Synthesis of various magnetic nanostructures and the microwave characterizations 3

Chapter 3 Synthesis and microwave absorbing properties of Fe/SiO2 particles with core/shell structure 3.1 Introduction In recent years, the magnetic core-shell structure composites hav

Chapter 3 Synthesis and microwave absorbing properties of Fe/SiO2

particles with core/shell structure

3.1 Introduction

In recent years, the magnetic core-shell structure composites have attracted intense attention for their novel performance in various applications, such as magnetic resonance imaging,[1] optoelectronic devices[2] and microwave absorbing[3] One

of the promising shell materials used in magnetic core-shell particles is silica (SiO2) The use of silica shell provides several characteristics such as biocompatibility in biological system, high suspension stability in various solvents, surface conductivity modification and improved chemical stability of magnetic core materials.[4]

As is well known, iron particles are promising candidate for electromagnetic (EM) wave absorption application Iron as magnetic metal can maintain high electromagnetic parameters at high frequency due to its large saturation magnetization and higher Snoek’s limit.[5] However, serious energy loss caused by the eddy current limits its application at higher frequency To overcome this problem, it is necessary to introduce an insulating shell to form a core/shell structure.[6,7] Besides the advantages mentioned above, silica shell is chosen for another reason, i e the synthesis methods Among the various methods for core/shell composites fabrication,

such as sol-gel,[8] heterogeneous precipitation,[9] arc discharge[10] and electroless plating,[11] the Stöber process[12-14] is more popular and suitable for massive product The Stöber process is specially designed for the coating of silica on other core materials

The significance of an insulating shell layer to the microwave performance of Fe particles was predicted by using the simulation models, which are given as

(ε eff −εe)

3εeff = p

(ε1−εe)+(2ε1+εe+2(εeff−εe)) (ε2−ε1)𝑎2 3/𝑎1

(ε2+2ε1+2(εeff−εe))

(ε 1 +2ε eff )+2(ε 1 −ε e ) (ε2−ε1)𝑎23/𝑎1

(ε2+2ε1+2(εeff−εe))

(3.1)

(μ eff −μe)

3μeff = p

(μ 1 −μ e )+(2μ 1 +μ e +2(μeff−μ e )) (μ2−μ1)𝑎2 3/𝑎1

(μ2+2μ1+2(μeff−μe))

(μ 1 +2μ eff )+2(μ 1 −μ e ) (μ2−μ1)𝑎23/𝑎1

(μ2+2μ1+2(μeff−μe))

(3.2)

The calculation models were developed based on Landau-Lifshitz-Gilbert (LLG)

theory by our group members C P Neo et al.[15] It could be used to calculate the

effective electromagnetic parameters (εeff, μeff) of composite consisting of spherical magnetic particles with core/shell structure dispersed in a medium In this work, the epoxy resin was used as the medium Therefore, εe ≅ 3 and μe ≅ 1 The schematic illustration core/shell structure is shown in Fig 3.1 As we can see, 𝑎1 and 𝑎2 are the radius of the shell layer and the core particles, respectively Similarly, the parameters in Eq (3.1) and Eq (3.2) denoted as ε1 and μ1 are for the shell layer materials, while those denoted as ε2 and μ2 are for the core materials When SiO2 is used as the shell material, the parameters are known to be ε1 = 3.9 and μ1= 1.0 p

refers to the volume concentration of magnetic materials in the composite

In this chapter, Fe/SiO2 core/shell structure was prepared through the Stöber process The electromagnetic performance was performed on Fe/SiO2/Epoxy composite with three different volume concentrations The experimental results were used to compare with the calculated results to verify the effectiveness of the calculation models Furthermore, the microwave absorbing performance of as-synthesized core/shell structure was investigated

3.2 Experimental results

3.2.1 Characterizations on the Fe/SiO 2 core/shell structure

The particle sizes of commercial carbonyl iron particles are in the range of 1.29±0.58

μm, as evaluated from electron micrographs by counting over two hundred particles The SEM images of Fe and Fe/SiO2 particles are shown in Fig 3.1a&b As seen from the images, the surface morphology changed obviously due to the silica coating The presence of Si on the surface of Fe particles is also evidenced by the EDS spectrum and elemental mapping The chemical analysis from the EDS spectrum in Fig 3.2d

Fig 3.1 An inhomogeneous sphere with a spherical core and a spherical layer.

provides an atomic ratio of Fe to Si about 5.4 Fig 3.2(e1, e2, e3 and e4) are elemental mappings of C, O, Fe and Si, respectively, as labeled in the images The carbon shown in both of the EDS spectrum and the mapping images is mainly from the conductive carbon paste Fig 3.2c shows the TEM image of Fe/SiO2 particles which clearly reveals the core/shell structure The thickness of the shell layer is uniform and measured to be around 140±12 nm Compared with the uncoated Fe particles, the saturation magnetization of silica coated Fe decreases by 12.8%, as indicated by the magnetic hysteresis loops in Fig 3.2f

3.2.2 Investigations on the electromagnetic parameters

The filler volume concentrations of as prepared composites (Fe/Epoxy, Fe/SiO2/Epoxy) are 8.6%, 15.5% and 26.3% Similar trend is observed in the

Fig 3.2 SEM images of (a) Fe particles and (b) Fe/SiO 2 particles; (c) TEM images of Fe/SiO 2 particles; (d) EDS spectrum of Fe/SiO 2 particles; (e1) ~ (e4) are elemental mapping corresponding to C, O, Fe and Si, respectively; (f) Magnetic hysteresis loops of Fe Fe/SiO 2 particles The scale bars in (a), (b) and (c) stand for 1μm

Fig 3.3 Comparison of experimental results: (a) the complex permittivity and (b) complex permeability of Fe/Epoxy and Fe/SiO 2 /Epoxy composites The volume concentration of magnetic filler is 8.6%.

Fig 3.4 Comparison of experimental results: (a) the complex permittivity and (b) complex permeability of Fe/Epoxy and Fe/SiO 2 /Epoxy composites The volume concentration of magnetic filler is 15.5%.

Fig 3.5 Comparison of experimental results: (a) the complex permittivity and (b) complex permeability of Fe/Epoxy and Fe/SiO 2 /Epoxy composites The volume concentration of magnetic filler is 26.3%.

experimental results, as shown in Fig 3.3, Fig 3.4 and Fig.3.5 The effective permeability of Fe/SiO2/Epoxy composite change little compared with that of Fe/Epoxy composite, however, an obvious decrease of the real part of permittivity ε′ due to the silica coating layer could be found This phenomenon exists for all the samples with different filler concentrations The results might suggest two advantages

of Fe/SiO2 core/shell particles to be used as microwave absorbing materials The one

is that the skin effect is reduced by the insulating coating layer To the best of my knowledge, the skin effect usually limits the application of iron particles, especially at relative GHz frequency range Hence the suppression of the skin effect may increase the resonance frequency The other is that the lower permittivity value of Fe/SiO2/Epoxy composite makes its impedance closer to that of the free space, so the microwave can permeate into the composite more easily, resulting in a more effective attenuation of incident microwaves

3.2.3 The comparison between the measured electromagnetic performance and the calculated results

The effective permeability (μ′, μ′′) and effective permittivity (ε′, ε′′) were measured and calculated based on Eq (3.1) and Eq (3.2) The figures in Fig 3.6 are shown to present the comparison of computed effective permeability and permittivity of three volume concentrations of Fe/SiO2 particle composites with the measurement results

As expected, the permittivity and permeability increase with increasing volume

fraction The computed results agree quite well with the measurement results The agreement is better for smaller volume concentration, which may be explained by the better dispersion of Fe/SiO2 particles in the medium at a smaller volume concentration The effective permeability and the effective permittivity are important parameters to obtain the reflection loss, therefore the prediction on these parameters is important to study the microwave absorption performance

3.2.4 Evaluation on the microwave absorbing performance

To find out the effect of the silica coating layer on the microwave absorbing performance, the reflection loss (RL) curves of Fe/Epoxy and Fe/SiO2/Epoxy composites are calculated from the measured complex permeability (μr = μ′+ jμ′′) and permittivity (εr = ε′+ jε′′) of the composites The reflection loss (RL) was

Fig 3.6 Comparisons between calculated and experimental results of the effective electromagnetic parameters for Fe/SiO 2 composites The filler concentration is labeled on top of figures.

calculated from the measured μr and εr at given frequency and absorber thickness

t according to Eq (2.5) and Eq (2.6)

The measured complex permeability and permeability shown in Fig 3.4a&b are used

to obtain the RL curves of Fe/Epoxy and Fe/SiO2/Epoxy composites with volume concentration of 15.5%, and the resultant RL curves are shown in Fig 3.7 As seen from the results, the optimal thickness of Fe/Epoxy composite is 3 mm, corresponding

to a lowest RL value of -20.5 dB at the frequency of 8.5 GHz, and the frequency band

with RL lower -10 dB is around 4 GHz The RL value of -10 dB means that 90% of incident microwave is absorbed by the absorbing materials, and the frequency band with RL lower than -10 dB is an important criterion to evaluate the microwave performance The broader the frequency band, the better the microwave performance For the Fe/SiO2/Epoxy composite, the optimal thickness does not show up in the calculated thickness range of 2.2 mm to 3.4 mm When the thickness t of the

Fig 3.7 The calculated reflection loss curves for (a) Fe/Epoxy composite and (b) Fe/SiO 2 /Epoxy composite The thicknesses of the microwave absorbers are assumed to be 2.2 mm, 2.6 mm, 3 mm and 3.4 mm for calculation.

absorber gets smaller, the lowest RL value keeps decreasing towards to higher frequency range With t = 2.2 mm, the lowest RL value of Fe/SiO2/Epoxy composite

is -17 dB at 13.6 GHz, and the frequency band with RL lower than -10 dB is around 5.12 GHz These results can illustrate the improvement of the insulating SiO2 shell layer on the microwave absorption performance of Fe particles

3.3 Summary

In this chapter, the commercial carbonyl iron particles were coated by 140nm-silica shell layer through Stöber method Compared with Fe composite, the effective permittivity of Fe/SiO2 composite decreases dramatically, while the effective permeability changes little The results indicate that the insulating coating layer of SiO2 on the Fe particles is effective to reduce the skin effect The improvement is also shown by the reflection loss curves at relative high frequency (above 12 GHz) The absorption peak of Fe/SiO2/Epoxy composite corresponding to the lowest reflection loss shifts to higher frequency comparing with Fe/Epoxy composite Furthermore, the optimal thickness t of Fe/SiO2/Epoxy composite is smaller than that of Fe/Epoxy composite, which renders it suitable for thinner and lighter microwave absorber Furthermore, the calculation models on the effective permeability and the effective permittivity are significant to the microwave absorption performance estimation

3.4 References

[1] S Cheong, P Ferguson, K W Feindel, I F Hermans, P T Callaghan, C Meyer, A Slocome, C H Su, F Y Cheng, C S Yeh, B Ingham, M F Toney, R D Tilley, Angew Chem Int Ed., 50, 4206-4209 (2011)

[2] Y L Chueh, C H Hsieh, M T Chang, L J Chou, C S Lao, J H Song, J Y Gan, Z L Wang, Adv Mater., 19, 143-149 (2007)

[3] C Wang, R T Lv, Z H Huang, F Y Kang, J L Gu, J Alloys Compd., 509, 494- 498 (2011)

[4] V K Varadan, L Chen, J Xie, Magnetic Nanoparticles, in Nanomedicine: Design and Applications of Magnetic Nanomaterials, Nanosensors and Nanosystems John Wiley & Sons, Ltd, p108 (2008)

[5] X M Ni, Z Zheng, X Hu, X L Xiao J Colloid Interface Sci., 341, 18-22 (2010) [6] X Yan, G Z Chai, D S Xue, J Alloys Compd., 509, 1310-1313 (2011)

[7] V Sunny, D S Kumar, P Mohanan, M R Anantharaman, Mater Lett., 64, 1130-1132 (2010)

[8] X J Wei, J T Liang, L Zhen, Y X Gong, W Z Shao, C Y Xu, Mater Lett., 64, 57-60 (2010)

[9] X F Meng, X Q Shen, W Liu, Appl Surf Sci., 258, 2627-2631(2012)

[10] X F Zhang F, X L Dong, H Huang, Y Y Liu, W N Wang, Appl Phys Lett., 89,

053115 (2006)

[11] H J Zhang, X W Wu, Q L Jia, X L Jia, Mater Des., 28,1369-1373 (2007)

[12] X G Lu, G Y Liang, Y M Zhang, Mater Lett., 61,4928-4931(2007)

[13] X M Ni, Z Zhong, X K Xiao, L Huang, L He, Mater Chem Phys., 120, 206- 212 (2010)

[14] Y C Qing, W C Zhou, S Jia, F Luo, D M Zhu, Physica B, 406, 777-780 (2011) [15] C P Neo, Calculations of Microwave Permeability, Permittivity and Absorption Properties of Magnetic Particle Composites, PhD Thesis, National University of Singapore,

2010