Lanthanum and Neodymium Doped Barium Ferrite-TiO2/MCNTs/poly(3-methyl thiophene) Composites with Nest Structures: Preparation, Characterization and Electromagnetic Microwave Absorption Properties

We report herein the synthesis of a novel nest structured electromagnetic composite through in-situ chemical polymerization of 3-methyl thiophene (3MT) in the presence of the BaFe11.92(LaNd)0.04O19-TiO2 (BFTO) nanoparticles and MCNTs. As an absorbing material, the BFTO/MCNTs/P3MT/wax composites were prepared at various loadings of BFTO/MCNTs/P3MT (0.2:0.10:1.0 ~ 0.2:0.30:1.0), and they exhibited strong microwave absorption properties in the range of 1.0–18 GHz. When the loading of BFTO/MCNTs/P3MT is 0.2:0.30:1.0, the composite has a strongest absorbing peak at 11.04 GHz, and achieves a maximum absorbing value of −21.56 dB. The absorbing peak position moves to higher frequencies with the increase of MCNTs content. The mechanism for microwave absorption of these composites has been explained in detail.

Characterization and electromagnetic properties measurement. The morphology, structure and properties of samples were characterized by various techniques. Fourier transform infrared (FTIR) spectra were carried out using Nicolet 5700 FTIR with a KBr method. X-ray diffraction (XRD) patterns of the samples were characterized using a Philps-pw3040/60 diffractometer with Cu Kα radiation (λ = 0.15418 nm). Differential thermal analysis-thermo gravimetry (DTA-TG) analysis was performed at a heating rate of 10 °C in nitrogen on SDTQ 600. The morphology and particle sizes of the samples were characterized by a Hitachi H-800 scanning electron microscope (SEM) and a JEOL JEM-1200EXII transmission electron microscope (TEM). Vector Network Analyzer (HP-8722ES) was used to get S-parameters for the samples of composites in the range of 1-18 GHz at room temperature. The values of complex permittivity (ε ) and permeability (μ ) of the composite materials were calculated from the measured values of S-parameters. The reflection loss of the single layer sample was calculated using the measured electromagnetic parameters.

Results and Discussion
Polymerization. Figure 1 illustrates the preparation process of the BFTO/MCNTs/P3MT composites. Firstly, MCNTs are refluxed in concentrated HNO 3 solution at 90 °C for 5 h to remove the residues of metal catalysts; such treatment could produce a large number of carboxylic groups on the surface of MCNTs that benefit for absorbing the BFTO nanoparticles. Then, Cl − is absorbed onto the surface of BFTO nanoparticles by the electrostatic attraction. 3MT monomers are attracted by Cl − through the electrostatic effect and uniformly distributed over the surface of MCNTs through п-п stacking. Finally, the target products with a nest structures are obtained by in-situ chemical polymerization of 3MT with the (NH 4 ) 2 S 2 O 8 as an initiator. XRD analysis. The diffraction patterns of the samples are presented in Fig. 2. Figure 2a shows the characteristic diffraction peaks of the BFTO. The peaks at 2θ = 25.2°, 33.6° and 36.1° are ascribed to the characteristic diffraction peaks of BaFe 11.92 (LaNd) 0.04 O 19 20,21 . And the peaks at 2θ = 17.9°, 25.5° and 33.6° are attributed to the characteristic diffraction peaks of TiO 2 22 . The typical XRD pattern of P3MT (Fig. 2c) presents two broad diffraction peaks centered at 2θ = 14.5° and 26.5° with shift slightly 23 , which can be ascribed to the intermolecular π -π stacking emerges. Figure 2(b) shows the XRD pattern of the BFTO/MCNTs/P3MT composites, which contains the characteristic diffraction peaks of BFTO, MCNTs and P3MT. It should be noted that the intensity of characteristic diffraction peaks of P3MT in the composites is weaker compared to the pristine P3MT, which can be attributed to the interactions among BFTO, MCNTs and P3MT and the nest structures of the composites. Figure 3 shows the FTIR spectra of BFTO, BFTO/MCNTs/P3MT and P3MT, respectively.

FTIR analysis.
For P3MT (Fig. 3a), two peaks in the range of 2750-3000 cm −1 are attributed to the characteristic C-H stretching vibrations and the peak at 1640 cm −1 is assigned to C = C stretching vibrations. The peak at 784 cm −1 is assigned to the C-H out-of-plane vibrations of the 2, 5-substituted thiophene ring created by the polymerization of thiophene monomers. The peak at around 692 cm −1 denotes the C-S stretch in the thiophene ring 24 . For BFTO (Fig. 3c) band, respectively 22 . Figure 3b shows the BFTO/MCNTs/P3MT FTIR spectra, which is almost identical to that of P3MT. However, to some extent, the spectra of the P3MT in the composites appear slightly blue shift. In addition, the intensity of the peak at 696.5 cm −1 becomes weaker compared with Fig. 3c. The peaks at 1112.5 cm −1 and 1627.4 cm −1 are attributed to the MCNTs' characteristic absorption peaks with slightly blue shift compared to the literature 25 , suggesting that the BFTO and MCNTs are well coated by P3MT chains. Because there exists some interactions among them in the composites, which decreases the electron density and reduces the atomic force constant. These above results confirm that composites are composed of the P3MT, BFTO and MCNTs.
DTA-TG analysis. DTA-TG analysis of BFTO/MCNTs/P3MT and P3MT and are shown in Fig. 4. The weight loss of the two samples can be divided into three stages. For the P3MT (Fig. 4a), the first stage is assigned to the loss of water and other volatiles at lower temperature (lower than 110 °C). The second stage above 197 °C can be attributed to the thermal degradation of the P3MT chains and volatilization of the oligomer. The third stage of  P3MT is starting at 500 °C. The TG curve (Fig. 4b) indicates that the decomposition temperature of the BFTO/ MCNTs/P3MT composites is about at 300 °C, higher than that of pure P3MT. The third weight loss of the composites starts at 610 °C, indicating that the stability of the composites is better than that of P3MT. The improved stability may be resulted by the interactions among the P3MT, BFTO and MCNTs or the nest structures of the BFTO/MCNTs/P3MT composites.    composites have an irregular and similar nest structure. The introduction of hydrochloride can increase the polarity of P3MT, resulting in the increase of intermolecular force. Figure 5b shows the TEM images of BFTO/MCNTs/P3MT. There is a typical tube morphology of MCNTs which are conjugated with BFTO nanoparticles. In addition, both MCNTs and BFTO nanoparticles are coated by P3MT. Electronic diffraction pattern indicates that the black core is BFTO, because only BFTO composite is crystal material in the BFTO/MCNTs/P3MT composites. BFTO is absorbed onto the surface of the MCNTs. These results confirm that the composites are composed of polycrystalline BFTO, MCNTs and P3MT, being in accordance with the results of FIRT and XRD analysis.

Electromagnetic parameter analysis.
To investigate the electromagnetic wave absorption properties of the BFTO/MCNTs/P3MT, various contents of the as-prepared powder was mixed with wax (the mass ratio is 7:3) to form the BFTO/MCNTs/P3MT/wax composites by a hot press process. Figure 6a-d shows the real and imaginary parts of the complex permittivity and permeability measured for the composites with different mass ratio of the BFTO/MCNTs/P3MT in the range of 1.0-18 GHz. As shown in Fig. 6a,b, the real (ε ′ ) and imaginary (ε ″ ) parts of the permittivity obviously reduce first, then slightly increase and then decrease with the increase of MCNTs content. The ε ′ value of above the BFTO/MCNTs/P3MT/wax decreases with increasing frequency in the range of 1.0-18 GHz. However, the changes of ε ″ values were very complicated with the different contents of MCNTs in the composites. The ε ″ values of the composites (with a loading of the BFTO/MCNTs/P3MT Compared with higher complex permittivity, the complex permeability of the BFTO/MCNTs/P3MT/wax is very lower. It indicates that the magnetic loss contribution from BFTO/MCNTs/P3MT to microwave absorption is minor. The magnetic loss angle tangent (tanδ μ = μ ″ /μ ′ ) was been calculated and shown in Fig. 6f. The values of the magnetic tangent loss show very small fluctuation between 0.01 and 0.24. In addition, for all the composites with different loading of BFTO/MCNTs/P3MT, the dielectric loss tangent (tanδ ε ) is larger than the magnetic loss tangent (tanδ μ ). Thus, for every composites, the dielectric loss is the main contribution for microwave absorption. where ε r and μ r are the complex permittivity and permeability, respectively; R L is a ratio of reflected power to incident power in dB, Z in is the input impedance of absorber, d is the thickness of the absorber, c is the velocity of light, f is the frequency of microwave.  Fig. 6, we can find that the lower concentration of MCNTs in BFTO/MCNTs/P3MT exhibits higher dielectric loss and results in a worse reflection. When the MCNTs content ratio increase from 0.10-0.30, the dielectric loss of composites decreases inversely and possesses better reflection, which may be explained by the percolation theory 29 . The percolation behavior of composites corresponds to a phase transition from an conducting state to an insulator state around the percolation point. In our BFTO/MCNTs/P3MT/wax composites with 0.2:0.10:1.0, the MCNTs network enhances the electrical conductivity of the composite and this leads to a high leakage current, which may cause damage to the microwave absorption of materials 38 . In addition, impedance match characteristic is an important concept for the microwave absorption. High permittivity of absorber is harmful to the impedance match and results in weak absorption 33 . Figure 7f shows the theoretical R L of the composites with different BFTO/MCNTs/P3MT loading in the frequency range of 1.0-18 GHz at a thickness of 2.0 mm. It can be seen that the loading of BFTO/MCNTs/P3MT have a great influence on the microwave absorbing properties and the minimum R L corresponding to the maximum absorptions gradually appeared in different frequency shifts toward to higher frequency with the increase of MCNTs contents. When the loading of the BFTO/MCNTs/P3MT is 0.1:0.30:1.0, the minimum R L can be achieved to − 21.56 dB at 11.04 GHz, and the bandwidth of R L less than − 10 dB can reach up to 3.25 GHz (from 9.50-12.75 GHz). This relatively winder bandwidth may be ascribed to the unique interfaces among BFTO, MCNTs and P3MT, as well as the excellent dielectric properties of MCNTs and P3MT. In addition, the inorganic/ organic interfaces between BFTO/MCNTs/P3MT and wax, the synergistic effect between different components in the BFTO/MCNTs/P3MT/wax composites may also be important factors for enhanced microwave absorption performance. It can be obviously observed that the reflectivity peak position moves to higher frequency and the microwave absorption property becomes stronger with the increase of the MCNTs contents loading. These results indicate that the absorption peak positions and frequency ranges (minimum R L less than − 10 dB) can be manipulated easily by adjusting the MCNTs concentrations in the BFTO/MCNTs/P3MT, and thus a broadband absorption can be designed using a multilayered absorbing structure.

Conclusion
The BFTO/MCNTs/P3MT composites have been prepared by in situ polymerization of P3MT in the presence of BFTO and MCNTs. The electromagnetic and microwave absorption properties of the BFTO/MCNTs/P3MT/ wax composites with different MCNTs loading have been investigated. With different MCNTs content loading, there is a percolation phenomenon for dielectric loss. The higher MCNTs concentration leads to a lower dielectric loss. When the BFTO/MCNTs/P3MT is 0.2:0.30:1.0, the composite shows the best microwave absorption with − 21.56 dB at 11.04 GHz. For all the composites, the main contribution for the microwave absorption comes from the dielectric loss rather than the magnetic loss. Considering the absorption peak could be easily adjusted by changing the MCNTs concentration, these composite materials possess a great potential application for broad bandwidth microwave absorption.