Formation and Transformation of TiO2 (Anatase) Solid Solution in the System TiO2—Al2O3

In the system TiO₂—Al₂O₃, TiO₂ (anatase, tetragonal) solid solutions crystallize at low temperatures (with up to ∼ 22 mol% Al₂O₃) from amorphous materials prepared by the simultaneous hydrolysis of titanium and aluminum alkoxides. The lattice parameter a is relatively constant regardless of composition, whereas parameter c decreases linearly with increasing Al₂O₃. At higher temperatures, anatase solid solutions transform into TiO₂ (rutile) with the formation of α-Al₂O₃. Powder characterization is studied. Pure anatase crystallizes at 220° to 360°C, and the anatase-to-rutile phase transformation occurs at 770° to 850°C.     

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.     

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.     

Homogeneous Precipitation of TiO2 Ultrafine Powders from Aqueous TiOCl2 Solution

Crystalline TiO₂ powders were prepared by the homogeneous precipitation method simply by heating and stirring an aqueous TiOCl₂ solution with a Ti⁴⁺ concentration of 0.5 M at room temperature to 100°C under a pressure of 1 atm. TiO₂ precipitates with pure rutile phase having spherical shapes 200-400 nm in diameter formed between room temperature and 65°C, whereas TiO₂ precipitates with anatase phase started to form at temperatures >65°C. Precipitates with pure anatase phase having irregular shapes 2-5 µm in size formed at 100°C. Possibly because of the crystallization of an unstable intermediate product, TiO(OH)₂, to TiO₂ x H₂O during precipitation, crystalline and ultrafine TiO₂ precipitates were formed in aqueous TiOCl₂ solution without hydrolyzing directly to Ti(OH)₄. Also, formation of a stable TiO₂ rutile phase between room temperature and 65°C was likely to occur slowly under these conditions, although TiO₂ with rutile phase formed thermodynamically at higher temperatures.     

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.     

A Study of the Phase Relations of ZrO2–TiO2 and ZrO2–TiO2–SiO2

The phase relations of the systems ZrO₂–TiO₂ and ZrO₂–TiO₂–SiO₂ were investigated. X-ray diffraction techniques served as the principal means of analysis. The binary system ZrO₂–TiO₂ was found to be one of partial solid solutions with no intermediate compounds. A eutectic point was found to exist at 50 to 55 weight % ZrO₂ and 1600°C. A preliminary investigation of the ternary system ZrO₂–TiO₂–SiO₂, although not extensive, resulted in a better understanding of this system, with a fairly accurate location of some of its boundary lines. A eutectic point was located at 2% ZrO₂, 10% TiO₂, and 88% SiO₂ at approximately 1500°C.     

Preparation of TiO2 Nanoparticles on Different SiO2 Supports by the Adsorption Phase Technique

The influence of supports on the preparation of TiO₂ nanoparticles by the adsorption phase technique is studied in detailed. Series temperature experiments of two types of supports (named as SiO₂ A and B) were used. Energy-dispersive analysis by X-ray indicates that the concentration of TiO₂ on both supports decreases with temperature increasing. TiO₂ quantity on SiO₂ A decreases sharply between 40° and 60°C, whereas the temperature range for SiO₂ B is between 30° and 50°C. X-ray diffraction (XRD) shows that grain size of TiO₂ particles on two SiO₂ surfaces is all below 7 nm. It is also shown by XRD that particles on SiO₂ A decrease sharply as in the quantity curve of TiO₂, but particles on SiO₂ B all change gradually and TiO₂ particles on SiO₂ B are more uniform in transmission electron spectroscopy. The similarly of both supports is considered to be the reason for the similar changes in Ti concentration, and the different characteristics of the internal/external surface lead to variant quantity and grain size, as well as characteristics of TiO₂.     

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.     

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.     

Reactions in the System Li2O–MgO–Al2O3–SiO2: I, The Cordierite-Spodumene Join

Phase equilibria along the nonbinary join between cordierite (2MgO · 2Al₂O₃· 5SiO₂) and spodumene (Li₂O · Al₂O₃· 4SiO₂) were investigated in the temperature range 800° to 1550°C. using the quench technique on fourteen compositions. The phase diagram at high temperatures is characterized by a very small region of solid solution on the cordierite side, appreciable solid solution on the spodumene side, and regions of three and four phases toward the center of the system, including liquid, α-cordierite, mullite, spinel, corundum, and β-spodumene and its solid solutions. The liquidus has a flat minimum between 40 and 50% cordierite at 1347°, and rises on one side to the congruent melting point of β-spodumene (1421°) and on the other side to the temperature of complete melting of cordierite (1530°). The lowest temperature at which liquid appears is 1325°. At low temperatures a complete series of metastable solid solutions exists between μ-cordierite and β-spodumene. The significance of the data in the preparation of thermal-shock-resisting bodies is discussed.     

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.     

Revised Phase Diagram for the System Al2O3—SiO2

On the basis of 190 runs made up to 1860°C in sealed noble-metal containers the following revisions have been made in the equilibrium diagram for the system A1₂O₃–SiO₂. Mullite melts congruently at 1850°C. The extent of equilibrium solid solution in mullite at solidus temperature is from approximately 60 mole % Al₂O₃ (3/2 ratio) to 63 mole % A1₂O₃. Metastable solid solutions can be prepared up to about 67 mole % Al₂O₃. There is no evidence for stable solubility of excess SiO₂ beyond the 3/2 composition at pressures below 3 kbars. Refractive indices are presented for glasses containing up to 60 mole % Al₂O₃ and from them the composition of the eutectic is confirmed at 5 mole % SiO₂. The variation in lattice constants of the mullite solid solution is not an unequivocal guide to composition since mullites at one composition produced at different temperatures show differences in spacing, no doubt reflecting Al-Si ordering phenomena. The possibility of quartz and corundum being the stable assemblage at some low temperatures and pressures cannot be ruled out. A new anhydrous phase in the system is described, which was previously thought to be synthetic andalusite; it is probably a new polymorph of the Al₂SiO₅ composition with ortho-rhombic unit-cell dimensions a =7.55 A, b =8.27 A, and c = 5.66 A.     

Scandium-Doped Anatase (TiO2) Nanoparticles Directly Formed by Hydrothermal Crystallization

Anatase-type TiO₂ solid solutions doped with 0–10 mol% scandium were formed by hydrothermal crystallization under weak basic conditions above 180°C for 5 h from amorphous co-precipitates that were obtained from the aqueous precursor solutions of TiOSO₄ and Sc(NO₃)₃ using aqueous ammonia. The anatase particles were spindle-like and consisted of nanosized-crystallites (23–25 nm). The lattice parameter c ₀ of anatase and the length and width of the spindle-like anatase gradually increased when the scandium content was increased. The diffuse reflectance spectra of the as-prepared TiO₂ doped with scandium showed that the onset of absorption slightly shifted to longer wavelengths with increasing scandium content. The band gap of anatase was slightly increased by making solid solutions with scandium oxide.     

Thermal Oxidation Kinetics of MoSi2-Based Powders

The oxidation process of MoSi₂ is very complex, and controversial results have been reported, especially for the early-stage oxidation before the formation of passive SiO₂ film. Most oxidation studies have been carried out on bulk consolidated samples, and the early stage of oxidation has not been studied. In this investigation, very fine MoSi₂ powder with an average particle size of 1.6 μm was used. Such a fine particle size makes it easier to study the early stages of oxidation since a significant portion of the powder is oxidized before the formation of passive SiO₂ film. The oxidation kinetics of commercial MoSi₂-SiC and MoSi₂-Si₃N₄ powder mixtures were also studied for comparison. Weight changes were measured at discrete time intervals at 500° to 1100°C in 0.14 atm of oxygen. X-ray diffraction was used to identify the phases formed during oxidation. Our results show the formation of MoO₃ phase and an associated weight gain at low temperatures (500° and 600°C). At temperatures higher than 900°C, Mo₅Si₃ phase formed first and was subsequently oxidized to solid SiO₂ and volatile MoO₃, resulting in an initial weight gain followed by subsequent weight loss. A model based on the assumption that oxidation kinetics of both MoSi₂ and Mo₅Si₃ are proportional to their fractions in the system describes the experimental data well.     

Effects of Preparation History on TiO2· SiO2 Glasses

Differences in the Raman spectra of various heat-treated TiO₂· SiO₂ glasses could be related to their thermal and chemical histories. For instance, while rutile could be detected in batch-prepared glasses heated at 1100°C, only α-cristobalite could be detected in heat-treated devitrified flame-prepared glasses with comparative TiO₂-concentrations. Thermal expansion coefficients increased for batch-prepared glasses upon heat treatment due to exsolution of rutile from the glasses. Earlier work had noted similar behavior at lower temperatures due to exsolution of anatase.     

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.     

Phase Equilibrium in a Portion of the Ternary System Ba OA-I2,O3,-SiO2,

Phase-equilibrium relations on the liquidus surface in the system Ba0-A1₂O₃-SiO₂ have been investigated by the quenching method. The compositions investigated within the ternary area were those containing less than 30%, A1₂O₃ and more than 20% SiO₂ by weight. Petrographic and X-ray techniques were employed in the determination of the crystalline phases.
The crystal phases that separate from melts within the area investigated are barium orthosilicate (2BaO. SiO₂,), barium metasilicate (BaO 2SO₂,), solid solutions, sanbornite (BaO 2SiO₂), tridymite and cristobalite (SO₂), mullite (3A1₂O₃ 2SiO₂), and celsian (BaO A1₂O3_.2SiO₂). Diagrams show the isotherms and indices of refraction of the glasses.
Five quintuple points and eleven boundary curves have been determined within = .5yo compositional variations. The liquidus-surface temperatures have been obtained within limits of ± 125°C.     

Synthesis of Nanocrystalline TiO2 Particles by Hydrolysis of Titanyl Organic Compounds at Low Temperature

Fourier transform infrared analysis, nuclear magnetic resonance, and thermogravimetric analysis show that most of the solid product prepared from the reaction of Ti(OC₄H₉)₄ and excess (CH₃CO)₂O is a mixture of titanyl organic compounds. Nanocrystalline TiO₂ particles, which include anatase TiO₂, rutile TiO₂, and a mixture of anatase and rutile, can be obtained from hydrolysis of the titanyl organic compounds under normal pressure at 60°C. The particle size, shape, and formation process of the crystals have been studied using X-ray diffraction and transmission electron microscopy. The specific-surface-area data for a rutile TiO₂ sample and the powders obtained after calcination at different temperatures have been measured by the Brunauer–Emmett–Teller method.     

The System Al2O3-Cr2O3-Sio2

Liquidus phase equilibrium data are presented for the system Al₂O₃-Cr₂O₃-SiO₂. The liquidus diagram is dominated by a large, high-temperature, two-liquid region overlying the primary phase field of corundum solid solution. Other important features are a narrow field for mullite solid solution, a very small cristobalite field, and a ternary eutectic at 1580°C. The eutectic liquid (6Al₂O₃-ICr₂O₃-93SiO₂) coexists with a mullite solid solution (61Al₂O₃-10Cr₂O₃-29SiO₂), a corundum solid solution (19Al₂O₃-81Cr₂O₃), and cristobalite (SO₂). Diagrams are presented to show courses of fractional crystallization, courses of equilibrium crystallization, and phase relations on isothermal planes at 1800°, 1700°, and 1575°C. Tie lines were sketched to indicate the composition of coexisting mullite and corundum solid solution phases.     

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.