Lightweight Gadolinium Hydroxide@polypyrrole Rare Earth

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Functional Nanostructured Materials (including low-D carbon)

Lightweight Gadolinium Hydroxide@polypyrrole Rare Earth Nanocomposites with Tunable and Broadband Electromagnetic Wave Absorption Wei Wei, Xiao-Guang Liu, Wanli Lu, Han Zhang, Jun He, Haicheng Wang, and Yanglong Hou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21516 • Publication Date (Web): 08 Mar 2019 Downloaded from http://pubs.acs.org on March 10, 2019

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ACS Applied Materials & Interfaces

Lightweight Gadolinium Hydroxide@polypyrrole Rare Earth Nanocomposites with Tunable and Broadband Electromagnetic Wave Absorption Wei Wei a, Xiaoguang Liu b, Wanli Lu a, Han Zhang a, Jun He c, Haicheng Wang a*, Yanglong Hou d

a National Center for Materials Service Safety, University of Science and Technology Beijing, Beijing 100083, China. b Department of Inorganic Nonmetallic Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. c Institute of Functional Materials, Central Iron & Steel Research Institute, Beijing, 100081, P.R. China.

d Beijing Key Laboratory for Magnetoelectric Materials and Devices (BKLMMD), Beijing Innovation Center for Engineering Science and Advanced Technology (BIC-ESAT), Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China

ACS Paragon Plus Environment

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Key words: gadolinium hydroxide, rare earth nanocomposites, electromagnetic wave absorber, light weight, impedance matching

ABSTRACT

Lightweight and highly efficient nanocomposite absorbing materials are gaining tremendous interest in recent years. Due to the unique electronic structure characteristics, nanoscale rare earth materials are of great significance in the development of advanced functional materials. Herein, gadolinium hydroxide/polypyrrole（Gd(OH)3@PPy）nanocomposites were synthesized by a facial chemical solution route. The composites could achieve absorbing performance of 51.4 dB at 16.2 GHz, with a bandwidth 4.8 GHz covering the entire Ku band at a thickness of only 2.2 mm. Furthermore, the absorption intensity and bandwidth can be effectively tuned by adjusting the concentration of Gd(OH)3 in the composite. Due to the improvement of impedance matching, dual loss mechanism and the synergistic effect of rare earth hydroxide and conductive polymer, lightweight gadolinium hydroxide@polypyrrole composites are promising candidates for strong and broadband electromagnetic wave absorption.

1. INTRODUCTION Electromagnetic (EM) waves absorbing materials can significantly improve the survivability and penetration ability of the weapon systems, eliminate the electromagnetic interference of communication and navigation systems, and protect the physical security of humans. 1-4 With the development of military stealth technology and the electro-magnetic compatibility technology in the internet of things ， electromagnetic wave absorbing coating materials have attracted more and more attention. During recent years, promising improvements have been achieved about the


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absorbing materials from either the material category, design or synthesis point of view, by the promotion of application requirement and technical innovation.5-9 The microwave absorption performances are strongly associated with the complex permittivity (εr = ε' − jε′′) and complex permeability (μr = μ′ − jμ′′) and the impedance matching between them. Traditional electromagnetic wave absorption materials, 10-14 such as ferromagnetic metals, carbons, ceramics and conducting polymers, cannot meet the requirements thoroughly owing to their high density, poor stability and bad impedance matching when applied as a single EM absorbing material.15-19 With the rapid development of the high-frequencies communication technology, the electromagnetic wave absorption materials have been designed to suit four conditions: lightweight, strong absorption property, broad frequency bandwidth and high service stability.20-22 Graphene has shown attractive potential for electromagnetic wave absorption due to their much lower density in recent years. Zhang et al used a graphite-based material prepared by insitu method to achieve absorbing properties of -22 dB at 5.3 GHz.23 Wang et al successfully synthesized NiO@graphene material which achieved absorbing performance of -59.6dB at 14.16 GHz, and the corresponding absorbing thickness was only 1.7mm.24 However, the application of graphene is still restricted because of the difficulty of industrial scale-up production. Polypyrrole, as a kind of conductive polymer, not only have unique physical and chemical properties but also lower density and easier large scale industrial production, which will benefit for the practical application.25-26 However, the pure conductive polymer has a single performance and cannot meet the requirements of wide frequency band and strong absorption. The magnetic/dielectric nanocomposites with controllable structures are of attractive candidates for advanced absorbing materials due to the multi-loss mechanism (magnetic loss,


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dielectric loss and interfaces polarization loss) and better impedance matching. For example, Zhang et al successfully synthesized CoNi@SiO2@PPy composite material, and obtained the best absorbing performance of -34.19 dB at 9.59 GHz, and the corresponding thickness is only 2.12 mm.27 Li et al successfully synthesized rGO-CNT-Fe3O4 composites, and achieved the best absorbing properties of -50.5 dB at 1.42 mm.28 Our group successfully synthesized Co@ZIF-67 composite material, and realized the best absorbing performance of -30.31 dB at 3 mm.29 Although these great progress has been made in the field of absorption materials, the problems such as narrow bandwidth and thick absorbing coating still need a better improvement. 30 Rare earth elements have typical magneto-crystalline anisotropy due to the unpaired 4f electrons and the strong spin–orbit coupling of their angular momentum, which result in the improved electrical and magnetic properties.31 Some studies have shown the effects of rare earth elements on static magnetic properties and electromagnetic wave absorbing properties of M-type ferrite in recent years.32-33 The unique magnetic property could attribute to the absorption to the electromagnetic wave.34 However, the higher density of the ferrite still impedes the applications of the rare earth based composites.35 Light weight still remains a challenge to the advanced absorbing materials.36 Therefore, nanoscale rare earth based composites show great potential in the practical application of the advanced absorbers. To our best knowledge, gadolinium hydroxide and polypyrrole nanocomposites for absorbing materials have not been reported in the literature. In this paper, gadolinium hydroxides are synthesized by a hydrothermal method, and then composited with polypyrrole by in-situ chemical oxidation. A schematic diagram of synthesis of gadolinium hydroxide polypyrrole composites is illustrated in Figure 1.


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Figure 1. Schematic diagram of synthesis of Gd(OH)3@PPy composites. 2. EXPERIMENTAL SECTION 2.1 Materials. Gadolinium nitrate hexahydrate (Gd(NO3)3·6H2O, 99%), laurylamine (C12H27N,AR), pyrrole (C4H4N,99%), ammonium persulfate ((NH4)2S2O8,AR), sodium dodecyl benzene sulfonate (C18H29NaO3S, 90%), polyvinylpyrrolidone (PVP, AR) were purchased from Acros. Ethanol (CH3CH2OH, AR) was purchased from Sinopharm Chemical Reagent. All chemicals were not further purified. 2.2 Preparation and Modification of gadolinium hydroxide. 0.4 mmol of gadolinium nitrate hexahydrate was dissolved in 20 mL of deionized water, and 10 mL of laurylamine was dissolved in 20 mL of absolute ethanol. After stirring for 15 min, the two solutions were mixed and transferred to a 100 mL reaction vessel and incubated at 180 ℃ for 18 hours. After centrifugation, the acquired precipitate was washed by deionized water and absolute ethanol several times and then dried for use. 0.1 g of Gd(OH)3 was dissolved in 50 mL of absolute ethanol and stirred. 1g of polyvinylpyrrolidone (PVP) was dissolved in 20mL of absolute ethanol, and then injected slowly into the Gd(OH)3 solution. After stirred magnetically and reacted for 24 hours, the precipitation were centrifuged thoroughly with deionized water and ethanol, and then dried for use.


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2.3 Preparation of Gd(OH)3@PPy composite. 0.7 g of ammonium persulfate was dissolved in 40 mL of deionized water and cooled down to 0 ℃. Different amounts of modified Gd(OH)3 were added to 20 mL of deionized water. After being diluted to 200 mL, 0.1 g of sodium cetylbenzene sulfonate was dissolved into the solution. 2 mL of pyrrole monomer was dissolved in 8 mL of absolute ethanol and then added to the above 200 mL of aqueous solution. The mixed solution was transferred to a low temperature tank, stirred at 0℃ for 10 min, and then pre-cooled ammonium persulfate solution was added dropwise, followed by reacting at 0℃ for 7 h. After the reaction, the precipitation was suction filtered with deionized water and absolute ethanol, and dried for use. 2.4 Characterization. The magnetic properties of the samples were measured using a Physical Properties Measurement System (MMPS-VSM-7T). The chemical bond of the composite was characterized by Fourier Transform Infrared Spectroscopy (SPECTRUMONE, 7800-350 CM). The morphology, crystal structure and composition of the particles were determined by Transmission Electron Microscopy (FEI TECNAI F30 and FEI Titan ETEM (Cscorrected)). 2.5 Electromagnetic Performance Measurement. An Agilent N5230C vector network analyzer was used to measure the electromagnetic parameters (complex permittivity and complex permeability) of the samples in the frequency of 1-18 GHz according to the coaxial reflection/transmission method. A cylindrical sample, with thickness of 2.00 mm and with inner and outer diameter of 3.00 mm and 7.00 mm respectively, was fabricated by mixing paraffin wax with gadolinium hydroxide@polypyrrole nanocomposite absorbent (20% wt). On the basis of the transmission line theory, the reflection loss (RL) value of the absorbing material can be calculated according to Equation 1.37


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RL  20log

Z i  Z0 Z i  Z0

(1)

Zi is the normalized input impedance of the absorbing material:

Zi  Z0

r   2fd   tanh i  r r  r  c  

（2）

Where f is the frequency of the electromagnetic wave, d is the thickness of the absorbing material, c is the speed of light, μr and εr are the complex permeability and complex permittivity, Z0 is the impedance of free space, and Zi is the impedance of the absorbing material. 3. RESULTS AND DISSCUSSION As shown in Figure 2a), the gadolinium hydroxide shows good crystallinity and rod-like structure with a length of about 100 nm and a width of about 10 nm. The measured interplanar spacing is 0.315 nm, which agrees well with the (110) crystal orientation of Gd(OH)3. Figure 2b) is a TEM image of pure polypyrrole ， which shows that polypyrrole are nanoplates with a diameter of about 80 nm. From Figure 2c) it can be seen that the rod-shaped gadolinium hydroxides composited well with polypyrrole. Figure 2d-h) shows the STEM-HAADF image and EDS mapping images of elements and the corresponding distribution of Gd(OH)3@PPy composite. This compositional structure has large specific surface area, which will increase the process of interfacial polarization and multiple scattering during absorbing.38 Besides, the magnetic units, gadolinium hydroxides, would also have magnetic resonance loss to electromagnetic wave.39-40 The synergistic effect of magnetic Gd(OH)3 and dielectric polypyrrole will endow the composites with strong electromagnetic absorption.


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Figure 2. a-c) TEM bright field images of Gd(OH)3, polypyrrole, Gd(OH)3@PPy, d) STEMHAADF image of Gd(OH)3@PPy, e~h) EDS mapping of elements and corresponding distribution. Figure 3 a) shows that pure gadolinium hydroxide is paramagnetic at room temperature, and the maximum magnetic susceptibility is 11.6×10-5 emu/g·Oe. It can also be seen that the magnetic susceptibility of the pure polypyrrole is negative, indicating that the polypyrrole material is diamagnetic. When 0.025 g of Gd(OH)3 is doped, the composite is still diamagnetic,


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and the maximum magnetic susceptibility is -8.88×10-6 emu/g·Oe. As the concentration of Gd(OH)3 increases, the material exhibits paramagnetic property. The maximum magnetic susceptibility of the composites with different Gd(OH)3 concentration is 2.95 × 10 -6 emu/g·Oe, 10.2 × 10

-6

emu/g · Oe and 14.8 × 10

-6

emu/g · Oe, respectively. The magnetic property of

Gd(OH)3@PPy composites can be controlled by the concentration of gadolinium hydroxide.

Figure 3. Magnetization curves the samples. a) pure polypyrrole and pure Gd(OH)3, b) composites with different Gd(OH)3 concentration. The complex permittivity (εr = ε' − jε′′) and complex permeability (μr = μ′ − jμ′′) of the absorber are key parameters for characterizing the electromagnetic properties.41 To achieve better impedance matching and excellent absorption, the electromagnetic parameters (ε, μ) of the material should be properly designed and selected. Here, the effect of concentration of Gd(OH)3 on the electromagnetic parameters of the composites were investigated. Figure 4 shows the real and imaginary parts of the complex permittivity and complex permeability (ε', ε", μ', μ") of Gd(OH)3@PPy composites with different gadolinium hydroxide concentration. Their ε' value of the five samples tends to decrease continuously throughout the whole range of 1-18 GHz at various extents. Specifically, the ε' value dropped from 14.58 to 5.23, from 19.59 to 5.77, from


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12.36 to 4.70, from 11.08 to 4.29 and from 10.78 to 4.71 for the corresponding five samples. Similarly, the ε′′ value decreases from 20.31 to 3.17, from 26.64 to 4.56, from 13.58 to 2.53, from 8.82 to 2.17, and from 7.42 to 1.95, respectively. Both ε' and ε" of the samples decrease faster firstly at 1~8 GHz and then relatively slower at 8~18 GHz. With increasing of the concentration of Gd(OH)3, the dielectric constant of the composite increase first and decrease afterwards. This can be explained that the addition of rod-shaped Gd(OH)3 could increase the dielectric polarization relaxation. However, with further increasing the concentration of Gd(OH)3, the proportion of polypyrrole in the composite is reduced, eventually leading to a gradual decrease in the real and imaginary parts of the dielectric constant. At the meantime, Figure 4d-e) illustrates the fluctuation of μ' and μ" of the five samples with increasing of frequency. Figure 4c) and f) shows the dielectric loss tangent and the magnetic loss tangent of the composites. It can be seen that, the magnetic loss tangent increased with the increasing concentration of Gd(OH)3. Though the magnetic loss tangent is less than the dielectric loss tangent, the impedance matching is improved, and also the dual loss mechanism will benefit for the absorption to electromagnetic wave.


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Figure 4. Gd(OH)3@PPy composites with different gadolinium hydroxide content correspond to a) ε'; b) ε''; c) tan δE; d) μ'; e) μ''; f) tanδM. The dielectric loss of materials mainly originates from conductivity loss and polarization loss.42 Polypyrrole is a kind of conductive polymer material. The conductivity of this material is provided by the non-locality of the π-electrons of the double bonds in the molecule.43 After


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doping with Gd(OH)3, the delocalization of π electrons in polypyrrole is reduced, and the electron migration ability is thus weakened, which may cause the decreasing of the electrical conductivity of composites. ε′′ =

ωτ(𝜀𝑆 ― 𝜀∞) 1 + 𝜔2𝜏2

(3)

+𝜎/𝜔𝜀0

In Equation 3, ω, τ, εs, ε∞ and σ represent the loss angle, polarization relaxation time, static dielectric constant, relative permittivity and conductivity at high frequencies, respectively. Generally, as the conductivity decreases, the imaginary part of the dielectric constant of the material also decreases. It can be seen from Fig. 4b that the imaginary part of the dielectric constant of the material is generally reduced with doping of gadolinium hydroxide. At the meantime, the decreasing of dielectric constant is beneficial to the improvement of impedance matching. The polarization loss of the absorbers mainly includes several mechanisms such as ion polarization, dipole polarization, electron polarization and interfacial polarization.44 However, in general, the ion polarization process has not kept up with the change of the frequency of the 105 Hz electromagnetic field,45 and the electron polarization is only present in the band 1013-1016 Hz.46 Therefore, the polarization loss of the composites synthesized in this paper may have dipole polarization and interfacial polarization. In order to further characterize the mechanism of electrical loss in materials, the Debye dipole relaxation model is introduced in this paper.47-48 In this model, the complex permittivity ε can be expressed by Equation 4: 𝜀𝑆 ― 𝜀∞

ε = ε′ ― 𝑖ε′′ = ε∞ + 1 + 𝑖𝜔𝜏0

(4)


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Where τ0 is the relaxation time, ω=2πf is the angular frequency, εS is the static dielectric constant, and ε∞ is the dielectric constant of the frequency at infinity. According to Equation 4, it can be concluded that: 𝜀𝑆 ― 𝜀∞

𝜀′ = 𝜀∞ + 1 + (𝜔𝜏 )2 0

ε′′ =

𝜔𝜏0(𝜀𝑆 ― 𝜀∞) 1 + (𝜔𝜏0)2

(5) (6)

From equation 5 and Equation 6, equation 7 can be derived, as shown below : (𝜀′ ― 𝜀∞)2 +(𝜀′′)2 = (𝜀𝑆 ― 𝜀∞)2

(7)

Each semi-circle in the curve corresponds to one polarization loss mechanism. Figure 5 illustrates Cole-Cole curves of Gd(OH)3@PPy composites. It can be seen that the composite consists of two semicircles and a long smooth tail, which indicates that the composite has multiple relaxation processes and conductive processes in the 1-18 GHz band.49 In terms of the relaxation process, there are mainly dipole polarization and interfacial polarization in the given frequency range. Dipole polarization is a polarization phenomenon caused by the electric field under the action of the electric field, during which the dipole moment inherent in the material is rearranged along the electric field, and the macro dipole moment is not zero. Besides, the interface polarization is due to the formation of electrons or ions at the interface under the action of an applied electric field. There are plenty of interfaces between gadolinium hydroxide and polypyrrole, which will be beneficial to the occurrence of interfacial polarization. Therefore, it can be concluded that the polarization of the composite include dipole polarization, the interfacial polarization, and also the conductive process could contribute a better wave absorption.


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Figure 5. Cole-Cole curves of composites with different gadolinium hydroxide doping concentration. a) pure PPy, b) Gd(OH)3(0.025g)@PPy,

c) Gd(OH)3(0.050g)@PPy, d)

Gd(OH)3(0.075g) @PPy, e) Gd(OH)3( 0.100g). As for the main magnetic loss mechanisms, it are generally divided into hysteresis loss, domain wall resonance loss, eddy current loss and ferromagnetic resonance loss.50-53 The hysteresis effect is originated from the change of the magnetic induction caused by the irreversible rotation of the magnetic moment with the magnetic field. However, as shown in Figure 3, the Gd(OH)3@PPy composite exhibits reverse ferromagnetism at room temperature, and there is no Rayleigh region (reversible wall-shifted magnetization region of static magnetization). Therefore, it can be inferred that no hysteresis loss occurs in this composite. Domain wall resonance means that under the action of an alternating magnetic field, the domain wall is subjected to a force to vibrate at an equilibrium position. However, this loss generally occurs in the low frequency band (MHz), not in the measurement range of 1-18 GHz, thus it can also be ignored.54 Considering if its magnetic loss comes largely from the eddy current loss, C0 (C0 = μ"(μ')-2 f-1) should be a constant.55-57 Figure 6 shows that the C0 curve of Gd(OH)3@PPy composites with different Gd(OH)3 concentration. It can be seen that C0 value of these composites varies between -0.010 to 0.005. Therefore, it can be deduced that the main loss mechanism of the material is eddy current loss in the range of 4-18 GHz. Besides, an apparent peak in the 1-4 GHz may correspond to the ferromagnetic resonance loss.52


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Figure 6. C0 curve of Gd(OH)3@PPy composite with different gadolinium hydroxide. As a kind of rare earth material, gadolinium hydroxide material has good paramagnetism, and the rod-shaped gadolinium hydroxide material prepared in this paper has shape anisotropy, which leads to good magnetocrystalline anisotropy. The magnetic properties of magnetically absorbing materials at high frequencies are mainly subject to natural resonance. For isotropic magnetic materials, there is a Snoek limit:58-59 1

(8)

(μ𝑖 ― 1)𝑓𝑟 = 3𝜋γ𝑀𝑠

Where γMs is the Snoek constant, γ is the gyromagnetic ratio, Ms is the saturation magnetization, fr is the natural resonance frequency, and μi is the initial permeability. It is known from Snoek that it is difficult to simultaneously increase the magnetic permeability and resonance frequency of a magnetic material. Hence, anisotropy is a more effective way to break through the snoek limit. For anisotropic magnetic materials, the snoek limit can be rewritten as:60 1

(μ𝑖 ― 1)𝑓𝑟 = 3𝜋γ𝑀𝑠

𝐻𝜃 𝐻𝛷

(9)


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Among them, Hθ is an anisotropic equivalent field of easy magnetization direction, and HΦ is an anisotropic equivalent field of hard magnetization direction. When Hθ is much larger than HΦ, the magnetic permeability and resonant frequency of the material can be simultaneously improved, thereby improving the absorbing properties of the material. Therefore, the introduction of the rod-shaped gadolinium hydroxide will be greatly beneficial to the regulation of the absorbing properties of the composite. From the point of view of electromagnetic wave absorption, with the incorporation of gadolinium hydroxide, the conductive network of polypyrrole is broken, and the conjugation effect of polypyrrole is weakened, resulting in less conductive loss. While at the same time, the interface between gadolinium hydroxide and polypyrrole is increased, and the interfacial polarization effect of the composite is enhanced. From the perspective of electromagnetic matching, with increasing of concentration of gadolinium hydroxide in the composite, the dielectric constant decreases gradually whereas the magnetic permeability gradually increases, which is beneficial to better impedance matching. As the concentration of gadolinium hydroxide further increased, the dielectric properties of the composite material are reduced sharply accompanying with a slight increase of magnetic permeability, which will result in a bad impedance matching. The electromagnetic matching of the composite could be thus controllable by tuning the concentration of gadolinium hydroxide. Figure 7 shows the reflection loss value of Gd(OH)3@PPy composites with different Gd(OH)3 concentration calculated according to the transmission line theory.61 It can be seen that Gd(OH)3(0.075g)@PPy composite can reach a maximum reflection loss of -61.8dB at 10.4GHz with a bandwidth of 4.9 GHz (8.4~13.3 GHz). Meanwhile, Gd(OH)3(0.050g)@PPy can achieve reflection loss of -51.4 dB at 16.2 GHz with a bandwidth 4.8 GHz (13.2-18 GHz) covering the


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entire Ku band with a thickness of only 2.2 mm. It can be seen from Figure 8 that as the concentration of Gd(OH)3 increases, the effective bandwidth shifts from the Ku band to the X band, and finally moves to the C band, realizing the regulation of the effective bandwidth of the composites. Therefore, it can be concluded that the concentration of gadolinium hydroxide can not only effectively enhance the absorption intensity to electromagnetic wave, but also control the absorption frequency and bandwidth.


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Figure 7. The 3D reflection loss contour maps of Gd(OH)3@PPy composite with different concentration of gadolinium hydroxide and the optimal reflection loss of the best matching thickness

of

the

corresponding

samples.

a~e):

PPy,

Gd(OH)3(0.025g)@PPy,

Gd(OH)3(0.050g)@PPy, Gd(OH)3(0.075g)@PPy and Gd(OH)3( 0.100g). f~j): corresponding reflection loss map at the optimal thickness for each sample.

Figure 8. a) Gd(OH)3(0.050g)@PPy, b) Gd(OH)3(0.075g)@PPy and c) Gd(OH)3(0.100g) @PPy reflection loss at optimum thickness. According to the theory of quarter-wave impedance matching, the material has the following quantitative relationship:61 𝑑𝑚 = 4𝑓

𝑚

𝑛𝑐

|𝜀𝑟𝜇𝑟|,(𝑛 = 1,2,3···)

（10）

Where dm is the thickness of the material coating, c is the speed of light, fm is the frequency corresponding to the optimum thickness of the matching loss, and εr and μr are the complex


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permittivity and complex permeability. When the electromagnetic wave incident on the material is 180 degrees out of phase with the reflected electromagnetic wave, the electromagnetic wave is attenuated. In order to confirm that the composite material obtained by doping 0.05 g Gd(OH)3 has the best comprehensive absorbing property at 2.2 mm, the calculated value of the thickness and the experimental value obtained by the Equation (10) are plotted in Figure 9 where tmexp is the experimental value and the curve tmfit is the simulated value. The two values are well matched, which indicates that this material is consistent with the quarter-wavelength theory. It can also be derived from Equation (10) that dm is inversely proportional to fm, that is, in order to regulate the optimal absorbing properties of the material to low frequencies, the material needs to increase the coating thickness accordingly. Up to now, it is still a great challenge to obtain excellent absorption performance at lower frequency band. It can be seen that, Gd(OH)3(0.10g) @PPy can achieve a reflection loss value of -61.8 dB with a bandwidth of 3.3 GHz nearly covering the whole X-band despite of having a relatively thicker thickness. Lightweight Gadolinium hydroxide/PPy composites could be great potential candidates for advanced absorbers with strong absorption, broad frequency bandwidth and high service stability.


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Figure 9. a) Gd(OH)3(0.050g) @PPy absorption performance under different thickness; b) simulated thickness based on 1/4 wavelength theory; c) controllable absorption bandwidth under different thickness.


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4. CONCLUSIONS Series of gadolinium hydroxide@polypyrrole composites are synthesized by hydrothermal method and in situ polymerization process. The absorption of the composites could cover the cband, X-band and Ku-band of electromagnetic waves by tuning the concentration of gadolinium hydroxide. Gd(OH)3(0.05g)@polypyrrole material (20 wt%) can obtain a maximum RL of 51.4dB at 16.2 GHz with a thickness of 2.2mm, accompanying with bandwidth 4.8GHz (13.2GHz~18GHz). When the concentration of Gd(OH)3 is increased to 0.075 g, the absorption value of -61.8 dB can be achieved with a bandwidth 4.9 GHz covering most of the X-band. The introduction of gadolinium hydroxides endows the composites with better oxidation resistance, good shape anistropy and magnetocrystalline anistropy which will be benefit for improving the permeability and resonant frequency of the materials. The dielectric loss of the composite mainly originates from the dipole polarization, interface polarization and conduction loss. Meanwhile, magnetic loss is mainly resulted from the eddy current loss and the natural ferromagnetic resonance loss. These gadolinium hydroxide@polypyrrole nanocomposites with well-designed constituents are promising candidates for electromagnetic wave absorbers due to the good impedance matching and strong attenuation capability originating from the dual-loss mechanism and synergistic effect of the components. It is believed that this work could provide new inspiration and strategy for lightweight, broadband and highly efficient electromagnetic wave absorbing materials.

ASSOCIATED CONTENT Supporting Information


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Figure S1 FT-IR curves of pure polypyrrole and gadolinium hydroxide @polypyrrole composite. Figure S2 Reflection loss curves of pure Gd(OH)3, pure PPy and the composites. Table S1 Comparison of reflection loss of pure PPy and Gd(OH)3@PPy composites. AUTHOR INFORMATION Corresponding author: [email protected] Notes § The authors declare no competing financial interest.

ACKNOWLEDGMENT H.W. acknowledges financial support from the National Natural Science Foundation of China (51101013, 21374009) and the Fundamental Research Funds for the Central Universities (FRFTP-15-007A3, FRF-GF-17-B16). Y.H. acknowledges financial support from the National Natural Science Foundation of China (51590882, 51631001) and the State key Project of Research and Development of China (2017YFA0206301).

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Graphical Abstract


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Lightweight Gadolinium Hydroxide@polypyrrole Rare Earth

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