Selected Materials Properties

The availability of fine, pure mullite powders and novel processing routes have made it possible to obtain dense polycrystalline mullite with higher deformation resistance and hardness at higher temperatures than most other ceramics, including alumina [44,45]. Mullite has good chemical stability and a stable temperature-independent oxygen vacancy structure up to the melting point [46], making mullite particularly creep-resistant. It should be noted that the majority of studies on high temperature mechanical properties of mullite have concentrated on measurements of strength or the creep deformation under testing conditions of four point bending or compression under static loading [47,48]. These testing procedures are useful as an initial evaluation of failure strength or creep resistance but the complexity of the stress makes it difficult to interpret the effect of the material variables on the creep mechanisms [49]. Nevertheless, to cite one representative study, creep may occur by a diffusional mechanism for grain sizes <1.5 pm with stresses of less than 100 MPa at temperatures between 1,365 and 1,480°C. High activation energy of 810 kJ mol-1 was determined for this process. Larger grain sizes and higher stresses indicate creep occurs by slow crack growth [48]. Selected mechanical properties are provided in Table 2. In general, creep resistance increases with sintering temperature, while flexural strength decreases [50].

With a low thermal conductivity of 0.06 W cm-1 K-1 and a low thermal expansion coefficient a ~ 4.5 x 10-6°C-1, mullite is useful for many refractory applications [49]. According to Schneider, most mullites display low and nonlinear thermal expansions below, but larger and linear expansion above, ~300°C. The volume thermal expansion

Table 2 Values of fracture toughness (K ), fracture strength (of), flexural strength, and microhard­ness for 3:2 mullite at different temperatures

T (°C)

Kic (MPa m1/2)

of (MPa)

Flexural strength (MPa)

Microhardness (GPa)

22

2.5 ± 0.5a

15b

1000

10b

1200

3.6 ± 0.1

260 ± 15

Ul

О

О

1300

3.5 ± 0.2

200 ± 20

1400

3.3 ± 0.2

120 ± 25

360c

From [49] (specimens had apparent density of 2.948 Mg m-3 and grain size of 4.0 pm) a Value from [58] b Values from [45] c Values mentioned in [8]

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 ZrO2 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]:

Dox = 1.32 x 10-2 exp[-397kJ/ RT]cm2 s-1 (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 < AHSi4+ < 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].