The effect of polymerization temperature and reaction time on microwave absorption properties of Co-doped ZnNi ferrite/polyaniline composites

This study presents the systematic potential effects of reaction parameters on the synthesis of Co-doped ZnNi ferrite/polyaniline composites prepared via novel interfacial polymerization. Through intensive experiments and analysis, optimum reaction conditions including the polymerization temperature and reaction time are proposed so that the performance of the material is significantly improved. The structure, functional groups and morphologies of composites are investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform-infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). In addition, the electromagnetic properties and microwave absorption properties of Co-doped ZnNi ferrite/polyaniline composites are examined by a vibrating sample magnetometer (VSM), Quantum Design (MPMS-VSM and MPMS-XL), the superconducting quantum interference device (SQUID) magnetometer and vector network analysis. Based on these analyses, it is found that by tuning the reaction conditions, i.e., polymerization temperature and reaction time, microwave absorption capabilities in terms of the maximum reflection loss (RL) value and absorber thickness can be readily optimized. The results show that the composite with an optimized polymerization condition of 20 °C for 12 h displays remarkable microwave absorption properties with maximum reflectivity of −54.3 dB, and the effective bandwidth (RL < −10 dB) is about 6.02 GHz at a thickness of 6.8 mm. Furthermore, the discussion shows that the promising microwave absorption may be due to the uniform urchin-like structure of the composites.


Introduction
In the modern world, with a growing number of wireless technologies in our daily life, scientists are becoming increasingly concerned about the consequences of electromagnetic (EM) pollution that has caused very serious health and environmental problems such as Lyme disease, chronic fatigue syndrome, and damage to other various human physiological systems. 1,2 Therefore, in recent years, high-performance EM wave absorbing materials having low thickness, wide bands, light weight, and strong absorption properties have been intensively studied. 3 The EM wave can be absorbed by some organic/inorganic materials (such as graphene, carbonyl iron powders, etc.) through dielectric (and magnetic) losses, and the material can exhibit the best EM wave absorption while satisfying the impedance match. 4 Among the various EM wave absorbing materials, spinel ferrites have been widely explored due to their excellent magnetic spectrum and thin absorbing layers. 5 Spinel ferrites belong to the cubic crystal system with high symmetry and small anisotropy of magnetic crystals. They are typically represented as MeFe 2 O 4 (Me ¼ Zn 2+ , Ni 2+ , Co 2+ , Mg 2+ , etc.). 5 The EM wave absorption properties of spinel ferrites are not only related to their elemental composition, but are also affected by their microstructure, particle size, and other factors. 6 For example, Xie et al. 7 synthesized (Ni 0.407 Co 0.207 Zn 0.386 )Fe 2 O 4 ferrite that exhibited an effective absorption frequency band (reection loss below À10 dB) from 8.64 to 11.2 GHz. In another report, Sozeri et al. 8 studied Mn-Co-substituted Ni-Zn ferrite nanoparticles with a simple format Ni x Zn 0.8Àx Mn 0.1 Co 0.1 Fe 2 O 4 (0 # x # 0.8), and the results showed that Ni 0.6 Zn 0.2 Mn 0.1 Co 0.1 Fe 2 O 4 has the best EM wave absorption properties with the maximum reection of À25 dB at frequency 10 GHz. In that study, Ni-Zn-Co ferrite was used due to its high magnetic loss, and EM wave was absorbed by eddy current loss, hysteresis loss and natural resonance. Compared with Ni-Zn ferrite, the doped Co 2+ ferrite could increase the saturation magnetization strength, coercivity, and dielectric constant. 9 However, issues such as narrow EM wave absorption bands and high density limit their application potential in electronic communication and radar stealth industries. One of the effective ways to solve such problems is to combine conductive polymer materials with ferrites as they have low density, a strong designed structure, easy processed form, and unique electrical properties.
Polyaniline (PANI) is a typical dielectric material among conducting polymers known for its excellent environmental stability, facile synthesis condition, and controlled electrical conductivity. 10 The conductive PANI/ferrite composites have increasingly attracted more attention because of the synergistic effect between PANI and ferrite. 11,12 For example, Ting et al. 13 synthesized an NiZn ferrite material coated with different ratios of PANI, which showed that a wider EM wave absorption band could be achieved by adding various ratios of PANI in the frequency range of 2-40 GHz. Wang et al. 14  It is well-known that pure PANI is almost an insulator, but appropriate acid doping can improve its conductivity. In addition, interfacial polymerization has many advantages as compared with traditional in situ polymerization. For instance, the polymerization occurs at the interface of two dissolvable solvents because aniline monomers can only contact the oxidant on the interface and subsequently, the reaction occurs. Upon formation of hydrophilic PANI nanobers, they can quickly leave the interface and diffuse to the aqueous phase, thus avoiding the secondary growth of nanometer bers. In this context, the current report proposed facile synthesis of Zn 0.4 Ni 0.4 Co 0.2 Fe 2 O 4 (ZNCF) particles via the cost-effective sol-gel method. Thereaer, ZNCF/PANI nanocomposites were prepared by the interfacial polymerization procedure. In addition to this, to the best of our knowledge, this is the rst time that the effects of polymerization temperature and reaction time on EM wave absorption properties are explored. Furthermore, the mechanism of enhanced EM absorption behavior is discussed in detail on the basis of structural, morphological, and electromagnetic properties.  16 A typical ZNCF preparation process is as follows: rst appropriate stoichiometric ratios of all nitrates were dissolved in 120 mL deionized water with constant stirring for 5 min, followed by quantitative citric acid accession under continuous stirring for another 10 min. Then, NH 3 $H 2 O was added to the above solution with continuous stirring to adjust the pH value to 7. The suspension was then poured into a dry glass-beaker and heated in an oil-bath at 80 C for 7 h to form a gel-state mixture. Subsequently, two heat treatments were used to relax the mixture: it was dried in an oven at 130 C for 11 h and heated at 210 C for 2 h. Finally, the material was annealed at 1090 C for 2 h and cooled in the air. ZNCF was used for further preparation.

Synthesis of PANI/ZNCF composites
In a typical procedure, 0.2 g aniline (An) was dissolved in 50 mL carbon tetrachloride (CCl 4 ) with stirring for 20 min to form a solution, which was named solution A. Then, ratable (NH 4 ) 2 S 2 O 8 (APS) and ZNCF (molar ratio of the APS/aniline was 1 : 1) were added to 50 mL distilled water, which was named as solution B, along with continuous stirring for 10 min. There-aer, appropriate content of HCl (1 mol L À1 ) was dissolved in the above solution B with stirring for 20 min at room temperature. Polymerization started aer the dropwise addition of solution B onto the surface of solution A, resulting in the formation of layered solutions under controlled reaction times and temperatures, which are listed in Table 1. Finally, the suspension was ltered and cleaned with distilled water until the lter liquor became clear, followed by drying in a drying cabinet at 60 C for 24 h. The schematic diagram of the preparation of PANI/ZNCF composites is shown in Fig. 1.

Characterization
Heteroatom functional groups and element identication were characterized by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI). X-ray diffraction (XRD) of prepared samples was performed by using a Bruker D8 X-ray diffractometer with Cu Ka radiation (l ¼ 0.15406 nm) in the 2q range (2q ¼ 10-80 ) followed with a scanning rate of 2 min À1 . The chemical bonds and functional groups of samples were analyzed by Fourier transform-infrared (FT-IR) spectra in the range of 2000-400 cm À1 using an infrared spectrophotometer  The test circular ring was composed of wax and samples with a mass ratio of 7 : 3 following an articial hot pressing progress in the mold, and it was cooled at room temperature. Finally, the test ring with inner diameter 3.01 mm, outer diameter 7.02 mm and thickness 2 mm was obtained.   Ni 2p 1/2 and Ni 2p 3/2 peaks at 873.1 eV and 854.9 eV, respectively, indicate the presence of Ni 2+ in the system (Fig. 2c). In the Co spectra (Fig. 2d), the binding energies of 796.5 eV and 780.6 eV are assigned to Co 2p 1/2 and Co 2p 3/2 , respectively. 17 The presence of Fe in the samples is conrmed by the peaks at 724.5 eV and 711.3 eV, which are assigned to Fe 2p 1/2 and Fe 2p 3/2 , respectively, and related to Fe 3+ ions in tetrahedral sites (plots 1 and 3 in Fig. 2e). 20 Moreover, curves 2 and 4 show binding energies of 726.7 eV and 714.2 eV, which are assigned to Fe 2p 1/2 and Fe 2p 3/2 and related to Fe 3+ ions in octahedral sites. 7,19,20 The O 1s spectrum (Fig. 2f) can be resolved into two peaks centered at 531.6 eV and 529.8 eV, which are assigned to the surface hydroxyl and oxygen groups, respectively, in the composite. 21,22 The analysis of the N 1s spectrum (Fig. 2g) reveals two binding energies: 400.1 eV (curve 2) and 399.3 eV (curve 1). The binding energy in curve 2 is assigned to the benzene-diamine units, whereas that in curve 1 is due to the quinine-diimine units. 23 The C 1s spectrum (see Fig. 2h) can be resolved into four peaks at different binding energies. The curve 1 with a peak at 284.3 eV corresponds to C-C or C-H bonds. The curve 2 shows a peak at 285.6 eV, which is assigned to C-N or C]N bonds, and the third one (curve 3) at 286.5 is due to the C-O bond; the forth one (curve 4) at 287.5 corresponds to C]N + or C]O bonds. 18,23 The XRD patterns of the obtained PANI/ZNCF composites are shown in Fig. S1. † The analysis of XRD patterns indicates the presence of typical spinel cubic-structure ferrites in the composites. The FT-IR spectrum of pure PANI, ZNCF and their composites is shown in Fig. 3. The spectrum of ZNCF indicates two characteristic peaks, whereas the peaks at 590 and 410 cm À1 can be assigned to the coupling between metal and oxygen (M-O) stretching modes of the spinel structure. 13,17 The peaks at 1587 and 1496 cm À1 indicate the C]C bond of the benzenoid ring and C]N bond of the quinoid ring, respectively. 24 The characteristic peaks at 1306 and 1233 cm À1 indicate the C-N stretching vibration of benzenoid ring. 24,25 The distinct peak at 1150 cm À1 is described as"electronic-like band", 26 which is assigned to the N ¼ Q ¼ N mode (Q on behalf of the quinonictype rings). The PANI/ZNCF composites exhibit both PANI and ZNCF characteristic peaks, whereas there is a clear shi at 564 cm À1 because the PANI chains are tightly covered with ZNCF. 27 This result is consistent with the XRD results, indicating that the PANI/ZNCF composites are successfully synthesized.

Results and discussion
To investigate the effect of temperature on polymerization of PANI/ZNCF composites, SEM results of ZNCF and composites synthesized at 0 C, 20 C and 40 C for 12 h are obtained. Fig. 4a shows the SEM micrograph and EDS spectrum of ZNCF, where ZNCF particles exhibit a cubic-like structure with a smooth surface, and they are tightly bound because magnetic particles attract each other. 11 The diameter of the ZNCF granules ranges from 400 nm to 800 nm. The EDS spectrum indicates that Zn, Ni, Co, and Fe exhibit 2 characteristic peaks, which are consistent with XRD results. As seen in Fig. 4b, PANI molecular chains agglomerate and disperse unevenly on the surface of ZNCF particles, due to which some ZNCF particles are exposed to air, which causes negative effect on the absorption of electromagnetic waves. 21,22,28 Fig. 4c and d exhibit similar urchin-like structures that are ZNCF granules covered with uniformly distributed PANI chains and a number of salient forms on the surface of particles. However, some accumulation of PANI chains is observed for both samples, and sample T-3 agglomerates seriously because the molecular thermodynamic movement is promoted with the increase in temperature.
Compared with the above samples, the sample dried at 20 C (T-2) has the most homogeneous stable structure, which has a signicant effect on its performance. To conrm the optimum reaction conditions, a series of contrast experiments under different reaction times at 20 C are carried out.
Morphological characteristics of formed PANI/ZNCF composites under different reaction times (4 h, 8 h, 12 h, 16 h, 20 h, and 24 h) can be observed in Fig. 5. It can be seen that PANI/ZNCF composites exhibit a rough surface, which demonstrates that the composites are compounded successfully by interfacial polymerization. With increasing reaction time (Fig. 5a-c), the ZNCF particles are wrapped by more PANI chains on the surface. As shown for sample D-1 (Fig. 5a), there are few PANI chains on ZNCF; on the other hand, the polarization between PANI chains induces strong agglomeration, which leads to the smooth surface of ZNCF. 13,29 Samples D-2 and T-2 ( Fig. 5b and c) exhibit similar urchin-like structures, but the PANI chains of T-2 disperse better. Furthermore, aer increasing the reaction time for D-4, D-5, and D-6 ( Fig. 5d-f), the agglomeration increases to a great extent. Low-molecularweight PANI chains polymerize into larger spheres (the diameter of the PANI particles is increased from 300 nm to 700 nm), which results in ZNCF particle formation. In this study, the PANI/ZNCF composite prepared with the reaction conditions of 12 h and polymerization temperature of 20 C has a uniformly distributed urchin-like structure, which can be helpful to improve EM wave absorption properties.
The structure and morphology of sample T-2 are investigated by TEM, and the results are shown in Fig. 6. As shown in Fig. 6a and b, it is clear that some brous PANI chains cover ZNCF To further conrm the crystalline structure of ZNCF, HRTEM image of PANI/ZNCF composites is presented in Fig. 6c and d. Clear lattice fringes can be observed, and the lattice fringe spacing of ZNCF particles is approximately 0.268 nm, corresponding to the (311) plane of ZNCF (Fig. S1 †). 21 The inset shows the SAED pattern of sample T-2, and the (311) and (400) planes can be observed.
Hysteresis loops of PANI/ZNCF composites under different reaction temperatures are shown in Fig. 7a. The samples ZNCF, T-1, T-2, and T-3 exhibit ferromagnetic behaviors with saturation magnetization values of 81.1, 42.1, 38.1, and 35.5 emu g À1 , respectively. The coercivity of all samples is negligible, which can be used in so magnetic eld. 21,27 The saturation magnetization of ZNCF is much higher than that of composites owing to magnetism that is reduced by additional nonmagnetic PANI. 30 Furthermore, Fig. 7b shows the magnetic properties of samples ZNCF, D-1, D-2, T-2, D-4, and D-6 with saturation magnetization values of 81.1, 45.6, 41.2, 38.1, 23.3, and 18.1 emu g À1 , respectively. A possible explanation is that ZNCF is covered with increasing nonmagnetic PANI chains, leading to decreased magnetic properties. Besides, there may be a critical value of additional reaction time for which saturation magnetization is almost unchanged. 31 The coercivity values of these samples are low ($10-20 Oe). Fig. 8 shows the magnetization of sample T-2 versus temperatures in an applied eld (100 Oe). The result shows a clear transformation phase from magnetic order to disorder from 300 K to 700 K. The inset shows that the composite has a Curie temperature of about 593 K. The sample T-2 shows relatively stable magnetization at low temperatures (300-400 K). A similar result has been reported by Li et al. 32 for Ni 0.4 Zn 0.5 -Co 0.1 Fe 2 O 4 ferrite prepared by the sol-gel method. However, in this study, the Curie temperature of the composite is higher than that reported, which may be due to different ratios of ferrite. Fig. 9a and b show the real (3 0 ) and imaginary part (3 00 ) values of complex permittivity under different polymerization temperature conditions in the frequency range of 2-18 GHz. It is well-known that 3 0 represents the storage and 3 00 represents the loss ability of electric energy. The 3 0 value of ZNCF is almost unchanged, and the 3 00 value is fairly low (approaching almost zero); this indicates that ZNCF is not a dielectric lossabsorbing material. 33 Compared with ZNCF, PANI/ZNCF composites exhibit relatively better dielectric properties due to the doped PANI chains. A number of p-p conjugated bonds found in PANI are favorable to decrease collision of electrons. 12 According to the free electron theory, more conductive PANI chains increase the complex permittivity value. 34,35 As shown in Fig. 9a and b, sample T-2 exhibits higher 3 0 and 3 00 values compared with T-1 and T-3, conrming that dielectric properties can be inuenced by reaction temperature and there exists an optimal temperature (20 C). According to the authors, the urchin-like coated structures of PANI/ZNCF composite and graed PANI chains offer extra contact surfaces and junctions as compared with other composites. 36 The dissipation factors varying with frequency can be seen as dielectric loss tangent (tan d 3 ¼ 3 00 /3 0 ) (Fig. 9c). The T-2  composite has the maximum tan d 3 value. Meanwhile, the values of tan d 3 for three PANI/ZNCF composites are greater than that for pure ZNCF. Fig. 9d and e exhibit the real part (m 0 ) and imaginary part (m 00 ) values of complex relative permeability under different polymerization temperature conditions (frequency ranges from 2 to 18 GHz). As shown in Fig. 7d, the m 0 values of T-1 and T-3 are slightly lower than that of ZNCF, whereas T-2 exhibits higher m 0 value as compared with ZNCF in frequency range of 2-7 GHz. Fig. 9e demonstrates decreased trends of m 00 values with the increase in frequency. Sample T-2 exhibits higher m 00 , which represents the best magnetic loss. However, the m 00 values of T-1 and T-3 are lower than that of ZNCF owing to excessive or minimal non-magnetic PANI chains that reduce the magnetism of the composites. The tan d m value is calculated based on the m 0 and m 00 values (tan d m ¼ m 00 /m 0 ) (Fig. 9f); as seen in this gure, changing the polymerization temperature has clear effects on the magnetic loss of PANI/ZNCF composites. The value of tan d m for sample T-2 is greater than that of ZNCF in the range 2-12 GHz, which ascribes to the charge transfer between the ZNCF  surface and PANI. 37 Sample T-2 exhibits best tan d m as compared with other composites. Besides, the comparison of all the samples indicates that sample T-2 exhibits similar tan d m and tan d 3 values, which can increase the degree of impedance matching and promote the EM wave absorption properties of materials. Fig. 10 shows complex permittivity and complex permeability of ZNCF and its composites under different reaction times in the frequency range of 2-18 GHz. As shown in Fig. 10a and b, sample D-6 has a relatively outstanding 3 0 value, which decreases from 6.7 to 4.8, and the 3 00 value decreases from 2.8 to 0.7 in the frequency range from 2 to 18 GHz. It is reasonable to show that the different dielectric behaviors of this sample are related to its special morphology and structure. Fig. 10c shows the dielectric loss (tan d 3 ) of composites. All composites show similar tan d 3 values, exhibiting that the change in reaction time has negligible effects on dielectric loss. Sample D-6 has a slightly higher value of tan d 3 compared with others in low frequency, which may contribute to microwave absorption. Fig. 10d and e show m 0 and m 00 of PANI/ZNCF composites. It can be seen that both values of m 0 and m 00 for all samples exhibit decreased tendencies with increased frequencies. For ZNCF particles, m 0 and m 00 retain the relatively little decrease trends as compared with that for PANI/ZNCF composites in the frequency range of 2-18 GHz, and m 0 of ZNCF is higher than those of other samples in the range of 7-14 GHz. This specic phenomenon is due to the addition of PANI, reducing the ability of magnetic energy. 38 m 00 generally represents the loss ability of the magnetic material. m 00 of ZNCF is lower than that of the composites in the frequency range of 2-11.5 GHz, but it is higher in the range of 11.5-18 GHz. Nearly all PANI/ZNCF composites exhibit similar decreased tendencies of m 0 and m 00 except samples D-1 and D-6, which show prominent peaks in the range of 10-11 GHz. Fig. 10f shows tan d m of composites. It can be seen that changing the reaction time has negligible effects on magnetic loss for PANI/ZNCF composites. Sample D-1 and D-6 have narrow resonance peaks in the range of 10-12 GHz. Samples D-2 and T-2 exhibit better electromagnetic loss tangent values compared with other composites, which can promote the absorption of EM wave.
The complex relative permeability and magnetic loss tangent (tan d m ) exhibit clear decrease in the frequency range of [11][12][13][14][15][16][17][18] GHz, which may be due to exchange resonance, dimensional resonance, and eddy current resonance. 19,39 For spinel ferrites such as ZNCF, exchange resonance has negligible contribution to the magnetic loss in the high frequency range (11)(12)(13)(14)(15)(16)(17)(18). 16,18 Moreover, the dimensional resonance can be explained as follows: 19,38 here, d is the thickness of sample, and l is the wavelength of the electromagnetic wave entering the samples. From eqn (1) and (2), it can be found that the wavelength decreases with the increase in frequency. If the physics origin of the peaks arises from the dimensional resonances, the calculated d should be larger than 3 mm, which is much larger than the real sample thickness of about 2 mm. Hence, the dimensional resonance can be excluded. 19 If the magnetic losses of composites are caused by eddy current resonance, it can be expressed by the equation: 16,19 C here, m 0 is the permeability of vacuum, s is the conductivity and C 0 is the eddy current coefficient. The change in C 0 represents that the magnetic loss is not induced by the eddy current loss. 18,19,30 As can been seen in Fig. 11, the decrease in C 0 at 2-14 GHz indicates that the magnetic loss in this region is not caused by eddy current resonance. When the frequency is higher than 14 GHz, C 0 tends to be stable, proving that the magnetic loss in this region is mainly caused by eddy current resonance.
The microwave absorption properties of as-synthesized PANI, ZNCF and PANI/ZNCF composites can be dened as the reection loss (R L ), which can be simulated by the complex permittivity and permeability at set thickness according to the transmission theory, as shown in the following equations: 40,41 here, the input impedance Z in of the absorber is given by the following equation: here, the velocity of EM waves in free space is c, d is the thickness of absorbent layer and f is the microwave frequency. The calculated R L values of composites in the frequency range of 2-18 GHz with varied absorber thicknesses of 1-10 mm under different preparation temperatures with 12 h reaction time are shown in Fig. 12a-d. Usually, when R L < À10 dB, it results in efficient microwave absorption, which can be used in actual applications. The absorption peak of ZNCF only reaches À17.5 dB with a thickness of 8.9 mm, and the effective Fig. 11 The eddy current data of pure ZNCF and PANI/ZNCF composites with different polymerization temperatures and reaction times.
bandwidth (R L < À10 dB) is about 2.67 GHz. Compared with ZNCF, PANI/ZNCF composites have better microwave absorption properties and especially, the composite prepared at temperature 20 C is the best. The sample T-2 has an excellent absorption peak, which reaches À54.3 dB with a thickness of 6.8 mm, and the effective bandwidth is about 6.02 GHz. The maximum peak of the T-1 composite reaches À26.3 dB with a thickness of 7.7 mm, and the peak for the T-3 composite reaches À17.2 dB with a thickness of 8.9 mm. Clearly, the urchin-like structure of T-2 has an active effect on the EM wave absorption property. Fig. 12(e-i) show the EM wave absorption properties of PANI/ ZNCF composites with different reaction conditions in terms of preparation time. It is clear that samples D-1, D-2 and T-2 exhibit better microwave absorption properties, and they display decreased trends with the increase in reaction time. The effective bandwidth of sample D-1 is 6.11 GHz, and the maximum R L reaches À32.2 dB with a thickness of 7.7 mm. The optimal absorption peak of the D-2 composite can reach À44.1 dB at 15.1 GHz with a thickness of 6.8 mm, and the effective bandwidth can reach 5.71 GHz. Sample T-2 has the best EM wave absorption properties compared to others. From Fig. 12, we can see that the maximum R L shis to the low frequency region with increasing thickness of the sample. Usually, the thicker the sample, the higher the R L value for all frequencies.
Thus, it is necessary to nd a balance between thickness and microwave absorption properties. 42 The excellent microwave absorption properties of PANI/ ZNCF composites can be explained as follows (Fig. 13). First, perfect EM wave absorption properties depend on the impedance matching characteristics of composites, which are inuenced by permittivity and permeability. 7,8 If there are clear differences between the values of permittivity and permeability, the EM wave can be reected from the surface of the composites. In contrast, the EM wave can pass through the surface with slight reection and then, it enters into the composites with strong absorption. 7,24 It is well-known that PANI mainly exhibits dielectric loss, whereas ZNCF mainly exhibits magnetic loss, which can be helpful for the impedance matching of their respective composites. Second, because of the large differences of polarity or conductivity between PANI and ZNCF, the electrons or ions in dielectric medium can concentrate at interfaces and exhibit interfacial polarization under the effect of an external electromagnetic eld. 12,13 Moreover, there are a number of gaps and defects in PANI molecular chains, which can introduce dipole polarization. 18,20,25 The dipole polarization and interfacial polarization can enhance the electromagnetic loss and improve the EM wave absorption. Third, the PANI chains can form a conductive network due to their reasonable conductivity and intensive distribution, which can be benecial for the transfer of the EM wave. 19 Finally, when the EM wave passes through the composites, the dense distribution of ZNCF particles and PANI/ZNCF particles can result in multiple scattering and reection, nally enhancing EM wave absorption. Therefore, the control of PANI chains onto the surface of ZNCF particles is an effective way to enhance microwave absorption applications.

Conclusion
Novel Zn 0.4 Ni 0.4 Co 0.2 Fe 2 O 4 graed with PANI is synthesized via the interfacial polymerization method. The XRD and FT-IR results conrm that both NZCF and PANI coexist in PANI/ ZNCF composites. The results show that the composites with polymerization at 20 C for 12 h display excellent EM wave absorption properties; the effective bandwidth (R L < À10 dB) is 6.02 GHz, and the maximum R L reaches À54.3 dB with a thickness of 6.8 mm. The enhanced microwave absorption properties of PANI/ZNCF composites are mainly related to a special urchinlike structure, which contributes to high dielectric loss and improved impedance matching. These PANI/ZNCF composites with enhanced properties can be efficiently applied in microwave absorption applications.

Conflicts of interest
There are no conicts to declare.