Development of Mischmetal-(FeCo)-B Ribbons with Improved Magnetic Properties by Addition of Si

Abstract

In order to develop mischmetal-based permanent magnets with a high performance/cost ratio, Si addition was employed to enhance the magnetic performance of the MM₁₆Fe_76-xCo₂Si_xB₆ (x = 0–1.5%) ribbons. The ribbons were manufactured by a melt-spinning technique at different velocities. Samples were studied in an as-cast state and after annealing. It was found that the addition of Si in the MM₁₆Fe_76-xCo₂Si_xB₆ ribbons increased the exchange interaction between Fe atoms in the 2:14:1 phase, reduced the crystal grain size, and increased the exchange coupling between the crystal grains; as a result, the magnetic properties were improved. The coercivity and Curie temperature increased with the increasing Si content from x = 0 to x = 1.5 at%, while the remanence and energy product increased with the addition of Si up to 1% and decreased with further Si addition. The best combination of magnetic properties, such as coercivity of H_c = 8.9 kOe, remanence Mr = 98 emu/g, Curie temperature T_c = 257 °C, and energy product (BH)_max = 13.84 MGOe, were obtained in ribbons with 1.0 at. % Si. Thus, it is demonstrated that the addition of Si leads to an improvement of the magnetic properties of MM₁₆Fe_76-xCo₂Si_xB₆ ribbons, making them good candidates as precursors for the preparation of permanent magnets with an energy product between that of ferrites and Nd-Fe-B magnets.

1. Introduction

Nd-Fe-B-based magnets are widely used in various applications due to their excellent magnetic properties at room temperature [1,2,3,4,5,6]. The demand for magnets with high magnetic performance has accelerated even more, with the rapid development of the sector dedicated to clean energy, especially electric vehicles, wind power generators, and other emerging applications [7,8]. Thus, due to the fact that rare earths (RE) are generally mined together, the accelerated demand for permanent magnets has led, on the one hand, to the overconsumption of critical rare earth elements, namely Dy, Nd, and Pr, and, on the other hand, to the overstocking of cheap and abundant rare earths such as Ce or La [8,9]. According to the Motion Control & Motor Association (MCMA), the world’s population is expected to reach 9 billion by 2050, and every human on Earth will be served by three service robots. Additionally, each robot is expected to contain more than 100 motors. As a result, the demand for permanent magnets is expected to grow exponentially in the coming years. Therefore, increasing the demand for permanent magnets, growing concerns about environmental degradation as a result of the exploitation of rare earths, increasing costs, and availability problems of rare earths have led to a global concern to search for new solutions for magnetic materials that can be used for the preparation of permanent magnets with a performance as close as possible to that of magnets based on Nd-Fe-B. The first solution was to replace or reduce critical rare earth elements with other more abundant and less expensive ones (for example, Ce or La) [10,11,12,13]. Unfortunately, although convenient from an economic point of view, the total replacement of the critical rare earths by noncritical rare earth (La or Ce) leads to a sharp degradation of the magnetic properties, because the room temperature saturation magnetization and the anisotropy field of Ce₂Fe₁₄B and La₂Fe₁₄B compounds are lower than those of the Nd₂Fe₁₄B compound [14,15,16]. In addition, the extraction and separation processes of rare earths are accompanied by severe environmental pollution [17,18,19,20]. In this regard, due to the fact that mischmetal (MM) is mined as a raw ore consisting of a mixture of the rare earth elements La, Ce, Pr, and Nd, which does not require separation to obtain pure elements, its use as a total substituent for Nd leads to a reduction in (i) the final cost of the magnets, (ii) the unbalanced use of rare earths, and (iii) environmental pollution. In addition, the magnetic properties are less degraded than in the case of total replacement of Nd with La or Ce, due to the fact that there is a significant percentage of Nd and Pr in the composition of the mischmetal ore. Because mischmetal is a raw ore, the ratio of its component elements La, Ce, Pr, and Nd can be very different depending on the mine from which it is extracted. For example, the typical composition of MM from the Bayan Obo mine in China is (25 to 35 wt. % La, 45 to 55 wt. % Ce, 4 to 10 wt. % Pr, and 14 to 18 wt. % Nd), while the composition of MM from Jiangxi in the south of China is (25 to 30 wt. % La, 10 to 15 wt. % Ce, 10 to 15 wt. % Pr, 42 to 47 wt. % Nd). For this reason, the magnetic properties of alloys from the MM-Fe-B system reported by various authors may appear to be inconsistent even for the same mischmetal content.

Gong et al. deem that MM-Fe-B magnets can reach a maximum energy product (BH)_max in the range of 10–20 MGOe, which represents half of the energy product of Nd-Fe-B magnets [21]. Otherwise, a compromise situation for an energy product greater than 20 MGOe continues to remain the partial replacement of Nd with MM such as (Nd_100-xMM_x)-Fe-B [22]. Although MM-Fe-B compounds have inferior magnetic performance compared to Nd-Fe-B compounds, they can have higher magnetic properties than hard ferrites or alnico-type permanent magnets, thus filling the energy gap between them. However, to date, obtaining high energy density (BH)_max = 20 MGOe in MM-Fe-B-based magnets is still a great challenge. Zuo et al. have tested a wide composition range for MM-Fe-B ribbons and have found that a large energy product (BH)_max of 12 MGOe is obtained in ribbons with the composition of MM₁₃Fe₈₁B₆ [23]. Zhang et al. [24] showed that (BH)_max = 10.14 MGOe (80.69 kJ/m³) and H_cj = 6.28 Oe (500 kA/m) can be achieved with melt-spun MM₂.₄Fe₁₄B. The highest value for (BH)_max = 14.57 MGOe (116 kJ/m³) was obtained by Hu et al. in MM₂.₂Fe₁₄B ribbons [25]. Also, for the total replacement of Nd with MM, different researchers have reported that the Curie temperature (Tc) of MM-Fe-B alloys, compared to Nd-Fe-B alloys, decreases from 320 °C to 250 °C according to [21] or even up to 210 °C according to [26], while a partial replacement of Nd by MM can maintain a high Curie temperature of, e.g., 310 °C [27]. A compromise solution in maintaining or even increasing the Curie temperature is the well-known replacement of Fe by Co, a situation in which, with a 50% replacement of Fe, the Tc can increase up to 450 °C [28]. In order to develop industrially viable MM-Fe-B-based alloys with medium performance, sufficiently high magnetic properties such as coercivity, remanence, and Curie temperature are required. For this reason, solutions to improve them are continuously being sought. As in the case of Nd-Fe-B alloys, where a successful solution to improve the magnetic properties was the addition of a small but critical concentration of elements, in the case of the proposed MM-Fe-B alloys, the same path is followed. Although studies have reported on improving the coercivity of MM-Fe-B alloys by adding elements such as Ti, PrCo, TbH, or Ho elements [29,30,31,32], to date there are no known studies on the improvement of the Curie temperature. The Si element was employed in Nd-Fe-B-based alloys to improve magnetic properties and thermal stability [33,34]. Therefore, we expect Si to have a positive impact on the magnetic properties of the MM-Fe-B alloy.

In this paper, we investigate the effects of Si addition on the microstructure and magnetic properties of MM₁₆Fe_76-xSi_xCo₂B₆ (x = 0–1.5) ribbons with the aim of using them as precursors for medium-performance and low-cost magnets.

2. Materials and Methods

Starting from the M₁₆Fe₇₆Co₂B₆ composition optimized in our previous study [35], a series of Si-doped ingots with nominal compositions of MM₁₆Fe_76-xCo₂Si_xB₆ (with x varying from 0 to 1.5 at%) were prepared by arc melting under an argon atmosphere using the raw materials of mischmetal with a purity of 99.5% along with Fe, Co, and B with a purity higher than 99.99% %. The composition of the mischmetal used by us was La-25%, Ce-53%, Pr-5%, and Nd-17%. Each ingot was re-melted five times to obtain a homogeneous composition and further used to prepare ribbons by a melt-spinning technique. The ribbons were obtained after a series of experimental tests to establish the optimal technological parameters of the melt-spinning process, such as the ejection pressure (Δp), the circumferential speed (v) of the copper disc, the ejection temperature (T), the diameter of the ejection nozzle (D), and the nozzle–disc distance (d). The schematic structure of the melt-spinning plant is shown in Figure 1.

The alloy was melted by electromagnetic induction in a quartz tube equipped with a rectangular nozzle at the bottom, 300 µm wide and 1 mm long. After melting the alloy at a temperature of 1300 °C, the tube was lowered to the ejection distance d = 0.4 mm from the cooling disk rotating at speeds between 15 and 35 m/s. The alloy was ejected by the application to the upper part of the quartz tube of an ejection pressure Δp = 0.3 bar, measured against the pressure in the enclosure where the molten alloy was located. The ribbon thicknesses were typically in the range 20–45 μm depending on the disc velocity. The as-spun ribbons presented amorphous or nanocrystalline structures depending on the velocity of the disc, set for the process. In order to obtain the optimum nanocrystalline structure, as-spun ribbons with an amorphous structure were annealed in a vacuum of about 5 × 10⁻⁶ mbar for different periods of time at temperatures ranging between 600 and 700 °C. The structural analysis of the melt-spun ribbons was carried out by X-ray diffraction (XRD) in a Bruker AXS D8-Advance diffractometer (Mannheim, Germany) over the angular range 20–70°, with a step of 0.02° and a counting time of 2 s per step, using conventional Cu-Kα incident radiation. The crystalline grain’s size was determined using the Debye–Scherrer method. Phase identification and analysis were performed using X’Pert High Score Plus software, version 3.0e (3.0.5). Magnetic measurements were performed using a vibrating sample magnetometer (VSM) (Lake Shore VSM 7410) in a maximum applied field of 20 kOe for M-H measurements made at room temperature, while for M-T measurements a constant field of 10 kOe was applied. The microstructure of the sample was investigated using a scanning electron microscope, FIB/FE-SEM Cross-Beam Carl Zeiss NEON 40 EsB (Oberkochen, Germany), equipped with an energy dispersive X-ray spectroscopy (EDS) module.

3. Results and Discussions

The X-ray diffraction patterns of the MM₁₆Fe_76-xCo₂Si_xB₆ as-cast ribbons with variable Si content prepared at two different circumferential disc velocities are shown in Figure 2. For the ribbons prepared at a low speed of 15 m/s, the results indicate a structure well crystallized, and the characteristic diffraction peaks correspond well to those of the tetragonal phase RE₂(FeCo)₁₄B (space group P42/mnm) as a main phase, traces of RE₂O₃ (space group P321), REO₂ (space group Fm-3m), and a cubic phase αFe as secondary phases. The diffraction peaks of the 2:14:1 hard magnetic phase and the soft magnetic α-Fe phase became weaker and broadened with the addition of Si, indicating that Si has a grain size refining effect on the melt-spun ribbons. For ribbons prepared at a high disc velocity of 35 m/s, the structure was found to be completely amorphous regardless of the Si content.

Table 1. Evolution of magnetic hysteresis parameters and crystalline grain size for MM₁₆Fe_76-xCo₂Si_xB₆ as-cast ribbons as a function of Si content.

Si (at. %)	v = 15 m/s				v = 35 m/s
	Hc (Oe)	Ms (emu/)	Mr/Ms	Grain Size of RE2(FeCo)14B (nm)		Hc (kOe)	Ms (emu/g)	Mr/Ms
x = 0.0	6.6	118	0.56	275		0.2	111	0.13
x = 0.5	6.9	113	0.60	171		0.25	108	0.10
x = 1.0	7.1	107	0.58	132		0.24	106	0.14
x = 1.5	7.3	100	0.61	88		0.26	102	0.18

presents the magnetic hysteresis parameters and the evolution of the size of the 2:4:1 crystalline grains versus Si content for the MM₁₆Fe_76-xCo₂Si_xB₆ as-cast ribbons prepared at 15 m/s and 35 m/s, respectively. For the easy visualization of the variation trends of coercivity and saturation magnetization, their corresponding data from Table 1 are also plotted in Figure 3. It can be seen that the nanocrystalline ribbons prepared at 15 m/s show hard magnetic characteristics, while the amorphous ribbons prepared at 35 m/s reveal soft magnetic properties. Such behavior was also observed by other authors [36].

The coercivity of the nanocrystalline ribbons gradually increases with Si content, while the saturation magnetization decreases. The increase in coercivity can be attributed to the refinement of the crystalline grains in agreement with the XRD results, while the decrease in saturation is the result of the magnetic dilution effect, because of the replacement of the Fe atom (2.2 µB) with the non-magnetic Si atom. Therefore, we can conclude that Si plays a key role in influencing the magnetic properties of the ribbons by hindering the growth of the grain in the main phase.

The desired magnetic properties of MM₁₆Fe_76-xCo₂Si_xB₆ ribbons can be controlled not only by the composition but also by the parameters of the annealing treatment process, i.e., the time and annealing temperature. Therefore, in order to suppress the unwanted secondary phases and for a controlled refinement of the crystalline grain sizes of the main phase 2:14:1, the as-spun ribbons with the amorphous structure were thermal annealed. Considering that the influence of the treatment conditions on the magnetic properties is outside the scope of this paper, we will only mention that the heat treatment conditions, for which the optimal magnetic properties were obtained, were found at 650 °C for 20 min. In the following, annealed ribbons will mean ribbons annealed at 650 °C for 20 min.

Figure 4 shows the demagnetization curves of the annealed MM₁₆Fe_76-xCo₂Si_xB₆ ribbons, and their corresponding magnetic hysteresis parameters are summarized in

Table 2. The hysteresis parameters for annealed MM₁₆Fe_76-xCo₂Si_xB₆ ribbons.

Si (at%)	Hc (kOe)	Mr (emu/g)	Squareness	(BH)max (MGOe)
x = 0.0	8.17	90.72	0.76	10.42
x = 0.5	8.51	95.43	0.80	11.86
x = 1.0	8.98	98.25	0.88	13.84
x = 1.5	9.13	93.63	0.87	13.21

.

A slight improvement in the coercivity with Si addition can be seen, while the remanence and squareness show a significant improvement of up to 1% at. Si addition and a decrease with further Si addition. Although the coercivity increases continuously with the addition of Si, the decrease in the remanence of 1.5 at. % Si causes the energy product to decrease with the addition of 1.5 at. % Si. Therefore, a better combination of magnetic properties such as coercivity of about 8.9 kOe, remanence of about 98 emu/g, and energy product of about 13.84 MGOe was obtained for x = 1 at. % Si.

In an attempt to find the connection between the improvement of the magnetic properties and the addition of Si, XRD and SEM investigations were carried out on the thermally annealed ribbons. Figure 5 shows the X-ray diffraction patterns of annealed MM₁₆Fe_76-xCo₂Si_xB₆ ribbons.

It can be seen that the annealed ribbons have a structure consisting of a tetragonal phase 2:14:1 (space group P42/mnm) and a cubic phase α-Fe, without other additional phases. The intensity of the diffraction peaks decreases and the half-width of the diffraction peaks increases progressively with increasing Si content, similar to the case of the as-cast strips prepared at a speed of 15 m/s. The evolution of the size of the crystalline grains of the main phase with the addition of Si is presented in

Table 3. Evolution of lattice constants and crystal grain size of the 2:14:1 phase in thermally annealed MM₁₆Fe_76-xCo₂Si_xB₆ ribbons.

x (at.%)	Grain Size of RE2(FeCo)14B (nm)	a (Å)	c (Å)
0.0	194	8.7913	12.2154
0.5	87	8.7844	12.2056
1.0	61	8.7767	12.1951
1.5	39	8.7684	12.1847

.

The size of the crystalline grains decreases from 194 nm for the ribbons with x = 0% at Si to 39 nm for the ribbons with x = 1.5% Si. Therefore, the increase in the coercivity of the MM₁₆Fe_76-xCo₂Si_xB₆ ribbons, along with the increase in the Si content, can be attributed to the decrease in the size of the crystalline grains. To investigate whether the Si atoms enter the main phase lattice, the crystal lattice parameters are calculated by full-spectrum fitting with Jade 6.0 software for XRD patterns. Values of lattice constants a and c are given in Table 3. It is observed that the lattice constants decrease and there is a slight shift of the diffraction peaks toward large angles, which illustrates that Si atoms enter into the lattice of the main phase to occupy the Fe sites. The decrease in the crystal lattice constants of the main phase leads to an increase in the exchange interaction between Fe atoms, thus explaining the increase in remanence. The improvement of the exchange interaction between Fe atoms should also be reflected in the value of the Curie temperature, which is an intrinsic property of the MM₁₆Fe_76-xCo₂Si_xB₆ ribbons and is mainly determined by the Fe–Fe exchange interaction in the 2:14:1 main phase. The temperature dependence of magnetization for the annealed MM₁₆Fe_76-xCo₂Si_xB₆ ribbons with x = 0–1.5 at. % Si was measured in an applied external field of 10 kOe and is shown in Figure 6a. It can be observed that the Curie temperature, which was determined from the minimum of the first derivative of the magnetization in relation to the temperature, Figure 6b, shows a slight increase with the Si content, from 248 °C for x = 0 at. % Si, to 251 °C for x = 0.5 at. % Si, 257 °C for x = 1 at. % Si, and up to 262 °C for x = 1.5 at. % Si. Therefore, the increase in the Curie temperature confirms that Si atoms enter the main phase lattice to partially replace Fe atoms, resulting in an improvement in the exchange interaction between Fe atoms.

The microstructural investigation of MM₁₆Fe_76-xCo₂Si_xB₆ ribbons with x = 0–1.5 at. % Si is presented by backscattered electron (BSE) SEM images in Figure 7. It can be clearly seen that the ribbons’ grain is refined dramatically with Si addition, which is consistent with the XRD results. The average grain size of the Si-free ribbons is about ̴ 204 nm, while the average grain size of the ribbons decreases with Si addition, being below 36 nm when Si content is more than 1.5 at. %, which is in good agreement with the results presented in Table 3. As shown in SEM images, the irregular-shaped grains have decreased in number, while the microstructure is more uniform compared with the Si-free sample. This is probably the reason why the Hc is enhanced with the refined microstructure.

In order to investigate the intergrain interactions, the δM variation as a function of the applied field was employed. The δM curve is defined as: $\delta M=m_r^d-(1-2m_r^i)$ where $m_r^d=M_r\left(H\right)/M_r\left(Hmax\right)$ is the reduced value of the remanence at different amplitudes of the applied demagnetizing field, and $m_r^i=M_r\left(H\right)/M_r\left(Hmax\right)$ is the reduced remanence at different amplitudes of the applied magnetizing field. The interactions between grains in magnetic materials include long-range and short-range interactions. The short-range interactions (exchange coupling interactions) are characterized by positive values of δM, whereas the long-range interactions (magnetostatic interactions) are characterized by negative values of δM. Figure 8 shows the δM curves of MM₁₆Fe_76-xCo₂Si_xB₆ annealed ribbons.

All ribbons show δM curves, with positive maximum values, which means that the predominant interactions between the crystal grains are exchange-coupling interactions. The peak height, δM, is a measure of the strength of the exchange interaction between the magnetic grains. Clearly, the maximum value of δM appears for the ribbons with a 1 at. % Si addition, indicating the strongest exchange coupling between the crystalline grains. Consequently, we can conclude that the improved magnetic properties of the annealed MM₁₆Fe₇₅Co₂Si₁B₆ ribbons can be attributed to the synergistic actions of increasing the exchange interaction between Fe atoms in the 2:14:1 phase, decreasing the size of the crystalline grains and increasing the exchange coupling between the crystalline grains.

The investigation of the thermal stability of the annealed ribbons was carried out by means of the temperature coefficients of remanence (α) and coercivity (β). In this sense, hysteresis curves were recorded at different temperatures between 25 and 125 °C.

The temperature coefficients represent the rate of change of remanence, Mr and coercivity, Hc, respectively, within a specified temperature range and are defined as:

$$\alpha =\frac{M_r\left(T_f\right)-M_r\left(T_i\right)}{{[M}_r\left(T_i\right)\left(T_f-T_i\right)]}\times 100\%\mathsf{\beta }=\frac{H_c\left(T_f\right)-H_c\left(T_i\right)}{{[H}_c\left(T_i\right)\left(T_f-T_i\right)]}\times 100\%,$$

where $M_r\left(T_f\right)$, $M_r\left(T_i\right),$ and $H_c\left(T_f\right),H_c\left(T_i\right)$ are remanence, respectively, coercivity at the final temperature, $T_f$, respectively, at the initial temperature $T_i$.

Figure 9 shows the variation in the remanence temperature coefficient α and coercivity temperature coefficient β of the MM₁₆Fe_76-xCo₂Si_xB₆ with the Si content at 25–125 °C. It can be seen that with the increase in the Si content from 0 to 1.5%, α decreases from −0.21%/°C to −0.155%/°C, while β decreases from −0.37%/°C to −0.31%/°C. These findings indicate that both the temperature stability of remanence and the temperature stability of coercivity are improved by the addition of Si.

Therefore, the thermal stability by the temperature coefficients α and β of the MM₁₆Fe_76-xCo₂Si_xB₆ ribbons can be controlled by adjusting the Si content.

4. Conclusions

The effect of Si addition on the magnetic properties and the microstructure of MM₁₆Fe_76-xSi_xCo₂B₆ ribbons was investigated. It was found that the addition of Si to the MM₁₆Fe_76-xSi_xCo₂B₆ ribbons was effective, not only in improving the coercivity as a result of the refinement of the 2:14:1 phase grain but also in increasing the Curie temperature, since the Si atom enters the cell of the 2:14:1 phase and changes the interatomic distance between the positions of Fe atoms, leading to increased interactions between them. It was also found that adding Si can enhance the thermal stability of MM₁₆Fe_76-xSi_xCo₂B₆ ribbons. Thus, the coercivity increased from 8.17 to 9.1 kOe, while the Curie temperature of the 2:14:1 phase increased from 248 °C to 262 °C when the Si content increased from 0 to 1.5 at%. The remanence and energy product (BH)_max increased with the addition of Si up to 1% and decreased with further addition of Si, due to the fact that the effect of magnetic dilution by the addition of a non-magnetic element becomes more important compared to the positive effect produced by the exchange interaction between atoms of Fe in the 2:14:1 phase and exchange coupling between crystalline grains. Overall, the best magnetic properties, such as coercivity of about 8.9 kOe, remanence of 98 emu/g, energy product of 13.84 MGOe, and Curie temperature of 257 °C, were obtained in annealed MM₁₆Fe₇₅Co₂Si₁B₆ ribbons. The results obtained in this paper indicate that the MM₁₆Fe₇₅Co₂Si₁B₆ ribbons can be used as precursors in the powder metallurgy technique for the preparation of permanent magnets with medium performance and low costs.