Equation of State of He-H2 and He-D2 Dense Fluid Mixtures at High Pressures and Temperatures

The fluid variational theory is used to calculate the Hugoniot equation of state (EOS) of He, D2, He + H2, and He + D2 fluid mixtures with different He:H2 and He:D2 compositions at high pressures and temperatures. He, H2, and D2 are the lightest elements. Therefore, the quantum effect is included via a first-order quantum correction in the framework of the Wigner-Kirkwood expansion. An examination of the reliability of the above computations is performed by comparing experiments and calculations, in which the calculation procedure used for He and D2 is adopted also for He + D2 and He + H2, since no experimental data for the mixtures are available to conduct these comparisons. Good agreement in both comparisons is found. This result may be seen as an indirect verification of the calculation procedures used here, at least, in the pressure and temperature domains covered by the experimental data for He and D2 used for comparisons, which is nearly up to 40 GPa and 105 K. Also, the equation of state of He + H2 fluid mixtures with different compositions is predicted over a wide range of temperatures and pressures.