A. Larrea

ZrO2–Al2O3 eutectic plates produced by laser zone melting

Abstract This paper describes a procedure for preparing large surfaces of eutectic composites directionally solidified from the melt, with very fine and uniform microstructures. Large surface plates of the Al 2 O 3 –ZrO 2 eutectic, with thickness up to 250 μm, have been grown by solidification using a modified laser zone melting method suitable for the preparation of large area samples. The surface of a ceramic precursor is scanned with a rectangular CO 2 laser beam of 20×0.5 mm 2 size which induces surface melting. The resulting microstructure is colony free and it consists of fine and alternating interpenetrating Al 2 O 3 and ZrO 2 single crystal lamellae. This microstructure is the basis for improvement of mechanical behaviour in the processed material. The interspacing, crystal structure and orientation relationship between the phases has been determined. Solidification fronts were studied as a function of the processing conditions. A mapping of the residual stresses has also been performed along the transverse plate sections using micro-luminescence techniques.

Microstructure and physical properties of some oxide eutectic composites processed by directional solidification

Abstract Eutectic composites of lamellar ZrO 2 –CaO and ZrO 2 –NiO and fibrous Al 2 O 3 –ZrO 2 , Al 2 O 3 –ZrO 2 (Y 2 O 3 ), ZrO 2 –MgO and CaF 2 –MgO wide gap materials have been grown from the melt by unidirectional solidification using laser floating zone and Bridgman techniques. The unique microstructure and interface morphology of these composites led to some remarkable mechanical (strength and toughness), optical (light guiding) and transport (ionic conduction) properties. The underlying relationships between microstructure and properties were briefly discussed in each case in the light of some possible applications of these eutectics.

Flux pinning improvement in Bi-2212 silver sheathed tapes with submicron SrZrO3 inclusions

Abstract Silver sheathed tapes of Bi-2212 undoped and doped with SrZrO 3 have been prepared, using co-precipitated oxalates as precursors. Thermal treatment conditions have been optimised, varying the maximum processing temperature T max . A comparative study on thermal treatment conditions, microstructure, and flux pinning properties for doped and undoped samples has been performed in order to evaluate the effect of SrZrO 3 addition. The doped samples contain SrZrO 3 submicron particles, appearing as aggregates included between Bi-2212 lamellae with a Cu-free phase and the rest of the liquid, and also distributed inside Bi-2212 grains. The presence of SrZrO 3 surplus leads to higher critical current densities in samples prepared in a wide T max range; in addition, the lowest T max , at which high J c can be obtained, becomes 5 deg lower (875°C). At T = 5 K critical currents for all the samples are of the order of 10 5 A/cm 2 and are independent of doping, while at 60 K doped samples exhibit roughly two times higher J c in zero field and more than an order of magnitude higher J c at H = 40 mT. A magnetisation decay study reveals that samples with SrZrO 3 surplus have a larger fraction of pinning centres with high energy. The enhanced pinning properties can be related to the finely dispersed zirconate inclusions found in the superconductor matrix.

Spectroscopic properties of Er3+ and Nd3+ doped glasses with the 0.8CaSiO3–0.2Ca3(PO4)2 eutectic composition

Abstract Rods of glass with the composition 56CaO–35SiO 2 –9P 2 O 5 (in mol%) were produced by the fast solidification of laser float zone melted precursors with the wollastonite (CS) and tricalcium phosphate (TCP) eutectic composition. This inverted glass with a high content of CaO modifier presents a high transparency optical window from 4 to 0.35 μm and is not hygroscopic. Its refractive index is n =1.65. The glass can be doped with rare-earth oxides up to relatively high doping levels without impurity crystallisation or aggregation effects. The Er 3+ and Nd 3+ absorption and emission spectra were measured at temperatures from 10 to 300 K for different doping concentrations between 0.07 and 4 wt%. The energy of the 4f levels was determined from the absorption and emission spectra. Judd–Ofelt intensity parameters were calculated and compared with those of other glass systems. The Nd 3+ emissions from the 4 F 3/2 level were studied in detail. The lifetime in the less concentrated sample at 300 K is 330 μs and the peak cross-section of the 1.062 μm emission is 2.2×10 −20 cm 2 . The decay time of the Er 3+ emission at 1.539 μm is 7.75 ms and the emission cross-section 0.6×10 −20 cm 2 . These values are well retained up to the highest doping levels.

Microstructure of laser floating zone (LFZ) textured (Bi, Pb)SrCaCuO superconductor composites

Abstract Directionally solidified high temperature superconducting (Bi, Pb)SrCaCuO pure ceramics and composites were obtained using a laser floating zone (LFZ) apparatus. The presence of secondary non-superconducting and metallic phases as well as their solidification habit have been analysed. The influence of the LFZ growth conditions and the precursor composition on the microstructure of the final products was studied using optical and electron microscopies.

Solution-based synthesis routes to (Bi 1− x Pb x ) 2 Sr 2 Ca 2 Cu 3 O 10+δ

A comparison of several solution syntheses with a solid-state route to the leadcontaining high Tc phase of the Bi-Sr-Ca-Cu-O system has been performed using DSC, TGA, X-ray diffraction, electron microscopy, magnetic ac susceptibility, and critical-current measurements. A novel polymer solution synthesis route is shown to yield an increased percentage of the 2223 high Tc phase when sintering is performed at the low temperature end of this phases stability range. Under the best preparative conditions given in the literature, however, the properties of samples obtained with the different synthetic methods described in the present study are shown to be similar. Samples derived from solution syntheses have been found to contain higher amounts of carbon precipitates, while samples derived from conventional solid-state synthesis contain CuO precipitates, according to energy dispersive spectroscopy analyses.

Fabrication Methods and Performance in Fuel Cell and Steam Electrolysis Operation Modes of Small Tubular Solid Oxide Fuel Cells: A Review

Higher energetic density, better resistance to thermal stresses and smaller starting times as compared with conventional planar stacks, make the so-called microtubular SOFC (mT-SOFCs with diameters in the millimeter size region) devices suitable for portable applications in the sub kW energy range. However, fabrication of mT-SOFCs is a challenging process where a number of ceramic layers with different compositions and characteristics have to be placed together in the cylindrical device. Several co-sintering processes have to be performed at different temperatures and using distinct atmospheres to complete cell fabrication. In this review we summarize recent activity in the field of fabrication and characterization of mT-SOFCs, including the use of mT-SOFCs for steam electrolysis.

Superparamagnetic particles in ZSM-5-type ferrisilicates

As-synthesized, low iron content, ferrisilicates of ZSM-5-type contain well-separated Fe(III) ions in a tetrahedral environment and display paramagnetic behavior. After hydrothermal treatment, the iron ions are partially extracted from the framework, generating nanosize iron oxide or oxyhydroxide ferrimagnetic particles. This process has been studied by transmission electron microscopy (TEM), Mossbauer spectroscopy, magnetic ac susceptibility (chi(ac)), and field dependent magnetization, on samples containing up to 6.7 wt. % Fe. The experiments evidence the growth of nonaggregated particles, with a typical size around 3 nm, presumably located at the surface of the ferrisilicate crystallites, From a thorough granulometric analysis involving TEM and chi(ac) data, it is concluded that, in the range from 1.5 to 4.6 wt. % Fe, the particle size distributions are significantly independent of the iron content.

Directionally solidified calcia stabilised zirconia–nickel oxide plates in anode supported solid oxide fuel cells

Abstract We present here a new manufacturing procedure for the anode Ni–zirconia cermet. It is based on the modification of the surface of a NiO–CaSZ (calcia stabilized zirconia) pellet of eutectic composition by surface laser melting and resolidification. A smooth, continuous and dense NiO–CaSZ layer is obtained on top of the ceramic pellet. Its depth can be varied from less than 200 μm to more than 570 μm, depending on the processing conditions. Its microstructure consists mainly of lamellar eutectic grains with interlamellar spacing ranging from 0.4 to 1.6 μm. The interspacing diminishes towards the surface, where a very fine microstructure is developed. Chemical reduction treatment transforms NiO to metallic Ni with the accompanying volume reduction. Complete reduction results into a cermet with 43 CaSZ+33.5 Ni+23.5 pores (%vol). Electrical conductivity is mainly electronic and proceeds along NiO lamellae or through percolating Ni particles.

The secret of early nanomaterials is revealed, thanks to transmission electron microscopy

Lustre ceramic decoration was revealed by analytical electron microscopy to have been the first nanostructured metallic thin film made by man. Considering this type of decoration in the context of cultural heritage, this is a remarkable discovery in the history of technology, because nanocrystal films have been produced empirically since medieval times. This meant that a high level of technological knowledge of materials science was required to obtain and to reproduce lustre layers. An analytical approach by means of transmission electron microscopy (TEM), with energy-dispersive X-ray spectroscopy (EDS) and electron diffraction (ED) facilities, allowed us to resolve the microstructure and chemical composition of the lustre.