R.I. Merino

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.

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.

Raman spectroscopic study of cation disorder in poly- and single crystals of the nickel aluminate spinel

The Raman spectrum of NiAl(2)O(4) inverse spinel has been studied in quenched polycrystalline pellets produced by solid-state reaction and in single crystals grown by the floating zone method. The lattice parameters and inversion degrees were determined by x-ray diffraction. Polarization measurements in single crystals allow mode symmetry assignment. Then, a correlation is established between the bands observed in polycrystalline samples and those of single crystals. Both kinds of sample present more bands than the five expected (A(1g)+E(g)+3T(2g)) in a cubic Fd3m spinel. This multiplicity is attributed to the almost fully inverted cation distribution in NiAl(2)O(4), with inversion parameter x≈0.9. The multiplicity of the high-frequency A(1g) band, in particular, is attributed to the different possible configurations of Ni(2+) and Al(3+) cations occupying the three octahedral sites close to a given oxygen ion. A strong downshift of the E(g) mode frequency, as compared to the normal spinel MgAl(2)O(4), is attributed to the longer bonding distance between oxygen and octahedral cations in inverse II-III spinels. Due to the small range of variation of x upon thermal treatment in NiAl(2)O(4), no significant differences were found between the spectra of samples quenched at different temperatures, from 800 to 1200 °C.

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.

Raman and x-ray study of perovskite solid solutions

La and Nd gallates are currently being used as substrate materials for high temperature superconductor thin film deposition. In this work we have synthesized and characterized by means of XRD and Raman spectroscopy solid solutions of the type . Polarized Raman study of and single crystals is also performed. Our results confirm the Pbnm space group for both La and Nd perovskites at room temperature and the centrosymmetric for in the high temperature phase. The orthorhombic to rhombohedral first order phase transition ( in ) is studied as a function of Nd substitution. increases quadratically with x for . Beyond x = 0.17 the phase transition, if present, is above the limit of our experimental setup (870 K). The detection of a soft mode in the rhombohedral phase is interpreted as due to the existence of a virtual second order transition to cubic symmetry at high temperature, analogous to that observed in rare earth aluminates.

Ce3+↔Ce4+ conversion in ceria-doped zirconia single crystals induced by oxido-reduction treatments

Abstract Optical absorption and resonant Raman techniques have been used to study the Ce3+↔Ce4+ conversion in cerium-doped yttria-stabilized zirconia single crystals (YSZ). The reduced material presents an intense orange coloration associated with a broad absorption band at 460 nm that we assign to a 4f→5d transition. Its experimental oscillator strength is ≈1×10−2. The Raman spectra consist of the lattice phonon bands at 600 cm−1 and below together with an intense electronic Raman peak at 2086 cm−1. The anomalous intensity of this peak is due to a resonance effect by the excitation in the 4f→5d transition. The peak is associated with the 2F 7 2 →2F 5 2 electronic transition within the 4f1 configuration of Ce3+. Heating in ai r at 1000°C produces a complete discoloration of the sample. The effect of the oxidation can be fully reversed by reducing the sample at the same temperature. Optical spectroscopy has been used to determine the kinetics of the oxido-reduction process. Isothermal annealing in the temperature range 750–950°C were performed in both oxidizing and reducing atmospheres. The results indicate that the oxidation process is diffusion-limited with an activation energy of ≈1.7 eV. For the reduction two alternative limiting processes are proposed: electron diffusion and ionization reaction.

Spectroscopic characterization of Er3+ in stabilized zirconia single crystals

Erbium-doped yttria-stabilized zirconia (YSZ) single crystals of 84ZrO2/16YO1.5 composition have been studied by electron paramagnetic resonance (EPR), optical absorption and photoemission techniques at temperatures between 10 and 680 K. At 10 K the Stark-structure of the 4I15/2 and 4S3/2 multiplets has been resolved and it corresponds to a statistical distribution of nearly-cubic octocoordinated (CN8) and trigonal hepta-coordinated (CN7) Er3+ ions. The 300 K spectroscopic properties are studied in the frame of the Judd-Ofelt theory. From the calculated radiative transition probabilities and the measured lifetimes the authors have determined the nonradiative transition rates and luminescence efficiencies, and compared them with previous results using YSZ of a different composition. The influence of the matrix composition on the spectroscopic characteristics of the Er3+ ion is discussed.

Self-organization approach for THz polaritonic metamaterials

In this paper we discuss the fabrication and the electromagnetic (EM) characterization of anisotropic eutectic metamaterials, consisting of cylindrical polaritonic LiF rods embedded in either KCl or NaCl polaritonic host. The fabrication was performed using the eutectics directional solidification self-organization approach. For the EM characterization the specular reflectance at far infrared, between 3 THz and 11 THz, was measured and also calculated by numerically solving Maxwell equations, obtaining good agreement between experimental and calculated spectra. Applying an effective medium approach to describe the response of our samples, we predicted a range of frequencies in which most of our systems behave as homogeneous anisotropic media with a hyperbolic dispersion relation, opening thus possibilities for using them in negative refractive index and imaging applications at THz range.

The effects of the NaF flux on the oxidation state and localisation of praseodymium in Pr-doped zircon pigments

Abstract The role that NaF plays in the preparation of Pr-doped zircon pigments was studied through the analysis of the nature and localisation of the Pr cations into the zircon matrix in samples prepared in the absence and in the presence of NaF. As previously observed, the addition of NaF caused a decrease of the minimum temperature required for zircon formation from 1400 to 1100°C, and an increase of the yellow colour intensity. In the absence of NaF, the Pr cations mainly presented a threefold oxidation state, being located out of the zircon lattice as Pr2Zr2O7, whereas in the presence of this flux, most of the Pr cations showed a fourfold valence and formed a solid solution with the zircon lattice, which was then the main responsible for the stronger yellow colour observed in this case. After heating this pigment at 1400°C, we detected an exsolution of the Pr (IV) cations as Pr8Si6O24 which was accompanied by a decrease of the yellow colour intensity. Therefore, it was concluded that the main role of NaF in the preparation of yellow Pr-zircon pigments is to decrease the temperature of zircon formation to the range in which the chromophore responsible for the bright yellow colour, i.e. the Pr (IV)-zircon solid solution, is stable.