Hydrothermal Synthesis and Microwave Absorption Properties of Nickel Ferrite/Multiwalled Carbon Nanotubes Composites

: It is well accepted that the microwave absorption performance of ferrite can be enhanced via the hybridization. However, it is still very challenging to design the hierarchical nanostructure of ferrite hybrids to fabricate wave absorbing composites with both the high efﬁciency and lightweight. Herein, we successfully realize the in-situ synthesis of nickel ferrite/multiwalled carbon nanotubes (NiFe 2 O 4 /MWCNTs) hybrids with a large-scale production by the hydrothermal method. The structural characteristics, morphology, electromagnetic and microwave absorption properties were analyzed by X-ray diffraction, scanning electron microscope and vector network analyzer. The mor-phological study shows that NiFe 2 O 4 nanoparticles with a small size (tens of nanometers) are coated on the MWCNTs, leading to a three-dimensional hierarchical nanostructure. The NiFe 2 O 4 /MWCNTs hybrids show satisﬁed microwave absorption properties. Typically, the optimized sample shows the minimum reﬂection loss of − 19 dB at 11.3 GHz, and the bandwidth of the reﬂectivity below − 10 dB is 2.5 GHz with a thin thickness of 1.5 mm. This result shall be due to the improved dielectric losses or interface polarization etc. Our results demonstrate a facile approach for the design of ferrite-based microwave absorber to meet the requirements of lightweight, thin-thickness and high efﬁciency.


Introduction
To date, the development of radar stealth technology has been attached great importance to many countries [1,2]. As one of the main ways to achieve radar stealth, the development of radar absorbing materials has become a major issue of the various countries' military technology. The comprehensive requirements of stealth technology for absorbing materials (primarily about absorbing coating) can be referred to as: thin, wide, light and strong [3][4][5].
As the microwave absorbing material, ferrite has excellent magnetic properties and high electrical resistivity, and outstanding magnetic loss can appear in a wide frequency range, into which electromagnetic wave is easy to enter and decay rapidly [6][7][8]. Moreover, high curie temperature and low eddy current loss of ferrite make it suitable for being used as radar absorbing materials especially at high frequencies. However, it also has many shortcomings, such as lower impedance matching, onefold loss absorption mechanism and higher density, which greatly limit the ferrite in the actual applications [9,10].
Carbon nanotubes (CNTs), with large length to diameter ratio, low density, large specific surface area and high electrical conductivity, show good absorbing properties [11][12][13][14]. However, the high conductivity of CNTs has also led to the poor absorption caused by the mismatch of interface impedance. Thus, other dielectric or magnetic materials have been used to regulate the impedance of CNTs, such as conductive polymers, magnetic metal or metal oxides. Due to the special structure and dielectric properties of carbon nanotubes, the absorbing properties of the composite materials synthesized by carbon nanotubes and other materials has shown good application prospect, such as architecture and aircraft coatings [15,16]. For example, CNTs has been combined with Fe 3 O 4 to enhance their electromagnetic wave absorption performance [17,18]. Lu et al. [17] reported that the reflection loss of Fe 3 O 4 /CNTs can reach −12.62 dB at 17.72 GHz. The interface introduced by Fe 3 O 4 can not only enhance the magnetic loss, but also create resonance in complex permittivity. Hou et al. [18] also fabricated Fe 3 O 4 /CNTs hybrids by a chemical synthesishydrothermal treatment method and obtained a minimum reflection loss of −18.22 dB at 12.05 GHz. Despite substantial efforts have been devoted to the investigation of CNTs hybrids, the design of the hierarchical nanostructure of the hybrids to take full advantage of the conduction and magnetic loss, as well as the interfacial loss is still very challenging.
Nickel ferrite has been extensively investigated for applications in ferrofluids, microwave devices and magnetic materials etc. Moreover, hydrothermal route has been demonstrated to be a low-cost, rapid and large-production method of synthesizing ferrite powders. As a result, in this contribution, we selected nickel ferrite (NiFe 2 O 4 ) as a model ferrite material, and successfully synthesized NiFe 2 O 4 /multiwalled carbon nanotubes (MWCNTs) composite absorbing material through the hydrothermal method. The effect of different reaction conditions for structure and morphology was studied, and the microwave characteristics of the prepared samples was evaluated. Reflection loss evaluations indicated that the composites display good absorbing properties.

In-Situ Synthesis of NiFe 2 O 4 / MWCNTs Hybrids
Spinel nickel ferrite powders with the nominal composition of NiFe 2 O 4 were hydrothermally synthesized. MWCNTs were added in-situ during the process. First, 1 mmol Ni(NO 3 ) 2 ·6H 2 O, 2 mmol Fe(NO 3 ) 3 ·9H 2 O and a certain amount of purified MWCNTs were dissolved in 30 mL distilled water and ultasonicated for 1 h to form aqueous solutions. Then, 0.8 g NaOH was slowly added to the above solution to coprecipitate Ni 2+ and Fe 3+ ions by constant stirring and then a brown precipitate was formed. The mixture was stirred vigorously for 30 min, transferred and sealed in a 50 mL Teflon-lined stainless-steel were autoclaved, and maintained at 100-200 • C for 2-8 h. The system was cooled to room temperature naturally. The products of NiFe 2 O 4 / MWCNTs hybrids with the black color were collected by a magnetic field, rinsed with absolute ethanol and deionized water for several times and dried at 60 • C for 24 h. For comparison, nickel ferrite powders were also prepared using the same procedure.

Characterizations
The crystalline structure of produced materials were examined by X-ray diffractometer (X'Pert PRO, PANalytical, The Netherlands) with Cu Kα radiation (λ = 0.154 nm). Scanning electron microscopy (SEM) (VEGAIIXMUINCN, TESCAN company, Brno, Czech republic) with an acceleration voltage of 20 kV was used to examine the sample morphology and microstructure; electromagnetic parameters were examined by vector network analyzer (HP 8720ES, Agilent Technologies, Santa Clara, CA, USA) with coaxial-line method.
Preparation of samples and testing procedure are as follows: sample powders of the desired substrate paraffin and absorbing agents were weighted, and then absorbents powder was added to the molten paraffin wax and stirred sufficiently, cooled and grinded using a mortar and pestle and then melt-mixing. After repeating this procedure for 3 to 4 times, an appropriate amount of alcohol was added and the mixture was sheared with the high-speed emulsifying machine. After being dried, it was milled into powders and pressed into the circular coaxial sample (outer diameter Φ7 mm, inner diameter Φ3 mm, length 2-5 mm). Samples were placed in a coaxial test fixture. Calibrated HP 8720ES vector network analyzer were then used to test specimens at 2-18 GHz.

Results and Discussion
Effects of temperature and reaction time on the sample formation were investigated first. Figure 1 shows the XRD patterns of samples synthesized at different temperature with a reaction time of 2 h. According to the results, samples at lower temperature show a weak, broad peak; when the temperature increases from 100 • C to 200 • C, the sharp diffraction peaks form for both α-Fe 2 O 3 and NiFe 2 O 4 , as labeled by filled diamond and filled circle, respectively, in Figure 1. Some characteristic peaks from NiFe 2 O 4 are fairly weak, which means that the obtained sample is basically a mixture of α-Fe 2 O 3 and NiFe 2 O 4 .
Preparation of samples and testing procedure are as follows: sample powders of the desired substrate paraffin and absorbing agents were weighted, and then absorbents powder was added to the molten paraffin wax and stirred sufficiently, cooled and grinded using a mortar and pestle and then melt-mixing. After repeating this procedure for 3 to 4 times, an appropriate amount of alcohol was added and the mixture was sheared with the high-speed emulsifying machine. After being dried, it was milled into powders and pressed into the circular coaxial sample (outer diameter Φ7 mm, inner diameter Φ3 mm, length 2-5 mm). Samples were placed in a coaxial test fixture. Calibrated HP 8720ES vector network analyzer were then used to test specimens at 2-18 GHz.

Results and Discussion
Effects of temperature and reaction time on the sample formation were investigated first. Figure 1 shows the XRD patterns of samples synthesized at different temperature with a reaction time of 2 h. According to the results, samples at lower temperature show a weak, broad peak; when the temperature increases from 100 °C to 200 °C, the sharp diffraction peaks form for both α-Fe2O3 and NiFe2O4, as labeled by filled diamond and filled circle, respectively, in Figure 1. Some characteristic peaks from NiFe2O4 are fairly weak, which means that the obtained sample is basically a mixture of α-Fe2O3 and NiFe2O4. Characteristic peaks for α-Fe2O3 and NiFe2O4 are labeled by filled diamond and filled circle, respectively. Figure 2 shows the XRD patterns of sample powders synthesized at 200 °C with different reaction time. Comparing the diffractograms with the powder data of α-Fe2O3 (JCPDS card no. 33-0664) and NiFe2O4 (JCPDS card no. 10-0325), it can be seen that with the increase of reaction time, the diffraction peaks of α-Fe2O3 become less and the peak intensity become weaker gradually. When the reaction time is 8 h, the diffraction peaks of α-Fe2O3 disappear. The sample powder exhibits similar diffraction peaks which correspond to the cubic inverse spinel type lattice of NiFe2O4. The sharp diffraction peaks corresponding to (111) [2,19]. Figure 3 shows the SEM images of the MWCNTs and NiFe 2 O 4 /MWCNTs composites produced at 150 and 200 • C. It is obvious that as the temperature rises, more and more ferrites grown on the MWCNTs under the same conditions. The NiFe 2 O 4 shall be anchored on the surface of MWCNTs through electrostatic interaction during the reaction [20]. Moreover, the transformation of metal ion may occur causing the reduction of the oxygen-containing group on the oxidized CNTs surface and lead to a stronger chemical interaction between NiFe 2 O 4 and MWCNTs [21]. When the temperature is 200 • C, MWC-NTs are completely covered with NiFe 2 O 4 nanoparticles. This is because the reaction rate is accelerated with the increasing temperature, which is facilitated to the formation and  Figure 3 shows the SEM images of the MWCNTs and NiFe2O4/MWCNTs composites produced at 150 and 200 °C. It is obvious that as the temperature rises, more and more ferrites grown on the MWCNTs under the same conditions. The NiFe2O4 shall be anchored on the surface of MWCNTs through electrostatic interaction during the reaction [20]. Moreover, the transformation of metal ion may occur causing the reduction of the oxygencontaining group on the oxidized CNTs surface and lead to a stronger chemical interaction between NiFe2O4 and MWCNTs [21]. When the temperature is 200 °C, MWCNTs are completely covered with NiFe2O4 nanoparticles. This is because the reaction rate is accelerated with the increasing temperature, which is facilitated to the formation and coated of ferrite particles.    It is obvious that as the temperature rises, more and more ferrites grown on the MWCNTs under the same conditions. The NiFe2O4 shall be anchored on the surface of MWCNTs through electrostatic interaction during the reaction [20]. Moreover, the transformation of metal ion may occur causing the reduction of the oxygencontaining group on the oxidized CNTs surface and lead to a stronger chemical interaction between NiFe2O4 and MWCNTs [21]. When the temperature is 200 °C, MWCNTs are completely covered with NiFe2O4 nanoparticles. This is because the reaction rate is accelerated with the increasing temperature, which is facilitated to the formation and coated of ferrite particles.  The absorption performance of single-absorbing coating is determined by the thickness and the electromagnetic parameters. The theoretical calculations of the single-absorbing coating must consider both the reflection coefficient and the matching of electromagnetic parameters. According to the theory of electromagnetic wave transmission line, when the uniform plane electromagnetic wave whose frequency is f incident on a vertical conductor surface coated by single-absorbing materials, the reflection loss is related to the normalized input impedance Z in as [22,23] Z in represents the input impedance at the absorber/free space interface, which can be expressed as follows: where c is the velocity of light, f is frequency, d is the thickness of the composite in mm unit, ε and µ are the measured data corresponding to the relative complex permittivity (ε = ε − jε ) and the relative complex permeability (µ = µ − jµ ), respectively. Figure 5 shows the electromagnetic parameters of the NiFe 2 O 4 and NiFe 2 O 4 /MWCNTs composites. The NiFe 2 O 4 /MWCNTs sample synthesized using 0.15 g MWCNTs is used. Obviously, for all frequencies between 2 and 18 GHz, the real and imaginary parts of complex permittivity of the composite samples increase significantly. The real part of the complex permeability increases, while the imaginary part decreases.  The absorption performance of single-absorbing coating is determined by the thickness and the electromagnetic parameters. The theoretical calculations of the single-absorbing coating must consider both the reflection coefficient and the matching of electromagnetic parameters. According to the theory of electromagnetic wave transmission line, when the uniform plane electromagnetic wave whose frequency is f incident on a vertical conductor surface coated by single-absorbing materials, the reflection loss is related to the normalized input impedance Zin as [22,23] 20lg 1 1 Zin represents the input impedance at the absorber/free space interface, which can be expressed as follows: where is the velocity of light, is frequency, is the thickness of the composite in mm unit,  and are the measured data corresponding to the relative complex permittivity (   ) and the relative complex permeability ( ), respectively. Figure 5 shows the electromagnetic parameters of the NiFe2O4 and NiFe2O4/MWCNTs composites. The NiFe2O4/MWCNTs sample synthesized using 0.15 g MWCNTs is used. Obviously, for all frequencies between 2 and 18 GHz, the real and imaginary parts of complex permittivity of the composite samples increase significantly. The real part of the complex permeability increases, while the imaginary part decreases.  Figure 6 shows the color map of reflectivity values calculated from the measured electromagnetic parameters of NiFe2O4 sample and NiFe2O4/MWCNTs composite. It is obvious that the matching thickness of NiFe2O4 sample is 8-10 mm, the absorption band is narrow and concentrated in the high frequency region or low frequency region. This is because that NiFe2O4 belongs to the magnetic absorbing materials. While the electromagnetic wave can effectively enter the material inside, because of its small relative complex permittivity, larger thickness is required for effective electromagnetic wave attenuation. Meanwhile, the matching thickness of the NiFe2O4/MWCNTs composite sample decreased obviously, which is 1-3 mm, the absorption bandwidth increased relatively.  Figure 6 shows the color map of reflectivity values calculated from the measured electromagnetic parameters of NiFe 2 O 4 sample and NiFe 2 O 4 /MWCNTs composite. It is obvious that the matching thickness of NiFe 2 O 4 sample is 8-10 mm, the absorption band is narrow and concentrated in the high frequency region or low frequency region. This is because that NiFe 2 O 4 belongs to the magnetic absorbing materials. While the electromagnetic wave can effectively enter the material inside, because of its small relative complex permittivity, larger thickness is required for effective electromagnetic wave attenuation. Meanwhile, the matching thickness of the NiFe 2 O 4 /MWCNTs composite sample decreased obviously, which is 1-3 mm, the absorption bandwidth increased relatively.
The relationship between reflectivity and frequency of the NiFe 2 O 4 /MWCNTs composite sample at different thickness (0.5-3 mm) is given in Figure 7. As the thickness of the composite absorber increases, the reflectivity peak moves to lower frequency. In particular, the maximum reflectivity is −19 dB obtained at 11.3 GHz and bandwidth of the reflectivity below −10 dB is 2.5 GHz when sample thickness is 1.5 mm. The thickness leads to a variation in reflectivity and the peak of reflectivity reaches a minimum value at a matching thickness 1.5 mm. The microwave absorption properties of NiFe 2 O 4 /MWCNTs composite in this work is superior or comparable to previous results reported in the literature. For example, Zhao et al. [24] successfully prepared NiFe 2 O 4 -polystyrene composites, which shows a minimum reflection of −13 dB at 11.5 GHz with a −10 dB bandwidth over the frequency range of 10.3-13 GHz for the thickness of 2 mm. Hou et al. [18] also fabricated Fe 3 O 4 /CNTs hybrids by a chemical synthesis-hydrothermal treatment method and obtained a minimum reflection loss of −18.22 dB at 12.05 GHz. Chakradhary et al. [25] synthesized cobalt nickel ferrite/carbon nano-fiber composite achieving minimum reflection loss of −19.41 dB in X-band. In addition, Fu et al. [26] reported a novel fabrication of NiFe 2 O 4 /graphene composite with a minimum reflection loss of −29.2 dB at 16.1 GHz with a thickness of 2.0 mm. The absorbing properties is attributed to the conductive network formed by two-dimensional sheet of graphene with a large specific surface area, the outstanding magnetic properties of NiFe 2 O 4 and possible interface scattering [26]. To sum up, the approach used in the present work to design NiFe 2 O 4 /MWCNTs composite shall be a cost-effective way to fabricate the microwave absorber. Compared with other wave absorbers, the sound wave absorbing ability of NiFe 2 O 4 /MWCNTs can be due to the enhanced absorption of multiple heterogeneous interfaces between NiFe 2 O 4 and MWC-NTs because of the heteronanostructure, which facilitates the enhancement of conduction loss, interface polarization, dielectric relaxations polarization and multiple reflections and scattering [27,28]. Specifically, carbon nanotube can not only form bridges in the hybrids network and enhance the conductivity of the absorbing materials, but also create interfaces and facilitate the interfacial polarizations.  Figure 6 shows the color map of reflectivity values calculated from the measured electromagnetic parameters of NiFe2O4 sample and NiFe2O4/MWCNTs composite. It is obvious that the matching thickness of NiFe2O4 sample is 8-10 mm, the absorption band is narrow and concentrated in the high frequency region or low frequency region. This is because that NiFe2O4 belongs to the magnetic absorbing materials. While the electromagnetic wave can effectively enter the material inside, because of its small relative complex permittivity, larger thickness is required for effective electromagnetic wave attenuation. Meanwhile, the matching thickness of the NiFe2O4/MWCNTs composite sample decreased obviously, which is 1-3 mm, the absorption bandwidth increased relatively. The relationship between reflectivity and frequency of the NiFe2O4/MWCNTs composite sample at different thickness (0.5-3 mm) is given in Figure 7. As the thickness of the composite absorber increases, the reflectivity peak moves to lower frequency. In particular, the maximum reflectivity is −19 dB obtained at 11.3 GHz and bandwidth of the reflectivity below −10 dB is 2.5 GHz when sample thickness is 1.5 mm. The thickness leads to a variation in reflectivity and the peak of reflectivity reaches a minimum value at a matching thickness 1.5 mm. The microwave absorption properties of NiFe2O4/MWCNTs composite in this work is superior or comparable to previous results reported in the literature. For example, Zhao et al. [24] successfully prepared NiFe2O4-polystyrene composites, which shows a minimum reflection of −13 dB at 11.5 GHz with a −10 dB bandwidth over the frequency range of 10.3-13 GHz for the thickness of 2 mm. Hou et al. [18] also fabricated Fe3O4/CNTs hybrids by a chemical synthesis-hydrothermal treatment method and obtained a minimum reflection loss of −18.22 dB at 12.05 GHz. Chakradhary et al. [25]  synthesized cobalt nickel ferrite/carbon nano-fiber composite achieving minimum reflection loss of −19.41 dB in X-band. In addition, Fu et al. [26] reported a novel fabrication of NiFe2O4/graphene composite with a minimum reflection loss of −29.2 dB at 16.1 GHz with a thickness of 2.0 mm. The absorbing properties is attributed to the conductive network formed by two-dimensional sheet of graphene with a large specific surface area, the outstanding magnetic properties of NiFe2O4 and possible interface scattering [26]. To sum up, the approach used in the present work to design NiFe2O4/MWCNTs composite shall be a cost-effective way to fabricate the microwave absorber. Compared with other wave absorbers, the sound wave absorbing ability of NiFe2O4/MWCNTs can be due to the enhanced absorption of multiple heterogeneous interfaces between NiFe2O4 and MWCNTs because of the heteronanostructure, which facilitates the enhancement of conduction loss, interface polarization, dielectric relaxations polarization and multiple reflections and scattering [27,28]. Specifically, carbon nanotube can not only form bridges in the hybrids network and enhance the conductivity of the absorbing materials, but also create interfaces and facilitate the interfacial polarizations.
. Figure 7. The relationship between reflection loss and frequency of the NiFe2O4/MWCNTs composite with different thicknesses.

Conclusions
The NiFe2O4 and NiFe2O4/MWCNTs composite absorbing materials were successfully synthesized by hydrothermal method. The sample powders were characterized with