles made from yarns or fibers already possessing electronic properties. The creation of passive devices, such as textile electrodes that measure body parameters, has proved successful. However, there is a great need for the development of textiles possessing additional active functions. Accordingly, we investigated the possibility of developing a textile substrate possessing integrated switching and amplification functions by depositing parts of an organic thin-film transistor on fibrous substrates of varying geometries and origins. This article relates the initial steps we employed to develop a textile-based thin-film transistor. It reports the development of a gate layer from the deposition of electroless copper, as well as the deposition of a polyimide dielectric layer using dip coating. Further, it discusses the layer’s properties in terms of thickness and electrical characteristics. A copper layer of 350 nm thickness deposited on polyester tape and polyamide fibers displayed excellent electro-conductive properties. A smooth gate dielectric layer was achieved with a polyimide concentration of 15 w% and a withdrawal speed of 50 mm/min. As a result, optimum conditions for producing thin functional gate and dielectric layers were found. The transistor properties, the deposition of a semiconductive layer, and the production of drain and source electrodes remain the focus of future work.