Wideband and enhanced microwave absorption ...

Journal of Magnetism and Magnetic Materials 385 (2015) 407–411

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Wideband and enhanced microwave absorption performance of doped barium ferrite Pingyuan Meng a, Kun Xiong a, Kui Ju b, Shengnan Li a, Guangliang Xu a,n a State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, PR China b Guizhou Institute of Metallurgy and Chemical Engineering, Guiyang 550002, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 9 October 2014 Received in revised form 26 December 2014 Accepted 27 February 2015 Available online 28 February 2015

To achieve stronger microwave attenuation and larger bandwidth in electromagnetic absorber, the nickel ions (Ni2 þ ) and manganese ions (Mn2 þ ) were employed to partially replace the cobalt ions (Co2 þ ) in BaCoTiFe10O19, and the doped barium hexaferrite (Ba(MnNi)0.2Co0.6TiFe10O19 and Ba(MnNi)0.25Co0.5 TiFe10O19) powders were synthesized via the sol–gel combustion method. Subsequently, the microwave absorbing composites were prepared by mixing the ferrite powders with the paraffin. The X-ray diffraction (XRD) patterns of the doped ferrites confirmed the formation of the M-type barium ferrite, and no other types of barium ferrite could be found. Based on the electromagnetic parameters measured by the vector net-analyzer, it was found that the composite (Ba(MnNi)0.2Co0.6TiFe10O19) possessed a minimum reflection loss of 52.8 dB at 13.4 GHz with a matching thickness of 1.8 mm and the bandwidth below 15 dB was 5.8 GHz. Moreover, the maximum attenuation of Ba(MnNi)0.25Co0.5TiFe10O19 could reach 69 dB when its thickness was 1.8 mm, and also the bandwidth less than 20 dB was ranging from 13.2 GHz to 18 GHz. Thus, Ba(MnNi)0.2Co0.6TiFe10O19 and Ba(MnNi)0.25Co0.5TiFe10O19 could be the good microwave absorbers, which have great potentials to be applied in the high frequency fields of the microwave absorbing materials. & 2015 Published by Elsevier B.V.

Keywords: Barium ferrite Sol–gel combustion Wideband Reflection loss

1. Introduction Concern about the electromagnetic interference and electronic countermeasures have become crucial issues in military fields, and as a result, much more researches in recent years have focused on electromagnetic-absorber technology [1–6]. The electromagnetic wave absorbers can effectively reduce the reflection of electromagnetic signals, which could improve the battlefield survivability of military aircrafts and vehicles, as electromagnetic wave absorbing materials coating on the surfaces of military equipments. However, the absorbers for a single frequency cannot meet the demands of the updated technology, while the materials which possess broad bandwidth, minimum reflection loss (RL) and lightweight mass (or small thickness) have been extensively studied recently [7]. M-type barium hexagonal ferrite (BaFe12O19, BaM) is considered as one of the promising candidate materials for up-to-date microwave absorption in the GHz range, which is due to its low cost, good stability, high natural resonance frequency (about n

Corresponding author. E-mail address: [email protected] (G. Xu).

http://dx.doi.org/10.1016/j.jmmm.2015.02.059 0304-8853/& 2015 Published by Elsevier B.V.

47.6 GHz) and excellent microwave magnetic loss [8–11]. As is well known, it is an effective way to change the microwave absorption performance of BaM by using other metal ions to replace the ferric ions (Fe3 þ ) [12–17]. Tehrani and his co-workers [4] have studied the substituted M-type barium hexagonal ferrite BaMg0.25 Mn0.25Co0.5Ti1.0Fe10O19, and the bandwidth of 4.5 GHz below 20 dB at a thickness of 2.7 mm was obtained. Sun and his partners [3] have researched Ce-substituted barium ferrite BaCe0.05Fe11.95O19, and the results showed a minimum reflection loss value of 37.4 dB with a matching thickness of 3.5 mm. Dong et al. [8] have reported that the BaCo0.3Ti0.3Fe11.4O19 exhibits a minimal RL ( 47 dB) at a thickness of 0.8 mm. Thus, choosing appropriate ions to substitute BaM is significance to improve the absorption performance of the ferrite. In the previous work, it had been demonstrated that the Mn2 þ and Ni2 þ could enhance the absorption properties of BaCoTiFe10O19 composites [18,19]. Especially, it could also broaden the absorbing bandwidth as the Ni2 þ substituted [7]. Thus, the microwave absorption performances of Ba(MnNi)0.2Co0.6TiFe10O19 (MNCF1) and Ba(MnNi)0.25Co0.5TiFe10O19 (MNCF2) were preliminary researched in present work, as Mn2 þ along with Ni2 þ was employed to partially occupy the place of cobalt ions (Co2 þ ) in BaCoTiFe10O19. The phase composition and crystalline structure of

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P. Meng et al. / Journal of Magnetism and Magnetic Materials 385 (2015) 407–411

MNCF1 and MNCF2 were evaluated. Moreover, the magnetic properties and the microwave absorption properties of doped ferrites were detailedly investigated in the frequency range of 0.5– 18 GHz.

complex permittivity and permeability of the composite, respectively, f is the microwave frequency and c is the velocity of light in free space. From the reflection loss, the microwave performance of the composites was investigated.

2. Experimental

3. Results and discussion

2.1. Preparation of doped ferrites powders

3.1. Microstructure and magnetic characteristics

The powders of Ba(MnNi)0.2Co0.6TiFe10O19 and Ba(MnNi)0.25Co0.5TiFe10O19 were synthesized via the sol–gel combustion method. All of the raw materials in this experiment are analysis reagents and made in Aladdin Industrial Corporation of China. As starting materials, appropriate amount of Fe(NO3)3 9H2O, Ba(NO3)2, Co(NO3)2 6H2O, Ni(NO3)2 6H2O, Ti(OC4H9)4 and Mn(NO3)2 were dissolved in 100 mL deionized water by stirring it constantly using a magnetic stirrer. After they were dissolved completely, 100 mL citric acid solution (0.65 M) was added into the solution, and then ammonia solution was added dropwise with vigorous stirring to maintain the pH value of the solution at 7. Subsequently, the neutralized solution was heated at 100 °C with continuous magnetic stirring to obtain the dried gel. With further heating, the dried gel would burn up in a self-propagating combustion manner, and some brown powders could be obtained. Finally, these brown powders were pre-heated at 450 °C for 4 h, and then calcined at 1100 °C for 4 h to obtain the Ba(MnNi)0.2Co0.6TiFe10O19 (MNCF1) and Ba(MnNi)0.25Co0.5 TiFe10O19 (MNCF2).

The XRD patterns of MNCF1 and MNCF2 powders are presented in Fig. 1. All the diffraction peaks are very consistent with those of BaM, and no second phase can be detected, indicating that the substitution of Mn–Ni seems to arrange in the hexagonal structure and also will not affect its original phase. The size and morphology of the synthesized MNCF1 and MNCF2 powders were further examined by the FE-SEM. Overall view of the SEM imagines in Fig. 2, the ferrite particles appeared fine grain growth with some agglomeration and many small ferrites grains had a size ranging in 100–400 nm. The majority of the doped ferrites grains displayed an irregular shape except some bigger crystals, which have a significant hexagonal-shape. The hysteresis loops of doped BaM ferrites are presented in Fig. 3. It has been reported that the coercive force (Hc) of pure BaM ferrite can reach 4500 Oe due to its strong uniaxial magnetocrystalline anisotropy along c-axis [4]. Fig. 3 demonstrates the Hc decreases to about 1000 Oe with the substitution of Mn, Ni, Co and Ti, and also slightly increases when the amount of Mn2 þ Ni2 þ varied from 0.2 to 0.25, which may be attributed to the enhancement of uniaxial anisotropy along c-axis resulting from the different replacing sites of Fe3 þ [12].

2.2. Preparation of doped ferrites composites The ferrites composites were prepared by mixing doped ferrites powders with the paraffin according to a mass ratio of 85:15. Subsequently, the mixture was dissolved in xylene and ultrasonicated for 30 min. Finally, the mixture was kept in the oven at 70 °C to remove the solvent completely and the dried mixture was hot pressed at 220 °C under 5.5 MPa for 30 min into a circular cylinder with an inner diameter of 3.0 mm, outer diameter of 7.0 mm and thickness ranged from 1.5 mm to 2.0 mm. 2.3. Measurement of properties

3.2. Microwave absorption characteristics Generally, the real parts of complex permittivity and permeability symbolize the storage capability of electric and magnetic energy, and the imaginary parts represent the loss of electric and magnetic energy [12,21]. To reveal the microwave absorbing property, the complex relative permittivity (ε ¼ ε′þ jε′′) and the complex relative permeability (μ ¼ μ′þjμ′′) for the ferrites–paraffin composites are investigated in Figs. 4 and 5. As shown in Fig. 4a, the real parts of complex permittivity (ε′) for both

The phase composition of doped ferrites powders was identified by the X-ray diffraction (XRD, X'Pert PRO, PANalytical B.V., The Netherlands) equipment, with Cu Kα radiation (λ ¼ 1.540598 Å, 35 kV and 25 mA) in the range of 20–70° and a scan rate of 6°/min. The size and morphology of doped ferrites powders were observed by using the field emission scanning electron microscopy (FE-SEM, Zeiss Ultra 55, Germany). The vibrating sample magnetometer (VSM) was used to measure magnetic hysteresis (M–H) loops of the ferrites powders. Finally, the complex permittivity and permeability of MNCF1 and MNCF2 composites in the frequency range of 0.5–18 GHz were determined by a network analyzer (Agilent Technologies, E8363A) using the coaxial measurements, and the reflection loss of the samples was calculated from the relative complex permeability and permittivity with a given frequency range and a given absorber thickness (d) by the following equation [20]:

RL = 20 log (Zin − 1)/(Zin + 1)

(1)

Zin is determined as: 1/2

Zin = (μ r /εr )

tan h ⎡⎣j (2πfd/c)( μ r εr )1/2⎤⎦

where Zin is the input impedance of absorber,

(2)

εr and μr are

Fig. 1. X-ray diffraction patterns for the powders: (a) Ba(MnNi)0.2Co0.6TiFe10O19 and (b) Ba(MnNi)0.25Co0.5TiFe10O19.


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Fig. 2. SEM micrographs for the samples: (a, c) Ba(MnNi)0.2Co0.6TiFe10O19 and (b, d) Ba(MnNi)0.25Co0.5TiFe10O19.

Fig. 3. Magnetic hysteresis loops for Ba(MnNi)0.2Co0.6TiFe10O19 (MNCF1) and Ba(MnNi)0.25Co0.5TiFe10O19 (MNCF2).

composites had less fluctuation in the range of 0.5–10 GHz and exhibited decreases from 10 to 18 GHz. The imagine parts of complex permeability (ε′′) were displayed in Fig. 4b, from which it could be seen that the ε′′ cures kept relative stable in the range of 0.5–6.4 GHz and presented rises in the range of 6.4–18 GHz. According to dielectric physics, the dielectric properties of ferrite composites are closely associated with the dielectric polarization. When the frequency of the applied electric field is high, the dielectric polarization cannot keep up with the varying of the applied electric field, which finally results in dielectric loss [22]. Consequently, ε′ decreases and ε′′ increases in the range of 10– 18 GHz. The real part (μ′) and the imaginary part (μ′′) of the relative complex permeability of the composites are given in Fig. 5. As

illustrated by Fig. 5a, the μ′ initially had a slight decline and then increased with the increasing frequency. Peaks appeared around 11 GHz and 12 GHz for two composites, respectively, after which the curve decreased. The decreases of μ′ could be attributed to the limited speed of spin and domain wall movement (displacement/ rotation) in the sample [23]. Since the wavelength in microwave absorber decreases as the frequency increases, the μ′ declined after it reaches the maximum, which is favorable for the microwave surface impedance match [24]. From Fig. 5b it could be seen that the value of μ′′ hardly had any obvious fluctuation in the range of 0.5–9 GHz, and then rose to a maximum value around 13 GHz. The observed magnetic spectra are in agreement with the mechanism of natural magnetic resonance. The natural magnetic resonance involves the domain wall displacement and domain rotation, and those motions lag behind the applied magnetic field, which will cause magnetic losses in the magnetic materials [22,25]. In general, the resonance frequency (fr) of BaM is closely associated with the magnetic anisotropy field (HA) by following relation [3]:

2πfr = γH A

(3)

where γ is the absolute gyromagnetic ratio. As previously mentioned, the magnetic anisotropy field was slightly enhanced, which finally resulted in a slight increase of the resonance frequency (fr). This is the reason why the peak of μ′′ moved slightly to higher frequency from MNCF1 to MNCF2. In addition, the peaks of μ′′, the loss of magnetic energy, exactly appeared in the frequency where the μ′ decreased, which indicated the magnetic loss could be occurred in the frequency range of 12–15 GHz. Combining with the mentioned dielectric loss, the maximum of reflection loss might appeared in 12–15 GHz. The RL spectra of the composites with different thickness are exhibited in Fig. 6. Fig. 6a and b shows the curves of RL versus frequency for MNCF1 and MNCF2, respectively, and Fig. 6c displays the variation of RL at the thickness of 1.8 mm for the composites. From Fig. 6a and b, it can be seen that the attenuations of two

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Fig. 4. Permittivity response of the ferrites Ba(MnNi)0.2Co0.6TiFe10O19 (MNCF1) and Ba(MnNi)0.25Co0.5TiFe10O19 (MNCF2): (a) real part and (b) imaginary part of complex permittivity.

samples could reach the maximum at the thickness of 1.8 mm. Moreover, it also could be indicated that the maximal attenuations for MNCF1 and MNCF2 appeared at about 13.4 GHz and 14 GHz, which was consistent with the previous speculation. As detailed in Fig. 6c, it can be noticed that the minimum RL for two samples could reach 52.8 dB and 69.2 dB at 13.4 GHz and 14 GHz, respectively. Furthermore, the bandwidth less than 15 dB can reach 5.8 GHz (12.2–18 GHz) for MNCF1, and the composite of MNCF2 has a bandwidth below 20 dB ranging from 13.2 to 18 GHz. From above results, the composites of doped BaM ferrites showed relative excellent absorption performance. According to the electromagnetic energy conversion principle, apart from dielectric loss and magnetic loss, a proper matching between the dielectric loss and magnetic loss also determines the reflection and attenuation characteristics of electromagnetic absorbers [26]. The BaM possesses dielectric loss and magnetic loss, and the incorporation of Mn, Ni, Co and Ti may improve the impedance matching between dielectric loss and magnetic loss, which finally reduce the reflection and enhance the attenuation of the ferrites. However, this is preliminary research for Mn–Ni substituted in BaCoTiFe10O19 and much work should be done in the next step. Besides, the results are not completely display right now owing to

Fig. 5. Permeability response of the ferrites Ba(MnNi)0.2Co0.6TiFe10O19 (MNCF1) and Ba(MnNi)0.25Co0.5TiFe10O19 (MNCF2): (a) real part and (b) imaginary part of complex permeability.

the limit of the testing frequency (no more than 18 GHz for the laboratory). Thus, a more perfect and systematic work will be continued.

4. Conclusions M-type barium hexagonal ferrite structured Ba(MnNi)0.2 Co0.6TiFe10O19 and Ba(MnNi)0.25Co0.5TiFe10O19 were successfully obtained via the sol–gel combustion reaction method. The XRD confirmed the phase of M-type barium ferrite and the VSM revealed the magnetic properties of doped ferrites. The grain size of the ferrite powders was distributed in the range of 100–400 nm. Moreover, the doped ferrites showed good microwave absorption performance. For Ba(MnNi)0.2Co0.6TiFe10O19, it had a 5.8 GHz bandwidth (12.2–18 GHz) below 15 dB at the thickness of 1.8 mm, and also displayed a minimum reflection loss RL¼ 52.8 dB. The bandwidth less than 20 dB for Ba(MnNi)0.25Co0.5TiFe10O19 was from 13.2 GHz to 18 GHz when the thickness of the composite is 1.8 mm, and the maximum reflection loss could reach 69 dB. The excellent absorption performances of Ba(MnNi)0.2Co0.6TiFe10O19 and Ba(MnNi)0.25Co0.5TiFe10O19 indicated they had great significances to be applied in the high


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Fig. 6. Reflection loss spectra for ferrites Ba(MnNi)0.2Co0.6TiFe10O19 (MNCF1) and Ba(MnNi)0.25Co0.5TiFe10O19 (MNCF2): (a) RL for MNCF1 with different thicknesses (1.7– 1.9 mm), (b) RL for MNCF2 with different thicknesses (1.7–1.9 mm), (c) RL for MNCF1 and MNCF2 at thickness of 1.8 mm.

frequency fields of microwave absorbers.

Acknowledgment This work is supported by the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (No. 11zxfk24).

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