Rare earth aluminates

In the 1990s a breakthrough was achieved in the development of long-lived afterglow phosphors. It was discovered that by co-doping rare earth aluminates, especially strontium, with europium and dysprosium gave phosphors with around ten times the afterglow of copper activated zinc sulfide and also with ten times the... 

Ceramics in aluminate systems are usually formed from cubic crystal systems and this includes spinel and garnet. Rare earth aluminate garnets include the phase YAG (yttrium aluminium garnet), which is an important laser host when doped with Nd(III) and more recently Yb(III). Associated applications include applications as scintillators and phosphors. 

Aluminate glasses are transparent in the infrared region and these too have specialist applications, although the glass-forming ability is poor. Recently, rare earth aluminate glasses have been developed commercially in optical applications as alternatives to sapphire for use in, for example, infrared windows. 

In high radiation fields, the spinel crystal structure has been shown to change. The structure, while still cubic, becomes disordered with a reduction in lattice parameter. The disordered rock-salt structure has a smaller unit cell reflecting the more random occupation of the octahedral sites by both trivalent and divalent ions. Increased radiation damage results in the formation of completely amorphous spinels. Radial distribution functions (g(r)) of these amorphous phases have Al-0 and Mg-O radial distances that are different from equivalent crystalline phases. The Al-0 distance in the amorphous form is reduced from Al-O of 0.194nm in the crystalline phase to 0.18nm in the amorphous phase, while the Mg-O distance is increased (0.19nm in the crystal to 0.21 nm in the amorphous phase). Differences between the Al-O distances of crystalline and amorphous phases are a characteristic of both calcium and rare earth aluminates. 

Rare earth aluminates are also important commercially as ceramics and ceramic composites for scintillation applications. The importance of the optical properties of rare earth aluminates is underscored by the used of Nd-doped YAG as a laser host. 

C.J. Howard, B.J. Kennedy, and B.C. Chakoumakos, Neutron powder diffraction study of rhom-bohedral rare-earth aluminates and the rhombohedral to cubic phase transition. J. Phys.-Conden. Matter. 12(4), 349-365 (2000). 

M.C. Wilding, P.F. McMillan, and A. Navrotsky, Thermodynamic and structural aspects of the polyamorphic transition in yttrium and other rare-earth aluminate liquids. Phys. Stat. Mech. Appl. 314(1—4), 379-390 (2002). 

We previously proposed an interpretation of thermal stabilization of supported catalysts by rare-earth elements [10, 11, 18] by means of interfacial structural coherence between alumina and rare-earth aluminates. This model, developped for lanthanum and neodymium, can be easily extended to cerium since LaAlOs, NdAlOs and CeAlOs are iso-structural mixed oxides with identical lattice parameters (a = 7.6 A). 

Porous matrices have been discussed in this chapter as a mechanism for providing toughness to a CMC. A porous coating can provide a similar function, with the porosity localized around the fiber within a coating. Various porous coatings have been examined, including zircon [108], Z1O2 [48], and rare-earth aluminates [109]. 

M. K. Cinibulk, T. A. Parthasarathy, K. A. Keller, and T. Mah, Porous Rare-Earth Aluminate Fiber Coatings for Oxide-Oxide Composites, Ceram. Eng. Sci. Proc., 21 [4] 219-228 (2000). 

To better imderstand the physical nature of the rare earth perovskite materials availability of detailed information about their crystal structures is the key. The above-mentioned physical and chemical properties of rare earth aluminates... 

In the last decade, a polymer complex method, corresponding to the sol-gel method, had been widely applied for the preparation of various mixed oxide compositions, including rare earth aluminates (Rao, 1996 Lo and Tseng, 1998). By using this technique, RAIO3 compounds of La Ho except for Ce and Pm can be... 

In contrast to orthorhombic aluminates, RAIO3 compounds with rhombohe-dral structures display no anomalies both in the low- and high-temperature expansion (Figure 50). For rhombohedral perovskites, the thermal expansion in the fl-direction is smaller compared with the c-direction (0.75% and 1% at 1200 K, respectively). The Pa and pc values are practically identical for all rhombohedral rare earth aluminates (Figure 50B and D). The analysis of some representatives of Ri xR x AIO3 solid solutions shows that the character of their thermal expansion is... 

From Table 29 and Figure 52A, it is evident that the linear thermal expansion coefficients for orthorhombic rare earth aluminates in the temperature range of 300-1200 K lie within the limits a = (8.9-11.3) x 10 = (3.8-7.S) x 10 ... 

Perovskite-type rare earth aluminates are also prospective materials for deposition of GaN and AIN layers and manufacturing of blue light semiconductor lasers, as well as epitaxial layers of HTSC materials. LaAlOs and LSAT (Lai j Srj Ali yTay03) are well known in this respect. The application of a material as a substrate requires the knowledge of its physical characteristics and their temperature evolution in addition to the standard condition, which is the minimum mismatch between the cell parameters of the deposited film and the substrate. For details refer to O Bryan et al. (1990). 

Dielectric data for microwave dielectric materials, namely, the relative dielectric permittivity (e,), the product of the quality factor Q and the frequency (Q x/), the frequency of the measurement (/), and the temperature coefficient of the resonance frequency (xf) based on rare earth aluminates are listed in Table 30. Measurements carried out using low-frequency (MHz) impedance methods are... 

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