MEASUREMENTS OF THE ISOCHORIC HEAT C(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 heat c(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 of T - T-c = +/- 6 K. Approaching T-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 of c(v). The inhomogenities introduced by a finite ramp rate were greatly reduced by the fast dynamic temperature propagation (''critical speeding up''). The c(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 any c(v) measurements done so far. The preliminary value for the critical exponent alpha is 0.107 +/- 0.02, and for the amplitude ratio we obtained A(-)/A(+) = 1.94 +/- 0.07. In contrast to the cooling runs, the heating runs showed a strong hysteresis of c(v). A comparison to 1 g measurements is provided.