Oxidation Resistance of Alumina–Titanium Nitride and Alumina–Titanium Carbide Composites

The presence of TiC or TiN paritcles in an Al₂O₃ matrix affects the thermal stability of the composites in oxidizing environments. In isothermic oxidation tests at 700°, 800°, 900°, 1000°, and 1100°C for up to 20 h, two different oxidation regimes have been observed at T < 900°C and at 900°C ≤ T ≤ 1100°C. At low temperatures ( T < 900°C), the oxidation follows a phase-boundary reaction; the reaction product initially consists of aggregates of submicrometer needlelike TiO₂ rutile crystals that subsequently grow and coalesce. When a continuous TiO₂ rutile layer is formed ( T ≥ 900°C), the oxidation kinetics change to parabolic, and the diffusion of O₂ through a thick TiO₂ layer is proposed as the governing step.     

Formation of Titanium Carbide/Aluminum Oxide Nanocomposite Powder by High-Energy Ball Milling and Subsequent Heat Treatment

The preparation of titanium carbide/aluminum oxide (TiC/Al₂O₃) nanocomposite powders from a mixture of titanium, carbon, and Al₂O₃ powders via a high-energy ball milling process and subsequent heat treatment was investigated. The microstructure development of the powder mixtures was monitored by X-ray diffraction and transmission electron microscopy. The ball milling of an elemental carbon, titanium, and Al₂O₃ powder mixture at ambient temperature resulted in the formation of TiC within 15 h of milling. With further milling of up to 25 h, the resulting powder mixture was composed of nanosized TiC particles and nanocrystalline carbon, titanium, and Al₂O₃. The nanocrystalline titanium and carbon were transformed into nanosized TiC particles after subsequent heat treatment. The final product was composed of nanosized TiC and microcrystalline Al₂O_3. Most of the nanosized TiC particles were located within Al₂O₃ grains.     

Auger Electron Spectroscopy Analysis of Cross-Section Surface of Oxidized Titanium Carbide Single Crystal

A TiC monocrystal was heated by three thermal cycles with isotherm at 1108 K while exposed to Ar/O₂ mixtures respectively with O₂ contents of 239, 1.0, and 324 ppm. The reactivity was detected with a homemade device based on two identical solid electrolyte oxygen sensors connected to a quadrupole mass spectrometer (QMS). The oxidized sample was cleaved by impact bending under high vacuum and the cross section investigated by Auger electron spectroscopy (AES) and scanning electron microscopy (SEM) techniques. Both area and profile AES spectra were acquired by using a 70 nm diameter beam. Spectral changes have been analyzed to identify chemical species present at the TiC/TiO₂ interface. A model for high-temperature oxidation of TiC has been proposed. It implies oxygen diffusion through a protective TiO₂ layer and the existence of two inner interfaces: TiC/TiC_{1− x} O _x and TiC_{1− x} O _x /TiO₂.     

Effect of Composition on Properties of Alumina/Titanium Silicon Carbide Composites

In the present study, the room-temperature properties of Al₂O₃-Ti₃SiC₂ composites with different Ti₃SiC₂ contents are determined. The composites are prepared by attrition milling Al₂O₃ and Ti₃SiC₂ mixture powders followed by spark plasma sintering (SPS) under vacuum. From a closer examination of the dependencies of the electrical conductivity on compositions in this system, we determined the percolation threshold at which an interconnected network of electrically conductive phase arises. Since the hardness of Ti₃SiC₂ is lower than that of Al₂O₃, the Vickers hardness decreased with the increasing of Ti₃SiC₂ content while the fracture toughness and the strength increased. The maximum strength (673 MPa) and the maximum toughness (9.3 MPa·m^1/2) were reached in the pure Ti₃SiC₂ material.     

Stability Domains of TiO2–Ti3O5–Ti2O3–TiC/Ti(CN) Systems During Reduction Process

The phase domain of Ti₃O₅–Ti₂O₃–Ti(CO) at 1580 K was determined from the formation energies of Ti(C _x O _y ), as calculated via the Gibbs–Duhem equation. An extensive Ti(CO) domain is attributed to the high affinity between TiC and TiO. The phase domain of Ti₃O₅–Ti₂O₃–Ti(CN) was obtained at 1673 K using the formation energies of Ti(C _x N _y ). This study shows that the stable region for Ti₂O₃ is significantly small in the Ti₃O₅–Ti₂O₃–Ti(CN) phase domain. It demonstrates the absence of TiO and Ti₂O₃ in the normal syntheses of TiC and Ti(CN) from TiO₂, respectively.     

Photoluminescence Probing of the Formation of Titanium Dioxide Sols from a Titanium Peroxide Solution

Photoluminescence spectroscopy was used to probe the formation of colloidal TiO₂ sols from a titanium peroxide solution, which were prepared by reaction of H₂O₂ and titanium hydrate gel that was precipitated from acidic TiOCl₂ solution neutralized by NH₄OH, during aging at 100°C for 0–64 h. Emission spectra revealed broad band luminescence which is interpreted as the superposition of a multiple photoluminescence process involving TiO₂ precursors. The maximum peak position of the luminescence band was shifted to a shorter wavelength by the generation of the dominant emission peak as aging time increased, and a peak at ∼390 nm, indicating the anatase phase, was observed after aging for >48 h. However, sols aged for 2 h already were of the anatase phase, according to X-ray diffraction analysis. It was deduced that the interfacial layers between the particles and the liquid in an aqueous solution caused luminescence quenching or accelerated sink.     

Ambient- and High-Temperature Properties of Titanium Carbide–Titanium Boride Composites Fabricated by Transient Plastic Phase Processing

Ambient- and high-temperature properties of a class of titanium carbide-titanium boride composites that have been produced by transient plastic phase processing are presented. The composites produced are comprised of Ti₃B₄, TiB₂, and TiC_0.65 at their equilibrium composition (34.5, 30.5, and 34.9 vol%, respectively), and the Ti₃B₄ phase in these composites occurs either as equiaxed grains or as platelets, depending on the starting mixture composition. Measurements of the ambient- and high-temperature flexure strength and fracture toughness, thermal shock susceptibility, oxidation resistance, and wear resistance of this class of composites are presented. The role of various microstructural parameters—such as the morphology of the Ti₃B₄ phase, the length scale of the microstructure, and the volume fraction of borides—on these properties has been identified.     

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.     

Synthesis of Nanosized Titanium-Chromium Oxynitride Powders by Ammonolysis of Coprecipitated TiO2/Cr2O3 Precursors

Interstitial titanium-chromium oxynitrides in the solid solution series Ti_{1− z} Cr _z (O _x N _y ) ( z = 0.2, 0.4, 0.5, 0.6, 0.8) have been obtained by ammonolysis of the TiO₂/Cr₂O₃ precursors resulting from the coprecipitation method. The precursors and the resulting oxynitrides were characterized by auger electron spectroscopy, X-ray diffraction analysis, electron probe microanalysis, transmission electron microscopy, and BET surface area techniques. Compounds in the Ti_{1− z} Cr _z (O _x N _y ) series are prepared as single phases by nitridation at 1073 K for 8 h. The as-synthesized oxynitride powders contain only Ti_{1− z} Cr _z (O _x N _y ) with cubic structure and the particle size is in the nanometer scale.     

Structure Formation in the Combustion Synthesis of Al2O3–TiC Composites

Structure formation in the combustion synthesis of Al₂O₃–TiC composites from TiO₂, Al, and graphite powders was investigated using cylindrical samples and cone-shaped "quenching samples." It is shown that the phases Ti and Ti₃Al exist as intermediates in the combustion synthesis process. Titanium carbide forms in a secondary step through reactions between graphite and liquid Ti or Ti₃Al, then nucleates from a liquid mixture of the three phases Ti, Ti₃Al, and alumina. The nucleated particles grow in the postcombustion stage. Liquid alumina, containing TiC as a dissolved phase, solidifies into corundum grains in the postcombustion stage. Moreover, it is shown that the temperature gradient in the postcombustion stage markedly affects the microstructures of the products. Higher-temperature gradients, typical at the surface of the samples, give rise to the formation of corundum whiskers and TiC agglomerates. In contrast, lower gradients, typical in the center of the samples, lead to the formation of relatively large TiC particles and corundum grains.     

Dopants in Flame Synthesis of Titania

The effect of dopants on the characteristics of titania particles made by oxidation of TiCl₄ in a laminar diffusion flame reactor is presented. Introduction of dopant SiCl₄ inhibits the transformation of anatase to rutile, due to the formation of interstitian solid solution of SiO₂ and TiO₂. Silica decreases the sintering rate of titania and decreases the primary particle size, and, as a result, the specific surface area increases. Intruduction of SnCl₄ enhances the transformation of anatase to rutile, due to the similar crystalline structure of SnO₂ and rutile titania. However, the presence of SnO₂ and rutile titania. However, the presence of SnO₂ does not affect the primary particle size or the specific surface area of titania particles. Introduction of AICI₃ enhances the transformation of anatase to rutile, due to the formation of excess oxygen vacancies as Al₂O₃ and TiO₂ form a substitutional solid solution.     

In Situ Processing and High-Temperature Properties of Ti3Si(Al)C2/SiC Composites

Ti₃SiC₂ has many salient properties including low density, high strength and modulus, damage tolerance at room temperature, good machinablity, and being resistant to thermal shock and oxidation below 1100°C. However, the low hardness and poor oxidation resistance above 1100°C limit the application of this material. The poor oxidation resistance at temperatures above 1100°C was because of the absence of protective layer in the scale and the presence of TiC impurity phase. TiC impurity could be eliminated by adding a small amount of Al to form Ti₃Si(Al)C₂ solid solutions. Although the high-temperature oxidation resistance was significantly improved for the Ti₃Si(Al)C₂ solid solutions, the strength at high temperatures was lost. One important way to enhance the high-temperature strength is to incorporate hard ceramic particles like SiC. In this article, we describe the in situ synthesis and simultaneous densification of Ti₃Si(Al)C₂/SiC composites using Ti, Si, Al, and graphite powders as the initial materials. The effect of SiC content on high-temperature mechanical properties and oxidation resistance were investigated. The mechanisms for the improved high-temperature properties are discussed.     

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.     

Effect of Particle Dispersion on the Mechanism of Combustion Synthesis of Titanium Silicide

The combustion synthesis (SHS) of titanium silicide (Ti₅Si₃) was investigated. The main combustion reaction (thermal explosion) occurs in the presence of a liquid phase and is usually preceded by a relatively small reaction in the solid state. The influence of Ti particle size on the dominance of each of these types of reactions was investigated. SEM observations and X-ray diffraction analyses indicate that the combustion synthesis of Ti₅Si₃ proceeds according to the following sequence: TiSi₂→ TiSi → Ti₅Si₄→ Ti₅Si₃.     

Enhancing Effect of Iron Chlorides on the Anatase-Rutile Transition in Titanium Dioxide

The effect of solid Fe₂O₃ and gaseous iron chlorides on the anatase-rutile phase transition was investigated in the temperature range of 750°-950°C via X-ray diffractometry and scanning electron microscopy. In both cases, iron diffusion in the anatase lattice decreased the transition temperature and increased the anatase-rutile transformation rate, in comparison with that in TiO₂ fired in air. The enhancement effect of iron on the anatase-rutile transition is understood on the basis of oxygen-vacancy formation, which favors rutile nucleation. Solid-state iron diffusion between the contact points of TiO₂ and Fe₂O₃ particles and vapor mass transport through gaseous chlorides were the primary mechanisms of iron mass transport to the TiO₂ surface in the presence of both Fe₂O₃ and gaseous iron chlorides, respectively. The transformation rate at a given reaction temperature increased in the following order of reaction conditions: pure TiO₂ in air, TiO₂ in the presence of Fe₂O₃ in air, TiO₂ in the presence of Fe₂O₃ in chlorine, and TiO₂ in the presence of gaseous iron chlorides.     

Phase Relations in the Pseudobinary System Na2TiSi4O11-Na2Ti2Si2O9

The quenching technique was used to study subliquidus and subsolidus phase relations in the pseudobinary system Na₂ Ti₂Si₂ O₁₁-Na₂ Ti₂ Si₂ O₉. Both narsarukite (Na₂TiSi₄O₁₁) and lorenzenite (Na₂Ti₂Si₂O₉) melt incongruently. Narsarsukite melts at 911°±°C to SiO₂+liquid, with the liquidus at 1016°C. Lorenzenite melts at 910°±5°C to Na₂ Ti₆ O₁₃+liquid; Na₂ Ti₆ O₁₃ reacts with liquid to form TiO₂ and is thus consumed by 985°±5°C. The liquidus occurs at 1252°C.     

Reinvestigation of the Phase Diagram for the System Titanium–Oxygen

Samples of the titanium oxides with O/Ti ratios between 0.5 and 1.67 were prepared by heating mixtures of Ti and TiO₂ in high vacuum to 1500°C and annealing them in silica ampuls at 900° to 1100°C. Results of chemical and X-ray analyses are combined with previously published data to form a complete phase diagram. The melting points of Ti₂O₃ and Ti₃O₅ were 1839°° 10°C and 1774°° 10°C, respectively.     

Preparation of Titanium Nitride: Microwave-Induced Carbothermal Reaction of Titanium Dioxide

Carbothermal reduction and nitridation of TiO₂ was performed in a 2 kW, 2.45 GHz microwave furnace. Carbon, generally added for the removal of oxygen from TiO₂ lattice also served as the low-temperature susceptor in these experiments. At temperatures >1200°C, the mixtures started reacting vigorously (self-burn), which was never observed with the respective pure compounds. In the self-burning state, the minimum duration required for complete titanium nitride transformation was ∼20 min with stoichiometric amounts of carbon. With excess C, the transformation duration dropped to just 1 min. CO removal and subsequent Ni fixation occur more directly in microwave-induced reactions than in conventional nitridation procedures. Intermediate formation of successive Magneli phases (Ti_{2 n} O_{2 n −1}) was not found in the microwave-induced reactions. The combination of microwave processing and combustion makes this route of economical interest.     

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.     

Origin of Microwave Dielectric Loss in ZnNb2O6-TiO2

(1− x )ZnNb₂O₆· x TiO₂ ceramics were prepared using both anatase and rutile forms of TiO₂. At a composition of x = 0.58, a mixture region of ixiolite (ZnTiNb₂O₈) and rutile was observed and the temperature coefficient of resonant frequency (τ_f) was ∼0 ppm/°C. We found that although ɛ_r and τ_f were comparable, the quality factor ( Q × f , Q ≈ 1/ tan δ, f = resonant frequency) of 0.42ZnNb₂O₆·0.58TiO₂ prepared from anatase and rutile was 6000 and 29 000, respectively. The origin of the difference in Q × f of both samples was investigated by measuring electrical conductivity and by analysis of the anatase–rutile phase transition. The anatase-derived sample had higher conductivity, which was related to the reduction of Ti⁴⁺. It is suggested that the increase of dielectric loss originates from an increase in Ti³⁺ and oxygen vacancies due to an anatase–rutile phase transition.