Li Qiushi, Antoniou Antonia, Pierron Olivier N
G.W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30318, United States.
ACS Appl Nano Mater. 2024 Nov 25;7(23):27173-27184. doi: 10.1021/acsanm.4c05133. eCollection 2024 Dec 13.
The reliability of nanocomposite conductive inks under cyclic loading is the key to designing robust flexible electronics. Although resistance increases with cycling and models exist, the exact degradation mechanism is not well understood and is critical for developing inks. This study links cracking behavior to changes in electrical resistance by performing in situ cyclic stretch experiments in scanning electron microscopy (SEM) with synchronized resistance measurements. Two screen-printed conductive inks, PE874 and 5025, on thermoplastic polyurethane (TPU) and polyimide (PI) substrates, respectively, were tested using the in situ technique. The obtained SEM images were analyzed with digital image correlation (DIC) to map the strain across cycles. The strain maps show that fatigue damage mainly occurred within the cracks formed during the initial monotonic stretch. There was no delamination at the ink-substrate interface or crack extension along the surface with cycling. Instead, fatigue damage resulted from a combination of crack widening and local shearing within the existing cracks. Crack depth varied based on the ink and substrate properties. The cracks in the 5025 ink on the PI substrate were only partially through the ink thickness, while fully through-thickness cracks were more prevalent in the PE874 ink on the TPU substrate. The 5025 ink showed a faster resistance increase with cycling than the PE874 ink because fatigue damage affected more bridging ink material for partial through-thickness cracks. Higher strain amplitudes caused greater crack widening and shearing and therefore faster resistance increase per cycle.
纳米复合导电油墨在循环载荷下的可靠性是设计坚固耐用的柔性电子产品的关键。尽管电阻会随着循环次数增加，且已有相关模型，但确切的降解机制仍未完全了解，而这对于开发油墨至关重要。本研究通过在扫描电子显微镜（SEM）中进行原位循环拉伸实验并同步测量电阻，将开裂行为与电阻变化联系起来。分别在热塑性聚氨酯（TPU）和聚酰亚胺（PI）基板上的两种丝网印刷导电油墨PE874和5025，使用原位技术进行了测试。利用数字图像相关（DIC）分析获得的SEM图像，以绘制整个循环过程中的应变图。应变图表明，疲劳损伤主要发生在初始单调拉伸过程中形成的裂纹内。随着循环次数增加，油墨与基板界面处没有分层现象，表面也没有裂纹扩展。相反，疲劳损伤是由现有裂纹内的裂纹加宽和局部剪切共同作用导致的。裂纹深度因油墨和基板特性而异。PI基板上5025油墨中的裂纹仅部分贯穿油墨厚度，而TPU基板上PE874油墨中全厚度裂纹更为普遍。5025油墨比PE874油墨在循环过程中电阻增加更快，因为对于部分贯穿厚度的裂纹，疲劳损伤影响了更多的桥接油墨材料。更高的应变幅值会导致更大的裂纹加宽和剪切，因此每循环电阻增加更快。
