Dec 19 2017
Thin (left) and thick films made of porous nanoparticles of calcium and silicate reacted differently under pressure as tested in a Rice University lab. Particles in the thin films moved out of the way for a nanoindenter and allowed the film to stay intact, while thick films cracked. (Courtesy of the Multiscale Materials Laboratory)
Porous particles of silicate and calcium exhibit potential as building blocks for numerous applications like bone-tissue engineering, self-healing materials, drug delivery, ceramics, insulation, and construction materials according to Rice University engineers who decided to check out how well they perform at the nanoscale.
Based on earlier research on self-healing materials using porous building blocks, Rice materials scientist Rouzbeh Shahsavari and graduate student Sung Hoon Hwang made a broad range of porous particles between 150 and 550 nm in diameter — several times smaller than the thickness of a sheet of paper — with pores approximately the width of a strand of DNA.
They then assembled the particles into micron-sized sheets and pellets to observe how well the arrays managed under pressure from a nanoindenter, which tests the hardness of a material.
The results of over 900 tests, reported this December in the American Chemical Society’s ACS Applied Materials and Interfaces, revealed that bigger individual nanoparticles were 120% tougher compared to smaller ones.
This, Shahsavari said, was clear indication of an intrinsic size effect where particles between 300 and 500 nm went from brittle to ductile, or flexible, even though they all possessed the same small pores that were 2 to 4 nm. But they were amazed to discover that when the same big particles were stacked, the size effect did not carry over completely to the larger structures.
The principles discovered should be significant to engineers and scientists exploring nanoparticles as building blocks in several kinds of bottom-up fabrication.
With porous building blocks, controlling the link between porosity, particle size and mechanical properties is essential to the integrity of the system for any application. In this work, we found there is a brittle-to-ductile transition when increasing the particle size while keeping the pore size constant. This means that larger submicron calcium-silicate particles are tougher and more flexible compared with smaller ones, making them more damage-tolerant.
Rouzbeh Shahsavari, Materials Scientist, Rice University
The lab examined self-assembled arrays of the miniature spheres and arrays compacted under the equivalent of 5 tons inside a cylindrical press.
Four sizes of spheres were permitted to self-assemble into films. When these were underwent nanoindentation, the team discovered the intrinsic size effect mostly disappeared as the films exhibited variable stiffness. Where it was thin, the feebly bonded particles just allowed the indenter to sink through to the glass substrate. The film cracked in areas where it was thick.
We observed that the stiffness increases as a function of applied indentation forces because as the maximum force is increased, it leads to a greater densification of the particles under load,” Shahsavari said. “By the time the peak load is reached, the particles are quite densely packed and start behaving collectively as a single film.
Rouzbeh Shahsavari, Materials Scientist, Rice University
Pellets composed of compacted nanospheres of varying diameters deformed under pressure from the nanoindenter but exhibited no evidence of becoming tougher under pressure, they stated.
As a next step, we’re interested in fabricating self-assembled superstructures with tunable particle size that better enable their intended functionalities, like loading and unloading with stimuli-sensitive sealants, while offering the best mechanical integrity.
Rouzbeh Shahsavari, Materials Scientist, Rice University
The research is supported by the National Science Foundation.