MgO-Al2O3 System

The binary phase diagram for MgO-Al2O3 is simpler than that for the CaO-Al2O3 system (Fig. 2). There is only one stable intermediate compound that of the spinel phase (Mg2AlO4) [60]. Spinel melts at 2,105°C, but there is a eutectic at 1,995°C and a limited solid solution between stoichiometric spinel and MgO (periclase), up to 6 wt% MgO, can be dissolved into the spinel structure without exsolution. This limited solid solution is an important property that is utilized in manufacture of spinels for use in reducing conditions [70].

The cubic spinel crystal structure (Fd3m) is a close-packed array of oxygen ions, which has the general form AB2O4. A is a divalent cation and B trivalent [60, 71].

MgO-Al2O3 System

Fig. 2 The MgO-Al2O3 phase diagram [59]

The metal cations occupy two sites: divalent cations (A) are in tetrahedral coordi­nation, while trivalent ions (B) occupy octahedral sites. The oxygen ions form a face-centered cubic close-packed arrangement and the unit cell consists of 32 oxygen ions, 8 divalent (A), and 16 trivalent ions (B) with dimensions of 0.80832 nm. There are a large number of natural forms of spinel structure, which include Cr2O3 and Fe2O3 forms. The lattice parameter Ao is 0.80832 nm, and in synthetic spinels, the limited solid solutions with both Al2O3 and MgO end-members are accommo­dated in this cubic structure, although there is slight increase in the lattice parame­ter [16].

There are two types of spinel, normal and inverted. Normal spinels have all the A ions in tetrahedral sites and all B ions in octahedral coordination. When the structures are inverted, the divalent A ions and half of the trivalent B ions are in the octahedral sites while the remaining B ions have tetrahedral coordination. Both normal and inverted spinels have the same cubic structure (space group Fd3m).

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-O and Mg-O radial distances that are different from equivalent crystalline phases. The Al-O distance in the amorphous form is reduced from Al-O of 0.194 nm in the crystalline phase to 0.18 nm in the amorphous phase, while the Mg-O distance is increased (0.19 nm 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.

The MgO-Cr2O3 binary is closely related to the equivalent Al2O3 system. Here too the only stable compound is a spinel-structured phase MgCr2O4, which has a high melting point (2,350°C). The chrome-bearing ceramics have similar applications but have a significant drawback environmentally. There is a risk that chrome-bearing ceramics in furnace waste will interact and contaminate ground water. Cr[VI] ions leached from remnant refractory materials in wastes into ground water are a serious contaminant and have been linked to skin ulceration and carcinoma. MgO-Al2O3 ceramics are, therefore, much more desirable.