Particles with dimensions less than 100 nm that display special magnetic properties different from the bulk material are known as magnetic nanoparticles. Due to the effects of smaller size and greater surface area, this type of nanoparticles demonstrates fascinating size-dependent magnetic properties. Particularly, special magnetic property of superparamagnetism has been observed in some small ferromagnetic or ferrimagnetic nanoparticles. Similar to a paramagnetic material, superparamagnetic nanoparticles can be magnetized by an external magnetic field. However, their magnetic susceptibility is much larger than that of paramagnets. In fact, superparamagnetism is a distinction between the behavior of the small magnetic moment of a single paramagnetic atom and that of the much larger magnetic moment of a nanosized magnetic particle which arises from the coupling of many atomic spins. After elimination of magnetic field, the particles no longer show magnetic interaction; a feature that is important for their usability. Synthesis of magnetic nanoparticles via water in oil microemulsion method is under attention due to its tendency for synthesis the wide range of materials with controllable sizes and morphologies. In this method, nanoparticles are synthesized in small water droplets distributed in an organic phase by the means of a suitable surfactants. Different parameters that may affect size and morphology of microemulsion synthesized particles such as type of organic phase and surfactant, aqueous/organic/surfactant ratio, type and concentration of the reactants, temperature, pH, mixing method, rate of mixing, stirring method and stirring time. In microemulsion method, synthesis of particles with desirable size and morphology is related to finding the effects of process parameters and their interactions. Due to the variety of parameters affected the process, a proper approach required to study simultaneous effects of process parameters on the desirable outputs. Design of experiments (DOE) is a collection of mathematical and statistical useful techniques for developing, improving and optimizing the processes. This paper summarized microemulsion synthesis of Ni nanoparticles and investigated the effects of main synthesis parameters on the size of particles. Statistical method of Fractional Factorial Design (FFD) was used to evaluate effects of process parameters and find the most significant parameter affecting synthesis of Ni nanoparticles. A liner model with high correlation of 96.84% presented for predicting the mean size of synthesized Ni nanoparticles based on the microemulsion parameters. Accordingly, synthesis temperature and NiCl2.6H2O concentration have the most effects on the size of Ni nanoparticles, respectively. The next important parameters are pH, N2H4.H2O concentration, synthesis time and NiCl2.6H2O-temperature interaction. Mean particle size of Ni nanoparticles decreases with increasing the pH and N2H4.H2O concentration and decreasing the NiCl2.6H2O concentration, synthesis temperature and its time. As well, higher synthesis temperatures lead to more growth of Ni nanoparticles at increased concentrations of NiCl2.6H2O. Nanoparticles synthesized at optimum conditions have spherical morphology and average size of 4 nm. Magnetic properties measurement using vibrating sample magnetometer (VSM) indicated interesting relationship between the magnetic properties and size of nanoparticles, is such a way that decreasing the size of the nickel nanoparticles to 4 nm, transferred ferromagnetic nanoparticles to superparamagnetic ones.

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