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.     

Stability of metastable tetragonal ZrO2 in compound powders and nucleation arguments

ZrO₂-SiO₂ (11) mixtures and ZrO₂ particles were prepared by a sol-gel method from the solutions of ZrOCl₂·8H₂O (ZOC) + Si(OC₂H₅)₄ (TEOS) + C₂H₅OH and ZOC + C₂H₅OH + H₂O + NH₄Cl systems, respectively. Quantitative changes of phase crystallized by heating were compared with those of ZOC + TEOS + H₂O and/or ZrO(NO₃)₂ · 2H₂O + H₂O systems, respectively. The stability of metastable tetragonal (mt)-ZrO₂ particles depends on the Young's modulus of the SiO₂ matrix. Transition metal oxides existing on the interface assisted in stabilizing mt-ZrO₂. The order of the assistance agreed with that of the relative field strengths of their oxides. A relationship between the ionic radius and the volatility of anionic groups, and the difficulty of nucleation for the martensitic transformation accompanying a shear stress, is suggested.     

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.     

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.     

Determination of the monoclinic, tetragonal and cubic phases in mechanically alloyed ZrO2-Y2O3 and ZrO2-CoO powder mixtures by Raman spectroscopy

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

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.     

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

Abstracts are not published in this journal     

Optical spectroscopy of Eu3+ ions in tetragonal ZrO2 nanocrystals

A facile solvothermal method was introduced to incorporate Eu3+ ions into the monodisperse tetragonal ZrO2 nanocrystals (NCs) with small size of approximately 4 nm. The optical properties for Eu3+ doped ZrO2 NCs were investigated in detail by using the photoluminescence (PL) spectroscopy at room and low temperatures. Intense red emissions from Eu3+ ions could be achieved via the host sensitization, which was found to be much more efficient than the direct excitation of lanthanide ions. Moreover, multiple sites of Eu3+ as well as the host-to-Eu3+ energy transfer were also revealed based on the PL analyses.     

Mechanochemical synthesis of ultrafine ZrO2 powder

The synthesis of ultrafine zirconia powders by mechanochemical reaction of ZrCl₄ with CaO has been investigated using x-ray diffraction, TEM and DSC measurements. Mechanical milling resulted in a nanoscale mixture of CaO and amorphous ZrCl₄. with no evidence of any reaction having occurred. Subsequent heat treatment at temperatures above 300 °C resulted in the formation of separated particles of cubic ZrO₂,5–10 nm in diameter, within an CaCl₂ matrix. Measurements of the effect of particle size on the crystal structure of ZrO₂ are also reported.     

Texture and surface morphology effects on tetragonal phase stabilization in ZrO2 films deposited by metal-organic chemical vapor deposition

Tetragonal zirconia coatings (ZrO2) without doping any trivalent impurities have been deposited by metal-organic chemical vapor deposition (MOCVD) on (100) Si single crystals, using Zr(thd)4 precursor. The surface and cross-section morphologies were observed with Field-Emission-Gun Scanning Electron Microscopy (FEG-SEM). The crystalline structures were characterized by grazing incident X-ray diffraction (GIXRD). Crystallographic textures of these films were studied for both {011}t and {110}t planes by pole figure recording by X-ray diffraction under a 4-circle goniometer. The internal stresses were measured with the use of sin2 psi method. In order to study the relationship of microstructures and tetragonal phase stabilization in ZrO2 films, annealing experiments were taken at different temperature. The results show that the critical crystallite size for tetragonal to monoclinic phase transformation is different for samples with different initial microstructures. Besides the critical crystallite size and the residual stress, the texture and crystallite morphology of the ZrO2 films are responsible for the stabilization of the metastable tetragonal phase.     

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.