A printed, recyclable, ultra-strong, and ultra-tough graphite structural material

doi:10.1016/j.mattod.2019.03.016

Materials Today

Volume 30, November 2019, Pages 17-25

https://doi.org/10.1016/j.mattod.2019.03.016 Get rights and content

Abstract

The high mechanical performance of common structural materials (e.g., metals, alloys, and ceramics) originates from strong primary bonds (i.e., metallic, covalent, ionic) between constituent atoms. However, the large formation energy of primary bonds requires high temperatures in order to process these materials, resulting in significant manufacturing costs and a substantial environmental footprint. Herein, we report a strategy to leverage secondary bonds (e.g., hydrogen bonds) to produce a high-performance and low-cost material that outperforms most existing structural compounds. By dispersing graphite flakes and nanofibrillated cellulose (NFC) in water at room temperature to form a stable and homogeneous solution with a high solid concentration (20 wt%), we demonstrate this slurry can be scalably printed to manufacture a graphite-NFC composite that exhibits a high tensile strength (up to 1.0 GPa) and toughness (up to 30.0 MJ/m³). The low density of graphite and cellulose leads to a specific strength of the composite (794 MPa/(g cm⁻³)) that is significantly greater than most engineering materials (e.g., steels, aluminum, and titanium alloys). We demonstrate how hydrogen bonds between the graphite flakes and NFC play a pivotal role in the superb mechanical performance of the composite, also enabling this low-cost material to be recyclable for an environmentally sustainable solution to high performance structural materials.

Graphical abstract

Introduction

A widely used strategy in the design of structural materials featuring high mechanical performance is to leverage strong primary bonds between constituent atoms [1], [2], [3], [4]. For example, the carbon–carbon covalent bonds that make up carbon fibers result in a tensile strength of up to 4 GPa [5]; strong metallic bonds lead to the high melting points of metals; and the high stiffness and hardness of ceramics are dictated by strong ionic bonds. The high formation energy of primary bonds enables these kinds of desirable mechanical properties, however, it also requires the use of high processing temperatures and significant energy consumption during manufacture. As a result, the high performance of structural materials often comes at a price of adverse environmental impact. Furthermore, while the energy barrier required to break a primary bond is high, once broken, it is also difficult to recover, which can result in undesirable material properties, such as the mutual conflict between strength and toughness (i.e., stronger materials are often brittle, such as ceramics) [6].

Secondary bonds (e.g., hydrogen bonds) have modest bonding energy, but can readily form between atoms or functional groups [7]. For example, a hydrogen bond can easily form when two hydroxyl groups come within proximity of each other. When such a hydrogen bond is broken (e.g., by separating the two hydroxyl groups apart), new hydrogen bonds can easily re-form after the hydroxyl groups move to the vicinity of other neighboring hydroxyl groups. This unique feature of secondary bonds inspires the material design strategy to achieve highly desirable material properties. For example, cellulose nanopaper made of densely packed nanofibrillated cellulose (NFC) can be made orders of magnitude stronger and tougher than regular paper made of cellulose microfibers due to the significantly increased number of hydrogen bonds between the rich hydroxyl groups along neighboring cellulose molecular chains [8].

Herein, we demonstrate a material design strategy that utilizes secondary bonding to achieve a high performance structural material at low cost and with a significantly reduced environmental footprint. Using a room temperature, scalable, and surfactant-free process, we show how few-layer, highly crystalline graphite flakes can be directly exfoliated from commercial graphite powder in an aqueous solution using NFC as the dispersing agent. The resulting graphite-NFC slurry can then be printed in large area and cast-dried into an ultra-strong and ultra-tough composite that outperforms most metals and alloys.

Section snippets

Results and discussion

NFC contains both hydrophilic functional groups and hydrophobic C–H moieties [9]. The hydrophobic sites interact with the hydrophobic plane of the graphite flakes while the hydrophilic hydroxyl groups form hydrogen bonds with the defective edges, enabling NFC to directly exfoliate graphite as a dispersant in a manner similar to surfactant aided graphite dispersion and exfoliation [10], [11], [12]. The presence of adsorbed NFC fibers on the surface prevents the re-stacking of graphite flakes due

Conclusion

In summary, a secondary bonding strategy was developed to produce a mechanically robust graphite-NFC composite via a room-temperature, scalable, and surfactant-free solution process. Commercial graphite powders can be directly exfoliated into few-layer graphite flakes by the aqueous solution of native NFC, forming an ultra-high concentrated (20 wt%) and stable graphite-NFC dispersion. A large-scale (120 cm × 30 cm) graphite-NFC composite with a laminated structure can be achieved through an

Preparation of graphite-NFC composites

Commercial graphite powder (Asbury Carbons 3061) and 2 wt% NFC solution were mixed together with a solid mass ratio of 1:1 for graphite to NFC. All the samples have a graphite to NFC mass ratio of 1:1. The dispersion process was performed using a Vibra-Cell ultrasonic liquid processor for 5 min, and then bath sonicated for 15 min (FS110D, Fisher Scientific). After sonication, the graphite flakes were well dispersed in the NFC solution. The obtained graphite-NFC slurry was degassed for 20 min in

Acknowledgments

We acknowledge the support of the Maryland Nanocenter, its Surface Analysis Center, and the AIMLab. We acknowledge the Dynamic Effects Lab under William L. Fourney in the Mechanical Engineering department at the University of Maryland for conducting the ballistic tests using their air-gun ballistic tester. The authors acknowledge the University of Maryland supercomputing resources (http://hpcc.umd.edu) made available for conducting the research reported in this work. The in situ AFM pulling

Author contributions

Y. Zhou, C. Chen, and S. Zhu contributed equally to this work. L. Hu and Y. Zhou contributed to the idea and experimental design. Y. Zhou and D. Liu contributed to the graphite slurry preparation and the film formation. Y. Zhou and C. Chen contributed to the mechanical measurements. U. Ray, N. Quispe, U. Leiste, H. Bruck, and T. Li contributed to the mechanical tensile and ballistic tests. C. Sui, C. Wang, H. Guo and J. Lou contributed to in situ AFM pulling tests. Y. Kuang contributed to the

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^†: These authors contributed equally to this work.

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Materials Today

ResearchA printed, recyclable, ultra-strong, and ultra-tough graphite structural material

Abstract

Graphical abstract

Introduction

Section snippets

Results and discussion

Conclusion

Preparation of graphite-NFC composites

Acknowledgments

Author contributions

Carbohydr. Polym.

Compos. B Eng.

Carbon

Mater. Des. (1980–2015)

Mater. Sci. Eng., A

Carbohydr. Polym.

Carbohydr. Polym.

Carbohydr. Polym.

Science

Science

Science

Nature

Adv. Mater.

Nat. Commun.

Science

Proc. Natl. Acad. Sci.

Chem. Rev.

Research
A printed, recyclable, ultra-strong, and ultra-tough graphite structural material