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Direct Measurement of Metal-Hydride Formation during Ejecta Particle Transport in Reactive Gases Using Raman Spectroscopy

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Home / Mission / Site-Directed Research and Development / FY 2023 SDRD Annual Report Index / Direct Measurement of Metal-Hydride Formation during Ejecta Particle Transport in Reactive Gases Using Raman Spectroscopy

Project # 23-090 | Year 1 of 3

Jason Mance, Thomas Myers, Rick Allison, Chusia Moua, Mark Morey, Jerry Stevens

Special Technologies Lab (STL)
This work was done by Mission Support and Test Services, LLC, under Contract No. DE-NA0003624 with the U.S. Department of Energy, the NNSA Office of Defense Programs, and supported by the Site-Directed Research and Development Program. DOE/NV/03624–1923.

Abstract

There are important and unresolved questions about the properties of ejecta that are relevant to the weapons community. These questions have been, and still are, an ongoing topic of emphasis in the Nevada National Security Sites (NNSS) experimental program. Results of recent studies suggest that ejecta properties are likely influenced by chemical reactions when propagating in reactive gases. However, the presence of these reactions has thus far only been inferred through pyrometric temperature measurements. No direct evidence has been presented. We are proposing experiments that can provide direct evidence of these chemical reactions.

Background

New methods for measuring temperature and composition in ejecta experiments are needed. Several diagnostic techniques have shown success in these areas; pyrometry is often used to estimate ejecta temperature and Laser Induced Breakdown Spectroscopy (LIBS) has been used to identify the atomic content of ejecta clouds. But neither of these techniques can confirm or deny the presence of metal hydride compounds and a parallel method for measuring ejecta temperatures is needed to compliment the pyrometry work.

Technical Approach

We have been working towards using spontaneous Raman spectroscopy to both identify and measure temperature of cerium hydrides in ejecta/gas mixtures. This technique has the potential to provide direct evidence of byproducts from the proposed chemical reaction. It will also offer a measurement of temperature independent of emissivity, which can be used to support pyrometry measurements.

We are using a single 532 nm Q-switched laser pulse and a spectrometer coupled to an Intensified Charge-Coupled Device (iCCD) camera to measure the Raman spectra. We began by purchasing cerium hydride powders and attempted to measure the Raman spectra; however, there was not a measurable Raman spectrum with these powders using the Q-switched laser system. We then tried to measure the Raman spectrum using a Renishaw Raman microscope because it has a much higher sensitivity than the Q-switched system. The Renishaw also could not detect a Raman signal. The Raman spectrum of cerium hydrides has been published; however, the method for producing the hydrides was different than the method for producing the hydride powders we purchased. Next year, we plan to try again to obtain a static Raman measurement on cerium hydrides by following the methods outlined in Avisar and Livneh1.

We also purchased cerium oxide powders and were able to clearly measure the Raman spectrum. We then moved on to attempt these measurements after launching the particles into gases with explosively driven shock waves. In our first measurement, we launched cerium oxide powders into oxygen gas. This created a cloud of ejecta travelling at 2.5 km/s. We hit the ejecta cloud with our laser and measured the Reyleigh scattered 532 nm light; however, no Rayleigh peaks were visible. We hypothesized that the ~3000 K temperatures created by the shocked oxygen gas had caused the cerium oxides to decompose, so we repeated the experiment in helium since shocked helium temperatures are much lower. This resulted in a lower temperature, as expected; however, there were still no peaks visible in the Raman spectrum. In the second year of this project, we plan to repeat this experiment in vacuum. We also plan to attempt Raman measurements on conventional ejecta clouds that are formed from shocking defects on the surface of metals.

Results and Technical Accomplishments

We succeeded in building and fielding a system capable of measuring spontaneous Raman spectra in dynamic shock experiments driven by high explosives. We have designed and constructed cerium targets and chambers that will allow us to make dynamic measurements under vacuum. We are also in the process of measuring the infrared (IR) absorption spectrum in cerium hydrides and other metal oxide or hydride compounds. If the IR signature is strong enough, we will attempt to identify metal oxide and/or hydride formation dynamically using IR spectroscopy.

Conclusions and Path Forward

It has been challenging to measure the Raman spectrum of cerium hydrides. Publications have reported that CeH2.0 has metallic properties that cause the Raman spectrum to disappear, while CeH2.3 to CeH2.9 has a measurable spectrum. Possibly, our samples have been dominantly CeH2.0 and thus we have been unable to measure the Raman spectrum. The Raman spectra of cerium oxide is strong; however, initial attempts to measure the spectrum dynamically have shown that the Raman signal disappears when the material is shocked. Next year, we will try new methods for both producing our oxide or hydride compounds as well as examine additional optical methods (such as IR absorption) for identifying these compounds.

  • [1] D. Avisar and T. Livneh, J. Alloys Cmpd. 494, 1–2 (2010)
Raman spectrum in cerium oxide and hydride powders purchased from American Elements. The oxide powders show a strong Raman signal while the hydride powders show no discernable peaks. Measurements were made with a Q-switched Nd:YAG laser (532 nm) and a spectrometer coupled to an intensified CCD camera.

Publications

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