**4. Conclusion**

252 Materials Science and Technology

determined was considerably larger than the activation energy in conventional liquid-phase reaction systems because the impact energy was the maximum mechanical energy which α-FeOOH is able to receive. When both the net mechanical energy transferred from the balls to α-FeOOH and the amount of α-FeOOH receiving the energy are known, the true activation

Fig. 18. Variation in accumulative impact energy required for completing Fe3O4 formation

In general mechanochemical processes, when the mechanical energy caused by shear, compression and friction actions of balls applies to particulate materials under dry condition, the surface energy of particles can increase due to the physical change such as distortion of the crystal lattice, increase of the surface area, and appearance of newly formed crystal surface. This causes the mechanochemical activation of particles. Under wet condition, however, the particles are difficult to undergo the mechanochemical activation because the increased surface energy is reduced by the solvent. Furthermore, it is very difficult to apply the mechanical energy effectively to nanoparticles. Accordingly, in this synthesis process, the mechanochemical activation of nanoparticles is difficult to occur, and the solid phase reaction from α-FeOOH to Fe3O4 may hardly proceed by direct contribution of the mechanical energy. However, at the rotational speed of 35 rpm, i.e. in a low mechanical energy field, the Fe3O4 formation reaction surely proceeded while the reaction rate was relatively low. There is no doubt that the applied mechanical energy promotes the reaction; the reaction mechanism in this synthesis process is considered that the reaction may proceed not by the mechanochemical activation of α-FeOOH but by local and rapid heating and/or through a different reaction path. In the conventional methods

energy of the mechanochemical reaction can be determined.

reaction with rotational speed.

A novel process for preparing superparamagnetic Fe3O4 nanoparticles with high crystallinity in water system has been developed, in which a cooled tumbling ball mill is used as the reaction field. It has been confirmed that this method provides successfully the Fe3O4 nanoparticles having a size of less than 15 nm without using any conventional heating techniques. This mechanochemical process was kinetically analyzed, indicating that the Fe3O4 formation reaction obeys the 0.6th-order rate equation. In addition, the mechanical energy (i.e., the impact energy of balls) promoting the Fe3O4 formation reaction was also analyzed using the numerical simulation method. The rate constant of the reaction was investigated based on the mechanical energy. As a result, the apparent activation energy of the reaction was estimated. This mechanochemical process may contribute to the production of superparamagnetic Fe3O4 nanoparticles under environmentally friendly conditions and be applied to another reaction systems synthesizing functional nanoparticles.
