Preparation of NiAl2O4/SiO2 and Co2+-Doped NiAl2O4/SiO2 Nanocomposites by the Sol–Gel Route

NiAl₂O₄/SiO₂ and Co²⁺-doped NiAl₂O₄/SiO₂ nanocomposite materials of compositions 5% NiO – 6% Al₂O₃– 89% SiO₂ and 0.2% CoO – 4.8% NiO – 6% Al₂O₃– 89% SiO₂, respectively, were prepared by a sol–gel process. NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals were grown in a SiO₂ amorphous matrix at around 1073 K by heating the dried gels from 333 to 1173 K at the rate of 1 K/min. The formations of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in SiO₂ amorphous matrix were confirmed through X-ray powder diffraction, Fourier transform infrared spectroscopy, differential scanning calorimeter, transmission electron microscopy (TEM), and optical absorption spectroscopy techniques. The TEM images revealed the uniform distribution of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in the amorphous SiO₂ matrix and the size was found to be ∼5–8 nm.     

Formation and Sintering of Yttria-Doped Tetragonal Zirconia with 50 mol% Alumina Prepared by the Hydrazine Method

Al₂O₃/Y₂O₃-doped ZrO₂ composite powders with 50 mol% Al₂O₃ are prepared by the hydrazine method. As-prepared powders are mixtures of AlO(OH) gel and amorphous ZrO₂ solid solutions containing Y₂O₃ and Al₂O₃. The formation process leading to α-Al₂O₃- t -ZrO₂ composite powders is examined. Hot isostatic pressing is performed for 2 h at 1400°C under 196 MPa using θ-Al₂O₃- t -ZrO₂ composite powders. The resulting dense, sintered α-Al₂O₃- t -ZrO₂ composites show excellent mechanical strength.     

Tensile Ductility in Zirconia-Dispersed Alumina at High Temperatures

High-temperature plastic flow in Al₂O₃-10 wt% ZrO₂ (2.5 mol% Y₂O₃) has been examined at temperatures between 1400° and 1500°C. Al₂O₃-10 wt% ZrO₂ (2.5 mol% Y₂O₃) exhibits much higher flow stress and smaller tensile elongation below about 1450°C than 0.1 wt% MgO-doped single-phase Al₂O₃. The suppression of grain growth with ZrO₂ dispersion into Al₂O₃ is not effective for improving the tensile ductility. The limited ductility in Al₂O₃-10 wt% ZrO₂ (2.5 mol% Y₂O₃) is associated with the increment of flow stress caused by ZrO₂. The ZrO₂ dispersion or segregation in Al₂O₃/Al₂O₃ boundaries suppresses the grain boundary sliding and hence results in the increased flow stress at high temperatures.     

Phase Transformation and Densification of an Attrition-Milled Amorphous Yttria-Partially-Stabilized Zirconia–20 mol% Alumina Powder

Phase transformations during consolidation treatments of an attrition-milled amorphous yttria-partially-stabilized zirconia (Y-PSZ: ZrO₂–3 mol% Y₂O₃)–20 mol% Al₂O₃ powder and the resulting microstructures have been investigated. A metastable cubic phase ( c -ZrO₂ solid solution) together with an α-Al₂O₃ phase is formed in the amorphous matrix by consolidation at temperatures below 1204 K. The metastable cubic phase transforms to a stable tetragonal phase ( t -ZrO₂ solid solution) with an increase in the consolidation temperature. Fully dense bulk samples consisting of extremely fine tetragonal grains together with a small amount of α-Al₂O₃ particles could be obtained by consolidation at temperatures above 1432 K. Important features concerned with the densification behavior are as follows: (1) Marked increase in the relative density occurs after cubic crystallization and subsequent cubic-to-tetragonal transformation. (2) All of the consolidated bulk samples show extremely fine grain structure with grain sizes of several tens of nanometers, irrespective of the consolidation temperature. (3) The regularity of the lattice fringe contrast in each tetragonal grain seems to be kept in the vicinity of grain boundaries. These results suggest that densification of the attrition-milled amorphous powder proceeds via superplastic flow and/or diffusional creep, rather than viscous flow of the initial amorphous phase before crystallization.     

Influence of the Y2O3 Content and Temperature on the Mechanical Properties of Melt-Grown Al2O3–ZrO2 Eutectics

The effect of Y₂O₃ content on the flexure strength of melt-grown Al₂O₃–ZrO₂ eutectics was studied in a temperature range of 25°–1427°C. The processing conditions were carefully controlled to obtain a constant microstructure independent of Y₂O₃ content. The rod microstructure was made up of alternating bands of fine and coarse dispersions of irregular ZrO₂ platelets oriented along the growth axis and embedded in the continuous Al₂O₃ matrix. The highest flexure strength at ambient temperature was found in the material with 3 mol% Y₂O₃ in relation to ZrO₂(Y₂O₃). Higher Y₂O₃ content did not substantially modify the mechanical response; however, materials with 0.5 mol% presented a significant degradation in the flexure strength because of the presence of large defects. They were nucleated at the Al₂O₃–ZrO₂ interface during the martensitic transformation of ZrO₂ on cooling and propagated into the Al₂O₃ matrix driven by the tensile residual stresses generated by the transformation. The material with 3 mol% Y₂O₃ retained 80% of the flexure strength at 1427°C, whereas the mechanical properties of the eutectic with 0.5 mol% Y₂O₃ dropped rapidly with temperature as a result of extensive microcracking.     

Mechanical Properties of Hot Isostatically Pressed Zirconia-Toughened Alumina Ceramics Prepared from Coprecipitated Powders

Amorphous Al₂O₃–ZrO₂ composite powders with 5–30 mol% ZrO₂ have been prepared by adding aqueous ammonia to the mixed solution of aqueous aluminum sulfate and zirconium alkoxide containing 2-propanol. Simultaneous crystallization of γ-Al₂O₃ and t -ZrO₂ occurs at 870°–980°C. The γ-Al₂O₃ transforms to α-Al₂O₃ at 1160°–1220°C. Hot isostatic pressing has been performed for 1 h at 1400°C under 196 MPa using α-Al₂O₃– t -ZrO₂ composite powders. Dense ZrO₂-toughened Al₂O₃ (ZTA) ceramics with homogeneous-dispersed ZrO₂ particles show excellent mechanical properties. The toughening mechanism is discussed. The microstructures and t / m ratios of ZTA are examined, with emphasis on the relation between strength and fracture toughness.     

Surface Strengthening of Alumina-Zirconia Composite by Addition of Molybdenum Dioxide

MoO₂ is found to stabilize m -ZrO₂ Because of the addition of MoO₂ into ZrO₂ at the surface region of an Al₂O₃-15 vol% ZrO₂composite, the volume fraction of m -ZrO₂ at the surface increases from 0.3 to 0.5–0.6 without changing the size of the ZrO₂grains. This enhanced transfomation results in an increase in flexural strength of up to 30% because of compressive stresses built in at the surface. The increase in flexural strength is proportional to the difference in volume fraction of m -ZrO₂ between the surface layer and the bulk matrix, in agreement with a previous analysis.     

Nd2O3-Doped Sialons with ZrO2/ZrN Additions Formed by Sintering and Hot Isostatic Pressing

β-sialon and Nd₂O₃-doped α-sialon materials of varying composition were prepared by sintering at 1775° and 1825°C and by glass-encapsulated hot isostatic pressing at 1700°C. Composites were also prepared by adding 2–20 wt% ZrO₂ (3 mol% Nd₂O₃) or 2–20 wt% ZrN to the β-sialon and α-sialon matrix, respectively. Neodymium was found to be a fairly poor α-sialon stabilizer even within the α-phase solid solution area, and addition of ZrN further inhibited the formation of the α-sialon phase. A decrease in Vickers hardness and an increase in toughness with increasing content of ZrO₂(Nd₂O₃) or ZrN were seen in both the HIPed β-sialon/ZrO₂(Nd₂O₃) composites and the HIPed Nd₂O₃-stabiIized α-sialons with ZrN additions.     

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.     

Phase Equilibration in ZrO2-Y2O3 Alloys by Liquid-Film Migration

The tetragonal ( t ) and cubic ( c ) ZrO₂ solid solutions in two-phase ZrO₂-8 wt% Y₂O₃ ceramics have low and high solute content, respectively. Annealing samples sintered at 1600°C between 700° and 1400°C requires a change in the volume fraction of the coexisting phases, as well as their equilibrium Y₂O₃ content. The enrichment in Y₂O₃ content of the c -ZrO₂ grains is accomplished by liquid-film migration involving the ubiquitous silicate grain-boundary phase, while the volume fraction of t -ZrO₂ increases by the nucleation and growth of cap-shaped t -ZrO₂ lenses. The interfaces between the c -ZrO₂ matrix and the growing t -ZrO₂ lenses are semicoherent.     

Thermodynamics and Phase Equilibria in the Sr─Cu─O System

Phase equilibria in the SrO─CuO─Cu system were determined at 1173 K from the results of X-ray diffraction measurements using specimens annealed under low oxygen partial pressure ( p o₂= 0.21–10⁻⁸ atm). Electromotive force (EMF) measurements using ZrO₂ solid electrolyte cells were carried out in the ternary phase equilibria. Gibbs free energies for five reactions in the cell electrodes were summarized by equations with linear temperature dependence. The standard free energies of formation for Sr₂CuO₃, SrCuO₂, Sr₁₄Cu₂₄O₄₁, and SrCu₂O₂ were derived. The stability conditions of all oxides are displayed in the p-T-x diagram using the new data.     

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.     

Particle Interactions in Zirconia-Toughened Alumina

In order to determine the effect of the coarse tail of the ZrO₂ size distribution of the monoclinic ZrO₂ content of zirconia-toughened alumina, quantitative hot-stage X-ray measurements were made on (1) A1₂O₃-10% Zr0₂ (submicrometer with a coarse tail >1 μm), (2) A1₂O₃-10% ZrO₂ with the coarse tail of the ZrO₂ carefully removed by centrifugation, and (3) Al₂O₃-10% ZrO₂ in which 10% of Zr0₂(1) and 90% of Zr0₂(2) were used. The monoclinic content of (3) was compared with the weighted average of(1) and (2). A disproportionate amount of monoclinic ZrO₂ was found in (3) and it was concluded that transformation of the coarse particles promotes transformation in finer particles. Microstructural examination supports this conclusion. Results were interpreted as being due both to autocatalytic transformation and to constraint relief from microcracking.     

Influence of Cesium Nitrate and Heating Rate on Densification and Microstructure of Cs-Deficient Pollucite Sintered Body

Cs-deficient pollucite, Cs_0.9Al_0.9Si_2.1O₆, a sintered body with a low thermal expansion coefficient of 1.26 × 10⁻⁶ K⁻¹ in the temperature range of 298 to 1273 K, has been fabricated using calcined powders with an amorphous phase. The calcined powder was prepared by heating mixtures of CsNO₃, γ-Al₂O₃, and amorphous SiO₂ powders at 798 K in air for 20 h, repeated 10 times. The repetition of the heat treatment at 798 K was performed to decompose CsNO₃ thoroughly in the mixed raw powders and to obtain calcined powders with a high specific surface area. The relative density of the sintered bodies fabricated from the green compact of the amorphous calcined powder increased with increasing sintering temperature and heating rate. The sintered body with a relative density of ca. 96%, of a well-crystallized pollucite single phase and with the grain size in the range of 0.75–4.75 μm, was fabricated by heating the green compact of the amorphous powders at 1673 K in air for 20 h at a heating rate of 20 K/min.     

Anatase-Type TiO2 and ZrO2-Doped TiO2 Directly Formed from Titanium(III) Sulfate Solution by Thermal Hydrolysis: Effect of the Presence of Ammonium Peroxodisulfate on their Formation and Properties

Anatase-type TiO₂ powder containing sulfur with absorption in the visible region was directly formed as particles with crystallite in the range 15–88 nm by thermal hydrolysis of titanium(III) sulfate (Ti₂(SO₄)₃) solution at 100°–240°C. Because of the presence of ammonium peroxodisulfate ((NH₄)₂S₂O₈), the yield of anatase-type TiO₂ from Ti₂(SO₄)₃ solution was accelerated, and anatase with fine crystallite was formed. Anatase-type TiO₂ doped with ZrO₂ up to 9.8 mol% was directly precipitated as nanometer-sized particles from the acidic precursor solutions of Ti₂(SO₄)₃ and zirconium sulfate in the presence and the absence of (NH₄)₂S₂O₈ by simultaneous hydrolysis under hydrothermal conditions at 200°C. By doping ZrO₂ into TiO₂ and with increasing ZrO₂ content, the crystallite size of anatase was decreased, and the anatase-to-rutile phase transformation was retarded as much as 200°C. The anatase-type structure of ZrO₂-doped TiO₂ was maintained after heating at 1000°C for 1 h. The favorable effect of doping ZrO₂ to anatase-type TiO₂ on the photocatalytic activity was observed.     

Formation, Characterization, and Hot Isostatic Pressing of Cr2O3-Doped ZrO2 (0,3 mol% Y2O3) Prepared by Hydrazine Method

In the ZrO₂-Cr₂O₃ system, metastable t -ZrO₂ solid solutions containing up to 11 mol% Cr₂O₃ crystallize at low temperatures from amorphous materials prepared by the hydrazine method. The lattice parameter c decreases linearly from 0.5149 to 0.5077 nm with increased Cr₂O₃ content, whereas the lattice parameter a is a constant value ( a = 0.5077 nm) regardless of the starting composition. At higher temperatures, transformation (decomposition) of the solid solutions proceeds in the following way: t (ss)→ t (ss) + m + Cr₂O₃→ m + Cr₂O₃. Above 11 mol% Cr₂O₃ addition, c-ZrO₂ phases are formed in the presence of Cr₂O₃. The t -ZrO₂ solid solution powders have been characterized for particle size, shape, and surface area. They consist of very fine particles (15–30 nm) showing thin platelike morphology. Dense ZrO₂(3Y)-Cr₂O₃ composite ceramics (∼99.7% of theoretical) with an average grain size of 0.3 μm have been fabricated by hot isostatic pressing for 2 h at 1400°C and 196 MPa. Their fracture toughness increases with increased Cr₂O₃ content. The highest K _Ic value of 9.5 MPa·;m^1/2 is achieved in the composite ceramics containing 10 mol% Cr₂O₃.     

Phase Diagram of the Alumina–Zirconia–Samaria System

The phase diagram of the Al₂O₃–ZrO₂–Sm₂O₃ system was constructed in the temperature range 1250°–2800°C. The phase transformations in the system are completed in eutectic reactions. No ternary compounds or regions of appreciable solid solution were found in the components or binaries in this ternary system. Two new ternary and one new binary eutectics were found. The minimum melting temperature is 1680°C and it corresponds to the ternary eutectic Al₂O₃+F-ZrO₂+SmAlO₃. The solidus surface projection, the schematic of the alloy crystallization path, and the vertical sections present the complete phase diagram of the Al₂O₃–ZrO₂–Sm₂O₃ system.     

Electrical Conductivity in the ZrO2-Rich Region of Several M2O3—ZrO2 Systems

The electrical conductivity of M₂O₃-ZrO₂ compositions containing 6 to 24 mole % M₂O₃, where M represents La, Sm, Y, Yb, or Sc, was examined. Only Sm₂O₃, Y₂O₃, and Yb₂O₃ formed cubic solid solutions with ZrO₂ over most of this substitutional range. Scandia forms a wide cubic solid solution region with ZrO₂ at temperatures above 130°C whereas the cubic solid solution region at room temperature is narrow (6 to 8 mole % Sc₂O₃). Lanthana additions to ZrO₂produced no fluorite-type cubic solid solutions within the compositional range investigated. Generally, the electrical conductivity of these cubic solid solutions increased as the size of the substituted cation decreased and the electrical conductivity for each binary system attained a maximum at about 10 to 12 mole % M₂O₃.     

Creep in Yttria- and Scandia-Stabilized Zirconia

Compression creep measurements at constant load on ZrO₂-6 mol% Sc₂O₃ (grain size ∼1 μm), ZrO₂-6 mol% Y₂O₃ (grain size ∼17 μm), and heat-treated ZrO₂-6 mol% Sc₂O₃ (grain size ∼2 μm) yield activation energies of 89, 86, and 74 kcal/mol, respectively. The creep rates are linearly proportional to the inverse square of the grain size of the material. A stress exponent, n , of 1.5 was found for the scandia-doped zirconia and two regimes, with n =1 and 6, were found for the yttria-doped zirconia. These data, supported by metallographic evidence, are interpreted as showing that n =1 is associated with cation diffusion control of creep, n =6 with local propagation of inter-crystalline cracks, and n =1.5 with a transition region.     

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.