Kinetics and mechanisms of nanoparticle formation and growth in vapor phase condensation process A. Simchi * , R. Ahmadi, S.M. Seyed Reihani, A. Mahdavi 1 Department of Materials Science and Engineering, Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O. Box: 11365-9466, Azadi Avenue, 14588 Tehran, Iran Received 25 April 2005; accepted 28 October 2005 Available online 20 December 2005 Abstract Design of nanoparticle synthesis by inert-gas condensation process was studied according to the mechanisms and kinetics of nucle- ation and growth in the vapor phase. The effect of process parameters, e.g., source temperature, evaporation rate, and the inert-gas pres- sure, on the particle size and particle shape was examined at the example for silver and copper–tin alloy. The synthesized nanopowders had near spherical shape with particle size range from 10 to 60 nm dependent on the processing condition. Scanning and transmission electron microscopy (SEM and TEM) analyses showed that the crystallites are subunits of larger agglomerate particles, and relatively large particles display crystal habit. Based on the experimental results and theoretical principles, nucleation, growth, coagulation and coalescence of the particles were analyzed. Accordingly, the kinetics and mechanisms of nanoparticle synthesis in low-pressure gas phase was determined. A simple operating map for nanoparticle synthesis was presented. The results may serve as a guide for design of exper- imental studies on the effect of process parameters on nanoparticle characteristics. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Nanoparticles; Synthesis, inert-gas condensation; Kinetics; Nanomaterials; Silver; Copper–tin alloy 1. Introduction A variety of metal nanoparticles are made today for a spectrum of niche applications. The production methods are rather versatile and a great development has been achieved during the past few years. Among different exist- ing methods, condensation of the corresponding metal vapors is the predominant technology for synthesis of metal nanoparticles [1]. The process is so-called inert-gas consolidation (IGC) method with which Gleiter and co- workers [2] first generated the materials that demonstrated the exciting properties of nanostructured materials [3]. In this method, a metal is typically vaporized into a low den- sity gas by Joule heating, although other ways of evapora- tion such as thermal plasma [4] and laser ablation [5] are also being used. Vapors migrate from the hot source into a cooler gas by a combination of convective flows and dif- fusion. The decreasing temperature leads to a far more rapid decrease in the equilibrium vapor pressure and corre- spondingly high supersaturation [3]. At high supersatura- tion, the vapors rapidly nucleate, forming very large numbers of extremely small particles. The particles then grow by Brownian coagulation [6]. The product particles are generally collected by thermophoretic deposition. In order to enhance the deposition efficiency, a substrate sur- face cooled with liquid nitrogen may be used. The produc- tion rate is typically low, i.e., in the order of 1–2 kg per day [7]. Recently, some modifications such as forced gas flows [1] were proposed for production of large quantities of nanoparticles by IGC method. The other changes were concerned with the introduction of different evaporation sources to adopt for different starting materials such as high melting point materials [8] or gaseous precursors [9]. 0261-3069/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2005.10.017 * Corresponding author. Tel.: +98 21 616 5262; fax: +98 21 616 5261. E-mail address: simchi@sharif.edu (A. Simchi). 1 Civil Aviation Technology College, Mehrabad International Airport, P.O. Box: 13445-418 Tehran, Iran. www.elsevier.com/locate/matdes Materials and Design 28 (2007) 850–856 Materials & Design