Is there a way to contain fluorine gas for long term so that it ...

In addition to the suggestion of using metal fluorides like CaF2, I want to mention the possibility of using carbon-based fluoropolymers, such as polytetrafluoroethylene (PTFE), perfluoroalkoxy alkanes (PFA), and fluorinated ethylene propylene (FEP). These materials are commonly recognized by the brand name "Teflon," which many people associate primarily with PTFE. These plastics are widely used to manage highly reactive fluorine compounds like HF, UF6, and HSbF6, as well as various other reactive substances. While all these plastics are colorless, PTFE is primarily translucent and usually appears white, while FEP and PFA are transparent.

According to the chemical resistance chart, PTFE is deemed "resistant" to dry fluorine (F2) at temperatures up to 60°C, along with various other substances listed. PFA demonstrates similar properties. FEP shows compatibility with F2 up to at least 60°C; however, it is noted for its "better gas and vapor permeability," which raises concerns about storing gaseous fluorine. "Better" could imply lower permeability, as a PDF about FEP films from Teflon.com highlights its exceptionally low gas permeability.

Some vague comparisons of these three materials suggest that they may be more affordable than metal fluorides, though I cannot confirm this with certainty. Notably, these polymers are not fully fluorinated and can indeed react with F2, as illustrated in the equation:

(CF2)n(s) + nF2(g) → nCF4(g)

A manufacturer of these materials notes that "[t]he extremely potent oxidisers, fluorine (F2) and related compounds (e.g., chlorine trifluoride, ClF3), can only be handled by PTFE/PFA with great care due to potential hazards." The quoted statement suggests that Teflon can ignite in the presence of fluorine, similar to how conventional oxide-based glass behaves. This is consistent with the knowledge that Teflon can ignite in oxygen, although it typically does not combust in air. Notably, the "Teflon fire" incidents in liquid oxygen tanks are believed to have contributed to the Apollo 13 disaster. Chemical resistance data indicates that FEP, PTFE, and PFA are all "not recommended" for use with F2 at 100°C. The Teflon.com PDF specifically mentions that FEP reacts with "molten alkali metals, fluorine at elevated temperatures, and certain complex halogenated compounds, such as chlorine trifluoride, at elevated temperatures and pressures." With these polymers melting and chemically decomposing into somewhat toxic gases at temperatures common for plastics, their fire resistance is not particularly strong. As such, attempts to seal F2 in an FEP ampule by melting its tip might not be advisable, although it could work if the F2 is in liquid form, distanced from the melting section. FEP has been noted for its performance in cryogenic conditions without becoming brittle, making it a potential candidate for containing liquid F2 effectively.

For further insights, a question exists on a relevant community forum about possible chemical agents capable of damaging PTFE: Is there ANY chemical that can destroy PTFE, or Teflon?.

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In contrast, metal fluorides are significantly more refractory, although not as highly refractory as oxides. Particularly, CaF2 has one of the highest melting points, with the best number for the melting point recorded at 1418°C according to my CRC Handbook. Materials such as ScF3, LaF3, and CeF3 boast higher melting points but lower boiling points. Moreover, CaF2 and many other metal fluorides cannot undergo further fluorination or combustion. A few may react to form higher fluorides which could be gaseous, but those are not typically necessary. A potential concern with CaF2 and various metal fluorides is their non-negligible solubility in water; however, the solubility of CaF2 remains quite low—similar to that of CaCO3—indicating minimal dissolution unless submerged in significant water volume, exposed to prolonged rainfall, or subjected to acid treatment, which is the conventional method for HF production. When in pure form, CaF2 and other metal fluorides can be highly transparent, often utilized in fiber optics, lenses, and laboratory equipment windows. However, it's unclear if this transparency extends to fluoropolymers.

Compounds like SiO2, Al2O3, and several other non-fluorine alternatives are known for high transparency, refractory nature, water insolubility, and reasonable resistance to oxidation by F2, though they theoretically do remain susceptible to corrosion by F2:

SiO2(s) + 2F2(g) → SiF4(g) + O2(g)

ΔG°298K,1atm = -172.55 kcal/mol for glass or -171.13 for quartz (indicating spontaneity)

2Al2O3(s) + 6F2(g) → 4AlF3(s) + 3O2(g)

ΔG°298K,1atm = -303.0 kcal/mol for corundum to form crystalline AlF3 (indicating spontaneity)

It is essential to note that SiF4(g) is a gas (with a boiling point around -86°C or -90.3°C), while AlF3 stands as a colorless refractory solid (with a sublimation point near 1270°C) which could create a protective coating on the inner surface of a containment vessel. This is akin to the NiF2 layer forming inside nickel containers typically used for storing F2 and other highly reactive fluorine compounds. Thus, I postulate that Al2O3 might present a superior alternative to SiO2. This advantage might likewise be observed in "aluminosilicate" glasses, constructed from aluminum, silicon, and oxygen, and in any event, this material should function comparably to fused quartz (pure SiO2 glass).

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