Coprecipitation Synthesis of Fe-doped ZnO Powders with Enhanced Microwave Absorption Properties

Transcription

1 NANO: Brief Reports and Reviews Vol. 11, No. 12 (2016) (12 pages) World Scienti c Publishing Company DOI: /S Coprecipitation Synthesis of Fe-doped ZnO Powders with Enhanced Microwave Absorption Properties Ruiwen Shu*, Xin Wang, Yingying Yang, Xiayu Tang, Xian Zhou and Yangfan Cheng School of Chemical Engineering, Anhui University of Science and Technology Huainan , P. R. China *austshuruiwen@126.com NANO Downloaded from Received 26 June 2016 Accepted 30 August 2016 Published 14 October 2016 In this work, the Fe-doped ZnO powders have been synthesized by a facile chemical coprecipitation method. The structure, morphology and magnetic properties of the as-prepared powders were characterized by X-ray di raction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectrum (EDS), X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometer (VSM). The results showed that the Fe ions were well incorporated into the crystal lattice of ZnO and had a valence state of þ3. The magnetization curves indicated the Fe-doped ZnO presented the ferromagnetic behavior at room temperature. Moreover, the electromagnetic (EM) parameters and microwave absorption properties of Fe-doped ZnO/para±n wax in the frequency range of 2 18 GHz were explored. The minimum re ection loss reached 38:4 db at 6.6 GHz, and the re ection loss less than 10:0 db was 4.0 GHz (from 11.0 GHz to 15.0 GHz) with a thickness of only 2.5 mm. Signi cantly, the enhanced microwave absorption of the as-prepared powders could be achieved by doping with Fe 3þ ions or varying the thickness of the absorbers. The mechanism of microwave absorption were attributed to the good impedance matching, the dielectric loss resulted from the crystal lattice defects and the magnetic loss originated from the natural resonance. It is believed that the Fe-doped ZnO powders could be used as potential microwave absorbers. Keywords: ZnO; doping; coprecipitation; ferromagnetic; microwave absorption. 1. Introduction Microwave absorption materials have attracted much attention due to the increasingly electromagnetic (EM) pollution problems, which not only in uence the performance of electronic devices, but may also be harmful to the health of human beings. 1 3 In the past decades, extensive e orts have been made to develop EM wave absorption materials that meet the requirements of strong absorption, wide frequency band, thin thickness and light weight. 4 8 Zinc oxide (ZnO) is one of the most important n-type semiconductor with a wide bandgap of 3.37 ev at room temperature. 9,10 It exhibits many excellent properties, such as good chemical stability, low cost, high optical transparency and electrical * Corresponding author

2 NANO Downloaded from R. Shu et al. conductivity, which made it become an attractive material for application in optoelectronic devices, 11 catalysis 12 and gas-sensing. 13 Recently, ZnO has attracted signi cant attention owing to its remarkable microwave absorption properties. 4 6 In order to further improve the microwave absorption performance of ZnO, scientists have made much e orts and their interest is mainly focused on the two aspects. On the one hand, ZnO with unique microstructure is fabricated, such as tetra-needlelike ZnO whiskers, 6 ZnO nanorods, 9 ZnO nanotrees, 14 ZnO dendritic nanostructures, 15 nanoneedlelike ZnO, 16 crossed ZnO netlike micro-/nanostructures, 17 etc. For example, Liu et al. fabricated nanoneedlelike ZnO via a facile combustion synthesis route and the materials exhibited enhanced permittivity and multi-region microwave absorption in the X-band at elevated temperature and the minimum re ection loss (RL min ) reached 44 db. 16 Li et al. prepared three-dimensional (3D) ZnO micro/nanorod networks through the direct evaporation of metal zinc and graphite powders in Ar and O 2 at 910 C without any catalyst. The value of RL min for the composites with 50 vol.% ZnO netlike structures was 37 db at 6.2 GHz with a thickness of 4.0 mm. 17 On the other hand, ZnO is complexed or hybridized with other components to fabricate composites, such as ZnO/reduced graphene oxide (ZnO/ RGO), 4,18,19 ZnO/multi-wall carbon nanotubes (ZnO/MWCNTs), 5,20 ZnO/Fe 3 O 4 composites 21,22 etc. For instance, Wu et al. fabricated 3D ZnO/ RGO nanocomposites using a two-step reduction process and found that the composites exhibited excellent microwave absorbing properties, i.e., the maximum e ective absorption bandwidth of RL lower than 10 db could reach 6.4 GHz when the ller loading was 10 wt.% and the thickness of the absorber was 2.5 mm. 4 Lu et al. synthesized ZnO/MWCNTs nanocomposites by a mild solutionprocess synthesis method and found that the composites exhibited highly e±cient microwave absorption at elevated temperature. 5 However, there are some issues in the production of ZnObased microwave absorption materials, such as uncontrollable morphology, size, and complex processing, need to be addressed. Therefore, it is essentially necessary to develop a facile or controllable way to improve the microwave absorption performance of ZnO. As is well known, a good EM wave absorbent needs to meet the two requirements, i.e., EM impendence matching and strong loss tangent tan (dielectric, conductive and magnetic loss, etc.). 23 For the semiconductor material ZnO, the lack of permeability leads to the poor EM impedance matching. In recent years, numerous studies have reported that transition-metal (Fe, Co, Ni, etc.) doped ZnO could endow a certain ferromagnetism to ZnO For example, Liu et al. 24 synthesized Fe-doped ZnO nanoparticles by a sol gel method and found that the nanoparticles showed typical ferromagnetism. El-Hilo et al. 26 synthesized Nidoped ZnO powders by thermal co-decomposition of a mixture of bis(acetylacetonato)zinc(ii)hydrate and bis(dimethylglyoximato)nickel(ii) complexes and observed that the powders presented a ferromagnetic behavior at room temperature. Therefore, transition-metal doping may be an e ective way to enhance the magnetic loss of ZnO and the impendence mismatching problem will be solved. In our previous work, the Ni-doped ZnO/Al/para±n wax composites were prepared by a chemical co-precipitation method and found that the composites exhibited strong microwave absorption (RL min reached 32.5 db at 13.6 GHz with a thickness of 4.5 mm) and low infrared emissivity (0.37). 29 Since the Fe element also belongs to the VIII group, it will present the similar e ect on the microstructure, dielectric and magnetic properties of ZnO as the Ni element possibly. Compared with Ni and Co, Fe possesses the lower density and better magnetic performance. Thus, the Fe-doped ZnO powders are potential microwave absorption materials with a light weight and strong absorption performance. In the present study, the Fe-doped ZnO powders were prepared by a chemical coprecipitation method, using zinc nitrate as the staring material, urea as the precipitator, and ferric nitrate as the doped source, respectively. The structure, morphology and magnetic performance of the as-prepared powders were investigated. Moreover, the EM parameters and microwave absorption properties of Fe-doped ZnO/para±n wax were explored. 2. Experimental Section 2.1. Materials Zinc nitrate (Zn(NO 3 Þ 2 6H 2 O), ferric nitrate (Fe(NO 3 Þ 3 9H 2 O) and urea (CO(NH 2 Þ 2 Þ were purchased from Sinopharm Chemical Reagent Co. Ltd. All the chemical reagents were analytical grade

3 NANO Downloaded from and were used without further puri cation. Water was puri ed by deionization and ltration with a Millipore puri cation apparatus (18:2 M cm) Sample preparation The Fe-doped ZnO powders were prepared by a facile chemical coprecipitation method. In a typical procedure, the Zn(NO 3 Þ 2 6H 2 O, Fe(NO 3 Þ 3 9H 2 O and CO(NH 2 Þ 2 were rst separately dissolved in de-ionized water. Then, appropriate amounts of Fe(NO 3 Þ 3 9H 2 O solutions were added into the Zn (NO 3 Þ 2 6H 2 O solution according to the experimental design. In the present study, the molar ratio of Fe to (Zn þ Fe), i.e., n Fe /(n Fe þ n Zn Þ was Next, the diluted urea aqueous solution was added drop-wise into the reaction mixture to adjust the ph equal to 9 and then reacted at 100 C for 2 h. After that, the resulting precipitates were separated by centrifuging and washed with de-ionized water and ethanol several times, and then dried in air at 100 C in stove for 24 h. Lastly, the powders were obtained after the calcination of precipitates at 600 C for 1 h under N 2 atmosphere. For comparison, the un-doped ZnO powder was also prepared by similar procedures without adding dopant Fe (NO 3 Þ 2 9H 2 O Characterization The crystalline structure of the as-prepared samples was characterized by X-ray di raction (XRD) using a LabX XRD-6000 (Shimadzu, Japan) with Cu-K radiation ( ¼ 0:154 nm) in the scattering range (2) of25 80 with a scan rate of 2 /min. The micromorphology analysis of the materials was observed by a eld emission scanning electron microscopy (FESEM, FEI-Sirion200, Netherlands) and eld emission transmission electron microscopy (FETEM, JEM-2100F, Japan). The surface compositions and chemical state of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250, USA). The magnetic properties were carried out on a vibrating sample magnetometer (VSM, Nanjing NanDa Instrument Plant HH-20, China) at room temperature (298 K). The EM parameters were measured at room temperature by using a vector network analyzer (VNA, AV3629D, China) in the frequency range of 2 18 GHz. Specimens for EM parameters Coprecipitation Synthesis of Fe-doped ZnO Powders measurements were prepared by uniformly dispersing the prepared powders in molten para±n wax (the weight percentage of the powders in specimens was 50 wt.%), and then the mixture was pressed into a toroid with an outer diameter of 7.0 mm and inner diameter of 3.0 mm. 3. Results and Discussions 3.1. Structural analysis The crystal structure of the as-prepared Fe-doped and un-doped ZnO powders were examined using XRD, as shown in Fig. 1. From Fig. 1, the characteristic di raction peaks of un-doped ZnO located at 2 ¼ 31:6 ; 34:3 ; 36:2, 47:4, 56:5, 62:7 and 67:8 can be assigned to the (100), (002), (101), (102), (110), (103) and (112) crystal planes, Fig. 1. XRD patterns of the Fe-doped ZnO and un-doped ZnO powders, and comparison of the (100), (002), (101) peaks from XRD patterns between the two samples

4 NANO Downloaded from R. Shu et al. corresponding to the hexagonal wurtzite structure of ZnO. 5,10 To investigate the e ect of doping on the crystallinity of the ZnO, the (100), (002), and (101) di raction peaks were monitored. Figure 1 shows that there was a slightly left shift among the (100), (002), and (101) di raction peaks of the Fe-doped ZnO compared with those of the un-doped ZnO. This proved that Fe ions incorporation led to the lattice deformation in the doped ZnO. However, no characteristic peaks of other impurity phases, such as Fe 2 O 3,Fe 3 O 4, or ZnFe 2 O 4 were observed for the Fe-doped ZnO, as shown in Fig. 1. All of these indicated that Fe ions systematically entered into the crystal lattice of ZnO without deteriorating the original crystal structure. In addition, the mean particle sizes of the sample were also estimated to be 36 nm using the Scherrer formula. The lattice constants a and c calculated by Bragg formula of the Fe-doped ZnO and un-doped ZnO powders are depicted in Table 1. It can be found that both a and c of Fe-doped ZnO are slightly larger than that of un-doped ZnO. This indicated that Fe ions were incorporated into the crystal lattice of ZnO, leading to the lattice deformation. Because the ionic radius of Fe 3þ (0.645 A) is smaller than that of Zn 2þ (0.74 A), 29 substitution of Fe atoms for Zn atoms in the ZnO lattice will lead to a smaller lattice constant for the doped sample compared with the un-doped sample. However, this is inconsistent with the change of lattice constants a and c in the present system. Recently, Liu et al. 27 synthesized the Ni-doped ZnO nanoparticles by a sol gel method and found that the lattice constant c rstly decreased with increasing doping Ni 2þ molar concentration (x) when x 0:02, while increased with Ni 2þ concentration until the maximum concentration (0.08). They interpreted the experimental results as the fact that the Ni 2þ substituted Zn 2þ from ZnO lattice when x 0:02 and took interstitial sites in ZnO as x > 0:02. Because our doping Fe ions molar concentration equal to 0.05, which are just Table 1. Lattice constant of the as-prepared Fe-doped ZnO and un-doped ZnO powders. Lattice constant Samples a ¼ b (A) c (A) un-doped ZnO Fe-doped ZnO located at the range of , the increase of lattice constants in the present system may also be explained by the interstitial doping Fe ions into ZnO. In order to further investigate the surface compositions and chemical state of Fe-doped ZnO sample, XPS measurements were performed as shown in Fig. 2. Figure 2 shows the wide scan XPS pro les, indicating that there are characteristic peaks of Zn, Fe, O, and C. All spectra were calibrated by C 1s peak (284.8 ev) to compensate the charge e ect, and the C 1s spectrum is showed in Fig. 2. Form Fig. 2(c), it can be found that the O1s peak locates at ev. From Fig. 2(d), the Zn 2p 3=2 and Zn 2p 1=2 are located at ev and ev, respectively. Moreover, the energy gap of Zn 2p 3=2 and Zn 2p 1=2 is 23.0 ev, which is just equal to that of ZnO. 27 These indicate that the valence state of Zn in the Fe-doped ZnO sample is þ2. As depicted in Fig. 2(e), the Fe 2p 3=2 and Fe 2p 1=2 are located at ev and ev, respectively. They are di erent either from the 2p 3=2 and 2p 1=2 peaks for metallic Fe located at ev and ev or Fe 2þ at ev and ev, but close to Fe 3þ at ev and ev. Meanwhile, the energy gap of Fe 2p 3=2 and Fe 2p 1=2 is 13.4 ev, which is di erent from that of Fe (13.2 ev) or Fe 2þ (12.0 ev), while it is very close that of Fe 3þ (13.3 ev). 24 Therefore, the valence state of Fe ions in the Fe-doped ZnO sample is believed to be þ Morphological analysis The morphological features of the un-doped and Fe-doped ZnO samples were observed by SEM. As depicted in Figs. 3 and 3, the un-doped ZnO sample presents an irregular shape and has lateral dimension in the range of nm. In addition, the ZnO particles exhibit well-de ned pro le and good dispersity. Figures 3(c) and 3(d) show the low magni cation and high magni cation SEM images of the Fe-doped ZnO sample, respectively. It is clearly seen that the sample exhibits branchlike morphology. Therefore, doping by Fe 3þ could signi cantly change the micromorphology of ZnO. In order to further observe the microscopic morphology of the Fe-doped ZnO sample, we perform the TEM characterization, as shown in Figs. 3(e) and 3(f). The low-resolution TEM micrograph reveals that the average size of the Fe-doped ZnO particles is about 100 nm, which is obviously larger than the calculated result from XRD (ca. 36 nm)

5 Coprecipitation Synthesis of Fe-doped ZnO Powders Intensity (a.u.) Zn 3d Zn 3p 3/2 Zn 3s O 1s Zn LMM Zn 2p 3/2 Zn 2p 1/2 C 1s Fe 2p OKLL Binding Energy (ev) C 1s O 1s NANO Downloaded from Intensity (a.u.) Intensity (a.u.) Binding Energy (ev) Zn 2p 3/ Zn 2p 1/ Intensity (a.u.) Intensity (a.u.) Binding Energy (ev) (c) Fe 2p 3/2 Fe 2p 1/ Binding Energy (ev) Binding Energy (ev) (d) (e) Fig. 2. XPS spectra of the Fe-doped ZnO sample: wide scan, C 1s spectrum, (c) O 1s spectrum, (d) Zn 2p spectrum and (e) Fe 2p spectrum. High-resolution TEM micrograph shows that the interplanar distance of fringes is nm, which is in good agreement with the (101) crystal plane of wurtzite ZnO. In addition, the crystal plane of Fe or iron oxide could not be founded. According to the results of XRD patterns and HRTEM images, we believe that the Fe 3þ ions are well incorporated into the crystal lattice of ZnO

6 NANO Downloaded from R. Shu et al. Figure 3(g) represents the EDS pattern of the Fe-doped ZnO sample. It can be found that only Zn, O, Fe, C and Au elements are detected. The peak at 0.53 kev belongs to O, the peaks at 1.03 kev, 8.64 kev and 9.58 kev belong to Zn, and the peak at 6.42 kev belongs to Fe. The C (0.25 kev) and Au (2.16 kev) come from the conductive adhesive and coating Au lm, respectively, indicating that there is no impurity element in the sample. Therefore, the result of EDS analysis is qualitatively consistent with the XPS survey in Fig Magnetic properties The magnetic performance is very important to investigate the microwave absorption properties. The eld dependence of magnetization for the Fe-doped ZnO and un-doped ZnO samples were measured at room temperature by a VSM, as shown in Fig. 4. The un-doped ZnO sample hardly presents any magnetic performance. However, after doping with Fe 3þ ions, the Fe-doped ZnO sample exhibits a typical S type hysteresis loop in the M H curves, indicating a ferromagnetic characteristic. 24 The saturation magnetization (M s ), remanent magnetization (M r ), and coercivity (H c Þ values are about 4.4 emu/g, 0.7 emu/ g and 96.0 Oe, respectively. The result of magnetic measurement demonstrates that the Fe-doped ZnO sample is ferromagnetic, suggesting that it may be used to attenuate EM wave Microwave absorption properties The microwave absorption properties of Fe-doped ZnO/para±n wax can be characterized by the RL. According to the transmission line theory, RL can (c) (d) Fig. 3. SEM images of the un-doped ZnO sample: low magni cation and high magni cation; SEM images of the Fe-doped ZnO sample: (c) low magni cation and (d) high magni cation; TEM images of the Fe-doped ZnO sample: (e) low magni cation and (f) high magni cation; (g) EDS pattern of the Fe-doped ZnO sample

7 Coprecipitation Synthesis of Fe-doped ZnO Powders (e) (f) NANO Downloaded from Fig. 3. be calculated by the following equations 6,8,30 33 : RLðdBÞ ¼20 lg Z in Z 0 Z in þ Z ; ð1þ 0 rffiffiffiffiffi Z in ¼ Z r 0 tanh j 2fd p ffiffiffiffiffiffiffiffiffi " r c r " r ; ð2þ where Z in is the input impedance of absorber, Z 0 is the impedance of free space, " r is the relative complex permittivity, r is the relative complex permeability, d is the thickness of the absorber, c is the velocity of light in free space, and f is the frequency. Figure 5 shows the calculated RL of Fe-doped ZnO/para±n wax and un-doped ZnO/para±n wax samples with an absorber thickness of 3.0 mm in the range of 2 18 GHz. It is observed that the RL value of the Fe-doped ZnO sample ( 21.0 db) is much larger than the un-doped ZnO sample ( 10.1 db). Figure 5 depicts the 3D presentation of the RL of the Fe-doped ZnO/para±n wax with di erent (g) (Continued) thicknesses. The RL min of the Fe-doped ZnO sample reaches 38.4 db at 6.6 GHz, and the e ective absorption bandwidth (RL 10.0 db) is 4.0 GHz (11: GHz) with a thickness of only 2:5 mm. M / (emu/g) ZnO Fe-doped ZnO H / Oe Fig. 4. Magnetization curves of the Fe-doped ZnO and un-doped ZnO samples

8 R. Shu et al. Reflection Loss (db) db d = 3.0 mm db ZnO Fe-doped ZnO Frequency (GHz) 0 ZnO NANO Downloaded from Reflection Loss (db) However, the RL min of un-doped ZnO sample is only 10.1 db [Fig. 5(c)], indicating much weak microwave absorption. Furthermore, the microwave absorption ability of the Fe-doped ZnO/para±n wax at di erent frequencies can be controlled by adjusting the thickness of the absorbents. In addition, the RL peaks of Fe-doped ZnO samples shifts to lower frequency band with the increasing of the thickness of the absorbers. To make a more clear presentation of the microwave absorption properties of the Fe-doped ZnO/para±n wax sample, the thickness dependence of absorption peak frequencies (the frequency corresponding to the absorption peak value) and effective absorption bandwidth are calculated and the results are displayed in Fig. 6. Generally, when the EM wave is transmitting into a microwave absorption medium, the peak -2-4 d / mm db Frequency (GHz) (c) Fig. 5. Re ection loss of the Fe-doped ZnO/para±n wax and un-doped ZnO/para±n wax with a thickness of 3.0 mm, 3D presentation of the re ection loss of the Fe-doped ZnO/para±n wax and (c) re ection loss of the un-doped ZnO/para±n wax with di erent thicknesses. frequency f m can be expressed as follows 34,35 : nc f m ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffi ðn ¼ 1; 3; 5;...Þ; ð3þ 4t m j" r r j where f m and t m are the peak frequency and matching thickness of re ection loss peak, respectively, and c is the velocity of light. Equation (3) illustrates that the thickness of absorber varies inversely with the peak frequency, which is in accordance with the result in Fig. 6. FromFig. 6, the e ective absorption bandwidth is 4.0 GHz with a thickness of only 2.5 mm, suggesting that the Fe-doped ZnO/para±n wax exhibits excellent microwave absorption performance. In addition, the e ective absorption bandwidth decreases with the increasing of the thicknesses of the absorbers in the range of 2:5 4.5 mm. To investigate the possible EM wave absorption mechanism of samples, the frequency dependence of

9 Coprecipitation Synthesis of Fe-doped ZnO Powders Peak frequency (GHz) Fe-doped ZnO d (mm) Absorption bandwidth (GHz) Fe-doped ZnO d (mm) Fig. 6. Thickness dependent peak frequency and e ective absorption bandwidth of the Fe-doped ZnO/para±n wax. NANO Downloaded from relative complex permittivity and permeability for materials are displayed in Fig. 7. In general, the real part (" 0 ) and imaginary part (" 00 ) of relative complex permittivity of the Fe-doped ZnO decrease with the increasing of frequency, while that of the un-doped ZnO show little change with frequency. Signi cantly, it can be seen that the values of " 0 and " 00 for the Fedoped ZnO are much larger than the un-doped ZnO, which indicates that doping with Fe 3þ can greatly improve the dielectric properties of ZnO. Speci cally, the un-doped ZnO has the value of " 0 about 3.87 at 2 GHz and holds a very slight declination with the increasing of frequency (3.32 at 18 GHz), while for the Fe-doped ZnO, the value of " 0 rises to at 2 GHz, which is about three times larger than that of the un-doped ZnO, as shown in Fig. 7. From Fig. 7, the value of " 00 for the Fedoped ZnO decreases with the increasing of frequency, while that of the un-doped ZnO exhibits little change. Furthermore, the value of " 00 for the Fe-doped ZnO is about nine times of the un-doped ZnO at 2 GHz. Figure 7(c) demonstrates the real part of relative complex permeability ( 0 ) as a function of frequency. The values of 0 are all in the range of and have the tendency of decreasing with the rising of frequency and there are no obvious di erences between the two samples. Figure 7(d) presents the frequency dependence of the imaginary part of complex permeability ( 00 ). It is clearly observed that the values of 00 for Fe-doped ZnO are signi cantly larger than that of un-doped ZnO in the lower frequency band range (2 4 GHz), suggesting better magnetic loss characteristic. However, there are no obvious di erences between the two samples in the higher frequency band range (4 18 GHz). Generally, the tan is applied to characterize the loss properties of the incident EM wave in an absorber. The dielectric loss tangent (tan e ¼ " 00 =" 0 ) and magnetic loss tangent (tan m ¼ 00 = 0 ) based on the measured complex permittivity or complex permeability of the two samples are shown in Fig. 8. From Fig. 8, a pronounced tan e peak can be observed at 15.5 GHz for the un-doped ZnO sample, which is in accordance with the result of Fig. 7. By doping with Fe 3þ ions, the tan e values of Fedoped ZnO sample show an obvious enhancement compared with un-doped ZnO sample. For undoped ZnO sample, the tan e value is about in the whole testing frequency range. As the Fe-doped ZnO sample, the tan e value increases to 0.34 at 2 GHz and increases to 0.52 at 18 GHz, respectively. This result can be attributed to the increased permittivity by doping with Fe 3þ ions. As shown in Fig. 8, the frequency dependence of magnetic loss tangent tan m exhibits a similar variation tendency of 00, which indicates that the Fe-doped ZnO sample shows better magnetic loss characteristic than the un-doped ZnO sample. It can be concluded that the values of tan e are much higher than tan m for the both samples, which means that the main loss mechanism for microwave absorption is dielectric loss. However, we notice that there is also magnetic loss for the Fe-doped ZnO sample. According to

10 R. Shu et al. NANO Downloaded from (c) (d) Fig. 7. Frequency dependence of real and imaginary parts of relative complex permittivity; (c) real and (d) imaginary parts of relative complex permeability of the Fe-doped ZnO/para±n wax and un-doped ZnO/para±n wax. Fig. 8. Frequency dependence of the dielectric loss tangent tan e and magnetic loss tangent tan m of the Fe-doped ZnO/para±n wax and un-doped ZnO/para±n wax

11 NANO Downloaded from Pan et al.'s research achievement, 36 the eddy current e ect and natural resonance may be responsible for the attenuation of EM waves over 2 18 GHz frequency range. The eddy current loss can be calculated by the following equation 36 : ð 0 Þ 2 d 2 f=3; ð4þ where 0 (H/m) and (S/cm) are the electric permeability and the conductivity in vacuum, respectively. If the re ection loss is caused by eddy current loss e ect, the values of C 0 (C 0 ¼ 00 ð 0 Þ 2 f 1 ) are constant when the frequency varies. From Fig. 9, it can be observed that the value of C 0 presents obvious uctuation in the testing frequency range, which suggests that the eddy current e ect should not be the mechanism of microwave energy dissipation for the Fe-doped ZnO sample. The natural resonance mechanism for magnetic loss can be expressed in the following equations 36,37 : 2f r ¼ rh a ð5þ H a ¼ 4jK 1 j=3 0 M s ; ð6þ where r is the gyromagnetic ratio, H a is the anisotropy energy, and jk 1 j is the anisotropy coe±cient, M s is the saturation magnetization. The resonance frequency depends on the e ective anisotropy eld, which is associated with coercivity value of the materials. 36,38 As shown in Fig. 4, the coercivity value of the Fe-doped ZnO is about 96.0 Oe, which is bene cial to EM wave absorption. The enhanced microwave absorption properties of Fe-doped ZnO/para±n wax could be explained as follows. Because of the doping with Fe 3þ, the " 0 values are signi cantly increased and the impedance matching condition is improved. Moreover, the Coprecipitation Synthesis of Fe-doped ZnO Powders crystal lattice defects induced by doping make the bound electrons around the surrounding migration in an alternating EM eld, resulting in relaxation polarization and dielectric loss. Thus, the echo return are reduced, which leads to the enhancement of re ection loss. 10 In addition, Fe-doped ZnO sample presents ferromagnetic behavior and thus the magnetic loss enhances, which originated from the natural resonance mechanism. As a consequence, the Fe-doped ZnO/para±n wax exhibits much better microwave absorption performance than the undoped ZnO/para±n wax. The analysis above indicates that the enhanced microwave absorption properties of ZnO can be achieved by doping with Fe 3þ ions or varying the thickness of the absorber. Therefore, it is believed that the Fe-doped ZnO powders could be used as potential microwave absorption materials. 4. Conclusions In summary, the Fe-doped and un-doped ZnO powders were synthesized by a chemical coprecipitation method. HRTEM, XPS, EDS and XRD results revealed that Fe ions were successfully incorporated into the crystal lattice of ZnO and had a valence of þ3. SEM and TEM results demonstrated that the doping could signi cantly change the microscopic morphology of ZnO. Magnetic measurements indicated that Fe-doped ZnO powders presented ferromagnetic behavior at room temperature. The results of EM measurements showed that the Fe-doped ZnO exhibited improved microwave absorption performance in comparison with the undoped ZnO. Moreover, the excellent microwave absorption properties of ZnO could be facilely achieved by doping with Fe 3þ ions or varying the thickness of the absorbers. Our results demonstrated that the Fe-doped ZnO powders were very promising for application as microwave absorbers. Fig. 9. The C 0 -f curve of the Fe-doped ZnO/para±n wax. Acknowledgments This work was nancially supported by the National Natural Science Foundation of China (Grant No ), the National Training Program of Innovation and Entrepreneurship for Undergraduates (Grant No ), the Doctor's Start-up Research Foundation of Anhui University of Science and Technology (Grant No. ZY537) and the Natural Science Foundation of Anhui Province (Grant No QA15)

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