J.I. Peña

Mechanical properties of directionally solidified Al2O3–ZrO2(Y2O3) eutectics

Abstract The relationship between microstructure and mechanical properties was studied in Al2O3–ZrO2 eutectic rods. The material, produced by directional solidification using the laser-heated float zone method, was formed mainly of colonies consisting of a fine interpenetrating or ordered network of ZrO2 and α-Al2O3 surrounded by a thick boundary region that contained pores and other defects. The flexure strength of the eutectic rods was excellent (>1.1 GPa) owing to the small critical defect size and the high toughness (7.8 MPa m ). No microstructural changes were observed after about 1 h of exposure at 1700 K, and the eutectic oxide maintained a very high strength up to this temperature. The nature of the critical defects that led to fracture, the toughening micromechanisms, and the differences between the longitudinal and transverse strength are discussed in the light of the microstructural features of the material.

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.

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.

Efficient Nd(3+)-->Yb(3+) energy transfer in 0.8CaSiO(3)-0.2Ca(3)(PO(4))(2) eutectic glass.

In this work we report the study of energy transfer between Nd(3+) and Yb(3+) ions in glasses with the 0.8CaSiO(3)-0.2Ca(3)(PO(4))(2) eutectic composition at room temperature by using steady-state and time-resolved laser spectroscopy. The Nd(3+)?Yb(3+) transfer efficiency obtained from the Nd(3+) lifetimes in the single doped and codoped samples reaches 73% for the highest Nd(3+) concentration. The donor decay curves obtained under pulsed excitation have been used to establish the multipolar nature of the Nd(3+)-->Yb(3+) transfer process and the energy transfer microparameter. The nonradiative energy transfer is consistent with an electric dipole-dipole interaction mechanism assisted by energy migration among donors. Back transfer from Yb(3+) to Nd(3+) is also observed.

Growth rate effects on thin Bi2Sr2CaCu2O8+δ textured rods

Abstract Cylindrical polycrystalline textured Bi-2212 samples of lengths up to 10 cm and 1 mm of diameter have been grown using a Laser Float Zone (LFZ) melting technique. In order to improve their transport critical current density ( J c ) at 77 K, the influence of growth rate on their microstructure has been analysed. The final transport properties of the textured materials are determined by the balance between better grain alignment and larger grain size, that takes place at the lowest growth rates, and the simultaneous appearance of cracks. The potential of LFZ grown BSCCO rods for the development of current leads at low fields is suggested. Optimum transport properties correspond to thin rods obtained using intermediate growth rates of 15 to 30 mm/h, with J c (77 K) values up to 5500 A/cm 2 in the self field. These also exhibit minimum resistivity values above T c and irreversibility lines shifted towards higher temperatures and fields.

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.

Piezo-spectroscopy at low temperatures: residual stresses in Al2O3–ZrO2(Y2O3) eutectics measured from 77 to 350 K

Abstract High-resolution, piezo-spectroscopic studies were performed in alumina/zirconia eutectic composites with different Y 2 O 3 contents and microstructures at different temperatures using the ruby R-line luminescence. Measurements at 77 K allowed the precise determination of the average stress and its distribution in the alumina phase. A normal distribution function was obtained in most of the cases. In some composites the highest stresses are relaxed by micro cracking, giving an asymmetrical distribution function. In the composites with stabilized zirconia the residual stresses originate from the differences in the thermal expansion coefficients of the component phases. A linear dependence of the thermostresses with temperature was obtained in the 77–350 K temperature range. The effective elastic modulus for thermostresses was 118±2.7 GPa. Using the coefficients of thermal expansion of the component phases and making an extrapolation of the low temperature values the stress-free temperature of 1270±35 K was determined.

Self-Supporting Thin Yttria-Stabilised Zirconia Electrolytes for Solid Oxide Fuel Cells Prepared by Laser Machining

A novel procedure to make self-supporting thin yttria-stabilised zirconia (YSZ) membranes by laser machining is shown. We have used a galvanometric controlled laser beam to machine the surface of a conventional sintered YSZ plate and achieved thin areas up to 10 lm thick, but also maintaining thicker support beams to ensure the structural strength of the membrane. The outer areas of the plate are left unaltered to facilitate the sealing of the cell. This kind of thin membrane is ideal for preparing electrolytesupported Solid Oxide Fuel Cells (SOFC) operating at intermediate temperatures. The membranes have been characterized by optical profilometry, Raman Spectroscopy and Electrochemical Impedance Spectroscopy. The YSZ properties, except those derived from membrane thinning, remain unaltered by processing. Using the laser machined YSZ electrolyte a conventional electrolyte supported YSZ-Ni/YSZ/LSM-YSZ planar single cell with average electrolyte thickness of less than 50 lm has been fabricated and characterized. Performance of the cell is improved as a result of the thinning process.

Conductivity anisotropy in directionally solidified CaZrO3CaSZ and MgO-MgSZ eutectics

The results of the impedance spectroscopy measurements on eutectic samples based on zirconium oxide are presented here. Samples of CaZrO3ZrO2(cubic) and MgOZrO2(cubic) have been grown by a directional solidification procedure such that the different phases appear nearly oriented along the growth direction (lamellae in the system of CaZrO3-ZrO2(cubic) and fibers of MgO in a ZrO2 matrix in the other system). The DC electrical conductivity has been measured by impedance spectroscopy along and across the growth axis. For CaZrO3ZrO2 the coductivity is clearly anisotropic. The following values for σT have been obtained: the conductivity at 600 °C equals 2.0 × 10−6 Ω−1 cm−1 perpendicular to the fiber axis and 1.4 × 10−5 Ω−1 cm−1 parallel to it and with an activation energy of 1.3 eV for σT. For MgOZrO2(cubic) the isotropic value of the conductivity at 600 °C is 10−4 Ω−1 cm−1 with activation energy for σT of 1.5 eV. The anisotropic conductivity in the CaZrO3ZrO2 (cubic) system has been explained by a model of an ordered stacking of oxygen conducting (cubic ZrO2) and non-conducting (CaZrO3 or MgO) phases.

Inkjet printing of functional materials for optical and photonic applications

Inkjet printing, traditionally used in graphics, has been widely investigated as a valuable tool in the preparation of functional surfaces and devices. This review focuses on the use of inkjet printing technology for the manufacturing of different optical elements and photonic devices. The presented overview mainly surveys work done in the fabrication of micro-optical components such as microlenses, waveguides and integrated lasers; the manufacturing of large area light emitting diodes displays, liquid crystal displays and solar cells; as well as the preparation of liquid crystal and colloidal crystal based photonic devices working as lasers or optical sensors. Special emphasis is placed on reviewing the materials employed as well as in the relevance of inkjet in the manufacturing of the different devices showing in each of the revised technologies, main achievements, applications and challenges.