Enhancement of Practical Energy Product by Mircostructure Modeling

Abstract

Devices such as electric vehicles and wind power generators utilize the magnetic energy of permanent magnet. To meet the increasing demand for such devices, researchers have actively developed permanent magnets with higher efficiencies and lower cost. Magnetic energy is represented by the energy product BH, which is twice the energy stored in the stray field outside the magnet. It can be obtained from the volume integral of the square of the stray field outside the magnet, or from the volume integral of the dot product between the demagnetizing field Hd and the internal magnetic flux density B. The maximum energy product, (BH)max is widely used as a figure of merit for evaluating the performance of hard-magnetic materials. (BH)max should be evaluated rigorously from the hysteresis loop by considering the exact working point, which is determined by the shape of the magnet. However, many researchers still use (BH)max obtained from the hysteresis loop of a certain shape as a representation of the material, even though the only information BH from a certain shape of the magnet is an energy product at the remanent state. Thus, correctly calculated (BH)max was compared with that of obtained from conventional way through several models by using micromagnetic simulation. To obtain high energy product for permanent magnet, high saturation magnetization and coercivity is essential. However, there is still large discrepancy between theory and practice. This clear contradiction is called Brown’s paradox. One of the main reasons for the paradox is known as microstructure including grain properties, grain boundary, crystalline anisotropy distribution, and so on. Despite this importance, the effect of microstructure on magnetic properties have not much been studied. Thus, the effect of various microstructure has been discussed by theoretical and numerical methods, and then the microstructures having high energy product are proposed. Furthermore, cylindrical core/shell exchange-coupled magnet having diverse conditions by microstructure modeling has been investigated.