Micropillar compression of LiF [111] single crystals: effect of size, ion irradiation and misorientation

The mechanical response under compression of LiF single crystal micropillars oriented in the [111] direction was studied. Micropillars of different diameter (in the range 1–5 lm) were obtained by etching the matrix in directionally-solidified NaCl–LiF and KCl–LiF eutectic compounds. Selected micropillars were exposed to high-energy Ga+ ions to ascertain the effect of ion irradiation on the mechanical response. Ion irradiation led to an increase of approximately 30% in the yield strength and the maximum compressive strength but no effect of the micropillar diameter on flow stress was found in either the as-grown or the ion irradiated pillars. The dominant deformation micromechanisms were analyzed by means of crystal plasticity finite element simulations of the compression test, which explained the strong effect of micropillar misorientation on the mechanical response. Finally, the lack of size effect on the flow stress was discussed to the light of previous studies in LiF and other materials which show high lattice resistance to dislocation motion

Interacting plasmon and phonon polaritons in aligned nano- and microwires

The availability of macroscopic, nearly periodic structures known as eutectics opens a new path for controlling light at wavelength scales determined by the geometrical parameters of these materials and the intrinsic properties of their component phases. Here, we analyze the optical waveguiding properties of eutectic mixtures of alkali halides, formed by close-packed arrangements of aligned cylindrical inclusions. The wavelengths of phonon polaritons in these constituents are conveniently situated in the infrared and are slightly larger than the diameter and separation of the inclusions, typically consisting on single-crystal wires down to submicrometer diameter. We first discuss the gap mode and the guiding properties of metallic cylindrical waveguides in the visible and near-infrared, and in particular we investigate the transition between cylinder touching and non-touching regimes. Then, we demonstrate that these properties can be extended to the mid infrared by means of phonon polaritons. Finally, we analyze the guiding properties of an actual eutectic. For typical eutectic dimensions, we conclude that crosstalk between neighboring cylindrical wires is small, thus providing a promising platform for signal propagation and image analysis in the mid infrared

Microstructural and mechanical study of AL2O3/Er3Al5O12 eutectic rods grown by the laser floating zone method

Eutectic rods of Al2O3–Er3Al5O12 were grown by directional solidification using the laser-heated floating zone method at rates in the range 25–1500 mm/h. Their microstructure and mechanical properties (hardness, toughness and strength) were investigated as a function of the growth rate. A homogeneous and interpenetrated microstructure was found in most cases, and interphase spacing decreased with growth rate following the Hunt–Jackson law. Hardness increased slightly as the interphase spacing decreased while toughness was low and independent of the microstructure. The rods presented very high bending strength as a result of the homogeneous microstructure, and their strength increased rapidly as the interphase spacing decreased, reaching a maximum of 2.7 GPa for the rods grown at 750 mm/h. The bending strength remained constant up to 1300 K and decreased above this temperature. The relationship between the microstructure and the mechanical properties was established from the analysis of the microstructure and of the fracture mechanism

Superplastic deformation of directionally solidified nanofibrillar Al2O3-Y3Al5O12-Zr O2 eutectics

Nanofibrillar Al2O3–Y3Al5O12–ZrO2 eutectic rods were manufactured by directional solidification from the melt at high growth rates in an inert atmosphere using the laser-heated floating zone method. Under conditions of cooperative growth, the ternary eutectic presented a homogeneous microstructure, formed by bundles of single-crystal c-oriented Al2O3 and Y3Al5O12 (YAG) whiskers of ˜100 nm in width with smaller Y2O3-doped ZrO2 (YSZ) whiskers between them. Owing to the anisotropic fibrillar microstructure, Al2O3–YAG–YSZ ternary eutectics present high strength and toughness at ambient temperature while they exhibit superplastic behavior at 1600 K and above. Careful examination of the deformed samples by transmission electron microscopy did not show any evidence of dislocation activity and superplastic deformation was attributed to mass-transport by diffusion within the nanometric domains. This combination of high strength and toughness at ambient temperature together with the ability to support large deformations without failure above 1600 K is unique and shows a large potential to develop new structural materials for very high temperature structural applications

Microestructura y propiedades mecánicas de fibras cerámicas eutécticas Al2O3/Er3Al5O12/ZrO2 procesadas por fusión zonal por láser.

Se han estudiado las fibras eutécticas Al2O3-Er3Al5O12-ZrO2 procesadas por fusión zonal por láser a distintas velocidades. Con el aumento de la velocidad de procesado se ha observado un refinamiento de la microestructura así como un cambio en la morfología de la misma. Se ha determinado mediante piezoespectroscopía que la fase alúmina se encuentra a compresión como consecuencia de las tensiones residuales que aparecen en la solidificación tras el fundido. El módulo de la componente hidrostática de dichas tensiones residuales se reduce conforme lo hace el tamaño de las fases. Se ha estudiado la dureza, la tenacidad de fractura y la resistencia a la flexión. Las dos primeras magnitudes han resultado independientes del tamaño de las fases, mientras que la resistencia a la flexión aumenta conforme disminuye el tamaño de las mismas. Finalmente, también se ha estudiado la dependencia de la resistencia a la flexión con la temperatura, observándose un comportamiento superplástico a 1700 K para la fibra procesada a mayor velocida

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

CATION COLLOIDAL PARTICLES IN ALKALINE-EARTH HALIDES

Nous avons étudié la formation des colloïdes de calcium, strontium et barium par une forte irradiation électronique et par coloration additive des cristaux de CaF2, SrF2 et BaF2. Nous avons obtenu des renseignements détaillés sur la dépendance avec la température de l'efficacité de la formation des colloïdes par irradiation. La croissance des particules métalliques en échantillons colorés additivement et contenant des centres F et M a été étudiée pour différentes concentrations des centres colorés et températures de recuit. En utilisant la théorie de Mie on a calculé les bandes d'absorption optique dues aux colloïdes métalliques. En considérant la pression qui fait la matrice sur les particules métalliques on a introduit diverses corrections aux constantes optiques des métaux. Les résultats expérimentaux obtenus sont en accord avec les calculs théoriques. L'évolution des colloïdes a été étudiée selon différentes expériences de recuit. Un modèle de diffusion limitée a été utilisé pour rendre compte de la dépendance du rayon des colloïdes par rapport au temps de recuit.The formation of calcium, strontium and barium colloids both in heavily electron irradiated samples and in additively colored crystals of CaF2, SrF2 and BaF2 has been investigated. Detailed data on the temperature dependence of the efficiency of colloid formation by irradiation have been obtained. The growth of metallic particles in additively colored samples containing F and M centers has been studied for different color center concentrations and annealing temperatures. The optical absorption bands due to metallic colloids have been calculated using the theory of Mie. To take account of the pressure exerted by the matrix on the metallic particles several corrections to the optical constants of the metals have been introduced. A good agreement between theoretical calculations and experimental results has been obtained. The evolution of colloids along several thermal annealing experiments has also been investigated. A diffusion-limited model has been used which accounts for the dependence of the colloid radii with the annealing time

Structured YBa2Cu3O7-δ thin films grown on aligned calcium stabilized zirconia-calcium zirconate lamellar eutectic substrates

In this paper we present a new approach to obtain structured biepitaxial films by using eutectic substrates. In particular we have deposited YBa2Cu3O7-δ (YBCO) thin films grown by Metal-Organic Chemical Vapor Deposition (MOCVD) on eutectic Ca-stabilized Zirconia - CaZrO3 (CSZ-CZO) substrates structured in the form of parallel lamellae. The films reproduce the pattern of the substrate and show ordered arrays of alternating c-axis and (103)-oriented YBCO strips. Crystallographic characterisation reveals that YBCO films grow epitaxially on both eutectic phases. The structured films were analysed by EPMA, and micro-Raman. The films show anisotropic transport properties in both normal (resistivity anisotropy ρT/ρL (300K) ≈ 1.9) and superconducting states (critical current anisotropy at 77K JcL/JcT ≈ 13) when measured along (L) or transverse (T) to the substrate lamellae. Transport along transverse direction showed a considerably higher sensitivity to applied magnetic field than measurements in the longitudinal direction. This was attributed to the weak-link behaviour of the artificially-induced grain boundaries between strips