Ambient aging of rhenium filaments used in thermal ionization mass spectrometry: Growth of oxo-rhenium crystallites and anti-aging strategies

Abstract

Degassing is a common preparation technique for rhenium filaments used for thermal ionization mass spectrometric analysis of actinides, including plutonium. Although optimization studies regarding degassing conditions have been reported, little work has been done to characterize filament aging after degassing. In this study, the effects of filament aging after degassing were explored to determine a “shelf-life” for degassed rhenium filaments, and methods to limit filament aging were investigated. Zone-refined rhenium filaments were degassed by resistance heating under high vacuum before exposure to ambient atmosphere for up to 2 months. After degassing the nucleation and preferential growth of oxo-rhenium crystallites on the surface of polycrystalline rhenium filaments was observed by atomic force microscopy and scanning electron microscopy (SEM). Compositional analysis of the crystallites was conducted using SEM-Raman spectroscopy and SEM energy dispersive X-ray spectroscopy, and grain orientation at the metal surface was investigated by electron back-scatter diffraction mapping. Spectra collected by SEM-Raman suggest crystallites are composed primarily of perrhenic acid. The relative extent of growth and crystallite morphology were found to be grain dependent and affected by the dissolution of carbon into filaments during annealing (often referred to as carbonization or carburization). Crystallites were observed to nucleate in region specific modes and grow over time through transfer of material from the surface. Factors most likely to affect the rates of crystallite growth include rhenium substrate properties such as grain size, orientation, levels of dissolved carbon, and relative abundance of defect sites; as well as environmental factors such as length of exposure to oxygen and relative humidity. Thin (∼180 nm) hydrophobic films of poly(vinylbenzyl chloride) were found to slow the growth of oxo-rhenium crystallites on the filament surfaces and may serve as an alternative carbon source for filament carburization.

Understanding the Conversion Process of Magnetron-Deposited Thin Films of Amorphous ReOx to Crystalline ReO3 upon Thermal Annealing

“…These nanocrystals were relatively easy to displace with the AFM tip during the scan, indicating that they were not embedded into the film but instead were situated on the surface. These crystals were probably Re 2 O 7 , which had grown on the rhenium oxide surface after its exposure to ambient moisture (Re 2 O 7 is nonconductive, and it is known that Re 2 O 7 nanocrystals grow on the metallic rhenium surface upon exposure to the atmosphere). After heating to 200 °C, the number of nonconductive nanocrystals on the surface greatly decreased (a few nonconductive spots are still seen in Figure d).…”

“…ReO 2 is a dark blue or black solid and has a monoclinic phase α-ReO 2 below 300 °C. 1 When heated above 300 °C, it irreversibly turns into the orthorhombic phase β-ReO 2 , which is stable in a vacuum to 850−1000 °C but oxidizes in air to Re 2 O 7 above 400 °C. Both phases of ReO 2 have metallic conductivity.…”

“…Rhenium oxides are known to exist in the three main phases ReO 2 , ReO 3 , and Re 2 O 7 , corresponding to the oxidation states Re 4+ , Re 6+ , and Re 7+ , respectively. ReO 2 is a dark blue or black solid and has a monoclinic phase α-ReO 2 below 300 °C . When heated above 300 °C, it irreversibly turns into the orthorhombic phase β-ReO 2 , which is stable in a vacuum to 850–1000 °C but oxidizes in air to Re 2 O 7 above 400 °C.…”

Metallic Re3O2 with mixed-valence states

“…24,25 However, up to now, no mixed-valence compounds are reported. At ambient pressure, Re can react with oxygen to form ReO 2 , [26][27][28] ReO 3 , [29][30][31] and Re 2 O 7 . 32 ReO 3 has drawn a great deal of attention due to its high conductivity comparable to Ag 33,34 and the negative thermal expansion property.…”

Measurement of PM10, PM2.5 and Black Carbon and assessment of their health effects in Agra, a semiarid region of India

“…Selections of metal matrices are primarily based on the reported data from different AMS facilities. However, additional Re metal powder was chosen here due to its high work function, higher melting points and enhance ionization efficiency of other elements during thermal ionization mass spectrometry (Studier et al, 1962;Mannion et al, 2017). The outcome of this study will be useful in quick and precise 10 Be AMS measurement of the natural samples.…”