Exploring crucible designs for differential scanning calorimetry measurements of fluoride salts

Abstract

One important consideration when performing Differential Scanning Calorimetry (DSC) measurements of fluoride salts is the crucible used to hold the specimen. The design of such a crucible is complicated by the high chemical reactivity of fluorides, whereby the crucible should be made of an inert material to avoid chemical interaction with the specimen. Also, the crucible should be able to maintain a hermetic seal to prevent potential damage to the instrument or potentially harm the user as a result of off-gassing. Furthermore, it is desirable to have a max use temperature of at least 1000 °C to accommodate many salt compositions. Unfortunately, a DSC crucible that meets the foregoing desgin requirements is not commercially available. Three custom crucible designs were explored in this work for use in a Netzsch Jupiter 449 F1 Simultaneous Thermal Analyzer. The first design involved a custom designed body, gasket, and screw-top lid, each machined out of Ni-201. The second design retrofitted a commercially available stainless steel body and lid pair with a custom machined boron nitride liner and a custom gasket punched from Ni-201 shim stock. The third design leveraged the same commercial crucible but was retrofitted with a custom liner made of Ni-200 and a gasket out of Ni-201. No noticeable evidence of wall-salt interaction was observed with this crucible with optical microscopy. Thermogravimetric analyses recorded during DSC measurements and visual inspection indicate that a hermetic seal was maintained. The third crucible design was subsequently qualified and benchmarked for DSC by successfully reproducing melting point temperatures reported in the literature for LiF, KF, NaF, CsF, and FLiNaK. Furthermore, some new DSC measurements are reported on the LiF-CsF system. This crucible design was found to be quite practical, affordable, and performed well in measuring phase transition temperatures of fluoride salts up to 1200 °C.