Immiscibility Area in the System TiO2–ZrO2–SiO2

A furnace for use in conjunction with the X-ray spectrometer was developed which was capable of heating small powdered specimens in air to temperatures as high as 1850°C. This furnace was also used for the heating and quenching of specimens in air from temperatures as high as 1850°C. An area of two liquids coexisting between 20 and 93 weight % TiO₂ above 1765°± 10°C. was found to exist in the system TiO₂–SiO₂, which is in substantial agreement with the previous work of other investigators. The area of immiscibility in the system TiO₂–SiO₂ was found to extend well into the system TiO₂–ZrO₂–SiO₂. The two liquids were found to coexist over a major portion of the TiO₂ (rutile) primary-phase area with TiO₂ (rutile) being the primary crystal beneath both liquids. The temperature of two-liquid formation in the ternary was found to fall about 80°C. with the first additions of ZrO₂ up to 3%. With larger amounts of ZrO₂ the change in the temperature of the boundary of the two-liquid area was so slight as to be within the limits of error of the temperature measurement. Primary-phase fields for TiO₂ (rutile), tetragonal ZrO₂, and ZrTiO₄ were found to exist in the system TiO₂–ZrO₂–SiO₂. SiO₂ as high cristobalite is known to exist in the system TiO₂–ZrO₂–SiO₂.     

Fluoride Model Systems: II, The Binary Systems CaF2–BeF2, MgF2–BeF2, and LiF–MgF2

Data obtained by quenching, thermal, and high-temperature X-ray techniques are presented for the three binary systems CaF₂–BeF₂, MgF₂–BeF₂, and LiF–MgF₂. The systems CaF₂–BeF₂ and MgF₂–BeF₂ are presented as weakened models of the systems ZrO₂–SiO₂ and TiO₂–SiO₂, respectively. The compound CaBeF₄ is a model of ZrSiO₄ (zircon). New data obtained for the system LiF–MgF₂ explain many discrepancies among the results of previous authors. Solid solution is almost complete between LiF and MgF₂ at elevated temperatures, but a small gap occurs at the eutectic (735°C.) with extensive exsolution at lower temperatures.     

Phase Equilibrium in the System PbO-TiO2–ZrO2

Phase equilibrium relations in the system PbO–TiO₂–ZrO₂ were studied by quenching in the range where the PbO content is 50 mole % and more. Isotherms were examined at 1100°, 1200°, and 1300°C and tie lines were determined between the liquid and solid solution in equilibrium. The incongruent melting point of PbZrO₃ was 1570°C and the equilibrium between liquid, PbO-type solid, and PbZrO₃ is peritectic. Pb(Zr,Ti)O₃ solid solutions containing more than 14 mole % PbZrO₃ decomposed to liquid, ZrO₂, and Pb(Zr,Ti)O₃ and the decomposition temperature rises from 1340° to 1570°C with increasing PbZrO₃ content. The system PbTiO₃–PbZrO₃ should not be treated as a binary, but as a section of the ternary system.     

SiO2–PbO–CaO–ZrO2–TiO2–(B2O3–K2O), A New Zirconolite Glass–Ceramic System: Crystallization Behavior and Microstructure Evaluation

Zirconolite (CaZrTi₂O₇) is a mineral that has a high containment capacity for actinides and lanthanides and is considered to be a good candidate for the immobilization of radioactive wastes. The glass–ceramic technique seems to be a very suitable and convenient method to produce zirconolite crystals by precipitating them in a specific glass matrix. In this study, development of a new zirconolite-based glass–ceramic belonging to SiO₂–PbO–CaO–ZrO₂–TiO₂–(B₂O₃–K₂O) system was investigated. The presence of PbO, together with B₂O₃ and K₂O, allowed the preparation of a X-ray diffraction (XRD) amorphous glass with a relatively high concentration of ZrO₂ and TiO₂, which was successfully converted to a glass–ceramic containing 34 wt% of zirconolite after heating at 770°C for 4 h. Differential thermal analysis, XRD, scanning electron microscope, and energy dispersive X-ray spectroscopy were used to determine the crystallization conditions, identify the crystallized phases, determine their compositions and quantities and observe and analyze the microstructures. The zirconolite crystals showed a platelet morphology with a monoclinic structure characterized by a =1.246 nm, b =0.7193 nm, c =1.128 nm, and β=100.508°.     

Sintering and Crystallization of CaO–Al2O3–ZrO2–SiO2 Glasses Containing Different Amount of Al2O3

In this work several complementary techniques have been employed to carefully characterize the sintering and crystallization behavior of CaO–Al₂O₃–ZrO₂–SiO₂ glass powder compacts after different heat treatments. The research started from a new base glass 33.69 CaO–1.00 Al₂O₃–7.68 ZrO₂–55.43SiO₂ (mol%) to which 5 and 10 mol% Al₂O₃ were added. The glasses with higher amounts of alumina sintered at higher temperatures (953°C [lower amount] vs. 987°C [higher amount]). A combination of the linear shrinkage and viscosity data allowed to easily find the viscosity values corresponding to the beginning and the end of the sintering process. Anorthite and wollastonite crystals formed in the sintered samples, especially at lower temperatures. At higher temperatures, a new crystalline phase containing ZrO₂ (2CaO·4SiO₂·ZrO₂) appeared in all studied specimens.     

Liquid–Solid Equilibria in the System NiO–TiO2–SiO2 in the Temperature Range 1430° to 1660°C

Equilibrium relations in the system NiO–TiO₂–SiO₂ in air have been investigated in the temperature range 1430° to 1660°C. The most conspicuous feature of the phase relations is the existence of a cation-excess spinel-type phase, in addition to NiO and NiTiO₃, on the liquidus surface and at subsolidus temperatures down to 1430°C. Three invariant points have been located on the liquidus. There is a peritectic at 1540°C characterized by coexisting NiO ( ss ), spinel( ss ), cristobalite, and liquid of composition 47 wt% NiO, 29 wt% TiO₂, and 24 wt% SiO₂. Two eutectics are present, one at 1480°C, with spinel( ss ), NiTiO₃, cristobalite, and liquid (42 wt% NiO, 43 wt% TiO₂, and 15 wt% SiO₂), as the coexisting phases. The other is at 1490°C with NiTiO₃, rutile, cristobalite, and liquid (32 wt% NiO, 56 wt% TiO₂, and 12 wt% SiO₂). A liquid miscibility gap extends across the diagram from the two bounding binary systems NiO–SiO₂ and TiO₂–SiO₂.     

Al2O3–ZrO2–SiO2 Ternary Glasses for Molybdenum Oxidation Barriers

The wettability of binary and ternary glasses belonging to SiO₂–Al₂O₃–ZrO₂ diagram has been studied using the sessile drop technique at 1750° and 1800°C. The ternary SiO₂–Al₂O₃–ZrO₂ (90–5–5 wt%) glass has proved to be well appropriated as a molybdenum oxidation barrier coating. The addition of 5 wt% of MoO₂ slightly improves its wettablity at higher temperatures without affecting its oxidation barrier properties. The Mo comes into the glass network as a mixture of Mo⁵⁺, Mo⁴⁺, and Mo⁶⁺. After oxidation at 1000°C in oxygen atmosphere, the molybdenum remains in the glass network as Mo⁶⁺.     

Nanostructure of TiO2 Nano-Coated SiO2 Particles

We characterized SiO₂–TiO₂ nano-hybrid particles, prepared using the sol–gel method, using high-resolution transmission microscopy. A few nanometer-ordered TiO₂ anatase crystallites could be observed on the monodispersed SiO₂ nanoparticle surface. The quantum size effect of the TiO₂ anatase crystallites is attributed to the blue shift of the absorption band. The rough surface of the SiO₂–TiO₂ nano-hybrid particles was derived from the developed growth planes of the TiO₂ anatase crystallites, grown from fully hydrolyzed Ti alkoxide that did not react with acetic acid during the crystallization process at 600°C thermal annealing.     

Solid-State Diffusive Amorphization in TiO2/ZrO2 Bilayers

Thin films of crystalline TiO₂ were deposited on self-assembled organic monolayers from aqueous TiCl₄ solutions at 80°C; partially crystalline ZrO₂ films were deposited on top of the TiO₂ layers from Zr(SO₄)₂ solutions at 70°C. In the absence of a ZrO₂ film, the TiO₂ films had the anatase structure and underwent grain coarsening on annealing at temperatures up to 800°C; in the absence of a TiO₂ film, the ZrO₂ films crystallized to the tetragonal polymorph at 500°C. However, the TiO₂ and ZrO₂ bilayers underwent solid-state diffusive amorphization at 500°C, and ZrTiO₄ crystallization could be observed only at temperatures of 550°C or higher. This result implies that metastable amorphous ZrTiO₄ is energetically favorable compared to two-phase mixtures of crystalline TiO₂ and ZrO₂, but that crystallization of ZrTiO₄ involves a high activation barrier.     

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.     

Influence of TiO2 on Properties of Glasses in the System K2O–PbO–SiO2–TiO2 and Its Relation to Structure

By a progressive weight percent substitution of TiO₂ for SiO₂ at various rations of concentration of K₂O and PbO, the entire region of glass formation in the quaternary system K₂O–PbO–SiO₂–TiO₂ was covered with 51 glass compositions. The properties of these glasses were determined and studied with respect to the role of TiO₂ in the system. The results indicated that the dielectric constant increased progressively with increasing TiO₂ concentration whereas the dissipation factor showed an overall decrease, when measured at 1 Mc and 25°C. Density and the refractive index increased progressively with increasing TiO₂ concentration but deviated from the additive relation. Chemical durability, expansivity, and softening temperature vs. composition curves showed definite inflections. The effect of TiO₂ on oxygen packing indicated that Ti⁴⁺ strengthens the network in lower concentrations and weakens the network in higher concentrations in this system. It appears to be likely that Ti⁴⁺ changes its coordination number form 4 to 6.     

Mechanism of ZrTiO4 Synthesis by Mechanochemical Processing of TiO2 and ZrO2

High-energy ball milling initiates a solid-state reaction in an equimolar mixture of TiO₂ and ZrO₂. The first stage of ball milling induced the transformation of anatase TiO₂ to high-pressure phase TiO₂ (II), isostructural with ZrTiO₄. The formation of solid solutions monoclinic ZrO₂/TiO₂ and TiO₂ (II)/ZrO₂ was observed in the intermediate stage. Afterward, a nanosized ZrTiO₄ phase was formed in the milled product from the TiO₂ (II)/ZrO₂ solid solution. The sintering of the milled product at a temperature <1100°C was examined in situ by Raman spectroscopy. The full solid-state reaction toward ZrTiO₄ ceramic is completed at a temperature considerably lower than reported in the literature.     

Solubility of TiO2 in ZrO2

The solubility of TiO₂ in tetragonal ZrO₂ is 13.8±0.3 mol% ui 1300°C, 14.9±0.2 mol% at 1400°C, and 16.1±0.2 mol% at 1500°C. These solid solutions transform to metastable monoclinic solid solutions without compositional change on cooling to room temperature.     

Chemical Interactions Promoting the ZrO2 Tetragonal Stabilization in ZrO2–SiO2 Binary Oxides

Metastable tetragonal ZrO₂ phase has been observed in ZrO₂–SiO₂ binary oxides prepared by the sol–gel method. There are many studies concerning the causes of ZrO₂ tetragonal stabilization in binary oxides such as Y₃O₂–ZrO₂, MgO–ZrO₂, or CaO–ZrO₂. In these binary oxides, oxygen vacancies cause changes or defects in the ZrO₂ lattice parameters, which are responsible for tetragonal stabilization. Since oxygen vacancies are not expected in ZrO₂–SiO₂ binary oxides, tetragonal stabilization should just be due to the difficulty of zirconia particles growing in the silica matrix. Furthermore, changes in the tetragonal ZrO₂ crystalline lattice parameters of these binary oxides have recently been reported in a previous paper. The changes of the zirconia crystalline lattice parameters must result from the chemical interactions at the silica–zirconia interface (e.g., formation of Si–O–Zr bonds or Si–O⁻ groups). In this paper, FT-IR and ²⁹Si NMR spectroscopy have been used to elucidate whether the presence of Si–O–Zr or Si–O⁻ is responsible for tetragonal phase stabilization. Moreover, X-ray diffraction, Raman spectroscopy, and transmission electron microscopy have also been used to study the crystalline characteristics of the samples.     

Degradation of Bioactive Polydimethylsiloxane–CaO–SiO2–TiO2 and Poly(tetramethylene oxide)–CaO–TiO2 Hybrids in a Simulated Body Fluid

It has been shown that polydimethylsiloxane (PDMS)–CaO–SiO₂–TiO₂ and poly(tetramethylene oxide) (PTMO)–CaO–TiO₂ hybrids form apatite on their surfaces in a simulated body fluid (SBF) and show mechanical properties similar to those of human cancellous bones. In the present study, changes, caused by soaking in SBF, were measured in the mechanical properties of PDMS–CaO–SiO₂–TiO₂ hybrids with different CaO and TiO₂ contents and PTMO–CaO–TiO₂ hybrids with different CaO contents. Significant decreases in the strength and strain at failure of the hybrids were observed for the PDMS–CaO–SiO₂–TiO₂ hybrids with high CaO or TiO₂ contents and PTMO–CaO–TiO₂ hybrids with a high CaO content after soaking in SBF for 4 w. This indicates that incorporation of a large amount of CaO component into the hybrids should result in the deterioration of the hybrids in the body environment.     

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.     

Fabrication of TiO2–SiO2 Photonic Crystals with Diamond Structure

Ceramic photonic crystals with diamond structure were fabricated using stereolithography and successive sintering. The green body of an epoxy resin incorporating 10 vol% TiO₂–SiO₂ was formed by stereolithography and then heated in air at 1100°–1400°C for 2 h. The sintered products maintained the diamond structure with a linear shrinkage ratio of about 57% and a porosity of 38%. The ceramic photonic crystal with eight unit cells showed a photonic band gap at the center frequency of 23.5 GHz. This fabrication method of three-dimensional (3D) ceramic photonic crystals is applicable to other 3D structural ceramics and does not require any molding techniques.     

Sol–Gel Route to Synthesis of Microporous Ceramic Membranes: Thermal Stability of TiO2–ZrO2Mixed Oxides

In this paper concerning the synthesis of microporous ceramic membranes, we focus on the preparation and ther- mal stability of unsupported microporous TiO₂-ZrO₂, mixed-oxide membranes. It has been observed that, by add- ing up to 20% ZrO₂, into TiO₂, or up to 10% TiO₂, into ZrO₂, these microporous membranes display improved thermal stability. They can be fired up to 500°C for 0.5 h without closing micropores. However, membranes containing almost equal percentages of each component have lost microporous features and have low surface areas and low porosities. A phase diagram of a two-component TiO₂- ZrO₂, mixed-oxide membrane has been prepared based on DTA and X-ray diffraction data in order to better under- stand the microstructure changes upon firing.     

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.     

Phase Equilibria and Structural Species in NdCl3–NaCl, NdCl3–CaCl2, PrCl3–NaCl, and PrCl3–CaCl2 Systems

Equilibrium phase diagrams for the systems NdCl₃–CaCl₂ and NdCl₃–NaCl were determined by differential thermal analysis. A simple eutectic was observed at 59 ± 1 mol% CaCl₂ and 600°± 2°C in the NdCl₃–CaCl₂ system. A compound NaCl.3NdCl₃ which melts incongruently at 545°± 5°C to NdCl₃ and a liquid containing approximately 47 mol% NaCl, and a eutectic at 68 mol% NaCl and 439°± 2°C were found in the NdCl₃–NaCl system. On the basis of agreements between the activities calculated by the Clausius–Clapeyron equation and Temkin's model using the present data for the NdCl₃–CaCl₂ system and the literature data for the PrCl₃–CaCl₂ system, the melts in the former system consist of Nd³⁺, Ca²⁺, and Cl⁻ ions and in the latter system of Pr³⁺, Ca²⁺, and Cl⁻ ions. The above approach indicates the presence of Na⁺, Cl⁻, and NdCl^2-₅ ions in the NaCl-rich melts and Nd³⁺, Cl⁻, and NdCl⁻₄ in the NdCl₃-rich melts in the NdCl₃–NaCl system. Analogous ions were indicated in the melts of the PrCl₃–NaCl system.