基于表面张力的超薄薄膜力学性能表征方法研究

MECHANICAL CHARACTERIZATION OF ULTRATHIN FILM BASED ON SURFACE TENSION

邵碧琪

11849252

硕士

材料工程

郭传飞

2020-05-28

2020-07-01

哈尔滨工业大学

深圳

薄膜材料被广泛应用于柔性显示、太阳能电池、柔性传感器等器件，了解薄膜材料的力学性能，如弹性模量、屈服强度、断裂强度等，对器件设计和性能优化具有极其重要的意义。目前测试薄膜力学性能主要通过拉伸薄膜或通过原子力探针或纳米压痕仪等对薄膜进行按压，然而上述方法用于研究超薄薄膜整体的力学性能具有一定的局限性。超薄薄膜往往难以自支撑，在基底上进行测试通常误差较大；按压的方式仅仅局限在很小的测量区域，难以反映出超薄薄膜整体的情况。有待提出新的研究方法来测量薄膜的力学性能。水的表面张力高达 72 mN/m，在室温下水的粘度仅为 0.89 mPa s。因此，密度较低的物体在水面上能保持漂浮的状态，且漂浮的固体可以进行几乎无摩擦的滑动。研究表明超薄膜材料能在水的表面张力作用下发生拉伸、弯曲或失稳起皱。因此，可以通过在水面上对薄膜材料进行拉伸的方法，来测量薄膜材料的力学性能。本文研究了一种基于液体表面张力的薄膜力学性能表征方法，重点研究了微米级厚度的聚二甲基硅氧烷（PDMS）圆环薄膜的弹性模量，证明了该方法的可行性。我们让 PDMS 圆环薄膜自由漂浮在水面上，在圆环中心处滴加表面活性剂降低圆环内液面的表面张力，使得 PDMS 圆环薄膜的内外边界因表面张力差而发生受拉变形。针对这种方法，本文研究了影响 PDMS 圆环薄膜的受拉变形的主要因素（PDMS 的配比、薄膜厚度、表面活性剂液滴的浓度），总结了变形量随时间变化的规律。结合圆环所受液面表面张力的测量结果，进一步讨论了圆环受拉变形过程。最后基于实验中测量得到的变形量和表面张力的数据，使用超弹性材料的本构方程计算出 PDMS 的弹性模量。本论文研究的是一种新颖的力学性能表征方法，利用液体表面张力的变化使漂浮在液面的固体薄膜发生拉伸变形。使用水面作为支撑基底，避免了普通拉伸测试中固体基底对测试结果的影响。该方法实验操作简单，且无需大型精密仪器的辅助，具有测量各类薄膜材料力学性能的潜力。同时该方法便于完整转移难以自支撑的薄膜材料，为大尺寸柔性器件制备提供了新的方法和思路。因此，基于液体表面张力的薄膜力学性能表征方法具有广泛的应用价值和重要的研究意义。

Thin films are widely used in flexible displays, solar cells, flexible sensors and other devices. Therefore, understanding the mechanical properties of thin films, such as elastic modulus, yield strength and fracture strength, is of great importance to device design and performance optimization. At present, thin films are stretched or indented by atomic force probes or nanoindentation instruments to access their mechanical properties. However, the above methods have certain limitations in studying the overall mechanical properties of ultrathin films. It is often difficult to test ultrathin films in a free-standing state. For ultrathin film on a substrate, the substrate effect during testing cannot be ignored. Indentation is limited to a local area, which is doubted to represent the overall mechanical property of the film. Therefore, a new method is needed to measure the mechanical properties of thin films. Due to the hydrogen bonding between water molecules, the surface tension of water can reach 72 mN/m. Therefore, objects of low density can remain afloat on the surface of water. In addition, the viscosity of water at room temperature is only 0.89 mPa s. In this case, objects floating on the liquid surface can slide almost without friction.Studies have shown that ultrathin films can be stretched, bent, or crumpled under the surface tension of water. Therefore, the mechanical properties of thin films can be measured by stretching on water. This work studies a method to characterize the mechanical properties of thin films based on liquid surface tension. The elastic modulus of polydimethylsiloxane (PDMS) ring films with micron thickness was studied. The PDMS annulus floats freely on the surface of water. The surface tension of the inner liquid surface of the ring is reduced by dropping surfactant. In this case, the inner and outer boundaries of the PDMS ring film are subjected to differential surface tension, and the annulus are stretched. In this work, the main factors influencing the stretches of the PDMS annulus are studied, such as the ratio of PDMS, film thickness and the concentration of surfactant droplets. The relation of stretches dependent of time is summarized. Combined with the measurement results of the surface tension of the ring, the process of the tension deformation of the annulus is further discussed. Finally, based on the stretches and surface tension measured in the experiment, the elastic modulus of PDMS is calculated by using a constitutive model of hyperelastic material. This work has proposed a novel characterization method to measure mechanical properties of thin films, which introduces a difference of the surface tension of water to deform the afloat thin film. Using the water surface as the underlying support avoids the substrate effect of the test results. The method proposed here is simple to operate, and does not need the assistance of large precision instruments. The method has the potential to measure the mechanical properties of various kinds of thin films.What’s more, this method is convenient for the complete transfer of free-standing thin film materials, and provides a new method and idea for the fabrication of large-size flexible devices. Therefore, the method of mechanical properties characterization of thin films based on liquid surface tension has wide application value and important research significance.

超薄薄膜 表面张力 表面活性剂 弹性模量

ultrathin film surface tension surfactant elastic modulus

中文

联合培养

南方科技大学

邵碧琪. 基于表面张力的超薄薄膜力学性能表征方法研究[D]. 深圳. 哈尔滨工业大学,2020.