A new tentative phase equilibrium diagram for the ZrO2-CeO2 system in air

The phase relationships over a wide range of temperature and compositions in the ZrO₂-CeO₂ system have been reinvestigated. From DTA results, thermal expansion measurements andK _IC determinations it was established that additions of CeO₂ to ZrO₂ decreases the monoclinic to tetragonal ZrO₂ transition temperature, from 990 ° C to 150 50 ° C, and an invariant eutectoid point at approximately 15 mol% CeO₂ exists. The extent of the different single- and two-phase fields were determined with precise lattice parameter measurements on quenched samples. Evidence for the existence of a binary compound Ce₂Zr₃O₁₀ (ø-phase) was obtained by X-ray diffraction. The ø-phase was stable below approximately 800 ° C, above which it decomposes into tetragonal zirconia + fluorite ceria solid solutions. Taking into account the polymorphic tetragonal-cubic transition and the narrowness of the two-phase tetragonal zirconia + fluorite ceria field above 2000 ° C, the existence of a new invariant eutectoid point was assumed, in which the metastable fluorite zirconia solid solution decomposes into tetragonal zirconia + fluorite ceria solid solutions. From the results obtained, the phase diagram also incorporates a eutectic point located at approximately 2300 ° C and 24 mol % CeO₂.     

Formation and hot isostatic pressing of ZrO2 solid solution in the system ZrO2-Al2O3

In the system of ZrO₂-Al₂O₃, cubic ZrO₂ solid solutions containing up to 40 mol% Al₂O₃ crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminium alkoxides. At higher temperatures, they transform into tetragonal solid solutions. Metastable ZrO₂ solid solution powders containing 25 mol% Al₂O₃ have been sintered at 1000–1150 °C under 196 M Pausing the hot isostatic pressing technique. The solid solution ceramics consisting of homogeneous microstructure with an average grain size of 50 nm exhibited a very high fracture toughness of 23 MN m ^–1.5. They have been characterized by X-ray diffraction and electron probe surface analyses.     

Mechanochemical Synthesis of ZrO2–CeO2 Solid Solutions

ZrO₂–CeO₂ solid solutions (12 and 13 mol % CeO₂) were prepared by mechanochemical synthesis in different media. The mechanical activation medium was shown to have a significant effect on the phase composition and particle size of the resultant powders. The conditions for sintering the powders were optimized.     

Preparation of an Alkali-Element or Alkali-Earth-Element-Doped CeO2–Sm2O3 System and Its Operation Properties as the Electrolyte in Planar Solid Oxide Fuel Cells

Ceria–samaria (CeO₂–Sm₂O₃) is one of the most interesting fluorite oxides since its ionic conductivity is higher than that of yttria-stabilized zirconia in air. However, these CeO₂ -based oxides are partially reduced and develop electronic conductivity under fuel cell operating conditions. In their application to the SOFC system, their current densities and power densities are not at a satisfactory level. For the development of high-performance CeO₂ electrolytes, it is important that the fluorite lattice of CeO₂-based oxide be improved from the viewpoint of crystallography. In this study, it is assumed that the reduction of Ce⁴⁺ in the fluorite lattice was inhibited by expansion of the CeO₂ lattice. In order to investigate the contribution of the expanded CeO₂ lattice to reduction resistance, CeO₂–Sm₂O₃ solid solution, calcia-doped CeO₂–Sm₂O₃ solid solution, and a small amount of alkali element-doped CeO₂–Sm₂O₃ -based oxide were prepared for comparison. It was found that the calcia or a small amount of alkali element-doped CeO₂ solid solution enhanced the oxide ionic conductivity. The power density of the latter showed a high value at 800°C. It is concluded that the improved fuel cell performance can be attributed to the good reduction resistance in the fuel cell atmosphere.     

Phase Composition, Electrical Conductivity, and Stability of ZrO2–Sc2O3–Cr2O3 Solid Electrolytes

The phase composition, electrical conductivity, and structural and electrical stability of ZrO₂–Sc₂O₃–Cr₂O₃ solid electrolytes prepared by solid-state reactions involving three-step firing at 1350, 1850 (vacuum), and 1300°C were studied for compositions along two lines: x(0.91ZrO₂ + 0.09Sc₂O₃)–yCr₂O₃ (I) andx(0.89ZrO₂ + 0.11Sc₂O₃)–yCr₂O₃ (II), x + y = 1, y = 0–0.04. The results indicate that the ternary solid solutions withy= 0.01–0.02 retain a cubic structure in a broad temperature range, down to room temperature. This increases the low-temperature (<600°C) conductivity of the solid electrolytes, especially in system II. In both systems, Cr₂O₃ solubility is about 3 mol %. Stability tests at 900°C for 200 h reduce the conductivity of the solid electrolytes, particularly at the lower Sc₂O₃ content and in the presence of Cr₂O₃. The reduction in conductivity is due to the decomposition of the high-temperature tetragonal phase and the formation of a tetragonal phase with a low stabilizer content.     

Preparation of ZrO2-CeO2 and HfO2-CeO2 nanopowders

We prepared weakly agglomerated powders of ZrO₂-CeO₂ and HfO₂-CeO₂ solid solutions 5–8 nm in particle size, consisting of monoclinic and tetragonal phases. After heat treatment at 1200°C, the crystallite size was 30 and 14 nm, respectively. We also examined the effect of precipitate freeze drying on the crystallization of hafnia-based solid solutions containing up to 20 mol % CeO₂.     

Effect of Preparation Conditions on the Phase Composition of ZrO2–Al2O3–CeO2 Powders

Data are presented on the phase composition of ZrO₂–Al₂O₃–CeO₂ powders containing 10, 30, and 70 mol %, prepared via coprecipitation and successive precipitation. The precipitation procedure is shown to have a strong effect on the phase composition of the heat-treated powders and the state (inter- or intracrystalline) of the T-ZrO₂ phase, thereby changing its stability. At an Al₂O₃ content of 30 mol %, the M-ZrO₂ content is independent of the precipitation procedure.     

Preparation of glass fibres of the ZrO2-SiO2 and Na2O-ZrO2-SiO2 systems from metal alkoxides and their resistance to alkaline solution

Glass fibres of the ZrO₂-SiO₂ and Na₂O-ZrO₂-SiO₂ systems containing up to 33 wt% ZrO₂ were prepared by a non-melting technique using zirconium n-propoxide, sodium methoxide and silicon tetraethoxide as raw materials. The mixed alkoxide solutions were exposed to moist air for hydrolysis. The fibrous gels were drawn from these solutions in the course of hydrolysis, and converted to the corresponding oxide glass fibres by heating at 500 to 700° C. It was found that chemical durability of the fibres toward alkaline solution increased with ZrO₂ content. The weight loss due to the corrosion by 2 N NaOH solution at 96° C for 4 h was around 14 mg dm^–2 for the fibres containing 17 to 26 wt% ZrO₂, which was comparable to the alkali-resistant glasses so far obtained by the conventional melting technique. The glass fibres containing 33 wt% ZrO₂ showed higher resistance.     

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.     

Crystallization process of cubic ZrO2 in calcium acetate solution

Ultrafine powders of cubic ZrO₂ were obtained at about 270° C by heating hydrated amorphous ZrO₂ in greater than 0.2 molal calcium acetate solutions. Ca²⁺ ions played a role as nucleii for crystallization and were introduced into distinct sites of the crystalline phases, that is, substituted for Zr⁴⁺ ions. Mn²⁺ ions produced almost the same effects on the crystallization of ZrO₂·EPR spectra for powder samples containing Mn²⁺ ions apparently showed two types as follows: for tetragonal ZrO₂ with a trace of monoclinic ZrO₂, the central fine structure transitions (M=+1/2–1/2) showed a well-resolved hyperfine structure. In addition to the m=0 transition, forbidden m=±1 transitions were observed. For cubic ZrO₂, the broad underlying response was observed as well as the hyperfine structure composed of six main peaks.     

Kinetics and mechanism of the solid-state synthesis of fluorite in ZrO2-Y2O3-Ln2O3 (Ln=Ce,Nd,Er) systems

The mechanisms and kinetics of the solid-state reactionx ZrO₂+yY₂O₃+(1–x–y) Ln₂O₂ ternary fluorite solid solution was studied in the temperature range 1350 to 1650° C by quantitative X-ray diffraction analysis. In the ZrO₂-Y₂O₃-CeO₂ system the fluorite formation process starts with the simultaneous interaction of CeO₂ and Y₂O₃ with ZrO₂, although the reaction rate of ceria with zirconia is more rapid. In the ZrO₂-Y₂O₃-Nd₂O₃ system, the formation of a pyrochlore, Nd₂Zr₂O₇, responsible for the formation process of the ternary fluorite solid solution. Finally, in the ZrO₂-Y₂O₃-Er₂O₃ system, a competitive interaction of yttria and erbia occurred in the formation process of the ternary solid solution. The kinetic data were treated using the Avrami equation, and activation energies for the processes studied calculated.     

The size effect of the martensitic transformation in ZrO2-containing ceramics

The relationship between the starting temperature of the martensitic transformation, M _s, and the grain size of the parent phase, d, in ZrO₂-containing ceramics was investigated. The experimental results showed that in tetragonal zirconia polycrystals doped with CeO₂ (8 mol%) and Y₂O₃ (0.25 mol%) (8Ce, 0.25Y-TZP), the M _s temperature displays a linear relationship with d ^–1/2, its slope being negative. A new explanation for this phenomenon, the so-called the size effect, has been presented, in which the grain size of the parent phase affects the M _s temperature through the strength of the parent phase. Thermodynamic calculation of the relationship between M _s and d gives a result consistent with the experimental ones.     

Enhancement of blue-green photoluminescence in B2O3 fritted ZrO2:Ce3+ phosphor

Blue-green emission of ZrO₂:Ce³⁺ phosphor, prepared by solid-state reaction, is demonstrated. The phosphor presents a strong and broad photoluminescence band centered at 496 nm with excitation at 291 nm. The optimized Ce content is 2.5 mol% for the strongest emission of ZrO₂:Ce³⁺ phosphors prepared without B₂O₃. The PL intensity is enhanced by at least 3 dB by adding 5.0 mol% B₂O₃ within the ZrO₂:Ce³⁺ containing 5.0 mol% Ce during synthesis. Increase of the B₂O₃ flux effectively induces the Ce ions to substitute the Zr ions in ZrO₂ lattice and causes the ZrO₂ lattice distortion. The formation of Ce_0.75Zr_0.25O₂ compound within the ZrO₂:Ce³⁺ occurred when the Ce content is greater than or equal to 2.5 mol% for the phosphors prepared without B₂O₃ and leads to a degradation of the phosphor PL intensity due to the host effect. The addition of B₂O₃ during the preparation of phosphors containing Ce ions lower than or equal to 5.0 mol% essentially restrains the Ce_0.75Zr_0.25O₂ formation and then enhances the blue-green PL.     

The ZrO2-rich region of the ZrO2-MgO system

The solubility limits of MgO in tetragonal zirconia were studied by combining the differential thermal analysis data and X-ray disappearing phase method. From these experiments a eutectoid reaction, tetragonal ZrO₂ solid solution monoclinic ZrO₂ solid solution + MgO, at 1120±10 °C and 1.6±0.2 mol% MgO was established. The solubility of MgO in tetragonal ZrO₂ diminished as the temperature increased, and at 1700 °C the solubility was less than 0.5 mol% MgO. The extent of the cubic zirconia solid solution single field was determined by using precise lattice parameter measurements and SEM observations. In this way an invariant eutectoid point, cubic ZrO₂ solid solution tetragonal ZrO₂ solid solution + MgO, was located at 1420±10 °C and 14.8±0.5 mol% MgO.     

Zr-Hf interdiffusion in polycrystalline Y2O3-(Zr+Hf)O2

Lattice and grain-boundary interdiffusion coefficients were calculated from the concentration distributions determined for Zr-Hf interdiffusion in polycrystalline 16Y₂O₃·84(Zr_1–x Hf _x )O₂ withx=0.020 and 0.100. The lattice interdiffusion coefficients were described byD=0.031 exp [–391 (kJ mol^–1)/RT] cm² sec^–1 and the grain-boundary diffusion parameters byD=1.5×10^–6exp [–309(kJ mol^–1)/RT] cm³ sec^–1 in the temperature range 1584–2116° C. Comparison of the results with those for the systems CaO-(Zr+Hf)O₂ and MgO-(Zr+Hf)O₂ indicated that the Zr self-diffusion coefficient was insensitive to the dopants in the fluorite-cubic ZrO₂ solid solutions.     

Effect of additives on the structure characteristics,thermal stability,reducibility and catalytic activity of CeO<Subscript>2</Subscript>–ZrO<Subscript>2</Subscript> solid solution for methane combustion

M _y O _x -modified CeO₂–ZrO₂ (M = Al, Ba, Cu, La, Nd, Pr, Si) solid solutions with the atomic ratio of Zr/Ce = 1 were prepared by the reverse microemulsion method, and the effect of different additives on the structure characteristics, thermal stability, reducibility, and catalytic activity of CeO₂–ZrO₂ solid solution for methane combustion were investigated. According to their different effects, M _y O _x can be classified into three groups. The first group includes SiO₂ and Al₂O₃ which do not vary the crystalline phase of CeO₂–ZrO₂ solid solution but distort the crystal lattice obviously. They are the most effective additives for improving the surface area, thermal stability, and reducibility of CeO₂–ZrO₂, and they can also promote the catalytic activity of Pd/CeO₂–ZrO₂ for methane combustion. The second group includes La₂O₃, Pr₂O₃, and Nd₂O₃, which can also keep the same crystalline phase, distort the crystal lattice, and improve the surface area and thermal stability of the solid solution, but their effects are much weaker and they decrease the reducibility of the solid solution. The third group includes BaO and CuO, whose effects on the property of CeO₂–ZrO₂ are much different. BaO and CuO, especially CuO, can decrease the thermal stability, and reduction extent of CeO₂–ZrO₂. CuO-modified CeO₂–ZrO₂ calcined at 550 °C shows the comparable high activity for the methane combustion, but after being calcined at 900 °C, CuO-modified CeO₂–ZrO₂ would separate into three phases as CeO₂, ZrO₂, and CuO, resulting in the much lower activity for the methane catalytic combustion.     

Reactivity of layered vanadium pentoxide hydrate with ultrafine metal oxide, TiO2 and ZrO2, particles in an aqueous system

The interactions between vanadium pentoxide hydrate (V₂O₅·nH₂O) sol and colloid solutions of ultra fine titanium dioxide TiO₂ and zirconium dioxide particles ZrO₂ were studied. When mixed with an intrinsic V₂O₅·nnH₂O sol, TiO₂ particles in the mixed sol are sandwiched by V₂O₅·nH₂O layer sheets to form intercalation compounds. An Interlayer distance of V₂O₅·nH₂O was increased by this treatment and the surface area was also increased from 7.9 m² g^–1 for the V₂O₅·nH₂O to ca. 50 m² g^–1. When the TiO₂ sol was contacted with K-type V₂O₅·nH₂O, microporous nature appeared in the sample and the surface area incrased up to ca. 100 m² g^–1. The porous structure was maintained up to 300°C, above which materials were separated into two phases, anhydrous V₂O₅ and anatase type TiO₂. Ultrafine ZrO₂ particles were intercalated stoichiometrically in both intrinsic and K-type V₂O₅·nH₂O giving ZrO₂-V₂O₅·nH₂O for all the mixing ratios from ZrO₂/V₂O₅ = 5 to 20. Physico-chemical properties were almost unvaried and the materials were nonporous. Their surface areas are around 50 m² g^–1 for the former and around 60 m² g^–1 for the latter. The layered structure was maintained up to 300°C above which the sample was crystallized into ZrV₂O₇. The reaction temperature is about 150°C lower than that the heated mixture of ZrO₂ and V₂O₅ powders. The electron microscope observations of the prepared materials showed that the number of the stacked layers was decreased from more than 10 sheets for the sample before intercalation to about 2–4 sheets by exfoliation. This indicates that V₂O₅·nH₂O is exfoliated by ion exchangeably reacting to ultrafine titanium oxide and zirconium oxide particles.     

Phase relations, lattice thermal expansion in CeO2-Gd2O3 system, and stabilization of cubic gadolinia

The phase relation in CeO₂-Gd₂O₃ system has been established under slow cooled conditions from 1400 °C. Two phase regions, namely F-type cubic and C-type cubic, were observed in this system. A striking observation of this investigation is the stabilization of C-type gadolinia after Ce⁴⁺ substitution, which is attributed to decrease in average cationic size on Ce⁴⁺ substitution at Gd³⁺ site. The lattice thermal expansion behaviour of C-type gadolinia samples was investigated by High temperature-XRD. The lattice thermal expansion coefficient was found to gradually increase on increasing the Ce⁴⁺ content, within the C-type homogeneity range, in Ce_1−xGd_xO_2−x/2 series.     

Acoustic emission study of phase relations in low-Y2O3 portion of ZrO2-Y2O3 system

The (metastable) tetragonal phase in 3–4 mol% Y₂O₃-ZrO₂ alloys undergoes a transition to the monoclinic form in the 200–300 °C temperature range. Microcracking due to the volume change at this transition has been detected in these compositions by sharp acoustic emission during heating. The phase change was confirmed by X-ray diffraction, dilatometry and scanning electron microscopy. The monoclinic tetragonal transition in ZrO₂-1 mol% Y₂O₃ alloy at 850–750 °C and the same phase change in 2, 3, 4 and 6 mol% Y₂O₃ compositions at the eutectoid temperature of about 560 °C was also clearly signalled by the acoustic emission counts during heating and cooling. There was no significant acoustic emission activity on heating and cooling the 9 and 12 mol% Y₂O₃ compositions, which are cubic. The acoustic emission data thus confirm the phase relations in the 1–12 mol% Y₂O₃ region, established by conventional methods such as differential thermal analysis, dilatometry and X-ray diffraction.     

Biological responses of neonatal rat calvarial osteoblasts on plasma-sprayed HA/ZrO2 composite coating

Plasma-sprayed hydroxyapatite (HA) coating, applied to metal substrates, can induce a direct chemical bond with bone and hence achieve a biological fixation of the implant. However, the poor bonding strength between the HA coating and the substrate has been a concern for the orthopedists. In a previous study, the zirconia-reinforced hydroxyapatite composite coatings (HA/ZrO₂) could significantly improve the mechanical strength before and after soaking in simulated body fluid. This study aims to investigate the biological responses of osteoblasts on plasma-sprayed HA/ZrO₂ coating. The osteoblasts derived from neonatal rat calvarial were cultured in Dulbeccos modified Eagle medium (DMEM) with fetal bovine serum (FBS) on the surface of plasma-sprayed HA coating, HA/ZrO₂ coating, and ZrO₂ coating, respectively. The biological responses were investigated by the cell growth (1, 3, 5, and 10 days) and the cell morphology under scanning electron microscopy (SEM) (3, 6, 12, 24 and 48 h). Examination by SEM revealed that osteoblasts on HA coatings exhibit less spreading during the medium phase (6 and 12 h), while, better morphologies were observed at the latter phases (24 and 48 h). This should be derived by the dissolution of HA coating in the culture medium. On HA/ZrO₂ coating, the cells showed the poor morphologies at the latter phases (24 and 48 h). This could be explained by the no apatite formed at the surface HA/ZrO₂ coating after soaking in simulated body fluid. The lower contents of ZrO₂ coating in HA coating and the addition of other solid solution (ZrO₂–MgO, CaO–ZrO₂, ZrO₂–CeO₂) in HA coating are the two possible methods to improve the cytocompatibility of HA/ZrO₂ coating.