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.     

Mineralizing Effect of Li2B4O7 and Na2B4O7 on Magnesium Aluminate Spinel Formation

Lithium borate (Li₂B₄O₇) and sodium borate (Na₂B₄O₇) mineralize spinel formation from stoichiometric MgO and Al₂O₃ between 1000° and 1100°C. Mineralization with both compounds is shown to be mediated by B-containing liquids which form glass on cooling. However, the liquid compositions depend on the type of mineralizer and temperature, suggesting that templated grain growth or dissolution–precipitation mechanisms are operating, one dominating over the other under certain conditions. Na₂B₄O₇-mineralized compositions show predominantly templated grain growth at 1000°C, which changes to dissolution–precipitation at 1100°C, whereas Li₂B₄O₇-mineralized compositions show dissolution–precipitation from 1000°C. Li₂B₄O₇ is a stronger mineralizer as spinel formation is complete with 3 wt% Li₂B₄O₇ at 1000°C and with ≥1.5 wt% addition at 1100°C, whereas Na₂B₄O₇-mineralized compositions are found to retain some unreacted corundum even at 1100°C.     

Hot Corrosion Mechanism of Composite Alumina/Yttria-Stabilized Zirconia Coating in Molten Sulfate–Vanadate Salt

In order to improve the hot corrosion resistance of yttria-stabilized zirconia (YSZ), an Al₂O₃ overlay has been deposited on the surface of YSZ by electron-beam physical vapor deposition. Hot corrosion tests have been performed on the YSZ coatings with and without an Al₂O₃ overlay in the molten salt mixture (Na₂SO₄+0–15 wt% V₂O₅) at 950°C. The presence of V₂O₅ in the molten salt exacerbates degradation of both the monolithic YSZ coating and the composite YSZ/Al₂O₃ system. The formation of a low-melting Na₂O–V₂O₅–Al₂O₃ liquid phase is responsible for degradation of the Al₂O₃ overlay. The Al₂O₃ overlay acts as a barrier against the infiltration of the molten salt into the YSZ coating during exposure to the molten salt mixture with <5 wt% vanadate.     

Space Charge-Mediated Ionic Transport in Yttria-Stabilized Zirconia–Alumina Composite Membranes

This paper reports ionic conductivity of yttria-stabilized zirconia (YSZ)–Al₂O₃ composite membranes. The tape cast specimens were subjected to binder burnout (500°C) and sintering (1550°C) processes to obtain 200–300 μm thick membranes. The ionic conductivity and microstructure of the membranes were characterized and are discussed in this paper. The ionic conductivity of the composite specimens was enhanced and was correlated with the number of charge carrier and their mobility. The solubility of Al₂O₃ in YSZ was minimal and nanosize Al₂O₃ of the batch sintered into microsize and existed as a distinct phase. The scanning electron microscopy micrographs revealed that YSZ and Al₂O₃ grains were strained.     

Reactions and Microstructure Development in Mullite Fibers

Microstructural and compositional changes during heat treatment of sol–gel-derived mullite fibers with additions of 2 wt% B₂O₃, 2 wt% P₂O₅, 2 wt% Cr₂O₃, and (1 wt% P₂O₅+ 1 wt% Cr₂O₃) were compared with those of undoped mullite fibers. For all compositions the sequence of phase development was the crystallization of a spinel phase (†-Al₂O₃ or Al–Si spinel) from amorphous material, followed by the formation of mullite at higher temperatures. Differential thermal analysis showed that additions of B₂O₃ and P₂O₅ increased the temperature of spinel formation and that B₂O₃ significantly decreased the temperature of mullite formation. After 1 h at 1200°C, the size of mullite grains in fibers that contained B₂O₃ was less than 1000 Å the grains in fibers of other compositions were 6000 to 12000 Å. After 60 h at 1400°C, fibers modified with B₂O₃ had a grain size less than 2000 to 3000 Å the grains in fibers of other compositions were 6000 to 12000 Å. B₂O₃ was the most volatile additive.     

Chemical Interactions between La-Sr-Mn-Fe-O-Based Perovskites and Yttria-Stabilized Zirconia

Orthoferrite-based perovskites are of interest as materials for the cathode in solid oxide fuel cells (SOFCs). Therefore, the chemical compatibility between perovskites of the composition (La_1−xSr_x)_zFe_1−yMn_yO_3−δ (0 # x # 0.3; 0.2 # y # 1; z = 0.90, 0.95, 1.00) and the solid electrolyte zirconia (ZrO₂) doped with 8 mol% yttria (Y₂O₃) (8YSZ) has been investigated. Powder mixtures of the two materials have been annealed at different temperatures. The formation of monoclinic ZrO₂ at 1000°C, as well as of La₂Zr₂O₇ and SrZrO₃ at 1400°C, has been determined in some samples. The reactions that are observed are discussed, with respect to the thermodynamic activities, tolerance factor, and oxygen-ion migration energies. Some perovskite compositions seem to be compatible with Y₂O₃-stabilized ZrO₂ (YSZ), thereby offering the possibility to use orthoferrite-based perovskites in SOFCs with a solid electrolyte made of YSZ.     

Scavenging of Siliceous Grain-Boundary Phase of 8-mol%-Ytterbia-Stabilized Zirconia without Additive

The grain-boundary conductivity (ς_gb) of 8-mol%-ytterbia-stabilized zirconia increased markedly with heat treatment between 1000° and 1300°C with a slow heating rate (0.1°C/min) before sintering. The extent of the ς_gb improvement was the same or larger than that via Al₂O₃ addition. The heat treatment did not affect the grain-interior conduction when sintered at 1600°C, while Al₂O₃-derived scavenging significantly did, given the larger increment of total conductivity in the heat-treated sample. The formation of a silicon-containing phase in a discrete form was suggested as a possible route of scavenging the resistive phase from the correlation between average grain size and ς_gb.     

Alumina Thin Films as Optical Waveguides

Amorphous AI₂O₃, fabricated by reactive rf magnetron sputtering, has been evaluated as a planar waveguide material. The microstructure and optical properties of planar waveguides were examined as a function of deposition (substrate temperature, O₂ flow rate) and annealing conditions. X-ray and electron diffraction verified that as-deposited films were amorphous for substrate temperatures up to 500°C and for a wide range of O₂ flow rates. This amorphous phase was stable through anneals up to 800°C, but crystallized to " y -Al₂O₃ at 1000°C and to α-Al₂O₃ at 1200°C. The amorphous films had transmission windows that extended from 200 nm to 7 (Am with an average refractive index of 1.65 and reproducible losses as low as ∼1 dB/cm at 632.8 nm. The refractive index increased with substrate temperature, but was independent of O₂ flow rate. The losses decreased with substrate temperature and increased as a function of O₂ flow rate. As a final check on the amorphous structure, Cr-doped films were prepared by codeposition. Fluoresence was detected only in annealed crystalline films.     

Imaging Secondary-Ion Mass Spectroscopy Observation of the Scavenging of Siliceous Film from 8-mol%-Yttria-Stabilized Zirconia by the Addition of Alumina

The scavenging of a resistive siliceous phase via the addition of Al₂O₃ was studied, using imaging secondary-ion mass spectroscopy (SIMS), given the improved grain-boundary conductivity in 8-mol%-yttria-stabilized zirconia (8YSZ). The grain-boundary resistivity in 8YSZ decreased noticeably with the addition of 1 mol% of Al₂O₃. Strong SiO₂ segregation at the grain boundaries was observed in a SIMS map of pure 8YSZ that contained 120 ppm of SiO₂ (by weight). The addition of 1 mol% of Al₂O₃ caused the SiO₂ to gather around the Al₂O₃ particles. The present observations provided direct and visual evidence of SiO₂ segregation at the grain boundaries (which had a deleterious effect on grain-boundary conductivity) and the scavenging of SiO₂ via Al₂O₃ addition.     

Electrical Conduction in Single-Crystal and Polycrystalline Al2O3 at High Temperatures

The electrical conductivity and ion/electron transference numbers in Al₃O₃ were determined in a sample configuration designed to eliminate influences of surface and gas-phase conduction on the bulk behavior. With decreasing O₂ partial pressure over single-crystal Al₂O₃ at 1000° to 1650°C, the conductivity decreased, then remained constant, and finally increased when strongly reducing atmospheres were attained. The intermediate flat region became dominant at the lower temperatures. The emf measurements showed predominantly ionic conduction in the flat region; the electronic conduction state is exhibited in the branches of both ends. In pure O₂ (1 atm) the conductivity above 1400°C was σ≃3×10³ exp (–80 kcal/ RT ) Ω⁻¹ cm⁻¹, which corresponds to electronic conductivity. Below 1400°C, the activation energy was <57 kcal, corresponding to an extrinsic ionic condition. Polycrystalline samples of both undoped hot-pressed Al₂O₃ and MgO-doped Al₂O₃ showed significantly higher conductivity because of additional electronic conduction in the grain boundaries. The gas-phase conduction above 1200°C increased drastically with decreasing O₂ partial pressure (below 10⁻¹⁰ atm).     

Studies in Lithium Oxide Systems: XII, Li2O-B2O3-Al2O3

Tentative phase relations in the binary system BnOa-A1₂O₃ are presented as a prerequisite to the understanding of the system Li₂O-B₂O₃-Al₂O₃. Two binary compounds, 2A1₂O₃.B₂O₃ and 9A1₂O₃.-2B₂O₃, melted incongruently at 1030° f 7°C and about 144°C, respectively. Two ternary compounds were isolated, 2Li₂O.A1₂O₃.B₂O₃ and 2Li₂O. 2AI₂O₃. 3B₂0₃. The 2:1:1 compound gave a melting reaction by differential thermal analysis at 870°± 20° C, but the exact nature of the melting behavior was not determined. The 2:2:-3 compound melted at 790°± 20° C to Li_zO.-5Al₂O₃ and liquid. X-ray diffraction data for the compounds are presented and compatibility triangles are shown.     

Reducing the Sintering Temperature for MgO–Al2O3Mixtures by Addition of Cryolite (Na3AlF6)

The preparation of near stoichiometric spinel and alumina-rich spinel composites from Al₂O₃and MgO powders with the addition of Na₃AlF₆up to 4 wt% in the temperature range 700°–1600°C was studied; 98 wt% spinel containing 72 wt% Al₂O₃can be produced from the mixture of 72 wt% (50 at.%) Al₂O₃+ 28 wt% (50 at.%) MgO powders with the addition of 1 wt% Na₃AlF₆fired at 1300°C for 1 h. Spinels containing 81–85 wt% Al₂O₃can be produced from either the mixture of 90 wt% (78 at.%) Al₂O₃+ 10 wt% (22 at.%) MgO or the mixture of 95 wt% (88 at.%) Al₂O₃+ 5 wt% (12 at.%) MgO powders with the addition of 4 wt% Na₃AlF₆in the temperature range 1300°–1600°C by using a torch-flame firing for 3 min, followed by quenching in water, while the same system under slow cooling in a furnace results in spinel containing 74–76 wt% Al₂O₃. Microscopic studies indicate that the alumina-rich spinel composites consist of a continuous majority spinel phase and an isolated minority corundum phase, regardless of slow cooling in a furnace or quenching in water.     

Suppression of Rhombohedral-Phase Appearance and Low-Temperature Sintering of Scandia-Doped Cubic-Zirconia

Inhibition of cubic-rhombohedral phase transformation and low-temperature sintering at 1000°C were achieved for 10-mol%-Sc₂O₃-doped cubic-ZrO₂ by the presence of 1 mol% Bi₂O₃. The powders of 1-mol%-Bi₂O₃–10-mol%-Sc₂O₃-doped ZrO₂ were prepared using a hydrolysis and homogeneous precipitation technique. No trace of rhombohedral-ZrO₂ phase could be detected, even after sintering at 1000°–1400°C. The average grain size of the ZrO₂ sintered at 1200°C was >2 μm because of grain growth in the presence of Bi³⁺. Cubic, stabilized Bi-Sc-doped ZrO₂ sintered at 1200°C had sufficient conductivity at 1000°C (0.33 S/cm) to be used as an electrolyte for a solid-oxide fuel cell (SOFC) and at 800°C (0.12 S/cm) for an intermediate-temperature SOFC.     

Effect of CaO Content on the Hot Strength of Alumina-Spinel Castables in the Temperature Range of 1000° to 1500°C

The modulus of rupture of Al₂O₃-spinel castables containing 20 wt% Al₂O₃-rich MgO-Al₂O₃ spinel and 1.36-2.04 wt% CaO generally increases with an increase in both CaO content and temperature from 1300° to 1500°C, but it remains virtually constant from 1000° to 1300°C. Microscopic observation of the castable fired at 1500°C for 3 h reveals the growth of some CA₆ crystals out of the Al₂O₃-rich spinel grains in the bonding matrix of the castable. The bond linkage between the CA₆ and spinel grains in the matrix is believed to cause both the CaO content and temperature dependence of the hot strength of the Al₂O₃-spinel castables as well as the hot strength enhancement of high-Al₂O₃ castables with addition of Al₂O₃-rich spinel.     

Grain Growth of ZnO in ZnO-Bl2O3 Ceramics with Ai2O3 Additions

Grain growth of ZnO during liquid-phase sintering of a ZnO-6 wt% Bi₂O₃ ceramic was investigated for A1₂O₃ additions from 0.10 to 0.80 wt%. Sintering in air for 0.5 to 4 h at 900° to 1400°C was studied. The AI₂O₃ reacted with the ZnO to form ZnAl₂O₄ spinel, which reduced the rate of ZnO grain growth. The ZnO grain-growth exponent was determined to be 4 and the activation energy for ZnO grain growth was estimated to be 400 kJ/mol. These values were compared with the activation parameters for ZnO grain growth in other ceramic systems. It was confirmed that the reduced ZnO grain growth was a result of ZnAl₂O₄ spinel particles pinning the ZnO grain boundaries and reducing their mobility, which explained the grain-growth exponent of 4. It was concluded that the 400 kJ/mol activation energy was related to the transport of the ZnAl₂O₄ spinel particles, most probably controlled by the diffusion of O^2- in the ZnAl₂O₄ spinel structure.     

High-Temperature Strength and Fracture Toughness of Y2O3-Partially-Stabilized ZrO2/Al2O3 Composites

The temperature dependence of bending strength, fracture toughness, and Young's modulus of composite materials fabricated in the ZrO₂ (Y₂O₃)-Al₂O₃ system were examined. The addition of A1203 enhanced the high-temperature strength. Isostatically hot-pressed, 60 wt% ZrO₂ (2 mol% Y₂O₃)/40 wt% Al₂O₃ exhibited an extremely high strength, 1000 MPa, at 1000°C.     

Reaction Kinetics and Mechanisms between La0.65Sr0.3MnO3 and 8 mol% Yttria-Stabilized Zirconia

The reaction kinetics and mechanisms between 8 mol% yttria-stabilized zirconia (YSZ) and 30 mol% Sr-doped lanthanum manganite (La_0.65Sr_0.30MnO₃, LSM) with A-site deficiency for the application of planar solid oxide fuel cells (SOFCs) were investigated. The LSM/YSZ green tapes were cofired from 1200° to 1400°C for 1 to 48 h and then annealed at 1000°C for up to 1000 h. The results showed that the diffusion of manganese cations first caused the amorphization of YSZ, and then the formation of small La₂Zr₂O₇ (LZ) or SrZrO₃ (SZ) crystals if treated for a longer time at 1400°C. The ambipolar diffusion of the Mn–O pair, transported through the migration of oxygen vacancy, plays an important role in the formation of secondary phases. The diffusion of LSM to YSZ and substitution of Mn for Zr both result in the enhanced concentration of oxygen vacancy, leading to the formation of a void-free zone (VFZ). No additional reaction products in annealed LSM/YSZ specimens, treated at 1000°C for 1000 h, were detected. The interfacial reactions, detailed reaction kinetics, and mechanisms are reported.     

Fabrication and Characterization of Anode-Supported Tubular Solid-Oxide Fuel Cells by Slip Casting and Dip Coating Techniques

High-performance anode-supported tubular solid-oxide fuel cells (SOFCs) have been successfully developed and fabricated using slip casting, dip coating, and impregnation techniques. The effect of a dispersant and solid loading on the viscosity of the NiO/Y₂O₃–ZrO₂ (NiO/YSZ) slurry is investigated in detail. The viscosity of the slurry was found to be minimum when the dispersant content was 0.6 wt% of NiO/YSZ. The effect of sintering temperature on the shrinkage and porosity of the anode tubes, densification of the electrolyte, and performance of the cell at different solid loadings is also investigated. A Ni/YSZ anode-supported tubular cell fabricated from the NiO/YSZ slurry with 65 wt% solid loading and sintered at 1380°C produced a peak power output of ∼491 and ∼376 mW/cm² at 800°C in wet H₂ and CH₄, respectively. With the impregnation of Ce_0.8Gd_0.2O₂ (GDC) nanoparticles, the peak power density increased to ∼1104 and ∼770 mW/cm² at 800°C in wet H₂ and CH₄, respectively. GDC impregnation considerably enhances the electrochemical performance of the cell and significantly reduces the ohmic and polarization resistances of thin solid electrolyte cells.     

Dense Alumina–Zirconia Coatings Using the Solution Precursor Plasma Spray Process

For the first time, dense coatings have been made by the solution precursor plasma spray (SPPS) process. The conditions are described for the deposition of dense Al₂O₃–40 wt% 7YSZ (yttria-stabilized zirconia) coatings; the coatings are characterized and their thermal stability is evaluated. X-ray diffraction analysis shows that the as-sprayed coating is composed of α-Al₂O₃ and tetragonal ZrO₂ phases with grain sizes of 72 and 56 nm, respectively. The as-sprayed coating has a 95.6% density and consists of ultrafine splats (1–5 μm) and unmelted spherical particles (<0.5 μm). The lamellar structure, typical of conventional plasma-sprayed coatings, is absent at the same scale in the SPPS coating. The formation of a dense Al₂O₃–40 wt% 7YSZ coating is favored by the lower melting point of the eutectic composition, and resultant superheating of the molten particles. Phase and microstructural thermal stabilities were investigated by heat treatment of the as-sprayed coating at temperatures of 1000°–1500°C. No phase transformation occurs, and the grain size is still in the nanometer range after the 1500°C exposure for 2 h. The coating hardness increases from 11.8 GPa in the as-coated condition to 15.8 GPa following 1500°C exposure due to a decrease in coating porosity.     

Temperature-Dependent Iron Solubility in Mullite

Sample disks prepared from Al₂O₃ (61 wt%), SiO₂ (28 wt%), and Fe₂O₃(II wt%) powders were sintered at 1270° and 1440°C and then annealed between 1300° and 1670°C. The annealed samples consisted of mullite as the main compound with minor amounts of glass and sometimes magnetite. The iron content of the mullites decreases strongly from ∼ 10.5 wt% Fe₂O₃ at 1300°C to ∼ 2.5 wt% Fe₂O₃ at 1670°C. A complex temperature-controlled exsolution mechanism of iron from mullite is considered.