Fatigue and Fracture of Stretchable Transparent Conducting Films

柔性和可拉伸的透明导电薄膜在显示器，光伏以及可穿戴和身体安装的传感器方面具有潜在的应用。该博士项目将研究一类透明导电薄膜在循环载荷下的基本失效机制，该薄膜由沉积在聚合物基板上的银纳米线网络组成。EPSRC先前在曼彻斯特资助的研究表明，这些网络的薄片电阻是表面覆盖率和网络电阻的简单函数，而网络电阻是导线的长度、纵横比、电阻率和导线间接触电阻的函数。薄片电阻在反复应变循环后增加，并且网络电阻的变化与表面覆盖无关，而是应变大小和应变循环次数的函数。该项目旨在帮助实现我们的总体目标，了解导致这些透明导电膜在循环机械载荷下电性能下降的机制。将探讨以下研究问题：-纳米线断裂、纳米线结失效或纳米线固有电阻增加的主要机制是什么？-在循环变形过程中，纳米线内部发生微结构变化的机制是什么？-银纳米线生长过程中自然形成的晶体孪晶等缺陷对上述两个主要研究点的影响是什么？学生将使用大学内的设备通过喷雾沉积制造银西北网络，并使用既定程序表征薄片电阻。我们已经开发了新的实验程序，以确定在控制应变循环后可以通过电子显微镜分离和成像的网络的特定区域。先前的工作表明，这些薄膜的片电阻，可以由单晶或五孪晶NWs制成，随着网络变得越来越疲劳，随着应变循环次数的增加而增加。这种电阻的增加也被证明主要是由于NW本身的失效（而不是结的失效或NW固有特性的变化）。NW网络的这种微观行为与薄膜的薄片电阻之间的关系是如何通过统计模型（用于随机线网络）来捕获的。然而，文献也表明，五晶孪晶NWs在疲劳方面的表现与单晶NWs一样好，尽管由于五晶孪晶产生的更大的几何约束，人们期望前者更容易断裂。这意味着在五孪晶NWs中存在一组更基本的纳米级变形行为，这些行为延长了它们的功能寿命——这些行为尚未被完全识别。在这个项目中，主要的重点将放在理解五孪晶NWs的这种变形上。该项目将重新评估该研究小组之前所做的工作，该研究表明，这些NWs中的弯曲和扭转变形可能通过高角度晶界孪晶操作实现，使用TEM与扫描纳米束衍射和电子束进动。然后，这项工作将扩展到研究其他类型的变形——压缩、拉伸、剪切等——以建立一个更全面的变形生态系统的图片，这些生态系统分布在给定的西北网络中。该项目将使用扫描电镜筛选经历循环加载的NW网络，以获得不同变形类型频率的面积统计数据，从而弥合观察到的纳米和微观尺度行为之间的差距。这可能反过来告知上述统计模型可以改进的方式，给出更完整和紧急的薄膜系统视图。

Flexible and stretchable transparent conducting films have potential applications for displays, photovoltaics and in wearable and body-mounted sensors. This PhD project will investigate the fundamental failure mechanisms, under cyclic loading, of a class of transparent conducting thin films that consist of a Ag nanowire network deposited onto polymeric substrates. Prior EPSRC funded work in Manchester has shown that the sheet resistance of these networks is a simple function of the surface coverage and the network resistance, which is a function of the length, aspect ratios and electrical resistivity of the wires and the resistance of interwire contacts. The sheet resistance has been shown to increase after repeated strain cycling and that the change in network resistance is independent of surface coverage but is a function of both the strain magnitude and number of strain cycles.This project aims to help achieve our overall objective of understanding the mechanisms that lead to this degradation in the electrical properties of these transparent conducting films under cyclic mechanical loading. The following research questions will be explored:- Is the main mechanism for the increase in electrical resistance nanowire fracture, nanowire junction failure or an increase in the intrinsic resistance of the nanowires.- What are the mechanisms of microstructural change that occur within the nanowire during cyclic deformation.- What is the influence of defects such as crystallographic twins, which form naturally during the growth of Ag nanowires, on the two major points of study listed above.The student will use the facilities within the University to manufacture Ag NW networks by spray deposition and characterize the sheet resistance using established procedures. We have developed novel experimental procedures to identify specific regions of the network that can be isolated and imaged through electron microscopy after controlled strain cyclingPrior work shows that the sheet resistance of these thin films, which can be made with single-crystal or penta-twinned crystal NWs, increases with the number of strain cycles as the network becomes increasingly fatigued. This increase in resistance is also shown to be due largely to the failure of the NWs themselves (as opposed to the failure of junctions or changes in intrinsic NW properties). How this micro-scale behaviour of the NW network relates to the sheet resistance of the thin film is captured by a statistical model (for random line networks). However, the literature also shows that penta-twinned NWs perform as well as single-crystal NWs with regards to fatigue, despite the expectation for the former to fracture more easily due to the greater geometric constraints that result from penta-twinning. This implies a set of more fundamental, nanoscale deformation behaviours in penta-twinned NWs that prolong their functional lifetime - behaviours that have yet to be fully discerned.In this project, the main focus will be on understanding such deformations of penta-twinned NWs. The project will re-evaluate previous work done by this research group, which suggests bending and twisting deformations in these NWs might be enabled by high-angle-grain-boundary twinning operations, using TEM alongside scanning nanobeam diffraction and electron beam precession. This work will then be expanded to investigate other types of deformation - compressive, tensile, shear, etc - to build up a more comprehensive picture of the ecosystem of deformations that populate a given NW network. The project will screen NW networks that have undergone cyclic loading using SEM, to obtain area statistics on the frequency of different deformation types, thus bridging the gap between the nano- and micro-scale behaviours observed. This may in turn inform of ways in which the aforementioned statistical model could be improved, given this more complete and emergent view of the thin film system.