Formation of Al2O3–TiC Composite Nano-Particles Synthesized from Carbon-Coated Precursors

The formation of nano-sized alumina–titanium carbide (Al₂O₃–TiC) composite powders from a carbon-coated titanium dioxide–aluminum (TiO₂–Al) mixture was investigated. The carbon-coated TiO₂–Al mixture altered the mechanism of the reaction, compared with standard mixtures, to produce high-quality nano-sized Al₂O₃–TiC powders. Data synthesized from intermediate temperatures indicate that these products form via Ti₂O₃ and Al₃Ti. TEM images of the Al₂O₃–TiC powders showed fine size (50–100 nm), narrow size distribution, and lack of agglomeration. DSC data for the carbon-coated TiO₂–Al mixture showed a single endothermic and four successive weak exothermic reactions as the carbon coating moderated the heat release during the reaction.     

Near-Net-Shape Fabrication of Ti3AlC2-Based Composites

A porous ceramic preform was fabricated by printing a powder blend of TiC, TiO₂, and dextrin. The presintered preforms contained a bimodal pore size distribution with intra-agglomerate pores ( d ₅₀≈0.7 μm) and inter-agglomerate pores ( d ₅₀≈30 μm), which were subsequently infiltrated by aluminum melt spontaneously in argon above 1050°C. A redox reaction at 1400°C resulted in the formation of dense Ti–Al–O–C composites mainly composed of Ti₃AlC₂, TiAl₃, Al, and Al₂O₃, which attained a bending strength of 320 MPa, a Young's modulus of 184 GPa, and a Vicker's hardness of 2.5 GPa.     

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.     

Molten Salt Synthesis of Magnesium Aluminate (MgAl2O4) Spinel on Ti3AlC2 Substrate

A MgAl₂O₄ (MA) spinel layer was synthesized on Ti₃AlC₂ substrate through the molten salt synthesis (MSS) method. The Ti₃AlC₂ substrate was immersed in MgCl₂·6H₂O powders and treated at 800°, 850°, and 900°C for 4 h in air. A continuous and 10-μm-thick MgAl₂O₄ layer was obtained at 900°C, by which the surface hardness of Ti₃AlC₂ can be effectively improved. The combined scanning electron microscopy observations and crystal morphology simulation further revealed that the as-formed MgAl₂O₄ presents tetragonal bipyramids morphology with (400)-orientation.     

Preparation and Properties of Ternary Ti3AlC2 and its Composites from Ti–Al–C Powder Mixtures With Ceramic Particulates

A layered ternary carbide phase, Ti₃AlC₂, was synthesized by hot pressing from the starting materials of Ti, aluminum, and activated carbon at 1400°C for 2 h. Its composites were also fabricated through addition of micro-sized SiC and partially stabilized zirconia particulates to the pulverized Ti₃AlC₂ powders. The polycrystalline Ti₃AlC₂ ceramic obtained has a flexural strength of 172 MPa and a fracture toughness of 4.6 MPa·m^1/2, respectively. This compound is relatively soft (Vikers hardness of 2.7 GPa) and exhibits good electrical conductivity with an electrical resistivity of 8.2 μΩ·m. Both the Ti₃AlC₂/SiC and Ti₃AlC₂/ZrO₂ composites are superior to the monolithic Ti₃AlC₂ ceramic in strength, fracture toughness, and micro-hardness.     

Synthesis of Ti3AlC2 Powders Using Sn as an Additive

Nearly pure Ti₃AlC₂ powders have been synthesized by calcining a mixture of titanium, aluminum, and graphite powders using tin powders as additives. Four recipes with different mole ratios of Ti:Al:C:Sn were examined at calcining temperatures from 1300° to 1500°C. The addition of Sn effectively inhibited the generation of thermal explosion when the volume of the starting materials is larger, and considerably reduced the lower-limit calcining temperature. The nearly pure Ti₃AlC₂ powders can be obtained reliably on a large scale by calcining the starting materials with a mole ratio of 3Ti:1Al:1.8C:0.2Sn at temperatures from 1350° to 1500°C.     

Inter-Conversion of Mn+1AXn Phases in the Ti–Al–C System

It is demonstrated that the M _{n +1}AX _n phase Ti₃AlC₂ may be readily synthesized by sintering a stoichiometric mixture of the lower order MAX phase Ti₂AlC mixed with a stoichiometric amount of TiC in the temperature range 1350°–1450 °C. High-quality Ti₃AlC₂ was readily produced using sintering times in the range 2–5 h. In general, <2% of unwanted or remnant phases were found to be present and in some samples none could be detected at all.     

Pressure-Assisted Combustion Synthesis of Dense Layered Ti3AlC2 and its Mechanical Properties

A near-single-phase Ti₃AlC₂ ternary carbide was synthesized from 3Ti–1.1Al–1.8C powder blend, both by the wave propagation and thermal explosion (TE) modes of self-propagating high-temperature synthesis. The application of a moderate (28 MPa) pressure immediately after TE at 800°C (reactive forging) yielded a 95% dense material containing, in addition to Ti₃AlC₂, an appreciable amount of TiC_{1− x} . By adjusting the starting composition, a 99% dense material containing up to 90 vol.% Ti₃AlC₂ was obtained. The material had a fine-layered microstructure with Ti₃AlC₂ grain size not exceeding 10 μm. The samples were readily machinable and had a high compressive strength of ∼800 MPa up to 700°C.     

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.     

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.     

Tribophysical Properties of Polycrystalline Bulk Ti3AlC2

A nearly pure, dense, polycrystalline bulk Ti₃AlC₂ sample was prepared by reactively hot pressing the element titanium, aluminum, and graphite powders. The tribophysical properties were investigated by sliding a Ti₃AlC₂ block dryly against a low-carbon steel disk. It was found that the friction coefficient is as low as ∼0.1, and the wear rate of Ti₃AlC₂ is only ∼2.5 × 10⁻⁶ mm³/N·m for the highest sliding speed of 60 m/s and the largest normal pressure of 0.8 MPa. These unusual properties are attributable to the presence of a compact self-generating film, which covers uniformly over the friction surface of Ti₃AlC₂ with a thickness of ∼0.5 μm.     

Surface Chemistry, Dispersion Behavior, and Slip Casting of Ti3AlC2 Suspensions

The surface chemistry and dispersion properties of aqueous Ti₃AlC₂ suspension were studied in terms of hydrolysis, adsorption, electrokinetic, and rheological measurements. The Ti₃AlC₂ particle had complex surface hydroxyl groups, such as ≡Ti–OH,=Al–OH, and −OTi–(OH)₂, etc. The surface charging of the Ti₃AlC₂ particle and the ion environment of suspensions were governed by these surface groups, which thus strongly influenced the stability of Ti₃AlC₂ suspensions. PAA dispersant was added into the Ti₃AlC₂ suspension to depress the hydrolysis of the surface groups by the adsorption protection mechanism and to increase the stability of the suspension by the steric effect. Ti₃AlC₂ suspensions with 2.0 dwb% PAA had an excellent stability at pH=∼5 and presented the characteristics of Newtonian fluid. Based on the well-dispersed suspension, dense Ti₃AlC₂ materials were obtained by slip casting and after pressureless sintering. This work provides a feasible forming method for the engineering applications of MAX-phase ceramics, wherein complex shapes, large dimensions, or controlled microstructures are needed.     

Phase Relations in the Pyrochlore-Rich Part of the Bi2O3−TiO2−Nd2O3 System

In this study we used solid-state synthesis to determine the phase relations in the pyrochlore-rich part of the Bi₂O₃−TiO₂−Nd₂O₃ system at 1100°C. The samples were analyzed using X-ray powder diffraction and scanning electron microscopy with energy- and wavelength-dispersive spectroscopy. A single-phase pyrochlore ceramic was obtained with the addition of 4.5 mol% of Nd₂O₃. We determined the solubility limits for the three solid solutions: (i) the pyrochlore solid solution Bi_{(1.6–1.08 x )}Nd _x Ti₂O_{(6.4+0.3 x )}, where 0.25< x <0.96; (ii) the solid solution Bi_{4− x} Nd _x Ti₃O₁₂, where 0< x <2.6; and (iii) the Nd_{2− x} Bi _x Ti₂O₇ solid solution, where 0< x <0.35. The determined phase relations in the pyrochlore-rich part are presented in a partial phase diagram of the Bi₂O₃−TiO₂−Nd₂O₃ system in air at 1100°C.     

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.     

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.     

Effect of Vacuum Annealing on the Phase Stability of Ti3SiC2

The effect of vacuum annealing on the thermal stability and phase transition of Ti₃SiC₂ has been investigated by X-ray diffraction (XRD), neutron diffraction, synchrotron radiation diffraction, and secondary ion mass spectroscopy (SIMS). In the presence of vacuum or a controlled atmosphere of low oxygen partial pressure, Ti₃SiC₂ undergoes a surface dissociation to form nonstoichiometric TiC and/or Ti₅Si₃C _x that commences at ∼1200°C and becomes very pronounced at ≥1500°C. Composition depth profiling at the near surface of vacuum-annealed Ti₃SiC₂ by XRD and SIMS revealed a distinct gradation in the phase distribution of TiC and Ti₅Si₃C _x with depth.     

Synthesis, Densification, and Phase Evolution Studies of Al2O3–Al2TiO5–TiO2 Nanocomposites and Measurement of Their Electrical Properties

Alumina–aluminum titanate–titania (Al₂O₃–Al₂TiO₅–TiO₂) nanocomposites were synthesized using alkoxide precursor solutions. Thermal analysis provided information on phase evolution from the as-synthesized gel with an increase in temperature. Calcination at 700°C led to the formation of an Al₂O₃–TiO₂ nanocomposite, while at a higher temperature (1300°C) an Al₂O₃–Al₂TiO₅–TiO₂ nanocomposite was formed. The nanocomposites were uniaxially compacted and sintered in a pressureless environment in air to study the densification behavior, grain growth, and phase evolution. The effects of nanosize particles on the crystal structure and densification of the nanocomposite have been discussed. The sintered nanocomposite structures were also characterized for dielectric properties.     

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.     

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.     

Subsolidus Phase Relationships in the System Fe2O3–Al2O3–TiO2 between 1000° and 1300°C

Subsolidus phase equilibria in the system Fe₂O₃–Al₂O₃–TiO₂ were investigated between 1000° and 1300°C. Quenched samples were examined using powder X-ray diffraction and electron probe microanalytical methods. The main features of the phase relations were: (a) the presence of an M₃O₅ solid solution series between end members Fe₂TiO₅ and Al₂TiO₅, (b) a miscibility gap along the Fe₂O₃–Al₂O₃ binary, (c) an α-M₂O₃( ss ) ternary solid-solution region based on mutual solubility between Fe₂O₃, Al₂O₃, and TiO₂, and (d) an extensive three-phase region characterized by the assemblage M₃O₅+α-M₂O₃( ss ) + Cor( ss ). A comparison of results with previously established phase relations for the Fe₂O₃–Al₂O₃–TiO₂ system shows considerable discrepancy.