Densification Behavior of Single-Crystal and Polycrystalline Spherical Particles of Zirconia

Micrometer size polydispersed spheres of zirconia were produced via electrostatic atomization and pyrolysis of aqueous zirconium acetate-yttrium acetate precursors. Varying the precursor composition in the ZrO₂-rich region of the ZrO₂-Y₂O₃ binary system resulted in the production of either single-crystal (0 and 10 mol% Y₂O₃O or dense polycrystalline (3 mol% Y₂O₃) zirconia spheres of similar size distribution for densification studies. Powders of either the singlecrystal or the polycrystalline particles exhibited contrasting densification behavior; viz., powder compacts composed of polycrystalline particles obtained significantly higher endpoint densities than their single-crystal counterparts. Microstructural observations showed that while necks between single-crystal particles reached a stable size, necks between polycrystalline particles continued to grow.     

Preparation of Porous Glass-Ceramics with a Skeleton of NASICON-Type Crystal CuTi2(PO4)3

A novel porous glass-ceramic with a skeleton of CuTi₂(PO₄)₃ was prepared by controlled crystallization of a glass and subsequent chemical leaching of the resulting dense glass-ceramic. A volume-crystallized dense glass-ceramic composed of CuTi₂(PO₄)₃ and Cu₃(PO₄)₂ whose surface was covered by a CuO thin layer was prepared by reheating a glass with a nominal composition of 50CuO·20TiO₂30P₂O₅ (in mol%) glass in air. When the resultant glass-ceramic was leached with dilute H₂SO₄, Cu₃(PO₄)₂ and CuO phases were dissolved out selectively, leaving a crystalline CuTi₂(PO₄)₃ skeleton. The specific surface area and the average pore radius of the porous glass-ceramic obtained were approximately 45 m₂g_-1 and 9 nm, respectively. The porous glass-ceramic showed catalytic activity in the conversion reaction of propene into acrolein.     

Microporous Glass-Ceramics in NASICON-Type Copper(II) Titanium Phosphate

A novel porous glass-ceramic with a skeleton of a NASICON-type copper(II) titanium phosphate was prepared via the controlled crystallization of a glass and the subsequent chemical leaching of the resulting dense glass-ceramic. A volume-crystallized dense glass-ceramic comprised of CuTi₂(PO₄)₃ and Cu₃(PO₄)₂, whose surface was covered by a thin layer of CuO, was prepared by reheating a glass with a nominal composition of 50CuO20TiO₂30P₂O₅ (in mol%) in air. When the resulting glass-ceramic was leached with dilute HCl, the Cu₃(PO₄)₂ and CuO phases were dissolved out selectively, and a cuprous NASICON crystal of CuTi₂(PO₄)₃ was converted to its cupric type, CuTi₄(PO₄)₆, which was left as a skeleton of the porous materials. The specific surface area and the average pore radius of the porous glass-ceramic obtained were ∼70 m²/g and ∼7 nm, respectively. The porous glass-ceramic showed high catalytic activities for the dehydration of 2-propanol.     

Preparation of Monodisperse Zirconia Particles by Thermal Hydrolysis in Highly Concentrated Solutions

Monodisperse zirconia particles were prepared by the thermal hydrolysis of mixtures of zirconyl chloride, zirconium hydroxide, and water at high concentrations corresponding to about 5 mol/L Zr. The particles, as first prepared, were temporarily agglomerated spheres composed of primary ultrafine zirconia crystals. The agglomerated particles collapsed and dispersed in water to form a translucent sol. When vacuum dried and followed by heat treatment, they were not dispersible. The size of the agglomerated particles increased with increasing molar ratio of the zirconium chloride in the starting mixture, varying from about 0.2 to 0.6 μm. Using the sample thus obtained, monodisperse tetragonal zirconia particles of about 0.35 μm containing 3 mol% Y₂O₃ with a relatively uniform composition were obtained by homogeneous precipitation of YOHCO₃ by heating with urea and calcination at 800°C.     

Preparation of Zirconia-Toughened Bioactive Glass-Ceramic Composite by Sinter–Hot Isostatic Pressing

High-strength bioactive ceramics, MgO-CaO-SiO₂-P₂O₅ glass-ceramic composites toughened by zirconia, were prepared by sinter-hot isostatic pressing (sinter-HIPing) to achieve easier mass production and higher reliability. Raw materials for preparing a dense presintered body were investigated to obtain almost complete densification by sinter-HIPing. It was found that the densification of a presintered body was influenced by residual glassy phases in the crystallized glass particles. By using a controlled crystallized glass powder and a zirconia powder as raw materials, a presintered body with a relative density higher than 94% was prepared, and then it was densified to near its theoretical density by sinter-HIPing. This bioceramic exhibited an extremely high bending strength of 400 to 1000 MPa and fracture toughness of 3 to 5 MPa.m^1/2 for 30 to 80 vol% of zirconia.     

High-Strength Mica-Containing Glass-Ceramics

Glass-ceramics containing barium–mica in the system Ba_0.5 Mg₃ (Si₃AIO₁₀)F₂–2MgO · 2Al₂O₃· 5SiO₂–Ca₃ (PO₄)₂ are two to three times stronger than conventional mica-containing glass-ceramics. Moreover, the barium-mica glass-ceramics are easier to machine, as confirmed by a drilling test using conventional steel tools. Such mechanical properties are attributable to the microstructure of the barium–mica glass-ceramics. Very fine, interlocking mica crystals are precipitated in the glass, and a crack-deflection mechanism is observed by scanning electron microscopy.     

A Novel Supercritical CO2 Synthesis of Amorphous Hydrous Zirconia Nanoparticles, and Their Calcination to Zirconia

A novel synthesis of amorphous hydrous zirconia nanoparticles was performed in a supercritical carbon dioxide (scCO₂) reverse microemulsion, converting a high concentration of a very inexpensive starting material (zirconyl nitrate hydrate) into a product that was then calcined to yield monoclinic zirconia nanoparticles. The amorphous hydrous zirconia precursor particles were obtained by simply adding a precipitating agent to [Zr⁴⁺_(aq)]/perfluoropolyether/scCO₂. Calcination converts the amorphous hydrous zirconia precursor into the oxide, and the corresponding phase changes that occur were confirmed by differential thermal analysis. Some control of particle size and shape (ellipticity) could be achieved by selecting the reaction pressure from within the range over which stable microemulsions are obtained (13.9–17.3 MPa): a higher reaction pressure yields smaller and more spherical particles. This novel route for the synthesis of zirconia nanoparticles is both "green" (environmentally friendly) and economical.     

Isomorphous Substitution for Silicon in Fluorphlogopite

Attempts to substitute for silicon in the fluorphlogopite structure resulted in the synthesis of a fluorgermanium mica and a boron phosphate micalike orthorhombic crystal. The batch material contained Al₂O₃, MgO, MgF₂, K₂CO₃, plus GeO₂ for forming the fluorgermanium mica and BPO₄ for forming the boron phosphate crystal. Optical analysis with a polarizing microscope and by X-ray diffraction verified the mica structure of the germanium crystal and disproved the mica structure of the boron phosphate crystal. The indices of refraction and unit-cell constants were determined for both crystals. A study of the electrical properties up to temperatures of 1000°F showed that the dielectric properties of the fluorgermanium mica were very similar to that of fluorphlogopite.     

Partial Stabilization of Tetragonal Zirconia in Oxynitride Glass-Ceramics

Nitrogen (via a polymeric AlN precursor) and ZrO₂ are introduced into a MgO-CaO-Al₂O₃-SiO₂-glass. Subsequent crystallization of the glass results in a fine-grained oxynitride glass-ceramic. The microstructure of the latter is found to be entirely different for nitrogen concentrations of 4.85 and 9.94 mol%. Not only do the phase contents differ, but also tetragonal zirconia is more effectively stabilized at higher nitrogen concentrations. Partial stabilization of tetragonal zirconia is not due to nitrogen incorporation but is based on an indirect effect: the spherical morphology of tetragonal zirconia precipitated at higher nitrogen contents suppresses the nucleation of the martensitic transformation. The beneficial effects of the introduction of nitrogen and the simultaneous incorporation of zirconia into a glass-ceramic result in overall improved mechanical properties.     

Joining of Calcium Phosphate Invert Glass-Ceramics on a β-Type Titanium Alloy

A bioactive calcium phosphate invert glass-ceramic containing β-Ca₃(PO₄)₂ crystals could be joined strongly with a Ti–29Nb–13Ta–4.6Zr alloy consisting of a β-titanium phase by heating the metal on which the mother glass powders with a composition 60CaO·30P₂O₅·7Na₂O·3TiO₂ (mol%) were placed, at 800°C for 1 h in air; the tensile joining strength was estimated to be ∼26 MPa on average. A compositionally gradient layer was developed on the metallic substrate during the heating. When the metal with glass powders on it was heated at 850°C in air, the phosphate glassy phase flowed viscously, permeating the oxide layer formed around the surface of the metal, which was thicker than that formed by heating at 800°C; a compositionally gradient layer was not developed, and a strong joining was not realized. The joining between the glass-ceramic and the metal is suggested to be controlled by viscous flow of the glassy phase in the glass-ceramic and by reaction of the glassy phase with the oxide phase formed around the surface layer of the metal.     

Strengthening Glass-Ceramics by Application of Compressive Glazes

Glasses in the Na₂O–Ba0–A1₂O₃-Si0₂ system, nucleated with TiO₂, were heat-treated to effect controlled crystallization. Resulting materials consisted of a dense, micro-crystalline mixture of nepheline (Na₂0–A1₂O₃-2SiO₂) and barium feldspar (BaO-A1₂O₃-2Si0₂) in a glassy matrix. Thermal expansion coefficients (O° to 300° C) of these bodies ranged from 75 to 125 × 10 –7/°C. Glazes in the Na₂O-CaO-PbO-B₂O₂-A1₂0₃-SiO₂ system having expansion coefficients of about 40 to 80 × 10 ^-7/0°C were applied to the glass-ceramics. On firing, the glazes matured well and reacted with the bodies to form interlocking crystals at the interface. This interfacial region was investigated using several instrumental techniques, and the crystals were identified as plagioclase feldspar. Applying these compressive glazes resulted in modular of rupture up to five times that of the initial glass-ceramic. Calculated strengths correlated well with experimental values.     

Biaxial Strength of a ZrO2/(Ni+Al2O3) Nanocomposite

Previous studies demonstrated that the strength of zirconia (ZrO₂) could be enhanced or reduced by respectively adding micrometer-sized alumina (Al₂O₃) or nickel (Ni) particles. In the present study, 5 vol% micrometer-sized Al₂O₃ particles and 1 vol% nanometer-sized Ni particles are incorporated into the ZrO₂ matrix, which is subsequently densified by pressureless sintering. The biaxial strength of the ZrO₂/(Ni+Al₂O₃) nanocomposite is nearly double that of the monolithic ZrO₂. The increase in strength correlated with a reduction in the critical flaw size and not with any change in toughness, which may be a result of grain boundary strengthening.     

Preparation of Zirconia Whiskers from Zirconium Hydroxide in Sulfuric Acid Solutions under Hydrothermal Conditions at 200°C

Mixtures of Zr(OH)₄ and ZrO₂ particles, ∼10 nm in size, were hydrothermally treated in 0.25–1.5 mol/L H₂SO₄ solutions at a temperature of 200°C. After 3 h, very short ZrO₂ fibers, 10–30 nm in length, were obtained, with no other zirconium compounds observed. The particles grew with treatment time and resulted in whisker particles. In a higher concentration (3 mol/L) H₂SO₄ solution, ZrO₂ whiskers were not obtained, and clear solutions resulted with the starting ZrO₂ particles remaining. It was concluded that Zr(OH)₄ was useful as a starting material and that nanosized ZrO₂ particles served as seed crystals for whisker formation.     

Preparation of Fine, Spherical Yttria-Stabilized Zirconia by the Spray-Pyrolysis Method

Fine yttria-stabilized zirconia powders were prepared by the spray pyrolysis of aqueous solutions of ZrOCl₂·8H₂O and Y(NO₃)₃·5H₂O (3 mol%). An appropriate thermal treatment resulted in slightly porous spherical particles with a narrow size distribution. The sintering ability of these powders was evaluated.     

Transformation Toughening in Sol–Gel-Derived Alumina-Zirconia Composites

The microstructures of α-Al₂O₃ seeded sol–gel-derived alumina-zirconia composites containing 20 wt% unstabilized zirconia (processed from zirconium n-propoxide) were very fine, with submicrometer alumina grains and small, mainly intergranular zirconia particles, the latter having a critical size for the tetragonal-to-monoclinic phase transformation of 0.45 μm. The corresponding ratios of the toughening contribution to the matrix toughness are relatively low (δK_c/K₀<1). This finding is confirmed by an analysis of the tetragonal zirconia particle size dependence of the stress-induced transformation toughening.     

Raman Spectroscopic Studies on the Formation Mechanism of Hydrous-Zirconia Fine Particles

The Raman spectra of hydrous-zirconia fine particles produced by the hydrolysis of various ZrOCl₂ solutions were investigated. The Raman spectra of hydrous zirconia synthesized at HCl concentrations below 1 mol/L were similar to those of monoclinic, crystalline ZrO₂; those of hydrous zirconia synthesized at HCl concentrations greater than 1 mol/L showed a crystal structure change. The line width of the Raman bands increased with increasing H⁺ ion concentration. Analyzing the relationship between Raman band width and particle size revealed that the primary particle size of hydrous zirconia was controlled by the H⁺ and Cl⁻ ions, because these ions interfered with the polymerization in a hydrolysis reaction. Based on the experimental results, the formation mechanism for primary particles of hydrous zirconia was determined.     

Direct Observation of the Crystallization of Li2O-Al2O3-SiO2 Glasses Containing TiO2

The nucleation and crystallization of Li₂O-Al₂O₃-SiO₂ glasses containing TiO₂ were investigated using transmission electron microscopy of thin sections produced from bulk samples. Phase separation occurs during cooling from the melt, and on heating, a large number of titanium-aluminum crystals approximately 50 A in diameter are formed. These crystals are the heterogeneous nuclei for the crystallization of the remaining glass. Photomicrographs of various stages of crystallization show the development of the fine-grained glass-ceramic.     

Preparation and Characterization of Fine-Grained 3Y-TZP/BaFe12O19 In Situ Composites

Composites of BaFe₁₂O₁₉ particles dispersed throughout a 3-mol%-yttria-doped zirconia (3Y-TZP) matrix have been prepared using the pressureless reactive sintering of 3Y-TZP, BaCO₃, and γ-Fe₂O₃ powders. The reaction behavior of the mixed powder was studied with an in situ , high-temperature powder X-ray diffraction technique. The composite that was sintered at 1300°C consisted of submicrometer-sized 3Y-TZP grains and BaFe₁₂O₁₉ particles; the size of the 3Y-TZP grains was ∼100-300 nm, and the size of the BaFe₁₂O₁₉ particles was ∼200-500 nm. Based on the grain size, most of the BaFe₁₂O₁₉ grains presumably had a single-magnetic-domain structure. The 3Y-TZP/20-wt%-BaFe₁₂O₁₉ composite showed high magnetization and coercivity values, despite the low concentration of ferromagnetic phase. Preliminary mechanical tests revealed that the composite possessed moderately good mechanical properties.     

Internal Stresses and the Martensite Start Temperature in Alumina-Zirconia Composites: Effects of Composition and Microstructure

n alumina-based composites containing ceria-stabilized tetragonal zirconia, the martensite start temperature ( M_s ) of the tetragonal-to-monoclinic zirconia phase transformation exhibits a grain size dependence that becomes increasingly pronounced as the zirconia content decreases. Neutron diffraction experiments confirm earlier dilatometry measurements of M _s in composites containing ≥20 vol% ZrO₂ and were instrumental in obtaining M _s values in lower zirconia content (i.e., 10 vol%) composites. The dependence of M _s on zirconia content is related to the internal stresses that arise from differences in thermal expansion coefficients between the two phases. Neutron diffraction measurements show that the internal tensile stresses in the zirconia grains increase with decreasing zirconia content. The measured internal stresses are in quantitative agreement with predictions based on models assuming isolated ZrO₂ particles at low zirconia contents and a continuous ZrO₂"matrix" phase at higher zirconia contents. This assumption is consistent with the observed microstructural development in which the low zirconia contents result in isolated zirconia grains, whereas higher zirconia contents result in more interconnected zirconia grains.     

Critical Microstructures for Microcracking in Al2O3-ZrO2 Composites

The internal strains asSociated with the martensitic phase transformation of zirconia were used to introduce microcracks into Al₂O₃/ZrO₂ composites. The degree of transformation was found to be dependent on the volume fraction of ZrO₂ and its size, the latter of which could be controlled by suitable heat treatments. The microstructural changes that occurred during the heat treatments were studied using quantitative microscopy and X-ray diffraction. For materials containing more than 7.5 vol% Zr0₂, the ZrO₂ particles were found to pin the Al₂O₃ grain boundaries, thus limiting the Al₂O₃ grain growth. The limiting grain size was found to be dependent on size and volume fraction of ZrO₂. Heat treatments for the higher volume fraction materials (>7.5 vol% ZrO₂) caused micro-structural changes which resulted in increased amounts of monoclinic ZrO₂ at room temperature; elastic modulus measurements indicated that this was occurring concurrently with microcracking. By combining the ZrO₂ grain-size distributions with the X-ray analysis it was possible to calculate the critical ZrO₂ size required for the transformation. The critical size was found to decrease with increasing amounts of ZrO₂. Hardness and indentation fracture toughness were measured on the composites. Grain fragmentation was observed at the edge of the indentations and microcracks were observed directly, using an AgNO₃ decoration technique, near the indentations.