Electromagnetic Wave Absorption Properties of ZnO-Based Materials

Jan 11, 2013 - This work was financially supported by the funding from the State Key Laboratory of Solidification Processing in NWPU (No. ... Saleh , ...
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Electromagnetic Wave Absorption Properties of ZnO-Based Materials Modified with ZnAl2O4 Nanograins Luo Kong, Xiaowei Yin,* Fang Ye, Quan Li, Litong Zhang, and Laifei Cheng Science and Technology on Thermostructure Composite Materials Laboratory, Northwestern Polytechnical University, West Youyi Rd., No. 127, Xi’an, Shaanxi 710072, People’s Republic of China ABSTRACT: Considering the widespread presence of electromagnetic interferences (EMI), it is necessary to develop new electromagnetic wave (EM) absorbing materials with low reflection coefficient and large operating frequency band. The well-known EM absorbing materials have a microstructure combining a low permittivity phase with a high electrical conductivity phase. In the present work, a phase in nanoscale with medium permittivity is added into the well-known EM absorption materials to obtain an EM absorption material with low EM reflection coefficient and wide absorption band. Composite powders with special microstructure have been synthesized via sol−gel process, which are composed of submicrometer-sized ZnO acting as electrically lossy phase and ZnAl2O4 nanograins acting as a medium permittivity phase. When the composite powders are mixed with paraffin, the as-received materials exhibit appropriate permittivity and electrical conductivity, which can be attributed to the high carrier concentration and mobility at the interfaces in nanoscale. The high absorption coefficient, small reflection coefficient, and wide absorption band can be obtained. Absorption coefficient per unit thickness increases from 0.01 to 0.13/mm, the minimum reflection coefficient reaches −25 dB, and the effective absorption bandwidth covers the whole X-band (8.2−12.4 GHz). The ZnO/ZnAl2O4 composite materials exhibit excellent EM absorption properties.

I. INTRODUCTION In recent years, electromagnetic wave (EM) absorbing materials have aroused great interests because of more and more civil and military applications in electromagnetic interference (EMI) shielding and the reduction of radar cross section (RCS) in the gigahertz (GHz) band range. For example, Doppler, weather radar, TV picture transmission, and telephone microwave relay systems lie in frequency range of 8.2−12.4 GHz (X-band).1 When the reflection coefficient (RC) of an EM absorbing material is smaller than −10 dB, only 10% of the EM power is reflected and the left 90% is attenuated. The corresponding frequency range within which reflection coefficient is smaller than −10 dB is defined as the effective absorption bandwidth. The conventional EM absorbing materials have larger density, inferior environmental stability, weak absorption and narrow effective absorption bandwidth. These disadvantages have severely inhibited their future applications, and EM absorbing materials with relatively low weight and strong absorption in a wide band are in highly demand nowadays. Therefore, researchers are urged to find new types of EM absorbing materials to deal with various types of EM radiation. Recently, nanostructure materials, such as carbon nanotubes, graphene, SiC, Mn2O3, and ZnO nanoparticle composites have attracted great interest as EM absorbing and EMI shielding materials in the GHz frequency range due to their unique chemical and physical properties.2−9 The EM absorption properties of Feencapsulated CNTs mixed with epoxy have been studied, and RC lower than −10 dB was obtained in CNTs/epoxy composites with matching thickness of 2 mm.10,11 The EM © 2013 American Chemical Society

absorption properties of ZnO with dendritic nanostructures and 3D sponge-like porous networks of Mn3O4 were studied by mixing them with paraffin.12,13 The above materials possess interesting EM absorption properties at a thickness of 5.0 mm, and the absorption bandwidth with a RC lower than −10 dB is about 2.6 GHz. EM absorption properties of cagelike ZnO/ SiO2 composite ceramic have been also studied.14 The composites with 20 wt % ZnO had a minimum RC of −10.7 dB at 12.8 GHz, and the microcurrent network mechanism on the EM attenuation in X-band was proposed. The absorption bandwidth with a RC lower than −10 dB is about 0.5 GHz. To obtain improved EM absorption properties, absorbing materials need to have a suitable permittivity and dielectric loss. The increasing dielectric loss is beneficial to increase the absorption coefficient. However, the problem is that, with the increase of dielectric loss, the permittivity increases, and the EM reflection increases consequently. It is revealed that the materials with high EM absorption capability require a low real part of permittivity and an appropriately high electrical conductivity (around 1 S/m).15,16 The increases in imaginary part of the permittivity (ε″) and dielectric loss are attributed to the increase of electrical conductivity according to eq 1: σ = 2πfε0ε″

(1)

Received: October 9, 2012 Revised: December 30, 2012 Published: January 11, 2013 2135

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Figure 1. Relationship between permittivity and reflection coefficient.

where σ is the electrical conductivity (S/m), ε0 is the free space permittivity (8.854 × 10−12F/m), and f is the frequency (Hz). The EM absorption properties are evaluated using RC (dB), which is based on the transmission-line theory and metal backpanel model.17 RC expresses the ratio of the total reflected EM power against the incident EM power. RC is determined by the measured relative complex permeability and permittivity according to the following equations: RC = 20· log10|(Z in − 1)/(Z in + 1)| Z in =

μ tanh(j2π με fd /c) ε

ε = ε ′ − jε ″

(4)

In the present work, μ is taken as 1 because of the weak magnetic property of the studied materials. The relation of permittivity and reflection coefficient calculated according to eq 2 is exhibited in Figure 1a. Figure 1b is the top view of Figure 1a. At a frequency of 10 GHz and a thickness of 2.86 mm, the optimum real and imaginary parts of the permittivity are equal to 7.3 and 3.3 to get the lowest RC. The minimum value of RC changes with the frequency and thickness, as shown in Figure1c. The permittivity corresponding to a minimum reflection coefficient decreases with the increase of thickness and frequency, which can be seen from the four color lines in the figure. The baby blue color line shows the minimum reflection coefficient when the thickness is 2 mm, and the complex permittivity of the minimum reflection coefficient is reduced from 20−5j to 10−3j when the frequency changes from 8.2 to 12.4 GHz. When the thicknesses are 2.86, 4, and 5 mm, respectively, they show a similar tendency, which are represented with orange, yellow, and red lines, respectively.

(2)

(3)

where Zin, ε, and μ are the normalized input impedance, permittivity, and permeability of the material, respectively; d and c represents thickness (m) and the light velocity in vacuum (3 × 108m/s), respectively. Permittivity is composed of a real part (ε′) and an imaginary part (ε″). It is expressed by the following equation: 2136

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Table 1. Absorber Composition and Absorption Properties of the Dielectric Materials in the Literature phase composition A

B

silica epoxy resin porous Si3N4 porous Si3N4 zirconia felt, pore paraffin paraffin paraffin porous ZrSiO4

caron fiber SWCNT SiBC SiC SiC Mn3O4 TiC ZnO nanotrees flower-like ZnO cagelike ZnO

SiO2

absorbent content

optimum thickness (mm)

optimum frequency (GHz)

RCmin (dB)

effective absorption bandwidth (GHz)

ref

20 wt % 3 wt % 6 vol % 11 vol % 86.9 wt % 50 vol % 50 wt % 60 vol % 10 wt %

5 9.7 3.4 1.2 5 2 2 2 3.5

9.9 10 10.2 12.2 16.5 10.3 12.8 9 9.3

−10.22 −19 −23.3 −6.7 −26.6 −18 −28 −30 −32

0.8 1.8 3.5 0 5 2.6 3.4 2 3.8

20

20 wt %

3

12.79

−10.68

1.3

14

9 21 22 23 12 24 13,25 26

The C phase is a low dielectric loss material, it has larger permittivity than A phase, but smaller than B phase. In the literature,15 a hierarchical architecture involving a metallic honeycomb filled with carbon nanotube-reinforced polymer foam has been developed. A novel hybrid material with carbon nanotube, polymer, and pore exhibits an excellent EM absorption capability. In our work, we use the other way to achieve this purpose, which starts from the material design itself. The C phase may exist in two types of microstructures, which is shown in Figure 2. When the B phase is surrounded by

The complex permittivity of EM absorbing materials with a dielectric absorbent in the literatures is also exhibited in Figure1c. It can be noticed that there is no material in the literature possessing the best combination of the real and imaginary parts of permittivity to meet the objective of minimizing RC and maximizing effective absorption bandwidth. To improve absorption properties, it is necessary to design a special composition and microstructure to obtain the proper permittivity.

II. COMPOSITION AND MICROSTRUCTURAL DESIGN The EM absorption materials are composed of an electrically insulating matrix, denoted as A phase, and an electrically lossy phase denoted as B phase,18,19 and materials with this kind of microstructure reported in some literatures are listed in Table 1. The permittivity of A phase is close to the air, and it can not absorb EM. B phase acts as EM absorbent, which has appropriate permittivity and can absorb EM when it is dispersed in the A phase. Hybrid materials composed of A and B phases naturally emerge as the solution when seemingly antagonist properties must be combined. In Table 1, we can observe the absorber composition, optimum thickness, optimum frequency, RC, and effective absorption bandwidth of the absorption materials in the literature. The bandwidth corresponding to a RC less than −10 dB is seldom larger than 3 GHz when the sample thickness is smaller than 3 mm in the Xband. When the equivalent complex permittivity is increased with the absorbent content, it is difficult to obtain the optimum real and imaginary parts of the permittivity, simultaneously. The EM power is reflected on the surface of absorption materials because of the increasing impedance mismatch between the air and the materials. Therefore, it is necessary to design a microstructure that can not only increase the imaginary part of permittivity, but also keep a relatively lower real part. In recent years, core/shell nanostructures are developed to attenuate EM radiation and attract more and more attention.27−32 Core/shell nanostructure materials exhibit very good EM absorption properties due to the interfacial polarization, confinement effect, and effective complementarities between the dielectric and magnetic losses. In the present work, we borrow and modify the idea of core/shell nanostructure, and a novel hierarchical architecture is designed which is beneficial to meet the requirement of impedance match. The third phase, which is denoted as C phase, is added into the EM absorbing materials composed of A and B phases.

Figure 2. (a−c) Schematic illustrations exhibiting the different model of A/B/C microstructures; (d) schematic illustration of the ZnOZnAl2O4/paraffin microstructure.

nanoparticles of the C phase (Figure 2c), the hybrid materials have more interface than the one that the B phase is coated by a layer of the C phase (Figure 2b) so that the better interfacial polarization capability can be obtained. Figure 2d is the schematic illustration of ZnO-ZnAl2O4/paraffin structure. The EM absorption capability can be attributed to the interfacial polarization effect in EM field. Gahnite (ZnAl2O4) exhibits high melting temperature (1950 °C), low thermal expansion coefficient (20−900 °C, α = 7.0 × 10−6 /°C), and high chemical inertness at both low and high temperatures. ZnAl2O4 has much lower permittivity than Aldoped ZnO.33 Microstructure and distribution of the absorbent have a great influence on the EM absorbing properties. Therefore, ZnAl2O4 can be a suitable host material for ZnO, which acts as an EM absorbent and plays a crucial role in altering the overall dielectric properties. 2137

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transmission electron microscopy (HRTEM) images of the samples were conducted on a 200 KV LaB6 TEM. The TEM specimens were prepared by dispersing the samples in ethanol and placing drops of the dispersion onto Cu TEM grid covered with a holey carbon film. 3.3. Dielectric Property Characterization. The samples used for dielectric properties measurement were prepared by mixing the 40 wt % ZnO/ZnAl2O4 composite powders with paraffin. The relative complex permittivity (ε) of the asreceived samples with dimensions of 22.86 × 10.16 × 2.86 mm was measured by a vector network analyzer (VNA, MS4644A, Anritsu, Atsugi, Japan) using the waveguide method in the Xband. The reflection coefficient can be calculated using relative complex permittivity.

Polarization and electrical conductivity are closely related. Dielectric in an EM field can be equivalent to a capacitor and resistor in parallel. In Figure 2d, composite powders are composed of ZnO and nano-ZnAl2O4, ZnO is dispersed in nano-ZnAl2O4, and the composites are prepared by mixing the composite powders with paraffin. Paraffin is an EM-transparent material, it can be used for an EM-transparent matrix to adjust the overall dielectric properties of composite, which reduces surface reflection and permits more EM power to penetrate into the absorbing materials. Nano-ZnAl2O4 covers submicrometer-ZnO, which can make ZnO dispersed in composite powders, enhance interfacial polarization effect, and raise dielectric loss. It can alter the overall dielectric properties of composite to meet the requirement of impedance match. Based on the above analysis, it is attractive to prepare the composite in which submicrometer-ZnO is dispersed in ZnAl 2 O 4 nanograins.

IV. RESULTS AND DISCUSSION 4.1. Crystalline Phase and Morphology. The X-ray diffraction patterns of the ZnO/ZnAl2O4 composite powders with different ZnO content are presented in Figure 3. It can be observed that the composite powders are composed of ZnO and ZnAl2O4. The Al2O3 peaks can not be found, which implies that Al2O3 is completely turned into ZnAl2O4 due to the

III. EXPERIMENTAL DETAILS 3.1. Preparation Process. ZnO/ZnAl2O4 composite powders were synthesized by sol−gel process. All the chemicals in our experiments were analytical grade reagents. Analytic 2methoxyethanol (CH3OCH2CH2OH, purity 99.0%, Fuchen chemical reagents, Tianjin, China) and analytic monoethanolamine (MEA, HO (CH2)2 NH2, purity 98.0%, Hengxing Chemical Preparation Co. Ltd., Tianjin, China) were used as a solvent and stabilizer, respectively. An appropriate amount of analytic zinc acetate dehydrate (Zn(CH3COO)2·2H2O, purity 99.0%, Fuchen chemical reagents, Tianjin, China) was dissolved in a mixture of 2-methoxyethanol and MEA solution at room temperature. The mole ratio of the zinc acetate and the MEA was 1:1, and the molarity of the solution was 1 mol/L. After that, an ethanol solution of analytic aluminum nitrate nonahydrate (Al(NO3)3·9H2O, purity 99.0%, Fuchen chemical reagents, Tianjin, China) was added into the above solution to obtain a desired ZnO content. ZnO content increased from 50 mol % to 100 mol %, while Al2O3 content dropped correspondingly from 50 mol % to 0, which were designated as samples A−F in Table 2. The nanocomposite powders were obtained by drying the solution and annealing in nitrogen at 800 °C for 2 h. Table 2. Relative Contents (mol %) of ZnO, Al2O3, and ZnAl2O4 in the Different Samples ZnO/ZnAl2O4 nanocomposite powders

reactants sample

ZnO content

Al2O3 content

ZnO content

ZnAl2O4 content

A B C D E F

100 90 80 70 60 50

0 10 20 30 40 50

100 88.9 75 57.1 33.3 0

0 11.1 25 42.9 66.7 100

3.2. Phase Composition and Microstructure Characterization. The crystal structure of the composite powders was identified by X-ray diffractometer (X′ Pert Pro, Philips, Heracles Almelo, The Netherlands), using Cu Kα (λ = 1.54 Å) radiation. The morphology of the composite powders was observed by a scanning electron microscope (SEM, S-4700, Hitachi, Tokyo, Japan) and transmission electron microscope (TEM, G-20, FEI-Tecnai, Hillsboro, U.S.A.). High-resolution

Figure 3. X-ray diffraction patterns of the ZnO/ZnAl2O4 composite powders: (a) 2θ from 25 to 75°; (b) 2θ from 30 to 38°. 2138

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Figure 4. TEM (a, c, e, g, i, and k) and HRTEM (b, d, f, h, j, and l) photographs corresponding to the ZnO/ZnAl2O4 composite powder samples (A−F).

reaction with ZnO in annealing process. Meanwhile, it can be noticed that the ZnO peaks become higher with the increase of ZnO content on the whole. The above result reveals that Al2O3 completely reacted with ZnO, resulting in more ZnO remaining

in the composite powders. Figure 3b shows the diffraction peaks at 2θ ranging from 30 to 38°. As shown in Figure 3b, the diffraction angles of pure ZnO are smaller than the one of ZnO in ZnO/ZnAl2O4 composite powders. The above result 2139

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indicates that the ZnO has been doped by Al atoms in ZnO/ ZnAl2O4 composite powders, which is consistent with our previous work.26 The microstructures of the EM absorbing materials have important influence on their EM absorption properties.34 Figure 4 shows the morphologies of the ZnO/ZnAl2O4 composite powders with different ZnO contents. Detailed microstructural and morphology information about the ZnO/ ZnAl2O4 nanocomposites are provided by HRTEM. The thermal decomposition of precursor first led to the formation of ZnO and amorphous alumina phase; with the increase of the calcination temperature, a solid phase reaction between ZnO and alumina occurred to form ZnAl2O4 nanoparticles that were dispersed in the ZnO phase. ZnAl2O4 nanograins surrounding ZnO particles can potentially have direct contact/interaction with ZnO. In addition, considering that the metal cations are uniformly distributed on a molecular level in the precursor, without segregation area of separate cations, such ZnAl2O4 nanograins may be homogeneously dispersed inside the ZnO micronetwork. In Figure 4a, it shows that pure ZnO particles are submicrometer-sized and the diameter is about 400 nm. Doping of ZnO by Al atoms restrains the growth of ZnO grains. The average particle size of ZnO is 100 nm for the sample B, while particle size of ZnAl2O4 ranges from 20 to 25 nm, and ZnAl2O4 nanograins are dispersed in the ZnO phase (Figure 4c). When the Al content continues to increase, a larger amount of ZnAl2O4 nanograins appear and ZnO particles gradually decrease, as shown in Figure 4e. In Table 1 we can observe that the ZnO particles content in sample D and E is decreasing significantly. A large amount of ZnAl2O4 nanograins make a pinning effect and restrict the growth of ZnO crystal. In Figure 4g,i, the particles size is close to 50 nm and the ZnO nanograins are uniformly dispersed in ZnAl2O4 nanograins. The sample F is pure ZnAl2O4 phase. The particles size is uniform and also close to 50 nm (Figure 4k). Corresponding to each sample, the HRTEM images reveal that both ZnO and ZnAl2O4 phases are highly crystallized. The spacing values of 0.24 and 0.26 nm depict the lattice-resolved (101̅1) and (0002) crystalline plane of ZnO phase, respectively, and the spacing value of 0.46 nm corresponds to the (111) facets of ZnAl2O4 phase. Figure 5a shows the SEM morphology of sample C. We can see the submicrometer-sized ZnO particles dispersed in ZnAl2O4 nanoparticles. The diameter of ZnO particles is about 200 nm, and the diameter of ZnAl2O4 is about 30 nm. Figure 5b shows the morphology of sample B. It is obvious that the content of the ZnO particles increases significantly in sample B. ZnO and ZnAl2O4 particles are uniformly mixed together. The measured grain size of ZnAl2O4 is consistent with the calculation one by using Scherrer expression.35 4.2. Dielectric Properties. Figure 6 illustrates the dielectric properties and dielectric loss (tan δ = ε″/ε′) in X-band for the ZnO/ZnAl2O4/paraffin composite with different ZnO contents. As shown in Figure 6a,b, with the increase of the ZnO content, both ε′ and ε″ show first increase and then decrease across the whole X-band. The ε′ of sample C and ε″ of sample B are the highest, which are in the range of 7−8.7 and 3.1−3.9, respectively. The permittivity of the sample A is small, because the permittivity of pure ZnO is smaller than Al-doped ZnO.26 The theoretical density of composite powders can be calculated by the rule of mixture. The theoretical density of composite powder C is lower than powder B and the volume of powders C is larger than powders B with the same mass. Therefore, the ε′ of sample C is largest according to the rule of mixture. The

Figure 5. SEM photographs of the ZnO/ZnAl2O4 composite powders: (a) sample C, (b) sample B.

imaginary part of permittivity is influenced by interface polarization effect, so the ε″ of sample B is the highest. However, too high permittivity is harmful to the impedance match and results in strong reflection and weak absorption.36 This is revealed in Figure 1. Because the EM attenuation capability of a material is influenced by both real and imaginary parts of permittivity, it is more appropriate to analyze the properties using dielectric loss. When the permittivity meets the impedance match requirement, higher dielectric loss implies better EM absorption properties. As an EM absorbent, larger imaginary part of complex permittivity and larger dielectric loss are expected. Figure 6c illustrates the dielectric loss of the ZnO/ZnAl2O4 composite in X-band. It is observed that the dielectric loss first increases and then decreases with the increase of ZnO, and the dielectric loss of sample B is the highest. It can be attributed to the high carrier concentration and enhanced mobility in the heterojunction of ZnO/ZnAl2O4 composite. To enhance electrical conductivity and keep a relatively lower permittivity, it is necessary to introduce ZnAl2O4, which has lower permittivity, similar crystal structure, and chemical stability with ZnO. It can obtain more crystal interface and enhance interfacial polarization effect, when ZnO phase is combined with ZnAl2O4 phase. First, the introduction of medium permittivity C phase into A + B dielectric material can make more grain interface. It results in the increase of dielectric loss. Suppose that the C phase plays a role of pore, the effect of the C phase volume fraction on the dielectric loss is shown in eq 4.37 ⎛ P ⎞2/3 ⎟ tan δ = (1 − P) tan δ0 + AP ⎜ ⎝1 − P ⎠

(4)

where P is the volume fraction of the C phase, tan δ0 is the loss tangent of the full dense B phase materials, A is the coefficient, tan δ0 = 1.565 × 10−5, and A = 9.277 × 10−3. Second, owing to the lattice mismatch, the coherent interface between dissimilar structures (in this case, wurtzite and spinel 2140

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disorder of atom in the interface results in the introduction of a large number of interfacial vacancies and a substantial decrease in the activation energy for vacancies migration. In fact, the absence of ion−ion correlations may produce a decrease in the activation energy of the conductivity. The large in-plane expansive strain on the interface plane, together with the high concentration of vacancies and probable positional disorder, surely contributes to the reduction in the activation energy and the resulting huge enhancement in ionic conductivity.38 The free electron in semiconductor can be mainly attributed to the impurities. The electronic moving activity decreases due to the impact between free electron and impurities. However, the electron of impurities can fall into the middle layer of heterojunction, because the impurities in middle layer of heterojunction have low energy. Therefore, electrons and impurities are separated in space and the electronic move is not influenced by the impact between each other, which provides a greatly enhancement with carrier mobility. The combination of epitaxial strain and suitable heterogeneous interfaces appears to be a key step in the design of artificial nanostructures with high electrical conductivity and dielectric loss.35,38 As a consequence, with raising interface area, the increased electrical conductivity can enhance the dielectric loss of the ZnO/ZnAl2O4 composite. Appropriate ZnO content can make the material achieve reduced surface reflection and improved absorption capability to EM. 4.3. EM Absorption Properties. 4.3.1. Reflection Coefficient and Absorption Coefficient. To reveal the EM absorption properties of the ZnO/ZnAl2O4 composite, reflection coefficient of the composite is determined at a given frequency and thickness using the relative complex permeability and permittivity according to eqs 2 and 3. Figure 7a shows the reflection coefficient of the ZnO/ZnAl2O4/ paraffin composite at a thickness of 2.86 mm. It is observed that the effective absorption bandwidth becomes broader and the minimum reflection coefficient becomes larger with increasing ZnO content. When the ZnO content is less than 90 mol %, the EM absorption properties decrease due to the increase of the ZnAl2O4 content. The increase in the content of ZnAl2O4 reduces the free-electronic mobility, which makes the absorption of electromagnetic energy become less. However, the appropriate ZnAl2O4 content exhibits considerably enhanced EM absorption capability because of the interactions of electromagnetic radiation with charge multipoles at the nanograin interfaces. It is worth noticing that sample B exhibits the best EM absorption properties, and the minimum reflection coefficient reaches −25 dB at 9.7 GHz with an effective absorption bandwidth across the whole X-band. The effective absorption bandwidth is one of the most important parameter for absorption materials. In Table 1 we can observe the absorber composition and absorption properties of the dielectric materials in the literatures. Effective absorption bandwidth is across the whole X-band, which has not been reported. Different from RC, which is a measure of EM absorption property based on metal back-panel model, the definition of absorption coefficient (A) exhibits the capability of an EM absorption material to absorb EM itself. A is defined as the ratio between absorbed power PA and incident power PI without the reflection of metal back-panel when the EM transmits out the materials, that is, A = PA/PI = 1 − |S11|2 − |S21|2,20,39 which is different from metal back-panel model. It is, thus, an alternate formulation, compared to reflection coefficient, for character-

Figure 6. Frequency dependence of (a) real and (b) imaginary part of relative complex permittivity and (c) dielectric loss of ZnO/ZnAl2O4/ paraffin composite with different ZnO content.

in ZnO/ZnAl2O4 heterojunction) provides both a high carrier concentration and, simultaneously, a reduction of activation energy, achieving a greatly enhanced carrier mobility that accounts for the many orders of magnitude increase in the electrical conductivity. Heterojunction is constituted by the interface area of two semiconductors, which have a similar crystal structure, spacing, and thermal expansion coefficient. The interface microstructure of the heterojunction has noticeable changes, compared to the atom edge from the middle of the crystal. These changes are consistent with an enhanced density of vacancies. The partial occupancy and high 2141

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entirely attributed to the interface polarization and dielectric loss. 4.3.2. Destructive Interference. Known from the above analyses, to obtain a minimum reflection coefficient, RC, it is necessary to improve EM absorption capability of material itself, achieving a higher absorption coefficient, A. To employ destructive interference is also an effective method. The wellknown metal back-panel model combines effects of material absorption and destructive interference. It is mostly used to study the EM absorption property.9,12−14,20−26 The schematic illustration of the EM absorbing materials based on the metal back-panel model is shown in Figure 8. For

Figure 8. Schematic illustration showing destructive interference of the EM absorbing materials based on the metal back-panel model.

Figure 7. (a) Reflection and (b) absorption coefficients of the ZnO/ ZnAl2O4 composite with a thickness of 2.86 mm as a function of frequency.

the incident EM, one part can be absorbed, and the other leads to the occurrence of surface reflection. When the angular phase of the upper and bottom surface reflection EM is opposite, it leads to the destructive interference. Transmission coefficient T = 0 because EM is reflected by back-panel, and it is absorbed twice in the materials, 1 = 2A + R. PR includes upper surface reflection power and bottom surface reflection power. Reflection efficient R can be expressed as R = R upper + R bottom = 1 − 2·A (5)

izing the performances of an absorption material.40 The scattering parameters, S11 and S21, were obtained from the VNA by waveguide cavity. The EMI shielding reflection coefficient R, absorption coefficient A, and transmission coefficient T can be calculated. Here, R and T are primarily given as R = |S11|2 and T = |S21|2, respectively. We can use the relation A + R + T = 1 to obtain A. Variation of absorption coefficient of the ZnO/ZnAl2O4/ paraffin composite for different ZnO contents in X-band is given in Figure 7b. When the pure ZnO was combined with paraffin into A + B type material, the ZnO/paraffin has a smaller absorption coefficient, 0.03, implying a weak absorption capability, due to the fewer carriers in the pure ZnO. When 11.1 mol % nanosize ZnAl2O4 is added into sample A, the formed sample B become an A + B + C type material, and sample B has the highest absorption coefficient, 0.37. Because the higher absorption coefficient and thin sample thickness are the requirements on an EM absorption material, absorption coefficient per unit thickness of sample is used to evaluate the absorption properties of the materials. Absorption coefficient per unit thickness increases from 0.01 of sample A to 0.13 of sample B. With the further increase in ZnAl2O4 content, the free-electronic mobility in the A + B + C type material is reduced, which makes the absorption capability of electromagnetic energy decrease. Absorption coefficient per unit thickness decreases from 0.13 of sample B to 0.01 of sample F, which contains 100 mol % ZnAl2O4. Because the composite is a nonmagnetic material, the EM absorbing property of ZnO/ZnAl2O4/paraffin composite is

PR = PI − PA = (1 − 2A)PI

(6)

where Rbottom and Rupper are defined as the ratio of the reflected wave power of bottom (PR‑bottom) and upper surfaces (PR‑upper) to the PI, A is absorption coefficient, which is defined as the ratio of the absorbed EM power (PA) in the materials to the PI. For the PR‑upper and PR‑bottom, there is an angular phase difference. The angular phase difference (Δφ) is decided by the thickness of absorbing materials. When the thickness of EM absorbing materials, d, is approximately a quarter of the propagating wavelength (λ) multiplied by an odd number, that is, d = nλ/4 (n = 1, 3, 5, 7, 9, ...), the signal reflected by the upper surface has a phase opposite to the signal coming from the back reflection, resulting in destructive interference of EM when EM is incident on the surface of the sample.20,23 When the thickness of EM absorbing materials is thick enough (d ≥ dc, dc is critical thickness), the EM entering the EM absorbing materials is completely absorbed, PR‑bottom = 0, PR = PR‑upper. RCupper (dB) is defined as the reflection coefficient at the upper surface of the EM absorbing materials. Because RCupper is not influenced by thickness of EM absorbing materials, 2142

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Figure 9. (a) RC of the EM absorbing materials at different sample thickness and frequency, (b) RC of the EM absorbing materials as a function of thickness at 10 GHz. 2143

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⎛ PR−upper ⎞ RC = RCupper = 10 log10⎜ ⎟ ⎝ PI ⎠

power can be absorbed, and the reflected power of bottom surface decreases gradually. Finally, it can match with the reflected power of upper surface, and the destructive interference effect is gradually enhanced, which can be observed in Figure 9a. For the EM absorbing materials with smaller real part of permittivity, it requires a greater thickness in order to achieve a smaller reflection coefficient. However, for sample B, it can get a smaller reflection coefficient in a relatively thin thickness. Figure 9(b) shows the reflection coefficient with different thickness of EM absorbing materials at 10 GHz. Reflection coefficient is undulate with the increase of the sample thickness. The wavelength is determined by permittivity of EM absorbing materials. When the thickness is a quarter-wavelength, the EM has strong destructive interference. The optimum thickness and the corresponding reflection coefficient in a given frequency are exhibited in Figure 9(b). For sample B, the reflection coefficient is −21 dB, when the thickness is 2.86 mm. It shows the best balance between the lowest thickness and the best absorption. For other samples, attenuation of EM is weak at a quarterwavelength thickness. 4.4. EM Absorption Mechanism. The frequency dependence of the EM absorption properties is attributed to various relaxation processes. Dipolar reorientation processes and interfacial polarization relaxation effects should be considered. The dispersion of electric conductive regions in a lowconducting or nonconducting medium is known to lead to the Maxwell−Wagner−Sillars (MWS) effect. The exact mechanisms can be drawn from the calculation of relaxation time, activation energy and analysis of dielectric relaxation spectra of the composite.39 Nanograin interfaces have a high density of point defects and dangling bonds that may be 3 orders of magnitude higher than the conventionally sized particles. Defects can act as polarization centers, which will generate polarization relaxation under the alternating electromagnetic field and absorb EM. Dangling bonds induce electric dipole polarization. The electron motion hysteresis in the dipole under alternating electromagnetic field induces additional polarization relaxation process which is favorable in enhancing the EM absorbing capability. The interfacial electric polarization should be also considered. The multi-interfaces of A + B + C ternary dielectric materials are beneficial to absorb EM. Submicrometer-ZnO particles are dispersed in ZnAl2O4 nanoparticles, which possess more complicated interfaces which are beneficial to enhance the EM absorption capability.13 Consequently, it is reasonable that the ZnO/ZnAl2O4/paraffin composite possess excellent EM absorption properties. In the composite, the ZnO particles are distributed in the ZnAl2O4 nanoparticles, which can not contact with each other. Therefore, the dispersed ZnO forms discrete local micronetwork structure in the composite powders under the alternating electromagnetic field. The discrete micronetwork structure makes the microcurrent exhaust until it disappears completely in the micronetwork structure. In the present work, micronetwork structure generates a stronger conductive loss, which leads to the increase of EM absorption capability and higher absorption coefficient. An important concept relating to the EM absorbing materials is the impedance match characteristic. The Al-doped ZnO has higher electric conductivity, which may lead to stronger reflection. ZnAl2O4 has lower permittivity and dielectric loss compared with Al-doped ZnO. It can be a suitable host material of Al-doped ZnO, which acts as an EM absorbent distributed in

(7)

When d < dc, part of the EM can not be absorbed completely, leading to the occurrence of secondary reflection, PR = (PR−upper 2 + PR−bottom 2 + 2PR−upper·PR−bottom· 0.5

cos Δφ)

PR PI ⎡ = 10 log10⎢(PR−upper 2 + PR−bottom 2 + 2PR−upper· ⎣

RC = 10 log10

⎤ 0.5 PR−bottom·cos Δφ) /PI ⎥ ⎦

(8)

where Δφ = cos(4πd/λ) and λ = (λ0/(|ε||μ|) ). λ and λ0 are the wavelengths in EM absorbing materials and free space, respectively, |ε| and |μ| are the moduli of ε and μ of the EM absorbing materials, respectively. When cos((4πd(|ε||μ|)1/2)/λ0) = −1, RC = 10 log10((PR−upper − PR−bottom)/PI), RC can take the minimum value. Known from eqs 6 and 8, the key to decrease the reflection coefficient is to enhance the PA and absorption coefficient A as large as possible, and to make the PR−upper match well with the PR−bottom. When the PR−bottom and PR−upper have equal value and inverse phase, it results in strong destructive interference and weak reflection coefficient. Based on the above analysis, designing the thickness of EM absorbing materials and using the destructive interference are useful to improve absorption properties. Three-dimensional graphics of reflection coefficient for different ZnO contents and thickness of absorbing materials are given in Figure 9, which are calculated by eq 2. Attenuation in the dielectric absorbing materials consists of absorption and destructive interference effects. Incident EM wavelength in absorbing material varies with the permittivity. As shown in Figure 9a, the thickness of the EM absorbing materials, which is an odd multiple of a quarter-wavelength, result in the destructive interference. This is caused by the inverse phase of the reflection EM from the upper and bottom surfaces. The EM absorbing materials with different permittivity have different upper surface reflection coefficients, and incident EM will have different power attenuation. The remaining EM is reflected from the bottom surface. The destructive interference effect is strongest when the upper surface reflection power matches well with the bottom surface reflection power. When the EM absorbing materials are thick enough, the reflection coefficient tends to be a stable value. As shown in Figure 9a, when the thickness of sample B is a quarter-wavelength, it results in the strongest destructive interference and the minimal reflection coefficient. It shows that the reflection power of the upper and bottom surface is matching. For other samples, due to the weak absorption capability of the EM, the reflection coefficient of upper surface is small, and the absorption of incident EM is weak too, when the thickness is a quarterwavelength. Therefore, the power reflected by the bottom surface is greater than that reflected by the upper surface. As a result, the destructive interference effect is not obvious. As the thickness of the EM absorbing materials increases, more EM 1/2

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an EM-transparent matrix, paraffin, and plays a crucial role in altering the overall dielectric property to meet the requirement of impedance match. Therefore, it is beneficial for the EM to penetrate into composite material formed by the ZnO particles dispersed in ZnAl2O4 nanoparticles and paraffin matrix. The EM energy will be induced into a dissipative current, and then the current will be consumed, which leads to the EM power absorption.

V. CONCLUSIONS In summary, a novel EM absorbing material composed of low, medium, and high permittivity phases have been established. The increase of EM absorption capability can be attributed to the interface effect on nanointerface between the high permittivity phase and the medium permittivity phase. As an example, ZnO/ZnAl2O4/paraffin composite materials have been synthesized by the sol−gel process. SubmicrometerZnO grains are dispersed in ZnAl2O4 nanoparticles, which are uniformly distributed in an EM-transparent matrix, paraffin. High carrier concentration and the high mobility in the heterojunction of ZnO/ZnAl2O4 composite are beneficial to increase interfacial polarization capability and dielectric loss. The lower surface reflection and strong absorption capability can well meet the requirement of impedance match, and the destructive interference plays an important role in EM absorbing properties. The absorption coefficient per unit thickness increases from 0.01 to 0.13/mm, minimum reflection coefficient reaches −25 dB, and effective absorption bandwidth is cross the whole X-band. The ZnO/ZnAl2O4/paraffin composite with excellent EM absorption properties exhibit a promising prospect as a kind of EM absorbing materials.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 29 88494947. Fax: +86 29 88494620. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the funding from the State Key Laboratory of Solidification Processing in NWPU (No. KB200920), the Natural Science Foundation of China (Grant: 50972119), and the 111 Project (B08040).

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