Weather-manipulated smart broadband electromagnetic

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Functional Nanostructured Materials (including low-D carbon)

Weather-manipulated smart broadband electromagnetic metamaterials Kai-Lun Zhang, Xiao-Dong Cheng, Ya-Jing Zhang, Mingji Chen, Haosen Chen, Yazheng Yang, Wei-Li Song, and Daining Fang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15643 • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 4, 2018

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ACS Applied Materials & Interfaces

Weather-Manipulated Smart Broadband Electromagnetic Metamaterials

Kai-Lun Zhang,a,b,c,ǂ Xiao-Dong Cheng,a,b,ǂ Ya-Jing Zhang,a,b Mingji Chen,a,b Haosen Chen,a,b Yazheng Yang,a,b Wei-Li Song, a,b,* Daining Fanga,b

a

Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing

100081, P. R. China. b

Beijing Key Laboratory of Lightweight Multi-functional Composite Materials and

Structures, Beijing Institute of Technology, Beijing 100081, P. R. China. c

School of Materials Science & Engineering, Beijing Institute of Technology, Beijing

100081, China.

* Corresponding author: E-mail: [email protected] ǂ These authors made equal contribution to this work.

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Abstract Smart materials and structures with tunable electromagnetic (EM) properties are highly demanded for active environmental sensitive systems. As polar molecules in the environment, in this work, we utilize water and ice as wetting and freezing conditions to manipulate the electromagnetic response behaviors in a graphene-based composite material, aiming to achieving a smart weather-manipulated EM metamaterial. Owing to the introduced polar water and ice phase in the self-assembled porous electromagnetic attenuating networks, energy consumption of EM wave is significantly altered via multiple scattering of polar induced interfaces. In frozen condition, a wide absorption band (2 ~ 18 GHz) with efficient absorption (reflection loss < -10 dB) has been obtained. Additionally, the mechanical feature of the as-assembled metamaterials could be also manipulated via altering the weather conditions in terms of changing the phase of the introduced water. Interestingly, the mechanical properties could be massively changed while the broadband absorption capability has rarely been impacted. Implication of the results highlights an efficient method for fabricating smart EM metamaterials that are able be manipulated by the environment.

Key words: Smart materials; Metamaterials; Weather manipulation; Electromagnetic; Microwave absorption.

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1. Introduction Smart electronic technologies, which is the key part of smart life, mainly focus on the interaction between devices and environment/human beings. Many smart materials and structures with sensitive response to the change of environment are widely used as sensors or switches in the smart devices and systems.1-5 For a typical smart material, the external stimuli like mechanical force, thermal energy, electricity, chemical molecules, etc., could induce the change of structures and components,6-10 leading to variations of intrinsic physical and chemical properties.11-13 In the fabrication of smart devices, the electromagnetic (EM) wave possesses advantages in detection, scanning, and communication technologies. For the control of EM energy, considerable efforts have been paid for the efficient absorption materials.1424

Owing to the unique electrical conductivity and microstructures, strong absorption

peaks with broadband reflection loss (RL) less than –10 dB (linked with the effective absorption bandwidth) have been realized in the graphene-based composites.25-28 In addition to porous materials,29-33 optimization of metamaterials exhibits significant advance in the fabrication of broadband absorption material, for substantially combining the advantages from various types of EM attenuation mechanisms.34-38 Consequently, the absorption performance would be highly dependent on the structure size, and the frequency of EM responses could be manipulated by structure variation.39 Additionally, the manipulation of EM properties could be realized in more convenient way.40-42 For examples, Lv et al. provide voltage-boosting strategy for the manipulation of EM properties, more than 85% absorption at 1.5 ~ 2.0 GHz was achieved through an 3



applied voltage of 16 V.41 He et al. proposed graphene metamaterial with tunable resonance transmission curves, resulting from manipulated Fermi level by applied electric field.42 For the developing of smart devices with self-adaptive EM properties, typical environment conditions are considered as the options for manipulating EM response behaviors. However, the climate conditions don’t have any influence on the performance of electromagnetic wave in the frequency range of 2 ~ 18 GHz, which drives us to find smart structures with tunable EM properties. Song and coworkers reported smart sandwich structures with hydro-sensitive EM interface shielding performance.11 In our previous work,12 a highly sensitive humidity graphene-based dielectric has been also well studied. For well utilizing the polar molecules of water in environment, in the present contribution, we demonstrate weather-manipulated broadband electromagnetic metamaterials, in terms of tailoring the intrinsic dielectric properties and mechanical features according to the phase states of water. Under various environmental circumstances, either wet and iced conditions were observed to substantially alter the electromagnetic absorption performance of the metamaterials via changing the polar components in the as-assembled porous graphene-based composite networks. Interestingly, the solid phase of water incorporated into the composite networks could not only broaden the entire absorption bandwidth (2~18 GHz) of the metamaterials, but also change the mechanical features from soft to rigid fashion. Based on the sustainable conditions of the environmental changes (Figure 1), such weatherdependent electromagnetic metamaterials highlight a new platform for designing and 4


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manufacturing smart materials with both manipulatable electromagnetic and mechanical properties.

Figure 1 Illustration of smart materials with weather-manipulated EM property.

2. Results and discussion In the design of the smart porous materials with weather-manipulated properties, polypropylene non-woven fabrics (PNF) were selected as the porous substrates for construct hierarchical graphene-based composites via self-assembling of graphene into the three-dimensional (3D) PNF frameworks. Briefly shown in Figure 2a, the cleaned 3D PNF framework was immersed into the mixture of graphene oxide (GO) solution and hydroquinone. The added hydroquinone acts as reduction agent in the following thermal treatment, and the RGO sheets were in-situ grown onto the 3D PNF framework. The microscale fibers of PNF were surrounded by RGO, forming 3D scattering networks for EM waves (Figure 2b). The PNF-RGO composites with microscale 3D 5



scattering network can promote the attenuation of EM energy, similar with the porous graphene-based materials29. According to the concentrations of used GO solutions (5 mg/mL and 8 mg/mL), the fabricated samples were assigned as PNF-RGO5 and PNFRGO8 separately.

Figure 2 Fabrication and characterization of samples: (a) fabrication process and microstructure of PNF-RGO; (b) schematic of the multiple reflection scattering for EM wave (right); cross-section SEM images of pure PNF (c, f), PNF-RGO5 (d, g) and PNFRGO8 (e, h) with different magnification.

The microstructures of the fabricated samples were characterized with scanning electron microscopy (SEM) technique. Cross-section SEM views of the pristine PNF (Figures 2c and 2f), PNF-RGO5 (Figures 2d and 2g) and PNF-RGO8 (Figures 2e and 2h) are given in Figure 2. In the pristine PNF, randomly interwoven polypropylene 6


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fibers constructs into 3D framework (Figures 2c and 2f). After immersing into GO mixture and thermal treatment, RGO nanosheets were easily wrapped on the surface of polypropylene fibers due to the large surface and interaction between RGO and polypropylene. Consequently, unique 3D scattering microstructures were selfassembled in PNF-RGO composites. As shown in Figures 2d and 2g, the polypropylene fibers were wrapped by thin RGO sheets, forming random reflective interfaces for scattering EM waves (Figure 2b). In Figures 2e and 2h, there are increased RGO sheets on the surface of polypropylene fibers due to higher concentration of GO solution used in the fabrication of PNF-RGO8. The complex permittivity of the samples was measured on a vector network analyzer (VNA). Figure 3 plots weather-sensitive complex permittivity of PNF and PNF-RGO composites. The initial dry samples including PNF, PNF-RGO5 and PNFRGO8 are assigned as PNF-D, PNF-RGO5-D and PNF-RGO8-D. Then, the asfabricated samples are exposed to wet conditions for increasing the water loadings in the materials, obtaining wet samples named as PNF-W, PNF-RGO5-W and PNFRGO8-W. Finally, the frozen samples PNF-F, PNF-RGO5-F and PNF-RGO8-F were obtained via transferring the wet samples into a refrigerator for 10 hours. The water loading of wet and frozen samples were fixed as ~ 50 wt%. The complex permittivity of the PNF-RGO5 and PNF-RGO8 was found to be higher than that of PNF. In Figures 3a and 3d, the complex permittivity of PNF-D is close to that of air (εair = 1 + 0i). Under wet and frozen conditions, polar water interfaces are introduced due to the increased water loading, both real and imaginary permittivity 7



of PNF increases. For PNF-RGO5 and PNF-RGO8, polar lossy interfaces are formed, which is attributed to the interaction between polar water interfaces and RGO frameworks. Therefore, pronounced increased permittivity was observed under wet and frozen conditions and real permittivity of the wet samples was close to that of the frozen samples. Due to the enhanced van der Waals interaction and hydrogen bond between water molecules, the imaginary permittivity of frozen samples (PNF-RGO5-F and PNF-RGO8-F) is lower than wet samples (PNF-RGO5-W and PNF-RGO8-W). Consequently, the complex permittivity of PNF-RGO shows variation with change of climatic conditions.

Figure 3 Complex permittivity of dry, wet and frozen samples: real permittivity of pure PNF (a), PNF-RGO5 (b) and PNF-RGO8 (c); imaginary permittivity of pure PNF (d), PNF-RGO5 (e) and PNF-RGO8 (f); the dry, wet and frozen samples are plotted as yellow, green and blue solid lines, respectively.

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For further discussing the weather-sensitive EM properties, the absorption performance of the fabricated samples was calculated according to the measured complex permittivity, as exhibited in Figure 4. With the addition of polar water molecules, the absorption performance of the pristine PNF has been slightly improved under wet and frozen conditions. Upon introducing polar self-assembled RGO interfaces, the absorption performance of PNF-RGO composites was obviously improved. For the PNF-RGO8-D with 6 mm thickness, a narrow absorption band with RL less than -20 dB was observed at 8 ~ 9 GHz. Due to the greatly increased permittivity, the absorption performance has been tuned under wet and frozen condition. The absorption peak of PNF-RGO8 shifted to lower frequency band and the absorption of materials increased at thicknesses of 2 and 4 mm. Therefore, absorption performance of PNF-RGO composites is also expected to be sensitive to the change of climatic condition, which is linked with the presence of the polar molecules of water or ice.

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Figure 4 Calculated absorption performance of the samples: absorption performance of dry (a), wet (b) and frozen (c) PNF; absorption performance of dry (d), wet (e) and frozen (f) PNF-RGO5; absorption performance of dry (g), wet (h) and frozen (i) PNF-RGO8.

For effectively broadening absorption bandwidth, multiple absorption peaks were obtained via constructing the fabricated PNF-RGO8 into metamaterials The structure size of the metamaterials is given in Figure 5a. Note that the optimizing process of structure size and its theoretical base have been discussed in our previous work43,44. According to the structure size, various pieces of the fabricated PNF-RGO8 were stacked and assembled into a bi-layer pattern. The cycle of climatic conditions is described in the top of Figure 5. Typically, the initial dry metamaterial has been transferred in wet environment to achieve two different wet conditions (with 50 wt% 10


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and 70 wt% water loading in the metamaterials separately). After settling in a refrigerator for 10 hours, the wet metamaterial (80 wt% water loading) was assigned as a frozen metamaterial. Finally, drying processes were applied to the frozen metamaterials to fully remove water and ice. The weight variation of metamaterials is exhibited in the experimental section, and the re-dried metamaterial possess the same weight as the initial dried state, suggesting a structure stability of the as-fabricated metamaterial in the entire cycle of various climatic conditions.

Figure 5 Experimental weather-manipulated absorption performance of metamaterials by arch method: illumination of structure (top) and measured absorption performance (bottom) at various conditions including dry (a), wet with 50 wt% water loading (b), wet with 80 wt% water loading (c), frozen (d) and re-drying (e).

Moreover, the absorption performance of the metamaterial upon various conditions were measured with arch method, as plotted in the bottom of Figure 5. In an EM absorbing material, frequency bands with RL less than -10 dB are considered as the effective absorption bandwidths (EABs). Originating from the coupling of multiple resonances34, 43, 44, the as-fabricated metamaterial possesses wide EABs under all the 11



dry, wet and frozen conditions. Specifically, the frozen metamaterial possesses an EAB of the entire 2 ~ 18 GHz, exhibiting broadband absorption performance in comparison with the other reported absorption materials and structures (Table 1). Moreover, this work provides a new strategy of achieving smart EM absorption materials under various weather conditions. Additionally, the metamaterial possesses stable EM performance in the changing cycle of weather conditions. In Figure 5e, the re-dried metamaterial exhibits both effective bandwidth in comparison with the initial dried condition. The switchable absorption performance under wet and freeze conditions can be reserved via the drying process to remove the introduced polar water molecules, suggesting a stable circulation of weather-manipulated performance. Additionally, Figure 6 shows that the metamaterial also presents weathermanipulated mechanical features. Because of enhanced van der Waals interaction and hydrogen bond between water molecules, the metamaterial was found to be hardening in the freezing process. Meanwhile, the wetting process have limited influence on soft feature of the dried metamaterial. Typically, the as-fabricated metamaterial exhibits flexible shape under wet condition, and various shapes could be programed under mechanical force (Figure 6a). Upon freezing, the mechanical strength of the metamaterial was reinforced, and various rigid shapes could be obtained. As a consequence, the frozen metamaterial was able to support loadings for the enhanced mechanical strength with presence of ice in the porous metamaterials. In Figures 6c and 6d, a stainless-steel autoclave (2.32 kg) can be loaded by the frozen metamaterial, different from the case for the wet condition. Thus, the mechanical feature of the 12


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metamaterial is also switchable between various conditions.

Table 1 Typical EM absorption materials and structures along with related performance. (MM: metamaterial; FSS: frequency selective surface; PNF: polypropylene non-woven fabrics; RGO: reduced graphene oxide; MWCNT: multi wall carbon nanotube; CI: carbonyl iron; NZFO: Ni0.5Zn0.5Fe2O4; GFRC: glass fiber reinforced composite; PVC: polyvinyl chloride).

Structures

Absorption band

Materials

Smart response to

(RL < -10 dB) Frozen

2-18 GHz

Dry MM

stimulus Water loaded

EM and Mechanical

This

PNF/RGO

responses

work

5.5-12.76; 15.2-

Water loaded

EM and Mechanical

This

18 GHz

PNF/RGO

responses

work

3-5.6; 8.5-18

PNF/RGO

EM and Mechanical

This

responses

work

-

45

MM Wet MM

GHz Dielectric

3.4 -18.0 GHz

material Magnetic

Ref.

MWCNT/CI/epox y silicone

4.9-18 GHz

BFO/NZFO

-

46

3.6-18.0 GHz

Dendrite-like

-

47

material Dendritelike

Fe/wax 13



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magnetic material 3D

3-16 GHz

glass/epoxy/MW

Honeycom

-

48

-

49

-

50

-

51

-

52

-

53

-

54

-

55

CNT

b Pyramid

2-8 GHz

periodic metallic

Array Single-

pyramid/MF-110 5.27-18 GHz

Coupling resistive

Layer FSS

pattern/dielectric substrate

Single-

2 - 11.3 GHz

PIN diodes-

Layer FSS Multi-

loaded FSS 4-17 GHz

Resistive

layer FSS

pattern/GFRC/PV C foam

double-

8-18 GHz

FSS film/glass

layer FSS

fiber composite

double-

2.00 - 17.07

resistor-loaded

layer FSS

GHz

double squareloop arrays.

Origami structure

3.6–11.4 GHz.

Arrays of folding planar resistive 14


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patch.

Figure 6 Weather-manipulated mechanical property of metamaterials: (a) flexible shapes of wet metamaterial; (b) free-standing metamaterials with different configurations including cylindrical, roof and arch at frozen condition; load-bearing capacity comparison of wet (c) and (d) frozen metamaterials.

Due to the efficient absorption performance under different weather, the asfabricated metamaterial could be used in different locations. Notably, the earth weather changes seasonally and the wet and frozen conditions are more common than dry condition in many regions.56 On one hand, there are more than 150 rainy days per year in the low latitude regions (Figure 7c), where the EM properties under wet conditions would be considerably concerned. On the other hand, the temperature decreases with the increase of latitude. In many districts with high latitudes like Tiksi (Russia), the 15



average temperature is below zero for over 8 months per year, and thus the material should be utilized for the applications under frozen conditions when ice and snow is considered (Figures 7e and 7f). Thus, the decoupled feature of mechanical properties and EM properties is critical in application of absorber, which means that the mechanical properties could be massively changed while the broadband absorption capability has rarely been impacted. In the present work, a new approach was established for fabricating metamaterial with smart EM response ability. Initially, the weather-sensitive EM and mechanical properties have been achieved due to the tunable interactions between the introduced polar water molecules and porous graphene composites. Moreover, broadband EABs are realized through the fabrication of metamaterial. As metamaterial with tunable EM performance, the manipulating mechanism is different from the previous studies. The porous PNF-RGO composite and introduced polar water molecules act as the tunable medium and stimulus, respectively. With the change of weather conditions, the asfabricated metamaterial exhibits broadband EABs.39, 42, 55 The absorption performance of metamaterial has been linked with the intrinsic microstructure of the material. More importantly, the smart metamaterial possesses stable changing cycles of manipulatable absorption properties. All the wetting, freezing and drying processes could change the range of strong absorption, with improvement on the bandwidth of effective absorption under frozen condition, promising as a smart broadband absorption metamaterial. These results highlight a new strategy for smart components with EM properties sensitive to the environment, which would be extend to other spectrum for the development of 16


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advanced electromagnetic technology.

Figure 7 Perspective of applications: (a) The world distribution of annual precipitation; (b) illumination of the applied smart metamaterial under wet weather; (c) average number of rainy days per year and annual precipitation in some representative cities (marked with stars in (a)); (d) the world distribution of average temperature; (e) illumination of the applied smart metamaterial under frozen condition; (f) evolution of the max and min mean monthly temperature in some representative cities (marked with stars in (d)).

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3. Conclusion In summary, graphene-based polypropylene fabrics were fabricated by in-situ selfassembling RGO sheets on the surface of 3D polymer frameworks. With the presence of tunable polar interfaces, the complex permittivity of the composite could be manipulated under different weather conditions. Moreover, the as-fabricated electromagnetic metamaterials present broadband absorption. In the changing cycle of dry, wet and freezing conditions, the metamaterial exhibits enlarged effective bandwidth for EM absorption. Simultaneously, the mechanical feature of the metamaterials could be substantially reinforced with the presence of ice. Discussion of the results suggests a weather-manipulated smart broadband EM metamaterial, which provide a new approach for the realization of smart EM devices for environment applications in various regions.

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Experimental section Polypropylene non-woven fabric/reduced graphene oxide (PNF-RGO) composites: The PNF-RGO composites were fabricated by in-situ growing RGO sheets on the polymer framework.15 Typically, graphene oxide (GO) solution was prepared according to modified Hummers method.57 GO solutions with different concentrations (5 mg/mL and 8 mg/mL) and mixed with hydroquinone (GO : hydroquinone = 1 : 5 wt%/wt%). Polypropylene non-woven fabrics (PNF) with a thickness of 3 mm were commercially provided by Jinhua Yuhuang Toys Company. After cleaned with ethanol, PNF was dried and immersed in mixed solution of GO and hydroquinone for 2 hours. Then the mixture was sealed and heated up to 100 ℃ for 10 hours in an oven. Though high temperature reduction, GO in the mixture was transferred into RGO and self-assembled in-situ to PNF-RGO with 3D framework structure. PNF-RGO samples were finally obtained after washed with water and dried in an oven. The as-fabricated samples were named as PNF-RGO5 and PNF-RGO8, according to the different concentrations of used GO solutions (5 mg/mL and 8 mg/mL). In the fabrication of wet and frozen composites, the as-fabricated materials were exposed to wet climatic condition for the addition of water loading. The wetting process was finished upon fixed water loading of 70 wt% was achieved. The water loading of material was calculated according to the change of weight: w

m m1  m0 ,  m0 m0

(1)

where m0 and m1 are weight of dry and wet materials, respectively. Then, the wet materials were put into a refrigerator for 10 hours until the samples were completely 19



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frozen. The weights of materials (with fixed sizes: 22.86×10.16×3 mm) under different climatic conditions are collected in Table 2.

Table 2 The weight of fabricated PNF and PNF-RGO composites at different conditions. Dry

Wet (70 wt%)

Frozen

PNF

0.0780 g

0.1327 g

0.1340 g

PNF-RGO5

0.0805 g

0.1374 g

0.1365 g

PNF-RGO8

0.0834 g

0.1423 g

0.1415 g

Scanning electron microscopy (SEM) Characterization: The SEM photos of PNF, PNF-RGO5 and PNF-RGO8 were obtained through a ZEISS supra 55 system.

Metamaterial: The as-fabricated PNF-RGO5 were used for fabricating metamaterial with excellent absorption performance according to the structure size shown in Figure 5a. Typically, 3 pieces of PNF-RGO5 (300×300×3 mm) were stacked as the substrate of metamaterial. Then, another piece of PNF-RGO5 was cut into small cubes (15×15×3 mm). Every 2 cubes were stacked and periodically arranged as the top patterned layer of metamaterial. The period of patterned layer was 20 mm and the metamaterial were stitched up with polyester fibers. In the measurement of absorption performance under different climatic conditions, orderly wetting, freezing and drying processes were experienced by the initial 20


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metamaterial. In the wetting process, two wet conditions including 50 and 70 wt% water loading were selected. Then, the frozen metamaterial was obtained after the freezing process. Finally, the introduced water was removed in the drying process, constructing cyclic change of climatic conditions. The weights of metamaterial under different climatic conditions were characterized in Table 3.

Table 3 The weight of fabricated metamaterial at different state. State

Dry

Weight (g) 228.65

Wet (50 wt%)

Wet (70 wt%)

Frozen

Re-dry

348.23

391.44

398.50

223.43

Measurement of complex permittivity: An X-band waveguide method was chosen for measuring complex permittivity of pure PNF, PNF-RGO5 and PNF-RGO8. The asfabricated samples were cut into matched size with rectangular waveguide of X-band (22.86×10.16 mm). Then, the prepared samples were put into measuring waveguide, which was connected to an Anritsu 37269D vector network analyzer.

Calculated EM absorption performance from complex permittivity: The asmeasured complex permittivity are used for calculating the absorption properties of materials with different thicknesses. Typically, the RL can be expressed as:

r  2  tanh  j r  r fd  , r  c 

Zin 

RL =20log

21


Zin  1 , Zin  1

(2)

(3)


Where Zin is the normalized input impedance, c the light velocity, f the frequency, εr the complex permittivity, and μr the complex permeability.

Arch method for practical absorption performance: In the measurement of practical absorption performance, the fabricated metamaterial (300 × 300 mm2) was placed on the metal plate of arch setup. The transmit and receive antennas were connected by a vector network analyzer (Anritsu 37269D).

Acknowledgements Financial support from National Natural Science Foundation of China (Grant Nos. 11672341 and 111572002) and Beijing Natural Science Foundation (Grant Nos. 16L00001 and 2182065) is gratefully acknowledged.

Supporting Information Available: Cycled permittivities of PNF-RGO8.

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219x174mm (300 x 300 DPI)



180x127mm (300 x 300 DPI)


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219x124mm (300 x 300 DPI)



189x176mm (300 x 300 DPI)


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299x100mm (300 x 300 DPI)



180x144mm (300 x 300 DPI)


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122x150mm (300 x 300 DPI)



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Weather-manipulated smart broadband electromagnetic

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