Vapor pressure measurements on mixtures of LiF, NaF, and BeF2

Knowledge of the vapor pressure of fluoride salts is necessary in order to assess the suitability of these salts as a liquid first wall for fusion energy schemes. A combination of Thermogravimetric Analysis (TGA) and Knudsen Cell Mass Spectrometry (KCMS) was used to characterize the vapor phase above mixtures of LiF, NaF, and BeF₂. The scope was limited to mixtures containing between 0.3 and 0.9 mole fraction BeF₂. Measurements were performed on pure BeF₂ as well as the binary systems of LiF-BeF₂ and NaF-BeF ₂. For BeF₂, values of DeltaH_vap were obtained by TGA and KCMS; and the vapor pressure was measured using TGA. Results were in agreement with literature values within experimental uncertainty. KCMS on the binary and ternary systems showed that the vapor phase was composed primarily of BeF₂, and to a lesser extent the mixed dimers LiBeF₃ and NaBeF₃. Data for the binary systems were in agreement with the available literature sources. The ternary system was treated as a pseudobinary system by fixing the ratio of LiF to NaF. Data for both the ternary and binary systems were correlated in terms of the BeF₂ activity coefficient