Appearance of the supercritical state of carbon in the laser evaporation of low-density graphite foil

The role of crystalline defects and the density of a graphite foil in the laser-induced degradation

“…It has been shown in ref. 6 that the degradation of the lowdensity high-defect GF sample no. 5 proceeds via hydrodynamic removal of the substance (carbon in the supercritical state) from the laser-heated surface layer of the target.…”

“…Because of the high porosity of the structure, the GF absorbance A(629 nm) = 0.9 in the near-surface region (up to 100 nm in thickness at a density of 0.7 g cm -3 ) approaches this parameter for the ideal black body, and the velocities of propagation of acoustic and thermal waves in the material dramatically decrease. 3 In the region of radiation absorption, quasi-equilibrium evaporation into the pores of the foil (the average pore diameter is shorter than the free path length of vapour particles) occurs, and depending on the ratio between the material density r and the density of carbon in the critical state (r cr = 0.64 g cm -3 ), 5 the subcritical (r £ r cr ), critical, and supercritical (r > r cr ) 6 states of the substance can occur in the pores. Due to the presence of nonequilibrium defects, the GF structure is metastable; therefore, the heats of laser-induced phase transitions in GF can differ from similar characteristics for crystalline graphite.…”

Production of carbon nanoclusters supported on a graphite foil by laser ablation under supercritical conditions

“…[7][8][9][10] Low bulk density and high porosity (near 70%) of the sample resulted in the black body absorbance (0.9) of visible light by a subsurface layer (100 nm) of the foil, very slow propagation of ultrasonic waves (V us » 450 m s -1 ) 8 in the sample and low thermal diffusivity. Quasi-equilibrium laser evaporation of the subsurface layer of the sample occurred in pores with an average size of 10-20 nm (comparable to a free path length under conditions of intense evaporation) and was followed by subsequent formation of critical and supercritical states of carbon 9,10 due to a relatively high bulk density of the foil sample (r > r crit , where the critical carbon density r crit is equal to 0.64 g cm -3 ). 11 A considerable energy (about 70 kJ mol -1 ) released under laser irradiation of the graphite foil due to thermal annealing of non-equilibrium thermally and chemically induced defects (vacancies etc.)…”

“…in graphite crystallites decreased the laser power density required for the generation of a supercritical carbon phase to 0.006 GW cm -2 . 9,10 A sample of the graphite foil (size of 1×1 cm, thickness of 0.06 cm) used for preparing carbon nanoclusters was placed on a square linear translation stage, which can be moved in the horizontal plane with a step of 5 µm. The surface of the lowdensity sample was accurately covered with a transparent glass plate to avoid any interface cavities.…”