Fluid-cell Raman Spectroscopy for operando Studies of Reaction and Transport Phenomena during Silicate Glass Corrosion

The overall aim is to improve our understanding of reaction and transport processes at solid water interfaces at elevated temperature and at a microscopic scale. With Fluid-Cell Raman Spectroscopy, we study the aqua corrosion of porous silicate glasses as they present a favorite material for the immobilization of high level nuclear waste. Current challenges exist in the potential entrapment of air pockets during closing the cell and the top side corrosion of the glass due to the solution-filled gap between the sapphire window and the glass sample itself.

Especially for long-term experiments, these corrosion products can reduce the signal to noise ratio of the spectra and the spatial resolution. With a new in situ method, we are particularly investigating the still debated rate-limiting reaction and transport processes controlling glass corrosion in aqueous environments over geological timescales. Existing glass corrosion models are highly controversial and therefore require further spatially resolvesd real-time data to improve analytical and numerical models predicting the long-term behavior of nuclear waste glasses, as well as any technique of glasses in aqueous solutions.

The latest results raised further question about how self-irradiated glasses corrode under distinct pH conditions and over longer timescale with respect to the currently debated glass corrosion mechanisms. To begin, grind the glass sample coupon using 600 grit silicon carbide paper on two opposite sides until it fits into the PTFE sample holder. Mount the PTFE holder containing the glass sample into a larger metallic sample holder to prepare for grinding the top side of the glass coupon until it is level with the PTFE holder.

Once the sample PTFE and metallic sample holder are nearly in one plane, grind the surface using a finer 1, 000 grit silicon carbide paper. Polish the top side of the sample within the PTFE holder with a three micrometer polishing cloth for at least 20 minutes. To measure the Raman mode's characteristic of the sample and solution, click on acquisition.

For the borosilicate glass sample, set the first spectral window range from 200 to 1, 735 inverse centimeters. To measure the Raman modes of molecular water, set the second window range from 2, 800 to 4, 000 inverse centimeters. For a sufficient intensity signal of the glass and water, measure the spectral windows for seven and two seconds respectively.

To achieve the best possible signal to noise ratio, set the accumulation to five rounds. Adjust the spectrometer entrance slit width to 200 micrometers, and the confocal hole to 600 micrometers to optimize depth resolution. Place the neon lamp alongside the beam path of the scattered light.

To begin, place the silicone washer on the inverted fluid cell lid. Then position the sapphire window and the PTFE sample holder with the top side of the sample facing the sapphire window. Fix the position of the silicone washer, sapphire window, and sample with the screw cap.

Insert the O-ring into the provided groove. Inject the reactive solution from both sides of the reactor until the outlet of the tubing inside the reactor is fully covered, ensuring no air is trapped. Then, close the valves before removing the syringe to prevent air accumulation in the tubing or valves.

Add the remaining solution from the top of the reactor vessel until the solution forms a convex meniscus. Fill the free spaces of the sample holding lid by carefully dripping the solution along the right and left sides of the sample coupon. Check the filled lid for possible air pockets.

Next, turn the lid upside down to place it on top of the reactor vessel. Quickly secure the cell using the six screws. Mount the fluid cell on the XYZ stage, and connect it to the heating stage.

Once the nominal temperature is reached, adjust the laser focus on the top of the sapphire window, centering it in the X and Y directions above the sample. Set the Z position to zero as a reference. Now move the laser focus in the Z direction until the first Raman signals of water or solution species, such as bicarbonate and carbonate are detected.

Continue moving the laser focus further downwards until a pure spectrum of the glass sample is identified using the real-time display function. Move the laser focus further into the Z direction, penetrating more than 30 to 50 micrometers into the sample for glass corrosion rate observation. Next, move the stage in the X direction to determine the sample solution interface based on the decreasing Raman signal intensity of the sample and the increasing intensity of the solution.

Set the position of the sample solution interface X to zero. Set the line scan from minus 60 to 40 to cover the glass solution interface at approximately zero micrometers in the X direction. Choose a step size of two micrometers, resulting in 51 steps by a line scan of 100 micrometers.

The glass solution interface continuously retreated within the first four hours, indicating congruent dissolution of the glass. The first amorous silica signals appeared after 8.3 hours, indicating the precipitation of the surface alteration layer. A water-rich interfacial zone began forming after approximately 80 hours, gradually developing into a distinct interfacial water layer with a width of about six to eight micrometers.