34 D.J. Duval et al.High-purity glass-free mullite monoliths have been obtained by at least three tradi-
tional methods:- Starting materials with alumina contents near the stoichiometry of 2:1 mullite may
be completely melted above 1,960°C and then cooled to about 1,890°C without
crystallizing. At the latter temperature (in the shifted solid solution region), infrared-
transparent mullite single crystals could be grown by the Czochralski method [32]. - Pask [29] reports that mullites with higher molar ratios of alumina to silica (i.e., >3:1)
have been prepared by homogenous melting of the constituents above the liquids
and subsequent quenching. As a note, mullites prepared by fusion are generally
weaker than those produced by sintering [33]. - Mullite powders obtained by various methods can first be crystallized near
1,200°C, and then sintered at temperatures below the eutectic. Highly pure mullite
and mullite composites have been obtained by hot pressing below 1,300°C with
this method [34].
When processed close to or above the eutectic temperature (~1,590°C), mullite with
bulk compositions of less than 72 wt% Al 2 O 3 (3:2 mullite) exhibits a microstructure
of elongated grains that is believed to be promoted by the presence of a glassy second
phase. For Al 2 O 3 concentrations greater than 72 wt% Al 2 O 3 , the amount of glassy
phase is less and the initially formed mullite grains are smaller and more equiaxial.
Further heat treatment results in rapid grain growth driven by a decrease of the high
grain boundary area associated with the fine grains in the initial system. This leads to
fast growth of the grains along the c-axis and a higher aspect ratio for the overall
grains. After this rapid decrease in the driving force, the grains grow more slowly and
the overall decrease in the free energy of the system dictates the development of a
more equiaxial microstructure [35]
An interesting approach in making mullite powders has been via combustion syn-
thesis [36]. An aqueous heterogeneous redox mixture containing aluminum nitrate,
silica fume (soot), and urea in the appropriate mole ratio is mixed together. When
rapidly heated to 500°C, the mixture boils, foams, and can be ignited with a flame.
The process yields weakly crystalline mullite powder in less than 5 min. Fully crystal-
line mullite can be obtained by incorporating an extra amount of oxidizer, such as
ammonium perchlorate in the solution.
Recent work on mullite synthesis has focused on variations of sol–gel methods,
which allow control of the local distribution and homogeneity of the precursor chemistry.
The microstructure of a sol–gel derived mullite is shown in Fig. 5. Along with an
understanding of kinetics, sol–gel methods look promising for use in the manufacture
of bulk materials, thin films, or fibers of mullite with almost any specified phase
purity, phase distribution, and grain morphology.
Three categories of gels are usually made [37]. Single-phase (type I) mullite
precursor gels have near atomic level homogeneous mixing. The precursors transform
into an alumina-rich mullite at about 980°C in the same way as rapidly quenched
aluminosilicate glasses. These are formed from the simultaneous hydrolysis of the
aluminum and silicon sources. Type I xerogels, for example, can be synthesized from
tetraethylorthosilicate (TEOS) or tetramethylorthosilicate (TMOS) and aluminum
nitrate nonahydrate [38]. Diphasic (type II) gels comprised two sols with mixing on
the nanometer level. These gels, after drying, consist of boehmite and noncrystalline
SiO 2 , which at ~350°C transform to γ-Al 2 O 3 and noncrystalline SiO 2. An example of