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Influence of heat treatment on the microwave absorption properties of flaky carbonyl iron powder
International Journal of Lightweight Materials and Manufacture xxx (xxxx) xxx
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Original Article
Influence of heat treatment on the microwave absorption properties of flaky carbonyl iron powder Hongyu Wei a, *, Zhiping Zhang a, Laishui Zhou a, Behzad Heidarshenas a, Chi Zhang a, Jun Xia a, Linyi Zhi a, Guozhu Shen b, Hongyan Wu b a b
College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 January 2020 Received in revised form 5 February 2020 Accepted 9 February 2020 Available online xxx
High-energy ball milling is a very common method to produce materials with high magnetic permeability due to its ability to produce flaky shape particles on the nano-scale. It is an efficient technique to improve microwave absorbing properties. Therefore, this study aims to achieve an ideal microstructure and improve microwave absorption properties of flake carbonyl iron powder fabricated by high-energy ball milling. The influence of pre-heating at different times/temperatures on magnetic absorbing properties is also investigated. Further, the relationship between heat treatment temperature, permeability is analyzed. Microstructural analyses reveal that the phase of the material does not change with the change of pre-heating temperature. However, the material made by ball-milling has a finer grain size and a higher flattening ratio. The results show that the length to diameter ratio of the flake material can be increased by pre-heating, and thus improve the magnetic permeability and absorbing performance of the material. The optimal parameters of pre-heating are obtained at the temperature and time of 200 C and 2 h, respectively, wherein the real part of the permeability reaches up to 3.20 at 2 GHz, the imaginary part reaches 1.61 at 6.2 GHz. © 2020 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).
Keywords: Pre-heat treatment Flaky carbonyl iron powder High-energy ball milling Lattice parameter Absorbing properties
1. Introduction Magnetic absorbing materials are widely used for defense applications. The ability to effectively absorb electromagnetic waves is a key performance indicator of these materials [1e3]. The magnetic materials with high permeability and high magnetic loss are deemed to ensure a higher absorption effect. The material processing parameters play a key role in determining the magnetic absorbing properties [4,5]. Many studies have shown that magnetic materials with flake morphology can exceed the Snoekds limit (one of the conditions to achieve the magnetic absorbing effect) and
* Corresponding author. E-mail addresses: [email protected] (H. Wei), [email protected] (Z. Zhang), [email protected] (L. Zhou), [email protected] (B. Heidarshenas), [email protected] (C. Zhang), [email protected] (J. Xia), [email protected] (L. Zhi), [email protected] (G. Shen), whyc3w3@ 126.com (H. Wu). Peer review under responsibility of Editorial Board of International Journal of Lightweight Materials and Manufacture
obtain a high permeability in the high-frequency range (2e18 GHz). It can meet the requirements of “thin, light, wide and strong” of microwave absorbing materials [6]. Equation (1) presents an expression on guiding how one can exceed the Snoek's limit [7,8]:
ðmi 1Þf0 ¼
2gMs
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Hha =Hea 3p
(1)
where mi, g, f0, and Ms are the relative initial permeability, the material's gyromagnetic ratio, the cut-off frequency, and saturation magnetization, respectively. Moreover, Hea is the anisotropic equivalent field when the magnetization vector (M) and the equilibrium position are apart by an angle 4; Hha is the anisotropic equivalent field of the magnetization vector (M) starting from the equilibrium position to the base plane. High-energy ball milling is used to prepare flaky magnetic absorbing materials, such as FeeSi, FeeSieAl, carbonyl iron powder (CIP), etc. Acceptable magnetic and microwave absorption properties of materials can be achieved through this method. However, the as-produced materials may have high lattice defects and
https://doi.org/10.1016/j.ijlmm.2020.02.001 2588-8404/© 2020 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article as: H. Wei et al., Influence of heat treatment on the microwave absorption properties of flaky carbonyl iron powder, International Journal of Lightweight Materials and Manufacture, https://doi.org/10.1016/j.ijlmm.2020.02.001
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internal strain which is likely to have a negative effect on their magnetic properties [9e11]. The proper heat treatment can recover lattice defects and reduce internal stress and strain induced in the materials induced while ball milling [12,13]. The annealing treatment of milled FeeCo, FeeSieAl, and other magnetic materials can effectively reduce the macro strain, increase the grain size, and improve permeability [14,15]. The magnetic permeability of flake carbonyl iron powder increases after approaching a specific heat treatment temperature. The temperature and time of heat treatment affect other properties such as reflectivity [16,17]. Many studies put the points on these. Wang et al. [18] proposed a heating wet ball-milling method by controlling the ambient temperature during high energy ball milling. Chen et al. [19] employed high-energy ball milling combined with heat treatment to produce FeeSieAl powder. They found that pre-heating and post-heating at 500 C decreased internal strain from 0.45% to 0.16% and enhanced both real and imaginary parts of magnetic permeability from 4.1 to 5.09, and 2.3 to 3.65, respectively. Zhou et al. [20] mixed ZrO2 with FeeSieAl micro-powder by the ball milling process. After post heat treatment at 573 K, the magnetic permeability increased resulting in a high peak Reflection Loss of 10 dB at the frequency of 1.3 GHz. The above literature analysis points out that heat treatment can significantly improve the magnetic properties of absorbing materials. Also, the nature and degree of heat treatment effect may vary from material to material. Limited efforts have been spent on heat treatment effects, specifically pre-heat treatment, of magnetic materials with flaky morphology of particles prepared by highenergy ball milling. Also, more investigations need to be performed on specific reasons or comparative analysis of why and how heat-treating impacts on the materials’ properties. This study focuses on the influence of preheating temperature (150 C/200 C/ 250 C) and preheating time 2 h/3 h/4 h on the magnetic permeability of Carbonyl Iron Powders (CIPs) prepared by high-energy ball milling. This research achieves an ideal flake carbonyl iron powder fabricated by high-energy ball milling and compared the influence of pre-heating at different times/temperatures on magnetic absorbing properties. Further, the relationship between heat treatment temperature, permeability is analyzed. 2. Experimental details Raw materials of spherical Carbonyl Iron Powder were purchased from Jiangsu Tianyi Ultra-fine Metal Powder Co., Ltd. with a mean diameter of about 2.5 mm. At first, the nitrogen as protective gas was injected which followed the insertion of 45 g of CIPs as a raw material in a Tube Furnace. Then flaky CIPs were produced by a High-energy Ball Milling (XQM-20) with the ball-to-powder weight ratio of 10:1 and speed of 500r/min for 12hr. During the milling process, 50 mL of alcohol was added as a process control agent. In order to assess the influence of preheating temperature and time on the microstructure evolution of the nanocrystalline CIPs during the micro-fabrication processing, the experiments were conducted over a range of temperatures (i.e., 150 C/200 C/250 C) and time (i.e., 2 h/3 h/4 h). The experimental settings are presented in Table 1. All the as-milled samples were separated by filtration after cooling down to room temperature in the atmosphere of nitrogen and then dried at 60 C in the air for further characterization. The research plan adopted in this research work is presented in Fig. 1. The microstructure and morphology of the samples were analyzed by a Su3500 scanning electron microscope (SEM) at an accelerating voltage of 10 kV and a Smart lab 9kw X-ray diffractometer equipped with graphite monochromatized Cu Ka radiation (k ¼ 1.5418 Å), respectively. The average width (L) and thickness (d) of the as-milled particles were calculated considering at least 100
Table 1 Pre-heating parameters. No.
Pre-heating temperature
Pre-heating time
High energy ball milling
1 2 3 4 5 6
e 150 200 200 200 250
e 2h 2h 3h 4h 2h
12 12 12 12 12 12
C C C C C
h h h h h h
particles via the SEM. To obtain the cylindrical toroidal samples for microwave measurement, 70 vol% of the as-obtained powders were fully mixed with paraffin wax. The samples were subsequently pressed into annular disks with a size of 7 3.04 3 mm. The complex permittivity (i.e., ε ¼ ε0 -jε00 ) and permeability (i.e., m ¼ m0 jm00 ) of the composite samples were measured using an Agilent 5245A vector network analyzer over the frequency range of 1e18 GHz. The reflection loss (RL) was calculated following equation (2):
RL ¼ 20 logjðZin Z0 Þ = Zin þ Z0 j
(2)
Z0 ¼ ðm0 =ε0 Þ1=2
(3)
i h Zin ¼ Z0 ðm=εÞ1=2 tanðhÞ j2pfdðmεÞ1=2 c
(4)
where Z0, Zin, f, d, and c are free space impedance shown as equation (3), input impedance shown as equation (4), frequency, the coating thickness and the speed of light in vacuum, respectively.
3. Results and discussion The SEM micrographs of ball-milled CIPs (for 12 h) preheated at different temperatures and times are presented in Fig. 2. These particles have a spherical shape with a mean size of about 2.5 mm. As can be seen from the morphological evolution of the CIPs (Fig. 2), the morphology of powders changes with the change of the temperature and time (1-untreated, 2e150 C/2 h, 3e200 C/2 h, 4e200 C/3 h, 5e200 C/4 h, 6e250 C/2 h). In this study, the diameter of each particle was measured in order to accurately evaluate the changes in the average aspect ratio. The average and maximum size of particles derived through ImageJ software are listed in Table 2. As can be seen from Fig. 2 (1), powders without heat treatment after ball milling show irregular morphology. After 150 C/2 h heat treatment (Fig. 2 (2)), the powders got more idea to flatten shape than untreated because of the difference plasticity of the powders with or without pre-heating [21]. There is a similar effect on 200 C and 250 C temperature (Fig. 2 (3) and (6)). Different process temperatures provide various levels of energy on particles. The results show that the plasticity of the particles is fully improved at 200 C/3 h, which confirms the considerable flakiness of the ballmilled powder. At the same time, in the 200 C/3 h group, the maximum flake size was 7.44 mm and the average value was 4.72 mm. However, the other two groups (i.e., 200 C/2 h and 200 C/4 h) were similar to the untreated group, with a relatively minor difference. Thus, this parameter plays an essential role in the plasticity and thus, the morphology of particles. By comparing the effects of temperature and time on the final morphology, it is revealed that a maximum and average size of 8.49 mm and 4.72 mm can be achieved at 150 C/2h and 200 C/3h preheating, respectively. This is due to the fact that the heat
Please cite this article as: H. Wei et al., Influence of heat treatment on the microwave absorption properties of flaky carbonyl iron powder, International Journal of Lightweight Materials and Manufacture, https://doi.org/10.1016/j.ijlmm.2020.02.001
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Fig. 1. The idea and objective of this research.
Fig. 2. SEM of samples obtained with different pre-heating times and temperatures ((1) untreated; (2) 150 C/2 h; (3) 200 C/2 h; (4) 200 C/3 h; (5) 200 C/4 h; (6) 250 C/2 h).
Table 2 Characteristic parameters of CIPs in each group. Pre-heating temperature
Pre-heating time
Mean size
Maximum size
Grain size
The internal strain
e 150 200 200 200 250
e
4.05 mm
6.84 mm
20 nm
0.102%
12 17 13 12 85
0.336% 0.301% 0.104% 0.185% 0.293%
C C C C C
2 2 3 4 2
h h h h h
4.35 3.89 4.72 3.93 3.73
mm mm mm mm mm
treatment temperature affects the maximum size of the particles, representing the improvement in the level of plasticity. However, the time of heat treatment affects the average value of particle size, i.e. the coverage of overall plasticity improvement. The XRD analysis of the flaky nanocrystalline CIPs fabricated at various temperatures and times is shown in Fig. 3. It can be seen
8.49 7.13 7.43 7.29 6.64
mm mm mm mm mm
nm nm nm nm nm
from the figure that all samples show three characteristic diffraction peaks at 2 h ¼ 44.6 , 65.0 , and 82.3 . They are respectively associated with the (110), (200), and (211) planes of cubic phase aFe. It demonstrates that the phase of the material after ball milling does not change with the change of pre-heating temperature and time. Table 2 shows grain size(D) and internal strain (x) of the
Please cite this article as: H. Wei et al., Influence of heat treatment on the microwave absorption properties of flaky carbonyl iron powder, International Journal of Lightweight Materials and Manufacture, https://doi.org/10.1016/j.ijlmm.2020.02.001
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Fig. 3. XRD of samples obtained by ball milling under different parameters.
milled flaky nanocrystalline CIPs as functions of temperature and time after ball milling. The grain size is calculated from the (110) peaks according to the Scherrer's formula, and the internal strain was obtained using the HalleWilliamson analysis as shown below [22,23]:
bhkl cos q ¼ K lD þ 4x sin q
(5)
where bhkl is the corrected full width at half maximum (FWHM) corresponding to each diffraction peak. Shape factor K equals 0.9, and the wavelength of Cu Ka radiation l ¼ 1.5418 Å. According to Table 2, with the increase of heat treatment temperature at a constant time of 2 h, the grain size of the material slightly decreases initially following an increase in a way to yield a size much greater at 250 C than that of the untreated material. This is due to the fact that the grain size grows up because of the high temperature. Also, the internal strain of the material initially increases, approaches to a maximum value at 150 C, and then gradually decreases with the increase of temperature. This shows that preheating has a significant influence on the grain size and internal strain of material and hence on its electromagnetic parameters. With increasing the duration of heat treatment (from 2 h to 4 h at 200 C), the grain size gradually decreases while the internal strain drops after gaining a maximum value at 2 h. It can be
due to a large change in plastic deformation in the material and no strain accumulation of the material. Figs. 4 and 5 show the complex permittivity and permeability, in a frequency range of 2e18 GHz, of the flaky CIPs, fabricated for 2 h at 150 C, 200 C, and 250 C. From Fig. 4, it is to notice that the real part of permittivity (ε0 ) of the powders, after heating, is significantly reduced. When the pre-heating conditions are 150 C/ 2h, the real part (ε0 ) is reduced to 13, which represents electrode polarization effects due to temperature change [24]. The changes in the imaginary part (ε00 ) of the permittivity over the considered range of frequency show an irregular trend. The change in ε00 of untreated powder is gentle, and the peak value of ε00 appears after 150 C/2h and 200 C/2h treatments. It slightly decreases in the low-frequency band but increases in the high-frequency band. While ε00 of the powders treated at 250 C/2h increases sharply and shows an obvious peak. This is due to the fact that heat treatment results in the alteration of the lattice parameters of the material thereby rendering a reduction in the internal relaxation effect and dielectric losses in a specific frequency band. It is mainly because of the change in size and internal strain of the grain and alteration in the particle morphology [25]. It can be seen from Fig. 5 that when the preheating temperature is 150 C, m0 and m00 of the magnetic permeability are lower than those of the untreated material. Moreover, m0 and m00 increase at the temperatures of 200 C and 250 C. At 200 C, m0 reaches to a maximum value of 3.2 at 2 GHz. This endorses a claim that the improvement of the initial permeability (m0) of the material can be achieved through a pre-heat treatment process. The intrinsic reason is that the grain size approaches nano-scale which can be confirmed by XRD. The maximum value of m00 is 1.61 which is obtained at 6.2 GHz. However, the peak value increases, that is, the breakthrough of the Snoek's limit is also achieved simultaneously. Yet, the effect is not obvious from time to time. In addition, it is worth noticing that in terms of appearance, the average size of the material pre-heated at 150 C is larger, and the flattening effect is better. Nevertheless, its magnetic permeability does not show a significant improvement owing to the fact that the high flattening ratio increases the eddy current loss effect and suppresses the magnetic permeability rise. Figs. 6 and 7 show the complex permittivity and permeability in 2e18 GHz frequency range for the flaky CIPs fabricated at 200 C for 2 h 3 h and 4 h. It is to observe from Fig. 6 that ε0 of the powders after 2 h and 3 h pre-heating at 2 GHz are reduced from its initial value of 16.8 to 15.5 and 16.2, respectively. It represents that the change in time results in the reduction of the dielectric ability of the material. After the processing time approaches 4 h ε0 suddenly
Fig. 4. Frequency dependencies of the complex permittivity with different temperatures for the flaky CIPs.
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Fig. 5. Frequency dependencies of the magnetic permeability with different temperatures for the flaky CIPs.
Fig. 6. Frequency dependencies of the complex permittivity with different time for the flaky CIPs.
Fig. 7. Frequency dependencies of the magnetic permeability with different time for the flaky CIPs.
increases to 18.7 and the polarization of the material is enhanced. According to Fig. 2 and Table 2, this phenomenon results from the flattening of the material and changes in the polarization mechanism. There is almost a similar trend for ε00. ε00 of the powder without pre-heating. However, there is a rise in ε00 after 2 h and 3 h processing. It slightly reduces in the low-frequency band but increases in the high-frequency band. After 4 h, ε00 increases overall
with prominent value in high-frequency bands. This mainly stems from the fact that the heat treatment alters the morphology and lattice parameters of the material, causing the internal strain relaxation and dielectric loss to rising significantly. It can be seen from Fig. 7 that m0 and m00 of the powders are reduced after heat treatment at 200 C at different times. After 200 C/2 h treatment, m0 reaches the maximum of 3.2 with a peak at
Please cite this article as: H. Wei et al., Influence of heat treatment on the microwave absorption properties of flaky carbonyl iron powder, International Journal of Lightweight Materials and Manufacture, https://doi.org/10.1016/j.ijlmm.2020.02.001
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Conflicts of interest
Table 3 Characteristic parameters of CIPs in each group. Pre-heating temperature
Pre-heating time
Magnetic permeability
e 150 200 200 200 250
e
mr0 : 2.73, mr0 : 1.19 mr0 : 2.61, mr0 : 1.03 mr0 : 3.20, mr0 : 1.61 mr0 : 2.71, mr0 : 1.16 mr0 : 2.96, mr0 : 1.33 mr0 : 2.93, mr0 : 1.42
C C C C C
2 2 3 4 2
h h h h h
2 GHz, and after 200 C/4 h treatment, it approaches a maximum value of 2.96 at 2 GHz. It proves that the improvement of the initial permeability of the material can be achieved through the pre-heat treatment. It is because the material structure is converted to nanocrystals (as can be inferred from Table 2). The maximum value of the imaginary part of 1.61 was obtained at 6.2 GHz while the cut off frequency remains constant. However, the peak value is increased, that is, the Snoek's limit exceeds simultaneously, but the effect was not obvious from time to time. The average size of the particles after 3 h and 4 h pre-heating is larger than that of the sample after 2 h, and the flattening is clearer. Yet, its magnetic permeability is not significantly improved. This is attributed to a fact that the higher flattening ratio tends to increase the eddy current loss effect thereby, in turn, suppressing the increase in magnetic permeability. Table 3 compares magnetic permeability at various experimental conditions. The following observations can be made from this table: (1) From the perspective of morphological evolution, the preheat treatment at a temperature of 150 C and a time of 3 h has a better effect on improving the particles size. The temperature affects the degree of plasticity improvement. Further, the length of the preheating time influences the average value of the particles size, that is, overall plasticity improvement. (2) From the microwave permeability improvement point of view, the preheating of 200 C/2 h can effectively improve the magnetic permeability of flaky powders after ball milling. Moreover, a very large morphological size is not very effective in improving magnetic permeability.
4. Conclusions The flaky CIPs with large size, low x, and appropriately small D are obtained by controlling the temperature and time of preheating and high-energy ball milling for 12 h. The following important conclusions can be derived: (1) The grain size and internal strain changes according to the temperature and time alteration. (2) The changes in morphology, grain size, and internal strain result in the change of magnetic permeability and permittivity as well as the morphology. (3) For the best magnetic absorbing property, the optimal level of particles size and internal strain need to be selected. The optimum pre-heating parameters for carbonyl iron powder are selected as 200 C/2 h, (4) It is found that at optimal pre-heating parameters, the magnetic permeability and absorption of powders approach to the highest level, such that mr0 and mr00 reach 3.20 at 2 GHz and 1.61 at 6.2 GHz respectively. The results here also provide an effective strategy to improve the microwave absorption property of MAMs in a wide frequency range via controlling the particle size, internal strain and grain size of its magnetic metallic Nanocrystalline absorbents.
The authors declare that there is no conflicts of interest.
Acknowledgments The necessary funding to realize this work in the research community was provided by Fundamental Research Funds for the Central Universities [Grant No. NS2015055], and National Natural Science Foundation of China [Grant No. 51105202] for which the authors are grateful.
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Original Article
Influence of heat treatment on the microwave absorption properties of flaky carbonyl iron powder Hongyu Wei a, *, Zhiping Zhang a, Laishui Zhou a, Behzad Heidarshenas a, Chi Zhang a, Jun Xia a, Linyi Zhi a, Guozhu Shen b, Hongyan Wu b a b
College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 January 2020 Received in revised form 5 February 2020 Accepted 9 February 2020 Available online xxx
High-energy ball milling is a very common method to produce materials with high magnetic permeability due to its ability to produce flaky shape particles on the nano-scale. It is an efficient technique to improve microwave absorbing properties. Therefore, this study aims to achieve an ideal microstructure and improve microwave absorption properties of flake carbonyl iron powder fabricated by high-energy ball milling. The influence of pre-heating at different times/temperatures on magnetic absorbing properties is also investigated. Further, the relationship between heat treatment temperature, permeability is analyzed. Microstructural analyses reveal that the phase of the material does not change with the change of pre-heating temperature. However, the material made by ball-milling has a finer grain size and a higher flattening ratio. The results show that the length to diameter ratio of the flake material can be increased by pre-heating, and thus improve the magnetic permeability and absorbing performance of the material. The optimal parameters of pre-heating are obtained at the temperature and time of 200 C and 2 h, respectively, wherein the real part of the permeability reaches up to 3.20 at 2 GHz, the imaginary part reaches 1.61 at 6.2 GHz. © 2020 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).
Keywords: Pre-heat treatment Flaky carbonyl iron powder High-energy ball milling Lattice parameter Absorbing properties
1. Introduction Magnetic absorbing materials are widely used for defense applications. The ability to effectively absorb electromagnetic waves is a key performance indicator of these materials [1e3]. The magnetic materials with high permeability and high magnetic loss are deemed to ensure a higher absorption effect. The material processing parameters play a key role in determining the magnetic absorbing properties [4,5]. Many studies have shown that magnetic materials with flake morphology can exceed the Snoekds limit (one of the conditions to achieve the magnetic absorbing effect) and
* Corresponding author. E-mail addresses: [email protected] (H. Wei), [email protected] (Z. Zhang), [email protected] (L. Zhou), [email protected] (B. Heidarshenas), [email protected] (C. Zhang), [email protected] (J. Xia), [email protected] (L. Zhi), [email protected] (G. Shen), whyc3w3@ 126.com (H. Wu). Peer review under responsibility of Editorial Board of International Journal of Lightweight Materials and Manufacture
obtain a high permeability in the high-frequency range (2e18 GHz). It can meet the requirements of “thin, light, wide and strong” of microwave absorbing materials [6]. Equation (1) presents an expression on guiding how one can exceed the Snoek's limit [7,8]:
ðmi 1Þf0 ¼
2gMs
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Hha =Hea 3p
(1)
where mi, g, f0, and Ms are the relative initial permeability, the material's gyromagnetic ratio, the cut-off frequency, and saturation magnetization, respectively. Moreover, Hea is the anisotropic equivalent field when the magnetization vector (M) and the equilibrium position are apart by an angle 4; Hha is the anisotropic equivalent field of the magnetization vector (M) starting from the equilibrium position to the base plane. High-energy ball milling is used to prepare flaky magnetic absorbing materials, such as FeeSi, FeeSieAl, carbonyl iron powder (CIP), etc. Acceptable magnetic and microwave absorption properties of materials can be achieved through this method. However, the as-produced materials may have high lattice defects and
https://doi.org/10.1016/j.ijlmm.2020.02.001 2588-8404/© 2020 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article as: H. Wei et al., Influence of heat treatment on the microwave absorption properties of flaky carbonyl iron powder, International Journal of Lightweight Materials and Manufacture, https://doi.org/10.1016/j.ijlmm.2020.02.001
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internal strain which is likely to have a negative effect on their magnetic properties [9e11]. The proper heat treatment can recover lattice defects and reduce internal stress and strain induced in the materials induced while ball milling [12,13]. The annealing treatment of milled FeeCo, FeeSieAl, and other magnetic materials can effectively reduce the macro strain, increase the grain size, and improve permeability [14,15]. The magnetic permeability of flake carbonyl iron powder increases after approaching a specific heat treatment temperature. The temperature and time of heat treatment affect other properties such as reflectivity [16,17]. Many studies put the points on these. Wang et al. [18] proposed a heating wet ball-milling method by controlling the ambient temperature during high energy ball milling. Chen et al. [19] employed high-energy ball milling combined with heat treatment to produce FeeSieAl powder. They found that pre-heating and post-heating at 500 C decreased internal strain from 0.45% to 0.16% and enhanced both real and imaginary parts of magnetic permeability from 4.1 to 5.09, and 2.3 to 3.65, respectively. Zhou et al. [20] mixed ZrO2 with FeeSieAl micro-powder by the ball milling process. After post heat treatment at 573 K, the magnetic permeability increased resulting in a high peak Reflection Loss of 10 dB at the frequency of 1.3 GHz. The above literature analysis points out that heat treatment can significantly improve the magnetic properties of absorbing materials. Also, the nature and degree of heat treatment effect may vary from material to material. Limited efforts have been spent on heat treatment effects, specifically pre-heat treatment, of magnetic materials with flaky morphology of particles prepared by highenergy ball milling. Also, more investigations need to be performed on specific reasons or comparative analysis of why and how heat-treating impacts on the materials’ properties. This study focuses on the influence of preheating temperature (150 C/200 C/ 250 C) and preheating time 2 h/3 h/4 h on the magnetic permeability of Carbonyl Iron Powders (CIPs) prepared by high-energy ball milling. This research achieves an ideal flake carbonyl iron powder fabricated by high-energy ball milling and compared the influence of pre-heating at different times/temperatures on magnetic absorbing properties. Further, the relationship between heat treatment temperature, permeability is analyzed. 2. Experimental details Raw materials of spherical Carbonyl Iron Powder were purchased from Jiangsu Tianyi Ultra-fine Metal Powder Co., Ltd. with a mean diameter of about 2.5 mm. At first, the nitrogen as protective gas was injected which followed the insertion of 45 g of CIPs as a raw material in a Tube Furnace. Then flaky CIPs were produced by a High-energy Ball Milling (XQM-20) with the ball-to-powder weight ratio of 10:1 and speed of 500r/min for 12hr. During the milling process, 50 mL of alcohol was added as a process control agent. In order to assess the influence of preheating temperature and time on the microstructure evolution of the nanocrystalline CIPs during the micro-fabrication processing, the experiments were conducted over a range of temperatures (i.e., 150 C/200 C/250 C) and time (i.e., 2 h/3 h/4 h). The experimental settings are presented in Table 1. All the as-milled samples were separated by filtration after cooling down to room temperature in the atmosphere of nitrogen and then dried at 60 C in the air for further characterization. The research plan adopted in this research work is presented in Fig. 1. The microstructure and morphology of the samples were analyzed by a Su3500 scanning electron microscope (SEM) at an accelerating voltage of 10 kV and a Smart lab 9kw X-ray diffractometer equipped with graphite monochromatized Cu Ka radiation (k ¼ 1.5418 Å), respectively. The average width (L) and thickness (d) of the as-milled particles were calculated considering at least 100
Table 1 Pre-heating parameters. No.
Pre-heating temperature
Pre-heating time
High energy ball milling
1 2 3 4 5 6
e 150 200 200 200 250
e 2h 2h 3h 4h 2h
12 12 12 12 12 12
C C C C C
h h h h h h
particles via the SEM. To obtain the cylindrical toroidal samples for microwave measurement, 70 vol% of the as-obtained powders were fully mixed with paraffin wax. The samples were subsequently pressed into annular disks with a size of 7 3.04 3 mm. The complex permittivity (i.e., ε ¼ ε0 -jε00 ) and permeability (i.e., m ¼ m0 jm00 ) of the composite samples were measured using an Agilent 5245A vector network analyzer over the frequency range of 1e18 GHz. The reflection loss (RL) was calculated following equation (2):
RL ¼ 20 logjðZin Z0 Þ = Zin þ Z0 j
(2)
Z0 ¼ ðm0 =ε0 Þ1=2
(3)
i h Zin ¼ Z0 ðm=εÞ1=2 tanðhÞ j2pfdðmεÞ1=2 c
(4)
where Z0, Zin, f, d, and c are free space impedance shown as equation (3), input impedance shown as equation (4), frequency, the coating thickness and the speed of light in vacuum, respectively.
3. Results and discussion The SEM micrographs of ball-milled CIPs (for 12 h) preheated at different temperatures and times are presented in Fig. 2. These particles have a spherical shape with a mean size of about 2.5 mm. As can be seen from the morphological evolution of the CIPs (Fig. 2), the morphology of powders changes with the change of the temperature and time (1-untreated, 2e150 C/2 h, 3e200 C/2 h, 4e200 C/3 h, 5e200 C/4 h, 6e250 C/2 h). In this study, the diameter of each particle was measured in order to accurately evaluate the changes in the average aspect ratio. The average and maximum size of particles derived through ImageJ software are listed in Table 2. As can be seen from Fig. 2 (1), powders without heat treatment after ball milling show irregular morphology. After 150 C/2 h heat treatment (Fig. 2 (2)), the powders got more idea to flatten shape than untreated because of the difference plasticity of the powders with or without pre-heating [21]. There is a similar effect on 200 C and 250 C temperature (Fig. 2 (3) and (6)). Different process temperatures provide various levels of energy on particles. The results show that the plasticity of the particles is fully improved at 200 C/3 h, which confirms the considerable flakiness of the ballmilled powder. At the same time, in the 200 C/3 h group, the maximum flake size was 7.44 mm and the average value was 4.72 mm. However, the other two groups (i.e., 200 C/2 h and 200 C/4 h) were similar to the untreated group, with a relatively minor difference. Thus, this parameter plays an essential role in the plasticity and thus, the morphology of particles. By comparing the effects of temperature and time on the final morphology, it is revealed that a maximum and average size of 8.49 mm and 4.72 mm can be achieved at 150 C/2h and 200 C/3h preheating, respectively. This is due to the fact that the heat
Please cite this article as: H. Wei et al., Influence of heat treatment on the microwave absorption properties of flaky carbonyl iron powder, International Journal of Lightweight Materials and Manufacture, https://doi.org/10.1016/j.ijlmm.2020.02.001
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Fig. 1. The idea and objective of this research.
Fig. 2. SEM of samples obtained with different pre-heating times and temperatures ((1) untreated; (2) 150 C/2 h; (3) 200 C/2 h; (4) 200 C/3 h; (5) 200 C/4 h; (6) 250 C/2 h).
Table 2 Characteristic parameters of CIPs in each group. Pre-heating temperature
Pre-heating time
Mean size
Maximum size
Grain size
The internal strain
e 150 200 200 200 250
e
4.05 mm
6.84 mm
20 nm
0.102%
12 17 13 12 85
0.336% 0.301% 0.104% 0.185% 0.293%
C C C C C
2 2 3 4 2
h h h h h
4.35 3.89 4.72 3.93 3.73
mm mm mm mm mm
treatment temperature affects the maximum size of the particles, representing the improvement in the level of plasticity. However, the time of heat treatment affects the average value of particle size, i.e. the coverage of overall plasticity improvement. The XRD analysis of the flaky nanocrystalline CIPs fabricated at various temperatures and times is shown in Fig. 3. It can be seen
8.49 7.13 7.43 7.29 6.64
mm mm mm mm mm
nm nm nm nm nm
from the figure that all samples show three characteristic diffraction peaks at 2 h ¼ 44.6 , 65.0 , and 82.3 . They are respectively associated with the (110), (200), and (211) planes of cubic phase aFe. It demonstrates that the phase of the material after ball milling does not change with the change of pre-heating temperature and time. Table 2 shows grain size(D) and internal strain (x) of the
Please cite this article as: H. Wei et al., Influence of heat treatment on the microwave absorption properties of flaky carbonyl iron powder, International Journal of Lightweight Materials and Manufacture, https://doi.org/10.1016/j.ijlmm.2020.02.001
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Fig. 3. XRD of samples obtained by ball milling under different parameters.
milled flaky nanocrystalline CIPs as functions of temperature and time after ball milling. The grain size is calculated from the (110) peaks according to the Scherrer's formula, and the internal strain was obtained using the HalleWilliamson analysis as shown below [22,23]:
bhkl cos q ¼ K lD þ 4x sin q
(5)
where bhkl is the corrected full width at half maximum (FWHM) corresponding to each diffraction peak. Shape factor K equals 0.9, and the wavelength of Cu Ka radiation l ¼ 1.5418 Å. According to Table 2, with the increase of heat treatment temperature at a constant time of 2 h, the grain size of the material slightly decreases initially following an increase in a way to yield a size much greater at 250 C than that of the untreated material. This is due to the fact that the grain size grows up because of the high temperature. Also, the internal strain of the material initially increases, approaches to a maximum value at 150 C, and then gradually decreases with the increase of temperature. This shows that preheating has a significant influence on the grain size and internal strain of material and hence on its electromagnetic parameters. With increasing the duration of heat treatment (from 2 h to 4 h at 200 C), the grain size gradually decreases while the internal strain drops after gaining a maximum value at 2 h. It can be
due to a large change in plastic deformation in the material and no strain accumulation of the material. Figs. 4 and 5 show the complex permittivity and permeability, in a frequency range of 2e18 GHz, of the flaky CIPs, fabricated for 2 h at 150 C, 200 C, and 250 C. From Fig. 4, it is to notice that the real part of permittivity (ε0 ) of the powders, after heating, is significantly reduced. When the pre-heating conditions are 150 C/ 2h, the real part (ε0 ) is reduced to 13, which represents electrode polarization effects due to temperature change [24]. The changes in the imaginary part (ε00 ) of the permittivity over the considered range of frequency show an irregular trend. The change in ε00 of untreated powder is gentle, and the peak value of ε00 appears after 150 C/2h and 200 C/2h treatments. It slightly decreases in the low-frequency band but increases in the high-frequency band. While ε00 of the powders treated at 250 C/2h increases sharply and shows an obvious peak. This is due to the fact that heat treatment results in the alteration of the lattice parameters of the material thereby rendering a reduction in the internal relaxation effect and dielectric losses in a specific frequency band. It is mainly because of the change in size and internal strain of the grain and alteration in the particle morphology [25]. It can be seen from Fig. 5 that when the preheating temperature is 150 C, m0 and m00 of the magnetic permeability are lower than those of the untreated material. Moreover, m0 and m00 increase at the temperatures of 200 C and 250 C. At 200 C, m0 reaches to a maximum value of 3.2 at 2 GHz. This endorses a claim that the improvement of the initial permeability (m0) of the material can be achieved through a pre-heat treatment process. The intrinsic reason is that the grain size approaches nano-scale which can be confirmed by XRD. The maximum value of m00 is 1.61 which is obtained at 6.2 GHz. However, the peak value increases, that is, the breakthrough of the Snoek's limit is also achieved simultaneously. Yet, the effect is not obvious from time to time. In addition, it is worth noticing that in terms of appearance, the average size of the material pre-heated at 150 C is larger, and the flattening effect is better. Nevertheless, its magnetic permeability does not show a significant improvement owing to the fact that the high flattening ratio increases the eddy current loss effect and suppresses the magnetic permeability rise. Figs. 6 and 7 show the complex permittivity and permeability in 2e18 GHz frequency range for the flaky CIPs fabricated at 200 C for 2 h 3 h and 4 h. It is to observe from Fig. 6 that ε0 of the powders after 2 h and 3 h pre-heating at 2 GHz are reduced from its initial value of 16.8 to 15.5 and 16.2, respectively. It represents that the change in time results in the reduction of the dielectric ability of the material. After the processing time approaches 4 h ε0 suddenly
Fig. 4. Frequency dependencies of the complex permittivity with different temperatures for the flaky CIPs.
Please cite this article as: H. Wei et al., Influence of heat treatment on the microwave absorption properties of flaky carbonyl iron powder, International Journal of Lightweight Materials and Manufacture, https://doi.org/10.1016/j.ijlmm.2020.02.001
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Fig. 5. Frequency dependencies of the magnetic permeability with different temperatures for the flaky CIPs.
Fig. 6. Frequency dependencies of the complex permittivity with different time for the flaky CIPs.
Fig. 7. Frequency dependencies of the magnetic permeability with different time for the flaky CIPs.
increases to 18.7 and the polarization of the material is enhanced. According to Fig. 2 and Table 2, this phenomenon results from the flattening of the material and changes in the polarization mechanism. There is almost a similar trend for ε00. ε00 of the powder without pre-heating. However, there is a rise in ε00 after 2 h and 3 h processing. It slightly reduces in the low-frequency band but increases in the high-frequency band. After 4 h, ε00 increases overall
with prominent value in high-frequency bands. This mainly stems from the fact that the heat treatment alters the morphology and lattice parameters of the material, causing the internal strain relaxation and dielectric loss to rising significantly. It can be seen from Fig. 7 that m0 and m00 of the powders are reduced after heat treatment at 200 C at different times. After 200 C/2 h treatment, m0 reaches the maximum of 3.2 with a peak at
Please cite this article as: H. Wei et al., Influence of heat treatment on the microwave absorption properties of flaky carbonyl iron powder, International Journal of Lightweight Materials and Manufacture, https://doi.org/10.1016/j.ijlmm.2020.02.001
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Conflicts of interest
Table 3 Characteristic parameters of CIPs in each group. Pre-heating temperature
Pre-heating time
Magnetic permeability
e 150 200 200 200 250
e
mr0 : 2.73, mr0 : 1.19 mr0 : 2.61, mr0 : 1.03 mr0 : 3.20, mr0 : 1.61 mr0 : 2.71, mr0 : 1.16 mr0 : 2.96, mr0 : 1.33 mr0 : 2.93, mr0 : 1.42
C C C C C
2 2 3 4 2
h h h h h
2 GHz, and after 200 C/4 h treatment, it approaches a maximum value of 2.96 at 2 GHz. It proves that the improvement of the initial permeability of the material can be achieved through the pre-heat treatment. It is because the material structure is converted to nanocrystals (as can be inferred from Table 2). The maximum value of the imaginary part of 1.61 was obtained at 6.2 GHz while the cut off frequency remains constant. However, the peak value is increased, that is, the Snoek's limit exceeds simultaneously, but the effect was not obvious from time to time. The average size of the particles after 3 h and 4 h pre-heating is larger than that of the sample after 2 h, and the flattening is clearer. Yet, its magnetic permeability is not significantly improved. This is attributed to a fact that the higher flattening ratio tends to increase the eddy current loss effect thereby, in turn, suppressing the increase in magnetic permeability. Table 3 compares magnetic permeability at various experimental conditions. The following observations can be made from this table: (1) From the perspective of morphological evolution, the preheat treatment at a temperature of 150 C and a time of 3 h has a better effect on improving the particles size. The temperature affects the degree of plasticity improvement. Further, the length of the preheating time influences the average value of the particles size, that is, overall plasticity improvement. (2) From the microwave permeability improvement point of view, the preheating of 200 C/2 h can effectively improve the magnetic permeability of flaky powders after ball milling. Moreover, a very large morphological size is not very effective in improving magnetic permeability.
4. Conclusions The flaky CIPs with large size, low x, and appropriately small D are obtained by controlling the temperature and time of preheating and high-energy ball milling for 12 h. The following important conclusions can be derived: (1) The grain size and internal strain changes according to the temperature and time alteration. (2) The changes in morphology, grain size, and internal strain result in the change of magnetic permeability and permittivity as well as the morphology. (3) For the best magnetic absorbing property, the optimal level of particles size and internal strain need to be selected. The optimum pre-heating parameters for carbonyl iron powder are selected as 200 C/2 h, (4) It is found that at optimal pre-heating parameters, the magnetic permeability and absorption of powders approach to the highest level, such that mr0 and mr00 reach 3.20 at 2 GHz and 1.61 at 6.2 GHz respectively. The results here also provide an effective strategy to improve the microwave absorption property of MAMs in a wide frequency range via controlling the particle size, internal strain and grain size of its magnetic metallic Nanocrystalline absorbents.
The authors declare that there is no conflicts of interest.
Acknowledgments The necessary funding to realize this work in the research community was provided by Fundamental Research Funds for the Central Universities [Grant No. NS2015055], and National Natural Science Foundation of China [Grant No. 51105202] for which the authors are grateful.
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Please cite this article as: H. Wei et al., Influence of heat treatment on the microwave absorption properties of flaky carbonyl iron powder, International Journal of Lightweight Materials and Manufacture, https://doi.org/10.1016/j.ijlmm.2020.02.001