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Guan et al., Sci. Adv. 2020; 6 : eaaz1114 1 May 2020

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M A T E R I A L S S C I E N C E

Lightweight, tough, and sustainable cellulose nanofiber-derived bulk structural materials with low thermal expansion coefficientQing-Fang Guan1*, Huai-Bin Yang1*, Zi-Meng Han1*, Li-Chuan Zhou2, Yin-Bo Zhu2, Zhang-Chi Ling1, He-Bin Jiang2, Peng-Fei Wang2, Tao Ma1, Heng-An Wu2, Shu-Hong Yu1†

Sustainable structural materials with light weight, great thermal dimensional stability, and superb mechanical properties are vitally important for engineering application, but the intrinsic conflict among some material prop-erties (e.g., strength and toughness) makes it challenging to realize these performance indexes at the same time under wide service conditions. Here, we report a robust and feasible strategy to process cellulose nanofiber (CNF) into a high-performance sustainable bulk structural material with low density, excellent strength and toughness, and great thermal dimensional stability. The obtained cellulose nanofiber plate (CNFP) has high specific strength [~198 MPa/(Mg m−3)], high specific impact toughness [~67 kJ m−2/(Mg m−3)], and low thermal expansion coeffi-cient (<5 × 10−6 K−1), which shows distinct and superior properties to typical polymers, metals, and ceramics, making it a low-cost, high-performance, and environmental-friendly alternative for engineering requirement, es-pecially for aerospace applications.

INTRODUCTIONSince the dawn of human existence, materials have been fundamental to the development of society. In all kinds of materials, structural materials, such as metals, ceramics, and polymers, are the most widely used (1). In recent years, designing structural materials with high performance on mutually exclusive properties (e.g., strength and toughness) at the same time, especially based on nanoscale building blocks, attracted more and more interest (2–8). When these nanoscale building blocks are assembled into macroscale materials, many extraordinary nanoscale properties can be scaled to macroscopic level and new macroscopic properties that are ascribable to the assembly of individual units emerge. In particular, it is of great importance and remains a huge challenge to construct highperformance, allgreen bulk structural materials from renewable and sustainable nanoscale building blocks (9–15).

Cellulose nanofiber (CNF), which can be derived from plant or produced by bacteria, is one of the most abundant allgreen resources on Earth (16, 17). Many attractive properties of CNF including low density, low thermal expansion coefficient, high strength, high stiffness, and easily modifiable surface make CNF an ideal nanoscale building block for constructing macroscopic highperformance materials. Although a variety of efforts have been made to scale those extraordinary nanoscale properties of CNFs to macroscopic level, only macro fibers and films can be prepared by different strategies up until now. For example, macro fiber was obtained from wood CNF by flowassisted organization, with a Young’s modulus of 86 GPa and a tensile strength of 1.57 GPa, exceeding any known natural or synthetic biopolymers (18). Macro fiber with high performance was also

prepared by a wetdrawing and wettwisting process of ultralong bacterial CNF (19). In addition, films consisting of CNF with high strength, high transparency, and low thermal expansion coefficient have been designed and used in many fields such as electron devices and flexible display (20–24). However, challenges remain in scaling those extraordinary nanoscale properties of CNFs to threedimensional bulk structural materials. If sustainable highperformance bulk structural materials can be constructed, it will certainly promote the development of CNF, expand its application fields, and provide more materials selection in engineering design.

Here, we report a robust and feasible strategy to process CNFs into a highperformance bulk structural material with low density, outstanding strength and toughness, and great thermal dimensional stability. The obtained CNF plate (CNFP) has high specific strength [~198 MPa/(Mg m−3)], high specific impact toughness [~67 kJ m−2/(Mg m−3)], and low thermal expansion coefficient (<5 × 10−6 K−1), which shows distinct and superior properties to typical polymers, metals, and ceramics, making it a lowcost, highperformance, and environmentalfriendly alternative for engineering requirement, especially as spacecraft materials. Moreover, CNFP also shows good serviceability under extreme temperature or rapid thermal shock and high energy absorption properties, and its mass production can be achieved in large scale at low cost.

RESULTSMaterial fabrication and characterizationFigure 1 shows a schematic of our bottomup approach to press multilayered pretreated CNF hydrogels into highperformance CNFP. CNF hydrogels with a robust threedimensional nanofiber network are produced from glucose by biosynthesis and then treated with a different polymer solution or by surface modification before hot pressing (Fig. 1, A to C). The CNFPs prepared from untreated, polyvinyl alcohol (PVA)–treated, and silicic acid solution–treated bacterial CNF hydrogels and polyacrylic acid (PAA)–treated surfaceoxidized bacterial CNF hydrogels are marked as CNFP0, CNFP1, CNFP2, and CNFP3, respectively (table S1).

1Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Chinese Academy of Sciences (CAS) Center for Excel-lence in Nanoscience, Hefei Science Center of CAS, Department of Chemistry, Insti-tute of Biomimetic Materials & Chemistry, University of Science and Technology of China, Hefei 230026, China. 2CAS Key Laboratory of Mechanical Behavior and De-sign of Materials, Department of Modern Mechanics, CAS Center for Excellence in Complex System Mechanics, University of Science and Technology of China, Hefei 230027, China.*These authors contributed equally to this work.†Corresponding author. Email: [email protected]

Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

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Fig. 1. Fabrication and structure analysis of CNFP. (A) CNF hydrogel can be produced by biosynthesis. (B) CNF hydrogel and its robust three-dimensional nanofiber network. (C) Numerous layers of treated CNF hydrogels are pressed at 80°C to fabricate CNFP. (D) The diagrammatic drawing of CNFP. (E) The multilayer structure of CNFP. (F) The robust three-dimensional nanofiber network of one layer in CNFP. (G) Cellulose molecular chains are tightly bonded together by hydrogen bonds and expose lots of –OH groups on the surface of CNF to form interfiber hydrogen bonds. (H) Photograph of large-sized CNFP with a volume of 320 mm by 220 mm by 27 mm. (I) Parts with different shapes of CNFP produced by a milling machine. Scale bar, 1 cm (I). (Photo credit: Zi-Meng Han, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China.)

Fig. 2. Structural characterization of CNFP. (A) SEM image of the marked area in the inset photograph, clearly showing the multilayer structure where each layer is about 20 m. The inset is a photograph of CNFP. (B) Magnified SEM image of a layer in CNFP, showing the microscopic layered structure of CNFs. (C) SEM image of the marked area in the inset photograph, showing the robust three-dimensional nanofiber network. Numerous CNFs are intertwined with each other and combined together by a strong hydrogen bond. The inset is a photograph of CNFP. (D) Profile of the fractured CNFP-0 showing the sliding between about 20-m layers. (E) Numerous CNFs are intertwined with each other and combined together between layers [enlarged micrograph of the marked area in (D)]. (F) SEM image of an oblique section of CNFP. Be-tween different layers, a large number of CNFs are pulled out from the layer and intertwine with each other. (Photo credit: Huai-Bin Yang, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China.)


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Our method can achieve largescale preparation directly by larger press equipment. A largesized CNFP was fabricated with a volume of 320 mm by 220 mm by 27 mm and with a weight of 2560 g (Fig. 1H). Since bacterial cellulose hydrogel is a kind of industrial product and its market price is lower than $0.01 per kilogram, CNFP can be prepared at low cost, which is lower than many typical structural materials (tables S2 and S3). Furthermore, CNFP can be processed into desired shape and size; for example, different shapes of parts were obtained by a milling machine, which shows its good processability (Fig. 1I).

The scanning electron microscopy (SEM) images reveal the multilayer structure of CNFP, and each layer is about 20 m, which is determined by the thickness of CNF hydrogels (Fig. 2A). Inside the layer, there are densified and robust threedimensional nanofiber networks (Fig. 2, B and C and fig. S1, G to I). Numerous CNFs are intertwined with each other and combined together by strong hydrogen bonds.

The interfacial failure of CNFP under external loading shows a large amount of connected CNFs, demonstrating the tightly hydrogen bonded adjacent layers in CNFP (Fig. 2, D to F, and fig. S1, A to F).

Thermal and mechanical propertiesThrough this strategy, many remarkable nanoscale properties of CNF can be successfully scaled to macro level on CNFPs, including low thermal expansion coefficient and high strength. Low thermal expansion coefficient is a vitally important feature for materials in many application areas, especially for aerospace applications. From −120° to 150°C, the average thermal expansion coefficients of CNFP are lower than 5 × 10−6 K−1 (parallel to layer) and 7 × 10−6 K−1 (perpendicular to layer), respectively, which is close to those of ceramic materials and much lower than those of typical polymers and metals (Fig. 3A and fig. S2). Meanwhile, the excellent mechanical properties

Fig. 3. Superb thermal and mechanical properties of CNFP. (A) Thermal expansion of CNFP (parallel to layer), polyamide (PA), Al alloy (7075 Al), and Al2O3. (B) Compar-ison of flexural strength and stiffness of different kinds of CNFPs. (C) Comparison of Charpy impact toughness of CNFP-0 with other widely used polymer-based materials. (D) Flexural stress–strain curves of CNFP-0 at different temperatures. (E and F) Comparison of CNFP-0 with other widely used polymer-based materials at (E) 30°C and (F) 200°C. (G) Schematic of rapid thermal shock for 10 times. (H) Flexural stress–strain curves of CNFP-0 before and after rapid thermal shock for 10 times. PMMA, polymethyl methacrylate; PVC, polyvinyl chloride; ABS, acrylonitrile butadiene styrene; PC, polycarbonate; PF, phenolic resin; POM, polyformaldehyde; PP, polypropylene. (Photo credit: Zi-Meng Han, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China.)


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of CNF are also successfully scaled to macro level on CNFPs (Fig. 3B; see also figs. S3 to S6). With a low density of ~1.35 g cm−3, the ultimate flexural strength and the flexural modulus of CNFP can be up to ~269 MPa and ~17 GPa, respectively (Fig. 3B). The organization of CNFs also provides other intriguing macroscopic properties. The Charpy impact test of notched CNFP yields an impact toughness of 87.6 ± 4.3 kJ m−2, which is much higher than those of typical plastics (Fig. 3C). Moreover, it shows no visible change at 200°C, indicating that the thermostability of CNFP is much better than those of other widely used polymerbased materials (Fig. 3, E and F, and fig. S7). After enduring rapid thermal shock for 10 times between two extreme temperature conditions (−196° and 120°C), CNFP still maintains a similar mechanical performance (Fig. 3, G and H). Notably, at 120° and −50°C, the flexural strength of CNFP does not change notably (Fig. 3D), which is vitally important for practical application in extreme environments. In addition, after having been subjected to 95% relative humidity for 60 hours, the thickness and flexural strength of CNFP change only slightly, showing good stability under moisture attack (fig. S8).

CNFP has multiple outstanding features in one material, including low thermal expansion coefficient, lightweight, high strength, and high toughness. To demonstrate the overall performance of CNFP, we put its properties into two Ashby maps to compare with different kinds of traditional structural materials (Fig. 4). In Ashby maps, despite the wide range of properties of metals (as an example), the clusters occupy a field that is distinct from that of polymers or that of ceramics. This fact shows that for different kinds of materials, the fields may overlap, but they always have a characteristic place within the whole picture (1). According to this pattern, CNFP appears on the maps as a whole new kind of material, which occupies a distinct field with low thermal expansion coefficient (lower than 5 × 10−6 K−1), high specific strength [up to 198 MPa/(Mg m−3)], and high specific impact toughness [up to 67 kJ m−2/(Mg m−3)]. The specific strength and specific impact toughness of CNFP are higher than those of traditional metals and alloys, making it an allgreen and highperformance alternative for engineering design. The position of the fields on Ashby maps can be understood in simple physical terms, as the nature of the fundamental building blocks and how they combine with each other determine the

position of the fields that different kinds of materials occupy. Considering CNFP, CNFs are its basic building blocks, while hydrogen bonds are the main interaction that bind them together. The strength (at least 2 GPa) and Young’s modulus (138 GPa) of individual CNFs can be almost as high as those of steel and Kevlar (22), and those superior nanoscale properties can be scaled to macro level (tables S4 and S5), mainly due to the strong interaction between CNFs (25–29).

Mechanism analysis of thermal and mechanical propertiesAccording to previous researches (16, 22, 30, 31), since the CNFs are aggregates of semicrystalline extended cellulose chains, its thermal expansion coefficient is as small as 1 × 10−7 K−1, which is lower than that of silica glass. When those CNFs are bound by strong interfiber hydrogen bonds, a low thermal expansion coefficient is achieved.

The ultrafine nanofiber network structure in CNFP results in more extensive hydrogen bonding, the high inplane orientation, and “three way branching points” of the microfibril networks (Fig. 2F) (17). Those structural features allow the CNFP to withstand high stress without breaking and also disperse stress and suppress crack formation and crack propagation. According to a photograph of a broken sample (fig. S9A), the main fracture mode of CNFP is interlaminar debonding, which means that the interlaminar shear strength between layers is the limiting factor of the strength of CNFP. Thus, we tune the interlaminar shear strength by treating the CNF hydrogel surface with different polymer solutions. For untreated CNFP0, the interaction between layers is hydrogen bond, while PVAtreated CNFP1 has weaker interlaminar shear strength, since PVA forms weaker hydrogen bonds between layers than cellulose itself. For CNFP2 treated by silicic acid solution, silicic acid dehydrates and provides covalent strong crosslinks during the hotpress process (32), which effectively improves the interlayer interaction compared to CNFP0 and CNFP1. For CNFP3, the oxidation process introduces an amount of carboxyl groups on the surface of CNF. Those carboxyl groups of CNFs are further crosslinked by the calcium ions and polyacrylic acid. The calcium ions and carboxyl groups form strong ionic bond and further improve the interaction between layers. The nuclear magnetic resonance (NMR)–13C and Fourier transform infrared spectroscopy (FTIR) results verify the mechanism above,

Fig. 4. Comparison of thermal and mechanical properties of CNFP with typical polymers, metals, and ceramics. (A) Ashby diagram of thermal expansion versus specific strength for CNFP compared with typical polymers, metals, and ceramics (1, 41–46). (B) Ashby diagram of thermal expansion versus specific impact toughness for CNFP compared with typical polymers, metals, and ceramics (1).


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Fig. 5. Superb toughness mechanism and impact resistance properties of CNFP. (A and B) FEM simulations of (A) laminated structure and (B) monolithic bulk for the SENB test. (C) Stress-strain curves of different kinds of interfaces for the SENB test. (D) Force-displacement curve of CNFP-0 for the drop hammer impact test. (E) Schematic of the drop hammer impact tester. (F) Photograph of CNFP-0 after the drop hammer impact test. Scale bar, 1 cm. (G) Schematic of the SHPB. (H) Compressive stress-strain curves of CNFP-0 for the SHPB test under different strain rates. (Photo credit: Huai-Bin Yang, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China.)


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while the xray diffraction (XRD) result shows that the crystallinity of CNFs has no visible change after pretreatment (figs. S10, S11, and S12) (33, 34). By tuning the interaction between layers, it can be seen that when the interaction is enhanced, the interlaminar shear strength and flexural strength increase simultaneously (Fig. 3B; see also fig. S9B).

A longstanding challenge in engineering material design is the conflict between strength and toughness, because these properties are, in general, mutually exclusive (25, 35–37). The high ultimate flexural strength of CNFP is not accompanied by low impact toughness (Fig. 3, B and C). This unique toughening mechanism can be understood at multiscale. At microscale, bending force initiated the sliding between ~20m layers (Figs. 1E and 2D; see also fig. S1, A to C), and the sliding dispersed the stress, avoiding stress concentration. At nanoscale, tightly hydrogenbonded CNFs at interface are pulled out from the opened layer during the interlaminar sliding (Figs. 1F and 2, E and F; see also fig. S1, D to F), which further disperse the stress and prevent the generation and propagation of crack. At molecular scale, owing to the rich hydroxyl groups in cellulose molecular chains, when CNFPs are subjected to deformation, relative sliding among CNFs involves an enormous number of hydrogenbond formation, breaking, and reformation (Fig. 1G) (25, 38). Owing to the above mechanisms, deformation of the CNF network can absorb a large amount of energy. To demonstrate the role of interfacial properties on toughness, we further investigated the failure behavior of CNFP under singleedge notched bend (SENB) simulation. As a comparison, the monolithic bulk shows the stress concentration around the crack tip and soon displays the brittle damage through the fracture section (Fig. 5B). With the decrease of interfacial strength, it is clear that laminar bulk illustrates brittletotough transition from the forcedisplacement curves (Fig. 5C). For an appropriate interfacial strength, the detailed failure behavior is given in Fig. 5A, where the deformation energy is dissipated by interfacial sliding and opening instead of brittle damage of laminae. For CNFP, therefore, suitable interfacial modification can enormously enhance the toughness and retain considerable strength at the same time.

Impact resistanceTo further evaluate the impact resistance of CNFP, dynamic mechanical tests including drop hammer impact and split Hopkinson pressure bar (SHPB) were carried out. The CNFP sample can generate a high resist force during the impact and remarkably decrease the velocity of hammer and absorb its energy (Fig. 5, D and E). The absorption energy in the drop hammer test can be calculated by the forcedisplacement curve, and for CNFP, it is ~2.7 kJ m–1 (Fig. 5D). CNFP under highvelocity impact can maintain its shape, instead of shattering or deforming. The damage pattern of CNFP demonstrates the stress localization of energy adsorption under highvelocity impact (Fig. 5F). SHPB is a powerful tool to study the behavior of material under high–strain rate deformation. Its structure is shown in Fig. 5G, and the strain gauge on the incident bar and the transmission bar can convert mechanical wave into electronic signal and yield the stressstrain curve. The highest compression stress of CNFP can reach ~1600 MPa during high–strain rate compression (14,000 s−1), and the energy absorption of compression is ~387.5, ~89.3, and ~37.0 MJ m−3 under the strain rates of 14,000, 10,000, and 7500 s−1, respectively (Fig. 5H). The stressstrain curves in the SHPB experiments illustrate anomalous decline after the first peak compared with the common plastic platform area of traditional metal materials and energy absorbing materials. Under initial low strain in the impact,

the CNF network and the layered plate as the structural frameworks resist the shock stress. Then, the following large strain would trigger the deformation of the CNF network at microscale and even the shear behavior of cellulose nanocrystals at nanoscale. Recent simulation studies demonstrated that breaking and reformation of interfiber hydrogen bonds as well as the dihedral rotation are nonnegligible mechanisms in the origin of outstanding mechanical properties of cellulose nanocrystals (38, 39). At last, the larger compression strain results in the densification of CNFP (CNF network at microscale). While the compression speed increases, adsorption energy increases. The light weight and high energy absorption properties of CNFP indicate that it can be a potential armored material for shock waves from blast (40).

DISCUSSIONIn summary, we have developed a robust and feasible strategy to process CNF into an environmentally friendly bulk structural material, CNFP. It has multiple unique features in one material, which are low thermal expansion coefficient, light weight, high strength and toughness, good impact resistance, and easy realization of largescale production, making it a lowcost and highperformance alternative for engineering design. For example, it can be a strong competitor for the lightweight material used for automobiles and aircraft, and remarkably for aerospace application, such as optical lens bracket for the lunar rover where light weight, high strength, and low thermal expansion coefficient are all vitally essential. From the perspective of environmental life cycle impact assessment (10), CNFP contributes almost nothing to extra greenhouse gas emissions, human health impairment, ecosystem toxicity, or resource depletion, indicating that it is a good example of an allgreen structural material. Furthermore, CNFP can be designed at multiscale according to application requirements. Various functional bulk structural nanocomposites can be achieved, and thus, the performance of this emerging sustainable structural material can be further explored.

MATERIALS AND METHODSFabrication of CNFPsAll reagents and raw materials were commercially available. Bacterial cellulose hydrogels were produced by Gluconacetobacter xylinus 1.1812 (China General Microbiological Culture Collection Center) on the liquid nutrient media at 30°C (24). One liter of liquid nutrient media consisted of glucose (50 g liter−1), yeast extract (5 g liter−1), citric acid (2 g liter−1), Na2HPO4·12H2O (4 g liter−1), and KH2PO4 (2 g liter−1). A part of bacterial cellulose hydrogels were surface oxidized by the 2,2,6,6Tetramethylpiperidine1oxyloxidized method. They were then immersed into CaCl2 solution (0.1 M) for 24 hours and rinsed three times with deionized water. The bacterial cellulose hydrogels (untreated/surface oxidation) were cut into sheets, and they were immersed in the surface treatment solution for 6 hours. Then, a certain number of bacterial cellulose sheets were stacked by hydrogel layer bylayer (HLBL) and were pressed with a pressure of ~1 MPa. Last, a hotpressing step with a pressure of 100 MPa was applied at 80°C until CNFP was completely dry. The treatment method of bacterial cellulose hydrogels, surface treatment solution, and the corresponding numbers are shown in table S1. For a typical CNFP0 with a thickness of 6 mm, it was pressed with a pressure of ~1 MPa for about 3 hours and then was hotpressed with a pressure of ~100 MPa at 80°C for about 1 hour.


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CharacterizationSEM images were taken with a Carl Zeiss Supra 40 field emission scanning electron microscope (5 kV), and all samples were gold sputtered for 30 s at a constant current of 30 mA before observation. XRD data were measured by a PANalytical X’pert PRO MRD Xray diffractometer equipped with Cu K radiation ( = 1.54056 Å), and the samples were prepared by cutting down a ~0.5mm layer of CNFPs. Thermogravimetric analysis data were measured in air atmosphere with a TA Instruments SDT Q600 thermogravimetric analyzer, and the samples were prepared by grinding CNFPs into powders. FTIR spectra were acquired by a Bruker Vector22 FTIR spectrometer in attenuated total reflectance mode, and the samples were prepared by cutting down a ~0.5mm layer of CNFPs. 13CNMR cross polarization magic angle spinning spectra were recorded at 100.62 MHz on a Bruker Avance III 400 WB spectrometer using a 90° pulse of 4 s, an acquisition time of 33.9 ms, a contact time of 3 ms, a recycle delay of 5 s, and a spin rate of 14 kHz.

Thermal and mechanical testingThreepoint bending test was performed on an Instron 5565A universal testing machine according to ASTM D79015e1. The specimens were carefully cut with a size of about 25 mm by 2 mm by 2 mm. The test was carried out at a loading rate of 1.0 mm min−1 with a support span of 12.5 mm. For all the mechanical tests, the applied loading direction was perpendicular to the layers, and each kind of specimen was tested at least five times, unless noted otherwise. Tensile test and compression test were performed on an MTS 809 material testing machine. The dimensions for tensile specimens (dogboneshaped specimens) were approximately 100 mm long, 10 mm wide, and 2 mm thick. The specimens were clamped at both ends and stretched along the specimen length direction with a constant test speed of 1.0 mm min−1 at room temperature. The dimensions for compression specimens were about 9 mm by 9 mm by 4.5 mm, and the specimens were compressed along the thickness direction (4.5 mm) at room temperature with a loading rate of 1 mm min−1. The Charpy impact test of CNFPs was performed on a Chengde Bao Hui XJJY5 pendulum impact tester, and the dimensions for Charpy Vnotch specimens were about 50 mm long, 10 mm wide, and 2 mm thick with a notch depth of 1 mm. The drop hammer impact test of CNFPs was performed on an Instron CEAST 9340 drop hammer tester. The hammer with a weight of 15 kg (total mass) was falling freely from a height of 30 cm, and the dimensions of the tested specimens were about 50 mm by 50 mm by 2 mm. For the SHPB test, the dimensions of the tested specimens were about 6 mm by 6 mm by 2 mm.

Thermal expansion coefficient of the CNFP was measured by NETZSCH TMA 402F3. The specimens were carefully cut with a size of about 20 mm by 4 mm by 2 mm, and the test was carried out from −130° to 150°C. Thermal expansion coefficient () was calculated by the equation = L/(L T) K−1. Density () of the CNFP was calculated by first treating the material into a cuboid and then using the equation = mass/volume. The specific values of the impact fracture toughness and the ultimate flexural strength were calculated by dividing the density. The shortbeam shear strength can be calculated by threepoint bending test according to ASTM D2344/D2344M16. Shortbeam shear strength (F sbs) was calculated by

F sbs = 0.75 P m ─ bh (1)

where Pm is the maximum load in threepoint bending test, b is the measured specimen width, and h is the measured specimen thickness.

Computational methodsThe SENB simulation for CNFPs is implemented by finite element method (FEM) with the software ABAQUS. The geometric model consists of three components: a plate specimen, an indenter, and two symmetrical fixtures. The plate specimen is precracked with a notch in the middle of the specimen. The thickness of the notch is half that of the specimen. The indenter and the fixtures are set as the rigid body in the simulation. The contact constraints are defined between the component surface pairs.

The failure behavior of monolithic bulk without lamination is simulated first. The property of CNFP elements is defined by the userdefined field subroutine. The elements are linear elastic at the initial stage. When the element stress is greater than the material strength max, the element Young’s modulus is degraded to 2 × 10−9 of the initial value as an approximate simulation of fracture damage. In the simulation, the indenter pushes down with a uniform speed. The reaction force and falling displacement of indenter are compared and adopted to plot the stressstrain curve (Fig. 5C). It can be seen that the curve grows linearly at first and then decreases rapidly after reaching the peak value. The overall failure of the specimen is due to the stress concentration at the precrack tip. The crack expands upward rapidly along the initial crack path, causing the specimen to break (Fig. 5B).

For multilayered CNFPs, the specimen is divided into isopachous multiple layers to consider the effect of interface on the bending property. Contact constraints with cohesive behavior are defined between the layers. The maximum shear stress max and fracture energy G of cohesive elements are adjusted with different values to observe the change of failure mode. When the values of max and G are big enough, the failure mode is almost unchanged, and the specimen is still broken from the middle. The flexural modulus is reduced due to interlayer slippage. With the decrease of interfacial strength, we can see that the failure mode varies, in which the interface reaches the critical strength earlier than the elements during the indenter falling process. The cracks appear at the middle layer first and expand to both ends gradually (Fig. 5A). Then, more and more interlaminar cracks appear inside the specimen, whereas the prefabricated cracks cannot expand upward because energy is dispersed through layers. It is found that the toughness of specimen is much higher according to the stressstrain curve. As the interface strength continues to decrease, the failure mode remains the same, while the total strength and toughness are gradually reduced.

In summary, the interface property has a significant impact on the SENB test of CNFPs. The toughness of CNFPs can be improved by adjusting the interface strength.

SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/18/eaaz1114/DC1

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Acknowledgments Funding: This work was supported by the National Natural Science Foundation of China (Grant 51732011), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant 21521001), Key Research Program of Frontier Sciences, Chinese Academy of Sciences (Grant QYZDJ-SSW-SLH036), the National Basic Research Program of China (Grant 2014CB931800), the Fundamental Research Funds for the Central Universities (WK2090050043), the Users with Excellence and Scientific Research Grant of Hefei Science Center of Chinese Academy of Sciences (2015HSC-UE007), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB22040402), the National Natural Science Foundation of China (11525211), and the USTC Research Funds of the Double First-Class Initiative (YD2480002002). Author contributions: S.-H.Y. and Q.-F.G. conceived the idea and designed the experiments. S.-H.Y. supervised the project. Q.-F.G., H.-B.Y., Z.-M.H., and Z.-C.L. carried out the synthetic experiment and analysis. H.-A.W., L.-C.Z., and Y.-B.Z. contributed to both mechanical simulations and analysis. H.-B.J. and P.-F.W. contributed to SHPB test and analysis. T.M. contributed to the 3D illustrations. Q.-F.G., H.-B.Y., Z.-M.H., and S.-H.Y. wrote the paper, and all authors discussed the results and commented on the manuscript. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.

Submitted 13 August 2019Accepted 10 February 2020Published 1 May 202010.1126/sciadv.aaz1114

Citation: Q.-F. Guan, H.-B. Yang, Z.-M. Han, L.-C. Zhou, Y.-B. Zhu, Z.-C. Ling, H.-B. Jiang, P.-F. Wang, T. Ma, H.-A. Wu, S.-H. Yu, Lightweight, tough, and sustainable cellulose nanofiber-derived bulk structural materials with low thermal expansion coefficient. Sci. Adv. 6, eaaz1114 (2020).


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low thermal expansion coefficientLightweight, tough, and sustainable cellulose nanofiber-derived bulk structural materials with

Tao Ma, Heng-An Wu and Shu-Hong YuQing-Fang Guan, Huai-Bin Yang, Zi-Meng Han, Li-Chuan Zhou, Yin-Bo Zhu, Zhang-Chi Ling, He-Bin Jiang, Peng-Fei Wang,

DOI: 10.1126/sciadv.aaz1114 (18), eaaz1114.6Sci Adv

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