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There is great demand for nanoparticles (NPs) dispersed in liquid phases for practical applications of functional NP materials. However, it is difficult to produce NP dispersions with specific particle sizes, concentrations, viscosities, and purities on an industrial scale (large mass production rate and low energy consumption). In this review, we highlight recent developments in NP dispersion using low-energy bead mill. Such processes enable the use of small beads (7-50 μm). Smaller beads reduce the collision and shear energies of NPs during agitation. This minimizes NP breakage/damage, and retains the shape and crystallinity of the NPs, which determine the inherent NP functions. This review starts with a brief explanation of the theory and current status of NP dispersion and describes the mechanism and experimental results for low-energy bead mill processes, i.e., using uniaxial, dualaxial, and all-separator bead mills, and selection of dispersing agent. Applications of NP dispersions, including nanocomposite materials, and methods for dealing with NP dispersion coloration are also discussed, along with future research directions.

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Magnetic materials such as α″-Fe16N2 and α-Fe, which have the largest magnetic moment as hard and soft magnetic materials, are difficult to produce as single domain magnetic nanoparticles (MNPs) because of quasistable state and high reactivity, respectively. The present work reports dispersion of agglomerated plasma-synthesized core-shell α″-Fe16N2/Al2O3 and α-Fe/Al2O3 in toluene by a new bead-mill with very fine beads to prepare single domain MNPs. As a result, optimization of the experimental conditions (bead size, rotation speed, and dispersion time) enables the break-up of agglomerated particles into primary particles without destroying the particle structure. Slight deviation from the optimum conditions, i.e., lower or higher dispersion energy, gives undispersed or broken particles due to fragile core-shell structure against stress or impact force of beads. The dispersibility of α″-Fe16N2/Al2O3 is more restricted than that of α-Fe/Al2O3, because of the preparation conditions. Especially for α″-Fe16N2/Al2O3, no change on crystallinity (98% α″-Fe16N2) or magnetization saturation after dispersion was observed, showing that this method is appropriate to disperse α″-Fe16N2/Al2O3 MNPs. A different magnetic hysteresis behavior is observed for well-dispersed α″-Fe16N2/Al2O3 MNPs, and the magnetic coercivity of these NPs is constricted when the magnetic field close to zero due to magnetic dipole coupling among dispersed α″-Fe16N2 MNPs.

Well-dispersed spherical core-shell α''-Fe16N2/SiO2 ferro-magnetic nanoparticles were successfully synthesized from core-shell α-Fe/SiO2 nanoparticles. Introduction of oxidation prior to the nitridation process gives 90% of α''-Fe16N2 phase contained in the core while no phase change is observed without oxidation. Saturation magnetization and coercivity are 148 emu g(-1) and 1.82 kOe, respectively.