D. Voll

Formation of aluminum rich 9:1 mullite and its transformation to low alumina mullite upon heating

The formation of aluminum rich mullites Al4 + 2x Si2 − 2xO10 − x with x > 23 has been studied at annealing temperatures between 700 and 1650 °C. Calcination of the amorphous precursor at 700 °C yields a mullite with 88 mol% Al2O3 corresponding to an x-value of 0·809. Simultaneously, a γ-alumina phase is formed. Further increase of the annealing temperature yields an increase in the aluminum incorporation up to 92·1 mol% Al2O3 at 1000 °C derived from the refined lattice constants. This is the highest amount of Al observed so far in a mullite except the supposed end member ι-Al2O3 which, however, has not yet been established unambiguously. Above 1000 °C, the aluminum content in mullite is reduced. This is accompanied by a transformation of the spinel-type phase to a superstructure of a θ-alumina like phase. The final product at 1650 °C consists of 34 mol% of a ‘normal’ mullite with x = 0·32 and 66 mol% corundum.

Mullite precursor phases

Abstract Admixtures of tetraethoxysilane (TEOS) and aluminium sec.-butylate (AlOBu) with stoichiometric 3Al2O3.2SiO2 mullite composition were used for the syntheses. Depending on the amount of H2O, the velocity of the hydrolysis process and the pH of the solvent, three different types of temperature-induced mullite formation processes are observed. Type I mullite precursors: produced by slow hydrolysis with very little H2O. From ≈350°C and up to ≈900°C these precursors are non-crystalline and show a homogeneous mixture on an atomic level. Above this temperature limit the precursors transform to Al2O3-rich mullite and non-crystalline SiO2. Type II mullite precursors: produced by rapid hydrolysis with excess H2O in a very basic environment (pH > 10). Above ≈350°C these precursors are more phasic, consisting of relatively large crystalline γ-Al2O3, and of non-crystalline SiO2-rich areas. Mullite formation is observed at ≳1200°C. Type III mullite precursors: produced by rapid hydrolysis with excess H2O in a moderately basic environment (pH ≤ 10). From ≈350°C and up to ≈900°C these precursors are non-crystalline. Above this temperature limit the precursors gradually transform to γ-Al2O3 and non-crystalline SiO2-rich areas. Mullite formation is observed at ≳1200°C.

Synthesis and structural characterization of non-crystalline mullite precursors

Abstract Two different types of non-crystalline mullite precursor with identical bulk composition (72 wt% Al 2 O 3 , 28 wt% SiO 2 ) were prepared from tetraethoxysilane and silicon chloride, respectively, and aluminium sec-butozide, by using different methods of hydrolysis. The precursors, designated as type I and III, display different crystallization processes above ≈ 900°C: type I precursors directly form mullite, while type III precursors yield crystallization of transient γ-alumina. Infrared (IR) spectroscopy, large angle X-ray scattering (LAXS) and 27 Al nuclear magnetic resonance spectroscopic studies, and 29 Si nuclear magnetic resonance (NMR) literature data give evidence for a high degree of structural mixing in type I precursors and for a beginning of segregation into Al 2 O 3 -rich domains in type III precursors prior to crystallization (⩽ 900°C). Both precursors are composed of (SiO) tetrahedra and of (AlO) octahedra, tetrahedra and pentahedra although pentahedra are dominant in type I while octahedra occur more frequently in type III precursors. The driving force for mullitization (type I) and γ-alumina formation (type III) taking place at the same temperature is believed to be the instability of pentahedrally coordinated Al above ≈ 900°C. The sudden disappearance of Al pentahedra probably depends on the formation of reactive network centers during dehydroxylation. This hypothesis is derived from the observation that dehydroxylation and condensation strongly take place in a similar temperature range prior to crystallization.

Dehydration and Structural Development of Mullite Precursors: An FTIR Spectroscopic Study

Abstract Mullite type, I and III precursors were prepared by a sol-gel process using tetraethoxysilane and aluminum sec.-butoxide as starting materials. The precursors were treated by 15 h heating steps in intervals of 100°C from 200 to 1000°C. Type III precursors are characterised by a more discontinuous decrease of the analytical water content compared to that of type I precursors. The FTIR powder spectra of the preheated precursors show weak absorption bands at 5160 and 4540 cm −1 which are due to H 2 O and OH combination modes, thus proving the presence of both, H 2 O molecules and OH groups as structural components of the precursors. (H 2 O, OH) stretching modes are centered at around 3430 cm −1 and H 2 O bending modes at 1635 cm −1 . The deconvolution of the stretching mode bands reveals non-bridging and bridging H 2 O molecules and OH groups. Close similarities in the pattern of the 1400–400 cm −1 vibrational region between type I and III precursors exist up to preheating temperatures of 800°C. Significant differences are evident at temperatures of 900°C, where the spectrum of the type III precursor still corresponds to that at 800°C, while the type I precursor reveals a spectrum with features present in the spectrum of mullite. Bands centered around 1110 and 1010 cm −1 are assigned to Si–O stretching vibrations of the SiO 4 tetrahedral units and are strongly shifted in 900°C treated type I precursors to higher wavenumbers. This band shift is a strong indication for an increasing degree of network condensation and for changes in the Si–O distances to tetrahedra dimensions similar to those of mullite. A significant absorption around 860 cm −1 is assigned to Al–O stretching modes of AlO 4 tetrahedral units, a band around 570 cm −1 is assigned to Al–O stretching vibrations of AlO 6 octahedral units. A slightly decreasing intensity of this band with increasing preheating temperatures, along with a strongly increasing intensity of the 860 cm −1 band demonstrates a clear preference of Al for a four-fold coordinated structural position in the precursors preheated at high temperatures. This process is correlated to the dehydration process occurring in the medium- to high-temperature field of network condensation starting at around 400°C.

Structural evolution in gel-derived mullite precursors

Abstract The evolution of mullite from organo-metal gel precursors above 700 °C is found to be strongly influenced in both gel pieces and powdered samples by the thermal pretreatment at lower temperatures. Under the present conditions, the optimum preheating temperature was found to be 350 °C, at which temperature an anomalously high concentration was found of an Al species with a characteristic 27 Al magic-angle spinning NMR resonance at about 30 ppm. Such Al sites are often described as pentaco-ordinated, but an alternative assignment is considered. The optimum temperature for the formation of this Al site is also optimal for the catalytic formation of aromatic molecules from the residual organic fragments and/or solvent present. Mass spectrometry shows that under the present reaction conditions, these aromatics are thermally stable up to at least 900 °C in air, and the prolonged presence of their decomposition products (CO and water) could facilitate the transformation of the gel to crystalline mullite. The 29 Si NMR spectra indicate at least three different Si environments, including one which may arise from the formation of silicon oxycarbide glasses in these gels.

Constitution of the γ-alumina phase in chemically produced mullite precursors

Abstract The temperature development of type II mullite precursor powders have been studied in the temperature range of 150°C (as-received) and 1150°C. X-ray diffraction (XRD) measurements, infrared (IR) and 29 Si and 27 Al 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 contributing to the frequently discussed question, whether Si is incorporated into the γ-alumina spinel being formed as a transient phase in type II mullite precursors. The as-received precursors consist of relatively large sperical particles (≤ 0·5 μm) of non-crystalline SiO 2 and of much finer-grained agglomerates of pseudo-boehmite crystals (γ-AlO(OH), ≈ 20 nm), which are embedded in a SiO 2 matrix. Above ≈ 350°C, pseudo-boehmite transforms to spinel type alumina (γ-Al 2 O 3 ). During this transformation, all Si existing in the SiO 2 matrix of the pseudo-boehmite agglomerates is incorporated into γ-Al 2 O 3 corresponding to a SiO 2 content of ≈ 12 mole% at 500°C. Up to 750°C, the SiO 2 content of the γ-alumina remains constant but above this temperature it gradually rises and reaches a maximum amount of ≈ 18 mole% at 1150°C. A marginal decomposition of the spherical non-crystalline SiO 2 particles may be the sources to provide diffusion of Si species into the γ-alumina during a temperature increase above 750°C. It is most likely that Si species diffuse into the γ-alumina crystals along the crystallite boundaries. The diffusion process and Si incorporation are facilitated with the temperature increase.

Infrared band assignment and structural refinement of Al-Si, Al-Ge, and Ga-Ge mullites

A new band assignment of the IR spectrum of mullite is proposed on the basis of FTIR powder spec- troscopy of Al-Si, Al-Ge, and Ga-Ge compounds and polarised FTIR single-crystal spectroscopy of oriented ultrathin Czochralski-grown Al-Si 2:1-mullite slabs. The structural parameters of the mullite compounds were obtained from a single-crystal data refinement (Al-Si 2:1) and from Rietveld powder data refinements in space group Pbam. The refined chemical compositions varied from x = 0.31 (Ga-Ge), x = 0.34 (Al-Si) to x = 0.36 (Al-Ge) and x = 0.41 (Al-Si 2:1) with respect to the general mul- lite formula VI M3+ 2( IV T3+ 2+2x IV T4+ 2-2x)O10-x (M = Al, Ga; T = Al, Si, Ga, Ge). The FTIR powder spectra in the 1400-400 cm-1 range of Al-Si, Al-Ge, and Ga-Ge mullite compounds are char- acterised by three groups of bands designated as (a), (b) and (c). The deconvolution of the absorption features in the whole spectral range requires a minimum number of nine fitted bands. For Al-Si mullite, group (a) bands centre in the 1200-1100 cm-1 range, group (b) in the 1000-700 cm-1, and group (c) in the 650-400 cm-1 region. A strong shift of group (a), (b), and (c) bands towards lower wavenumbers exist in Al-Ge and Ga-Ge mullite with respect to Al-Si mullite. This is explained with the increasing size of the polyhedra in replacing Si by Ge and Al by Ga. The orientation-dependent bands in the spectra of the Al-Si 2:1-mullite single-crystal slabs can be clearly corre- lated with the fitted bands of the powder spectra. Due to the band shift and the polarisation behaviour, group (a) bands are assigned to high-energy Si-O and Ge-O stretching vibrations occurring along the extremely short bonds of the respective tetrahedral units within the (001) plane. Group (b) bands are essentially determined by stretching vibra- tions of Al and Ga on T-sites and T-O-T bending vibrations, while group (c) bands are due to stretching vibrations of Al and Ga in octahedral coordination and to O-T-O bending vibrations. On the basis of the present band assign- ment the lattice vibrational region of sillimanite is shortly discussed.

Crystal structure of synthetic Al4B2O9 : a member of the mullite family closely related to boralsilite

Abstract The crystal structure of Al4B2O9, synthesized from Al(NO)3·9H2O and B(OH)3 via a sol-gel process, is studied and characterized by Rietveld refinements and grid search analyses combined with 11B and 27Al MAS NMR spectroscopy. The aluminum borate with a unit-cell composition of Al32B16O72 is closely related to the boralsilite (Al32B12Si4O74) structure with Si replaced by B and to mullite (Al4+2xSi2-2xO10-x). It crystallizes in the monoclinic space group C2/m, a = 14.8056(7) Å, b = 5.5413(2) Å, c = 15.0531(6) Å, β = 90.913(2)°, Z = 8 for Al4B2O9. The main structural units are isolated chains of edge-sharing AlO6-octahedra running parallel to b that is a characteristic feature of the mullite-type crystal structures. The octahedral chains are crosslinked by AlO4, AlO5, BO3, and BO4 groups with two B atoms and one O atom (O5′) disordered on interstitial positions. 27Al and 11B NMR studies confirm the presence of sixfold (octahedral), fivefold, and fourfold (tetrahedral) coordinated Al (sixfold:[fourfold + fivefold] = ~50%:50%) and of threefold and fourfold coordinated B (~80%:20%).

Temperature-dependent dehydration of sol-gel-derived mullite precursors: An FTIR spectroscopic study

Abstract Mullite type I precursors were prepared by a sol-gel process using tetraethoxysilane and aluminum sec.-butoxide as starting materials. The precursors were treated by 15 h heating steps in intervals of 100 °C from 200 to 900 °C and remain non-crystalline over the whole temperature range. The analytically determined water content of the preheated precursors decreases continuously with increasing preheating temperature. FTIR powder spectra of the preheated precursors show absorption bands in the region of the H 2 O and (Si,Al)-OH combination modes (5160 and 4540 cm −1 ), (H 2 O, OH) stretching modes (3430 cm −1 ) and H 2 O bending modes (1635 cm −1 ). FTIR results provide evidence for the presence of both H 2 O molecules and OH groups in the precursor structure. In precursors preheated up to 600 °C the OH/H 2 O ratio increases continuously with increasing preheating temperature. Above 600 °C, molecular H 2 O is the dominating component of the precursors, indicating a recombination of OH groups to H 2 O molecules. On the basis of wave-number positions of the deconvoluted stretching mode bands, non-bridging and bridging H 2 O molecules (3440 and 2961 cm −1 ) and OH groups (3585 and 3226 cm −1 ) are discerned. The formation of non-bridging H 2 O represents an initial stage to the complete dehydration of the mullite precursor phases. On the basis of FTIR data a mechanism of precursor dehydration is developed: Up to about 400 °C the molecular H 2 O which adheres at the surface and in open pores of the precursor leaves the network by evaporation together with organic residuals. Above 600 °C the thermal energy is high enough for dehydroxilation. The OH groups then recombine to molecular H 2 O. Since dehydroxilation takes place in a temperature field of strong network condensation, part of the ‘recombination-produced’ H 2 O is trapped in newly formed closed pores, giving rise to a relative increase of molecular H 2 O. The high vapour pressure of the entrapped H 2 O above about 800 °C causes microfracturing of the precursors. Along the formed microcracks H 2 O rapidly evaporates, leaving behind nearly water-free precursors at 900 °C.