4 Aluminates 65
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 μm
(Fig. 7). Rare earth aluminate glasses are also important medically [121]. Yttrium in
rare earth aluminates can be activated by neutrons to form short-lived radioactive
isotopes (^90 Y) for cancer therapy.
20 Conclusions
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 Al 2 O 3.
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.
Fig. 7 A 1-cm diameter, 2-mm thick optical window made from REAL™ glass