Structural Changes Induced on Mullite Precursors by Thermal Treatment: A ²⁷Al MAS‐NMR Investigation

Abstract

NMR study of mullite precursors has shown that local arrangement of A1 in samples synthesized by spray pyrolysis differs considerably from the one adopted by samples obtained by polymeric or colloidal routes. Aluminum is tetraand pentahedrally coordinated in the first type of samples but is tetra-and octahedrally coordinated in the second ones. Segregation of S O z and AIz03 is directly produced in colloidal preparation; however, this phenomenon occurs only in polymeric gels when they are heated between 980" and 1100°C. In polymeric samples, thermal treatment at -980°C produces the formation of y-A1203. A similar treatment in spray-pyrolized powders gives directly 3: 2 mullite. From these results, exothermic and expansive effects detected at -980°C were ascribed to changes in coordination of Al produced during the atomic rearrangement that accompanies formation of these two phases (y-A1203 or mullite). Above 1200"C, incorporation of Si in the Al-rich phase induces the formation of 3:2 mullite in polynieric and colloidal samples. [

Mullite: Crystal Structure and Related Properties

“…Schmücker et al furthermore suggested that weak Al/Si ordering occurs in the tetrahedra of mullite. 27 Al MAS NMR has frequently been used to probe for the existence of four‐, five‐, and sixfold coordinated Al in mullite, mullite precursors and in aluminosilicate glasses . Three different Al sites were described in mullite, one octahedral (near 0 ppm, M), one tetrahedral site like in sillimanite (near 60 ppm, T), and a distorted tetrahedral site assigned to Al in the tetrahedral triclusters (near 43 ppm, T*).…”

Evolution of aluminosilicate structure and mullite crystallization from homogeneous nanoparticulate sol–gel precursor with organic additives

“…Also, the structural breakdown of the PI/A seems to be underway, as the absorption band near 840-870 cm −1 for AlO 4 emerged. For PI/A structures, as well as for homogeneous mullite gels, the Al coordination is usually modified due to the dehydroxylation, 10,15,22,51,52,58,69 and according to Jaymes et al, [12][13][14][15] tions (of AlO 6 ) in imogolite 68 at ∼940 cm −1 were diminished (especially for M0), and the Si-OH stretches at 950-970 cm −1 were shifted to higher wavenumbers, indicating the formation of an amorphous aluminosilicate network as well. 58,70 For MV, the absorbance at 850-900 cm −1 was observed already at 200 • C, which suggests, together with the dilatometry results, that when plain PVP was used, the interparticulate surface reactions differed or were delayed during the alkoxy-group removal, which supports the microphase separation in the presence of both polymers and HCl.…”

Application of27Al NMR techniques to structure determination in solids

“…The early stages all show Al(4) and Al (6), with additionally Al(5) in small amounts by the polymeric route and large amounts for spray pyrolysis. The amount of Al(5) can be correlated with the size of the 980"C exotherm and the subsequent formation of u initially in the colloidal and polymeric preparations compared to the direct formation of a 3 : 2 mullite from the amorphous state in spray pyrolysis [152].…”