Researchers ‘watch’ molten salts carve tiny nooks and tunnels into steel alloys in 3D

A multidisciplinary staff of scientists has used the National Synchrotron Light Source II (NSLS-II), a U.S. Department of Energy (DOE) Office of Science User facility situated on the DOE’s Brookhaven National Laboratory, to analyze how high-temperature molten salts corrode steel alloys. The group discovered a novel strategy for utilizing molten salts to create porous metallic supplies with microscopic networks of voids and steel ligaments, which may have functions in quite a lot of fields, similar to power storage and sensing. Their work additionally helps the event of molten salt reactors (MSRs), a know-how that might produce safer, cheaper, and extra environmentally sustainable nuclear energy.

Molten salts are one of many main candidates as a medium for high-temperature warmth switch in quite a lot of functions, together with next-generation nuclear and concentrated solar energy vegetation. They have a number of options that make them fascinating, similar to excessive boiling factors, excessive particular heats, excessive thermal conductivities, and low vapor pressures. However, one of many challenges of molten salts is their corrosivity after they are available in contact with alloys.

In MSRs, the molten salt incorporates the nuclear gas in dissolved kind and in addition serves as the first warmth switch fluid, working at 500–900°C (about 930–1650°F). One of the important thing steps towards creating MSRs is to realize a agency understanding of the chemistry of molten salts and the way they work together with the structural supplies in a reactor at excessive temperatures, with their corrosive results being a foremost focus. This work helps to deal with that aim by offering perception into molten salt dealloying, a course of by which sure parts inside a steel alloy are preferentially leached away into the molten salt throughout corrosion. It is the primary examine that explores utilizing the corrosive nature of molten salts to dealloy and purposely create porous constructions.

The analysis, which is described in a paper revealed on June 9, 2021 in Nature Communications, outcomes from a collaboration between NSLS-II and the Brookhaven-led Molten Salts in Extreme Environments Energy Frontier Research Center (MSEE EFRC). EFRCs have been established by DOE’s Office of Basic Energy Sciences to convey giant groups collectively to deal with difficult and interdisciplinary basic analysis challenges for the development of power applied sciences. The MSEE staff on this work included members from Stony Brook University, Brookhaven’s Chemistry Division, and Oak Ridge National Laboratory.

“The mission of MSEE is to provide the fundamental molten salt science needed to enable MSR technology,” mentioned the director of MSEE and one of many paper’s authors, Brookhaven chemist James Wishart.

The work was completed at two NSLS-II beamlines, the Full-Field X-Ray Imaging (FXI) beamline and the Beamline for Materials Measurement (BMM).

“The FXI beamline features an imaging technique called 3D X-ray nanotomography, which yields a time series of 3D visualizations—essentially a 3D movie—of a sample’s internal structure with a resolution of tens of nanometers,” mentioned the lead scientist on the FXI beamline, Wah-Keat Lee, who can also be an creator. “Other facilities have similar instruments, but FXI can yield images 20 times faster. This is what makes this beamline so useful for studies like this one.”

Both FXI and BMM present one other method known as X-ray absorption near-edge construction (XANES) spectroscopy, which is used to yield data on the oxidation state and native construction of the alloy parts through the dealloying response. The experimental outcomes have been then complemented by computational modeling and simulation.

To have the ability to picture high-temperature molten salt corrosion, the FXI beamline employees, NSLS-II engineers, and the MSEE analysis staff collectively developed a particular miniature heater that permits real-time measurements whereas supplies are evolving at situations as much as 1000 °C. This was a significant accomplishment by itself that was documented in a latest paper, revealed in Journal of Synchrotron Radiation.

The staff used the FXI heater system to time-resolve the morphological evolution of a nickel-chromium alloy (80% Ni / 20% Cr) wire in a molten 50-50 combination of potassium chloride and magnesium chloride at 800 °C. As time progressed, chromium was leached out of the wire by corrosion and the remaining nickel restructured right into a porous community. This is the primary time that researchers have noticed the altering 3D construction of a fabric present process the dealloying course of as it’s occurring.

“We watched the sample change in front of our eyes and were able to take a video of every single step, which is remarkable,” mentioned Stony Brook Ph.D. candidate Xiaoyang Liu, one of many joint first authors of the paper.

The staff noticed that the dealloying course of first begins on the interface between the alloy and the salt and propagates by way of to the middle of the alloy, creating the pore community. As chromium is additional leached away into the molten salt, the pores and cavities change into bigger (which is named “coarsening”) because of the diffusion of Ni atoms on the floor of the alloy.

The three-dimensional morphology of the fabric fashioned on this examine is classed as “bicontinuous,” which means each phases—the alloy and the community of pores created by the salt corrosion—are steady and unbroken. Porous bicontinuous supplies are of nice curiosity to researchers attributable to their diminished weight, giant floor areas, skill for mass transport of fluids by way of the pores, and electrical or thermal conductivity by way of the fabric matrix. Bicontinuous metal alloys, particularly ones with advantageous pore sizes, have quite a few potential functions in a number of fields, together with power storage, sensing, and catalysis.

Several strategies have traditionally been employed to create these extremely sought-after supplies, together with acid etching of essentially the most simply corroded aspect, or selective dissolution in liquid steel. However, the molten salt strategy, which has not been beforehand explored, operates by completely different mechanisms and follows completely different guidelines that may present a better diploma of management of each the leaching and restructuring processes, probably leading to superior supplies. This diploma of management is feasible as a result of the imaging capabilities on the FXI beamline enable the researchers to quantify the charges of the dealloying and coarsening processes as they modify parameters similar to temperature and alloy and salt composition.

“The FXI beamline was absolutely critical to this work,” mentioned Stony Brook Ph.D. pupil Arthur Ronne, the opposite joint first creator and co-corresponding creator. “Its time resolution, with the ability to watch the structure change on the minute scale at an excellent nanoscale spatial resolution, along with the furnace we jointly built, made this study possible.”

This work, and its persevering with extension into the results of temperature and salt and alloy composition, is essential for the design of sturdy molten salt reactor programs, which span a spread of temperatures the place mechanisms of corrosion by these processes may very well be predicted to range in several areas, and in addition depend upon the contents of the gas salt. The staff will use the FXI beamline and different superior methods to acquire the mandatory mechanistic data to allow such predictions. In doing so, they are going to get hold of key data to information the deliberate preparation of bicontinuous alloy supplies with particular morphologies and properties for a variety of functions.

“Behind this work is a multitude of incredible scientists and engineers,” mentioned corresponding creator Karen Chen-Wiegart, an assistant professor in Stony Brook’s College of Engineering and Applied Sciences who holds a joint appointment at NSLS-II. “It was only through the partnership of a large research center like MSEE and a world-class facility like NSLS-II that we were able to take this step. We are really only at the beginning of a wonderful journey to further explore the complex and yet fascinating interactions between the materials and molten salts using advanced synchrotron techniques.”