Investigation on Crystallization and Magnetic Properties of (Nd, Pr, Ce)2Fe14B/α-Fe Nanocomposite Magnets by Microwave Annealing Treatment

In the present work, the structures and magnetic properties of (Nd, Pr, Ce) 2Fe14B/α-Fe nanocomposite magnets were thoroughly investigated. The microwave annealing was applied to achieve a uniform heating effect and uniform grains. Microwave annealing is more favorable to obtain α-Fe phase than conventional annealing, which leads to the enhanced coercivity of hysteresis loops. The coercivity of nanocomposite magnets was 245 kA/m after annealing at 2000 W for 10 min.


Introduction
Nd 2 Fe 14 B/α-Fe nanocomposite magnets have attracted considerable research attention owing to the highest theoretical maximum energy product [1][2][3]. As previously reported, adjustment of the alloy composition and control of the crystallization process, especially the substitution of the Fe and Nd elements [4][5][6] was applied to improve the microstructure and properties [7,8]. Many elements (such as Pr, Y, etc.) have been applied to tailor the grain size and the crystallization phase [9][10][11]. As two effective rare earth elements, Pr and Ce have been widely used to substitute the Fe and Nd elements of Nd 2 Fe 14 B/α-Fe nanocomposite magnets and adjust their magnetic properties. Pr 2 Fe 14 B and Ce 2 Fe 14 B have the same crystal structure as the Nd 2 Fe 14 B, and the replacement of Ce element to Nd is beneficial to reduce the cost of RE-Fe-B magnets [12]. Nevertheless, the substitution of Ce for Nd deteriorates the magnetic properties because of the formation of Ce 2 Fe 14 B with low intrinsic magnetic properties [13], so in this research, the addition of Pr element is necessary, because the Pr substitution for the Nd improves the hard magnetic properties of the nanocomposite magnets [14]. The magnetic properties are significantly improved after the substitution of Ce and Pr elements to Nd [15]. Diffusion-processing is the typical method to fabricate high magnetic performance Nd-Pr-Ce-Fe-B sintered magnets [16]. For nanocomposite magnets with Ce and Pr substitution, the uniform and fine microstructure become the critical factor in improving the magnetic properties.
As is well known, the optimization of the annealing process is one of the effective ways to obtain uniform and fine microstructure of nanocomposite magnets [17,18]. The microwave heating process has shown significant advantages against conventional heating procedures in structure controlling. Microwave heating can heat materials to high temperatures with ultra-fast heating rates, short time, and high energy efficiency [19][20][21][22][23][24]. A higher heating rate favors obtaining uniform and refined grains after the crystallization process [25], enhancing coercivity, and also adjusts the proportion of hard and soft phases. The DSC curves of the (Nd0.525Pr0.175Ce0.3)9Fe64.5Co3Cu0.5Ti1B22 as-spun ribbons at a heating rate of 5, 20, and 40 °C /min are shown in Figure 2. Only one exothermic peak is detected, which indicates the synchronized precipitation of soft and hard magnetic phases, and they were formed at the same temperature. The crystallization characteristic The DSC curves of the (Nd 0.525 Pr 0.175 Ce 0.3 ) 9 Fe 64.5 Co 3 Cu 0.5 Ti 1 B 22 as-spun ribbons at a heating rate of 5, 20, and 40 • C/min are shown in Figure 2. Only one exothermic peak is detected, which indicates the synchronized precipitation of soft and hard magnetic phases, and they were formed at the same temperature. The crystallization characteristic temperature increased with the increase of the heating rate due to the hysteresis effect of phase change and the superheat. Moreover, as the heating rate increased, the higher the phase change energy storage per unit time was, the higher the heat released during the phase change was, so the exothermic peak became larger. Activation energy during the crystallization is a crucial parameter of thermal stability for the amorphous alloy, which statistically reflects the average of the activation energy in the crystallization process. The crystallization tendency of the amorphous alloy decreases with the increase of effective activation energy. According to Kissinger equation [26]: where In is natural logarithm; B is the heating rate; T is the characteristic crystallization temperature (in this study, we select T = T p ); E is the crystallization activation energy; R is the molar gas constant; and C is constant. The linear relationship between T 2 B and 1 T is shown in Figure 3. The slope values of E/R represent crystallization activation energy, resulting from E = 121.1 KJ/mol.

Comparison of Conventional Annealing and Microwave Annealing
The XRD patterns of the (Nd0.525Pr0.175Ce0.3)9Fe64.5Co3Cu0.5Ti1B22 ribbons annealed at 650 °C for 10 min under conventional and microwave heating conditions, respectively, are shown in Figure 4. Apart from the diffraction peaks from (Nd, Pr, Ce)2Fe14B and α-Fe phases, a broad peak locates from 20 degrees to 25 degrees is detected, suggesting the presence of some amorphous phases after conventional annealing treatment. However, the ribbons were annealed by microwave by increasing the microwave power from 800 W to 2000 W, the intensity of the α-Fe peak became significantly stronger than the (Nd, Pr, Ce)2Fe14B peak. The grain size can be calculated from the line width (FWHM in radian) of

Comparison of Conventional Annealing and Microwave Annealing
The XRD patterns of the (Nd 0.525 Pr 0.175 Ce 0.3 ) 9 Fe 64.5 Co 3 Cu 0.5 Ti 1 B 22 ribbons annealed at 650 • C for 10 min under conventional and microwave heating conditions, respectively, are shown in Figure 4. Apart from the diffraction peaks from (Nd, Pr, Ce) 2 Fe 14 B and α-Fe phases, a broad peak locates from 20 degrees to 25 degrees is detected, suggesting the presence of some amorphous phases after conventional annealing treatment. However, the ribbons were annealed by microwave by increasing the microwave power from 800 W to 2000 W, the intensity of the α-Fe peak became significantly stronger than the (Nd, Pr, Ce) 2 Fe 14 B peak. The grain size can be calculated from the line width (FWHM in radian) of XRD [27]. It shows that with the increase of microwave power, the grain size of the α-Fe phase increased greatly; however, the grain size of the (Nd, Pr, Ce) 2 Fe 14 B phase increased slightly, as are shown in Table 1, because microwave annealing treatment promotes the diffusion of elements, accelerates the crystallization rate, and then promotes the growth of grains, especially the α-Fe phase. The average size of grains increases from 11.9 nm in traditional annealing to 40.3 nm in microwave annealing at 2000 W. It explained that the microwave annealing treatment increased the grain size. Moreover, the broad peak completely disappears, presenting the elimination of the residual amorphous phase. The desired microstructural modifications have been achieved in the (Nd, Pr, Ce) 2 Fe 14 B/α-Fe samples after microwave annealing for the full range of power. The diffusion rates of the elements were promoted by a microwave field, the proportion of hard and soft magnetic phases were adjusted as well. traditional annealing to 40.3 nm in microwave annealing at 2000 W. It explained that the microwave annealing treatment increased the grain size. Moreover, the broad peak completely disappears, presenting the elimination of the residual amorphous phase. The desired microstructural modifications have been achieved in the (Nd, Pr, Ce)2Fe14B/α-Fe samples after microwave annealing for the full range of power. The diffusion rates of the elements were promoted by a microwave field, the proportion of hard and soft magnetic phases were adjusted as well.  The hysteresis loop curves of the (Nd0.525Pr0.175Ce0.3)9Fe64.5Co3Cu0.5Ti1B22 ribbons annealed at 650 °C for 10 min under conventional heating and microwave heating under different powers are shown in Figure 5. Their hysteresis loops display low magnetic properties. As reported, the Ce2Fe14B phase presents a lower first-order anisotropy constant than the Nd2Fe14B phase at room temperature, which induced the decrease of coercivity [28][29][30]. Even though it is expected that microwave heat treatment will increase the residual magnetization, there is no significant residual magnetic enhancement effect, and there is a weak exchange coupling effect between the soft and hard magnetic phases. The coercivity was enhanced with higher microwave power. It shows that the ribbons' coercivity  Figure 5. Their hysteresis loops display low magnetic properties. As reported, the Ce 2 Fe 14 B phase presents a lower first-order anisotropy constant than the Nd 2 Fe 14 B phase at room temperature, which induced the decrease of coercivity [28][29][30]. Even though it is expected that microwave heat treatment will increase the residual magnetization, there is no significant residual magnetic enhancement effect, and there is a weak exchange coupling effect between the soft and hard magnetic phases. The coercivity was enhanced with higher microwave power. It shows that the ribbons' coercivity and remanence magnetic ratio under microwave annealing treatment with a power of 2000 W reaches 245 kA/m and 0.34, which are higher than that of conventional annealing (132 kA/m and 0.26), as are shown in Table 2.  Table 2.    The σ s is remanence magnetic, σ r is saturation value of magnetization, H c is coercivity, and σ r /σ s is remanence magnetic ratio.
The X-ray diffraction patterns of the (Nd 0.525 Pr 0.175 Ce 0.3 ) 9 Fe 64.5 Co 3 Cu 0.5 Ti 1 B 22 ribbons annealed for 5, 10, and 15 min at a microwave power of 2000 W, as are, respectively, shown in Figure 6. With the increase of crystallization time, the intensity of the pattern peaks has an increasing tendency. It shows that the grain size of the α-Fe phase increased greatly; however, the grain size of the (Nd, Pr, Ce) 2 Fe 14 B phase increased slightly, as is shown in Table 3. This is because the elements are difficult to fully diffuse in a short crystallization time, so the grains did not have enough time to grow. When annealing for 5 min, the average grain size of the α-Fe phase is only 26.9 nm. However, when the crystallization time is longer, the elements can fully diffuse, which promotes the grain growth; when annealing for 15 min, the average grain size of the α-Fe phase reached 60.2 nm. It indicated that the crystallization time affected the grain size and adjustment of the percentage of soft and hard magnetic phases.     Figure 7. It shows that annealing under the microwave power of 2000 W for 10 min could obtain a higher coercivity and residual magnetization, 245 kA/m and 21 Am 2 /kg, as are, respectively, shown in Table 4. There is no significant residual magnetic enhancement effect, and there is a weak exchange coupling effect between the soft and hard magnetic phases annealing under the microwave power of 2000 W for 10 min. The coercivity and remanent magnetic ratio could be enhanced under appropriate microwave annealing process.

Conclusions
This study investigated the crystallization and magnetic properties of (Nd, Pr, Ce)2Fe14B/α-Fe nanocomposite magnets through microwave annealing treatment. The results of the experiment indicate that compared with traditional annealing, even though microwave annealing slightly reduces the magnetic saturation strength of the magnet, it increases the coercivity and remanence ratio of (Nd, Pr, Ce)2Fe14B/α-Fe nanocomposite magnets. Coercivity and remanence ratio up to 245 kA/m and 0.38 for (Nd0.525Pr0.175Ce0.3)9Fe64.5Co3Cu0.5Ti1B22 ribbons were obtained after annealed at microwave field with 2000 W for 10 min.  The σ s is remanence magnetic, σ r is saturation value of magnetization, H c is coercivity, and σ r /σ s is remanence magnetic ratio.

Conclusions
This study investigated the crystallization and magnetic properties of (Nd, Pr, Ce) 2 Fe 14 B/ α-Fe nanocomposite magnets through microwave annealing treatment. The results of the experiment indicate that compared with traditional annealing, even though microwave annealing slightly reduces the magnetic saturation strength of the magnet, it increases the coercivity and remanence ratio of (Nd, Pr, Ce) 2 Fe 14 B/α-Fe nanocomposite magnets. Coercivity and remanence ratio up to 245 kA/m and 0.38 for (Nd 0.525 Pr 0.175 Ce 0.3 ) 9 Fe 64.5 Co 3 Cu 0.5 Ti 1 B 22 ribbons were obtained after annealed at microwave field with 2000 W for 10 min.