Synthesis of Magnetic Nanofibers for Their Potential Applications in Wireless Charging for Energy Efficient EVs

There is a growing demand to find new materials for flux guiding applications in the inductive power transfer systems. The inductive power transfer system (IPT) in electric vehicles has gained importance due to advantages including safety, efficiency, flexibility, and user-friendliness. The potential of IPT systems in dynamic flux transfer on-road requires the flux concentrators to maximize flux transfer at minimum power losses. Ferrite materials are the most commonly used materials in the IPT cores for flux-transfer applications due to their reasonable susceptibility, high resistivity, and semi-conducting nature. Their high resistivity is advantageous to minimize eddy-current losses. However, there are certain drawbacks to their use, for example, they can still result in some eddy-current losses as their resistivity is not high enough, they are very brittle in nature and are difficult to be made in thin sheets or in arbitrary shapes, and they are very heavy that increases the vehicle mass and energy consumption. An ideal material for wireless charging applications should be thin, and flexible that can be made in arbitrary shapes with a reasonable susceptibility and high resistivity for wireless charging applications.

This thesis aimed to investigate the preparation of bimetallic Ni1−xFex, semi-conducting MnFe2O4, and Sm3+ doped semi-conducting MnFe2−xSmxO4 nanofibers made by an electrospinning method for their potential applications in wireless charging. Ni1−xFex was selected because the bulk material shows a high permeability which is useful for flux-guiding, but it has a low resistivity. However, the preparation of thin Ni1−xFex nanofiber sheets can result in high resistivities in nanodimensions. The parameter x was studied by varying its value between 0.1-0.5 as Ni1−xFex show high susceptibility at x∼0.2, and high magnetic moment at x∼0.5. The characterizations showed the presence of Ni1−xFex nanoparticles formation within nanofibers for all the samples which can be advantageous to further increasing the resistivity and to reduce eddy-current losses. A bimodal particle size distribution was observed at x∼0.1, that became less bimodal at x∼0.2 and skewed with predominantly small nanoparticles at x∼0.5. Superparamagnetic behaviour was observed at x∼0.5 due to the formation of smaller nanoparticles. There was a systematic increase in the differential susceptibility with increasing x from 6 at x∼0.1 to 18 at x∼0.5. These results were encouraging for potential applications in wireless charging.

Semiconducting MnFe2O4 nanofibers were made by electrospinning method because bulk material has high resistivity. The thermal processing of these nanofibers at 700◦C resulted in large polycrystalline nanoparticles whereas, the thermal processing at lower temperature 620◦C resulted in a mixture of small nanoparticles within these nanofibers. The presence of some single crystal nanorods was also seen in both samples. The high field magnetization was largest for the sample processed at higher temperature, i.e., 57% of the total magnetization value at 700◦C as compared to 46% obtained at low temperatures at 620◦C. However, one sample processed at 620◦C showed the complete formation of MnFe2O4 nanorods with the highest saturation magnetization to 76% of the total magnetization. These results are encouraging as the potential of nanorod synthesis can be useful in the flux guiding applications. Electrospun MnFe2−xSmxO4 nanofibers with Sm3+(x) doping were also made to observe the change in the structural and magnetic properties at x=0.06-0.25. The results showed the successful incorporation of Sm3+ in the crystal structure of MnFe2O4 at x≤0.2. Polycrystalline nanoparticles were seen at low fractions of x≤0.1 but more smaller nanoparticles were formed at x=0.2 and x=0.25. Superparamagnetic behaviour was observed at high fractions of x (x=0.2 and x=0.25). The saturation magnetization for x≤0.2 was largest for x=0.06. This study has provided a solid foundation for a more in-depth exploration of magnetic electrospun nanofiber sheets for future applications in wireless charging systems.