Shock temperature of stainless steel and a high pressure — high temperature constraint on thermal diffusivity of Al 2 O 3

Time dependent shock temperatures were measured for stainless steel (SS) films in contact with transparent anvils. The anvil/window material was the same as the driver material so that there would be symmetric heat flow from the sample. Inferred Hugoniot temperatures, Th, of 5800-7500K at 232-321GPa are consistent with previous measurements in SS. Temperatures at the fihn-anvil interface (T,), which are more directly measured than Th, indicate that Ti did not decrease measurably during the approximately 250ns that the shock wave was in A1203 or LiF anvils. Thus an upper bound is obtained for the thermal diffusivity of A1203 at the metal/anvil interface at 230GPa and 6000K of ~ < O.O0096cm2/s. This is a factor of 17 lower than previously calculated values, resulting in a decrease of the inferred :/'a by 730k. The observed shock temperatures are combined with temperatures calculated from measured ttugonlots and are used to calculate thermal conductivities of A1203. Also we note that since there was no measurable intensity decrease during the time when the shock wave propagated through the window, we infer from this that A1203 remained transparent while in the shocked state. Thus sapphire is a good window material to at least 250GPa for shock temperature measurements for metals. I n t r o d u c t i o n There have been several studies on the shock temperatures of iron using optical measurements of the interface radiation to infer the Planck temperatures and emissivities. These interface temperatures, Ti, are used to infer the Hugoniot temperature, Th, as a function of shock pressure. Several authors have suggested that optical radiation in these experiments is not actually emitted from the metal-window interface and some light may be coming from the shocked anvil material[I]. There is also an uncertainty in the values of the constants, such as the thermal diffusivity of the anvil material, that are used in the calculation of Th from T/. Since the calculations of McQueen et al, 1970 [2] predict a lower shock temperature for 304 stainless steel (SS) than for iron, this study focuses on the shock temperatures and thermal properties of SS. If there is a systematic difference between the measured shock temperatures of SS and iron, and those differences are consistent with the predictions, then it can be inferred that the optical radiation is coming from the metal-anvil interface, rather than from within the shocked anvil. Also two different anvil materials are used and the consistency of the observed interface temperatures can be used to infer that a non-significant contribution of the radiation is originating within the anvil. Additionally, a new target configuration is used to constrain the thermal diffusivity of the anvil. The previous configuration [3] used metal drivers and 10pro thick films so that the there would be no predicted decrease in the interface temperature with time. Thus the Ti could be related to the Th as in Tan and Ahrens 199014]. The new, or "sandwich" configuration has a thinner film of 1 #rn and the same material for the anvil and the driver. This allows heat to diffuse out of the hot metal into the relatively colder anvil material so that the temperature is expected to decrease at the interface. The magnitude of the temperature decrease with time is dependent on the thermal diffusivities of the metal and the anvil. Since the thermal conductivity of the metal is well constrained by the Weideman-Franz law, we are effectively measuring the thermal diffusivity of the anvil at high pressure. We assume that specific heats axe well known for both media. It could be argued that some or all of the decrease in radiation with time could be due to the anvil material becoming opaque [5]. However, for film thicknesses of 10 pro, no measurable temperature decrease is expected due to thermal diffusion. Thus if we employ 10 prn films and observe no measurable temperature decrease, then we can conclude that the anvil material remains transparent.