Fabrication of transparent AlN ceramics

Abstract

Aluminum nitride (AlN) is mostly known for its high theoretical thermal conductivity (320 W mK−1). In present work, special attention has been given to the fabrication of transparent AlN ceramics. With a wide band gap (6.3 eV) [1], AlN can be made transparent, which when combined with its wear resistance make AlN attractive for some electrooptics applications [2]. AlN is also used as a phase contrast layer in optical disks and has potential as a phase shift material in lithographic photo mask [3]. These applications generate an interest in the optical properties of AlN. But in many cases, AlN ceramics are translucent or opaque because of the difficulty of obtaining high density because of its highly covalent bonding structure. The fabrication of transparent AlN ceramics is by hot pressing traditionally [4–7]. However, the major problem is the long densification time at high temperatures. Spark plasma sintering (SPS) is a newly developed densification technique by which ceramic powder can be very fast to full density at relatively lower temperature. Similar to hot pressing, SPS process is also carried out in a graphite die. But the heating is accomplished by spark discharges in voids between particles generated by the pulse current applied through electrodes at the top and bottom punches of the graphite die. Because of these discharges, the particle surface is activated and purified. The heat transfer and mass transfer can be completed in a very short time. In this paper, SPS was selected to reduce the need for long densification time and sinter transparent AlN ceramics rapidly. Commercially available AlN powder (Tokuyama Soda Co. Ltd., Japan) was used as the starting material. The properties of the AlN powder, according to manufacture’s date, were shown in Table I. Fig. 1 was the scanning electron microscope (SEM) microphotograph of the staring powder, which showed that AlN powder was uniformly spherical in shape with little agglomeration. Three samples were prepared through the following processes: