RESEARCH TOPICS

Background: Breaking common senses of soft magnetic materials

Materials with low core loss and high saturation flux density have acquired from markets, but this has not long been achieved to date. In developing soft magnetic materials, core loss and saturation flux density(Bs) are cooperative factors, demonstrating that Bs decreases/increases with decreasing/increasing core loss.
It is widely accepted that soft magnetic materials with high permeability and low core loss include ferrite, Co-based amorphous alloys, Ni-Fe alloys (Fig. 1). However, these materials generally possess Bs approximately only one tenth of the pure iron, which exhibits an extremely high Bs. In contrast, iron and silicon steel with high Bs exhibit as small permeability of the order of one tenth as Co-based amorphous alloys with high permeability.
The ultra high efficiency nano-crystalline soft magnetic material comprising iron-based alloy in the present research is an epoch-making material in point overturning common sense of soft magnetic materials for its achievements of low core loss as well as the conventional high permeability and high Bs as silicon steels.

Breaking common senses of soft magnetic materials

Fig. 1 Development road map of ultra low loss nanocrystalline alloy"NANOMET®"

Ultra High Efficiency Nano-Crystalline Soft Magnetic Material:"NANOMET®"

"NANOMET®" is a material with high iron-content of 93~94 wt.%, which has been developed by our group. In other words, "NANOMET®" is an ultra high efficiency nano-crystalline soft magnetic material with high iron content type and with structure of α-Fe with a diameter of 10 nm surrounded by a layer of non-magnetic materials. (trademark applied, "NANOMET®"). The non-magnetic layer is comprised of conventional elements, such as Si, B, P Cu and so forth. "NANOMET®"is an ideal material with low risk against remarkable rising up of the price of the elements owing to its feature — rare metal free.
"NANOMET®" provides expected applications that are demanded in the field using strong magnetic fields because of its low core loss and high Bs. An especial great contribution is expected for large current transformers for power grid and motors. Adopting the present nano-crystalline materials to transformers and motors enable us to saving energy due to reducing core loss.
Transformers and core materials, the latter of which are placed inside the coil of motors, are requested to meet flux density of 1.5 T, and thus, generally silicon steel is used in such applications. When comparing core loss under a condition of 50 Hz and 1.5 T, one can see that "NANOMET®" exhibits as mall core loss of the order of one tenth or slightly greater as the silicon steel, indicating that "NANOMET®" is high functional nano-crstalline soft magnetic material, which has not obtained yet (Fig. 2).

Ultra High Efficiency Nano-Crystalline Soft Magnetic Material

Fig. 2 Dependence of W on Bs of "NANOMET®"

Self-Assembly Nano-Heterogeneous Amorphous Structure

As-quenched Fe-based amorphous alloys and amorphous alloys as a precursor for nano-crystalline soft magnetic materials that are currently on applications to date should be formed into an amorphous single phase to achieve soft magnetic properties, restricting that Fe content of these alloys are ought to be 78 at.% or less. In reality, a single amorphous phase has not been formed for Fe-based alloys with high Fe content more than 80 at.%. Because of these circumstances, generally one can obtain the alloys with α-Fe with diameters ranging several tens to 100 nm and are inhomogeneously dispersed, leading to difficulty to exhibit high magnetic properties even though the alloys are prepared by melt-spinning.
In strong contrast, the present group has succeeded in finding out nano-crystallization caused by peculiar structure by taking into account the factors of mixing enthalpy among the constituent elements and small interfacial energy between CuP compound and α-Fe. As a result, as the first time in the world the present group has developed a new nano-crystalline FeBSiBPCu alloy (Fig. 3) in a ribbon shape with high Bs as silicon steel and excellent soft magnetic properties. The nano-crystalline phase precipitates with a help of α-Fe with a diameter of 3 nm or less as nuclei from heterogeneous amorphous phase when an optimal annealing is subjected to the alloy. Extremely excellent soft magnetic properties have been achieved through self-assembly structure comprising α-Fe as a major phase and residual amorphous phase at the grain boundaries as the minor phase.

Self-Assembly Nano-Heterogeneous Amorphous Structure

Fig. 3 Self-assembly hetero amorphous microstructure of FeSiBPCu nanocrystalline alloys

Minimizing Core Loss for Energy Saving:

It is said that core loss from transformers and motors accounts for 3.4% of the total electric power consumed domestically, and it reaches to 33.5 billion kWh a year. If the silicon steel used in all the transformers and motors is substituted for "NANOMET®", then, the loss of electric power can be reduced to 9.6 billion kWh (72% reduction) depending on the conditions of usage (Fig. 4). This reduction accounts for the seven thermal power stations. Furthermore, further reduction of core loss as small as to be 4.0 billion kWh is available by further improvement by the optimization of the materials.
In order to achieve this goal, the present group enthusiastically performs researches of (1) minimization of loss, (2) widening the width of the nano-crystalline ribbon materials and performing collaborative researches with aims to applications and mass production of the alloy, and (3) performing conventional researches on processing for application parts via bulk-shaping utilizing powder metallurgy and laminating layers, and increasing thickness of the ribbon materials as well. Furthermore, the following researches are also performed to clarify magnetic coupling and mechanism on generating core loss in nano-crystalline materials based on magnetic theories, structure and morphology analysis, as well as material production based on the optimal alloy design for mixing enthalpy. Besides, other researches on the development of new production method for powders and highly-precise layer-laminating technology are also investigated in the present project. In summary, the present project will contribute to human society by saving energy as a principal item by the achievement of Fe-based nano-crystalline soft magnetic materials with a highly flexibility.

Minimizing Core Loss for Energy Saving

Fig. 4 Dedication of "NANOMET®" to improvement of power loss