Enhanced microwave absorption properties in cobalt–zinc ferrite based nanocomposites

Enhanced microwave absorption properties in cobalt–zinc ferrite based nanocomposites

Journal of Magnetism and Magnetic Materials 416 (2016) 10–14 Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials j...

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Journal of Magnetism and Magnetic Materials 416 (2016) 10–14

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm

Enhanced microwave absorption properties in cobalt–zinc ferrite based nanocomposites A. Poorbafrani n, E. Kiani Electroceram Research Center, Malek Ashtar University of Technology, Shahin Shahr 83145-115, Iran

art ic l e i nf o

a b s t r a c t

Article history: Received 29 October 2015 Received in revised form 7 April 2016 Accepted 16 April 2016 Available online 20 April 2016

In an attempt to find a solution to the problem of the traditional spinel ferrite used as the microwave absorber, the Co0.6Zn0.4Fe2O4–Paraffin nanocomposites were investigated. Cobalt–zinc ferrite powders, synthesized through PVA sol–gel method, were combined with differing concentrations of Paraffin wax. The nanocomposite samples were characterized employing various experimental techniques including X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Alternating Gradient Force Magnetometer (AGFM), and Vector Network Analyzer (VNA). The saturation magnetization and coercivity were enhanced utilizing appropriate stoichiometry, coordinate agent, and sintering temperature required for the preparation of cobalt–zinc ferrite. The complex permittivity and permeability spectra, and Reflection Loss (RL) of Co0.6Zn0.4Fe2O4–Paraffin nanocomposites were measured in the frequency range of 1–18 GHz. The microwave absorption properties of nanocomposites indicated that the absorbing composite containing 20 wt% of paraffin manifests the strongest microwave attenuation ability. The composite exhibited the reflection loss less than –10 dB in the whole C-band and 30% of the X-band frequencies. & 2016 Elsevier B.V. All rights reserved.

Keywords: Microwave absorption Cobalt–zinc ferrite Nanocomposite Magnetic properties Reflection loss

1. Introduction In step with the development of GHz microwave communication, radar detection and other industrial applications, electromagnetic wave absorbing materials in the GHz range have attracted much attention in recent years. These absorbing materials can be manufactured by a number of magnetic and dielectric materials in powder forms, loaded in various kinds of polymeric binders. Various electromagnetic wave absorbing materials can be designed by using the dispersion characteristic of the complex permittivity and permeability [1–6]. Microwave absorption properties are highly dependent on processing parameters and chemical composition. These factors can greatly influence the crystal size, magnetic properties, complex magnetic permeability (mr) and complex permittivity (εr) [7–13]. Cobalt–zinc ferrites are well-known magnetic ceramics used in electrical equipment and microwave devices. When used in high frequency inductors or transformers, the core loss of ferrite is minimal because of their high electrical resistivity and magnetic softness [14,15]. Soft magnet cobalt–zinc ferrite with good mechanical hardness and chemical stability, has a large saturation magnetization and high Snoek's limit, resulting in highly complex permeability values at a wide frequency range. The above factor n

Corresponding author. E-mail address: [email protected] (A. Poorbafrani).

http://dx.doi.org/10.1016/j.jmmm.2016.04.046 0304-8853/& 2016 Elsevier B.V. All rights reserved.

makes cobalt-zinc ferrite highly useful as a thin absorber working at a high frequency band [16–18]. In the present work, an attempt was made to study microwave absorption properties of Cobalt Zinc Ferrite (CZF) nanocomposites. Ferrite phase was prepared, through sol–gel method. The process was carried out under appropriate stoichiometry, which enhanced magnetic properties of the phase. The magnetization curve of Co– Zn ferrite at room temperature showed high saturation magnetization of the magnetic phase. The formation of the Co0.6Zn0.4Fe2O4 phase, crystalline properties and morphology of the ferrite have been discussed through XRD analysis and FESEM images. In order to study the electromagnetic parameters and absorbing properties of the resulting powder, the Co0.6Zn0.4Fe2O4–paraffin wax composite was prepared. Paraffin wax is an insulating and nonmagnetic material. In addition, considering the zero value of the imaginary part of the complex permittivity and permeability, paraffin wax is transparent for electromagnetic wave. Thus, in the present research, paraffin wax was chosen as a matrix and binder to prepare toroidally shaped samples, which were subsequently used in measuring the complex permittivity and complex permeability of Co–Zn ferrite based nanocomposites.

2. Experimental procedures Synthesis of the CZF nanoparticles was based on our previous study [19], in which PVA solution was prepared by dissolving PVA

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1638

90

1460 1385

876

100

Transmittance

80

70

50 2000

1800

1600

1400

1200

1000

800

600

408 41

8

60

576

powders in deionized water (3% w/v) at temperatures between 70 and 80 °C and then the sols were prepared by dissolving ferric nitrate, cobalt nitrate and zinc nitrate in deionized water in stoichiometric ratio of 2:0.6:0.4. Being stirred constantly for 4 h, the sols were added drop wise to PVA solution. The subsequent mixture was then heated to 80 °C and subjected to constant stirring till a gel was obtained. The gel heated to 90 °C for 10 h to evaporate the water content. After that, the temperature was increased to 140 °C for 2 h until the gel was dried. The precursor was then sintered at 800 °C for 4 h. In order to prepare CZF nanocomposites, CZF powders were mixed with Paraffin wax at a concentration of CZF/paraffin by the weight ratios of 50/50, 60/40, 70/30 and 80/20. Subsequently, the composites were pressed into a toroidal shape. The X-ray diffraction (XRD) pattern of the sample was taken on Philips XPERT X-ray Diffractometer with Cu Kα radiation (λ ¼ 1.5406 A°) in the range of 2θ (25° o2θ o85°). The morphology of the sample was characterized using a Field Emission Scanning Electron Microscope (Hitachi S4160). Magnetic measurement was carried out at room temperature using an Alternating Gradient Force Magnetometer (AGFM: Meghnatis Daghigh Kavir Co., Iran) with a maximum magnetic field of 12 kOe. The real and imaginary parts of permittivity and permeability of nanocomposites were measured by a Vector Network Analyzer in the frequency range of 1–18 GHz. In order to measure the above parameters, the composites were pressed into toroidally shaped samples with different thicknesses having an inner diameter of 3.04 mm, and an outer diameter of 7.0 mm. The reflection loss (RL) of each composite was calculated from the complex relative permittivity and permeability at given frequencies and absorber thicknesses.

11

400

-1

Wavenumber( cm ) Fig. 2. FTIR spectrum of cobalt zinc ferrite nanoparticles.

3. Results and discussion Fig. 1 shows the XRD pattern of the Co0.6Zn0.4Fe2O4 powder synthesized through PVA sol–gel method. The prominent peaks were observed at 2θ values of 30°, 35.5°, 37°, 43°, 53.5°, 57°, 62.5° and at 74° were assigned to (220), (311), (222), (400), (422), (511), (440) and (533) planes, in the order mentioned. All these peaks confirm the cubic spinel type lattice of Co0.6Zn0.4Fe2O4 matching well with the standard XRD pattern (JCPDS card no. 22-1086). No additional peaks were observed in this XRD pattern, suggesting that no other phases besides cobalt–zinc ferrite structure were (311)

1600

Co0.6 Zn0.4Fe2O4

1400

(440)

1000

(511)

800

600

Fig. 3. FESEM Images of cobalt zinc ferrite nanoparticles at different length scales.

(444)

(551)

200

(620)

(222)

(422)

400

(533) (622)

(400)

(220)

Intensity(a.u.)

1200

0 25

30

35

40

45

50

55

60

65

70

75

2θ (degree[CuK α ])

Fig. 1. XRD pattern of cobalt zinc ferrite (CZF) nanoparticles.

80

85

detected in the sample. The average crystallite diameter of the CZF nanoparticles was determined from the major diffraction peak (311) and the calculated value was found to be about 30 nm. Fig. 2 shows the FTIR spectrum of the cobalt–zinc ferrite nanoparticles. The peaks at 408, 418 and 576 cm  1 are assigned to the characteristic Metal Oxide (M-O) stretching vibrations of cobalt–zinc ferrite which were located in the region between 400 cm  1 and 600 cm  1 [20]. In ferrites, according to the

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(a) CZF-P20 (b) CZF-P30 (c) CZF-P40 (d) CZF-P50

2.5

(I)

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μ 'r 1.5

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20

Frequency (GHz) 1.0 (a) CZF-P20 (b) CZF-P30 (c) CZF-P40 (d) CZF-P50

(II)

(a) Fig. 4. Magnetic hysteresis loop of cobalt zinc ferrite. 0.8 (a) CZF-P20 (b) CZF-P30 (c) CZF-P40 (d) CZF-P50

5.5

(I)

μ"r

(a)

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ε'

4.5

r

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0.2

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0.0

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3.5

0

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20

Frequency (GHz)

Fig. 6. Frequency dependence of the real parts mr′ (I) and the imaginary parts mr″ (II) of complex permeability for Co0.6Zn0.4Fe2O4/paraffin nanocomposites.

(d) 3.0 0

5

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Frequency (GHz) 0.20

(II)

(a) CZF-P20 (b) CZF-P30 (c) CZF-P40 (d) CZF-P50

(a)

0.15

(b)

ε"

r

0.10

(c)

(d) 0.05

0.00 0

5

10

15

20

Frequency (GHz)

Fig. 5. Frequency dependence of the real parts εr′ (I) and the imaginary parts εr″ (II) of complex permittivity for Co0.6Zn0.4Fe2O4/paraffin nanocomposites.

geometrical configuration of the oxygen nearest neighbors, the metal ions are situated in tetrahedral and octahedral sublattices. The studies on vibrational spectra of ferrites indicated that the high frequency band of 610–570 cm  1 is attributed to the intrinsic vibration of tetrahedral sites and low frequency band of 440–400 cm  1 is attributed to the octahedral sites [21,22]. As can be seen from Fig. 2, the peaks at 408 and 418 cm  1 are intrinsic vibrations of octahedral sites and the peak at 576 cm  1 is intrinsic

vibration of the tetrahedral site. The peak at 1638 cm  1 corresponds to the bending mode of the hydroxyls [20–22]. The FTIR spectra also confirm the formation of the cobalt–zinc ferrite nanoparticles. Fig. 3 shows the typical FESEM images of cobalt–zinc ferrite. The figure probably reveals that Co0.6Zn0.4Fe2O4 nanoparticles show the presence of very large lumps being agglomerates of small spherical particles with the average particle size of about 30 nm demanding more accurate observation through TEM. Maximum agglomeration size even extends up to 300 nm. Within single agglomerate, the particles are at close-packing limit, suggesting strong interaction resulting in high magnetization. The spherical particle shapes of the agglomeration may be a result of the preparation method and nanoparticles surface properties. In the sol–gel process, the presence of PVA chains makes up the network structure of agglomerated crystallites resulting in the formation of CZF nanoparticles with small grain size. To clarify the magnetic properties of the as-synthesized CZF nanoparticles, the hysteresis loop of the sample was measured using an Alternating Gradient Force Magnetometer (AGFM). The maximal magnetic field applied in the measurement was 12 kOe and the detection process was carried out at the room temperature. Fig. 4 shows the magnetization loop of the CZF sample prepared through sol–gel method. In the sol–gel process, metal ions can be homogenously distributed in the gel matrix with the result that impurity phases can be largely reduced. This can explain the high magnetization per mass unit obtained in the sample. In addition, using PVA coordinate agent for the preparation of CZF and

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(a) CZF & P20 - 3.5 mm (b) CZF & P20 - 4.3 mm (c) CZF & P20 - 5.3 mm (d) CZF & P20 - 7.1 mm (e) CZF & P20 - 7.0 mm

-25

(I)

(a) CZF & P40 - 4.1 mm (b) CZF & P40 - 5.5 mm (c) CZF & P40 - 7.2 mm (d) CZF & P40 - 8.0 mm

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-15 (a) CZF & P30 - 3.6 mm (b) CZF & P30 - 4.8 mm (c) CZF & P30 - 6.1 mm (d) CZF & P30 - 8.0 mm (e) CZF & P30 - 7.8 mm

(II) -20 0

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(a) CZF & P50 - 4.2 mm (b) CZF & P50 - 5.8 mm (c) CZF & P50 - 7.8 mm (d) CZF & P50 - 8.0 mm

-10 20

0

(IV)

5

10

15

20

Frequency (GHz)

Frequency (GHz)

Fig. 7. The frequency dependence of reflection loss (RL) for Co0.6Zn0.4Fe2O4/paraffin nanocomposites with Paraffin weight percentage of (I) 20%, (II) 30%, (III) 40% and (IV) 50% at different thicknesses.

employing appropriate sintering temperature could be the other reasons for the improvement in the magnetic properties of the obtained nanoparticles. The coercivity and saturation magnetization in the magnetic field of 12 kOe for the CZF sample were obtained to be about 72 Oe and 110 emu/g, respectively. Fig. 5(I) and (II) shows the frequency dependence of the real parts (ε′r) and imaginary parts (ε″r) of complex permittivity (εr ¼ ε′ r  jε″r) of Co0.6Zn0.4Fe2O4–paraffin wax composites in the range of 1–18 GHz. It can be observed that for each weight ratio of paraffin (20, 30, 40 and 50), the variation of the real part of permittivity with frequency was almost constant, whereas the imaginary part of permittivity showed a broad peak in the frequency range of 13– 18 GHz having a maximum peak value of about 0.18 at 15.4 GHz for the paraffin weight ratio of 20%. In the Co0.6Zn0.4Fe2O4–paraffin composites, the variation of the imaginary parts of permittivity were in the range of 0–0.18 showing small values. So, these composites do not have good absorption responses to the electric field of the electromagnetic wave in the range of 1–18 GHz. Fig. 6(I) and (II) shows the frequency dependence of the real parts (μ′r) and imaginary parts (μ″r) of complex permeability (μr ¼ μ′r jμ″r) of Co0.6Zn0.4Fe2O4–paraffin wax composites in the range of 1–18 GHz. It can be observed that for each weight ratio of paraffin (20, 30, 40 and 50), the imaginary part of permeability increases with an increases in frequency, presenting a peak at about 3.0 GHz with a maximum value of about 0.9, after which it shows a decline: in contrast, the real part of the complex permeability exhibits an obvious declining trend in the frequency range of 1–18 GHz.

As shown in Fig. 6(II), the frequency of the maximum peak values of the imaginary parts of permeability were found to move toward the higher frequency region by an increase in paraffin concentration in the composites. In Co0.6Zn0.4Fe2O4–paraffin composites, the variation of the imaginary parts of permeability are in the range of 0–0.9 and so these composites display good absorption responses to the magnetic field of the electromagnetic wave in the range of 1–18 GHz, especially in the S (2–4 GHz), C (4– 8 GHz) and X (8–12 GHz) frequency bands. In the above composites, the real and imaginary parts of permittivity and permeability decreases with an increase in paraffin concentration, which is due to a decrease in the magnetic phase of the composite. Therefore, the composite having 20 wt% of paraffin has the maximum values of the real parts and imaginary parts of permittivity and permeability in the frequency range of 1–18 GHz. The microwave absorption properties of Co0.6Zn0.4Fe2O4–paraffin nanocomposites were estimated using the reflection loss (RL). According to transmission line theory, under normal wave incidence at the surface of a single-layer material backed by a perfect conductor, the reflection loss of electromagnetic radiation can be defined by [23, 24]

RL = 20 log

( (

μr εr

tanh j

2πft c

μ r εr

μr εr

tanh j

2πft c

μ r εr

) )

−1 +1

(1)

In Eq. (1), c is the speed of electromagnetic waves in the free

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space, f is the frequency of the incident electromagnetic wave, t is the thickness of composite, mr and εr are the complex relative permeability and permittivity of the composite medium. Fig. 7 shows the frequency dependence of the calculated reflection loss of the Co0.6Zn0.4Fe2O4–paraffin wax nanocomposites at different weight percentage of paraffin 20, 30, 40 and 50 and at different thicknesses for each sample. By increasing paraffin concentration in the composites, the reflection loss and absorption values of composites decreased, which was attributable to a reduction in the magnetic phase of the composites. According to Fig. 7, the composites with different wt% of paraffin have one or two absorption peaks in the frequency range of 1–18 GHz. The first peaks are located in the frequency bands of S, C and X at which maximum values of μ″r of the composites occurred (magnetic loss) and the second peaks located at the frequency of about 15.0 GHz in the middle of Ku band (12–18 GHz), at which maximum values of ε″r of the composites took place (dielectric loss). As shown in Fig. 7(I), in the composite having 20 wt% of paraffin, the bandwidth with RL less than  10 dB reached a measure above 4.5 GHz at a matching thickness of 4.3 mm. This composite could cover the absorption of the electromagnetic waves in the whole C band and the first region of the X band. In particular, a minimum reflection loss value of  23.6 dB was observed at 7.0 GHz with a matching thickness of 4.3 mm in the composite having 20 wt% of paraffin. At this ratio, the composites have good compatibilities of dielectric and magnetic properties and hence, the electromagnetic wave absorbing properties show maximum value. These results suggested that Co0.6Zn0.4Fe2O4–paraffin nanocomposites show promising potential microwave application.

4. Conclusions We can say that, Co0.6Zn0.4Fe2O4 ferrite nanoparticles were successfully synthesized through PVA sol–gel method. The saturation magnetization and coercivity were enhanced utilizing appropriate stoichiometry, coordinate agent and sintering temperature. Co0.6Zn0.4Fe2O4–paraffin wax nanocomposites fulfilled the function of electromagnetic loss ability. Co0.6Zn0.4Fe2O4–paraffin containing 20 wt% of paraffin, showed excellent microwave absorbing properties with RL less than 10 dB in the whole Cband and 30% of the X-band frequencies. The minimum reflection loss value of this composite reached  23.6 dB at 7.0 GHz with a matching thickness of 4.3 mm. The present research was an attempt to solve the problem of the traditional spinel ferrite which is used as efficient microwave absorber. Our method synthesizing these Co0.6Zn0.4Fe2O4-based nanocomposites led to an enhancement in microwave reflection loss over extended frequency ranges.

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