methods of inspecting materials (autosaved)
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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
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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.
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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.
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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
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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
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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.
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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.
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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.