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The study on order behavior and electronic structure of flaky FeSiAl powder
Journal of Magnetism and Magnetic Materials 493 (2020) 165725
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
The study on order behavior and electronic structure of flaky FeSiAl powder
T
Xiaofei Niu Anhui Key Laboratory of Spintronics and Nanomaterials Research, Suzhou University, Suzhou 234000, Anhui, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Flaky FeSiAl Order behavior DO3 superlattice Electronic structure
In this paper, the grain growth, lattice strain, order behavior and electronic structure of FeSiAl powder were studied. Only the disorder bcc α-Fe peaks were exhibited after milling, and as the ball milling time increases, the internal strain rose and the grain size decreased. When the anneal temperature reached 773 K, we could see the obvious DO3 superlattice peak at (1 1 1), (2 0 0) and (3 1 1) plane. At the same time, the internal strain decreased and the grain size rose with the increment of annealing temperature. Annealing at temperatures above 773 K results in a long-range order (LRO) parameter about 0.67–0.76. This high LRO parameter further supports the observation of the formation of order DO3 structures. Furthermore, we found that the interaction between Fe and Si, Al atoms can decrease magnetic moment of the Fe atom. By the calculation of the empirical electron theory of solids and molecules, we confirmed that grinding process could change the electronic structure and the bond length difference ΔD of powder milled for 20 h had the smallest values.
1. Introduction The soft magnetic material having a nanocrystalline structure has exhibited improved magnetic properties when its grain size is less than the ferromagnetic exchange length of typically several tens of nanometers [1–5]. Nowadays, FeSiAl alloys have attracted great interests due to their cheap price, high magnetic permeability, high electrical resistivity, low coercivity, and relatively low loss [6,7]. The FeSiAl alloy consists of a disordered α-Fe structure and an ordered DO3 superlattice structure and the presence of DO3 superlattice structure can improve its soft magnetic properties [8]. The researchers of TOKIN Company studied water atomization FeSiAl powders made by ball milling. They found that the flake-shaped FeSiAl powders have good application prospects in electromagnetic interference (EMI) [9]. Therefore, flaky FeSiAl powders have been widely studied for microwave absorbing materials in recent years [10–12]. However, large cold deformation during grinding contribute to internal strain, so subsequent stress relief annealing is necessary to obtain good soft magnetic properties [13]. At the same time, the control of the annealing temperature is also the key for FeSiAl powders to the precipitation of the DO3 superlattice and the adjustment of the grain size. So far, a large number of studies on the structure, soft magnetic and microwave absorption properties of flaky FeSiAl have been reported [8,9,14]. However, further research is still required for sheetlike FeSiAl prepared by high-energy ball milling and thermal stability of the as-milled nanocrystal structure by anneal, especially considering the key issues of study the processing technology of the material from
E-mail address: [email protected]. https://doi.org/10.1016/j.jmmm.2019.165725 Received 18 July 2019; Accepted 20 August 2019 Available online 22 August 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
its structure, such as the relationship between the electronic structure and the milling time of the powder. In this paper, the flaky FeSiAl powder was prepared by high energy ball milling, and an order DO3 superlattice structure was precipitated by annealing. The morphology size, electronic structure, ordering transformation, grain size and lattice strain of the samples were investigated. 2. Experimental The composition of the gas atomized FeSiAl powder (Tiantong Co., Ltd.) is 9.7 wt% Si, 6 wt% Al, and the balance Fe. The FeSiAl powder has an average size of less than 20 μm. 100 g of powder was placed in a tempered steel cylinder while 1000 g of tempered ball mill balls were added, corresponding to a powder: ball mass ratio of 1:10. At the same time, 200 g of alcohol was adding as a solvent to prevent oxidation, which corresponds to a mass ratio of powder: alcohol of 1:2. The powder was milling in planetary ball mill at 300 rpm for 5, 10, 15, 20, 25, 30 h. Than the milling powder was annealed in a nitrogen-filled oven for 60 min. The annealing temperature ranged from 573 to 973 K. The surface morphology of the samples was observed by using a Hitachi S-4800 scanning electron microscopy (SEM). The phase structure of the powder in different stages was analyzed by X-ray diffraction (XRD, Rigaku D/max-2550V/PC). The powder composition was studied by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi).
Journal of Magnetism and Magnetic Materials 493 (2020) 165725
X. Niu
Fig. 1. The SEM micrographs of flaky paticles after grinding for different times: (a) 0 h, (b) 5 h, (c) 15 h, (d) 20 h, (e) 25 h and (f) 30 h.
Fig. 2. (a) X-ray diffraction spectra of various FeSiAl powder samples and (b) the effect of milling time on grain size and lattice strain of samples.
2
Journal of Magnetism and Magnetic Materials 493 (2020) 165725
X. Niu
3. Results and discussion
Specifically, at low temperatures (< 673 K), the grains show limited grain growth and the internal strain decreases slightly, and at higher temperature ranges (> 673 K), the order phase precipitates as the grain grows rapidly and the internal strain decreases significantly. The order degree can be quantified by long-range order (LRO) parameter, which is determined by the intensity of the reflection of superlattice and basic. The LRO parameter can be defined as [17]
3.1. Morphology and order behavior Fig. 1 presents the SEM micrographs of the milling FeSiAl powders. It can be easily found that the powder gradually changes from a spherical shape to a sheet shape. After ball milling for 15 h, the thickness of the powder is reduced to less than 1 μm. We can also find that as the ball milling time increases, the size of the powder first increases and then decreases. As can be seen from Fig. 1(a), the nearly spherical FeSiAl powder has a broad size distribution of several to several tens of microns, but the flaky powder has the bigger average size about 30 μm in Fig. 1(c). When the milling time exceeds 15 h, the length of powder decreases on the contrary and the powder begins to agglomerate. It can be explained that more and more powders crack under high internal stress and accumulate on the edges of uncracked sheets. Although the powder still maintains a large aspect ratio (> 10:1), a large number of smaller particles are formed, indicating that the extended milling has a detrimental effect on the fabrication of FeSiAl flakes. The XRD patterns of the samples at different milling time are shown in Fig. 2(a). We can find that the diffraction peaks of the DO3 superlattice appear at the positions of the (1 1 1) and (2 0 0) crystal faces, so the untreated powder has a disorder and order structure. In the bcc α-Fe phase, Fe, Si, and Al atoms randomly occupy positions and appear disorder, but in the DO3 superlattice structure (shown in Fig. 5(a)), Si and Al atoms mainly occupy alternate body center positions, presenting order state [8]. However, only the bcc α-Fe peaks are exhibited after milling. It can be explained that the Al and Si atoms are completely dissolved in the lattice of Fe [9]. Fig. 2(b) exhibits the grinding time dependence of the grain size and internal strain, and the specific data can be seen in Table 1. We can find that the grain size has a sharp drop and then tend to steady and the internal strain has an opposite trend. So it is reasonable that more and more powders crack under high internal stress. In addition, lattice distortion induced by internal stress is also the cause of the disappearance of DO3 superlattice. Fig. 3(a) shows the shape evolution model of FeSiAl powder under the condition of milling. As the ball milling time increases, the powder first becomes flat and larger, and then the edge breaks to form agglomeration, which is consistent with the result of SEM. At the same time, Fecht [15] found that a large number of dislocation could be formed during the milling process, which resulted in the nucleation of nanocrystalline, and the nucleation of nanocrystals further reduced grain size. In addition, the composition of the alloy will be converted into a pure iron component and cause changes in magnetic properties [16]. The XPS spectra of Fig. 3(b)–(d) can prove it. The arrow points to the peak of Fe0 and we can find the intensity of Fe0 increases with the milling process. The XRD diffraction patterns shown in the Fig. 4(a) reveal structure changes of the powder samples at different annealing temperature. We can find obvious DO3 superlattice phase at the (1 1 1), (2 0 0) and (3 1 1) crystal faces when the annealing temperature more than 673 K. Therefore, some disorder supersaturated bcc solid solutions have been transformed into DO3 order structures. From Fig. 4(b), the annealing temperature dependence of the grain size and internal strain is observed. We can easily find that the grain size increases and the lattice strain decreases with the increment of annealing temperature.
1/2
I s / I0s ⎤ LRO = ⎡ ⎢ I f /I f ⎥ 0 ⎦ ⎣ s
(1)
f
where I and I are the intensity of the superlattice line and the basic line after annealing, respectively, and I0s and I0f are the intensity of the same line in the fully order sample. A diffraction pattern using the ideal fully order DO3 structure that has been reported [18] is used to determine I0s and I0f . Using the prominent DO3 (1 1 1) superlattice peak and (2 2 0) base peak, the LRO parameters for different annealing conditions were calculated as shown in Table 1. Annealing at temperatures above 773 K results in an LRO parameter above 0.67. This excellent LRO parameter further supports the retionality of the formation of order DO3 structures. 3.2. Electronic structure research To explore the electronic structure of FeSiAl, the electron structure calculation was performed in Cambridge Serial Total Energy Package (CASTEP, version 2017). The following elements should be considered in the modeling process: for the minimum energy principle, one of the Fe atoms at the body center position is replaced by Si or Al atom, and a supercell (2 × 2 × 2) is generated to meet the crystal period, as shown in Fig. 5(a). The energy band can be found in Fig. 5(b). From the Fig. 5(b), the energy band structure of the crystal is calculated along the symmetry line Z-A; A-M; M-G; G-Z; Z-R; R-X; and X-G. It can be easily found that FeSiAl is a conductor. When the Fe atom is replaced randomly by a Si or Al atom, the electronic configuration of Fe in unit cell will changes. The interaction between Fe and Si, Al atoms can decrease magnetic moment of the Fe atom. In order to prove it, the Bohr magneton of Fe atom is analyzed by the Eq. (2):
m (μB ) = mσ3d (1 − x − y )
(2)
where x and y are the contents of Si and Al atoms, respectively. Since there is no 3d electron in the Al or Si atom, the magnetic moment of the FeSiAl comes only from the 3d electron of the Fe atom. An increase in the milling time increases the amount of Si and Al dissolved in Fe, causing an increases of contents of Si and Al atoms in Fe solid solution and a decrease in magnetic moment of Fe. At the same time, the statistical mean of the exchange split (Δx) can be calculated by considering the 60 bands near the Fermi surface. From previous research [18,20], the introduction of Al and Si will reduce Δx, which may contribute to a decrease in the Curie temperature Tc due to a decrease in magnetic moment. Furthermore, it is important to study the relationship between electronic structure and grinding. Due to the additive Al and Si atom replaces the Fe atom in the FeSiAl alloy and the primary bond in the αFe consists of the shortest bond of the (1 1 1) face and the sub-short bond of the (1 0 0) face. We can use the empirical electron theory of
Table 1 Grain size, lattice strain and LRO parameter of various FeSiAl powder samples. Milling time (h)
Grain size (nm) Lattice strain (%) LRO parameter
Annealing temperature (K)
0
5
10
15
20
25
30
573
673
773
873
973
29.2 0.2887 –
13.1 0.6953 –
12.3 0.7312 –
12.1 0.7447 –
12 0.7578 –
11.8 0.7594 –
11.9 0.7666 –
14.1 0.6235 –
15.8 0.5064 –
19.9 0.4452 0.6709
36.6 0.2456 0.7053
79.5 0.1064 0.7556
3
Journal of Magnetism and Magnetic Materials 493 (2020) 165725
X. Niu
Fig. 3. (a) The shape evolution model of FeSiAl powder under different milling time and (b)–(d) the XPS spectra of 0 h, 15 h and 30 h sample for Fe2p.
length difference of the two planes, and are listed in Table 2. By calculation, we can find that the value of bond length difference ΔD of different samples is very small, indicating that all the states are reasonable. The sample milled 20 h has the smallest bond length difference (ΔD = −0.0002) among the seven samples. At the same time, the pair number in the (1 0 0) plane is much smaller than the pair number in the (1 1 1) plane. This indicates that there is a stronger bonding tendency between the atoms of the bonds A (1 1 1) than the atoms of the bonds B (1 0 0) [21]. The DO3 superlattice structure disappears and the structure become disorder at various milling time, which is consistent of the previous work [8,14]. The internal strain,
solids and molecules to calculate the bond length difference [9,19].
nA =
∑ nC , nB = nA rB IA + IB rB
D (nα ) =
2Rs (1)
− β lg nα
(3) (4)
where,α represents the A (1 1 1) plane or the B (1 0 0) plane, β is a constant(β = 0.6), I is the number of bonds, Σnc is the number of covalent electrons, Rs(1) is the half length of a single bonds, D(n) is the bond length and ΔD is the bond length difference. Based on this theory, we can calculate the bond length, the pair number of the covalent electrons of the bond in the (1 1 1) and (1 0 0) planes, and the bond
Fig. 4. (a) X-ray diffraction spectra of FeSiAl powder samples on different anneal temperature and (b) the effect of anneal temperature on grain size and lattice strain of samples. 4
Journal of Magnetism and Magnetic Materials 493 (2020) 165725
X. Niu
Fig. 5. (a) The supercell (2 × 2 × 2) of α-Fe(Si, Al), in which the Si and Al atom substitutes Fe atom in the body-centered position to sayify energy minimum principle and (b) the energy band structure for FeSiAl. Table 2 Calculation of the electronic structure of the powder grinding at different times. Rs(1) = 1.1154 Å, Σnc = 3.7493 Time (h)
a (Å)
D (n)
Iα
By experiment
By empirical theory
nα
ΔD (Å)
|ΔD| % D¯ (nA )
0
2.8496
−0.0014
0.057
15
2.8557
−0.0033
0.134
20
2.8522
−0.0002
0.008
25
2.8544
−0.0022
0.089
30
2.853
0.39946 0.09227 0.39956 0.09214 0.39956 0.09214 0.39964 0.09204 0.39951 0.09221 0.39958 0.0921 0.39953 0.09217
0.065
2.8536
2.4699 2.8518 2.4699 2.8521 2.4699 2.8521 2.4698 2.8524 2.4699 2.8519 2.4698 2.8522 2.4699 2.852
−0.0016
10
2.4678 2.8496 2.4715 2.8538 2.4713 2.8536 2.4731 2.8557 2.4701 2.8522 2.472 2.8544 2.4708 2.853
0.085
2.8538
8 6 8 6 8 6 8 6 8 6 8 6 8 6
0.0021
5
D D D D D D D D D D D D D D
−0.0009
0.036
(nA) (nB) (nA) (nB) (nA) (nB) (nA) (nB) (nA) (nB) (nA) (nB) (nA) (nB)
disorder, vacancies, etc. caused by grinding change the structure of the powder and eventually lead to changes in the covalent electron pair number of the bond. In all, it shows that the electronic structure will be changed by the grinding process.
2019hx010) and Dual-ability Teaching Team of Suzhou University (No. 2019XJSN04).
4. Conclusion
[1] T. Takahashi, K. Yoshida, Y. Shimizu, A. Setyawan, M. Bito, M. Abe, A. Makino, FeSi-B-P-C-Cu nanocrystalline soft magnetic powders with high Bs and low core loss, AIP Adv. 7 (2017) 056111. [2] A. Makino, Nanocrystalline soft magnetic Fe-Si-B-P-Cu alloys with high of 1.8-1.9T contributable to energy saving, IEEE Trans. Magn. 48 (2012) 1331–1335. [3] A. Wang, C. Zhao, H. Men, A. He, C. Chang, X. Wang, R. Li, Fe-based amorphous alloys for wide ribbon production with high Bs and outstanding amorphous forming ability, J. Alloys Compd. 630 (2015) 209–213. [4] Z. Chen, X. Liu, X. Kan, Z. Wang, R. Zhu, W. Yang, K. Rehman, Improved soft magnetic properties of Fe83.5Si7.5B5Cr4 amorphous nanocrystalline powder cores by adding permalloy, J. Mater. Sci-Mater. 29 (2018) 19316–19321. [5] B. Zuo, T. Sritharan, Ordering and grain growth in nanocrystalline Fe75Si25 alloy, Acta Mater. 53 (2005) 1233–1239. [6] H. Shokrollahi, K. Janghorban, Soft magnetic composite materials (SMCs), J. Mater. Process Technol. 189 (2007) 1–15. [7] Y. Feng, C. Tang, T. Qiu, Effect of ball milling and moderate surface oxidization on the microwave absorption properties of FeSiAl composites, Mate. Sci. Eng. B-Adv. 178 (2013) 1005–1011. [8] T. Ma, M. Yan, W. Wang, The evolution of microstructure and magnetic properties of Fe-Si-Al powders prepared through melt-spinning, Scr. Mater. 58 (2008) 243–246. [9] D. Zhou, H. Zhou, F. Liang, L. Deng, Structure and electromagnetic characteristics of flaky FeSiAl powders made by melt-quenching, J. Alloys Compd. 484 (2009). [10] C. Zhang, J. Jiang, S. Bie, L. Zhang, L. Miao, X. Xu, Electromagnetic and microwave absorption properties of surface modified Fe-Si-Al flakes with nylon, J. Alloys Compd. 527 (2012). [11] Y. Qing, W. Zhou, F. Luo, D. Zhu, Thin-thickness FeSiAl/flake graphite-filled Al2O3 ceramics with enhanced microwave absorption, Ceram. Int. 43 (2017) 870–874. [12] J. Sun, H. Xu, Y. Shen, H. Bi, W. Liang, R. Yang, Enhanced microwave absorption
References
We had studied grain growth, lattice strain, order behavior and electronic structure of FeSiAl powder. The grain size and internal stress decreased and increased with the increase of ball milling time, but the opposite trend could be found with the increment of annealing temperature. The order DO3 superlattice coexisting with the disorder α-Fe matrix in the raw samples disappears with long time grinding, but appears again after relieaf annealing (> 773 K). Annealing at temperatures above 773 K results in LRO parameters above 0.67. This high LRO parameter further supports the observation of the formation of order DO3 structures. Furthermore, the change in the covalent electron pair number of the bond in the (1 1 1) plane could be ascribed to the milling process by the calculation of the empirical electron theory of solids and molecules. Acknowledgments This work was supported by Research on Quality Engineering Project Foundation of Anhui (Nos. 2016sjjd074, 2018mooc385), Academic Technical Leader of Suzhou University (No. 2018xjxs01), School-Enterprise Cooperation Project of Suzhou University (No. 5
Journal of Magnetism and Magnetic Materials 493 (2020) 165725
X. Niu
[13]
[14]
[15] [16]
properties of the milled flake-shaped FeSiAl/graphite composites, J. Alloys Compd. 548 (2013) 18–22. Z. Chen, X. Liu, X. Kan, et al., Phosphate coatings evolution study and effects of ultrasonic on soft magnetic properties of FeSiAl by aqueous phosphoric acid solution passivation, J. Alloys Compd. 783 (2019) 434–440. B. Zuo, N. Saraswati, T. Sritharan, H. Hng, Production and annealing of nanocrystalline Fe-Si and Fe-Si-Al alloy powders, Mater. Sci. Eng. A-Struct. 371 (2004) 210–216. J. Fecht, Nanostructure formation and properties of metals and composites processed mechanically in the solid state, Scr. Mater. 44 (2001) 1719–1723. S. Miraghaei, P. Abachi, R. Madaah-Hosseini, A. Bahrami, Characterization of mechanically alloyed Fe100-xSix and Fe83.5Si13.5Nb3 nanocrystalline powders, J. Mater.
Process. Technol. 203 (2008) 554–560. [17] H. Bakker, F. Zhou, H. Yang, Mechanically driven disorder and phase transformations in alloys, Prog. Mater. Sci. 39 (1995) 159–241. [18] X. Ma, J. Jiang, S. Bie, L. Miao, Electronic structure and magnetism of Fe3Al1−xSix alloys, Intermetallics 18 (2010) 2399–2403. [19] T. Zhou, D. Liang, L. Deng, D. Luan, Electron structure and microwave absorbing ability of flaky FeSiAl powders, J. Mater. Sci. Technol. 27 (2011). [20] V. Cardoso, M. Sandoval, J. Ardenghi, P. Bechthold, E. Gonzalez, P. Jasen, Electronic structure and magnetism on FeSiAl alloy: a DFT study, J. Magn. Magn. Mater. 389 (2015) 73–76. [21] Y. Ye, P. Li, L. He, Valence electron structure analysis of morphologies of Al3Ti and Al3Sc in aluminum alloys, Intermetallics 18 (2010) 292–297.
6
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
The study on order behavior and electronic structure of flaky FeSiAl powder
T
Xiaofei Niu Anhui Key Laboratory of Spintronics and Nanomaterials Research, Suzhou University, Suzhou 234000, Anhui, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Flaky FeSiAl Order behavior DO3 superlattice Electronic structure
In this paper, the grain growth, lattice strain, order behavior and electronic structure of FeSiAl powder were studied. Only the disorder bcc α-Fe peaks were exhibited after milling, and as the ball milling time increases, the internal strain rose and the grain size decreased. When the anneal temperature reached 773 K, we could see the obvious DO3 superlattice peak at (1 1 1), (2 0 0) and (3 1 1) plane. At the same time, the internal strain decreased and the grain size rose with the increment of annealing temperature. Annealing at temperatures above 773 K results in a long-range order (LRO) parameter about 0.67–0.76. This high LRO parameter further supports the observation of the formation of order DO3 structures. Furthermore, we found that the interaction between Fe and Si, Al atoms can decrease magnetic moment of the Fe atom. By the calculation of the empirical electron theory of solids and molecules, we confirmed that grinding process could change the electronic structure and the bond length difference ΔD of powder milled for 20 h had the smallest values.
1. Introduction The soft magnetic material having a nanocrystalline structure has exhibited improved magnetic properties when its grain size is less than the ferromagnetic exchange length of typically several tens of nanometers [1–5]. Nowadays, FeSiAl alloys have attracted great interests due to their cheap price, high magnetic permeability, high electrical resistivity, low coercivity, and relatively low loss [6,7]. The FeSiAl alloy consists of a disordered α-Fe structure and an ordered DO3 superlattice structure and the presence of DO3 superlattice structure can improve its soft magnetic properties [8]. The researchers of TOKIN Company studied water atomization FeSiAl powders made by ball milling. They found that the flake-shaped FeSiAl powders have good application prospects in electromagnetic interference (EMI) [9]. Therefore, flaky FeSiAl powders have been widely studied for microwave absorbing materials in recent years [10–12]. However, large cold deformation during grinding contribute to internal strain, so subsequent stress relief annealing is necessary to obtain good soft magnetic properties [13]. At the same time, the control of the annealing temperature is also the key for FeSiAl powders to the precipitation of the DO3 superlattice and the adjustment of the grain size. So far, a large number of studies on the structure, soft magnetic and microwave absorption properties of flaky FeSiAl have been reported [8,9,14]. However, further research is still required for sheetlike FeSiAl prepared by high-energy ball milling and thermal stability of the as-milled nanocrystal structure by anneal, especially considering the key issues of study the processing technology of the material from
E-mail address: [email protected]. https://doi.org/10.1016/j.jmmm.2019.165725 Received 18 July 2019; Accepted 20 August 2019 Available online 22 August 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
its structure, such as the relationship between the electronic structure and the milling time of the powder. In this paper, the flaky FeSiAl powder was prepared by high energy ball milling, and an order DO3 superlattice structure was precipitated by annealing. The morphology size, electronic structure, ordering transformation, grain size and lattice strain of the samples were investigated. 2. Experimental The composition of the gas atomized FeSiAl powder (Tiantong Co., Ltd.) is 9.7 wt% Si, 6 wt% Al, and the balance Fe. The FeSiAl powder has an average size of less than 20 μm. 100 g of powder was placed in a tempered steel cylinder while 1000 g of tempered ball mill balls were added, corresponding to a powder: ball mass ratio of 1:10. At the same time, 200 g of alcohol was adding as a solvent to prevent oxidation, which corresponds to a mass ratio of powder: alcohol of 1:2. The powder was milling in planetary ball mill at 300 rpm for 5, 10, 15, 20, 25, 30 h. Than the milling powder was annealed in a nitrogen-filled oven for 60 min. The annealing temperature ranged from 573 to 973 K. The surface morphology of the samples was observed by using a Hitachi S-4800 scanning electron microscopy (SEM). The phase structure of the powder in different stages was analyzed by X-ray diffraction (XRD, Rigaku D/max-2550V/PC). The powder composition was studied by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi).
Journal of Magnetism and Magnetic Materials 493 (2020) 165725
X. Niu
Fig. 1. The SEM micrographs of flaky paticles after grinding for different times: (a) 0 h, (b) 5 h, (c) 15 h, (d) 20 h, (e) 25 h and (f) 30 h.
Fig. 2. (a) X-ray diffraction spectra of various FeSiAl powder samples and (b) the effect of milling time on grain size and lattice strain of samples.
2
Journal of Magnetism and Magnetic Materials 493 (2020) 165725
X. Niu
3. Results and discussion
Specifically, at low temperatures (< 673 K), the grains show limited grain growth and the internal strain decreases slightly, and at higher temperature ranges (> 673 K), the order phase precipitates as the grain grows rapidly and the internal strain decreases significantly. The order degree can be quantified by long-range order (LRO) parameter, which is determined by the intensity of the reflection of superlattice and basic. The LRO parameter can be defined as [17]
3.1. Morphology and order behavior Fig. 1 presents the SEM micrographs of the milling FeSiAl powders. It can be easily found that the powder gradually changes from a spherical shape to a sheet shape. After ball milling for 15 h, the thickness of the powder is reduced to less than 1 μm. We can also find that as the ball milling time increases, the size of the powder first increases and then decreases. As can be seen from Fig. 1(a), the nearly spherical FeSiAl powder has a broad size distribution of several to several tens of microns, but the flaky powder has the bigger average size about 30 μm in Fig. 1(c). When the milling time exceeds 15 h, the length of powder decreases on the contrary and the powder begins to agglomerate. It can be explained that more and more powders crack under high internal stress and accumulate on the edges of uncracked sheets. Although the powder still maintains a large aspect ratio (> 10:1), a large number of smaller particles are formed, indicating that the extended milling has a detrimental effect on the fabrication of FeSiAl flakes. The XRD patterns of the samples at different milling time are shown in Fig. 2(a). We can find that the diffraction peaks of the DO3 superlattice appear at the positions of the (1 1 1) and (2 0 0) crystal faces, so the untreated powder has a disorder and order structure. In the bcc α-Fe phase, Fe, Si, and Al atoms randomly occupy positions and appear disorder, but in the DO3 superlattice structure (shown in Fig. 5(a)), Si and Al atoms mainly occupy alternate body center positions, presenting order state [8]. However, only the bcc α-Fe peaks are exhibited after milling. It can be explained that the Al and Si atoms are completely dissolved in the lattice of Fe [9]. Fig. 2(b) exhibits the grinding time dependence of the grain size and internal strain, and the specific data can be seen in Table 1. We can find that the grain size has a sharp drop and then tend to steady and the internal strain has an opposite trend. So it is reasonable that more and more powders crack under high internal stress. In addition, lattice distortion induced by internal stress is also the cause of the disappearance of DO3 superlattice. Fig. 3(a) shows the shape evolution model of FeSiAl powder under the condition of milling. As the ball milling time increases, the powder first becomes flat and larger, and then the edge breaks to form agglomeration, which is consistent with the result of SEM. At the same time, Fecht [15] found that a large number of dislocation could be formed during the milling process, which resulted in the nucleation of nanocrystalline, and the nucleation of nanocrystals further reduced grain size. In addition, the composition of the alloy will be converted into a pure iron component and cause changes in magnetic properties [16]. The XPS spectra of Fig. 3(b)–(d) can prove it. The arrow points to the peak of Fe0 and we can find the intensity of Fe0 increases with the milling process. The XRD diffraction patterns shown in the Fig. 4(a) reveal structure changes of the powder samples at different annealing temperature. We can find obvious DO3 superlattice phase at the (1 1 1), (2 0 0) and (3 1 1) crystal faces when the annealing temperature more than 673 K. Therefore, some disorder supersaturated bcc solid solutions have been transformed into DO3 order structures. From Fig. 4(b), the annealing temperature dependence of the grain size and internal strain is observed. We can easily find that the grain size increases and the lattice strain decreases with the increment of annealing temperature.
1/2
I s / I0s ⎤ LRO = ⎡ ⎢ I f /I f ⎥ 0 ⎦ ⎣ s
(1)
f
where I and I are the intensity of the superlattice line and the basic line after annealing, respectively, and I0s and I0f are the intensity of the same line in the fully order sample. A diffraction pattern using the ideal fully order DO3 structure that has been reported [18] is used to determine I0s and I0f . Using the prominent DO3 (1 1 1) superlattice peak and (2 2 0) base peak, the LRO parameters for different annealing conditions were calculated as shown in Table 1. Annealing at temperatures above 773 K results in an LRO parameter above 0.67. This excellent LRO parameter further supports the retionality of the formation of order DO3 structures. 3.2. Electronic structure research To explore the electronic structure of FeSiAl, the electron structure calculation was performed in Cambridge Serial Total Energy Package (CASTEP, version 2017). The following elements should be considered in the modeling process: for the minimum energy principle, one of the Fe atoms at the body center position is replaced by Si or Al atom, and a supercell (2 × 2 × 2) is generated to meet the crystal period, as shown in Fig. 5(a). The energy band can be found in Fig. 5(b). From the Fig. 5(b), the energy band structure of the crystal is calculated along the symmetry line Z-A; A-M; M-G; G-Z; Z-R; R-X; and X-G. It can be easily found that FeSiAl is a conductor. When the Fe atom is replaced randomly by a Si or Al atom, the electronic configuration of Fe in unit cell will changes. The interaction between Fe and Si, Al atoms can decrease magnetic moment of the Fe atom. In order to prove it, the Bohr magneton of Fe atom is analyzed by the Eq. (2):
m (μB ) = mσ3d (1 − x − y )
(2)
where x and y are the contents of Si and Al atoms, respectively. Since there is no 3d electron in the Al or Si atom, the magnetic moment of the FeSiAl comes only from the 3d electron of the Fe atom. An increase in the milling time increases the amount of Si and Al dissolved in Fe, causing an increases of contents of Si and Al atoms in Fe solid solution and a decrease in magnetic moment of Fe. At the same time, the statistical mean of the exchange split (Δx) can be calculated by considering the 60 bands near the Fermi surface. From previous research [18,20], the introduction of Al and Si will reduce Δx, which may contribute to a decrease in the Curie temperature Tc due to a decrease in magnetic moment. Furthermore, it is important to study the relationship between electronic structure and grinding. Due to the additive Al and Si atom replaces the Fe atom in the FeSiAl alloy and the primary bond in the αFe consists of the shortest bond of the (1 1 1) face and the sub-short bond of the (1 0 0) face. We can use the empirical electron theory of
Table 1 Grain size, lattice strain and LRO parameter of various FeSiAl powder samples. Milling time (h)
Grain size (nm) Lattice strain (%) LRO parameter
Annealing temperature (K)
0
5
10
15
20
25
30
573
673
773
873
973
29.2 0.2887 –
13.1 0.6953 –
12.3 0.7312 –
12.1 0.7447 –
12 0.7578 –
11.8 0.7594 –
11.9 0.7666 –
14.1 0.6235 –
15.8 0.5064 –
19.9 0.4452 0.6709
36.6 0.2456 0.7053
79.5 0.1064 0.7556
3
Journal of Magnetism and Magnetic Materials 493 (2020) 165725
X. Niu
Fig. 3. (a) The shape evolution model of FeSiAl powder under different milling time and (b)–(d) the XPS spectra of 0 h, 15 h and 30 h sample for Fe2p.
length difference of the two planes, and are listed in Table 2. By calculation, we can find that the value of bond length difference ΔD of different samples is very small, indicating that all the states are reasonable. The sample milled 20 h has the smallest bond length difference (ΔD = −0.0002) among the seven samples. At the same time, the pair number in the (1 0 0) plane is much smaller than the pair number in the (1 1 1) plane. This indicates that there is a stronger bonding tendency between the atoms of the bonds A (1 1 1) than the atoms of the bonds B (1 0 0) [21]. The DO3 superlattice structure disappears and the structure become disorder at various milling time, which is consistent of the previous work [8,14]. The internal strain,
solids and molecules to calculate the bond length difference [9,19].
nA =
∑ nC , nB = nA rB IA + IB rB
D (nα ) =
2Rs (1)
− β lg nα
(3) (4)
where,α represents the A (1 1 1) plane or the B (1 0 0) plane, β is a constant(β = 0.6), I is the number of bonds, Σnc is the number of covalent electrons, Rs(1) is the half length of a single bonds, D(n) is the bond length and ΔD is the bond length difference. Based on this theory, we can calculate the bond length, the pair number of the covalent electrons of the bond in the (1 1 1) and (1 0 0) planes, and the bond
Fig. 4. (a) X-ray diffraction spectra of FeSiAl powder samples on different anneal temperature and (b) the effect of anneal temperature on grain size and lattice strain of samples. 4
Journal of Magnetism and Magnetic Materials 493 (2020) 165725
X. Niu
Fig. 5. (a) The supercell (2 × 2 × 2) of α-Fe(Si, Al), in which the Si and Al atom substitutes Fe atom in the body-centered position to sayify energy minimum principle and (b) the energy band structure for FeSiAl. Table 2 Calculation of the electronic structure of the powder grinding at different times. Rs(1) = 1.1154 Å, Σnc = 3.7493 Time (h)
a (Å)
D (n)
Iα
By experiment
By empirical theory
nα
ΔD (Å)
|ΔD| % D¯ (nA )
0
2.8496
−0.0014
0.057
15
2.8557
−0.0033
0.134
20
2.8522
−0.0002
0.008
25
2.8544
−0.0022
0.089
30
2.853
0.39946 0.09227 0.39956 0.09214 0.39956 0.09214 0.39964 0.09204 0.39951 0.09221 0.39958 0.0921 0.39953 0.09217
0.065
2.8536
2.4699 2.8518 2.4699 2.8521 2.4699 2.8521 2.4698 2.8524 2.4699 2.8519 2.4698 2.8522 2.4699 2.852
−0.0016
10
2.4678 2.8496 2.4715 2.8538 2.4713 2.8536 2.4731 2.8557 2.4701 2.8522 2.472 2.8544 2.4708 2.853
0.085
2.8538
8 6 8 6 8 6 8 6 8 6 8 6 8 6
0.0021
5
D D D D D D D D D D D D D D
−0.0009
0.036
(nA) (nB) (nA) (nB) (nA) (nB) (nA) (nB) (nA) (nB) (nA) (nB) (nA) (nB)
disorder, vacancies, etc. caused by grinding change the structure of the powder and eventually lead to changes in the covalent electron pair number of the bond. In all, it shows that the electronic structure will be changed by the grinding process.
2019hx010) and Dual-ability Teaching Team of Suzhou University (No. 2019XJSN04).
4. Conclusion
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We had studied grain growth, lattice strain, order behavior and electronic structure of FeSiAl powder. The grain size and internal stress decreased and increased with the increase of ball milling time, but the opposite trend could be found with the increment of annealing temperature. The order DO3 superlattice coexisting with the disorder α-Fe matrix in the raw samples disappears with long time grinding, but appears again after relieaf annealing (> 773 K). Annealing at temperatures above 773 K results in LRO parameters above 0.67. This high LRO parameter further supports the observation of the formation of order DO3 structures. Furthermore, the change in the covalent electron pair number of the bond in the (1 1 1) plane could be ascribed to the milling process by the calculation of the empirical electron theory of solids and molecules. Acknowledgments This work was supported by Research on Quality Engineering Project Foundation of Anhui (Nos. 2016sjjd074, 2018mooc385), Academic Technical Leader of Suzhou University (No. 2018xjxs01), School-Enterprise Cooperation Project of Suzhou University (No. 5
Journal of Magnetism and Magnetic Materials 493 (2020) 165725
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