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Ce2O3 crystallizes in the trigonal P321 space group. The structure is two-dimensional and consists of one Ce2O3 sheet oriented in the (0, 0, 1) direction. Ce3+ is bonded in a distorted T-shaped geometry to three equivalent O2- atoms. All Ce–O bond lengths are 2.09 Å. O2- is bonded in a bent 120 degrees geometry to two equivalent Ce3+ atoms.
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 pct SiO2-8.3 wt pct-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.
Ce2O3 crystallizes in the trigonal P-3m1 space group. The structure is three-dimensional. Ce3+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Ce–O bond distances ranging from 2.31–2.67 Å. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded to four equivalent Ce3+ atoms to form OCe4 tetrahedra that share corners with six equivalent OCe6 octahedra, corners with six equivalent OCe4 tetrahedra, edges with three equivalent OCe6 octahedra, and edges with three equivalent OCe4 tetrahedra. The corner-sharing octahedra tilt angles range from 17–56°. In the second O2- site, O2- is bonded to six equivalent Ce3+ atoms to form OCe6 octahedra that share corners with twelve equivalent OCe4 tetrahedra, edges with six equivalent OCe6 octahedra, and edges with six equivalent OCe4 tetrahedra.
Ce2O3 crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. there are three inequivalent Ce3+ sites. In the first Ce3+ site, Ce3+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Ce–O bond distances ranging from 2.34–2.76 Å. In the second Ce3+ site, Ce3+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Ce–O bond distances ranging from 2.33–2.79 Å. In the third Ce3+ site, Ce3+ is bonded to six O2- atoms to form a mixture of distorted corner and edge-sharing CeO6 octahedra. The corner-sharing octahedral tilt angles are 0°. There are a spread of Ce–O bond distances ranging from 2.29–2.59 Å. There are five inequivalent O2- sites. In the first O2- site, O2- is bonded to four Ce3+ atoms to form distorted OCe4 trigonal pyramids that share a cornercorner with one OCe6 octahedra, corners with two equivalent OCe5 square pyramids, corners with nine OCe4 tetrahedra, corners with two equivalent OCe4 trigonal pyramids, edges with three equivalent OCe5 square pyramids, and edges with two equivalent OCe4 trigonal pyramids. The corner-sharing octahedral tilt angles are 34°. In the second O2- site, O2- is bonded to five Ce3+ atoms to form distorted OCe5 square pyramids that share corners with seven OCe4 tetrahedra, corners with two equivalent OCe4 trigonal pyramids, edges with two equivalent OCe6 octahedra, edges with two equivalent OCe5 square pyramids, edges with three OCe4 tetrahedra, and edges with three equivalent OCe4 trigonal pyramids. In the third O2- site, O2- is bonded to four Ce3+ atoms to form OCe4 tetrahedra that share corners with two equivalent OCe6 octahedra, corners with two equivalent OCe5 square pyramids, corners with four OCe4 tetrahedra, corners with six equivalent OCe4 trigonal pyramids, an edgeedge with one OCe6 octahedra, edges with two equivalent OCe5 square pyramids, and an edgeedge with one OCe4 tetrahedra. The corner-sharing octahedral tilt angles are 14°. In the fourth O2- site, O2- is bonded to four Ce3+ atoms to form OCe4 tetrahedra that share a cornercorner with one OCe6 octahedra, corners with five equivalent OCe5 square pyramids, corners with four OCe4 tetrahedra, corners with three equivalent OCe4 trigonal pyramids, edges with two equivalent OCe6 octahedra, an edgeedge with one OCe5 square pyramid, and edges with two equivalent OCe4 tetrahedra. The corner-sharing octahedral tilt angles are 51°. In the fifth O2- site, O2- is bonded to six Ce3+ atoms to form OCe6 octahedra that share corners with six OCe4 tetrahedra, corners with two equivalent OCe4 trigonal pyramids, edges with two equivalent OCe6 octahedra, edges with four equivalent OCe5 square pyramids, and edges with six OCe4 tetrahedra.
Glass compositions having a Young's modulus of at least 16 million psi and a specific modulus of at least 110 million inches consisting essentially of approximately, by weight, 20 to 43% SiO2, 8 to 21% Al2O3, 4 to 10% BeO, 27 to 58% of at least one oxide selected from a first group consisting of Y2O3, La2O3, Nd2O3, Ce2O3, Ce2O3, and the mixed rare earth oxides, and 3 to 12% of at least one oxide selected from a second group consisting of MgO, ZrO2, ZnO and CaO are described. The molar ratio of BeO to the total content of the first group oxides is from 1.0 to 3.0.
Mixed-valence cerium oxide nanoparticles (nanoceria) have been investigated with pronounced interest due to a wide range of biomedical and industrial applications that arises from its remarkable redox catalytic properties. However, there is no understanding of how to control the formation of these two types of nanoceria to obtain Ce3+/Ce4+ ratios required in various applications. In this work, using a soluble borate glass, nanoceria with specific ratios of Ce3+/Ce4+ are created and extracted via controlled glass-melting parameters. Glass embedded with nanoceria as well as nanoceria extracted from the glass were studied via XANES and fitted with the Multivariate Curve Resolution (MCR) technique to calculate the ratio of Ce3+/Ce4+. Results show that mixed-valence nanoceria with specific ratios are hermetically sealed within the glass for long durations. When the glass dissolves, the mixed-valence nanoceria are released, and the extracted nanoceria have unchanged Ce3+/Ce4+ ratios. Furthermore, TEM investigation on released nanoceria show that the nanoceria consist of several different structures. Although nanocrystal structures of Ce7O12, Ce11O20, and Ce2O3 contribute to the reduced state, a new quasi-stable phase of CeO1.66 has been observed as well.
Changes in the grain boundary (g.b.) phases of Si3N4 are analyzed, the effects of composition and thermal history on devitrification of the g.b. phases are determined, devitrification of the g.b. phases of Si3N are related to mechanical behavior and oxidation sensitivity of the material. The phase relationships that occur within the grain boundaries of Si3N4 containing various densification aids are reviewed. Comparisons of the effects of MgO, Y2O3, CeO2, and Y2O3 + AL2O3 are made in terms of the phase equilibria of the Si3N4 + SiO2 + additive compositional system. Two new equilibrium phase diagrams for the Si3N4-SiO2 and Y2O3 and Si3N4-SiO2-Ce2O3 systems are preented. The effects of Y2O3 vs CeO2 densification aids on the fracture surfaces of Si3N4 are compared. Auger electron spectroscopy shows that both oxides are concentrated within the fracture surface. Scanning electron microscopy shows evidence that Si3N4 with CeO2 formed an intergranular structure of fine grained oxynitride reaction products, as predicted by phase quilibria, whereas the Y2O3 containing sample shows evidence of an intergranular glassy phase.