Combining 3D printing and self-assembly to make the world’s lightest material

May 10, 2022

(Spotlight on Nanowerk) In nature, wood, shells and other structural materials are light, strong and resistant. These multi-scale structures allow a series of exceptional performances using a limited number of constituent materials with ordinary properties.

While natural creatures are able to fabricate these complex structures through bio-controlled assembly, engineers and materials scientists have struggled to develop man-made materials with multi-scale structures ranging from nano- to macro- ladder.

Significantly, these natural materials are made at the ambient temperature of the local environment – not the high temperatures at which man-made structural materials are typically processed.

Over the past decade, researchers have developed additive manufacturing techniques that produce bespoke micro-to-macro-scale lattice structures, resulting in amazing materials. Examples are ultralight metals which are 100 times lighter than polystyrene foam, ultralight and ultra-stiff metamaterials which could be made from a variety of materials, such as metals or polymers, and which could establish new stiffness records for a given weight. Researchers have also developed a bio-inspired approach to 3D printing recyclable liquid crystal polymers using conventional desktop printers that outperform state-of-the-art printed polymers and rival even the highest performing lightweight materials.

“Significant progress has been made in forming such 3D-printed structures, but structural control from the nanoscale to the macroscale remains a major challenge due to the limited ability of existing fabrication methods to control such structures. multi-scale hierarchies with great complexity”, Ling Qiu, an associate professor at the Tsinghua Berkeley Institute in Shenzhen, tells Nanowerk. “For example, even for the most advanced and tricky nanofabrication techniques like two-photon lithography, the length scale range for the controllable structure is limited to hundreds of nanometers to millimeters, not to mention the cost in important time.”

Based on their knowledge of 2D material chemistry, Qiu, Professor Hui-Ming Cheng and their scientific colleagues from the Shenzhen Geim Graphene Center realized that graphene – which has a variety of interesting properties such as ultra-high density low, high compressibility, specific surface area, and electrical conductivity – could be an ideal building block to generate versatile 3D networks from controlled self-assembly.

Manufacturing process of multi-scale graphene structures and demonstration of micro and macro scale structure. a) Diagram of the manufacturing process. Following the arrows: a designed resin model was cut from a 3D digital model into 2D images and then printed by the DLP method. After treatment with oxygen plasma to modify its hydrophilicity to allow ink injection, the hollow spacers of the jig were filled using a syringe pump. An rGO hydrogel was formed by self-assembly inside the jig, and the jig was chemically etched to obtain a pure rGO hydrogel after rinsing with DI water. After lyophilization and annealing, an autonomous rGO airgel was obtained. (Reproduced with permission from Wiley-VCH Verlag)

Meanwhile, rapidly advancing 3D printing techniques offer unprecedented structural design freedom and 3D printing of simple nanostructures with graphene has already been demonstrated. However, it remained difficult to fabricate 3D pure graphene structures with high structural complexity.

“Therefore, we would like to combine the strength of the manufacturing method and the characteristics of the materials, to preserve the pristine properties of functional materials (such as low density and high deformability of graphene structures) and study their structure-dependence,” explains Qiu. for the latest work of the team, reported in Advanced functional materials (“3D printed model-directed assembly of multi-scale graphene structures”).

In this work, the team used graphene as the building block of the model, just like a Lego® piece. Using a combination of 3D printed model and self-assembly technique, graphene “Lego pieces” can be assembled into a programmed structure from nano to macro scale.

This technique enables the fabrication of materials with complex Lego-like patterns with customizable feature size traversing a record seven orders of magnitude – from nanometers to centimeters. This has resulted in the fabrication of graphene structures with tunable mechanical properties, ranging from super stiff (an order of magnitude higher modulus than other ultralight materials) to superelastic (capable of recovering from extreme compression of 95%) .

Structure of multi-scale graphene structures with controllable characteristics over seven orders of magnitude Structure of multi-scale graphene structures with controllable characteristics over seven orders of magnitude. a) Optical images of multi-scale graphene structures of different sizes at the ≈1 cm scale. b–d) SEM images showing a structural hierarchy from ≈1 mm to tens of micrometers with highly tunable feature sizes. e) TEM images showing flakes of multi-scale graphene structures of different sizes and thicknesses (right image scale bar: 2 nm). (Reproduced with permission from Wiley-VCH Verlag)

The fascinating result is the lightest material in the world with a density of 0.08 mg/cm3 – which is 15 times less than the density of air.

This research not only provides an easy approach to fabricate programmed graphene structures, but reveals the possibility of customizing its properties through rational structure design at multiple scales towards a plethora of applications.

“Our work provides a potentially universal method to create various 3D structures with high-resolution multi-scale design using different functional nanomaterials, thereby expanding the fabrication capabilities of existing 3D printing techniques,” emphasizes Jingzhuo Zhou, first author of the item.

Although there have been several reports of macroscopic structures derived from 3D printed models, they have mainly focused on the compositions of the injected materials (e.g., magnetic fluid field responses) with the 3D printed polymer ( or the remaining charred skeleton) as a structural support. .

On the other hand, the novelty of this work lies in the controllable self-assembly of the graphene sheets inside the template, which allows a much more precise control of the structure than the previous works with pure functional materials (template completely took of).

In general, the mechanical strength (such as Young’s modulus) of bulk materials degrades exponentially with decreasing bulk density, which limits the development and application of lightweight materials. But thanks to this new manufacturing technique, the manufacture of structures with record density and record modulus becomes possible.

Ultralight Graphene Mesh on Feather The ultralight graphene network floats on a feather. (Image courtesy of the researchers)

The graphene structures demonstrated in this work can achieve ultra-low density (mainly in the range of 0.1-1 mg/cm3) and corresponding ultra-high stiffness (Young’s modulus is an order of magnitude higher than reported materials with the same density).

The team also used their graphene structures for the manufacture of ceramics. Generally, the fabrication of multilevel ceramic structures is difficult: the microstructure is not easily controllable and the macrostructure is difficult to process. Using the new graphene structures as templates, combined with deposition techniques, the researchers showed that it is possible to obtain pure ceramic structures that reproduce the hierarchical structures of graphene after air sintering.

The 3D printed graphene structure is picked up by a feather. (Video courtesy of researchers)

“Our 3D-printed model-directed assembly method can be applied to other polymer or nanoparticle colloidal dispersion systems, expanding the range of materials that can be fabricated into custom-designed architectures,” Qiu notes. “Achieving great design freedom of a multi-scale structure provides a materials platform for systematic research into structure-property relationships. Last but not least, the nearly unlimited structural design freedom for a range of Potential materials offer virtually endless possibilities for creating, optimizing, and customizing application-oriented architectures.”

For example, this method could provide a potential universal solution to the fabrication of 3D electrodes in energy storage devices with custom fine structures for easy electrolyte permeation and rapid ion diffusion, thus enabling higher density. of energy and an extraordinary power flux-density.

In ACS Nano (“Ti 3D printed model assisted assembly without additives3VS2JX MXene microgrids with custom fabrics for high areal capacity”), the team also reported successfully obtaining pure MXene architectures with a level of structural complexity comparable to that of this work.

With rational structure design, fabricated MXene architectures can achieve high areal capacity, areal energy density, and charge retention (87%) with high mass loading. By adjusting tortuosity, MXene architectures can achieve high throughput performance or capacity performance based on custom requirements.

This means that in principle this method is applicable to any nanomaterial that can be assembled from a stable suspension into a self-supporting structure.

Michael is the author of three Royal Society of Chemistry books:
Nano-society: pushing the limits of technology,
Nanotechnology: the future is tinyand
Nanoengineering: the skills and tools that make technology invisible
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