Review

Soft magnetic materials for a sustainable and electrified world

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Science  26 Oct 2018:
Vol. 362, Issue 6413, eaao0195
DOI: 10.1126/science.aao0195

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Faster switching for soft magnets

The most familiar magnets are permanent magnets like the ones on a refrigerator door. However, for applications in transformers and motors, soft magnets that can rapidly switch their magnetization in response to a magnetic field are used. In electronics, wide bandgap semiconductors such as silicon carbide will allow power conversion electronics and motor controllers to operate more efficiently, but soft magnets must be developed that can respond at higher frequencies. Silveyra et al. review the development of current soft magnetic materials and opportunities for improving their performance in high-frequency operation. Materials being explored include soft ferrites, amorphous and nanocrystalline alloys, and powder cores or soft magnetic composites.

Science, this issue p. eaao0195

Structured Abstract

BACKGROUND

Soft magnetic materials and their related devices (inductors, transformers, and electrical machines) are often overlooked; however, they play a key role in the conversion of energy throughout our world. Conversion of electrical power includes the bidirectional flow of energy between sources, storage, and the electrical grid and is accomplished via the use of power electronics. Electrical machines (motors and generators) transform mechanical energy into electrical energy and vice versa. The introduction of wide bandgap (WBG) semiconductors is allowing power conversion electronics and motor controllers to operate at much higher frequencies. This reduces the size requirements for passive components (inductors and capacitors) in power electronics and enables more-efficient, high–rotational speed electrical machines. However, none of the soft magnetic materials available today are able to unlock the full potential of WBG-based devices.

Since the 1800s, when iron was the only soft magnetic material available, metallurgists, materials scientists, and others have been periodically introducing improved materials. The invention of silicon (electrical) steel in 1900 was a notable event for soft magnetic materials. Silicon steel still dominates the global market of soft magnets and is the material of choice for large-scale transformers and electrical machines. However, its low electrical resistance makes it subject to large losses from eddy currents, particularly as operating frequencies are increased. This leaves the soft magnetic community looking to other materials to meet the needs of newer high-frequency devices.

ADVANCES

Several soft magnetic materials show promise for high-frequency operation. As oxides, soft ferrites stand out from other magnetic materials because they are insulating and therefore excel at reducing losses from eddy currents. However, soft ferrites suffer from a saturation magnetization (Ms) that is approximately one-fourth that of silicon steel. This substantially limits the power density in devices designed using soft ferrites and therefore limits their application. The current state-of-the-art materials are the amorphous and nanocrystalline alloys, which were invented in 1967 and 1988, respectively. Their distinctive nanostructures and extremely thin laminations work together to suppress eddy current losses, even at high frequencies. However, fabricating parts by cutting and then stacking or winding extremely thin and brittle laminations can be challenging. Composites are the newest class of soft magnetic materials, and in the field of soft magnetics, they are referred to as powder cores or soft magnetic composites. These materials are composed of micrometer-sized particles coated with an insulating binder and consolidated at high pressures. Although their performance is rather modest, their isotropic nature makes them well suited for use in rotating electrical machines.

OUTLOOK

The need for improved soft magnets capable of efficient operation at high frequencies is capturing the attention of a growing number of researchers. Improvements to existing materials are being developed by some, whereas others are exploring radically new approaches. Even though ferrites were invented in the 1940s, grain-boundary engineering and new syntheses compatible with integrated manufacturing are being pursued. The nanocrystalline and amorphous materials are constantly being improved through increases in Ms and the introduction of alloys that are more amenable to the fabrication of large-scale parts. Powder cores have opened the door for nanoparticle-based composites, which can be fabricated with top-down as well as bottom-up approaches. A well-designed nanocomposite has the potential for negligible losses over specific temperature and frequency ranges. In all materials, advanced characterization techniques will be paramount to understanding the relationship between nanostructure and magnetization reversal, which will then enable the design of improved soft magnets.

Two views of soft magnetic materials.

(Left) A row of three wound toroidal inductors mounted on a printed circuit board. (Right) Magneto-optical Kerr effect image of the magnetization pattern on the surface of an amorphous alloy soft magnet.

CREDITS: (LEFT TO RIGHT) BILLNOLL/ISTOCK; R. SCHAEFER ET AL., IEEE TRANS. MAGN. 37, 2245 (2001)

Abstract

Soft magnetic materials are key to the efficient operation of the next generation of power electronics and electrical machines (motors and generators). Many new materials have been introduced since Michael Faraday’s discovery of magnetic induction, when iron was the only option. However, as wide bandgap semiconductor devices become more common in both power electronics and motor controllers, there is an urgent need to further improve soft magnetic materials. These improvements will be necessary to realize the full potential in efficiency, size, weight, and power of high-frequency power electronics and high–rotational speed electrical machines. Here we provide an introduction to the field of soft magnetic materials and their implementation in power electronics and electrical machines. Additionally, we review the most promising choices available today and describe emerging approaches to create even better soft magnetic materials.

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