Kinematic Self-Replicating Machines
© 2004 Robert A. Freitas Jr. and Ralph C. Merkle. All Rights Reserved.
Robert A. Freitas Jr., Ralph C. Merkle, Kinematic Self-Replicating Machines, Landes Bioscience, Georgetown, TX, 2004.
B.7 Wall Sublimation and Mechanical Depassivation Contamination
Wall materials potentially can sublimate into the interior vacuum of the device, the solid establishing equilibrium with its vapor at a temperature-dependent vapor pressure. However, this process should produce negligible effluent at room temperature for likely assembler building materials such as diamond . A volumetric number density of less than one wall-material atom per Vint (0.00054 micron3) – e.g., a perfect vacuum inside the assembler – equals a contaminant partial pressure of <2 x 10-11 atm. To reach this vapor pressure, pure carbon (e.g., graphite ) must be heated to >2000 K; at an assembler operating temperature of ~273 K, the vapor pressure of fully bonded wall carbon can be estimated [3161, 3271] as only ~10-133 atm. (The vapor pressures of various fullerenes at elevated temperatures have been measured  as ~3 x 10-8 atm at 693 K  and ~10-6 atm at 773 K  for C60.) Drexler  estimates the characteristic thermal cleavage time for unstressed 556 zJ C-C bonds in diamond as ~1085 sec at 300 K and ~1012 sec at 700 K.
In a perfect vacuum, unstressed polished hydrogenated diamond surfaces remain chemically and mechanically stable up to ~1275 K, although above this temperature the passivating hydrogens are removed and crystallographic surface reconstruction begins . Thus it is unlikely that passivating hydrogens on the interior surfaces of assembler diamond walls will spontaneously debond, contaminating the interior environment. The energy barrier for hydrogen abstraction from hydrogen passivated diamond surface has been estimated as ~7.7 kcal/mole  or ~53.5 zJ/atom, roughly the energy liberated by the application of a force of ~270 pN applied through a distance of ~ one atomic diameter. Since this is only slightly higher than forces commonly applied by the Stewart platform manipulators, a sufficiently sharp impact between such a manipulator and the inside walls could mechanically depassivate hydrogen into the device interior. Hydrogen atom bombardment of hydrogenated diamond surface has also been shown to abstract surface hydrogens [3276, 3277]. However, this failure mode is unlikely to occur in a continuously positionally-controlled mechanosynthetic manufacturing environment.
Last updated on 13 August 2005