A LANTHANIDE LANTHOLOGY (.pdf) - Davidson Physics

ZIRCONATES, ZIRCONIUM OXIDEThis section covers compounds within the Ln-Zr-O system and Ln-doped ZrO 2 's.Ln 2 Zr 2 O 7 compounds, (1:2, La 2 O 3 :ZrO 2 ) can be made by a solid state reaction between theoxides or, preferably, by variations on the sol-gel Pechini process with citric acid and glycol.[1]The structures found, pyrochlore and fluorite, are very closely related[2] and, indeed, whichstructure is formed depends on the temperature reached during preparation. Ln zirconates are ofinterest for catalytic[3] and conductivity[4] properties. However, although defect fluorites can begood ionic conductors, ordering of theO 2- vacancies in the pyrochlore (A 2 B 2 O 8-1 ) version offluorite, (A 2 B 2 O 8 ≡O 2 ) limits this potential.Lanthanide ZirconatesLn 2 Zr 2 O 7Ln-range Structure type Relation to FluoriteLa - Eu Pyrochlore O-ordered vacanciesTb – Er "Fluorite" O-substochiometricZrO 2 's doped with the Ln's or Y can show exceptional mechanical and electricalproperties[5]. The cations of the Ln’s and Y can substitute for Zr 4+ in the zirconia lattice. Chargecompensation is made possible through the formation of oxygen vacancies; this influences ionicand electrical conductivity.Depending on the amount of dopant present the zirconia can be partially or full stabilized.Sub-micron grain size, tetragonal-phase Y 2 O 3 -zirconia (Y-TZP) ceramic exhibit a high fracturetoughness[6] and are used for wear components; yttria dopin is ≈3 mol % in order to confermechanical strength. In the fully stabilized form, YSZ's materials are the solid electrolyte in oxygensensors for exhaust emission control. With ceria a higher doping, ≈12 mole %, is needed to givedoped zirconias exceptional fracture toughness and hardness[7]. A significant use of Ln-dopedZrO 2 's is plasma-spray coatings for protection of metal surfaces at very high temperatures.[1] Preparation of La 2Zr 2O 7 by Sol-Gel Route, H.Kido et al., J.Am.Ceram.Soc., 1991,74(2),422:PhaseRelationships and Conductivity of Sr- and La-Zirconates, F.W.Poulsen et al., Solid State Ionics, 199 53-56,777[2] Crystal Structure Analyses of the Pyrochlore- and Fluorite-Type Zr 2Gd 2O 7 and Anti-Phase DomainStructure, T.Moriga et al., Solid State Ionics, 1989, 31, 319[3] Bond Energy Effects in Methane Oxidative Coupling on Pyrochlore Structures, C.Petit et al., J.Catal1993,140,328[4] Electrical and Catalytic Properties of some Oxides with the Fluorite or Pyrochlore Strycture, Part 1 :Synthesis, Characterization and Conductivity, M.P.vanDijk et al.,Mat.Res.Bull., 1984, 19, 1149[5] ZrO 2 Materials for Application in the Ceramics Industry, H.H.Zender et al., Interceram., 1990,39(6), 33[6] Transformation Toughening in ZrO 2-containing Ceramics, A.H.Heuer, J.Am.Ceram.Soc., 198 70(l0),689[7] Features of the Ceria-Zirconia System, S.Meriani, Mads. Sci. Eng., 1989, A109, 12158