Phase Relations in the System Al2O3─Ce2Si2O7 in the Temperature Range 900° to 1925°C in Inert Atmosphere

Equilibrium relationships in the system Al₂O₃-Ce₂Si₂O₇ in inert atmosphere have been investigated in the temperature range 900° to 1925°C. A simple eutectic reaction was found at 1375°C and 51 mol% Ce₂Si₂O₇. A high-low polymorphic transformation in Ce₂Si₂O₇ was observed at 1274°C. New XRD patterns are suggested for both polymorphs of cerium pyrosilicate. The melting point of Ce₂Si₂O₇ was found to be 1788°C. A value for ΔH°_m,Ce2Si2O7 of 36.81 kJ/mol was calculated from the initial slope of the experimentally determined liquidus in equilibrium with the pyrosilicate phase.     

Isothermal Oxidation Behavior of Mo-Si-B Intermetallics at 1450°C

Five Mo-Si-B multiphase intermetallic compositions were synthesized and oxidized isothermally at 1450°C in flowing air. Average mass change rates were strongly dependent on sample composition, particularly boron content. An Mo₅Si₃ matrix material containing 1.6 wt% boron exhibited parabolic mass gain with a rate of 5.3 × 10^-4 mg²(cm⁴.h), while a similar material with 0.14 wt% boron oxidized rapidly in a linear manner at a rate of -3.3 mg/(cm².h). Oxidation rates of the Mo-Si-B intermetallics were compared to that of MoSi₂ oxidized at 1450°C under identical conditions.     

X-ray Tomography Study on Green State Joining of Silicon Carbide Using Polymer Precursors

X-ray tomography has been used to investigate the density variations in SiC joints formed using polymer pastes. It has been demonstrated that X-ray tomography provides accurate bulk density measurements and volumetric density gradients. The results suggest that the magnitude of the applied pressure after green state joining and the amount of polymer (polycarbosilane, PCS) in the joining pastes influence the green density of the joints. All joints are prepared and applied in air atmosphere and at room temperature. The green densities of the joints increase from 54% to 66% of theoretical with the increase of the applied pressure from ambient to 138 MPa. Highest joint density without applied pressure is achieved using paste containing 50 vol% PCS. Furthermore, allylhydridopopolycarbosilane- (AHPCS-) containing pastes resulted in higher densities at the joint–matrix interface, indicating infiltration of polymer into the matrix.     

Green State Joining of Silicon Carbide Using Polycarbosilane

Green state joining of SiC was investigated using a paste consisting of polycarbosilane polymer and SiC powder. The joining process and densification were described. Initial experiments resulted in the formation of symmetrical black bands and cracks on both sides of the joint. However, with modifications in processing conditions, the cracks were eliminated and the resulting joints were indistinguishable from the matrix. The flexural strength of joined samples was measured to be 234 MPa, which was comparable to that of the control sample with similar density. As the applied pressure during joining was increased from 34 to 138 MPa, the strength of the joined samples increased from 180 to 250 MPa.     

Role of Ceria in Enhancing the Resistance of Aluminosilicate Refractories to Attack by Molten Aluminum Alloy

Rare-earth doping of aluminosilicate refractory compositions has been shown to impart improved resistance to attack by molten aluminum alloys. The basis for this improved alloy attack resistance was investigated. The addition of rare-earth oxide to bauxite was found to alter the microstructure to form large, uniformly shaped corundum crystals interlinked with cerium-containing strands. It was concluded that this unique microstructure was responsible for the enhanced attack resistance of the refractories. Other possible causes for improved alloy attack resistance, such as enhanced sintering, excess oxygen provided by the CeO₂ dopant, or higher thermodynamic stability of the phases formed in the doped refractory compositions, were not consistent with the experimental findings.     

Phase Relations in the System Ce2O3–Al2O3 in Inert and Reducing Atmospheres

The 1:1 compound, CeA1O₃, in the system Ce₂O₃–Al₂O₃ has been synthesized from the oxides and shown to have a perovskite-like tetragonal unit cell with the lattice parameters a = 3.763 and c = 3.792 Å. A new XRD pattern is suggested for CeA1O₃. This compound is shown to be stable up to 1950°C. The 1:11 compound, CeAl₁₁O₁₈, has also been synthesized and shown to possess a magnetoplumbite-like hexagonal unit cell with the lattice parameters α= 5.558 and c = 22.012 å. An XRD pattern is suggested for CeAl₁₁O₁₈ for the first time. The evolution of eutectic-like microstructures was observed and reported in the Ce₂O₃-rich side of this binary system.     

Synthesis of Nickel Sulfide by Homogeneous Precipitation from Acidic Solutions of Thioacetamide

Nickel sulflde powders were precipitated by utilizing the thermal decomposition of thioacetamide in acidic aqueous solutions. Precipitation was induced by a direct reaction between thioacetamide and nickel at 70° and 80°C with thioacetamide concentrations of 0.80 M . Precipitation was induced by reaction of sulfide ions with nickel at 90°C and at low pH. Direct reaction products were amorphous and contained significant amounts of carbon, excess sulfur, and hydrogen. Hydrolysis-controlled precipitation yielded faceted crystalline particles 2 to 4 μm in size. Crystalline, spherical particles with compositions near stoichiometric NiS were precipitated with high yields.     

Preparation of spherical yttrium oxide powders using emulsion evaporation

Water-in-oil type emulsions were used to prepare yttrium oxide powders by evaporation of the yttrium-containing aqueous phase in a hot oil bath. Emulsions were characterized with respect to emulsion type, droplet size distribution and stability. Y₂O₃ powders obtained by this method consisted of small (1–3 μm) spherical granules made up of 0·1 μm crystallites. The effects on the powder of emulsifier concentration, yttrium ion concentration, evaporation temperature and atmosphere were studied.