Abstract
MCo2O4 (M = Mn,Ni) microparticles were synthesised by a simple hydrothermal solvothermal method. The samples were characterised by X-ray diffraction, X-ray energy dispersive spectroscopy and scanning electron microscopy, which showed that MnCo2O4 microparticles with spherical particles aggregated by a large number of small cubes and cubic shaped NiCo2O4 microcubes were obtained. The microwave absorption properties of these products were systematically investigated by vector network analysis. Results indicated that the minimum reflection loss value of MnCo2O4 microparticles was −26.34 dB at 11.04 GHz with the absorber thickness of 2.5 mm, which was much lower than that of NiCo2O4 microcubes with the same absorber thickness. The possible mechanism was analysed, indicating that the geometry and size of MCo2O4 (M = Mn,Ni) microparticles played a key role in microwave absorption.
Keywords
Introduction
With the rapid development of wireless communication technology in modern society, the environmental and security problems caused by electromagnetic (EM) radiation and electromagnetic interference become more and more serious [1-5]. Thus the suitable materials for absorbing electromagnetic wave have attracted much attention, since they can be used to attenuate electromagnetic waves effectively. Among those microwave absorbing materials, ferrites have many good applications in the field of military and civilian [610] due to their high saturation magnetisation, antiperoxide and corrosion resistance. However, the magnetic loss of ferrite is always weak in high-frequency region due to the Snoek's limit. Therefore, it cannot effectively absorb electromagnetic wave [1113]. Thus it is important to explore new microwave adsorbents instead of traditional ferrite materials.
Owing to the wide range of magnetic, optical and electronic properties, some transition metal oxides including manganese, cobalt and nickel oxides have been widely used in the field of high-density magnetic storage, magnetic resonance imaging, catalysis, sensors, electronic and optical device [14-21]. MCo2O4 (M = Mn,Ni) nanomaterials had been widely investigated as the promising materials for battery or capacity device [2232]. At the same time, a large number of article have reported that Ni, Co-based composites as microwave absorbing materials, and this material shows excellent electromagnetic wave absorption performance [3337]. However, MnCo2O4 with unique surface structure has rarely been reported as a study of microwave absorbing materials.
In the current study, we prepared MCo2O4 (M = Mn,Ni) micromaterials by a simple hydrothermal solvothermal method. The microwave absorption characteristics of synthetic materials were systematically investigated as the function of composition, morphology and microstructure.
Experimental section
Synthesis of hollow MnCo2O4 microparticles and the NiCo2O4 microcubes
1 mmol of manganous acetate tetrahydrate (Mn(CH3COO)2·4H2O), 2 mmol of cobalt acetate tetrahydrate (Co(CH3COO)2·4H2O) and 0.06 mol of urea (H2NCONH2) was dissolved in 30 mL deionised water and ethylene glycol (1:5). After stirred for 30 min, the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave, sealed and maintained at 120°C for 12 h. After the reaction system was naturally cooled down to room temperature, the pink precipitates were separated from solution and thoroughly washed by deionised water and ethanol for several times. Then the obtained samples were dried in a vacuum oven at 60°C for 10 h. To prepare MnCo2O4 microparticles, the pink precipitates were further calcined at 450°C for 3.5 h under atmospheric condition. Following the similar synthesis route, we obtained NiCo2O4 microcubes by using nickel acetate tetrahydrate to replace manganese acetate tetrahydrate. All reagents were of an analytical grade and commercially available from Sinopharm Chemical Reagent Co., Ltd (China), which were used in the experiment without further purification.
Material characterisation
X-ray diffraction (XRD) patterns for the powder samples were recorded by Panalytical Empyrean X-ray diffractometer. The surface morphologies of the samples were examined by a JEOL-6610-LV scanning electron microscope (SEM) equipped with X-ray energy dispersive spectroscopy (EDS, X-Max, Oxford Instruments). The electromagnetic parameters of the obtained samples were examined by the vector network analyzer (VNA, AV3629D) under the classic Reflection/Transmission Nicolson Ross model.
Results and discussion
The crystal structures of the samples were recorded using powder XRD. As shown in Figure 1(a), the distinct peaks observed at 2θ values of 18.9°, 31.2°, 36.7°, 44.6°, 55.4°, 59.1°, 65.0°, and 77.0° were assigned to (111), (220), (311), (400), (422), (511), (440) and (533) plane reflections of the face-centered cubic structure NiCo2O4 (JCPDS#73-1702), respectively. No diffraction peak from impurity component was detected, indicating the formation of pure NiCo2O4 product. The typical XRD pattern of the as-prepared MnCo2O4 was shown in Figure 1(b). All diffraction peaks were indexed to spinel structured MnCo2O4 (JCPDS#84-0482) [38].
XRD pattern of MnCo2O4 and NiCo2O4.
The SEM images for MnCo2O4 sample were shown in Figure 2(a,b). We found that the prepared MnCo2O4 microparticles were spherical particles aggregated by a large number of small cubes. Figure 2(c,d) showed the typical SEM images of NiCo2O4 sample, which contained square-like microcubes with the diameter of about 0.5–1 μm. The individual microcube also consisted of a number of irregular nanoparticles with a diameter of 40–80 nm. In addition, it was observed that the obtained samples showed quite different morphologies and particle sizes under almost the same conditions except the replacement of the manganese element by nickel element.
SEM images of (a) and (b) MnCo2O4, (c) and (d) NiCo2O4.
With the weight ratio of powders/paraffin of about 3:1, the obtained samples were well dispersed in paraffin by a careful mixing process. Then the hybrid paraffin was compressed to circular composites with the inner diameter of 3.04 mm, outer diameter of 7.0 mm, and the thickness of 2.0 mm. The complex permittivity, complex permeability, dielectric loss tangent and magnetic loss tangent measurement were obtained combined with the calculation procedures. Before the sample test, a dual port calibration in the model of AV31101 was conducted in order to reduce the experiment error.
Typically, the RL value was used to assess the microwave absorption efficiency of absorbers. On the basis of the transmission line theory, the RL values can be calculated according to the measured relative complex permittivity (
) and relative complex permeability (
) by the following equations at a given frequency and thickness layer.
,
,
, and
represent the real and image parts of permittivity and permeability, respectively. The f value is the frequency of electromagnetic wave, d is the thickness of the absorber, Z0 is the impedance of free space, Zin is the normalised input impedance, and c is the velocity of light in free space.
According to formula (1)–(2), the reflection loss (RL) of −20 dB is related to the ∼99% absorption of the incoming electromagnetic wave by the absorber, which indicates the material can satisfy the actual application [19,31,39]. In the current experiment, the electromagnetic (EM) parameters of NiCo2O4 microcubes and MnCo2O4 microparticles were measured in the frequency range of 2.0–18.0 GHz. The real (
The frequency dependence of complex permittivities (a, b) and permeabilities(c, d) of NiCo2O4 and MnCo2O4.
) and imaginary permittivity (
) of NiCo2O4 microcubes and MnCo2O4 microparticles were shown in Figure 3(a,b). It was observed that the
values of NiCo2O4 microcubes decreased from 17.4 to 8.9 as the function of frequency. However, the
values showed quite different behaviour. It decreased slowly from 8.3 to 6.1 in the range of 2.0–10.6 GHz, and then increased gradually in the range of 10.6–18 GHz, showing typical dielectric response characteristics. Compared with NiCo2O4 microcubes, less changes of
and
values for MnCo2O4 microparticles were observed. For MnCo2O4 microparticles,
and
decreased from 11.3 to 7.2 and from 5.3 to 3.0 with the increase of frequency, respectively. The magnitude of the
values for NiCo2O4 microcubes was much larger than that of MnCo2O4 microparticles, which showed more potential applications in microwave absorption. At the same time, based on the free electron theory [19,40],
= 1/2ε0πρƒ (ε0, ρ, ƒ are the permittivity of a vacuum, the resistivity, and the frequency of the microwave, respectively), we could conclude that the high electronic conductivity would generate high
values. Therefore, the high
values of NiCo2O4 microcubes resulted in the high conductivity, compared with the MnCo2O4 microparticles. However, too high conductivity would easily lead to the mismatching between permittivity and permeability, which was not favourable to the microwave absorption performance.
Figure 3(c,d) showed the complex permeability in the frequency range of 2.0–18.0 GHz for NiCo2O4 microcubes and MnCo2O4 microparticles. The
and
intensively fluctuated with frequency, and varied in the range of 0.90–1.19 and −0.23–0.27, respectively. It was clear that pretty complex and fluctuations were appeared, which might be attribute to the size effect, exchange resonance and natural ferromagnetic resonance [39,41]. The
values of complex permeability were smaller than zero in the range of 7.6–18 GHz and 7.36–18 GHz, respectively. The results showed that the magnetism of NiCo2O4 microcubes and MnCo2O4 microparticles were small and negligible [19].
Further, it was well known that the imaginary parts of the relative complex permittivity and permeability were related to the energy dissipation and real parts were associated with the energy storage. The RL of absorbers depended on the magnetic loss (
Magnetic loss factor and dielectric loss factor of the NiCo2O4 and MnCo2O4.
) and dielectric loss (
). As shown in Figure 4, the dielectric loss tangent for NiCo2O4 microcubes was slightly more than MnCo2O4 microparticles at 2.0–18.0GHz. A maximal dielectric loss tangent of 0.46 was obtained at 4.8 GHz for MnCo2O4. We also observed a maximal dielectric loss tangent of 0.89 at 18 GHz for NiCo2O4. Therefore, it was concluded that the dielectric loss dominates the attenuation of EM energy over the whole frequency range.
According to the electromagnetic parameters obtained from the test samples, the relationship between RL and test frequency of different thickness was calculated by formula (1) and (2). As shown in Figure 5(a,c), it was seen clearly that MnCo2O4 microparticles exhibited the highest MA performance, followed by NiCo2O4 microcubes. These results reconfirmed the point that high conductivity and lower magnetic loss easily leaded to impedance mismatching. To reveal the influence of the thickness on the MA performance, 3D RL curves of MnCo2O4 microparticles and NiCo2O4 microcubes were obtained. The minimum RL value of MnCo2O4 microparticles in Figure 5(b) was −26.34 dB (more than 99.9% of effective absorption) at 11.04 GHz with absorber thickness of 2.5 mm. Compared with MnCo2O4 microparticles, the minimum RL values of the NiCo2O4 microcubes was −17.67 dB at 8.24 GHz with the same absorber thickness of 2.5 mm (Figure 5(d)). Moreover, the absorber bandwidth (RL<−10 dB) of MnCo2O4 microparticles varied from 12.05–16.52 GHz with absorber thickness at 2.0 mm. The RL value for NiCo2O4 microcubes less than −10 dB was obtained in the 9.33–12.74 GHz with absorber thickness at 2.0 mm.
Frequency dependence of microwave RL values of (a) MnCo2O4 and(c) NiCo2O4 with different thickness. 3D images of calculated RL values of (b) MnCo2O4, (d) NiCo2O4.
From the above testing results, it was clear that the MnCo2O4 exhibited excellent microwave absorption property than NiCo2O4. We present the relevant schematics to more intuitively illustrate the loss mechanism of MnCo2O4 microparticles on electromagnetic waves. As can be seen from Figure 6, the unique surface structure of MnCo2O4 microparticles can reflect and absorb the absorbed electromagnetic waves multiple times to enhance the loss of electromagnetic waves in the sample [42]. However, the NiCo2O4 microcubes prepared in the same manner do not have this special surface structure. As a result, it is impossible to obtain the same RL to electromagnetic wave as MnCo2O4 microparticles.
A schematic illustration of MnCo2O4 microparticles to electromagnetic wave attenuation mechanism.
Conclusions
In summary, MCo2O4 (M = Mn,Ni) microparticles were prepared via a hydrothermal solvothermal method and a subsequent annealing process at 450°C. Compared with NiCo2O4 microcubes, the MnCo2O4 microparticles with abundant surface structure aggregated by a large number of small cubes would absorb electromagnetic wave with much higher efficiency. Such a preferable absorption activity was mainly attributed to the unique surface structure, which would enable higher multiple electromagnetic wave reflections, resulting the enhanced EW-harvesting efficiency. It was believed that the MnCo2O4 microparticles should be good alternative for high-performance microwave absorbers in practical applications.
Footnotes
Disclosure statement
No potential conflict of interest was reported by the authors.
Correction Statement
This article has been republished with minor changes. These changes do not impact the academic content of the article.