CHINA PARTICUOLOGY Vol. 3, No. 3, 187-190, 2005
PREPARATION AND PROPERTIES OF ULTRAFINE COMPOSITE POWDERS OF Fe3O4 AND Ni/MICA Yucheng Wu1, 2,*, Jiaqin Liu1, Rujun Xue1, Guozhong Wang2 and Lide Zhang2 1
Faculty of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, P. R. China 2 Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P. R. China *Author to whom correspondence should be addressed. Tel: 0086-551-2901361, E-mail:
[email protected]
Abstract This paper presents the preparation of ultrafine powders of Fe3O4 and Ni by a chemical method, followed by mixing the prepared powders with mica and other ultrafine powders for synthesizing microwave absorption coatings. The microwave attenuation rate of the coatings was measured by the Microwave Network Analyzer in the frequency range of 8−12 GHz at room temperature. The results indicate that microwave could be absorbed by the coatings with an effectiveness strongly dependent on the powder sort and content and the coating thickness. Keywords
ultrafine powders, coating, microwave attenuation
1. Introduction With the development of radar, microwave communication technology and especially the need for anti-electromagnetic interference coatings, self-concealing technology and microwave darkrooms, the study of microwave absorbing materials has increased in recent years (Tzeng & Chang, 2002; Giannakopoulou et al., 2002). Microwave absorbing material is an important functional material, which can absorb and attenuate electromagnetic wave within a certain frequency range. It may make the electromagnetic wave disappear by interference or conversion of the electromagnetic power into thermal energy (Shin & Oh, 1993). Generally speaking, a microwave absorbing material is composed of a matrix and an absorber which could significantly affect its microwave absorbing properties (Li et al., 2002). Conductive metal powder or films are generally used for shielding microwaves in the MHz regions. Ferrites, carbon powder, metal-coated carbon fibers or synthetic organic fibers are also used for absorbing the MHz and lower frequency microwaves in the GHz regions (Motojima et al., 2003). It has been reported that ultrafine or nanoscaled metallic and ferrite powders are promising microwave absorbers (Sugimoto et al., 1999; Meshram et al., 2001). However, few studies on them have been carried out on these materials while much research has been done on the microwave absorbing properties of carbon powders, straight carbon fibers (CF) or metal-coated CF in the lower bands of 1−12 GHz (Chandrasekhar & Naishadham, 1999; Laiho et al., 2000; Ustinov et al., 1999). From the viewpoint of material design, one hopes that microwave absorbing material would meet two demands: first, the material may allow electromagnetic energy enter into its inside, namely, it has small surface energy reflectance, that is, the impedance of the material well matches the wave source; second, it has excellent absorption and attenuation effect on the electromagnetic wave. One effi-
cient way to solve these two problems is multi-layer design, which can achieve quasi-consecutive transition for incident electromagnetic wave (Meshram et al., 2004; Giannakopoulou et al., 2003; Cao et al., 1998; Cao et al., 2002). Presently, there are many methods for making metal and metallic oxides powders with ultrafine or nanoscaled size (Khollam et al., 2002; Zhang & Mou, 2001), both physically (PVD, ball milling etc.) and chemically (redox, electrolysis, sol-gel etc.). This paper reports the preparation and the properties of thin broadband microwave absorption coatings of multi-layer structure using ultrafine powders of Fe3O4 and Ni prepared by the redox method, which is a rapid process with convenient operation at room temperature.
2. Experimental 2.1 Preparation of Fe3O4 ultrafine powder Fe3O4 crystallite can be prepared by two different methods: neutralization and oxidation. In the present study we adopted the nuetralization method using Fe(NO3)3, FeSO4 and NaOH as raw materials for synthesizing Fe3O4 crystallite under definite conditions, according to the following reaction: 3+ 2+ 2Fe + Fe + 8OH− → Fe3O4↓ + 4H2O Fe(NO3)3 and FeSO4 solutions at a molar ratio of n(Fe3+):n(Fe2+)=2:1 were first mixed evenly at 80°C, then NaOH solution was gradually added into the mixture with stirring to arrive at a pH value 9.0, at which a black precipitate appeared. The black precipitate was washed successively with distilled water and acetone, and then filtered. After drying at 40°C, the filter cake was milled time and again to obtain the ultrafine Fe3O4 powder.
2.2 Preparation of Ni ultrafine powder After heating to 85°C in a water-bath, a 1 mol·L−1 NiSO4·6H2O solution and a 2 mol·L−1 NaOH solution were mixed together evenly. When the solution turned green,
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N2H4·H2O was added with stirring, upon which a violent reaction took place, producing a black precipitate, which was washed successively with distilled water and acetone and then filtered. After drying in vacuum, the filter cake was milled repeatedly to obtain the ultrafine Ni powder.
No.86-1345 (International Center for Diffraction Data), thus identifying it to be Fe3O4.
2.3 Preparation and measurement of wave absorbing coatings We used Ni and Fe3O4 as absorbers in a polyvinyl alcohol (PVA) matrix to prepare our organic-inorganic microwave absorbing composite coatings of different thickness, powder content and particle size and layer-sequence. The mixture was composed of 400 g·L−1 of the prepared Ni and Fe3O4 ultrafine powders and 34 g·L−1 of PVA in distilled water, which was coated on a sheet of blank cardboard or plastic board (2 cm×2 cm). Then microwave attenuation was measured on the Microwave Network Analyzer in the frequency range of 8−12 GHz at room temperature.
3. Results and Discussion 3.1 Morphology and microstructure of Fe3O4 ultrafine powder Figure 1 shows the scanning electron micrographs (SEM) of Fe3O4 ultrafine powder, showing its asymmetrical and irregular morphology, revealing its high specific surface energy.
2θ / °
Fig. 2
3.2 Morphology and microstructure of Ni ultrafine powder Figure 3 shows the SEM micrographs of Ni ultrafine powder with irregular sphericity and an average diameter of about 0.2 μm. The micrographs also show sintering of the particles, due primarily to their large specific surface area and therefore high specific surface energy (Ruan et al., 2000; Moumen, 1996).
10 μm
100 μm
10 μm
10 μm
Fig. 1
SEM micrographs of prepared Fe3O4 ultrafine powder.
Figure 2 shows that the XRD Pattern of the prepared Fe3O4 ultrafine powders well conforms the JCPDS:
XRD pattern for prepared Fe3O4 ultrafine powder.
Fig. 3
SEM Micrographs of prepared Ni ultrafine powder.
Figure 4 presents the XRD pattern of the prepared Ni ultrafine powders which well fits the JCPDS: No.04-0850, thus identifying it to be Ni.
Wu, Liu, Xue, Wang & Zhang: Preparation of Ultrafine Composite Powders of Fe3O4 and Ni/Mica
2θ / °
Fig. 4
XRD Pattern for prepared Ni ultrafine powder.
3.3 Microwave absorbing properties Tables 1 and 2 show the microwave attenuation rates of the coatings measured by the Microwave Network Analyzer in the frequency range of 8−12 GHz at room temperature. Twelve samples coated with the mixture of PVA and ultrafine powders were divided into two groups. The average primary voltages of the two groups were 30.0 mV. The coating thickness of Group 2 is much larger than that of Group 1. Table 1
Microwave attenuation of Group 1 coatings Samples
(1) Glass-board # (2) Cardboard # (3) 10 g mica * (4) 5 g Fe3O4 + 5 g mica * (5) 5 g Ni + 5 g mica * Table 2
Attenuation rate/%
7.3 9.7 2.0
Microwave attenuation of Group 2 coatings Samples
Attenuation rate/%
Blank cardboard # (6) toplayer: 5 g Ni + 5 g mica 11.7 underlayer: 5 g Fe3O4 + 5 g mica * (7) toplayer: 10 g Ni + 5 g mica 18.1 underlayer: 10 g Fe3O4 + 5 g mica * 15.2 (8) 5 g Fe3O4 + 5 g mica * 28 (9) 10 g Ni + 10 g mica * (10) toplayer: 10 g Fe3O4 + 5 g mica 42 underlayer: 10 g Ni + 5 g mica * 86 (11) 10 g Ni + 5 g mica + 5 g SiC * (12) toplayer: 5 g Fe3O4 + 2.5 g TiO2 + 5 g SnO2 +7.5 g mica * 60 underlayer: 4 g Ni + 2.5 g TiO2 + 2.5 g SnO2 + 7.5 g mica * # and * indicate respectively the measured samples were blank cardboard and plastic-board coated with absorbing mixture.
Comparing sample 4 (5 g Fe3O4 + 5 g mica) with sample 5 (5 g Ni + 5 g mica), we can conclude that the kind of doped powder has great importance on the microwave attenuation properties of the coating. Tables 1 and 2 show that sample 10 consisting of an Fe3O4 toplayer and a Ni underlayer has better microwave absorbing properties and
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high microwave attenuation rate than sample 7 in which the layer sequence is reversed. Sample 6 and sample 7 show the effect of powder content on microwave absorbing properties. That the microwave absorbing property of sample 8 surpasses that of sample 4 indicates the importance of coating thickness. Particle size also has important influence on microwave absorbing properties (Yacaman, 1998; Gerhard, 1998). Compared to conventional materials, ultrafine materials (<1 micron) possess not only tremendous specific surface area, but also profuse hanging bond atoms as well as surface defects. Both energy spectrum theory and quantum size effect tell us that the energy spectrum of electrons in metal is approximately consecutive, but the energy level will split when a particle is small enough, and the spacing between adjacent energy states increases inversely with the volume of the particle. If the particle size of absorber is small enough and the discrete energy level spacing is in the energy range of microwave (10−2−10−4 eV), electron can absorb energy as it leaps from one level to another, thus leading to increment of attenuation. Therefore, the microwave absorbing properties become better with decrease in the granularity of the ultrafine particles. From Tables 1 and 2, it could be seen that with different powder sort and content and coating thickness, the attenuation rate of the sample 11(86%)is the largest among all samples. Evidently the low attenuation rate samples could not realize industrialization. Further studies on the relationships among the microwave absorbing properties, coating thickness, powder sort and content, particle size as well as the layer-sequence are in progress.
4. Conclusions Ultrafine metallic and ferrite powders are promising microwave absorbers. Microwave absorbing properties could be manipulated by altering the coating thickness, powder sort and content, as well as the layer-sequence. Multi-layer structure could achieve quasi-consecutive transition for incident electromagnetic wave; consequently it could enhance microwave absorption ability.
Acknowledgement This work was supported by Anhui Natural Science Foundation (Grant No. 01044903) and Creative Program of Hefei University of Technology (Nanostructure and Functional Nanomaterials, Grant No. 103-037016).
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