High-Pressure Equation-of-State of Porous-Ta2O5

Highly-porous samples of tantalum pentoxide (Ta2O5) aerogel were compressed from initial densities of 0.1, 0.15 and 0.25 g/cm by shock-waves with strengths between 0.3 and 3 Mbar. The compressed material was between 5 and 15 times as dense as the pre-compressed aerogels with temperatures of about 5 eV (58,000 °K). These strong shock-wave loadings were produced by the OMEGA laser system at the Laboratory of Laser Energetics. Because these aerogels represent a small fraction of the full crystalline density of Ta2O5, 8.2 g/cm, the material response to shock-waves is heavily influenced by the large amount of shock-wave heating. This response is quite different than the crystalline material yielding insight into a broader range of material properties. The characterization of the aerogels indicate that the materials have one percent or less by mass of residual contaminants from the manufacturing process present when the experiments are performed; therefore, the material response is highly representative of Ta2O5. The shocked states are diagnosed with the OMEGA Velocity Interferometer System for Any Reflector (VISAR) and the Streaked Optical Pyrometer (SOP). The VISAR provides shock-wave velocity information yielding the compression of the shock-wave compressed state relative to a standard, and the SOP provides thermal information of the compressed material. When the compression measurements are compared to an available equation-of-state (EOS) model it is found that the model underestimates the level of compression achieved by shock-wave loading below one Mbar. This observation indicates that the material exhibits more energy degrees-of-freedom, such as molecular contributions, than have been encompassed in the model. The thermal measurements indicate less significant heating than the models predict; however, the shock-wave strength dependence could indicate that non-equilibrium effects require more attention when considering aerogel materials.