Stretching salt

Dr. Neil Canter, Contributing Editor | TLT Tech Beat January 2010

A phenomenon known as superplasticity allows a material to stretch to more than 100% of its original length.

KEY CONCEPTS
• Salt on the macroscale is a brittle material that readily shatters like glass when placed under pressure.
• Salt exhibits superplasticity on the nanoscale as it can be stretched to more than 100% of its original length.
• Low levels of sodium metal present in the salt may act to keep the nanowires flexible and fluid-like.

Salt or sodium chloride is a typical ionic solid that we all know well from both personal and professional perspectives. This compound contains sodium and chlorine ions organized in face-centered cubic crystals. Electrostatic interactions between these ions are the source of the bonding that holds this compound together.

The presence of salt as a contaminant is a concern because it promotes metal corrosion. Salt is readily soluble in water and facilitates the movement of ions through metal, which accelerates the corrosion process.

Salt is also a brittle material that readily shatters like glass when placed under pressure. But when placed in a humid environment, salt becomes more ductile. Nathan Moore, postdoctoral researcher at Sandia National Laboratories in Albuquerque, N. M., says, “It is possible to bend and twist large pieces of salt as long as they are not just single crystals but are made of smaller grains.”

Humidity facilitates the diffusion of atoms in the crystal, causing the salt structure to generate more defects such as steps, kinks and buried dislocations. This enables salt to exhibit bulk plasticity. Such a phenomenon is observed in large, underground rock salt deposits.

On the nanoscale, salt is organized into fewer ionic crystals or grains and so contains fewer defects, according to Moore. This means the likelihood is remote for salt to display any plasticity.

SUPERPLASTICITY
Increasing demand for better quality water has spurred research efforts to develop more efficient desalination technologies. As part of this effort, Moore, in collaboration with other researchers, initiated work to better understand how water bonds to salt surfaces. He adds, “We were originally seeking to understand how salt and water interact on a molecular level in order to determine how to improve the ability to filter salt out of water.”

In initial experiments, salt surfaces were studied under controlled humidity conditions using interfacial force microscopy. Freshly cleaved (100) crystal faces of 99. 9% salt were evaluated in this study. Moore says, “Our intention was to prod the salt surface with a diamond tip in order to detect water.”

As the tip approached the salt surface, the researchers were surprised to encounter an unknown and unexpected force that did not fit any classical models. Once this force is established, the tip starts to pull salt from the crystal. Moore says, “We were very surprised to see salt become stretched into nanowires as it was pulled from the salt surface.”

This behavior is known as superplasticity, which Moore defines as the ability of a material to be stretched to more than 100% of its original length. Initial work was conducted at a relative humidity of 0.1% at ambient temperature. No change in the superplasticity of salt was seen as the humidity was raised up to 60%.

Further studies were conducted using transmission electron microscopy (TEM) under high-vacuum conditions. Moore says, “In this procedure, a 100 keV (kiloelectron volt) electron beam was operated at a dose rate of 1 ampere per square centimeter.”

A gold scanning tunneling microscope probe is pushed against the salt surface generating what the researchers consider to be “debris” in the shape of nanowires. Figure 1 shows salt stretched in this fashion over 2,000 nanometers.


Figure 1. Salt at the nanoscale can be stretched to more than 100% of its original length. This effect is known as superplasticity. (Courtesy of Sandia National Laboratories)

Salt is hygroscopic, and Moore follows the general rule of thumb, “If salt is there, water is there.” But Moore believes the TEM experiments indicate that water does not contribute to the superplasticity. He says, “Under the conditions of the TEM work, the electron beam would blast most of the water away from the salt surface. A tiny bit of water may be present, but it is not believed to enhance the superplasticity effect.”

Salt was stretched up to a maximum of 2.2 microns, which represents an elongation effect of 280%. The researchers were limited by the range of the probe and could not stretch the salt further. Nanowire thickness is in the range of 210 nanometers, according to Moore. He adds, “When we are able to break the nanowire, it ruptures in the middle producing debris that comes close to the probe and to the salt surface.” It is clear that this phenomenon involves the unexpected breakage of salt’s crystal structure. An attractive force is present between the probe and the salt surface. Moore says, “As the probe withdraws from the crystalline structure, a bubble or meniscus of salt is formed. When further stretched, the meniscus follows the tip and becomes elongated.” In contrast, withdrawing an object from liquid water would form a meniscus that breaks under its own weight. Salt is also acting as a fluid, but the meniscus produced just becomes extended without rupturing.

Use of the electron beam seems to enhance this effect. Moore explains, “The electron beam facilitates the reduction of a small fraction of the sodium cations to neutral sodium metal with the accompanying loss of chlorine anions. Sodium metal may stabilize the nanowire structure.”

Moore believes that sodium metal can provide stability to the salt nanowires as they stretch. This is accomplished because sodium metal gets in the way of sodium and chloride ions that would otherwise try to reform into a unified, rigid crystal. Thus, the small fraction of sodium metal may be one key to keeping the nanowires flexible and fluid-like. Nanowires have also been made completely out of sodium metal.

This research shows that the properties of salt can be very different at the macroscale compared to the nanoscale. Further work is needed to determine how salt nanowires may influence the corrosion process.

Additional information on this research can be found in a recent article (1) or by contacting Moore at nwmoore@sandia.gov. A video showing the salt nanowire being stretched has been posted on YouTube and may be accessed here. 

REFERENCE
1. Moore, N., Luo, J., Huang, J., Mao, S. and Houston, J. (2009), “Superplastic Nanowires Pulled From the Surface of Common Salt,” Nano Letters, 9 (6), pp. 2295-2299.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.