**1. Introduction**

234 Materials Science and Technology

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Nano-sized magnetite (Fe3O4) powder has been widely used in various industrial products: for instance, pigments, recording materials, printing and electrophotography such as copying toner and carrier powders, etc. (Buxbaum & Pfaff, 2005; Hakata, 2002; Ochiai et al., 1994) because of the excellent physicochemical properties. In particular, Fe3O4 nanoparticles with a size of smaller than about 20 nm exhibit a unique magnetic property, namely, superparamagnetism. In recent years, its non-toxicity property and high chemical stability have attracted much attention, and the use of superparamagnetic Fe3O4 nanoparticles have rapidly expanded in biomedical fields, such as magnetic resonance imaging (Hu et al., 2010; Lee et al., 2009), drug delivery (Guo et al., 2009; Zhou et al., 2009), hyperthermia (Gao et al., 2010; Muzquiz-Ramos et al., 2010), immobilization of enzymes (Wang et al., 2008) and proteins (Can et al., 2009), and magnetic separation of cells (Li et al., 2009).

For industrial production of superparamagnetic Fe3O4 nanoparticles, chemical coprecipitation is often employed (Buyukhatipoglu et al., 2009; Compeán-Jasso et al., 2008; Iwasaki et al., 2011a; Mizutani et al., 2010; Yang et al., 2009; Yu et al., 2010). In coprecipitation methods, ferrous and ferric ions are simultaneously precipitated as ferrous hydroxide (Fe(OH)2) and goethite (α-FeOOH) in an alkaline solution, respectively, resulting in the formation of Fe3O4. In order to prepare superparamagnetic Fe3O4 nanoparticles with good dispersibility, not only the primary size of Fe3O4 nanoparticles must be controlled but also the aggregation of Fe3O4 nanoparticles must be avoided. Thus, anti-aggregation agents (e.g., surfactant) and toxic organic solvents are added to the staring solution in many cases (Cheng et al., 2005; Ge et al., 2007; Hua et al., 2008; Wan et al., 2007; Wen et al., 2008). Therefore, the Fe3O4 suspensions thus obtained should be washed enough before its use in the applications. In addition, for improving the crystallinity, heating treatments such as annealing and hydrothermal treatment are often performed, leading to the enhancement of the ferromagnetism (Mizutani et al., 2008; Wu et al., 2007, 2008; Zheng et al., 2006). Unfortunately, these required treatments complicate the preparation process and may increase the environmental impact and the production cost.

In order to overcome these problems, a novel synthesis method using a mechanochemical effect has been developed for the production of superparamagnetic Fe3O4 nanoparticles with

Novel Mechanochemical Process

under vacuum overnight.

for Aqueous-Phase Synthesis of Superparamagnetic Magnetite Nanoparticles 237

After replacement of air in the milling pot with argon, the milling pot is sealed. In order to promote the reaction between Fe(OH)2 and α-FeOOH, the ball-milling treatment is then carried out by rotating the milling pot at rotational speeds of 35 to 140 rpm (corresponding to 16% to 64% of the critical rotational speed (= 220 rpm) determined experimentally based on the behavior of balls containing the suspension) for a given time. During the ball-milling treatment, the milling pot is cooled from its outside in a water bath. Temperature of the water bath is kept at 1.0±0.1°C, and temperature of the suspension is between 1.6°C and 1.7°C within the rotational speed range; this means that the milling pot is cooled enough. After the ball-milling treatment, the obtained precipitate is washed and then dried at 30°C

The dried samples thus obtained were characterized according to standard methods. The powder X-ray diffraction (XRD) pattern of samples was measured with CuKα radiation ranging from 2θ = 10 to 80° at a scanning rate of 1.0°/min using a Rigaku RINT-1500 powder X-ray diffractometer. Fig. 2 shows the XRD pattern of samples obtained at various rotational speeds of the milling pot. In all the XRD patterns, clear diffractions indicating Fe(OH)2 phase were not observed because it tends to form amorphous phase. Before the ball-milling treatment, the sample contained amorphous α-FeOOH and Fe3O4 phases. As the milling time elapsed, α-FeOOH gradually disappeared and finally the single-phase of Fe3O4 formed. This reaction can be attributed to the application of the mechanical energy generated by collision of the balls to α-FeOOH and Fe(OH)2. The reaction rate depended strongly on the rotational speed. The time required for completing the Fe3O4 formation reaction was reduced with increasing in the rotational speed; the reaction almost completed in 12 h at 35 rpm, in 9 h at 70 rpm, in 7.5 h at 105 rpm, and in 6 h at 140 rpm. On the other hand, when the ball-milling treatment was not conducted, i.e., the suspension was kept cooling statically at below 2°C in the water bath, the XRD pattern was almost the same as that of the staring precipitate even after 12 h. It was confirmed that the ball-milling treatment promoted the solid phase reaction by the mechanochemical effect. At early stages of the ball-milling treatment, α-FeOOH seemed to increase from the initial. Actually,

Fig. 1. Schematic illustration of tumbling ball mill used in this work.

high crystallinity without using any environmental-unfriendly additives (Iwasaki et al., 2008, 2009, 2010, 2011b). This chapter describes the outline of this method and the kinetic analysis of the mechanochemical process.
