Processing and Applications

As mentioned in the previous section, the formation, phase purity, and morphology of mullite depend upon precursor materials and processing history. Mullite was first identified as the product of heating kaolinitic clays, resulting in a compound with an

MOLE %

Processing and Applications

Fig. 4 Phase diagram for the alumina-silica system. From [28]

approximate alumina-to-silica molar ratio of 3:2. The order of reaction proceeds as follows [30]:

Al2Si2O5(OH)4

Kaolinite

450°C

->

2(Al2O3-2SiO2) + 2H2O Metakaolin

2(Al2O3-2SiO2)

Metakaolin

925°C

->

2Al2O3-3SiO2 + SiO2 Silicon spinel

2Al2O3.3SiO2 Silicon spinel

1,100°C

2(Al2O3.SiO2) + SiO2

Pseudomullite

3(Al2O3.SiO2)

Pseudomullite

1,400°C

->

3Al2O3.2SiO2 + SiO2 Mullite + cristobalite

Excess corundum may be added, and the system heated at higher temperatures to minimize free SiO2. Toward this end, Goski and Caley [31] suspended grains of the mineral kyanite (a high-pressure form of Al2O3SiO2) with submicron alumina in water to provide intimate mixing of these mullite precursors. The alumina-kyanite suspension was slip cast to form a green body that was reaction-sintered to form an alumina-mullite composite. According to phase diagrams, a silica-rich glassy phase in 3:2 mullite is predicted when sintered at temperatures higher than the eutectic (1,587°C). Many common 3:2 mullite products are sintered between 1,600 and 1,700°C and may contain a glassy phase in the microstructure.

High-purity glass-free mullite monoliths have been obtained by at least three tradi­tional methods:

1. 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].

2. 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].

3. 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% Al2O3 (3:2 mullite) exhibits a microstructure of elongated grains that is believed to be promoted by the presence of a glassy second phase. For Al2O3 concentrations greater than 72 wt% Al2O3, 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 syn­thesis [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 crystal­line 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 SiO2, which at ~350°C transform to y-Al2O3 and noncrystalline SiO2. An example of

Processing and Applications

Fig. 5 Scanning electron micrograph of 3:2 mullite. Specimen was sintered at 1,700°C, hot isostatically pressed at 1,600°C, and thermally etched. From [54]

a type II gel would be a mixture of boehmite with a TEOS or TMOS sol [22]. Type III diphasic gels contain precursors that are noncrystalline up to 980°C and then form Y-Al2O3 and noncrystalline SiO2.

Subsequent heat treatments of the three types of gels result in very different micro­structures even if the alumina-silica molecular ratios are identical. Mullite conversion from powders or diphasic gels tends to be diffusion rate controlled. In the case of monophasic gels, conversion from the amorphous to crystalline phase appears to be nucleation rate dependent [39]. Such nucleation rate dependence would seem to indicate that it would be difficult to obtain very fine-grained mullite monoliths. However, some researchers have been successful in producing such monoliths. Monophasic xerogels prepared by slow hydrolysis (4-6 months) of hexane solutions of aluminum sec-butoxide and TMOS have been used to make optically clear mullite monoliths. The gel was heated in the range of ~1,000-1,400°C to form a completely dense crystalline material with glass-like mechanical properties (brittle and conchoidal fractures, rapid crack propagation, and no clear evidence of intergranular fracture) [40].

Seeding sol-gel precursors with nucleation sites for growth appears to be a method of making fine-grained monolithic optically transparent materials. Initially upon heating, gels formed by mixing a colloidal boehmite-silica sol with a polymeric aluminum nitrate-TEOS sol (a hybrid type I and type II gel) tend to crystallize, form­ing mullite seed crystals. Homoepitactic nucleation during continued heat treatment results in mullite monoliths. The introduction of the polymeric gel resulted in an increase in apparent nucleation frequency by a factor of 1,000 at 1,375°C, and a reduction in high-temperature grain size from 1.4 to 0.4 |jm at 1,550°C, with little or no intragranular porosity [41].

MacKenzie et al. [42] prepared type I gels to determine the role of preheat treat­ment temperature on subsequent mullite microstructure. They found that an optimal preheat temperature of about 250-350°C for a long period of time resulted in an optimal concentration of mullite in the final product. Concurrently, there was an increase in the 27Al nuclear magnetic resonance spectrum at about 30 ppm. The 30 ppm Al signal is often attributed to penta-coordinated Al, which may be located in the mullite precursor gels at the interface between Si-rich and Al-rich microdomains. MacKenzie et al. attribute this Al signal to the distorted tetrahedral Al environment in the region of O – deficient triclusters. They noted that the signal becomes increasingly strong just prior to mullitization. It was also noted that organic residues and hydroxyl groups were present up to 900°C. According to the analysis, the presence of these groups in the system at high temperatures could influence the structural evolution of the gel by pro­viding a locally reducing and/or humid atmosphere that could facilitate tricluster for­mation. These sites could influence subsequent mullite formation because they form an essential element of the mullite structure. In terms of the nature of the triclusters, Schmueker and Schneider [5] proposed that the triclusters of tetrahedra may compensate the excess negative charge in the network caused by Si+4-Al3+ substitution. Na+ doped into aluminosilicate gels can also compensate for the Si4+-Al3+ substitution. For this system, the formation of triclusters was no longer required, and a significant drop in the 30 ppm Al peak was observed.

Transparent mullite may have optical applications. With a scattering loss of less than 0.01 cm-1, it could be an excellent candidate for use in transparent windows in the mid­infrared range (3-5 pm wavelength). Furthermore, when mullite glass ceramics were formed with Cr3+ additions, significant differences in the luminescence spectra between the glassy phase and crystalline mullite were observed [43] Cr3+ was shown to reside in the mullite crystalline phase. The luminescence quantum efficiency increased from less than 1% to about 30% by the crystallization process. Further research is needed to estab­lish mullite as a candidate for high-energy laser applications.