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Electrically conductive and mechanically strong graphene/mullite ceramic composites for high-performance electromagnetic interference shielding Jianhong Ru, Yuchi Fan, Weiwei Zhou, Zhenxing Zhou, Tuo Wang, Ruiheng Liu, Jianping Yang, Xiaofang Lu, Jiancheng Wang, Chengchang Ji, Lianjun Wang, and Wan Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12933 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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ACS Applied Materials & Interfaces

Electrically conductive and mechanically strong graphene/mullite ceramic composites for high-performance electromagnetic interference shielding

Jianhong Rua, Yuchi Fana,b* , Weiwei Zhou c*, Zhenxing Zhoua,c, Tuo Wangd, Ruiheng Liud, Jianping Yanga, Xiaofang Lua, Jiancheng Wanga, Chengchang Jia, Lianjun Wanga and Wan Jianga,b,e

a. State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China b. Institute of Functional Materials, Donghua University, Shanghai 201620, China. c. Department of Materials Processing, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan d. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China e. School of Material Science and Engineering, Jingdezhen Ceramic Institute, Jindezhen 333000, China

*Corresponding

authors. E-mail addresses: [email protected] (WW Zhou)

[email protected]

1

ACS Paragon Plus Environment

(YC

Fan),

ACS Applied Materials & Interfaces 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

Abstract: Ceramic composites with good electrical conductivity and high strength that can provide electromagnetic interference (EMI) shielding are highly desirable for the applications in harsh environment. In this study, lightweight, highly conductive and strong mullite composites incorporated with reduced graphene oxide (rGO) are successfully fabricated by spark plasma sintering at merely 1200℃ using the core-shell structured γ-Al2O3@SiO2 powder as precursor. The transient viscous sintering induced by the γ-Al2O3@SiO2 precursor not only greatly reduces the sintering temperature to prohibit the reaction between mullite and rGO, but also induces a highly anisotropic structure in the rGO/mullite composite, leading to extremely high in-plane electrical conductivity (696 Sm-1 for only 0.89 vol.% of rGO) and magnitude lower cross-plane electrical conductivity in the composites. As a result, very large loss tangent and EMI shielding effectiveness (>32dB) can be achieved in the whole K band with extremely low rGO loading (less than 1 vol.%), which is beneficial to maintain good mechanical performance in ceramic matrix composites. Accordingly, the rGO/mullite composites show greatly improved strength and toughness when the rGO content is not high, which enables them to be applied as highly efficient EMI shielding materials while providing excellent mechanical performance.

Keywords: graphene, mullite, ceramic matrix composite, anisotropic structure, electromagnetic interference shielding, mechanical properties 2


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1. Introduction With the soaring development of telecommunication, electronic devices and sensors, the pollution induced by electromagnetic interference (EMI) has become a critical issue in modern society.1-3 For instance, the K band (i.e. 24 GHz) has widespread applications in the intelligent transportation system and automotive radar including lane change assist, pre-crash warning, blind spot detection etc., which is developing very fast due to the significance of safety in transportation. Therefore, the EMI shielding materials in K band have become increasingly important for protecting the electronic devices from the malicious influence. Carbon nanomaterials based composites are very attractive for EMI shielding due to their high electrical conductivity, large surface area and lightweight, exemplified by various composites filled with carbon nanotubes (CNTs) and graphene.4-9 However, CNTs tend to tangle with each other, leading to tremendous difficulty during composite processing.10 Graphene - the first 2D material discovered with outstanding electrical and mechanical properties - is relatively convenient for composite fabrication compared to carbon nanotubes owing to its 2D morphology.11 Moreover, scalable synthesis method of graphene such as reduction of graphene oxide (GO) derived from graphite has significantly reduced the cost of production, which is another advantage in comparison to other filler materials in EMI shielding composites.12 Therefore, intensive researches have been conducted on the graphene based composites, mostly with polymer matrices.13-15 Indeed, polymers could bring the merit of 3



lightweight and flexibility, which is beneficial to many applications. However, when hardness, strength, wear resistance and thermal stability are highly concerned, ceramics are definitely better options over polymers, though very few reports related to the graphene/ceramic composites have been revealed so far. Qing and coworkers 16 prepared graphene nanosheet(GN)/BaTiO3 composite with graphene content up to 4 wt.% (~10.7 vol.%), which exhibits a EMI shielding effectiveness (SE) value above 40 dB in X band when the filler fraction is high, thanks to the high electrical conductivity of GN and large dielectric constant of BaTiO3. However, BaTiO3 obviously cannot provide adequate mechanical support when applied in a field of high stress. The GN /Al2O3 composite studied by the same group seems more attractive, considering that both mechanical properties and EMI shielding function can be provided by one material.17 Nevertheless, selecting GN which contains few defects and yet plenty of graphene layers as the filler might not be the best choice compared with reduced graphene oxide (rGO) in terms of EMI shielding. It is well known that rGO is the analogue of graphene which derives from the reduction of graphene oxide (GO).18 While GO is completely insulating due to the severe oxidation process breaking most sp2 hybridized structure, rGO could be highly conductive depending on the method of reduction, albeit defects usually remain on the surface of rGO which can serve as dipoles when interacting with microwave. In fact it has been demonstrated that rGO exhibits superior EMI shielding performance compared with GN due to the enhanced conduction loss, polarization loss, and multiple scattering.19, 20 Therefore, rGO should be a more appropriate microwave absorbent if it 4


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can be successfully incorporated into the ceramic matrix. In this study, a lightweight, strong rGO/mullite composite with high EMI shielding efficiency in K band is demonstrated for the first time. It is well known that mullite shows high strength and thermal shock resistance at elevated temperature, and it also possesses the merit of low density which is very suitable to be exploited as the matrix of EMI shielding composite for lightening weight.21, 22 However, one major obstacle is the extremely high temperature for densification of mullite, which could be as high as 1600℃ even if atomic scale of homogeneity for Al and Si is realized in the precursor via sol-gel process .23 Given the fact that observed CNTs on the fracture surface of CNT/mullite composite suddenly decreased when sintered above 1600℃,24 it is highly possible that the high densification temperature of mullite could lead to the carbothermal reaction between rGO and mullite , noting that the defective rGO is much more reactive compared to GN, which has been proved in graphene/strontium titanate composites.25,

26

In addition, the

homogeneous dispersion of rGO in the mullite matrix is also critical to the mechanical behavior as well as the EMI shielding properties of composite. Herein, an extremely low densification temperature for mullite was realized by viscous flow sintering using a core-shell structural γ-Al2O3@SiO2 powder as precursor. After homogeneously mixed with GO via a heteroaggregation strategy, the dense rGO/mullite composites with very uniform and fine microstructure were obtained, whose EMI shielding and mechanical properties were investigated. 2. Experimental 5



2.1 Preparation of γ-Al2O3@SiO2 precursor In a typical process, the γ-Al2O3 power (1g) was modified by refluxing with (3-Aminopropyl) triethoxysilane (APTES, 1ml) in toluene solution under protection of argon at 150℃ for 6 hours. The modified γ-Al2O3 power was washed thoroughly with ethanol by vacuum filtration to remove excess APTES, and then dried in a vacuum oven at 80℃ for 5 hours. Next, the obtained γ-Al2O3 powder was coated with SiO2 by modified Stober method. Specifically, the modified γ-Al2O3 (0.3g) was dispersed into a mixture solution (300ml) of ethanol and water (4:1) via sonication for 30 min to form well-disperse solutions before 5ml of ammonia solution (27%) was added. Then 200μL of tetraethyl orthosilicate (TEOS) was injected and reacted for 4 h before another 200μL of TEOS was added. After 10h of reaction, the as-prepared γ-Al2O3@SiO2 was collected and washed with ethanol and water by vacuum filtration, and then dried for the next process. 2.2 Preparation of GO/γ-Al2O3@SiO2 hybrids GO was prepared by the modified hummers method reported elsewhere .27 The γ-Al2O3@SiO2 powder was modified with APTES again by the same method mentioned above. GO/γ-Al2O3@SiO2 hybrids were fabricated by liquid-phase deposition method as follows28: The obtained modified γ-Al2O3@SiO2 power (1g) was dispersed into deionized water (100ml) via adjusting pH value to 3.3 using hydrochloric acid, followed by sonication for 30 min to form uniform suspension. Various amount of GO colloid (0.8mg/ml) prepared in advance was then slowly added into suspension of the surface 6


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modified

γ-Al2O3@SiO2

dropwise

while

stirring.

The

precipitated

graphene/γ-Al2O3@SiO2 hybrid power was readily collected by vacuum filtration, and then dried in a vacuum oven at 80℃ for 8 h. 2.3 Sintering of monolithic mullite and rGO/mullite composites The pure mullite and rGO/mullite composites were consolidated by spark plasma sintering (SPS) from γ-Al2O3@SiO2 powder and GO/γ-Al2O3@SiO2 hybrid powder, respectively. The sintering was conducted at 1200℃ in vacuum (6 Pa) with heating rate of 100℃min-1, uniaxial pressure of 60MPa, and holding time of 10 mins. The hybrid powder with GO contents of 0.51, 0.76 and 1.1 wt.% were sintered using the same condition. The bulk samples for mechanical test were discs with diameter of 10mm and thickness of 0.5mm, while the samples for EMI shielding test were rectangular solids with size of 10.67×4.31×1.4mm. The processing route can be illustrated in Figure 1. After sintering, the apparent density of monolithic mullite and composites was measured by Archimedes’ principle. The content of the rGO in the composite was calculated from the carbon content in the composites, which was measured by high frequency IR carbon-sulfur analyzer (G4ICARUS).

7



Figure 1. Schematic diagram of the preparation process for rGO/mullite composites.

2.4 Structural characterization The constituent phases in pure mullite and composites were identified by X-ray diffraction (XRD) using Cu-Kα radiation with a scanning range of 10~90°.The quality of rGO was characterized by Raman spectroscopy (Tokyo Instruments Co.). The fracture surface of mullite and rGO/mullite composites were observed by field emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F). The elemental analysis was performed using the electron probe micro-analyzer (EPMA, JEOL JXA-8230). Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM, JEOL 200CX) were exploited to investigate the microstructure of powders and bulk composite sample. 2.5 Evaluation of electrical and EMI shielding properties

8


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The electrical conductivity at room temperature was measured by the Van der Pauw's method using Hall measurement system (Lake Shore 8400 Series). Disc samples with the dimension of 0.5 mm ×∅ 10 mm were used for this measurement. The transmission and reflection scattering parameters and the complex permittivity (ε = ε′ +jε") of the rGO/mullite ceramics at room temperature were measured by the wave-guide method in the frequency range of 18–26.5 GHz using a network analyzer (Agilent technologies E8362B:10 MHz–28 GHz), which was based on the measurements of the reflection and transmission module and on the fundamental wave-guide mode TE10. 2.6 Mechanical measurement Young’s modulus and Poisson’s ratio (Table S1) were determined by advanced ultrasonic material characterization system (UMS-100, France). The Vickers hardness was measured by indentation method using a load of 9.8N for 5s, and the indentation fracture toughness was calculated according to the equation proposed by Anstis. 29 0.5

𝐾𝐼𝐶 = 0.016(𝐸 𝐻𝑉) 𝑃𝐶 ―1.5

(1)

where E is the elastic modulus (GPa), Hv is the Vickers hardness, P is the load, and c is the crack half length. The strength for monolithic mullite and composites was measured by modified small punch (MSP) test, whose configuration can be illustrated by Figure S1. The breaking strength can be calculated by the following formula: δ = 3𝑃 2𝜋𝑡2[1 ― [(1 ― 𝛾2) 4] ∙ (𝑏2 𝑎2) + (1 + 𝛾)ln( 𝑎 𝑏)]

(2)

where P is the breaking load, γ is the Poisson’s ratio, a and b are the radius of the hole in 9



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the bearing mold and the radius of the cylindrical indenter, and t is the thickness of the sample.

3 Results and discussion 3.1 Preparation and microstructure of rGO/mullite composites In the preparation of mullite ceramics two key factors govern the quality of final compacts, namely mullitization and densification. When mullite ceramics are prepared by sol-gel route, the mullitization always occurs before densification process, which requires a high sintering temperature above 1500℃ for achieving 98% relative density even sintered by SPS, owing to the low sintering activity of mullite.30 In contrast, monophasic mullite ceramics with high relative density can be obtained at low temperature via transient viscous flow sintering (TVS) by using amorphous SiO2 coated -Al2O3 (-Al2O3@ SiO2) powder as precursor (Figure S2), since the densification induced by TVS occurs prior to the mullitization which takes place

below 1300℃ between SiO2

and -Al2O3 .31 Therefore, the -Al2O3@SiO2 powder was fabricated at first for the processing of monolithic mullite and corresponding composites. It has to be emphasized here that the quality of SiO2 coating has great influence on whether monophasic mullite can be obtained at low temperature. As shown in Figure 2A, most of the -Al2O3 particles possess a stick-like shape, which cannot be readily dispersed in the mixture solution of ethanol and water. Previous works used non-covalent surface modification method to get good dispersion of -Al2O3, which has not been reproduced successfully in this study due 10


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to the poor coating effect on the surface of -Al2O3. Instead, a covalent surface modification was conducted by using APTES as coupling agent, which can render -Al2O3 particle good stability in mixture solution and nucleation sites during the coating process. As a result, a core-shell structural -Al2O3@SiO2 powder with ~3.5nm thick SiO2 layer evenly coated on -Al2O3 was obtained, as shown in Figure 2B and D. Since the homogeneous dispersion of graphene in ceramic matrix composite is critical to electrical and mechanical properties, the approach for mixing precursor powder and GO should be carefully considered. Taking advantage of the wisdom in colloid science,32, 33 a heteroaggregation strategy was adopted, in which the precursor particles and GO were granted opposite surface charge for triggering a spontaneous precipitation upon mixing the two types of colloids together. To this end, APTES was exploited again to graft amino group on the surface of SiO2 shell, by which the -Al2O3@SiO2 particles became positively charged when dispersed in water. Given that GO was negatively charged, -Al2O3@SiO2 particles were attracted and decorated onto GO plane to form a GO/-Al2O3@SiO2 hybrid powder via dropwise addition of GO colloid, as proved by TEM observation in Figure 2C. The -Al2O3@SiO2 particles with a distinct core-shell structure can be observed clearly in the hybrid powder as well (Figure 2D).

11



A

-Al2O3

B

GO

-Al2O3@SiO2

20 nm

20 nm C

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D Core-shell structured precursor

10 nm

50 nm

Figure 2. TEM images of various powders as raw material or precursor of mullite ceramic and composite. (A) Pristine γ-Al2O3 powder. (B) As prepared γ-Al2O3@SiO2 powder. (C) GO/γ-Al2O3@SiO2 hybrid powder prepared by heteroaggregation, the arrow indicates the GO sheet. (D) Magnified area of the dash line square in (C), showing the core-shell structure of γ-Al2O3@SiO2 particle on GO.

The attainment of -Al2O3@SiO2 powder with high quality has significant meaning 12


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to depress the temperature for mullitiztion. According to the first report of mullite derived from SiO2 coated -Al2O3, the mullite phase did not show until 1275℃, and pure mullite phase could not be obtained until 1285℃.31 By contrast, monophasic mullite was obtained at sintering temperature of 1100℃, which is only 100℃ higher than the softening temperature of SiO2 (Figure 3A). The XRD results indicate that the mullitization temperature is between 1000 and 1100℃, which is almost 200℃ lower compared to the previous work. It is deduced that the lower mullitization temperature here can be ascribed to the applied high pressure during SPS, as well as the high-quality -Al2O3@SiO2 precursor with uniform SiO2 coating.

Raising the sintering temperature

to 1200℃ gave rise to a relative density of 97.5%, which is high enough as structural ceramics (Table S1). Based on the thermodynamic calculation, the reaction between carbon and mullite occurs in the range from 1200 to 1600℃ depending on the source of carbon (carbon black or graphite, Figure S3). Therefore, the sintering temperature for mullite composites was set at 1200℃ to reach relative density as high as possible while avoiding the possible reaction between rGO and mullite. As shown in Figure 3B, with the addition of GO there is no noticeable impurity from XRD analysis, suggesting that graphene did not change the process of mullitization. Meanwhile, the peak belonging to rGO cannot be identified from XRD pattern, probably owing to the extremely low graphene content in composites (less than 1 vol.%).

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A

B

Figure 3. XRD results for mullite precursor and sintered compacts. (A) XRD pattern for -Al2O3@SiO2 powder and corresponding bulks sintered at different temperature. (B) XRD patterns for rGO/mullite composites with different rGO content. (mullite PDF#:83-1881, -Al2O3 PDF#:50-741)

However, the structure of rGO in mullite composite can be revealed clearly by TEM observation from the pressure axis direction (Figure 4). In the mullite composite with rGO content of 0.61 vol.%, an extremely fine microstructure with average grain size less than 500nm is observed (Figure 4A). The absence of abnormally large grain in the composite reflects the homogeneous distribution of rGO throughout the composite. Under higher magnification, it can be seen that rGO lying along mullite grain boundaries are connected to construct a continuous network in the matrix, suggesting an electrically percolated behavior in this composite (Figure 4B). Besides, the selected area electron diffraction pattern reconfirms the formation of monophasic mullite phase from 14


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microscopic level (Figure 4C). Another distinct characteristic of the rGO is the highly wrinkled structure throughout the entire graphene plane, as shown in Figure 4D. It is known that the sintering process can lead to folding and curling of graphene flakes, but this folding structure can only be found at triple junctions in other graphene based ceramic composites as a result of pore elimination at the final stage of sintering.34 However, in rGO/mullite composite the rGO flakes are found to be folded or wrinkled almost everywhere rather than only at triple junctions, which may deeply relate to the TVS process. Above the softening temperature of SiO2 glass, the amorphous SiO2 layers became soft and easy to deform and flow under pressure, this TVS process facilitated the densification of composite before mullitization, and also could result in folding and curling of rGO flakes during the viscous flowing process, owing to the highly flexible feature of graphene under bending or shearing force.

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Figure 4. TEM and HRTEM images observed from the pressure axis direction for the rGO/mullite composite with graphene content of 0.61 vol.%. (A) Bright field TEM image of the composite. (B) the magnified area in (A); the white arrows indicate connected rGO flakes along one direction. (C) Wrinkled rGO flakes on the grain boundary; inset is selected area electron diffraction in the circled area. (D) HRTEM image illustrating the highly winkled structure of rGO.

A great number of strategies have been reported for converting GO into rGO, among 16


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which the thermal reduction is one of the most effective paths.35 Since the SPS process itself is a high temperature procedure (1200℃) in vacuum (6 Pa), relatively high level of reduction in rGO can be expected after SPS without additional reducing procedure. The quality of rGO in mullite composites was characterized by Raman spectroscopy (Figure 5A). As the main feature of sp2 carbon materials that represents the first-order scattering E2g vibration mode, the G-band located at the 1580cm-1 can be readily recognized from Raman spectra for all the rGO/mullite composites.36 Additionally, the prominent D band located at 1350cm-1 that reflects the disorder of the sp2 carbon structure can also be observed, which reveals the high level of defects in rGO.37 Although the Raman intensity ratio of D and G bands (ID/IG) for composites are higher than that of pristine GO, the presence of 2D band and well separated G and D bands suggest much higher sp2 carbon area in rGO/mullite composites compared to GO. The restored crystal structure in rGO is beneficial to the enhanced conductivity loss in composite, while the residual defects on rGO plane may construct localized states near the Fermi level to increase radiation attenuation.38 The remained group species on rGO in the mullite composite was further characterized by XPS. It can be seen that in pristine GO the C 1s envelop contains four components which can be assigned as C-C (254.7 eV) C-O (285.9 eV), C=O (287.1 eV) and O-C=O (288.3 eV) (Figure 5B). After thermal reduction at high temperature in vacuum during SPS, the main oxygen containing peak (C=O) disappeared while only much weaker C-O and O-C=O peaks can be decoupled from the single envelop, which results in largely increased C-C component in rGO (from 30.3% to 66%). On the other 17



hand, XPS spectra has confirmed the existence

and species of residual oxygen

containing groups, which is in agreement with the prominent D band in Raman spectrum.

A

B

Figure 5. The quality of GO and rGO in composite with 0.47 vol.% rGO. (A) Raman spectra (B) High resolution C1s XPS spectra.

3.2 Electrical properties and anisotropic structure The electrical conductivity of composite not only could have great impact on the EMI shielding performance, but also reflect the general dispersion status of filler material in the matrix. Although mullite is a well-known insulator whose resistivity is reported to be in the order of 1013 Ω/cm (~10-11 Sm-1),39 the incorporation of rGO renders rGO/mullite composite greatly improved electrical conductivity. For the mullite composite with only 0.47 vol.% of rGO, the electrical conductivity of in-plane direction (σ‖) is as high as 190 Sm-1 (Figure 6), which is two orders higher than that of rGO/Al2O3 18


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composite (1 Sm-1 at 0.42 vol.%) prepared via similar strategy .34 The much higher electrical conductivity in rGO/mullite composite is considered to be related to the TVS process, in which the viscous flow under high pressure could result in alignment of rGO perpendicular to the pressure direction. Further increasing the rGO content to 0.61 and 0.89 vol.% leads to even higher electrical conductivity of 600 and 696 Sm-1, respectively. Noting that the electrical conductivity for 1 vol.% GN/Al2O3 EMI shielding composite is merely 4.7 Sm-1,40 and for 7.8 vol.% GN/BaTiO3 EMI shielding composite is less than 600 Sm-1, 41 the realization of high conductivity at such low graphene content explicitly indicates the advantage of using rGO as microwave absorbent and the success of our processing strategy for achieving uniform dispersion of rGO in matrix. Furthermore, compared to the other graphene based ceramic composites reported so far, the most distinct characteristic in rGO/mullite composite is the highly anisotropic structure, which is revealed by the huge difference in electrical conductivity along the in-plane and cross-plane (σ ⊥ ) directions of sample (Figure 6). For the composite with 0.47 vol.% filler content, the σ ⊥ is only 0.55Sm-1 which is three orders lower than σ‖. Although the anisotropic electrical behavior has been found in many graphene based ceramic composites, none of them shows the great difference similar to the composite studied here,42,

43

suggesting a highly anisotropic microstructure in rGO/mullite

composite. In addition, the composites shows different conducting behavior along the in-plane and cross-plane directions, as evidenced by the different increasing rate of electrical conductivity, which can be understood from the percolation theory. The 19



electrical conductivity increases quickly after percolation at first and tends to rise slowly when the conducting filler content becomes high. Apparently, due to the anisotropic microstructure, the rGO/mullite composites show two different percolation thresholds in two directions, which leads to distinct increasing rates for the electrical conductivity. The anisotropic microstructure in rGO/mullite composite was characterized by back-scattered electron (BSE) images observed from directions parallel and perpendicular to the pressure (Figure 7A and B). It can be seen that the polished in-plane surface shows no preferred orientation of grains, which is in consistence with the TEM observation. In contrast, the polished cross-plane surface displays very obvious alignment of grains perpendicular to the pressure direction, thanks to the large contrast between rGO and mullite in BSE signal. To further confirm the origin of the contrast in BSE image, the secondary electron (SE) image and BSE image taken from the same area are compared, as shown in Figure 7C1 and C2. The laminate structure cannot be observed in SE image, while can be clearly identified in BSE image, indicating the contrast is originated from the composition difference. Moreover, the contrast in BSE image is in agreement with the EPMA mapping for C especially in the area containing large mullite grains (Figure 7C3), which is another solid evidence of the anisotropic structure in the rGO/mullite composites. The formation of anisotropic structure can be ascribed to the viscous flow during sintering, which greatly facilitates the rearrangement of rGO. Since rGO can inhibit the grain growth across the graphene plane, the mullite grains were forced to grow along the graphene plane direction, leading to the texture that may have great influences 20


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on the EMI shielding and mechanical properties.

Figure 6. Electrical Conductivity of rGO/mullite composites in the cross-plane and in-plane directions.

B

A

P

10 μm

10 μm C3

P

C2

P

C1

P

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


21

20 μm


20 μm

20 μm


Figure 7. Anisotropic structure in the rGO/mullite composite with rGO content of 0.61 vol.%. (A)BSE image observed from in-plane surface. (B)BSE image observed from cross-plane surface. (C1-C3) SE, BSE images and EPMA mapping of C observed from cross-plane surface in the same area; the arrows indicate the pressing direction during sintering.

3.3 EMI shielding performance The EMI shielding performance of rGO/mullite composite is mainly determined by its complex permittivity, since no magnetic material is included in the composite. In a dielectric material, the real part of permittivity (ε’) reflects the storage capacity of the electromagnetic energy, which is mainly stemmed from the polarization process in the material, while the imaginary part of permittivity (ε") represents the energy dissipation processes including the conduction and polarization relaxation loss. In the case of mullite, it has a moderate ε’ and low ε", leading to relatively high loss tangent (~0.3) in the whole K band (Figure 8A-C). It has been proved that high loss tangent of pure mullite is caused by the residual APTES in surface modification (Figure S5), which should be beneficial to the EMI shielding performance. With the addition of rGO, it is observed that both ε’ and ε" in the composites increase quickly with increasing filler fraction. The large ε’ in composite should be ascribed to the enhanced dipole orientation polarization and interfacial polarization, considering the other mechanisms such as electronic and ionic 22


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polarization usually occur at much higher frequency.44 As analyzed in the previous section, plenty of defects and oxygen containing groups were left on rGO in the composite, which is the main reason of strengthened dipole orientation polarization. Moreover, the rGO created abundant novel interfaces and the uneven distribution of space charges induced in the composites thereby are also deemed as an important factor of large ε′. In comparison, both electrical conduction loss and polarization relaxation contribute to ε", which can be expressed by the equation: ε" = 𝜀′′relax + 𝜎 𝜔𝜀0

(3)

where 𝜀′′𝑟𝑒𝑙𝑎𝑥 is the electron relaxation process in this case, σ is the electrical conductivity, 𝜀0 is the dielectric constant in a vacuum, and ω is the angular frequency.45 While the polarization relaxation term in ε" is attributed to the similar processes for enhanced ε’, the conduction loss term derived from highly conductive rGO may dominate the energy dissipation behavior in rGO/mullite composites. As a result, the loss tangent for mullite composites increases rapidly with increasing rGO content. The tanδ value for 0.47 vol.% composite is larger than 1, and reaches 2.5~3 for the composite with graphene content of 0.89 vol.%, suggesting high efficiency for microwave attenuation.

23



A

D

B

E

C

F

Figure 8. Dielectric and EMI shielding properties for mullite and rGO/mullite composites in K band. (A-C) Real part permittivity, imaginary part permittivity and loss tangent for mullite and composites with different rGO content, respectively. (D, E) EMI SET and SEA values for mullite and composites with different rGO content, respectively. (F) 24


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Correlation between SET, SEA, SER values and rGO volume fraction in mullite composites at 24 GHz.

The high efficiency of rGO/mullite composite (thickness d=1.4mm) for EMI shielding in K band is proved by the measurement of EMI shielding effectiveness (EMI SET), as shown in Figure 8D. It can be seen that the pure mullite has very mild value of EMI SET, which decreases monotonically with arising frequency in K band. By contrast, the EMI SET value is already above 15dB for the composite with 0.47 vol.% rGO, and exceeds 20dB when rGO content is 0.61 vol.%, which means more than 99% of microwave cannot pass through this material.

For the mullite composite with 0.89

vol.% rGO, the EMI SE achieves a high value larger than 30dB in average, which is comparable to the 3mm thick CNT/ polyurethane shape memory polymer composite with CNT content of 6.7 wt.%.46 The EMI SE value is almost unchanged for the rGO/mullite composite throughout the entire K band. This characteristic is highly desirable for EMI shielding, since it can provide reliable performance in a broadband range. To understand the mechanism of high EMI SE in rGO/mullite composite, the total EMI SET is divided into two components including reflection (SER) and absorption (SEA), which can be calculated by the following equations: SET= SEA+ SER

(4)

SER=10log [1 (1 ― |𝑆11|2)]

(5)

25



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2 SEA=10log [(1 ― |𝑆11| ) |𝑆21|2]

(6)

R = |𝑆11|2 = |𝑆22|2

(7)

A = |𝑆12|2 = |𝑆21|2

(8)

T=1―R―A

(9)

where Sij represents the power transmitting from port i to port j; R, A and T are reflected power, absorbed power and transmitted power, respectively. Very similar to the result for total EMI SE, SEA has a considerable leap upon even small amount of rGO loading, and continues to increase rapidly when rGO content becomes higher (Figure 8F). In addition, it can be seen that the trend observed for SEA is in very good agreement with that for loss tangent, which confirms that the high attenuation ability stems from the large loss tangent in the composite (Figure 8E). The value of SEA increases with increasing rGO content, achieving ~23dB for the composite with 0.89 vol.% rGO. It is considered that three factors mainly account for the high absorption value in the composite with large electrical conductivity. Firstly, the high electrical conductivity directly induces large dielectric loss, because the electrical conductivity is proportional to ε” according to the free electron theory (Eq. 3); secondly, the sample with higher electrical conductivity has larger graphene content, which leads to high interfacial relaxation due to the large surface area of rGO; thirdly, the multiple internal reflection effect induced by anisotropic structure will be strengthened with increasing graphene content, which also contributes to the absorption of microwave. Therefore, it can be also deduced that the EMI shielding effectiveness along the in-plane direction should be inferior to that along the cross-plane 26


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direction, because of the much lower electrical conductivity on the incident surface (cross-plane). Although large loss tangent usually implies the loss of impedance matching and thus highly increased SER for the EMI shielding materials, the increase of SER is quite mild for rGO/mullite composite (Fig. 8F). Compared with monolithic mullite, the SER value only has very limited increase with increasing rGO content at 24GHz, while most of the enhanced EMI SET comes from the improvement of SEA component, representing the high intrinsic ability to attenuate microwave that goes inside the material. However, due to the relatively high SER of the mullite matrix and increased reflection induced by large electrical conductivity, most of the incident energy is reflected back by the composite before absorption, as indicated by the values of R and A (Figure S4). From the mean value of T for composite with 0.89 vol.% rGO we can see that only 0.1% waves transmit the material, which means nearly 99.9% of the waves are shielded. This result is among the best performance of EMI shielding composites in K band, though the thickness is relatively large compared with some polymer based composites with much higher filler content (Table 1).7 More importantly, to realize a high EMI shielding efficiency, the filler loading required is the lowest in comparison to the listed ceramic and polymer based composites, which has critical meaning to preserve high mechanical performance especially in ceramic matrix composite.

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Table 1. EMI shielding performance of various carbon-based composites Filler

Filler content

Matrix*

Thickness (mm)

Electrical conductivity (Sm-1)

EMI SE (dB)

Frequency range

GN CNT CNT GN rGO GN GN rGO CNT

13 vol.% 6.4 vol.% 6.7 wt.% 19 vol.% 0.89 vol.% 7.8 vol.% 2 vol.% 20 vol.% 10.4 vol.%

PVDF epoxy SMP HDPE mullite BaTiO3 Al2O3 SiO2 SiO2

0.2 3 3 3 1.4 1.5 1.5 1.5 2.5

0.1 8.3 ~10-5 696 ~600 120 30 5

29 35 32 22.5 32 30 23 35 22

K K K K K X X X X

Reference 7 47 46 48

This work 16 17 49 50

*PVDF: poly- (vinylidene fluoride), SMP: shape memory polymer, HDPE: high-density polyethylene.

3.4 Mechanical properties of rGO/mullite composite With excellent EMI shielding performance, the mechanical properties of rGO/mullite composites were also investigated to find whether rGO would deteriorate the mullite as structural material. The strength of mullite and its composite with disc shape was measured by MSP method, which is very convenient and reliable for evaluating the strength of samples with small size. It is found that the MSP strength measured for monolithic mullite here (~410MPa)

is very close to the flexure strength of fully dense

mullite sample measured by 3-point bending test (466 MPa), indicating the strength value 28


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obtained by MSP test is quite reasonable .51 The MPS strength of rGO/mullite composite reaches a peak value of 506 MPa for the composite with 0.47 vol.% rGO content and then starts to decrease with increasing rGO content . The ~23% enhanced strength in the rGO/mullite composite with 0.47 vol.% rGO content can be ascribed to the improved fracture toughness (KIC) measured by indentation method (Figure 9A). Note that although the indentation method is not a standard approach to give fracture toughness, the indentation KIC has quite good consistency with the KIC value obtained by well-established Chevron Notch method when graphene content is lower than 5 vol.% .52 Compared to monolithic mullite, the composites with rGO content of 0.47 and 0.61 vol.% both show much improved indentation KIC, while the composite with rGO content of 0.89% has sudden decreased indentation KIC even lower than that of pure mullite. It is well known that graphene is very effective for toughening ceramics. Our previous research has demonstrated that rGO can toughen the alumina matrix via a stretched filler bridging mechanism, which is also applicable in the mullite composite due to the similar wrinkled rGO microstructure observed in the composite. In fact, the toughening effect from graphene should be more pronounced in the rGO/mullite composite owing to the highly anisotropic structure. Aligned rGO flakes along the in-plane direction of sample will intensify the toughening effect compared with the composite with randomly dispersed graphene. However, too much rGO could result in increased amount of rGO in the out-of-plane direction, which actually acts as micro-cracks in the composite, and thereby gives rise to the decreased indentation KIC for 0.89 vol.% rGO/mullite composite. 29



Therefore, the strengthening effect is much more pronounced for the mullite composite with low rGO contents, but completely disappeared when rGO content became high. Moreover, the increased pores induced by increasing rGO also undermines the strength of composite, which explains the lower MSP strength for the mullite composite with 0.61 vol.% rGO content compared with the one with 0.47 vol.% rGO content, even though the former exhibits higher indentation KIC. Concerning the anisotropy, the strength and toughness along the in-plane direction should be low, since the rGO will act as defects rather than reinforcement in this situation.

A

B

Figure 9. Mechanical properties for mullite and rGO/mullite composites. (A) MSP strength and indentation KIC (B) Vicker hardness and Young’s modulus for mullite composites with various rGO contents, respectively; the error bar for Young’s modulus is too small to be shown here.

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As another important mechanical property for structural materials, the hardness of mullite composite was also influenced by adding rGO. First of all, it is worth noting that the monolithic mullite ceramic prepared via TVS has a high Vickers hardness of 12 GPa (1 kgf), which is similar to that of the bulk sample (12.3 GPa, 0.1 kgf) sintered directly from mullite powder by SPS at 1700 ℃ , reflecting again the unparalleled advantage in sintering temperature by using TVS method.53 However, the hardness for composites has a mild decrease to ~10 GPa, and shows no obvious correlation to the rGO content. This phenomenon can be understood from two aspects. On one hand, the decreased hardness mainly comes from the drop of Young’s modulus in composites, which decreases monotonically with increasing rGO content (Figure 9B). The decline of stiffness in the mullite composites is also associated to the very unique wrinkled structure of rGO, since the highly folded structure could completely relax the load transfer to rGO, making rGO sheets behave like 2D pores in the matrix. On the other hand, the decreasing tendency in hardness could be countered by restrained grain size in mullite composites. As shown in Figure 10, the mullite composites show declining grain size with increasing rGO content, owing to the great capability of graphene in prohibiting the grain growth during sintering process. Finally, the hardness is rebalanced by the lowering Young’s modulus and finer microstructure, displaying a stable value for all the mullite composites in this study. To sum up, compared with monolithic mullite, the rGO/mullite composite exhibits enhanced strength, toughness and largely maintained hardness except for the one with highest rGO content. Considering the high performance in EMI shielding, it can be concluded that an 31



advanced mullite composite with both excellent EMI shielding efficiency and mechanical properties has been achieved.

Figure 10. SEM (secondary electron) images of fractured surface for (A) pure mullite and mullite composites with rGO content of (B) 0.47, (C) 0.61and (D) 0.89 vol.%, respectively; the inset in (D) is a magnified area showing the rGO on the surface.

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4 Conclusions In summary, the mullite based EMI shielding composites with highly oriented rGO as microwave absorbent have been successfully fabricated for the first time. To inhibit the reaction between rGO and mullite during sintering process, the highly dense rGO/mullite composite was prepared at merely 1200℃ by transient viscous sintering using -Al2O3@SiO2 powder as precursor. Due to the applied high pressure during viscous sintering, a highly anisotropic structure of rGO in the mullite matrix was constructed, resulting in abnormally large in-plane electrical conductivity of 696 Sm-1 at merely 0.89 vol.% rGO content in composite. It is found that the high conductivity, interfacial polarization loss and multi-reflection effect stemmed from the highly anisotropic structure are responsible for the large EMI SE (>32dB) at extremely low filler fraction in the K band, which is beneficial to maintain the outstanding mechanical properties of matrix material. Consequently, the rGO/mullite composite shows greatly improved strength and toughness when the rGO content is not high, albeit the hardness is slightly declined due to the decreasing stiffness. Therefore, a lightweight, strong and tough rGO/mullite composite with high EMI shielding efficiency has been realized in this study, which has great potential to be applied as an effective EMI shielding material in very harsh environment of high stress field and elevated temperature.

Supporting information Schematic diagram of the MSP device, schematic illustration of TVS for the formation of 33



mullite, the calculated Gibbs free energy for the reaction between carbon (graphite or carbon black) and mullite, power balance of the resultant samples and dielectric loss tangent of different mullite samples.

Acknowledgements This work was funded by the National Key R&D Program of China (2017YFB0703200), the National Natural Science Foundation of China (No. 51702045), the Natural Science Foundation of Shanghai (17ZR1400900) and the Pujiang Talent Program (17PJ1400200). The authors would like to thank Dr. Lingmin Zhang of Tongji University for the assistance of EPMA analysis. Acknowledgement is also given to Dr. Jixuan Liu for the calculation of Gibbs energy.

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