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Investigation of magnetic properties, phase evolution, and microstructure of melt spun PrFeTiBC nanocomposites
Journal of Alloys and Compounds 424 (2006) 376–381
Investigation of magnetic properties, phase evolution, and microstructure of melt spun PrFeTiBC nanocomposites C.H. Chiu a , H.W. Chang a,b , C.W. Chang a , W.C. Chang a,∗ a
Department of Physics, National Chung Cheng University, Taiwan, ROC Institute of Physics, Academia Sinica, NanKang, Taipei, Taiwan, ROC
b
Received 29 November 2005; received in revised form 23 December 2005; accepted 1 January 2006 Available online 8 February 2006
Abstract Microstructure, phase evolution, and magnetic properties of Pr-lean and B-excess Pr9 Febal. Tix B11−y Cy (x = 0 and 2.5; y = 0–11) ribbons have been investigated. For Pr9 Febal. B11−y Cy (Ti free) series ribbons, the effect of slight C addition not only forms the Pr2 Fe14 (B,C) phase but also refines the grain size, yielding an increase in the remanent magnetization (Br ), intrinsic coercivity (i Hc ) and energy product ((BH)max ). For Pr9 Febal. Ti2.5 B11−y Cy series ribbons, substitution of Ti suppresses the formation of metastable Pr2 Fe23 B3 phase and ensures the existence of large amount magnetically hard Pr2 Fe14 B phase. However, the increase of C substitution in the Pr9 Febal. Ti2.5 B11−y Cy (y = 0–11) ribbons degrades Br , i Hc , and (BH)max monotonically. It arises from the increase of volume fraction of Pr2 Fe17 Cz and ␣-Fe phases, and the rapid decrease of 2:14:1 phase. The attractive properties of Br = 9.7 kG, i Hc = 7.8 kOe, (BH)max = 13.1 MGOe and α = −0.130 %/◦ C, β = −0.615 %/◦ C are obtained in Pr9 Febal. B10.5 C0.5 nanocomposites, on the other hand, the optimal properties of Br = 9.5 kG, i Hc = 10.8 kOe, (BH)max = 17.8 MGOe and α = −0.135 %/◦ C, β = −0.576 %/◦ C are achieved in Pr9 Febal. Ti2.5 B11 nanocomposites. © 2006 Published by Elsevier B.V. PACS: 75.20.En; 75.50.Bb; 75.50.Kj; 75.50.Ww Keywords: Pr2 Fe14 BC; Melt spinning; Magnetic properties; Coercivity
1. Introduction High coercivity Nd–Fe–C magnets, prepared by melt spinning without long time annealing, have attracted much attention since last decade [1,2]. It was also found that Pr2 Fe14 C compound is isostructural with Pr2 Fe14 B or Nd2 Fe14 B. However, the magnetocrystalline anisotropy field of Pr2 Fe14 C (HA = 148 kOe) is larger than that of Pr2 Fe14 B (HA = 87 kOe) or NdFeB (HA = 67 kOe) [3,4]. Thus, partial replacement of C for B in Pr2 Fe14 (B,C)/␣-Fe type nanocomposites is anticipated for improving the coercivity of the ribbons. Unlike Nd2 Fe14 (B,C) phase, the Pr2 Fe14 (B,C) phase exhibits no spin reorientation phenomenon appeared for at low temperature (below 150 K), which suggests that the range of application temperatures of Pr2 Fe14 (B,C)-type nanocomposite magnets is wider than that of Nd2 Fe14 (B,C)-type ones [5]. Therefore, a partial replacement
∗
Corresponding author. Tel.: +886 5 2721091; fax: +886 5 2721091. E-mail address: [email protected] (W.C. Chang).
0925-8388/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jallcom.2006.01.002
of C for B in ␣-Fe/Pr2 Fe14 B [6,7] (i.e. Pr10−15 Febal. B5−8 ) or Fe–B/Nd2 Fe14 B-type [8] nanocomposites has shown its effect in improving the coercivity (i Hc ) and energy product ((BH)max ) of the ribbons. Even so, in our recent findings [9], when the rare-earth content is lower than 9 at.% and at a boron concentration of 10–11.5 at.%, the metastable R2 Fe23 B3 phase appears and the volume fraction of R2 Fe14 B reduces, which deteriorates the magnetic properties drastically. Nevertheless, a slight substitution of Ti for Fe in Pr2 (Fe0.975 Ti0.025 )23 B3 is effective in suppressing the formation of metastable Pr2 Fe23 B3 phase, which leads to the presence of large amount of Pr2 Fe14 B and ␣-Fe phases of fine grain sizes in the matrix even at lower annealing temperature (=650 ◦ C) [10,11]. To date, the literature systematically reported concerning the effect of substitution of C for B on the Pr lean and B-enriched PrFeB ribbons is not found. Therefore, the effect of carbon content on the microstructure, phase evolution and magnetic properties of rare earth lean and boron-enriched ribbons, i.e. Pr9 Febal. B11−y Cy (y = 0–11), are studied at the beginning, and
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then followed by Ti containing Pr9 Febal. Tix B11−y Cy (y = 0–11) ribbons. 2. Experiment Alloy ingots with nominal compositions of Pr9 Febal. Tix B11−y Cy (x = 0 and 2.5; y = 0, 0.5, 1, 2.5, 5.5, 7.5, and 11) were prepared by vacuum induction melting. Meltspun ribbons were produced from ingots with wheel speeds ranging from 10 to 30 m/s. The ribbons selected were annealed at 600–700 ◦ C for 10 min to optimize crystallization and to improve the permanent magnetic properties. Magnetic phases and their corresponding Curie temperatures were determined by a thermal gravimetric analyzer (TGA) with an externally applied magnetic field (conventionally referred as “TMA”). The magnetic properties, temperature coefficient and Henkel Plot [12,13] of the ribbons were measured by a vibrating sample magnetometer (VSM). The microstructures of the ribbons were observed by transmission electron microscopy (TEM). Fig. 1. Demagnetization curves of Pr9 Febal. B11−y Cy ribbons.
3. Results and discussion 3.1. Pr9 Febal. B11−y Cy ribbons (Ti free) Table 1 lists Br , i Hc , (BH)max values and the Curie temperature of 2:14:1 phase for melt-spun Pr9 Febal. B11−y Cy (y = 0–11) ribbons following their optimal crystallization treatment, along with the reversible temperature coefficients of induction (commonly referred to as α for Br and β for i Hc ) for the temperature range of 25–100 ◦ C. Fig. 1 shows the demagnetization curves of Pr9 Febal. B11−y Cy ribbons. Clearly, with increasing carbon content y, i Hc and (BH)max increase from 7.6 kOe to 12.2 MGOe for y = 0 to 7.8 kOe and 13.1 MGOe for y = 0.5, respectively, and then decrease monotonously. As a consequence, the optimal magnetic properties of Br = 9.7 kG, i Hc = 7.8 kOe, and (BH)max = 13.1 MGOe are obtained in Pr9 Febal. B10.5 C0.5 ribbons. On the other hand, for the temperature coefficients, the absolute values of α and β increase with increasing carbon content y, which reflects that substitution of C for B in the studied alloys degrades the thermal stability of the ribbons. Furthermore, the Curie temperature of 2:14:1 phase decreases with increasing carbon content y. It suggests that the higher substitution of C for B in Pr9 Febal. B11−y Cy ribbons, the higher amount of C enters the
crystalline structure of 2:14:1 phase in forming Pr2 Fe14 (B,C). Besides, the decrease of Curie temperature of 2:14:1 phase with C substitution may be the main factor to degrade the thermal stability of the ribbons. TMA scans of melt-spun Pr9 Febal. B11−y Cy (y = 0–11) ribbons after optimum thermal treatments are presented in Fig. 2. Apparently, the magnetically hard phase 2:14:1 coexists with soft phase ␣-Fe and metastable phase Fe3 B and Pr2 Fe23 B3 for y = 0, while additional small amount of soft phase Pr2 Fe17 Cz is found for y = 0.5 and 1, four phases, namely 2:14:1, Pr2 Fe23 B3 , Pr2 Fe17 Cz and ␣-Fe appear for y = 2.5. Besides, with increasing carbon content from y = 5.5 to 11, it seems that 2:14:1 phase dissociates into ␣-Fe and Pr2 Fe17 Cz phases, which results in the rapid decrease of coercivity. Fig. 3(a) and (b) show TEM micrographs of Pr9 Febal. B11−y Cy ribbons with y = 0 and 0.5, respectively. The average grain size is estimated to be 15–30 nm for y = 0 and 10–25 nm for y = 0.5, respectively. For Pr9 Febal. B11−y Cy series ribbons, the effect of slight C addition not only forms the Pr2 Fe14 (B,C) phase but also refines the grain size, yielding an increase in i Hc and (BH)max .
Table 1 Br , i Hc , (BH)max values and the Curie temperature of 2:14:1 phase of melt-spun Pr9 Febal. B11−y Cy (y = 0–11) ribbons following their optimal crystallization treatment, along with the reversible temperature coefficients of induction (commonly referred to as α for Br and β for i Hc ) for the temperature range of 25–100 ◦ C x
0 0 0 0 0 0 0
y
0 0.5 1.0 2.5 5.5 7.5 11
Wheel speed (m/s)
18 20 19 18 12 10 10
Annealed temperature (◦ C)
675 675 650 625 625 675 675
Br (kG)
9.3 9.7 10.0 10.3 10.9 4.2 2.6
i Hc (kOe)
7.6 7.8 6.8 5.2 2.0 0.2 0.1
(BH)max (MGOe)
12.2 13.1 11.3 11.2 5.6 0 0
25–100 ◦ C
TC(2:14:l) (◦ C)
α (%/◦ C)
β (%/◦ C)
−0.128 −0.130 −0.141 −0.149 – – –
−0.601 −0.615 −0.626 −0.635 – – –
287 285 281 272 – – –
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Fig. 4. The applied magnetic field dependence of δM of Pr9 Febal. B11−y Cy ribbons.
of exchange-coupling effect between magnetic grains. Clearly, with increasing C content, the maximum δM value first increases to reach a maximum value at y = 0.5, and then decreases. Due to the presence of the finer grain and more uniform distribution of grain size, the Pr9 Febal. B10.5 C0.5 ribbon exhibits the strongest exchange-coupling interaction in comparison with all other ribbons, resulting in the increase of Br and (BH)max . 3.2. Pr9 Febal. Ti2.5 B11−y Cy ribbons Fig. 2. TMA scans of melt-spun Pr9 Febal. B11−y Cy (y = 0–11) ribbons after optimal annealing treatment.
Fig. 4 illustrates the applied magnetic field dependence of δM = md (H)−(1−2mr (H)) [12,13], with md being the reduced magnetization and mr the reduced remanence, of Pr9 Febal. B11−y Cy ribbons. The positive δM-peak height indicates that the existence of exchange-coupling interaction either between magnetically hard phases or between magnetically hard and soft phases. The value of maximum δM reflects the strength
Table 2, Figs. 5 and 6 show magnetic properties, demagnetization curves, and TMA scans of melt-spun Pr9 Febal. Ti2.5 B11−y Cy (y = 0–11) ribbons, respectively. The Pr9 Febal. Ti2.5 B11 ribbon primarily consists of the magnetically hard phase 2:14:1 and soft phases ␣-Fe. It is again proven that addition of Ti in Pr lean and B enriched PrFeB nanocomposite without C substitution can suppress 2:23:3 and Fe3 B phases to improve permanent magnetic properties from i Hc = 7.8 kOe and (BH)max = 13.1 MGOe to i Hc = 10.8 kOe and (BH)max = 17.8 MGOe. In addition to 2:14:1 and ␣-Fe
Fig. 3. TEM micrographs of Pr9 Febal. B11−y Cy ribbons. (a) y = 0 and (b) y = 0.5.
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Fig. 5. Shows the demagnetization curves of Pr9 Febal. Ti2.5 B11−y Cy ribbons.
phase, the soft phase Pr2 Fe17 Cz is found for the ribbons with y = 0.5 and 1. Five phases, namely, 2:14:1, Fe3 B, Pr2 Fe17 Cy , ␣-Fe and small amount 2:23:3 phase appear in the ribbons with y = 2.5. With complete substitution of C for B in Pr9 Febal. Ti2.5 C11 alloy ribbons (y = 11), 2:14:1 phase almost disappears. From Table 2, for Pr9 Febal. Ti2.5 B11−y Cy series ribbons, with the increment of C content, i Hc and (BH)max of the ribbons decrease monotonously from 10.8 kOe to 17.8 MGOe for y = 0–5.2 kOe and 7.5 MGOe for y = 5.5, respectively, which is different from the results in free Ti content Pr9 Febal. B11−y Cy ribbons. Actually, for the Ti free Pr9 Febal. B11−y Cy alloy ribbons, C substitution mainly refines the microstructure, which leads to slightly improve the magnetic properties. On the other hand, for the Ti containing and C free Pr9 Febal. Ti2.5 B11 alloy ribbons, magnetic properties of the ribbons are higher than those of the Ti-free ribbons. It primarily results from the reduction of volume fraction of Pr2 Fe23 B3 and Fe3 B phases by the Ti substitution for Fe, as evidenced by their TMA scans in Fig. 6 [11]. However, with the addition and increase of C content in
Fig. 6. TMA scans of melt-spun Pr9 Febal. Ti2.5 B11−y Cy (y = 0–11) ribbons.
the Pr9 Febal. Ti2.5 B11−y Cy alloy ribbons, C may tend to conjunct with Ti in forming TiC [8] and consuming part of the Ti element. Consequently, it retards the effect of Ti for suppressing the formation of Pr2 Fe23 B3 and Fe3 B phases, displayed to x = 2.5 and y = 2.5 in Fig. 6. On the other hand, the absolute values of α and β increase and Curie temperature of 2:14:1 phase decreases with increasing carbon content y. Most importantly, magnetic properties of the Ti containing Pr9 Febal. Ti2.5 B11−y Cy alloy ribbons are higher than those of the Ti-free ribbons, and 2:14:1 disappears at higher C content (>7.5 at.%) for Ti substituted ribbons in comparison with that
Table 2 Magnetic performances of melt-spun Pr9 Febal. Ti2.5 B11−y Cy (y = 0–11) ribbons x
2.5 2.5 2.5 2.5 2.5 2.5 2.5
y
0 0.5 1.0 2.5 5.5 7.5 11
Wheel speed (m/s)
16 14 15 16 12 12 11
Annealed temperature (◦ C)
675 650 650 675 650 675 650
Br (kG)
9.5 9.5 9.3 8.9 8.8 8.6 3.1
i Hc
(kOe)
10.8 10.3 10.0 9.7 5.2 1.8 0.1
(BH)max (MGOe)
17.8 17.0 15.5 15.3 7.5 2.9 0
25–100 ◦ C
TC(2:14:1) (◦ C)
α (%/◦ C)
β (%/◦ C)
−0.135 −0.146 −0.153 −0.157 −0.161 – –
−0.576 −0.576 −0.592 −0.608 −0.627 – –
286 284 277 276 – – –
380
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Fig. 7. TEM micrographs of Pr9 Febal. Ti2.5 B11−y Cy ribbons. (a) y = 0, (b) y = 0.5 and (c) y = 7.5.
(>5.5 at.%) for Ti free series ribbons. It primarily results from the fact that the 2:23:3 and Fe3 B phases are suppressed to stabilize the formation of 2:14:1 phase with Ti substitution, as evidenced by TMA scans in Figs. 2 and 6. Shown in Fig. 7(a), (b), and (c) are the TEM micrographs of Pr9 Febal. Ti2.5 B11−y Cy ribbons with x = 0, 0.5, and 7.5, respectively, after optimum thermal treatments. The average grain size is estimated to be about 10–25 nm for y = 0 and 0.5, and 30–60 nm for y = 7.5, respectively. The different microstructure is found for the ribbons with y = 7.5. Many small precipitates appear and the form of 2:14:1 phases is not clear. It is presumed that the paramagnetic PrC2 phase form due to the dissociation of 2:14:1 phase [14]. Fig. 8 illustrates the applied magnetic field dependence of δM of Pr9 Febal. Ti4 B11−y Cy ribbons. Clearly, the trend of maximum δM value in Fig. 8 is similar to that of Br in Table 2. With increasing C content, the exchange-coupling interaction decrease monotonously, resulting in the decrease of Br and (BH)max . To sum up, in the Pr9 Febal. B11−y Cy ribbons, the effect of slight C addition not only forms the Pr2 Fe14 (B,C) phase but also refines the grain size, yielding an increase in coercivity. Besides, due to the presence of the finer grain and more uniform distribution of grain size, the Pr9 Febal. B10.5 C0.5 ribbon exhibits the strongest exchange-coupling interaction in comparison with all other ribbons, resulting in the increase of remanence. On the other hand, for Pr9 Febal. Ti2.5 B11−y Cy ribbons, with increasing carbon content, 2:14:1 phase dissociates into Pr2 Fe23 B3 , Fe3 B or Pr2 Fe17 Cz phases and the exchange-coupling interaction decrease monotonously, resulting in the decrease of Br and (BH)max .
Fig. 8. The applied magnetic field dependence of δM of Pr9 Febal. Ti2.5 B11−y Cy ribbons.
4. Conclusions For Pr9 Febal. B11−y Cy (Ti free) series ribbons, the effect of slight C addition not only forms the Pr2 Fe14 (B,C) phase but also refines the grain size, yielding an increase in i Hc and (BH)max . The attractive properties of Br = 9.7 kG, ◦ i Hc = 7.8 kOe, (BH)max = 13.1 MGOe and α = −0.130 %/ C,
C.H. Chiu et al. / Journal of Alloys and Compounds 424 (2006) 376–381
β = −0.615 %/◦ C are obtained in Pr9 Febal. B10.5 C0.5 nanocomposites. Besides, in comparison with the magnetic properties of Ti free Pr9 Febal. B11−y Cy (y = 0–5.5) ribbons (i Hc = 2.0–7.7 kOe and (BH)max = 5.6–13.1 MGOe), improved magnetic properties of (BH)max = 15.3–17.8 MGOe with higher coercivity of i Hc = 9.7–10.8 kOe have been obtained in Ti containing Pr9 Febal. Ti2.5 B11−y Cy (y = 0–5.5) ribbons, which suggests that sufficient Ti element is necessary to suppress the formation of metastable Pr2 Fe23 B3 phase to ensure larger amount of magnetically hard Pr2 Fe14 B phase in the ribbons. Importantly, for the Ti containing Pr9 Febal. Ti2.5 B11−y Cy alloy ribbons, magnetic properties of the ribbons are higher than those of the Ti-free ribbons. In this paper, the optimal properties of Br = 9.5 kG, ◦ i Hc = 10.8 kOe, (BH)max = 17.8 MGOe and α = −0.135 %/ C, ◦ β = −0.576 %/ C are achieved in Pr9 Febal. Ti2.5 B11 nanocomposites. Acknowledgement This paper was supported by National Science Council, Taiwan under Grant No. NSC-93-2112-M-194-001.
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Investigation of magnetic properties, phase evolution, and microstructure of melt spun PrFeTiBC nanocomposites C.H. Chiu a , H.W. Chang a,b , C.W. Chang a , W.C. Chang a,∗ a
Department of Physics, National Chung Cheng University, Taiwan, ROC Institute of Physics, Academia Sinica, NanKang, Taipei, Taiwan, ROC
b
Received 29 November 2005; received in revised form 23 December 2005; accepted 1 January 2006 Available online 8 February 2006
Abstract Microstructure, phase evolution, and magnetic properties of Pr-lean and B-excess Pr9 Febal. Tix B11−y Cy (x = 0 and 2.5; y = 0–11) ribbons have been investigated. For Pr9 Febal. B11−y Cy (Ti free) series ribbons, the effect of slight C addition not only forms the Pr2 Fe14 (B,C) phase but also refines the grain size, yielding an increase in the remanent magnetization (Br ), intrinsic coercivity (i Hc ) and energy product ((BH)max ). For Pr9 Febal. Ti2.5 B11−y Cy series ribbons, substitution of Ti suppresses the formation of metastable Pr2 Fe23 B3 phase and ensures the existence of large amount magnetically hard Pr2 Fe14 B phase. However, the increase of C substitution in the Pr9 Febal. Ti2.5 B11−y Cy (y = 0–11) ribbons degrades Br , i Hc , and (BH)max monotonically. It arises from the increase of volume fraction of Pr2 Fe17 Cz and ␣-Fe phases, and the rapid decrease of 2:14:1 phase. The attractive properties of Br = 9.7 kG, i Hc = 7.8 kOe, (BH)max = 13.1 MGOe and α = −0.130 %/◦ C, β = −0.615 %/◦ C are obtained in Pr9 Febal. B10.5 C0.5 nanocomposites, on the other hand, the optimal properties of Br = 9.5 kG, i Hc = 10.8 kOe, (BH)max = 17.8 MGOe and α = −0.135 %/◦ C, β = −0.576 %/◦ C are achieved in Pr9 Febal. Ti2.5 B11 nanocomposites. © 2006 Published by Elsevier B.V. PACS: 75.20.En; 75.50.Bb; 75.50.Kj; 75.50.Ww Keywords: Pr2 Fe14 BC; Melt spinning; Magnetic properties; Coercivity
1. Introduction High coercivity Nd–Fe–C magnets, prepared by melt spinning without long time annealing, have attracted much attention since last decade [1,2]. It was also found that Pr2 Fe14 C compound is isostructural with Pr2 Fe14 B or Nd2 Fe14 B. However, the magnetocrystalline anisotropy field of Pr2 Fe14 C (HA = 148 kOe) is larger than that of Pr2 Fe14 B (HA = 87 kOe) or NdFeB (HA = 67 kOe) [3,4]. Thus, partial replacement of C for B in Pr2 Fe14 (B,C)/␣-Fe type nanocomposites is anticipated for improving the coercivity of the ribbons. Unlike Nd2 Fe14 (B,C) phase, the Pr2 Fe14 (B,C) phase exhibits no spin reorientation phenomenon appeared for at low temperature (below 150 K), which suggests that the range of application temperatures of Pr2 Fe14 (B,C)-type nanocomposite magnets is wider than that of Nd2 Fe14 (B,C)-type ones [5]. Therefore, a partial replacement
∗
Corresponding author. Tel.: +886 5 2721091; fax: +886 5 2721091. E-mail address: [email protected] (W.C. Chang).
0925-8388/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jallcom.2006.01.002
of C for B in ␣-Fe/Pr2 Fe14 B [6,7] (i.e. Pr10−15 Febal. B5−8 ) or Fe–B/Nd2 Fe14 B-type [8] nanocomposites has shown its effect in improving the coercivity (i Hc ) and energy product ((BH)max ) of the ribbons. Even so, in our recent findings [9], when the rare-earth content is lower than 9 at.% and at a boron concentration of 10–11.5 at.%, the metastable R2 Fe23 B3 phase appears and the volume fraction of R2 Fe14 B reduces, which deteriorates the magnetic properties drastically. Nevertheless, a slight substitution of Ti for Fe in Pr2 (Fe0.975 Ti0.025 )23 B3 is effective in suppressing the formation of metastable Pr2 Fe23 B3 phase, which leads to the presence of large amount of Pr2 Fe14 B and ␣-Fe phases of fine grain sizes in the matrix even at lower annealing temperature (=650 ◦ C) [10,11]. To date, the literature systematically reported concerning the effect of substitution of C for B on the Pr lean and B-enriched PrFeB ribbons is not found. Therefore, the effect of carbon content on the microstructure, phase evolution and magnetic properties of rare earth lean and boron-enriched ribbons, i.e. Pr9 Febal. B11−y Cy (y = 0–11), are studied at the beginning, and
C.H. Chiu et al. / Journal of Alloys and Compounds 424 (2006) 376–381
377
then followed by Ti containing Pr9 Febal. Tix B11−y Cy (y = 0–11) ribbons. 2. Experiment Alloy ingots with nominal compositions of Pr9 Febal. Tix B11−y Cy (x = 0 and 2.5; y = 0, 0.5, 1, 2.5, 5.5, 7.5, and 11) were prepared by vacuum induction melting. Meltspun ribbons were produced from ingots with wheel speeds ranging from 10 to 30 m/s. The ribbons selected were annealed at 600–700 ◦ C for 10 min to optimize crystallization and to improve the permanent magnetic properties. Magnetic phases and their corresponding Curie temperatures were determined by a thermal gravimetric analyzer (TGA) with an externally applied magnetic field (conventionally referred as “TMA”). The magnetic properties, temperature coefficient and Henkel Plot [12,13] of the ribbons were measured by a vibrating sample magnetometer (VSM). The microstructures of the ribbons were observed by transmission electron microscopy (TEM). Fig. 1. Demagnetization curves of Pr9 Febal. B11−y Cy ribbons.
3. Results and discussion 3.1. Pr9 Febal. B11−y Cy ribbons (Ti free) Table 1 lists Br , i Hc , (BH)max values and the Curie temperature of 2:14:1 phase for melt-spun Pr9 Febal. B11−y Cy (y = 0–11) ribbons following their optimal crystallization treatment, along with the reversible temperature coefficients of induction (commonly referred to as α for Br and β for i Hc ) for the temperature range of 25–100 ◦ C. Fig. 1 shows the demagnetization curves of Pr9 Febal. B11−y Cy ribbons. Clearly, with increasing carbon content y, i Hc and (BH)max increase from 7.6 kOe to 12.2 MGOe for y = 0 to 7.8 kOe and 13.1 MGOe for y = 0.5, respectively, and then decrease monotonously. As a consequence, the optimal magnetic properties of Br = 9.7 kG, i Hc = 7.8 kOe, and (BH)max = 13.1 MGOe are obtained in Pr9 Febal. B10.5 C0.5 ribbons. On the other hand, for the temperature coefficients, the absolute values of α and β increase with increasing carbon content y, which reflects that substitution of C for B in the studied alloys degrades the thermal stability of the ribbons. Furthermore, the Curie temperature of 2:14:1 phase decreases with increasing carbon content y. It suggests that the higher substitution of C for B in Pr9 Febal. B11−y Cy ribbons, the higher amount of C enters the
crystalline structure of 2:14:1 phase in forming Pr2 Fe14 (B,C). Besides, the decrease of Curie temperature of 2:14:1 phase with C substitution may be the main factor to degrade the thermal stability of the ribbons. TMA scans of melt-spun Pr9 Febal. B11−y Cy (y = 0–11) ribbons after optimum thermal treatments are presented in Fig. 2. Apparently, the magnetically hard phase 2:14:1 coexists with soft phase ␣-Fe and metastable phase Fe3 B and Pr2 Fe23 B3 for y = 0, while additional small amount of soft phase Pr2 Fe17 Cz is found for y = 0.5 and 1, four phases, namely 2:14:1, Pr2 Fe23 B3 , Pr2 Fe17 Cz and ␣-Fe appear for y = 2.5. Besides, with increasing carbon content from y = 5.5 to 11, it seems that 2:14:1 phase dissociates into ␣-Fe and Pr2 Fe17 Cz phases, which results in the rapid decrease of coercivity. Fig. 3(a) and (b) show TEM micrographs of Pr9 Febal. B11−y Cy ribbons with y = 0 and 0.5, respectively. The average grain size is estimated to be 15–30 nm for y = 0 and 10–25 nm for y = 0.5, respectively. For Pr9 Febal. B11−y Cy series ribbons, the effect of slight C addition not only forms the Pr2 Fe14 (B,C) phase but also refines the grain size, yielding an increase in i Hc and (BH)max .
Table 1 Br , i Hc , (BH)max values and the Curie temperature of 2:14:1 phase of melt-spun Pr9 Febal. B11−y Cy (y = 0–11) ribbons following their optimal crystallization treatment, along with the reversible temperature coefficients of induction (commonly referred to as α for Br and β for i Hc ) for the temperature range of 25–100 ◦ C x
0 0 0 0 0 0 0
y
0 0.5 1.0 2.5 5.5 7.5 11
Wheel speed (m/s)
18 20 19 18 12 10 10
Annealed temperature (◦ C)
675 675 650 625 625 675 675
Br (kG)
9.3 9.7 10.0 10.3 10.9 4.2 2.6
i Hc (kOe)
7.6 7.8 6.8 5.2 2.0 0.2 0.1
(BH)max (MGOe)
12.2 13.1 11.3 11.2 5.6 0 0
25–100 ◦ C
TC(2:14:l) (◦ C)
α (%/◦ C)
β (%/◦ C)
−0.128 −0.130 −0.141 −0.149 – – –
−0.601 −0.615 −0.626 −0.635 – – –
287 285 281 272 – – –
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Fig. 4. The applied magnetic field dependence of δM of Pr9 Febal. B11−y Cy ribbons.
of exchange-coupling effect between magnetic grains. Clearly, with increasing C content, the maximum δM value first increases to reach a maximum value at y = 0.5, and then decreases. Due to the presence of the finer grain and more uniform distribution of grain size, the Pr9 Febal. B10.5 C0.5 ribbon exhibits the strongest exchange-coupling interaction in comparison with all other ribbons, resulting in the increase of Br and (BH)max . 3.2. Pr9 Febal. Ti2.5 B11−y Cy ribbons Fig. 2. TMA scans of melt-spun Pr9 Febal. B11−y Cy (y = 0–11) ribbons after optimal annealing treatment.
Fig. 4 illustrates the applied magnetic field dependence of δM = md (H)−(1−2mr (H)) [12,13], with md being the reduced magnetization and mr the reduced remanence, of Pr9 Febal. B11−y Cy ribbons. The positive δM-peak height indicates that the existence of exchange-coupling interaction either between magnetically hard phases or between magnetically hard and soft phases. The value of maximum δM reflects the strength
Table 2, Figs. 5 and 6 show magnetic properties, demagnetization curves, and TMA scans of melt-spun Pr9 Febal. Ti2.5 B11−y Cy (y = 0–11) ribbons, respectively. The Pr9 Febal. Ti2.5 B11 ribbon primarily consists of the magnetically hard phase 2:14:1 and soft phases ␣-Fe. It is again proven that addition of Ti in Pr lean and B enriched PrFeB nanocomposite without C substitution can suppress 2:23:3 and Fe3 B phases to improve permanent magnetic properties from i Hc = 7.8 kOe and (BH)max = 13.1 MGOe to i Hc = 10.8 kOe and (BH)max = 17.8 MGOe. In addition to 2:14:1 and ␣-Fe
Fig. 3. TEM micrographs of Pr9 Febal. B11−y Cy ribbons. (a) y = 0 and (b) y = 0.5.
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379
Fig. 5. Shows the demagnetization curves of Pr9 Febal. Ti2.5 B11−y Cy ribbons.
phase, the soft phase Pr2 Fe17 Cz is found for the ribbons with y = 0.5 and 1. Five phases, namely, 2:14:1, Fe3 B, Pr2 Fe17 Cy , ␣-Fe and small amount 2:23:3 phase appear in the ribbons with y = 2.5. With complete substitution of C for B in Pr9 Febal. Ti2.5 C11 alloy ribbons (y = 11), 2:14:1 phase almost disappears. From Table 2, for Pr9 Febal. Ti2.5 B11−y Cy series ribbons, with the increment of C content, i Hc and (BH)max of the ribbons decrease monotonously from 10.8 kOe to 17.8 MGOe for y = 0–5.2 kOe and 7.5 MGOe for y = 5.5, respectively, which is different from the results in free Ti content Pr9 Febal. B11−y Cy ribbons. Actually, for the Ti free Pr9 Febal. B11−y Cy alloy ribbons, C substitution mainly refines the microstructure, which leads to slightly improve the magnetic properties. On the other hand, for the Ti containing and C free Pr9 Febal. Ti2.5 B11 alloy ribbons, magnetic properties of the ribbons are higher than those of the Ti-free ribbons. It primarily results from the reduction of volume fraction of Pr2 Fe23 B3 and Fe3 B phases by the Ti substitution for Fe, as evidenced by their TMA scans in Fig. 6 [11]. However, with the addition and increase of C content in
Fig. 6. TMA scans of melt-spun Pr9 Febal. Ti2.5 B11−y Cy (y = 0–11) ribbons.
the Pr9 Febal. Ti2.5 B11−y Cy alloy ribbons, C may tend to conjunct with Ti in forming TiC [8] and consuming part of the Ti element. Consequently, it retards the effect of Ti for suppressing the formation of Pr2 Fe23 B3 and Fe3 B phases, displayed to x = 2.5 and y = 2.5 in Fig. 6. On the other hand, the absolute values of α and β increase and Curie temperature of 2:14:1 phase decreases with increasing carbon content y. Most importantly, magnetic properties of the Ti containing Pr9 Febal. Ti2.5 B11−y Cy alloy ribbons are higher than those of the Ti-free ribbons, and 2:14:1 disappears at higher C content (>7.5 at.%) for Ti substituted ribbons in comparison with that
Table 2 Magnetic performances of melt-spun Pr9 Febal. Ti2.5 B11−y Cy (y = 0–11) ribbons x
2.5 2.5 2.5 2.5 2.5 2.5 2.5
y
0 0.5 1.0 2.5 5.5 7.5 11
Wheel speed (m/s)
16 14 15 16 12 12 11
Annealed temperature (◦ C)
675 650 650 675 650 675 650
Br (kG)
9.5 9.5 9.3 8.9 8.8 8.6 3.1
i Hc
(kOe)
10.8 10.3 10.0 9.7 5.2 1.8 0.1
(BH)max (MGOe)
17.8 17.0 15.5 15.3 7.5 2.9 0
25–100 ◦ C
TC(2:14:1) (◦ C)
α (%/◦ C)
β (%/◦ C)
−0.135 −0.146 −0.153 −0.157 −0.161 – –
−0.576 −0.576 −0.592 −0.608 −0.627 – –
286 284 277 276 – – –
380
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Fig. 7. TEM micrographs of Pr9 Febal. Ti2.5 B11−y Cy ribbons. (a) y = 0, (b) y = 0.5 and (c) y = 7.5.
(>5.5 at.%) for Ti free series ribbons. It primarily results from the fact that the 2:23:3 and Fe3 B phases are suppressed to stabilize the formation of 2:14:1 phase with Ti substitution, as evidenced by TMA scans in Figs. 2 and 6. Shown in Fig. 7(a), (b), and (c) are the TEM micrographs of Pr9 Febal. Ti2.5 B11−y Cy ribbons with x = 0, 0.5, and 7.5, respectively, after optimum thermal treatments. The average grain size is estimated to be about 10–25 nm for y = 0 and 0.5, and 30–60 nm for y = 7.5, respectively. The different microstructure is found for the ribbons with y = 7.5. Many small precipitates appear and the form of 2:14:1 phases is not clear. It is presumed that the paramagnetic PrC2 phase form due to the dissociation of 2:14:1 phase [14]. Fig. 8 illustrates the applied magnetic field dependence of δM of Pr9 Febal. Ti4 B11−y Cy ribbons. Clearly, the trend of maximum δM value in Fig. 8 is similar to that of Br in Table 2. With increasing C content, the exchange-coupling interaction decrease monotonously, resulting in the decrease of Br and (BH)max . To sum up, in the Pr9 Febal. B11−y Cy ribbons, the effect of slight C addition not only forms the Pr2 Fe14 (B,C) phase but also refines the grain size, yielding an increase in coercivity. Besides, due to the presence of the finer grain and more uniform distribution of grain size, the Pr9 Febal. B10.5 C0.5 ribbon exhibits the strongest exchange-coupling interaction in comparison with all other ribbons, resulting in the increase of remanence. On the other hand, for Pr9 Febal. Ti2.5 B11−y Cy ribbons, with increasing carbon content, 2:14:1 phase dissociates into Pr2 Fe23 B3 , Fe3 B or Pr2 Fe17 Cz phases and the exchange-coupling interaction decrease monotonously, resulting in the decrease of Br and (BH)max .
Fig. 8. The applied magnetic field dependence of δM of Pr9 Febal. Ti2.5 B11−y Cy ribbons.
4. Conclusions For Pr9 Febal. B11−y Cy (Ti free) series ribbons, the effect of slight C addition not only forms the Pr2 Fe14 (B,C) phase but also refines the grain size, yielding an increase in i Hc and (BH)max . The attractive properties of Br = 9.7 kG, ◦ i Hc = 7.8 kOe, (BH)max = 13.1 MGOe and α = −0.130 %/ C,
C.H. Chiu et al. / Journal of Alloys and Compounds 424 (2006) 376–381
β = −0.615 %/◦ C are obtained in Pr9 Febal. B10.5 C0.5 nanocomposites. Besides, in comparison with the magnetic properties of Ti free Pr9 Febal. B11−y Cy (y = 0–5.5) ribbons (i Hc = 2.0–7.7 kOe and (BH)max = 5.6–13.1 MGOe), improved magnetic properties of (BH)max = 15.3–17.8 MGOe with higher coercivity of i Hc = 9.7–10.8 kOe have been obtained in Ti containing Pr9 Febal. Ti2.5 B11−y Cy (y = 0–5.5) ribbons, which suggests that sufficient Ti element is necessary to suppress the formation of metastable Pr2 Fe23 B3 phase to ensure larger amount of magnetically hard Pr2 Fe14 B phase in the ribbons. Importantly, for the Ti containing Pr9 Febal. Ti2.5 B11−y Cy alloy ribbons, magnetic properties of the ribbons are higher than those of the Ti-free ribbons. In this paper, the optimal properties of Br = 9.5 kG, ◦ i Hc = 10.8 kOe, (BH)max = 17.8 MGOe and α = −0.135 %/ C, ◦ β = −0.576 %/ C are achieved in Pr9 Febal. Ti2.5 B11 nanocomposites. Acknowledgement This paper was supported by National Science Council, Taiwan under Grant No. NSC-93-2112-M-194-001.
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