© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

9.3.5.3.4 Thermal Decomposition

Since carbon burns readily in oxygen, simple incineration in a high-pressure oxygen-rich atmosphere seems an obvious approach to diamond decomposition. A sapphire inner wall coating provides a flameproof combustion chamber with a ~200,000 atm failure strength (Table 9.3), and an externally insulated electrodynamic vacuum suspension (Section 6.3.4.4) prevents rapid thermal conduction and allows maintenance of high operating temperatures. The combustion temperature of diamond in air at 1 atm is usually given as 870-1070 K,⁶⁹¹ and higher oxygen pressures should lower this range considerably. Evans¹²⁷⁷ has studied the noncombustive oxidation of diamond in pure oxygen atmospheres as a function of temperature and pressure. Using data from Evans' oxidation rate/pressure and Arrhenius oxidation/temperature plots, the etch rate (in meters/sec) for the {111} diamond crystal face (which oxidizes most readily) may be crudely approximated as:

{Eqn. 9.55}

{Eqn. 9.56}

where p_oxy is oxygen pressure (atm), T is diamond temperature (K), k₁ = 3.6 x 10⁶ m/sec-atm, k₂ = 2.68 x 10⁴ K, k₃ = 3 x 10^-5 m/sec-atm, and the time required to completely oxidize a diamond cube of edge L is t_decomp = L / v_etch. For p_oxy = 100 atm, a 100 nm diamond cube is consumed in 1 second at T ~ 750 K; a 1 nm cube heated to T ~ 660 K decomposes to oxide in 1 second, or in ~1 hour at 530 K. Although not modeled using high-pressure data, Eqn. 9.55 hints that a pure oxygen atmosphere at p_oxy ~ 1000 atm might support active combustion at an ignition temperature as low as ~700 K. Oxygen ions at 100-1000 eV can machine the diamond {100} face at a rate of ~0.01 (nm/sec) / (ion/sec-nm²) that saturates at a gas pressure >10^-7 atm.²⁷⁰⁸

Extremely high temperatures are required to initiate and to sustain pure graphitization -- the transformation of hard diamond into relatively soft (and more easily disposable) graphite by simple heating. In vacuo, unstressed polished hydrogenated diamond surfaces remain chemically and mechanically stable up to ~1275 K, at which temperature the passivating hydrogens are removed and crystallographic surface reconstruction begins.¹²⁹¹ Drexler¹⁰ estimates the characteristic thermal cleavage time for unstressed 556-zJ C-C bonds in diamond as ~10⁸⁵ sec at 300 K, ~10¹² sec at 700 K, and ~10⁴ sec at 1000 K. At 1300 K, a shear stress >~0.18 nN/bond initiates plastic flow,¹²⁹² but diamond remains chemically stable up to ~1800 K in an inert atmosphere. Heating above 1800 K can result in extensive graphitization,¹²⁹⁰ an autocatalytic process that spreads outward from nucleation centers. The experimental graphitization rate of the {111} diamond crystal face at zero pressure is approximated by Evans¹²⁷⁷ as:

{Eqn. 9.57}

where the activation energy E_a = 1760 zJ/atom, k = 0.01381 zJ/K (Boltzmann constant), T = temperature (K), and k₄ = 5.4 x 10¹⁶ m/sec. At 1800 K, graphitization is very slow, about 1 nm/day; v_graphite ~ 1 nm/sec at T = 2150 K, ~1 micron/sec at T = 2440 K, well above the 2070 K softening point and the 2310 K melting point of sapphire.¹⁶⁰² Catalytic graphitization by Ni and Fe has been observed at >~1070 K.¹⁵⁹⁶ But because of the high temperatures involved, thermal decomposition of diamond by graphitization is impractical in vivo.

Last updated on 21 February 2003