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Methods of inspecting Materials

Microscopes

Prepared by : Karim Sami Mohamed Mahmoud

Supervisor: Professor Dr. Osama Mounir

Date : October 2012

Helwan University Faculty of Engineering class 2012-2013 Master

Degree preparation year


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Contents

1. Objective2. scope3. Introduction4. Types of microscopes:

4.1 Light microscopy

A. Single lens (simple) microscopeB. Compound microscope

4.2 X-ray diffraction analysis

4.3 Electron Microscopy

A. Transmission Electron MicroscopesB. Scanning Electron Microscopes
http://www.microscopemaster.com/transmission-electron-microscope.htmlhttp://www.microscopemaster.com/scanning-electron-microscope.htmlhttp://www.microscopemaster.com/scanning-electron-microscope.htmlhttp://www.microscopemaster.com/transmission-electron-microscope.html


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1.Objective

The objective of this report is to provide some information on the types of

microscopes used to investigate the structure of material. The types used

will be of the same order of magnitude of the structure to be studied.

Not all types of microscopes will be discussed but the most popular ones.

This report does not discuss in complex details of design of the types used

but gives a brief over view of the theory of operation.

2. Scope

This report will describe briefly few of the microscopes used in the field of

metallurgy engineering and will describe only the limitation of use or the

working range giving quick description of the theory ofoperation. The

report will discuss the light microscope, X-ray diffraction microscope, and

the electron microscope.

3. Introduction

As the microstructure of material is very important in determining the

properties of a material and studying the behavior of material during

processes and working , then it is required to find a method to enable

researcher and students to see and analyze the material structure in a clear

resolution and magnification .

The method to be used should be suitable to the order of magnitude of the

required view or display. It will be of no great use to use unsuitablemicroscope , not providing clear image of the required part. From here it is

essential to start by the size of the crystal structure, which depends on the

atom size.

The start will be determining the order of magnitude of the atom, the atom

size is determined by its radius.


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Definition :

The atomic radius of a chemical element is a measure of the size of itsatoms, usually the mean or typical distance from the nucleus to the

boundary of the surrounding cloud ofelectrons

The arrangement of the materials in the periodic table shows us the variation

in atomic size of element, see fig 1-1, it can be seen that as we move to the

left atomic radius increases, also as we move down the table the atomic

radius increases.

Fig 1-1 periodic table showing how atom size change

Fig 1-2 shows a schematic drawing showing the size variation among the

different element in the periodic table.
http://en.wikipedia.org/wiki/Atomic_radiushttp://en.wikipedia.org/wiki/Chemical_elementhttp://en.wikipedia.org/wiki/Atomic_nucleushttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Atomic_nucleushttp://en.wikipedia.org/wiki/Chemical_elementhttp://en.wikipedia.org/wiki/Atomic_radius


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Fig 1-2 atom size variation in periodic table

The atomic radius of some elements are shown in table 1-1

Table 1-1 atomic radius for some elements in picometre

Element Atomic radius (picometre)

Helium (He) 31 pm

Calcium (Ca) 197 pm

Silver (Ag) 144 pm

Sodium (Na) 190 pm

1 m = 1,000,000,000,000 picometre (pm)

1m = 1,000,000,000 nanometer (nm)

GrainsThe microstructure of metals and many other solids consists of grains. A

molten metal is poured into a sand mold and allowed to air cool slowly will

result in the production of coarse grains. Pouring a molten metal into a

metal mold with enhanced cooling produces finer grains. Introducing

forced circulation of water /oil in the metal mold produces even finer grain


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structures. Fig 1-3 schematic drawing showing atoms arranged in a crystal

lattice inside the grain boundaries

Fig 1-3 schematic drawing of grains

The order of magnitude of atoms in a single grain is in the order of 1018

atoms, which can indicate that the method used to examine the atomic

structure is different from that examining the grains .

Is it more important the resolution or the magnification?

The microscope, in its various forms, is the principal tool of the materials

scientist. The magnification of the image produced by an electron

microscope can be extremely high; however, on occasion, the modest

magnification produced by a light stereomicroscope can be sufficient to

solve a problem. In practical terms, the microscopist attaches more

importance to resolution than magnification that is, the ability of the

microscope to distinguish fine detail. In a given microscope, increasing the

magnification beyond a certain limit will fail to reveal further structural

detail; such magnification is said to be empty. Unaided, the human eye

has a resolution of about 0.1 mm: resolution of light microscopes and

electron microscopes are, respectively, about 200 and 0.5 nm. The

resolution is a function of wave length.


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4.Types of microscopes:

4.1 Light microscopy

The light microscope provides two-dimensional representation of structureover a total magnification range of roughly40 to1250

Examination will reveal structural features such as shrinkage or gas

porosity, cracks and inclusions of foreign matter

Fig 1-4 light microscope

There are two basic configurations of the conventional optical microscope:

the simple (single lens) and the compound (many lenses). The vast majority

of modern research microscopes are compound microscopes while some

cheaper commercial digital microscopes are simple single lens microscopes.

A magnifying glass is, in essence, a basic single lens microscope. In general,microscope optics are static; to focus at different focal depths the lens to

sample distance is adjusted, and to get a wider or narrower field of view a

different magnification objective lens must be used. Most modern research

microscopes also have a separate set of optics for illuminating the sample.
http://en.wikipedia.org/wiki/Researchhttp://en.wikipedia.org/wiki/Digital_microscopehttp://en.wikipedia.org/wiki/Magnifying_glasshttp://en.wikipedia.org/wiki/Magnifying_glasshttp://en.wikipedia.org/wiki/Digital_microscopehttp://en.wikipedia.org/wiki/Research


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A.Single lens (simple) microscope

A simple microscope is a microscope that uses only one lens for

magnification, and is the original design of light microscope. Van

Leeuwenhoek's microscopes consisted of a small, single converging lensmounted on a brass plate, with a screw mechanism to hold the sample or

specimen to be examined. Demonstrations by British microscopist have

images from such basic instruments. Though now considered primitive, the

use of a single, convex lens for viewing is still found in simple magnification

devices, such as the magnifying glass and the loupe.

B. Compound microscope

A compound microscope is a microscope which uses multiple lenses to

collect light from the sample and then a separate set of lenses to focus the

light into the eye or camera. Compound microscopes are heavier, larger

and more expensive than simple microscopes due to the increased number

of lenses used in construction. The main advantages of multiple lenses are

improved numerical aperture, reduced chromatic aberration and

exchangeable objective lenses to adjust the magnification. A compound

microscope also makes more advanced illumination setups, such as phase

contrast

Fig 1-5 Optical path in a typical microscope
http://en.wikipedia.org/wiki/Antonie_van_Leeuwenhoekhttp://en.wikipedia.org/wiki/Antonie_van_Leeuwenhoekhttp://en.wikipedia.org/wiki/Lens_(optics)#Types_of_simple_lenseshttp://www.brianjford.com/wavrbcs.htmhttp://en.wikipedia.org/wiki/Magnifying_glasshttp://en.wikipedia.org/wiki/Loupehttp://en.wikipedia.org/wiki/Phase_contrasthttp://en.wikipedia.org/wiki/Phase_contrasthttp://en.wikipedia.org/wiki/Phase_contrasthttp://en.wikipedia.org/w/index.php?title=File:Microscope-optical_path.svg&page=1http://en.wikipedia.org/wiki/Phase_contrasthttp://en.wikipedia.org/wiki/Phase_contrasthttp://en.wikipedia.org/wiki/Loupehttp://en.wikipedia.org/wiki/Magnifying_glasshttp://www.brianjford.com/wavrbcs.htmhttp://en.wikipedia.org/wiki/Lens_(optics)#Types_of_simple_lenseshttp://en.wikipedia.org/wiki/Antonie_van_Leeuwenhoekhttp://en.wikipedia.org/wiki/Antonie_van_Leeuwenhoek


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4.2 X-ray diffraction analysis

X-ray crystallography is a method of determining the arrangement ofatoms

within a crystal, in which a beam ofX-rays strikes a crystal and causes the

beam of light to spread into many specific directions. From the angles and

intensities of these diffracted beams, a crystallographer can produce a

three-dimensional picture of the density of electrons within the crystal.

From this electron density, the mean positions of the atoms in the crystal

can be determined, as well as their chemical bonds, their disorder and

various other information.

X-ray crystallography has been fundamental in the development of many

scientific fields. This method determined the size of atoms, the lengths and

types of chemical bonds, and the atomic-scale differences among various

materials, especially minerals and alloys.

The use of diffraction methods is of great importance in the analysis of

crystalline solids. Not only can they reveal the main features of the

structure, i.e. the lattice parameter and type of structure, but also other

details such as the arrangement of different kinds of atoms in crystals, thepresence of imperfections, the orientation, sub-grain and grain size, the

size and density of precipitates. X-rays are a form of electromagnetic

radiation differing from light waves (=400800 nm) in that they have a

shorter wavelength (0.1 nm). These rays are produced when a metal

target is bombarded with fast electrons in a vacuum tube. The radiation

emitted can be separated into two components, a continuous spectrum

which is spread over a wide range of wavelengths and a superimposed line

spectrum characteristic of the metal being bombarded. The energy of the

white radiation, as the continuous spectrum is called, increases as the

atomic number of the target and approximately as the square of the

applied voltage, while the characteristic radiation is excited only when a

certain critical voltage is exceeded. The characteristic radiation is produced
http://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Crystalhttp://en.wikipedia.org/wiki/X-rayhttp://en.wikipedia.org/wiki/Crystallographyhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Entropyhttp://en.wikipedia.org/wiki/Alloyhttp://en.wikipedia.org/wiki/Alloyhttp://en.wikipedia.org/wiki/Entropyhttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Crystallographyhttp://en.wikipedia.org/wiki/X-rayhttp://en.wikipedia.org/wiki/Crystalhttp://en.wikipedia.org/wiki/Atom


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when the accelerated electrons have sufficient energy to eject one of the

inner electrons (1s-level, for example) from its shell. The vacant 1s-level is

then occupied by one of the other electrons from a higher energy level, and

during the transition an emission of X-radiation takes place.

Fig 1-6 Workflow for solving the structure of a molecule by X-ray crystallography.

4.3 Electron Microscopy

An EM is a microscope that focuses beams of energetic electrons toexamine objects up to nano-scales.

They utilize the same principles behind an optical microscope, but rather

than photons or particles of light, concentrate electrons, charged particles

located on the outside of atoms, onto an object.

Additional differences include preparation of specimens before being

placed in the vacuum chamber, the use of coiled electromagnets instead of

glass lenses, the use of a thermionic gun as an electron source and the

image or electron micrograph is viewed on a screen rather than an

eyepiece.

All EMs use electromagnetic and/or electrostatic lenses, which consist of a

coil of wire wrapped around the outside of a tube, commonly referred to as

a solenoid.


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In addition, EMs use digital displays, computer interfaces, software for

image analysis and a low vacuum or variable pressure chamber, which

upholds the pressure differential between the high vacuum levels essential

to the gun and column area and the low pressure required in the chamber.

In this microscope, images are produced from the interaction between the

prepared samples in the vacuum chamber and energetic electrons.

The electron beam passes through one or more solenoids and, with the aid

of the thermionic electron gun, is directed down the column and onto the

sample.

Equivalent to the magnification that occurs from light refraction in an

optical microscope, the coils in an EM bend the electron beams to create an

image.

The following gives you a description of two types of EMs,the Transmission

(TEM) and Scanning Electron Microscope(SEM).

A.Transmission Electron Microscopes

The transmission electron microscope (TEM), the first type of EM, has many

commonalities with the optical microscope and is a powerful microscope,

capable of producing images 1 nanometer in size.

They require high voltages to increase the acceleration speed of electrons,

which, once they pass through the sample (transmission), increase the

image resolution.

The 2-d, black and white images produced by TEMs can be seen on a screen

or printed onto a photographic plate.

TEM technique : a beam ofelectrons is transmitted through an ultra thin

specimen, interacting with the specimen as it passes through. An image is

formed from the interaction of the electrons transmitted through the

specimen; the image is magnified and focused onto an imaging device, such

as a fluorescent screen, on a layer ofphotographic film, or to be detected

by a sensor such as a CCD camera.
http://www.microscopemaster.com/transmission-electron-microscope.htmlhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Focus_(optics)http://en.wikipedia.org/wiki/Fluorescenthttp://en.wikipedia.org/wiki/Photographic_filmhttp://en.wikipedia.org/wiki/CCD_camerahttp://en.wikipedia.org/wiki/CCD_camerahttp://en.wikipedia.org/wiki/Photographic_filmhttp://en.wikipedia.org/wiki/Fluorescenthttp://en.wikipedia.org/wiki/Focus_(optics)http://en.wikipedia.org/wiki/Electronhttp://www.microscopemaster.com/transmission-electron-microscope.html


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TEMs are capable of imaging at a significantly higher resolution than light

microscopes, owing to the small de Broglie wavelength of electrons. This

enables the instrument's user to examine fine detaileven as small as a

single column of atoms, which is tens of thousands times smaller than the

smallest resolvable object in a light microscope. TEM forms a major analysismethod in a range of scientific fields, in both physical and biological

sciences. TEMs find application in cancer research, virology, materials

science as well as pollution, nanotechnology, and semiconductor research.

Fig 1-7 Cross sectional diagram of an electron gun assembly, illustrating electron extraction

B. Scanning Electron Microscopes

Reflecting light microscopes are the optical counterpart to scanning

electron microscopes (SEM) and produce similar data.

SEMs are primarily used to obtain topographical information.

In this type of EM, a series of solenoids pulls the beam back and forthacross the sample, systematically scanning the surface; it detects secondary

electrons emitted from the surface and produces an image.

Although SEMs are approximately 10 times less powerful than TEMs, they

produce high-resolution, sharp, black and white 3D images.
http://en.wikipedia.org/wiki/Optical_resolutionhttp://en.wikipedia.org/wiki/De_Broglie_wavelengthhttp://en.wikipedia.org/wiki/Cancer_researchhttp://en.wikipedia.org/wiki/Virologyhttp://en.wikipedia.org/wiki/Materials_sciencehttp://en.wikipedia.org/wiki/Materials_sciencehttp://en.wikipedia.org/wiki/Pollutionhttp://en.wikipedia.org/wiki/Nanotechnologyhttp://en.wikipedia.org/wiki/Semiconductorhttp://www.microscopemaster.com/scanning-electron-microscope.htmlhttp://en.wikipedia.org/w/index.php?title=File:Electron-gun.svg&page=1http://www.microscopemaster.com/scanning-electron-microscope.htmlhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Nanotechnologyhttp://en.wikipedia.org/wiki/Pollutionhttp://en.wikipedia.org/wiki/Materials_sciencehttp://en.wikipedia.org/wiki/Materials_sciencehttp://en.wikipedia.org/wiki/Virologyhttp://en.wikipedia.org/wiki/Cancer_researchhttp://en.wikipedia.org/wiki/De_Broglie_wavelengthhttp://en.wikipedia.org/wiki/Optical_resolution


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The primary advantage of Electron Microscopy is its powerful

magnification.

SEM begins with an electron gun generating a beam of energetic electrons

down the column and onto a series of electromagnetic lenses. These lensesare tubes, wrapped in coil and referred to as solenoids. The coils are

adjusted to focus the incident electron beam onto the sample; these

adjustments cause fluctuations in the voltage, increasing/decreasing the

speed in which the electrons come in contact with the specimen surface.

Controlled via computer, the SEM operator can adjust the beam to control

magnification as well as determine the surface area to be scanned.

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