Comparison of OM,TEM and SEM
Light Microscope
Magnetic
lenses
detector
CRT
Light source Source of
electrons
Condenser
Specimen
Objective
Eyepiece Projector Specimen
Image Viewed
Directly
Image on
Fluorescent Screen
Scanning
Electron Microscope
Transmission
Electron Microscope
(Cathode Ray
Tube)
• The transmission electron microscope (TEM) was the first
type of Electron Microscope to be developed and is patterned
exactly on the light transmission microscope except that a
focused beam of electrons is used instead of light to "see
through" the specimen. It was developed by Max Knoll and
Ernst Ruska in Germany in 1931.
• The first scanning electron microscope (SEM) debuted in
1938 ( Von Ardenne) with the first commercial instruments
around 1965. Its late development was due to the electronics
involved in "scanning" the beam of electrons across the
sample.
Dates
Topography
The surface features of an object or "how it looks", its texture;
direct relation between these features and materials properties
Morphology
The shape and size of the particles making up the object; direct
relation between these structures and materials properties
Composition
The elements and compounds that the object is composed of
and the relative amounts of them; direct relationship between
composition and materials properties
Crystallographic Information
How the atoms are arranged in the object; direct relation
between these arrangements and material properties
1.1 Characteristic Information: SEM
e.g. Identification of Fracture Mode
SEM micrographs of fractured surface of two BaTiO3
samples.
Cracks Cracks
Pores
Grain
boundary
4µm 20µm
OM vs. SEM
2D 3D 5 µm
How Fine can You See?
• Can you see a sugar cube? The thickness of a sewing
needle? The thickness of a piece of paper? …
• The resolution of human eyes is of the order of 0.1 mm,
100µm ? 4 mils.
• However, something vital to human beings are of sizes
smaller than 0.1mm, e.g. our cells, bacteria,
microstructural details of materials, etc.
Microstructural Features which
Concern Us
• Grain size: from <µm to the cm regime
• Grain shapes
• Precipitate size: mostly in the µm regime
• Volume fractions and distributions of various
phases
• Defects such as cracks and voids: <µm to the cm
regime
Scale and Microscopy Techniques
Microstructure ranging from crystal structure to engine components
(Si3N4)
XRD,TEM SEM OM
Grain I
Grain II
atomic
Valve
Turbo
charge
Crystal Structure Microstructure Component
Structure Determination
Fracture Mechanics
Applied Research Basic Research
Advantages of Using SEM over OM
Mag Depth of Field Resolution
OM: 4x – 1400x 0.5mm ~ 0.2mm
SEM: 10x – 500Kx 30mm 1.5nm
The SEM has a large depth of field, which allows a large
amount of the sample to be in focus at one time and
produces an image that is a good representation of the
three-dimensional sample.
The combination of higher magnification, larger depth of
field, greater resolution, compositional and
crystallographic information makes the SEM one of the
most heavily used instruments in academic/national lab
research areas and industry.
2.1 Electron-Solid Interactions
When an electron beam strikes a sample, a large