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Fabrication of porous flaky electromagnetic particles by electroless plating of CoNiP on diatomite
Applied Surface Science 258 (2012) 9680–9684
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication of porous flaky electromagnetic particles by electroless plating of CoNiP on diatomite Liming Yuan, Jun Cai ∗ , Wenqiang Zhang, Zhiyang Lian, Deyuan Zhang Bionic and Micro/Nano/Bio Manufacturing Technology Research Center, School of Mechanical Engineering and Automation, Beihang University, Beijing 100191, PR China
a r t i c l e
i n f o
Article history: Received 26 October 2011 Received in revised form 7 May 2012 Accepted 4 June 2012 Available online 23 June 2012 Keywords: Diatomite Coupling processing Electroless plating Porous flaky particle Electromagnetic property
a b s t r a c t In order to fabricate lightweight flaky electromagnetic particles efficiently, the diatomite was used as the forming template to which the CoNiP coating was deposited by an electroless plating technique. The electroless plating of CoNiP on diatomite was accomplished with the coupling processing before the activation, in which 3-aminopropyltriethoxysilane (APTES) was used as the coupling agent. The effect of coupling processing on the activation and electroless plating of diatomite was characterized by SEM, FTIR and EDS analysis. The results indicated that with coupling processing before activation, the catalytic effect was improved remarkably than that of direct colloidal palladium activation, which resulted in superior electroless plating on diatomite, even on the surface of submicron holes. The XRD analysis showed that the CoNiP coating was a transitional structure between crystal and amorphousness. The measurement of complex permeability and permittivity in the range of 2–18 GHz suggested that the micro flaky electromagnetic particles had promising electromagnetic properties. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Electromagnetic wave absorbing materials had been playing an important role in the fields such as radar cross section (RCS) reduction, electromagnetic interference (EMI) and electromagnetic compatibility (EMC). The absorbing performance of the materials mainly depended on the dielectric and magnetic performances of the electromagnetic absorbing particles embedded. The researches concerning with the fabrication of electromagnetic particles mainly focus on two aspects, one is to develop new materials with promising electromagnetic performance by control of composition and particle shapes, the other is to reduce the density of the filler particles. Since the magnetic performance of flaky particles deviates from Snoek’s law, which would result in high permeability [1], it is a good choice to fabricate magnetic particles with flaky shape. At present, flaky electromagnetic particles were mainly fabricated by ball milling [2–6], though the promising performance was achieved, yet the density was heavy. Fabricating flaky particles with hollow structure was an effective way to reduce density of composites. Mostly, hollow-flake particles were prepared by surface metallization of flake templates, such as graphite sheet [7–9], but the density of the template was not light enough and the flake shape was irregular. Diatomite, the main component being amorphous SiO2 , is composed of the skeletons of diatoms, single-celled algae, which
∗ Corresponding author. Tel.: +86 10 82316603; fax: +86 10 82316603. E-mail address: jun [email protected] (J. Cai). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.06.009
accumulate in huge seabed or riverbed throughout the world. Diatomite had been widely used as filler, filter-aid, adsorbent, high temperature insulating material and catalytic support in virtue of its low cost, abundance, and intrinsic properties such as high porosity, thermal resistance and chemical stability [10–12]. Moreover, diatomite also has various shapes including regular flake shape with porous substructure. Therefore, taking diatomite as a template is one promising way to fabricate flake particles with lightweight and good electromagnetic properties. Electroless plating has been generally applied in metallization of non-conductive substrates because of its convenience, low-cost and high efficiency. Preliminary surface treatment is the precursor for effective absorption of the catalyst in standard electroless plating of non-conductive substrate, such as colloidal palladium activation. Yet for some certain substrate materials, such as glass, silicon dioxide, silicon-nitride, and polymer (tetrafluoroethylene), it is difficult to obtain the well-activated surface if we directly use the colloidal palladium activation. So there has been growing use of organosilane molecules before the activation process to improve the absorption of metallic nanoparticles/colloids to such material surfaces [13–17]. The diatomite is composed of silicon dioxide, and its structure was porous and its surface was not smooth, so the pretreatment process would be more critical for the following electroless plating process. In order to fabricate flaky light-weight particles with fine coating and promising electromagnetic properties by way of electroless plating of CoNiP onto diatomite, different pre-treatment process of the diatomite was carried out and optimized. Moreover, the effect of 3-aminopropyltriethoxysilane (APTES) on the electroless
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Table 1 Composition of electroless CoNiP plating solution. Composition Main salt Reducing agent Fig. 1. Process flow sheet of electroless plating on diatomite.
Complexing agent Buffer
plating was analyzed, and the electromagnetic properties of the micro flake porous particles fabricated by such way were investigated.
2. Experimental procedures 2.1. Preparation The diatomite was purchased from Linjiang Sailite Diatomite Co. Ltd., Jilin province, China. 3-Aminopropyltriethoxysilane with concentration of 99% was from Acros Organics, Geel, Belgium. All other chemical reagents were of analytic grade and used without further purification, which were bought from Beijing Chemical Reagents Factory. And all the aqueous solutions were prepared with deionized water during the process of preparation. The electroless plating of CoNiP on diatomite was carried out by the following process (as shown in Fig. 1): Firstly, diatomite was dispersed in ethanol and cleaned with ultrasonic washer for 40 min to remove the impurities. Then, the gravity sedimentation was carried out to improve the consistency of diatomite. After being dried in a vacuum drying chamber at 120 ◦ C for 6 h, 10 g dried diatomite was dispersed in 150 ml APTES ethanol solution (1.5 wt%), shaken at 30 ◦ C for 4 h, then the diatomite was filtered off and dried in a vacuum drying chamber at 120 ◦ C for 4 h. The diatomite needs to be treated with a noble metal catalyst before electroless plating since their surface was non-catalytic. This was accomplished with a colloidal Pd–Sn catalyst, after which the acceleration process was carried out with a solution containing 30 g/l NaH2 PO2 ·2H2 O for 30–60 s at room temperature. The activated diatomite was subsequently washed and stored in deionized water. The well-dispersed activated diatomite was then cast into self-prepared electroless plating solution of CoNiP. The composition of the plating solution was shown in Table 1, and the pH value of the solution was adjusted to 9 with sodium hydroxide. Electroless plating of CoNiP was carried out at 80 ◦ C in water bath. To avoid aggregation and sedimentation of diatomite, mechanical stir was used during the whole electroless plating process. To examine the effects of the coupling processing, electroless plating of CoNiP on diatomite without coupling processing by APTES was also prepared.
Agents
Concentration (mol/L)
NiSO4 ·6H2 O CoSO4 ·7H2 O NaH2 PO2 ·2H2 O C4 H4 Na2 O5 C3 H2 Na2 O4 ·H2 O C4 H4 Na2 O4 ·6H2 O (NH4 )2 SO4
0.04 0.06 0.2 0.4 0.3 0.5 0.1
To investigate the electromagnetic properties of flaky CoNiPplated diatomite, a sample containing 50 wt% coated diatomite in the power-paraffin was made into a toroidal mold with an outer diameter of 7.00 mm and an inner diameter of 3.00 mm. Moreover, another sample containing CoNiP-plated hollow glass microspheres, with the same CoNiP content, was also prepared, so as to investigate the different electromagnetic properties between the flaky particles and spherical particles caused by the particle shape effect. 2.2. Characterization Fourier-transform infrared spectra (FT-IR) of the sample in the range of 400–4000 cm−1 were made with Nicolet AVATAR 360 spectrometer. Measurements were performed in the transmission mode in KBr pellets. The morphology of the sample was observed using Nikon 4500 optical microscope with a photographic camera and Hitachi S-4800 scanning electron microscope (SEM). The composition and phase structure analysis were studied by using Oxford Link 860 energy dispersive spectrometer (EDS) and D/Max 2200 PC automatic X-ray diffraction (XRD) respectively. The complex permeability and permittivity were measured with a CETE AV3627 vector network analyzer in the range of 2–18 GHz. 3. Results and discussion 3.1. Morphology and structure of diatomite before and after electroless plating of CoNiP When the activated diatomite with coupling processing was cast into the heated electroless CoNiP plating solution, a great deal of fine air bubbles could be seen in solution in less than 2 min, which meant the deposition proceeded smoothly. Fig. 2 showed the optical image of the diatomite before and after electroless plating of CoNiP, respectively. It was noted that the raw diatomite was transparent, yet the coated diatomite had the metallic luster under the reflection flight of the microscope. Fig. 3 showed the SEM image of the cross section of diatomite before and after plating.
Fig. 2. Optical micrographs of diatomite before (a) and after electroless CoNiP plating (b).
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Fig. 3. SEM micrographs of cross section of diatomite before (a) and after electroless CoNiP plating (b).
Comparing Fig. 3(a) and (b), it was very clear that the diatomite surface was covered with smooth and uniform coating after electroless plating of CoNiP, and the smooth and uniform coating was also deposited onto the surface of sub-holes. The coating thickness could be adjusted by change of the ratio of solution to diatomite, so as to achieve the coated flake with the porous structure or the full deposition of the sub-holes. The EDS result indicated the coating was composed of Co, Ni and P, with the atom ratio of Co:Ni:P being 4.5:3.5:1. Fig. 4 showed the XRD patterns of raw diatomite and the electroless CoNiP-plated diatomite. The “hill-like” peak around 2Â = 22.5◦ in Fig. 4(a) indicated that diatomite was mainly composed of amorphous SiO2 . In Fig. 4(b), there was a “hill-like” peak around 2Â = 45◦ in addition to that of diatomite in Fig. 4(a), which indicated that the CoNiP coating was amorphous. Moreover, there were sharp diffraction peaks on the top of “hill-like” peak, which suggested there are micro crystal structure in the coating. So we can conclude that the CoNiP coating was a transitional structure between crystal and amorphousness. As a comparison, the electroless plating on the activated diatomite without coupling process was not that prosperous. There were only a few bubbles came into being during the process of electroless CoNiP plating. Even after several hours’ reaction, only a few diatomites were coated, and the coating was neither smooth nor uniform.
(a), the diatomite before coupled (b) and diatomite after coupled (c) between 400 and 4000 cm−1 , respectively. In Fig. 5(a), the peak located at 2974 cm−1 represented the asymmetric stretching vibration of C H in CH2 CH2 , and the double-peaks at 1080 cm−1 and 1104 cm−1 were assigned to the asymmetric stretching vibration of Si O C. In Fig. 5(b), the peak located at 1094 cm−1 and 800 cm−1 were both attributed to the stretching vibration of Si O. The band at 3438 cm−1 was caused by stretching vibration of OH, and it indicated the presence of OH on the surface of diatomite. Compared with Fig. 5(b), there were two new peaks in Fig. 5(c). The weak peak located at 525 cm−1 was attributed to the stretching vibration of Si O Si, and the peak at 857 cm−1 was assigned to the deformation vibration of N H in CH2 NH2 . The presence of the two peaks was argued in favor of the occurrence of the coupling reaction between APTES and OH on the surface of diatomite. It indicated that amino group was chemisorbed onto the surface of diatomite after coupling processing. 3.3. EDS analysis of activated diatomite
To find out the differences between the two samples during electroless plating, FT-IR spectrum was used to find out the effect of coupling by APTES. Fig. 5 showed the FT-IR spectra of APTES
After activation, the coupled diatomite showed pale brown, while the non-coupled diatomite showed yellow, almost the same as the color of raw diatomite. EDS analysis was conducted to further compare the activation effect of diatomite with or without coupling processing. The result showed that the diatomite directly activated without coupling processing showed no Pd peak, yet the diatomite activated after the coupling process had several weak Pd peaks (as shown in Fig. 6), which indicated that Pd came into being on the surface of the coupled diatomite after activated by Pd–Sn colloids. The composition analysis indicated that the fraction of Pd is 0.89 wt% (as shown in Table 2).
Fig. 4. XRD patterns of: (a) raw diatomite; (b) diatomite after electroless CoNiP plating.
Fig. 5. FT-IR spectra of: (a) APTES; (b) diatomite before coupled; (c) diatomite after coupled.
3.2. Characterization by Fourier-transform infrared spectroscopy (FT-IR)
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Table 2 Analysis on the composition of coupled diatomite surface after activation. Element
C
O
Si
Pd
Total
Mass fraction (%) Atom fraction (%)
29.06 42.08
30.81 33.48
39.24 24.29
0.89 0.15
100.00 100.00
3.4. The mechanism of coupling, activation and deposition
Fig. 6. EDS analysis of coupled diatomite’s surface after activation.
The effective absorption of catalyst was the prerequisite to deposit a metal coating on the nonconductive substrate’s surface. There were two different functional groups in APTES, (CH2 )3 NH2 and OCH2 CH3 . In the coupling process, the OCH2 CH3 group hydrolyzed, and self-assembled membranes of the coupling agent were formed on the surface of the diatomite through a condensation reaction. Furthermore, weak covalent bond was formed
Fig. 7. Diagrammatic sketch of the general procedure for electroless plating of diatomite.
Fig. 8. Complex electromagnetic parameters and dissipation factors for the NiCoP plated diatomite flake and hollow microsphere.
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between APTES and the surface of diatomite. As a result, the NH2 group was chemisorbed onto the surface of diatomite. During the activation process, the catalyst Pd–Sn colloids were negatively charged (probably due to SnCl3 − ions) [15,18]. Once the diatomite coupled by APTES was immersed in the catalyst solution, the amino group was protonated and became positively charged. Thus, the Pd–Sn colloids could electrostatically bind to the protonated amines. In other words, the bridge-function of the APTES could greatly improve the anchor effect of colloidal Pd–Sn to the diatomite and enhance the adsorption amount of the catalytic sites. As a result, superior deposition of CoNiP coating on diatomite could be achieved. The diagrammatic sketch of the general procedure for electroless plating of diatomite was shown in Fig. 7.
There are abundant diatomites with different shapes and structures by nature. Using these diatomites as templates, micro electromagnetic particles with various shapes and structures could be fabricated by similar method presented in this work, which would have potential application in many fields including electromagnetic shielding and microwave absorbing.
3.5. Electromagnetic properties of diatomite after electroless plating of CoNiP
References
The complex permittivity and permeability, along with dissipation factors of the CoNiP-plated diatomite and hollow microsphere were shown in Fig. 8. From the result we could see that the CoNiP-plated diatomite had a superior complex permittivity and electromagnetic dissipation factors, although the real part of permeability was a little smaller than that of CoNiP-plated hollow microsphere (as shown in Fig. 8(c)). The reason was that the good flake shape of the CoNiPcoated diatomite was good for enhancing the electromagnetic susceptibility, and easy to form conductive networks which could increase the permittivity of composites effectively; furthermore, the porous substructure of diatomite could enhance scattering effect of electromagnetic wave within the composites which was in favor of electromagnetic dissipation. Moreover, it was discovered that a higher highest-content of effective material would be obtained using flaky diatomite than hollow microspheres as a template when other conditions were the same, which was also a key factor to improve the electromagnetic performance of the composites.
4. Conclusions Electroless plating of CoNiP on diatomite was successfully accomplished with the help of coupling processing, which offered a novel method to prepare micro flaky electromagnetic particles with porous structure. FT-IR and EDS results showed that the coupling processing with APTES played a very important role to improve the activation effect: the diatomite coupled by APTES enhanced the anchor effect on colloidal Pd–Sn particles. SEM and XRD results showed that the morphology and structure of diatomite could be well preserved after electroless plating. And the CoNiP coating was smooth and uniform. The deposited CoNiP coating was a transitional structure between crystal and amorphousness. The investigation on the electromagnetic properties of these micro flaky electromagnetic particles indicated that they possessed superior electromagnetic performance.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 50805005), the National 863 Project of China (Grant No. 2009AA043804), the Foundation for the Author of National Excellent Doctoral Dissertation of PR China (Grant No. 2007B32).
[1] A.N. Lagarkov, K.N. Rozanov, High-frequency behavior of magnetic composites, Journal of Magnetism and Magnetic Materials 321 (2009) 2082–2092. [2] X. Wang, R. Gong, P. Li, L. Liu, W. Cheng, Effects of aspect ratio and particle size on the microwave properties of Fe–Cr–Si–Al alloy flakes, Materials Science and Engineering A 466 (2007) 178–182. [3] L.J. Deng, P.H. Zhou, J.L. Xie, L. Zhang, Characterization and microwave resonance in nanocrystalline FeCoNi flake composite, Journal of Applied Physics 101 (2007) 103916. [4] P.H. Zhou, J.L. Xie, Y.Q. Liu, L.J. Deng, Composition dependence of microstructure, magnetic and microwave properties in ball-milled FeSiB nanocrystalline flakes, Journal of Magnetism and Magnetic Materials 320 (2008) 3390–3393. [5] M.S. Kim, E.H. Min, J.G. Koh, Comparison of the effects of particles shape on thin FeSiCr electromagnetic wave absorber, Journal of Magnetism and Magnetic Materials 321 (2009) 581–585. [6] R. Han, L. Qiao, T. Wang, F. Li, Microwave complex permeability of planar anisotropy carbonyl-iron particles, Journal of Alloys and Compounds 509 (2011) 2734–2737. [7] M. Ananth Kumar, R.C. Agarwala, V. Agarwala, Synthesis and characterization of electroless Ni–P coated graphite particles, Bulletin of Materials Science 31 (2008) 819–824. [8] Q. Li, G. Zeng, W. Zhao, G. Chen, Preparation and characterization of nickelcoated graphite nanosheets, Synthetic Metals 160 (2010) 200–202. [9] Y. Yang, S. Qi, J. Wang, Preparation and microwave absorbing properties of nickel-coated graphite nanosheet with pyrrole via in situ polymerization, Journal of Alloys and Compounds 520 (2012) 114–121. [10] X. Li, X. Li, N. Dai, G. Wang, Large-area fibrous network of polyaniline formed on the surface of diatomite, Applied Surface Science 255 (2009) 8276–8280. [11] F. Akhtar, P.O. Vasiliev, L. Bergstrom, Hierarchically porous ceramics from diatomite powders by pulsed current processing, Journal of the American Ceramic Society 92 (2) (2009) 338–343. [12] O. Hadjadj-Aoul, R. Belabbes, M. Belkadi, M.H. Guermouche, Characterization and performances of an Algerian diatomite-based gas chromatography support, Applied Surface Science 240 (2005) 131–139. [13] E. Delamarche, J. Vichiconti, S.A. Hall, Matthias, Electroless deposition of Cu on glass and patterning with microcontact printing, Langmuir 19 (17) (2003) 6567–6569. [14] E. Delamarche, M. Geissler, R.H. Magnuson, H. Schmid, B. Michel, Patterning NiB electroless deposited on glass using an electroplated Cu mask, microcontact printing, and wet etching, Langmuir 19 (14) (2003) 5892–5897. [15] A. Holtzman, S. Richter, Electroless plating of silicon nitride using (3aminopropyl) triethoxysilane, Journal of the Electrochemical Society 155 (3) (2008) D196–D202. [16] S. Asakura, S. Fukutani, A. Fuwa, Fabrication of built-in copper microstructures on epoxy resin, Microelectronic Engineering 75 (2004) 375–382. [17] M.C. Zhang, E.T. Kang, K.G. Neoh, K.L. Tan, Electroless plating of copper and nickel on surface-modified poly(tetrafluoroethylene) films, Journal of the Electrochemical Society 148 (2) (2001) C71–C80. [18] R.L. Cohen, K.W. West, Generative and stabilizing processes in tin–palladium sols and palladium sol sensitizers, Journal of the Electrochemical Society 120 (4) (1973) 502–508.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication of porous flaky electromagnetic particles by electroless plating of CoNiP on diatomite Liming Yuan, Jun Cai ∗ , Wenqiang Zhang, Zhiyang Lian, Deyuan Zhang Bionic and Micro/Nano/Bio Manufacturing Technology Research Center, School of Mechanical Engineering and Automation, Beihang University, Beijing 100191, PR China
a r t i c l e
i n f o
Article history: Received 26 October 2011 Received in revised form 7 May 2012 Accepted 4 June 2012 Available online 23 June 2012 Keywords: Diatomite Coupling processing Electroless plating Porous flaky particle Electromagnetic property
a b s t r a c t In order to fabricate lightweight flaky electromagnetic particles efficiently, the diatomite was used as the forming template to which the CoNiP coating was deposited by an electroless plating technique. The electroless plating of CoNiP on diatomite was accomplished with the coupling processing before the activation, in which 3-aminopropyltriethoxysilane (APTES) was used as the coupling agent. The effect of coupling processing on the activation and electroless plating of diatomite was characterized by SEM, FTIR and EDS analysis. The results indicated that with coupling processing before activation, the catalytic effect was improved remarkably than that of direct colloidal palladium activation, which resulted in superior electroless plating on diatomite, even on the surface of submicron holes. The XRD analysis showed that the CoNiP coating was a transitional structure between crystal and amorphousness. The measurement of complex permeability and permittivity in the range of 2–18 GHz suggested that the micro flaky electromagnetic particles had promising electromagnetic properties. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Electromagnetic wave absorbing materials had been playing an important role in the fields such as radar cross section (RCS) reduction, electromagnetic interference (EMI) and electromagnetic compatibility (EMC). The absorbing performance of the materials mainly depended on the dielectric and magnetic performances of the electromagnetic absorbing particles embedded. The researches concerning with the fabrication of electromagnetic particles mainly focus on two aspects, one is to develop new materials with promising electromagnetic performance by control of composition and particle shapes, the other is to reduce the density of the filler particles. Since the magnetic performance of flaky particles deviates from Snoek’s law, which would result in high permeability [1], it is a good choice to fabricate magnetic particles with flaky shape. At present, flaky electromagnetic particles were mainly fabricated by ball milling [2–6], though the promising performance was achieved, yet the density was heavy. Fabricating flaky particles with hollow structure was an effective way to reduce density of composites. Mostly, hollow-flake particles were prepared by surface metallization of flake templates, such as graphite sheet [7–9], but the density of the template was not light enough and the flake shape was irregular. Diatomite, the main component being amorphous SiO2 , is composed of the skeletons of diatoms, single-celled algae, which
∗ Corresponding author. Tel.: +86 10 82316603; fax: +86 10 82316603. E-mail address: jun [email protected] (J. Cai). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.06.009
accumulate in huge seabed or riverbed throughout the world. Diatomite had been widely used as filler, filter-aid, adsorbent, high temperature insulating material and catalytic support in virtue of its low cost, abundance, and intrinsic properties such as high porosity, thermal resistance and chemical stability [10–12]. Moreover, diatomite also has various shapes including regular flake shape with porous substructure. Therefore, taking diatomite as a template is one promising way to fabricate flake particles with lightweight and good electromagnetic properties. Electroless plating has been generally applied in metallization of non-conductive substrates because of its convenience, low-cost and high efficiency. Preliminary surface treatment is the precursor for effective absorption of the catalyst in standard electroless plating of non-conductive substrate, such as colloidal palladium activation. Yet for some certain substrate materials, such as glass, silicon dioxide, silicon-nitride, and polymer (tetrafluoroethylene), it is difficult to obtain the well-activated surface if we directly use the colloidal palladium activation. So there has been growing use of organosilane molecules before the activation process to improve the absorption of metallic nanoparticles/colloids to such material surfaces [13–17]. The diatomite is composed of silicon dioxide, and its structure was porous and its surface was not smooth, so the pretreatment process would be more critical for the following electroless plating process. In order to fabricate flaky light-weight particles with fine coating and promising electromagnetic properties by way of electroless plating of CoNiP onto diatomite, different pre-treatment process of the diatomite was carried out and optimized. Moreover, the effect of 3-aminopropyltriethoxysilane (APTES) on the electroless
L. Yuan et al. / Applied Surface Science 258 (2012) 9680–9684
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Table 1 Composition of electroless CoNiP plating solution. Composition Main salt Reducing agent Fig. 1. Process flow sheet of electroless plating on diatomite.
Complexing agent Buffer
plating was analyzed, and the electromagnetic properties of the micro flake porous particles fabricated by such way were investigated.
2. Experimental procedures 2.1. Preparation The diatomite was purchased from Linjiang Sailite Diatomite Co. Ltd., Jilin province, China. 3-Aminopropyltriethoxysilane with concentration of 99% was from Acros Organics, Geel, Belgium. All other chemical reagents were of analytic grade and used without further purification, which were bought from Beijing Chemical Reagents Factory. And all the aqueous solutions were prepared with deionized water during the process of preparation. The electroless plating of CoNiP on diatomite was carried out by the following process (as shown in Fig. 1): Firstly, diatomite was dispersed in ethanol and cleaned with ultrasonic washer for 40 min to remove the impurities. Then, the gravity sedimentation was carried out to improve the consistency of diatomite. After being dried in a vacuum drying chamber at 120 ◦ C for 6 h, 10 g dried diatomite was dispersed in 150 ml APTES ethanol solution (1.5 wt%), shaken at 30 ◦ C for 4 h, then the diatomite was filtered off and dried in a vacuum drying chamber at 120 ◦ C for 4 h. The diatomite needs to be treated with a noble metal catalyst before electroless plating since their surface was non-catalytic. This was accomplished with a colloidal Pd–Sn catalyst, after which the acceleration process was carried out with a solution containing 30 g/l NaH2 PO2 ·2H2 O for 30–60 s at room temperature. The activated diatomite was subsequently washed and stored in deionized water. The well-dispersed activated diatomite was then cast into self-prepared electroless plating solution of CoNiP. The composition of the plating solution was shown in Table 1, and the pH value of the solution was adjusted to 9 with sodium hydroxide. Electroless plating of CoNiP was carried out at 80 ◦ C in water bath. To avoid aggregation and sedimentation of diatomite, mechanical stir was used during the whole electroless plating process. To examine the effects of the coupling processing, electroless plating of CoNiP on diatomite without coupling processing by APTES was also prepared.
Agents
Concentration (mol/L)
NiSO4 ·6H2 O CoSO4 ·7H2 O NaH2 PO2 ·2H2 O C4 H4 Na2 O5 C3 H2 Na2 O4 ·H2 O C4 H4 Na2 O4 ·6H2 O (NH4 )2 SO4
0.04 0.06 0.2 0.4 0.3 0.5 0.1
To investigate the electromagnetic properties of flaky CoNiPplated diatomite, a sample containing 50 wt% coated diatomite in the power-paraffin was made into a toroidal mold with an outer diameter of 7.00 mm and an inner diameter of 3.00 mm. Moreover, another sample containing CoNiP-plated hollow glass microspheres, with the same CoNiP content, was also prepared, so as to investigate the different electromagnetic properties between the flaky particles and spherical particles caused by the particle shape effect. 2.2. Characterization Fourier-transform infrared spectra (FT-IR) of the sample in the range of 400–4000 cm−1 were made with Nicolet AVATAR 360 spectrometer. Measurements were performed in the transmission mode in KBr pellets. The morphology of the sample was observed using Nikon 4500 optical microscope with a photographic camera and Hitachi S-4800 scanning electron microscope (SEM). The composition and phase structure analysis were studied by using Oxford Link 860 energy dispersive spectrometer (EDS) and D/Max 2200 PC automatic X-ray diffraction (XRD) respectively. The complex permeability and permittivity were measured with a CETE AV3627 vector network analyzer in the range of 2–18 GHz. 3. Results and discussion 3.1. Morphology and structure of diatomite before and after electroless plating of CoNiP When the activated diatomite with coupling processing was cast into the heated electroless CoNiP plating solution, a great deal of fine air bubbles could be seen in solution in less than 2 min, which meant the deposition proceeded smoothly. Fig. 2 showed the optical image of the diatomite before and after electroless plating of CoNiP, respectively. It was noted that the raw diatomite was transparent, yet the coated diatomite had the metallic luster under the reflection flight of the microscope. Fig. 3 showed the SEM image of the cross section of diatomite before and after plating.
Fig. 2. Optical micrographs of diatomite before (a) and after electroless CoNiP plating (b).
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Fig. 3. SEM micrographs of cross section of diatomite before (a) and after electroless CoNiP plating (b).
Comparing Fig. 3(a) and (b), it was very clear that the diatomite surface was covered with smooth and uniform coating after electroless plating of CoNiP, and the smooth and uniform coating was also deposited onto the surface of sub-holes. The coating thickness could be adjusted by change of the ratio of solution to diatomite, so as to achieve the coated flake with the porous structure or the full deposition of the sub-holes. The EDS result indicated the coating was composed of Co, Ni and P, with the atom ratio of Co:Ni:P being 4.5:3.5:1. Fig. 4 showed the XRD patterns of raw diatomite and the electroless CoNiP-plated diatomite. The “hill-like” peak around 2Â = 22.5◦ in Fig. 4(a) indicated that diatomite was mainly composed of amorphous SiO2 . In Fig. 4(b), there was a “hill-like” peak around 2Â = 45◦ in addition to that of diatomite in Fig. 4(a), which indicated that the CoNiP coating was amorphous. Moreover, there were sharp diffraction peaks on the top of “hill-like” peak, which suggested there are micro crystal structure in the coating. So we can conclude that the CoNiP coating was a transitional structure between crystal and amorphousness. As a comparison, the electroless plating on the activated diatomite without coupling process was not that prosperous. There were only a few bubbles came into being during the process of electroless CoNiP plating. Even after several hours’ reaction, only a few diatomites were coated, and the coating was neither smooth nor uniform.
(a), the diatomite before coupled (b) and diatomite after coupled (c) between 400 and 4000 cm−1 , respectively. In Fig. 5(a), the peak located at 2974 cm−1 represented the asymmetric stretching vibration of C H in CH2 CH2 , and the double-peaks at 1080 cm−1 and 1104 cm−1 were assigned to the asymmetric stretching vibration of Si O C. In Fig. 5(b), the peak located at 1094 cm−1 and 800 cm−1 were both attributed to the stretching vibration of Si O. The band at 3438 cm−1 was caused by stretching vibration of OH, and it indicated the presence of OH on the surface of diatomite. Compared with Fig. 5(b), there were two new peaks in Fig. 5(c). The weak peak located at 525 cm−1 was attributed to the stretching vibration of Si O Si, and the peak at 857 cm−1 was assigned to the deformation vibration of N H in CH2 NH2 . The presence of the two peaks was argued in favor of the occurrence of the coupling reaction between APTES and OH on the surface of diatomite. It indicated that amino group was chemisorbed onto the surface of diatomite after coupling processing. 3.3. EDS analysis of activated diatomite
To find out the differences between the two samples during electroless plating, FT-IR spectrum was used to find out the effect of coupling by APTES. Fig. 5 showed the FT-IR spectra of APTES
After activation, the coupled diatomite showed pale brown, while the non-coupled diatomite showed yellow, almost the same as the color of raw diatomite. EDS analysis was conducted to further compare the activation effect of diatomite with or without coupling processing. The result showed that the diatomite directly activated without coupling processing showed no Pd peak, yet the diatomite activated after the coupling process had several weak Pd peaks (as shown in Fig. 6), which indicated that Pd came into being on the surface of the coupled diatomite after activated by Pd–Sn colloids. The composition analysis indicated that the fraction of Pd is 0.89 wt% (as shown in Table 2).
Fig. 4. XRD patterns of: (a) raw diatomite; (b) diatomite after electroless CoNiP plating.
Fig. 5. FT-IR spectra of: (a) APTES; (b) diatomite before coupled; (c) diatomite after coupled.
3.2. Characterization by Fourier-transform infrared spectroscopy (FT-IR)
L. Yuan et al. / Applied Surface Science 258 (2012) 9680–9684
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Table 2 Analysis on the composition of coupled diatomite surface after activation. Element
C
O
Si
Pd
Total
Mass fraction (%) Atom fraction (%)
29.06 42.08
30.81 33.48
39.24 24.29
0.89 0.15
100.00 100.00
3.4. The mechanism of coupling, activation and deposition
Fig. 6. EDS analysis of coupled diatomite’s surface after activation.
The effective absorption of catalyst was the prerequisite to deposit a metal coating on the nonconductive substrate’s surface. There were two different functional groups in APTES, (CH2 )3 NH2 and OCH2 CH3 . In the coupling process, the OCH2 CH3 group hydrolyzed, and self-assembled membranes of the coupling agent were formed on the surface of the diatomite through a condensation reaction. Furthermore, weak covalent bond was formed
Fig. 7. Diagrammatic sketch of the general procedure for electroless plating of diatomite.
Fig. 8. Complex electromagnetic parameters and dissipation factors for the NiCoP plated diatomite flake and hollow microsphere.
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between APTES and the surface of diatomite. As a result, the NH2 group was chemisorbed onto the surface of diatomite. During the activation process, the catalyst Pd–Sn colloids were negatively charged (probably due to SnCl3 − ions) [15,18]. Once the diatomite coupled by APTES was immersed in the catalyst solution, the amino group was protonated and became positively charged. Thus, the Pd–Sn colloids could electrostatically bind to the protonated amines. In other words, the bridge-function of the APTES could greatly improve the anchor effect of colloidal Pd–Sn to the diatomite and enhance the adsorption amount of the catalytic sites. As a result, superior deposition of CoNiP coating on diatomite could be achieved. The diagrammatic sketch of the general procedure for electroless plating of diatomite was shown in Fig. 7.
There are abundant diatomites with different shapes and structures by nature. Using these diatomites as templates, micro electromagnetic particles with various shapes and structures could be fabricated by similar method presented in this work, which would have potential application in many fields including electromagnetic shielding and microwave absorbing.
3.5. Electromagnetic properties of diatomite after electroless plating of CoNiP
References
The complex permittivity and permeability, along with dissipation factors of the CoNiP-plated diatomite and hollow microsphere were shown in Fig. 8. From the result we could see that the CoNiP-plated diatomite had a superior complex permittivity and electromagnetic dissipation factors, although the real part of permeability was a little smaller than that of CoNiP-plated hollow microsphere (as shown in Fig. 8(c)). The reason was that the good flake shape of the CoNiPcoated diatomite was good for enhancing the electromagnetic susceptibility, and easy to form conductive networks which could increase the permittivity of composites effectively; furthermore, the porous substructure of diatomite could enhance scattering effect of electromagnetic wave within the composites which was in favor of electromagnetic dissipation. Moreover, it was discovered that a higher highest-content of effective material would be obtained using flaky diatomite than hollow microspheres as a template when other conditions were the same, which was also a key factor to improve the electromagnetic performance of the composites.
4. Conclusions Electroless plating of CoNiP on diatomite was successfully accomplished with the help of coupling processing, which offered a novel method to prepare micro flaky electromagnetic particles with porous structure. FT-IR and EDS results showed that the coupling processing with APTES played a very important role to improve the activation effect: the diatomite coupled by APTES enhanced the anchor effect on colloidal Pd–Sn particles. SEM and XRD results showed that the morphology and structure of diatomite could be well preserved after electroless plating. And the CoNiP coating was smooth and uniform. The deposited CoNiP coating was a transitional structure between crystal and amorphousness. The investigation on the electromagnetic properties of these micro flaky electromagnetic particles indicated that they possessed superior electromagnetic performance.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 50805005), the National 863 Project of China (Grant No. 2009AA043804), the Foundation for the Author of National Excellent Doctoral Dissertation of PR China (Grant No. 2007B32).
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