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.     

Anisotropic Abnormal Grain Growth in TiO2/SiO2-Doped Alumina

The effect of TiO₂/SiO₂ addition on the grain growth of alumina was reinvestigated. TiO₂ promoted the grain growth, but there was no abnormal grain growth. However, codoping of TiO₂ and SiO₂ resulted in a duplex microstructure consisting of large platelike grains, ∼800 μm long and ∼100 μm thick, and fine matrix grains. The observed anisotropic abnormal grain growth was explained in terms of liquid formation during heat treatment.     

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-Processed SiO2/TiO2/Methylcellulose Composite Materials for Optical Waveguides

SiO₂–TiO₂–methylcellulose (MC) composite materials processed by the sol-gel technique were studied for optical waveguide applications. Dense, crack-free and homogeneous films as thick as 2 μm were obtained via the organic binder MC-assisted sol–gel process and single coating with low-temperature treatment. Light waveguiding in such hybrid film was demonstrated at a wavelength of 650 nm. About 1.1 dB/cm or lower propagation loss for the SiO₂ (80 mol%)–TiO₂ (20 mol%)–MC (22 wt%) film can be achieved. The effects of thermal treatment on the structure and properties of the gel films were also investigated.     

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.     

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.     

Anisotropic Grain Growth in α-Al2O3 with SiO2 and TiO2 Additions

Microstructural changes caused by doping α-Al₂O₃ with small amounts of SiO₂ and TiO₂ added singly or together were investigated. When they were sintered at 1450°C for 120 min, singly doped samples developed equiaxed microstructures, but codoped material developed an anisotropic microstructure that contained platelike grains with an average aspect ratio of 3.4. The development of anisotropy thus resulted from a cooperative effect of silicon and titanium. Amorphous material was present at most grain boundaries in the silicon-doped sample. In the codoped sample, only boundaries that exhibited a basal facet were penetrated by amorphous material. Energy dispersive X-ray spectroscopy analysis showed strong titanium enrichment at the edges of platelets. Additional experiments demonstrated that the volume fraction of highly anisotropic platelike grains interspersed with equiaxed grains could be adjusted by using varying amounts of titanium with a constant amount of silicon content. The fracture toughnesses of such materials increased as the structure became more anisotropic.     

Formation of SiO2, Al2O3, and 3Al2O3. 2SiO2 Particles in a Counterflow Diffusion Flame

SiO₂, Al₂O₃, and 3Al₂O₃.2SiO₂ powders were synthesized by combustion of SiCl₄ or/and AlCl₃ using a counterflow diffusion flame. The SiO₂ and Al₂O₃ powders produced under various operation conditions were all amorphous and the particles were in the form of agglomerates of small particles (mostly 20 to 30 nm in diameter). The 3Al₂O₃.2SiO₂ powder produced with a low-temperature flame was also amorphous and had a similar morphology. However, those produced with high-temperature flames had poorly crystallized mullite and spinel structure, and the particles, in addition to agglomerates of small particles (20 to 30 nm in diameter), contained larger, spherical particles 150 to 130 nm in diameter). Laser light scattering and extinction measurements of the particle size and number density distributions in the flame suggested that rapid fusion leading to the formation of the larger, spherical particles occurred in a specific region of the flame.     

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.     

Crystallization Behavior of CaO–P2O5 Glass with TiO2, SiO2, and Al2O3 Additions

TiO₂ above 4 mol% is an effective nucleating agent for CaO–P₂O₅ glass which also contains substantial SiO₂ and Al₂O₃ additions. Glass ceramics can be made from this glass using a single slow heating ramp with no need for a nucleating heat treatment step. Powder of this composition crystallizes rapidly to β-Ca₂P₂O₇, whereas bulk glass crystallizes from diphasic nuclei consisting of a central cubic Ca-P-Ti-Si-Al oxide phase surrounded by impure AlPO₄ dendrites. Metastable calcium phosphate grows on the AlPO₄ dendrites and later transforms to β-Ca₂P₂O₇.     

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.     

Direct Formation and Photocatalytic Performance of Anatase (TiO2)/Silica (SiO2) Composite Nanoparticles

Anatase (TiO₂)/silica (SiO₂: 23.9–27.7 mol%) composite nanoparticles were directly synthesized from (i) the reaction of titanyl sulfate (TiOSO₄) and sodium metasilicate (Na₂SiO₃) under mild hydrothermal conditions, (ii) the acidic precursor solutions of TiOSO₄ and tetraethylorthosilicate (TEOS) by thermal hydrolysis, and (iii) the metal alkoxides, i.e., tetraisopropoxide (TTIP) and TEOS, by the sol–gel method. Their photocatalytic activities were evaluated by measurements of the relative concentration of methylene blue after UV irradiation. The as-prepared TiO₂/SiO₂ composite nanoparticles showed far more improved photocatalytic activity than the pure anatase-type TiO₂. The composite nanoparticles formed from (i) TiOSO₄ and Na₂SiO₃ as well as those from (ii) TiOSO₄ and TEOS showed fairly good photocatalytic activity, and it was better than that of those synthesized from (iii) the metal alkoxides, which was suggested to be due to the difference in crystallinity of the anatase.     

Formation of Nanometric TiB2 from TiO2

Titanium diboride can be produced by ball-milling a mixture of TiO₂, B₂O₃, and Mg metal for between 10 and 15 h. The reaction was found to be completed during the milling with no evidence of residual Mg. The unwanted phase, MgO, was readily removed by leaching in acid. The leached powder obtained after 15 h milling had a particle size of <200 nm and was highly faceted. The particle size decreased to ∼50 nm after 100 h milling and seemed to be relatively monodisperse. Scherrer calculation of the crystallite size showed that the product particles were probably single crystal.     

Co-Additions of TiO2 and SiO2 Crystalline Fillers to Tailor the Properties of BaO–ZnO–B2O3–SiO2 Glass for Application to Barrier Ribs of Plasma Display Panels

The influence of co-additions of crystalline TiO₂ and SiO₂ fillers (10 wt% addition in total) to BaO–ZnO–B₂O₃–SiO₂ glass on resultant properties was investigated from the viewpoint of applying the material to the barrier ribs of plasma display panels. The substitution of SiO₂ for TiO₂ reduced the dielectric constant significantly, while it maintained high optical reflectance and appropriate coefficient of thermal expansion (CTE) in the case when TiO₂ alone was used. A 5–7.5 wt% SiO₂ addition with 2.5–5 wt% TiO₂ under the constraint of 10 wt% total fillers demonstrated an optical reflectance of about 55%, a CTE of about 8.3 × 10⁻⁶ K⁻¹ (compatible with glass panels), and a dielectric constant of about 7.5, which are promising properties for the barrier rib application.     

An Evaluation of Vaporizing Rates of SiO2 and TiO2 As Protective Coatings for Ultrahigh Temperature Ceramic Composites

For temperatures >1973 K, the thermodynamic and kinetic analyses of the major gaseous species for a liquid titanate layer would vaporize significantly less than a silicate layer, when considering these layers as a protective barrier for ultrahigh temperature ceramic composites. At 2500 K, the major species is TiO( g ) with p _{TiO( g )}=0.1 kPa compared with SiO( g ) with p _{SiO( g )}=1.3 × 10³ kPa at the Ti/TiO₂ and Si/SiO₂ equilibrium, respectively. The SiO( g ) attains a partial pressure greater than ambient pressure at 2500 K even with a thermodynamic activity of 0.01 considering equilibration with a silicide (e.g., TiSi _x ). In addition, at 2500 K the TiO₂ layer would vaporize at a rate of 0.23 mm/s compared with the SiO₂ layer's loss rate of 207 mm/s. Although the oxygen diffusivity and permeability through titanate solutions must be further analyzed, the thermodynamic and kinetic analyses for vaporization indicate a longer duration for a liquid titanate than for a liquid silicate layer at ultrahigh temperatures.     

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.     

Solubility of TiO2 in BaTiO3

Scanning electron microscopy and electron probe micro-analysis were used to investigate the microstructure of both slow-cooled and quenched polycrystalline BaTiO₃ specimens with a small excess of TiO₂ (Ba/Ti=0.995 to 0.999) or of BaO (Ba/Ti=1.002 and 1.005). The electron micrographs of polished and etched TiO₂-excess BaTiOs samples, and of fracture surfaces of quenched samples, showed a second phase in the grain boundaries and triple-point regions, whereas no second phase was observed in samples having Ba/Ti=1.000. Microprobe analysis of the second phase gave compositions near that of the reported adjacent phase of higher TiO₂ content, Ba₆Ti₁₇O₄₀. The results indicate that the solubility of TiO₂ in BaTiO₃ is <0.1 mol%.     

Crystallization Kinetics and Dielectric Properties of Fresnoite BaO–TiO2–SiO2 Glass–Ceramics

Nucleation and crystallization kinetics of fresnoite (Ba₂TiSi₂O₈) crystals in BaO–TiO₂–SiO₂ glasses have been explored for dielectric applications. The volume fractions crystallized at different temperatures and times were tracked by XRD analysis. The activation energy of crystallization was estimated from DTA results to be about 528 kJ/mol, which is consistent with the value obtained by XRD results. The Avrami parameter values calculated at different temperatures from DTA results were found to be between 3.2 and 3.9, indicating that the growth is three dimensional and the mechanism of growth is interface-controlled. Additionally, because of compositional similarities, the dielectric contrast between the glass (ɛ_r∼15) and the resulting glass–ceramic (ɛ_r∼18) was minimal.