Formation and Transformation of Cubic ZrO2 Solid Solutions in the System ZrO2—Al2O3

In the system ZrO₂-Al₂O₃, cubic ZrO₂ solid solutions containing up to 40 mol% Al₂O₃ crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminum alkoxides. The values of the lattice parameter, a, increase linearly from 0.5095 to 0.5129 nm with increasing Al₂O₃ content. At higher temperatures, the solid solutions transform into tetragonal ZrO₂ and α-Al₂O₃. Pure ZrO₂ crystallizes in the tetragonal form at 415° to 440°C.     

Sol—Gel Fabrication of Pb(Zr0.52 Ti0.48)O3 Thin Films Using Lead Acetylacetonate as the Lead Source

Lead zirconium titanate (PZT) thin films of the morphotropic phase boundary composition [Pb(Zr_0.52Ti_0.43)O₃] were deposited on platinum-coated silicon by a modified sol-gel process using lead acetylacetonate as the lead source. The precursor solution for spin coating was prepared from lead acetylacetonate, zirconium n -butoxide, and titanium isopropoxide. The use of lead acetylacetonate instead of the widely used lead acetate trihydrate provided more stability to the PZT precursor solution. Films annealed at 700°C for 12 min formed well-crystallized perovskite phase of Pb(Zr_0.52Ti_0.48)O₃. Microstructures of these films indicated the presence of submicrometer grains (0.1 to 0.2 μm). The dielectric constant and loss values of these films measured at 10 kHz were approximately 1200 and 0.04, respectively, while the remanent polarization and coercive field were ∼ 13 μC/cm² and ∼ 35 kV/cm. Aging of the solution had almost no effect on the dielectric and ferroelectric properties of these films.     

Formation of Zirconia Titanate Solid Solution from Alkoxides

In the system ZrO₂–TiO₂, ZrTiO₄ solid solutions prepared by the simultaneous hydrolysis of zirconium and titanium alkoxides crystallize at low temperatures from amorphous materials between 30 and 70 mol% TiO₂. As zirconium is substituted for titanium, the solid solutions can be indexed in an orthorhombic unit cell with a and c decreasing linearly from 0.4832 to 0.4778 nm and from 0.5063 to 0.5002 nm, respectively, and b increasing linearly from 0.5401 to 0.5478 nm. The volume of the unit cell decreases continuously with increasing TiO₂ content. At higher temperatures the solid solutions decompose into ZrTiO₄ and either ZrO₂ (monoclinic) or TiO₂ (rutile), depeanding on the starting composition.     

Synthesis of ZrO2 and Y2O3-Doped ZrO2 Thin Films Using Self-Assembled Monolayers

Undoped or Y₂O₃-doped ZrO₂ thin films were deposited on self-assembled monolayers (SAMs) with either sulfonate or methyl terminal functionalities on single-crystal silicon substrates. The undoped films were formed by enhanced hydrolysis of zirconium sulfate (Zr(SO₄)·4H₄O) solutions in the presence of HCl at 70°C. Typically, these films were a mixture of two phases: nanocrystalline tetragonal- ( t -) ZrO₂ and an amorphous basic zirconium sulfate. However, films with little or no amorphous material could be produced. The mechanism of film formation and the growth kinetics have been explained through a coagulation model involving homogeneous nucleation, particle adhesion, and aggregation onto the substrate. Annealing of these films at 500°C led to complete crystallization to t -ZrO₂. Amorphous Y₂O₃-containing ZrO₂ films were prepared from a precursor solution containing zirconium sulfate, yttrium sulfate (Y₂(SO₄)₃8·H₂O), and urea (NH₂CONH₂) at pH 2.2–3.0 at 80°C. These films also were fully crystalline after annealing at 500°C.     

ZrO2-Transformation-Toughened Glass-Ceramics Prepared by the Sol-Gel Process from Metal Alkoxides

Glasses of composition 3ZrO₂O · 2SiO₂ were prepared by the sol-gel process from metal alkoxides. Tetragonal ZrO₂ was precipitated by appropriate heat treatment at 1000° to 1200°C. The fracture toughness of these glass-ceramics increased with increasing crystallite size of the tetragonal ZrO₂, reaching ∼5.0 MN/m^3/2 at a size of ∼40 nm. The higher fracture toughness was attributed to tetragonal → monoclinic ZrO₂ transformation toughening.     

Phase Transformation during Plasma Spraying of Hydroxyapatite–10-wt%-Zirconia Composite Coating

This study aims to explore phase transformation in plasma-sprayed hydroxyapatite (HA) + 10 wt% ZrO₂–8-mol%-Y₂O₃composite coating, using separately prepared HA and ZrO₂–8-mol%-Y₂O₃coatings as a control. Changes in the phase and chemistry of the coatings are characterized by X-ray diffractometry, with lattice-constant measurement (Cohen's method), and by transmission electron microscopy. Experimental results show evidence of diffusion, in the liquid state, of calcium ions from the HA matrix into the ZrO₂. This behavior causes the formation of the following structural features in the composite coating: (i) a CaO-doped ZrO₂solid solution (ZrO₂–7.7 mol% Y₂O₃–4.4 mol% CaO); (ii) a mixture of ZrO₂and CaZrO₃having a crystal-orientation relationship; (iii) an amorphous phase containing elements of calcium, phosphorus, zirconium, and yttrium; and (iv) a remaining CaO-poor HA matrix (Ca_{10− x} (HPO₄) _x (PO₄)_{6− x} (OH)_{2− x} ; x = 0.06). Rationales for the greatly decreased impurity phases of CaO and Ca₄P₂O₉found in the composite coating are discussed.     

Microstructure and Oxidation-Resistant Property of Sol-Gel-Derived ZrO2-Y2O3 Films Prepared on Austenitic Stainless Steel Substrates

The effect of Y₂O₃ addition to the oxidation resistance of sol-gel-derived zirconia films coated on austenitic stainless steel substrates was examined. The oxidation weight gain measurement and XRD analyses of oxides showed that addition of Y₂O₃ reduces oxide formation. TEM observations revealed that the films are joined to the substrates via an amorphous layer with concentrated Si, and the layer grew thicker by adding Y₂O₃ or elevating the firing temperature. Lattice constants of the films were shown to be more expanded than the zirconia powders prepared from the coating liquids.     

Processing and Performance of Hydroxyapatite/Fluorapatite Double Layer Coating on Zirconia by the Powder Slurry Method

A hydroxyapatite/fluorapatite (HA/FA) double layer was coated on ZrO₂ by a powder slurry method. The FA layer between the HA layer and the ZrO₂ substrate was effective in suppressing the reaction between HA and ZrO₂. The rheological properties of the slurry and the thermal treatment conditions were optimized. Addition of small amounts of tri-ethyl phosphate dispersant and polyvinylbutyral binder (up to 5 wt%) was effective in reducing the slurry viscosity, even at high loadings of HA powder. Each layer (HA, FA) was deposited on ZrO₂ repeatedly to induce a uniform layer, and the final heat treatment was carried out above 1200°C to consolidate the coating layer. During this process, without the FA-intermediate layer, a severe reaction between HA and ZrO₂ occurred to form tri-calcium phosphate and CaZrO₃ products. However, the presence of the FA layer between HA and ZrO₂ effectively suppressed the reaction up to 1300°C. The obtained HA/FA double coating layer was micro-porous and relatively rough, but was firmly adhered to the ZrO₂ substrate, having a bonding strength of approximately 25 MPa after heat treatment above 1200°C. The osteoblast-like cells cultured on the HA/FA coating layer spread and proliferated favorably, having a cell proliferation rate comparable to that of a plastic control and HA bulk ceramic.     

Effect of Na2SO4 on Structure and Corrosion Resistance of Ceramics Coatings Containing Zirconium Oxide on Ti–6Al–4V Alloy

A micro-plasma oxidation (MPO) technique has been developed in recent years, by which ceramic coatings are reported to possess improved properties and promising application prospects in many fields. The aim of this work was to study the effect of sodium sulfate, as an additive in the zirconate system, on the structure and corrosion resistance of ceramics coatings containing zirconium oxide grown on Ti–6Al–4V by MPO process. The phase composition, morphology, and element distribution in the coating were investigated by X-ray diffractometry, scanning electron microscopy, and energy distribution spectroscopy. Meanwhile, the corrosion resistance of the coated samples was examined by polarizing curves and potentiodynamic anodic curves in 3.5% NaCl solution. The results show that ceramic coatings were composed of m -ZrO₂, t -ZrO₂, and KZr₂(PO₄)₃. The Ti content in the coating near the substrate decreased sharply, and then remained at 5 wt% or so through the coating, while the Zr content near the substrate increased greatly, and then remained at about 55 wt% through the coating. The addition of sodium sulfate did not change the composition of the coatings, but increased the relative proportion of zirconium oxide to KZr₂(PO₄)₃ in the coating. Sodium sulfate decreased the thickness of the coating, while improving the density of the coatings. Moreover, the addition of the sodium sulfate improved the corrosion resistance of the coated samples in a 3.5% NaCl solution, whether considering localized pitting corrosion resistance or uniform corrosion resistance.     

Microwave Properties of Ba2Ti9O20 Doped with Zirconium and Tin Oxides

The effects of solid-solution additives, their concentration, and the thermal processing schedule on the microstructure evolution and microwave properties of Ba₂Ti₉O₂₀ were studied. The solubility of tin in Ba₂Ti₉O₂₀ was higher than that of zirconium. Both elements facilitated the formation of phase-pure Ba₂Ti₉O₂₀ resonators. Ba₂Ti₉O₂₀ formed most easily with low dopant concentrations (0.82 mol%) (most impressively for ZrO₂ substitutions). Extended heat treatment (16 h versus 6 h at a temperature of 1390°C) resulted in volatilization of the grain-boundary liquid phase, which leads to more-porous resonators that have correspondingly lower permittivities. Increasing the dopant concentration resulted in minor increases in the quality factor; doping with zirconium led to slightly higher values (a maximum of 13900 at a frequency of 3 GHz). Increasing the measurement temperature degraded the quality factor (most precipitously for BaTi₄O₉). The temperature coefficient decreased as the ZrO₂ substitution increased but was largely unaffected by the SnO₂ concentration. Excess TiO₂ in a batch with no other dopants demonstrated degraded microwave properties.     

Surface Precipitation Route for the Development of Cordierite-Zirconia Composites

Based on the interfacial electrochemical properties of aqueous cordierite dispersion, a novel interfacial scheme for fabricating cordierite-ZrO₂ composite with a dense and homogeneous microstructure was proposed and tested. In the present processing route, cordierite particles were first uniformly coated with a thin layer (∼40 nm) of aluminum hydroxide by an in situ surface-induced coating. Then, a controlled surface precipitation of ultrafine zirconium hydroxide precursor onto the coated cordierite particles was performed to obtain a composite with a uniform spatial distribution of the dispersed ZrO₂ particles throughout the matrix. The coated layer acted to suppress the formation of zircon phase during sintering. The enhanced fracture toughness observed in the presence of ZrO₂ was partly attributed to the t → m transformation toughening of the dispersed tetragonal ZrO₂ particles.     

Dependence of Oxidation Modes on Zirconia Content in Silicon Carbide/Zirconia/Mullite Composites

Two basic oxidation modes of silicon carbide/zirconia/mullite (SiC/ZrO₂/mullite) composites were defined based on the plotted curve of the gradient of the silica (SiO₂) layer thickness (formed on individual SiC particles) versus depth. Mode I, where oxygen diffusivity was much slower in the matrix than in the SiO₂ layer, exhibited a relatively large gradient and limited oxidation depth. Mode II, where oxygen diffusivity was much faster in the matrix than in the SiO₂ layer, displayed a relatively small gradient and an extensive oxidation depth. When the volume fraction of ZrO₂ was below a threshold limit, the composites exhibited Mode I behavior; otherwise, Mode II behavior was observed. For composites with a ZrO₂ content above the threshold limit, the formation of zircon (ZrSiO₄), as a result of the reaction between ZrO₂ and the oxidation product (i.e., SiO₂), might change the oxidation behavior from Mode II to Mode I.     

Compressive Surface Stresses Developed in Ceramics by an Oxidation-Induced Phase Change

Silicon nitride/zirconium oxide hot-pressed materials contain ZrO₂-_{2 x} N_{4 x /3} (0.25≤ x ≤0.43), which readily oxidizes at ≥ 500°C to monoclinic ZrO₂. The molar volume increase (∼4 to 5%) of this reaction was used to develop compressive surface stresses. The development of these useful surface stresses was demonstrated with an indentation technique used to measure the apparent critical stress intensity factor ( K_a ) on the surfaces of materials fabricated with 5 to 30 vol% ZrO₂ and subsequently oxidized at 600° to 800°C. The increase in K_a was directly related to the oxidation kinetics and the initial volume content of the unstable phase.     

Sol–Gel Synthesis and Characterization of Ag and Au Nanoparticles in SiO2, TiO2, and ZrO2 Thin Films

Silver and gold nanoparticles were synthesized by the sol–gel process in SiO₂, TiO₂, and ZrO₂ thin films. A versatile method, based on the use of coordination chemistry, is presented for stabilizing Ag⁺ and Au³⁺ ions in sol–gel systems. Various ligands of the metal ions were tested, and for each system it was possible to find a suitable ligand capable of stabilizing the metal ions and preventing gold precipitation onto the film surface. Thin films were prepared by spin-coating onto glass or fused silica substrates and then heat-treated at various temperatures in air or H₂ atmosphere for nucleating the metal nanoparticles. The Ag particle size was about 10 nm after heating the SiO₂ film at 600°C and the TiO₂ and ZrO₂ films at 500°C. After heat treatment at 500°C, the Au particle size was 13 and 17 nm in the TiO₂ and ZrO₂ films, respectively. The films were characterized by UV–vis optical absorption spectroscopy and X-ray diffraction, for studying the nucleation and the growth of the metal nanoparticles. The results are discussed with regard to the embedding matrix, the temperature, and the atmosphere of the heat treatment, and it is concluded that crystallization of TiO₂ and ZrO₂ films may hinder the growth of Ag and Au particles.     

Oxidation Behavior of Ferritic Steel Alloy Coated with Highly Dense Conducting Ceramics by Aerosol Deposition

Conducting La_0.8Sr_0.2MnO₃ (LSM) ceramic layers with a thickness of ∼10 μm were deposited on ferritic stainless steel (SS) by aerosol deposition for use as an oxidation resistance coating layer in the metallic interconnects of solid oxide fuel cell. The microstructural evolution and electrical properties of the LSM-coated SS were observed. The coated layers were fairly dense without pores or cracks, and maintained good adhesion even after oxidation at 800°C for 1000 h in air atmosphere. The surface of the bare SS after heat treatment at 800°C for 1000 h was covered with Cr-containing oxide scales, and the electrical conductivity was sharply decreased. However, the LSM-coated SS alloy showed a surface microstructure with almost no chromic oxide formation and maintained good electrical conductivity after the heat treatment. Close observation of the interface between LSM and SS indicated the presence of ∼500-nm-thick Cr₂O₃ and MnCr₂O₄ spinel phase, which may have caused the long-time deterioration of the interconnector performance. The area-specific resistance of the LSM-coated alloy after heat treatment at 800°C for 1000 h was 10.4 mΩ·cm².     

Novel Mg2Zr5O12/Mg2Zr5O12–ZrO2–MgF2 Gradient Layer Coating on Magnesium Formed by Microarc Oxidation

A novel Mg₂Zr₅O₁₂-based coating on magnesium was formed by microarc oxidation (MAO) in a K₂ZrF₆-containing electrolyte. The structure of the coating was examined by X-ray diffraction using the grazing angle method, scanning electron microscopy, and transmission electron microscopy. The friction and wear properties of the MAO-coated and -uncoated Mg samples were evaluated in a ball-on-disk testing system. The corrosion resistance of the coating in a 3.5% NaCl solution was investigated by the potentiodynamic polarization test. The coating is relatively dense and composed of a Mg₂Zr₅O₁₂–ZrO₂–MgF₂ inner layer and a nanocrystalline Mg₂Zr₅O₁₂ outer layer with a maximum hardness of 1240 Hv. The friction coefficient of the coating against Si₃N₄ is 0.35 under a dry-sliding condition. The corrosion resistance of the magnesium substrate is improved considerably by MAO treatment. The corrosion potential of the Mg₂Zr₅O₁₂-coated sample is −1.43 V with a current density as low as 7.06 × 10⁻⁸ A/cm². It is expected that the coating can considerably protect magnesium from wear and corrosion.     

Zirconium Oxide Structure Prepared by the Sol–Gel Route: I, The Role of the Alcoholic Solvent

Sol–gel-derived powder samples of zirconia (ZrO₂) prepared via the dissolution of zirconium n -propoxide in methanol, ethanol, and 2-propanol have been characterized mainly using perturbed angular correlations spectroscopy, as a function of temperature. Results indicate that the nanostructures and subsequent thermal evolution are alcohol dependent: the shorter the alcohol chain, the more hydrolyzed the product. ZrO₂ powder that has been obtained using ethanol as the solvent is the product that exhibits the better stabilization of the metastable tetragonal phase ( t -phase). It undergoes a clear and detailed t ₁-form → t ₂-form → monoclinic ZrO₂ thermal transformation and shows the highest activation energy against the transformation to the monoclinic phase.     

Effects of Oxygen Partial Pressure on the Nucleation Behavior and Morphology of Chemically-Vapor-Deposited Zirconia on Hi-Nicalon Fiber and Si

A ZrO₂ coating was prepared on Hi-Nicalon fiber and single-crystal Si by chemical vapor deposition (CVD) using ZrCl₄, CO₂, and H₂ as precursors at 1050°C. The effects of oxygen partial pressure on the nucleation behavior of the CVD-ZrO₂ coating were systematically studied by intentionally varying the controlled amount of O₂ into the CVD chamber. Characterization results suggested that the number density of tetragonal ZrO₂ nuclei apparently decreased with increasing the oxygen partial pressure from 4 × 10⁻³ to 1.6 Pa. Also, the coating layer became more columnar and contained larger monoclinic ZrO₂ grains. The observed relationships between the oxygen partial pressure and the nucleation and morphologic characteristics of the ZrO₂ coating were attributed to the grain size and oxygen deficiency effects, which have been previously reported to cause the stabilization of the tetragonal ZrO₂ phase in bulk ZrO₂ specimens.     

Growth of Monoclinic ZrO2 Thin, Flaky Crystals by Hydrothermal Decomposition of Zirconium Oxide Sulfate Crystals

Thin platelike hexagonal crystals of zirconium oxide sulfate (ZOS) with a chemical composition of Zr₃O₅SO₄. n H₂O were previously synthesized by the hydrolysis of an aqueous solution of 2 mol/L ZrOSO₄ at 240°C. The ZOS particles obtained were hydrothermally treated again in dilute sulfuric acid solution (<0.15 mol/L) at 240°C. The decomposition of ZOS and the nucleation and growth morphology of the monoclinic ZrO₂ crystals were automatically controlled with increasing sulfuric acid released by the decomposition of ZOS, and were influenced by the morphology of ZOS as precursors. Anisotropic monoclinic ZrO₂ single crystals, elongated thin and flaky particles (length: >2 (Am; width: about 0.1 μm; thickness:«0.01 μm), could be obtained under some conditions without the coexistence of another type of anisotropic ZrO₂ fibrous twin crystal.     

Effect of Zirconia Content on the Oxidation Behavior of Silicon Carbide/Zirconia/Mullite Composites

The oxidation of hot-pressed SiC-particle (SiC_p)/zirconia (ZrO₂)/mullite composites with various ZrO₂ contents, exposed in air isothermally at 1000° and 1200°C for up to 500 h, was investigated; an emphasis was placed on the effects of the ZrO₂ content on the oxidation behavior. A clear critical volume fraction of ZrO₂ existed for exposures at either 1000° or 1200°C: the oxidation rate increased dramatically at ZrO₂ contents of >20 vol%. The sharp transition in the oxidation rate due to the variation of ZrO₂ content could be explained by the percolation theory, when applied to the oxygen diffusivity in a randomly distributed two-phase medium. Morphologically, the composites with ZrO₂ contents greater than the critical value showed a large oxidation zone, whereas the composites with ZrO₂ contents less than the critical value revealed a much-thinner oxidation zone. The results also indicated that the formation of zircon (ZrSiO₄) at 1200°C, through the reaction between ZrO₂ and the oxide product, could reduce the oxidation rate of the composite.