Synthesis of Highly Porous Yttria-Stabilized Zirconia by Tape-Casting Methods

Porous ceramics of Y₂O₃-stabilized ZrO₂ (YSZ) were prepared by tape-casting methods using both pyrolyzable pore formers and NiO followed by acid leaching. The porosity of YSZ wafers increased in a regular manner with the mass of graphite or polymethyl methacrylate (PMMA) to between 60% and 75% porosity. SEM indicated that the shape of the pores in the final ceramic was related to the shape of the pore formers, so that the pore size and microstructure of YSZ wafers could be controlled by the choice of pore former. Dilatometry measurements showed that measurable shrinkage started at 1300 K, and a total shrinkage of 26% was observed, independent of the amount or type of pore former used. Temperature-programmed oxidation (TPO) measurements on the green tapes demonstrated that the binders and dispersants were combusted between 550 and 750 K, that PMMA decomposed to methyl methacrylate between 500 and 700 K, and that graphite combusted above 900 K. The porosity of YSZ ceramics prepared by acid leaching of nickel from NiO–YSZ, with 50 wt% NiO, was studied as a function of NiO and YSZ particle size. Significant changes in pore dimension were found when NiO particle size was changed.     

Reduction and Sintering of a Nickel–Dispersed-Alumina Composite and Its Properties

High-density nickel–dispersed-alumina (Al₂O₃/nickel) composites with superior mechanical properties were obtained by the hydrogen reduction and the hot pressing of alumina–nickel oxide (Al₂O₃/NiO) mixed powders. The mixtures were prepared by using NiO or nickel nitrate (Ni(NO₃)₂· n H₂O) as a dispersion source of nickel metal. Microstructural investigations of the composite fabricated using nitrate powder revealed that fine nickel particles, } 100 nm in diameter, dispersed homogeneously at the matrix grain boundaries, forming the intergranular nanocomposite. High strength (.1 GPa) and high-temperature hardness were registered for the composite that contained a small amount of nickel dispersion. The ferromagnetic properties of nickel, such as high coercive force, were observed, because of the fine magnetic dispersions, which indicates a functional value of structural composites.     

Dissolution and Reaction of Yttria-Stabilized Zirconia Single Crystals in Hydrothermal Solutions

Dissolution and reaction of yttria-stabilized zirconia (YSZ) single crystals were investigated in various solutions at 600° to 780°C under 100 MPa. YSZ crystals were not corroded in pure H₂O and neutral solutions such a LiF, LiCl, NaNO₃, KCl, KBr; K₂SO₄, and Na₂SO₄ even under severe conditions at 600°C, 100 MPa. They were, however, dissolved and reprecipitated in basic solutions such as NaF, K₂CO₃, KOH, NaOH, and LiOH with partial decompositon (destabilization in the last three solutions. YSZ crystals were completely decomposed into m -ZrO₂ in acidic solutions of Li₂SO₄, H₂SO₄, and HCl, whereas they reacted with the solution and formed other compounds in KF, NH₄F, and H₃PO₄ solutions.     

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.     

Uniformly Porous Al2O3/LaPO4 and Al2O3/CePO4 Composites with Narrow Pore-Size Distribution

Porous Al₂O₃/20 vol% LaPO₄ and Al₂O₃/20 vol% CePO₄ composites with very narrow pore-size distribution at around 200 nm have been successfully synthesized by reactive sintering at 1100°C for 2 h from RE₂(CO₃)₃· x H₂O (RE = La or Ce), Al(H₂PO₄)₃ and Al₂O₃ with LiF additive. Similar to the previously reported UPC-3Ds (uniformly porous composites with a three-dimensional network structure, e.g. CaZrO₃/MgO system), decomposed gases in the starting materials formed a homogeneous open porous structure with a porosity of ∼40%. X-ray diffraction, ³¹P magic-angle spinning nuclear magnetic resonance, scanning electron microscopy, and mercury porosimetry revealed the structure of the porous composites.     

Solubility and Transport of NiO Cathodes in Molten Carbonate Fuel Cells

Equilibrium NiO solubility measurements were made in Li₂CO₃/K₂CO₃ mixtures as a function of temperature, ambient gas environment, and salt composition for gases containing 3.1% H₂O. The equilibrium solubility was found to increase with increasing temperature, CO₂ partial pressure, and cation fraction Li⁺ and to decrease slightly with increasing O₂ partial pressure. These results were coupled with theoretical predictions of (1) O₂, H₂, and CO₂ activities and (2) compositional gradients which develop across the electrolyte structure of an operating carbonate fuel to explain the two distinct regions of nickel precipitation across the fuel cell. Precipitation occurs near the cathode because NiO solubility decreases commensurate with electrolyte compositional gradients, whereas the second region deeper within the electrolyte matrix results from a decreased O₂ activity.     

Fabrication of Long Lengths of Epitaxial Buffer Layers on Biaxially Textured Nickel Substrates Using a Continuous Reel-to-Reel Dip-Coating Unit

A low-cost, nonvacuum, solution precursor route has been developed to produce epitaxial oxide buffer layers of Eu₂O₃ or La₂Zr₂O₇ on biaxially textured Ni (100) tapes. A reel-to-reel continuous dip-coating unit consisting of a constant-tension tape transport system attached to a controlled atmosphere furnace was fabricated. Nickel tapes were pulled through a 2-methoxyethanol solution of europium methoxyethoxide/acetate or lanthanum zirconium methoxyethoxide. The double-sided dip-coated tapes were then annealed in a preheated furnace at 1000°–1100°C with a high flow rate of Ar/H₂ (4%) gas. The dip-coated buffers were dense, continuous, crack-free, and epitaxial with a single cube texture. A critical current ( J _c) of >1 MA/cm² at 77 K and self-field was obtained for YBa₂Cu₃O_7-δ (YBCO) films with a layer sequence of YBCO ( ex situ BaF₂ process)/CeO₂ (sputtered)/YSZ (sputtered)/Eu₂O₃ (dip-coated)/nickel. We have produced 1–2 m lengths of epitaxial buffer layers on textured nickel substrates using a nonvacuum process for the first time.     

Novel Method for Preparing Nickel/YSZ Cermet by a Vapor-Phase Process

A novel method for preparing a nickel/YSZ (yttria-stabilized zirconia) cermet has been developed. In the present work, YSZ was deposited onto a substrate containing NiO as an oxygen source by allowing it to undergo a phase reaction in a vapor mixture of ZrCl₄ and YCl₃. In the initial stage of the reaction, a porous YSZ layer was deposited onto an NiO pellet by chemical vapor deposition (CVD). The porous layer became finer and more gas-tight as the CVD reactions proceeded, and the process then changed from CVD to electrochemical vapor deposition (EVD), resulting in the growth of a dense YSZ layer on the NiO pellet.     

Zirconia/Alumina Functionally Gradiented Composites by Electrophoretic Deposition Techniques

An electrophoretic deposition and sintering route was used to prepare YSZ/Al₂O₃ composites with a compositional gradient. The YSZ content was continuously decreased from the YSZ-rich surface to the Al₂O₃-rich surface, Microstructural and Vickers hardness (16–24 GPa) evidence tracked the compositional development, and the indentation fracture toughness was found to vary across the section (10–3 MPa·m^1/2).     

Mechanism of Reaction between Lanthanum Manganite and Yttria-Stabilized Zirconia

The reaction of La_{1- x} Ca _x MnO₃ ( x = 0, 0.1, 0.2) with ZrO₂-8 mol% Y₂O₃ (YSZ) has been investigated at temperatures ranging from 1300° to 1425°C in air. Substitution of Ca for La in LaMnO₃ depresses the reactivity with YSZ. A layer of La₂Zr₂O₇ is formed at the La_{1- x} Ca _x MnO₃/YSZ interface after an induction period, and its formation is accelerated when the La_{1- x} Ca _x MnO₃ phase is porous. The reaction proceeds by unidirectional diffusion of La, Mn, and/or Ca ions, mainly Mn ions, into YSZ. The diffusion coefficients of La and Mn ions in YSZ, which are estimated using a LaMnO₃/single-crystal YSZ couple, are much lower than that of oxygen ion. From the experimental data, a reaction mechanism is proposed.     

Fabrication of Silicon Carbide Whisker/Alumina Composite by Thermal-Gradient Chemical Vapor Infiltration

SiC( w )/Al₂O₃ composites were made from an AlCl₃-H₂-CO₂ mixture by a thermal-gradient chemical vapor infiltration (CVI) method. Al₂O₃ was deposited from the reaction of AlCl₃ and H₂O, which was produced from the oxidation of H₂ by CO₂. The densification rate was measured at various reactant compositions and total pressures. When the reaction rate or total pressure increased, the rate-controling step shifted from H₂O production to AlCl₃ diffusion, which led to premature pore closing. To obtain dense composites in a short infiltration time, the diffusion rate of AlCl₃ had to be increased by decreasing the total pressure.     

Relationship between the Mesoporous Intermediate Layer Structure and the Gas Permeation Property of an Amorphous Silica Membrane Synthesized by Counter Diffusion Chemical Vapor Deposition

An amorphous silica membrane with an excellent hydrogen/nitrogen (H₂/N₂) permselectivity of >10 000 and a He/H₂ permselectivity of 11 was successfully synthesized on a γ-alumina (γ-Al₂O₃)-coated α-alumina (α-Al₂O₃) porous support by counter diffusion chemical vapor deposition using tetramethylorthosilicate and oxygen at 873 K. An amorphous silica membrane possessed a high H₂ permeance of >1.0 × 10⁻⁷ mol·(m²·s·Pa)⁻¹ at ≥773 K. The dominant permeation mechanism for He and H₂ at 373–873 K was activated diffusion. On the other hand, that for CO₂, Ar, and N₂ at 373–673 K was a viscous flow. At ≥673 K, that for CO₂, Ar, and N₂ was activated diffusion. H₂ permselectivity was markedly affected by the permeation temperature, thickness, and pore size of a γ-Al₂O₃ mesoporous intermediate layer.     

Effect of Alumina on Sintering Behavior and Electrical Conductivity of High-Purity Yttria-Stabilized Zirconia

The sintering behavior and electrical conductivity of high-purity 8-mol% Y₂O₃-stabilized ZrO₂ (8YSZ) with Al₂O₃ additions were investigated. The addition of 1 wt% AI₂O₃ to 8YSZ provided dense, sintered samples with 9.1% relative density at 1400°C without a holding time. Addition of 1 wt% SiO₂ enhanced the sinterability of 8YSZ. Na₂O addition of 0.1 wt% remarkably lowered it. Electrical conductivity at 1000°C in air increased slightly with increased Ai₂O₃ content up to 1 wt% and then monotonously decreased. 8YSZ with 1 wt% AI₂O₃ showed the maximum conductivity of 0.16 S/cm at 1000°C.     

Preparation of Yttria-Stabilized Zirconia Thin Films on Strontium-Doped LaMnO3 Cathode Substrates via Electrophoretic Deposition for Solid Oxide Fuel Cells

A yttria-stabilized zirconia (YSZ) thin film on an La_0.8Sr_0.2MnO₃ porous cathode substrate was prepared, using electrophoretic deposition (EPD) to fabricate a solid oxide fuel cell (SOFC). The electrical conductivity of an La_0.8Sr_0.2MnO₃ substrate is satisfactorily high at room temperature; therefore, YSZ powder could be deposited electrophoretically onto an La_0.8Sr_0.2MnO₃ substrate without any extra surface treatment, such as a metal coating. Successive repetition of EPD and sintering was required to obtain a film without gas leakage, because of the thermal expansion coefficient mismatch between the YSZ and the La_0.8Sr_0.2MnO₃ substrate. On the other hand, the electromotive force of the oxygen concentration in the cell that used YSZ film prepared via EPD increased and attained the theoretical value when the number of deposition and calcination cycles was increased. Six or more successive repetitions were required to obtain a YSZ film without gas leakage. A planar-type SOFC was fabricated, using nickel as the anode and YSZ film (∼10 μm thick) that had been deposited onto the La_0.8Sr_0.2MnO₃ substrate as the electrolyte and cathode. The cell exhibited an open circuit voltage of 1.0 V and a maximum power density of 1.5 W/cm². Thus, the EPD method could be used as a colloidal process to prepare YSZ thin-film electrolytes for SOFCs.     

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.     

Fabrication of NiO Nanoparticle-Coated Lead Zirconate Titanate Powders by the Heterogeneous Precipitation Method

NiO nanoparticle-coated lead zirconate titanate (PZT) powders are successfully fabricated by the heterogeneous precipitation method using PZT, Ni(NO₃)₂·6H₂O, and NH₄HCO₃ as the starting materials. The amorphous NiCO₃·2Ni(OH)₂·2H₂O are uniformly coated on the surface of PZT particles. XRD analysis and the selected-area diffraction (SAD) pattern indicate that the amorphous coating layer is crystallized to NiO after being calcined at 400°C for 2 h. TEM images show that the NiO particles of ∼8 nm are spherical and weakly agglomerated. The thickness of the nanocrystalline NiO coating layer on the surface of PZT particle is ∼30 nm.     

India as a Hot Corrosion-Resistant Stabilizer for Zirconia

After showing that india (In₂O₃) resisted high-temperature reaction with SO₃/Na₂SO₄ and vanadate melts, we prepared india-stabilized zirconia (ISZ) by a proprietary sol–gel process, and tested the material for corrosion resistance to 700–900°C molten vanadates. ISZ was superior to yttria-stabilized zirconia (YSZ) in vanadate resistance at 700°C, and essentially equivalent at 900°C. Certain differences were observed between the vanadate-induced corrosion/destabilization of ISZ and that of YSZ.     

Phase Equilibria and Physical Properties of Oxygen-Deficient Zirconia and Thoria

The compositions of anion-deficient zirconia and thoria in equilibrium with O₂ were measured from 1 to 10⁻⁶ atm and 1400° to 1900°C; for ZrO_{2- x} (po₂ in atm, and T in °K), log x∼−0.890-[(0.400×10⁴)/ T ]-[(log p )/6]; for ThO_{2- x} , log x∼−1.870-[(0.340×10⁴)/ T ]-[(log p )/6]. The ZrO_{2- x} -Zr boundary was located at x=0.014 at 1800°C; thoria was single-phase over the entire range. Consistent results were obtained when O₂/inert gas mixtures were used, but use of H₂/H₂O and CO/CO₂ at 1000° to 1200°C gave abnormal and, in the latter case, erratic data; side reactions in these atmospheres are inferred. The monoclinic-tetragonal phase change of ZrO₂ and the lattice thermal expansion, room-temperature Young's modulus, and strength properties of ZrO₂ and ThO₂ bodies were not appreciably altered by oxygen deficiency. The lattice dimensions decreased slightly with departure from stoichiometry.     

Influence of TiO2 Solid Solution on the Thermal Property and Ionic Conductivity Of Partially Stabilized Zirconia

TiO₂(0–20 mol%)-3 mol% yttria-stabilized zirconia (3YSZ) ceramics were prepared by a solid-state reaction. With increasing TiO₂ content in 3YSZ, the structure of the main phase changed from a monoclinic, tetragonal, and cubic mixture to a tetragonal single phase. Increasing TiO₂ content in 3YSZ caused an increase in the average grain size of these ceramics. The thermal conductivity decreased from 4.1 to 2.1 at room temperature with an increase in the TiO₂ content. The specific heat of non-TiO₂-doped 3YSZ was slightly larger than all the doped TiO₂–3YSZ at room temperature. When the TiO₂ content was >8 mol% in 3YSZ, no abrupt expansion, shrinkage, or cracks were observed on heating and cooling these samples; thus, the thermal stability of 3YSZ was improved by TiO₂ solid solution. The ionic conductivity of the samples decreased with increasing TiO₂ solid solution.     

Mechanism of Thermal Transport in Zirconia and Yttria-Stabilized Zirconia by Molecular-Dynamics Simulation

We present results of molecular-dynamics simulations of the thermal conductivity, κ, of ZrO₂ and Y₂O₃-stabilized ZrO₂ (YSZ). For both pure ZrO₂ and YSZ with low concentrations of Y₂O₃, we find that the high-temperature κ is typical of a crystalline solid, with the dominant mechanism being phonon-phonon scattering. With increasing Y₂O₃ concentration, however, the mechanism changes to one more typical of an amorphous system. In particular, phononlike vibrational modes with well-defined wave vectors appear only at very low frequencies. As in amorphous materials, the vast majority of vibrational modes, while delocalized, do not propagate like ordinary phonon modes but transport energy in a diffusive manner. We also find that the few highest frequency modes are localized and do not contribute to κ.