CoO microrods

Accepted Manuscript Facile preparation and microwave absorption properties of porous Co/CoO microrods Xiang Liu, Yulong Qiu, Yating Ma, Hongfei Zheng, Lai-Sen Wang, Qinfu Zhang, Yuanzhi Chen, Dong-Liang Peng PII:

S0925-8388(17)31985-0

DOI:

10.1016/j.jallcom.2017.06.011

Reference:

JALCOM 42079

To appear in:

Journal of Alloys and Compounds

Received Date: 14 March 2017 Revised Date:

16 May 2017

Accepted Date: 1 June 2017

Please cite this article as: X. Liu, Y. Qiu, Y. Ma, H. Zheng, L.-S. Wang, Q. Zhang, Y. Chen, D.-L. Peng, Facile preparation and microwave absorption properties of porous Co/CoO microrods, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.06.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Facile preparation and microwave absorption properties of porous Co/CoO microrods Xiang Liu, Yulong Qiu, Yating Ma, Hongfei Zheng, Lai-Sen Wang*, Qinfu Zhang, Yuanzhi Chen, and Dong-Liang Peng*

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Department of Materials Science and Engineering, Collaborative Innovation Center of Chemistry for Energy Materials, College of Materials, Xiamen University, Xiamen 361005, China

*Corresponding author: [email protected](Lai-Sen Wang); [email protected](Dong-Liang Peng)

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Tel: 86-592- 2180155; Fax: 86-592-2183515 ABSTRACT

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Electromagnetic wave absorption materials with the features of being low density and porosity have attracted a great deal of attentions. In this manuscript, porous Co/CoO microrods were successfully synthesized through a facial and simple method. The resultant porous Co/CoO microrods exhibited superior microwave absorption properties because of their special structure. The high saturation

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magnetization of ferromagnetic metal Co component made the as-prepared composites have large relative complex permeability. The porous structures of materials improved the impedance match and

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enhanced the multiple reflections within the materials. With the 70 wt% of functional filler loading, the relative complex permittivity and permeability had significant increase due to the percolation effect.

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When the thickness of absorber is only 1.20 mm, the effective bandwidth is from 10.83 to 17.84 GHz covered almost Ku band (12-18 GHz). The minimum value of RL (-47.96 dB) was obtained at the frequency of 8.08 GHz with the thickness of only 1.76 mm. The result shows that this material can be used as a candidate for microwave absorption materials at low frequency range with a small thickness.

Keywords: Microwave absorption; Percolation effect; Porous; Microrods

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1. Introduction Recently, microwave absorption materials (MAMs) are urgent in civil and military fields which can absorb redundant electromagnetic (EM) wave that regarded as a source of pollution in the human living

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space, or help the weapons escape from the radar tracking [1-6]. Outstanding impedance matching and strong EM energy attenuation characteristics are prerequisite in designing the broadband MAMs [7, 8]. Simultaneously, in consideration of practical application, MAMs should be low density and small

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thickness [9-15]. MAMs with strong EM wave absorption in the frequency range of C band (4-8 GHz)

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and X band (8-12 GHz) which is of great interest for weather radar and military radar at a thickness smaller than 2 mm can be very valuable for practical applications [16]. Although extensive research have been done to meet the above mentioned requirements, to fabricate the MAMs that have excellent microwave absorption properties in low frequency with a small thickness is still a challenge [17].

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According to the theory of quarter-wavelength cancellation [18-20], the frequency locality of absorption peak at a certain thickness is decided by the quarter-wavelength of traveling microwave in absorber. As shown in below formula,

f m = nc / 4d m ε r µr

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(1)

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Where fm and dm are matching frequency and matching thickness of RL peak, εr and µr are relatively complex permittivity and permeability at fm, c is the velocity of light in free space. To achieve effective microwave absorption in low frequency with a small thickness, the value of

ε r µr

should be larger.

Meantime, according to the perfect impedance matching condition [21-23], 1/2 1/2 Zin / Z0 = ( µr / ε r ) tanh  j ( 2π fd / c )( µrε r )  = 1  

(2)

Where Zin is the input impedance of absorber and Z0 is the impedance of free space, d is the thickness of absorber. According to equation (1) and (2)，a new formula can be obtained as:

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(3)

To improve the impedance matching between absorber and free space, it is requisite to make the relatively complex permittivity (εr) close to the complex permeability (µr) [24]. However, the real part

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of permeability presents unavoidably a sudden decrease to 1 at high frequency due to Snoek limit, which leads to the impedance mismatch. The Snoek limit can be express as [25-27]:

( µi − 1) f r = γ M s / 3π

(4)

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Where µi is static permeability, γ is the gyromagnetic factor, and Ms is the saturation magnetization. To

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maintain significant µi at gigahertz frequency, the microwave absorber should have superior magnetic properties.

Ferromagnetic metal material Co possesses high saturation magnetization (1430 G) and a large magnetocrystalline anisotropy (K1=5.2×105 J/cm3, K2=1.0×105 J/cm3) [28, 29]. As an important

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magnetic metal with comparatively large permeability which contributes to electromagnetic parameters matching, cobalt has been extensively studied the microwave absorption properties [30-35]. However, the high conductivity and skin effect of ferromagnetic metal materials lead to poor impedance matching

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and increase the reflection wave [36, 37]. A direct and efficient approach to resolve this contradiction is

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to design the porous materials and coat with a metal oxide layer. The unique porous structures not only improve the impedance matching between the absorbers and free space but also provide effective electromagnetic wave channels and increase the multiple reflections in MAMs [38-44]. The porous cobalt materials could be a promising candidate in low frequency range at a small thickness. As we all know, the electromagnetic parameters of microwave absorption materials are directly related to the mass fraction of fillers. According to the mixture theory, the Maxwell Garnet theory and Bruggeman’s effective medium theory, it is of great significance to investigate the EM properties of

ACCEPTED MANUSCRIPT composites with different mass ratios of fillers [45-46]. Konstantin et al proposed a mixing rule for predicting frequency dependence of material parameters in magnetic composites [47]. Jiang et al investigated the EM wave absorption properties dependent on the different mass ratio of LaNiO3

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powders [48]. Han et al found that microwave absorption properties of FeCo/CNTs-paraffin composite were enhanced when the mass ratio of the fillers approached percolation threshold [49]. Increasing the mass ratio of the fillers is an effective way to enhance the permittivity and permeability.

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In this paper, we demonstrated a successful preparation of porous Co3O4 and Co/CoO microrods

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through a facial approach in aqueous phase. It is found that Co/CoO microrods have larger permittivity and permeability than Co3O4 samples. The porous structure and cobaltous oxide coating layer are greatly helpful to suppress the high conductivity and the skin effect. As a result, Co/CoO microrods show superior microwave absorption properties than Co3O4. We investigated the EM parameters of

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composites with different mass ratios of fillers. It is worth point out that as the mass ratio of Co/CoO microrods/wax paraffin increasing, the RL peaks move to a lower frequency. The minimum value of RL (-47.96 dB) was obtained at the frequency of 8.08 GHz with the thickness of 1.76 mm and the effective

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bandwidth (RL<-10 dB) is from 6.96 GHz to 9.68 GHz. When the thickness of absorber is 1.20 mm,

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the effective bandwidth is from 10.83 GHz to 17.84 GHz covered almost Ku band (12-18 GHz).

2. Experimental

2.1 Synthesis of Co3O4 microrods In a typical procedure, 5.0 mmol cobalt nitrate hexahydrate (Co(NO3)2•6H2O), 2.5 mmol hexamethylenetetramine ((CH2)6N4) were dissolved into 500 mL deionized water and then adding 1.25 mmol oxalic acid (H2C2O4) which used as precipitant. Then keep the mixture into a water bath at the temperature of 90 oC for 20 minutes. The pink precipitate (CoC2O4) were washed with distilled water

ACCEPTED MANUSCRIPT and ethanol and then separated of centrifugalization for several times. Then the obtained powders were placed into an oven (70 oC) over the overnight to remove the water. Finally, the resulted products were calcined at 450 oC with a heating rate of 2 oC/min in air atmosphere and maintained for 4 h. Finally, the

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black products were obtained. 2.2 Synthesis of Co/CoO microrods

The cobalt oxalate precursor was obtained as the synthesis of Co3O4 microrods. Then the pink

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powders were calcined at 450 oC with a heating rate of 2 oC/min in Ar atmosphere and maintained for 4

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h. The Co/CoO microrods were fabricated. 2.3 Characterizations

X-ray diffraction (XRD) patterns were performed on Rigaku Ultima IV XRD with Cu Kα radiation (40 mA, 40 kV). Scanning eletron microscopy (SEM) images were obtained by Hitachi SU-70 field

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emission Scanning eletron microscope (FESEM) at the accelerated voltage of 10 kV. Transmission electron microscopy (TEM) images, high-solution TEM (HRTEM) and selected area electron diffraction (SAED) were recorded at accelerated voltage of 200 kV by using a JEM 2100 transmission

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electron microscope. The static magnetism was achieved on vibrating sample magnetometer (VSM,

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LakeShore 7404) at room temperature. 2.4 Measurements of electromagnetic properties Electromagnetic properties were obtained from a network analyzer (Agilent Technologies, N5222A) at the frequency ranging from 2 to 18 GHz. The tested paraffin composites were prepared by dispersing microrods powders into 40 wt%, 50 wt%, 60 wt% and 70 wt% wax paraffin uniformly and then pressed into coaxial rings with an outer diameter of 7.0 mm and an inner diameter of 3.04 mm at the thickness about 2 mm.

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3. Results and discussion

Figure 1. XRD patterns of the precursor, Co3O4 and Co/CoO microrods.

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As described in Figure 1, the XRD pattern of precursor (black line) shows that the pink precipitate is cobalt oxalate (CoC2O4). After annealing in air atmosphere, the precursor transforms to cobalt (II, III) oxide (red line), a spinel structure (PDF#43-1003). When the precursor is calcined in Ar atmosphere, two phases are obtained (blue line). The XRD pattern of the as-prepared product shows the diffraction o

and 42.40

o

are corresponding to the (111) and (200) planes of cubic CoO

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peaks at 36.50

(PDF#43-1004), respectively. The diffraction peaks at 44.22 o, 51.52 o and 75.85 o are assigned to (111),

47.57

o

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(200) and (220) planes of Co (cubic phase, PDF#15-0806), respectively. A tiny diffraction peak at is belong to (101) plane of hexagonal Co (PDF#05-0727). The surface composition and

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chemical states of the as-prepared products are examined by the X-ray photoelectron spectrum (XPS). Figure 2 shows the in-depth evolution of XPS spectrums of samples that both after Ar+ sputtering etching. In this study, the Co 2p3/2 bands were curve-fitted. The intensive main peak at 779.3 eV is corresponding to the Co 2p3/2 electronic orbit for the as-prepared Co3O4 porous microrods (Figure 2(a)). The peaks at 780.8 eV and 782.0 eV are observed in the spectrum of Co3O4, which contains Co(Ⅱ) and Co(Ⅲ) two oxidation states [50]. The XPS spectrum of Co/CoO porous microrods is shown in Figure 2(b). The main peaks of Co 2p3/2 at binding energy of 777.8eV are assigned to zero-valence Co.

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Meantime, the peaks at 780.4 eV and 779.1 eV are corresponding to Co2O3 and CoO [51].

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Figure 2. The XPS high-solution spectra of (a) Co3O4 and (b) Co/CoO.

The morphologies and microstructures of the as-synthesized Co3O4 and Co/CoO porous microrods are clarified by FESEM observation (Figure 3). It can be seen that both as-prepared products present rod-like shape. Further magnifying the microrods (Figure 3(b) and (d)) reveals that the rod-like

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structures consists of numerous small particles with the size of about 20 nm. Meantime, both microrods

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are characterized with a hierarchical porous structure.

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Figure 3. SEM images of the as prepared products (a-b) Co3O4 and (c-d) Co/CoO. The structures of the as-prepared materials were further examined by TEM. The TEM images

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(Figure 4(a) and (d)) show that the shapes of the products with porous micro-rod structures. The SAED patterns recorded from the round area in Figure 4(a) and (d) confirm that the microrods are good

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crystalline and the diffraction rings indicate the polycrystalline features. The magnifying images show that both samples have the porous structures (Figure 4(b-c) and (d-f)). As inset picture in Figure 4c, the

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HRTEM image indicates that the interplanar spacing of 0.467 nm could be assign to (111) plane of the Co3O4. The HRTEM image inset in Figure 4f manifests that the interplanar spacing of 0.205 nm and 0.213 nm are specified as (111) plane of Co and (200) plane of CoO. The result is according well with the XRD and XPS patterns. It is worth mentioning that the unique porous structure is helpful to improve the impedance match and increase the multiple reflections of EM wave, which contribute to achieve the goal of being strong absorption, wide bandwidth and lightweight.

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Figure 4. TEM images of (a-c) Co3O4 and (d-f) Co/CoO. The insets in (a) and (d) are the

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corresponding SAED patterns while those in (c) and (f) are the HRTEM images.

Figure 5. Hysteresis loop of Co/CoO porous microrods at room temperature. Figure 5 shows the hysteresis loop of Co/CoO porous microrods by the vibrating sample

magnetometer (VSM) at room temperature. The as-prepared Co/CoO porous microrods are confirmed to possess ferromagnetic property with a high saturation magnetization (Ms) value of 143.2 emu/g and coercive force (Hc) value of 100.0 Oe. According to equation (4), the large Ms is conducive to enhance the value of the relative complex permeability. Moreover, the large coercive force (Hc) and saturation

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magnetization (Ms) can improve the magnetic hysteresis loss.

Figure 6. Frequency dependence of the relative complex permittivity and permeability of (a) Co3O4 and (b) Co/CoO; The reflection loss curves of (c) Co3O4 and (d) Co/CoO.

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The relative complex permittivity ( ε r

= ε '+ jε '' ) and permeability ( µr = µ '+ j µ '' ) of Co3O4

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and Co/CoO porous nanorods (40 wt%) are shown in Figure 6(a) and (b). It is obviously seen that the values of ε r and µ r of Co/CoO sample are both larger than Co3O4. The theoretical reflection loss (RL) values are calculated as follow:

RL = 20log ( Z in − Z 0 ) / ( Z in + Z 0 )

(5)

where Zin is input impedance of absorber and Z 0 is the impedance of free space. The effective values of RL should be exceeded -10 dB that make the MAMs applied in practical application which means 90% EM wave energy attenuation. Based on the criteria, the Co3O4 porous microrods are out of

ACCEPTED MANUSCRIPT consideration in this study (Figure 6(c)). To the Co/CoO porous microrods, the minimum value of RL was -28.42 dB at the frequency of 14.64 GHz with the thickness of 5 mm and the effective bandwidth is from 13.84 GHz to 15.24 GHz (Figure 6(d)). The improved microwave absorption of Co/CoO

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porous microrods is mainly benefited from the specific magnetic properties and interface polarizations. However, it is still difficult to meet the requirements of thin and strong absorption at low frequency. A simple and effective strategy to enhance the microwave absorption is increasing the active absorber

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substances.

Figure 7. Frequency dependence of the real part (a) and image part (b) of the relative complex permittivity, the real part (c) and image part (d) of the relative complex permeability of 40 wt%, 50 wt%, 60 wt%, and 70 wt%, Co/CoO porous microrods in paraffin wax matrix, respectively. Figure 7 shows the real part (ε') and image part (ε'') of the relative complex permittivity and the real

ACCEPTED MANUSCRIPT part (µ') and image part (µ'') of the relative complex permeability of 40 wt%, 50 wt%, 60 wt%, and 70 wt%, Co/CoO porous microrods in paraffin wax matrix. The values of the relative complex permittivity and permeability of 70 wt% Co/CoO-paraffin wax have a significant increase which is ascribed to

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percolation effect due to the increase of Co/CoO filler. As seen in Figure 7(a), the value of ε' of 70 wt% Co/CoO-paraffin wax has an abrupt decrease from 17.59 to 10.81 at the frequency range from 7.92 to 16.24 GHz. Meanwhile, an obvious resonance peak at 12.72 GHz which ascribed to Debye relaxation

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is observed in Figure 7(b). The value of µ' decrease from 2.08 to 1.35and µ'' is around 0.3 and a resonance peak at 4.24 GHz assigned to nature resonance (Figure 7(c-d)). The real part and image part

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of relative complex permeability is larger than the majority of literatures ( µ ' ≈ 1 , µ '' ≈ 0 ) which contribute to enhance the microwave absorption properties at low frequency range. It is well known that the dielectric loss is mainly from the electric dipolar polarization and interfacial polarization at

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microwave frequencies, which can be expressed by the Debye dipolar polarization [52, 53].

ε '( f ) = ε ∞ +

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ε ''( f ) =

εs − ε∞ 1 + (2π f ) 2τ 2

(6)

2π f τ (ε s − ε ∞ ) 1 + (2π f )2τ 2

(7)

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Where f is the frequency of the electromagnetic wave, τ is the relaxation time, εs and ε∞ is the stationary and optical dielectric constant, respectively. From equation (6) and (7), it can be deduced that:

(ε ' −

ε s + ε∞ 2

2 ε −ε  ) 2 + (ε '' ) =  s ∞   2 

2

(8)

In this case, the plot of ε' vs ε'' would be a single semicircle, namely Cole-Cole semicircle, when the dielectric loss solely comes from the Debye polarization loss. When the sample filler is 70 wt%, an obvious semicircle is observed (Figure 8(a)) which indicated a complete Debye relaxation process. The magnetic losses originate mainly from eddy-current effect, natural resonance and exchange resonance

ACCEPTED MANUSCRIPT at microwave frequencies. As the literatures reported, the natural resonance of cobalt ferromagnetic metal is at 4.24 GHz. Due to the high electric conductivity, 70 wt% Co/CoO porous microrods probably induce the eddy current loss. The eddy-current effect can be presented by [54, 55]:

(9)

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µ '' µ ' fD 2 ∝ µ' ρ

Where D is the particle diameter, ρ is electric resistivity. It can be further arranged to be followed: −2

f −1

(10)

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C0 = µ '' ( µ ' )

If the magnetic loss only comes from the eddy current loss, the C0 should be a constant for a certain

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absorber. Seen from Figure 8(b), the value of C0 has a significant decrease from 2 to 6 GHz which can be deduced that the nature resonance is the dominated magnetic loss. The value of C0 has slight perturbation between 6 and 18 GHz which indicates that the magnetic loss is mainly from eddy-current

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loss.

Figure 8. The Cole-Cole plots (a) and the eddy-current modeling plots of 40 wt%, 50 wt%, 60 wt%, and 70 wt% Co/CoO porous microrods in paraffin wax matrix, respectively.

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Figure 9. Frequency dependence of the reflection loss (RL) at selected absorber thicknesses of 40 wt%,

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50 wt%, 60 wt%, and 70 wt% Co/CoO porous microrods in paraffin wax matrix, respectively. Figure 9 shows the calculated reflection loss plots of different filler of Co/CoO porous microrods in

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paraffin wax matrix at selected absorber thickness. To illustrate the application value of EM wave absorber with small thicknesses, the absorber thicknesses are selected less than 3.56 mm. The 40 wt%,

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50 wt% and 60 wt% Co/CoO-paraffin wax composites have very poor absorption with RL > -10 dB in the frequency range of 2-18 GHz. For the 70 wt% Co/CoO porous microrods composites, the microwave absorption properties have significant improvement (Figure 9(d)). When the thickness of absorber is only 1.20 mm, the minimum value of RL is -16.72 dB and the effective bandwidth is from 10.83 GHz to 17.84 GHz. As the thickness of absorber increasing, the absorption peak is shifting to lower frequency which could be explained by the quarter-wavelength cancellation model. The minimum value of RL is achieved -47.96 dB at the frequency of 8.08 GHz with the thickness of only

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absorption peak at the thickness of 1.5 mm is shifting 4.70 GHz towards lower frequency and the absorption value is greatly improved (Figure 10). The impedance mismatch could be effectively prevented and then the excellent microwave absorption in low frequency with small thickness can be

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achieved.

Figure 10. The reflection loss (RL) at 1.50 mm of 40 wt%, 50 wt%, 60 wt%, and 70 wt% Co/CoO

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porous micro-rods in paraffin wax matrix, respectively.

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Table 1 Microwave absorption performance of representative Co-based composite. Materials

wt%

Min RL (dB)

d (mm)

f range (GHz)

Refs

Co/C

50%

-43.4

2.3

8.7-18

56

NOMC-800-Co

70%

-29.4

1.5

11.4-15.3

57

Fe7Co3

70%

-53.6

1.55

11.2-18

58

Co@CoO

50%

-90.2

1.3

10.8-18

59

Co@CoO

60%

-30.5

1.7

12.6-17.3

60

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-22

2

13.8-18

41

Co/CoO/C

50%

-78.4

1.3

~10.4-17.6

61

Co

70%

-60.1

1.4

~11.9-17.3

62

Co/CoO

70%

-48

1.76

6.8-9.7

This work

Co/CoO

70%

-16.7

1.2

10.8-17.8

This work

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4. Conclusion

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Co/CoO

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In summary, we have fabricated the porous Co/CoO micro-rods via a facial and simple method. The relative complex permittivity and permeability of as-prepared porous Co/CoO micro-rods/paraffin wax composites were investigated by vector network analyzer (VNA). According to transmission line theory, the theoretical values of reflection loss were calculated. When the loading content of functional

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microrods was higher than percolation threshold, the values of both permittivity and permeability were effectively increasing. As a result, the porous Co/CoO micro-rods based composites could achieve the effective bandwidth of 6.96 GHz to 9.68 GHz and the minimum RL value of -47.96 dB at the frequency

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of 8.08 GHz with a thickness of only 1.76 mm. This work provides a candidate for microwave

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absorption materials at low frequency range with small thickness. Acknowledgements This work was partially supported by the National Natural Science Foundation of China (Grant Nos. 51371154, 51571167 and 51171158), the Fundamental Research Funds for the Central Universities (Grant No. 20720140547).

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ACCEPTED MANUSCRIPT Highlights: · The porous Co/CoO microrods have been successfully fabricated by a facial method. · Large values of permittivity and permeability of Co/CoO-paraffin wax were obtained. · The porous Co/CoO microrods have excellent performance of microwave absorption at low frequency range with a small thickness.

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· The features of being “low density, small thickness, strong absorption and wide bandwidth” of absorber were achieved.