Hydrothermal Synthesis of Weakly Agglomerated Nanocrystalline Scandia-Stabilized Zirconia

A pure cubic phase of weakly agglomerated, nanocrystalline, 8-mol%-Sc₂O₃-stabilized zirconia (8ScSZ) was synthesized by a two-step hydrothermal treatment in the presence of urea: a stock solution of metal nitrates and urea was heated at 80°C for 48 h and then at 180°C for 72 h. The omission of the first low-temperature treatment resulted in more monoclinic phase. The effects of urea concentration and calcining temperature on the crystallization of the as-synthesized nanocrystalline 8ScSZ are also discussed.     

Crystallite Size and Microstrain of Thermally Aged Low-Ceria- and Low-Yttria-Doped Zirconia

The final phases of zirconia powders depend on the synthesis method employed, and the amounts of stabilizer present. In this study, ceria- and yttria-doped zirconia powders were prepared by urea hydrolysis and subsequent hydrothermal treatment. The amount of tetragonal ( t ) vs monoclinic ( m ) phase in the powders increased with increasing stabilizer content, while the tetragonal phase size decreased and the microstrain of t crystals remained unchanged. The thermal degradation behavior of the metastable t phase in zirconia containing a low CeO₂ or YO_1.5 doping level was explored during aging treatment by means of X-ray line profile broadening analysis. Both ultrafine yttria- and ceria-doped zirconia powder pellets exhibit isothermal t → m transformation after aging at 900°C for various times. It is argued that a crystallite size effect, rather than the dopant valence, dictates the occurrence of the t → m transformation in ultrafine zirconia powders. The change in crystallite sizes of both t and m phases during aging depends significantly on the amount of stabilizer, aging time, and mechanism of t → m phase transformation. However, the change of microstrain in both t and m phases is related to the amount of stabilizers present and the matrix constraints.     

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.     

Crystalline Phase Change in Yttria-Partially-Stabilized Zirconia by Low-Temperature Annealing

Changes in the phase composition and microstructure of yttria-partially-stabilized zirconia by low-temperature annealing were investigated at 100° to 500°C using bodies sintered from coprecipitated fine ZrO₂-Y₂O₃ powders at varied temperatures. Tetragonal zirconia on the surfaces of bodies sintered at <1500°C transformed to the monoclinic phase at 100° to 400°C. Transformation behavior was strongly affected by grain size.     

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.     

Vanadate Hot Corrosion Behavior of India, Yttria-Stabilized Zirconia

Powders of In₂O₃–Y₂O₃"dual stabilized"ZrO₂ (IYSZ) were prepared using sol-gel procedures and tested for resistance to destabilization by molten NaVO₃ at 700° and 900°C. IYSZ powders with stabilizer oxide compositions ranging from 100% down to about 50% In₂O₃ were superior to 100%-Y₂O₃-stabilized ZrO₂ (YSZ) in resistance to destabilization, especially at 700°C. Small additions of Y₂O₃ were speculated to reduce the acidity of the ZrO₂ oxide anion lattice, and, therefore, improve bonding of In₂O₃ (more acidic than Y₂O₃) into the ZrO₂ lattice.     

Subsolidus Phase Equilibria and Ordering in the System ZrO2-Y2O3

The subsolidus phase relations in the entire system ZrO₂-Y₂O₃ were established using DTA, expansion measurements, and room- and high-temperature X-ray diffraction. Three eutectoid reactions were found in the system: ( a ) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y₂O₃ and ∼490°C, ( b ) cubic zirconia solid solutiow→δ-phase Y₄Zr₃O₁₂+hexagonalphase Y₆ZrO₁₁ at 45 mol% Y₂O₃ and ∼1325°±25°C, and ( c ) yttria C -type solid solution→wcubic zirconia solid solution+ hexagonal phase Y₆ZrO₁₁ at ∼72 mol% Y₂O₃ and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y₂O₃ with ideal formula Y₄Zr₃O₁₂, and another, a new hexagonal phase, at 75 mol% Y₂O₃ with formula Y₆ZrO₁₁. They decompose at 1375° and >1750°C into cubic zirconia solid solution and yttria C -type solid solution, respectively. The extent of the cubic zirconia and yttria C -type solid solution fields was also redetermined. By incorporating the known tetragonal-cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO₂-Y₂O₃.     

Zirconium Aluminum Carbides: New Precursors for Synthesizing ZrO2–Al2O3 Composites

ZrO₂–Al₂O₃ nanocrystalline powders have been synthesized by oxidizing ternary Zr₂Al₃C₄ powders. The simultaneous oxidation of Al and Zr in Zr₂Al₃C₄ results in homogeneous mixture of ZrO₂ and Al₂O₃ at nanoscale. Bulk nano- and submicro-composites were prepared by hot-pressing as-oxidized powders at 1100°–1500°C. The composition and microstructure evolution during sintering was investigated by XRD, Raman spectroscopy, SEM, and TEM. The crystallite size of ZrO₂ in the composites increased from 7.5 nm for as-oxidized powders to about 0.5 μm at 1500°C, while the tetragonal polymorph gradually converted to monolithic one with increasing crystallite size. The Al₂O₃ in the composites transformed from an amorphous phase in as oxidized powders to θ phase at 1100°C and α phase at higher temperatures. The hardness of the composite increased from 2.0 GPa at 1100°C to 13.5 GPa at 1400°C due to the increase of density.     

A Novel Method for Fabrication of Y2O3-Stabilized ZrO2 Electrolyte Films

Gas-tight Y₂O₃-stabilized ZrO₂ (YSZ) films were prepared on NiO–YSZ and NiO–SDC (Sm_0.2Ce_0.8O_1.9) anode substrates by a novel method. A cell, Ni–YSZ/YSZ(10 μm)/LSM–YSZ, was tested with humidified hydrogen as fuel and ambient air as oxidant. The maximum power densities of 1.64, 1.40, 1.06, and 0.60 W/cm² were obtained at 850°, 800°, 750°, and 700°C, respectively. With methane as fuel, a cell of Ni–SDC/YSZ (12 μm)/LSM–YSZ exhibited the maximum power densities of 1.14, 0.82, 0.49, and 0.28 W/cm² at 850°, 800°, 750°, and 700°C, respectively. The impedance results showed that the performance of the cell was controlled by the electrode polarization rather than the resistance of YSZ electrolyte film.     

Densification of Calcia-Stabilized Zirconia with Borates

Densification studies of submicrometer ZrO₂ powders stabilized with 6.5 wt% CaO (CSZ) showed borate additions (1 to 10 wt%) to be effective sintering aids. Estimated densities >99% of theoretical were obtained on sintering at 1200°Cfor 4 h with 2 wt% B₂O₃ or 5 wt% CaO·2B₂O₃ additions to the CSZ powders. Average grain sizes obtained were typically <1 μm. Partial development of a monoclinic ZrO₂ phase was observed in the sintered samples. The amount of this phase varied from ∼7 to 75 wt% and was approximately linearly dependent on the additive concentration. The effect was most marked for the B₂O₃ additions. Development of the monoclinic phase was attributed to progressive leaching of Ca from the CSZ phase by B₂O₃, in effect partially destabilizing the ZrO₂.     

Effect of Sodium on Crystallite Size and Surface Area of Zirconia Powders at Elevated Temperatures

Fine ZrO₂ powders were synthesized by an aqueous precipitation method using zirconyl nitrate. By adding the precursor salt to NaOH, single-phase ZrO₂ powders were formed, and the monoclinic phase did not appear upon heat treatment up to 1000°C. The samples were digested in NaOH for different amounts of time. Different levels of washing of digested samples produced surface area at 900°C for 4 h ranging from 8 to 100 m²/g. It was found that the properties of the powders at elevated temperatures were sensitive to the sodium content. The surface area decreased while the crystallite size and pore size of the samples increased with increased sodium content. Our results indicated that sodium is detrimental to the stabilization of surface area at elevated temperatures.     

Phase Formation and Stability in Reactively Sputter Deposited Yttria-Stabilized Zirconia Coatings

Yttria-stabilized zirconia (YSZ) coatings were produced by reactively cosputtering metallic zirconium and yttrium targets in an argon and oxygen plasma using a system with multiple magnetron sputtering sources. Coating crystal structure and phase stability, as functions of Y₂O₃ content, substrate bias, and annealing temperature, were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Results demonstrated that highly (111)-oriented tetragonal and cubic zirconia structures were formed in 2 and 4.5 mol% Y₂O₃ coatings, respectively, when the coatings were grown with an applied substrate bias. Conversely, coatings deposited with no substrate bias had random tetragonal and cubic structures. XRD analysis of annealed coatings showed that the cubic zirconia in 4.5 mol% Y₂O₃ coatings exhibited structural stability at temperatures up to 1200°C. Transformation of the tetragonal to monoclinic phase occurred in 2 mol% Y₂O₃ coating during high-temperature annealing, with the fraction of transformation dependent on bias potential and annealing temperature.     

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.     

Structural Evolution and Electrical Properties of Sc2O3-Stabilized ZrO2 Aged at 850°C in Air and Wet-Forming Gas Ambients

X-ray diffraction (XRD) and electron microscopy investigations have been performed on Sc₂O₃-stabilized ZrO₂ as-sintered and after aging in air or in wet-forming gas at 850°C for 1000 h. Some tetragonal to monoclinic transformation had occurred in the near-surface regions of 4 mol% Sc₂O₃ samples after aging; the phase transition was more severe for samples aged in the forming gas ambient. A decrease of ∼20% in electrical conductivity accompanied the aging. In 6 mol% Sc₂O₃ samples, although no cubic to tetragonal transformation was detected, both the electrical conductivity and the activation energy for ionic conductivity decreased significantly during aging. Ten mole percent Sc₂O₃ samples did not show appreciable change in electrical conductivity due to aging, although some near-surface cubic to rhombohedral transformation did occur. Sharpening of the (400)_t XRD peak of Sc₂O₃-stabilzed tetragonal ZrO₂ accompanies the change(s) in the electrical conductivity.     

Ceria–Yttria-Stabilized Zirconia Composite Ceramic Systems for Applications as Low-Temperature Electrolytes

Ceramic samples of Ce_{1− x} Gd _x O_{2− y} and yttria-stabilized zirconia (YSZ) were prepared using solid-state methods. Polished faces of disks of these materials were held in intimate contact in a reaction cell at temperatures ranging from 1000° to 1300°C for durations up to 72 h. XRD, SEM, and microprobe Raman techniques were used to analyze the resulting reactions and ion diffusion. No reaction was observed at 1000°C after 72 h between the 10-mol%- and 20-mol%-Gd-doped CeO₂ and the YSZ. However, at 1300°C, a mixing region 25 μ m wide occurred, resulting in a cubic phase, where Zr⁴⁺ ions diffused into the CeO₂.     

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.     

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.     

Stability of ZrO2 Phases in Ultrafine ZrO2-Al2O3 Mixtures

Mixtures of ultrafine monoclinic zirconia and aluminum hydroxide were prepared by adding NH₄OH to hydrolyzed zirconia sols containing varied amounts of aluminum sulfate. The mixtures were heat-treated at 500° to 1300°C. The relative stability of monoclinic and tetragonal ZrO₂ in these ultrafine particles was studied by X-ray diffractometry. Growth of ZrO₂ crystallites at elevated temperatures was strongly inhibited by Al₂O₃ derived from aluminum hydroxide. The monoclinic-to-tetragonal phase transformation temperature was lowered to ∼500°C in the mixture containing 10 vol% Al₂O₃, and the tetragonal phase was retained on cooling to room temperature. This behavior may be explained on the basis of Garvie's hypothesis that the surface free energy of tetragonal ZrO₂ is lower than that of the monoclinic form. With increasing A1₂O₃ content, however, the transformation temperature gradually increased, although the growth of ZrO₂ particles was inhibited; this was found to be affected by water vapor formed from aluminum hydroxide on heating. The presence of atmospheric water vapor elevates the transformation temperature for ultrafine ZrO₂. The reverse tetragonal-to-monoclinic transformation is promoted by water vapor at lower temperatures. Accordingly, it was concluded that the monoclinic phase in fine ZrO₂ particles was stabilized by the presence of water vapor, which probably decreases the surface energy.     

Strength and Phase Stability of Yttria-Ceria-Doped Tetragonal Zirconia/Alumina Composites Sintered and Hot Isostatically Pressed in Argon–Oxygen Gas Atmosphere

Yttria-ceria-doped tetragonal zirconia (Y,Ce)-TZP)/alumina (Al₂O₃) composites were fabricated by hot isostatic pressing at 1400° to 1450°C and 196 MPa in an Ar–O₂ atmosphere using the fine powders prepared by hydrolysis of ZrOCl₂ solution. The composites consisting of 25 wt% Al₂O₃ and tetragonal zirconia with compositions 4 mol% YO_1.5–4 mol% CeO₂–ZrO₂ and 2.5 mol% YO_1.5–5.5 mol% CeO₂–ZrO₂ exhibited mean fracture strength as high as 2000 MPa and were resistant to phase transformation under saturated water vapor pressure at 180°C (1 MPa). Postsintering hot isostatic pressing of (4Y, 4Ce)-TZP/Al₂O₃ and (2.5Y, 5.5Ce)-TZP/Al₂O₃ composites was useful to enhance the phase stability under hydrothermal conditions and strength.     

Role of Water Vapor in Crystallite Growth and Tetragonal-Monoclinic Phase Transformation of ZrO2

The role of water vapor in crystallite growth and the tetragonal-to-monoclinic phase transformation of ZrO₂ was studied using three specially prepared samples: an ultrafine powder of monoclinic ZrO₂ obtained by hydrolysis of ZrOCI₂, an aggregated powder of tetragonal ZrO₂ obtained by thermal decomposition of Zr(OH)₄ under reduced pressure, and an ultrafine powder of tetragonal ZrO₂ obtained by thermal decomposition of zirconyl acetate dispersed in caramel. The samples were heat-treated up to 1000°C in dry and wet atmospheres saturated with water vapor at 90°C. It was found that water vapor markedly accelerated crystallite growth for both monoclinic and tetragonal ZrO₂ and facilitated the tetragonal-to-monoclinic phase transformation. Water vapor increases surface diffusion and thus enhances crystallite growth and decreases surface energy, which leads to stabilization of the tetragonal phase.