FEDERAL UNIVERSITY OF TECHNOLOGY, OWERRI PMB. 1526 USE OF ELECTRON MICROSCOPY IN MATERIALS AND METALLURGICAL ENGINEERING BY OFUA ELDRED ESEOGHENE 20124767258 BEING A TERM PAPER SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE COURSE ELECTRON OPTICS AND MICROSCOPY MME 604 SUBMITTED TO PROF. O.O. ONYEMAOBI
USE OF ELECTRON MICROSCOPY IN MATERIALS AND METALLURGICAL ENGINEERING
by Eldred Ofua
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FEDERAL UNIVERSITY OF TECHNOLOGY, OWERRI PMB. 1526USE OF ELECTRON MICROSCOPY IN MATERIALS AND METALLURGICAL ENGINEERING
BYOFUA ELDRED ESEOGHENE
20124767258BEING A TERM PAPER SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE COURSE ELECTRON OPTICS AND MICROSCOPY MME 604SUBMITTED TO PROF. O.O. ONYEMAOBI
ABSTRACT
During the 70’s, metal industry was vital for development and therefore strongly
influencing research activities. It was not surprising that R&D (research&
development) activities were primarily directed toward the development of new,
and improvement of existing metallic materials particularly those processed using
machining, metal forming and casting technologies. Until year 2000 materials of
the primary concern were iron base and aluminum alloys including high value
ferrite martensite steels, micro alloyed high strength steels for welding, cast irons
in as-cast and austempered condition and high strength aluminum alloys. Other
research activities include plasma-nitriding and physical vapor deposition (PVD)
surface technologies, fracture mechanic methods for examination of welded
joints and “quick stop” method for chip formation during machining which is rapid
enough to freeze the cutting action.
After year 2000 research activities have been expanded in the areas of structural
ceramic materials and advanced glasses respectively. Electron microscopy is a
powerful technique widely used in materials science research for morphology,
microstructure, composition and chemical states study. It has further been used
in support of most of the mentioned research activities for which the results have
been presented and published in international and domestic journals and
conferences.
HISTORY OF ELECTRON MICROSCOPE
The word microscope is derived from the Greek mikros (small) and skopeo (look
at). From the dawn of science, there has been an interest in being able to look at
smaller and smaller details of the world around us. Biologists have wanted to
examine the structure of cells, bacteria, viruses, and colloidal particles. Materials
scientists have wanted to see inhomogeneities and imperfections in metals,
crystals, and ceramics. In geology, the detailed study of rocks, minerals, and
fossils on a microscopic scale provides insight into the origins of our planet and its
valuable mineral resources.
Nobody knows for certain who invented the microscope. The light microscope
probably developed from the Galilean telescope during the 17th century. One of
the earliest instruments for seeing very small objects was made by the Dutchman
Antony van Leeuwenhoek (1632-1723) and consisted of a powerful convex lens
and an adjustable holder for the object being studied. With this remarkably
simple microscope, Van Leeuwenhoek may well have been able to magnify
objects up to 400x; and with it he discovered protozoa, spermatozoa, and
bacteria, and was able to classify red blood cells by shape. The limiting factor in
Van Leeuwenhoek’s microscope was the single convex lens. The problem could be
solved by the addition of another lens to magnify the image produced by the first
lens. This compound microscope – consisting of an objective lens and an eyepiece
together with a means of focusing, a mirror or a source of light and a specimen
table for holding and positioning the specimen – is the basis of light microscopes
today.
By the middle of the 19th century, microscopists had accepted that it was simply
not possible to resolve structures of less than half a micrometre with a light
microscope because of the Abbe’s formula, but the development of the cathode
ray tube was literally about to change the way they looked at things – by using
electrons instead of light! Hertz (1857-94) suggested that cathode rays were a
form of wave motion and Weichert, in 1899, found that these rays could be
concentrated into a small spot by the use of an axial magnetic field produced by a
long solenoid. But it was not until 1926 that Busch showed theoretically that a
short solenoid converges a beam of electrons in the same way that glass can
converge the light of the sun, that a direct comparison was made between light
and electron beams. Busch should probably therefore be known as the father of
electron optics.
In 1931 the German engineers Ernst Ruska and Maximillion Knoll succeeded in
magnifying an electron image. This was, in retrospect, the moment of the
invention of the electron microscope but the first prototype was actually built by
Ruska in 1933 and was capable of resolving to 50 nm. Although it was primitive
and not really fit for practical use, Ruska was recognised some 50 years later by
the award of a Nobel Prize. The first commercially available electron microscope
was built in England by Metropolitan Vickers for Imperial College, London, and
was called the EM1, though it never surpassed the resolution of a good optical
microscope. The early electron microscopes did not excite the optical
microscopists because the electron beam, which had a very high current density,
was concentrated into a very small area and was very hot and therefore charred
any non-metallic specimens that were examined. When it was found that you
could successfully examine biological specimens in the electron microscope after
treating them with osmium and cutting very thin slices of the sample, the electron
microscope began to appear as a viable proposition. At the University of Toronto,
in 1938, Eli Franklin Burton and students Cecil Hall, James Hillier and Albert
Prebus constructed the first electron microscope in the New World. This was an
effective, high-resolution instrument, the design of which eventually led to what
was to become known as the RCA (Radio Corporation of America) range of very
successful microscopes.
Unfortunately, the outbreak of the Second World War in 1939 held back their
further development somewhat. However, within 20 years of the end of the war
routine commercial electron microscopes were capable of 1 nm resolution.
THE ELECTRON MICROSCOPE
An electron microscope (EM) is a type of microscope that uses an electron beam
to illuminate a specimen and produce a magnified image.
An EM has greater resolving power than a light microscope and can reveal the
structure of smaller objects because electrons have wavelengths about 100,000
times shorter than visible light photons. They can achieve better than
50 pm resolution and magnifications of up to about 10,000,000x whereas
ordinary, non-confocal light microscopes are limited by diffraction to about
200 nm resolution and useful magnifications below 2000x.
The electron microscope uses electrostatic and electromagnetic lenses to control
the electron beam and focus it to form an image. These electron optical
lenses are analogous to the glass lenses of a light optical microscope.
TYPES OF ELECTRON MICROSCOPE
The original form of the electron microscope is the transmission electron
microscope. Other forms of the electron microscope are the scanning electron
microscope and the scanning transmission electron microscope.
The Transmission Electron Microscope
There are four main components to a transmission electron microscope: an
electron optical column, a vacuum system, the necessary electronics (lens
supplies for focusing and deflecting the beam and the high voltage generator for
the electron source), and control software. A modern TEM typically comprises an