Mullite formation from nonstoichiometric slow hydrolyzed single phase gels
TL;DR: A comparative dynamic x-ray diffraction (DXRD) and differential thermal analysis (DTA) study was performed in the investigation of mullite and spinel formation from slowly hydrolyzed single phase gels with Al/Si ratios ranging from 1/1 to 14/1 as discussed by the authors.
Abstract: A comparative dynamic x-ray diffraction (DXRD) and differential thermal analysis (DTA) study was performed in the investigation of mullite and spinel formation from slowly hydrolyzed single phase gels with Al/Si ratios ranging from 1/1 to 14/1. Both metastable tetragonal mullite and spinel were observed to form at temperatures 1250 °C. As the Al/Si ratio increases, both the tetragonal mullite and spinel formation temperatures decrease while the orthorhombic mullite formation temperature increases. Based on the Al/Si composition where the formation extents of tetragonal mullite and spinel were maximum, their compositions are estimated to be 2Al 2 O 3 · SiO 2 and 6A12 O3 · SiO 2 , respectively.
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TL;DR: In this paper, the effect of the process parameters or starting precursor materials properties on the particle size, shape and structure have been examined by particle size measurements, TEM and scanning electron microscopy.
129 citations
TL;DR: The kinetics, microstructural changes, and crystal structure development for crystallization of amorphous, quenched, mullite composition glass (3Al 2 O 3 ·2SiO 2 ) were studied between 900 and 1400°C as discussed by the authors.
Abstract: The kinetics, microstructural changes, and crystal structure development for crystallization of amorphous, quenched, mullite composition glass (3Al 2 O 3 ·2SiO 2 ) were studied between 900 and 1400°C. The phenomena observed were characterized using non-isothermal differential scanning calorimetry (DSC), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), powder X-ray diffraction (XRD, with both a standard laboratory diffractometer, as well as with synchrotron radiation), and Rietveld analysis. Crystallization of amorphous mullite was observed to occur in two steps. The activation energy for crystallization was 892 kJ/mol for the first step and 1333 kJ/mol for the second step. From the amorphous state, the first phase(s) to crystallize were alumina-rich, pseudotetragonal mullite (∼70 mol% Al 2 O 3 ). These crystals were highly strained and contained numerous nanometer scale inclusions. With increasing temperature, the crystals were observed to incorporate increasing amounts of SiO 2 , and approach the equilibrium orthorhombic structure. By 1400°C the pseudotetragonal to orthorhombic transition was complete, the strain was eliminated, most of the inclusions had been assimilated, there was ∼67% reduction in grain size, and the crystals had attained the composition of the initial, bulk glass (∼60 mol% Al 2 O 3 ).
66 citations
TL;DR: In this article, the authors characterized nano-mullite powders using several spectroscopic and microscopy methods, including thermal gravimetric analysis, differential thermal analysis, diffuse reflectance infrared Fourier transform spectroscopy and transmission electron microscopy.
Abstract: The flame spray pyrolysis of alcohol-soluble precursors allows the synthesis of mullite-composition nanopowders (average size of ;60 ‐100 nm) that, when annealed carefully, provide processable nano-mullite powders. The powders have been characterized using several spectroscopic and microscopy methods, including thermal gravimetric analysis, differential thermal analysis, diffuse reflectance infrared Fourier transform spectroscopy, and transmission electron microscopy. Preliminary studies on the pressureless-sintering behavior of these powders are presented. Without additives or any efforts to optimize the process, powder compacts could be sintered to relative densities of >90%, with grain sizes of <500 nm at 1600°C.
64 citations
31 May 1999-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, a solgel process is used to synthesize a single phase mullite powder, which is then transformed to orthorhombic crystalline mullite at 1300°C.
Abstract: Mullite of stoichiometric composition has been synthesized by sol-gel process. Aluminium nitrate and ethyl silicate are used as the starting materials to prepare the single phase mullite powder. DTA shows that the mullitization starts at 971°C. It is observed from XRD analysis that the amorphous mullite completely transforms to orthorhombic crystalline mullite at 1300°C. Wet milling operation has enhanced the sintering density of pure mullite by 12% more than that of dry milling. The naturally available raw material, clay has been identified as one of the suitable sintering aids for mullite. The addition of clay to the mullite powder enhances the density up to 95% of TD even at a very low temperature, 1450°C by pressureless sintering. A viscous flow sintering mechanism during densification is one of the main criteria to achieve good densification.
43 citations
TL;DR: A simple, processable precursor to mullite can be synthesized in quantities of 100 g in a few hours by direct reaction of silica, aluminum hydroxide, and triethanolamine in ethylene glycol as mentioned in this paper.
Abstract: A simple, processable precursor to mullite can be synthesized in quantities of 100 g in a few hours by direct reaction of silica, aluminum hydroxide, and triethanolamine in ethylene glycol. To delineate a processing window whereby precursor shapes can be transformed into mullite, the chemical and phase microstructural evolution of this precursor on pyrolysis to selected temperatures in air is followed by thermal gravimetric analysis, differential thermal analysis, diffuse reflectance infrared Fourier transform spectroscopy, solid-state 27 Al and 29 Si nuclear magnetic resonance, X-ray diffractometry, and Brunauer‐Emmett‐ Teller analytical methods. The precursor behaves as a single-phase, atomically mixed material that initially transforms to a porous, amorphous aluminosilicate when heated to temperatures as high as 950°C. Above 950°C, the precursor first transforms to tetragonal mullite, based on comparison with the literature, and, on continued heating above 1200°C, to orthorhombic mullite with coincident loss of porosity.
40 citations
References
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TL;DR: In this paper, concentration profiles of Al/sub 2/O/sub 3/ in diffusion couples made from sapphire and fused silica were used to determine the stable equilibrium phase diagram of the system.
Abstract: Concentration profiles of Al/sub 2/O/sub 3/ in diffusion couples made from sapphire and fused silica were used to determine the stable equilibrium phase diagram of the system SiO/sub 2/--Al/sub 2/O/sub 3/. The intermediate compound mullite, 3Al/sub 2/O/sub 3/.2SiO/sub 2/, melts incongruently at 1828 +- 10/sup 0/C; its stable solid-solution region ranges from 70.5 to 74.0 wt percent Al/sub 2/O/sub 3/ below 1753/sup 0/C and from 71.6 to 74.0 wt percent at 1813/sup 0/C. The microstructures of diffusion zones and heat-treated specimens also indicate the incongruency of mullite. Additional information is given for 3 metastable systems: SiO/sub 2/-Al/sub 2/O/sub 3/ in the absence of mullite, SiO/sub 2/-ordered-mullite in the absence of alumina, and SiO/sub 2/-disordered mullite in the absence of alumina. Under metastable conditions, ordered mullite melts congruently at approximately 1880/sup 0/C and its solid-solution range extends up to approximately 77 wt percent Al/sub 2/O/sub 3/. The solid-solution range of disordered mullite extends to approximately equal to 83 wt percent Al/sub 2/O/sub 3/ with an estimated congruent melting temperature of approximately 1900/sup 0/C. The existence of metastable systems is associated with superheating of mullite above the incongruent melting temperature and with nucleation of alumina and mullite in supercooled aluminum-silicate liquids. (auth)
410 citations
TL;DR: Two kinds of xerogels were prepared by slow and rapid hydrolyses of tetraethoxysilane and aluminum nitrate nonahydrate dissolved in ethanol as discussed by the authors.
Abstract: Two kinds of xerogels were prepared by the slow and rapid hydrolyses of tetraethoxysilane and aluminum nitrate nonahydrate dissolved in ethanol. Xerogels prepared by slow hydrolysis crystallized mullite directly from the amorphous state on firing whereas those formed by rapid hydrolysis crystallized a spinel phase before mullite formation. The composition of the spinel phase was characterized by various methods to be near SiO2·6Al2O3. The process of mullite formation is discussed in relation to the structures of the starting materials.
282 citations
TL;DR: In this paper, two series of xerogels (desiccated hydrogels), one consisting of single phases and the other of two phases, were prepared from organic and inorganic precursors in the system Al2o3-Sio2.
Abstract: Two series of xerogels (desiccated hydrogels), one consisting of single phases and the other of two phases, were prepared from organic and inorganic precursors in the system Al2o3-Sio2. The effect of these nanostructural differences in the xerogel powders on the energy content and behavior on heating were studied using DTA, powder X-ray diffraction, and ultrafast heating on a strip furnace. The results demonstrate that diphasic xerogels are substantially different materials from single-phase xerogels of the same composition.
249 citations
TL;DR: Barium titanate precursors with Ba/Ti ratios ranging from 2:1 to 1:9 were prepared by controlled hydrolysis of mixed barium and titanium species in an alcohol medium.
Abstract: Barium titanate precursors with Ba/Ti ratios ranging from 2:1 to 1:9 were prepared by controlled hydrolysis of mixed barium and titanium species in an alcohol medium. Details of the synthesis and characterization of the resultant products are given. Amorphous powders precipitated by hydrolysis from ethanol solutions of barium and titanium alkoxides crystallize to single-or two-phase 1:2 and 1:5 compounds at ∼700°C. These compounds transform at higher temperatures to other known crystalline phases, the 1:5 phase being maintained metastably to ∼1100°C.
220 citations