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Ultra-light ceramic aerogels have the characteristics of low density, high porosity, large specific surface area, good thermal and chemical stability, etc., and have great applications in energy storage, catalysis, heat preservation, environmental protection, electromagnetic wave absorption and electromagnetic interference shielding. application potential. We fabricated three-dimensional (3D) porous, ultralight, and cross-linked SiC@SiO₂ nanofibrous aerogels (SiC@SiO₂ nanofibrous aerogels) using low-cost raw materials through a simple chemical vapor deposition (CVD) method and subsequent calcination process. NFA). The SiO₂ nanolayer is coated on the surface of SiC nanofibers through an oxidation process, which not only optimizes the impedance matching of the ceramic aerogel, improves the microwave absorption performance, but also enhances its high temperature thermal stability. 

Multifunctional SiC@SiO₂ Nanofiber Aerogel with Ultrabroadband Electromagnetic Wave Absorption

Limeng Song, Fan Zhang, Yongqiang Chen*, Li Guan, Yanqiu Zhu, Mao Chen, Hailong Wang, Budi Riza Putra, Rui Zhang*, Bingbing Fan*

Nano-Micro Letters (2022) 14: 152

https://doi.org/10.1007/s40820-022-00905-6

Highlights of this article

1. In this paper, a multifunctional SiC@SiO₂ ceramic aerogel has been successfully prepared, which has superelasticity, fatigue resistance, high temperature thermal stability, thermal insulation performance and remarkable strain piezoresistive sensing performance.

2. SiC@SiO₂ ceramic aerogel exhibits excellent electromagnetic wave absorption performance, the minimum reflection loss value is -50.36 dB, and the maximum effective absorption bandwidth is 8.6 GHz.

brief introduction

Traditional ceramic materials are generally fragile, lack flexibility, and have high production costs, which seriously hinder their practical applications. Multifunctional nanofibrous ceramic aerogels are well suited for applications in extreme environments, however, integrating multiple functionalities into one material during their preparation remains extremely challenging. In order to solve this problem, the research group of Professor Zhang Rui and Associate Professor Fan Bingbing of Zhengzhou University fabricated a multifunctional SiC@SiO₂ ceramic aerogel (SiC@SiO₂ ceramic aerogel with 3D porous cross-linked structure) by a simple chemical vapor deposition method and subsequent calcination process. SiO₂ NFA). The as-prepared SiC@SiO₂ NFAs exhibit ultra-low density (~11 mg cm⁻3), superelasticity, fatigue resistance, fire resistance, high-temperature thermal stability, thermal insulation properties, and remarkable strain-dependent piezoresistive sensing behavior. In addition, the SiC@SiO₂ NFA exhibits excellent electromagnetic wave absorption performance with a minimum reflection loss (RLmin) value of -50.36 dB and a maximum effective absorption bandwidth (EABmax) of 8.6 GHz. The successful preparation of this multifunctional aerogel material provides a promising prospect for the design and fabrication of cutting-edge ceramic materials.

Graphical guide

Preparation mechanism, structure and composition characterization of I SiC@SiO₂ NFA

As shown in Fig. 1, SiC@SiO₂ NFAs were prepared by a simple CVD method and subsequent calcination treatment process, which was divided into five steps: (1) SiO₂ and Si nanopowders as silicon sources, and CaCO₃ and activated carbon hybrid particles as carbon The source, the materials were mixed and poured into a graphite crucible, and kept in a nitriding furnace filled with argon at 1500 ° C for 5 hours; (2) as the reaction progressed, SiC nucleated and grew into fibers on the surface of the graphite cover, forming 3D network; (3) After the reaction is completed, the SiC nanofibers form an aerogel with a certain thickness, which is labeled as SiC NFA; (4) The SiC NFA is cut into circular sheets with a diameter of 2 cm; (5) Finally, the The prepared samples were calcined at 1100 °C for 30 minutes in an air atmosphere, during which the surface of the SiC nanofibers was oxidized, and the resulting product was SiC@SiO₂ NFA. The SiC@SiO₂ NFA prepared by this method has a density of only ~11 mg cm⁻3 and a porosity of about 99.6%, which can stand stably on leaves (as shown in inset 6 in Fig. 1).

Figure 1. Preparation process of SiC@SiO₂ NFA. ①Step 1: Incubate at 1500°C for 5 hours in an argon atmosphere; ②Step 2: Self-assemble into a 3D highly porous aerogel; ③Step 3: Calcination in an air atmosphere at 700°C for 2 hours Separation of SiC NFA from the graphite cover; ④ Step 4: Cut the aerogel into circular flakes with a diameter of 2 cm; ⑤ Step 5: Oxidize SiC NFA in an air atmosphere at 1100 °C for 30 minutes to form stable cross-links, And the surface of each nanofiber is coated with SiO₂, which is marked as SiC@SiO₂ NFA; ⑥ The prepared SiC@SiO₂ NFA has an ultra-low density (~11 mg·cm⁻3).

As shown in Fig. 2, the microstructure of SiC@SiO₂ NFA was characterized by SEM and TEM. It can be seen from the figure that after oxidation treatment, a SiC@SiO₂ nanofiber structure with the inner core of SiC and the outer layer of SiO₂ nanolayers is formed. The SiC@SiO₂ NFA is composed of cross-linked SiC@SiO₂ nanofibers with a fiber diameter of 200-400 nm and a length of tens to hundreds of microns. In addition, the main constituent elements of SiC@SiO₂ NFA are C, Si, and O.

Figure 2. Microstructure of SiC@SiO₂ NFA. (a) SEM image of the wire-cluster cross-linked microstructure of SiC@SiO₂ NFA; (b) SEM image of the cross-linking of three cross-linked SiC@SiO₂ nanofibers, and schematic diagram of SiC nanofibers coated with SiO₂ nanolayers ( Inset b); (c) SiC@SiO₂ nanofibers with smooth and rounded surfaces and the corresponding EDS spectra of the nanofibers (inset c), (d–f) EDS spectra corresponding to (c); (g) TEM image of the cross-section of SiC@SiO₂ nanofibers with core-shell structure; (h) enlarged cross-section of SiC@SiO₂ nanofibers; (i) boundary between SiC and SiO₂; (j-l) EDS spectra corresponding to h picture.

As shown in Figure 3, the XRD test results show that the prepared SiC@SiO₂ core is 3C-SiC, and the FTIR, Raman and XPS spectra show that SiO₂ and SiC components exist in the SiC@SiO₂ NFA. In addition, TGA tests showed that the SiO₂ nanolayer formed during the oxidation process can effectively slow down the rate of inward diffusion of oxygen in the nanofibers, thereby protecting the nanofibers from further oxidation. The BET specific surface area of SiC@SiO₂ NFA was determined to be 185.3 m2·g⁻1 and the mesopore pore size was 22 nm by N₂ adsorption/desorption isotherms and pore size distribution maps.

Figure 3. Crystal structure, thermal stability, pore structure and chemical composition analysis of SiC@SiO₂ NFA. (a) XRD pattern of SiC@SiO₂ NFA; (b) FTIR spectrum; (c) Raman spectrum and (d) TGA curve; (e) N₂ adsorption/desorption isotherms and corresponding adsorption pore size distribution (in e) Inset); (f) XPS total spectrum and (g) Si 2p, (h) C 1s, (i) O 1s XPS high-resolution spectra.

II Hyperelasticity of SiC@SiO₂ NFAs under Severe Temperature Changes

The elasticity of materials plays a key role in high-level electromagnetic wave absorption and piezoresistive sensing applications. At different temperatures (~25 °C, ~700 °C, ~ −40 °C and ~ −196 °C), after 1000 cycles, the SiC@SiO₂ NFA almost completely retained the original macroscopic shape with only a slight Permanent deformation. Furthermore, the Youngs modulus (E) of the aerogel at ~25 °C is about 41.17 kPa. The calculated specific elastic modulus (E/ρ) is about 3.74 kN m kg⁻1, which is significantly higher than that reported in other literatures (Fig. 4l). These results indicate that the as-prepared SiC@SiO₂ NFAs have excellent mechanical properties.

Figure 4. Superelasticity of SiC@SiO₂ NFA. (a) The SiC@SiO₂ NFA was placed in the flame of an alcohol burner (~ 700 °C) and immersed in liquid nitrogen (~ -196 °C), respectively; (b) The compression test of the SiC@SiO₂ NFA, which can be quickly recovered to Original shape; compressive stress-strain curves of SiC@SiO₂ NFA at (c)~25°C, (e)~700°C, (g)~-40°C and (i)~-196°C; Cyclic compressive stress-strain curves of SiC@SiO₂ NFA at (d)~25 °C, (f)~700 °C, (h)~ -40 °C and (j)~ -196 °C; (k) Maximum stress and Youngs modulus during compression test cycles; (l) Comparison of relative elastic moduli of SiC@SiO₂ NFA and other aerogels.

III Piezoresistive pressure sensor of SiC@SiO₂ NFA for monitoring human motion

The SiC core in SiC@SiO₂ nanofibers is a semiconductor, and its resistance value changes accordingly with compressive deformation. Therefore, a piezoresistive pressure sensor based on SiC@SiO₂ NFA is fabricated. Due to the excellent compressive recovery and fatigue resistance of SiC@SiO₂ NFA, the resistance can be fully recovered to the initial value, exhibiting excellent strain-sensing reversibility at every stage. Furthermore, the change in relative resistance and the increase in compressive strain showed a clear linear relationship, resulting in a gage factor (GF = (ΔR/R₀)/ε) of 1.23. Therefore, the amount of human motion can be effectively detected according to the real-time change of the resistance to ensure the health of the human body. Figure 5f shows the schematic diagram of the SiC@SiO₂ NFA sensor for monitoring human motion and the mechanism of the piezoresistive sensing performance. In practical applications, pressure sensors can be used to detect human activity and tiny pressures, and then transmit this information to a mobile phone.

Figure 5. Strain and pressure sensing behavior of SiC@SiO₂ NFA. (a) ΔR/R₀ of SiC@SiO₂ NFAs, strains from 5% to 40% at a compression rate of 6 mm min⁻1; (b) compression rates of 6 mm min⁻1, under different compressive strains The real-time ΔR/R₀ cycling test of , ΔR/R₀ varies linearly with strain (inset b, GF = 1.23); (c) ΔR/R₀ of SiC@SiO₂ NFA at different compression rates at 30% compressive strain; ( d) Stability test of the piezoresistive behavior of SiC@SiO₂ NFA under 1000 cycles at a compressive strain of 30% and a compression ratio of 6 mm min⁻1 (the inset is a partially enlarged curve); (e) in NaCl Real-time ΔR/R₀ response in the presence of aqueous solution (inset depicts the corresponding schematic for the NaCl aqueous solution droplet test); (f) SiC@SiO₂ NFA pressure sensor for detecting physical activity and micro-pressure.

IV for use as a super-insulation material at extreme temperatures

SiC@SiO₂ NFAs exhibit excellent chemical and thermal stability at extreme temperatures. Through ablation and thermal insulation tests, it is found that SiC@SiO₂ NFA has excellent ablation resistance and thermal insulation properties, and can be used as a potential high-performance thermal insulation material in the aerospace field. The thermal insulation mechanism mainly involves two aspects: (1) when heat is transferred in the 3D network structure of SiC@SiO₂ NFA, the limited contact surface between SiC@SiO₂ nanofibers can effectively reduce the solid-phase heat conduction; (2) SiC@SiO₂ nanofibers SiO₂ NFAs have a large number of mesoporous structures that can bind air molecules to reduce gas-phase thermal convection.

Figure 6. Ablation resistance, high and low temperature resistance, and thermal insulation properties of SiC@SiO₂ NFA. (a) Digital photo of SiC@SiO₂ NFA heated in alcohol flame; (b) thermal conductivity of SiC@SiO₂ NFA at different temperatures in argon atmosphere; (c) a petal placed on asbestos mesh for direct heating; (d) ) One petal placed on SiC@SiO₂ NFA for heating; (e) IR thermograms of SiC@SiO₂ NFA taken during heating on a heating platform and (g) during freezing on a cooling platform, (f, h) corresponding temperatures vs time; (i) Macroscopic schematic diagram of thermal insulation of SiC@SiO₂ NFA.

High absorption capacity and self-cleaning performance of V oil-modified SiC@SiO₂ NFAs

The pristine SiC@SiO₂ NFAs are superhydrophilic, and the hydrophilic SiC@SiO₂ NFAs can be converted into hydrophobic materials by the oil impregnation method on the aerogel surface. The oil-modified SiC@SiO₂ NFA showed excellent hydrophobic properties to aqueous solutions with different pH values, so self-cleaning and adsorption tests were carried out on it. The results showed that the oil-modified SiC@SiO₂ NFA could be used as an efficient Selective adsorption material. In addition, the excellent hydrophobicity of oil-modified SiC@SiO₂ NFA makes it a candidate material for high-performance electromagnetic wave absorption.

Figure 7. High absorption capacity and self-cleaning performance of oil-modified SiC@SiO₂ NFAs for organic liquids. (a-c) Digital photos and contact angles of the corresponding water droplets of oil-modified SiC@SiO₂ NFA at different pH values (~1, ~7, and ~14); (d-g) Self-cleaning process of oil-modified SiC@SiO₂ NFA ; (h-k) The process of oil-modified SiC@SiO₂ NFA absorbing methyl orange oil solution and subsequent combustion test: (l) Oil-modified SiC@SiO₂ NFA recyclability; (m) Oil-modified SiC The absorption capacity of @SiO₂ NFA for various organic liquids and the corresponding recyclability (n) of SiC@SiO₂ NFA.

Electromagnetic wave absorption properties of VI SiC@SiO₂ NFA

The reflection loss (RL) is an important index to evaluate the electromagnetic wave absorption performance of SiC@SiO₂ NFA, and its calculation formula is as follows:

As shown in Figure 8a, the EABmax of the SiC@SiO₂ NFA is 8.6 GHz, corresponding to a frequency range of 5.82-14.42 GHz, and the RLmin value of -50.36 dB corresponds to a frequency of 7.44 GHz and a thickness of 1.6 mm. Figure 8c shows |Zin/Z₀|

The dielectric polarization can be clearly revealed by off-axis electron holography, especially the potential orientation and charge density distribution can be intuitively and quantitatively characterized in specific interface regions. Figure 9a-d show the TEM images and corresponding charge density maps of the longitudinal section of SiC@SiO₂ nanofibers under stepwise amplification of the signal. It can be clearly observed that with the continuous amplification of the signal, the charges are concentrated at the SiC/SiO₂ and SiO₂/air interfaces, resulting in a strong interfacial polarization. Figure 9e-h shows the state of charge density distribution at the interface of SiC and SiO₂ in the cross-section of SiC@SiO₂ nanofibers. With the increase of signal intensity, an obvious local polarization field is formed, which will greatly consume the incident electromagnetic wave energy, Enhanced microwave absorption performance. Furthermore, it can be clearly observed from Fig. 9i that SiC and SiO₂ grow closely together, enabling the leakage and tunneling of electrons. Charges accumulated at the interface (Fig. 9j–l) can break the potential barrier under a local strong electric field, thereby expanding the electron transition path and further dissipating the energy of the electromagnetic wave.

Figure 9. Off-axis electron holographic image of SiC@SiO₂ NFA. (a) TEM image of the longitudinal section of SiC@SiO₂ nanofibers and the corresponding (b-d) charge density images; (e) TEM image of the cross-section of the junction node and the corresponding (f-h) charge density images; (i) SiC@SiO₂ TEM images of nanofiber cross-sections and corresponding (j-l) charge density images.

SiC is an excellent dielectric loss electromagnetic wave absorbing material, while SiO₂ is an electromagnetic wave transmitting material. As shown in Fig. 10, this paper comprehensively studies the electromagnetic wave absorption mechanism of SiC@SiO₂ NFAs from the perspective of dielectric losses such as multiple reflections, conduction losses, defect-induced polarization, interface polarization, and dipole polarization. When electromagnetic waves are incident on the surface of SiC@SiO₂ nanofibers, the SiO₂ nanolayer can lock the electromagnetic waves to avoid being reflected, while the SiC core can effectively convert the electromagnetic energy into heat or electricity. These results indicate that the synergistic effect of SiC cores and SiO2 nanolayers in SiC@SiO₂ nanofibers enables the aerogels to exhibit excellent electromagnetic wave absorption properties.

Figure 10. Schematic diagram of the electromagnetic wave absorption mechanism of SiC@SiO₂ NFA.