Resilient Si3N4 Nanobelt Aerogel as Fire-Resistant and

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Functional Inorganic Materials and Devices 3

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Resilient SiN Nanobelt Aerogel as Fire-Resistant and Electromagnetic Wave-Transparent Thermal Insulator Lei Su, Mingzhu Li, Hongjie Wang, Min Niu, De Lu, and Zhixin Cai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02869 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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ACS Applied Materials & Interfaces

Resilient Si3N4 Nanobelt Aerogel as Fire-Resistant and Electromagnetic Wave-Transparent Thermal Insulator Lei Su, Mingzhu Li, Hongjie Wang*, Min Niu, De Lu, Zhixin Cai State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, 710049, People’s Republic of China E-mail: [email protected] KEYWORDS: ceramic aerogels, electromagnetic wave-transparency, resilient compressibility, fire-resistance, thermal insulation ABSTRACT: With the prevailing energy challenges and the rapid development of aerospace engineering, high-performance thermal insulators with various functions are attracting more and more attention. Ceramic aerogels are promising candidates for thermal insulators to be applied in harsh environments because of their low thermal conductivity and simultaneously excellent thermal and chemical stability. In general, the effective properties of this class of materials depend on both their microstructures and the intrinsic properties of their building blocks. Herein, to enrich the family and broaden the application fields of this class of materials, we prepared ultralight α-Si3N4 nanobelt aerogels (NBAs) with tunable densities ranging from 1.8 to 9.6 mg cm-3. The α-Si3N4 NBA realized resilient compressibility (with a recoverable strain of 40% to 80%), fire-resistance (1200 oC butane blow torch), thermal insulation (0.029 W m-1K-1) and electronic wave-transparency (a dielectric constant of 1 to 1.04 and a dielectric loss of 0.001 to 0.004) in one material, which makes it a promising candidate for mechanical energy dissipative, fire-resistant and electronic wave-transparent thermal insulator to be applied in extreme conditions. The successful preparation of such resilient and multifunctional α-Si3N4 NBAs will open up a new world for the development and widespread applications of ceramic aerogels. 1 ACS Paragon Plus Environment

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1. INTRODUCTION The increasing energy challenges and rapid development of aerospace engineering require thermal insulators with high efficiency and multifunction.1,2 For instance, thermal insulators with good fire-resistance can not only help buildings to keep a comfortable indoor environment but also prevent them from fire disaster. Another example is that thermal insulators that are simultaneously mechanical robust, high-temperature-stable, and electromagnetic wave-transparent are of great significance for re-entry aerospace vehicles to withstand the serious mechanical shock and high heat flux, and to be capable of transmitting electromagnetic signals.2 Because of the ultralow thermal conductivity, excellent chemical stability and various functions, ceramic aerogels exhibit promising potential for harsh environment applications.3–5 However, traditional ceramic aerogels usually exhibit a nanoparticle assembled pearl-necklace-like architecture, which leads to poor mechanical property. As is known, the effective properties of aerogels depend on both their microstructures and the intrinsic properties of their frameworks.6–10 Using 1D or 2D ceramic nanostructures to construct well-interconnected and highly porous architecture has been proved to be an effective way to overcome the brittleness and to achieve the resilient compressibility of ceramic aerogels.4,5,11–13 However, for other functional properties, different materials exhibit their own characters. For examples, carbon nanostructures based aerogels are usually applied to mechanical sensors for wearable devices,14 TiO2 nanofiber sponge can be used as photocatalytic material for degrading organic matters,12 while SiC based foams are suitable for electromagnetic wave-absorption applications.15 Up to now, only several kinds of resilient ceramic aerogels have been reported. Therefore, enriching the family of ceramic aerogels are of great importance to broaden their further applications. Si3N4 is one of the most excellent structural ceramic with high mechanical modulus, hightemperature-stability, and high thermal shock resistance.16–19 Porous Si3N4 ceramics are also 2 ACS Paragon Plus Environment

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attractive to be used as electromagnetic wave-transparent materials for aerospace vehicles due to their low dielectric constant and loss.19,20 α-Si3N4 nanobelt (NB) possesses both the characteristics of Si3N4 ceramic and the flexibility of one-dimensional (1D) nanostructures.21– 23

Thus, it is very suitable to be fabricated into α-Si3N4 NB aerogels (NBAs) to achieve the

expected properties, which has never been reported. Herein, we prepared ultralight α-Si3N4 NBAs with tunable densities (1.8 to 9.8 mg cm-3) via a facile partial-hot-pressing treatment process by using a home-made free-standing αSi3N4 NBA paper as the starting material. The use of flexible yet strong 1D α-Si3N4 NBs to construct a highly porous and well-interconnected architecture enables the α-Si3N4 NBAs a unique combination of properties of resilient compressibility, mechanical energy dissipation, excellent fire-resistance, high-temperature stability in ambient air, good thermal insulation performance (0.029 W m-1K-1) and ultralow dielectric constant (1.01 to 1.04) and loss (0.001 to 0.004), which makes the aerogel a high-performance thermal insulator with multifunction for harsh environment applications. 2. EXPERIMENTAL SECTION Preparation of siloxane xerogel. Home-made siloxane xerogel was used as the raw materials to prepare the paper-like α-Si3N4 NBA. Commercial methyltrimethoxysilane, dimethyldimethoxysilane, ethanol (solvent), deionized water (hydrolytic agent) and nitric acid (gelation catalyst) with a weight ratio of 8:2:4:4:1 were mixed together to prepare a siloxane sol by magnetic stirring. After stirring for 20 min, a transparent and homogeneous mixed liquid was obtained. Then, the mixed liquid turned into siloxane gel after 2 hours at room temperature. The gel was then dried at 100 °C for approximately 2 hours to remove the residue water and ethanol, and the siloxane xerogel formed. Preparation of paper-like α-Si3N4 NBA. The preparation of the α-Si3N4 NBAs is illustrated in Figure 1. The siloxane xerogel was placed in a graphite crucible with four gas vents. The outer wall of the crucible was used as NBAs’ growth substrate. The furnace was 3 ACS Paragon Plus Environment


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Figure 1. Schematic illustration of the formation of the paper-like α-Si3N4 NBA. (a) Decomposition of siloxane xerogel, the location where the aerogel growth and the aerogel detached from the graphite crucible. (b) Growth process of the NBs assembled aerogel. heated up to 1500 °C at 5 °C min-1 in N2 atmosphere and held at the maximum temperature for 2 h. During this period, the SiO and CO gases were generated from pyrolysis of the siloxane xerogel. With gradual increase of gas pressure inside the crucible, the gases escaped from the vents on the crucible and gradually diffused into the environment atmosphere. When the SiO gas encountering with N2 at the surface of graphite substrate, they reacted to form αSi3N4 nucleates according to Equation 1. And then the α-Si3N4 NBs grew via the reaction of CO, SiO and N2 according to Equation 2. With continuous supply of these gases, NBs nucleated and grew on the surface of pre-existing NBs according to Equation 2, resulting in self-assembly of the α-Si3N4 NBs. No catalyst was added during the process and could not be found at the tip of the NBs (Figure S1 in the Supporting Information). Thus the growth of the NBs follows a catalyst free vapor-solid mechanism. After the furnace cooling down, a piece of paper-like α-Si3N4 NBA can be detached from the graphite substrate (Movie S1). 3SiO(g) + 3C(g) + 2N2(s)→ Si3N4(s) + 3CO(g)

(1)

3SiO(g) + 3CO(g) + 2N2(s)→ Si3N4(s) + 3CO2(g)

(2)

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Fabrication of bulk α-Si3N4 NBAs with various densities. The preparation of the α-Si3N4 NBAs with tunable densities is illustrated in Figure 2a. As shown in Step 1, the home-made free-standing paper-like α-Si3N4 NBA was used as the raw material. In step 2, the paper-like aerogel was treated in air at 1000 °C for 10 min to form a ∼10 nm amorphous SiO2 shell on the surface of NBs. Then the treated aerogel was cut into pieces by a die and assembled layer-by-layer to obtain aerogel bulks (Step 3). In Step 4, a partial-hot-pressing treatment was conducted at 1200 °C for 30 min to obtain bulk α-Si3N4 NBAs. During this process, the amorphous SiO2 shell formed in Step 2 acted as binding material between adjacent NBs, which brought junctions between NBs (Figure 2i). After Step 4, each layer is attached tightly to the adjacent layers, and no obvious gap can be observed in the microstructure of bulk α-Si3N4 NBAs (Figure S2 in the Supporting Information). The graphite mould used in Step 4 possesses a constant volume, thus we can adjust the densities of the bulk α-Si3N4 NBAs by controlling the mass of the aerogel bulk added in the mould. Characterization. The densities of the α-Si3N4 NBAs were calculated from the mass and volume of samples. The microstructure of the samples was analyzed by SEM (Quanta 600, FEI, United States) and TEM (JEM-2100, JEOL, Japan). The compositions of the samples were detected by XPS (ESCALAB Xi+, Thermo Fisher, England) and EDS. The crystal structures of samples were analyzed by an XRD analyzer (X’ Pert Pro, PANalytical B. V., Almelo, Holland). The dielectric constant and loss was measured by Agilent E5071C network analyzer at frequency range of 8-18 GHz. The resilient compressibility of α-Si3N4 NBAs was characterized by using a dynamic mechanical analyzer (Q800, TA, United States) at 0.5 mm min-1. And the high speed compression fatigue resistance was characterized by using Instron 5943 testing system at 80 mm min-1. The microstructure evolution during loading-unloading process was observed in SEM. TGA curve obtained in air atmosphere was performed with DSC/TGA thermal analyzer (TGA/DSC3+, METTLER TOLEDO, Switzerland) from room 5 ACS Paragon Plus Environment


temperature to 1400 °C at a heating rate of 10 °C min-1. Thermal conductivity of the aerogel was measured by using a transient hot-wire method via a Thermal Constants Analyzer (XIATECH, TC3000E, China) with a testing range of 0.001 to 10.0 W m-1K-1. Two pieces of the aerogels with a diameter of 30 mm were tightly placed below and above the 25 mm testing sensor to ensure good thermal contact. Infrared images of samples during the butane blow torch treating process were recorded by a thermal infrared imager (NEC, TH5100, Japan). 3. RESULTS AND DISCUSSION Figure 2b shows the macroscopic image of a piece of α-Si3N4 NBA with an ultralow density of 1.8 mg cm-3 (Movie S2) standing on a leaf. This ultralow density results in an ultrahigh porosity of ~99.94%. Figure 2c presents a typical scanning electron microscopy (SEM) image of the α-Si3N4 NBA, showing that the aerogel’s architecture is assembled by a large amount of NBs with a length up to several hundreds of micrometers. These NBs possess a typical thickness of 20 to 90 nm and a typical width of 0.5 to 1.6 μm (Figure 2d). The large aspect ratio of the NBs gives them high flexibility. Energy disperse spectrum (EDS) of a typical NB in Figure S3a in the Supporting Information indicates the Si3N4 nature of the NBs. O was detected by X-ray photoelectron spectroscopy (XPS, in Figure 2e) due to the existence of ∼10 nm surface amorphous SiO2 layer (Figure S3b in the Supporting Information). Figure 2f displays a typical TEM image of the α-Si3N4 NB. The [100] growth direction of the NB is confirmed by the corresponding selected area electron diffraction (SAED) analysis (Inset in Figure 2f). This growth direction for α-Si3N4 NBs has also been reported in other works.21,24 The SAED pattern and the high-resolution transmission electron microscopy (HRTEM) image displayed in Figure 2g indicates hcp structure of α-Si3N4, which was further corroborated by X-ray diffraction (XRD) pattern of the aerogel (Figure S4 in the Supporting Information). Figure 2h and i present morphologies of two kinds of junctions between NBs among the NBAs’ architecture: NB branch and NB cross, respectively. TEM images in Figure S5 in the Supporting Information indicate these junctions are an atomic level connection. The branch 6 ACS Paragon Plus Environment

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Figure 2. Preparation process and highly porous architecture of the α-Si3N4 NBA. (a) The preparation process of α-Si3N4 NBAs. (b) Photograph of a 5 cm3 α-Si3N4 NBA with a density of 1.8 mg cm-3 standing on a dandelion. (c) Highly porous architecture of the NBA. (d) Size distribution histogram of the NBs’ thickness and width, respectively. (e) XPS spectrum of NBs after being treated at 1000 °C in ambient air for 10 min. (f) TEM image of NB and the corresponding SAED pattern in the inset. (g) HRTEM of the NB in (f). (h, i) SEM images of two kinds of NB junctions in the architecture: (h) NB branch and (i) NB cross. formed as a result of the nucleation of new generated NB on the surface of pre-existing one, thus they are connected with each other by crystalline α-Si3N4. And the NB cross formed due to the welding between the NBs during the partial-hot-pressing treatment, thus the binder between them is the amorphous SiO2 layer. As a result, a large amount of these chemical junctions guarantee the α-Si3N4 NBAs a well-interconnected architecture. Compression experiments on the α-Si3N4 NBAs showed resilient compressibility. For example, as displayed in the inset of Figure 3a, α-Si3N4 NBA with a density of 4.0 mg cm-3 can recovery to its original state from compression strain of 60%. The consecutive compression stress–strain (σ–ε) curves exhibit two different regimes: a linear elastic regime at ε < 20% with a compression modulus of ~ 2.5 kPa and a nonlinear elastic regime above 20% 7 ACS Paragon Plus Environment


Figure 3. Mechanical properties of α-Si3N4 NBA. (a) σ−ε curve obtained by consecutive compression at set strains of 20%, 40%, 60%. Inset: photographs of the NBA under compressing and releasing (4 mg cm-3). (b) σ−ε curves of 100 cyclic compressions at set strains of 40% (4 mg cm-3). (c) History of compression modulus, maximum stress and energy dissipation coefficient during the cyclic compression (4 mg cm-3). (d) Dependence of the relative modulus on the relative density for the NBAs and other reported materials. strain (Figure 3a). Compression modulus was calculated by fitting the σ–ε curve in the linear elastic range. During the consecutive compression, the later loading curve threads the maximum stress of the prior cycle, illustrating a good shape recovery effect.25 During the unloading process, the stress decreases rapidly but does not reach zero until to almost 0 % strain, indicating nearly complete recovery. The aerogel also showed good loading–unloading (with a loading rate of 80 mm min-1) cyclic fatigue resistance (Movie S3). The corresponding σ–ε curves are plotted in Figure 3b. Compared with the first compression cycle, the second cycle shows a ~10% decrease in the compression modulus and the maximum stress. After the tenth cycle, the compression modulus and maximum stress keep almost constant at 2.0 kPa and 0.75 kPa, respectively (Figure 3c). Significant hysteresis is observed in the σ–ε curves, indicating effective energy dissipation. The work done in the loading process of the first cycle 8 ACS Paragon Plus Environment

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is calculated to be 0.20 mJ cm-3, and the energy dissipation to be 0.11 mJ cm-3, resulting in an energy loss coefficient of 0.40. After the tenth cycle, the coefficient remains constant at 0.37 (Figure 3c), making the α-Si3N4 NBAs a good candidate for energy damping materials.9,10 These deformation features are similar to other resilient lightweight materials, such as SiO2PAN aerogels,9 metallic microlattice,10 BN foam,25 graphene/ceramic metamaterial26 and graphene microlattice.27 To understand the deformation mechanism thoroughly, we studied the deformation behaviour of α-Si3N4 NBAs with different densities. With the increase of density from 1.8 to 9.7 mg cm-3, the maximum reversible compression strain of the α-Si3N4 NBAs decreased from 80% to 40%, but the elastic modulus of α-Si3N4 NBAs increased from 0.3 kPa to 8.4 kPa (Figure S6 in the Supporting Information). Similar to other ultralight and highly porous structures,9,10,13,28 the relative compression modulus, E/Es, of the α-Si3N4 NBAs follows a power law scaling with relative density, ρ/ρs, as (E/Es) ∼ (ρ/ρs)m (Figure 3d). The scaling exponent of 2 indicates a bending-dominated deformation mechanism and represents an efficient load transfer between the building blocks.8–10 In situ observation of the microstructure evolution during the loading-unloading process was further conducted under SEM to illustrate the deforming process. As shown in Figure 4 and Movie S4, upon compression, the NBs in the NBA moved along the loading direction and exhibited elastic bending and buckling deformation (directed by the yellow arrows). With the increase of compression strain, NBs connected with the bent NBs (marked by the white dash line in the original state in Figure 4a) deformed and moved synergistically. Upon 60% compression strain, NBs moved to their lowest position and their bending curvature became larger. This large bending deformation of the NBs is benefit from the flexibility of the NBs and the large deformation space in the highly porous architecture. During the whole deformation process, the junctions between NBs (marked by yellow dash circle in Figure 4a) remained well, holding the integrity of the aerogel architecture and guaranteeing effective 9 ACS Paragon Plus Environment


Figure 4. In situ observation of the microstructure evolution during the loading and unloading process. (a) Original state. (b–e) Morphologies of the NBA at compression strain of 15%, 30%, 45% and 60%, respectively. (f) The recovery state after releasing. load transfer between NBs. Therefore, the resilient compressibility of the α-Si3N4 NBAs is attributed to the inherent flexibility of NBs, the highly porous architecture of NBAs and the robustness of NB junctions. In addition, some fracture in aerogel’s architecture was observed in Figure 4 (marked by the white dash circle). This damage might be responsible for the decrease in the maximum stress and compression modulus after the first compression cycle. Simultaneously, due to the random distribution of the NBs among the aerogel’s architecture, frictional sliding between the contacting NBs could occur easily, thus resulting in energy dissipation during compression cycles. As a result, the attractive mechanical properties, including resilient compressibility, good fatigue resistance and mechanical energy dissipation, of the α-Si3N4 NBAs mainly stem from the flexible yet strong α-Si3N4 NBs assembled wellinterconnected highly porous architecture. As a lightweight material that is made solely from ceramics, α-Si3N4 NBAs exhibits good fire-resistance and thermal and chemical stability. We characterized the fire-resistance of αSi3N4 NBAs by subjecting the material to a butane blow torch to simulate a high-temperature ablation condition. A thermal infrared imager was used to record the infrared images of the 10 ACS Paragon Plus Environment

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Figure 5. Fire-resistance of the α-Si3N4 NBAs. (a) Illustration of the experiment set-up using a butane blow lamp. (b) Fire-resistance of the aerogel sample under the heating of a butane blow torch. (c) Infrared image of the front side subjected to the butane blow torch. (d–f) Infrared images of the back side during the 30 min heating process: (d) 2 min, (e) 10 min and (f) 30 min. Photograph of the back side (g) and front side (h) after 30 min fire-resistance test. (i) SEM of front side after 30 min fire-resistance test. front and back side of the sample (Figure 5a). Although the blow torch reached up to more than 1200 °C (Figure 5c) and the part of tweezers in the front side of the sample was heated to red hot, the aerogel kept nonflammable during the whole 30 min heating process (Figure 5b and Movie S5). After being subjected to the butane flame for 30 min, the subsurface (∼ 0.5 mm away from to the front surface, Figure S7 in the Supporting Information) and the back side (Figure 5g) maintained almost the same, and no obvious change was found in the shape and size of the aerogel except slight ablation traces in the front side (Figure 5h and i). These 11 ACS Paragon Plus Environment


observations illustrate excellent fire-resistance of the α-Si3N4 NBAs which we attributed to the intrinsic ceramic nature of the α-Si3N4 NBAs. This was further confirmed by thermosgravimetric analysis (TGA) and isothermal oxidation experiment. The TGA curve (Figure 6a) obtained at a heating rate of 10 °C min-1 shows that the weight changes very little until ∼1000 °C, and then gradually increases with the further increase of temperature. The weight increase results from the increased thickness of the surface amorphous SiO2 layer, which was verified by comparing the TEM image in Figure S8 (∼15 nm) and that in Figure S3b (∼10 nm) in the Supporting Information. The increased amorphous layer can protect the NBs from further oxidation, thus leading to slower weight gain rate above 1200 °C. After 1250 °C, the weight gain rate starts rising, owing to the decrease of protective capability of the surface oxide layer at higher temperatures. These results can be explained by the XRD analysis. As shown in Figure 6b, the peaks of α-Si3N4 crystal were decreased after being treated at 1200 °C, corresponding to the increased amorphous SiO2 layer. While the crystal structure of the sample after being treated at 1300 °C was transformed into crystalline SiO2, indicating the decreased protective capability of oxide layers at high temperature. Isothermal oxidation test shows that the macroscopic morphology and size of the sample after being treated at 1200 °C for 30 min remained almost the same as the untreated one (Figure S9a in the Supporting Information). While for the sample being treated at 1300 °C for 30 min, it showed obvious volume shrinkage (Figure S9b in the Supporting Information). Therefore, the maximum working temperature of the α-Si3N4 NBAs in ambient air can be reached as high as 1200 °C. Thermographic recordings of the back side demonstrated efficient thermal insulation of the α-Si3N4 NBAs (Figure 5d to f and Figure S10 in the Supporting Information). The backside temperature maintained at a maximum temperature of around 480 °C during the testing process. Measured results show that the thermal conductivity is only 0.029 W m-1K-1 for the sample with the density of 1.8 mg cm-3 and reaches slightly to 0.035 W m-1K-1 at the density 12 ACS Paragon Plus Environment

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Figure 6. Thermal properties of the α-Si3N4 NBAs. (a) TGA curve showing the thermo-oxidative stability of the α-Si3N4 NBAs. (b) XRD pattern of the as-prepared aerogel, aerogel after being treated at 1200 °C in air for 30 min and aerogel after being treated at 1300 °C in air for 30 min. (c) Thermal conductivities of the α-Si3N4 NBAs with different densities, showing good thermal insulation performance. (d) Thermal conductivity versus the maximum working temperature of thermally insulating materials. of 9.8 mg cm-3 (Figure 6c). The extremely low thermal conductivity is mainly attributed to the high porosity of the aerogel, and the slight increase of thermal conductivity is due to the slight decrease of the porosity.12 As presented in Figure 6d, commercial polymeric thermal insulators, such as expanded polystyrene, usually possess a thermal conductivity of 0.0300.040 W m-1K-1, but their maximum working temperature is lower than 200 °C.29 And they are highly inflammable, which brings potential safety hazard. Fiberglass wool is fire-resistant, but it shows a higher thermal conductivity of 0.033-0.044 W m-1K-1 and its maximum working temperature is limited at 1000 °C.29 Conventional silica aerogel possesses a lower thermal conductivity (0.017-0.021 W m-1K-1) than polymeric thermal insulators and fiberglass wool, but they are brittle.30 Spaceloft and Pyrogel realize both flexibility and low thermal 13 ACS Paragon Plus Environment


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conductivity (0.014 W m-1K-1 and 0.020 W m-1K-1 for Spaceloft and Pyrogel, respectively) of commercial aerogel products through forming fiber toughened silica aerogel composites. But their maximum working temperatures (200 °C and 650 °C for Spaceloft and Pyrogel, respectively) are still low. Comparing with these commercial thermal insulation materials, the present α-Si3N4 NBAs combine the reversible compressibility, low thermal conductivity, and fire-resistance, and high-temperature stability together. The maximum working temperature of our α-Si3N4 NBAs can reach up to 1200 °C, which is also higher than the 1100 °C of the recently reported SiC nanowire aerogel5 and SiO2 nanofiber aerogel.11 Thus the α-Si3N4 NBAs are promising to be used as fire- and high-temperature-resistant thermal insulation materials in buildings, industry and aerospace vehicles. Another attractive property is its ultralow dielectric constant (κ) and loss (tanδ). We measured the κ of the α-Si3N4 NBAs using microwave frequencies of 8-18 GHz at room temperature and the results are shown in Figure 7a. The aerogels with densities from 1.8 to 9.8 mg cm-3 possess ultralow κ of 1.01–1.04. And the κ follows a linear relationship with its relative densities, ρ/ρs, as Equation 3 (Figure 7b). This linear law has been observed in silica

(κ-1) = 14.3ρ/ρs

(3)

aerogels, which indicates the gas-like dielectric behavior of the aerogel and confirms the low κ mainly comes from its high porosity.31 We further examined the κ and tanδ of the α-Si3N4 NBAs with a density of 7.5mg cm-3 at elevated temperatures in N2 and the results are reported in Figure 7c and d, respectively. The κ (1 to 1.04) and tanδ (0.001 to 0.004) change very little with the increase of temperature, even at 1200 °C, showing a good stability of the electromagnetic wave-transparent property at elevating temperatures. In comparison, the κ of our α-Si3N4 NBAs is comparable or lower than those of BN foam (κ = 1.03),4 silica aerogel (κ= 1.008),31 BN nanosheet aerogel (κ =1.24),32 and traditional porous Si3N4 ceramics (κ > 14 ACS Paragon Plus Environment

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Figure 7. Electromagnetic wave-transparent property of the α-Si3N4 NBAs at room and high temperatures. (a) Dielectric constant of the NBAs with various densities at frequencies of 8−18 GHz measured at room temperature, showing good electromagnetic wave-transparent performance of the α-Si3N4 NBAs. (b) Dielectric constant of α-Si3N4 NBAs with different densities at the frequency of 8 GHz versus the relative density, showing a linear relationship between dielectric constant and relative densities. (c) Dielectric constant and (d) Dielectric loss of the α-Si3N4 NBA with a density of 7.5 mg cm-3 at frequencies of 8−18 GHz from room temperature to 1200 °C, showing stable electromagnetic wave-transparent performance at different temperatures. 3)20, showing a promising potential to be used as electromagnetic signal transmitting thermal insulator in both room and high temperatures. 4. CONCLUSIONS In summary, α-Si3N4 NBAs with ultralow densities (tunable densities below 10 mg cm-3), high compressibility and resilience, mechanical energy dissipation (a coefficient of 0.37), excellent fire-resistance, high-temperature-resistance (1200 °C in ambient air), thermal insulation performance (0.029 Wm-1K-1) and ultralow dielectric constant (1.01–1.04) at room 15 ACS Paragon Plus Environment


and high temperatures, have been achieved through a facile partial-hot-pressing process by using a home-made free-standing paper-like α-Si3N4 NBA as starting material. The mechanical resilience is attributed to the flexibility of the NBs, high porosity of the NBAs and robustness of the chemical junctions. The ultralow dielectric constant and loss and ultralow thermal conductivity mainly result from the high porosity of the NBAs. The excellent fireand high-temperature-resistance in ambient air is due to the intrinsic thermal stability of the αSi3N4 constitute. The combination of these characteristics makes the α-Si3N4 NBAs a promising candidate for mechanical shock dissipative, fire- resistant and electromagnetic wave-transparent thermal insulator to be applied in harsh environments. This material enriched the family of ultralight and resilient ceramic aerogels and broadened the application area of this class of materials. The successful fabrication of such resilient and multifunctional α-Si3N4 NBAs will provide new perspectives for the development of ceramic aerogels for various applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. SEM image of the tip of a α-Si3N4 NB, cross section microstructure of α-Si3N4 NBA, analysis of the elemental composition of the NBs, XRD pattern of the α-Si3N4 NBA, TEM images of the chemical junctions, mechanical properties of the α-Si3N4 NBAs with various densities, morphologies of the subsurface (~ 0.5mm away from to the front surface) of NBA after 30 min heating by butane blow torch, typical HRTEM of a NB after being treated at 1100 oC in ambient air for 1 h, isothermal test results and the time dependent temperature profile of the highest temperature on the back side during the 30 min heating of butane blow torch (PDF) Movie S1. Detaching α-Si3N4 NBAs from graphite substrate (AVI) 16 ACS Paragon Plus Environment

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Movie S2. Measuring the density of α-Si3N4 NBAs (AVI) Movie S3. The α-Si3N4 NBA under compression and releasing process (AVI) Movie S4. In situ observation of the microstructure evolution of the α-Si3N4 NBA during compression and releasing process (AVI) Movie S5. The α-Si3N4 NBA under the heating of ~ 1200 °C butane blow torch (AVI)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Lei Su: 0000-0003-1707-3102 Hongjie Wang: 0000-0003-0043-2885 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge the funding support from the National Natural Science Foundation of China (No. 51772237), Shaanxi Science & Technology co-ordination & Innovation project (No. 2015KTCL01-13), Shaanxi Innovation Capacity Support Program (2018TD-031) and Independent Innovation Capacity Improvement Plan of Xi’an Jiaotong University (PY3A033). We thank Dr. Chaowei Guo from CAMPNANO of Xi’an Jiaotong University, Prof. Jianhua Wang and Dr. Fei He from University of Science and Technology of China for the help in the present work.

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Figure 1 119x86mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 2 160x113mm (300 x 300 DPI)



Figure 3 159x112mm (300 x 300 DPI)


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Figure 4 139x82mm (300 x 300 DPI)



Figure 5 160x149mm (300 x 300 DPI)


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Figure 6 159x116mm (300 x 300 DPI)



Figure 7 160x123mm (300 x 300 DPI)


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TOC Figure 54x44mm (600 x 600 DPI)


Resilient Si3N4 Nanobelt Aerogel as Fire-Resistant and

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