Effect of a capillary force induced network on the development of cracks in nanoparticle films
Final Report Abstract
We investigated the drying behavior of capillary suspensions in this project. This type of suspension consists of a three-phase liquid-liquid-solid dispersion. The secondary liquid, which is only present in amounts of a few volume percent, forms a particle-bridging network. These linkages change the rheological properties from fluid-like to paste-like behavior upon the addition of up to 7 percent secondary fluid, depending on the chosen system. These pastes were shown to exhibit fewer cracks after drying, but the degree of crack reduction and the cause was heretofore unknown. The suppression of crack formation is crucial in many applications, such as printed electronics. For these applications, the addition of polymeric binders and surfactants are typically used to suppress cracking and tune the rheology for better printing. However, these additives require subsequent energy intensive and potentially substrate destructive heat treatments. By employing the capillary suspension concept, these subsequent annealing steps may become superfluous. Cracking in coatings can be directly related to stresses formed during drying. We built a new apparatus to investigate the drying stresses of capillary suspensions. Additionally, we improved the concept of cantilever beam bending with a temperature and humidity controlled drying chamber in a unique way, which allows us to simultaneously measure the stresses and the weight loss while drying. This novel set-up allows us to not only measure the stresses within the film, but also deduce information about the porosity of the investigated film. Previous methods required two different samples with the assumption that they are identical – an assumption that usually cannot be met with paste-like formations. In this study, we found that capillary suspensions reduce drying stresses by up to 30% at higher initial particle loadings. Moreover, we could show that increasing the relative humidity in the drying atmosphere can decrease drying stresses. The additionally gained information about the film density enabled us to clearly proves that the stress reduction for capillary suspensions is not solely attributed to an increase in film porosity, but that the induced network structure itself withstands and counters stresses acting on the particles. This reduction in stresses has an impact on applications, such as printed electronics, as it renders subsequent processing steps obsolete and offering a new pathway to print conductive circuits on flexible substrates without destroying them. Furthermore, the link between the stress reduction and induced network structure shows that it is possible for other interparticle forces to reduce cracking as long as this force persists into the final drying stages – a hypothesis that must be tested in follow-up studies.
Publications
- (2017). Suppressing crack formation in particulate systems by utilizing capillary forces. ACS Applied Materials and Interfaces, 9 (12), 11095-11105
Schneider M., Maurath J., Fischer S.B., Weiß M., Willenbacher N., Koos E
(See online at https://doi.org/10.1021/acsami.6b13624)
