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.     

Formation of Zirconia Solid Solutions Containing Alumina Prepared by New Preparation Method

In the system ZrO₂–Al₂O₃, a new method for preparing ZrO₂ solid solutions from ZrCl₄ and AlCl₃ using hydrazine monohydrate is investigated. c -ZrO₂ solid solutions containing up to ∼40 mol% Al₂O₃ crystallize at low temperatures from amorphous materials. The formation mechanism is discussed from IR spectral data. The values of the lattice parameter α increase linearly from 0.5072 to 0.5105 nm with increasing Al₂O₃ content. At higher temperatures, transformation of the solid solutions proceeds as follows: c ( SS ) → t ( ss ) → t ( ss ) +α-Al₂O₃→ m +α-Al₂O₃. m -ZrO₂–α-Al₂O₃ composite ceramics are fabricated by hot isostatic pressing for 2 h at 1250°C and 196 MPa. Microstructures and mechanical properties are examined, in connection with increasing Al₂O₃ content.     

Stability of Ultrafine Tetragonal ZrO2 Coprecipitated with Al2O3 by the Spray-ICP Technique

The transformation of ultrafine powders (particle size, 0.01 to 0.04 μm) of the system ZrO_2–Al₂O₃, prepared by spraying their corresponding nitrate solutions into an inductively coupled plasma (ICP) of ultrahigh temperature, was investigated. The powders were composed of metastable tetragonal ZrO₂ ( mt- ZrO₂) and γ-Al₂O₃. On heating, the mt- ZrO₂ (or tetragonal ZrO₂, t -ZrO₂) was retained up to 1200°C. At 1380°C the transformation to monoclinic ZrO₂ ( m -ZrO₂) occurred and the amount of the m -ZrO₂ decreased with the increase in Al₂O₃ content, thus indicating the stabilization of the t -ZrO₂ by the Al₂O₃, which seems to be explained in terms of the retardation of grain growth.     

Formation of Alumina/Zirconia (3 mol% Yttria) Composite Powders Prepared by the Hydrazine Methods

Alumina/3 mol% yttria-doped zirconia composite powders have been prepared by the hydrazine method. As-prepared powders are AlO(OH) gel solid solutions and the mixtures of this and amorphous ZrO₂ below and above 10 mol% ZrO₂, respectively. The formation process leading to α–Al₂O₃– t -ZrO₂ composite powders is examined.     

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.     

Conversion from β-Ce2O3·11Al2O3 to α-Al2O3 in Tetragonal ZrO2 Matrix

Composites of β-Ce₂O₃·11Al₂O₃ and tetragonal ZrO₂ were fabricated by a reductive atmosphere sintering of mixed powders of CeO₂, ZrO₂ (2 mol% Y₂O₃), and Al₂O₃. The composites had microstructures composed of elongated grains of β-Ce₂O₃·11Al₂O₃ in a Y-TZP matrix. The β-Ce₂O₃·11Al₂O₃ decomposed to α-Al₂O₃ and CeO₂ by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce₂O₃·11Al₂O₃ divided into several grains of α-Al₂O₃ and ZrO₂ doped with Y₂O₃ and CeO₂. High-temperature bending strength of the oxygen-annealed α-Al₂O₃ composite was comparable to the β-Ce₂O₃·11Al₂O₃ composite before annealing.     

Metastable Phase Selection and Partitioning for Zr(1−x)AlxO(2−x/2) Materials Synthesized with Liquid Precursors

Aqueous solutions of zirconium acetate and aluminum nitrate were spray pyrolyzed at 250°C and upquenched to different temperatures to yield metastable solid solutions of composition Zr_{(1− x )}Al_xO_{(2− x /2)}. An amorphous oxide forms first during pyrolysis which subsequently crystallizes as a single phase for x ≤ 0.57 (≤40 mol% Al₂O₃). The crystallization temperature increased with Al₂O₃ content. Electron diffraction, supported by Raman spectroscopy, indicates that the initial phase is tetragonal. At higher temperatures, the initial solid solation partitions to other metastable phases, viz., t -ZrO₂+γ-Al₂O₃, prior to achieving their equilibrium phase assemblage, m -ZrO₂+α-Al₂O₃. Partitioning yields a nanocomposite microstructure with grain sizes of 20–100 nm, compared to the 3 to 5 nm in the initial, single phase. Compositions containing 45 to 50 mol% Al₂O₃ concurrently crystallize and partition. The structure selected during crystallization and the partitioning phenomena are discussed in terms of diffusional constraints during crystallization, which are conceptually similar to those operating during rapid solidification.     

Low-Temperature High-Pressure Consolidation of Amorphous Al2O3–15 mol% Y2O3

This study examined pressure consolidation of amorphous Al₂O₃–15 mol% Y₂O₃ powders prepared by co-precipitation and spray pyrolysis. The two amorphous powders had similar true densities and crystallization sequences. Uniaxial hot pressing was carried out at 450°–600°C with a moderate pressure of 750 MPa. The co-precipitated powder could be hot pressed to a maximum relative density of 98% and remained amorphous. Pressure adversely affected the densification of the spray-pyrolyzed powder by favoring an early crystallization of γ-Al₂O₃ phase at 580°C. Plastic deformation of the amorphous phase is believed to be responsible for the large densification of the amorphous powders.     

Metastable Phase Formation in Plasma-Sprayed ZrO2 (Y2O3)–Al2O3

Rapidly solidified ZrO₂ (Y₂O₃)–Al₂O₃ powders were prepared by melting fine-particle aggregates in a high-enthalpy plasma flame and then rapidly quenching them in cold water or on a copper chill plate. To ensure complete melting and homogenization of all the particles before quenching, the water-quenching treatment was often repeated two or even three times. The resulting melt-quenched powders and splats displayed a variety of metastable structures, depending on composition and cooling rate. ZrO₂-rich material developed an extended solid solution phase, whereas eutectic material formed a nanofibrous or amorphous structure. Under high cooling rate conditions, the ZrO₂-rich material developed a nanocomposite structure ( t -ZrO₂+α-Al₂O) directly by melt-quenching, whereas, more typically, such a structure was developed only after postannealing of the as-quenched metastable material.     

Preparation of Nanometer-Sized α-Alumina Powders by Calcining an Emulsion of Boehmite and Oleic Acid

This study proposes a method to form ultrafine α-Al₂O₃ powders. Oleic acid is mixed with Al(OH)₃ gel. The gel is the precursor of the Al₂O₃. After it is mixed and aged, the mixture is calcined in a depleted oxygen atmosphere between 25° and 1100°C. Oleic acid evaporates and decomposes into carbon during the thermal process. Residual carbon prevents the growth of agglomerates during the formation of α-Al₂O₃. The phase transformation in this process is as follows: emulsion →γ-Al₂O₃→δ-Al₂O₃→θ-Al₂O₃→α-Al₂O₃. This process has no clear θ phase. Aging the mixed sample lowers the formation temperature of α-Al₂O₃ from 1100° to 1000°C. The average crystallite diameter is 60 nm, measured using Scherrer's equation, which is consistent with TEM observations.     

Preparation of Larnthanum β-Alumina with High Surface Area by Coprecipitation

Mixtures of La₂O₃ and Al₂O₃ with various La contents were prepared by co-precipitation from La(NO₃)₃ and Al(NO₃)₃ solutions and calcined at 800° to 1400°C. The addition of small amounts of La₂O₃ (2 to 10 mol%) to Al₂O₃ gives rise to the formation of lanthanum β-alumina (La 2 O₃·11–14Al₂O₃) upon heating to above 1000°C and retards the transformation of γ-Al₂O₃ to α-Al₂O₃ and associated sintering.     

Development of Nano-Composite Microstructures in ZrO2-Al2O3 via the Solution Precursor Method

Aqueous mixtures of zirconium acetate and aluminum nitrate were pyrolyzed and crystallized to form a metastable solid solution, Zr_{1- x} Al _x O_{2− x /2} ( x < 0.57). The initial, metastable phase partitions at higher temperatures to form two metastable phases, viz., t −ZrO₂+γ-Al₂O₃ with a nano-scale microstructure. The microstructural observations associated with the γ- →α-Al₂O₃ phase transformation in the t -ZrO₂ matrix are reported for compositions containing 10, 20, and 40 mol% A1₂O₃. During this phase transformation, the α-Al₂O₃ grains take the form of a colony of irregular, platelike grains, all with a common crystallographic orientation. The plates contain ZrO₂ inclusions and are separated by ZrO₂ grains. The volume fraction of A1₂O₃ and the heat treatment conditions influence the final microstructure. At lower volume fractions of A1₂O₃, the colonies coarsen to single, irregular plates, surrounded by polycrystalline ZrO₂. Interpenetrating microstructures produced at high volume fractions of A1₂O₃ exhibit very little grain growth for periods up to 24 h at 1400°C.     

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.     

Crystallization of MgO-Al2O3-SiO2-ZrO2 Glasses

The crystallization of MgO-Al₂O₃-SiO₂-ZrO₂ glasses at 1000°C was studied. Isothermal heat treatments of a cordierite-based glass (2MgO.2Al₂O₃.5SiO₂= Mg₂Al₄Si₅O₁₈) with 7 wt% ZrO₂ produced surface crystallization of α-cordierite and tetragonal ZrO₂ ( t -ZrO₂). These phases advanced into the glass by cocrystallization of t -ZrO₂ rods in an α-cordierite matrix with a well-defined orientation relation. The t -ZrO₂ rods were unstable with respect to diffusional breakup (a Rayleigh instability) and decomposed into rows of aligned ellipsoidal and spheroidal particles. The t -ZrO₂ was very resistant to transformation to monoclinic symmetry. With a similar glass containing 15 wt% ZrO₂, surface crystallization of α-cordierite and t -ZrO₂ was accompanied by internal crystallization of t -ZrO₂ dendrites. Transformation of the dendrites to mono-clinic symmetry was observed under some conditions.     

Synthesis of AlN Nanopowder from γ-Al2O3 by Reduction–Nitridation in a Mixture of NH3–C3H8

Aluminum nitride (AlN) powders were synthesized by gas reduction–nitridation of γ-Al₂O₃ using NH₃ and C₃H₈ as the reactant gases. AlN was identified in the products synthesized at 1100°–1400°C for 120 min in the NH₃–C₃H₈ gas flow confirming that AlN can be formed by the gas reduction–nitridation of γ-Al₂O₃. The products synthesized at 1100°C for 120 min contained unreacted γ-Al₂O₃. The ²⁷A1 MAS NMR spectra show that Al–N bonding in the product increases with increasing reaction temperature, the tetrahedral AlO₄ resonance decreasing prior to the disappearance of the octahedral AlO₆ resonance. This suggests that the tetrahedral AlO₄ sites of the γ-Al₂O₃ are preferentially nitrided than the AlO₆ sites. AlN nanoparticles were directly formed from γ-Al₂O₃ at low temperature because of this preferred nitridation of AlO₄ sites in the reactant. AlN nanoparticles are formed by gas reduction–nitridation of γ-Al₂O₃ not only because the reaction temperature is sufficiently low to restrict grain growth, but also because γ-Al₂O₃ contains both AlO₄ and AlO₆ sites, by contrast with α-Al₂O₃ which contains only AlO₆.     

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.     

Mechanochemical Synthesis of Magnesium Aluminate Spinel Powder at Room Temperature

MgAl₂O₄ spinel was successfully synthesized using a mechanochemical route that avoided the formation and calcination of its precursors at high temperatures. The method involved a single step in which γ-Al₂O₃–MgO, AlO(OH)–MgO, and α-Al₂O₃–MgO mixtures were milled at room temperature under air atmosphere. The formation of MgAl₂O₄ occurred faster with γ-Al₂O₃ than with AlO(OH) or α-Al₂O₃. After 140 h, the mechanochemical treatment of the γ-Al₂O₃–MgO mixture yielded 99% of MgAl₂O₄.     

Mechanical-Activation-Triggered Gibbsite-to-Boehmite Transition and Activation-Derived Alumina Powders

Mechanical activation of monoclinic gibbsite (Al(OH)₃) in nitrogen led to the formation of nanocrystalline orthorhombic boehmite (AlOOH) at room temperature. The boehmite phase formed after merely 3 h of mechanical activation and developed steadily as the mechanical-activation time increased. Forty hours of mechanical activation resulted in essentially single-phase boehmite, together with α-alumina (α-Al₂O₃) nanocrystallites 2–3 nm in size. The sequence of phase transitions in the activation-derived boehmite was as follows: boehmite to γ-Al₂O₃ and then to α-Al₂O₃ when flash-calcined at a heating rate of 10°C/min in air. γ-Al₂O₃ formed at 520°C, and flash calcination to 1100°C led to the formation of an α-Al₂O₃ phase, which exhibited a refined particle size in the range of 100–200 nm. In contrast, the gibbsite-to-boehmite transition in the unactivated gibbsite occurred over the temperature range of 220°–330°C. A flash-calcination temperature of 1400°C was required to complete the conversion to α-Al₂O₃ phase, with both δ-Al₂O₃ and θ-Al₂O₃ as the transitional phases. The resulting alumina powder consisted of irregularly shaped particles 0.4–0.8 μm in size, together with an extensive degree of particle agglomeration.     

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.     

New Sol–Gel Route for the Preparation of Pure α-Alumina at 950°C

An anhydrous alumina (Al₂O₃) sol was prepared from aluminum isopropoxide and an organic solvent, using an acetic acid stabilizer. The complete conversion of the dried sol to α-Al₂O₃ was accomplished at a temperature of 950°C by a single transition via γ-Al₂O₃. Al₂O₃ that was deposited via dip coating resulted in amorphous films, even after annealing at 1100°C, because of the silicon diffusion from the substrate. This phenomenon was avoided using a rapid thermal treatment in a flame after dip coating, which resulted in uniform thin films that are converted to α-Al₂O₃ via heat treatment.