Final Technical Report
OAK-B135 Shock-induced super-heating is definitely observed for five materials: (Mg3Fe)2SiO4 (olivine), SiO2 (crystal quartz and fused quartz), CsBr, and Kbr. Experimental data for Fe and NaCl indicate but do not clearly demonstrate the occurrence of the phenomenon in the latter two media. We have theoretically examined superheating of a crystalline solid with homogeneous nucleation theory. Using a dimensionless energy barrier for nucleation (beta), involving solid-liquid interfacial energy, heat of fusion, and heating rate. For a number of elements and compounds, beta varies from 0.2 to 10 at ambient pressure and is of the same order at megabar pressures. This energy barrier range corresponds to maximum superheating of 0.05-0.35 and 0.06-0.50 at heating rates of 1 K/s and 10{sup 15} K/s, respectively. Superheating by homogeneous nucleation is more pronounced and achievable with high heating rates, such as shock compression induced ultrafast internal heating at 10{sup 12}-10{sup 15} K/s. We demonstrated that superheating comparable to the prediction of the proposed theory has been achieved in planar impact and intense laser irradiation experiments. Experiments were conducted with light pulses from the N2 laser are assumed to simulate particle impact. These light pulses irradiate the sample with a power density up to 10{sup 11} W/cm{sup 2}. We examined mass spectra for cosmochemically important minerals, kamacite (Fe-Ni), pyrrhotite (FeS), olivine [(Mg,Fe)SiO4], serpentine [Mg3Si2O7 {sup 2}H2O], and Murchison meteorite. Shock-wave experiments on iron preheated to 1573 K from 14 to 73 GPa, yield sound velocities of the gamma- and liquid-phases. Melting is observed in the highest pressure ({approx}71 ? 2 GPa) experiments at calculated shock temperatures of 2775 ? 160 K. This single crossing of the gamma-liquid boundary agrees with the gamma-iron melting line of Boehler [1993], Saxena et al. [1993], and Jephcoat and Besedin [1997]. This gamma-iron melting curve is {approx}300 C lower than that of Shen et al. [1998] at 80 GPa. In agreement with Brown [2001] the discrepancy between the diamond cell melting data and the iron shock temperatures require the occurrence of yet another sub-solidus phase along the principal Hugoniot at {approx}200 GPa. This would reconcile the static and dynamic data for iron's melting curve. Upward pressure and temperature extrapolation of the gamma-iron melting curve to 330 GPa yields 5300 ? 400 K for the inner core-outer core boundary temperature.
- Research Organization:
- California Institute of Technology (CalTech), Pasadena, CA (United States)
- Sponsoring Organization:
- USDOE Office of Defense Programs (DP)
- DOE Contract Number:
- FG03-99DP00295
- OSTI ID:
- 806599
- Country of Publication:
- United States
- Language:
- English
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