Fire Protection Laboratory Methods Day

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Presentation transcript:

Fire Protection Laboratory Methods Day June 25, 2014 Optical Diagnostics: Transmission Electron Microscopy & Laser Extinction Paul M. Anderson Graduate Research Assistant University of Maryland Department of Fire Protection Engineering

Transmission Electron Microscopy Basic Principles Beam of electrons thermionically ejected from electron gun Beam travels through series of apertures and magnetc lenses

Transmission Electron Microscopy Basic Principles Beam of electrons thermionically ejected from electron gun Lanthanum Hexaboride LaB6 crystal tip of a thermionic electron gun

Transmission Electron Microscopy Basic Principles Beam travels down evacuated column (10-4 Pa) through aperture and electro-magnetic lens before reaching specimen Lanthanum Hexaboride Electromagnetic lens schematic

Transmission Electron Microscopy Basic Principles The electron beam interacts with the specimen held on a 3.05 mm TEM grid. Lanthanum Hexaboride Typical TEM grids with copper wire mesh and carbon substrate

Transmission Electron Microscopy Basic Principles Electron beam is transmitted through the specimen, travels through additional lenses & apertures and imaged onto a fluorescent screen or CCD camera Lanthanum Hexaboride

Transmission Electron Microscopy Lanthanum Hexaboride Soot at 25,000 magnification

Transmission Electron Microscopy Lanthanum Hexaboride Soot at 500,000 magnification

Transmission Electron Microscopy Key Concepts Beam must transmit - requires very thin specimen (about 100-150nm for a 200kV TEM) UMD Nanocenter TEM can achieve good resolution up to magnification of about 800,000x Atomic resolution achievable with High Res or Field Emission TEM Specimen composition can be analyzed concurrently if TEM is equipped with x-ray spectrometer (available in UMD Nanocenter) Lanthanum Hexaboride

Transmission Electron Microscopy Key Concepts Diffraction pattern due to Bragg scattering of electrons with specimen can be imaged. Gives info about crystalline structure of sample Different structures of a specimen affect the diffraction pattern. Lanthanum Hexaboride

Transmission Electron Microscopy Aerosol sample collection methods (Koylu et al., 1997) Thermophoretic probe Thermophoretic precipitator Lanthanum Hexaboride

Transmission Electron Microscopy Additional information: P.M. Anderson, The Transmission Electron Microscopy Lab Companion, (2012). D. B. Williams and C. B. Carter, Transmission Electron Microscopy: A Textbook for Materials Science, New York: Springer Science & Business Media, (2009). R.A. Dobbins and C.M. Megaridis, C.M., Morphology of flame- generated soot as determined by themophoretic sampling. Langmuir, 2 (1987), 254-259. A.D. Maynard, The Development of a New Thermophoretic Precipitator for Scanning Transmission Electron Microscope Analysis of Ultrafine Aerosol Particles. Aerosol Science and Technology, 23 (1995), 521-533. Lanthanum Hexaboride

Laser Extinction Theory Aerosols attenuate incident light intensity. Beer’s Law: For particles much smaller than wavelength of incident light, Rayleigh scattering holds. For spherical particles this is: E(m) = Refractive index absorption fuction fs = aerosol volume fraction For soot, E(m)=0.26 for a refractive index of m = 1.57 – 0.56 i I0 I L Incidence Transmittance

Laser Extinction Theory Laser extinction is a line-of-sight method. We must integrate over the path length: Aerosol volume fraction can be found from: A is the deconvolution operator (Abel Transform), which is needed in the radially axisymmetric case. Laser is a good choice owing to monochromaticity

Laser Extinction Experimental Line Filter Flame Parabolic Mirror Lens Decollimator Neutral Density Filter Planar Mirror Vibrator Diffusers Pinhole 7 mW He-Ne Laser at 632.8 nm Camera Lens Camera

Laser Extinction Experimental Diffuser and vibrator Laser is highly coherent and results in interference patterns (i.e., laser fringes). Diffuser is used to reduce the laser coherency. Vibrator is used to obtain temporal averaging. Laser background with fringes (contrast enhanced)

Wavelength and Spatial Filters Laser line filter: 632.8 nm bandpass filter 1 nm FWHM Removes most flame irradiance Spatial filter (objective + pinhole)