Mullite Whiskers Grown from Erbia‐Doped Aluminum Hydroxide–Silica Gel

Abstract

Mullite whiskers and anisotropic grains that were derived from erbia-doped aluminum hydroxide-silica gel were studied. Firing 3.0-mol%-erbia-doped isostatically pressed pellets at 1600°C for 1.0 -8.0 h resulted in a high surface concentration of mullite whiskers. Their c-axes were aligned preferentially along the pellet surface; the maximum length was 50 m, and the maximum aspect ratio was 23. The pellet surface was fully covered by mullite whiskers, and small anisotropic grains with a low aspect ratio were observed in the bulk. The voids that were observed in the fracture surfaces were covered fully by mullite whiskers. The large number of voids resulted in an apparent density of 1.60 g/cm 3 in the sintered pellets. The molar ratio of alumina to silica in the whiskers was in the range of 1.30 -1.45 (an average value of 1.31), regardless of whether the alumina/silica powder compositions were mixed in a 3:2 or 2:1 ratio.

Molten salt synthesis of mullite whiskers from various alumina precursors

“…In previous research, anisotropic grains were observed in Aldeficient mullite prepared by solid-state method [20,21]. However, anisotropic grains were observed in Al-rich mullite by MSS.…”

“…Besides the sort of starting materials, Al/Si ratio of starting materials is another factor that has a significant effect on the composition and morphology of mullite [19]. In mullite whiskers synthesized by solid-state reaction [20,21], Si-rich composition was suggested to facilitate the growth of anisotropic mullite that contained low Al 2 O 3 content (i.e. <60 mol% Al 2 O 3 ).…”

Influence of silica sources on morphology of mullite whiskers in Na2SO4 flux

“…Particle size of the amorphous silica, quartz and fused silica powders was 20-30 m. First, Al2(SO4)3·18H2O were calcined at 300 • C for 3 h to obtain Al2(SO4)3. According to the former research results that the Si-rich composition is beneficial for needlelike mullite grains formation [15,16], the constant composition [Al2(SO4)3:SiO2 = 2.5:1(wt%)] was employed in all samples. For the samples with various Na2SO4 contents, Al2(SO4)3 and Na2SO4 by weight percentage (wt%) as 2.5:14, 2.5:7, 2.5:3.5, and 2.5:1.75 were used and present by solid circles in Al2(SO4)3-Na2SO4 phase diagram [17] (Fig.…”

Preparation of mullite whiskers from topaz decomposition

“…The raw topaz was processed either by wet ball milling for several hours with 10-mm-high alumina balls, then dried and crushed into powders or just grinding by a mortar and pestle, followed by mixing with 0.5 wt.% of AlF 3 1, the decomposition experiments were carried out at a programmed heating profile with maximum temperature 1300 jC for 4 h and some for 1 h under continuous flow of air, which was supplied by compressed air pump and measured by counting the bubble in a paraffin oil, or sometimes, water. NaOH water solution (5%) was used to absorb the off-gases from the system.…”