Effect of a capillary force induced network on the development of cracks in nanoparticle films

裂纹是由纳米颗粒分散体在刚性基底上制造均匀薄膜的主要问题。颗粒的移动性和裂纹的形成会降低膜的性能，甚至导致功能的完全丧失。裂缝通常是通过干燥前沿附近的颗粒运动（如咖啡渍）或溶剂蒸发时与收缩相关的累积应力（如干泥）形成的。通常通过复杂的温度和湿度曲线降低蒸发速率来防止裂纹，或者使用重复退火来固定裂纹;这两种方法都增加了制造时间和成本。我们提出，在毛细悬浮液中发现的强颗粒网络可用于增加材料强度并降低颗粒流动性，从而减少快速干燥条件下裂纹的形成。初步研究表明，向悬浮液中加入少量的二次流体，形成所谓的毛细悬浮液，可以减少薄膜的破裂。在本项目中，将研究开裂程度与二次流体含量、干燥条件和材料特性的关系。开裂将与两种流体的蒸发速率以及使用颗粒跟踪监测的微观结构变化相关。这种破裂也将与悬浮液的流变学测量相关。然后，我们将研究开裂的机制，并确定控制裂纹的形成和尺寸的参数。最后，这些控制参数将被应用于创建可印刷薄膜太阳能电池和柔性电子产品的模型无裂纹膜。

Cracks are a major problem in the manufacture of uniform thin films from nanoparticle dispersions onto rigid substrates. Particle mobility and formation of cracks can degrade the film properties or even result in a total loss of functionality. Cracks are usually formed through the movement of particles near the drying front (as in coffee stains) or from the accumulated stress associated with the shrinkage as the solvent is evaporated (as in dried mud). Cracks are often prevented by reducing the rate of evaporation with sophisticated temperature and humidity profiles or fixed using repeated annealing; both methods increase the time and cost of manufacturing. We propose that the strong particle network found in capillary suspensions can be used to increase the material strength and reduce particle mobility, thereby reducing the formation of cracks under fast drying conditions. Preliminary investigations have revealed that the addition of small amounts of a secondary fluid to a suspension, forming so-called capillary suspensions, can reduce cracking in thin films. In this project, the extent of cracking will be investigated as a function of the secondary fluid content, drying conditions, and material properties. The cracking will be correlated to the rate of evaporation of the two fluids as well as microstructural changes as monitored using particle tracking. This cracking will also be correlated with rheological measurements of the suspension. We will then investigate the mechanism of cracking and determine the parameters controlling the formation and size of cracks. Finally, these control parameters will be applied towards the creation of model crack-free films for printable thin film solar cells and flexible electronics.