MSE306Chap2-Diffraction

MSE306Chap2-Diffraction - Structural and Chemical...

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Unformatted text preview: Structural and Chemical Characterization in Materials Science Diffraction Techniques based upon... X-rays Electrons TEM and SEM Neutrons wont discuss, but can be useful because of high penetration depths and because sensitive to magnetic structure. Diffraction occurs when the structural grating has a periodicity close to the wavelength of an impinging wave. E.g. visible light (400-800 nm) can be diffracted through gratings (1 - 2 m) Outline Electron source Electron Sample Interaction Including X-ray generation Diffraction X-ray Diffraction Techniques TEM SEM The Electron Source The filament has historically been a sharply bent W wire (a hairpin) The reduced cross-section at the tip creates a region of higher resistance (higher T) By applying a current the filament is heated to extremely high T. Some electrons will receive sufficient thermal energy to overcome the work function of the W/vacuum interface e- leave the surface (Thermionic emission) A bias (of 10-1000 kV) between the filament and a target material draws the electrons to an anode. Electron Interactions When an electron beam strikes a sample, a large number of signals are generated. We can divide the signals into two broad categories: electron signals photon signals Example of a typical Interaction Volume for Sample is predominately atomic number 28 Accelerating Voltage is 20 kV Incident beam is normal to specimen surface noting the approximate maximum sampling depths for the various interactions. Specimen Interaction Volume Types of Electron Scattering There are two broad categories to describe electron scattering: Elastic direction of electron is changed (up to 180), but velocity (energy) remains essentially same. Types of Electron Scattering Inelastic energy of the electron is changed (reduced). The excess energy is released as a continuous spectrum of X-rays called braking radiation or bremsstrahlung X-rays Part of the electromagnetic radiation spectrum Ionizing radiation high energy (danger) Characteristic X-rays and Auger Electrons N N E x-ray Auger Weve already discussed how an electron collision can create an x-ray that is characteristic of the sample. This same process also produces Auger electrons Generally, in the SEM we are only concerned with the characteristic x-ray signal. Characteristic X-rays The same sequence of events which produces a characteristic x-ray for use in a diffractometer applies to a sample in the SEM. In this case instead of the electrons from a tube striking a target of a single element, electrons from the SEM strike samples that may be of any composition....
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This note was uploaded on 11/24/2009 for the course MSE 306 taught by Professor Smith during the Spring '09 term at UVA.

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MSE306Chap2-Diffraction - Structural and Chemical...

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