Synthesis of Hard Magnetic Materials For Rare-Earth-Free Permanent Magnets Applications

Abstract

Permanent magnets have been attracting increasing attention due to their wide applications in our daily life such as electric motors, wind turbines, speakers, and magnetic storage, etc. Rare-earth permanent magnets, such as NdFeB and SmCo, are widely used in these areas because of their good magnetic performances. However, due to environmental degradation concerns on mining rare-earth elements, it is very important to search and investigate potential candidates for permanent magnets without rare-earth elements. Furthermore, it is better that these candidates are composed of earth-abundant elements besides possessing required high magnetic anisotropy and large saturation magnetization (Ms) to obtain high maximum energy product (BH)max and large remanent magnetic flux density (Br). Body-centered tetragonal (bct) Fe and α”-Fe16N2 are promising candidates because of the good magnetic properties and abundance of Fe and N in nature. Iron is known as a soft magnetic material when it is with a body-centered cubic (bcc) crystalline structure. Soft magnetic iron and iron alloy are widely used in power generators and transformers. When the iron lattice is distorted along its c axis to form a bct structure, the asymmetry of the crystal structure induces a much higher magnetocrystalline anisotropy than that of bcc Fe. Meanwhile, the Ms of bct Fe is similar to or even higher than that of bcc Fe, which makes bct Fe a promising candidate for rare-earth-free magnets. There is no report on the synthesis of bct Fe nanoparticles (NPs) before this thesis work. In this thesis, bct Fe NPs are synthesized by a sputtering-based gas-phase condensation (GPC) system that can provide proper growth conditions and cooling rates for the formation of bct Fe NPs. The magnetic anisotropy of bct Fe NPs is about seven times higher than that of bcc Fe. The GPC system is under high vacuum conditions and can make both NPs and thin films, which is an ideal system for making a nanocomposite where NPs are embedded in a thin-film matrix. Nanocomposite samples are made and investigated, in which bct Fe NPs are embedded in an antiferromagnetic MnN matrix. The exchange coupling between NPs and MnN matrix helps enhance the coercivity and remanent ratio of bct Fe NPs, which can provide an alternative way to design magnets. Another promising rare-earth-free magnet candidate is α”-Fe16N2, which is known for its giant Ms. It also has a relatively large magnetocrystalline anisotropy of ~1.8×107 erg/cm3. Besides, both Fe and N are earth-abundant elements. In this thesis, α”-Fe16N2 ribbons and foils (~20-25 μm thick) synthesized by a low-temperature nitriding approach are reported for the first time. Compared to NPs, ribbons and foils are easy to process and good for mass production. However, it is difficult to make ribbons and foils with a high phase ratio of α”-Fe16N2 by low-temperature nitriding because their thicknesses are several hundred times larger than the diameters of NPs. The critical requirements to prepare α”-Fe16N2 ribbons and foils are to enhance the nitrogen diffusivity in these samples. Oxidation and hydrogen reduction processes are used as pretreatments on the ribbon and foil samples to modify their microstructures. After these pretreatments, a porous structure is created. This porous structure facilitates the diffusion of nitrogen into the samples. Besides, nano-size channels can also be created inside iron grains of the hydrogen reduced samples, which can improve the reactivity of Fe and nitrogen and obtain a high phase ratio of α”-Fe16N2. The Ms of the obtained α”-Fe16N2 foil samples is up to 222 emu/g, which is slightly higher than that of bcc Fe. And the achieved coercivity is 1.1 kOe, which can be further enhanced by decreasing the exchange interactions between α”-Fe16N2 grains. An ultralow positive temperature coefficient of coercivity (0.4 Oe/K) is obtained from the synthesized α”-Fe16N2 foils in the temperature range from 300 to 425 K. Rare-earth permanent magnets have a large negative temperature coefficient of coercivity, like NdFeB (~70-80 Oe/K) in the similar temperature range. The α”-Fe16N2 foils with an ultralow positive temperature coefficient of coercivity are very attractive for the applications at varying temperatures such as high-performance audio devices.