The zirconium–platinum phase diagram

Phase relationships were studied in Pt-rich, near-equiatomic Zr–Pt alloys. The composition range of the previously unreported rhombohedral compound Zr₃Pt₄, isomorphous with Zr₃Pd₄ above room temperature, extends to the Zr-rich composition Zr₉Pt₁₁ by the formation of lattice vacancies on certain Pt sites. A metastable tetragonal Zr₉Pt₁₁ compound is formed, however, when vacancy formation is inhibited. The rhombohedral structure undergoes a displacement transformation on cooling between 90 and 140 °C to a low-temperature structure that is presumably triclinic. The orthorhombic compound ZrPt is stable from room temperature to 1590 °C where it transforms to a cubic B2-type structure. Structural data are given for the compounds ZrPt, Zr₉Pt₁₁, Zr₃Pt₄ and Zr₇Pt₁₀, and a complete Zr–Pt phase diagram is presented.     

A laser spectroscopic study of Nd-doped zirconia

High-surface-area rare-earth (RE) modified zirconia powders prepared by solution methods can be used as catalytic support of noble metals and as electrolyte oxygen sensors in an automobile exhaust-emission-control system. A previous neutron-scattering study showed that substituting zirconium with trivalent RE ions not only stabilizes the cubic and tetragonal phases over a wide range of temperatures, but also creates oxygen vacancies in the RE–Zr oxide solid solution. This work focuses on the fluorescence of Nd in Nd_0.1Zr_0.9O_1.95 powders under laser excitation of the Nd³⁺ ground state to the ⁴G_7/2 states. Distinct features were observed at 8 K in the ⁴I_9/2→⁴G_7/2 excitation and ⁴F_3/2→⁴I_9/2 emission spectra using two sets of incident and emission frequencies, respectively. The results are discussed in terms of site-sensitive local structures surrounding the Nd ions in the two-phased oxide structure.     

Thermal properties of zirconia co-doped with trivalent and pentavalent oxides

Zirconia doped with 6–8 wt% (3.2–4.2 mol%) yttria (6–8YSZ), the most common thermal barrier coating material, relies mostly on oxygen vacancies to provide the phonon scattering necessary for low thermal conductivity. The present study examines whether specific substitutional defects—in addition to, or instead of, oxygen vacancies—can provide similar or greater reductions in conductivity. To this end a series of zirconia samples co-doped with varying levels of yttrium (trivalent) and tantalum/niobium (pentavalent) oxides were synthesized, thereby allowing oxygen vacancy and substitutional atom concentration to be varied independently. The results show that Nb–Y and Ta–Y co-doped zirconia samples containing only substitutional defects produce stable single-phase tetragonal materials with thermal conductivities very close to that of the conventional 6–8YSZ. In these samples, Nb⁵⁺ and Td⁵⁺ are similarly effective in lowering thermal conductivity, in contradiction to phonon scattering theories that consider primarily mass effects and thereby predict significantly greater conductivity reduction due to Ta⁵⁺ doping than Nb⁵⁺ doping. Finally, Nb⁵⁺/Ta⁵⁺–Y³⁺ doped samples, which contain both oxygen vacancies and substitutional defects, are found not to be stable in single-phase form; however, the thermal conductivities of the two-phase tetragonal+cubic mixtures are again as low as that of the conventional 6–8YSZ.     

Mechanical loss of cubic zirconia

Cubic zirconia can be stabilized by doping with lower valent oxides such as Y₂O₃ or CaO. Oxygen vacancies are then created as charge compensating defects. Mechanical loss measurements were performed in the temperature range 300–1600 K using both free decaying (3 Hz, 3 kHz) and forced vibrations (10⁻²–10 Hz). The influence of Y₂O₃ (10–24 mol%) and CaO (10–16 mol%) on the loss spectra was studied. The low temperature spectra (<1000 K) show a composite loss maximum which can be decomposed into two peaks, I and I_A (I′, I_A′). Both peaks, with activation enthalpy values between 1 and 2 eV, rely on local jumps of oxygen vacancies which are trapped by dopant cations. Submaxima I (I′) are assigned to defect pairs of oxygen vacancies and dopant ions (Y or Ca) forming elastic (electric) dipoles. These are oriented parallel to 111 (trigonal symmetry) with the vacancies on nearest neighbor sites. Maxima IA (I_A′) are assigned to relaxation of vacancies within larger clusters and interaction effects (Y or Ca). The high temperature loss spectra (≥1000 K) show various relaxation phenomena. In Y₂O₃ stabilized ZrO₂ we observe around 1400 K a loss peak with frequency independent temperature position which is assigned to a structural phase transition. ZrO₂–CaO shows a high temperature peak of Debye type (H=4.0±0.5 eV) which is related to local diffusional jumps of cations via vacancies.     

Synthesis and optical properties of yttria-doped ZrO2 nanopowders

Yttria-doped zirconia (YDZ) nanopowders were synthesized via a solvothermal route using ethanol as solvent. Evolution of crystal phases for different amount of yttria-doped samples were studied by X-ray diffraction (XRD). Morphology and component of the as-synthesized cubic YDZ were characterized by scanning electron microscopy (SEM) and energy dispersion spectrum (EDS). Defects of the sample were detected using ultraviolet–vis (UV–vis) absorption spectrum and photoluminescence (PL) spectrum. The results indicated that cubic structured nanocrystals can be obtained through doping 4 mol% Y₂O₃ into ZrO₂ lattice. The particles had sphere morphology with an average crystal size of 10 nm and agglomerated into bigger spheres with a diameter of about 120 nm. Mechanism of the agglomeration was also discussed. UV spectra showed two absorption peaks, red shift for both of the adsorption edges was observed. PL spectra with excitation wavelength of 260 and 420 nm revealed six fluorescence peaks which were regarded as various energy levels in the band gap and as the evidence of existence of oxygen vacancies in the as-synthesized sample.     

Development of Nano-tetragonal Zirconia-Incorporated Ni–P Coatings for High Corrosion Resistance

Pure tetragonal 10-20-nm-size zirconia-based Ni-P composite coating was developed. The physicochemical and electrochemical characteristics including corrosion resistance of the coating were investigated. The Ni-P-nano-tetragonal zirconia coating was partially crystalline having face-centered cubic phase. The coating had very high corrosion resistance due to its dense compact morphology and low surface roughness. The Ni-P-nano-tetragonal zirconia coatings exhibited a cathodic shift of open-circuit potential(OCP) in the range from-0.340 to-0.520 V. A high polarization resistance of the order of 13.2 kΩ/cm~2 and low corrosion current density of 3.9 μA/cm~2 were achieved due to the effective incorporation of zirconia into the coating.     

The hot corrosion resistance of 20 mol% YTaO4 stabilized tetragonal zirconia and 14 mol% Ta2O5 stabilized orthorhombic zirconia for thermal barrier coating applications

Zirconia stabilized with 3.2–4.2 mol% (6–8 wt.%) yttria (3–4YSZ), the current material of choice for thermal barrier coating applications, is susceptible to hot corrosion by acidic oxides such as vanadia in the 700–900 °C range. The current study is a preliminary examination of the hot corrosion resistance to NaVO₃–V₂O₅ mixtures in the above temperature range of two alternative materials: a tetragonal zirconia co-doped with 10 mol% yttria+10 mol% tantala (20YTaO₄Z) and an orthorhombic zirconia doped with 14 mol% tantala (14TZ). Results show that the 20YTaO₄SZ is resistant to destabilization by NaVO_3, but is attacked at higher V₂O₅ activities, resulting in the formation of YVO₄ and orthorhombic zirconia. Studies on the 14TZ itself then indicated that it is substantially more resistant than the YSZ to attack by environments more acidic (specifically V₂O₅ rich) than pure NaVO₃. However, it is less suitable than either 20YTaO₄SZ or 3–4YSZ for environments that are more basic. A comparison of the resistance of the 14TZ, the 3–4YSZ and the 20YTaO₄Z shows that the 20YTaO₄SZ is more resistant to acidic oxides than the YSZ and more resistant to the basic oxides than the 14TZ.     

Solid-state chemical synthesis of nanoparticulate zirconia

Mechanically activated reaction of anhydrous chloride precursors with Li₂O has been used to synthesise ultrafine powders of ZrO₂, Mg–PSZ, and Y–TZP. In each case, milling of the reactant mixtures resulted in a combination of amorphisation and overall microstructural refinement. Chemical reaction, with the consequent formation of nanoparticulate zirconia and LiCl, only occurred during subsequent low temperature heat treatment. The mechanism of reaction was found to depend on the heating rate and also on the presence of LiCl diluent in the initial reactant mixture. In addition to altering the reaction mechanism, the addition of diluent decreased the average crystallite size and increased the proportion of single crystal particles in the final zirconia product. Accompanying the decrease in average particle size with dilution was an increase in the tetragonal volume fraction.     

Synthesis and properties of lithium ion conductors Li3−2x(Sc1−xZrx)2(PO4)3

Lithium ion conductors, Li_3−2x(Sc_1−xZr_x)₂(PO₄)₃ (0 x 0.3), were prepared by a solid-state reaction. TG–DTA analysis indicated no phase transition in the samples with x superior to 0.05. X-ray powder diffraction analysis of these samples clearly showed the stabilization of a superionic conduction phase at room temperature with an orthorhombic system Pbcn. The highest conductivity was observed for the sample with x=0.05, and ascribed to the stabilization of the superionic conduction phase and the introduction of vacancies on the Li⁺ sites by substituting Zr⁴⁺ for Sc³.