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Browsing by Author "Lehman, John H."

Now showing 1 - 4 of 4
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    Foam-based optical absorber for high-power laser radiometry
    Ramadurai, Krishna; Cromer, Christopher L.; Li, Xiaoyu; Mahajan, Roop L.; Lehman, John H. (Optical Society of America, 2007-12-01)
    We report damage threshold measurements of novel absorbers comprised of either liquid-cooled silicon carbide or vitreous carbon foams. The measurements demonstrate damage thresholds up to 1.6 x 104 W/cm(2) at an incident circular spot size of 2 mm with an absorbance of 96% at 1.064 mu m. We present a summary of the damage threshold as a function of the water flow velocity and the absorbance measurements. We also present a qualitative description of a damage mechanism based on a two-phase heat transfer between the foam and the flowing water. (c) 2007 Optical Society of America.
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    High-performance carbon nanotube coatings for high-power laser radiometry
    Ramadurai, Krishna; Cromer, Christopher L.; Lewis, Laurence A.; Hurst, Katherine E.; Dillon, Anne C.; Mahajan, Roop L.; Lehman, John H. (American Institute of Physics, 2008-01-01)
    Radiometry for the next generation of high-efficiency, high-power industrial lasers requires thermal management at optical power levels exceeding 10 kW. Laser damage and thermal transport present fundamental challenges for laser radiometry in support of common manufacturing processes, such as welding, cutting, ablation, or vaporization. To address this growing need for radiometry at extremely high power densities, we demonstrate multiwalled carbon nanotube (MWCNT) coatings with damage thresholds exceeding 15 000 W/cm(2) and absorption efficiencies over 90% at 1.06 mu m. This result demonstrates specific design advantages not possible with other contemporary high-power laser coatings. Furthermore, the results demonstrate a performance difference between MWCNTs and single-walled carbon nanotube coatings, which is attributed to the lower net thermal resistance of the MWCNT coatings. We explore the behavior of carbon nanotubes at two laser wavelengths (1.06 and 10.6 mu m) and also evaluate the optical-absorption efficiency and bulk properties of the coatings. (c) 2008 American Institute of Physics.
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    Laser-induced exfoliation of amorphous carbon layer on an individual multiwall carbon nanotube
    Singh, G.; Rice, P.; Hurst, K. E.; Lehman, John H.; Mahajan, R. L. (AIP Publishing, 2007-07)
    Pulsed laser treatment of an individual multiwall carbon nanotube induced selective exfoliation of the amorphous carbon contamination layer. The multiwall carbon nanotube (MWCNT) was exposed to a 248 nm excimer laser. After the treatment, transmission electron microscopy images show that the amorphous layer has expanded and separated from the crystalline MWCNT walls. This interesting observation has implications for laser cleaning and possible thinning of MWCNTs to reduce the radial dimensions. (C) 2007 American Institute of Physics.
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    Raman and electron microscopy analysis of carbon nanotubes exposed to high power laser irradiance
    Ramadurai, Krishna; Cromer, Christopher L.; Dillon, Anne C.; Mahajan, Roop L.; Lehman, John H. (American Institute of Physics, 2009-05-01)
    High power laser radiometry requires efficient and damage-resistant detectors. The current study explores the evolving nature of carbon nanotube coatings for such detectors upon their exposure to incrementally increasing laser power levels. Electron microscopy images along with the D-band to G-band intensity ratios from the Raman spectra from eight irradiance levels are used to evaluate changes before and after the exposure. Electron microscopy images of the exposed multiwalled carbon nanotubes revealed the formation of intermittent pockets of moundlike structures at high power densities exceeding 11 kW/cm(2). Raman spectroscopy measurements also demonstrated higher values for the ratio of the D-band intensity to that of the G-band, suggesting the possible transformation of nanotubes into structurally different forms of carbon. Exposure to a sample of single-walled nanotubes did not demonstrate the evolution of structural changes, which could be due in part to the higher irradiance levels relative to the damage threshold, employed in the experiment. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3116165]