carbon black composite absorption coatings

Accepted Manuscript Title: A Discrete Structure: FeSiAl/Carbon Black Composite Absorption Coatings Author: Duan Yuping Liu Wei Song Lulu Tongmin Wang PII: DOI: Reference:

S0025-5408(16)31217-X http://dx.doi.org/doi:10.1016/j.materresbull.2016.12.015 MRB 9057

To appear in:

MRB

Received date: Revised date: Accepted date:

29-9-2016 7-12-2016 9-12-2016

Please cite this article as: Duan Yuping, Liu Wei, Song Lulu, Tongmin Wang, A Discrete Structure: FeSiAl/Carbon Black Composite Absorption Coatings, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2016.12.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A Discrete Structure: FeSiAl /Carbon Black Composite Absorption Coatings Duan Yuping*, Liu Wei, Song Lulu, Tongmin Wang

Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116085, P.R. China



Corresponding author. Tel: +86 411 84708446; fax: +86 411 84708446

E-mail address: [email protected]; [email protected];

Highlights 1. Flaky FeSiAl (FFSA) absorption coatings and flaky FeSiAl (FFSA)/carbon black (CB) composite absorption coatings were fabricated. A kind of discrete structure was applied on coatings to improve the absorption performance. 2. The effects of absorber content and structure on the absorption properties were studied. 3. The coatings with discrete structure possessed better absorption properties. 4. The equivalent circuit model was constructed to analyze the improvement mechanisms of absorption properties.

Abstract We fabricated flaky FeSiAl (FFSA) absorption coatings and flaky FeSiAl (FFSA)/carbon black (CB) composite absorption coatings, and studied the effects of a discrete structure on the absorption performance of both coatings. The electromagnetic parameters of FFSA and CB were measured in the frequency range of 2–18GHz using transmission/reflection technology. We also investigated the electromagnetic loss mechanisms of FFSA and CB. The microwave absorption properties of the coatings were determined by measuring the reflection loss (RL) using the arch method. We then analyzed the effects of the contents and the discrete structure on absorption properties. The results showed that, for the FFSA coatings, discrete structures markedly improved absorption properties. When the discrete unit sizes decreased, the absorption properties increased. The addition of CB further improved the absorption performance of the FFSA absorption coating. And when FFSA/CB composite coatings were cut into discrete structures, the absorption peak values decreased and the absorption peak frequencies shifted toward higher frequency ranges, which were consistent with those of the FFSA coating. Because of the conductivity differences between FFSA and CB, the FFSA/CB composite coatings with discrete structures had significant improvements in absorption. The improved absorption weakened when the CB content achieved a certain value. The equivalent circuit models were built to analyze the improvement mechanisms of absorption properties when the coating was cut into a discrete structure.

Keywords: Microwave absorption; Coating; Flaky FeSiAl; Carbon black; Discrete structure

1. Introduction The reduction of electromagnetic wave radiation is crucial to combat the hazards of radiation emitted from electronic products used in daily life. The most common method to reduce radiation is microwave absorbing coating because it is cost effective and efficient. Prior researched focused on designing microwave absorbing coatings with sufficiently wide bandwidths and strong absorption properties. FeSiAl alloys (FSA), are soft metallic magnetic materials and are commonly used in absorbing materials because of their excellent magnetic properties, good temperature stability and low cost. Researchers found that flake-shaped FeSiAl powders (FFSA) have a higher Snoek’s limit because of the shape anisotropy [1, 2], which makes them good microwave absorbers in the higher GHz range. Carbon black (CB) is a typical resistance type absorber and is commonly used in electromagnetic interference shielding applications because of its electrical conductivity, chemical resistance and low density [3]. However, as a absorbing coating with single absorber, it has several disadvantages, such as a small absorption peak value and a narrow absorption bandwidth [4]. Further, when the content of CB exceeds the percolation threshold, it leads to the rapid increase of electrical conductivity and results in the deterioration of absorption properties. Composite absorbers were extensively studied in recent years [5, 6, 7]. However, the effects of different fillers on the absorption performances of composite coating are not fully understood. Structure optimization is also an effective way to improve the absorption performance of composite coating. Common methods of structure optimization are double-layer absorption coatings[8, 9], sandwich structures [10, 11] and frequency selective surfaces [12, 13]. We developed a discrete slab absorber with excellent absorber properties by isolating conductive medium in an absorbing matrix [14]. By conducting a comprehensive analysis of microwave absorption materials, the structure and fillers of the absorption coating played a key role in improving the absorption performance. In this study, a new discrete absorption coating was prepared using FFSA and composites FFSA/CB as absorbers. The absorption performance was compared with that of continuous coatings with the same absorbers content. We also

studied the effect of the absorber content and discrete unit size on the coating absorption performance. Finally, we investigated the microwave loss mechanisms in the absorption coating with the discrete structure.

2. Experiment 2.1 Materials and preparation of microwave-absorbing coatings Flaky FeSiAl (FFSA) was purchased from the Zhejiang Yuanbang Material Technology Co., Ltd., China. Carbon Black (CB) was purchased from the Fushun Dongxin Chemical Co., Ltd., Liaoning, China. To reduce the size and activate the surface of CB particles, CB particles were ball milled for 5 hours before use. The polymer matrix used was polyurethane (PU), and was purchased from the Zuoronggong Industrial Co., Ltd., Dongguan, China. The procedures for the fabrication of the microwave absorbing coating are as follows. First, a polyurethane solution was mixed with diluent (a mixture of ethyl acetate and n-Butyl acetate) by a certain percentage. Next, different absorbers were gradually filled into the prepared polyurethane solution. The mixtures were uniformly stirred at 480 rpm for 30 minutes in a homemade blender, and then were subjected to ultrasonic-vibrating for 30 minutes to obtain the sticky coating solution. Finally, a curing agent was added into coating solution to a scale. After stirring, the uniform mixtures were coated on a square aluminum substrate (200mm×200mm×1.5mm). The thicknesses of the coatings were established at 1.5mm. After the polyurethane hardened, the as-prepared coatings were ready for the microwave properties test. After the testing, these coatings were cut into discrete structures with different discrete unit sizes for subsequent tests. Photographs of the coatings with the discrete structures are shown in Fig. 1. The component proportion, structure of coatings and the discrete unit size are shown in Table 1 and Table 2.

2.2 Characterization The morphologies of the FFSA and CB particles and the fracture surface of coatings were observed by scanning electron microscopy (SEM) and transmission

electron microscopy (TEM). The electromagnetic parameters of FFSA and CB particles were measured by a HP8722 vector network analyzer in the 2–18GHz range using the transmission/reflection method. Coaxial specimens for the electromagnetic parameters were prepared by dispersing different materials in molten paraffin wax with given weight fractions, and then were molded into toroid–shaped samples with a 7.00mm outer diameter and a 3.04mm inner diameter. The reflection loss (RL) versus frequency of the microwave absorbing coating was tested by a HP8720B vector network analyzer in the 2–18GHz range using the arch method.

3. Results and discussion 3.1 Morphology Fig. 2(a) shows the SEM image of the FFSA particles. The FFSA particle has an irregular flake shape with a zigzag boundary. The average diameter is 100–150μm and the average thickness is approximately 1μm, which indicated that the FFSA particle had a high aspect ratio. Fig. 2(b) shows the TEM morphology of CB particles that are nearly spherical with diameters approximately 20–30nm, and most of the particles aggregated to form botryoidal shapes because of the high surface energies of the CB particles. Also, vacancies and pores existed among the CB particles. Fig. 3 shows the SEM microstructures of the fracture surfaces of (a) FFSA coating and (b) composite coating. The image in Fig. 3(a) reveals uniformly dispersed FFSA particles in the polyurethane matrix without significant agglomeration. Because of the high aspect ratio, the FFSA particles are prone to contact mutually. As shown in Fig. 3(b), FFSA and CB particles are uniformly separated in the PU matrix. Because of the difference in particle size, CB particles exist in the spacing among FFSA particles.

3.2 Electromagnetic characteristics To investigate the intrinsic reasons for microwave absorption of the composite coating, the electromagnetic parameters of FFSA were measured in the frequency range of 2–18 GHz as shown in Fig. 4(a). Fig. 4(b) shows the loss tangents calculated

according to the permittivity and permeability of FFSA. The real parts of permittivity and permeability represent the capability of storing electric and magnetic energy. The imaginary parts stand for the loss of electric and magnetic energy. And the loss tangent signifies the loss properties of incidence electromagnetic wave in the microwave absorber [15]. As illustrated in Fig. 4(a), the real part (ε') of the permittivity of FFSA showed less variation from 2 to 6 GHz, but decreased significantly above 6 GHz. At the same time, the imaginary part (ε") showed little change over the whole range of frequency, distributing in 0–0.6. In general, the dielectric loss consisted of the polarization loss and the conductance loss. However, the polarization loss depended on the frequency and reached its maximum as the frequency increased. Therefore, ε" for FFSA, that is independent of total frequency, was the result of conductance leakage [16]. Fig. 4(a) also shows the permeability of FFSA, in which the real part (μ') decreased gradually from 2.2 to 1 as the frequency increased. Meanwhile, the imaginary part (μ") exhibited a resonance peak at 8.2 GHz. The microwave magnetic loss of magnetic materials primarily originated from the domain wall resonance, natural resonance and eddy current loss [17]. The domain wall resonance occurred only in the megahertz range. Eddy current loss was related to the thickness (d) and conductivity (σ) of the absorber [18], as follows: 𝜇𝑟 " = 3𝜋𝜇0 (𝜇 ′ )2 𝑑 2 𝑓𝜎

(1)

where 𝜇0 was permeability of the vacuum. If the magnetic loss only results from the eddy current loss, then 𝜇 " (𝜇 ′ )−2 𝑓 −1 should be constant at the varying frequency. Fig. 5 shows the frequency dependences of 𝜇 " (𝜇 ′ )−2 𝑓 −1 of FFSA. We found that 𝜇 " (𝜇 ′ )−2 𝑓 −1 changed from 0.04 to 0.10 with the increase of the frequency. Therefore, the eddy current loss was excluded and the magnetic loss originated from natural resonance. Fig. 4(b) shows the calculated loss tangent of FFSA within the measured frequencies. The magnetic loss tangent (𝑡𝑎𝑛 𝛿𝜇 = 𝜇 " ⁄𝜇 ′ ) was much greater than the dielectric loss tangent (𝑡𝑎𝑛 𝛿𝜀 = 𝜀 " ⁄𝜀 ′ ), which suggested that the magnetic loss

dominated the microwave absorption. Fig. 6 shows the corresponding electromagnetic parameters of CB within the measured frequencies. As shown in Fig. 6(a), the real part (ε') and imaginary part (ε") of permittivity had the same variation tendency in the whole range of frequency. Both decreased significantly below 7 GHz and smoothly beyond 7 GHz. At the same time, the real part (μ') and imaginary part (μ") of permeability showed little variation over the whole range of frequency. The values of μ' and μ" fluctuated around 1 and 0. We calculated the loss tangent of CB using the measured value, as shown in Fig. 6(b). The results indicated that CB possessed excellent dielectric properties and weak magnetic loss, so the absorption properties of CB primarily originated from dielectric loss. The loss mechanisms of CB are summarized as follows [19]: (1) CB was a type of conductive powder that attenuated the electromagnetic wave during the process of damped vibration; (2) CB had a large specific surface area and more dangling bonds because of its small particle size, and when the electromagnetic wave entered the absorber, they were attenuated by multi-scattering and reflection; (3) The electromagnetic wave was attenuated by eddy current loss which resulted from the formed conductive network in CB particles.

3.3 Microwave absorbing property 3.3.1 Theory According to the transmission line theory, the RL (dB) of electromagnetic radiation under normal incident waves at the surface of single-layer materials backed by a perfect conductor is defined by [20]: 𝑍 −𝑍

𝑅𝐿(𝑑𝐵) = 20 𝑙𝑜𝑔 |𝑍𝑖𝑛+𝑍0 | 𝑖𝑛

𝜇

2𝜋𝑓𝑑

𝑍𝑖𝑛 = √ 𝜀 𝑟 𝑡𝑎𝑛ℎ [𝑗 ( 𝑟

𝑐

0

) √𝜇𝑟 𝜀𝑟 ]

(2) (3)

where, 𝑍𝑖𝑛 is the input impedance of absorber, 𝑍0 is intrinsic impedance of free space with a value of 377Ω, d is the thickness of absorber, c is the velocity of light, f is the frequency, 𝜇𝑟 and 𝜀𝑟 are the permeability and permittivity. 3.3.2 Absorption properties of FFSA coating

Fig. 7 shows the microwave absorption properties of coatings with continuous and discrete structures in different FFSA contents. We found that discrete structures possessed better absorption properties. The absorption peak values significantly decreased, and the peak frequency shifted towards a higher frequency range compared to that of the continuous structure. These changes become increasingly obvious with the decrease in the discrete unit size. When the discrete unit size was 5mm×5mm, the absorption properties were the best. The absorption peak values changed from -12.3dB at 14.4GHz to -18.9dB at 15GHz (shown in Fig. 7a), from -25.5dB at 12.3GHz to -30.4dB at 12.6GHz (shown in Fig. 7b), from -9.5dB at 11GHz to -17.5dB at 11.2GHz (shown in Fig. 7c), from -8.8dB at 9GHz to -12.5dB at 9.8GHz (shown in Fig. 7d), from -7.1dB at 7.1GHz to -11.7dB at 7.9GHz (shown in Fig. 7e) and from -6.7dB at 5.7GHz to -9.2dB at 6.9GHz (shown in Fig. 7f). In addition, improved absorption properties were also reflected in the broadening of the absorption bandwidth (RL<-8dB), as shown in Fig. 8. In the coatings with different contents, the absorption bandwidths (RL<-8dB) widened to some extent. When the FFSA content was 0.7:1(FFSA:PU mass ratio), the absorption bandwidths (RL<-8dB) of the coating with a 5mm×5mm discrete unit size achieved 4GHz. However, the corresponding continuous coating did not have absorption properties better than -8dB. The absorption properties improved after the coating was cut into discrete structures. We assumed that the coating could be divided into several small units as shown in Fig. 9. These units connected with each other and formed a whole before being divided into several small units, but existed independently without contact when the coating was cut into discrete structures. Electromagnetic waves have two vectors of electric and magnetic fields and are vertical to the direction of wave propagation. When electromagnetic wave was vertical in relation to the absorption coating, the conductive current was generated along the direction of the electric field. We choose two adjacent units for this study. The equivalent series circuits used to manifest the flow of current along the direction of the electric field are shown in the inset in Fig. 9, and R is the resistance of each small unit. When the coating was cut into discrete structures, the existence of the gap impeded electron migration and resulted in the

gathering of electrons in the borders. Both migration conductance and hopping conductance occurred in the coating along the direction of the electric field. When the coating was continuous, the migration conductance was dominant but the hopping conductance was insignificant. But for the discrete coating, because the migration was blocked, the hopping conductance was significant in the total conductance. Compared to electrons migration, electron hopping requires a higher activation energy. Therefore, a secondary reflection wave excited on the coating with a discrete structure was significantly reduced under the same incident electromagnetic wave. Moreover, we can simplify the gap into equivalent capacitance C. Because of the existence of capacitance, the total impedance increased from 2R to 2𝑅 + 1/𝑗𝜔𝐶. The increase of impedance was helpful for the improvement of the impedance matching condition. Therefore, the reflected electromagnetic wave weakened because of the improved impedance match, and more electromagnetic waves were transmitted into the coating. As a result, the absorption properties of discrete structure coatings were better than that of the continuous structure. When the discrete unit size decreased, the total number of gaps in the coating increased, which led to further increases of total impedance. This is the primary reason for the better absorption properties of coatings with smaller discrete unit sizes. The total impedance decreased as the frequency increased. In other words, the impedance matching conditions of discrete structures deteriorated gradually as the frequency increased. However, we found that FFSA filling had higher magnetic loss tangent values at high frequencies when compared to the values at low frequencies, as shown in Fig. 4(b). As such, the absorption properties of discrete structures of high frequency areas do not diminish. The shifts in peak frequencies are dependent on the conductivity of the coating. Lower conductivity resulted in higher inputs of wave impedance in discrete structures. The complex permittivity (𝜀) was calculated as [21]: 𝜀 = 𝜀 ′ − 𝑖𝜀" = 𝜀 ′ − 𝑖𝜎⁄𝜔𝜀0

(4)

where ε' was the real part of complex permittivity, ε" was the imaginary of complex permittivity, 𝜀0 was the permittivity of the free space, 𝜎 was the conductivity and

𝜔 was the angular frequency of the electromagnetic wave. Lower conductivity decreased ε". According to the equation below, the matching frequency was determined by the thickness of structural entity and the electromagnetic parameters [22]:

𝑓𝑚 = 𝑛𝑐 ⁄4𝑡𝑚 √|𝜇𝑟 ||𝜀𝑟 |

(n=1, 3, 5, 7…)

(5)

where 𝑓𝑚 was the matching frequency, tm was the matching thickness, 𝜀𝑟 and 𝜇𝑟 were the relative permittivity and the relative permeability. As a result, the matching frequency shifted toward the higher frequency range.

3.3.3 Absorption properties of FFSA and CB composite coating We also fabricated composite coatings using both FFSA and CB as absorbers. The effects of the CB content on the microwave absorption properties are shown in Fig. 10. The absorption bands shifted towards the lower frequency range with increasing CB content, and the absorption peak values decreased to the minimum value and then increased as the CB content increased. The absorption peak values for 20# was the minimum at -18.4 dB at 11GHz, and the bandwidth that was less than -8dB and was approximately 4.5GHz. When the content of CB was greater than this ratio (CB: FFSA: PU mass ratio= 0.15:0.3:1), the absorption peak value increased significantly, even though the bandwidth slightly narrowed. Compared to the FFSA coating, the addition of CB improved the absorption performance of the coating. Since CB is a type of conductive material, when added to the coating, it formed a local conductive net and resulted in increased eddy current loss. On the other hand, the increased dielectric polarization because of the addition of CB was favorable for attenuating additional input electromagnetic waves. However, when the content of CB in the coating increased to a certain extent, it led to electromagnetic wave impedance mismatches at the air-coating interface [5]. To test the impact of discrete structure on the absorption performance of composite coatings, we fabricated FFSA/CB composite coatings with different discrete unit sizes. Reflection loss contrasts between continuous and discrete structures with different

discrete unit sizes and CB contents are shown in Fig. 11. Detailed data is listed in Table 3. The absorption peak values decreased obviously and the peak frequency trended towards a higher frequency range when the coatings were cut into discrete structures. These changes were obvious when the discrete unit size decreased, which was consistent the FFSA coatings. Further comparisons demonstrated that the addition of CB influenced the impact of the discrete structures on the absorption performance. When the FFSA continuous coating was cut into discrete structures (the discrete unit size was 5mm×5mm), the absorption peak value decreased by 6.7dB (FFSA:PU mass ratio=0.3:1) as shown in Fig. 7a. When CB was added in the FFSA coating, the variation first increased (7.7dB for CB:FFSA:PU mass ratio =0.1:0.3:1 and 7.9dB for CB:FFSA:PU mass ratio =0.15:0.3:1), and then markedly decreased (2.6dB for CB:FFSA:PU mass ratio =0.2:0.3:1). Because of the excellent conductivity of CB particles, the addition of CB into FFSA coating increased the conductivity of the coating and led to the changes of impedance when the continuous coating was cut into the discrete structure. However, when the CB content achieved a certain level, the CB particles had mutual contact and formed small conductive nets. Although the discrete process impeded electron movement between the two adjacent discrete units, the small conductive nets in the discrete unit still existed, which weakened the effect of the discrete structure on impedance. This indicated that the discrete structure was an effective method to obtain better absorption performance for conductive composite coatings at specific frequency ranges.

4. Conclusion We fabricated discrete absorption coatings with different discrete unit sizes using FFSA and FFSA/CB as absorbents. Compared to conventional continuous absorption coating methods, our discrete structure improved the absorption performances in both kinds of absorption coatings. The absorption peak values decreased significantly and the absorption peak frequency shifted towards higher frequency ranges. For the FFSA/CB conductive composite coatings, the improvement in absorption performance was significant. In addition, as the discrete unit size decreased, the

improvement of the absorption performance increased. When the content of discrete absorption coating was 0.15:0.3:1(CB:FFSA:PU mass ratio) in the FFSA/CB composite coating, and the discrete unit size was 5mm×5mm, the absorption peak value reached -28dB at 11.2GHz, and the absorption bandwidth (RL<-8dB) was 5.1GHz, which was an improvement over the performance of the corresponding continuous coatings. Therefore, the discrete structure was effective in improving the absorption properties of coatings.

Acknowledgements The authors acknowledge the Supported by Program for the National Natural Science Foundation of China (No. 51577021), the New Century Excellent Talents in University [No. NCET-13-0071], the Fundamental Research Funds for the Central Universities (DUT14YQ201, DUT15LAB24).

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Fig. 1 Photos for absorption coatings with discrete structure (a) sketch map and (b) actual picture

Fig. 2 Morphologies of (a) FFSA and (b) CB. The insets are the macro features of FFSA and CB.

Fig. 3 SEM micrographs of fracture surface of (a) FFSA coating (FFSA: PU mass ratio=0.3:1) and (b) composite coating (CB: FFSA: PU mass ratio=0.2:0.3:1)

Fig. 4 Electromagnetic parameters of FFSA: (a) permittivity(ε) and permeability(μ) and (b) loss tangent

Fig. 5 Values of 𝜇 " (𝜇 ′ )−2 𝑓 −1 vs. frequency for FFSA

Fig. 6 Electromagnetic parameters of CB: (a) permittivity(ε) and permeability(μ) and (b) loss tangent

Fig. 7 Reflection loss contrasts between continuous and discrete structures with different discrete unit sizes and FFSA contents

Fig. 8 The detailed data of absorption bandwidth comparison between continuous and discrete structures

Fig. 9 Schematic illustration showing the small units in coating. The inset shows the change of relationship between two adjacent units and the corresponding equivalent circuit models when the coating was cut into discrete structures.

Fig. 10 Absorption curves for the samples with varied CB contents

Fig. 11 Reflection loss contrasts between continuous and discrete structures with different discrete unit sizes and CB contents

Table 1 Subject parameters of FFSA absorption coatings Samples

Component proportion (mass ratio)

Coating structure

Discrete unit size (mm×mm)

1#

FFSA:PU 0.3:1

Continuous

none

2#

FFSA:PU 0.3:1

Discrete

20×20

3#

FFSA:PU 0.3:1

Discrete

10×10

4#

FFSA:PU 0.3:1

Discrete

5×5

5#

FFSA:PU 0.4:1

Continuous

none

6#

FFSA:PU 0.4:1

Discrete

20×20

7#

FFSA:PU 0.4:1

Discrete

10×10

8#

FFSA:PU 0.4:1

Discrete

5×5

9#

FFSA:PU 0.5:1

Continuous

none

10#

FFSA:PU 0.5:1

Discrete

20×20

11#

FFSA:PU 0.5:1

Discrete

10×10

12#

FFSA:PU 0.5:1

Discrete

5×5

13#

FFSA:PU 0.6:1

Continuous

none

14#

FFSA:PU 0.6:1

Discrete

20×20

15#

FFSA:PU 0.6:1

Discrete

10×10

16#

FFSA:PU 0.6:1

Discrete

5×5

17#

FFSA:PU 0.7:1

Continuous

none

18#

FFSA:PU 0.7:1

Discrete

20×20

19#

FFSA:PU 0.7:1

Discrete

10×10

20#

FFSA:PU 0.7:1

Discrete

5×5

21#

FFSA:PU 0.8:1

Continuous

none

22#

FFSA:PU 0.8:1

Discrete

20×20

23#

FFSA:PU 0.8:1

Discrete

10×10

24#

FFSA:PU 0.8:1

Discrete

5×5

Table 2 Subject parameters of FFSA/CB absorption coatings Samples

Component proportion (mass ratio)

Coating structure

Discrete unit size (mm×mm)

25#

CB:FFSA:PU 0.1:0.3:1

Continuous

none

26#

CB:FFSA:PU 0.15:0.3:1

Continuous

none

27#

CB:FFSA:PU 0.2:0.3:1

Continuous

none

28#

CB:FFSA:PU 0.1:0.3:1

Discrete

20×20

29#

CB:FFSA:PU 0.15:0.3:1

Discrete

20×20

30#

CB:FFSA:PU 0.2:0.3:1

Discrete

20×20

31#

CB:FFSA:PU 0.1:0.3:1

Discrete

10×10

32#

CB:FFSA:PU 0.15:0.3:1

Discrete

10×10

33#

CB:FFSA:PU 0.2:0.3:1

Discrete

10×10

34#

CB:FFSA:PU 0.1:0.3:1

Discrete

5×5

35#

CB:FFSA:PU 0.15:0.3:1

Discrete

5×5

36#

CB:FFSA:PU 0.2:0.3:1

Discrete

5×5

Table 3 Detailed data Frequency of absorption Absorption peak

Absorption bandwidth

peak (GHz)

value (dB)

(GHz) RL<−8dB

1#

14.4

-12.1

4.4

25#

11.8

-14.1

4.1

26#

11

-18.4

4.5

27#

9.2

-16.8

4.7

28#

11.8

-15.2

4.6

29#

11

-19.5

4.7

30#

9.4

-17.6

3.6

31#

11.8

-18.4

5.1

32#

11

-20.1

5.2

33#

9.8

-17.9

3.8

34#

11.8

-21.8

4.7

35#

11.2

-28

5.1

36#

10.4

-19.4

5

Samples