Phthalonitrile-Based Carbon Foam with High Specific Mechanical

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Phthalonitrile-Based Carbon Foam with High Specific Mechanical Strength and Superior Electromagnetic Interference Shielding Performance Liying Zhang, Ming Liu, Sunanda Roy, Eng Kee Chua, Kye Yak See, and Xiao Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12072 • Publication Date (Web): 24 Feb 2016 Downloaded from http://pubs.acs.org on March 1, 2016

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ACS Applied Materials & Interfaces

Phthalonitrile-Based Carbon Foam with High Specific Mechanical Strength and Superior Electromagnetic

Interference

Shielding

Performance Liying Zhang†, Ming Liu*†, Sunanda Roy‡, Eng Kee Chu§, Kye Yak See§ and Xiao Hu*‡ †Temasek Laboratories, Nanyang Technological University, 50 Nanyang Drive, 637553, Singapore ‡School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore §School of Electrical & Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore

ABSTRACT: Electromagnetic interference (EMI) performance materials are urgently needed to relief the increasing stress over electromagnetic pollution problem arising from the growing demand for electronic and electrical devices. In this work, a novel ultralight (0.15 g/cm3) carbon foam was prepared by direct carbonization of phthalonitrile (PN) based polymer foam aiming to simultaneously achieve high EMI shielding effectiveness (SE) and deliver effective weight reduction without detrimental reduction of the mechanical properties. The carbon foam prepared by this method had specific compressive strength of ~ 6.0 MPa·cm3/g. High EMI SE of ~ 51.2 dB was achieved, contributing from its intrinsic nitrogen containing structure (3.3 wt% of nitrogen atoms). The primary EMI shielding mechanism of such carbon foam was determined to be

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absorption. Moreover, the carbon foams showed excellent specific EMI SE of 341.1 dB·cm3/g, which was at least two times higher than most of the reported materials. The remarkable EMI shielding performance with the combination of high specific compressive strength indicated that the carbon foam could be considered as a low density and high performance EMI shielding material used in areas where mechanical integrity is desired.

KEYWORDS: phthalonitrile, carbonization, carbon foam, compressive strength, EMI shielding

1. INTRODUCTION All electrical and electronic devices emit electromagnetic (EM) fields at various frequencies. Electromagnetic interference (EMI) occurs when electronic devices are subject to EM radiation from unexpected sources at the same frequency ranges that these sources operate.1 This may cause the degradation of the electronic devices performance as well as do harm to humans. The tragedy happened at USS Forestall of Vietnam in July, 1967 which claimed 134 lives and caused $72M of damage was due to undesirable EM waves interference.2 Nowadays, with the rapid development of modern electronic devices market, great effort has been stimulated for the development of high performance EMI shielding materials to minimize harmful EMI originated from unwanted surrounding systems. In addition to high EMI shielding performance, lightweight and high mechanical properties are two additional important technical requirements for potential applications especially in the areas of automobile and aerospace. Comparing with metal based


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materials, electrically conductive polymer composites have emerged as attractive EMI shielding candidates owing to theirs high strength-to-weight ratio, corrosion resistance, flexibility, and lower cost.3-9 Such polymer composites are made by compounding nonconductive polymer matrix with electrically conducting filler to achieve the EMI shielding effectiveness (SE). Carbon-based materials including carbon black (CB), carbon fibers (CFs), carbon nanofibers (CNFs), carbon nanotubes (CNTs) and graphene were commonly used as conductive fillers for the fabrication of composite materials to achieve desirable EMI shielding performance.10-12 A good dispersion and high content are the prerequisites to obtain a conducting network in the insulting polymer matrix in order to enhance the electrical conductivity and thus EMI SE of the composites. However, high loading may cause agglomeration of the fillers, which in turn decreases the mechanical properties and challenge the processibility. In addition, increased density, caused by the high loading of fillers, is undesirable. Carbon foam is a class of three-dimensional (3D) architecture consists of a spongelike interconnected network of porous carbon.13 It emerges as an attractive candidate in the field of realistic EMI shielding applications due to its outstanding properties such as low density, high thermal and electrical conductivity, protection and resistance against chemical corrosion and survival ability at high service temperature. Recently, several carbon based foams were developed, such as CNT sponge14 and graphene foam.15-16 These types of carbon foams faced the challenges of complex preparation processes, large-scale production and poor mechanical properties. On the other hand, direct carbonization of polymer foams offers an alternative approach to fabricate carbon foams.17 The simplicity, low cost and scalability make it a more attractive approach.



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Chen et al.18 reported that elastic carbon foam can be prepared via direct carbonization of melamine foam. However, the mechanical property e.g., compressive strength, was in the range of KPa, which was relatively low. R. Kumar et al.19 reported carbon foam with compressive strength of 4 MPa via direct carbonization of phenolic-based foam. Although the compressive strength was quite high for foams, the EMI SE was around 20 dB, which was not attractive. Thus, it is extremely challengeable to fabricate carbon foams with the combination of high mechanical properties and high EMI SE. Phthalonitrile (PN) polymer was identified and proven to be an excellent thermally stable void free thermoset resin by Keller from U.S Naval Research Laboratory (NRL).20 The heterocyclic macromolecular structure of the highly crosslinked PN resin makes it very unique in terms of the mechanical strength as well as thermal stability (Figure S1 in the Supporting Information). The high char yield (> 70%) (Figure S2 in the Supporting Information) and intrinsic nitrogen content (13.5 wt% shown in Table S1 in the Supporting Information) make it an even more interesting material, especially after carbonization because the presence of nitrogen enables the possibility to design or tailor functionalities.21-22 In our recent work, for the first time, we have developed a simple method to fabricate PN foams with adjustable densities and easy scalability by a one-step synchronized foaming polymerization method with the use of chemical blowing agent.23 Herein, we take the advantages of the unique properties of PN resin, for the first time, to fabricate a novel carbon foam by direct carbonization of PN foams, aiming to achieve light weight, mechanical strong and superior EMI shielding properties which are of paramount importance for modern aircraft and automobile applications.


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2. EXPERIMENTAL SECTION Raw materials. Rresorcinol, 4-nitrophthalonitrile, anhydrous potassium carbonate, anhydrous dimethyl sulfoxide (DMSO) were purchased from Sigma Aldrich. 1,4-bis (4aminophenoxy) benzene (p-APB) was purchased from Tokyo Chemical Industry. Azodicarbonamide was used as the chemical blowing agent (CBA) and purchased from Hubei Fine Chemical Import & Export Co., Ltd. All chemicals were used as received. PN Prepolymer Preparation. PN monomer was prepared following a modified procedure described by Keller et al.24 Resorcinol, 4-nitrophthalonitrile and pulverized anhydrous potassium carbonate, at molar ratio of 1:2:3 were added into three-neck round bottom flask and mechanically stirred for 24 h in DMSO solvent before poured slowly into dilute hydrochloric acid solution. The pale yellow precipitate was washed with deionized water till the pH of the filtrate was neutralized. Prepolymer was prepared by adding p-APB as curing agent, to the molten PN monomer in round bottom flask with nitrogen purging, and was quenched after 10 to 15 min of stirring. Preparation of PN based polymer foam. CBA was properly mixed with the PN prepolymer and sealed in mould. The mould was placed in conventional oven preheated to 220 °C for 2 h. The gas released through thermal decomposition within the molten PN resin led to bubble initiation. Bubble growth through gas diffusion and coalescence and stabilization by the crosslinking of PN resin occurred simultaneously. The foams were removed from the mould and post-cured in conventional oven at 250 °C for 2 h, and 280 °C for 2 h, labelled as C280. Preparation of PN-based carbon foams. Heat-treatment process plays a key role in the carbon foams formation. The post-cured polymer foams were subjected to heat



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treatment at 400 °C and dwelt for 0.5 h under continuous argon purge and then cooled to room temperature, (C400). C400 was heated to 600 °C, dwelt for 0.5 h under continuous argon purge and then cooled to room temperature, (C600). C600 was carbonized at 800 °C for 0.5 h, followed by 1000 °C for 0.5 h under a continuous purge of argon, labelled as C800 and C1000 respectively. The preparation scheme was illustrated in Scheme 1. Characterization. Microstructures of the specimens were examined using a Jeol JSM 6360 scanning electron microscope (SEM). All SEM specimens were gold sputtered before examination. Compression properties were obtained using an Instron Tester (Model 5567), equipped with a maximum capacity of 30 KN. The specimens were polished to dimensions of 25.0 × 25.0 × 25.0 mm3. The cross-head speed applied was 0.5 mm/min. The averages values of at least five tests were presented. X-ray diffraction (XRD) patterns of the specimens were acquired by a X-ray diffractometer (Bruker D8) using CuKα radiation (λ = 0.15406 nm). The electrical resistance was recorded by Hewlett Packard 4140B pA Meter/DC Voltage source. The I-V curves were measured, at ambient condition, over a range of 0.2 to -0.2 V with a step of -0.01 V across the parallel side of the specimen with a length of 3 cm. The electrical conductivities were calculated according to Ohm’s law with the geometric dimension (30.0 × 10.0 × 10.0 mm3) and the measured electrical resistance. The average values of five tests were presented. Raman spectra were recorded under spectra resolution of 1 cm-1 by a WITEC CRM200 Confocal Raman Microscope. The objective lens used was 100 × with numerical aperture of 0.95. The elemental compositions of different specimens were measured by CHNOS Elemental Analyzer. The scattering parameters (S11, S22, S21 and S12) were measured using a


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FieldFox Microwave Analyzer (N9917A) in the waveguide method for the frequency range 8.2 – 12.4 GHz (X band). The schematic sketch for the scattering parameters measurements setup was illustrated in Figure S3 in the Supporting Information. The specimens were fabricated to rectangle plates of 25.4 × 12.7 × 2.0 mm3 to fit the waveguide WR90. The reflectance (R), absorbance (A), and transmittance (T) of the 2

2

2

2

incident EM wave were determined as follows. R = S11 = S 22 , T = S12 = S 21 , and A = 1 – R - T, where S11, S22, S12 and S21, and are input reflection, output reflection, reverse transmission and forward transmission, respectively. The overall of EMI SE can be calculated using equation SEoverall = -10logT.25 SE is generally expressed in decibels (dB). The complex permittivity ɛ' and ɛ" were retrieved using the Keysight 85071E Materials Measurement Software.

3. RESULTS AND DISCUSSION Microstructural analysis of PN-based carbon foam. Figure 1 shows the digital image and SEM micrograph of the foam. The foams were prepared using chemical blowing agent in an enclosed mould. The pressure released from the chemical blowing agent reached saturation and led to bubble growth in all direction due to pressure difference inside and outside the bubble. As can be seen from the SEM micrograph (Figure 1), the bubbles were observed as spherical regardless of the foaming direction. The effect of heating temperatures on the morphological structures of the foams (C280, C400, C600, C800 and C1000, the numerical numbers indicates the heating temperature) was demonstrated in Figure S4 in the Supporting Information. Pore size was measured directly from the SEM images using the automated scale bar function. It was observed 7 ACS Paragon Plus Environment


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that the PN-based polymer foam (C280) was made up of randomly distributed closed cells isolated from its neighbouring cells. Subscale porosity (microvoids on the cell wall) was clearly observed on the cell wall. Heat treatment at 400 °C led to size reduction of the closed cells and caused shrinkage and breakage of the subscale pores. When treatment temperature increased to 600 °C, the size of the closed cells remained almost unchanged for C600, while the size of broken subscale pores grew larger leading to generate open cell foam structure. The progressive volume shrinkage and weight loss observed during carbonization were mainly contributed by the thermal decomposition of volatile organic components. Increase of the subscale porosity in the structure of C800 and C1000 was observed, causing the collapse of closed cells as a result of forming open cells in the structure. It is worth noting that less weight loss (only 4 %) and volume shrinkage (only 3.2 %) occurred between 800 and 1000 °C due to the formation of the continuous stable carbon structure. Compared with C280, C1000 contained randomly distributed partly open and partly closed cells. The total volume shrinkage and weight loss of C1000 was approximately 43.3 % and 42.6 % compared to its C280 counterpart (Figure 2). As a result, the density of C1000 foams remained almost constant (Table 1). Compressive properties of PN-based carbon foam. Table 1 lists the specific compressive strength, density and porosity of different samples. Specific compressive strength is the value of the compressive strength normalized by density. It was observed that the specific compressive strength decreased sharply from 12.4 to 2.1 MPa·cm3/g between 280 and 400 °C due to thermal decomposition of PN resin. In addition, the broken and partial broken subscale pores (Figure S4 in the Supporting Information) might act as defects further decrease the compressive strength. No noticeable change in the


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strength was observed after heat treated at 600 °C. However, radical increase was observed for samples treated at 800 and 1000 °C and the specific compressive strength reached ~ 6.0 MPa·cm3/g as shown in Table 1. The increase in the specific compressive strength could be attributed to the formation of more crystalline carbon domains in the systems. In order to understand the structural transformation during heating process, XRD analysis was performed and the spectra are presented in Figure 3. C280 and C400 comprised of two broad peaks at 22.5 (002) and 43.5o (100), indicating the characteristic 2θ value for graphite-like carbon material.26 When carbonized above 600 °C, peak shift of (002) from 22.5 to 24.3° was noticed and the intensity of (100) diffraction peak was enhanced noticeably accompanied with slight shift towards 44.4°. These changes indicated the transformation of carbon atoms from disorder to order structure i.e. the formation of crystalline domains which would consume more energy when being compressed, leading to the enhancement of compressive strength of the carbon foams. In case of C600, the specific compressive strength remained almost unchanged compared with its C400 counterpart, attributed to the fact that the drop in the compressive strength due to the increasing open porosity (see Table 1) was compensated by the formation of better load bearing crystalline carbon. Therefore, it can be concluded that the interplay between the increasing open cell porosity and the increasing crystallinity of carbon structure preserved the mechanical properties during carbonization. EMI shielding performance of PN-based carbon foams. The EMI SE of different samples as a function of frequency was presented in Figure 4a. The inset table lists the electrical conductivity of the foams, demonstrating the strong correlation between electrical conductivity and SE value. Poor EMI SE was observed for the insulating C280



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and C400 foams because the matrix retained its polymeric nature after heat treated to 400 °C. A drastic change in conductivity value was observed for C600, due to the formation of crystalline carbons caused by heat treatment. After carbonization at 1000 °C, ~ 490 times enhancement in electrical conductivity was observed as compared with C600. Raman spectroscopy measurement was conducted to further understand the structural changes with increasing temperature and their relationship with conductivity enhancement. Two distinct bands centred at 1350 (D-band) and 1590 cm-1 (G-band), were observed for all the samples in Figure 4b. The D-band is assigned to the vibrational mode of a tetrahedral sp3 configuration and the G-band indicates the sp2 carbon in-plane vibration.26 The Raman peaks shifts were virtually unchanged from C280 to C1000 while the drastic intensity increased indicated that the formation of more sp3 and sp2 carbon structure after carbonization. Chhowalla et al.27 demonstrated that sp2 carbon structures as the materials predominantly species enhancing the electronic and transport properties. The ID/IG values shown in the inset of Figure 4b suggested that an increasing sp2 carbon structures were formed in the matrix, which was the main contributing factor to the enhancement in electrical conductivity. Similar results have also been reported in our previous work.28 Apparently, the increment of the EMI SE was due to the formation of the conducting network in the carbon matrix. The growth of sp2 carbon structures increased the EMI SE by forming interconnected electrical network within the matrix which interacts with the incident radiation. The average SE of C1000 over the frequency range of 8.2 to 12.4 GHz was measured to be 51.2 dB, indicating that approximately 99.999% of the incident EM wave was shielded by C1000. 20 dB of SE is generally accepted as satisfactory for most practical applications29 and as our C1000 foam


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possessed about 2.5 times higher SE value, we thus believe that our foam can be a promising candidate to dominant many existing materials in this particular area. It is of interest that some recent studies focused on the addition of a second phase, such as CNTs, graphene, silver and magnetic iron oxides (see Table 2), into polymer matrix to improve EMI SE. Promising results were demonstrated by other groups intended to enhance EMI shielding performance through sulphur,30 boron and nitrogen31 doping. In our work, C1000 exhibited outstanding EMI SE without the need of additional constituent or complex doping process. The contributions for high EMI SE come from not only the conducting carbon matrix formed after fully carbonization, but also the polarization effects induced by the intrinsic nitrogen containing structure. Because nitrogen atoms possess more valence electrons than carbon atoms, hence the extra electrons from the intrinsic nitrogen atoms increase the density of electrons and charge concentration. The intrinsic nitrogen atoms (3.3 % of nitrogen in C1000, see Table S1 in the Supporting Information) create an extra electronic cloud, which interacts with the incoming EM waves. Moreover, oxygen and other residual defects present in C1000 also serve as polarization centers inducing defect polarization relaxation and group electronic dipole relaxation that further strengthens EMI SE.32 In contrast, high amounts of nitrogen present in a non-conductive structure do not influence the EMI SE. Although C280 contains 13.5 % nitrogen, the insulating structure prohibits the movement of the valence electrons from nitrogen atoms, as a result, no interaction with the incident EM wave. The EMI shielding mechanism of the carbon foams were also analysed. It was reported that incident EM wave became insensitive when the sensing wavelength is smaller than a particular particle or structural feature size.33 The carbon foams (C600,



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C800 and C1000) can then be treated as a continuous effective medium. When EM radiation is incident on a piece of material, the reflectance (R), absorbance (A) and transmittance (T) sum to the value 1 and can be expressed as R+A+T=1

(1)

SEoverall is the total value of contributed by absorption (SEA), reflection (SER), and multiple reflections (SEMR). It can be expressed as SEoverall = SEA + SER + SEMR (dB)

(2)

where SER and SEA are the SE due to reflection and absorption, respectively whereas SEMR is additional effects caused by multiple reflections. According to the theory of EMI shielding,25 SER, SEA and SEMR can be expressed as

SE R = 39.5 + 10 log

SE A = 8.7

σ 2πfµ

d

δ

SEMR = 20 log1 − e −2d / δ

(3)

(4)

(5)

where σ is the electrical conductivity, f is the frequency of the radiation, µ is the magnetic permeability (µ=µ0µr), µ0 equals to 4π × 10-7 H/m and µr is the relative magnetic permeability. For non-magnetic materials, µr = 1.34 d is the thickness of the specimen. δ is the skin depth, which is defined as the distance up to which the EM wave drops to 1/e of the incident strength. It can be expressed as δ = ( πf µσ ) −1

(6)

Multiple reflections can be ignored if the sample thickness is more than the skin depth. The skin depth and SEMR of C600, C800 and C1000 were calculated using equations (5)


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and (6) and theirs variation with frequency were shown in Figure 5. Since the skin depth of C600 was thicker than the thickness of the sample (2 mm), thus SMR cannot be ignored. The influence of multiple reflections became significant in SEoverall. As shown in Figure 5, the average SEMR of C600 was -7.3 dB, substantially decreasing the SEoverall. In the case of C800 and C1000, the skin depth was thinner than the thickness of the sample. SEMR became negligible (the average SEMR of C800 and C1000 were -0.4 and -4.3×10-5 dB, respectively) and can be safely ignored. Hence, SEoverall = SER + SEA. Figure 6 shows the SER and SEA of C600, C800 and C1000 over the measuring frequency range. It is worth noting that equation (3) will predict negative value for SER when the conductivity of the material is lower than 0.9 S/m.25 Hence, SER of C600 should be calculated using the equation (2) (SER = SEoverall - SEA - SEMR). It can be seen that for C600, SER > SEA, indicated reflection was the dominant shielding mechanism. In the case of C800 and C1000, SEA and SER were calculated using the equations (3) and (4) and found to be SEA > SER, indicated that the contribution of absorption to SEoverall surpassed that from reflection. The absorption dominant shielding materials are capable of preventing EM waves from reflect back to the shielding device or disturb the performance of surrounding components, which is an attribute for practical applications, such as electronic circuits where the EM radiation emitting component needs EMI shielding at the same time. In order to understand the reasons for EM wave absorption and reflection by shielding materials, the intrinsic parameters of the materials, that is, complex permittivity, εr = ε’-jε’’, were used for analysis. Figure 7 shows relative complex permittivity of C600, C800 and C1000 over the measured frequency. The real part (ε’) of complex permittivity of a material represents its polarization capability at a certain frequency of EM waves.



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The imaginary part (ε’’) of complex permittivity corresponds to the leakage current arising from dielectric loss. It can be seen that ε’’ of C1000 was higher than that of C800 and C600. The higher number of free electrons increases the intensity of leakage current that gives rise to high ε’’. According to the theory of EMI shielding, for absorption of EM radiation, the shielding material should have electrical or magnetic dipoles which interact with the EM waves. The electrical dipoles can be provided by materials with a high dielectric constant.34 It was also reported that direct nitrogen atoms substitution for carbon atoms enhances the polarizability.35 In our work, the presence of 3.3 % of nitrogen was found remained in the carbon structural framework (Table S1 in the Supporting Information) after carbonization at 1000 °C, inducing localized charges on the carbon backbone that gives strong polarization effect. The air-shielding material is an important factor for consideration when determine the attenuation of the incident EM wave caused by reflection or absorption. When an incident EM wave strikes a shielding material, two types of waves will be generated at the external surface; a reflected wave and refracted wave, respectively as illustrated in Scheme 2. As the refracted wave propagates through the shielding material, absorption by the materials causes exponential decrease of the wave. The amplitude of reflected wave and refracted wave depends on the intrinsic impedance of the shielding material and the impedance of the domain in which the incident EM wave was propagating before reaching the external surface of the shielding material. The reflectance (R) of the incident EM wave can be expressed as

R = 20 log

Z1 − Z 0 Z1 + Z 0


(7)

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where Z0 is the impedance of free space (Z0 = (µ0/ε0)0.5 = 377 Ω.) and Z1 presents the intrinsic impedance of the shielding material.36 The condition maximizing absorption of incident EM wave happens when Z1 equals to Z0. The complex permittivity of the shielding material is an important factor to determine the ratios of reflection for an incident EM wave. As the complex permittivity of the samples increased significantly with increasing heat-treatment temperature, Z1 of C1000 with higher permittivity mismatched more drastically with Z0 than that of C800 and C600, leading to the high reflection of the incident EM wave. We also calculated loss tangent (ε’’/ε’) which implies the energy loss in the material and plotted it versus frequency in inset figure in Figure 7. The lower value of the loss tangent in the case of C1000 indicated the higher reflection of the incident wave. Here, it could be concluded that in the case of C1000, the amount of signal blocked by reflection was higher than being absorbed, i.e, correlated with the high value ε’, impedance mismatch and lower loss tangent. However, it appeared to be the conflict with the results obtained from Figure 6. In this work, we think the contribution of absorption to the overall shielding should be based on the ability of the material to attenuate the incident signal that has not been reflected at the air/material interface, ie, the signal transmitted into the sample. Similar interpretation were also stated by other researchers.25 SEA was the amount of signal absorbed by the specimen with respect to the refracted signal at the boundary between air and the specimen (see Scheme 2). When the refracted signal travelled to the other boundary of the specimen and air, the specimen would absorb large amount of signal and left small amount of signal to transmit out of the specimen. Hence, we concluded that absorption was the dominant shielding mechanism for the carbon foam.



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The specific EMI SE (SE normalized by density) is more commonly used in advanced applications and to compare the shielding performance among different materials. In this work, the electrical conductivity, EMI SE, specific EMI SE, thickness and specific compressive strength of different materials were summarized and compared in Table 2. Metals, typically solid copper, showed excellent electrical conductivity and EMI SE. However, its high density leads to relatively low specific EMI SE.37 Composites containing large amounts of conductive fillers, such as CFs, nickel fibers, or graphene,38 were developed to achieve sufficiently high conductivities, but often at the expense of the mechanical properties and density. The excessive fillers agglomeration jeopardized the mechanical advantages and the specific EMI SE. Foam-structured polymer composites containing CNTs,39-40 graphene,41-42 silver-coated hollow spheres43 and magnetic and dielectric nanoparticles19 were employed as a form of weight reduction, but associated with high production cost. In our previous work, attempts of inclusion of CNFs into epoxy based foams were aimed to simultaneously increase mechanical properties and EMI shielding performance.44-45 High specific compressive strength was achieved, while the EMI SE was unsatisfactory due to the small amounts of CNFs loading. Recently, the highest specific EMI SE (500 dB·cm3/g) was reported based on a lightweight, flexible graphene foam.16 However, the real applications of the graphene foam were limited to systems where mechanical properties are not critical and will only be realized up on addressing the issues related to the intricate preparation pathway and the fragile nature. Here, we would like to highlight that the thickness of the shielding materials is crucial to the EMI SE and should be taken into consideration while comparing the specific EMI SE. According to the equations (3) and (4), SEA increases with increasing thickness, while the


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thickness does not influence the SER. Apparently, EMI SE could be enhanced with increasing the thickness of shielding materials. It would be more appropriate to compare of different shielding materials in terms of specific EMI SE associated with the thickness. Agnihotri et al.46 proposed to compare the efficiency of different shielding materials by specific EMI SE per unit thickness based on the assumption that the relationship between EMI SE and thickness for a single piece of isotropic material was linear. In our work, C1000 showed super high specific EMI SE (341.1 dB·cm3/g) as compared with other recently reported materials. The specific EMI SE/thickness of C1000 was found to be at least two times higher than most of the reported materials. In addition, although the specific EMI SE of C1000 was lower than that of the above graphene foam, the absolute EMI SE/thickness was higher. Therefore, considering its high specific compressive property along with superior EMI SE, C1000 can be an outstanding candidate as EMI shielding materials for high-performance applications.

4. CONCLUSIONS This work reported a superior EMI shielding carbon foam of respectable mechanical properties. The carbon foam was obtained through direct carbonization of the PN-based polymer foam. The foam structure provided thermal stress relief up on pyrolysis and making carbonization while maintaining mechanical integrity possible. The high EMI SE comes from not only the high graphitic carbonaceous species converted from the heterocyclic macrostructures but also the intrinsic nitrogen-containing structure. The specific EMI shielding effectiveness remained the best (341 dB·cm3/g) reported so far when mechanical property was concerned.



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With obtaining high EMI shielding materials with straightforward preparation method as the main motivation, the one step synchronized foaming process made foam preparation appealing, especially since the foam can be formed in any desirable mould shape. The ease in preparation and versatility has definitely made leap progress towards real application. Despite the various strategies on design principles and materials selection suggested for optimized EM performance, only limited success were realized due to challenging processibility, insufficient electrical conductivity and poor mechanical properties. Comparing with filler-containing carbon foams, the PN-based carbon foam eliminated the usage of conducting fillers hence the related processing issues, such as (i) non-uniform dispersion of functional fillers (ii) insufficient interaction between matrix and fillers, (iii) limited filler loading content, (iv) altered viscosity, and (x) high intrinsic nitrogen content. In addition, the tunable density of the PN foams allows the tailoring of EMI shielding and mechanical properties to suit various advanced applications. The high service temperature of the carbon foam makes it an unique class of material which is unreplaceable by both metals and polymers. Hence, it is expected that the results provided in this work would be beneficial for guiding future design of EMI shielding materials.

ASSOCIATED CONTENT Supporting Information. Possible macromolecular structures present in crosslinked PN resin; TGA curve of PN polymer; Schematic sketch of the set-up used to evaluate the EMI SE; (a) - (e) SEM images of the microstructures of C280, C400, C600, C800 and


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C1000; Comparison of elemental compositions analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected] [email protected]

ACKNOWLEDGEMENT The authors would like to acknowledge the funding supported by Nanyang Technological University (NTU) with the grant number M4061124 and the support from School of Materials Science and Engineering and Electromagnetic Effect Research Laboratory at NTU on the present work. The authors also express appreciation to Mr. Lai Chee Hoong Patrick for the Raman measurement and Dr. Cheah Jun Wei Jason for the electrical conductivity measurement.

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FIGURES

Figure 1. Digital image (left) and typical SEM micrograph (right) of PN-based carbon foams

Figure 2. Typical sample volume shrinkage (%) and weight loss (%) after various heat treatments


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C1000

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C800

C600 C400 C280

10

20

30

40

50

60

70

80

2θ

Figure 3. XRD patterns of C280, C400, C600, C800 and C1000

(b)

(a)

Figure 4. (a) EMI SE as a function of the frequency range from 8.2 to 12.4 GHz for C280, C400, C600, C800 and C1000. (b) Raman spectra of C280, C400, C600, C800 and C1000



Figure 5. Skin depth and contribution of multiple reflections of C600, C800 and C1000 in the frequency range of 8.2 – 12.4 GHz 70

SER (C600) SER (C800) SER (C1000)

60

SER and SEA (dB)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

50

SEA (C600) SEA (C800) SEA (C1000)

40 30 20 10 0

8

9

10

11

12

Frequency (GHz)

Figure 6. Contribution of reflection and absorption of C600, C800 and C1000 in the frequency range of 8.2 – 12.4 GHz


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Incident wave Shield Refracted wave

Reflected wave Transmitted wave

Multiple reflections

Z0

Z1

Z0

d

Scheme 2. Schematic representation of EM wave attenuation by a shield (thickness of shield = d)

TABLE Table 1. Comparison of the specific compressive strength, density and porosity of C280, C400, C600, C800 and C1000 Specific compressive strength Sample

3

Density (g/cm3)

Porosity (%)

(MPa·cm /g) C280

12.3 (±2.14)

0.152 (±0.004)

89.1

C400

2.2 (±0.29)

0.159 (±0.005)

90.8

C600

2.0 (±0.18)

0.150 (±0.008)

92.2

C800

3.5 (±0.43)

0.156 (±0.004)

92.3

C1000

6.0 (±0.75)

0.154 (±0.003)

92.9


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Table 2. Comparison of electrical conductivity, EMI SE, specific EMI SE, thickness and specific compressive strength of different materials with the results in this work

Samples Solid copper 7 vol% 2 µm Ni fiber/PES composites 19 vol% carbon fiber/PES composites 2 wt% aligned rGO/epoxy composites 7 wt% CNT/PS foam MWCNTs incorporated carbon foam 1.8 vol% graphene PMMA foam Graphene/PS foam Ag-hollow sphere/epoxy foam Fe3O4-ZnO/carbon foam 2 vol% CNF/epoxy foam Graphene foam C1000

Electrical conductivity (S/cm) 5.8 × 105

EMI SE (dB)

Specific EMI SE (dB·cm3/g)

Thickness (mm)

90.2 (±5.0)

10 (±0.5)

3.1

Specific compressive strength (MPa·g/cm3) -

-

58.1 (±4.2)

31 (±3)

2.85

-

37

-

73.9 (±5.1)

50 (±4)

2.85

-

37

~ 0.01

38

35.3

>0.1

-

38

-

18.2-19.3

33.1

1.2

-

39

150

85

163

2.75

17.9

40

3.1

13-19

25

4

-

41

1.25× 10-2

29

64.4

2.5

-

42

-

60.2

46.3

1.5

-

43

13.5 0.1 2

48.5 25 ~ 20

125 24.6 ~ 500

2.75 3 1

20 35.2 (±4.0) -

2.4

~ 51.2

341.1

2.0

~ 6.0

19 44,45 16 Present work


Ref 37


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Table of Contents Graphic


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Phthalonitrile-Based Carbon Foam with High Specific Mechanical

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