New Way to Prevent Cracking

The research was led by Jacob Prosser and Daeyeon Lee, both of the Department of Chemical and Biomolecular Engineering in Penn’s School of Engineering and Applied Science.

To generate a nanoparticle film, the desired particles are suspended in a suitable liquid, which is then thinly and evenly spread over the surface through a variety of physical methods. The liquid is then allowed to evaporate, but, as it dries, the film can crack like mud in the sun.

“One method for preventing cracking is modifying the suspension’s chemistry by putting binding additives in there,“ Prosser said. “But that is essentially adding a new material to the film, which may ruin its properties.”

This dilemma is highlighted in the case of electrodes, the contact points in many electrical devices that transfer electricity. High-end devices, like certain types of solar cells, have electrodes composed of nanoparticle films that conduct electrons, but cracks in the films act as insulators. Adding a binder to the films would only compound the problem.

“These binders are usually polymers, which are insulators themselves,” Lee said. “If you use them, you are not going to get the targeted property, the conductivity, that you want.”

Engineers can prevent cracks with alternative drying methods, but these involve ultra-high temperatures or pressures and thus expensive and complicated equipment. A cheap and efficient method for preventing cracks would be a boon for any number of industrial processes.

The ubiquity of cracking in this context, however, means that researchers know the “critical cracking thickness” for many materials. The breakthrough came when Prosser tried making a film thinner than this threshold, then stacking them together to make a composite of the desired thickness.

“I was thinking about how, in the painting of buildings and homes, multiple coats are used,” Prosser said. “One reason for that is to avoid cracking and peeling. I thought it could work for these films as well, so I gave it a try.”

The method the researchers used to make the films is known as “spin-coating.” A precise amount of the nanoparticle suspension — in this case, silica spheres in water — is spread over the target surface. The surface is then rapidly spun, causing centrifugal acceleration to thin the suspension over the surface in a uniform layer. The suspension then dries with continued rotation, causing the water to evaporate and leaving the silica spheres behind in a compacted arrangement.; Source: University of Pennsylvania