Oxidation Behavior of Titanium Boride at Elevated Temperatures

The oxidation behavior of dense TiB₂ specimens was investigated. Hot-pressed TiB₂ with 2.5 wt% Si₃N₄ as a sintering aid was exposed to air at temperatures between 800° and 1200°C for up to 10 h. The TiB₂ exhibited two distinct oxidation behaviors depending on the temperature. At temperatures below 1000°C, parabolic weight gains were observed as a result of the formation of TiO₂( s ) and B₂O₃( l ) on the surface. The oxidation layer comprised two layers: an inner layer of crystalline TiO₂ and an outer layer mainly composed of B₂O₃. When the oxidation temperatures were higher than 1000°C, gaseous B₂O₃ was formed along with crystalline TiO₂ by the oxidation process. In this case, the surface was covered with large TiO₂ grains imbedded in a highly textured small TiO₂ matrix.     

Synthesis, Properties, and Oxidation of Alumina-Titanium Nitride Composites

Al₂O₃-TiN composites varying from 60 to 66.6 mol% TiN were prepared by an in situ reaction between TiO₂ and AlN. N₂ or O₂ evolution takes place, depending on the composition selected. A pseudobrookite (PB) phase appears in the reaction product, the amount decreasing as the TiO₂:AlN ratio becomes poor in AlN. The in situ reaction product can be pressureless sintered to 94% to 97% theoretical density at 1600°C in N₂. The four-point flexural strength varies from 280 to 430 MPa at room temperature. The fracture toughness is 3 to 4.7 MPa.m^1/2. Oxidation of a 94% dense TiN-Al₂O₃ composite in the temperature range 710° to 1050°C was also studied. A layer of TiO₂ (rutile) protects the composite at 710°C from further oxidation with a weight gain of 0.08 mg/cm² in 90 min. In the temperature range 820° to 1050°C, the initial oxidation kinetics are parabolic, with an activation energy of 216.5 kJ/mol. Linear oxidation kinetics with an activation energy of 113.7 kJ/mol pertain at longer times.     

Titanium Incorporation in Zn2TiO4 Spinel Ceramics

Compositions in the Zn₂TiO₄+ x TiO₂ system ( x = 0–0.43) were synthesized via the solid-state reaction route, using high-purity (≥99.99%) metal-oxide powders. The incorporation of titanium, in the form of TiO₂, in Zn₂TiO₄ spinel ceramics was investigated by analyzing the crystal structure and measuring the dielectric properties. The results of the crystal structure analyses suggested that TiO₂ levels of x ≤ 0.33 could be incorporated into the Zn₂TiO₄ spinel at temperatures of T > 945°C, whereas the solid solubility of titanium in Zn₂TiO₄ decreased for T < 945°C. When x ≥ 0.28, the Zn₂Ti₃O₈ phase formed in the Zn₂TiO₄ grain interior while cooling after heat treatment. Measurement of the microwave dielectric properties also supported the conclusion that the solubility limit of titanium in Zn₂TiO₄ was close to x = 0.33, as determined through analysis of the crystal structure.     

Effects of Thermal Annealing on Formation of Micro Porous Titanium Oxide by the Sol–Gel Method

We investigated the structural and optical properties of microporous titanium oxide (TiO₂) fabricated by the sol–gel method using templates of colloidal crystals with polystyrene spheres when the annealing temperature was changed between 600° and 1000°C. From X-ray diffraction patterns and SEM images, the rutile TiO₂ annealed at a high temperature did not form periodic porous bodies, while the anatase TiO₂ annealed at lower than 800°C formed periodic porous bodies. The porous TiO₂ obtained acts as an air-sphere/TiO₂ photonic crystal with an FCC structure. It is suggested that TiO₂ sol annealed at a lower temperature do not lead to phase transition from the anatase phase to the rutile phase to obtain the air-sphere/TiO₂ photonic crystal by the sol–gel method using templates of colloidal crystals.     

Subsolidus Phase Relationships in the Ga2O3–Al2O3–TiO2 System

Subsolidus phase relationships in the Ga₂O₃–Al₂O₃–TiO₂ system at 1400°C were studied using X-ray diffraction. Phases present in the pseudoternary system include TiO₂ (rutile), Ga_{2−2 x} Al_{2 x} O₃ ( x ≤0.78 β-gallia structure), Al_{2−2 y} Ga_{2 y} O₃ ( y ≤0.12 corundum structure), Ga_{2−2 x} Al_{2 x} TiO₅ (0≤ x ≤1 pseudobrookite structure), and several β-gallia rutile intergrowths that can be expressed as Ga_{4−4 x} Al_{4 x} Ti _{n −4}O_{2 n −2} ( x ≤0.3, 15≤ n ≤33). This study showed no evidence to confirm that aluminum substitution of gallium stabilizes the n =7 β-gallia–rutile intergrowth as has been mentioned in previous work.     

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.     

Antibacterial Activity of Photocatalytic Titanium Dioxide Thin Films with Photodeposited Silver on the Surface of Sanitary Ware

The antibacterial activity of photocatalytic titanium dioxide (TiO₂) thin films with photodeposited silver on the surface of sanitary ware was studied. Samples were prepared by coating a TiO₂ sol that was calcined at 880°–980°C and photodeposited with silver ions onto the glazed layer of the sanitary ware. The relationships between the antibacterial activity and the fabrication conditions were investigated by X-ray diffractometry, scanning electron microscopy, and colorimetry. The phase of TiO₂ identified in the thin films was a mixture of anatase and rutile. The amount of rutile phase increased as the calcination temperature increased, and grain growth of the TiO₂ particles was observed. The activity was dependent on the TiO₂ thickness, the calcination temperature, and the amount of silver. These results suggest that the antibacterial activity was strongly affected by the amount of anatase in the thin films.     

In Situ Diffraction Study of Self-Recovery in Aluminum Titanate

Aluminum titanate (Al₂TiO₅) is an excellent refractory and thermal shock resistant material due to its relatively low-thermal expansion coefficient and high melting point. However, Al₂TiO₅ is only thermodynamically stable above 1280°C and undergoes a eutectoid decomposition to α-Al₂O₃ and TiO₂ (rutile) in the temperature range of 900°–1280°C. In this paper, we describe the use of high-temperature neutron diffraction to study the properties of self-recovery in Al₂TiO₅ when it is annealed at ≥1300°C in air. It is shown that the process of decomposition in Al₂TiO₅ is reversible and that self-recovery occurs readily when decomposed Al₂TiO₅ is reheated above 1300°C. It is further shown that the existence of a temperature range (900°–1280°C) in which Al₂TiO₅ is prone to decomposition can be explained by the competing dominance of self-recovery at ≥1280°C and decomposition at ≤1280°C.     

Creep of Duplex Alumina–Titanium Nitride Polycrystalline Microstructures

Compression creep results in an inert atmosphere are presented for fine-grained Al₂O₃–TiN composites deformed under a stress range of 10–50 MPa stress and a temperature range of 1250°–1400°C. The effects of TiN content (10–70 vol%) and the preparation route (i.e., reactive versus nonreactive hot pressing) are considered. Al₂O₃ and TiN both are efficient grain-growth inhibitors, relative to each other; thus, deformation develops in stable equiaxed microstructures. Except for low TiN contents (10 and 20 vol%), the processing route does not markedly influence the observed strain rates, at least up to a strain of −0.16. In all cases, deformation occurs mainly via interfacial/grain-boundary sliding, and its rate is enhanced in approximate proportion to the amount of Al₂O₃/TiN interfacial area. The deformation rate seems to be limited by either some interface reaction or a complex boundary diffusional mechanism that occurs at the Al₂O₃ grain boundaries.     

Oxidation of HIPed TiC Ceramics in Dry O2, Wet O2, and H2O Atmospheres

Isothermal oxidation of dense TiC ceramics, fabricated by hot-isostatic pressing at 1630°C and 195 MPa, was performed in Ar/O₂ (dry oxidation), Ar/O₂/H₂O (wet oxidation), and Ar/H₂O (H₂O oxidation) at 900°–1200°C. The weight change measurements of the TiC specimen showed that the dry, wet, and H₂O oxidation at 850°–1000°C is represented by a one-dimensional parabolic rate equation, while the oxidation in the three atmospheres at 1100° and 1200°C proceeds linearly. Cross-sectional observation showed that the dry oxidation produces a lamellar TiO₂ scale consisting of many thin layers, about 5 μm thick, containing many pores and large cracks, while H₂O-containing oxidation decreases pores in number and diminishes cracks in scales. Gas evolution of CO₂ and H₂ with weight change measurement was simultaneously followed by heating the TiC to 1400°C in the three atmospheres. Cracking in the TiO₂ scale accompanied CO₂ evolution, and the H₂O-containing oxidation produced a small amount of H₂. A piece of single crystal TiC was oxidized in ¹⁶O₂/H₂¹⁸O to reveal the contribution of O from H₂O to the oxidation of TiC by secondary ion mass spectrometry.     

Thermal Treatment of Titanate Derivatives Synthesized by Ion-Exchange Reaction

TiO₂ fibers were formed by thermal treatment of layered H₂Ti₄O₉ (hydrous titanium dioxide) and KHTi₄O₉ synthesized by ion-exchange reactions. The calcination of the former at 900° and 1050°C for 3 h yielded TiO₂ fibers with anatase and rutile phases, whose length and diameter were 15–20 and 2–5 μm and 10–15 and 3–5 μm, respectively. The thermal treatment of the latter at temperatures of 250° to 500°C yielded pure K₂Ti₈O₁₇, which tended to decompose to K₂Ti₆O₁₃ and TiO₂ at temperatures >600°C. At 1050°C, K₂Ti₆O₁₃ phase was formed with rutile TiO₂ fibers, whose length and diameter were 10–20 and 1–3 μm, respectively.     

A New Microwave Dielectric Ceramic for LTCC Applications

A new low-sintering temperature microwave dielectric ceramic, the Li₂TiO₃ solid solution, was found and investigated in the Li₂O–Nb₂O₅–TiO₂ system. The compound with the composition of Li_2.081Ti_0.676Nb_0.243O₃ crystallizes as a monoclinic structure. This new microwave dielectric ceramic shows a relatively low permittivity (∼20), high Q × f values up to 50 000 (7.8 GHz), and near-zero temperature coefficients (13 ppm/°C), which were obtained via sintering at 1100°C. The addition of ≤2 wt% B₂O₃ was very effective in lowering the sintering temperature ( T _s), and dense ceramics could be obtained at T _s≤900°C. The addition of B₂O₃ does not induce apparent degradation in the microwave properties but lowers the τ_f value to near zero. It is obvious that the ceramics could be promising candidates for multilayer low-temperature co-fired ceramics applications.     

Electrical Properties of Manganese-Doped Titanium Dioxide

The electrical conductivity and Seebeck coefficient for TiO₂ doped with varying amounts of MnO₂ have been measured over the temperature range of 900°–1100°C. The Hall coefficient was also measured over the temperature range of 300°–500°C. The electrical conductivity and carrier density were found to decrease initially, go through a minimum at approximately x = 0.0001–0.001, and then increase with increasing content of MnO. For dopant levels above 0.01 at.%, p -type electrical conduction behavior was observed.     

Nitridation of the Sol–Gel-Derived Titanium Oxide Films by Heating in Ammonia Gas

Dip-coated sol–gel-derived TiO₂ films on an alumina substrate were converted to nonstoichiometric titanium nitride (TiN _x ( x ≦ 1)) films by heating at approxmately 1000°C in NH₃ gas. TiO₂ films made from TiO₂ sols prepared from Ti(O– i -C₃H₇)₄ and stabilized by diethanolamine were more easily nitrided than those from sols containing HCl as a deflocculant reagent. This appears to be a result of the more porous structure of the former films.     

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.     

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.     

Anatase-to-Rutile Transformation in Titania Powders

Titania (TiO₂) is an important electronic ceramic material for use in diverse applications such as gas sensors, catalysts, dielectrics, and ceramic membranes. TiO₂ exists as several polymorphic phases, most commonly as rutile or anatase. This paper investigates the microstructural evolution of anatase-based commercial TiO₂ powders, with an average size of 100 nm, at high temperatures. These powders transform to the rutile structure at 1000°C. The characteristics of the anatase-to-rutile transformation have been studied using transmission electron microscopy analysis, and new information regarding the nature and mechanisms of this polymorphic reaction has been revealed.     

Oriented Barium Titanate Thick Films by Reactive Coating on Rutile

A mechanically stable [110] oriented tetragonal barium titanate layer of ≤50 μm thickness on a rutile substrate has been obtained by reactive coating of screen printed barium titanate submicrometer powder on the substrate and following a two-step firing cycle at 1400°C for 6 min and 1310°C for 60 min. The results have been explained taking into account the BaTiO₃–TiO₂ equilibrium phase diagram.     

Fabrication of Free-Standing Titania-Based Gas Sensors by the Oxidation of Metallic Titanium Foils

A simple method for fabricating TiO₂-based sensors of CO( g ) is demonstrated: the oxidation of Ti-bearing foils. Metallic foils (35 μm thick) were converted into free-standing, porous rutile foils (60 μm thick) by exposure to O₂( g ) at 800°—965°C. The oxidized foils contained thin (0.5—1 μm thick), regularly spaced oxide layers oriented parallel to the external surface. The exposure of such porous foils to increasing concentrations of CO( g ) resulted in a monotonic increase in the steady-state electrical resistance. Rutile foils sensitive to 50 ppm changes in CO( g ) content with response times of a few minutes were produced. The effects of oxidation conditions and copper doping on sensing performance are discussed.