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Project 3 : Nanostructured Permanent Magnets for Magnetic Refrigeration

 

            Permanent magnets are vital components in electrical appliances and generators. Magnets with high energy products increase the energy generated and the efficiency in energy conversion. Types of permanent magnets commonly available in the market are ferrites which are inexpensive but moderately efficient and rare earth magnets which have high energy products and magnetic field supplied. However, the development of these magnets approaches the saturation and rare earth materials such as neodymium (Nd) and dysprosium (Dy), exclusively supplied by China, are decreasing. The shortage of raw materials for Nd-Fe-B and Sm-Co magnets is therefore impending. The new technology is therefore needed for electrical appliances, generators, wind mills, energy harvesters as well as future magnetic refrigerators. The research and development on novel magnets is in good agreement with the strategic development plan to encourage new industry based on nanotechnology in Thailand.

 

           The concept of novel permanent magnets is based on the combination of nanostructured hard magnets with soft magnetic materials, hence referred to as hard/soft magnetic nanocomposites. The soft phase impedes the domain wall movement of the hard phase. Whereas the rare earth magnets exhibit the maximum energy product and coercive field, the synthesis of rare earth nanostructures prove unsuccessful. Since typical materials lose the ferromagnetism when their size is reduced below the superparamagnetic limit, the research on new nanostructured magnet has been focused on three materials as follows;

 

           Iron-Platinum (FePt) under 10 nm in diameter still retains ferromagnetism. It has been under research and development for ultrahigh density hard disk drive. FePt nanoparticles of uniform size and shape can be synthesized. The thermal treatment enhances the energy product of as-synthesized nanoparticles without excessive agglomeration. However, the composition cannot be fully controlled and Fe(CO)5 used as starting reagent is highly toxic. The cost of materials is also a limiting factor for the commercial use in future magnetic refrigerator.

 

            Manganese-Bismuth (MnBi) is an intermetallic compound and its ferromagnetism is related to the hexagonal structure with low temperature phase. The coercive field of MnBi is high due to magnetocrystalline anisotropy. In contrast to conventional trend of decreasing coercive field at high temperature, MnBi uniquely has the positive temperature coefficient. It can be prepared by several techniques including arc melting, melt spinning, solid state sintering and ball milling. The ball milling can also enhance the coercive field by virtue of grain size reduction. However, the synthesis to obtain desirable phase with sufficient yield for commercial production requires further research and development.

 



Figure 3.1 Hysteresis loops of FePt nanoparticles before (a-b) and after (c-d) the thermal treatment.



Figure 3.2 Hysteresis loops of pressed MnBi pellets in the ratio Mn:Bi = 1:1, 2:1 and 3:1 after sintering at 1000 °C.

            Cobalt Carbide (ConC, n=1-6) has received less attention than FePt and MnBi. There are a few reports on successful chemical synthesis of nanoparticles with high energy product. It is therefore challenging to improve the yield for the commercial scale process since ConC is much less expensive than FePt. When the appropriate process is realized, the hard phase will be used in the compaction into hard/soft magnetic composite.

 

 

Principal Investigator: Associate Professor Dr.Chitnarong Sirisathitkul 1)
Collaborators: Dr. Thanida Charoensuk 1), Assist. Prof. Dr. Phimphaka Harding 2), Dr. Komkrich Chokprasombat 3), Dr. Chat Pholnak3)
Affiliated Institutes: 1) Department of Physics, Walailak University, 2) Department of Chemistry, Walailak University, 3) Department of Physics, Thaksin University

 

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