Preparation and property modulation of stretchable conductive composites for extrusion 3D printing
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摘要: 多层柔性和可拉伸电子因其在生物医疗、可穿戴设备、电子皮肤等领域具有较大的应用前景。然而，适用于多层电子的拉伸导电材料存在导电性差、拉伸性低以及层间互联导线制造难等问题，限制了多层柔性和可拉伸电子的进一步发展和应用。本文通过在纳米银颗粒(AgNP)、多壁碳纳米管(MWCNT)和聚二甲基硅氧烷(PDMS)复合导电材料内灵活的添加二甘醇(DEG)助剂，制备了一种适用于层间互联导线直接挤出3D打印的高导电可拉伸复合材料。受益于二甘醇的高沸点，固化过程中导电复合材料中的AgNP能够有效聚集并析出表面，从而提高导电性。溶剂良好的润湿性能平衡材料结构塌陷和3D打印机喷嘴堵塞的问题，有利于层间互联导线的3D打印。同时，大长径比的MWCNT在拉伸过程中能够稳定AgNP之间的电连接。最终制备的导电材料具备优异的导电性能(104 S·cm−1)和拉伸性能(在40%应变下循环拉伸
1000 次以上)，并能基于材料挤出3D打印技术实现可拉伸层内互联导线、自支撑3D垂直互联导线以及2.5D弧形导线的打印。本文制造的拉伸复合浆料在柔性电加热以及柔性显示灯阵实现了良好的应用，充分验证了可拉伸导电复合材料在柔性和可拉伸电子领域的应用前景，为3D打印多层柔性和可拉伸电子的发展铺平了道路。Abstract: Multi-layer flexible and stretchable electronics have great potential in fields such as biomedicine, wearable devices, and electronic skin. However, the development and application of multi-layer flexible and stretchable electronics are hindered by issues such as poor conductivity, low stretchability, and difficulties in manufacturing interlayer interconnects. In this paper, a highly conductive stretchable composite material suitable for direct extrusion 3D printing of interlayer interconnecting wires was prepared by flexibly adding diethylene glycol (DEG) additives within the composite conductive materials of silver nanoparticles (AgNP), multi-walled carbon nanotubes (MWCNT) and polydimethylsiloxane (PDMS). Benefiting from the high melting point of diethylene glycol, the AgNP in the conductive composites can effectively aggregate and precipitate out of the surface during the curing process, thus improving the electrical conductivity. The good solvent wettability balances the issues of material structure collapse and 3D printer nozzle clogging, facilitating the 3D printing of interlayer interconnects. Meanwhile, the large aspect ratio of MWCNT can stabilize the electrical connection between AgNP during the stretching process. The resulting conductive material exhibits excellent conductivity (104 S·cm−1) and stretchability (cycling over1000 times at 40% strain), which can achieve the printing of stretchable intralayer interconnects, self-supporting 3D vertical interconnects, and 2.5D curved interconnects based on material extrusion 3D printing technology. The stretchable composite paste developed in this study demonstrates good performance in flexible electrothermal heating and flexible display arrays, confirming the promising application prospects of stretchable conductive composite materials in the field of flexible and stretchable electronics, paving the way for the development of 3D printed multi-layer flexible and stretchable electronics. -
图 1 可拉伸导电复合材料的材料：(a)内部形态示意图；(b)渗流形态示意图；(c-d)微观形貌电镜图；(e-f)多壁碳纳米管(MWCNT)分布电镜图；(g)元素分析
Figure 1. Materials of stretchable conductive composites: (a) Schematic internal morphology;; (b) Schematic percolation morphology; (c-d) Electron microscopy of the micromorphology; (e-f) Electron microscopy of the distribution of of multi-walled carbon nanotubes (MWCNTs); (g) Elemental analyses
图 2 (a)可拉伸导电复合材料固化前后表面的微观形貌变化图；(b)可拉伸导电复合材料添加二甘醇(DEG)前后表面的组分质量分数比例
Figure 2. (a) Micro-morphological changes of the surface of stretchable conductive composites before and after curing; (b) Component mass fraction ratio of the surface of stretchable conductive composites before and after addition of diethylene glycol (DEG)
图 3 纳米银颗粒(AgNP)含量对可拉伸导电复合材料导电性能的影响
Figure 3. The influencet of silver nanoparticles (AgNP) content on the electrical conductivity of stretchable conductive composites
图 4 不同AgNP含量导电薄膜SEM图：(a)2.5 g；(b)3 g；(c)4 g；(d)5 g；(e)6 g
Figure 4. The SEM images of conductive films with different AgNP contents:(a) 2.5 g; (b) 3 g; (c) 4 g; (d) 5 g; (e) 6 g
图 5 DEG含量对可拉伸导电复合材料导电性能的影响
Figure 5. The influence of DEG content on the electrical conductivity of stretchable conductive composites
图 6 AgNP含量对可拉伸导电复合材料流变性能的影响：(a)黏度曲线；(b)流动曲线
Figure 6. The influence of AgNP content on the rheological properties of stretchable conductive composites: (a) viscosity profile; (b) flow profile
图 7 MWCNT含量对可拉伸导电复合材料黏度的影响
Figure 7. The influence of multi-walled carbon nanotube content on the viscosity of stretchable conductive composites
图 8 DEG含量对可拉伸导电复合材料黏度的影响
Figure 8. The influence of DEG content on the viscosity of stretchable conductive composites
图 9 四氢呋喃(THF)含量对可拉伸导电复合材料黏度的影响
Figure 9. The influence of tetrahydrofuran (THF) content on the viscosity of stretchable conductive composites
图 10 可拉伸导电复合材料热重分析
Figure 10. Thermogravimetric analysis of stretchable conductive composites
图 11 固化温度和固化时间对导电复合材料的导电性能的影响
Figure 11. The influence of curing temperature and curing time on the electrical conductivity of conductive composites
图 12 可拉伸导电复合材料拉伸性能对比(a)未退火(b)退火后；(c)直线拉伸效果；(d)蛇形曲线拉伸效果
Figure 12. Comparison of tensile properties of stretchable conductive composites (a) unannealed (b) after annealing; (c) straight line tensile effect; (d) serpentine curve tensile effect
图 13 可拉伸导电复合材料打印的层间互联导线拉伸和弯曲性能测试
Figure 13. Tensile bending performance test of interlayer interconnecting wires printed from stretchable conductive composites
图 14 打印速度对打印层内互联导线线宽的影响规律
Figure 14. The influence of printing speed on the line width of interconnecting wires in the printing layer
图 15 打印气压对打印层内互联导线线宽的影响规律
Figure 15. The influence of printing air pressure on the line width of interconnecting wires in the printing layer
图 16 打印喷头内径大小对打印层内互联导线线宽的影响规律
Figure 16. The influence of printhead inner diameter size on the line width of interconnecting wires in the printing layer
图 17 垂直互联导线(VIA)的打印原理图及流程图
Figure 17. Printed schematic and flowchart for Vertical Interconnect Access (VIA)
图 18 打印速度对VIA成形高度和横截面面积的影响
Figure 18. The influence of print speed on VIA forming height and cross-sectional area
图 19 打印气压对VIA成形高度和横截面面积的影响
Figure 19. The influence of print air pressure on VIA molding height and cross-sectional area
图 20 打印喷头内径大小对VIA成形高度和横截面面积的影响
Figure 20. The influence of the inner diameter of the printing nozzle on the VIA forming height and cross-sectional area
图 21 (a)弧形互联导线的打印原理图及流程；(b-e)VIA的高度对弧形互联导线的跨越宽度和高度的影响规律
Figure 21. (a) Printing schematic diagram and process of arc-shaped interconnection wires; (b-e) The influence of the height of VIA on the span width and height of arc-shaped interconnection wires.
图 22 弧形互联导线：(a)打印效果图；(b)SEM图像；(c)俯视图；(d)用镊子按压效果图
Figure 22. Curved interconnecting wires: (a) printed effect; (b) SEM image; (c) top view; (d) effect of pressing with tweezers
图 23 打印的多种不同尺寸、不同图案的导电薄膜：(a)花朵；(b)分形；(a)凤凰；(a)雪花；(e-f)不同图案的导电薄膜形貌特征和拉伸效果
Figure 23. A variety of printed conductive films with different sizes and patterns: (a) flower; (b) fractal; (a) phoenix; (a) snowflake; (e-f) morphological characteristics and tensile effects of conductive films with different patterns
图 24 (a)透明电加热效果图；(b)2.5 D结构柔性显示灯阵的效果图及性能展示：(c)2.5 D结构柔性显示灯阵的拉伸变形测试；(d)2.5 D结构柔性显示灯阵的弯曲变形测试
Figure 24. (a) Transparent electric heating effect diagram; (b) Effect diagram and performance demonstration of 2.5 D structure flexible display light array: (c) Tensile deformation test of 2.5 D structure flexible display light array; (d) Bending deformation test of 2.5 D structure flexible display light array
表 1 导电复合材料各组分配比
Table 1. Distribution ratios for each group of conductive composites
Materials Mass fraction/wt% AgNP 71.3 MWCNT 0.18 PDMS 17.82 THF 7.1 DEG 3.6 Notes：AgNP is silver nanoparticles, MWCNT is multi-walled carbon nanotubes, PDMS is polydimethylsiloxane, THF is tetrahydrofuran solvent, DEG is diethylene glycol solvent 
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表 2 不同AgNP含量的可拉伸导电复合材料黏附力
Table 2. Adhesion of stretchable conductive composites with different AgNP contents
AgNP content 2.5 g 3 g 4 g 5 g 6 g Adhesion 5 B 5 B 5 B 2 B 0 B
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表 3 不同MWCNT含量导电复合材料的黏附力
Table 3. Adhesion of conductive composites with different MWCNT contents
MWCNT content 0 mg 10 mg 20 mg Adhesion 5 B 5 B 5 B
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