Porous graphite plates, cylinders and cones with densities of 1.55-1.82 g/cm3 were irradiated by a 10 kW fiber laser at 0.075 –3.525 kW/cm2 for 120 s to study mass removal and crater formation. Surface temperatures reached steady state values as high as 3767 K. The total decrease in sample mass ranged from 0.06 to 6.29 g, with crater volumes of 0.52 - 838 mm3, and penetration times for 12.7 mm thick plates as short as 38 s. Minor contaminants in the graphite samples produced calcium and iron oxide to be re-deposited on the graphite surface. Significantly increased porosity of the sample is observed even outside of the laser-irradiated region. Total mass removed increases with deposited laser energy at a rate of 4.83 g/MJ for medium extruded graphite with an apparent threshold of 0.15 MJ. Visible emission spectroscopy reveals C2 Swan and CN red, CN violet bands and Li, Na, and K 2P3/2,1/2 – 2S1/2 doublets. The reacting boundary layer is observed using a mid-wave imaging Fourier transform spectrometer (IFTS) at 2 cm-1 spectral resolution, 0.5 mm/pixel spatial resolution, and 0.75 Hz data cube rate. A two-layer radiative transfer model was used to determine plume temperature, CO, and CO2 concentrations from spectral signatures. The new understanding of graphite combustion and sublimation during laser irradiation is vital to the more complex behavior of carbon composites.
The various effects of laser heating of carbon materials are key to assessing laser weapon effectiveness. Porous graphite plates, cylinders, and cones with densities of 1.55 to 1.82 g/cm3 were irradiated by a 10-kW fiber laser at 0.075 to 3.525 kW/cm2 for 120 s to study mass removal and crater formation. Surface temperatures reached steady state values as high as 3767 K. The total decrease in sample mass ranged from 0.06 to 6.29 g, with crater volumes of 0.52 to 838 mm3, and penetration times for 12.7-mm-thick plates as short as 38 s. Minor contaminants in the graphite samples produced calcium and iron oxide to be redeposited on the graphite surface. Dramatic graphite crystalline structures are also produced at higher laser irradiances. Significantly increased porosity of the sample is observed even outside the laser-irradiated region. Total mass removed increases with deposited laser energy at a rate of 4.83 g/MJ for medium extruded graphite with an apparent threshold of 0.15 MJ. At ∼3.5 kW/cm2, the fractions of the mass removed from the cylindrical samples in the crater, surrounding trench, and outer region of decreased porosity are 38%, 47%, and 15%, respectively. Graphite is particularly resistant to damage by high power lasers. The new understanding of graphite combustion and sublimation during laser irradiation is vital to the more complex behavior of carbon composites.
Bidirectional reflectance distribution has been measured from an atomic-force-microscope (AFM)-patterned surface
for the first time. The AFM was used to generate a two-dimensional square array of sub-wavelength surface features
from a single material at a scale large enough to permit optical characterization. A diamond nano-indentation AFM
probe was used to produce a 325-μm by 200-μm array of indentations in a 120-nm-thick polystyrene film deposited on
silicon. Indentation spacing of 400 nm produced well-defined surface features with a maximum height of 140 nm. The
full size array was achieved by tiling together single arrays, limited in size by the AFM scanner range, through the use of
the AFM's translation stage. An SMS Complete Angle Scatter Instrument (CASI) was used to measure in-plane
bidirectional reflectance at incident angles ranging from 0 to 80 degrees. Because of the small array size, the CASI beam
was focused to approximately 140 μm and recalibrated using a 10-μm AFM calibration standard. Two wavelengths were
investigated, 633 and 544 nm, at both s and p incident polarizations. Negative-first-order diffraction peaks were
observed that were consistent with feature spacing. An anomalous scatter peak, believed to be associated with guidedmode
resonance of the structure, was also observed. This is the first demonstration of an AFM-patterned polymer
surface to behave as a 2D photonic crystal. The ability to construct and image arrays of optically active nano-features
has potential DoD applications in laser eye protection and anti-reflection coatings for high power laser optics.
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