GRAIN GROWTH IN NANOCRYSTALLINE SnO 2 PREPARED BY SOL-GEL ROUTE C.H. Shek*, J.K.L. Lai and G.M. Lin Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong (Received June 30, 1999) (Accepted September 23, 1999) Abstract—The isothermal grain growth of nanocrystalline SnO 2 , prepared by the sol-gel route was investigated at various temperatures between 500°C and 800°C. Grain growth data were analyzed using two different models. A conventional grain growth model for polycrystalline materials yields an extremely low activation energy of 47 kJ/mol, but large grain growth exponent n from 5 to 11. These values exceed the rational region deduced from conventional theory. An alternative model is based on the assumption that the ordering of the interface regions in nanocrystalline SnO 2 occurs simultaneously with grain growth by structural relaxation. This structural relaxation model describes the grain growth kinetics satisfactorily and also yields a low activation energy of 31 kJ/mol appropriate for the rearrangement of atoms. ©1999 Acta Metallurgica Inc. 1. Introduction Nanocrystalline (nc) materials with average grain size of less than 50 nm have attracted considerable scientific interest because of the improvements in a variety of properties that result from grain-size refinement on the nanometer scale. The investigation of the thermal stability or grain growth behavior is therefore important from the technological point of view as well as for scientific interest. Neverthe- less, the dynamics of grain growth and their mechanism for nc-materials are not clear entirely. The detailed information especially for nc-ceramics is even rarer. For conventional polycrystalline or coarse grain materials, the driving force for grain growth results from the decrease of the system energy by decreasing the total grain boundary energy. The rate of grain growth is proportional to the radius of curvature of the grain and the kinetic equation of grain growth is [1]: D n - D 0 n = Kt (1) where D is the average grain size after annealing, D 0 the initial average grain size, t the annealing time and K a temperature- (T) dependent rate constant. The constant K can be expressed in an Arrhenius type equation, K  exp(-Q/RT) with Q being the activation energy for isothermal growth and R the gas constant. The index n in equation (1) is called the grain growth exponent. According to previous works [2,3], the n value depends on the microstructure and growth mechanism. n equals 2 for normal grain growth in a pure, single phase system; 3 for grain growth in the presence of solutes, and 4 in the presence of pores. If the rate of growth is assumed to be proportional to the difference between the grain * Corresponding author. Pergamon NanoStructured Materials, Vol. 11, No. 7, pp. 887– 893, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 0965-9773/99/$–see front matter PII S0965-9773(99)00387-6 887