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L05D: Observation of defects Many methods of observing defects in and on solids, with improvements and new techniques being developed all the time. What.

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Presentation on theme: "L05D: Observation of defects Many methods of observing defects in and on solids, with improvements and new techniques being developed all the time. What."— Presentation transcript:

1 L05D: Observation of defects Many methods of observing defects in and on solids, with improvements and new techniques being developed all the time. What technique is best depends on the size of the feature you want to see and the type of material. Surface features seen with the naked eye can reveal a lot, e.g.: −The symmetry of the crystal lattice may be manifested in the crystal shape. −The shape may reveal the presence of grain boundaries. −Very small features can be seen by looking at light reflected off the surface, such as grains down to ~ 1 mm and the emergence of screw dislocations. Roughening the surface mechanically or by etching can accentuate orientation differences in reflected light, as well as preferential etching of grain boundaries to outline them. For small grains might need an optical microscope with reflected light at different angles. Also known as incident light or metallurgical microscopy. No color. Last modified by W.R. Wilcox, Clarkson University, on October 20, 2013

2 2 Useful up to 1000X magnification. Depth of field decreases as mag increases Polishing removes surface features (e.g., scratches) Etching or sand blasting changes reflectance depending on grain orientation. Optical microscopy by reflected light (e.g. a metallurgical microscope with light from below) Micrograph of brass (a Cu-Zn alloy) 0.75mm crystallographic planes

3 Incident light microscopy Basic limitation is the wavelength of light, ~0.2  m = 200 nm But interference effects can be used to see much smaller step heights in incident light. For opaque engineering materials, the solid is first cut, polished mechanically, possibly etched, and then can be viewed by various methods: e.g., stainless steel :http://www.cartech.com/techarticles.aspx?id=1450 Brass : For some materials, the polarization of reflected light depends on orientation, so a polarizing microscope can be used to reveal grains and twins in color without a colorizing stain.

4 Transmitted light microscopy For transparent materials, shine the light up through the sample. Many variations in the optics are possible. The colors obtained in a polarizing microscope reflect both the grain orientation and thickness, e.g.:  Alkalic Syenite

5 5 Other methods of optical microscopy Polarized light –metallographic microscopes often use polarized light to increase contrast using reflected (incident) lighting. –Also used for transparent samples such as some ceramics and polymers using transmitted light. Can see strain. Dark-field –Light comes in only from an angle. Only the light reflected in the direction of the objective lens is seen. All else is dark. Ways to beat the resolution and depth of field limits imposed by the wavelength of light and/or the need to focus: –Interference microscopy (several types) –Scanning optical microscopy: –Confocal microscopy:

6 6 Other types of microscopy For higher resolution need shorter wavelength than light: −X-Rays? Difficult to focus, but x-ray topography useful for dislocations −Transmitted electrons –wavelengths about 3 pm (0.003 nm) depending on energy (Magnification up to ~1,000,000X) –Electron beam focused by magnetic lenses –Requires very thin flat samples for electrons to go through & focus –Can make carbon replicas of the surface of thick materials or particles

7 Transmission electron microscopy Very thin samples so that electrons can pass through them. High resolution TEM allows resolution of atoms. Can see individual dislocation lines, e.g. in Ti: Si EBSP (electron backscatter diffraction)

8 Scanning electron microscopy (SEM) A serious limitation of optical and electron microscopy is that the depth of field is on the order of the resolution. As you go up in magnification the depth that is in focus becomes smaller and smaller so only view planar surfaces. Modern electronics enables imaging of non-planar surfaces. In SEM a finely focused electron beam is raster scanned across the surface in synchronization with a display device such as a monitor. A detector is used to detect scattered electrons reflected by the sample, or secondary electrons or x-rays emitted by the sample. The detector signal is fed to the display device, thereby yielding an image of the surface. For example: ms-facilities/rcsms-sem fracture surfaces:

9 Scanning electron microscopy of surfaces −1 nm to 20 nm resolution. Several detection methods: Secondary electrons Back-scattered electrons Because focusing not required, sample need not be planar. To avoid charging of the sample, it must either be an electrical conductor or be coated with a thin conducting film, such as gold.

10 Energy-dispersive x-ray spectroscopy Detection of x-rays emitted in an SEM enables analysis of composition (acronyms EDS, EDX, XEDS). For example, Portland cement: Image is 256 µm x 200µm. C 3 S C 2 S C 3 A C 4 AF gypsum K 2 SO 4 lime and/or calcite silica periclase (with Mg) C=CaO, S=SiO 2, A=Al 2 O 3, F=Fe 2 O 3 all hydrated

11 Scanning probe microscopy https://en.wikipedia.org/wiki/Scanning_probe_microscopy https://en.wikipedia.org/wiki/Scanning_probe_microscopy Atomically sharp probe that is raster scanned along a surface in synchronization with a display. Many types of tips, which may tap the surface or be slightly above it. Can sense many types of atomic fields – electronic, magnetic, etc. Can see individual atoms and even move them around. Tend to be slow, cover a small area, and require a very planar surface. Tilt grain boundary in SrTiO 3 [001](210) by STM Vacancy on Ge (111) by STM

12 Atoms and molecules can be imaged and also moved on a surface! CO on platinum (111) Fe on copper (111). These Kanji characters represent the word “atom.” Scanning Tunneling Microscopy (STM or STEM) Many other sensing methods have been developed for surface scanning.  CO on copper

13 Dislocation etch pits With some etchants, pits are formed where dislocations emerge at a surface. For example, InP. Line is 0.1 mm. If the numbers are not large and not aligned in low-angle grain boundaries, the number of pits per unit area can be counted to give an etch pit density, which is often referred to as the dislocation density. It’s equivalent to the total lengths of all dislocations per unit volume. May be as high as /cm 2

14 Characterization of grain size ASTM International standard methods: M%20standards/ASTM%20E112-10%20Average%20Grain%20Size.pdfASTM International M%20standards/ASTM%20E112-10%20Average%20Grain%20Size.pdf ASTM grain size number, n = (log N/log 2) + 1 where N is the number of grains per square inch at 100X. Methods to measure N: –Planimetric: Count grains in given area –Intercept: Count the number of grain boundaries intersecting a line. –Above by hand or with image analysis software. –For equiaxed grains compare image at 100X with n N


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