Journal of Advanced Materials in Engineering (Esteghlal)

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The effects of temperature, time and atmosphere on microstructure and magnetic properties of NiFe2O4 glassceramic were investigated utilizing differential thermal analysis, X-ray diffraction, vibrating sample magnetometer and scanning electron microscope techniques. Various compositions were studied in the Na2O-NiO-Fe2O3-B2O3-SiO2 system to obtain amorphous phase. The sample heat-treated in graphite bed at 510°C for 1 hr showed higher magnetization than the one heattreated in the air under the same condition. XRD analysis showed the presence of nickel ferrite and some non-magnetic phases such as sodium borate and silicate phases in the heat treated samples. The maximum magnetization of samples reduced by increasing the holding time from 1hr to 3 hr at 510°C. Increment of temperature to 700°C increased the amount of NiFe2O4 and maximum magnetization.

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Nickel ferrite based cermets and their relevant composites have been widely used as inert anodes for aluminum electrolysis due to their good combination of chemical resistance, thermal stability and mechanical properties. In this study, various NiO/NiFe2O4 composites consisting of 5, 10 and 15% NiO in conjunction with Cu/NiFe2O4 cermets containing 0.5, 10 and 15% Cu were prepared by powder metallurgy method. The degradation resistance of the developed inert composites was examined under hot corrosion condition by plunging samples in to the molten electrolyte at 1000ºC. The strength, toughness, hardness, relative density, microstructural observation, phase analysis and electrical resistivity were evaluated by 3-points bending tests, Vickers method, Archimedes method, scanning electron microscope, x-ray diffraction and conventional direct current four-probe techniques, respectively. The experimental results for NiO/NiFe2O4 composites showed that a significant improvement of toughness and degradation resistance continuously occurred with a moderate decrease in strength by increasing NiO content, while the relative density was increased only up to 5%NiO content. By increasing the Cu content in the cermet samples, all the properties such as strength, toughness and electrical conductivity were improved considerably but the degradation resistance decreased.

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In the present investigation, spherical nanoparticles of nickel ferrite with uniform structure were successfully produced by hydrothermal method in the presence of polyethylene glycol (PEG) as a polymeric surfactant at 180°C for 12 hour aging time and the effects of the synthesis time, temperature and surfactant were investigated. According to the X-ray analysis, conversion of nickel oxide and hematite to nickel ferrite was a way to produce NiFe2O4. At 140‌°C, agglomerated particles without specific shape were formed, but at 180°C particles were homogenous with spherical shape. Saturation magnetization increased by increasing the hydrothermal process aging time.

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In this research, nickel ferrite nanoparticles were synthesized by sol-gel auto-combustion route, and the effect of calcination temperature on phase constituents, magnetic properties and microstructure of the synthesized nanoparticles was evaluated using X-ray Diffraction (XRD), Vibrating Sample Magnetometer (VSM) and Scanning Electron Microscopy (SEM). XRD results were submitted to quantitative analysis. Microstructural studies and crystallite size calculations showed formation of nanoparticles. XRD results showed that the combustion product consisted of NiFe₂O₄, α-Fe₂O₃, NiO, and FeNi₃ phases. FeNi₃ was eliminated by calcination, and the amounts of NiO and α-Fe₂O₃ were modvlated by changing in calcination temperature. Saturation magnetization changed from 37emu/g in combustion product to 30emu/g by calcination at 600°C, due to decomposition of FeNi₃ magnetic phase and formation of higher amount of antiferromagnetic hematite phase. Also, the coercivity values increased, that could be due to increasing the amount of nickel ferrite phase and eliminating FeNi₃ phase. Saturation magnetization reached to 43emu/g in calcinated sample at 1000°C due to the reaction between hematite and NiO phases that led to formation of higher amount of nickel ferrite to 43emu/g. Coercivity value dropped out to 127Oe by calcination at 1000°C, the reason of which could be incresing of particle size and formation of multi domain magnetic particles.