Structural, microstructural, magnetic and electromagnetic absorption properties of spiraled multiwalled carbon nanotubes/barium hexaferrite (MWCNTs/BaFe12O19) hybrid

Microwave absorption properties were systematically studied for synthesised barium hexaferrite (BaFe12O19) nanoparticles and spiraled multiwalled carbon nanotubes (MWCNTs) hybrid. BaFe12O19 nanoparticles were synthesised by a high energy ball milling (HEBM) followed by sintering at 1400 °C and structural, electromagnetic and microwave characteristics have been scrutinized thoroughly. The sintered powders were then used as a catalyst to synthesise spiraled MWCNTs/BaFe12O19 hybrid via the chemical vapour deposition (CVD) process. The materials were then incorporated into epoxy resin to fabricate single-layer composite structures with a thickness of 2 mm. The composite of BaFe12O19 nanoparticles showed a minimum reflection loss is − 3.58 dB and no has an absorption bandwidth while the spiraled MWCNTs/BaFe12O19 hybrid showed the highest microwave absorption of more than 99.9%, with a minimum reflection loss of − 43.99 dB and an absorption bandwidth of 2.56 GHz. This indicates that spiraled MWCNTs/BaFe12O19 hybrid is a potential microwave absorber for microwave applications in X and Ku bands.

www.nature.com/scientificreports/ on the literature, many researchers reported on the preparation of CNTs mixture with ferrite [7][8][9] . In the present research work, spiraled MWCNTs/BaFe 12 O 19 hybrid will be synthesised via CVD process by using BaFe 12 O 19 sintered powder as a catalyst. MWCNTs/BaFe 12 O 19 will be attached to each other, and this will increase the energy transfer from one medium to another medium. Hypothetically, the EM wave absorption becomes more efficient. The spiraled MWCNTs/BaFe 12 O 19 hybrid will be prepared to investigate its performance on the EM wave absorption with a wideband frequency capability. The spiraled MWCNTs/BaFe 12 O 19 hybrid may be used as a potential microwave absorber as well as a protective coating.

Experimental procedure
Preparation of BaFe 12 O 19 . The details of extraction and purification of Iron oxide (Fe 2 O 3 ) from mill scale waste have been reported elsewhere 10 . Fe 2 O 3 obtained by oxidation process from mill scale waste was mixed and weighed with barium carbonate (BaCO 3 ) (99.8%, Alfa Aesar) according to stoichiometry formula. Then it underwent a high energy ball milling with 10:1 ball to powder ratio (BPR) using a SPEX8000D HEBM milling machine for 3 h. Then the milled powders were sintered at 1400 °C for 6 h in an ambient atmosphere.
Preparation of spiraled MWCNTs/BaFe 12 O 19 hybrid. The sintered powder of BaFe 12 O 19 was used as a catalyst and ethanol solution (C 2 H 5 OH) (96%, Sigma Aldrich) as a carbon source to synthesis spiraled MWCNTs via chemical vapour deposition (CVD) method ( Fig. 1). 1.0 g of BaFe 12 O 19 was inserted into the middle of the furnace with an argon flow of 100 sccm. As the furnace reached the targeted synthesis temperature of 750 °C, the evaporated ethanol solution at 100 °C temperature was flowed in for 30 min. Then the furnace was left to cool down to room temperature under an argon environment before the sample was taken out for further analysis. The synthesised temperature using CVD was at 750 °C while the temperature of BaFe 12 19 with an average size of approximately ~ 3.19 µm were homogeneously distributed with agglomeration particles due to the magnetic attraction behaviour (Fig. 3a). The hybridisation of spiraled MWCNTs/BaFe 12 O 19 after the CVD process showed that the carbon structures were mostly formed in a straightlike, spiral and twisted fibre structures (Fig. 3b). However, we observed net-like fibres which were created by the aggregation of excellent fibres and particle-like carbon, and we also noticed that the fibres were highly aggregated and favoured in spiral coil form. The average outer diameter of the spiraled MWCNTs/BaFe 12 O 19 hybrid synthesised was approximately ~ 120.74 nm (Fig. 3b). Furthermore, the presence of BaFe 12 O 19 nanoparticles structures in the enlarged image can be clearly seen in (Fig. 3c). Absorption and dissociation of a carbon precursor on the surface of a catalyst particle and dissolution of carbon into the catalyst particle are the commonly suggested mechanisms for carbon fibre growth. The carbon crystallises the metal particle after the catalyst particle is loaded with carbon and extruded to form CNTs or CNF 12,13 . CNTs are commonly found as cylinders of rolled-up graphene sheets 14 , resulting in single-walled, double-walled, and multi-walled entities. Coiled tubes occur in one of the two types of helical materials where an inner hollow occurs along the length of the coil 15 . Both straight, spiral fibres and dark spot which BaFe 12 O 19 particles can be seen from the HRTEM image have an average outer diameter of approximately ~ 142.45 nm (Fig. 4). The presence of hollow and tube-like structures confirms the tubular nature structure of MWCNTs (Fig. 4), and we believe that the coil growth process involves a core cluster formation followed by a helical tube formation. As shown in the FESEM and HRTEM morphology images (Figs. 3b, c, 4), the BaFe 12 O 19 nanoparticles are strongly attached to the surface and tips of MWCNTs. The HRTEM morphology image displayed the conductive MWCNTs pathways formed by an interconnected network of spiraled MWCNTs/BaFe 12 O 19 hybrid embedded in the composites resin matrix. The network structure of a nanocomposite with a large surface area is expected to result in a variety of interfacial polarisation in the hybrid nanocomposite. The difference in electrical conductivity between spiraled MWCNTs and BaFe 12 O 19 nanoparticles also create high interfacial polarization in the hybrid nanocomposite. The interfacial polarisation occurs when the motion of moving charge is impeded at the interfaces of a material. There could be multiple interfacial polarisation due to the large specific areas of both MWCNTs and BaFe 12 O 19 nanoparticles, which can lead to an increase in complex dielectric permittivity values (Fig. 9). Based on the previous article, a researcher reported on the detailed mechanism of the reaction 28 .
Most of the synthesised MWCNTs fibres show a spiraled structure. A regularly orientated nucleation of pentagonal, hexagonal and heptagonal carbon rings along the nanotube body plays essential roles in producing a coiled nanotube, and details of its growth mechanisms have been proposed and described previously 16,17 . This spiral structure brings advantages in attenuating and EM wave within the material as reported by 18 . On the other hand, short fibre structure improves the performance of EM wave absorption as compared to microparticles 19 . It www.nature.com/scientificreports/ is due to their properties of high surface-to-volume ratio, quantum size effects and the network structure effect of the material in the composite 3  the Raman spectrum in a frequency range of 500-2500 cm −1 (Fig. 5). The stretching of sp 2 hybridised carbon in MWCNTs is reflected in two influential bands which appeared at around 1350 and 1600 cm −1 as the defect (D) band and graphite (G) band respectively. The ratio of the intensity of G-to D-bands in the spectrum of spiraled  hybrid was employed using energy-dispersive X-ray (EDX) to confirm the chemical composition in those samples (Fig. 6a, b). The EDX spectra of BaFe 12 O 19 nanoparticles revealed the presence of oxygen (O), barium (Ba), iron (Fe) and carbon (C) peak. The appearance of carbon (C) peak may be due to the carbon tape from the preparation of a sample for characterisation (Fig. 6a) Magnetic properties. Figure 8 shows the hysteresis curves for BaFe 12 (Table 1). This is due to the presence of MWCNTs as a phase with a very low saturation magnetisation structural distortion in the surface of hexaferrite nanoparticles caused by the interaction of the metal ions transi-    (Table 1). This could also be related to the improvement of some of the surface homogenities of ferrite nanoparticles and the surface pinning of the magnetic moment. The surface magnetic anisotropy of nanoparticles could be increased in the interface of CNTs and nanoparticles that may lead to the increase of coercivity.
Microwave characteristics. The dielectric and magnetic properties of the composites were investigated by measuring the complex permittivity (ɛr = ɛ′ − jɛ″) and the complex permeability (μr = μ′ − jμ″) in a frequency range of 8-18 GHz. The variation of real (ε′) and imaginary (ε″) parts of complex permittivity (ɛ r ) with a frequency of BaFe 12 O 19 and spiraled MWCNTs/BaFe 12 O 19 hybrid with different filler contents are presented in Fig. 9. The value of complex permittivity is higher at low frequencies and lowers down at high frequencies for all samples. These behaviours could be explained by electronic and ionic polarisation considerations, intrinsic electric dipole polarisation and electron hoping 24 . As expected, the imaginary (ε″) parts of complex permittivity of barium hexaferrite sample could significantly be improved by adding spiraled MWCNTs (Fig. 9b) 25 . Generally, the complex permittivity (ε r ) values of spiraled MWCNTs/BaFe 12 O 19 hybrid decrease and remain almost constant with the increase of frequency, which is in agreement with the regular rules of polarisation relaxation 26 . The polarisation of the inner electric dipole in dielectrics was unable to keep pace with the change of frequency. Due to the rising of the frequency, the polarisation weakened, and the complex permittivity (ε r ) values diminished 27 . Therefore, a significant enhancement is achieved in both ε′ and ε″ with the increasing filler content of spiraled MWCNTs/BaFe 12 O 19 hybrid loading, ranging from 2 to 10 wt%. The enhancement of ε r further confirms the shift of obtained RL peak as shown in Fig. 12a for as-prepared composites. Prior to the 10 wt%, the increment of εr may be attributed to the enhanced dipole polarisation, interfacial polarisation or both 27 . The frequency dependency of real (μ′) and imaginary (μ″) parts of the complex permeability (µ r ) values for BaFe 12 O 19 and spiraled MWCNTs/BaFe 12 O 19 hybrid with different filler contents are presented in Fig. 10. Based on the    www.nature.com/scientificreports/ findings of this study, we suggest that MWCNTs do not have a significant impact on the complex permeability (µ r ) of spiraled MWCNTs/BaFe 12 O 19 hybrid. This is due to an expected behaviour resulted from negligible or a low magnetic property of MWCNTs. However, the values of μ′ and µ″ decrease with the increased frequency of samples between 2 to 10 wt%. Generally, the decreasing trend values of µ′ and µ″ were attributed to the relaxation of magnetisation induced by domain wall displacement at a lower frequency and spin rotation at an upper frequency in the samples 28 . However, in this experiment, the domain wall contribution did not happen, and it depended on spin relaxation since we were measuring the frequency at between 8 and 18 GHz. The magnetic spin resonance governed the relaxation of BaFe 12 O 19 as shown by these formulae 29 : where M s is the saturation magnetisation and K is the total anisotropy. This is because the size of BaFe 12 (Fig. 10b) Fig. 12a. The EM wave absorption parameters of the prepared samples are tabulated in Table 2. According to the transmission line theory, a general expression for normalised input impedance, Z in , of the metal-backed microwave absorption layer is 30 : (1) X spin = 2πMs 2/K  www.nature.com/scientificreports/ where Z 0 = (μ 0 /ε 0 ) 1/2 is the impedance of vacuum, c is the velocity of light in free space, d is the thickness of the absorber, f is the frequency of the electromagnetic wave while ɛ r and µ r are the complex permittivity and complex permeability of the composite medium. The impedance matching condition representing the perfect absorbing properties is given by Z in /Z o = 1. The reflection loss (RL) of the electromagnetic radiation for a single microwave absorbing layer can be expressed as shown in Eq. 3 10,31 .
When the characteristic impedance of free space is matched with the input characteristic impedance of an absorber; where Z in = Z o , the impedance matching condition may occur. In addition, electromagnetic energy can be absorbed completely and dissipated into heat through magnetic and dielectric losses. RL results of BaFe 12 O 19 and spiraled MWCNTs/BaFe 12 O 19 hybrid with different filler content (wt%) composites are described in Fig. 12a. The calculation of the reflection loss was made for composites thickness (d) of 2 mm. The maximum reflection loss (RL) (~ − 43.99 dB) and the bandwidth was observed to cover 2.56 GHz for the sample with 10 wt% and a matching frequency of 12.96 GHz (Fig. 12a). We also observed that the RL value increases with the increasing filler content (wt%) of spiraled MWCNTs/BaFe 12 O 19 hybrid, ranging from RL = − 1.54 dB, − 5.68 dB, − 13.88 dB, − 34.85 dB and − 43.99 dB with filler content (wt%) = 2,4,6,8 and 10 respectively. Those RLs are higher than that of BaFe 12 O 19 sample without MWCNTs (maximum RL = − 3.58 dB at 2 mm thickness) in the measuring frequency range as shown in Table 2. This indicates that the hybridisation of spiraled MWCNTs/BaFe 12 O 19 hybrid helps in enhancing the EM wave performance.
As the filler content (wt%) of spiraled MWCNTs/BaFe 12 O 19 hybrid increases, the MWCNTs spontaneously form larger aggregates and agglomerates. Meanwhile, the permittivity of the spiraled MWCNTs/BaFe 12 O 19 hybrid samples increases as the MWCNTs wt% filler content increases. It can be seen that the imaginary part (energy www.nature.com/scientificreports/ loss) of permittivity from (Fig. 9b) and dielectric loss tangent from (Fig. 11a) of spiraled MWCNTs/BaFe 12 O 19 hybrid with MWCNTs loadings from 2 to 10 wt% are substantially increased, and this also shows the frequencydependence that resembles the absorption ratio of the spiraled MWCNTs/BaFe 12 O 19 hybrid. Hence, we conclude that the primary enhancement of microwave absorption of the spiraled MWCNTs/BaFe 12 O 19 hybrid samples is due to the dielectric loss of the composites. The bandwidth RL = − 10 dB (The RL = − 10 dB) is an indicator of 90% absorption of the EM wave as reported previously 32 . It is also well known that the enhancement of microwave absorption performance can mainly be ascribed to the good impedance matching ratio, high values of tan δɛ and tan δµ, and good compensation between the dielectric loss and magnetic tangent loss 33 . Based on our results, we found that the enhancement of microwave absorption abilities of the spiraled MWCNTs/BaFe 12 O 19 hybrid (10 wt%) composite has resulted in a maximum absorption loss of RL − 43.99 dB with a bandwidth of 2.56 GHz at a frequency of 12.96 GHz. The microwave absorption of the sample with a filler content of 10 wt% has been perceived to be increasing due to a suitable matching between the magnetic loss and dielectric loss with a strong attenuation characteristic 10 . The enhancement of microwave absorption properties of the spiraled MWCNTs/BaFe 12 O 19 hybrid originated from the good impedance matching between the magnetic and dielectric losses parameter 34 .
Besides, a normalised impedance of spiraled MWCNTs/BaFe 12 O 19 hybrid sample with 10 wt% filler content in the frequency range from 12 to 14 GHz is approximate to 1 (Fig. 12b). Meanwhile, a normalised impedance of sample with a filler content of 6 wt% and 8 wt% in the frequency range from 12 to 15 GHz and 11 GHz and 13.50 GHz respectively shows a nearly matched impedance which is approximately equal to 1 within their frequency range.
To further evaluate the microwave absorption properties of the spiraled MWCNTs/BaFe 12 O 19 hybrid, the attenuation constant (α) is introduced 35 : The attenuation constants of the samples versus frequency are shown in Fig. 12c. The attenuation constants of all hybrid sample are higher than that of pure BaFe 12 O 19 . Therefore, the hybrid samples exhibit higher microwave www.nature.com/scientificreports/ absorption performance due to their optimal impedance matching and larger attenuation constant. As predicted, the sample with a filler content of 10 wt% has the highest value of attenuation constant compared to other samples. Therefore, this sample is the best candidate for the electromagnetic wave absorption compared to others because of its excellent impedance matching features with the largest value of attenuation constant. Generally, the magnetic loss originated from the domain wall resonance and natural resonance 36 . At microwave frequencies, eddy current and ferromagnetic resonance are factors that contribute to magnetic loss. The magnetic loss comes only from the eddy current when C o is constant in the considered frequency range. The effect of eddy current coefficient (μ″(μ′) −2f.−1 ) is investigated by introducing the parameter C o = (μ″(μ′) −2f.−1 ) 36,37 . The values of C o versus frequency are shown in Fig. 13. The values of the quantity for all the samples fluctuated quite distinctly with frequency, except 4 wt%. These results indicated there is just a small contribution from the eddy current loss and it does not play a major role in the magnetic loss within this range of frequency. The C o for the 4 wt% samples decreased with frequency and showed a more stable behaviour beyond about 13 GHz, ascribing the phenomenon of eddy current effect. Therefore, that the resonance in all of the samples were principally due to natural ferromagnetic resonance within the frequency range. According to previous literature 38, 39 , the eddy  www.nature.com/scientificreports/ current loss contribution to the imaginary part of permeability is related to the thickness (d) and the electric conductivity (σ) of the composite: μ″ = 2πμ o (μ′) 2 d 2 fσ where μ o is the permeability of vacuum. If the magnetic loss only results from the eddy current loss, the values of (μ″μ′) −2f.−1 should be constant when frequency is varied 40 . Single semicircle (Cole-Cole semicircle) represents the presence of Debye relaxation (Fig. 14); in particular, a distorted semicircle is due to the combined effect of other loss mechanisms 41 . According to the characteristics of the curves, microwaves have multiple relaxations in BaFe 12 O 19 and spiraled MWCNTs/BaFe 12 O 19 hybrid. The result in Fig. 14 shows that the dielectric relaxation of BaFe 12 O 19 and spiraled MWCNTs/BaFe 12 O 19 hybrid samples could serve as the centre of polarisation and could provide dipole polarisation.
As summary, the mechanism of this microwave absorption can be ascribed to MWCNTs network. MWCNTs bear more defects and have more degrees of functionalisation. This defect structure, which has multiple bonding linkages, causes interfacial electric polarisation and thus energy dissipation in an alternating electromagnetic field by generating continuous current losses. Next, due to the discontinuity of energy states for the BaFe 12 O 19 nanoparticles on the surface of MWCNTs, the well-known quantum confinement effect allows electrons to hop from a lower energy state to a higher energy state and hence increasing microwave absorption. Then, for improved microwave performance is also from a suitable combination of epoxy, BaFe 12 O 19 nanoparticles and MWCNTs concentration with an adequate thickness which forms a multiple scattering network.