Subsolidus Phase Relationships in Si3N4–AlN–Rare-Earth Oxide Systems

The subsolidus phase relationships in Si₃N₄–AlN–rare-earth oxide (Me₂O₃ where Me=Nd, Sm, Gd, Dy, Er, and Yb) systems were studied. Solid-solution regions with the α-Si₃N₄ structure were delineated along the Si₃N₄–"Me₂O₃:9AIN" joins for all of the rare-earth oxide systems studied. The solubility limits of these solid solutions increased with decreasing size of the rare-earth ions.     

Phase Relationships in the ZrO2-Sc2O3 and ZrO2-In2O3 Systems

Zirconia-rich subsolidus phase relationships in the ZrO₂–Sc₂O₃ and ZrO₂–In₂O₃ systems were investigated. Phase inconsistencies in the ZrO₂–Sc₂O₃ system resulted from a diffusionless cubic-to-tetragonal ( t' ) phase transformation not being recognized in the past. Through three different measuring techniques, along with microstructural observations, the solubility limits of the tetragonal and cubic phases were determined.     

Phase Stability and Physical Properties of Cubic and Tetragonal ZrO2 in the System ZrO2–Y2O3–Ta2O5

The cubic ( c -ZrO₂) and tetragonal zirconia ( t -ZrO₂) phase stability regions in the system ZrO₂–Y₂O₃–Ta₂O₅ were delineated. The c -ZrO₂ solid solutions are formed with the fluorite structure. The t -ZrO₂ solid solutions having a c/a axial ratio (tetragonality) smaller than 1.0203 display high fracture toughness (5 to 14 MPa · m^1/2), and their instability/transformability to monoclinic zirconia ( m -ZrO₂) increases with increasing tetragonality. On the other hand, the t -ZrO₂ solid solutions stabilized at room temperature with tetragonality greater than 1.0203 have low toughness values (2 to 5 MPa · m^1/2), and their transformability is not related to the tetragonality.     

Effect of AlN and Al2O3 additions on the phase relationships and morphology of SiC Part I Compositions and properties

X-ray diffraction was utilized to follow the transformation from -SiC (3C) to the various -SiC polytypes in the presence of AlN and Al₂O₃ additives after hot pressing from 1700 to 2100°C. The 2H- and 6H-polytypes of -SiC were the predominate polytypes with additions of only AlN or Al₂O₃, respectively. The amount of 2H- and 6H-polytypes, and subsequently the microstructural morphology of the SiC materials, were found to be controlled by varying the amount of AlN and Al₂O₃. Improvements in fracture toughness to 9 MPa-m were achieved with flexural strengths ranging from 600 to 900 MPa. These results suggest that accurate control of the polytypic make-up of SiC-based materials, along with their mechanical properties, can be achieved through AlN and Al₂O₃ additions.     

The determination of optical absorption and scattering in translucent porcelain

Reflectance and transmittance theories have been used to calculate the optical absorption and scattering within materials. For these calculations, the incorporation of corrections for interfacial reflections has not been universally applied, and when corrections have been incorporated, the value used for the diffuse internal reflection coefficient has often been based on theoretical considerations. A method of incorporating interfacial-reflection corrections is presented. The importance of the incorporation of such corrections, with the proper value for the internal reflection coefficient, is clearly demonstrated for both reflectance and transmittance theories. The limitation of transmittance theory to materials with a scattering power (SX) of greater than 0.5 is also demonstrated for collimated illumination, unless an independent determination is made of the internal reflection coefficient.     

X-ray Absorption Studies of Ceria with Trivalent Dopants

The local structural environments of Sc³⁺ and Gd³⁺ in ceria were studied for the first time by using EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) spectroscopy. Our results indicate that Sc³⁺ in 10-mol%-Sc₂O₃-doped ceria has a 7-fold-coordinated Sc-ordered structure with two different Sc─O distances (2.13 and 2.26 Å). An octahedrally coordinated Sc─O cluster is present in 5 mol% Sc₂O₃─5 mol% Gd₂O₃─CeO₂, because of the strong scavenging effect of Sc³⁺ ions on oxygen vacancies created by Gd³⁺. The Gd³⁺ in this sample is largely unassociated. Sc₂O₃ (5 mol%) and Gd₂O₃ (5 or 10 mol%) form random solid solutions with CeO₂. These results are consistent with the current concept of defect structures in this structural family.     

Formation of β-Silicon Nitride Crystals from (Si,Al,Mg,Y)(O,N) Liquid: II, Population Dynamics and Coarsening Kinetics

Precipitation, growth, and coarsening of Si₃N₄ crystals in (Si,Al,Mg,Y)(O,N) liquids at 1680°C has been studied. Contrary to the common observation in kinetics, coarsening rates of crystals in length and width are found to accelerate when the total volume of crystals remains little changed. This is attributed to the concomitant β-Si₃N₄ to β'-SiAlON transformation, which introduces an additional driving force for crystal dissolution and reprecipitation. As a result of the additional driving force, which has a nonmonotonic size dependence, the normalized size distribution is expected to evolve with time, initially broadening, then shifting skewing as the transformation passes the midpoint, and finally converging to a sharp distribution as the transformation completes. These evolutions have been observed in all the compositions studied.     

Solid-Liquid Reactions in the System Si3N4–β-SiAlON–Y3Al5O12

Solid-liquid equilibria in the system Si,Al,Y/N,0 were determined for the compatibility triangle bounded by the β-SiAlON solid-solution line and the compound Y₃Al₅O₁₂. X-ray diffraction was used to determine the crystalline phases present in the equilibrated, rapidly cooled specimens. The liquid phase was quantified with volume fraction measurements performed on scanning electron micrographs. The solid-liquid tie lines at 1650° and 1750°C were established from lattice parameters of the β-SiAlON phase and from the amount of liquid phase in equilibrium with the crystalline solid. The liquid phase was crystallized to verify the location of the starting composition.