Ceramic and Glass Materials

2 Mullite 37

decreases with alumina content, and the anisotropy of thermal expansion is reduced

simultaneously [51].

Given that mullite is a defect structure, one would expect high ionic conductivity.

Rommerskirchen et al. have found that mullite has ionic conductivity superior to that

of the usual CaO-stabilized ZrO 2 solid electrolytes at temperatures from 1,400 to

1,600°C [52]. The oxygen self diffusion coefficient in the range 1,100 < T < 1,300°C

for a single crystal of 3:2 mullite has been given by [53]:

DRTox=× −1 32 10. −−^2 exp[ 397 kJ/ ]cm s^21 (2)

Grain boundary diffusion coefficients are about five orders of magnitude higher than
volume diffusion in the same temperature range. The activation energy for grain
boundary diffusion [54] is 363 ± 25 kJ mol−1 – a remarkably similar value compared
with that of volume diffusion.
The activation energy for silicon diffusion during the formation of mullite from
fused couples at 1,600 < T < 1,800°C [55] is in the range of 730 < ∆HSi4+ < 780 kJ
mol−1. There is support for the idea that Al3+ diffusion coefficients are much higher
than those of silicon at temperatures above the mullite–silica eutectic [56].

References

1. H. Schneider and K. MacKenzie, J. Eur. Ceram. Soc. 21 , iii (2001).


2. M. Tokonami, Y. Nakajima, and N. Morimoto, The diffraction aspect and a structural model of
   mullite, Al(Al1+2xSil−2x)O5−x,Acta Cryst.A36, 270–276 (1980).


3. J. L. Holm, On the energetics of the mullite solid-solution formation in the system Al 2 O 3 −SiO 2 ,
   J. Mat. Sci. Lett. 21 , 1551–1553 (2002).


4. W.M. Kriven, M.H. Jilavi, D. Zhu, J.K.R. Weber, B. Cho, J. Felten, and P. C. Nordine, Synthesis
   and microstructure of mullite fibers grown from deeply undercooled melts, in Ceramic
   Microstructures: Control at the Atomic Level, A. P. Tomsia and A. M. Glaeser (eds.), Plenum, New
   York, NY, (1998) pp. 169–176.


5. M. Schmuecker and H. Schneider, Structural development of single phase (type I) mullite gels,
   J. Sol–Gel Sci. Tech. 15 , 191–199 (1999).


6. R.X. Fischer, H. Schneider, and M. Schmuecker, Crystal structure of Al-rich mullite, Am.
   Mineral., 79 (9–10), 983–990 (1994).


7. S. Freimann and S. Rahman, Refinement of the real structures of 2:1 and 3:2 mullite, J. Eur.
   Ceram. Soc. 21 , 2453–2461 (2001).

Table 2Values of fracture toughness (KIc), fracture strength (sf), flexural strength, and microhard-

ness for 3:2 mullite at different temperatures

T (°C) Kic (MPa m1/2) sf (MPa)

Flexural strength

(MPa) Microhardness (GPa)

22 2.5 ± 0.5a 15 b

1000 10 b

1200 3.6 ± 0.1 260 ± 15 500 c

1300 3.5 ± 0.2 200 ± 20

1400 3.3 ± 0.2 120 ± 25 360 c

From [49] (specimens had apparent density of 2.948 Mg m−3 and grain size of 4.0 μm)

a Value from [58]

b Values from [45]

c Values mentioned in [8]