Light and Strong Hierarchical Porous SiC Foam for Efficient

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Light and Strong Hierarchical Porous SiC Foam for Efficient Electromagnetic Interference Shielding and Thermal Insulation at Elevated Temperatures Caiyun Liang, Zhenfeng Wang, Lina Wu, Xiaochen Zhang, Huan Wang, and Zhijiang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07735 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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ACS Applied Materials & Interfaces

Light and Strong Hierarchical Porous SiC Foam for Efficient Electromagnetic Interference Shielding and Thermal Insulation at Elevated Temperatures

Caiyun Liang a, Zhenfeng Wang b, Lina Wu c, Xiaochen Zhang,b Huan Wang d, Zhijiang Wang*,a

a

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and

Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China; b

Beijing Institute of Nearspace Vehicle’s Systems Engineering, Beijing 100076, China c

Molecular Imaging Research Center (MIRC), Harbin Medical University, Harbin, Heilongjiang 150001, China d

FAW-Volkswagen automotive Co., Ltd, Changchun 130011, China.

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ABSTRACT A novel light but strong SiC foam with hierarchical porous architecture are fabricated by using dough as raw material via carbonization and followed carbothermal reduction with silicon source. A significant synergistic effect is achieved by embedding meso and nano pores in micro-sized porous skeleton, which endows the SiC foam with high-performance electromagnetic interference (EMI) shielding, thermal insulation and mechanical properties. The micro-sized skeleton withstands the high stress. The meso- and nano-sized pores enhance the multiple reflection of the incident electromagnetic waves and elongate the path of heat transfer. For the hierarchical porous SiC foam with 72.8% porosity, EMI shielding can be higher than 20 dB and specific EMI effectiveness exceeding 24.8 dB cm3 g−1 in the frequency of 11 GHz at 25−600 °C, which is 3 times higher than dense SiC ceramic. The thermal conductivity reaches as low as 0.02 W m−1 K−1, which is comparable to aerogel. But the compressive strength is as high as 9.8 MPa. Considering the chemical and high-temperature stability of SiC, the fabricated SiC foam is a promising candidate for modern aircraft and automobile applications.

KEYWORDS SiC, hierarchical porous foam, electromagnetic interference shielding, thermal insulation, mechanical property

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1. Introduction Hierarchical porous materials containing structure-within-structure, are currently attractive and play important roles in environmental protection or energy related applications profile ranging from electromagnetic interference (EMI) shielding1, water purification2,

batteries3,

supercapacitors4,

catalyst

supports5

and

solar

cells6.

Understanding the structure-property relationships at a detailed level could provide the guideline for the successfully design and integrating multi-functional materials not available in nonhierarchical counterparts, which can expand their potential applications. Recently, materials with lightweight, high-strength, excellent thermal stability and high-efficiency EMI shielding performance are highly desired for practical EMI shielding applications, especially in areas of aerospace and aircraft7,8. The EMI shielding materials applied to those vehicles are always in service under high-temperature environments. In addition, these kinds of materials should be mechanical strong but being light in order to save energy. Hence, it is particularly urgent to explore new EMI shielding materials that can fulfill these harsh requirements. The development of porous structure is a useful way to reduce the density of materials. However, materials with high porosity is usually at the expense of mechanical stability. Aerogel with nanometer pores is now the subject of much investigation, which suffering from brittle mechanical properties. Hierarchical designs can offer effective solutions, because the cellular architectures within a materials have excellent mechanical tolerance9 and the load-bearing can be dispersed in different length scales in these hierarchical porous structure10,11. Therefore, materials can be designed into hierarchical porous

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structures to avoid fracture during compression and retain a more efficient relationship between mechanical response and density. In addition, the hierarchical structures in materials play an important role in creating different functions. It has been proposed that the hierarchical porous structure encompassing different pore scales is beneficial to enhance

impedance

matching

and

multiple

reflection-absorption

under

the

electromagnetic field.12,13 Recent developments also show that the hierarchical structure will lengthen the path of heat transfer and hinder the gaseous heat conduction, leading to a low thermal conductivity14,15. Therefore, hierarchization of materials with proper porosities and structures may endow materials with superior EMI shielding performance and thermal insulation property simultaneously. On the other hand, different from traditional metal-based and polymer-based EMI shielding materials, which suffer from either high mass density, or undesirable oxidation and corrosion, silicon carbide (SiC) has sparked much of the current interest due to its advantages in relative low mass density, chemical inertness and outstanding thermal stability.16,17 As a consequence, light and strong porous SiC structures are appealing for many advanced technological applications. Recently, there have been many reports in the synthesis of porous SiC materials, such as direct foaming, gelcasting and sacrificial template methods.18,19 However, these methods faced the challenges of complicated procedures, high sintering temperature and additional sintering additive. In this regard, the development of a facile and environmentally friendly pathway to fabricate porous SiC materials is still of great challenge. Considering that biomass is a promising renewable resource and may make the strategy

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of fabricating porous materials highly economical, in the present study, we develop a novel bimass-derived strategy to synthesize hierarchical porous SiC foam for EMI shielding and thermal insulation at high temperature. The synthesis is facile, reproducible, green, and using inexpensive and widely available biomass materials, dough, as starting materials. By the incorporation of porosity at multiple length scale, the as-obtained hierarchical porous SiC foam showed excellent compressive strength and exhibited a unique combination of properties. It had a better specific EMI shielding performance than dense SiC ceramic at the temperature of 25−600 °C in X-band and presented very low thermal conductivity. The influences of the hierarchical porous structure of SiC foam and temperature on the EMI shielding effectiveness were explored. The mechanical properties and thermal conductivity of SiC foam were also discussed.

2. Experimental section 2.1 Materials The flour and yeast were obtained from local market. Silicon powder and silicon dioxide were supplied by Sinopharm Chemical Reagent Co., Ltd, China. β-SiC powder are purchased from North Star Special Ceramics Co., Ltd.

2.2 Fabrication of hierarchical porous carbon foam 100 g of flour and 1 g of dry yeast were grinded to form a uniform mixture powders. 55 mL of water was poured into the well mixture powders of flour and yeast, which was kneaded slowly for about 20 min until the wet flour cohered to a dough. The dough was allowed to ferment for about 30 min at 35 °C and the porous structures was formed during this process. Then, the dough was placed in a freezer at −20 °C for 12 h and further dried

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in a freeze-dryer (model:FD-1-50, Beijing Boyikang Co., Ltd.) for 24 h. Finally, the dry dough was carbonized at 800 °C under a protective flow of nitrogen to generate a hierarchical porous carbon. Heating rate was set as 10 °C min−1 and maintained at 800 °C for 2 h.

2.3 Fabrication of hierarchical porous SiC foam The hierarchical porous carbon was put in a corundum crucible, and then covered by mixed powders of silicon and silicon dioxide. The mole ratio of C: Si : SiO2 was fixed at 1:1:1. The corundum crucible was placed into the center of a sintering furnace. The temperature of the furnace was heated to 1500 °C at a rate of 5 °C per minute and a dwell time of 5 h under an argon flow( 10 L h−1). Finally, the sample was cooled down to room temperature. In order to remove the residual unreacted carbon, the sample was calcinated in the air at 700 °C for 12 h.

2.4 Fabrication of dense SiC ceramic β-SiC powders and sintering additive of SiO2 powders were mixed in ethanol by milling in a planetary ball attritor using SiC balls as grinding media. After milling, the ethanol was subsequently evaporated in an oven at 60 °C for 24 h. The dried powders were crushed in an agate mortar and sieved through a 100-screensieve, then the powders were cold pressed into cylindrical green bodies with diameter of 50 mm and height of 4 mm under a pressure of 70 MPa. Finally, the green bodies were sintered at 2000 °C for 2 h under a pressure of 40 MPa in argon atmosphere.

2.5 Characterizations X-ray diffraction (XRD) patterns of the products were collected using a Rigaku

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D/max-gB diffraction system equipped with a rotating anode and Cu Kα source. The morphologies of samples were studied on a scanning electron microscope (SEM, Helios NanoLab 600i). The nanometer pore diameter distribution was obtained from the absorption isotherms, according to the Barrett-Joyner-Halenda (BJH) model. The macropore size distribution was evaluated by mercury porosimeter (AutoPore IV 9500, Micromeritics, USA). The density and porosity of SiC foam was calculated from the AutoPore IV by measuring the volume and weight of the sample. The apparent density of dense SiC ceramic was measured by Archimedes method in water. The electrical conductivity of samples were obtained using a two-probe method with a Keithley 2400 electrometer (Keithley, Cleveland, USA), at temperatures between 25 and 600 °C. Raman spectra were obtained on a confocal Raman spectroscopic system (Renishaw, In Via) using a 633 nm laser. Compression properties were measured by Instron testing machine (Instron 5596) at a compression speed of 1 mm/min and the samples were polished into dimensions of 10 × 10 × 5 mm3 for the compression test. Thermal conductivity analysis was conducted on a Nezstch LFA−427 Laser Flash Apparatus from room temperature to 800 °C. UV−vis light transmittance spectra were recorded with a U3900-H UV-Vis Spectrophotometer (Hitachi). The EMI shielding properties of samples at 25−600 °C were measured using a vector network analyzer (VNA, MS4644A, Japan) in the frequency range of 8.2−12.4 GHz by waveguide method. The samples were cut into rectangular blocks with dimensions of 22.5 × 10.0 × 3 mm3 to well fit the waveguide holders. The scattering parameters (S11 and S21) of each sample was recorded and the values of EMI sheilding effectiveness (EMI SE) was calculated based on the measured scattering

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parameters.

3. Results and Discussion 3.1 Microstructure analysis The hierarchical porous SiC foam was obtained by using dough as raw materials via carbonization and followed carbothermal reduction with silicon source, as shown in Figure 1a. A dough was used as carbon source whose shape and original hierarchical porous structure can be remained after fermented and freeze-dried. The dried dough was then carbonized and reacted with silicon source at the temperature of 1500 °C to obtain hierarchical porous SiC foam. A little shrink of the sample takes place at each step of carbonization and carbothermal reduction. During carbonization step, the dried dough composed of (C6H10O5)n are converted into solid carbon and gaseous water. This is a weight-reduced and size-shrunk reaction. The further carbothermal reduction process is fulfilled by the reaction between carbon and silicon powder under a high temperature of 1500 °C. Part of carbon will be consumed. Meanwhile, the surface tension also results in the sample shrinking. The SEM images of the dough-derived carbon foam reveal its hierarchical porous structure. The low magnification SEM images in Figure 1b indicate that a lot of round or elliptic open pores with diameters ranging from 5 to 50 µm present in the porous carbon foam. A high magnification image in Figure 1e shows that large quantities of smaller pores ranging from a few to several ten microns are present in the skeletons. Figure 1c and 1f are the SEM images of the as-synthesis hierarchical porous SiC foam, which confirm that the sample is highly porous and possesses an interconnected

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three-dimensional network. The pore size distribution is mostly in the range of 5 to 50 µm and some smaller pores also exist in the skeletons. The skeletons are constituted of crystalline SiC grains with a diameter of 100−200 nm. Although shrinking on the size taken place, it is obvious that the hierarchical porous morphology, pore arrangement and size distribution of porous SiC are almost replicated from those of the carbon foam. From Figure 1d and g, it can be observed that the dense SiC ceramic has a highly compact structure and the SiC grains in the size of 0.5−1 µm are bonded to each other. The XRD patterns of the dough-derived porous carbon foam has two broad peaks appeared at 22.3°and 43.8°, which are corresponding to the (002) and (100) planes of graphite, respectively (Figure S1). In the porous SiC foam, all diffraction peaks match very well with the standard peak positions for a cubic zincblended 3C-SiC structure (β-SiC JCPDS Card 75–0254) (Figure 1h). The small diffraction peak locating at 33.6°, marked with SF, is attributed to the spontaneous stacking fault during SiC growth. No impurities peak, such as residual silicon dioxide and carbon, is detected indicating that the reaction between carbon and silicon source is completely processed. The XRD pattern of the dense SiC ceramic is quiet similar to the hierarchical porous SiC foam. There are five diffraction peaks belonging to β-SiC, except for a peak at 2θ = 21.9° which is corresponding to SiO2. The existence of SiO2 is caused by using SiO2 powders as sintering additive during sintering process. Dough-derived SiC foam has a hierarchical porous structure from nanometer sizes to micrometer sizes. The mercury porosimeter was used to investigate the macropore structure of the hierarchical porous SiC foam. The hierarchical porous SiC foam has a

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bulk density of 0.87 g cm−3 and a porosity of 72.8%. Figures 1i exhibits the pore distribution curves of the hierarchical porous SiC foam in which pores with a size of 5-35 µm domain in the macropores range. Some larger macropores range from 35 to 60 microns are also present, but to a relatively low content. This result indicates that the fabricated hierarchical porous SiC foam contains numerous macropores and is in good agreement with the analysis of SEM. The porosity of hierarchical porous SiC foam was further determined by N2 physical adsorption-desorption measurements. Figure 1j illustrates the corresponding pore size distribution of the SiC foam calculated by BJH method. The pores range from 1.7 to 40 nm and most of the pores are in the scope from 2 to 5 nm. These characterizations indicate that micro-, meso- and macro-pores coexist in hierarchical porous SiC foam. Such a unique hierarchical architecture with low density is highly desirable for environmental protection, energy storage and conversion. On the contrary, the density and porosity of dense SiC ceramic measured by Archimedes’ method are 3.18 g cm−3 and 1.9%, respectively.

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Figure 1. Schematic representation of fabrication process for hierarchical porous SiC (a). SEM images of dough-derived carbon foam (b), (e); hierarchical porous SiC foam (c), (f); and dense SiC ceramic (d), (g). XRD patterns of hierarchical porous SiC foam and dense SiC ceramic (h). Macropore size distribution measured by mercury porosimetry (i) and nanopore size distribution calculated by BJH method (j) for hierarchical porous SiC foam.

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3.2 EMI shielding performance Strikingly, the hierarchical architecture endows the SiC foam with a high-performance EMI shielding. Significantly different with previous reported metal-based and polymer-based EMI shielding materials, the EMI shielding performance of SiC foam is increased with the rising of the temperature. The values of EMI SE are related to the ability of materials to attenuate EM waves and is generally expressed in decibel (dB).20 The total EMI SE (SET) is the summation of the reflection from the surface of shielding materials (SER), the absorption energy of EM waves (SEA) and the multiple reflection (SEM).

21,22

The higher the dB level of EMI SE, the more incident EM waves are

obstructed by materials. For example, the value of SET being 20 dB is corresponding to that 99% of the incident EM waves are blocked. When SET is above 15 dB, SEM can be neglected. 23 Hence the SET can be simplified as:

SET ≈ SE R +SEA (1) Figure 2a and 2b show the EMI SE of dense SiC ceramic and hierarchical porous SiC foam versus frequency at different temperatures. As observed in Figure 2a, the EMI SE of dense SiC ceramic decreased dramatically from 25.2−33.8 dB to 22.7−29.3 dB with the increase of temperature at 25−300 °C in X-band. The downward trend become weak in the high temperature region between 400 to 600 °C. With the rising of frequency from 8.2 to 12.4 GHz, the EMI SE was declined rapidly at 25−300 °C and then showed weaker frequency dependent at high temperature. However, the variation trend of EMI SE for porous SiC versus frequency and temperature was opposite to the dense SiC ceramic. This could be seen in Figure 2b that the EMI SE of porous SiC increased with the elevation of

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frequency and temperature, respectively. Owing to this inversing trend, the porous SiC foam exhibited a better EMI shielding performance under the conditions of high temperature and high frequency. For example, in the frequency range of 11.4−12.4 GHz, the observed shielding performance (23.4−24.1 dB) of hierarchical porous SiC foam was larger than the shielding performance (23.0−23.4 dB) of dense SiC ceramic at 200 °C. When the temperature was increased to 600 °C, the outstanding shielding performance (23.5−24.2 dB) of porous SiC foam was further enhanced, compared to the shielding performance of dense SiC ceramic (21.8-23.5 dB) in a broader frequency range being 9−12.4 GHz. It is worth noticing that the hierarchical porous SiC has the best EMI shielding efficiency at 600 °C, even though the density is only 0.87 g cm−3. The maximum value of EMI SE reaches 24.2 dB which is larger than the target value of EMI SE for commercial application (about 20 dB). This result demonstrates that the porous SiC foam can be used as an effective and lightweight EMI shielding material at high temperature. Figure S2a and S2b show the plots of SET, SEA, SER of the dense SiC ceramic and porous SiC as a function of temperature at the frequency of 11 GHz. It can be clearly seen that the contribution of SER to total EMI SE was relatively low compared with SEA in dense SiC ceramic and hierarchical porous SiC, which suggested an absorption-dominant EMI shielding mechanism for porous SiC foam.24 The specific EMI SE calculated by the rate of total EMI SE to the sample density is also a significant criterion, especially for the applications that required lightweight shielding materials.25 The comparison of specific EMI SE between dense SiC ceramic and

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hierarchical porous SiC foam at 11 GHz and temperature range of 25−600 °C is presented in Figure 2c. The specific EMI SE of hierarchical porous SiC foam was all above 24.8 dB in the whole temperature range which was three times higher than that of the dense SiC ceramic (6.9−8.7 dB). This result proves that the hierarchical porous structure facilitates to improve the specific EMI SE, hence the hierarchical porous SiC foam bearing a better material utilizing efficiency than dense SiC ceramic. The specific EMI shielding effectiveness of hierarchical porous SiC foam is higher than 23.5 dB cm3 g−1 at the temperature range of 25−600 °C in X-band, which is far higher than those of typical metals (10 dB cm3 g−1 for solid copper26) and other SiC-based composites (9.6 dB cm3 g−1 for YSZ/SiC27, 14.3 dB cm3 g−1 for SiCf/SiC28).

Figure 2. EMI shielding efficiency of samples: dense SiC ceramic (a) and hierarchical porous SiC foam (b). The specific EMI SE of dense SiC ceramic and hierarchical porous

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SiC foam at 11 GHz (c) and the proposed EMI shielding mechanism (d). Electrical conductivity is of great importance for EMI shielding performance, because it is an intrinsic ability of a material to attenuate electromagnetic radiation.29 SiC is a typical semiconductor whose electrical conductivity (σ) is determined by30,31

σ (T ) = σ 0e

− Eg /2 kT

(2)

Here T is the temperature, σ0 is a constant, Eg is the band gap energy of the crystal and k is the Boltzmann constant. From Equation 2, it can be concluded that the electrical conductivity of SiC is increased with the raising temperature. The electrical conductivity of dense SiC ceramic and porous SiC foam is plotted against temperature in Figure 3. As can be seen, the electrical conductivity of dense SiC ceramic was as low as 3.5×10−9 S cm−1 at room temperature, which can be ascribed to the addition of insulated SiO2 enveloping the semi-conductive SiC grains.32 Interestingly, the electrical conductivity of dense SiC ceramic remained unchanged at 25−300 °C and exhibited a little increase at 300−600 °C. Compared to dense SiC ceramic, hierarchical porous SiC foam possessed a higher electrical conductivity of 0.04 S cm−1 at room temperature which was closed to the target electrical conductivity value (0.01 S cm−1)33 required for EMI shielding application. With the rising of temperature, the electrical conductivity of porous SiC foam continually increased and reached 0.23 S cm−1 at 600 °C. The higher of conductivity of SiC foam than dense SiC may be owing to the carbon-rich state of SiC grain in porous foam. As shown in Raman spectrua (Figure S3), the D band and G band signals of carbon are observed both in dense SiC ceramic and porous SiC foam. Even though the porous SiC foam has been calcinated in air under

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700 °C for 12 h to burn off the unreacted carbon, the signals of D band and G band are still much higher than dense SiC ceramic. This result indicates that prepared hierarchical porous SiC foam is comprised of carbon-rich SiC grains. The larger conductivity is related to the carbon-rich state of porous SiC which can form conductive network within the hierarchal porous structure. The carbon-rich state of SiC has also been investigated by X-ray absorption near-edge structures in our previous study, where plenty of C-C bonds (sp3 and sp2 bonded carbon) were detected in SiC nanowires.34 For electrically thick materials (t≥δ), the absorption loss is calculated by equation 3. 7,35

SE A = 20log et /δ = 8.68t / δ = 8.68t πµσ f

(3)

Where t is the sample thickness, δ is the skin depth, σ is the conductivity, f is frequency,

µ is magnetic permeability (µ=µoµr), µo = 4π ×10−7, µr is relative magnetic permeability. For SiC material, µr =1. According to this equation, it can find that SEA is increased with the increasing of conductivity. Our results indicate that the large electrical conductivity and the remarkable enhancement of electrical conductivity at high temperature may endow SiC foam with good EMI shielding performance, especially at high temperature.

Figure 3. The electrical conductivity of dense SiC ceramic and hierarchical porous SiC 16


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foam at 25–600 °C Furthermore, the hierarchical porous structure of SiC foam is beneficial for the attenuation of EM waves. As presented at Figure 2d, when EM waves strike the surface of shielding materials, some EM waves is reflected directly because of the impedance mismatch between air and shielding materials. However, the macropores on the near surface of hierarchical porous SiC foam can reduce the obstruction of EM waves infiltrating into the foam, greatly improving the condition of impedance match. The incident waves then interact with the SiC grains on the cell walls that will lead to ohmic losses, causing a dissipation in energy of the EM waves. After that, the surviving EM waves was trapped in the hierarchical porous structure and reflected numerous times on the cell walls. Finally, the EM waves are difficult to escape from the sample before being absorbed or transferred into heat completely.36 This is in marked contrast to the dense SiC ceramic that the internal reflecting surface was unavailable. Therefore, a better EMI shielding performance can be achieved in hierarchical porous SiC foam with improved impedance match and internal multiple reflection process.

3.3 Thermal insulation performance Notably, the hierarchical porous SiC foam presents superior thermal insulation performance. We used laser flash analysis to investigate the thermal conductivity of hierarchical porous SiC foam from room temperature to 800 °C. Thermal conductivity (K) is calculated from the following equation:

K = α ⋅ ρ ⋅ CP

(4)

Here, α is the thermal diffusivity coefficient, ρ is the density of sample and Cp is the heat

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capacity of hierarchical porous SiC foam. The hierarchical porous SiC foam remained stable in the whole range of temperature with low thermal conductivity. As shown in Figure 4, the K values of hierarchical porous SiC foam at room temperature was as low as 0.02 W m−1 K−1, which was comparable to those of previously reported insulating silica37 and alumina aerogels38. The thermal conductivity displayed a rapid climbing tendency from 0.02–0.5 W m−1. K−1 at 25–400 °C, then retained in a steady range being 0.5−0.7 at 400–800 ℃.The phenomenon that the thermal conductivity increases with temperature has also been found in other highly porous structures which could be ascribed to the radiative contribution39. A fresh flower was put on the hierarchical porous SiC foam with thickness of about 4 mm, under which a fire was burning with temperature of 800 °C. The flower remained intact after firing for 1 min indicating the hierarchical porous SiC foam has a superior heat resistance property and hold great promise for the application in thermal insulation. This excellent thermal insulation performance of hierarchical porous SiC foam could be explained as follows: (i) the low density and hierarchical porous structure of SiC foam has enriched channels and high specific areas, which is equivalently to infinite thermal insulation panels. Hence, the path for heat transfer inside the materials become tortuous and lengthy. (ii) there are many nanometer pores with pore diameter of 1.7–40 nm in hierarchical porous SiC foam, which is smaller than the average free path (about 70 nm) of dominated molecules (N2, O2) in the air. This makes it difficult for the gas molecules in the pores to contact with each other and the energy exchange between the gas molecules becomes inefficient. As a result, a low thermal conductivity is achieved due to the

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intricate hierarchical porous structure of SiC foam.

Figure 4. Thermal conductivity of hierarchical porous SiC foam at various temperatures.

3.4 Mechanical property Apart from excellent EMI shielding and thermal performance, the mechanical properties of porous materials are other important factors for their practical applications. Figure 5 shows the stress–strain curves of the fabricated hierarchical porous carbon foam and SiC foam. The maximum compressive strength of carbon foam was about 4.8 MPa and its compressive modulus could be about 259.5 MPa. For the resulting SiC foam, the maximum compressive strength and compressive modulus were 9.8 MPa and 546.8 MPa, respectively. It was striking that the maximum compressive strength and compressive modulus of the presenting hierarchical porous SiC foam were two times larger than its carbon foam template. Moreover, the compressive strength of the hierarchical porous SiC foam is much higher than other reported SiC foams40-42 with a similar porosity which is summarized at Table 1. Conventional SiC foams, especially in high porosity, have a porous network with

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relatively weak adhesive interaction, because they are stacked by solid SiC particles. However, the fabricated hierarchical porous SiC foam in this study was a direct transformation from the carbon foam whose strength was high enough under the carbon thermal reaction. The continuous carbon thermal process from outside to inner resulted in covalently bonded SiC structure43,44. As a consequence, the struts of the macropore walls in hierarchical porous SiC foam could endure higher stresses without breakage. Aerogels are one of the lightest solid materials and well recognized for their substantially excellent thermal insulation properties. However, the poor mechanical property is a fatal weakness of aerogels. For example, the maximum compression strength of silica aerogels is lower than 0.5 MPa45,46, which is far from satisfaction for practical application. Different from most of the aerogels with low compressive strength, in which the pore sizes always fall on a narrow nanometer range, the sophisticated hierarchical structure of SiC foam is facilitated for the mechanical efficiency by reducing microdefects and introducing load-bearing structural units at multiple length scales. In this three-level hierarchical porous SiC foam, the skeletons of micron pores, the skeletons of the mesopores and the skeletons of micropores constitute the load-bearing structural units at gradually smaller length scales, which provides a means for dissipating strain. Therefore, the introduction of hierarchical porous architecture offers the opportunity to improve the mechanical strength of materials, allowing us to achieve high strength and low density simultaneously.

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Figure 5. Compressive strength of hierarchical porous SiC foam and hierarchical porous carbon foam. Table 1. Compressive strength and compressive modulus of some representative SiC foams Samples

porosity

Compressive

Compressive

strength (MPa)

modulus (MPa)

Ref.

SiC foam 1

72.8%

9.8

546.8

current study

SiC foam 2

74.6%

2.61

-

44

SiC foam 3

77.42

5.04

-

45

SiC foam 4

87%

0.75

11.7

46

4. Conclusion In summary, we have developed a hierarchical porous SiC foam through direct carbonization and followed carbothermal reduction with silicon by using dough as precursor which is easy accessible from natural sources. The as-obtained hierarchical porous SiC foam has micro-sized porous skeleton embedded with meso and nano pores, showing a synergistic effect on their multi-functional properties. It exhibits a unique 21



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combination of functional properties such as outstanding EMI shielding at high temperature, excellent thermal insulation and high mechanical property. When the hierarchical porous SiC foam is in a porosity of 72.8%, EMI shielding reaches higher than 20 dB and specific EMI effectiveness exceeding 24.8 dB cm3 g−1 in the frequency of 11 GHz at 25−600 °C, which is 3 times higher than dense SiC ceramic. The thermal conductivity reaches as low as 0.02 W m−1 K−1, which is comparable to aerogel. But the compressive strength is as high as 9.8 MPa. The unique hierarchical porous structure combined

with

high

electrical

conductivity,

internal

multi-reflection

and

absorption-dominant shielding mechanism is responsible for the high EMI shielding efficiency. The effective of infinite thermal insulation panels and tremendous nanometer pores in hierarchical porous SiC foam lead to a very low thermal conductivity. Our understanding on the interrelationship between the hierarchically porous structures and their performances can provide a guideline for designing and fabricating new structures with multiple advanced properties.

ASSOCIATED CONTENT Supporting Information The following materials are available free of charge. (1) XRD pattern of hierarchical porous carbon foam, (2) the comparison of SET, SEA and SER at 11 GHz for dense SiC ceramic and hierarchical porous SiC foam,（3）Raman spectra of hierarchical porous SiC foam and dense SiC ceramic, (4) schematic illustrations of electrical conductivity measurement and (5) UV−vis light transmittance spectra of

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hierarchical porous SiC foam. .

Author information *Corresponding author: Tel.&Fex: +86 451 86418409. E-mail address: [email protected] (Z. Wang) Notes The authors declare no competing financial interest.

ACKNOLEDGMENTS The authors highly appreciate for the financial support of National Natural Science Foundation of China (No. 51572062), Natural Science Foundation of Heilongjiang Province (No. B2015002), CALT Foundation of Beijing, Heilongjiang Postdoctoral Scientific

Research

Developmental

Fund

(No.

LBH-Q16079,

LBH-Q15090),

Heilongjiang Province Foundation for Returness (No. LC2016034) and Wuliande Foundation of Harbin Medical University (Grant No. WLD-QN1404).

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Table of Contents Graphic

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Light and Strong Hierarchical Porous SiC Foam for Efficient

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