Deterministic, Nanoscale Fabrication of Mesoscale Objects
Neither LLNL nor any other organization has the capability to perform deterministic fabrication of mm-sized objects with arbitrary, {micro}m-sized, 3-dimensional features with 20-nm-scale accuracy and smoothness. This is particularly true for materials such as high explosives and low-density aerogels. For deterministic fabrication of high energy-density physics (HEDP) targets, it will be necessary both to fabricate features in a wide variety of materials as well as to understand and simulate the fabrication process. We continue to investigate, both in experiment and in modeling, the ablation/surface-modification processes that occur with the use of laser pulses that are near the ablation threshold fluence. During the first two years, we studied ablation of metals, and we used sub-ps laser pulses, because pulses shorter than the electron-phonon relaxation time offered the most precise control of the energy that can be deposited into a metal surface. The use of sub-ps laser pulses also allowed a decoupling of the energy-deposition process from the ensuing movement/ablation of the atoms from the solid, which simplified the modeling. We investigated the ablation of material from copper, gold, and nickel substrates. We combined the power of the 1-D hydrocode ''HYADES'' with the state-of-the-art, 3-D molecular dynamics simulations ''MDCASK'' in our studies. For FY04, we have stretched ourselves to investigate laser ablation of carbon, including chemically-assisted processes. We undertook this research, because the energy deposition that is required to perform direct sublimation of carbon is much higher than that to stimulate the reaction 2C + O{sub 2} => 2CO. Thus, extremely fragile carbon aerogels might survive the chemically-assisted process more readily than ablation via direct laser sublimation. We had planned to start by studying vitreous carbon and move onto carbon aerogels. We were able to obtain flat, high-quality vitreous carbon, which was easy to work on, experimentally and relatively easy to model. We were provided with bulk samples of carbon aerogel by Dr. Joe Satcher, but the shop that would have prepared mounted samples for us was overwhelmed by programmatic assignments. We are pursuing aligned carbon nanotubes, provided to us by colleagues at NASA Ames Research Center, as an alternative to aerogels. Dr. Gilmer started modeling the laser/thermally accelerated reactions of carbon with H{sub 2}, rather than O{sub 2}, due to limited information on equation of state for CO. We have extended our molecular dynamics models of ablation to include carbon in the form of graphite, vitreous carbon, and aerogels. The computer code has features that allow control of temperature, absorption of shock waves, and for the ejection of material from the computational cell. We form vitreous carbon atomic configurations by melting graphite in a microcanonical cell at a temperature of about 5000K. Quenching the molten carbon at a controlled rate of cooling yields material with a structure close to that of the vitreous carbon produced in the laboratory. To represent the aerogel, we have a computer code that connects ''graphite'' rods to randomly placed points in the 3-D computational cell. Ablation simulations yield results for vitreous carbon similar to our previous results with copper, usually involving the transient melting of the material above the threshold energy density. However, some fracturing in the solid regions occurs in this case, but was never observed in copper. These simulations are continuing, together with studies of the reaction of hydrogen with vitreous graphite at high temperatures. These reactions are qualitatively similar to that of oxygen with the carbon atoms at the surface, and the simulations should provide insight into the applicability of the use of chemical reactions to shape the surfaces of aerogels.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15015101
- Report Number(s):
- UCRL-TR-206947; TRN: US200508%%353
- Resource Relation:
- Other Information: PBD: 24 Sep 2004
- Country of Publication:
- United States
- Language:
- English
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