— At times it becomes necessary in metallurgical research to examine metal structures at very high magnifications.
— Unfortunately the highest magnification possible with an ordinary optical metallurgical microscope (Fig. 42.1) is in the region of X2000.
— For very high-power microscopy (i.e., between X2000 and X200,000) light rays are replaced by a beam of electrons and this way developed an electron microscope.
(i) Preparation of metal/alloy specimen. Most electron microscopy is carried out by using transmission-type instruments that produce images of either transparent replicas of the etched specimen or of very thin metal films obtained by various techniques. It is necessary for electron microscope specimen to be transparent to the electron beam.
Replicas are produced in plastic or some other suitable material, which reproduces faithfully the contours of the polished and etched specimen.
Thin foilsot the metals which work as (electron microscope) specimen are of 100-2000. A° thickness and are prepared by several available methods, one of which is Ion Bombardment technique and another is Electropolishing method.
(ii) Construction and Operation
— An electron microscope consists of an electron gun and condenser and projector lens. Vacuum is necessary to allow passage of the electron beam.
— Electrons emitted by a hot tungsten-filament cathode are accelerated, to form a high velocity beam, by the anode.
— Depending upon the density and thickness of the replica (specimen) at each point, some of the electrons are absorbed or scattered while the remainder pass through, i.e., transmit.
— The magnetic field of the objective lens focuses and enlarges the electron beam that has passed through the replica.
— Some of the electrons in this image are brought into a second focus on a fluorescent screen by the projector lens.
For detail refer to Techniques of Electron Microscopy by D. H. Kay, Blackwell Scientific Publications. 1965.