Behavior of chemically zoned acicular mullite fibers annealed at high temperature

The synthetic analogue of the rare mineral mullite (solid solution VIAl2IV[Al2+2xSi2-2x]O10-x) is widely used in ceramics as “3/2-mullite” (x~0.25), whereas single crystals grown from melt are “2/1-mullites” (x~0.4) [1].

The investigated acicular mullite ceramic consists of single-crystalline fibers with high-Al mullite (M1) in the core and low-Al mullite (M2) in the rim [2,3], appearing as two distinct mullite phases in powder XRD patterns [2]. On annealing the as-prepared sample at 1550°C (24, 96, 192 and 384 h) the two zones gradually merge into one homogeneous phase [2]. Here, we present the details of this merging, investigating cross sections of fibers via TEM.

Observed changes in a) width of the transition zone, b) relative lattice orientations of the two zones and c) lattice parameter a document progressive homogenization of the mullite, which is complete after 384h. Concurrently, we observed Si-rich, round to rectangular amorphous regions, probably from partial melting, most strikingly after 384 h. An increasing amount of Si-rich glass phase with annealing time would explain the overall increase of average Al-content in mullite which we observed in our XRD data [2].

We thank Prof. K.T. Faber (NW University, Evanston, IL/USA) for the samples, and the DFG for funding (Grant FI442/24-1).

[1] R.X. Fischer & H. Schneider (2005), in Mullite (eds. H. Schneider, S. Komarneni), Weinheim: Wiley-VCH 1-46

[2] J. Birkenstock, Y. Igami, R.X. Fischer & H. Schneider (2019), Abstract GeoMünster 2019

[3] A.J. Pyzik, C.S. Todd & C. Han (2008), J. Eur. Cer. Soc. 28, 383-391