http://SaturnianCosmology.Org/ mirrored file For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== 28th ICPIG, July 15-20, 2007, Prague, Czech Republic Evidence for Nanoparticles in Microwave-Generated Fireballs by Synchrotron X-Ray Scattering J.B.A. Mitchell1, J.L. LeGarrec1, M. Sztucki2, T. Narayanan2, V. Dikhtyar3, E. Jerby3* 1P.A.L.M.S., U.M.R. No. 6627 du C.N.R.S., Université de Rennes I, 35042 Rennes cedex, France 2European Synchrotron Radiation Facility, BP-220, 38043 Grenoble Cedex, France 3Dept. of Engineering, University of Tel-Aviv, Ramat-Aviv, 69978, Israel * e-mail: jerby@eng.tau.ac.il The small-angle X-ray scattering (SAXS) method has been applied to study fireballs ejected from molten hotspots in borosilicate glass by localized microwaves [Dikhtyar & Jerby, PRL 96 045002 (2006)]. The fireball's particle size distribution, density, and decay rate in atmospheric pressure were measured. The results show that the fireballs contain particles with diameters of ~10 nm in clusters of ~64 nm with particles' average densities of ~109 cm-3. Hence, fireballs can be considered as a dusty plasma which consists of an ensemble of charged nanoparticles in the plasma volume. This finding is likened to the ball-lightning phenomenon explained by the formation of an oxidising particles' networks liberated by lightning striking to the ground [Abrahamson and Dinniss, Nature 403, 519 (2000)] 1. Introduction The ball-lightning enigma has attracted scientists for more than a century. This rare natural phenomenon has been observed accidentally by eyewitnesses, mostly during electrical storms, as glowing luminescence objects floating in the air and lasting for up to several seconds [1-3]. Kapitza produced fireballs by high-power microwaves [4] suggesting accordingly an external-energy mechanism for ball-lightning objects in nature. Ohtsuki and Ofuruton et al. obtained fireballs using microwaves in an air-filled cavity, and demonstrated fireball motions similar to those observed in nature [5, 6]. Abrahamson and Dinniss [7] proposed a mechanism for self-sustaining ball-lightning in which a lightning strike to the ground ejects a plume of silicon nanoparticles whose oxidation generates the heat that sustains the reaction. The oxide layer on the particles' surfaces slows the oxidation rate in the interior owing to the higher melting point of the oxide, thus yielding the delayed time structure of the phenomenon (thus the ball lighting seems to be a hybrid effect between plasma and combustion phenomenon). Dikhtyar and Jerby [8] have demonstrated experimentally the ejection of fireballs from molten hotspots induced by electrode-localized microwaves in solid substrates. These fireballs float in the air, hence demonstrating the buoyancy feature of ball lightning. These fireballs can be sustained for minutes in the microwave field. After the microwave supply is turned off, the fireball lasts for another ~30 ms (in accordance with Boichenko's ball-lightning lifetime model [9] and Kapitza's estimates [4]). Recently, Paiva et al. [10] have demonstrated a similar electrode mechanism operated at 50 Hz to ignite ball-lightning-like objects from silicon. The lifetime of these luminous objects exceeded 8 seconds but they did not demonstrate the ball lightning buoyancy feature. The experimental ability presented recently enables us to reproduce ball-lightning-like effects from solid hotspots in a controlled fashion in the laboratory, and it allows the experimental characterization of these luminous objects in order to study their mechanism of formation and other features. A key factor in the ball lightning modelling, theoretically and experimentally, is the size distribution of the particles involved. The Abrahamson and Dinniss' theory [7] predicts, for instance, a significant content of nanoparticles. Therefore, one may consider the ball lightning as a form of dusty plasma [11] subjected to an inner oxidation process. In this paper, we report results obtained using the small-angle X-ray scattering (SAXS) method for measuring the size distribution of particles contained in fireballs generated from molten hotspots [8]. The application of this method to dilute systems in extreme environments has been demonstrated before in measurements of particle size distributions diffusion flames [12-15] (the density of particles in the fireball is considered to be similar to or higher than that of soot particles in a hydrocarbon flame and so the contrast in X-ray scattering intensity signals is measurable similarly). The results obtained may elucidate the relevance of these fireballs [8] to the natural ball-lightning phenomenon. 2. Experimental The microwave part of the experimental setup is similar to that described in Ref. [8] and is illustrated schematically in Fig. 1. It consists of a microwave cavity fed by a 0.6-kW magnetron. The microwave energy is concentrated by the movable electrode in the substrate material (e.g. borosilicate glass) to form in it a molten hot-spot, from which the fireball evolves. A mirror made of vanes under cutoff conditions allows a direct optical view into the cavity for visual observation and video recording. The fireball is formed when the electrode made of tungsten or copper is brought into contact with the dielectric substrate within the waveguide. An initial hotspot is created by the microwave-drill mechanism [16] in a thermal-runaway process [17]. As the hotspot becomes molten, the electrode is lifted up and the molten drop is detached from the substrate and evaporated, and blown up to form a confined glowing fireball floating in air [18]. Figure 1: The experiment setup of the microwave cavity in which the fireball is created [8] adapted for the X-ray beam line scattering experiment. Figure 2 shows an optical image of a fireball created from a substrate of borosilicate glass. The fireball may last for several minutes, as long as it is irradiated by the microwave power. Reference [19] presents recent experimental results including optical spectroscopy and microwave measurements of these fireballs. The microwave fireball apparatus was installed in Beamline ID09 at the ESRF. The X-ray beam crossed the fireball through metallic tubes (under microwave cutoff) in the cavity walls, as shown in Fig. 1. By illuminating the fireball with 12 keV x rays, we performed X-ray scattering experiments using the SAXS pin-hole camera detector [20]. The fireballs in these experiments were ignited by a copper electrode from borosilicate glass substrates. Figure 2: A fireball generated in the setup shown in Figure 1. The nipple at the bottom of the fireball demonstrates its origin from a drop of liquid detached from a solid substrate. 3. Results and Discussion Figure 3 shows the detected scattered X-ray intensity versus the parameter q, the magnitude of the momentum transfer or scattering vector, given by [21] ( ) ( ) q 4 sin 2 = (1) where and are the wavelength of the incident X-ray radiation and the scattering angle, respectively. The scattering results can be quite well fitted using a Debye-Beuche function with a fractal structure factor [15]. The good fit to the Debye Bueche function means that the particle size distribution in the fireball is very polydisperse and that it consists of particles with a solid surface. The addition of the fractal structure factor improves the fit and from this we can say that the scattering objects consist of small particles with diameters of about 10 nm that are aggregated into larger units with diameters of about 64 nm. This is similar to what we have found for soot particles in diffusion flames [22], though the fireball's particle sizes are larger. We cannot say whether the aggregates that we see are themselves sub-units of larger aggregate structures (as is the case for mature soot particles) since our technique is not sensitive to objects larger than about 150 nm. With glass as the initial substrate, we can assume that the particles are predominantly silicon dioxide. X-ray spatial detector View through cutoff vanes Microwave input port Rectangular cavity Electrode Hotspot in solid Fireball Synchrotron X-ray beam ~5 cm Figure 3: X-ray Intensity vs. the parameter q given in Eq. (1) for a typical fireball in our experiment. Once the microwaves are cut off, the ball disappears. The X-ray data shows that the particle density falls to background levels after about 2 seconds (note that the visible fireball's lifetime was measured as ~30-ms [8]). This period is shorter than expected from the Abrahamson and Dinniss model [7] and from the observations of Paiva et al. [10], and it is shorter also from the typical neutral particle Brownian dispersion rates. Our results agree, however, with Boichenko's model for ball-lightning lifetimes [9] and with Kapitza's estimates [4]. Given the intense light emission seen during the fireball, and in view of our previous analyses [8, 19], it is reasonable to suppose that the particles' temperature is in the order of ~103 K, and therefore they emit electrons due to thermionic emission (as do soot particles in a flame). The free electrons will also be interacting with the microwaves, and thus these luminous objects containing ~64-nm macroparticle networks could be considered as dusty plasma. Similar experiments could be conducted also with other substrate materials, such as germanium and alumina, for the sake of comparison with silicon. The experimental results may contribute to the comprehension of ball-lightning enigma that has fired the imagination of scientists for generations. Acknowledgements The authors acknowledge the European Synchrotron Radiation Facility (ESRF) for provision of synchrotron radiation facilities and financial assistance and Grant no. 1270-04 of the Israeli Academy of Science References [1] B. M. Smirnov, Phys. Rep. 224, 151 (1993). [2] J. Abrahamson (Ed.), Phil. Trans. R. Soc. Lon. A 360 (2002); and references therein. [3] V. A. Rakov, M. A. Uman, Lightning: Physics and Effects, Cambridge Univ. Press (2003). [4] P. L. Kapitza, Sov. Phys. JETP 30, 973 (1970); also in P. L. Kapitza, Dokl. Acad. Nauk. USSR 101, 245 (1955). [5] Y. H. Ohtsuki, H. Ofuruton, Nature 350, 139 (1991). [6] H. Ofuruton, N. Kondo, M. Kamogawa, M. Aoki, Y. H. Ohtsuki, Geophys. Res. J. 106, 12367 (2001). [7] J. Abrahamson, J. Dinniss, Nature 403, 519 (2000). [8] V. Dikhtyar, E. Jerby, Phys. Rev. Lett. 96, 045002 (2006). [9] A. M. Boichenko, Phys. Wave Phenom. 13, 104 (2005). [10] G. S. Paiva, A. C. Pavao, E. A. de Vasconcelos, O. Mendes Jr., E. F. da Silva Jr., Phys. Rev. Lett. 98, 048501 (2007). [11] V. N. Tsytovich, G. E. Morfill, H. Thomas, Plasma Phys. Reports 30, 816 (2004). [12] J. P. Hessler, S. Seifert, R. E. Winans, Proc. Combustion Institute, 29, 2743 (2002) [13] G. Beaucage, H. K. Kammler, R. Mueller, R. Strobel, S.E. Pratsinis, T. Narayanan, Nature Materials 3, 370 (2004) [14] S. di Stasio, J. B. Mitchell, J. L. LeGarrec, L. Biennier, M. Wulff, Carbon 44, 1267 (2006) [15] J. B. A. Mitchell, A. I. Florescu-Mitchell, S. di Stasio, J. Courbe, T. Weiss, J. Appl. Phys. 100, 124918 (2006). [16] E. Jerby, V. Dikhtyar, O. Actushev, U. Grosglick, Science 298, 587 (2002). [17] E. Jerby, O. Aktushev, V. Dikhtyar, J. Appl. Phys. 97, 034909 (2005). [18] www.eng.tau.ac.il/~jerby/Fireballs.html. [19] E. Jerby, V. Dikhtyar, Microwave Discharges: Fundamentals and Applications, 227 232, Y. A. Lebedev (Ed.), Yanus-K, Moscow 2006, [20] http://www.esrf.fr/UsersAndScience/Experi ments/SCMatter/ID02 [21] A. Guinier, X-Ray Diffraction in Crystals, Imperfect Crystals and Amorphous Bodies, Dover Publications Inc., New York, 1994. [22] J. B. A. Mitchell, S. di Stasio, J.L. LeGarrec, A.I. Florescu-Mitchell, T. Narayan, M. Sztucki, Submitted to J. Appl. Phys.