Thermophysical Properties of Gaseous Tungsten Hexafluoride From Speed-of-Sound Measurements
John J. Hurly
The speed of sound was measured in gaseous WF6 using a highly precise acoustic resonance technique. The data span the temperature range from 290 to 420 K and the pressure range from 50 kPa to the lesser of 300 kPa or 80 % of the sample s vapor pressure. At 360 K and higher temperatures, the data were corrected for a slow chemical reaction of the WF6 within the apparatus. The speed-of-sound data have a relative standard uncertainty of 0.005%. The data were analyzed to obtain the ideal-gas heat capacity as a function of the temperature with a relative standard uncertainty of 0.1 %. These heat capacities are in reasonable agreement with those determined from spectroscopic data. The speed-of-sound were fitted by virial equations of state to obtain the temperature dependent density virial coefficients. Two virial coefficient models were employed, one based on square-well intermolecular potentials, and the second based on a hard-core Lennard-Jones intermolecular potential. The resulting virial equations reproduced the sound-speed data to within 0.005 % and may be used to calculate vapor densities with relative uncertainties of 0.1 % or less. The hard-core Lennard-Jones potential was used to estimate the viscosity and the thermal conductivity of dilute WF6. The predicted viscosities agree with published data to within 5 % and can be extrapolated reliably to higher temperatures.