Nanocrystals are crystalline particles with at least one dimension measuring at less than 1000 nm (nm)1. Nanocrystals have a wide variety of potential applications such as manufacturing the filter that refines crude oil into diesel fuels, the development of solar panels, the removal of pollutants and toxins, medical imaging, protein analysis, etc1.
Materials in the nanoscale have physicochemical properties that differ greatly from their bulk counterparts as they typically exhibit superior qualities of strength and durability.
Many theories have been proposed to explain why metals get stronger as their dimensions approach the micrometer (mm) or even smaller scale2. In metal objects comprised of larger crystals, the plastic deformation of a metal object is controlled by the way the defects, known as dislocations, move along the planes within their crystal structure2.
Mechanical deformation therefore usually leads to an increase in the number of dislocations in the material2. For example, the bending of a paper clip results in the tangling of its trillions of dislocations per square centimeter and the multiplication of these dislocations as they slide along one another across numerous slip planes2.
The phenomenon of mechanical deformation in nanostructures is very different as a result of the increased surface area to volume compared to their bulk counterparts2. Even though the nanocrystals have dislocations, compressing these crystals often results in driving these dislocations out of the material, reducing the dislocation density by fifteen orders of magnitude and finally producing a perfect crystal2.
The dislocations present in nanocrystals escape from the crystal at the surface by a phenomenon referred to as dislocation starvation. Microstructures can therefore sustain the deformation as a result of the dislocations escaping the microcrystals before they can interact and multiply, which does not leave enough active dislocations for the deformation to happen. This process of mechanical annealing further supports the concept that “smaller is stronger” 2.
As impressive as it sounds, these materials often become brittle as they become stronger. Silver wires follow the same trend up to 40 nm, while a thickness of less than 10 nm changes this behavior3. Research by other scientists has shown that silver wires with a thickness of less than 10 nm becomes very soft when compressed, behaving like a Jello gelatin dessert3.
The reason for this peculiar liquid-like behavior at room temperature is believed to be as a result of the smaller size of the silver crystals, where most of the atoms are on the surface leave only a few atoms present in the core or interior surface of the wire3. This arrangement of silver atoms in the nanocrystals allows for the diffusion of the individual atoms to dominate the behavior of the metal instead of the cracking and slipping the organized lattices within the atoms3.
Scientists of the Department of Mechanical Engineering and Materials Science of both University of Vermont and University of Pittsburgh recently discovered a super strong and stretchable silver, which can potentially open-up new possibilities for the development of bendable touchscreen phones and tablets.
Frederic Sansoz and his team determined that silver nanowires of just a few hundred atoms thick are both strong and stretchable like gum, and can therefore be shaped into a mesh that can conduct electricity and allow light to shine through it3. The silver nanowire exhibited an unusual room temperature elongation without softening in face-centered-cubic silver nanocrystals3.
Using in situ transmission electron microscopy and atomistic models on a super computer, Sansoz’s team determined that the two mechanisms of “smaller is stronger” and the liquid-like behavior shown in other nanomaterials coexisst in the nanosilver wire with a 10 nm thickness4. Source-controlled dislocation plasticity of the silver nanocrystal was determined by in situ transmission electron microscopy tensile tests of submicrometer aluminum single crystals.
The results showed that the dislocations emitted by the source were escaping the crystal before being able to multiply and the dislocation nucleation and loss rates were counterbalanced at about 0.2 events per second4.
At this thickness, diffusion of the surface atoms fill and heals the defects when the atoms are pulled apart allowing it to stretch up to 200 percent4. Therefore, the dislocation density remains statistically constant throughout the deformation at strain rates of about 10-4 per second4.
However, when the strain rate is increased suddenly to 10-3 per second, the nucleation rate outweighed the loss rate resulting in a noticeable surge in dislocation density showing that the deformation of submicrometer crystals is strain-rate sensitive4.
Silver nanowires are expected to have a wide variety of applications in electronics including conductive electrodes for touchscreen displays3. Sansoz’s team are hopeful that their discovery should give chemists and industrial engineers a target size for creating silver wires that could lead to the first foldable phone3.
- “What Is Nanocrystal?” WhatIs.com, http://www.whatis.techtarget.com/definition/nanocrystal.
- “Smaller Is Stronger — Now Scientists Know Why.” Research News, 2 Jan. 2008, http://www2.lbl.gov/Science-Articles/Archive/MSD-mech-annealing.html.
- “Super-Strong, Stretchy Silver.” Phys.org , 31 Mar. 2017. http://www.phys.org/news/2017-03-super-strong-stretchy-silver.html.
- Li Zhong et al. Slip-activated surface creep with room-temperature super-elongation in metallic nanocrystals, Nature Materials (2016).
- Image Credit: shutterstock.com/ Forance