Measurements of the isochoric heatc v at the critical point of SF6 under microgravity: Results of the German Spacelab Mission D2

During the Second German Spacelab Mission D2 (April 26 to May 6, 1993) the isochoric specific heatc_v of SF₆ was measured along the critical isochore under microgravity conditions with a newly developed scanning radiation calorimeter. This calorimeter provided the possibility to perform comparable heating and cooling runs with variable ramp rates since the spherical sample cell was heated and cooled only by radiation. During the experimental time of 220 h, 11 heating and cooling runs with different ramp rates were performed in a temperature range ofT–T_c=±6 K. ApproachingT_c by cooling from the homogeneous one-phase region avoided significant temperature and density gradients in the fluid, which would have distorted the integral measurement ofc_v. The inhomogenities introduced by a finite ramp rate were greatly reduced by the fast dynamic temperature propagation (critical speeding up). Thec_v data achieved with slow cooling runs are in remarkably good agreement with the theoretical prediction more than one order of magnitude closer to the critical point than anyc_v measurements done so far. The preliminary value for the critical exponent is 0.107±0.02, and for the amplitude ratio we obtainedA^–/A⁺=1.94±0.07. In contrast to the cooling runs, the heating runs showed a strong hysteresis ofc_v. A comparison to 1g measurements is provided.Invited paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, USA.