Optimizing Ply Pattern and Composition of Layered Composites based

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Optimizing Ply Pattern and Composition of Layered Composites Based on Cyanate Ester, Carbon Nanotube and Boron Nitride: Toward Ultralow Dielectric Loss and High Energy Storage Chunqing Lu, Li Yuan, Qingbao Guan, Guozheng Liang, and Aijuan Gu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12117 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Optimizing Ply Pattern and Composition of Layered Composites based on Cyanate Ester, Carbon Nanotube and Boron Nitride: Toward Ultralow Dielectric Loss and High Energy Storage

Chunqing Lu, Li Yuan, Qingbao Guan, Guozheng Liang,* and Aijuan Gu*

State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application Department of Materials Science and Engineering College of Chemistry, Chemical Engineering and Materials Science Soochow University, Suzhou, China.

*Corresponding Authors No. 199 Ren-Ai Road, Suzhou Industrial Park, Suzhou, Jiangsu 215123, China. Tel: +86 512 65880967. Fax: +86 512 65880089. Current institution e-mail: [email protected] (G.L. Liang), [email protected] (A.J. Gu).

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ABSTRACT High dielectric loss and low breakdown strength have been two bottlenecks restricting applications of conductor/polymer composites for high energy storage. Herein, three kinds of layered composites, named BC, BCB and CBC, were fabricated through stacking up hexagonal boron nitride (hBN)/cyanate ester (CE) (B layer) on carbon nanotube (CNT)/CE with 0.4 wt% CNTs (C layer). The effects of ply pattern and composition on dielectric properties, breakdown strength and energy density of composites were investigated, and also compared with those of single-layer 0.4CNT/CE composite containing 0.4wt% of CNTs. Results show that three kinds of layered composites have significantly reduced dielectric loss, improved breakdown strength and energy density. Especially, for the CBC composite with 20 wt% hBN in B layer (C20BC), it’s dielectric constant is as high as 323 (100 Hz) and keeps a value larger than 270 (1-103 Hz), it is the highest value reported so far among multi-layered composites based on conductor/polymer and insulating layers; moreover, it’s breakdown strength and energy density are about 1.5 and 25 times of that of 0.4CNT/CE composite, respectively; note that C20BC composite has very low dielectric loss (0.049 at 100 Hz) or much less at increased frequencies (> 100 Hz), only about 2.6×10-3 times of that at 100 Hz of 0.4CNT/CE composite. The origin behind these attractive properties was intensively discussed.

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1. INTRODUCTION As a passive component, dielectric capacitors are not only environmental-friendly, but also possess intrinsic high power density due to their ultrafast charge-discharge capability.1 Therefore, they have wide applications in hybrid electric vehicles, medical devices and electrical weapon systems.2-4 In recent years, the great demand on miniaturization and package application asks for increasing requirement on capacitors with high energy storage. However, the energy density of dielectric capacitors is greatly limited by the low dielectric constant of available dielectric materials.5 Many studies have shown that greatly improved dielectric constant of conductor/polymer composites have been achieved at a very low conductor loading because of an insulator-conductor transition called percolation effect,6-8 meeting the requirement of high dielectric constant for dielectric capacitors, especially for embedded capacitors (>100, 100 Hz).9 On the other hand, for polymers and their composites, the energy density is proportional to dielectric constant and the square of breakdown strength,10,11 so high energy density can be obtained by increasing either dielectric constant or breakdown strength. But, for traditional conductor/polymer composites, high dielectric constant is usually accompanied by significantly reduced breakdown strength and suddenly increased dielectric loss;12-14 this not only limits the improvement of energy density, but also damages the operational reliability and service life of capacitors.15 Therefore, how to fabricate high energy density polymer composites with both high dielectric constant and low dielectric loss is still a big challenge. Recent researches prove that introducing electrical inter-barrier layer is beneficial to suppress the increase of dielectric loss and improve the breakdown strength owing to avoiding the formation of conductive paths,16,17 however, the dielectric constant is generally lower than 100 at 100 Hz. Introducing an insulating layer into conductor/polymer 3



composites is another method for improving energy density, however, high filler loading is needed to achieve high energy density and dielectric constant. Lin et al.18 prepared a sandwich structure, in which polyamide (PI) simultaneously acts as the bottom and top layers, and amino modified carbon nanotube (CNT)/PI is the intermediate layer. When the CNT content is 5 wt%, the sandwich structure PI-CNT/PI-PI shows the highest energy density, about 1.3 times of PI; and the corresponding dielectric loss is very low (0.0016, 100 Hz), however, the dielectric constant is only 25 (100 Hz). When the CNT content is 10 wt%, the dielectric constant of PI-CNT/PI-PI gets the maximum, but the value is only 32 (100 Hz), and the energy density is slightly lower than PI. Shi’s group19 fabricated tri-layered composites with barium titanate/polyvinylidene fluoride (BT/PVDF) as the bottom and top layers, graphite/PVDF (GR/PVDF) as the intermediate layer. When the content of GR in the intermediate layer is 20.3 wt%, and the content of BT in the out layer is 40.6 wt%, the tri-layered composites possess the highest energy density, about 8 times of single-layered graphite/PVDF composite. Meanwhile, the dielectric constant also gets the maximum (92 at 100 Hz), but still can’t meet the demand of high dielectric constant for embedded dielectric capacitors;9 besides, such high loadings of BT and GR tend to damage the processing characteristics and mechanical properties.20,21 This paper reports new multi-layer structure composites consisting of CNT/cyanate ester (CE) with 0.4 wt% of CNTs (coded as C layer) and boron nitride (BN)/CE (coded as B layer). We choose CE resin as matrix because it not only has high thermal resistance, but also exhibits extremely low dielectric loss over a wide frequency range compared with other thermosetting resins. Through designing and optimizing ply patterns and compositions of layered composites, unique high-k composites with ultralow dielectric loss, high breakdown strength and high energy storage have been developed. The origin behind these attractive integrated performances was revealed. 4


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2．．EXPERIMENTAL 2.1 Materials CNTs used herein were multi-walled carbon nanotubes with a purity of more than 97% (the average out diameter is 7.15 nm, and the length is greater than 5 µm), which were bought from Shenzhen Nanoport Company, China. Hexagonal boron nitride BN (hBN) with a purity of 99.9% (the average diameter is 50 nm) was purchased from Shanghai Chaowei Nano Technology Co., Ltd, China. Diglycidyl ether of bisphenol A (EP) with an epoxide equivalent weight of 196 g/mol was obtained from Nantong Xingchen Synthetic Material Co., Ltd, China. CE used here is 2, 2’- bis(4-cyanatophenyl)propane, which was got from Yangzhou Techia Material Co. Ltd, China. 2.2 Preparation of CNT/CE composites An appropriate amount of CE was melted at 80 °C to get a liquid, to which CNTs were added with vigorous stirring under sonication for 30 min, followed by prepolymerizing at 150 °C for 3-6 h with stirring. Then, EP was added with stirring for 30 min to get a prepolymer, which was then cast into a mold for curing and postcuring taking the protocols of 150 °C/2 h + 180 °C/2 h + 200 °C/2 h + 220 °C/2 h, and 240 °C/4 h, successively. After slowly cooled to room temperature, a composite was obtained, coded as xCNT/CE, where x is the weight fraction of CNTs in the composite. 2.3 Preparation of two-layered structure composites Table 1 summaries the compositions and thicknesses of layered structure composites. CE was melted at 80 °C to get a liquid, into which 0.4 wt% of CNTs were added with vigorous stirring and under sonication for 30 min, followed by prepolymerizing at 150 °C for 3-6 h with stirring. Then, appropriate amount of EP was added with stirring for about 30 min to get CNT/CE suspension, which was then cast into a mold for curing at 150 °C for 1 h, followed by cooling naturally to get a B stage composite (0.4CNT/CE). 5



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Appropriate amounts of CE, EP and hBN were blended at 100 °C for 1 h with vigorous stirring to form a blend, into which 30 mL acetone were added to get a hBN/CE suspension. The suspension was then spin-coated on the surface of 0.4CNT/CE (2500 r/min, 20 s) to get an uncured bi-layered composite. The uncured bi-layered composite was put into an oven for curing and postcuring taking the protocols of 150 °C/2 h + 180 °C/2 h + 200 °C/2 h + 220 °C/2 h, and 240 °C/4 h, successively. After slowly cooled to room temperature, a two-layered composite was obtained, coded as yBC, where y is the hBN loading in the B layer.

Table 1. Compositions and thicknesses of multi-layered structure composites

Each C layer

Each B layer

CNT/CE/EP (w/w/w)

hBN/CE/EP (w/w/w)

10BC

0.3800/79.70/19.92

10.00/72.00/18.00

3.0±0.2

2.00±0.20

20BC

0.3800/79.70/19.92

20.00/64.00/16.00

5.0±0.2

2.00±0.20

30BC

0.3800/79.70/19.92

30.00/56.00/14.00

8.0±0.2

2.00±0.20

40BC

0.3800/79.70/19.92

40.00/48.00/12.00

14.0±0.2

2.00±0.20

10BCB

0.3800/79.70/19.92

10.00/72.00/18.00

1.5±0.1

2.00±0.20

20BCB

0.3800/79.70/19.92

20.00/64.00/16.00

2.5±0.1

2.00±0.20

30BCB

0.3800/79.70/19.92

30.00/56.00/14.00

4.0±0.1

2.00±0.20

40BCB

0.3800/79.70/19.92

40.00/48.00/12.00

7.0±0.1

2.00±0.20

C10BC

0.3800/79.70/19.92

10.00/72.00/18.00

3.0±0.2

2.00±0.20

C20BC

0.3800/79.70/19.92

20.00/64.00/16.00

5.0±0.2

2.00±0.20

C30BC

0.3800/79.70/19.92

30.00/56.00/14.00

8.0±0.2

2.00±0.20

C40BC

0.3800/79.70/19.92

40.00/48.00/12.00

14.0±0.2

2.00±0.20

Composite

6


Thickness of each B layer (µm)

Total thickness of composite (mm)

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2.4 Preparation of three-layered structure composites The uncured bi-layered composite described in Section 2.3 was pre-cured at 150 °C for 2 h. After cooling naturally, the opposite surface of 0.4CNT/CE was spin-coated with hBN/CE suspension (2500 r/min, 20 s), followed by curing and postcuring taking the protocols of 150 °C/2 h + 180°C/2 h + 200 °C/2 h + 220 °C/2 h, and 240 °C/4 h, successively. After slowly cooled to room temperature, a three-layered composite was obtained, coded as yBCB. The uncured bi-layered composite described in Section 2.3 was pre-cured at 150 °C for 2 h. After cooling naturally, the 0.4CNT/CE suspension described in Section 2.3 was cast into the surface of hBN/CE in the pre-cured bi-layered composite, followed by curing and postcuring taking the protocols of 150 °C/2 h + 180 °C/2 h + 200 °C/2 h + 220 °C/2 h, and 240 °C/4 h, successively. After slowly cooled to room temperature, a tri-layered composite was obtained, coded as CyBC. 2.5 Characterizations A scanning electron microscope (SEM, Hitachi S-4700, Japan) was employed to observe the morphologies of cross-sections of composites. Dielectric and electric conductivities were measured on a broadband dielectrics spectrometer (Novocontrol Concept 80, Hundsangen, Germany) at room temperature over a frequency range from 1 to 107 Hz. The dimensions of each sample were (25±0.2) × (25±0.2) × (2±0.2) mm3. Three samples of each composite were tested. The breakdown strengths were measured using a breakdown strength tester (HT-50c, Guilin Electric Research Institute, China) with a voltage ramp rate of 1000 Vs-1. The dimensions of each sample were (50±0.02) × (50±0.02) × (1.8±0.1) mm3. Six samples of each composite were tested. 3．．RESULTS AND DISCUSSION 7



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3.1 Design and preparation of multi-layered structure composites According to the percolation theory, there is an insulator-conductor transition in conductor/polymer

at

percolation

threshold

(fc).22,23

This

effect

endows

a

conductor/polymer with high dielectric constant, and also brings high dielectric loss and low breakdown strength due to the formation of conductive paths. Besides, it has been found that the huge difference of dielectric constant between conductor and polymer is another important factor that damages the breakdown strength.24,25 Similar problem also appears in ceramic/polymer composites, that is, an increased dielectric constant is obtained at the expense of breakdown strength.26,27 To simultaneously overcome the shortcomings, some multi-layered polymer composites were designed.28-30 But these researches have a common drawback that the improved dielectric constant is still not high (εr100, 100 Hz). In order to fabricate high energy density polymer composites with both high dielectric constant and low dielectric loss, and intensively study the influence of different space structure on dielectric properties and energy density, three layered-structure composites (BC, BCB and CBC) were designed as shown in Figure 1, the B layer in three layered structures have same thickness. In these multi-layered structures, C layer based on CNTs is used to greatly increase the dielectric constant according to the percolation effect; the insulating layer (B layer) contains hBN owing to its unique advantages. Firstly, hBN with strong B-N covalent bonds is regarded as white graphene, showing high mechanical strength;31,32 secondly, hBN has high thermal conductivity, and thus can improve heat dissipation ability of composites by inhibiting the occurrence of surface breakdown strength;33 thirdly, hBN has strong insulation performance, its electric conductivity ranges from 10-16 to 10-18 Ω·cm-1 at room 8


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temperature, so it endows the insulting layer with big ability to hinder the formation of conductive paths.

Figure 1. Schematic diagrams (left) and SEM images (right) of cross-sections of three layered structure composites.

Note that dielectric properties of a filler/polymer composite are also affected by the dispersion of fillers in the polymer.15,34,35 To guarantee excellent electric properties of CNTs, no surface treatment on CNTs was conducted in many investigations.36-39 To avoid the agglomeration of CNTs and get good dispersion in polymer matrix, ultrasonic dispersion and suitable parameters of prepolymerization were used.35,40,41 In this work, the two techniques were used to prepare CNT/CE and hBN/CE samples. On the other hand, hBN has some active groups that can react with CE, further enhancing the dispersion of hBN in CE matrix. From SEM images as shown in Figure S1 in Supporting Information, it can be seen that hBN particles have good dispersion in CE matrix. 3.2 Electrical conductivities 9



Figure 2 depicts the frequency dependence of AC conductivities of CNT/CE composites with different contents of CNTs. When the content of CNTs is smaller than 0.4 wt%, the composites show linear dependence of AC conductivity on frequency, this is the typical insulator feature, while other composites exhibit conductor feature (Figure 2a). For example, when the content of CNTs increases from 0.3 to 0.4 wt%, the AC conductivity at low frequencies (BC>BCB, indicating that composite with the B layer on both sides (BCB) has stronger insulating properties.

Figure 3. Frequency dependence of AC conductivity of composites 3.3 Dielectric properties Figure 4 shows the frequency dependence of dielectric properties of BC, BCB and CBC structures. BC has higher dielectric constant than 0.4CNT/CE (Figure 4a) over the whole frequency range tested, this is opposite to the results calculated from Lichtenecker’s Equation as shown in Eqn. (1):46

ε L−1 = f 1ε 1 −1 + f 2ε 2 −1

(1)

Where f1 and f2 are the volume fractions of B layer and C layer in BC structure, respectively, and f1 + f2 = 1; ε1, ε2 and εL are dielectric constants of B layer, C layer and BC structure, respectively. Since multi-layered structure composites have the same surface area, Eqn. (1) can be simplified as: 11



ε L−1 = d 1ε 1 −1 + d 2ε 2 −1

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(2)

where d1 and d2 are the thickness ratio of B layer and C layer in BC structure, respectively, and d1 + d2 = 1.

Figure 4. Frequency dependence of dielectric properties of composites. C layer has much larger dielectric constant than B layer, so the dielectric constant of BC structure should fall in those of B layer and C layers, and decreases with the increase of thickness of the B layer according to Eqn. (2). However, in fact, compared with 0.4CNT/CE composite, BC structures have about 1.5-2.6 times larger dielectric constant over the whole frequency range tested. With regard to BC structure, as the content of hBN enlarges, its dielectric constant initially increases and reaches the maximum at 20 wt% of hBN; with further increased hBN loading, the dielectric constant of BC decreases and then almost levels off at 30 wt%. The reason behind is that the interfacial effect is not considered in Lichtenecker’s Equation. In fact, because of the huge difference in conductivity between B and C layers, there is interfacial polarity at the interface, resulting in accumulation of charges,47-49 and thus endowing BC structure with high dielectric constant. On the other hand, the conductivity of hBN is very low, and with the increase of hBN content, the dielectric constant of BC structure initially increases and then reduces. Similar phenomenon also 12


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appears in BCB and CBC structures. Among three layered structure composites, both BCB and CBC are tri-layered structures with interfacial polarization, but the position of B layer plays a big effect on the dielectric constant. Specifically, the two outer layers of BCB are insulating B layer, making BCB structure has the lowest conductivity (Figure 3); meanwhile the strong insulating property of B layer has big ability to inhibit the charge movement, leading to lower dielectric constant of BCB structure. Differently, as shown in Figure 4a, CBC structure has the highest dielectric constant over the whole frequency tested due to its strong interfacial polarization. Especially, all CBC structures have higher dielectric constants, which are larger than 100 when the frequency is lower than 5800 Hz, meeting the requirement of embedded capacitors. It is worth noting that when the content of hBN is 20 wt%, the dielectric constant of C20BC structure is as high as 323 (100 Hz) and keeps a value larger than 270 (1-103 Hz), which is the highest value among layer structure composites based on both conductor/polymer and insulating layers (see Table S1 in Supporting Information); moreover, the dielectric loss is only 0.049 at 100 Hz, or much less at increased frequencies (> 100 Hz). Figure 4b provides frequency dependence of dielectric loss for 0.4CNT/CE composite and layered structure composites. As the content of CNTs in 0.4CNT/CE composite is larger than fc, the dielectric loss of 0.4CNT/CE composite is as high as 19 (100 Hz). Differently, three layered structure composites have much decreased dielectric loss, lower than 0.4 (100 Hz). Specially, BCB structure with the B layers on both sides has the lowest dielectric loss (0.0018, at 100 Hz) which is only 9.5×10-5 times of that of 0.4CNT/CE composite, indicating that this structure prevents the formation of conductive paths, this result is consistent with conductivities shown in Figure 3. As a comparison, (hBN+CNT)/CE composites were also prepared, which have the 13



same contents of hBN and CNTs as layered structure composites. The plots reflecting frequency dependence of dielectric properties are shown in Figure 5. It can be seen that dielectric constants of (hBN+CNT)/CE are not larger than 22 at the whole frequency tested, and the dielectric losses of (hBN+CNT)/CE lie between 0.00254 and 0.00311 at 100 Hz or less at increased frequencies (> 100 Hz). These values demonstrate that the introduction of a small amount of hBN particles also blocks the formation of conductive network,45 however, layer-structure has its unique feature of increasing dielectric constant. This statement is confirmed by the insulation feature of the plots reflecting frequency dependence of conductivity (Figure 5c).

Figure 5. Dependence of frequency on dielectric constant (a), dielectric loss (b) and AC conductivity (c) for 0.4CNT/CE and (hBN+CNT)/CE composites.

3.4 Equivalent circuits Figure 6 presents the Nyquist plots of 0.4CNT/CE (Figure 6a) and layered-structure 14


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composites (Figure 6b-6d). The Nyquist plot of 0.4CNT/CE composite shows a semi-circle, indicating the formation of conductive paths in the composite;50,51 however, it’s not a standard semicircle, but a semi-ellipse with the center below the X axis, showing a non-Debye behavior.52,53 This phenomenon is also found in many other conductor/polymer composites when the content of conductor larger than fc.7,54 In reality, few material can be expressed by an ideal resistors and capacitors rather than a constant phase element (CPE).

Figure 6. Nyquist plots of 0.4CNT/CE (a), BC (b), BCB (c) and CBC (d) composites.

Compared with the Nyquist plot of 0.4CNT/CE composites, those of BC and BCB structures (Figure 6b-6c) show insulator feature, and this is the reason why BC and BCB structures have low dielectric loss. In addition, it is a remarkable fact that the Nyquist plot of each CBC structure is arcs, which doesn’t have intersection with X axis, meaning that the conductive paths are not completely formed in CBC. This kind of arcs also exists in 15



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conductor/polymer composites of which the content of the conductors approaches fc .55,56 Figure 6 demonstrates that the conductive paths are gradually interrupted from CBC to BC and BCB structures. In order to further discuss the mechanism about the difference in polarization and conduction processes between three multi-layered structures,ZSimpWin software was used to simulate the impedance spectra of these composites.57 Figure 7 gives equivalent circuits of various composites. The equivalent circuit for CNT/CE composite (Figure 7a) consists of parallel circuits in series, one contains a constant phase element (CPE1), a capacitance (C1) and a resistance (R1), while the other has a constant phase element (CPE2) and a resistance (R2). In addition, CNTs have large length/diameter ratio and coil structure, so inductance (L) is also one component in the circuit.

Figure 7. Equivalent circuits of composites (a: 0.4CNT/CE, b: BC, c: CBC, d: BCB).

For getting the best fitting results, CPE is defined as: ZCPE = Z 0

1 ( jω)n

(3)

where ZCPE is the impedance of the constant phase angle component; Z0 and n are constants; 16


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ω is the angular frequency; and j is the imaginary unit. When n=1, -1, and 0, CPE can be regarded as an ideal capacitor, inductance and resistance, respectively, otherwise, a CPE is divided into a capacitance (C) and a resistance (R) in parallel according to Eqn. (4): 1 n

CV = (ZCPE )(R

1   −1  n 

)

(4)

Different from the equivalent circuit of CNT/CE composite (Figure 7a), that of BC structure (Figure 7b) has additional parallel circuit induced by the interfacial polarization effect between B and C layers. The equivalent circuit of CBC structure (Figure 7c) is more complicate, which contains two additional parallel circuits in series, this is because there is an additional interface in CBC structure; what’s more, the conductive C layers on the top and bottom layers provide many polarized electrons, they significantly enhance the interfacial polarization in CBC structure. Although BCB structure also has two interfaces, the B layers on both sides of C layer weaken its response to external electric field of BCB structure, so that there is one extra resistance in the equivalent circuit (Figure 7d). Based on parameters of equivalent circuits for 0.4CNT/CE and multi-layered structure composites as summarized in Table S2-S4 in Supporting Information, the total resistance (Rt) and total capacitance (Ct) reflecting the conductivity and dielectric constant of composites, respectively, are obtained. Ct values of all composites fall in the magnitude of 10-11 F, and CBCs structure have slightly higher Ct values (3.0×10-11~5.6×10-11 F) than 0.4CNT/CE (1.41×10-11 F), BC structure (1.69×10-11~1.92×10-11 F) and BCB structure (1.14×10-11~1.30×10-11 F). This is because CBC structure can be regarded as a capacitor, of which the two C layers are conductors, divided by an dielectric material (B layer); in other word, the capacitor also makes contribution to the dielectric constant of CBC structure. In addition, four composites have greatly different Rt values. The magnitude of order in Rt is 0.4CNT/CE < CBC

Optimizing Ply Pattern and Composition of Layered Composites based

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