Solid solutions of metastable tetragonal ZrO2 and Ce3ZrO8 in the system ZrO2-CeO2

In the system ZrO₂-CeO₂, metastable t-ZrO₂ solid solutions containing up to 30 mol% CeO₂ crystallize at temperatures of 385–430 °C from amorphous materials prepared by the hydrazine method. Crystalline Ce₃ZrO₈ solid solutions are formed in as-prepared powders between 30–75 mol % CeO₂. The variation of the lattice parameters of both solid solutions is determined as a function of CeO₂ content. The value of the lattice parameter of pure Ce₃ZrO₈ (cubic) is a = 0.5342 nm. Detailed characterization of the Ce₃ZrO₈ powder has been performed. Crystallite size and particle size are strongly dependent on the heating temperature. Specific surface areas do not drop below 40 m²g^–1 until the heating temperature is above 1000°C.     

Preparation of tetragonal ZrO2-Gd2O3 powders

ZrO₂-Gd₂O₃ alloys containing 2,3,5 and 8 mol.% Gd₂O₃ have been prepared by mixed oxide (MO), hybrid sol-gel (SG), and co-precipitation (CP) routes. No tetragonal (t) phase is retained in the MO method, while 100% t phase is obtained in the calcined CP samples; the SG method leads to only partial stabilization of the t phase. Washing of the CP powders with propan-2-ol leads to unagglomerated powders with increased specific surface area (145 versus 89 m²g^–1) and sintered density (98% versus 79%). Cubic and t phase also appear on sintering the samples with >2 mol.% Gd₂O₃.     

CeO1.5-stabilized tetragonal ZrO2

Ceria-stabilized tetragonal zirconia polycrystals show high toughness and high resistance to the low-temperature ageing degradation. However, ceria is less effective in stabilizing tetragonal zirconia compared to yttria and other trivalent oxides. The tetravalent oxide of CeO₂ can be easily reduced to the trivalent CeO_1.5, but phase separations occur leading to the destabilization of the tetragonal phase or the stabilization of the cubic phase. A procedure of high-temperature sintering and low-temperature reduction has been developed for preparing CeO_1.5-stabilized tetragonal zirconia. It was found that 9 mol% CeO_1.5 could stabilize the tetragonal zirconia to room temperature and that the stability region in the CeO_1.5-ZrO₂ system was extended to the lower dopant content region. The CeO_1.5-stabilized tetragonal zirconia had a lower tetragonality and lower transformability compared to the CeO₂-stabilized tetragonal zirconia with the same dopant mole percentage. The changes in the phase composition, tetragonality and stability caused by the reduction of CeO₂ to CeO_1.5 have been discussed in relation to the changes of oxygen stoichiometry, which is considered of the firstorder importance in the stabilization of polymorphous zirconia.     

Hot isostatic pressing of tetragonal ZrO2 solid-solution powders prepared from acetylacetonates in the system ZrO2-Y2O3-Al2O3

In compositions having ZrO₂/Y₂O₃=(74.25–71.25)/(0.75–3.75) (mol% ratio) with 25 mol% Al₂O₃, metastable t-ZrO₂ solid solutions crystallize at 780° to 860°C from amorphous materials prepared by the simultaneous hydrolysis of zirconium, yttrium and aluminium acetylacetonates. Hot isostatic pressing has been performed for 1 h at 1130 and 1230°C under 196 MPa using their powders. Two kinds of material are fabricated: (i) perfect ZrO₂ solid-solution ceramics and (ii) composites of ZrO₂ solid solution and -Al₂O₃. Their mechanical properties are examined, in connection with microstructures and t/m ZrO₂ ratios. Composites with a homogeneous dispersed -Al₂O₃ derived from solid-solution ceramics result in a remarkable increase of strength.     

Stability of tetragonal ZrO2 phase in Al2O3 prepared from Zr-Al organometallic compounds

Zr-Al organometallic compounds have been spray-dried and heated at temperatures 600 to 1400°C to prepare ZrO₂-Al₂O₃ composite powders. The powders consist of balloon-like particles 0.5 to 2 m in diameter with homogeneously dispersed tetragonal ZrO₂ grains 0.1 to 0.2 m in diameter. The tetragonal fraction of ZrO₂ in the composite powders is higher than that in the powders prepared from sols of Zr(OBuⁿ)₄ and Al[OCH(CH₃)₂]₃. The fraction is affected by the organofunctional group in the Zr-Al compounds.Zr(OBuⁿ)₄ = Zr(OC₄Hgⁿ)₄; Al[OCH(CH₃)₂]₃ = Al(OPrⁱ)₃.     

Fracture of metastable tetragonal zirconia crystals

Crystals of single-phased metastable tetragonal zirconia (MTZ) were prepared from skullmelting at the composition ZrO₂-3 mol % M₂O₃ (M = Y, Yb or Gd). The fracture of the tetragonal crystals occurred differently than for the cubic stabilized zirconia ones. The toughness was much higher for the tetragonal specimens and the cleavage planes were not the same as for the fluorite crystals. The results were interpreted by considering the domain microstructure induced by the cubic → tetragonal phase transformation undergone by the crystals.     

Preparation of monodispersed Y-doped ZrO2 powders

3 mol% Y-doped ZrO₂ powders prepared by the controlled hydrolysis of metal alkoxides were monodispersed and grown to 0.5m after 5 h ageing. The as-prepared powder was amorphous and hydrate but transformed into a tetragonal single phase by heating. Furthermore, the Y₂O₃ concentration of each particle was almost the same in all particles. The synthesis conditions such as ageing time, ageing temperature and water concentration greatly affected the particle morphology. The refluxing of the alkoxide solution was particularly necessary to prepare the monodispersed particles. On the basis of the variation of size distribution with ageing time, the mechanism of particle growth was discussed.     

Preparation of ZrO2 coated graphite powders

Graphite powders were coated with ZrO₂ by the controlled hydrolysis of a zirconium oxychloride aqueous solution. Thehydrolysis process was carried out with temperature control because of the low wettability of ZrO₂ to the surface of thegraphite. PVA was added to the solution for the enhancement of Zr ion adsorption. The surface of the graphite particles coatedwith ZrO₂ was observed by TEM. There are two types of ZrO₂ particles; (a) primary particles a few nm in size and (b) secondary particles with 0.1 m size were obtained. The data of oxidation weight loss and surface potentialshow that the graphite surface was successfully modified by the forced hydrolysis of the zirconium chloride aqueous solution.     

Crack-tip transformation zones of CeO2-stabilized tetragonal ZrO2 polycrystals

Abstracts are not published in this journal     

Synthesis and luminescent properties of ZrO2 and Dy3+-activated ZrO2 powders

Journal of Materials Science: Materials in Electronics - ZrO2 and ZrO2:Dy3+ (0.25, 0.5, 1, 1.5, 2, 2.5, and 3 mol%) powders are synthesized via solution combustion method. Structural,...     

Preparation and characterization of dispersed Rh2O3 on tetragonal ZrO2

Samples of well-dispersed hexagonal Rh₂O₃ on tetragonal ZrO₂ have been prepared by the code composition of the nitrates at 900°C. A comparison of the stability towards reduction of the bulk and dispersed Rh₂O₃ products demonstrates the influence of an interaction between the dispersed metal oxide and the support.     

Microstructure and mechanical properties of ZrO2-Gd2O3 tetragonal polycrystals

The phases, transformability, microstructure and mechanical properties of ZrO₂-Gd₂O₃ polycrystals containing 1.75–8 mol% Gd₂O₃ were studied. The samples were prepared by a coprecipitation route followed by sintering at 1400°C for 2 hours. The grain size was in the range of 0.1–0.2 m except for some large grains at high Gd₂O₃ contents. Only a tetragonal phase was observed between 2–4 mol% Gd₂O₃ and a cubic phase for compositions containing 9.6 mol% Gd₂O₃. A peak K _IC of 12 MPa m^1/2 and a strength of 800 MPa were obtained in the 2 mol% Gd₂O₃ alloy for which the t m transformation on the fracture surface was also found to be maximum. Transformation toughening is able to account for most of the toughness of the samples.     

Phase-transformation study of metastable tetragonal zirconia powder

The rate of the transformation from the metastable tetragonal to the monoclinic phase of ZrO2 was measured in air from 850°C to 1000°C by neutron diffraction. This rate was found to be temperature dependent, and its measured values were considerably lower then those reported previously. The kinetics of this phase transformation is discussed in terms of a modified crystallite growth-martensitic transformation model' that includes the distribution of crystallite sizes.