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Engineering Materials and ProcessesLecture 10 – Practical Microscopy

Silicon atoms

Metallographic Sample Preparation: (Higgins Ch 10)

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Following closely with Higgins Ch 10

Selecting and mounting a specimen: (Higgins Ch 10.2)

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Grain orientation is important.

Take care not to heat while cutting.

Higgins

Selecting and mounting a specimen: (Higgins Ch 10.2)

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Specimens cast into thermoset in order to hold them easily and in the right orientation.

http://www.lambdaphoto.co.uk

Grinding and polishing the specimen: (Higgins Ch 10.3)

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Successively finer grades until specimen is finally polished with diamond paste.

The most important points to be observed during the grinding andpolishing processes are:• Absolute cleanliness is necessary at each stage.• Use very light pressure during both grinding and polishing.

Grinding and Polishing Machine for Metallographic Sample Preparationhttp://www.youtube.com/watch?v=yYiXfYwX1Ls

Etching the specimen: (Higgins Ch 10.4)

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When the specimen is clean and free from grease, it is etched by plunging it into the etching solution (acid), and agitating it vigorously for a few seconds. The specimen is then very quickly transferred to running water.

The highly polished surface will now appear dull. Grains may be visible if they are large (steel specimens need a microscope).

Different etchants are used for different metals.

The metallurgical microscope: (Higgins Ch 10.5)

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Metallurgical microscope is lit on the front of the specimen, rather than through the back of a thin slice like a biological microscope.

Optical microscope can only go so far – at x1000 magnification the resolution is down to the wavelength of light, which is the theoretical limit.

Well before the x1000 limit, the definition depends on the objective lense (nearest the specimen). So several hundred magnification is more likely the maximum.

Read Higgins:10.5.1 Using the microscope10.5.2 The care of the microscope

Photomicrographs:

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Images from a metallurgical microscope.(refer website)

Photographs though a microscope are called MetallographicsMetallurgical photosMicrographic images

watlas.mt.umist.ac.uk/internetmicroscope/micrographs/microstructures.html

Revision From Lecture 1Nanometer scale

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FDA

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Online Scale of the Universehttp://scaleofuniverse.com/Interactive flash animation from 10-35 m to 1027 m. (But more manageably; picameters to terameters)

50 nanometer diameter optical fibre transmits light while wrapped around a human hair.

Optical microscope image: Limin Tong/Harvard University

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Optical microscope limited to a resolution of about 200nm.

The electron microscope: (Higgins Ch 10.6)

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The electron microscope can magnify x2000 and up to about x200 000.

To see atoms, the magnification would need to be about twenty million times: x20 000 000 – a hundred times higher than the limit of the SEM.

When did scientists first start to see atoms?"We are crossing that threshold where we can really see atoms clearly for the first time ever” This was in 2004.

Human hair is about 50µm or 0.05 mm.

This nanowire (a fibre-optic glass wire) is about 50 nanometers, or 0.05 µm or 0.00005 mm.

SEM image: LiminTong/Harvard University

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That last image was done on a Scanning Electron Microscope, which is more powerful than optical microscope.

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Wikipedia

The electron microscope: (Higgins Ch 10.6)

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When did scientists first start to see atoms? "We are crossing that threshold where we can really see atoms clearly for the first time ever” This was in 2004.

We can’t really “see” them, but we can visualize or “feel” them. But we are just starting to get pictures of molecules and atomic lattices.

Here, an STM image of silicon atoms arranged on a face of a crystal. STM: http://www.sljus.lu.se/stm/NonTech.html

The STM microscope:

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Scanning Tunneling Microscope STM: http://www.sljus.lu.se/stm/NonTech.html

Using electronic feedback system to keep the current (and hence the gap) constant as the tip moves sideways across the surface. Because the current detection is so sensitive the tip actually has to ride up over the atoms of the surface in exactly the same way that a record player's stylus tracks the groove of an LP. By recording the tip's vertical position at points on a grid we can make a 3D map of the surface. A computer then takes this map and turns it into a picture.

The microscope is rather like an extremely accurate record player. It works by holding a very fine needle or tip approximately a billionth of a meter (1 nanometer) from the sample's surface. When the tip is this close, electrons can jump the gap between it and the sample. This 'tunnel current' can be amplified and used to measure the size of the gap with tremendous accuracy.

(SEM) A: an ant 'looking' on a microchip (but it’s dead), B: cantilevers on a microchip touch a carbon nanotube substrate. C: closeup of carbon nanotubes electron beam deposition welded to the microscantilevers; (TEM) D: multiwalled carbon nanotube actually suspended between two microcantilevers. E: closeup of shell structure of the carbon nanotube.

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Opensource Handbook of Nanoscience and Nanotechnology

Carbon Nanostructures.Nanotechnology is usually 10 to 100 nanometers.

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Wikipedia

PolymorphismAllotropes of Carbona) Diamondb) Graphitec) Lonsdaleited) C60 (Buckminsterfullerene)e) C540 (see Fullerene)f) C70 (see Fullerene)g) Amorphous carbonh) single-walled nanotube

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Graphical comparison of materials properties.

Wikipedia: Materials properties

Online Properties Resources.

DoITPoMS: Dissemination of IT for the Promotion of Materials Science

watlas.mt.umist.ac.uk/internetmicroscope/micrographs/microstructures.html

Scales of material structure

GLOSSARY

MetallographicMetallurgicalMicrographicMicrographic specimenEtchingOptical MicroscopeScanning Electron Microscope (SEM)MagnificationObjective LenseScanning Tunnelling Microscope (STM)Nanometer

QUESTIONS

1. Define the glossary terms2. Explain how atoms can be “seen” using STM.3. What causes the theoretical limit of the magnification of an optical

microscope?4. Describe some of the limitations when working with a SEM.5. Why is it important not to overheat a metallic specimen when cutting it

in preparation for metallographic imaging?6. Why is cleanliness important in polishing metallurgical specimens for

microscopy?