- Email: [email protected]
epoxy coatings
Physica B 405 (2010) 3611–3615
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Electroless plating preparation and microwave electromagnetic properties of Ni-coated carbonyl iron particle/epoxy coatings Jia Shun, Luo Fa, Qing Yuchang, Zhou Wancheng, Zhu Dongmei State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
a r t i c l e in fo
abstract
Article history: Received 4 March 2010 Received in revised form 16 May 2010 Accepted 17 May 2010
The morphology, composition, electromagnetic and microwave absorbing properties of the electroless Ni plated carbonyl iron (CI) particles were characterized using electron scanning microscope (SEM), energy dispersive spectrometer (EDS) and microwave network analyzer. The SEM and EDS results indicate that the CI particles were coated with uniform Ni coating. After the CI particles were coated with Ni, the real part of complex permittivity of the Ni-coated CI particles decreased while the complex permeability and the imaginary part of complex permittivity kept almost constant. The complex permittivity and complex permeability of the Ni-coated CI particles filled epoxy coatings increased with increase in weight content of Ni-coated CI particles. Compared with the raw CI particles/epoxy coatings with the same coating thickness and particles content, the Ni-coated CI particles/epoxy coatings possessed higher microwave absorption in the X-band and the microwave absorbing peak shifted to a higher frequency range. & 2010 Elsevier B.V. All rights reserved.
Keywords: Carbonyl iron particles Electroless plating Electromagnetic property Reflection loss
1. Introduction Carbonyl iron (CI) particles have attracted many researchers’ attention for a long time as magnetic components of polymeric composites for a number of applications, such as magnetic storage media, plastic encapsulated inductor cores [1,2] and electromagnetic wave absorbers [3,4]. And it is reported that CI particles have large saturation magnetization, high Curie temperature and a very wide bandwidth of microwave absorption, which contributed to the frequency dispersion of CI particles generated on the high frequency side (especially above 4 GHz) [5,6]. However, a common drawback of the CI particles is their oxidation at high temperatures to form a non-magnetic material Fe2O3, which leads to the decrease in magnetic properties of CI particles. Thus, some researchers have focused on the use of surface coatings to protect the CI particles from oxidation. Abshinova et al. [1] prepared CI particles coated with polyaniline (PANI) by using surface-stabilized PANI colloids in chloroform, which prevented further oxidation of the CI particles. Jay et al. [6] prepared Fe particles with Al by a fluidized bed metal organic chemical vapour deposition (FB-MOCVD) and the result demonstrated that this coating offered an efficient barrier to protect iron particles against oxidation. Electroless plating is a convenient method to obtain thin and uniform metallic layers on conductive and nonconductive substrates without an external current source, and the layers have
n
Corresponding author. Fax: + 86 29 8849 4574. E-mail address: [email protected] (S. Jia).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.05.050
good corrosion and wear resistances [7,8]. Therefore, some studies prepared Ni, Co, Zn and Cu layers by electroless plating to protect the magnetic particles from oxidation [9,10]. Ulicny and Mance [11] indicated that CI particles coated with Ni by electroless deposition is an effective way to reduce oxidation by the thermogravimetric analysis (TGA) results. However, the effects of Ni coating on electromagnetic properties of CI particles were unclear. Therefore, in this paper we mainly investigated the influence of electroless deposition Ni coating on the morphology, electromagnetic and related microwave absorption properties of CI particles.
2. Experimental The CI particles used in this study were fabricated by decomposition of Fe(CO)5. The main characteristics of CI particles are: the content of a-iron 499.5 wt%, thin flakes of 1–5 mm in diameter and below 1 mm in thickness and polycrystalline microstructure. In order to remove organic remains stuck on the CI particles surface, the CI particles were heated at 600 1C for 30 min and then cooled to room temperature in a vacuum furnace. Then the CI particles were degreased in dilute solution of hydrochloric acid at pH-value of 3 to remove oxidation films. The cleaned particles were washed three times in distilled water and then dried at 80 1C in vacuum oven, which were used as raw CI particles for electroless deposition Ni coatings. The composition of the bath for electroless deposition Ni coatings is shown in Table 1. Sigma-Aldrich reagents were used to
3612
S. Jia et al. / Physica B 405 (2010) 3611–3615
technologies E8362B: 10 MHz–20 GHz). The samples for electromagnetic measurement were of the size of 10.16 22.86 2 mm3 and put into the brass holder (10.16 22.86 mm2) to measure the complex permittivity and permeability, which were based on the measurements of the reflection and transmission module between 8.2 and 12.4 GHz in the fundamental wave-guide mode TE10. Three samples for each type were prepared and the data scatter among these samples was below 5%. In addition, the morphology and composition of the raw and Ni-coated CI particles were examined under a scanning electron microscope (SEM; JEOL JSM-5800 LV SKANNING) attachment with a Links Systems energy dispersive spectrometer (EDS).
prepare the bath. The nickel ion source was nickel sulfate hexahydrate (NiSO4 6H2O), and lactic acid (C3H6O3) and sodium citrate (C6H5Na3O7 2H2O) were used as complexing agents. The reducing substance of nickel ion was sodium hypophosphite (NaH2PO2 H2O). Succinate (C4H6O4) was responsible for buffering the bath and sulfourea (CH4N2S) was the stabilizer. Nickel coating deposition was performed in a beaker heated by thermostat water bath. A rate of 200 rmp of mechanic stirring ensured a homogenous suspension bath so that the particles did not deposit on the bottom of the beaker or form aggregated particles. The operating temperature was 70 1C and the deposition time was set for 60 min. The pH-value of the bath for electroless was adjusted to 5 with 10 wt% NaOH. Commonly, the quantity of Ni and P can be controlled through the content of NiSO4 6H2O and NaH2PO2 H2O, respectively, and the thickness of Ni coating can be controlled by the plating time [12,13]. In this work, the final weight ratio of Ni/CI particles was about 5:95 and the thickness of the Ni coating was below 0.1 mm. The raw and Ni-coated CI particles were homogeneously dispersed into epoxy resin to fabricate microwave absorbing coatings. The complex permittivity e(f) and permeability m(f) of the coatings were tested by network analyzer (Agilent
3. Results and discussion 3.1. Morphology and component of carbonyl iron particles before and after electroless nickel deposition Fig. 1 shows the SEM images and EDS spectra of the CI particles before and after electroless nickel deposition. From Fig. 1(a),
Table 1 The chemical composition of the electroless bath. NiSO4 6H2O (g/L)
NaH2PO2 H2O (g/L)
C6H5Na3O7 2H2O (g/L)
C4H6O4 (g/L)
C3H6O3 (g/L)
CH4N2S (g/L)
40
25
30
8
25
1
Fe
500 400 300 200 100
Fe
Fe
0 2
4
6
8
10
Kev 600 500 400 300 200 100 0
Fe
Fe
Ni
Ni
P 2
Ni 4
6
Ni 8
10
Kev Fig. 1. The SEM and EDS of CI particles before and after coated with Ni.
S. Jia et al. / Physica B 405 (2010) 3611–3615
it could be found that raw CI particles are flake shaped and exhibit a smooth surface. After electroless nickel plating, the surface of the CI particles become much rough and the average diameter of the particles grows larger than that of raw particles, as shown in Fig. 1(b). The EDS analysis indicates the presence of Ni and P on the surface of CI particles after the electroless deposition, as shown in Fig. 1(c) and (d).
3.2. The influence of electroless Ni deposition on the electromagnetic property of CI particles The complex permittivity e(f) and permeability m(f) of the coatings filled with the raw and Ni-coated CI particles (the content of the raw and Ni-coated CI particles are 75 wt%) are presented in Fig. 2. From Fig. 2(a), it could be seen that the values of the real part of permittivity of the coatings filled with raw CI particles kept almost unvaried (e0 E14) in the frequency range 8.2–12.4 GHz, and the real part of permittivity decreased (from E14 to E11) after Ni coating deposition on the surface of CI particles. It can be seen clearly from Fig. 2 that the imaginary part of permittivity and the complex permeability of the composites between 8.2 and 12.4 GHz almost remained unchanged before and after electroless Ni deposition. In general, the real part of permittivity is proportional to the quantity of charge stored on the capacitor surface, according to
Fig. 2. The electromagnetic properties of CI particles before and after coated with Ni.
3613
the principle of equivalent circuit [14]. After the eletroless nickel deposition, the diameter of CI particles was larger than that of raw CI particles. It means that the interfaces of Ni-coated particles/ resin reduced when the composites were filled with the same content of particles. Therefore, the real part of permittivity of coatings filled with Ni-coated CI particles was expected to be lower than that filled with raw CI particles. According to the free-electron theory, the imaginary part of permittivity of the metal particles filled coatings is mainly related to the conductivity loss [15]. The Ni-coated CI particles were encapsulated by conducting Ni shells. Thus, the raw and Ni-coated CI particles possessed the same electrical conductivity due to their similar metallic nature of Fe and Ni. From this point of view, the same electrical conductivity can give rise to the same value of the imaginary part of permittivity. 3.3. The effect of weight content on the electromagnetic property of Ni-coated CI particles The complex permittivity of the coatings with different weight contents of Ni-coated CI particles is presented in Fig. 3. When the weight contents of Ni-coated CI particles are 60, 65, 70, 75 and
Fig. 3. The effect of Ni-coated CI particles content on the (a) real and (b) imaginary parts of complex permittivity of the microwave absorbing coatings.
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S. Jia et al. / Physica B 405 (2010) 3611–3615
80 wt%, the values of real part permittivity are 5.6, 6.3, 7.3, 11 and 18 at the frequency of 8.2 GHz, respectively. The results showed a rapid increase of both the real and imaginary parts of permittivity as the content of Ni-coated CI particles increased from 70 to 80 wt%. The result may be attributed to higher multipole interaction that can be obtained between the particles when the filling fraction reaches the closest packing fraction and caused a big permittivity enhancement [16]. It is found that when the content of Ni-coated CI particles is lower, the permittivity of the coatings almost keeps unvaried in the frequency range 8.2–12.4 GHz. As the Ni-coated CI particles’ content is increased, the fluctuation of the real and imaginary parts’ permittivity of coatings is clearly visible. The result is attributed to the higher multipole interaction and interfacial polarization between the Ni-coated CI particles and epoxy resin with increase in the content of particles. Accordingly, the coatings with the lower Ni-coated CI particles content essentially show no fluctuation of real and imaginary parts of permittivity [17]. Fig. 4 shows the effect of the weight content of Ni-coated CI particles on the complex permeability of the coatings. Both the real and imaginary parts of permeability increased when the weight content of Ni-coated CI particles increased. According to the Lichtenecker law [18]: ln(m) ¼pln(mc)+ (1 p)ln(mm), where m, mc and mm are the permeabilities of the composite, Ni-coated CI particles and epoxy resin, respectively, and p is the volume fraction of the particles. Because the mc is far greater than mm, it is reasonable that the permeability of the coatings increased with the increase in p. 3.4. The effect of electroless deposition Ni on the microwave absorption of CI particles filled coatings According to the transmission line theory, the reflection loss curves can be calculated from the complex permittivity and complex permeability at a given frequency and the thickness of the microwave absorbing materials. For a single-layer absorber rffiffiffiffiffi mr 2p pffiffiffiffiffiffiffiffiffi Zin ¼ Z0 tanh j mr er fd ð1Þ er c where Z0 is the impedance of the free space; Zin is the input impedance; mr and er are the relative permeability and permittivity of the absorber, respectively; f is the frequency of the electromagnetic wave; d is the thickness of the absorber; and c is the velocity of the light in free space. The reflection loss RL is given by the following relation: RLðdBÞ ¼ 20 logðZin Z0 Þ=ðZin þ Z0 Þ ð2Þ Thus, using Eqs. (1) and (2) and the experimental values of mr and er shown in Fig. 2, the theoretical values of reflection loss of the coatings with 1.7 mm thickness can be obtained. Fig. 5 shows the effect of electroless deposition Ni on the microwave absorption of CI particles filled coatings (the content of particles are 75 wt%). When the thickness of the coatings is 1.7 mm, the reflection loss values less than 5 dB can be obtained in the frequency range 8.2–12.4 GHz for both the raw and Ni-coated CI particles filled coatings. In addition, the strong microwave absorption ( 410 dB, for over 90% microwave absorption) can be obtained in the frequency ranges 8.2–11 and 8.2–12.4 GHz, when the coatings filled with the raw and Ni-coated CI particles, respectively. It is also clearly found that the minimum reflection loss move toward high frequency region from 8.8 to 10.2 GHz after CI particles are coated with Ni. The reflection loss results indicated that the Ni-coated CI particles possess higher microwave absorption in the X-band. As we know, the design of EM waves absorbing materials with low reflection requires two important conditions: impedance matching
Fig. 4. The effect of Ni-coated CI particles content on the (a) real and (b) imaginary parts of complex permeability of the coatings.
Fig. 5. Microwave absorption of the raw and Ni-coated CI particles filled coatings.
S. Jia et al. / Physica B 405 (2010) 3611–3615
characteristic and attenuation characteristic. It can be noticed from relations (1) and (2) that the combinations of six parameters (e0 , e00 , m0 , m00 , f and d) mainly determined the microwave absorption of the composites, and the minimizing reflection of incident plane wave can happen when Zin is close to the constant Z0. As shown in Fig. 2, after the CI particles were coated with Ni, the real part of complex permittivity of the coatings decreased while the complex permeability and the imaginary part of complex permittivity kept almost constant. The reflection loss results indicate that the lower real part of permittivity can result in Zin of Ni-coated CI particles close to Z0. So, it is reasonable that the Ni-coated CI particles filled coating shows high microwave absorption in the X-band. Furthermore, the dielectric loss (tan(de)¼ e00 /e0 ) also increased after the CI particles were coated with Ni, which also act as another important factor for the increase of microwave absorption of the Ni-coated CI particles filled coating.
4. Conclusions After the electroless nickel plating, the surface of the CI particles has become much rough and the average diameter of the CI particles larger than raw CI particles. The real part permittivity decreased and the imaginary part and complex permeability almost remained unchanged after electroless nickel plating. In the frequency range 8.2–12.4 GHz, the values of complex permittivity and permeability increased with the increase in weight concentration of Ni-coated CI particles, and both the real and imaginary parts of permittivity increased rapidly as the content of Ni-coated CI particles increased from 70 to 80 wt%. When the thickness of
3615
the coating is 1.7 mm and the content of particles is 75 wt%, the reflection loss values less than 10 dB can be obtained in the frequency range 8.2–12.4 GHz after the CI particles are coated with Ni.
Acknowledgements This work was supported by the fund of the State Key Laboratory of Solidification Processing in NWPU, No. KP200901.
References [1] M. Abshinova, N. Kazantseva, P. Saha, et al., Polym. Degrad. Stabil. 93 (2008) 1826. [2] B. Zhang, Y. Feng, J. Xiong, et al., IEEE Trans. Magn. 42 (2006) 1778. [3] Y. Qing, W. Zhou, F. Luo, et al., J. Magn. Magn. Mater. 321 (2009) 25. [4] M. Abshinova, A. Lopatin, N. Kazantseva, et al., Compos. Part. A 38 (2007) 2471. [5] F. Wen, W. Zuo, H. Yi, et al., Physica B 404 (2009) 3567. [6] F. Jay, V. Brunet, F. Pailloux, et al., Surf. Coat. Technol. 202 (2008) 4302. [7] J. Gao, F. Tang, J. Ren, Surf. Coat. Technol. 200 (2005) 2249. [8] E. Rudnik, K. Kokoszka, J. Lapsa, Surf. Coat. Technol. 202 (2008) 2584. [9] M. Sarret, C. Muller, A. Amell, Surf. Coat. Technol. 201 (2006) 389. [10] S. Alirezaei, S. Monirvaghefi, M. Salehi, et al., Wear 262 (2007) 978. [11] J. Ulicny, A. Mance, Mat. Sci. Eng. A 369 (2004) 309. [12] M. Abdullah, S. Ahmad, S. Jusoh, et al., J. Appl. Phys. 92 (2002) 876. [13] S. Shiue, C. Yang, R.S. Chu, et al., Thin Solid Films 485 (2005) 169. [14] W. Doyle, I. Jacobs, Phys. Rev. B 42 (1990) 9319. [15] R. Martel, T. Schmidt, H. Shea, et al.Appl. Phys. Lett. 73 (1998) 2447. [16] A. Ma"ecki, A. Micek-Ilnicka, Surf. Coat. Technol. 123 (2000) 72. [17] P. Watts, D. Ponnampalam, W. Hsu, et al., Chem. Phys. Lett. 378 (2003) 609. [18] S. Ramo, J. Whinnery, T. Duzer, in: Fields and Waves in Communication ElectronicsWiley, New York, 1984.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Electroless plating preparation and microwave electromagnetic properties of Ni-coated carbonyl iron particle/epoxy coatings Jia Shun, Luo Fa, Qing Yuchang, Zhou Wancheng, Zhu Dongmei State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
a r t i c l e in fo
abstract
Article history: Received 4 March 2010 Received in revised form 16 May 2010 Accepted 17 May 2010
The morphology, composition, electromagnetic and microwave absorbing properties of the electroless Ni plated carbonyl iron (CI) particles were characterized using electron scanning microscope (SEM), energy dispersive spectrometer (EDS) and microwave network analyzer. The SEM and EDS results indicate that the CI particles were coated with uniform Ni coating. After the CI particles were coated with Ni, the real part of complex permittivity of the Ni-coated CI particles decreased while the complex permeability and the imaginary part of complex permittivity kept almost constant. The complex permittivity and complex permeability of the Ni-coated CI particles filled epoxy coatings increased with increase in weight content of Ni-coated CI particles. Compared with the raw CI particles/epoxy coatings with the same coating thickness and particles content, the Ni-coated CI particles/epoxy coatings possessed higher microwave absorption in the X-band and the microwave absorbing peak shifted to a higher frequency range. & 2010 Elsevier B.V. All rights reserved.
Keywords: Carbonyl iron particles Electroless plating Electromagnetic property Reflection loss
1. Introduction Carbonyl iron (CI) particles have attracted many researchers’ attention for a long time as magnetic components of polymeric composites for a number of applications, such as magnetic storage media, plastic encapsulated inductor cores [1,2] and electromagnetic wave absorbers [3,4]. And it is reported that CI particles have large saturation magnetization, high Curie temperature and a very wide bandwidth of microwave absorption, which contributed to the frequency dispersion of CI particles generated on the high frequency side (especially above 4 GHz) [5,6]. However, a common drawback of the CI particles is their oxidation at high temperatures to form a non-magnetic material Fe2O3, which leads to the decrease in magnetic properties of CI particles. Thus, some researchers have focused on the use of surface coatings to protect the CI particles from oxidation. Abshinova et al. [1] prepared CI particles coated with polyaniline (PANI) by using surface-stabilized PANI colloids in chloroform, which prevented further oxidation of the CI particles. Jay et al. [6] prepared Fe particles with Al by a fluidized bed metal organic chemical vapour deposition (FB-MOCVD) and the result demonstrated that this coating offered an efficient barrier to protect iron particles against oxidation. Electroless plating is a convenient method to obtain thin and uniform metallic layers on conductive and nonconductive substrates without an external current source, and the layers have
n
Corresponding author. Fax: + 86 29 8849 4574. E-mail address: [email protected] (S. Jia).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.05.050
good corrosion and wear resistances [7,8]. Therefore, some studies prepared Ni, Co, Zn and Cu layers by electroless plating to protect the magnetic particles from oxidation [9,10]. Ulicny and Mance [11] indicated that CI particles coated with Ni by electroless deposition is an effective way to reduce oxidation by the thermogravimetric analysis (TGA) results. However, the effects of Ni coating on electromagnetic properties of CI particles were unclear. Therefore, in this paper we mainly investigated the influence of electroless deposition Ni coating on the morphology, electromagnetic and related microwave absorption properties of CI particles.
2. Experimental The CI particles used in this study were fabricated by decomposition of Fe(CO)5. The main characteristics of CI particles are: the content of a-iron 499.5 wt%, thin flakes of 1–5 mm in diameter and below 1 mm in thickness and polycrystalline microstructure. In order to remove organic remains stuck on the CI particles surface, the CI particles were heated at 600 1C for 30 min and then cooled to room temperature in a vacuum furnace. Then the CI particles were degreased in dilute solution of hydrochloric acid at pH-value of 3 to remove oxidation films. The cleaned particles were washed three times in distilled water and then dried at 80 1C in vacuum oven, which were used as raw CI particles for electroless deposition Ni coatings. The composition of the bath for electroless deposition Ni coatings is shown in Table 1. Sigma-Aldrich reagents were used to
3612
S. Jia et al. / Physica B 405 (2010) 3611–3615
technologies E8362B: 10 MHz–20 GHz). The samples for electromagnetic measurement were of the size of 10.16 22.86 2 mm3 and put into the brass holder (10.16 22.86 mm2) to measure the complex permittivity and permeability, which were based on the measurements of the reflection and transmission module between 8.2 and 12.4 GHz in the fundamental wave-guide mode TE10. Three samples for each type were prepared and the data scatter among these samples was below 5%. In addition, the morphology and composition of the raw and Ni-coated CI particles were examined under a scanning electron microscope (SEM; JEOL JSM-5800 LV SKANNING) attachment with a Links Systems energy dispersive spectrometer (EDS).
prepare the bath. The nickel ion source was nickel sulfate hexahydrate (NiSO4 6H2O), and lactic acid (C3H6O3) and sodium citrate (C6H5Na3O7 2H2O) were used as complexing agents. The reducing substance of nickel ion was sodium hypophosphite (NaH2PO2 H2O). Succinate (C4H6O4) was responsible for buffering the bath and sulfourea (CH4N2S) was the stabilizer. Nickel coating deposition was performed in a beaker heated by thermostat water bath. A rate of 200 rmp of mechanic stirring ensured a homogenous suspension bath so that the particles did not deposit on the bottom of the beaker or form aggregated particles. The operating temperature was 70 1C and the deposition time was set for 60 min. The pH-value of the bath for electroless was adjusted to 5 with 10 wt% NaOH. Commonly, the quantity of Ni and P can be controlled through the content of NiSO4 6H2O and NaH2PO2 H2O, respectively, and the thickness of Ni coating can be controlled by the plating time [12,13]. In this work, the final weight ratio of Ni/CI particles was about 5:95 and the thickness of the Ni coating was below 0.1 mm. The raw and Ni-coated CI particles were homogeneously dispersed into epoxy resin to fabricate microwave absorbing coatings. The complex permittivity e(f) and permeability m(f) of the coatings were tested by network analyzer (Agilent
3. Results and discussion 3.1. Morphology and component of carbonyl iron particles before and after electroless nickel deposition Fig. 1 shows the SEM images and EDS spectra of the CI particles before and after electroless nickel deposition. From Fig. 1(a),
Table 1 The chemical composition of the electroless bath. NiSO4 6H2O (g/L)
NaH2PO2 H2O (g/L)
C6H5Na3O7 2H2O (g/L)
C4H6O4 (g/L)
C3H6O3 (g/L)
CH4N2S (g/L)
40
25
30
8
25
1
Fe
500 400 300 200 100
Fe
Fe
0 2
4
6
8
10
Kev 600 500 400 300 200 100 0
Fe
Fe
Ni
Ni
P 2
Ni 4
6
Ni 8
10
Kev Fig. 1. The SEM and EDS of CI particles before and after coated with Ni.
S. Jia et al. / Physica B 405 (2010) 3611–3615
it could be found that raw CI particles are flake shaped and exhibit a smooth surface. After electroless nickel plating, the surface of the CI particles become much rough and the average diameter of the particles grows larger than that of raw particles, as shown in Fig. 1(b). The EDS analysis indicates the presence of Ni and P on the surface of CI particles after the electroless deposition, as shown in Fig. 1(c) and (d).
3.2. The influence of electroless Ni deposition on the electromagnetic property of CI particles The complex permittivity e(f) and permeability m(f) of the coatings filled with the raw and Ni-coated CI particles (the content of the raw and Ni-coated CI particles are 75 wt%) are presented in Fig. 2. From Fig. 2(a), it could be seen that the values of the real part of permittivity of the coatings filled with raw CI particles kept almost unvaried (e0 E14) in the frequency range 8.2–12.4 GHz, and the real part of permittivity decreased (from E14 to E11) after Ni coating deposition on the surface of CI particles. It can be seen clearly from Fig. 2 that the imaginary part of permittivity and the complex permeability of the composites between 8.2 and 12.4 GHz almost remained unchanged before and after electroless Ni deposition. In general, the real part of permittivity is proportional to the quantity of charge stored on the capacitor surface, according to
Fig. 2. The electromagnetic properties of CI particles before and after coated with Ni.
3613
the principle of equivalent circuit [14]. After the eletroless nickel deposition, the diameter of CI particles was larger than that of raw CI particles. It means that the interfaces of Ni-coated particles/ resin reduced when the composites were filled with the same content of particles. Therefore, the real part of permittivity of coatings filled with Ni-coated CI particles was expected to be lower than that filled with raw CI particles. According to the free-electron theory, the imaginary part of permittivity of the metal particles filled coatings is mainly related to the conductivity loss [15]. The Ni-coated CI particles were encapsulated by conducting Ni shells. Thus, the raw and Ni-coated CI particles possessed the same electrical conductivity due to their similar metallic nature of Fe and Ni. From this point of view, the same electrical conductivity can give rise to the same value of the imaginary part of permittivity. 3.3. The effect of weight content on the electromagnetic property of Ni-coated CI particles The complex permittivity of the coatings with different weight contents of Ni-coated CI particles is presented in Fig. 3. When the weight contents of Ni-coated CI particles are 60, 65, 70, 75 and
Fig. 3. The effect of Ni-coated CI particles content on the (a) real and (b) imaginary parts of complex permittivity of the microwave absorbing coatings.
3614
S. Jia et al. / Physica B 405 (2010) 3611–3615
80 wt%, the values of real part permittivity are 5.6, 6.3, 7.3, 11 and 18 at the frequency of 8.2 GHz, respectively. The results showed a rapid increase of both the real and imaginary parts of permittivity as the content of Ni-coated CI particles increased from 70 to 80 wt%. The result may be attributed to higher multipole interaction that can be obtained between the particles when the filling fraction reaches the closest packing fraction and caused a big permittivity enhancement [16]. It is found that when the content of Ni-coated CI particles is lower, the permittivity of the coatings almost keeps unvaried in the frequency range 8.2–12.4 GHz. As the Ni-coated CI particles’ content is increased, the fluctuation of the real and imaginary parts’ permittivity of coatings is clearly visible. The result is attributed to the higher multipole interaction and interfacial polarization between the Ni-coated CI particles and epoxy resin with increase in the content of particles. Accordingly, the coatings with the lower Ni-coated CI particles content essentially show no fluctuation of real and imaginary parts of permittivity [17]. Fig. 4 shows the effect of the weight content of Ni-coated CI particles on the complex permeability of the coatings. Both the real and imaginary parts of permeability increased when the weight content of Ni-coated CI particles increased. According to the Lichtenecker law [18]: ln(m) ¼pln(mc)+ (1 p)ln(mm), where m, mc and mm are the permeabilities of the composite, Ni-coated CI particles and epoxy resin, respectively, and p is the volume fraction of the particles. Because the mc is far greater than mm, it is reasonable that the permeability of the coatings increased with the increase in p. 3.4. The effect of electroless deposition Ni on the microwave absorption of CI particles filled coatings According to the transmission line theory, the reflection loss curves can be calculated from the complex permittivity and complex permeability at a given frequency and the thickness of the microwave absorbing materials. For a single-layer absorber rffiffiffiffiffi mr 2p pffiffiffiffiffiffiffiffiffi Zin ¼ Z0 tanh j mr er fd ð1Þ er c where Z0 is the impedance of the free space; Zin is the input impedance; mr and er are the relative permeability and permittivity of the absorber, respectively; f is the frequency of the electromagnetic wave; d is the thickness of the absorber; and c is the velocity of the light in free space. The reflection loss RL is given by the following relation: RLðdBÞ ¼ 20 logðZin Z0 Þ=ðZin þ Z0 Þ ð2Þ Thus, using Eqs. (1) and (2) and the experimental values of mr and er shown in Fig. 2, the theoretical values of reflection loss of the coatings with 1.7 mm thickness can be obtained. Fig. 5 shows the effect of electroless deposition Ni on the microwave absorption of CI particles filled coatings (the content of particles are 75 wt%). When the thickness of the coatings is 1.7 mm, the reflection loss values less than 5 dB can be obtained in the frequency range 8.2–12.4 GHz for both the raw and Ni-coated CI particles filled coatings. In addition, the strong microwave absorption ( 410 dB, for over 90% microwave absorption) can be obtained in the frequency ranges 8.2–11 and 8.2–12.4 GHz, when the coatings filled with the raw and Ni-coated CI particles, respectively. It is also clearly found that the minimum reflection loss move toward high frequency region from 8.8 to 10.2 GHz after CI particles are coated with Ni. The reflection loss results indicated that the Ni-coated CI particles possess higher microwave absorption in the X-band. As we know, the design of EM waves absorbing materials with low reflection requires two important conditions: impedance matching
Fig. 4. The effect of Ni-coated CI particles content on the (a) real and (b) imaginary parts of complex permeability of the coatings.
Fig. 5. Microwave absorption of the raw and Ni-coated CI particles filled coatings.
S. Jia et al. / Physica B 405 (2010) 3611–3615
characteristic and attenuation characteristic. It can be noticed from relations (1) and (2) that the combinations of six parameters (e0 , e00 , m0 , m00 , f and d) mainly determined the microwave absorption of the composites, and the minimizing reflection of incident plane wave can happen when Zin is close to the constant Z0. As shown in Fig. 2, after the CI particles were coated with Ni, the real part of complex permittivity of the coatings decreased while the complex permeability and the imaginary part of complex permittivity kept almost constant. The reflection loss results indicate that the lower real part of permittivity can result in Zin of Ni-coated CI particles close to Z0. So, it is reasonable that the Ni-coated CI particles filled coating shows high microwave absorption in the X-band. Furthermore, the dielectric loss (tan(de)¼ e00 /e0 ) also increased after the CI particles were coated with Ni, which also act as another important factor for the increase of microwave absorption of the Ni-coated CI particles filled coating.
4. Conclusions After the electroless nickel plating, the surface of the CI particles has become much rough and the average diameter of the CI particles larger than raw CI particles. The real part permittivity decreased and the imaginary part and complex permeability almost remained unchanged after electroless nickel plating. In the frequency range 8.2–12.4 GHz, the values of complex permittivity and permeability increased with the increase in weight concentration of Ni-coated CI particles, and both the real and imaginary parts of permittivity increased rapidly as the content of Ni-coated CI particles increased from 70 to 80 wt%. When the thickness of
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the coating is 1.7 mm and the content of particles is 75 wt%, the reflection loss values less than 10 dB can be obtained in the frequency range 8.2–12.4 GHz after the CI particles are coated with Ni.
Acknowledgements This work was supported by the fund of the State Key Laboratory of Solidification Processing in NWPU, No. KP200901.
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