Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes 4.1 Introduction Ferromagnetic metal-based materials display high saturation magnetizations which make them of interest for microwave applications, namely higher working frequencies and a broader working frequency band than bulk ferromagnetic oxides.[1] However, the skin effect is always a problem of metal-based materials, such as Fe, especially in GHz applications. Chapter introduced a core/shell structure, of which an insulating SiO2 shell layer was coated on the surface of Fe particles, to reduce the skin effect. An optional route to reduce the skin effect is to shape Fe particles into some special shapes to make sure that the particle size in a certain dimensional is comparative with the skin depth. Therefore, the microwave absorption performance could be improved. In recent literatures, by virtue of the optimized particle shape and microwave permeability performance, ferromagnetic flakes are reported as promising candidates for electromagnetic wave absorption in GHz.[2-6] Theoretically, the metallic thin flakes could provide relative high permeability in gigahertz frequencies compared with the materials constrained by the traditional Snoek’s limit. With taking the shape anisotropy into consideration, the Snoek’s law becomes the following equation[7] 55 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes 1/2 H (μi − 1)ƒr = γ4πMs ( ) H (Eq. 1.11 in Chapter 1) ea Where μi , ƒr , γ and Ms are the initial permeability, resonance frequency, gyromagnetic ratio and saturation magnetization of the material, Hea represents the in-plane anisotropy field, and Hha represents the out-of-plane anisotropy field. It could be found that the product of μi and ƒr for flake-shaped particles can be much higher than that of isotropic particles because the out-of-plane anisotropy Hha of flake-like particles is much larger than the in-plane anisotropy Hea (Hha ≫ Hea ), while γ and Ms remain unchanged compared to isotropic particles.[8] Therefore the resonance frequency of flake-like magnetic materials could exceed the traditional Snoek’s law limitation ( (μi − 1)ƒr = γ4πMs ) and achieve a higher resonance frequency. Moreover, the skin effect could be effectively suppressed by the flakes because the thickness of as-prepared flakes could be comparative with the skin depth. As a consequence, higher permeability can be obtained. Through the studies on carbonyl iron particles with different morphology, Wen et al. found that higher value of magnetic permeability and permittivity could be obtained in the composites for thin flake carbonyl iron than spherical powders.[9] Zhou et al.[10] reported a comparison work on flake-like and spherical Fe3Co2 particles, and found that flake-like Fe3Co2 particles owned higher permeability value corresponding to a higher resonance frequency. All of these works show the improvement of the flake-shape on the electromagnetic properties of particles. 56 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes As seen from the above works, most of them have adopted the mechanical milling method to fabricate the flake-like particles. Mechanical milling turns to be an effective way to fabricate metallic alloys in a large quantity, and the morphology of as-milled metallic particles is much dependent on the milling time.[11-13] However, the product after mechanical milling is usually with a broad size distribution. To make an improvement on the morphology of as-milled products, jet milling was further used in the current study. The principle of jet milling will be briefly introduced in the third part of this work. According to our best knowledge, nanocrystalline alloy flakes have not been fabricated by using jet milling. The poor chemical stability of Fe particles also limits the applications when the particle is small. In our work, FeM alloys were studied instead of pure Fe. FeM flakes with micron- and submicron-sizes were prepared by two steps milling, i.e. high energy ball milling and jet milling. Among the FeM alloys, Fe90Al10 was mainly studied due to its relative high saturation magnetization and its relative good chemical stability. The results show that when Fe90Al10 flake size reaches submicron-scale, the resonance frequency shifts to higher frequency band. Through the calculation of the reflection loss by using electromagnetic wave transmission line theory, the as-prepared submicron-scale flakes are found to be very promising to make lightweight absorber with effective absorbing property at high frequency band. 57 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes 4.2 Experimental results 4.2.1 Effect of jet milling on the morphology of different materials Fig. 4.1 is the schematic diagram of the major working part of jet mill. The milling process is started with the tangential feed of the powder particles from the feed funnel into the flat circular grinding chamber by the compressed feed air. The powder particles are then accelerated in a spiral movement inside the grinding chamber. The pulverization of raw material takes place due to the collision between particles and the Feed funnel Compressed feed Grinding chamber Compressed grind air Fig. 4.1 Schematic diagram of the major part of jet mill. collision between particles and the wall of the chamber. The larger particles of the product get retained at the periphery of the chamber by centrifugal force and the smaller particles exit from the central port of the chamber. After jet milling, the products could be collected from a receiver. Based on the weight difference, the milled particles accumulate at the bottom or the top part of the receiver. We tried commercial iron particles with different sizes on the jet mill. As seen from the SEM images in Fig. 4.2, the initial iron particle sizes range from μm to μm. 58 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes Fig. 4.2 The SEM images of iron particles: (a) commercial iron particles with sizes ranging from μm to 5μm; (b) and (c) are jet milled iron particles at bars, which are collected from the bottom and top part of the receiver, respectively. The scale bar for μm is for all three images. After jet milling, the particles changes little. The products collected from the bottom part and the top part of the receiver showed few difference. When iron particles with size larger than 20 μm were used, the morphology of iron particles changed a lot after jet milling process. As seen from Fig. 4.3b, all the particle sizes were reduced to less than μm. Fig. 4.3c showed some more small pieces of particles. Hence we can know that jet mill is more effective on the pulverization of large size particles (˃ μm). Fig. 4.3 The SEM images of iron particles: (a) commercial iron particles with sizes larger than 20 μm; (b) and (c) are jet milled iron particles at bars, which are collected from the bottom and top part of the receiver, respectively. We further tried to some Fe-based alloys on the jet mill. The alloys were iron solid solutions (Fe90M10; M=Si, Co and Al) prepared by high energy ball milling. The 59 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes formation of solid solutions was proved through XRD, VSM and EDS measurement. While the XRD patterns in Fig. 4.4a demonstrated that there was not any peak brought by the solute elements but only BCC Fe phase (JCPDS no. 87-0722) was observed in these solid solutions. The existence of elements Si, Co and Al in different solid solutions was confirmed by the EDS spectra, as shown in Fig. 4.5 to Fig. 4.7. As revealed by the magnetic hysteresis loops (Fig. 4.4b and its inset), the saturation of Fe is around 220 emu/g. The addition of the solute into iron would reduce its magnetism in some extent. The Fe90Co10 solid solution still processes a very high saturation Fig. 4.4 (a) X-ray diffraction patterns and (b) M-H loops of Fe-based solid solutions; the inset is for a clear observation of saturation magnetization. magnetization of 212 emu/g, while the saturation magnetization values of Fe90Si10 and Fe90Co10 are reduced to 185 emu/g and 184 emu/g. Before jet milling, all of the solid solutions showed irregular shapes and nonuniform sizes. The SEM images in Fig. 4.5 to Fig. 4.7 also show the morphology modification of three kinds of Fe-based alloys after jet milling. In our studies, we fixed the parameters during jet milling process, such as the feed speed of raw materials and the grind air pressure. Hence the differences in the fineness of the milling products are probably due to the different 60 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes Fig. 4.5 The SEM images of Fe90Si10: (a) ball milled particles, which are further pulverized by jet milling; (c) and (d) jet milled products, which are collected from the bottom and top part of the receiver, respectively. (b) the EDS spectrum. Fig. 4.6 The SEM images of Fe90Co10: (a) ball milled particles, which are further pulverized by jet milling; (c) and (d) jet milled products, which are collected from the bottom and top part of the receiver, respectively. (b) the EDS spectrum. Fig. 4.7 The SEM images of Fe90Al10: (a) ball milled particles, which are further pulverized by jet milling; (c) and (d) jet milled products, which are collected from the bottom and top part of the receiver, respectively. (b) the EDS spectrum. 61 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes mechanical properties of the feed material, such as the hardness. As reported,[14] materials which have relatively low hardness show the higher rate of breakage in particles. Jet milling is possible to reduce the size of ball milled products to less than μm for all the alloys under study. If particles in smaller sizes are desired, we should collect the light-weight product on the top layer of the receiver. These products are much less than those products accumulated at the bottom part of the receiver. As we can see from Fig. 4.6d and Fig. 4.7d, the Fe/Co flakes and Fe/Al flakes in submicron scale could be produced by jet milling. This finding aroused us great interesting in the fabrication of Fe-based alloy flakes with different sizes. The chemical stability is always a problem for alloys. Thermogravimetric analysis (TGA) was used to check the stability of the produced Fe solid solutions at elevated temperature. The TGA Fig. 4.8 TGA plots and the derivative curves for (a) commercial iron with particles size ranging from μm to μm; (b) jet milled Fe90Co10 flakes and (c) jet milled Fe90Al10 flakes. measurement was performed with the temperatures ranging from room temperature to 62 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes 1000 ℃. Although nitrogen gas with purity of 99.9% was used as the protection gas, when the temperature rose to 350 ℃, the weight of commercial Fe (size: μm to μm) began to increase rapidly, as displayed by the relationship of the weight loss against the temperature in Fig. 4.8a. The only reason could be the very tiny amount of oxygen impurity in nitrogen gas. At elevated temperatures, Fe is found to be much more sensitive to the atmosphere.[15] In this work, the temperature at which the weight of the sample starts to increase is used to evaluate the stability. The higher the temperature, the better the stability. For jet milled samples, the stability of Fe90Al10 and Fe90Co10 submicron flakes were studied. The weight increase in percentage against temperature was plotted and shown in Fig. 4.8b&c. The derivative of mass was also plotted to show the mass change rate. Fe(Al) flakes showed a better stability than the commercial iron particles. While Fe(Co) flakes showed the worst stability. Therefore, Fe90Al10 was selected for further investigations on the size-controllable synthesis and the microwave absorption performance. 4.2.2 Fabrication and characterizations of micron and submicron Fe/Al flakes Sample S0 was obtained after mechanical milling of the mixture of Fe and Al metallic powders for 12 h using a Spex mixer (high energy ball mill machine). From the SEM micrographs (Fig. 4.9a), the particles of sample S0 have irregular and isotropic shapes with a broad size distribution from m to 10 m. After the first step of dry-milling process, the powder (Sample S0) was subsequently mixed with anhydrous ethanol and 63 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes milled for 0.5 h and h, respectively. The process is so called wet-milling. The corresponding products after wet-milling were named as Sample S1 (milled for 0.5 h) and Sample S2 (milled for h). As shown in Figs. 3.9c and d, the particles after wet milling process are flakes with average lateral size around 50 m for S1 and 100 m for S2, respectively. The thicknesses for the two samples are similar, about 0.5 m. Furthermore, the jet milling was employed to reduce the particle size. For jet mill, the pressure of the compressed grind air is a vital parameter which could affect the size and shape of resultant products.[16] In this work, the pressure, depending on the attached air compressor system, was set at bar. After the jet-milling, two sorts of particles could be obtained, as they were separated based on different masses (different particle sizes). The final product of small particles (Sample S3) showed a Fig. 4.9 SEM images of as-prepared Fe90Al10 samples: (a) Sample S0 after high energy ball milling for 12 h; (b) Sample S4 - spherical particles (large particles at the bottom part of the receiver) after jet milling; (c) Sample S1 obtained after wet ball milling for 0.5 h; (d) Sample S2 is obtained after wet ball milling for 5h; (e) Sample S3 - submicron flakes (small particles on the top part of the receiver) after jet milling; (f) a typical EDS spectrum. 64 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes flake-like structure in submicron-scale with average lateral size less than 500 nm and a thickness of 50 nm, as shown in Fig. 4.9e. The bigger particles were named as Sample S4. Compared with Sample S0, Sample S4 (shown in Fig. 4.9b) had quasi-spherical shape with particle sizes ranking in the range of m to m. A typical EDS spectrum shown in Fig. 4.9f illustrates that the atomic ratio of Fe to Al for as-prepared flakes is almost 9:1 for all samples obtained, and the composition changes little with different sizes of the flakes. As it is well known, fracture and welding are the two major processes during high-energy ball-milling. If anhydrous ethanol is added, the milling energy of the wet-milling is strongly reduced. If we use the isotropic particles as the starting material, the initial isotropic particles (with a particle size ranking form to 10 m) were flattened during the first wet-milling. After 0.5 h, flakes with a lateral size of 50 m and a thickness of 50 nm were formed. A prolonged wet-milling, the flakes were welded together and form larger flakes with a lateral size of 100m, while the thickness was kept unchanged (50 nm). To investigate the phase of the as-prepared samples after milling process, XRD patterns were taken (as shown in Fig. 4.10a), only the peaks for bcc-Fe could be observed for all the four samples. By using the Scherrer equation [Eq. (2.2)], the estimated value of grain size of as-milled particles is about nm, which indicates the formation of Fe(Al) solid solution phase with a nanocrystalline structure after milling 65 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes process. The magnetic properties of these samples were studied by VSM. The M-H loops are shown in Fig. 4.10b. Comparing with spherical sample (Sample S4), the Fig. 4.10 (a) XRD patterns and (b) M-H loops of as-prepared Fe90Al10 flakes. flake-like samples (S1, S2 and S3) possess lower saturation magnetization Ms and higher coercivity Hc . With the lateral size of the as-prepared Fe/Al flakes decreasing, especially to submicron-scale, the value Ms obviously decreases. The magnetic properties (saturation magnetization Ms and coercivity Hc ) are summarized and listed in Table 4.1. The spherical particles (Sample S4) possess a high magnetization value (182 emu/g). After wet-milling in anhydrous ethanol for 0.5 h, the saturation magnetization was reduced to 170 emu/g (Sample S1). A further reduction to 162 emu/g was observed after pro-longed wet-milling for h (Sample 2). One of possible mechanisms is that oxidation occurs during wet-milling. For submicron-sized flakes (Sample S3), saturation magnetization was reduced to 150 emu/g. This is probably due to slight surface oxidation during high-energetic jet-milling, if the particle size is reduced to 500 nm. It is to note that no oxide phase or significant oxygen content was observed in our XRD and EDS examinations probably due to possible surface 66 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes oxidation. Another possible mechanism is the formation of possible disordered Table 4.1 Summarized properties of the as-prepared samples. (m) 𝐌𝐬 (emu/g) ƒ𝐫 (GHz) 𝛍′′ S1 ~50 (flakes) 170 6.8 0.51 S2 ~100 (flakes) 162 4.5 0.62 S3 ~0.5 (flakes) 150 9.7 0.40 S4 ~5 (spherical) 182 5.5 0.29 Property Samples Average lateral Size structure (such as amorphous-like structure). The static magnetic properties (saturation magnetization Ms and coercivity Hc ) have been summarized and listed in Table 4.1. 4.2.3 Microwave absorption property of as-prepared Fe90Al10 flakes with different sizes In order to study microwave absorbing property of the Fe/Al flakes, the as-prepared samples (S1, S2, S3 and S4) were dispersed in epoxy resin with a volume concentration of 9%. The complex permittivity and permeability spectra are shown in Fig. 4.11 and the significant values (resonance frequency ƒr and maximum value μ"MAX obtained in Fig. 4.11d) are also listed in Table 4.1. As it can be seen from Fig. 4.11a&b, the flake-like samples have higher real and imaginary permittivity values compared to the spherical powder (S4). Among the flake-like samples, both real part 67 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes Fig. 4.11 The relationship of electromagnetic parameters and frequency in the range of 0.1~18 GHz:(a) and (b) are the real and imaginary part of measured complex permittivity, respectively; (c) and (d) are the real and imaginary part of measured complex permeability, respectively. and imaginary part of the complex permittivity increase with particle size. This is probably due space-charge polarization n, as reported previously. [17] As we know, the resonance frequency is an important factor for microwave application; however, the frequency position is usually limited by traditional Snoek’s law.[18] In this work, we have succeeded in the improvement of resonance frequency peak to higher frequency band, from 5.5 GHz for spherical Fe90Al10 particles (Sample S4) to 9.7 GHz for 500 nm flakes (Sample S3). The results show a clear size effect on the permeability and resonance frequency of the as-prepared flake-like samples: a) compared with the isotropic particles, the flakes provide higher permeability value at higher frequency range; b) with the lateral size of the flake-like sample decreasing, the resonance peak shifts to higher frequency band. Generally, the microwave 68 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes magnetic loss of magnetic particles originates from hysteresis, domain wall resonance, eddy current effect, natural resonance.[19] In our case, the contributions of magnetic hysteresis can be excluded due to weak applied field. The domain wall usually occurs below gigahertz range.[17] According to the previous report,[20] the estimated skin depth for iron-based composite is in the scale of several micron meters when frequency is over GHz. So the eddy current effect may exist for the flakes with sizes of 50 m and 100 m. The natural resonance plays an important role on the magnetic permeability of the as-prepared samples. As indicated by Eq. (3.1) in the introduction part, the natural resonance frequency could be significantly affected by the saturation magnetization and effective anisotropy of the materials. As shown in Table 4.1, Sample S3 (submicron flakes with size of 500 nm) possesses the lowest saturation magnetization Ms. Hence its permeability value is lower than those of micron flakes (Samples S1 and S2), but still higher than the isotropic one (S4), indicating that the effective anisotropy contributes a lot to the good microwave absorption property of the flakes. The similar trend was shown by the relationship between the reflection loss (RL) and the frequency. The reflection loss of the composite samples of magnetic particles dispersed in epoxy resin matrix was been calculated based on Eq. (2.5) and Eq. (2.6). To make a comparison among the as-prepared flake-like samples, the reflection loss values for Sample S1, S2 and S3 were shown in Fig. 4.12. When the thickness of the 69 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes Fig. 4.12 The calculated theoretical reflection loss in the frequency range of 0.1 GHz to 18 GHz for the as-prepared Fe90Al10 flakes. absorber layer is adjusted in the range of 1.9 mm to 3.7 mm, the optimal thickness shown by the results is 1.9 mm (with a maximum at 15 GHz), 3.1 mm (at 7.6 GHz) and 3.7 mm (at 4.5 GHz) for Sample S3 (~500 nm flakes), Sample S1 (~50 m flakes) and Sample S2 (~100 m flakes), respectively. With increasing thickness, the resonance peak of Sample S2 shifts to lower frequency with lager absolute value of reflection loss. For Sample S3, resonance peak shifts to higher frequency band with increasing absolute value of RL, when the thickness decreases. The results have shown that Fe/Al submicron flakes are promising to be used as thin microwave absorber. 4.3 Summary In this part, jet milling was found to be effective to refine the size of raw materials to 70 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes less than μm. By using high energy ball milling and jet milling, Fe90Al10 flake-like particles with an average lateral size around 100 m, 50 m and 0.5 m (500 nm) were successfully fabricated. The complex permeability was obtained by dispersing the magnetic particles into epoxy resin matrix in a volume ratio of 9%. All the flakes show better dynamic magnetic property than the isotropic particles. The 500 nm Fe/Al flakes show the highest resonance frequency probably due to the suppression of skin effect and change in magnetic shape anisotropy. Moreover, the reflection loss based on transmission line theory was calculated. The calculated reflection loss in the frequency range of 0.1 to 18 GHz showed that Fe90Al10 submicron flakes could be a good candidate for making lightweight microwave absorbers with effective absorbing property. 4.4 References [1] P. Toneguzzo, G. Viau, O. Acher, F. F. Vincent, F. Fiévet, Adv. Mater., 10, 1032-1035 (1998). [2] P. H. Zhou, J. L. Xie, Y. Q. Liu, L. J. Deng, J. Magn. Magn. Mater., 320, 3390-3393 (2008). [3] Z. H. Yang, Z. W. Li, L. Liu, L. B. Kong, J. Alloys Compd., 509, 3038-3041 (2011). [4] G. Z. Xie, X. L. Song, B. S. Zhang, D. M. Tang, Q. Bian, H. X. Lu, Powder Technology, 210, 220-224 (2011). [5] L. N. Jing, G. Q. Wang, Y. P. Duan, Y. Z. Jiang, J. Alloys Compd., 475, 862-868 (2009). [6] D. Y. Zhang, W. Q. Zhang, J. Cai, J. Magn. Magn. Mater., 323, 2305-2309 (2011). [7] J. Q. Wei, T. Wang, F. S. Li, J. Magn. Magn. Mater., 323, 2608-2612 (2011). [8] J. Liu, Y. B. Feng, T. Qiu, J. Magn. Magn. Mater. 323, 3071-3076 (2011). [9] F. S. Wen, W. L. Zuo, H. B. Yi, N. Wang, L. Qiao , F. S. Li, Physica B, 404, 3567-3570 (2009). [10] P. H. Zhou, L. J. Deng, J. L. Xie, D. F. Liang, J. Alloys Compd., 448, 303-307 (2008). [11] Z. Bensebaa, B. Bouzabata, A.Otmani, A. Djekoun, A. Kihal, J. M. Grenèche, J. Magn. 71 Chapter Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes Magn. Mater., 322, 2099-2103 (2010). [12] M. Mhadhbi, M. Khitouni, M. Azabou, A. Kolsi, Mater. Charact., 59, 944-950 (2008). [13] S. H. Kim, Y. J. Lee, B. H. Lee, K. H. Lee, K. Narasimhan, Y. D. Kim, J. Alloys Compd., 424, 204-208 (2006). [14] O. D. Vegt, H. Vromans, J. D. Toonder, K. V. D. V. Maarschalk, Powder Technol., 191, 72-77 (2009). [15] B. Panicaud, J. L. Grosseau-Poussard, C. Huvier, S. Rebeyrat, J. F. Dinhut, Mater. Sci. Eng., A, 356, 434-442 (2003). [16] W. Schlocker, S. Gschlieẞer, A. B. Schnürch, Eur. J. Pharm. Biopharm., 62, 260-266, 2006. (Evaluation of the potential of air jet milling of solid protein-poly(acrylate) complexes for microparticle preparation) [17] L. G. Yan, J. B. Wang, X. H. Han, Y. Ren, Q. F. Liu, F. S. Li, Nanotechnology 21 (2010) 095708. [18] O. Acher, S. Dubourg, Phys. Rev. B 77 (2008) 104440. [19] J. Ma, J. G. Li, X. Ni, X. D. Zhang, J. J. Huang, Appl. Phys. Lett. 95 (2009)102505. [20] L. Z. Wu, J. Ding, H.B. Jiang, L.F. Chen, C.K. Ong, J. Magn. Magn. Mater.285 (2005) 233-239. 72 [...]... permeability value is lower than those of micron flakes (Samples S1 and S2), but still higher than the isotropic one (S4), indicating that the effective anisotropy contributes a lot to the good microwave absorption property of the flakes The similar trend was shown by the relationship between the reflection loss (RL) and the frequency The reflection loss of the composite samples of magnetic particles dispersed... Table 4. 1 As it can be seen from Fig 4. 11a&b, the flake-like samples have higher real and imaginary permittivity values compared to the spherical powder (S4) Among the flake-like samples, both real part 67 Chapter 4 Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes Fig 4. 11 The relationship of electromagnetic parameters and frequency in the. .. on Eq (2.5) and Eq (2.6) To make a comparison among the as-prepared flake-like samples, the reflection loss values for Sample S1, S2 and S3 were shown in Fig 4. 12 When the thickness of the 69 Chapter 4 Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes Fig 4. 12 The calculated theoretical reflection loss in the frequency range of 0.1 GHz to... milling 65 Chapter 4 Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes process The magnetic properties of these samples were studied by VSM The M-H loops are shown in Fig 4. 10b Comparing with spherical sample (Sample S4), the Fig 4. 10 (a) XRD patterns and (b) M-H loops of as-prepared Fe90Al10 flakes flake-like samples (S1, S2 and S3) possess... compared with the isotropic particles, the flakes provide higher permeability value at higher frequency range; b) with the lateral size of the flake-like sample decreasing, the resonance peak shifts to higher frequency band Generally, the microwave 68 Chapter 4 Microwave properties of micron and sub-micron Fe90Al10 flakes fabricated via ball milling and jet milling routes magnetic loss of magnetic particles... property of as-prepared Fe90Al10 flakes with different sizes In order to study microwave absorbing property of the Fe/Al flakes, the as-prepared samples (S1, S2, S3 and S4) were dispersed in epoxy resin with a volume concentration of 9% The complex permittivity and permeability spectra are shown in Fig 4. 11 and the significant values (resonance frequency ƒr and maximum value μ"MAX obtained in Fig 4. 11d)... size of 100m, while the thickness was kept unchanged (50 nm) To investigate the phase of the as-prepared samples after milling process, XRD patterns were taken (as shown in Fig 4. 10a), only the peaks for bcc-Fe could be observed for all the four samples By using the Scherrer equation [Eq (2.2)], the estimated value of grain size of as-milled particles is about 6 nm, which indicates the formation of. .. our case, the contributions of magnetic hysteresis can be excluded due to weak applied field The domain wall usually occurs below gigahertz range.[17] According to the previous report,[20] the estimated skin depth for iron-based composite is in the scale of several micron meters when frequency is over 4 GHz So the eddy current effect may exist for the flakes with sizes of 50 m and 100 m The natural... natural resonance plays an important role on the magnetic permeability of the as-prepared samples As indicated by Eq (3.1) in the introduction part, the natural resonance frequency could be significantly affected by the saturation magnetization and effective anisotropy of the materials As shown in Table 4. 1, Sample S3 (submicron flakes with size of 500 nm) possesses the lowest saturation magnetization Ms... in the range of 1 m to 5 m A typical EDS spectrum shown in Fig 4. 9f illustrates that the atomic ratio of Fe to Al for as-prepared flakes is almost 9:1 for all samples obtained, and the composition changes little with different sizes of the flakes As it is well known, fracture and welding are the two major processes during high-energy ball-milling If anhydrous ethanol is added, the milling energy of . pulverization of raw material takes place due to the collision between particles and the collision between particles and the wall of the chamber. The larger particles of the product get retained at the. Fe 90 Al 10 was selected for further investigations on the size-controllable synthesis and the microwave absorption performance. 4. 2.2 Fabrication and characterizations of micron and submicron Fe/Al flakes. decreasing, the resonance peak shifts to higher frequency band. Generally, the microwave Fig. 4. 11 The relationship of electromagnetic parameters and frequency in the range of 0.1~18 GHz:(a) and (b)