Highly Porous Zirconia Ceramic Foams with Low Thermal Conductivity from Particle‐Stabilized Foams

Zirconia ceramic foams with ultra‐high porosity of 96%–98% have been fabricated using sodium dodecyl sulfate (SDS) as the particle stabilizer of zirconia particles for the first time. The wet foams stabilized by zirconia particles are ultra‐stable due to partially hydrophobic zirconia particles modified by SDS. Zirconia foams exhibit close cells with thin cell wall and small grain size. Increasing SDS concentration favors the foamability of the suspension, and further increases the porosity of ceramic foams. Zirconia ceramic foams with porosity of 98.1% have compressive strength of 0.26 ± 0.05 MPa. Decreasing solid loading leads to the porosity of ceramic foams. The compressive strength could be improved significantly by increasing the sintering temperature. Zirconia ceramic foams with porosity of 97.9% has low thermal conductivity of 0.027 ± 0.004 W·(m·K)^?1, which could be used as thermal insulation and refractory material.     

Crystal Structure of Orthorhombic Zirconia in Partially Stabilized Zirconia

A neutron powder diffraction investigation confirms that, in tough magnesiapartially-stabilized zirconia cooled to 30 K, most of the tetragonal zirconia transforms to an orthorhombic phase. This phase is retained on heating to room temperature; the lattice parameters at 295 K are a = 0.5068, b = 0.5260, and c = 0.5077 nm. The room-temperature crystal structure (space group Pbc2₁) is determined by multiphase Rietveld refinement from the neutron diffraction pattern. This orthorhombic structure is compared with the parent tetragonal structure and with the structure of monoclinic zirconia, which it closely resembles.     

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.     

Anisotropic Ionic Conductivity Observed in Superplastically Deformed Yttria-Stabilized Zirconia/Alumina Composite

Ionic conductivity measurements on a yttria-stabilized tetragonal zirconia polycry stall alumina composite subjected to superplastic deformation demonstrate anisotropic character. Parallel to the pressing direction, the grain-boundary resistance to oxygen ion mobility is 25% to 30% higher than that measured perpendicular to the pressing direction. The same directional dependency on the volume conductivity is observed but is less pronounced, showing approximately a 9% difference. Microstructural evidence reveals an agglomeration and elongation of alumina particles perpendicular to the pressing direction, and it is suggested that this phenomenon restricts the passage of ions parallel to the compression direction, giving rise to the anisotropic nature of the conductivity measurements.     

Morphology and Phase Stability of Nitrogen–Partially Stabilized Zirconia (N-PSZ)

The surface layer of yttria-doped tetragonal zirconia materials that have been heat-treated with zirconium nitride was observed to consist of a nitrogen-rich cubic matrix with nitrogen-poor tetragonal precipitates. The precipitates had a thin, oblate-lens shape, similar to those observed in magnesia–partially stabilized zirconia. Because of the fast diffusion of N⁴⁻ ions, the precipitates grew rather large, up to ∼5 μm in length, and remained stabilized in the tetragonal form at room temperature. Because the nitrided layer grew in the two-phase field, the size and distribution of the precipitates each was very irregular. The nitrogen content was observed to determine the proportion of cubic and tetragonal phases in the same way as in conventional cation-stabilized partially stabilized zirconia. A ternary phase diagram for the zirconium(yttrium)–nitrogen–oxygen system was suggested to explain the concentration gradient in the cubic matrix and the phase distribution of the nitrided layer.     

Roles of Alumina in Zirconia for Functional Applications

The Al₂O₃ addition to stabilized ZrO₂ has been studied for more than 20 years. In this article, literature and new results on the positive and negative effects of Al₂O₃ additions on the electrical properties of ZrO₂ are summarized and analyzed. In particular, a comprehensive grain-boundary conduction model is proposed. The Al₂O₃ addition always increases the bulk resistivity, mainly because of the formations of defect associates and insulating Al₂O₃ second-phase particles. The Al₂O₃ addition within the solubility limit increases the grain-boundary resistivity, as a result of increased grain-boundary space-charge potential; the Al₂O₃ addition above the solubility limit, however, scavenges the silicon-rich second phase from the grain boundaries, thereby decreasing the grain-boundary resistivity.     

Thermal Conductivity of Yttria–Zirconia Single Crystals, Determined with Spatially Resolved Infrared Thermography

The thermal diffusivity of yttria–zirconia (Y₂O₃–ZrO₂) single crystals, with YO_1.5 concentrations in the range of 0–60 mol%, has been determined using a new method that is based on spatially resolved (20 μm) infrared mapping of a modulated thermal field. The decreasing trend of the thermal conductivity ( K ), as a function of the YO_1.5 content (up to 20 mol%), can be described using a model based on a Debye approach that has been modified by introducing a cut-off length for the phonon mean free path. At higher concentrations, K increases, as a result of a possible ordering of the point defects.     

Crystallographic Texture and Thermal Conductivity of Zirconia Thermal Barrier Coatings Deposited on Different Substrates

The crystallographic texture and thermal conductivity of zirconia coatings deposited by electron beam evaporation on a variety of substrates have been measured. It was found that the thermal conductivity of coatings deposited at the same temperature was independent of whether they were deposited on polycrystalline alumina, single-crystal sapphire, single-crystal zirconia, or fused silica. The room-temperature thermal conductivity of the coatings deposited at 700°C was 0.32 W/(m·K), increasing to 1.36 W/(m·K) for coatings deposited at 1150°C. Similarly, the crystallographic texture was also independent of the substrate and had a (111) fiber texture at 700° and 900°C, switching to a (200) fiber texture by 1050°C. The exception was the coating deposited at 1150°C on (111) single-crystal zirconia which was epitaxial and exhibited a thermal conductivity of 2.46 W/(m·K). It is concluded that the properties of zirconia thermal barrier coatings are determined by the growth conditions rather than those associated with nucleation on the underlying substrate.     

Densification of 8Y‐Tetragonal‐Stabilized Zirconia Optoceramics with Improved Optical Properties by Y Segregation

8% Yttria‐stabilized zironcia (8YSZ) transparent ceramics have a wide technological applications. Segregation of the Y around the grain boundaries is favored by slow heating rate. The optimized sintering parameters helped in obtaining transparent ceramics of 8YSZ with a high percentage of cubic phase in addition to the presence of tetragonal phase. HRTEM was used to verify the grain growth suppression and to observe the presence of the cubic phase. The presence of cubic phase has suppressed the grain growth, which increased the transparency in the visible and infrared region without the addition of dopants or by utilizing high pressure.     

Microstructure and Ionic Conductivity of Yttria‐Stabilized Zirconia Thin Films Deposited on MgO

A number of reports have suggested that nanometric thin films of yttria‐stabilized zirconia (YSZ) deposited on MgO can support high ionic conductivity, but the results remain controversial and difficult to repeat. In this work, sub‐100‐nm‐thick YSZ films have been deposited on single‐crystal MgO substrates with different crystallographic orientations and sourced from different companies. The growth of YSZ on MgO (100) was found to be unstable: both (111)‐oriented films with polycrystalline structure and (100)‐oriented films with cube‐on‐cube epitaxy were observed despite seemingly identical deposition conditions. On MgO (110) and MgO (111) substrates, the growth of YSZ was more stable with high degrees of texture in the (110) and (111) film directions, respectively. Ionic conductivities of the films were measured with impedance spectroscopy, and conductivity values were consistently near or slightly below that of a YSZ single crystal.     

Phase Transformation in EB-PVD Yttria Partially Stabilized Zirconia Thermal Barrier Coatings during Annealing

Electron-beam physical-vapor-deposited thermal barrier coatings consisting of ZrO₂ stabilized by 7 wt% Y₂O₃ were investigated in regard to phase transformation after annealing. Free-standing ceramic layers were heat-treated in air, for up to 200 h, in the temperature range 1200°—1400°C and then analyzed by X-ray diffractometry. Based on information obtained from the {111} and {400} peaks, the phase composition and the Y₂O₃ content in the phases were calculated. At the start of transformation, small grains of a low-Y₂O₃ t phase and a c phase formed. After >30 h at 1300°C and at 1400°C, a mixture of a t phase deficient in Y₂O₃, an m phase, and a c phase formed after cooling, with the Y₂O₃ contents in the phases roughly predicted by the phase diagrams. The results of the present study are discussed here in detail and compared with data for plasma-sprayed coatings.     

Colloidal Processing and Ionic Conductivity of Fine-Grained Cupric-Oxide-Doped Tetragonal Zirconia

CuO-doped tetragonal ZrO₂ (3-mol%-Y₂O₃-doped tetragonal zirconia, 3Y-TZ) green bodies were consolidated from zirconia slurries with Cu²⁺ by a pressure filtration method. The slurries were prepared by dispersing 3Y-TZ powder in a solution of [NH₄OH + NH₃NO₃] = 0.1 M at pH 11 and adding an appropriate amount of Cu(NO₃)·3H₂O solution. Green bodies with narrow pore-size distribution were obtained after cold isostatically pressing the pressure-filtrated bodies. Small amounts of CuO-doped samples were densified fully at 1200°C. The size of a grain of 0.16-mol%-CuO-doped 3Y-TZ sintered at 1200°C was 84 nm. Bulk and grain-boundary conductivities are measured by a complex impedance method. The bulk conductivity of the CuO-doped 3Y-TZ was almost equal to the undoped one, but the grain-boundary conductivity decreased with CuO addition.     

Low-Temperature Ionic Conductivity of 9.4-mol%-Yttria-Stabilized Zirconia Single Crystals

Direct current and alternating current electrical conductivity measurements are used to determine the activation energies for the formation of oxygen vacancies by breakup of bound defect complexes (0.32 eV) and their migration (0.84 eV) at low temperatures (<600°C) in Y₂O₃-fully-stabilized ZrO₂ single crystals. The defect clusters break up between 360° and 450°C.     

Structural Characteristics and Electrical Conductivity of Spark Plasma Sintered Ytterbia Co‐doped Scandia Stabilized Zirconia

The effect of replacing Sc₂O₃ with Yb₂O₃ on the structural and electrical properties of xYb₂O₃–(12–x)Sc₂O₃–88ZrO₂ has been investigated. Spark plasma sintering technique is employed to fabricate dense bulk samples from the nano‐sized powders. X‐ray diffraction and transmission electron microscopy performed on pellets indicate the existence of cubic and rhombohedral phases in 12ScSZ, and a single cubic phase in all the co‐doped compositions. However, Raman spectroscopic studies suggest the presence of a metastable tetragonal t″‐phase along with rhombohedral phases in 12ScSZ, whereas a single cubic phase in all the co‐doped compositions. Significant enhancement in the conductivity of grain and grain boundary is observed on replacing Sc₂O₃ with Yb₂O₃. In the intermediate temperature range, 1Yb11ScSZ exhibits the highest, while 12ScSZ shows the lowest conductivity values, which is attributed to corresponding phases present in that range. Through co‐doping with >1 mol% Yb₂O₃ leads to conductivity decrease, but the value remains higher than that of 12ScSZ. A sharp conductivity change is observed in 12ScSZ and 1Yb11ScSZ samples, which is attributed to partial phase transition as well to the formation of cation‐vacancy complexes. In this work, the beneficial effect of Yb₂O₃ co‐doping in 12ScSZ on the phase and conductivity has been highlighted.     

Controlling the Diameter of Electrospun Yttria‐Stabilized Zirconia Nanofibers

This work reports the precise diameter control of electrospun yttria‐stabilized zirconia (YSZ) nanofibers from 200 to 900 nm after calcination. Fabricated YSZ nanofibers showed porous nanocrystalline structures with high aspect ratios of more than 500:1 and high surface‐to‐volume ratios with a specific surface area of 43.32 m²/g. The diameter of the YSZ nanofibers increased with the viscosity of the precursor solution, which was controlled by the concentrations of either polymers (polyacrylonitrile) or ceramic precursors (YSZ). We present a modified correlation between the diameter of a nanofiber and the synthetic conditions, as the observed behavior for calcined ceramic nanofibers deviated from the expected behavior. Our results demonstrate a modified but simple approach to fabricate ceramic nanofibers with desired diameters, providing a new design guideline for many electrochemical applications.     

Long-Term Behavior and Application Limits of Plasma-Sprayed Zirconia Thermal Barrier Coatings

Investigations of changes in phase composition, mechanical properties, and microstructure of ZrO₂-based plasma-sprayed thermal barrier coatings (TBCs) with 8 mol% CeO₂, 19.5 mol% CeO₂/1.5 mol% Y₂O₃, 35 mol% CeO₂, and 4.5 mol% Y₂O₃ after long-term heat treatments at typical operation temperatures (1000°–1400°C) are presented. Experimental studies include X-ray diffractometry, mechanical testing, and scanning electron microscopy. Thermal cycling experiments also have been performed. TBCs with 8 mol% CeO₂ contain mainly the tetragonal equilibrium phase and, therefore, show rapid failure because of the high amount of tetragonal → monoclinic phase transformation, even after relatively short heat treatments (1250°C/1 h). In the case of the other systems that consist mainly of the tetragonal or cubic nonequilibrium phases, TBCs with 19.5 mol% CeO₂/1.5 mol% Y₂O₃ or 35 mol% CeO₂ reveal a smaller amount of monoclinic phase after long-term heat treatments (1250°C/1000 h) compared with TBCs containing 4.5 mol% Y₂O₃. TBCs containing 35 mol% CeO₂ show a higher degree of sintering than the TBCs with 19.5 mol% CeO₂/1.5 mol% Y₂O₃ and, therefore, a greater increase of the elastic modulus. Among the systems investigated, TBCs containing 4.5 mol% Y₂O₃ exhibit the highest resistance to failure in thermal-cycling experiments.     

Heat Capacity of Monoclinic Zirconia between 2.75 and 350 K

We have measured the constant-pressure heat capacity C _p of pure monoclinic ZrO₂ between 2.75 and 350 K. At low temperatures we find heat capacity in excess of the thermal acoustic phonon contribution, which we attribute to the ingress of low-lying optic mode vibrations. We have calculated the heat capacity at constant volume C _v at the high-temperature end of our measurement range and used these values with the harmonic model to derive the moments of the phonon spectrum. Comparing the moments of the monoclinic phase with theoretically derived corresponding moments for the tetragonal phase, we find that a downward shift of optic mode frequencies accompanies the tetragonal-to-monoclinic transformation.     

Zirconia ceramics with additions of Alumina for advanced tribological and biomedical applications

The results of an investigation on slip cast and sintered Y₂O₃ (3 wt%)- stabilized ZrO₂ with additions of 5, 10, 15 wt% Al₂O₃ are reported. The surface roughness, porosity and density of the samples were measured. The hardness HRc and Hv, fracture toughness K_1C, and friction coefficients were also measured using standard methods. The structural properties of the samples were observed by Scanning Electron Microscopy (SEM). The surface topography was evaluated by means of Chromatic White Light Interferometry using MicroSpy® Topo of FRT Rauheit Kontur before and after tribological tests. The phase and chemical composition were analyzed by X-Ray Diffractometry (XRD), Energy Dispersive X-ray (EDX) spectroscopy, and Raman spectroscopy. Results show that the addition of Al₂O₃ into YSZ ceramics in the range of 5–10% allows the mechanical and tribological characteristics of the material that can be applied in different mechanical machines for different metallurgical processes to be improved, as well as in chemical engineering or medicine.     

Effect of Dynamic Compaction on the Retention of Tetragonal Zirconia and Mechanical Properties of Alumina/Zirconia Composites

Dynamic consolidation techniques were employed to investigate the retention of tetragonal zirconia and degree of consolidation in alumina/zirconia powder compacts. Heating the specimens prior to explosive shock compaction increased the tetragonal-phase retention significantly. Low shock pressures yielded no macrocracking, although final densities were low (60% to 70% of the theoretical density). Heat treatment following dynamic consolidation enhanced the retention of the tetragonal zirconia polymorph regardless of the shock pressure employed. Compact densities were increased to over 90% of theoretical at relatively low sintering temperatures (1300°C). Hardness, toughness, and Young's modulus of the compacts were comparable to those achieved in composites that were synthesized using more conventional techniques. Dynamic compaction offers an alternative method for the fabrication of zirconia-toughened alumina ceramics.     

Effective Thermal Conductivity of a Packed Bed of Hollow Zirconia Microspheres, under Vacuum and under 100 kPa of Argon

Measurements have been made of the effective thermal conductivity of a packed bed of hollow, yttria-stabilized zirconia microspheres, under vacuum and under 100 kPa of argon gas. Above 1400 K the spheres begin to sinter together. Before this occurs, the conductivity is given under vacuum by A ₁ T ³+ A ₂ with A ₁= 2 × 10⁻¹¹ W · m⁻¹· K⁻⁴ and A ₂= 0.01 W · m⁻¹· K⁻¹. The thermal conductivity increases strongly with both the gas pressure and the degree of sintering of the spheres. The measured values can be fitted reasonably well by a model developed by Takegoshi et al. These results may have some applicability to the development of high-temperature thermal insulation.