Correlation of the precursor type with densification behavior and microstructure of sintered mullite ceramics

Abstract

The effect of alumina component in diphasic mullite precursors containing alkoxy derived silica on the crystallization and sin tering behavior of compacts was studied. The phenomena observed were characterized using differential thermal analysis (DTA), powder X ray diffraction (XRD), dilatometry and transmission and scanning electron microscopy (TEM, SEM). In order to change the characteristics of as prepared gels, the alumina source was varied while keeping the silica source constant. Al(NO 3) 3. 9H 2 O, g Al 2 O 3 and boehmite (g AlOOH) were used as the alumina source and TEOS as the silica source. Clear differences were found in the microstructure of sintered samples derived from the precursors with aluminum nitrate nonahydrate in comparison to the sam ples containing g Al 2 O 3 or boehmite (g AlOOH). The former exhibited elongated mullite grains embedded into the ''equiaxial mullite matrix''. This morphology is due to the overlapping of mullite crystallization and viscous flow sintering temperatures. Transient alumina, either added as g Al 2 O 3 or formed in situ by decomposition of boehmite, shifts the mullite formation above the sintering temperature, and enables formation of equiaxial mullite. The smaller are the transient alumina particles, the smaller are mullite grains of sintered bodies.

Polymer-derived mullite–SiC-based nanocomposites

“…2(b) indicates the formation of ␦-and -alumina and a subsequent crystallization of mullite, respectively. 21,26 Additionally, the X-ray spectra for this sample reveal that a significant crystallization process takes places between 1300 and 1400 • C ( Fig. 3(b)).…”

Advanced ceramics from a preceramic polymer and nano-fillers

“…The beginning of conversion at 1250 • C is likely due to homogeneous nucleation of mullite (active only above 1250 • C, as observed in the literature). 8 In the system with clay, heterogeneous nucleation reasonably occurred at the mullite crystals developed upon the transformation of clay, and amorphous silica reacted with the Al 2 O 3 nano-filler even below 1200 • C. In fact, the conversion at 1200 • C cannot be attributed only to mullite from clay ("primary mullite", or mullite I, see Table 1). To our calculations, this contribution is very limited, being about 13 wt%; if we consider the amorphous silica from silicone to be fully active with nano-Al 2 O 3 (yielding "secondary mullite ", or mullite II), the mullite conversion reaches about 75 wt% corresponding to about 76 vol.%, very close to the conversion calculated from the fitting of XRD patterns (78 vol.%).…”

Transparent mullite ceramic from single-phase gel by Spark Plasma Sintering

“…a lower surface area of powders and hard agglomerates, both of which are unfavorable for packing. 5,22 On the other hand, it should be noted that the chemical composition of mullite of near 3:2 causes absence of amorphous silica, which contributes to the viscosity flow mechanism in lower calcinations temperature.…”

“…4 In contrast, precursors mixed at the nanometer scale are better suited for low-temperature (1250-1500 • C) densification through viscous deformation. 5,6 Klug prepared IR-infrared transparent mullite ceramic by hotpressing at 1630 • C from 72 wt% Al 2 O 3 composition and hot isostatic pressing at 1650 • C from 76 wt% Al 2 O 3 composition. 7 Ohashi et al fabricated visible spectrum to infrared translucent mullite by pressureless sintering at 1750 • C. 8 The precursors prepared by the above researchers were all diphase xerogels.…”