Synthesis of ZnFe 2 O 4 nanocrystallites by mechanochemical reaction Huaming Yang * , Xiangchao Zhang, Chenghuan Huang, Wuguo Yang, Guanzhou Qiu Department of Inorganic Materials, School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China Received 17 November 2003; accepted 4 March 2004 Available online 7 May 2004 Abstract Mechanochemical reaction of ZnO and a-Fe 2 O 3 in a planetary mill formed an amorphous precursor, which was subsequently heated to successfully produce zinc ferrite (ZnFe 2 O 4 ) nanocrystallites. The amorphous precursor and nanocrystallites were characterized by differential thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Calcination of the precursor powder at 600 8C led to the formation of ZnFe 2 O 4 nanocrystallites of about 22 nm in crystal size, and most of particle was about 10 – 50 nm in diameter. Effect of calcination temperature on the crystal size of the nanoparticles was investigated. The mechanism of nanocrystallite growth was primarily investigated. The activation energy of ZnFe 2 O 4 nanocrystallite formation during thermal treatment was calculated to be 18.5 kJ/mol. q 2004 Elsevier Ltd. All rights reserved. Keywords: B. Crystal growth; C. X-ray diffraction; D. Crystal structure 1. Introduction Zinc ferrite (ZnFe 2 O 4 ) has a normal spinel structure with a tetrahedral A-site occupied by Zn 2þ ions and octahedral B-site by Fe 3þ ions [1]. It is of interest not only to basic research in magnetism, but also has great potential in technological application [2]. Recently, ZnFe 2 O 4 has been widely applied in various fields of industry including magnetic materials, gas sensor and semiconductor photo- catalyst [3–7]. The properties of ZnFe 2 O 4 in above applications are highly affected by the particle size, it has been indicated that nanocrystalline ZnFe 2 O 4 is especially good for the property promotion. Many methods have been developed to prepare ZnFe 2 O 4 nanoparticles, including sol–gel route [8], chemical co-precipitation [9–11], hydrothermal synthesis [12,13], thermal plasma synthesis [14] and microemulsion method [15], but through all of the above methods, nanocrystalline ZnFe 2 O 4 is more difficult and inconvenient to obtain. Mechanochemical processing is a novel method for the production of nanosized materials, where separated nano- particles can be prepared. Recently, a wide variety of nanoparticles have been synthesized by mechanochemical technique, including ZnS, CdS, LiMn 2 O 4 , SiO 2 , CeO 2 , Co 3 O 4 and SnO 2 [16–23]. Mechanochemical synthesis is particularly suitable for large-scale production because of its simple process and low cost. However, few report has focused on the mechanochemical preparation of ZnFe 2 O 4 nanocrystallites. In this paper, the synthesis of nanocrystal- line ZnFe 2 O 4 by mechanochemical reaction is investigated and the mechanism for ZnFe 2 O 4 nanocrystallite growth is primarily discussed. 2. Experimental The starting materials were AR-grade ZnO and a-Fe 2 O 3 . The powders should be dried at 150 8C in air for 12 h prior to use. Dried ZnO and a-Fe 2 O 3 were mixed with a molar ratio of 1:1 to form the mixture. 30 g of the mixture and nine stainless steel balls of 2.5 mm in diameter, forty stainless steel balls of 0.4 mm in diameter were sealed in a 300 ml stainless steel vial. Mechanochemical milling was per- formed with a KM-10 type planetary mill for 0–8 h at 600 rpm using a ball-to-powder mass ratio of 10:1. The as- milled powder was dried at 100 8C in air to produce the nanoparticle precursor. The precursor was subsequently calcined at 600 8C in air in a porcelain crucible for 1 h to obtain ZnFe 2 O 4 nanocrystallites. 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.03.001 Journal of Physics and Chemistry of Solids 65 (2004) 1329–1332 www.elsevier.com/locate/jpcs * Corresponding author. Tel.: þ86-731-883-0549; fax: þ 86-731-871- 0804. E-mail address: hmyang@mail.csu.edu.cn (H. Yang).