K. Lebbou

Microstructure of Y2O3 doped Al2O3/ZrO2 eutectic fibers grown by the micro-pulling-down method

Al2O3/ZrO2 eutectic crystal fibers containing various amounts of Y2O3 were grown by the micro-pulling-down method. The eutectic microstructures and some mechanical properties were investigated as a function of growth rate and doping amount of Y2O3. Doped with Y2O3, Al2O3/ZrO2 eutectic fibers 0.3–2 mm in diameter and 500 mm in length have been grown over the range of pulling rate 0.1–15 mm/min. The dominant zirconia phase was changed from monoclinic to cubic as the doping amount of Y2O3 increased. The eutectic microstructures were oriented to a common direction which depended on the growth rate and doping amount of Y2O3. The uniform lamellar structure obtained at a lower growth rate below 1 mm/min transformed to an ellipsoidal cellular structure at a growth rate above 10 mm/min, and circular or triply facetted cellular patterns were observed at intermediate growth rates. The intercellular spacing increased as the doping amount of Y2O3 increased. The hardness value reached 20 GPa for 9 mol% Y2O3 doped fibers grown at a rate of 15 mm/min. The highest tensile strength reached 2000 MPa at room temperature and 560 MPa at 1500°C for the 3 mol% Y2O3 doped fibers grown at a rate of 15 mm/min.

Structural and Spectroscopic Characterization of Nominal Yb3+:Ca8La2(PO4)6O2 Oxyapatite Single Crystal Fibers Grown by the Micro-Pulling-Down Method

Yb-doped Ca8La2(PO4)6O2 (CLPA) single crystals with the apatite-type structure and having orientation were grown by the micro-pulling-down (μ-PD) method. The apatite structure is represented by the monophased field of Ca8(La2–xYbx)-(PO4)6O2 (CLYPA) where it is assumed that 2 Ca2+ sites are substituted by La3+ and Yb3+ cations. Its monophased range was found to be from x = 0.0 to 0.2. The segregation of Yb3+ in CLPA single crystals and the maximum Yb3+ concentration are discussed. The crystallinity was studied using X-ray rocking curve analysis. Absorption, emission and fluorescence decay studies of Yb3+ ions in CLPA were also carried out both at low temperature and room temperature. Spectroscopic data reveal Yb3+ ion occupation within different crystallographic sites of the apatite-type structure. The potential for a diode-pumped Yb3+ laser is evaluated.

Effect of Yb3+ content on purity and crystal growth of Ba2NaNb5O15

Abstract Growth by the micro-pulling-down technique (μ-PD) of homogeneous and crack-free fiber single crystals with composition Ba 2 Na 1− x Yb x Nb 5 O 15 ( 0 ) is reported. The effect of Yb 3+ addition to barium sodium niobate (BNN, x=0 ), having the tungsten bronze-type structure, was examined by room temperature X-ray powder diffraction and differential thermal analysis coupled to thermogravimetry. In the region of the monophased field the structure is tetragonal from x=0.02 to 0.16. Volume change is mainly by variation of the a -axis length. Addition of Yb 3+ to BNN could be effective for the production of high optical quality, crack-free, bulk crystals by the Czochralski technique.

Yb3+-doped Ba2NaNb5O15 (BNN) growth, characterization and spectroscopy

Abstract Low-temperature spectroscopy of host barium sodium niobate – Ba 2 NaNb 5 O 15 (BNN) crystal fibers doped with Yb 3+ ion has been studied. The fibers were grown by the laser-heated pedestal growth (LHPG) technique, and were characterized among others by X-ray diffraction, micro-Raman spectroscopy, X-ray fluorescence and optical microscopy. The results showed very homogeneous and defect-free materials with pseudotetragonal structure appropriate for optical applications. Absorption and emission spectroscopic studies at low temperature were used to determine the energy-level diagram of the Yb 3+ ion in host BNN.

Effect of gadolinium (Gd3+) addition on the monophased field and crystal growth of Ba2NaNb5O15 (BNN)

Ba2Na1� 3xGdxNb5O15 compounds with (0.024x40.2) have been prepared. The limit of the monophased composition field was determined as a function of Gd substitution. The effect of Gd on barium sodium niobate (BNN) was examined by X-ray powder diffraction at room temperature as a function of temperature. The melt behavior was studied by DTA. The homogeneity of single-crystal fibers doped with Gd and grown by micro-pulling down technique was investigated by EPMA. No cracks or other inhomogeneities could be detected, as already shown for Yb- and Nd-doped BNN. In this study, Gd addition was effective for growth of bulk crystals by Czochralski technique. # 2001 Elsevier Science B.V. All rights reserved.

Superconductor Bi2212 fiber growth from the melt by micro-pulling down technique

Bi Sr CaCu O fibers have been grown using the micro-pulling down m-PD technique. The fibers were produced 2 x 22 y . from melts containing different bismuth concentrations. Growth rates about 0.4 mmrmin were found that allowed . preparation of superconducting fibers with high critical temperature T f 90 K. The fibers contained plate like single c crystals each crystals oriented with the a-b plane parallel to the fiber axis which is the easy growth condition. q 2000 Elsevier Science B.V. All rights reserved.

Molecular beam epitaxy of rare-earth trifluoride, RF3 (R=Ce, Nd, Er, Lu) layers

Abstract RF3 (R=Ce, Nd) layers of hexagonal structure and RF3 (R=Er, Lu) layers of orthorhombic structure were grown on Si(1xa01xa01) substrates at 700 °C by molecular beam epitaxy (MBE). Their surface characteristics such as layer crystallinity, epitaxial relationship and surface morphology were investigated by reflection high-energy electron diffraction (RHEED), high-resolution X-ray diffractometer (HRXRD) and atomic force microscope (AFM). Both CeF3 and NdF3 layers of hexagonal structure were grown very conformably on Si(1xa01xa01) substrates. The ErF3 layer of the native orthorhombic structure could be grown relatively conformably with its crystallites aligned along one of six hexagonally symmetric orientations, whereas the LuF3 layer was degraded in crystalline quality probably due to structural incompatibility and large lattice mismatch with Si(1xa01xa01) substrate.

Solid state equilibrium in the doubly substituted (TI, Pb)(Sr, Ba)O3 system

Abstract The phase relations in the doubly substituted Tl x Pb 1− x Sr y Ba 1− y O 3 system show a solubility range of the perovskite phase BaPbO 3 from x =0, y =0.5 to x =0.8, y =0. The crystallographic structure of the solid solution is well described with a tetragonal cell, I 4/ mcm space group. The electrical conductivity shows a semiconducting behaviour with an activation energy of 7.2 kJ mol − in the temperature range 600–850°C. The Sr-based perovskite SrPbO 3 , with orthorhombic Pbnm structure does not accept extended Tl or Ba substitutions.

Improvement of the Irreversibility Line of the 1212 Compound Tl 0.5 Pb 0.5 Sr 2 CaCu 2 O 7−δ

The superconducting properties and the irreversibility line of the Tl0.5Pb0.5Sr2CaCu2O7−δ were studied by ac susceptibility on both ceramic and powder samples prepared in sealed quartz tube at 960°C. In parallel, carefull investigations by DTA/Tg, X rays diffraction and plasma emission spectroscopy were performed on each sample after each thermal treatment. It is shown that the superconducting properties are strongly dependent on further heat treatments and that the irreversibility line may be optimized. Nevertheless, the results obtained suggest that further improvement of the irreversibility line might be expected.

Effect of Ba Substitution on the Superconducting Properties and the Irreversibility Line of the Compounds TIPb 1223: Tl 0.5 Pb 0.5 (Sr 2−x Ba x )Ca 2 Cu 3 O 9−δ

The superconducting properties and the irreversibility line of the Tl0.5Pb0.5(Sr2−xBax)Ca2Cu3O9−δ were studied by ac susceptibility on both ceramic and powdered samples. The samples were prepared by solid state reaction in flowing oxygen at 880°C or in sealed quartz tube at 960°C. The irreversibility lines were measured up to 4 Tesla and were determined by the maximum of the X″ peak, using the same ac field to allow comparison between them. The presence of the 1212 phase as a minor superconducting phase was clearly evidenced in all cases. The case of samples of TlPbl223 with Tl/Pb ratio different from unity was also examined.