Determination of HF partial pressure during ex situ conversion of YBCO precursors

Abstract

The importance of HF gas behavior during ex situ YBCO formation is well known, however no experimental data on the partial pressure of HF gas exists due to difficulties in measuring this fundamental quantity. The objective of this study is to measure the partial pressure of HF above TFA-MOD derived coated conductor during processing. The method used was based on a modified Knudsen cell technique. Fluorine loss from the Knudsen cell was monitored by measuring residual fluorine in quenched films. Fluorine loss showed two separate regimes corresponding to the oxidation of YF3 and of BaF2. The partial pressure of HF was measured at 525, 575 and 625°C. Partial pressures of HF measured at 575°C were 0.16±0.02Pa for the YF3 reaction and 0.02±0.002Pa for the BaF2 reaction. Measured pressures gave ΔG values of the reactions, which were compared to calculated values for several possible reactions that literature suggests can occur during the processing. Experimental results indicate the most probable reaction for each step.