La2Ti2O7 Ceramics

La₂Ti₂O₇ powders were prepared using three different techniques. Single-phase material was obtained at 1150°C by calcination of mixed oxides, at 1000°C by molten salt synthesis, and at 850°C by evaporative decomposition of solutions. Particle sizes and morphologies of the powders differed substantially, as did the sintered microstructures and dielectric properties. Very dense (99%), translucent, grain-oriented lanthanum titanate was fabricated by hot-forging at 1300°C under a 200-kg load. Anisotropy was demonstrated by X-ray diffraction, scanning electron microscopy, thermal expansion, and dielectric measurements.     

Synthesis of Er3+ Doped Y2O3 Nanophosphors

The synthesis and characterization of yttrium hydroxyl carbonate (Y(OH)CO₃²⁻) and yttrium nitrate hydroxide hydrate (Y(OH)NO₃H₂O) precursor materials as well as Y₂O₃ nanoparticles are reported. The resultant precursor particle size is about 10–12 nm with a narrow size distribution by the enzymatic decomposition method, whereas the particle size was smaller than those acquired by the homogeneous and alkali precipitation methods. The formation of Y(OH)CO₃²⁻ and Y(OH)NO₃H₂O species was also evident from the fourier-transform infrared spectrometry (FT-IR) analysis. Precipitated Y(OH)CO₃²⁻ precursors have an amorphous nature whereas Y(OH)NO₃H₂O precursors have a crystalline nature, which was manifested from the XRD analysis. Moreover, precipitated Y(OH)NO₃H₂O precursors were found in the agglomerated form and Y(OH)CO₃²⁻ was established in the monodispersed form, as determined from the FE-SEM, TEM and DLS measurements. It was demonstrated that calcination of precursor materials at 900°C eventually removed the inorganic anions from the precursors and consequently produced crystalline Y₂O₃ nanoparticles, which was evident from the XRD and FT-IR analysis. The EDS analysis confirms Er³⁺ doping in the Y₂O₃ nanoparticles. The morphology and the size of the Y₂O₃ nanoparticles are almost unchanged before and after the calcination.     

Ta2O5/Nb2O5 and Y2O3 Co-doped Zirconias for Thermal Barrier Coatings

Zirconia doped with 3.2–4.2 mol% (6–8 wt%) yttria (3–4YSZ) is currently the material of choice for thermal barrier coating topcoats. The present study examines the ZrO₂-Y₂O₃-Ta₂O₅/Nb₂O₅ systems for potential alternative chemistries that would overcome the limitations of the 3–4YSZ. A rationale for choosing specific compositions based on the effect of defect chemistry on the thermal conductivity and phase stability in zirconia-based systems is presented. The results show that it is possible to produce stable (for up to 200 h at 1000°–1500°C), single (tetragonal) or dual (tetragonal + cubic) phase chemistries that have thermal conductivity that is as low (1.8–2.8W/m K) as the 3–4YSZ, a wide range of elastic moduli (150–232 GPa), and a similar mean coefficient of thermal expansion at 1000°C. The chemistries can be plasma sprayed without change in composition or deleterious effects to phase stability. Preliminary burner rig testing results on one of the compositions are also presented.     

Microstructural Evolution in a ZrO2 Wt% Y2O3 Ceramic

Several unusual microstructural features, i.e., 90° tetragonal ZrO₂ twins containing antiphase domain boundaries, tetragonal ZrO₂ precipitates in a colony morphology, and precipitate-free zones at the perimeter of cubic ZrO₂ grains containing fine tetragonal ZrO₂ precipitates, were observed in a single ZrO₂-12 wt% Y₂O₂ ceramic annealed at 1550°, 1400°, and 1250°C, respectively. The type of phase transformation responsible for each microstructural feature is described.     

Tentative Phase Relationships in the System CaHfTi2O7-Gd2Ti2O7 with up to 15 mol% Additions of Al2TiO5 and MgTi2O5

The phase relationships in the CaHfTi₂O₇-Gd₂Ti₂O₇ (zirconolite-pyrochlore) pseudobinary system were investigated, after heating at 1500°C, because of their importance in the design of pyrochlore-rich titanate ceramics for immobilization of impure surplus plutonium. Up to 15 mol% of MgTi₂O₅ and Al₂TiO₅ were added to CaHfTi₂O₇-Gd₂Ti₂O₇ compositions to elucidate the effects of divalent and trivalent impurities on the phase stability within these systems. From X-ray diffractometry analysis, scanning electron microscopy, and energy dispersive X-ray spectrometry, phase formation and compositional stability limits were evaluated. The main phases observed in these systems were pyrochlore, perovskite, and polytypes of zirconolite. The formation of the 2 M -, 4 M -, and 3 O -zirconolite polytypes was dependent on the amount of aluminum or magnesium present. In the magnesium system, a large area of pyrochlore-only was observed, which indicated that divalent impurities of appropriate ionic size could be readily incorporated in the eightfold site of the pyrochlore. The locations of the tentative phase boundaries are discussed with respect to the chemical composition.     

Oxygen Nonstoichiometry and Defects in Mn-Doped Gd2Ti2O7+x

The oxygen nonstoichiometry in Mn-doped Gd₂Ti₂O₇, Gd₂(Ti_0.975Mn_0.025)₂O_7+x, was measured electrochemically, as a function of temperature and oxygen partial pressure, with the aid of an oxygen titration cell. The analysis of the data shows that the defect equilibrium can be described by considering the dominant point defects to be neutral oxygen interstitials, doubly charged oxygen vacancies, and trivalent and quadrivalent Mn ions substituted in the Ti sites. The enthalpies for the formation of neutral oxygen interstitials and trivalent Mn are determined.     

Eutectic Composition in the Pseudobinary of Y4Al2O9 and Y2O3

The eutectic composition between Y₄Al₂O₉ and Y₂O₃ was determined using electron probe microanalysis (EPMA) on directionally solidified specimens with hypo- and hypereutectic compositions. The microstructures of the specimens as a function of composition differ considerably with small deviation from the eutectic composition (70.5 mol% Y₂O₃ and 29.5 mol% Al₂O₃). Based on the current results and other published data, the pseudobinary system between Al₂O₃ and Y₂O₃ is revised.     

Novel Plutonium Titanate Compounds and Solid Solutions Pu2Ti2O7-Ln2Ti2O7: Relevance to Nuclear Waste Disposal

The compounds SrPu₂Ti₄O₁₂, Pu₂Ti₃O_8.79, and Pu₂Ti₂O₇, where plutonium is in the (III) oxidation state, were prepared and identified via X-ray diffraction (XRD). The solid solubility limit of Pu₂Ti₂O₇ in Ln₂Ti₂O₇ (Ln = Gd, Er, or Lu) was also studied via XRD; it was determined that the solubility of Pu₂Ti₂O₇ increased as the radius of the lanthanide ion in the host compound decreased. Attempts to synthesize Sr₂Pu₂ Ti₅O₁₆ and Sr₂Ce_2–yPu_yTi₅O₁₆ solid solutions, where plutonium is in the (IV) oxidation state, were unsuccessful.     

Effects of B2O3 on the Phase Stability of Ba2Ti9O20 Microwave Ceramic

Preparation of dense and phase-pure Ba₂Ti₉O₂₀ is generally difficult using solid-state reaction, since there are several thermodynamically stable compounds in the vicinity of the desired composition and a curvature of Ba₂Ti₉O₂₀ equilibrium phase boundary in the BaO–TiO₂ system at high temperatures. In this study, the effects of B₂O₃ on the densification, microstructural evolution, and phase stability of Ba₂Ti₉O₂₀ were investigated. It was found that the densification of Ba₂Ti₉O₂₀ sintered with B₂O₃ was promoted by the transient liquid phase formed at 840°C. At sintering temperatures higher than 1100°C, the solid-state sintering became dominant because of the evaporation of B₂O₃. With the addition of 5 wt% B₂O₃, the ceramic yielded a pure Ba₂Ti₉O₂₀ phase at sintering temperatures as low as 900°C, without any solid solution additive such as SnO₂ or ZrO₂. The facilities of B₂O₃ addition to the stability of Ba₂Ti₉O₂₀ are apparently due to the eutectic liquid phase which accelerates the migration of reactant species.     

Photoluminescence Enhancement of (La0.95Eu0.05)2Ti2O7 Nanophosphors via Li+ Doping

Li⁺ ions have been successfully doped into the La sites of (La_0.95Eu_0.05)₂Ti₂O₇ nanocrystals through a facile citric acid sol–gel method. The doping concentration of Li⁺ ions can be as high as 15 mol%. Photoluminescence (PL) performances of the obtained samples have been investigated. The results showed that a doping with small number of Li⁺ ions improves the PL intensity of the synthesized La₂Ti₂O₇:Eu³⁺ nanophosphors. The highest emission intensity was observed using the formula of (La_0.92Eu_0.05Li_0.03)₂Ti₂O₇, whose brightness was increased by almost 20% in comparison with that of (La_0.95Eu_0.05)₂Ti₂O₇.     

Pseudobinary Phase Relations of Li2Ti3O7

The Li₂O-TiO₂ pseudobinary phase diagram was determined from 50 to 100 mol% TiO₂ by DTA, microscopy, and X-ray analysis; Li₂Ti₃O₇ effectively melts congruently at 1300° and decomposes eutectoidally at 940°C. A solid solution based on Li₂TlO₃ from 50 to ∼65 mol% TiO₃ was observed to exist at >930°C. A new metastable phase was discovered with a composition of ∼75 mol% TiO₂ and with a hexagonal unit cell (8.78 by 69.86 × 10⁻¹nm). Discrepancies in the literature regarding some of these phase equilibria are reconciled.     

High-Temperature Phase Relations in the System Y2O3-Ta2O5

The phase relations for the system y₂o₃–Ta₂o₅ in the composition range 50 to 100 mol% Y₂O₃ have been studied by solid-state reactions at 1350°, 1500°, or 17000C and by thermal analyses up to the melting temperatures. Weberite-type orthorhombic phases (W2 phase, space group C222₁), fluorite-type cubic phases (F phase, space group Fm3m )and another orthorhombic phase (O phase, space group Cmmm )are found in the system. The W2 phase forms in 75 mol% Y₂O₃ under 17000C and O phase in 70 mol% Y₂O₃ up to 1700°C These phases seem to melt incongruently. The F phase forms in about 80 mol% Y₂O₃ and melts congruently at 2454° 3°C. Two eutectic points seem to exist at about 2220°C 90 mol% Y²O₃, and at about 1990°C, 62 mol% Y₂O₃. A Phase diagram including the above three phases were not identified with each other.     

New Red-Shade Ceramic Pigments Based on Y2Sn2−xCrxO7−δ Pyrochlore Solid Solutions

New high-stability red-shade ceramic pigments based on pyrochlore solid solutions Y₂Sn_{2− x} Cr _x O_7−δ ( x = 0–1, 0 < δ= 0–1/2 x ) were developed employing conventional solid-state reaction synthesis. The relationship between their optical properties and microstructure was analyzed by X-ray diffractometry and ultraviolet–visible-light spectroscopy. Decomposition of the structure with formation of the perovskite YCrO₃ observed for chromium substitutions x ≥ 5 was attributed mainly to high concentration of defects. The structural and optical parameters of the materials were found to correlate with the concentration of a species with higher oxidation state than Cr(III), such as Cr(IV) or Cr(V).     

Cation Self-Diffusion in Polycrystalline Y2O3 and Er2O3

A tracer sectioning technique was used to measure cation self-diffusion coefficients in fully dense polycrystalline Y_aO₃ and Er₂O_s under oxidizing conditions. The results are described by the relations for Y₂O₃ (1400° to 1670°C), and for Er₂O₃ (1400° to 1700°C). The greater activation energy for erbium diffusion in erbia may be partly attributable to a mass effect.     

Phase Stability and Electrical Conductivity in Gd2Ti2O7-Gd2Mo2O7 Solid Solutions

Solid solutions in the Gd₂(Ti1-yMoy)2O7 pyrochlore system, with y = 0.1, 0.3, 0.5, and 0.7, have been investigated as potential candidates for the anode material of solid-oxide fuel cells. The electrical conductivity, stability range, and optimal synthesis conditions of the pyrochlore phase have been explored by electrical, coulometric titration, and X-ray diffractometry techniques. As the molybdenum content increases, the maximum conductivity increases to a value of 70 S/cm for y = 0.7 at 1000°C, whereas the stability range in the oxygen partial pressure (pO2) decreases to just over two orders of magnitude in pO2 in the temperature range of 600°-1000°C. A decomposition reaction that leads to decreased conductivity in oxidizing atmospheres is proposed.     

Effect of Ta2O5, Nb2O5, and HfO2 Alloying on the Transformability of Y2O3-Stabilized Tetragonal ZrO2

The addition of Ta₂O₅, Nb₂O₅, and HfO₂ enhanced the transformability of Y₂O₃-stabilized tetragonal ZrO₂ polycrystal (Y-TZP), which was indicated by an increase in phase transformation temperatures and fracture toughness of Y-TZP. Comparison of the alloying effects of these oxides on the transformability and crystal structure of Y-TZP suggested that an alloying oxide which increases the c/a axial ratio (tetragonality) of TZP also increases the transformability. Empirical equations to predict the tetragonality are proposed. Calculated tetragonalities showed good agreement with measured values in the systems ZrO₂-Y₂O₃-Ta₂O₅, -Nb₂O₅, and -HfO₂.