【正文】 y. After drying at 623 K for 2 h in Ar, the resultant powders were characterized by xray diffraction (XRD), and the microstructures were analyzed by a highresolution scanning electron microscopy (HRSEM).The magnetization hysteresis curves were recorded by a vibration sample magnetometer. Epoxy resin posites were prepared by homogeneously mixing epoxy resin with vol% powders and pressing into cylindricalshaped pacts. These pacts were cured at 453 K for 30 min, and then cut into toroidal shaped samples (Фout: mm, Фin: mm). In the – GHz, the relative permeability () and permittivity () values were measured on the toroidal shaped samples using a network analyzer(Agilent Technologies E8363A). The re?ection loss(RL) curves were calculated from the relative permeability and permittivity at the given frequency and absorber thickness, according to the following equations[13]: （1） （2）where f is the frequency of the electromagnetic wave, d is the thickness of an absorber, c is the velocity of light, Z0 is the impedance of free space, and Zin is the input impedance of absorber. The RL value of ?20 dB is parable to the 99% of EM wave absorption according to Eqs. (1) and (2), and thus “RL?20 dB” is considered as an adequate EM absorption.. XRD patterns of（a）Fe,（b） ,and（c）,（d）,（e）Fe/ nanoposite powders with 38, 70, or 85 vol % Fe, respectively.Figure 1 shows the typical XRD patterns measured on theFe, , and Fe/ powders. From (b), it was found that pound was formed by the solidstate reaction. All the peaks could be indexed as the hexagonal lattice of (JCPDS 1997). After ball milling the mixture of Fe with powders at 200 r/min for 30 h in hexane and subsequent drying, only the peaks of Fe were observed. The peaks of Fe exhibited the wider line broadening and was transformed into amorphous phase. Mean crystallite sizes of Fe were evaluated to be about 60 nm (asobtained) and 20 nm (after ball milling) from the line broadening of the XRD peaks by using the Scherrer’s formula. On the HRSEM photographs, Fe(325 mesh) particle size varied from 10 to 100 μm, but Fe/ nanoposite particles showed a size distribution from 100 to 900 nm and the grain size of Fe varied from 20 to 30 nm.FIG. 2. Frequency dependences of relative permittivity εr(a), real part (b), and imaginary part (c) of relative permeability for the resin posites with vol% of Fe, , and Fe/(38, 70, or 85 vol% Fe)nanoposite powders, respectively.Figure 2(a) shows that the real part () and the imaginary part () of relative permittivity for the resin posites with vol% Fe/ powders containing 38, 70, or 85 vol% Fe were almost constant between 2 and 18 GHz, for which the relative permittivity
() showed less variation (=10,10,11 and =, , , respectively). For the resin posites with vol% powders, the and values were low constant and almost independent of frequency in the 218 GHz (= and =). The real part （）and the imaginary part（）of relative permeability are plotted as a function of frequency in Figs. 2(b) and 2(c). The values declined from , , and to , , and gradually with increasing frequency in the GHz for the resi posites of Fe/ (38, 70, or 85 vol% Fe), respectively. For resin posites, the value decreased from to sharply in the GHz. The increased from to in the GHz, and then decreased rapidly in the higher GHz range. After the addition of 38 vol% Fe, the showed a broad peak in the 4–18 GHz and the maximum value of shifted to the higher frequency ( GHz), but it decreased from to . However, one can observe that the value increased with increasing the addition amount of Fe,and the maximum value of Fe/ nanoposites with 85 vol% Fe was up to at GHzFIG. 3. Frequency dependences of RL for the resin posites with vol% of(a) , and（b）Fe/（70 vol % Fe）powdersFigure 3(a) shows the typical relationship between RL and frequency for the resin posites with vol% powders. The RL values less than ?20 dB were obtained in the GHz with absorber thickness of mm. For the resin posites with vol% Fe/（70 vol% Fe）powders, the RL values less than ?20 dB were recorded in the GHz with absorber thickness of mm. In particular, a minimum RL of ?51 dB was obtained at GHz with a thickness of mm [(b)]. The saturation magnetization (Ms), coercivity（Hc）, and remanence（Br）ofFe, , and Fe/ nanoposites （38, 70, 85 vol% Fe）and the EM wave absorption properties of their resin posites prepared under the optimal conditions are summarized in Table I. The measured Ms values of Fe/ nanoposites increased with increasing the concentration of Fe because of the higher Ms value of Fe（～ T） than that of
（～ T）. Fe/ nanoposites showed higher Hc values than Fe and , and the maximum Hc （ KA/m）was observed for the 38:62 volume ratio. The possible reasons can be explained as follows: After ball milling with powders, Fe e particles（325 mesh）got successively ?ned and regularly ?akelike morphology was formed[14], which were surrounded by nanoparticles of . Therefore, the ?akelike Fe nanoparticles exhibited large shape anisotropy ?eld, which results in coercivity enhancement. On the other hand, the microstructure of nanoposites induces exchange bias between Fe and nanoparticles, which is probably another factor for the coercivity enhancement[15]. In addition, the Hc and Br values of nanoposites decrease with increasing the concentration of Fe. This behavior seems to indicate that the increasing interparticle interactions between ?akelike Fe particles weaken the effects of shape anisotropy and exchange bias[16]. Comparing with the resin posites of or other ferrites[17,18], the thicknesses （dm） ofFe /（70 or 85 vol% Fe）absorbers decreased by about 30%50% in the same frequency region. This is mainly ascribed to the strong increase of
and as expected after the addition of 70