MgO-Al2O3 Aluminates

Magnesium aluminate phases have high melting points and like calcium aluminates are used in refractory ceramic applications. These applications include the linings of ladles in steel plants and linings for cement kilns. In these applications, ceramics are used either in the form of castables, in case of linings to labels, or as bricks (kiln linings) [50-57]. Having low phonon energy and good mechanical properties, magnesium aluminates are also emerging as an infrared window material [20].

The only stable compound in the MgO-Al2O3 system [58, 59] is spinel [16, 60] (MgAl2O4), which has a melting point of 2,105°C and in addition to being a refractory compound has high resistance to chemical attack and radiation damage [56, 61-64]. Spinel ceramics have potential use for a variety of applications in the nuclear industry because of their high resistance to radiation and are candidates for potential ceramic waste host [65-68] and have also been suggested for use within new types of nuclear reactors. Ceramic-glass composites made from Mg-Al spinels and borosilicate glass can be used for ceramic boards for large scale integrated circuits used at high temperatures [69].

The presence of Al(III) ions is believed to inhibit formation of the SiO2 polymorph cristobalite, which degrades the mechanical and electrical properties of these specialized ceramics. Glass-spinel ceramics have the chemical and thermal resistance usually associated with aluminates and also low thermal expansion and a low dielectric constant. If there is formation of cristobalite in these types of composites, then the thermal expansion can be uneven. Temperature-dependent formation of additional SiO2 polymorphs can lead to micro-fracturing and mechanical degradation. Decreased ceramic contents of composites improve signal transfer by further lowering of the dielectric constant and so ideally the material will have a balance of spinel and glass optimized for the improved electrical properties and minimal cristobalite formation.