Re: What causes plasticity in Silica nanowires on an atomic level?

Dear Grace,

You sure picked a field of interest that's still full of unknowns! This should at least get you started. Your question has a few separate elements, so I'll try to address them one at a time.

First: deformation and fracture
A quick summary of this is worthwile, because it applies to crystalline structures large and small, to some extent. In crystalline structures, deformation takes on two forms: elastic and plastic. Elastic deformation is when bonds within the crystal lattice are stretched, but not broken - when you let go, the bonds return to their original state, and the energy that went into stretching the bonds is recovered. Plastic deformation is when bonds within the crystal lattice are broken (also called slip or dislocation formation/motion) and atoms are forced to rebond with their new neighbors - when you let go, the bonds that are broken stay broken. You would have to put MORE energy in to return the structure to its original shape, since you would have to rebreak the new bonds and move the atoms back to their original configuration. When these dislocations move, it's pretty orderly. After all, it's an organized lattice of atoms, and motion is only likely to occur in a few different directions. If dislocations pile up against each other or against discontinuities in the crystal structure, a crack can form, and this is one way in which fracture can occur.

In amorphous structures, things are a little different. There is no order to the crystal structure, so there are no dislocations moving on orderly paths. Instead, plastic deformation occurs locally in microscopic shear bands (for example, see this paper). The Center for the Science and Engineering of Materials at CalTech has some good pictures comparing fracture and plasticity in crystalline and amorphous metals here (it's a couple of pages into the document).

Second: deformation and fracture in nanostructures
OK, now we're getting into stuff that people are still learning about. Some researchers have been able to demonstrate that the principles of fracture in macroscopic crystalline structures still apply at the microscopic scale (for example, see Nature Materials v.4, pp.525– 529 (2005)). The big difference here is that in nanostructure, you don't have nearly as many atoms in the structure, and moving them around is a much bigger deal. All it takes to get fracture in a single crystal is enough dislocation motion in one direction. In polycrystalline materials, you need to generate dislocation motion in many different crystals with many different orientations all at the same time. This is why single crystal materials can be weaker than their polycrystalline counterparts. Aside from that, the mechanisms of deformation and fracture are similar.

Third: manufacturing silica nanowires
There are a couple of different ways in which nanowires can be manufactured. The first is by growing them (kind of like evaporating water off to leave salt crystals behind, only using Ga, Si, and O instead of water and salt...). There's a paper with some good illustrations and explanation from Physics Letters A, Volume 335, Issue 4 , 14 February 2005, pp. 304-309 by Dai, et al. In contrast, nanowires can also be formed by stretching out thicker wires until they are nano-sized - I think this might be the method you are asking about in your question. This process involves heat - the scientists heat the material, and can then plastically deform it much more than at room temperature. There's a brief description of the process here and a more detailed description of these scientists' work here (Limin Tong et al 2005 Nanotechnology v.16, pp.1445-1448). This process is not unique to nanostructures - many macroscopic structures are heated up and deformed to a much larger degree than they could be at room temperature, too, in the process called hot working.

Fourth: trying to tie it all together
There's an article you may be interested in from Molecular Dynamics on deformation and fracture in silica nanorods (a rod is basically like a really short wire). It shows the mechanisms of breaking bonds during fracture in crystalline nanowires. Basically, the bonds break, just like they do in macroscopic crystalline or amorphous materials - when you read about scientists plastically stretching silica nanowires to their final diameter, they probably used heat to increase the ductility of the material.

I hope this answers your question, or at least points you in the right direction to find out more about the fields of nanotechnology and materials!