Effect of Monovalent Cation Additives on the γ‐Al₂O₃‐to‐α‐Al₂O₃ Phase Transition

Abstract

were added by an impregnation method, using the appropriate nitrate solution. ␤-Al 2 O 3 was the crystalline aluminate phase that formed by reaction between these additives and Al 2 O 3 in the vicinity of the ␥-to-␣-Al 2 O 3 transition temperature, with the exception of Li ؉ . The transition temperature increased as the ionic radii of the additive increased. The change in specific surface area of these samples after heat treatment showed a trend similar to that of the phasetransition temperature. Thus, Cs ؉ was concluded to be the most effective of the present monovalent additives for enhancing the thermal stability of ␥-Al 2 O 3 . Because the order of the phase-transition temperature coincided with that of the formation temperature of ␤-Al 2 O 3 in these samples, suppression of ionic diffusion in ␥-Al 2 O 3 by the amorphous phase containing the added cations must have played an important role in retarding the transition to ␣-Al 2 O 3 . Larger cations suppressed the diffusion reaction more effectively.

Reactivity of aluminum sulfate and silica in molten alkali-metal sulfates in order to prepare mullite

“…The presence of alkali-metal cations, even in very low amounts, is likely to considerably decrease the crystallization temperature of silica 31 and to increase the temperature of the phase transformation of ␥-to ␣-alumina. 32 The ␣-alumina obtained and cristobalite were intimately mixed in the proportion 3Al 2 O 3 /2SiO 2 and reacted first in the solid state at 1500 • C (heating rate 150 • C/h, dwell time 2 h) and second at 950 • C for 6 h in the presence of a molten Na 2 SO 4 flux. The formation of mullite was never observed.…”

Crystallographical analysis of surface layers of refractory ceramics formed using combined flame spray and simultaneous laser treatment

“…This is probably attributed to larger the Cr ionic size that retarded the transition to the a-Al 2 O 3 phase. 22 The amount of eskolaite in the FCA coating was greater than that in the starting powder. This may be ascribed to more effective melting of the Cr 2 O 3 particles than the Al 2 O 3 particles during flame spraying, because Cr 2 O 3 has a higher thermal conductivity than Al 2 O 3 .…”

Titanium effect on phase transformation and sintering behavior of transition alumina

“…It increases much more (50 • C) with zirconium and yttrium doping which have a low solubility in ␣-alumina (<15 ppm). An evoked explanation for this behavior, based on the role of the ionic radius, can be ruled out 47,48 as the ionic radii of magnesium and zirconium are similar. 49 Finally, the relative density variation ρ R during the phase transformation is slightly reduced with Ti-doping; the higher the doping level, the smaller ρ R .…”