This paper reports a method for producing -Al 2 O 3 at low temperature, which uses a seeding technique. White precipitate of aluminum hydroxide, which was prepared with a homogeneous precipitation method using aluminum nitrate and urea in aqueous solution, was peptized by using acetic acid at room temperature and then transformed to a transparent alumina sol. To the alumina sol -Al 2 O 3 particles were added as a seed, and then the sol containing -Al 2 O 3 particles was transformed to an -Al 2 O 3 -seeded alumina gel by drying the sol at room temperature. The nonseeded alumina gel was amorphous or fine crystallites even after being annealed at 600 ∘ C and was crystallized to -Al 2 O 3 at 700 ∘ C. The -Al 2 O 3 seeding promoted crystallization of alumina gel toAl 2 O 3 . The promotion of crystallization was made remarkable with a decrease in -Al 2 O 3 particle size and an increase in -Al 2 O 3 particle content in weight for the final seeded alumina gel. With an -Al 2 O 3 particle size of 150 nm and an -Al 2 O 3 particle content of 5%, the seeded alumina gel was partially crystallized to -Al 2 O 3 by annealing at a temperature as low as 700 ∘ C and mostly at 900 ∘ C.

This paper reports fabrication of transparent self-supporting alumina films by using homogeneous precipitation and peptization processes. White precipitate of aluminum hydroxide was prepared with a homogeneous precipitation method using aluminum nitrate and urea in aqueous solution. The obtained aluminum hydroxide precipitate was peptized by using acetic acid at room temperature, which resulted in production of a transparent alumina sol. The alumina sol was transformed to an alumina gel film by drying the sol at room temperature. The alumina film was amorphous or fine crystallites even after annealed at a temperature as high as 500°C, and was crystallized to £-Al 2 O 3 at 900°C.

a b s t r a c tThis paper reports a method for producing ␣-Al 2 O 3 at low temperature using a seeding technique. A white product obtained by hydrolyzing aluminum isopropoxide in water at 80 • C was peptized using acetic acid at 80 • C, which transformed the white product to a transparent alumina sol. ␣-Al 2 O 3 particles were added to the alumina sol as seed material; the sol containing ␣-Al 2 O 3 particles was then transformed to an ␣-Al 2 O 3 -seeded alumina gel by drying the sol at room temperature. The non-seeded alumina gel remained boehmite after annealing at 300 • C and crystallized into ␥-Al 2 O 3 and ␣-Al 2 O 3 at temperatures between 300 and 500 • C and between 900 and 1100 • C, respectively. The ␣-Al 2 O 3 seeding promoted crystallization of the alumina gel into ␣-Al 2 O 3 . The promotion of crystallization was significant with an increase in ␣-Al 2 O 3 particle content by weight in the final seeded alumina gel. With an ␣-Al 2 O 3 particle content of 5%, the seeded alumina gel was partially crystallized into ␣-Al 2 O 3 by annealing at a temperature as low as 900 • C.

Please cite this article in press as: K. Yamamura, et al., Effect of hydrothermal process for inorganic alumina sol on crystal structure of alumina gel, J. Asian Ceram. Soc. (2016), http://dx. a b s t r a c tThis paper reports the effect of a hydrothermal process for alumina sol on the crystal structure of alumina gel derived from hydrothermally treated alumina sol to help push forward the development of low temperature synthesis of ␣-Al 2 O 3 . White precipitate of aluminum hydroxide was prepared with a homogeneous precipitation method using aluminum nitrate and urea in aqueous solution. The obtained aluminum hydroxide precipitate was peptized by using acetic acid at room temperature, which resulted in the production of a transparent alumina sol. The alumina sol was treated with a hydrothermal process and transformed into an alumina gel film by drying at room temperature. Crystallization of the alumina gel to ␣-Al 2 O 3 with 900 • C annealing was dominant for a hydrothermal temperature of 100 • C and a hydrothermal time of 60 min, as production of diaspore-like species was promoted with the hydrothermal temperature and time. Excess treatments with hydrothermal processes at higher hydrothermal temperature for longer hydrothermal time prevented the alumina gel from being crystallized to ␣-Al 2 O 3 because the excess hydrothermal treatments promoted production of boehmite.

This paper reports a method for producing α-Al2O3 at low temperature from aluminum alkoxide using a combination of seeding of α-Al2O3 nanocryatallites and adding of inorganic alumina sol. An alkoxide alumina sol was obtained by hydrolyzing aluminum isopropoxide in water at 80°C and then peptizing the hydrolyzed aluminum isopropoxide using acetic acid at 80°C. An inorganic alumina sol was obtained by producing aluminum compound with a homogeneous precipitation method using aluminum nitrate and urea in aqueous solution and then peptizing the aluminum compound using acetic acid at room temperature. α-Al2O3 nanocrystallites were added to the alkoxide alumina sol containing the inorganic alumina sol. The addition of inorganic alumina sol provided successful fabrication of a crack-free α-Al2O3-seded alumina film by a spin-coating technique. The sol containing α-Al2O3 nanocrystallites was transformed to an α-Al2O3-seeded alumina gel by drying the sol at room temperature. The non-seeded alumina gel was crystallized into γ-Al2O3 at a temperature below 900°C. In contrast, the alumina seeded at 1% α-Al2O3 nanocrystallites content began to be transformed to α-Al2O3 by annealing at the temperature. The seeding and the adding promoted crystallization of the alumina gel into α-Al2O3. The promotion of crystallization was significant with an increase in α-Al2O3 nanocrystallites content by weight in the final seeded alumina gel.