4 Aluminates 59ceramic requires a high mechanical and thermal stability so aluminates are often
considered; because the operating conditions require diffusion of^3 H through pores,
special synthesis conditions are required.
Lithium aluminates are also important in the development of molten carbonate fuel
cells (MCFC) [82, 83]. In these fuel cells, a molten carbonate salt mixture is used as an
electrolyte. These fuel cells operate through an anode reaction, which is a reaction between
carbonate ions and hydrogen. A cathode reaction combines oxygen, CO 2 , and electrons
from the cathode to produce carbonate ions, which enter the electrolyte. These cells oper-
ate at temperatures of ~650°C and the electrolyte, which is usually lithium and potassium
carbonate, is suspended in an inert matrix, which is usually a lithium aluminate.
14 Li 2 O−Al 2 O 3 System
As with the other aluminate systems, the binary Li 2 O−Al 2 O 3 is characterized by the two
refractory oxide end-members, Li 2 O (which melts at 1,000°C) and Al 2 O 3. There are
three stable compounds in the Li 2 O−Al 2 O 3 system: Li 5 AlO 4 , LiAlO 2 , and LiAl 5 O 8. The
phases of most interest for materials application are the LiAlO 2 compounds that have α,
β, and γ form (Fig. 3). The α-LiAlO 2 is orthorhombic and has the space group r3m,
while the γ form has even lower (tetragonal) symmetry. Both Li and Al are in tetrahedral
coordination in the γ phase. The γ phase can be produced irreversibly by sintering the α
Fig. 3 The Li 2 O−Al 2 O 3 phase diagram [83]T, KLL
+
Li 2 OL
+
γ- LiAIO 2γ- LiAIO 2
+
LiAI 5 O8(yn)α- LiAIO 2
+
LiAI 5 O8(yn)LiAIO 5
8(yn)+AIO 2
3AI 2 O3,%mLiAIO 5
8(Heyn)+AIO 2L 3
+
LiAI 5 O8(Heyn)β-Li 5 AIO 4
+
Li 2 Oα-Li 5 AIO 4
+
Li 2 O
Li 2 O 20 40 60 80 AI 2 O 3β-Li 5 AIO 4 +γ- LiAIO 2β-Li 5 AIO 4 +α- LiAIO 2β-Li 5 AIO 4 +α- LiAIO 2LiAI 5 O8(Heyn)
+
γ- LiAIO 22200200018001600140012001000