孟紅杰, 于　萍, 趙曉偉, 余來貴, 賈勇帥, 劉浩杰, 呂曉燕, 龔春紅*

(1. 河南大學(xué) 化學(xué)化工學(xué)院，河南 開封 475004;　2. 河南大學(xué) 納米材料工程研究中心，河南 開封 475004;

3. 河南大學(xué) 物理與電子學(xué)院，光伏材料省重點(diǎn)實(shí)驗(yàn)室，河南 開封 475004)

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一維鈷納米材料的簡(jiǎn)單制備及其磁性和電磁性能研究

孟紅杰1, 2, 于萍3, 趙曉偉1, 余來貴2, 賈勇帥2, 劉浩杰1, 呂曉燕2, 龔春紅1*

(1. 河南大學(xué) 化學(xué)化工學(xué)院，河南 開封 475004;2. 河南大學(xué) 納米材料工程研究中心，河南 開封 475004;

3. 河南大學(xué) 物理與電子學(xué)院，光伏材料省重點(diǎn)實(shí)驗(yàn)室，河南 開封 475004)

摘要：在常壓下采用簡(jiǎn)單的液相還原法制備了一維鈷納米材料，并對(duì)比了一維納米材料在研磨前后的形貌變化。結(jié)果表明，外加磁場(chǎng)的存在更利于鈷納米晶體的取向生長(zhǎng)，得到的一維鈷納米材料在被充分研磨后仍然保持有較大的長(zhǎng)徑比，并且具有更好的電磁吸波性能.

關(guān)鍵詞：鈷;磁性材料；晶體生長(zhǎng)；外加磁場(chǎng)； 電磁性能

Received date：2015-04-30.

Foundation item：The Natural Science Foundation of China (50902045).

Biography：MENG Hongjie (1990-), female, postgraduate, majoring in nano-composite absorbing materials.*Corresponding author, E-mail: gong@henu.edu.cn.

In recent years, morphological control of metals has become a subject of intensive research due to its importance in fundamental scientific research and potential technological applications[1-2]. Numerous reports have focused on the controllable preparation of ferromagnetic nanomaterials with one-dimensional (1D) assembled structures, because of their distinctive effect on common nanoparticles and high magnetic anisotropy[3-4]. Moreover, the well defined shapes make it possible for the magnetic materials to control the microwave absorbing properties. For example, the anisotropic magnetic particles can be used as the ideal material for electromagnetic wave absorption in a relatively high frequency range because of their higher resonance frequency which exceeds the Snoek’s limit in the gigahertz frequency range owing to their large shape anisotropy[5].

As typical magnetic materials, Co materials are promising electromagnetic wave absorbing materials in high temperature environment due to their higher Curie temperature (1 130 ℃) than Ni (353-360 ℃) and Fe (770 ℃)[6-8]. Unfortunately, very few investigations are currently available about the relation between the microstructures and the properties of 1D cobalt nanocrystals as candidates of microwave absorbers, though various methods such as electro-deposition method[9], structure-directing template-assisted method[10], and magnetic-field-induced growth route[11]have been developed for the synthesis of 1D cobalt nanocrystals. Furthermore, it should also be noted that a disperse treatment is usually needed to realize the measurement and applications[12], which induces the decrease in the length of 1D nanostructures after being treated under the shear stress generated in dispersion process[13]. However, the change of morphology after the grinding of the 1D nanostructure is often ignored[14].

In the present research, 1D Co nanostructures were prepared facially under both external magnetic field and self-generated magnetic field at ambient condition. Our results indicated that the Co nanostructure obtained under external magnetic field retained a large aspect ratio after being fully ground and exhibits a better microwave absorption performance than that obtained without external magnetic field. The reason might be ascribed to the better anisotropy and electromagnetic matching between dielectric loss and magnetic loss. This study is hoped to contribute to acquire more insights into the crystal growth and morphology-dependent electromagnetic absorption properties of magnetic materials.

1Experimental

1.1　Preparation of one-dimensional cobalt nanostructures

All the reagents are of analytical grade (Tianjin Kermel Chemical Company Ltd., Tianjin, China) and were used as received. Briefly, 0.6 g CoCl2·6H2O, 50 mL 1, 2-propylene glycol, 0.75 g of NaOH, and 3 mL of N2H4·H2O (80%, mass fraction) were uniformly mixed in a 250 mL beaker and then a 0.5 T NdFeB magnet was placed under the bottom of the beaker under the external magnetic field and the resultant mixture was heated to 60 ℃ and held there for 1-2 h. The resultant products were fully washed with ethanol, followed by drying in vacuum to afford anticipated Co nanostructure denoted as S2. Another Co nanostructure denoted as S1 was also prepared in the same manners for comparative studies while no external magnetic field was applied.

1.2　Characterization

The phase microstructures and compositions were analyzed by means of scanning electron microscope (SEM, JSM-5600LV; JEOL) and X-ray diffraction (XRD;λ= 0.154 18 nm). Magnetic measurements were conducted with a vibrating sample magnetometer (VSM, Lake Shore 7400). For the measurement of electromagnetic properties, as-synthesized Co nanostructures were mechanically mixed with the same weight of paraffin and pressed into cylindrically shaped compacts[15]; and then the complex permittivity and permeability were recorded at 2.0-18.0 GHz with an Agilent N5230A network analyzer.

2Results and discussion

2.1　Structure and morphology analyses of cobalt nanostructures

Fig.1a shows the XRD patterns of Co nanostructures of S1 and S2. It can be seen that both S1 and S2 exhibit four characteristic peaks at 2θ= 41.68, 44.46, 47.36 and 75.93°, being ascribed to the hexagonal-phase Co (JCPDS 05-0727)[16]. Evidently, the externally applied magnetic field has no effect on the composition and crystal structure of Co nanostructures. Moreover, the relative intensity of (002) peak of S2 is much higher than that of S1, indicating that the externally applied magnetic field promotes the preferred growth of Co nanostructure along the [001] direction[17].

Fig.1(b and c) shows the SEM images of as-prepared S1 and S2 and the same products after grinding. All Co nanostructures were found to exhibit chain-like morphology and have a length of about 10 μm whether in the absence or presence of external magnetic field. The as-prepared S1 and S2 were grinded in an agate mortar to examine the effect of the grinding on their morphology. It was found that although grinding led to the decrease in the length of the Co chains, S2 retained a large aspect ratio after being fully ground. This demonstrates that, under the applied magnetic field, Co particles tend to grow together rather than merely aggregate one another, owing to the enhanced nucleation and growth of Co nanocrystallite along the direction of the applied magnetic force. Different from S2, the chain-like S1 is almost fully broken into particles after grinding. This implies that the self-generated magnetic field, with a weaker intensity than that of the external one, can only induce weaker preferred orientation of Co crystalline.

Fig.1　XRD patterns of sample S1 and S2 (a); SEM images of S1 (b) and S2 (c),

2.2　Magnetic properties

Fig.2 shows the room temperature hysteresis loops of S1 and S2 in the frequency range of -5 kOe

Fig.2　Hysteresis loops of S1 and S2 in a frequency range of -5 kOe < H < 5 kOe at 298 K

2.3　Electromagnetic properties

The microwave absorbing properties of materials are mainly dictated by their relative permittivity (εr=εr′ +iεr″) and permeability (μr=μr′ +iμr″). As illustrated in Fig.3, the dielectric loss (tanδE=εr″/εr′) and magnetic loss (tanδM=μr″/μr′) of the two Co-filled paraffin composites change with the variation of electromagnetic wave frequency in very similar manners, which should be attributed to the intrinsic nature of the fillers. Besides, in the frequency range of 11.5-13.2 GHz, S2-paraffin composite has a higher dielectric loss than S1-paraffin composite. Thus it can be inferred that S2-paraffin composite exhibits a lower electric resistivity than S1-paraffin composite, givingεr″ ≈ 1/2πε0ρf(ρis the resistivity) according to the free electron theory. The reason might be that the chain-like S2 with a high aspect ratio tends to form conducting networks which promote the increase in conductivity and enhancement of space-charge polarization, allowing Co particles to have more chances to interact with electromagnetic waves and to provide enhanced dielectric loss. Different from the above mentioned, S1 has smaller aspect ratio than S2, and the magnetic particles of S1 can be more efficiently isolated by paraffin. As a result, Co magnetic particles in S1-paraffin composite can hardly form conducting network, for which a lower dielectric loss is obtained.

Fig.3　Frequency dependence of tanδE (a) and tanδM (b) of S1-paraffin and S2-paraffin composites

2.4　Microwave absorbing properties

The microwave absorbance capacity of metal-backed slabs can be evaluated by reflection loss (RL, in dB; a low reflection loss corresponds to a high absorption).The directly-measured and calculatedRLvalues are usually in good agreement due to the same underlying physical origin. To investigate the potential application of as-fabricated Co nanostructures as microwave absorbers, we use transmit line theory to calculate the microwave absorption capability by measuring the values ofεr′,εr″,μr′ andμr″[20].

Fig.4 shows the two-dimensional (2D)RLplots of S1-paraffin and S2-paraffin composites as the functions of the electromagnetic wave frequency and the thickness of absorber layer. As being expected, S1-paraffin and S2-paraffin composites exhibit very similar microwave absorbing properties, with S2-paraffin composite of a small absorber thickness (< 3 mm) possessing better microwave absorption capability at 13 GHz than S1-paraffin composite. This difference could be attributed to the different electromagnetic matching for the different morphology of S1 and S2.

Fig.4　2D plots showing frequency dependence of the RL of S1-paraffin composite (a) and S2-paraffin composite (b)

3Conclusions

To sum up, we have prepared 1D Co microstructuresviafacile liquid-phase reduction under normal pressure. The externally applied magnetic field promotes the preferred orientation of Co nanostructures, endowing Co nanocrystal large aspect ratios. The chain-like Co nanostructure obtained in the presence of external magnetic field, S2, possesses better absorption performance than that obtained without external magnetic field S1, probably due to the enhanced dielectric loss and the proper matching between the dielectric loss and magnetic loss of S2. We hope the present work can shed light on the growth and self-assembly mechanisms of 1D Co nanostructures, and further to promote their applications in electromagnetic wave absorption field.

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