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Connecting Humidity with Corrosion
Bare aluminum surfaces immediately react with air to form aluminum oxide, which then becomes confined under a nanoscopic film of condensed water vapor. Scientists at Lawrence Livermore performed simulations with Ruby, a Livermore supercomputer, to uncover why atmospheric corrosion of aluminum metal is controlled by the relative humidity in the air. Their research is featured in the June 14, 2023, issue of the ACS Journal of Applied Materials and Interfaces.
The team used all-atom molecular dynamics simulations to demonstrate how aluminum ions diffuse through condensed surface water and lead to the formation of corrosion pits. They found that aluminum ions tended to localize near the air-water interface and were completely absent near the oxide. This tendency contributed to height-dependent transport properties within the water film, with atoms diffusing more quickly as the air-water interface was approached. The height of the surface water itself depends on humidity in the air, linking humidity with diffusion rate and, therefore, corrosion rates.
These first-ever findings into the explicit role of humidity on aqueous ion transport will lead to better assessments of corrosion rates used to predict aluminum component lifetimes. “The results highlight that capturing unusual nanoscale effects and their dependence on humidity is essential when modeling atmospheric corrosion at larger length scales,” says Livermore scientist Jeremy Scher, the paper’s lead author.
Contact: Jeremy Scher (925) 423-5826 (scher1@llnl.gov).
Metasurface Optics Design Breakthrough
Metasurface optics design involves carefully altering the surface of a material to have different properties than the bulk of the material. A team of Laboratory researchers have refined a metasurface process to create taller surface alterations without increasing the space between them, which has important implications for anti-reflective (AR) optics. The team aims to create more durable and stable optics by removing the need for broadband AR coatings, instead achieving anti-reflectivity by creating nanofeatures in the surface of the optic itself. Livermore scientist Nathan Ray is first author on a paper presenting the team’s results, which appears in the June 19, 2023, issue of Advanced Optical Materials.
To achieve anti-reflectivity over a range of wavelengths, the features need to be spaced closer together than the shortest wavelength, and about as deep as half the longest wavelength. The team created a technique called “seeded dewetting” to build up mask nanoparticle height without compromising the necessary spacing between them. The result is a silica glass AR technology capable of eliminating reflected light over an unprecedented wavelength range and at a large span of incidence angles.
Applications for the new AR technology include uses in lasers at the National Ignition Facility and photovoltaic cells, among others. “We can now cover bandwidth range all the way from ultraviolet to wavelengths larger than 2 microns, which was not possible with existing technology,” says Eyal Feigenbaum, the study’s principal investigator.
Contact: Eyal Feigenbaum (925) 423-1343 (feigenbaum1@llnl.gov).
Observing Confined Water’s Behavior
Carbon nanotubes (CNTs) are useful for studying confinement effects on water at scales comparable to the diameter of a single water molecule. Confinement modifies water properties by altering the structure of the hydrogen bond network. However, explaining the experimentally observed differences between the hydrogen bond network of confined water and the bulk liquid has remained a challenge. Livermore scientists revealed the unique hydrogen bonding behavior of water confined in CNTs by combining large-scale molecular dynamics simulations with interatomic potentials constructed using machine-learning methods. Their findings, appearing on the cover of the June 22, 2023, edition of The Journal of Physical Chemistry Letters, have the potential to advance energy storage and ion-selective membranes for water desalination.
The team computed the infrared spectrum of confined water then compared it with existing experimental measurements. They found different effects on water structure between CNTs with diameters larger than 1.2 nanometers and smaller-diameter CNTs. The larger-diameter CNTs saw more disruptive effects on hydrogen bonding, leading to a more disordered structure than the bulk liquid. Smaller-diameter CNTs led to the formation of exotic phases of ordered water, such as nanotube ices and single-file structure.
This study improves simulations of confined water for future exploration. “Our work offers a general platform for simulating water in CNTs with quantum accuracy on time and length scales beyond the reach of conventional, first-principles approaches,” says lead author Marcos Calegari Andrade.
Contact: Marcos Calegari Andrade (925) 423-7381 (calegariandr1@llnl.gov).
