Processing and microstructure of high-pressure consolidated ceramic nanocomposites Ren-Guan Duan a,1 , Guo-Dong Zhan a , Joshua D. Kuntz a , Bernard H. Kear b , Amiya K. Mukherjee a, * a Department of Chemical Engineering and Materials Science, University of California, One Shields Avenue, Davis, CA 95616, USA b Center for Nanomaterials Research, Rutgers University, 607 Taylor Road, Piscataway, NJ 08854, USA Received 2 June 2004; received in revised form 20 August 2004; accepted 22 August 2004 Abstract Al 2 O 3 /TiO 2 nanocomposites were prepared by high-pressure sintering (HPS). The addition of nano-MgO powder improved the densification and retarded grain growth. The pressure decreased the energy barriers for nucleation, and caused the decomposition of Al 2 TiO 5 and the phase transformation of Al 2 O 3 to take place at lower temperatures. Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Sintering; High-resolution electron microscopy (HREM); X-ray diffraction (XRD); Nanocrystalline materials; Phase transformation kinetics 1. Introduction Nanocrystalline materials have been drawing atten- tion due to the expectations of enhanced mechanical and other functional properties [1]. Alumina-based nanocomposite ceramics demonstrate novel and attrac- tive properties when compared with their microsized counterparts [2,3]. However, it is difficult to get a fully dense ceramic while maintaining a nanocrystalline grain size. In the preparation of nanocrystalline ceramics from nanopowders, when the densities are larger than 90% of theoretical, grain coarsening becomes particularly severe [4,5]. Therefore, it is necessary to explore new consolida- tion techniques that can accelerate sintering without increasing grain growth. The conventional method of preparing nanocrystal- line ceramics is achieved by consolidation of the consti- tuted nanoscale powders. Another technique, transformation assisted consolidation [6], is also being used to prepare nanocrystalline ceramics. This method possesses an advantage in that a nanosized starting pow- der is not needed in order to prepare nanocrystalline bulk ceramics, and has been successfully applied to fab- ricate nano-Al 2 O 3 and nano-TiO 2 single phase materials with a density higher than 99% of theoretical and with a grain size as small as 18nm when consolidated at pres- sures above 5 GPa [6]. Sintered aluminum titanate (Al 2 TiO 5 , tialite) is a ceramic that can have many potential applications, such as components of internal combustion engines, thermal barriers, etc., due to its low thermal expansion coeffi- cient (0.2 · 10 6 to 1 · 10 6 K 1 ), low thermal conduc- tivity (0.9 to 1.5Wm 1 K 1 ), and excellent thermal shock resistance (500 Wm 1 ) [7]. Additionally, Al 2 -TiO 5 is a suitable additive as a second phase particle that can improve the thermal properties of ceramic matrix composites [8]. 1359-6462/$ - see front matter Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2004.08.022 * Corresponding author. Tel.: +1 530 752 1776; fax: +1 530 752 9554. E-mail address: akmukherjee@ucdavis.edu (A.K. Mukherjee). 1 Now at Center for Materials Research, Norfolk State University, 700 Park Avenue, Norfolk, VA 23504, USA. www.actamat-journals.com Scripta Materialia 51 (2004) 1135–1139