Measurement of the Velocity of Sound in Liquid Argon and Liquid Nitrogen at High Pressures

Abstract

An ultrasonic interferometer, consisting of a barium titanate ceramic cylinder suspended in a high-pressure vessel, has been used to measure the velocity of sound in liquid argon and liquid nitrogen at temperatures near their boiling points and at pressures up to 135 atm. The apparatus is suitable for measurements at much higher pressures, since it is rugged, compact, and has no moving parts. An analysis of the resonances obtained over a frequency range 0.2f0 to 1.7f0 (where f0 is the thickness resonant frequency of the ceramic cylinder) has shown that a simple relation exists between the velocity in the liquid and the zeros of the Bessel functions J0 and J1 for frequencies near 0.5f0 and 1.5f0. The velocity data obtained agree with previous measurements on liquid argon and liquid nitrogen at atmospheric pressure, but show that the velocity of sound in these liquids is not a linear function of pressure, as had been supposed by Itterbeek. Using known density data, the adiabatic compressibility and ratio of the specific heats γ have been found for liquid argon and liquid nitrogen at 90°K. For argon, γ falls slightly from 1.97 to 1.89 as the pressure rises to 135 atm, but for nitrogen γ rises from 1.50 to 1.79 with the applied pressure, suggesting the hindering of intramolecular motions at the higher pressures.