Hydrothermal Synthesis of Advanced Ceramic Powders Wojciech L. Suchanek 1, a and Richard E. Riman 2, b 1 Sawyer Technical Materials, LLC, 35400 Lakeland Boulevard, Eastlake, OH 44095, USA 2 Rutgers University, Department of Materials Science and Engineering, 607 Taylor Road, Piscataway, NJ 08855, USA a wls@SawyerLLC.com, b riman@email.rci.rutgers.edu Keywords: Hydrothermal synthesis, review powder synthesis, morphology control, hydroxyapatite, PZT, alumina, zinc oxide, carbon nanotubes. Abstract. This paper briefly reviews hydrothermal synthesis of ceramic powders and shows how understanding the underlying physico-chemical processes occurring in the aqueous solution can be used for engineering hydrothermal crystallization processes. Our overview covers the current status of hydrothermal technology for inorganic powders with respect to types of materials prepared, ability to control the process, and use in commercial manufacturing. General discussion is supported with specific examples derived from our own research (hydroxyapatite, PZT, Į-Al 2 O 3 , ZnO, carbon nanotubes). Hydrothermal crystallization processes afford excellent control of morphology (e.g., spherical, cubic, fibrous, and plate-like) size (from a couple of nanometers to tens of microns), and degree of agglomeration. These characteristics can be controlled in wide ranges using thermodynamic variables, such as reaction temperature, types and concentrations of the reactants, in addition to non-thermodynamic (kinetic) variables, such as stirring speed. Moreover, the chemical composition of the powders can be easily controlled from the perspective of stoichiometry and formation of solid solutions. Finally, hydrothermal technology affords the ability to achieve cost effective scale-up and commercial production. Introduction Hydrothermal research was initiated in the middle of the 19 th century by geologists and was aimed at laboratory simulations of natural hydrothermal phenomena. In the 20 th century, hydrothermal synthesis was clearly identified as an important technology for materials synthesis, predominantly in the fields of hydrometallurgy and single crystal growth [1]. However, the severe (supercritical) reaction conditions required particularly for growing single crystals have discouraged extensive research and commercialization for many materials. In recent years, commercial interest in hydrothermal synthesis has been revived in part because a steadily increasing large family of materials, primarily ceramic powders, has emerged that can be prepared under mild conditions (T<350ºC, P<100 MPa). The growing number of scientific papers on hydrothermal synthesis of ceramic powders, which almost quadrupled between 2000 and 2004 (Fig. 1a), illustrates the rising interest in this area, with China, Japan, and USA publishing most extensively (Fig. 1b). Hydrothermal Synthesis as a Materials Synthesis Technology Process Definition. Hydrothermal synthesis is a process that utilizes single or heterogeneous phase reactions in aqueous media at elevated temperature (T>25ºC) and pressure (P>100 kPa) to crystallize ceramic materials directly from solution. However, researchers also use this term to describe processes conducted at ambient conditions. Syntheses are usually conducted at autogeneous pressure, which corresponds to the saturated vapor pressure of the solution at the specified temperature and composition of the hydrothermal solution. Upper limits of hydrothermal synthesis extend to over 1000ºC and 500 MPa pressure [2]. However, mild conditions are preferred for commercial processes where temperatures are less than 350ºC and pressures less than Advances in Science and Technology Vol. 45 (2006) pp. 184-193 online at http://www.scientific.net © (2006) Trans Tech Publications, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net . (ID: 12.45.174.100-24/10/06,17:51:02)