**1. Introduction**

Permanent magnet materials are important for industry, military, and information technology [1–4]. At present, in an environment where green and clean energy is promoted, the demand for permanent magnet materials in the fields of wind power, electric vehicles, electric motors, robots, and aerospace is continuously increasing [5]. Among the permanent magnets, the demand for Nd-Fe-B increased the most, reaching 2.1×104 t production in 2020. However, after 2011, the price of rare earth soared due to fluctuations in prices [1]. At the same time, Ce-containing dual-main phase (DMP) Nd-Fe-B magnets emerged and became popular due to their balanced utilization of high-abundance rare earth elements [6]. DMP magnets are made by adding Ce-Fe-B during the preparation of Nd-Fe-B magnets to form magnets with both Nd-Fe-B and Ce-Fe-B main phases, and they have excellent magnetic properties compared to single main phase Nd-Fe-B magnets [7,8].

As the temperature increases, all magnets inevitably face the problem of irreversible demagnetization, and the above means only alleviating the problem to a certain extent. After the high temperature demagnetization of the magnet, it still has enough magnetic properties for use. The ideal magnet should have a high magnetic energy product and a square demagnetization curve, which undoubtedly describes Nd-Fe-B magnets. However, the Curie temperature of 315 ◦C limits its usable condition, which has to be lower than 80 ◦C (N grade); otherwise, irreversible demagnetization will occur. Although the coercivity is increased by adding heavy rare earth elements such as Dy and Tb, and although the Curie temperature is increased by adding Co, the maximum service temperature is not more than 240 ◦C (AH grade) [3]. When the Nd-Fe-B magnet rises from 20 ◦C to 100 ◦C, the magnetic energy product will decrease by half, and the motor power will decrease [9], which will cause a waste of magnetic energy.

Because the resistivity of rare earth permanent magnets is very small, the eddy current generated in them is the main factor for the heating of magnets. Reducing the eddy current loss in magnets and thus reducing the temperature of magnets during operation is a

**Citation:** Guo, Y.; Zhu, M.; Wang, Z.; Sun, Q.; Wang, Y.; Li, Z. Design of Glass Fiber-Doped High-Resistivity Hot-Pressed Permanent Magnets for Reducing Eddy Current Loss. *Metals* **2023**, *13*, 808. https://doi.org/ 10.3390/met13040808

Academic Editors: Andrea Di Schino and Claudio Testani

Received: 11 March 2023 Revised: 12 April 2023 Accepted: 18 April 2023 Published: 20 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

problem that needs to be considered. Through the optimization of motor structure designs, although a certain effect has been achieved [9–11], the magnet resistivity is often used as a fixed value, so the optimization results are not satisfactory. In addition to optimizing the design of the motor structure, Aoyama [12] noted that increasing the resistivity of the magnet can also effectively reduce the eddy current loss. Considering the skin effect of the eddy current and the insulation of the main phase grains or raw material particles in the magnet, the electron transport in the magnet can be reduced by the heat source, and the resistivity can be increased. Thus, the working temperature of the magnet can be decreased. Gabay [13] introduced CaS for insulation and isolation, and it was found that adding sulfur (phosphorus) compounds would form NdS, which would reduce the metallurgical bonding of the cladding layer. Although the introduction of SiO2 increases the resistivity to a certain extent, Nd will combine with O to form NdO, resulting in the loss of the Nd-rich phase and the reduction of magnetic properties [14]. Fluoride is suitable for addition to permanent magnet materials because of its inertia. The addition of NdF increased the magnetic resistivity by 200% [15]. Komuro et al. [16] prepared a magnet with a fluoride coating with a resistivity of 1.4 mΩ·mm. The surface resistivity of the magnet increased by 10 times, and the rotor temperature decreased by 50%. Although these works have made some progress, some problems still occur. In the above methods, most of the materials involved are inorganic compounds. In the process of magnet insulation and coating, these inorganic compounds cannot deform with the magnet; thus, spalling occurs during the deformation process of the magnet, which cannot effectively isolate the magnet. In addition, it is often necessary to complete the coating process with the help of liquid re-rich phase flow. The participation of the rare-earth rich phase in the coating will not only reduce the insulating ability of the coating layer but also cause the loss of the rare-earth rich phase and reduce the magnetic properties.

The method of adding insulating materials and the form of insulating materials play an important role in the resistivity of composite materials. McLachlan [17] gave the general effective media equation and described the different wetting and coating conditions between the conductive phase and the insulating phase. For calculating the resistivity of composite materials with continuous isolation layers:

$$
\rho\_{\mathfrak{m}} = \rho\_{\mathfrak{h}} (1 - \Phi)^{\mathfrak{m}\varphi}
$$

ρ<sup>m</sup> is the resistivity of the composite, ρ<sup>h</sup> is the resistivity of the low-conductivity phase, Φ is the volume fraction of the high-conductivity phase, and m<sup>Φ</sup> is the exponent for randomly oriented high-conductivity ellipsoids.

Depending on the value range of mΦ, a thin layer of insulation distributed continuously helps to isolate electron transport between adjacent particles, thus increasing the resistivity of the composite magnet. As seen from the metal binary phase diagram, most of the crystalline compound insulating materials have very high melting points, which cannot follow the deformation of the magnet during the particle coating process. Moreover, spalling occurs during the deformation process. At the same time, liquid re-rich phase flow is often needed to complete the coating process. The isolation layer formed by these crystalline compounds is still dispersed rather than continued under the non-wetting or intermediate wetting case discussed for the general effective media equation.

In this work, to overcome the shortcomings of the insulating layer, we designed a composite magnet doped with amorphous materials. In contrast to crystalline materials, amorphous materials, which can be deformed well with magnetic powder, will exhibit viscous flow with an increasing temperature. Through the systematic study of magnets with amorphous glass fibers, it is verified that this design can significantly improve the resistivity of the magnets and facilitate the preparation of magnets with oriented textures. The new design idea provides a new way to develop high-resistivity rare earth permanent magnets with oriented textures.
