Low Temperature Synthesis of α-Alumina with a Seeding Technique

This paper reports a method for producing α-Al2O3 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 α-Al2O3 particles were added as a seed, and then the sol containing α-Al2O3 particles was transformed to an α-Al2O3-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 γ-Al2O3 at 700°C. The α-Al2O3 seeding promoted crystallization of alumina gel to α-Al2O3. The promotion of crystallization was made remarkable with a decrease in α-Al2O3 particle size and an increase in α-Al2O3 particle content in weight for the final seeded alumina gel. With an α-Al2O3 particle size of 150？nm and an α-Al2O3 particle content of 5%, the seeded alumina gel was partially crystallized to α-Al2O3 by annealing at a temperature as low as 700°C and mostly at 900°C. 1. Introduction Not only electroconductive materials such as gold, silver, and copper but also electric-insulating materials are quite essential for producing electronic devices such as integrated circuits [1–3]. Alumina is one of representative electric-insulating materials. Since it has high thermal conductivity and high chemical stability at high temperature compared to other electro-insulating materials such as glass, plastic, and paper [4–6], it is promising as the electric-insulating material used in the electronic devices. There are various crystal structures in the alumina. Among the various crystal structures, alumina that reveals electric insulation is of α type. The α-alumina has been conventionally produced by annealing aluminum hydroxide derived from aluminum salts or minerals at temperatures higher than 1000°C [7–10]. The α-alumina can be also produced by fabricating amorphous alumina with methods using liquid phase such as precipitation and sol-gel process and then annealing it at high temperature [11–13]. These methods need the high temperatures, which brings about much consumption of energy. Accordingly, low temperature processes for producing α-alumina are desired for saving energy. Our research group has studied effects of nanocrystallites seeding on various titanates such as PZT and BST fabricated by the sol-gel method for the last decade [14–17]. As a result, their crystallization temperatures were