Liquids in the Y2O3-Al2O3 system do not form glasses very easily, a feature that is not just coincidental with the polyamorphic transition [115-117]. Poor glass-forming ability is a feature of “fragile” liquids and the polyamorphic transition in Y2O3-Al2O3 liquids is interpreted as a transition from a fragile to strong liquid. This has important implications for materials applications of YAG and closely-related glasses.
Y2O3-Al2O3 glasses have high mechanical strength and desirable optical properties so there are potential commercial applications for YAG fibers in composite materials and optical devices. Glasses can be made from YAG using container-less levitation [13, 105, 107, 118] with fibers drawn from the beads. The lengths of the fibers are limited and this is most likely due to the changes in rheology that occur as the transition is intercepted. Furthermore, when there is formation of a second glassy phase, different properties occur leading to a “necking” and breaking of the fiber.
The unusual behavior of Y2O3-Al2O3 glasses had resulted in extensive calorimetric and diffraction studies. These suggest that the glass structure is very different from the crystalline phases . Although the crystalline phases are dominated by octahedral aluminum (VIAl), this short range order is reduced in the amorphous phases and the mean Al-O coordination number is close to 4 (4.15) . A similar change in Y-O coordination is reported and the glass phases are not simply disordered forms of the crystalline equivalents. This observation is consistent with the fragility of Y2O3-Al2O3 liquids, and indeed molecular dynamics simulations (which are at much higher temperatures than the fictive temperatures of the glass) suggest that the Al-O coordination number increases with temperature, as further suggested by studies of levitated liquids. A structural model for a single phase (high density) Y2O3-Al2O3 glass is shown in Fig. 6.
Glasses of Y3Al5O12 composition can be made by levitation techniques  and other glass compositions can be made at compositions close to that of the metastable eutectic (Fig. 4) using a Xe-arc image furnace. The precursors are made via sol-gel route .
Commercial products are being developed from single phase aluminate glasses [107, 120]. These “REAL™ Glass” materials were first made using levitation melting. Subsequently, formulations that can be cast from melts formed in platinum crucibles have been developed. The glasses are hard, strong, and environmentally stable and
Fig. 6 Structural model of 20% Y2O3-80% Al2O3 glass
Fig. 7 A 1-cm diameter, 2-mm thick optical window made from REAL glass
they can be doped with high concentrations of rare earth ions for use in optical devices. Initial commercial applications are as an alternative to sapphire for use in infrared windows and optics that transmit to a wavelength of approximately 5 |rm (Fig. 7). Rare earth aluminate glasses are also important medically . Yttrium in rare earth aluminates can be activated by neutrons to form short-lived radioactive isotopes (90Y) for cancer therapy.
Aluminate glasses and ceramics include a range of compositions and crystal structures. Most of the important physical properties of aluminates are similar to those of Al2O3. These properties include high melting point and high mechanical strength. Aluminate ceramics are frequently binary systems and have intermediate compounds that, while retaining the relatively high melting point, can be easily synthesized.
Aluminate compositions include calcium aluminate cements, which have high chemical resistance, especially to sulfate, and is also used in refractory applications where ordinary Portland cements would be unsuitable. These same cements are used in bioceramic applications. The bioceramic applications reflect both the high mechanical strength of the calcium aluminate cements and also the “biocompatibility” of Ca-bearing phases, which bond well with, for example, bone.
Other binary aluminates include magnesium spinels that are used extensively as castable refractory ceramics. Lithium aluminates are used, potentially, in fuel cells and as materials for new types of nuclear reactors. Again, these applications reflect the refractory nature of aluminates and their chemical resistance.
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.
Synthesis of aluminates is in the most part a solid-state process using purified components and requiring high temperatures. Sol-gel techniques are also used, since this is a lower temperature route and also because in many applications grain size and porosity need to be controlled.
Glasses can be formed from aluminates, but the glass-forming ability is poor. This reflects the fragility of aluminate liquids which, in Y2O3-Al2O3 systems leads to anomalous thermodynamic properties. As a result, exotic techniques are used to make aluminate glasses, most importantly container-less levitation.
Acknowledgements Dr J. K. R Weber from Materials Development Inc. kindly commented on an earlier draft of this manuscript and also provided details of the REAL™ glasses and Fig. 7. I also thank Dr. J. F. Shackelford for his support and encouragement to ensure completion of this chapter.