Materials that are both strong and resilient are very desirable in the field of electronics, especially in the development of flexible electronics such as wearable devices and flexible tablets. Graphene aerogels are one of the ideal candidates for creating such versatile and light weight electronics that are flexible in nature, yet remain strong1.
Due to the lack of proper design guidelines, as well as the inability to achieve structure control over multiple length scales, developing flexible graphene aerogels which possess both properties of high strength and resilience, has remained a challenge until now1.
Scientists from the Zhejiang University’s Department of Polymer Science and Engineering have developed a biomimetic graphene-based aerogel that exhibited exceptional strength and resilience. This biomimetic graphene aerogel’s design was inspired from the architecture of the stem of Thalia dialbat, a perennial aquatic plant that is also known as the powdery alligator flag plant1. Thalia dialbata’s stem measures at only 6 – 10 millimeters (mm) in diameter, despite its length often measuring to about 2 meters (m) long1.
The porous architecture of the stem is responsible for the plant’s ability to survive the often harsh wind conditions that are present in its native areas of South America and Mexico, despite its large height/diameter ratio (200 - 350) 1. The Zhejiang Researchers created a graphene aerogel to mimic the structure of the stem of Thalia dialbata, as it could potentially be useful in developing flexible electronic devices in the future1.
Scanning electron microscope (SEM) images of the Thalia dialbata stem revealed a multi-structural architecture comprised of 100 micrometer (µm) thick oriented lamellar layers that are connected by interlayer bridges, measuring at a length of approximately 1 mm1. To obtain a graphene aerogel architecture that is similar to the make-up of the stem of Thalia dialbata, Hao Bai’s team used a bidirectional freezing technique.
In this technique, a low thermal conductive wedge is placed between the graphene oxide (GO) and poly vinyl alcohol (PVA) suspension and the cooling stage to simultaneously generate horizontal and vertical temperature gradients during freezing1. The ice crystals formed as a result of the bidirectional freezing serve as a template for the biomimetic graphene aerogel formation following sublimation and thermal reduction.
The biomimetic aerogel produced here was found to support more than 6,000 times its own weight with 50% strain, while also showing no signs of permanent deformation as evidenced by the complete recovery of the aerogel to its actual height upon removal of the applied weight1. The aerogel exhibited a high strength recovery ratio of 85% after 1,000 cycles of 50% strain, thereby proving that the aerogel is highly irrepressible when subjected to repeated stresses1.
Bai’s team further compared the microstructures and compression recovery behavior of their synthesized biomimetic aerogel using a bidirectional freezing method with a random graphene aerogel. After compressing to 50%, the biomimetic aerogel retained 92% of its original strength after 100 cycles, while the random aerogel was only able to retain 45% of its original strength after just 10 cycles1.
Increasing the compression to 90% resulted in 77% retention of strength after 100 cycles by the biomimetic aerogel, while the random aerogel exhibited a mere 36% retention in strength after just 10 cycles1. SEM images revealed that the biomimetic aerogel maintained its structural integrity of the lamella and bridges after 100 cycles of 50% compression, while cracks were observed in the microstructure of the random aerogel following 100 cycles of 50% compression1.
Furthermore, the biomimetic aerogel developed by Bai’s team exhibited a pressure sensitive conductivity when the graphene aerogel was connected to a light emitting diode (LED) in a circuit1. Upon application of stress onto the aerogel causing 50% strain, the LED was found to become brighter, while release of strain during recovery resulted in the LED becoming darker, therefore suggesting that this biomimetic aerogel could be used in flexible electronics such as sensors1.
The final architectures formed by the bidirectional freezing method can be fine-tuned by varying parameters such as suspension concentration, viscosity, slope angle of the wedge and cooling rate1. Even though GO/PVA suspension was used to create the graphene aerogel used in this study, the bidirectional freezing method could be utilized to prepare a wide variety of aerogels utilizing other materials such as polymers and polymer composites1.
- “Biomimetic architecture graphene aerogel with exceptional strength and resilience” M. Yang, N. Zhao, et al. ACS Nano. (2017). DOI: 10.1021/acsnano.7b01815.
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