H. Schneider et al., CONSTITUTION OF THE GAMMA-ALUMINA PHASE IN CHEMICALLY PRODUCED MULLITE PRECURSORS, Journal of the European Ceramic Society, 13(5), 1994, pp. 441-448
The temperature development of type II mullite precursor powders have
been studied in the temperature range of 150-degrees-C (as-received) a
nd 1150-degrees-C X-ray diffraction (XRD) measurements, infrared (IR)
and Si-29 and Al-27 nuclear magnetic resonance (NMR) spectroscopy and
analytical transmission electron microscopy (ATEM) have been performed
on the heat-treated precursors. The investigations had the aim of con
tributing to the frequently discussed question, whether Si is incorpor
ated into the gamma-alumina spinel being formed as a transient phase i
n type II mullite precursors. The as-received precursors consist of re
latively large sperical particles (less-than-or-equal-to 0.5 mum) of n
on-crytalline SiO2 and of much finer-grained agglomerates of pseudo-bo
ehmite crystals (gamma-AlO(OH), almost-equal-to 20 nm), which are embe
dded in a SiO2 matrix. Above almost-equal-to 350-degrees-C, pseudo-boe
hmite transforms to spinel type alumina (gamma-Al2O3). During this tra
nsformation, all Si existing in the SiO2 matrix of the pseudo-boehmite
agglomerates is incorporated into gamma-Al2O3 corresponding to a SiO2
content of almost-equal-to 12 mole% at 500-degrees-C Up to 750-degree
s-C, the SiO2 content of the gamma-alumina remains constant but above
this temperature it gradually rises and reaches a maximum amount of al
most-equal-to 18 mole% at 1150-degrees-C A marginal decomposition of t
he spherical non-crystalline SiO2 particles may be the sources to prov
ide diffusion of Si species into the gamma-alumina during a temperatur
e increase above 750-degrees-C It is most likely that Si species diffu
se into the gamma-alumina crystals along the crystallite boundaries. T
he diffusion process and Si incorporation are facilitated with the tem
perature increase.