Formation Mechanism of Hydrous-Zirconia Particles Produced by Hydrolysis of ZrOCl2 Solutions: II

The primary and secondary particle sizes of monoclinic hydrous-ZrO₂ particles produced by the hydrolysis of various ZrOCl₂ solutions, with and without the addition of NaCl, CaCl₂, or AlCl₃, were measured using X-ray diffraction and transmission electron microscopy in order to clarify the formation mechanisms of primary and secondary particles. The primary particle size of hydrous ZrO₂, under a constant ZrOCl₂ concentration, decreased monotonously with increasing Cl⁻-ion concentration. On the contrary, the secondary particle size increased monotonously with increasing Cl⁻-ion concentration. The present experimental results revealed that the primary and secondary particle sizes of hydrous ZrO₂ are controlled primarily by the concentrations of H⁺ and Cl⁻ ions produced during hydrolysis, and are independent of the type of added metal ions. The formation mechanisms of the primary and secondary particles of hydrous ZrO₂ were determined on the basis of the present experimental results.     

Formation Mechanism of Hydrous-Zirconia Particles Produced by Hydrolysis of ZrOCl2 Solutions

The secondary particle size of hydrous-zirconia fine particles that have been produced by the hydrolysis of various ZrOCl₂ solutions, with and without the addition of NH₄OH or HCl, was measured using transmission electron microscopy to investigate the effects of the H+ and Cl−ion concentrations on the formation of secondary particles. The average secondary particle size of hydrous zirconia increased as the H+ion concentration increased, attaining a maximum of 200 nm at an H−ion concentration of 0.44 moldm-3. Further increases in the H+ion concentration then caused a decrease in the average particle size. The present experimental results revealed that the secondary particle size of hydrous zirconia is controlled primarily by the concentration of H+ ions that are produced during hydrolysis. The formation mechanism for the secondary particles in hydrous zirconia was determined on the basis of the present experimental results.     

Measurement of the Crystallographically Transformed Zone Produced by Fracture in Ceramics Containing Tetragonal Zirconia

During fracture of ceramics containing tetragonal zirconia particles, a volume of zirconia material on either side of the crack irreversibly transforms to the monoclinic crystal structure. Transformation zone sizes, measured using Raman microprobe spectroscopy, are presented for three sintered ceramics. In a single-phase ZrO₂−3.5 mol% Y₂O₃ material, an upper bound measurement of 5 μm is obtained for the zone size. In the Al₂O₃/ZrO₂ composites studied, the zone size is deduced to correspond to ∼1 grain in diameter. On the basis of the monoclinic concentrations derived from the Raman spectra it is further concluded that only a fraction of the ZrO₂ grains within the transformation zone transform, providing indirect evidence for the effect of particle size on the propensity for transformation.     

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.     

Synthesis of Titanate Derivatives Using Ion-Exchange Reaction

Two types of titanate derivatives, layered hydrous titanium dioxide (H₂Ti₄O₉· n H₂O) and potassium octatitanate (K₂Ti₈O₁₇) with a tunnellike structure, were synthesized using an ion-exchange reaction. Fibrous potassium tetratitanate (K₂Ti₄O₉· n H₂O) was prepared by calcination of a mixture of K₂CO₃ and TiO₂ with a molar ratio of 2.8 at 1050°C for 3 h, followed by boiling-water treatment of the calcined products for 10 h. The material then was transformed to layered H₂Ti₄O₉· n H₂O through an exchange of K⁺ ions with H⁺ ions using HCl. K₂Ti₈O₁₇ was formed by a thermal treatment of KHTi₄O₉· n H₂O. Pure KHTi₄O₉· n H₂O phase was effectively produced by a treatment of K₂Ti₄O₉ with 0.005 M HCl solution for 30 min. Thermal treatment at 250°–500°C for 3 h resulted in formation of only K₂Ti₈O₁₇.     

Electroless Copper Coating of Zirconia Utilizing Palladium Catalyst

Electroless Cu coating of zirconia particles using a Pd⁰ catalyst is investigated in this paper. Uncoated and Cu-coated ZrO₂ particles are characterized by various analytical techniques to understand the mechanism of electroless Cu coating of ZrO₂ particles, which involves reduction of Pd²⁺ on ZrO₂ particle surfaces by Sn²⁺ to produce pure metallic Pd⁰ clusters, which provide catalytic sites for Cu deposition. The concept of XPS core-level binding energy shift due to small cluster size is utilized to predict the size of pure Pd⁰ clusters. The electroless Cu-coating process can be utilized in the future to produce Cu-doped ZrO₂ particles required for various potential applications.     

Formation Mechanism of Hydrous Zirconia Particles Produced by the Hydrolysis of ZrOCl2 Solutions: III, Kinetics Study for the Nucleation and Crystal-Growth Processes of Primary Particles

The formation rate and primary particle size of monoclinic, hydrous zirconia particles produced by the hydrolysis of various ZrOCl₂ solutions (with and without the addition of HCl, NH₄OH, NaCl, CaCl₂, or AlCl₃) were measured to clarify the effects of the H⁺ and Cl⁻ ion concentrations on the nucleation and crystal-growth processes of primary particles of hydrous zirconia. Chemical-kinetic analyses, to which Avrami–Erofeev's equation was applied, and XRD measurements revealed that both the rate constant and the primary particle size of the hydrous zirconia decreased as the concentrations of H⁺ and/or Cl⁻ ions produced by hydrolysis increased. The nucleation rate per unit of ZrOCl₂ concentration and the crystal-growth rate of the primary particles of the hydrous zirconia were determined by analyzing the relationships between the rate constant and primary particle size. The nucleation rate per unit of ZrOCl₂ concentration revealed almost no change and remained constant as the H⁺ and/or Cl⁻ ion concentrations increased, except in the case of a slight increase for ZrOCl₂ solutions with added HCl. The crystal-growth rate decreased as the H⁺ and/or Cl⁻ ion concentration increased. The present kinetic analyses revealed that the decrease in rate constant with increasing H⁺ and/or Cl⁻ ion concentrations resulted from the decrease in the crystal-growth rate. The decreasing tendency of the crystal-growth rate was attributed to interference with crystal growth by the Cl⁻ ions attracted onto the particle surface through the formation of an electric double layer. The formation mechanisms for the primary particles of hydrous zirconia were determined based on the present experimental results.     

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.     

Effect of Tetravalent Dopants on Raman Spectra of Tetragonal Zirconia

Tetragonal zirconia solid solutions were prepared by the additions of ceria, titania, germania, and cassiterite into 2-mol%-Y₂O₃-stabilized t -ZrO₂ (2Y-TZP), and the effect of the dopant size on the changes in the Raman spectra of t -ZrO₂ was investigated. The frequencies of the Raman modes that were associated with Zr-O bond stretching decreased as doping with the oversized cation (Ce⁴⁺) increased and increased linearly as doping with the undersized dopants (Ti⁴⁺, Ge⁴⁺, Sn⁴⁺) increased within the solubility limits of the dopants in 2Y-TZP. The frequency of the 640 cm⁻¹ Raman band decreased as the square root of the unit-cell volume increased, suggesting that the ionic size of dopants governs the Raman mode that is related to the short ZrO bond between two ZrO bonds in t -ZrO₂ solid solutions. Although there was no consistent relationship between the change in the tetragonality of t -ZrO₂ solid solutions and the tetravalent cation size, the intensity ratio of the Raman modes at 609 and 640 cm⁻¹ ( I ₆₀₉/ I ₆₄₀) decreased as the ionic size decreased.     

Synthesis and Growth Kinetics of Monodispersive Indium Hydrate Particles

Nano- or submicron In(OH)₃ and In₂O₃ particles of different morphologies were synthesized from a nitrate solution by a homogeneous precipitation process. By using X-ray diffractometer, thermogravimetric analysis, transmission and scanning electron microscopes, and inductively coupled plasma-optical emission spectrometry, the properties of particle growth were analyzed. The results indicated that the kinetics of the hydrolysis reaction of In³⁺ was a zero-order reaction with an activation energy of 128 kJ/mol, which implied that the reaction was controlled by the decomposition kinetic of urea additive. The growth anisotropic of particles, pH value of reaction solutions, residual In³⁺ concentration relative to aging time with different temperatures and starting concentrations were reported in this study. Calcination of the hydrate to form In₂O₃ particles between 300° and 900°C did not greatly change the morphologies of the particles.     

Formation of Ultrafine Tetragonal ZrO2 Powder Under Hydrothermal Conditions

Ultrafine tetragonal ZrO₂ powder was prepared by hydrothermal treatment at 100 MPa of amorphous hydrous zirconia with distilled water and LiCl and KBr solutions. The resulting powder consisted of well-crystallized particles; at 200°C, the particle size was 16 nm and at 500°C, 30 nm. Under hydrothermal conditions tetragonal ZrO₂ appears to crystallize topotactically on nuclei in the amorphous hydrous zirconia.     

Formation of Sm2+ lons in Sol-Gel-Derived Glasses of the System Na2 O-Al2O3-SiO2

Samarium ions (Sm²⁺) incorporated into aluminosilicate glasses by a sol-gel process showed persistent spectral hole burning at room temperature. Gels of the system Na₂O-Al₂O₃SiO₂ synthesized by the hydrolysis of Si(OC₂H₅)₄, Al(OC₄H₉)₃, CH₃ COONa, and SmCl₃·6H₂O were heated in air at 500°C, then reacted with H₂ gas to form Sm²⁺ ions. Whereas Al³⁺ ions effectively dispersed the Sm³⁺ ions in the glass structure, Na⁺ ions were not effective. The Al₂O₃-SiO₂ glasses proved appropriate for reacting the Sm³⁺ ions with H₂ gas and exhibited the intense photoluminescence of Sm²⁺ ions. The reaction of Sm³⁺ ions with H₂ in the Al₂O₂-SiO₂ glasses was determined by first-order kinetics, and the activation energy equaled 95 kJ/mol. At 800°C, the maximum photoluminescence of the Sm²⁺ ions was achieved within 20 min.     

含镁铝杂质硫酸氧钛溶液水热水解规律

探究了含镁铝杂质硫酸氧钛溶液[1.0 mol/L TiOSO₄+0.5 mol/L MgSO₄+0.25 mol/L Al₂（SO₄）₃+2.4 mol/L H₂SO₄]的水热水解规律。热力学计算表明：110~150 ℃,加入MgSO₄ 和Al₂（SO₄）₃的总体效应使钛的理论水解率略有降低[150 ℃时,纯硫酸氧钛溶液和含镁铝杂质硫酸氧钛溶液的理论水解率分别为99.8%和99.7%,镁铝杂质分别以MgSO_4（aq）和Al（OH）²⁺为主];升高温度有利于含钛组分水解,从110 ℃升高到150 ℃含镁铝杂质硫酸氧钛溶液的理论水解率由99.1%升高到99.7%。实验结果表明：加入MgSO₄ 和Al₂（SO₄）₃后钛水解率略有降低（150 ℃时,纯硫酸氧钛溶液和含镁铝杂质硫酸氧钛溶液的水解率分别为99.2%和98.3%）,升高温度可显著强化含镁铝杂质硫酸氧钛溶液的水解（110 ℃→150 ℃,68.6%→98.3%）,与热力学计算结果相符。在150 ℃水热反应10 h,可制得原始粒径为100~300 nm、团聚粒径为1~3 μm的不规则状偏钛酸,含钛组分水解率达到98.3%。     

Influence of Minor Ions on the Stability and Hydration Rates of β-Dicalcium Silicate

The effects of Al³⁺, B³⁺, P⁵⁺, Fe³⁺, S⁶⁺, and K⁺ ions on the stability of the β-phase and its hydration rate were studied in reactive dicalcium silicate (C₂S, Ca₂SiO₄) synthesized using the Pechini process. In particular, the dependences of the phase stability and degree of hydration on the calcination temperature (i.e., particle size) and the concentration of the stabilizing ions were investigated. The phase evolution in doped C₂S was determined using XRD, and the degree of hydration was estimated by the peak intensity ratio of the hydrates to the nonhydrates in ²⁹Si MAS NMR spectra. The stabilizing ability of the ions varied significantly, and the B³⁺ ions were quite effective in stabilizing the β-phase over a wide range of doping concentrations. The hydration results indicated that differently stabilized β-C₂S hydrated at different rates, and Al³⁺- and B³⁺-doped C₂S exhibited increased degree of hydration for all doping concentration ranges investigated. The effect of the doping concentration on degree of hydration was strongly dependent on the stabilizing ions.     

Raman Investigation of the Nitridation of Yttria-Stabilized Tetragonal Zirconia

Raman spectroscopy has been used to obtain Raman spectra of yttria-stabilized tetragonal zirconia subject to surface nitridation induced by contact with zirconium nitride. Raman spectra recorded from regions at increasing distance from the source of nitridation have been used to obtain diffusion profiles from samples treated at different times and temperatures. The coupling of X-ray diffraction data previously taken and of the Raman spectra shows that in the samples there is a two-phase region (tetragonal + cubic) near the nitrided surface and that, at larger distance inside the samples, there is only one phase (tetragonal). Fitting of the diffusive profiles in the single-phase tetragonal region with an appropriate diffusion function allows the determination of the diffusion coefficient of nitrogen in tetragonal zirconia which is expressed in terms of the preexponential factor, D ₀= (3.98 ± 0.5) × 10⁻³ cm²/s, and the activation energy, Q = 170 ± 10 kJ/mol.     

Physicochemical Mechanism for the Continuous Reaction of γ-Al2O3-Modified Aluminum Powder with Water

Previous experiments showed that γ-Al₂O₃-modified Al powder could continuously react with water and generate hydrogen at room temperature under atmospheric pressure. In this work, a possible physicochemical mechanism is proposed. It reveals that a passive oxide film on Al particle surfaces is hydrated in water. OH⁻ ions are the main mobile species in the hydrated oxide film. When the hydrated front meets the metal Al surface, OH⁻ ions react with Al and release H₂. Because of the limited H-soluble capacity in small Al particles and the low permeability of the hydrated oxide film toward H⁺ species, H₂ molecules accumulate and form small H₂ gas bubbles at the Al:Al₂O₃ interface. When the reaction equilibrium pressure in H₂ bubbles exceeds a critical gas pressure that the hydrated oxide film can sustain, the film on the Al particle surfaces breaks and the reaction of Al with water continues. As the surface oxide layer on modified Al particles has a lower tensile strength, the critical gas pressure in H₂ bubbles is lower so that under an ambient condition, the reaction of modified Al particles with water is continuous. The proposed mechanism was further confirmed by a new experiment showing that the as-received Al powder could continuously react with water at temperatures above 40°C and under low vacuum, because the vacuum makes the critical gas pressure in H₂ bubbles decrease as well.     

Phase Transformation of Hydrous-Zirconia Fine Particles Containing Cerium Hydroxide

Monoclinic hydrous-zirconia fine particles that contained cerium(IV) hydroxide (Ce(OH)₄) were heated from 200°C to 600°C, to investigate the phase transformation to CeO₂-doped tetragonal ZrO₂. Both ZrOCl₂·8H₂O and CeCl₃·7H₂O were dissolved in aqueous solutions and then boiled to prepare the hydrous-zirconia particles. The Ce(OH)₄-containing hydrous-zirconia particles were prepared by adding aqueous ammonia into the boiled solutions. The monoclinic-to-tetragonal ( m right arrow t ) phase transformation of the Ce(OH)₄-containing hydrous zirconias was observed at 300°C using X-ray diffraction (XRD). XRD and Brunauer-Emmett-Teller (BET) specific surface area measurements revealed that the Ce(OH)₄-containing hydrous zirconias had a tendency to transform from the monoclinic phase to the tetragonal phase at lower temperatures as the primary particle size of the hydrous zirconia decreased and the Ce(OH)₄ content increased. These tendencies for the m right arrow t phase transformation agree with the conclusions that have been derived from thermodynamic and kinetic considerations.     

Raman Spectra of Rare-Earth Phosphate Glasses

Raman spectra have been recorded for glasses in the binary systems CeO₂-P₂O₅ and Pr₂O₃-P₂O₅. The cerium phosphate glasses were prepared having different concentrations of CeO₂ and the praseodymium phosphate glasses with different ratios of Pr³⁺ to Pr⁴⁺. The spectra indicate that both cerium and praseodymium enter the glass in modifying sites. We see no changes in the Raman spectra with Pr³⁺/Pr⁴⁺ ratio. Measurements of the density and glass transition temperature are also reported.     

Photocatalytic Activity of Monodispersed Spherical TiO2 Particles with Different Crystallization Routes

Monodispersed spherical TiO₂ particles were prepared by hydrothermal crystallization and/or calcination of spherical amorphous particles, synthesized by thermal hydrolysis of TiCl₄. The crystallized spherical particles were secondary agglomerates of primary nanocrystallites. Different crystallization routes and conditions provided the spherical TiO₂ particles with wide particle characteristics, such as the fraction of crystallization, the size and shape of the primary nanocrystallites, and the specific surface area. The photocatalytic activity showed complex dependence on the crystallization routes and conditions. The complex dependence behavior could be explained by combining the effects of the fraction of crystallization, the specific surface area, and the adsorption ability for hydroxyl ions. Especially, in the present study, the hydrothermally crystallized TiO₂ particles with large primary nanocrystallites showed the highest photocatalytic activity. The high photocatalytic activity mainly resulted from the high surface adsorption ability for hydroxyl ions, which was closely related to the well-developed (flat and faceted) morphology of primary nanocrystallite.     

Effect of the Presence of Ammonium Peroxodisulfate on the Direct Precipitation of Ceria and Ceria–Zirconia Solid Solutions from Acidic Aqueous Solutions

Direct precipitation of nanometer-sized particles of ceria–zirconia (CeO₂–ZrO₂) solid solutions with cubic and tetragonal structures was successfully attained from acidic aqueous solutions of cerium(III) nitrate (Ce(NO₃)₃) and zirconium oxychloride (ZrOCl₂) through the addition of ammonium peroxodisulfate ((NH₄)₂S₂O₈), because of promotion of the hydrolysis via the oxidation of Ce³⁺ ions, together with the simultaneous hydrolysis of ZrOCl₂ under hydrothermal conditions. Ultrafine CeO₂ particles also could be formed from relatively concentrated aqueous solutions of the same trivalent cerium salt in the presence of (NH₄)₂S₂O₈ via hydrolysis. The crystallite size and lattice strain of as-precipitated solid solutions varied, depending on the composition within the CeO₂–ZrO₂ system. Creation of a solid solution of ZrO₂ into a fluorite-type CeO₂ lattice clearly introduced lattice strain, as a consequence of the decreasing crystallite size. Both the direct precipitation process and the effectiveness of the presence of (NH₄)₂S₂O₈ for the synthesis of CeO₂–ZrO₂ solid solutions were discussed.