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Project 4: Advanced Permanent Magnets for Magnetic Refrigeration

 

            Permanent magnets play an important role in modern society because of the outstanding properties of the magnets that can generate magnetic field without the need for power from any source. Permanent magnets are applied in a variety of devices in everyday life. A variety of permanent magnet applications result in different permanent magnet material requirements, depending on the application. The main considerations in choosing the permanent magnets is the strength of the magnetic field, and the ability of the magnetic material to maintain magnetic properties when operating in a reverse strong magnetic field or in hostile environments. More importantly, the key consideration is the cost. Bonded strontium ferrite ( SrFe12O19 , SrO6Fe2O3) permanent magnet is commonly used as magnetic stickers on a fridge wall. Sintered ferrite magnets used in magnetrons help to generate microwaves in the microwave ovens. The permanent magnet known as Alnico, which contains Fe, Al, Ni and Co as the main composition, is a magnet that is generally used in various sensors, for examples, in anti-theft devices and in many vehicle subsystems. Expensive permanent magnets with high energy density generate a relatively high magnetic field and is known as “Neo” magnet ( Nd2Fe14B, Nd is Neodymium, Fe Iron and B are elements of Boron.) Bonded Neo magnets are used in motors to rotate CDs, DVDs and Blue ray discs. Sintered Neo magnet is used to drive the read-write head across current hard drives in computers. Permanent magnet with high power density are critical to the development of multiple technologies, such as technology-driven electric motors with high efficiency, and energy harvesting technologies using high performance generators to produce electricity from natural energy sources. The demand for permanent magnets with high energy density, both in terms of quantity and performance, is likely to increase.

 



Figure 4.1 Some applications of permanent magnet, (a) magnetic stickers, (b) magnetic sensors,
(c) electricity-generating wind turbine and (d) magnetic refrigerator.


           At present, Neo magnet is the most used material in the production of high performance permanent magnets because it has a highest energy density than other commercially produced magnetic materials. The development of permanent magnet material at present emphasizes the addition of some additives to suit the application, such as the addition of Dy (Dysprosium) to increase efficiency when the temperature of the magnet increases. However increasing the use of rare earth elements will result in higher prices as the demand for permanent magnets due to rare earths is limited. Therefore, the development of permanent magnets for medium-term applications, which are in the period of not more than 10 years from now, must focus on reducing the use of rare elements. The permanent magnets shall be rare-earth free permanent magnets. Advances in nanotechnologyled to the success to synthesize high energy density nanoparticles of rare-earth free magnetic materials, such as Fe16N2, MnBi and FePt nanoparticles. With limited availability of Bi and Pt in nature, mass production of MnBi and FePt nanoparticles at low cost is questionable. On the other hand, iron and nitrogen are naturally abundant elements. Thus, the magnetic Fe16N2 nanoparticle is quite attractive. In addition, there has been impressive calculated results showing that Fe16N2 nanomaterials has the energy density up to 130 Megagauss-Oersted (MGOe), which is more than twice the highest measured value ever reported for the well-known Nd2Fe14B.

 

           Iron nitride based compounds are chemically diverse and thus are expected to be used in various applications. For its magnetic properties, it has been reported that epitaxial films of Fe16N2 have the highest magnesium saturation (Ms, saturation magnetization) among all metal compounds. Thus, Fe16N2 films were once expected to be used as a storage medium for hard disks. Meanwhile, nanoparticles of Fe16N2 is also a promising material for permanent magnets with high energy density. Recently, a lot of efforts have been made to synthesize Fe16N2 by various techniques. Thin film of Fe16N2 is generally prepared by magnetron sputtering technique. The production of Fe16N2 nanoparticles is by means of a chemical process called thermal nitration. It has been reported that Fe16N2 nanoparticles are unstable in normal atmosphere at a temperature of 200°C and above. Recent theoretical calculations has shown the possibility of structural stabilization by adding transition elements to the compounds Fe15TMN2 and Fe14TM2N2 (TM is a transition element). These compounds still exhibit magnetic properties. In addition to magnetic compounds, there has been reported on the development of nanocomposite between magnetic nanoparticles with high saturated magnetization and high coercivity. As long as the permanent magnets are important components that change the mechanical force into electrical force, and vice versa, and most importantly, the scope of magnetic applications is expanded to cover the cooling system in the future, such as a magnetic refrigerator, the demand for high performance permanent magnets will be increased inevitably.

 

           The main goal of this research project is to develop rare-earth free permanent magnets with high energy density and high stabilities in structural and magnetic properties. The initial phase of this work emphasizes on the investigations of phases changes of iron nitride thin films upon changes in temperature. A real-time monitoring techniquefor the study of phase changes is conducted under a special electron microscope known as LEEM/PEEM (low-energy /photoemission electron microscope). The iron nitride films are prepared by magnetron sputtering technique. It is expected that the information from LEEM/PEEM will be useful in determining the conditions for the preparation of iron nitride based magnetic nanoparticles. In addition, a time-resolved synchrotron X-ray absorption spectroscopic technique will be employed for real-time monitoring for mass production of iron nitride based permanent magnetic nanomaterials.

 

References

[1] J.-S. Chen, C. Yu and H. Lu, “Phase Stability, Magnetism, Elastic Properties and Hardness of Binary Iron Nitrides from First Principles”, J. Magn. Magn. Mater. 625 (2015) 230.
[2] J. Huang, W. Xie and X. Li, “The Stability, Magnetism and Electronic Structure of Fe15TMN2 and Fe14TM2N2 (TM = Cr, Mn, Co, and Ni)", J. Magn. Magn. Mater. 364 (2014) 1.
[3] T. Ogawa et. al., “Challenge to the Synthesis of α''-Fe16N2 Compound Nanoparticle with High Saturation Magnetization for Rare Earth Free New Permanent Magnetic Material”, Appl. Phys. Express 6 (2013) 073007.
[4] S. Yamamoto et. al., “Quantitative Understanding of Thermal Stability of α''-Fe16N2”, Chem. Commun. 49 (2013) 7708.
[5] N. Ji et. al."N site ordering effect on partially ordered Fe16N2", Appl. Phys. Lett. 98 (2011) 092506.
[6] S. Kikkawaet. al., “Fine Fe16N2 Powder Prepared by Low-temperature Nitridation”, Mater. Res. Bull. 43 (2008) 3352.
[7] E. Kitaet. al., “Magnetic Properties of Core–shell Type Fe16N2 Nanoparticles”, J. Magn. Magn. Mater. 310 (2007) 2411.

 

Principal Investigator: Associate Professor Dr. Prayoon Songsiriritthigul 1)
Collaborators: Dr. Mati Horprathum 2), Assoc. Prof. Dr. Supree Pinitsoontorn 3), Dr. Yingyot Puarporn 4), Dr. Hideki Nakajima 4)
Affiliated Institutes: 1) School of Physics, Institute of Science, Suranaree University of Technology, 2) National Electronics and Computer Technology Center (NECTEC), 3) Department of Physics, KhonKaen University, 4) Synchrotron Light Research Institute

 

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