Chapter 13
Downloaded by UNIV MASSACHUSETTS AMHERST on August 10, 2012 | http://pubs.acs.org Publication Date: September 21, 2006 | doi: 10.1021/bk-2006-0943.ch013
Investigation of Nanoparticle Formation During Surface Decontamination and Characterization by Pulsed Laser 1
Meng-Dawn Cheng and Doh-Won Lee
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Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 Oak Ridge Institute for Science and Education, Oak Ridge, TN 37830
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The production of ultrafine and nanoparticles from a surface is dependent on the laser energy and laser wavelength used to treat and on the material used to construct the surface. Under dry conditions, the minimal laser fluence (mJ cm- ) required to produce a detectable amount of particles was found to be the greatest for a pure material, alumina, then for a complex mixture, concrete, with the least for a simple mixture, stainless steel, using both visible (532-nm) and U V (266-nm) laser wavelengths. The threshold energy requirement was found to be significantly higher when a shorter laser wavelength was used. The results indicate that for a given amount of laser energy used, there are more than twice the particles produced when a 532-nm wavelength is used than a 266-nm, although a 266-nm photon has 2 times more energy than a 532-nm. For both wavelengths, the total number concentration of produced particles is found to be linearly proportional to laser fluence. The correlation of the log-log linearity is excellent, indicated by a R value close to 1 for all materials. The models were derived, empirically, for predicting the amount of particles that could be removed from the surface of different materials using different lasers operated at low fluence conditions. 2
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© 2006 American Chemical Society In Nuclear Waste Management; Wang, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.
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Downloaded by UNIV MASSACHUSETTS AMHERST on August 10, 2012 | http://pubs.acs.org Publication Date: September 21, 2006 | doi: 10.1021/bk-2006-0943.ch013
Introduction Decontamination and decommissioning (D&D) of a large number of nuclear facilities is a major effort at the U.S. Department of Energy (DOE) complexes across the country. Use of laser plasma for surface decontamination/cleaning is a new and effective technique. A large quantity of very small particles is produced during the decontamination process. Effective production of particles is critical in determining the surface cleaning efficiency. However, the particles could contain surface contaminants like toxic heavy metals (e.g., Cr, Hg, Pb, and Ni), radionuclides (e.g., Th, Cs, and U), and organic solvents that all might cause health concerns. In this project sponsored by the DOE Environmental Management Science Program (EMSP), we investigated the relationships of nanoparticle formation by the laser decontamination process using laboratory-prepared target surfaces made from Portland cement (concrete), stainless steel 316, and pure alumina. The first two samples are commonly found in DOE installations, while the last sample is used for understanding of fundamental processes. The data were correlated among the particles, surface chemistry, and the laser characteristics. The experiments were conducted to determine the threshold energy needed to remove particles from the surface materials. This is important in understanding the cleaning efficiency and the photon-material interaction at the material surface. The objectives of this study are to determine the minimal laser energy required to decontaminant a surface, which is surface dependent, and to characterize the particles generated during the decontamination processes.
Material and Method Two targets were selected to represent the range of surfaces commonly found in DOE installations, and one pure substance was selected for contrasting the complex surfaces of the other targets listed. The two complex composition targets were Portland cement or concrete and Stainless Steel 316. The pure substance was alumina. The chemical compositions of the targets are listed in Table I. Shown in the table are the identified compounds and the relative weight proportions of the compounds. Concrete consists of several oxides and some trace elements in which 45% is CaO as the major oxide and Si0 16.7%. Portland cement specimens of about 5-cm or 2-in.-diameter circular blocks were prepared initially without aggregate, cured under moist conditions for a minimum of 30 days, and then air dried for a minimum of 1 week before use (7). Stainless steel consists of Fe, Cr, and Ni with weight percentages of 70, 19, and 11, respectively. Figure 1 shows the experimental setup used in this study. An airtight chamber was used to contain a rotating target disc 5-cm in diameter. The disc 2
In Nuclear Waste Management; Wang, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.
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Table I. Chemical Composition of Targets Used in This Study Alumina A1 0 Downloaded by UNIV MASSACHUSETTS AMHERST on August 10, 2012 | http://pubs.acs.org Publication Date: September 21, 2006 | doi: 10.1021/bk-2006-0943.ch013
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Weight % Stainless Steel Weight % 100 70 Fe Cr 19 Ni
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Concrete Ai 0 CaO 2
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Fe 0 K0 MgO Mn0 2
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Na 0 Si0 2
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SrO Ti0 Loss-on-Ignition (1400 °C) Sum of Oxides Minor Elements 2
Weight % 3.35 44.79 2.66 0.49 2.01 0.06 0.14 16.65 0.04 0.21 28.53 98.95
Ag As Ba
