Effect of TiB2 and Boron Additions on the Stability of Wurtzite-Type Boron Nitride at High Temperatures and Pressures

Shock-synthesized wurtzite-type BN(w-BN) and mixtures of w-BN–TiB₂ and w-BN–ß were treated at high temperatures and pressures. The effect of TiB₂ and boron additions on the transformation of w-BN to zinc blende-type BN(z-BN) was studied at 1000° to 2200°C and 55 to 68 kbars. The formation region of z-BN was shifted toward higher temperatures by TiB₂ additions and toward lower temperatures by boron additions.     

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.     

Crack Formation and Swelling of TiB2-Ni Ceramics in Liquid Aluminum

TiB₂ ceramics are of interest for use as cathodes in aluminum reduction cells. The behavior of hot-pressed TiB₂ ceramics (containing 0 to 10 wt% nickel with a 5- to 12-μm boride grain) in liquid aluminum at 950°C is reported.     

Effect of Hot-Pressing Temperature on Densification and Mechanical Properties of Titanium Diboride with Silicon Nitride as a Sintering Aid

The effect of hot-pressing temperature on the densification behavior and mechanical properties of titanium diboride (TiB₂) was investigated. TiB₂ specimens were hot-pressed for 1 h at temperatures in the range of 1500°–1800°C, with an addition of 2.5 wt% of silicon nitride (Si₃N₄) as a sintering aid. The density increased markedly at temperatures in the range of 1500°–1600°C and remained constant thereafter. The formation of a eutectic liquid at 1550°C was attributed to the steep increase in density. The hot-pressing temperature also improved the mechanical properties, such as the flexural strength, Vickers hardness, and fracture toughness of the specimens. Similar to the density, the mechanical properties improved remarkably at ∼1550°C, so that optimum properties were obtainable at temperatures as low as 1600°C.     

Processing and Properties of TiB2 with MoSi2 Sinter-additive: A First Report

The densification of non-oxide ceramics like titanium boride (TiB₂) has always been a major challenge. The use of metallic binders to obtain a high density in liquid phase-sintered borides is investigated and reported. However, a non-metallic sintering additive needs to be used to obtain dense borides for high-temperature applications. This contribution, for the first time, reports the sintering, microstructure, and properties of TiB₂ materials densified using a MoSi₂ sinter-additive. The densification experiments were carried out using a hot-pressing and pressureless sintering route. The binderless densification of monolithic TiB₂ to 98% theoretical density with 2–5 μm grain size was achieved by hot pressing at 1800°C for 1 h in vacuum. The addition of 10–20 wt% MoSi₂ enables us to achieve 97%–99%ρ_th in the composites at 1700°C under similar hot-pressing conditions. The densification mechanism is dominated by liquid-phase sintering in the presence of TiSi₂. In the pressureless sintering route, a maximum of 90%ρ_th is achieved after sintering at 1900°C for 2 h in an (Ar+H₂) atmosphere. The hot-pressed TiB₂–10 wt% MoSi₂ composites exhibit high Vickers hardness (∼26–27 GPa) and modest indentation toughness (∼4–5 MPa·m^1/2).     

Molten Salt Synthesis of a Complex Perovskite, Pb(Fe0.5Nb0.5)O3

Pure, single-phase stoichiometric Pb(Fe_0.5Nb_0.5)O₃ (PFN) powders were successfully formed by molten salt synthesis using mixtures of NaCl and KCl salts. Lower temperatures and shorter times (1/2 h at 800°C) were needed for singlephase PFN formation from molten salts relative to those required for solid-phase methods (4 h at 1000°C). A systematic study indicating the effects of process parameters, such as temperature, time, and amount of flux with respect to starting oxides, on the PFN formation mechanism and its resulting powder characteristics is reported. The particle size increased with increasing synthesis temperature; the rate of increase was greatest above 900°C, which is close to the melting point of PbO. PFN powders formed by molten salt synthesis are spheroidal, free from aggregates, and sintered to good densities at temperatures as low as 1000°C.     

Oxidation Kinetics and Composite Scale Formation in the System Mo(Al,Si)2

The oxidation kinetics of hot-pressed Mo(Al_0.01Si_0.99)₂ and Mo(Al_0.1Si_0.9)₂ were measured at 480°C, and between 1200° and 1600°C. The qualitative oxidation of arc-melted Mo(Al_0.1Si_0.9)₂, Mo(Al_0.3Si_0.7)₂, Mo(Al_0.5Si_0.5)₂, and Mo₃Al₈ was examined after 600°C for 1000 h in air. At all temperatures, the compositional difference between the materials yielded very different oxidation rates and scale microstructures. At 1400° and 1500°C, microstructural evolution of the oxide scales resulted in improved oxidation resistance at long times (>400 h). At these temperatures, a significant reduction in the long-time oxidation kinetics was correlated with the in situ formation of an inner mullite scale. At 480° and 600°C, oxidation resistance improved significantly with increasing aluminum concentration. Contrary to the behavior of MoSi₂, samples of Mo(Al_0.01Si_0.99)₂ did not demonstrate catastrophic oxidation, and samples of Mo(Al_0.1Si_0.9)₂ were very oxidation resistant.     

Role of Microstructure and Intergranular Phases in Stress Corrosion of TiB2 Exposed to Liquid Aluminum

The resistance of several polycrystalline TiB₂ materials to penetration by liquid aluminum at 970°C was investigated, and their microstructures were characterized. The grain-boundary properties of individual diborides rather than the intrinsic properties of TiB₂ are thought to control stress corrosion susceptibility in liquid metal environments.     

Nano α-Al2O3 Powder Preparation by Calcining an Emulsion Precursor

An experimental study has been conducted to evaluate the formation of nano α-Al₂O₃ under various conditions, such as different calcining temperatures and emulsion ratios of aqueous aluminum nitrate solutions and oleic acid with a high-speed stirring mixer. Four batches of the precursor powders were calcined at three different temperatures of 1000°, 1050°, and 1100°C for 2 h and a terminal product of nano α-Al₂O₃ powders was obtained. The products have been identified by X-ray diffraction (XRD), specific surface area measurement scanning electron microscope, and transmission electron microscope (TEM). The XRD results show that the phase of powders is determined to be α-Al₂O₃, indicating that the overall process has been effective. The optimum calcination temperature of the precursor powder for crystallization of nano α-Al₂O₃ was found to be 1000°C for 2 h. The TEM image indicates that the particle grains have a sub-spherical shape with a mean size of 50–100 nm.     

Crack-Healing Behavior of Al2O3 Toughened by SiC Whiskers

Al₂O₃ reinforced by SiC whiskers (Al₂O₃/SiC-W) was hot-pressed to investigate the crack-healing behavior. Semielliptical surface cracks of 100 μm in surface length were introduced using a Vickers indenter. The specimens containing precracks were crack-healed at temperatures between 1000° and 1300°C for 1 h in air, and their strengths were measured by three-point bending tests at room temperature and elevated temperatures between 400° and 1300°C. The results show that Al₂O₃/SiC-W possesses considerable crack-healing ability. The surface cracks with length of 2 c = 100 μm could be healed by crack-healing at 1200° or 1300°C for 1 h in air. Fracture toughness of the material was also determined. As expected, the SiC whiskers made their Al₂O₃ tougher.     

Kinetics and Mechanism of Hot Corrosion of γ-Y2Si2O7 in Thin-Film Na2SO4 Molten Salt

γ-Y₂Si₂O₇ is a promising candidate material both for high-temperature structural applications and as an environmental/thermal barrier coating material due to its unique properties such as high melting point, machinability, thermal stability, low linear thermal expansion coefficient (3.9 × 10⁻⁶/K, 200°–1300°C), and low thermal conductivity (<3.0 W/m·K above 300°C). The hot corrosion behavior of γ-Y₂Si₂O₇ in thin-film molten Na₂SO₄ at 850°–1000°C for 20 h in flowing air was investigated using a thermogravimetric analyzer (TGA) and a mass spectrometer (MS). γ-Y₂Si₂O₇ exhibited good resistance against Na₂SO₄ molten salt. The kinetic curves were well fitted by a paralinear equation: the linear part was caused by the evaporation of Na₂SO₄ and the parabolic part came from gas products evolved from the hotcorrosion reaction. A thin silica film formed under the corrosion scale was the key factor for retarding the hot corrosion. The apparent activation energy for the corrosion of γ-Y₂Si₂O₇ in Na₂SO₄ molten salt with flowing air was evaluated to be 255 kJ/mol.     

Experimental Evidence for the Existence of the Ti3B4 Phase

Experimental evidence for the formation of Ti₃B₄ with an orthorhombic Ta₃B₄-type structure is presented. This phase forms by a peritectic reaction between TiB₂ and liquid at ∼2200°C and is stable to temperatures at least as low as 1690°C, the lowest temperature at which samples were annealed. No evidence was found to indicate the presence of a second high-temperature form of this phase, as has been reported in the literature.     

Effect of SiC on Interfacial Reaction and Sintering Mechanism of TiB2

Compacts of TiB₂ with densities approaching 100% are difficult to obtain using pressureless sintering. The addition of SiC was very effective in improving the sinterability of TiB₂. The oxygen content of the raw TiB₂ powder used in this research was 1.5 wt%. X-ray photoelectron spectroscopy showed that the powder surface consisted mainly of TiO₂ and B₂O₃. Using vacuum sintering at 1700°C under 13–0.013 Pa, TiB₂ samples containing 2.5 wt% SiC achieved 96% of their theoretical density, and a density of 99% was achieved by HIPing. TEM observations revealed that SiC reacts to form an amorphous phase. TEM-EELS analysis indicated that the amorphous phase includes Si, O, and Ti, and X-ray diffraction showed the reaction to be TiO₂+ SiC → SiO₂+ TiC. Therefore, the improved sinterability of TiB₂ resulted from the SiO₂ liquid phase that was formed during sintering when the raw TiB₂ powder had 1.5 wt% oxygen.     

Sc2O3 Nanopowders via Hydroxyl Precipitation: Effects of Sulfate Ions on Powder Properties

Hydroxyl-type Sc₂O₃ precursors have been synthesized via precipitation at 80°C with hexamethylenetetramine as the precipitant. The effects of starting salts (scandium nitrate and sulfate) on powder properties are investigated. Characterizations of the powders are achieved by elemental analysis, X-ray diffractometry (XRD), differential thermal analysis/thermogravimetry (DTA/TG), high-resolution scanning electron microscopy (HRSEM), and Brunauer-Emmett-Teller (BET) analysis. Hard-aggregated precursors (γ-ScOOH·0.6H₂O) are formed with scandium nitrate, which convert to Sc₂O₃ at temperatures ≥400°C, yielding nanocrystalline oxides of low surface area. The use of sulfate leads to a loosely agglomerated basic sulfate powder having an approximate composition of Sc(OH)_2.6(SO₄)_0.2·H₂O. The powder transforms to Sc₂O₃ via dehydroxylization and desulfurization at temperatures up to 1000°C. Well-dispersed Sc₂O₃ nanopowders (∼64.3 nm) of high purity have been obtained by calcining the basic sulfate at 1000°C for 4 h. The effects of SO₄²⁻ on powder properties are discussed.     

Preparation of Submicrometer A12O3 Powder by Gas-Phase Oxidation of Tris (acetylacetonato) aluminum (111)

Fine A1₂O₃ powder was prepared by the gas-phase oxidation of aluminum acetyl-acetonate. The reaction products were amorphous material at 600° and 800°C, γ-Al₂O₃ at 1000° and 1200°C, and δ-Al₂O₃ at 1400°C. The powders consisted of spherical particles from 10 to 80 nm in diameter; particle size increased with increasing reaction temperature and concentration of chelate in the gas.     

Process-Structure-Reflectance Correlations for TiB2 Films Prepared by Chemical Vapor Deposition

The process-structure-reflectance interrelationships for TiB₂ films prepared by CVD were determined using statistically designed experiments. A hot wall CVD reactor employing graphite substrates and the TiCl₄+ BCl₃+ H₂ reagent system were used at pressures of 2.7 and 6.7 kPa. Single-phase polycrystalline TiB₂ films were obtained. An increasing percentage of the grains were oriented with their (001) planes parallel to the substrate as the deposition temperature was increased and as the BCl₃:TiCl₄ ratio decreased. Grain size increased from ∼0.5 to 3 µm as the deposition temperature was increased from 900° to 1100°C and as the coating rate was decreased from 0.6 to 0.1 µm/min. Fine-grained, smooth, highly reflective films were obtained at low deposition temperatures and high BCl₃:TiCl₄ ratios.     

Characteristics of Si3N4-SiO2-Ce2O3 Compositions Sintered in High-pressure Nitrogen

Full-density Si₃N₄-SiO₂-Ce₂O₃ compositions were prepared by sintering with 2.5 MPa nitrogen pressure at temperatures of 1900° and 2090°C. Room-temperature flexural strengths near 700 MPa for sintered material compared favorably with the strength of hot-pressed material. At 1370°C, where flexural strengths as high as 363 MPa were obtained, it was observed that the coarsest structure was the strongest and the finest structure was the weakest. One of the compositions tested, Si₃N₄-8.7 wt% SiO₂-8.3 wt%-Ce₂O₃, was found to have excellent 200-h oxidation resistance at 700°, 1000°, and 1370°C, without incidence of 700° to 1000°C phase instability and cracking.     

Sintering of in-Situ Synthesized Sic–TiB2 Composites with Improved Fracture Toughness

TiB₂-particle reinforcement is one of the most successful methods for improving the fracture toughness of SiC ceramics.^1–3 Commercially available TiB₂ powders, however, have a large particle size and/or are highly reactive so that they are not favorable as a starting powder. In the present work, TiB₂ particles are formed by an in situ reaction between TiC and boron. The reaction takes place during sintering between 1000° and 1600°C and is accompanied by a large volume expansion. Under optimum conditions, dense composites (> 98% of theoretical) can be obtained by pressureless sintering using B and C as sintering additives. The in situ reaction method enables, for the first time, a complete densification of SiC-particulate composites by pressureless sintering. The fracture toughness of the composites was approximately 30% higher than that of the monolithic SiC ceramic.     

Phase Relations in the System UO2-UO3– Y2O3

The phase relations in the system U02-U03-Yz03, particularly in the Y203-rich region, were examined by X-ray and chemical analyses of reacted powders heated at temperatures up to 1700°C in H₂, CO₂-CO₂ and air. Four phases were identified in the system at temperatures between 1000° and 1700°C: U308, face-centered cubic solid solution, body-centered cubic solid solution, and a rhombohedral phase of composition (U,Y)₇O₂ ranging from 52.5 to 75 mole % Y₂O₃. The rhombohedral phase oxidized to a second rhombohedral phase with a nominal composition (U,Y), at temperatures below 1000°C. This phase transformed to a face-centered cubic phase after heating in air above 1000° C. The solubility of UO, in the body-centered cubic phase is about 14 mole % between 1000° and 1700°C but decreases to zero as the uranium approaches the hexavalent oxidation state. The solubility of Yz03 in the face-centered cubic solid solution ranges from 0 to 50 mole % Y2O3 under reducing conditions and from 33 to 60 mole % Y2O3 under oxidizing conditions at 1000°C. At temperatures above 1000° C, the face-centered cubic solid solution is limited by a filled fluorite lattice of composition (U,Y)O₂. For low-yttria content, oxidation at low temperatures (<300°C) permits additional oxygen to be retained in the structure to a composition approaching (U,Y)O_2.25 A tentative ternary phase diagram for the system UO₂-UO₃-Y₂O₃ is presented and the change in lattice parameter and in cell volume for the solid-solution phases is correlated with the composition.     

Preferential Grain Orientation in Hot Pressed TiB2

Preferential grain growth is reported for hot pressed titanium diboride (TiB₂) prepared at 1800°C and 50 MPa. Orientation imaging microscopy and X-ray diffraction revealed that the grains in the material were preferentially orientated with the [001] direction parallel to the mechanical field. The experimental findings are discussed, with emphasis on the anisotropic properties of TiB₂.