Tianjin University realizes 4D printing of highly conductive metals at room temperature

On July 5, 2023, the team of Professor Qi Wei and Professor Huang Renliang of Tianjin University collaborated with Professor Michael D. Dickey of North Carolina State University in the United States to publish a research result entitled “Metallic Gels for conductive 3D and 4D printing” in the journal Matter, which was selected as a featured article in the current issue. At the same time, the results were selected as Research Highlight in Nature and reported under the title “‘4D printed’ objects morph and flex thanks to a metallic ink”.

In this study, a liquid metal/copper/water three-phase metal gel with Pendular structure was prepared for the first time by using the unique wetting between liquid metal and copper. The gel has shear-induced alignment properties and exhibits anisotropic shrinkage after solvent evaporation. At the same time, the end-to-end connection structure of liquid metal and copper makes the material exhibit high conductivity of metalloid properties (1.05×105 S/m). Through pattern programming, 4D printing of highly conductive metals at room temperature is realized, which opens up new ideas for green and intelligent processing and manufacturing of metal materials.

The corresponding authors of the paper are Professor Huang Renliang, Professor Qi Wei and Professor Michael D. Dickey, and the first author is Dr. Xing Ruizhe.

Since metal powders often require high-temperature bonding or sintering to form, 3D printing of metal materials at room temperature is very challenging. Traditional metal 3D printing processes require a lot of energy and cannot be processed at room temperature. Liquid metal (LM) has low melting point and high electrical/thermal conductivity, and has great application potential in electricity, energy, catalysis, biology and other fields. At the same time, the low viscosity characteristics of LM also make it suitable for various processing methods, such as 3D printing, screen printing, inkjet printing, etc. In recent years, research has found that LM can be compatible with a variety of 3D printing technologies, such as direct writing, light curing, laser-assisted 3D printing technology, etc. However, these technologies inevitably use a large number of LMs to achieve high conductivity of the final product, which greatly limits their application. In addition, the reported LM conductive materials themselves also require post-processing (such as acid, alkali, mechanical force, laser, etc.) to achieve the conduction of internal circuits.

Figure 1: Preparation process of liquid metal three-phase gel, three-phase phase diagram and its structural properties.

Liquid metals can infiltrate the surface of most metals (such as Cu, Fe, Ag, etc.). In this work, inspired by the solid-liquid-liquid three-phase gel system, the authors successfully obtained a three-phase metal gel with the characteristics of the Pendular network by blending the ratio between LM, Cu particles and water. It was found that the gel was highly sensitive to the pH value of the aqueous solution, mainly because the oxide film on the surface of the LM was etched in a strong acid/alkali environment, which promoted the contact between the LM and the Cu particles to form a more tightly connected network. Through phase diagram analysis, it was found that the minimum LM content of the gel formed by the LM/Cu/water three-phase system was only 0.1 vol%. More importantly, LM three-phase gels have a yield modulus of more than 1kPa and have the characteristics of 3D printing.

Figure 2: 3D printing process of a liquid metal three-phase gel.

Furthermore, it is found that LM three-phase gels have significant shear-induced arrangement behavior during extrusion 3D printing, which is mainly due to the liquid properties of LM itself. At the same time, a small amount of methylcellulose was added to the aqueous solution as the skeleton, and it was found that the 3D printed lines showed significantly different axial and radial shrinkage during the drying process. Based on the above findings, a series of characteristic structures with different spatial curvatures were printed by reverse programming, and the zonal control of curvature was realized by controlling the drying temperature. In particular, the structure printed with liquid metal three-phase gel has high conductivity (1.05×105 S/m) and flexural modulus (0.89 GPa) of metalloids.

Figure 3: Liquid metal three-phase gel programmable 4D printing process.

In order to verify the diversity and flexibility of liquid metal gel in 4D printing, the basic units of different curvatures were combined to realize the bionic printing of complex structures of spider shapes, and a “spider robot” was developed that could sense the vibration direction in real time.

Figure 4: Bionic 4D printing prepares a spider robot that senses vibration direction in real time.

This work realizes the 4D printing of highly conductive metals at room temperature for the first time, opening up new ideas for green and intelligent processing and manufacturing of metal materials. The research was supported by grants from the National Natural Science Foundation of China (52203101) and the China Scholarship Council (201906250075). (Source: Science Network)

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